1 Broadband ISP Overview

Internet popularity is creating demand for ever-faster access, driving innovation and exerting downward pressure on price. Connection between end user and ISP is often called the last-mile. This implies there is a magical entity out there called “the Internet” and customers are passive consumers of Internet goodness. I prefer the term first-mile. It better denotes Internet value being the result each person’s connection as both contributor and consumer. Today most citizens of industrialized countries have access to some form of high-speed access and more is coming on line every day. Broadband is increasing being seen as a utility without which citizens are unable to fully participate in society.

Broadband is a much abused and inexact term. United States Federal Communication Commission (FCC) recently redefined broadband. Basic broadband is now defined as speed of 768 - 1500 kbps toward customer (downstream). Previously it was 200 kbps. Until a few years ago high-speed access was only available to large companies and government, individuals were limited to dialup. Dialup modems represent a tremendous technical achievement squeezing out every last bit of performance from public switched telephone network (PSTN). But due to the way telephone calls are digitized dialup is limited to less then 64 kbps.

Most of us utilize an Internet Service Provider (ISP). ISP owns, leases or otherwise has access to a data path to each customer. The picture at left provides a high level overview of how ISP connects customers to the Internet. Diagram is somewhat dated in that 1.544 Mbps T1 links would be something faster today.

Connecting to an ISP does not have much value if the only people you can communicate with have to be connected to the same ISP. To provide worldwide connectivity ISPs connect to other ISPs at Peering points. This allows traffic destined for hosts not local to the ISP be delivered anywhere in the world.

Each host on the Internet requires a unique address. ISPs typically provide residential customers with a single IP address. Large customers may obtain their address block directly from Internet Corporation for Assigned Names and Numbers (ICANN). Current version of Internet protocol is IPv4. IPv4 defines a 32-bit address space yielding about 4-billion possible addresses. That was a large number back when the Internet was limited to educational and government institutions but has become a critical problem today. As a result IPv4 addresses are in very short supply. Next generation Internet protocol IPv6 increases address space to 128-bits, a truly humongous number. Unfortunately conversion to IPv6 represents a wholesale upgrade to the Internet, a slow and difficult process

ISPs exert a great deal of control over how customer is able to use the Internet. Much is made of Internet robustness and redundancy. That is certainly true of the Internet in general but for most of us our ISP controls the Internet on ramp and acts as gatekeeper. In most locations broadband competition is nonexistent or extremely limited. ISP business policy has significant impact on how customers use the Internet and how new services are deployed.

There are several essential functions that must be provided by the ISP, as they are the only entity capable of doing so. There are many services often associated with ISPs that can be provided by anyone. The distinction between essential and non-essential functions is important when discussing Network Neutrality. As broadband access becomes more pervasive ISPs need to balance business considerations with public interest.

Essential Core Services

· First-Mile Access

· Customer Authentication

· IP Address Allocation

· Packet Routing

· Peering

· Multicast (IGMP)

· Quality of Service

· Service Level Agreement

· Acceptable Use Policy

· Customer Service

· Billing

Non-Essential Services

· Virtual Private Network

· Name Resolution (DNS)

· E-mail

· Usenet

· Web Hosting

· Voice over IP

· Fixed Mobile Convergence

· IP Radio

· IP TV

1.1 Essential Core Functions

For most of us Internet access means signing up with an ISP. The ISP provides either a routed or bridged customer connection. Residential accounts are typically bridged; the user connects to the ISP as if they were part of the ISP LAN. VLAN techniques are used to prevent users from seeing each other’s traffic. ISP business model originated during the heyday of dialup and has been adapted to broadband world.

1.1.1 First-Mile Access

Some ISPs own the First-Mile access network; Cable Companies and FTTP are examples of this type of ISP. Some ISPs rent physical access. This is typical with DSL where copper circuit is rented from Incumbent Local Exchange Carrier (ILEC). Dialup ISPs use Public Switched Telephone Network PSTN to connect customers. Wireless ISP does not provide a physical connection at all other then obtaining FCC radio license to use the airwaves.

Customer interface requirements differ greatly depending on type of service and whether or not ISP provides access network interface unit. For example Cable, DSL and FTTP ISPs typically provide customer with standard’s based modem with either Ethernet or USB as customer interface. T-1 is a tariffed telecommunication service. In the US FCC requires customer interface as two pair copper circuit typically implemented by a smart-jack. Dialup ISPs require customer obtain a V.90/92 or ISDN modem. Wireless ISPs typically supply and install customer antenna and radio.

1.1.2 Customer Authentication

ISPs need a mechanism to insure only authorized customers have access to service. For some types of service link between customer and ISP is hardwired so any traffic appearing on the link is assumed to originate from customer. This is common with T1, SDSL and FTTP. Shared media such as Cable Broadband and wireless need a way to identify customer. DOCSIS modems include digital signatures to prevent unauthorized access. ADSL ISPs typically use Point-to-Point Protocol over Ethernet (PPPoE) to authenticate customers. Telco’s like PPPoE because it facilitates support for third-party ISPs. Dial up ISPs typically utilize Point-to-Point Protocol (PPP) to authenticate customers using same RADIUS servers as PPPoE.

1.1.3 IP Address Allocation

Current Internet version is IPv4. A salient feature of IPv4 is its 32-bit address space. 32-bit binary address can support about 4 billion individual hosts. While that seems like a large number limited addressing potential of IPv4 is a concern. Explosive growth of all forms of Internet access over the last few year’s means IPv4 address space will be completely exhausted in the 2011-2012 timeframe. Broadband popularity is contributing to address exhaustion. It is an always-on connection and each customer requires at least one IP address.

IP addresses serve multiple functions. They denote a specific Internet host; each host needs at least one IP address. IP addresses also facilitate routing because they are allocated in blocks. If IP addresses were issued randomly each router would need to potentially look through four billion addresses to determine how to handle each packet. By aggregating addresses into large blocks router only need look at a few high order address bits to know how to forward packets.

Business accounts are typically assigned one or more static addresses. Static allocation is preferred for commercial accounts. With a static IP address customer settings are configured manually, based on information provided off-line by the ISP. This eliminates possibility of address change interfering with remote access.

Most residential ISP accounts use some form of dynamic IP address allocation. This is convenient because it eliminates need for non-technical customers to manually configure IP address, subnet mask, gateway address and DNS server address. Because dynamically assigned address may change at any time it makes it difficult to operate servers. Due to critical IPv4 address shortage residential ISP customers are typically only assigned a single IP address.

1.1.4 Packet Routing

The term Internet is a contraction of the term Inter network. Internet is literally a network of networks. Routers are used to forward packets between networks. Devices on the local network know if host they are trying to reach is local or not. To access a remote host packet is forwarded to a gateway router attached to the local area network LAN. Router in turn uses knowledge of connection topology to make intelligent decision about how to forward each packet. This process is repeated multiple times until packet finally reaches its ultimate destination. Routers learn connection topology by exchanging routing information.

Customer’s connection to ISP may be either Routed or Bridged. Bridged connections are extremely popular for residential accounts. Routed service is popular for commercial accounts assigned a block of IP addresses. Routed connections are more flexible and are essential if customer has multiple Internet connections either for backup or performance reasons.

In a bridged connection customers are literally part of the ISP local network. ISP prevents accidental file and print sharing between customers by blocking direct connection between customers.

With routed connection customer is assigned one or more IP addresses. Edge router, within ISP network, communicates with customer’s edge router and forward packets destined for customer network to it. Customer’s edge router forwards packets to ISP.

1.1.5 Peering

ISP would not be very valuable if a customer could only communicate with other users directly connected to it. The early Internet consisted of a few nodes interconnected by point-to-point links rented from the Bell System. As the Internet grew it became apparent there was a need for high-speed data network to interconnect high usage nodes. Several companies began to specialize as Interexchange carriers (IXC). IXC span continents and oceans providing the Internet backbone network. Peering points allow IXCs to exchange traffic among each other and accept user traffic. Large companies, ISPs and governments connect directly to peering points. Smaller ISPs purchase bandwidth from third party wholesale suppliers. The end result is regardless how one connects it is almost always possible to communicate with anyone else on the Internet.

1.1.6 Multicast (IGMP)

Internet is a powerful communication medium. A computer is able to connect to another anywhere in the world virtually instantly. As powerful as this type of communication is it is not well served to broadcasting, delivering one program to many subscribers. Traditional broadcast business model grew out of the technical limitation of radio. Once a station is built owner acquires programs and broadcasting begins. Everyone within receiving range hears the same program.

The one-to-one connection model used by the Internet makes it difficult to cost effectively broadcast programs since each listener requires a separate connection and session. Internet Group Management Protocol (IGMP) creates the infrastructure to deliver a single stream to multiple users. At each branch a decision is made whether or not to forward the stream. If an active listener is downstream packets are forwarded, if not they are dropped. This conserves channel capacity by suppressing streams no one is listening to. IGMP dramatically reduces server load since only a single copy of the material needs to be transmitted. Internet broadcasting is still in its infancy and IGMP is not commonly implemented by ISPs. For multicast to function each router between sender and receiver needs to support IGMP.

1.1.7 Quality of Service (QoS)

Internet was designed as an egalitarian best effort network. This works amazing well for transferring large chunks of data from point A to point B. The network continues to operate in the presence of all sorts of impairments and failures. However: best effort does not work as well with latency critical applications such as telephony and streaming media when dealing with congestion. For example during a Voice over IP (VoIP) phone call round trip latency should be under 150ms. Excessive delay makes carrying on a conversation difficult and when extreme virtually impossible. On the other hand if a print job is delayed a little no one is likely to notice as long as it completes successfully.

When switch or router encounters congestion it buffers incoming packets until it is able to forward them. Quality of Service (QoS) metrics allows latency critical packets go receive priority service. This simple strategy works well if latency critical traffic is a small percent of total. QoS marks packets with a (Diffserv) priority level. If congestion occurs higher value packets are delivered as quickly as possible. Lower value packets are delayed during congestion or discarded during periods of extreme congestion. QoS service allows more graceful degradation by moving high priority packets to the head of the queue.

As discussed in a later section traffic shaping and preferential packet treatment is controversial. Network Neutrality proponents are concerned ISPs will strike business deals with partners to preferential deliver their data at the expense of competitors. It is important to remember Quality of Service mechanisms do not provide additional channel capacity. They simply redefine winners and losers. When channel capacity does not meet “offered load” (an old telecom term) some policy must be in place to deal with congestion. The PSTN managed congestion by withholding dial tone or returning an “all trunks busy” message when call could not be completed. The Internet handles it by delaying packets. QoS controls which packets get delayed. Many argue deploying additional capacity is more cost effective then implementing a complex differential service mechanism.

To be maximally effective QoS requires end-to-end deployment. Technical and business problems facing QoS is much the same as IGMP. There is little value until “everyone” deploys it and little incentive to be an early adopter. ISP and all intermediate nodes need to monitor packet privilege level and treat them accordingly. Controls at each level need to monitor statistics to prevent “tragedy of the commons.” If too many packets ask for priority handling they all suffer.

Most residential broadband service is asymmetric; download is much faster then upload. There is benefit in shaping upload traffic so higher priority traffic is treated preferentially at the edge of the customer’s network. Customer’s edge router examines outbound packets and prioritizes them. Many residential routers already do this to a limited extent giving TCP/IP ACKs preferential treatment. Similar treatment may be applied to VoIP or critical gaming packets.

1.1.8 Service Level Agreement (SLA)

The main difference between residential and business accounts is Service Level Agreement (SLA). SLA defines things like: minimum speed, maximum latency, service reliability and mean time to repair. SLA guarantees are one of the reasons business class service is so expensive. Residential accounts are best effort. If connection fails or experiences congestion ISP is under no obligation to correct problem on an expedited basis. SLA imposes performance guarantees ISP must meet and penalties if they do not.

1.1.9 Acceptable Use Policy (AUP)

Acceptable use policy (AUP) defines customer responsibility, how service may be used and penalty for misuse. For example, residential customers are typically prohibited from reselling access or running servers and ISP blocks certain types of traffic. In an attempt to reduce cost some residential ISPs impose usage caps to limit monthly download and upload. Most ISP’s reserve the right to revise AUP at any time making for a pretty one-sided contract. If ISP does not provide required core services will have to look elsewhere.

1.1.10 Customer Service

Regardless of how good service is on occasion will be necessary to contact technical support to resolve problems. Tech support responsiveness dramatically affects overall customer satisfaction.

Due to hyper competitive nature of residential broadband most providers offer only limited help in troubleshooting problems. Finger pointing can be frustrating when a customer is trying to resolve a complex interaction and ISP does not consider it their responsibility. Specialized web sites such as Broadband Reports can be an effective alternative. Broadband Reports is a good example of an Internet community, members post questions and assist each other.

1.1.11 Billing

ISPs would not stay in business long if they could not bill for service. Most ISPs offer flat rate billing based on speed. Some ISPs, typically Cable and Wireless, set monthly quota that if exceeded results in extra (sometimes substantial) bandwidth charge or reduced connection speed. During the Dotcom era many dialup ISPs offered advertising supported free access. Most of these companies are long gone.

There is no comparable notion of telephone long distance in the Internet world. It does not cost any more to send a message across the street as around the world.

1.2 Non Essential Functions

This section examines services often provided by ISPs but that can be provided by third parties and in some cases even customer. This distinction is important in the Network Neutrality debate. If an ISP decides to offer a non-standard or value-add service and customer or third party is able to supply similar service result is dramatically different then if ISP implements proprietary core services.

1.2.1 Virtual Private Networking (VPN)

Virtual Private Network (VPN) uses public Internet to create private communication paths. Depending on how it is implemented it may be a feature that only the ISP is able to deliver or something customer or third-party is able to engineer.

Large companies make extensive use of MPLS VPN to implement geographically dispersed corporate LAN. To users, regardless of location, resources appear to be on the LAN. Service provider configures edge routers such that data presented to it is delivered to the correct physical location. ISP protects each company’s traffic so in is invisible to other companies.

More MPLS/VPN details in this Network World article.

It is also possible for customers to create their own VPN IPsec. In this case customer, rather then service provider, creates a secure end-to-end path through the public Internet. IPsec is used extensively to support satellite offices and telecommuters.

1.2.2 DNS

The Domain Name System (DNS) provides translation of Uniform Resource Locator (URL) to IP addresses. Without DNS web sites would have to be accessed by IP address. By specification if DNS is unable to resolve a domain name it returns an error message. Some ISPs have attempted to monetize entry of incorrect URLs by returning advertising supported web page if URL cannot be resolved.

DNS redirection is controversial. Some customers may find redirection useful, other not. For those who do not it may be possible to disable this feature or run their own DNS Resolver. For those who want to run their own DNS Resolver TreeWalk is popular

1.2.3 E- mail

As with DNS just about all ISPs provide email. It is wise to consider ISP e-mail account a throwaway. If you change ISP or the ISP is sold email address changes making it difficult for folks to stay in touch. We are currently going through an email change with sale of Verizon’s VT/NH/ME territory to FairPoint Communication. For a more permanent address use one of the free e-mail services such as Yahoo or Gmail or better yet register a domain name.

1.2.4 Usenet

Usenet Newsgroups are a valuable source of up to date information. Most ISPs include Usenet access. Due to declining interest in Usenet and legal attacks related to pornography many ISPs are taking easy way out and eliminating or scaling back support of Usenet. Usenet access is also available from a number of specialized companies. Newsadmin has a nice comparison list of newsgroup providers.

1.2.5 Web Hosting

Many ISPs provide web site hosting for residential customers. This allows customers to have an Internet presence without need to register a domain name or run their own server. ISP runs virtual server enabling many web sites to run on a single computer. ISP web hosting is a boon to residential customers by providing a painless way to create web presence. As with email and Usenet there are many hosting alternatives unconnected with specific ISP.

1.2.6 Voice over IP telephony (VoIP)

Public switched telephone network (PSTN) represents a hundred years of engineering. Recently packet based telephony has become a serious contender. Rather then traditional circuit switching Voice over IP (VoIP) uses packet-based communication to deliver two-way real time voice. Voice communication is very demanding. Voice data rate is low by Internet standards only 8-64 kbps in each direction. However latency is critical. If packets are delayed more then a few hundred milliseconds voice quality is seriously degraded.

As with any new technology many players have entered the market. Most will fail but a few will succeed. If your ISP offers VoIP check out the service thoroughly. The asymmetric nature of most residential service, upload being much lower then download, makes it easy to saturate the connection. Some form of Quality of Service (QoS) may be required to mark VoIP packets, as high priority so they get preferential treatment.

In the US FCC mandated telephone number portability. In most cases you will be able to transfer your existing phone number to new phone service.

1.2.7 Fixed/Mobile Cellular Convergence

There is tremendous interest in multimode cellular phones able to utilize both traditional cellular network and opportunistically, WiFi networks. Fixed Mobile Convergence (FMC) represents a win-win situation for both customer and wireless provider. For providers it utilizes the vast potential of the Internet and private LANs to remove traffic from expensive cellular radio networks. For customer it represents potentially lower cost and better performance. For business it represents a way to eliminate traditional PBX wired telephone infrastructure without paying extravagant per minute charges. Depending on legal restrictions it may also offer arbitrage possibility for multinational corporations to treat voice much like email, bypassing local phone companies and eliminating per minute charges.

An alternative to WiFi is femtocells being offered by several Cellular phone companies. Femtocells are low power cellular base stations that utilize customer’s broadband connection to deliver coverage to a single home. As with WiFi Cellular providers like it because it moves traffic off cell stations.

At this stage it is unclear as to ISP’s role in FMC. Theoretically any packet-based network can be used to transport voice. However, Cellular providers may be reluctant to have just anyone connect directly to their network. They may want to limit access to a few chosen providers. Only time will tell.

1.2.8 IP Radio

ISPs do not appear interested in becoming content aggregators for radio the way they are for TV. But other then much lower bandwidth Internet radio is not all that different then Internet TV. Radio-Locator is a convenient way to find over the air and Internet radio.

1.2.9 IP Television (IPTV)

Over-the-air (OTA), Cable and DBS TV all use basically the same transmission scheme. Individual programs are assigned a channel within allocated RF spectrum. US channels are 6 MHz wide, in Europe 8 MHZ. Channels were initially specified to carry a single analog TV program. Migration to digital TV enables multiple programs over a single RF channel due to increased transmission efficiency.

IPTV represents a fundamentally different way to deliver TV leveraging packet-based technology. IPTV opens the door to demand based programming. Instead of changing channels (VoD) is more like going to the library. Standard Definition TV (SDTV) program requires 2-3 Mbps, High Definition (HDTV) about 15 Mbps. These rates are result of data compression that process images both spectrally (within the picture) and temporally (over time). Raw data is too high to be delivered economically.

IPTV places strain on the network. Residential usage model is one of bursty traffic such as loading web sites or email. Streaming TV and to a lesser extent streaming radio lock up significant bandwidth for extended periods of time.

ISPs are interested in so-called triple-play services: data, telephone, and TV as a way to increase revenue per customer. IPTV opens the door to third-party as well as the ISP. IPTV dramatizes the disruptive nature of the Internet. Since end of WWII Cable companies wired cities to deliver broadcast TV over coax and more recently fiber. Cable network was intimately bound to TV delivery. As residential broadband speed increases the door opens for new providers to bundle content and deliver it without need to either build or own the means of delivery.

1.3 Connection Sharing

Historically when a customer contracted with an ISP they were given a block of IP addresses large enough to take care of their needs. IPv4 address shortage forced ISPs to rethink how they allocate scarce addresses. Most residential broadband ISPs restrict customer to a single IP address. This creates a quandary; how to cost effectively connect multiple hosts to the Internet? The most common workaround is Network Address Translation (NAT) coupled with private IP addresses. IETF RFC 1918 reserved three blocks of IP addresses guaranteed not used on the Internet. Because these addresses are not used on the public Internet they can be reused multiple times.

Combining NAT, more properly Network Address Port Translation since both address and port number are modified, and private addresses allow an unlimited number of computers to share an Internet connection even though ISP only provides a single IP address. NAT provides translation between private addresses on LAN and single public address issued by ISP on WAN.

LAN traffic is not affected by NAT. When a request cannot be serviced locally it is passed to NAT router, called a gateway. Router modifies packet by replacing private address with public address issued by ISP and if needed modifies port number to support multiple sessions and calculates new checksum. Router sends modified packet to remote host as-if-it-originated-from-the-router. When router receives reply modifications are reversed and packet is forwarded to LAN. Router tracks individual sessions so multiple computers are able to share a single address. As far as Internet hosts are concerned entire LAN looks like a single computer.

1.4 Blocked Ports

Internet was designed as a transparent end-to-end bit delivery network. This means any host on the Internet is able to communicate with any other host. TCP/IP and UDP/IP use ports to allow host support multiple simultaneous sessions. Ports are 16-bit unsigned values yielding up to 65,535 ports for each connection type. When a service is defined a port number is selected for initial contact. This is called the “well known port.” For example the well know port for HTTP Web access is 80. When a remote user attempts to connect it sends the request to TCP port 80. Once initial connection is established both computers agree to a different combination of ports to use for ongoing communication. An analogy is to think of well-known port as a doorbell. If access to well-known port is blocked remote users are unable to connect. The doorbell has been disconnected.

It is common practice for residential ISPs to block port 80 to prevent customers from running web sites, port 25 for mail servers to prevent spam, and ports 137, 138, 139, and 445 to prevent remote access to Windows LAN based file sharing. In an effort to reduce file trading some ISPs throttle or block ports used for peer-to-peer (P2P) file trading applications. Impact of blocked ports varies. For example: I have a registered domain. I use outgoing TCP port 25 to communicate with my SMTP mail server to send email. If my ISP blocks outgoing port 25 I will not be able to send outgoing mail to my SMTP mail server.

To get around blocked port it is easy to reconfigure server to use a non-standard port. If access is limited to known group it is easy enough to simply inform everyone which port to use. If goal is wider public access use of nonstandard ports is a problem. Without knowing port number remote user is unable to connect. URL forwarding is a technique to work around this restriction.

1.5 Servers and Dynamic IP Allocation

Most residential accounts are configured automatically each time customer connects. Dynamically assigned IP address makes it difficult to run a server because address can change at any time preventing remote users from connecting until they learn new address. Dynamic DNS service provides a workaround to run servers on dynamic accounts. A daemon runs on either router or server to detect addresses changes and notify dynamic DNS service. Even with automatic DNS update there is still a period of time after the address changes where server is not accessible and active sessions are aborted. Dynamic DNS services are really only suitable for casual personal servers, not business use.

1.6 Latency vs Speed

In the quest for ever-faster speed it is important not to loose sight of the interplay between speed and latency. As an example a truck carrying DVDs exhibits high speed (bits per second) once it arrives but also high latency because it takes hours or days for the data to arrive. Latency is defined as time it takes a packet to go from source to destination. Factors affecting latency are: connection speed, modem overhead, distance, speed of propagation and network congestion.

Modems operate on “chunks” of data increasing latency because entire block must be processed before being passed to next stage. Data cannot be used until the last bit in the bock is received. DSL modems often use a technique called interleave to reduce sensitivity to transient noise. This is effective in maximizing robustness by reducing effect of errors but adds latency because it operates over larger data block. Low speed connections such as dialup often used smaller packet size to minimize this effect.

Light travels about 186,000 miles per second in vacuum. Optical fiber is somewhat slower about 70% of light in vacuum. A packet traveling the 3,000 from New York to LA takes about 25 ms in each direction. To this one must add delay at each router between source and destination. Normally this delay is negligible but if network becomes congested router must temporally store incoming packets until outgoing path is free. In extreme cases router will discard packets. When that occurs upper level protocols either request retransmission (TCP/IP) or in the case of streaming data (UDP/IP) fake missing data.

Impact of latency is heavily dependant on type of traffic. Interactive use such as gaming and Voice over IP (VoIP) telephony place stringent demands on latency but do not require much bandwidth. File transfer on the other had is relatively insensitive to latency but places great importance on bandwidth.

Typical first-hop latency: T1 or FTTP 1ms, Cable/DSL 10-30ms, Dialup 100 ms, Satellite 500ms. For a more in-depth explanation see “It’s the Latency Stupid.”

1.7 Asymmetric Speed

Most residential broadband service is asymmetric: download is much faster then upload. This is done for technical and business reasons. The business goal is to position residential broadband as primarily consumption based and discourage customer from running servers. Asymmetric speed allows ISP to position residential service differently then business and charge higher fee for business class service.

Low upload speed makes it difficult to run a server or use Voice over IP since upload pipe is easily saturated.

1.8 Traffic Shaping

Internet is designed as an egalitarian best effort network. This means as packets arrive they are processed on a first come first serve basis. When enough channel capacity is available incoming packets never wait long.

Residential ISPs made assumptions about typical customer usage when they set monthly charges and designed infrastructure. Business model assumed bursty data flows predominantly web browsing, email, and occasional file download. Proliferation of Peer-to-Peer (P2P) file trading and streaming video services, such as YouTube and IPTV upset these assumptions. ISPs are struggling to carry more traffic then anticipated.

Some ISPs are responding with traffic quotas. When customer exceeds quota either speed is reduced or additional charge incurred. Especially in the case of cellular providers this has generated press coverage of to customer bills for thousands of dollars in overage charges. Some ISPs detect undesirable traffic and throttle it.

1.9 Measuring Speed

LAN performance is rarely a speed determinate. Speed is typically limited by first-mile WAN connection. It can be a challenge teasing out various components of end-to-end performance to see if ISP is working as advertised.

IP transmission splits data into 1500 byte chunks called packets (1-byte = 8-bits). Some of the 1500 bytes are used for network control so are not available for user data. TCP/IP uses 40 of the 1500 bytes for control. NOTE: this analysis assumes use of maximum size packets. Since overhead is fixed using smaller packet incurs higher overhead. With 40-bytes reserved for control out of every 1500-bytes sent only 1460 are available for data. This represents 2.6% overhead.

Some ISPs, typically phone companies, use an additional protocol called Peer to Peer Protocol over Ethernet (PPPoE) to transport DSL data. This is an adaptation of PPP used by dialup ISPs. Telco’s like PPPoE because it facilitates support of third party ISPs as mandated by FCC. PPPoE appends an additional 8-bytes to each packet increasing overhead to 48-bytes reducing payload to 1452. Where PPPoE is used overhead is increased to 3.2%.

Many phone companies use IP over Asynchronous Transfer Mode (ATM) (AAL5) to carry DSL traffic. ATM was designed for low latency voice telephony. When used for data it adds significant overhead. ATM transports data in 53-byte Cells of which only 48 are data the other 5 used for ATM control. Each 1500-byte packet is split into multiple ATM cells. A 1500-byte packet requires 32 cells (32 x 48 = 1,536 bytes). The extra 36-bytes are padded, further reducing ATM efficiency. 32 ATM cells require modem transmit 1,696 bytes of which only 1452 carry payload. Where ATM/PPPoE is used overhead is increased to 14.4%.

TCP/IP overhead 2.6% efficiency 97.4%

TCP/IP/PPPoE overhead 3.2% efficiency 96.8%

TCP/IP/PPPoE over ATM overhead 14.4%, efficiency 85.6%

It is easy to determine best-case file transfer rate if modem data rate is known. Broadband marketing rate may not the same as modem transfer rate. This may be done to simplify marketing by presenting a nice round number. Some Telco’s set transfer rate higher then marketing speed. When customer performs speed test they receive value close to marketed speed. Most broadband modems have status page allowing user to observe true transfer rate. Keep in mind this is rate modem connects to ISP not speed computer connects to modem or router with is typically 10 or 100 Mbps.

As an example our FairPoint 3000/768 ADSL service has a sync rate of 3360/864, 3360 kbps toward customer, 864 kbps toward Internet. FairPoint uses PPPoE and ATM yielding an overhead of 14.4%. Best-case transfer rate is 85.6% of sync rate, resulting in 2,876 kbps down 740 kbps up. Typical file transfer speed reported by Broadband Reports or Speedtest.net is shown below.

NOTE: This is best-case speed. Errors, transmission delays, etc will reduce speed from this value. The higher the speed the more impact even modest impairments will have on thru put. Some ISPs offer temporary boost during file transfer may result in overly optimist report.

1.10 Speed Optimization

There are many urban myths about magical performance improving tweaks. System tuning can be difficult because measurements are hard to duplicate. Many factors are outside user’s direct control.

TCP requires receiver to periodically send an Acknowledge to let sender know everything is OK. If the transmitter has not received acknowledgement after it sends a number of packets it stops transmitting and waits. This is called the receive window. For high speed connection or where latency is high default receive window (RWIN) should be increased to prevent pauses in transmission.

The other important tweak is packet size, called the maximum transmission unit (MTU). Maximum packet size is typically limited to1500 bytes. Normally this setting is fine for broadband access, dialup uses a much lower MTU typically 576. PPPoE encapsulation adds 8 bytes to each packet. This reduces maximum packet size to 1492 bytes. If sender attempts to transmit a larger packet it will either be rejected or fragmented into two parts, with attendant performance degradation.

A suite of optimization tools is available at Broadband Reports Tools. Once the optimum setting is determined DrTCP utility can used to effect change.

1.11 When “Unlimited” Doesn’t Mean “Unlimited”

There has been much press about residential ISPs marketing unlimited service and then imposing usage caps for heavy users. Some ISPs have gone so far as to call heavy users bandwidth hogs.

Controversy is not over ISP’s right to set terms of use it about misleading marketing. It is about calling a service unlimited then throttling or disconnecting customer if they use it too much.

1.12 When “Always On” Doesn’t Mean “Always On”

Broadband service is marketed as “always on.” Exactly what this means is subject to interpretation. The most “on” service is a bridged or routed connection configured with a static IP address. Once service is configured connection is permanent and always available until the next time the ISP needs to reallocate IP addresses or power fails.

Dynamic Host Configuration Protocol (DHCP) assigns client an IP address for a limited period called a lease. Before lease expires client automatically attempts to renew lease. DHCP simplifies task of managing customer settings. From customer’s perspective service is always on, lease renewal is transparent. Some ISPs bind IP address to hardware MAC address. This results in same IP address being assigned as long as customer does not change equipment.

Point-to-Point-Protocol over Ethernet (PPPoE) or ATM (PPPoA) emulates traditional PPP dialup connection. This type of service is common for ADSL. It leverages ISP investment in RADIUS authentication and billing equipment. Once customer is authenticated ISP issues IP address. If connection becomes idle the user is disconnected. Most residential routers include a keep-alive mechanism so connection is never disconnected.

Some ISPs, typically dialup, limit maximum continuous connect time. After a certain number of hours connection is dropped and must be reestablished.

1.13 Security and Privacy

Internet is a rough and tumble world often likened to the Wild West. The power of worldwide connectivity means anyone on the planet with an Internet connection is in a position to attack your computer. ISPs often block certain ports to reduce danger to unsophisticated users. Port blocking is a double edge sword as it may interfere with customer’s use of the Internet. Some ISP's go further acting as firewalls to protect customers from hostile attack and examine email for dangerous content or attachments. Some users consider this a great feature in the battle against spam and viruses. Others see it as an unwelcome intrusion in what should be individual control of network access.

The ISP is privy to all traffic that flows through its system. This raises two concerns, nosey ISPs and subpoenas. ISP can easily monitor how customers use the Internet, what sites they go to, what email they send and receive and in some cases even snoop usernames and passwords if they are sent in the clear. Privacy concerns have been exacerbated with expanded government snooping due to war on terrorism. Recent news reports indicate US government requested ISPs to monitor customer traffic without a court order. In most cases ISPs complied.

Privacy policy determines how customer information is used and protected. It is reasonable to expect ISP to collect and use information for diagnostic purposes and to improve service. However, some ISPs sell or otherwise make use of customer’s browsing habits. For example as a way to created targeted ads. In addition governments are pressuring ISPs to retain customer usage logs and make it available to law enforcement years after the fact.

Popularity of wireless networks raises security concerns. In a wired network attacker must physical connect to network. With wireless an attacker can eavesdrop from some distance away. This is especially worrisome with WiFi hot spots since they are in public places and integrity of owner is often unknown. To protect against this when setting up WiFi at home use wireless protected Access WPA2 mode. This provides over the air encryption. When using public Hot Spots one should be careful accessing any resource over a wireless network where passwords are exchanged in the clear.

1.14 Network Neutrality

As Internet access becomes pervasive there is growing tension between ISPs business practices and public good. ISPs are concerned being relegated to commodity bandwidth provider. As such they are frantically trying to create business relationships with select third parties to offer bundled services.

Network Neutrality proponents are concerned ISPs will created walled gardens and be in position to favor some companies and disadvantage others. Opponents of Network Neutrality argue ISPs ought to be able to due anything they want with their own network.

The reason I went into so much detail earlier about required and optional ISP services was to identify those services that only an ISP is able to deliver. Network Neutrality ought to insure network transparency is maintained, innovation encouraged and ISP allowed to offer value add services while being prevented from acting as gatekeeper. Internet’s rapid rise in popularity is the result of its open architecture. Entrepreneurs need to be able to create new business models and interact with customers without requiring permission or cooperation of network owner.

1.15 Finding an ISP

It can be difficult finding information about ISPs servicing a specific location. For residential use first place to check is local phone and cable companies. Some State Public Utility Commission’s (PUC) maintain broadband provider information.

For WISP checkout Broadband Wireless Exchange Magazine: WISP locator service.

2 Plain Old Telephone Service (Dialup)

Dialup has come a long way from early Bell 103 modem operating at 300 bps to current crop of V.90/92 modems capable of over 50,000 bps. Dialup Internet access is available to anyone with telephone service. Dialup modems can be used with both wired and cellular phone service. Data rate is significantly slower over cellular and per minute connect charges are the norm. Almost all Dialup ISPs support ITU-T V.90 modem standard. V.90 modems deliver up to 56 kbps over the PSTN. V.90 requires ISP modem connect to phone company digital trunk. Only a single digital to analog conversion can exist between ISP and user. In the US FCC power limitation reduces effective maximum speed to about 53.333 kbps. V.90 transmission from subscriber to ISP uses V.34 mode limiting maximum upload speed to 33.6 kbps. V.92 increased maximum upload speed to 48 kbps. If modem cannot connect in V.90 mode it automatically falls back to V.34 mode in both directions with a maximum speed of 33.6 kbps. Phone lines are analog only between customer and central office or remote terminal. At that point they are digitized at 64 kbps. This means POTS modem technology has reached its theoretical maximum speed. To obtain higher speed requires use of different technology.

V.92 is a minor enhancement to V.90. Upload speed is increased slightly to 48 kbps and implements faster auto negotiation to reduce call setup time. V.44 compression improved compression of reference test data to 6:1 vs 4:1 with V.90. Compression increases apparent speed because it reduces the number of bits transmitted over the slow phone line. Modem on Hold (MOH) allows modem to park a data session allowing user to answer a short incoming call. This works in conjunction with Phone Company Call Waiting feature and requires support from the ISP. V.92 modems are readily available but dialup ISP’s have been slow to upgrade.

At connect time modem probes phone line to determine noise and attenuation characteristics in order to set initial connect speed. Speed is constantly adjusted in response to varying line conditions. After connect the ISP performs user authentication and assigns an IP address. The most common method used to traverse the dialup connection is Point-to-Point Protocol (PPP). This allows Internet Protocol (IP) packets be transported over serial point-to-point telephone link between user and ISP. Once PPP process is complete user is able to access the Internet.

To obtain maximum speed V.90 and V. 92 modems require phone circuit exceed minimum FCC requirements. There are many effects that reduce dialup modem speed while not interfering with voice quality. Dialup modem impairments are discussed in a separate paper.

2.1 Dial Up Networking (DUN)

Dialup networking (DUN) establishes connection to ISP. DUN support automatically dials ISP phone number, waits for modem to connect then establishes a point-to-point-protocol (PPP) session to ISP. Once PPP session is set up ISP automatically configures IP parameters.

2.2 MultiLink

In the quest for higher speed some dialup ISPs support Multilink. Multilink binds two separate dialup connections into a single faster connection. If customer typically connects at say 44 kbps multilink doubles speed to 88 kbps. Multilink requires two modems; two phone lines, and an ISP that supports it. Where available it is a useful technique to obtain better performance from dialup.

Software at each end of the link splits date evenly between each connection effectively doubling speed. Unfortunately because data is still traveling over low speed dialup multilink does not improve latency.

Dialup ISP business model assume customers connect for relatively short periods of time. To enforce this most ISP’s enforce idle and maximum connect timeouts.

2.3 Dial Up Optimization

Compatibility - Proprietary ISP software makes it difficult or impossible to use a router to share the connection.

Cost Tip – make sure ISP has access numbers, Points of Presence (POP), close enough so calls are unmetered. Failure to do so will result in a rude surprise when the phone bill arrives.

Windows Performance - in dial up networking uncheck "Log on to Network." Most ISP’s use RADIUS authentication, eliminating Windows network login speeds up ISP connection process.

Windows Performance - Depending on Windows version uncheck NetBEUI and IPX in dialup networking. TCP/IP is the only protocol required.

Security - If computer is directly connect to dialup modem unbind file and print sharing from dialup. This prevents remote hosts on the Internet from gaining access to shared files

Call Waiting – Call waiting can be temporally disabled at the beginning of a call. The sequence varies by locale, in our area it is *70. Unfortunately sending the disable sequence to a line not equipped with call waiting is interpreted as part of the dialed number, resulting in an incorrect connection. This is a problem if the modem uses multiple lines and not all are equipped with Call Waiting.

Shared Line – if dialup modem shares a phone line there is possibility of mutual interference. If modem is in use picking up a phone will cause modem to disconnect. One can use a privacy device that monitors phone line voltage to prevent this. When phone is idle open circuit voltage is high around 48 volts, when a phone/modem is in use voltage drops to less than 10 volts. Privacy adapters measure line voltage to prevent phone use if a call is already in place. There are a couple of inconvenient side effects to this. Privacy device prevents calls being transferred from one phone to another and it confuses line use indicators built into many phones. I designed a Modem Access Adapter to prevent interference when modem and phone share the same line. Details are posted on the Writings page

3 T-1 and E-1 Carrier

The US Bell System developed T-1 digital carrier during the early 60’s to reduce interoffice transmission cost. Prior to T-1 analog frequency division multiplexing (FDM) was used to carry voice traffic between telephone switching centers. FDM carrier used a 4-wire circuit to carry 24 voice channels, one pair in each direction. T-1 was designed to carry 24 voice channels, facilitating transition from FDM to TDM. E-1 digital carrier, used in Europe, is similar transporting 30 voice channels. Each voice channel is digitized resulting in a 64 kbps data rate. 24 channels require 1.536 Mbps plus an 8 kbps control channel resulting in data rate of 1.544 Mbps (E1 is 2.048 Mbps). T-1 has a DS-1 channel speed of 1.544 Mbps and is carried over a 4-wire copper facility. Popular usage has corrupted this distinction. T-1 is now commonly used to mean any 1.544 Mbps service.

In the early 1980’s T-1 was trarrifed and made available to customers. T-1 continues to be extremely popular in commercial service carrying both voice and data. Prices for T-1 have dropped dramatically as technology improves and competitive pressure by alternative high-speed services.

3.1 Converting Voice to Digital Bits

Voice grade phone service occupies the frequency band of 300-3000 Hz. Low frequencies are suppressed to eliminate interference from power line noise. Increasing upper frequency bound beyond 3000 Hz does little to improve intelligibility, at the expense of greater bandwidth. Digital sampling must be performed at least twice the highest frequency of interest, a sample rate of 8,000 times a second was chosen. It was found sampling to 12-bits, resulting in 4096 possible values, produced excellent voice quality. This required a 96 kbps per channel data stream resulting in a composite data rate that exceeded what 1960s technology could deliver. To reduce data rate engineers decided to use only 8-bits or 256 values per sample, resulting in a 64 kbps data stream. To minimize quality degradation, conversion is performed logarithmically. When sound level is low samples are close together. During loud passages samples are farther apart. This masks quantizing noise generated by the conversion process. u-LAW conversion is used in US and A-LAW in Europe. The resulting digital signal is called Pulse Code Modulation (PCM). 24 phone calls in US (T-1) or 30 Europe (E-1) are interleaved using Time Division Multiplexing (TDM) combined with 8 kbps signaling channel resulting in a composite 1.544 Mbps (US) or 2.048 Mbps data stream.

PCM coding scheme developed for T-1 is what makes V.90 and V.92 dialup modems possible and also the reason dialup is limited to 56 kbps. Logarithmic sampling minimizes effect of audible noise but only allows 7 of the 8 bits be used for data. 8,000 samples per second times 7-bits per sample results in maximum data rate of 56,000 bits per second. Dialup modems have reached their theoretical limit.

3.2 Channelized vs. Unchannelized

When used for Internet access voice channelization is neither required nor desired. T-1 data circuits are unchannelized this exposes total channel capacity to the IP layer. IP, rather than physical layer, performs multiplexing. Some circuits are provisioned to allow flexible control of channelization. This allows an Integrated Access Device (IAD) to dynamically allocate bandwidth between voice and data.

3.3 Provisioning

The original implementation of T-1 required regenerators spaced every 6,000 feet. Regenerators recreate bipolar signals, allowing T-1 to deliver very low error rates compared to analog carrier. Regenerators can be powered from the T-1 line, called a span, eliminating need for local power. T-1 bipolar signaling is relatively noisy. This requires care during circuit provisioning to prevent interference between T-1 and other services, including other T-1s and DSL in the same cable.

Early T1 required 4-wire circuit, 1-pair in each direction. Newer T1 deployments using HDSL2 use a single pair for both directions. Digital signal processing techniques similar to that used with DSL reduce outside plant cable requirement and increases distance between regenerators.

4-wire T-1 circuit can be up to 50 miles, with regenerator every mile. Very long T-1 circuits are rare nowadays as fiber is more cost effective.

3.4 CSU and DSU

Channel Service Unit (CSU) is connected directly to the 4-wire facility. The CSU regenerates T-1 bipolar signals before handing them off to Data Service Unit (DSU). The CSU provides keep alive and Loopback testing enabling Telco to monitor line quality.

T-1 uses bipolar plus and minus 3-volt pulses, between pulses line voltage returns to zero. The Digital Service Unit (DSU) converts bipolar signals to a synchronous interface such as V.35 using both RS232 single ended and RS422 differential signaling to connect to customer equipment.

In the US CSU and DSU are built into customer premise equipment (CPE), such as a T-1 router. In the rest of the world CSU is owned by service provider, CPE includes only the DSU.

3.5 Smart Jack

When T-1 was developed the interface between CSU and DSU, called DSX-1, was designated the demarcation point between Telco and customer. DSX-1 is still the demarcation point in rest of the world. During US deregulation the FCC defined the 4-wire facility as the demarcation point. This caused problems for service providers as now management and quality assurance functions were no longer under their control but provided by customer premise equipment (CPE).

The solution was the Smart Jack. It presents a 4-wire (2-pair) interface to customer and implements service provider Loopback test function. This allows Telco to perform testing and maintenance functions while complying with FCC regulations.

Smart Jacks can be also used to deliver T-1 service to customers by converting other transmission schemes, typically optical, to T-1

3.6 Beyond T-1

The telephone network is almost entirely digital except 2-wire loop connection to analog POTS equipment. The digital carrier hierarchy is based on the notion of voice channels. The lowest level, called Digital Service 0 (DS-0), is a single PCM digitized voice circuit of 64 kbps. Next in the hierarchy is DS-1 (24 voice circuits over T-1 carrier) operating at 1.544 Mbps, then DS-2 (T-2) operating at 6.312 Mbps equivalent to 4 T-1 circuits. Then DS-3 (T-3) at 44.736 Mbps equivalent to 28 T-1 circuits.

Higher speed transmission is optical using Synchronous Optical Network (SONET) and ITU Synchronous Digital Hierarchy (SDH). Optical Carrier 1 (OC-1) and Synchronous Transport Signal Level 1 (STS-1) operate at 51.84 Mbps. Next in the hierarchy is STS-3 (OC-3) 155.52 Mbps. Then STS-12 (OC-12) operating at 622.08 Mbps and so forth. 10 Gbps STS-192 (OC-192) is an interesting convergence point. It is the first time Ethernet and SONET/STS operate at the same speed opens the door for Ethernet being carried directly over SONET.

For years 1.544 Mbps T-1 service was considered very fast, and compared to dialup it is. Advent of low cost residential broadband is driving demand for ever faster commercial service at the same time it is exerting downward price pressure. The good news for carriers is the tremendous capacity of fiber. Once fiber is installed increasing speed is often just a matter of changing access setting – cable plant and electronics are not affected. Even when electronics needs to be upgraded existing cable plant remains unchanged.

Telco Digital Carrier Hierarchy
301.336 Gbps	OC-7144F
159.252 Gbps	OC-3072 STS-3072
100.000 Gbps		Proposed 100 Gig Ethernet (not part of digital hierarchy)
79.626 Gbps	OC-1536
39.812 Gbps	OC-768 STS-768	Telco convergence with proposed 40 Gig Ethernet
9.953 Gbps	OC-192 STS-192	10G Sonet Telco convergence with 10 Gig Ethernet
4976 Gbps	OC-96
2.488 Gbps	OC-48 STS-48	2.5G Sonet
1.244 Gbps	OC-24 STS-24
1.000 Gbps		Gig Ethernet (not part of digital hierarchy)
622.080 Mbps	OC-12 STS-12
155.520 Mbps	OC-3 STS-3
100.000 Mbps		Fast Ethernet (not part of digital hierarchy)
51.840 Mbps	OC-1 STS-1
44.736 Mbps	T-3 DS-3 North America
34.368 Mbps	E-3 Europe
10.000 Mbps		Ethernet (not part of digital hierarchy)
8.448 Mbps	E-2 Europe
6.312 Mbps	T-2 DS-2 North America
2.048 Mbps	E-1 DS-1 Europe	30 DS-0 voice channels
1.544 Mbps	T-1 DS-1 North America	24 DS-0 voice channels, Primary Rate ISDN
144 kbps	Basic Rate ISDN	2B (64 kbps bearer channel) + D (16 kbps data channel)
64 kbps	DS-0	(DS) Digital Signal, Single PCM voice channel

4 Integrated Service Digital Network (ISDN)

Success of T1/E1 prompted Telephone industry to look for a way to deliver high-speed digital service directly to customer. Integrated Service Digital Network (ISDN) was supposed to be the next big thing poised to revolutionize telephone industry. Things have not played out that way. ISDN has been relegated to niche service.

Basic rate ISDN provides two 64 kbps bearer channels (B channels), and a 16 kbps data control channel (D channel). Primary Rate ISDN is basically a T-1 connection. ISDN is a circuit switched technology with very fast call setup time. Being digital full 64 kbps is available.

ISDN requires a Terminal Adapter (TA). TA connects to ISDN line, provides two POTS analog phone lines, and serial data connection. Both B channels may be bonded into a single faster 128 kbps connection. Combined with automatic dialing and incoming call detection both channels can be used for maximum speed when needed, while automatically falling back to single channel during voice call. ISP needs to support multilink in order to bond both B channels for 128 kbps service.

ISDN, like T1/E1, allows use of regenerators to extend distance between customer and central office.

ISDN was touted as the next big thing by the telephone industry in early 1990s. However deployment missteps and high cost have slowed deployment. ISDN is viable where other forms of high-speed access are not unavailable but its window of opportunity has passed.

Cost Tip – ISDN is a switched service make sure ISP has access numbers, Points of Presence (POP), close enough so calls are flat rate. Failure to do so will result in a rude surprise when the phone bill arrives. Telco’s offer different types of ISDN service, for Internet access unmetered service is ideal.

4.1 IDSL

A variation of ISDN, called ISDN digital subscriber line (IDSL), uses ISDN signaling to deliver 144 kbps data only service at greater distance than typical DSL.

5 Digital Subscriber Line (xDSL)

Digital Subscriber Line (DSL) technology utilizes telephone copper wiring between subscriber and phone company central office (CO) or Remote Terminal (RT) to deliver high-speed data. This allows local exchange carrier (LEC) to generate additional revenue by leveraging its massive investment in copper outside plant cabling. Several types of DSL have been developed hence the xDSL moniker. The most common types are Asymmetric DSL (ADSL) G992.1, ADSL2 (G.992.3), ADSL2+ (G.9925) and Symmetric DSL (SDSL). Telco’s like DSL not only as another revenue source but because it gets long duration data calls off the Public Switched Telephone Network (PSTN). This minimizes need for expensive upgrades to circuit switched phone network.

ADSL was initially developed for video on demand and has been recycled for broadband Internet access. ADSL has higher download speed, toward the subscriber, than upload. It uses frequencies above those used with Plain Old Telephone Service (POTS) allowing it to coexist with voice service. This minimizes cost by allowing a single copper pair to be used for both voice and data. A single line residence can be equipped with both phone and high-speed data service over the same copper circuit. Typical ADSL speed is 768 – 8,000 kbps downstream (toward customer) and 128 - 800 kbps upstream (toward Internet).

The Digital Subscriber Line Access Multiplexer (DSLAM) at the Telephone Central Office or Remote Terminal is connected to the customer’s phone line. The voice portion is passed through a low pass filter and delivered to POTS phone switch. DSLAM recovers customer data and uses Asynchronous Transfer Mode (ATM) to link customer to ISP. Telco’s use ATM because it facilitates support of third party ISPs. At the customer location a similar filter is used to separate DSL from POTS. This can be a single whole house POTS/DSL splitter or filter connected ahead of each non-DSL device.

Symmetric DSL (SDSL) is typically marketed as a business service. It requires a dedicated copper pair not shared with POTS. Being symmetric makes it suitable for use with servers.

Maximum DSL speed is a function of line length, wire gauge and line quality. ADSL service is limited to about 18,000 feet, with closer customers eligible for higher speed. Remote DSLAMs, called Remote Terminals (RT), shorten loop distance by moving the DSLAM closer to the customer. This increases number of potential customers within range and increases maximum speed.

FCC regulations require Incumbent Local Exchange Carriers (ILEC) to allow third party Competitive exchange carriers (CLECS) access to copper access network. CLECs need to rent collocation space in the central office to install DSLAMS and backhaul facilities to transport data. Unlike Cable there are many ISPs that resell DSL. In that case customer data is transported over ATM network eliminating need to physically install DSLAMs. As such they evolved a complex interconnect scheme consisting of 1) physical copper loop, 2) Asynchronous Transfer Mode (ATM) virtual circuits to transport customer packets over the First-Mile network to the respective ISP 3) typical ISP functions. DSL may involve three different companies, ILEC supplying copper service, CLEC first mile transport, and ISP doing the rest.

5.1 Splitter vs Inline filter

ADSL and Voice telephone share a single copper circuit. At each end filters prevent DSL from interfering with voice phone service. To reduce cost ADSL service providers include inline filters as part of a customer self-install kit. Customer is instructed to installed filter at each non-DSL device. Having customer self-install filters eliminates expense of a truck roll.

An alternative to per device filtering is a whole house POTS/DSL splitter. Splitter provides low pass filter isolating voice from high frequency DSL. Splitter has two outputs; “Data” connected directly to DSL modem and “Voice” connected to inside phone wiring. The splitter contains a half-ringer test circuit after the low pass POTS filter. This allows removal of half-ringer in NID, minimizing DSL signal loading.

Splitter Advantage

Splitter Disadvantage

Single device for entire house
Better electrical characteristics
Isolates inside wiring from DSL
Isolates half-ringer from DSL
Works with home Alarm dialer

Installation required
Dedicated run from splitter to DSL modem
Have to purchase separately

5.2 Impairments

DSL uses 100-year old copper telephone network to carry high-speed data. This is an impressive engineering accomplishment. Unfortunately not all phone lines are suitable for DSL. Assuming the local central office (CO) or remote terminal (RT) is equipped for DSL it may not be available for a number of reasons. This section discusses common problems and where applicable workarounds.

5.2.1 Loop Carrier

Digital Loop Carrier (DLC), Digital Added Main Line (DAML) et al are techniques to allow multiple customers share a single circuit. Phone companies use loop carrier when there are no available circuits and in rural areas where cost of active electronics is less then running dedicated circuit to customer. Unfortunately most forms of multiplexing are incompatible with DSL unless designed to support it.

5.2.2 NID, MTU and Half Ringer

In the bad old days before US telecom divestiture (1880 to early 1980’s) phone company supplied service, installed and leased all required telephone equipment. Customer was prohibited from connecting anything to the telephone network. With divestiture Phone Company regulated responsibility was limited to delivering service to premise. Inside wiring and equipment became customer’s responsibility.

This created a dilemma for the Phone Company, how to determine if a problem was their responsibility or that of the customer? Enter the Network Interface Device (NID). NID is the demarcation point, between Phone Company and customer. It incorporates lightning protection and a method to easily disconnect customer premise equipment (CPE) from the telephone network. Early NIDs used modular jack connected to old style carbon block lightning protector. Over time NIDs evolved into an integrated package.

The phone company uses automatic test equipment called mechanized loop test (MLT) to periodically test copper phone lines. They wanted a device; built into the NID, that allows MLT to determine where the network ended and where customer responsibility began. There have been two different approaches to this: MTU and Half-Ringer.

5.2.2.1 Maintenance Terminating Unit (MTU)

The MTU was the first device used during early deregulation. It is a pretty clever device. It consists of a series pass voltage controlled switch on each leg of the Plain Old Telephone Service (POTS) circuit. During normal phone usage switch conducts and POTS equipment operate normally. Testing, done at a lower voltage, does not trigger the series element thus isolating CPE side from the telephone network.

Unfortunately MTU’s are incompatible with DSL. The DSL modem does not seize the line: that is cause current to flow, which is what turns on the voltage-sensing switch. The MTU effectively isolates DSL modem from the telephone network.

If your line has an MTU it must be removed. For reasons too involved to go into here MTU’s caused other problems and have not been used for years. In most cases MTU have been removed from phone line long ago, but it is possible to still have one. Automated testing should flag the existence of an MTU, but not always. MTU being a series pass device has four leads, two connect to the Telco side the other two to CPE.

5.2.2.2 Half-Ringer

Half-Ringer is a simple circuit that emulates old style electromechanical Western Electric ringer providing test signature for automatic test equipment. It consists of a capacitor, resistor, and back-to-back zener diodes. ADSL is designed to operate in the presence of a half ringer so in most cases it will have no effect on ADSL. It does represent a small load so if signal is marginal removing it may help. SDSL is not able to operate in the presence of a Half-Ringer and it must be removed for SDSL service.

Excerpt from DSL Forum Technical Report 013.

It has been standard practice in many areas of the United States to install, at the Network Interface Device (NID), a network termination device called a half ringer. It is an example of an AC termination device since it is detected using AC techniques.

A normal POTS mechanical ringer is made up of an inductor and capacitor in series bridged between Tip and Ring. The half ringer is a capacitor in series with a zener diode and resistor. This, in the U.S., is a 0.47 micro Farad capacitor without the addition of the inductive part of the circuit, hence the name ‘half’ ringer.

5.2.3 Distance

ADSL service is limited to about 18,000 feet. Some ILECs are installing Remote Terminals (RT) to reduce cable distance allowing them to serve more customers at higher speed. Newer DSL equipment has slightly extended maximum distance. Obtaining accurate pre order distance estimate can be a difficult. The effective wire distance between DSLAM and customer is often substantially longer than driving distance making map based estimates questionable.

Aware published a great Whitepaper reviewing differences between the various versions of ADSL.

5.2.4 Bridged Taps

When telephone feeder cable is installed it is not known how many customer circuits will be needed at each location. The solution is to run a large feeder cable past many customers. As service is installed the technician selects an unused cable pair and splices it to the drop cable. The circuit feeding the drop may continue for hundreds or thousands of feet beyond the customer, this results in a bridged tap. Bridged taps are of no consequence for telephone service but can degrade DSL. The presence of a bridged tap causes DSL signal split at the tap going down both paths. When it reaches the end it is reflected back into the line, creating interference. DSL is designed to tolerate some amount of bridged tap, but if circuit is marginal it may cause problems or push customer over distance limit. SDSL providers typically pay Telco to remove bridged taps. This is expensive and is not ordinarily done for low cost residential ADSL.

5.2.5 Load Coils

Resistance and impedance attenuate signals. This effect is more pronounced at high frequencies and long circuits. Load coils are used on long loops to cancel these harmful effects resulting in better voice characteristics. Load coils are typically installed on loops over 18,000 feet. H88 load coils, the most common type, are spaced every 6,000 feet beginning 3,000 feet from the central office.

Unfortunately load coils are incompatible with DSL. They flatten response over the voice frequency range but severely attenuate higher frequencies used by DSL. If Load coils are present they must be removed prior to getting DSL.

5.2.6 Noise and Crosstalk

Telco feeder cable carries many different services: POTS, ISDN, DSL and T-1. Phone circuits often closely parallel power lines picking up power line noise. Imperfections cause unintentional coupling from one circuit to another. This raises the noise floor. If noise becomes excessive speed is impacted.

Residential DSL is not typically warranted for speed, it is a best effort service. If noise is present during phone call customer is more likely to get problem resolved than if it only affects DSL or dialup.

5.2.7 Interleave vs Fastpath

DSL lines are susceptible to noise bursts from many sources such as: lightning, ignition noise, radio transmissions and power line faults. DSL spec writers were aware of this and included forward error correction (FEC). FEC adds redundant check bits to data stream. If noise corrupts data check bits are used to recover from error. As long as only a few bits are damaged receiver is able to correct errors avoiding need for retransmitssion.

The key is only a few damaged bits can be recovered. If noise burst is long it corrupts too many bits for receiver to undo damage. In that case bad packet is passed to higher layer protocol. TCP/IP TCP requests retransmission. Needless to say that takes a "long" time. UDP/IP, used with VoIP and streaming data does not have a retransmission scheme. There is not enough time to retransmit data before it is needed by application. Streaming applications have provisions to fake missing data. How badly missing data affects quality depends on application and how extensive the damaged.

When interleave is turned on bits from several frames are interleaved in time. If noise burst is long relative to bit time it corrupts many bits. When receiver deinterleaves data corrupt bits are now spread across multiple frames - increasing chance FEC is able to correct them. This eliminates need for retransmission or application having to fake missing data.

As speed increases number of bits affected by a given noise burst increases. Lets say a noise burst corrupts a single bit at 768 kbps. At 1500 two bits and at 3000 the same pulse affects four. As transmission speed increases signal to noise margin decreases making transmission more susceptible to noise corruption.

Downside of Interleave is slightly higher first hop Ping, because multiple frames are processed as a single entity. The penalty for Interleave goes down as speed goes up since a given frame takes less time to transmit at higher speed. Unless you are an avid gamer interested in absolute lowest possible ping time Interleave is transparent. Other network issues usually swamp out the slight increase (10-20 ms) in first hop ping. Telco’s did not implement Interleave to annoy gamers; they did it to improve overall customer satisfaction.

6 Fiber to the Curb (FTTC)

VDSL2 is optimized for very high-speed service over short loops. The sweet spot for VDSL2 is 50 Mbps @3,000ft. To deploy VDSL Carriers are building so called fiber to the curb (FTTC) networks. Video ready access device (VRAD) cabinets are deployed in the field and linked to telephone central office via fiber. Cabinets are located close to customer to keep wire distances under 3,000 feet.

To shorten customer loop DSLAM must be located in the field near customer. This requires active electronics to deliver triple-play services (phone data video) and backhaul traffic to Central Office over fiber.

VSDL is fast enough to deliver limited television service while at the same time providing high-speed Internet. Standard definition TV (SDTV) requires 2-3 Mbps per program while high definition (HDTV) about 15 Mbps.

VRAD are controversial because they are rather large and targets for graffiti. VRADs are typically located near existing crossconnect boxes to gain access to customer copper circuit. VRADs require both AC power and backup power. AT&T experienced early battery problems, since resolved, that resulted in several explosions.

7 Data Over Cable Service Interface Specification (DOCSIS)

Cable TV (CATV) started in the 1950’s as Community Access TV in areas where roof top antennas did not provide adequate reception. Early pioneers found they could locate a large antenna on a local mountaintop and distribute broadcast TV over coax cable. By the 1990’s the industry was looking for new revenue opportunities and ways to fend off inroads being made by Direct Broadcast Satellite (DBS).

Historically Cable TV has been a one-way medium. TV signals originate at the CATV Head End (HE) and carried over coaxial cable to subscribers. To accommodate Internet service the Industry needed to upgrade unidirectional one way “broadcast” cable distribution with a bidirectional system. This involved replacing distribution amplifies with bidirectional amps. Previous upgrades had replaced the coaxial network with Hybrid Fiber Coax (HFC). Fiber is deployed deep into the CATV network. Redundant fiber loops interconnect the Head End to hubs. The hubs in turn connect to local nodes that convert fiber to coax. Coax is only used for relatively short distance connecting individual subscribers to HFC network.

CATV industry worked hard to standardize cable modems so they can be purchased retail like dialup modems. The industry is rapidly adopting DOCSIS created by Cable Labs. DOCSIS 1 delivers per segment bandwidth of up to 40 Mbps toward customer and 10 Mbps upload. DOCSIS 2 increases upload to about 30 Mbps. DOCSIS 3 increases downstream to 150 Mbps and upstream to 100 Mbps. This is the total data rate for a particular node that may consist of a 100 customers. Typical CATV service offerings are 2-20 Mbps down (toward the customer) and 2-5 up. Peak data rate is typically much higher then that available over DSL.

DOCSIS delivers about 40 Mbps over each RF channel allocated to Internet use. A TV channel (6 MHz wide in the US) is reserved for data service toward subscribers. DOCSIS 3 bonds multiple channels to obtain greater speed. Upstream data path is more challenging. Upload is carried in band below lowest TV channel. Distribution amplifiers had to be replaced with ones capable of amplifying signals in both directions. The Head End recovers these signals and routes them to Internet exchange carriers.

Some early Cable Internet deployments were unidirectional. Cable network was used for downstream transmission and dialup for upstream. This allowed CATV operators to quickly offer high-speed Internet service prior to upgrading cable facility to carry bi-directional data.

The industry is actively courting commercial customers. While DOCSIS can be used to service commercial accounts the preferred method is Wavelength Division Multiplexing. WDM uses different “colors” to share single fiber.

At customer premise coax transmission line is split to two drops. One drop feeds DOCSIS modem other feeds one or more TVs. This insures maximum signal strength for cable modem. DOCSIS uses one or more RF channels to deliver data to customer and low frequency band below first TV channel to upload traffic. In modern HFC CATV networks coax is used for a relatively short distance, between hub and customer. Fiber is used as inter hub backbone because of its superior transmission capacity.

7.1 Impairments

The CATV network, much like the phone network, has been pressed into service to deliver high speed Internet connectivity. There are a number if issues that interfere with obtaining maximum possible speed.

7.1.1 Shared Medium

Cable is a shared medium. Each user competes with others on the same segment. While all Internet access is shared at some point Cable is shared in the first-mile. As more customers subscribe Cable provider, called Multi System Operator (MSO) must reduce number of subscribers per segment to deliver acceptable service.

This is why Cable industry has been so aggressive going after “bandwidth-hogs,” customers that either upload or download a lot. It is not uncommon to have daily/monthly cap on residential cable accounts. If cap is exceeded speed is throttled or service discontinued.

7.1.2 Limited Upload

DOCSIS uses a time slot mechanism, called Time Division Multiplexing (TDM), to facilitate equitable upload over the shared cable segment. The Cable industry assumed customers would primarily use download capacity. Customers are taking advantage of Internet peer-to-peer capabilities to create and host their own data, use Cable for Voice over IP (VoIP) telephone service, and peer-to-peer file sharing. This creates a strain on limited Cable upload capability.

8 Cellular

Cellular phone service is hugely popular. What started out as an expensive lunchbox sized 2-way radio phone a couple of decades ago has shrunk to smaller then a pack of cigarettes and is considered an essential part of everyday life by much of the population. Some customers, especially younger ones, use cellular as their primary phone forgoing wired phone service altogether. The attraction of wireless connectivity is not limited to voice. Almost from the beginning Cellular was pressed into data service, typically with less then stellar results. The landscape is changing as Cellular providers offer a verity of fast wireless data services.

United States situation is somewhat unique compared to rest of the world where national cellular standards exist. FCC has not mandated a single US cellular standard. This resulted in a confusing patchwork of competing standards but it also allowed companies to rapidly bring innovative services to market. Early cellular protocol was analog: Advanced Mobile Phone System (AMPS). Modern cellular networks are digital. Most of the rest of the world uses Global System for Mobile Communication (GSM). In the US some carriers use GSM others Code Division Multiple Access (CDMA2000).

8.1 AMPS - Cellular Digital Packet Radio (CDPD)

Cellular Digital Packet Data CDPD is the granddaddy of wireless data service. It uses analog AMPS to deliver an anemic 9.6 or 14.4 kbps. That was not too bad a decade ago but is painfully slow today. With AMPS being phased out CDPD is also at end of life.

8.2 CDMA2000 - Evolution Data Optimized (EvDO)

Modern Cellular network is digital capable of faster data transport then previous analog generation. CDMA2000 cellular networks have several planned data enhancement called Evolution-Data Optimized EvDO. EvDO delivers download speed in the 3.1 Mbps range with 1.8 Mbps up.

8.3 GSM – Enhanced Data Rates for GSM (EDGE)

General Packet Radio Service (GPRS) was the first data enhancement to GSM. Typical GPRS speed is about 32-40 kbps. An enhanced version of GPRS Enhanced Data Rates for GSM Evolution (EDGE) triples data rate.

8.4 GSM - High Speed Download Packet Access (HSDPA)

High-Speed Download Packet Access (HSDPA) is the latest flavor of data service provided by GSM. HSDPA delivers about 3.6 Mbps.

8.5 GSM 3GPP Long Term Evolution (LTE)

LTE is part of the Third Generation partnership project for GSM networks. This is still a work in progress though trials have begun. Focus is migrating cellular data to packet based, rather then circuit switched network and delivering substantially higher speed then today’s cellular network. LTE will deliver Multi megabit speed with very low latency.

9 Wireless (WISP)

In areas not served by wired high-speed access Wireless ISPs (WISP) are rushing to fill the void. The first question that comes to mind is what is the difference between Cellular and WISP? Cellular began life as a voice centric service then added data as an enhancement. Cellular services are optimized for use while customer is in motion; WISPs are optimized for fixed location use. WISPs use either Point-to-Point or Point-to-Multipoint distribution network. In some cases customer’s equipment acts as a router creating a mesh network to expand service footprint. WISPs use radios that operate in both licensed and unlicensed bands. Optical units do not have to be licensed but must meet safety standards as pertains to light source.

9.1 Multipoint Radio

Wireless ISPs use a central radio to cover a large territory eliminating need to run cable to customer’s premise. Radio technology is ideal for rural areas where low population density makes installing copper or fiber too costly. As picture shows signal may take a direct path or if obstructions ISP may deploy Repeaters. Repeater acts as router forwarding packets and extending coverage area. Directional antenna can create multiple sectors increasing total bandwidth.

Radios may be either proprietary, Motorola Canopy gear is very popular or IEEE 802.16 World Interoperability for Microwave Access (WiMAX). WiMax trade association is promoting this evolving standard and hopes make it as successful as WiFi has become for Wireless LANs. WiMAX is specified to operate in 2.3-2.7 GHz and 3.3-3.8 GHz licensed bands and 5.725-5.85 GHz unlicensed band. Distance is about 10 miles in Non Line of Sight (NLOS) and 30 miles over line of sight (LOS). Maximum data rate is about 50 Mbps. Clearwire is marketing WiMax service.

9.2 White space Broadband

White Space refers to unused TV channels. As part of conversion to Digital TV FCC is investigating allowing unused TV channels be used for low power Internet access. Equipment vendors and service providers created the Wireless Innovation Alliance to exploit unused TV channels for data communication.

The Cable industry expressed concern nearby transmitters will cause unacceptable levels of interference with consumers AV gear due to signal leakage.

9.3 WiFi Hot Spots

The tremendous popularity of IEEE 802.11 WiFi Wireless LANs created the phenomena of WiFi hot spots. Shops and hotels and some government agencies use WiFi Access Points to create WiFi hot spots. Customers near hot spot have high speed Internet access. In some cases service is free in others it is pay to play.

WiFi was designed as a short-range wireless LAN. Attempts to provide citywide coverage using WiFi have met with great difficulty.

9.4 Optical Point-to-Point

Point-to-Point links are often optical. Optical interfaces are inexpensive compared to RF and very fast. The downside is birds, fog and snow obstruct transmission path. Canobean products are typical of free-space optical gear.

10 Satellite

Satellites act as a very tall antenna vastly expanding coverage area. Geosynchronous satellite use extremely high orbit so they appear to be stationary. Low Earth orbit satellites require a large number to cover the entire globe.

10.1 Geosynchronous

Science Fiction author Arthur C Clark is generally credited with proposing the notion of geosynchronous satellites in his 1945 paper Extra-Terrestrial Relays. On Earth this distance is about 22,300 miles, now called the Clark orbit. Clark’s idea has been a boon to Radio and TV broadcasting.

Orbital time is a function of distance. The farther a satellite is from earth the longer the orbit duration. Clark realized that at a certain distance orbital time would equal 24 hours. If the satellite is in equatorial orbit a 24-hour orbit means the satellite appears to stay positioned over the same spot on Earth permanently.

The great height of geosynchronous satellite creates continent sized signal footprint for each satellite. Since satellite appears fixed in space expensive antenna tracking mechanisms are not required.

When small aperture Direct Broadcast Satellite (DBS) TV became popular it was natural to adopt this technology to high-speed Internet access. One-way implementation uses satellite link for high-speed download and dialup modem for upload. Two-way service uses satellite link in both directions. Unfortunately the great height of geosynchronous orbit adds significant latency making this type of service more appropriate for file transfer than interactive browsing. One-way latency Ground—Sat--Ground is about ¼ second (250 ms). If the satellite is used in both directions latency is about 500 ms. When dialup is used for upload total latency is reduced to about 350 ms.

Satellite capacity is shared by many uses. Service providers implant Fair Access Policy (FAP) to allocate capacity equally to all customers.

10.2 Low Earth Orbit (LEO)

To reduce high latency satellite has to be closer to Earth. There have been several attempts to use Low Earth Orbit (LEO) satellites to provide Internet communication service but they have not been commercially successful. Covering the globe requires a constellation of hundreds of expensive satellites. The two most famous attempts were Iridium and Teledesic.

11 Broadband over Power Line (BPL)

In the quest for high speed Internet access what about electric utilities? One would think they are well positioned to take advantage of demand for broadband access. A speaker mentioned in a seminar I attended several years ago electric companies are ideally positioned to be broadband providers because they have: 1) Rights of way, 2) Guys with trucks, and 3) know how to send out a bill every month. In short they appear to be well positioned to roll out high speed Internet access. Alas except for a few isolated cases this has not happened.

Recently the media has been playing up news about Broadband over Power Lines (BPL). Rather then string fiber several companies are looking at feasibility of using power line itself to carry bidirectional data. The same technique that shoehorns megabit DSL onto phone lines can be used to send bits over power lines. Electric utilities have some experience with this technology. They use a much slower version to transport telemetry SCADA data from remote substations for years.

As with all new technology there are many players and many press releases about how this will revolutionize broadband access. The skeptical observer should keep several questions in mind. What is the benefit of using this compared to “real” wireless? What sort of performance can be expected when thousands of customers use the network? Will this cause unacceptable level of interference to existing radio users? The ARRL has been very vocal with concerns BPL will increase the noise floor negatively impact amateur radio operations. This is an important consideration as Amateur Radio Operators often are called to provide emergency communication during disasters.

12 Fiber to the Premise (FTTP)

The holy grail of broadband is fiber optic service all the way to the customer, called Fiber-to-the-Premise (FTTP). A fiber network costs $1000 to $2500 per home passed. To put that number in perspective it is about twice what a copper plant costs and about three times that of Cable. Service providers are faced with the difficult business decision of choosing to invest in technology to extend life of existing copper network or take the plunge and install fiber. Deploying fiber puts the company in a very strong competitive position but demands tremendous capital investment. Triple-play service: voice, data video takes advantage of fiber capacity to deliver converged services.

High cost of deploying FTTP has been an impediment to adoption. Companies are working hard to reduce both labor and component cost. As more systems are installed cost is falling rapidly. These efforts range from use of fiber optic ribbon cable and preterminated cable assemblies to installing fiber in sewer mains or abandoned water and gas pipes. Vendors are working to reduce cost of components needed to convert between electrical and optical signaling.

FTTP can emulate analog plain old telephone service (POTS) by reserving channel capacity and encoding phone calls optically. To subscriber service is identical to existing phone service. Implemented this way phone service is invisible to broadband connection. It is also possible to implement as Voice over IP (VoIP).

FTTP facilitates delivery of TV programs in addition to data and voice. Legacy Cable Hybrid Fiber Coax (HFC) network can be emulated by encoding TV programs and using another “color” lambda to transmit them over fiber. At the residence optical signal is converted to RF and delivered over standard coax used for Cable and over the air reception.

Speed of optical networks makes them ideally suited for Video on Demand (VoD). Video on Demand requires tremendous network capacity, especially for HDTV. Each HDTV program requires about 20 Mbps. 100 Mbps broadband is enough to deliver individual HDTV programs to a family of four with enough left over to Internet.

12.1 Controversy

FTTP represents a complete rethinking of how wired communication services are delivered. Building a FTTP network is a major construction project involving installation of fiber cabling, termination facilities and customer premise equipment.

12.1.1 Power Outage

Legacy analog POTS phone network is powered by telephone switching office. During power outages batteries and diesel generators maintain system power indefinitely. It is not feasible to deliver power over an optical network. Customer’s terminal equipment is battery backed so during power outage it continues to operate. Backup time is a function of battery size. Larger the battery longer service stays operational during a power failure. Batteries are relatively short lived components and need to be replaced every few years at customer’s expense.

12.1.2 Copper Decommissioning

Fiber outside plant (OSP) is much more reliable then copper dramatically reducing maintenance costs. Phone Companies have made no secret long-term goal is to discontinue use of copper outside plant. Current FCC regulations require Incumbent Local Exchange Carriers (ILEC) to share certain copper unbundled network elements (UNE) with third party service providers.

12.1.3 ONT Installation

There have been numerous horror stories about damage to other utilities and homeowner property during installation of FTTP. There have also been problems where CPE was installed in violation of National Electrical Code (NEC) requirements.

12.1.4 Competitors

In the US telecommunication deregulation requires phone companies to share certain portions of their network with competitors. That is no longer the case with fiber.

Once a locality is wired with FTTP it makes little sense for a competitor to do so. First-mile is the most expensive and lest profitable portion of the global telecommunication network. This creates policy issues as to the role of first-mile providers to society as a whole.

12.1.5 Municipal Broadband

Some municipalities frustrated by the slow roll out of high-speed service are installing their own fiber and renting it to third party service providers or delivering data, video and phone service (triple play) themselves. Currently this is a hotly debated topic. Should municipalities build their own fiber network or is this is best left to private enterprise?

12.2 Point-to-Point Ethernet

IEEE Ethernet in the First Mile working group developed both PON and Switched Ethernet version of the specification. With Switched Ethernet a customer is directly connected to a port on an Ethernet edge switch, typically located in a remote enclosure relatively close to customer. The advantage of this approach is costly electro/optical interfaces only need operate at link rate, typically 100 Mbps rather then aggregate PON rate. Customer premise equipment is cheaper since it only has to convert a point-to-point optical interface to Ethernet.

Switched Ethernet simplifies provisioning. Once a customer is connected, increasing or decreasing access speed can be performed by a command sent to the edge switch. Whereas with PON customer provisioning may require sending a craftsperson out to physically modify split ratios. Privacy is very good, as only traffic destined for the customer is visible at customer’s drop. Down side is requirement for remote equipment huts to house Ethernet switches and back up power.

12.3 Passive Optical Network (PON)

Passive Optical Network (PON) uses a single optical fiber to deliver services to 32 or more customers. Traffic toward customer is broadcast to all endpoints. Upstream traffic utilizes a time division-multiplexing (TDM) scheme to insure access fairness. Traffic toward customer and toward Internet is carried by different colors, called Lambdas. Advantage of PON is elimination of active electronics in the field and reduction of number of fibers used to connect customers. Passive convergence points house optical splitter used to connect multiple customers to single trunk fiber.

PON networks are able to deliver TV by using a different Lambda to carry RF CATV signal over the same fiber used for data. This technique is called Wavelength Division Multiplexing (WDM). WDM is the optical equivalent of Frequency Division Multiplexing (FDM) used at lower frequencies. At customer location an optical/electro converter translates PON optical signal to traditional CATV coax electrical interface, much as a node does in CATV network. This preserves backward compatibility with legacy CATV network – as well as its technical limitations.

12.3.1 Ethernet PON

IEEE Ethernet in the First Mile working group developed an Ethernet version of PON that does away with ATM and delivers Gigabit speed. E-PON is faster then B-PON (622/155 Mbps) but not as fast as new ITU GPON (2.4/1.2 Gbps). Second generation Ethernet PON increases speed to (10/1Gbps).

12.3.2 A-PON B-PON

ATM PON uses ATM to provide data and voice virtual circuits over single fiber. APON specification delivers aggregate bandwidth 622 Mbps down and 155 Mbps up. Maximum fiber distance is 20 km (65 kft). B-PON uses a third optical wavelength to emulate legacy CATV network for triple play service. ATM is used for transport reducing effective IP payload by about 10% due to ATM overhead. One also needs to factor in AAL2 POTS voice channels at 64 kbps each. Assuming a 1:32 split ratio B-PON delivers about 18 Mbps down and 4.5 up to each customer.

1550 nm is used to emulate CATV Hybrid Fiber Coax (HFC) network. In the US TV channels are 6 MHz wide. Each channel can be used to send a single analog SDTV channel or up to 40 Mbps of data. Data can be digitally compressed SDTV, HDTV or radio.

1490 nm is used to transmit data toward the customer. Each ONT “sees” all packets on the cable. However only those destined to the customer are forwarded to customer’s Ethernet connection.

1310 nm is used to transmit data from customer to ONL. Upstream traffic is based on a time division-multiplexing scheme to insure fairness. Unused slots are reclaimed and are available to other customers.

The system being deployed by Verizon includes 4 emulated POTS channels. This is not Voice over IP. POTS channels are carried over ATM, making them invisible to Internet traffic. Voice quality is identical to regular POTS, typically better due to short length of copper circuit.

Verizon residential PON installation consists of an Optical Network Terminal (ONT) typically mounted on exterior wall and a battery backed power supply inside the home.

A single fiber connects ONT to the PON network. Verizon is making heavy use of preterminated fiber to reduce installation cost. Fiber and UPS wiring connect to left hand Telco side of the ONT. The right hand customer side has four analog POTS interfaces, an F connector for TV and a RJ45 Ethernet connector for data.

During power failure Internet and TV portions are shutdown after a few minutes to conserve battery life. The uninterruptible power supply (UPS) keeps voice service active for about 12 hours when idle and about 4 –5 “talk” hours.

ONT cable TV emulation is unidirectional – toward customer. This creates a problem for smart set-to-boxes that need to communicate with head-end. Initially set-top-box required both coax and Ethernet connection. Recent deployments make use of Multimedia over Coax Alliance (MoCA) technology to utilize RG6 TV wiring for data.

12.3.3 G-PON

GPON ITU-T G984.1 and G.984.2 standard increases speed to 2.5 Gbps down and 1.25 Gbps up. GPON does away with ATM eliminating so-called ATM cell tax. Higher speed of GPON makes it better suited to IP based Video on demand (VoIP) then first generation BPON.

Closing Thoughts

These are dynamic times. High speed Internet represents a new way of communicating.

Never before have creators and patrons been so closely linked.

Never before have ordinary citizens “owned the printing presses.”

Never before has it been so easy and inexpensive to communicate with anyone on the planet.

Never before has it been so inexpensive to create audio, visual, and written works.

The next decade will profoundly change human civilization. I can’t wait.