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What is DSL?

DSL (Digital Subscriber Line) is a service that offers a faster Internet connection than a standard dial-up connection. DSL technology uses existing 2-wire copper telephone wiring to deliver high-speed data services to homes and businesses. 

DSL uses the existing phone line and in most cases does not require an additional phone line. This gives "always-on" Internet access and does not tie up the phone line. No more busy signals, no more dropped connections, no more waiting for someone in the household to get off the phone. DSL offers users a choice of speeds ranging from 144 Kbps to 1.5Mbps. This is 2.5x to 25x times faster than a standard 56Kbps dial-up modem.

This digital service can be used to deliver bandwidth-intensive applications like streaming audio/video, online games, application programs, telephone calling, video conferencing and other high-bandwidth services.

Today DSL is for the first time putting high-speed Internet access within the reach of the home, small and medium-size businesses. DSL takes existing voice cables that connect customer premises to the phone company's central office (CO) and turns them into a high-speed digital link.

Over any given line, the maximum DSL speed is determined by the distance between the customer site and the Central Office (CO). Most ISP's offer Symmetric DSL (SDSL) data services at speeds that vary from 144 Kbps to 1.54 Mbps, and now even faster up to 6.0 Mbps--so customers can choose the rate that meets their specific needs. At the customer premises, a DSL router or modem connects the DSL line to a local-area network (LAN) or an individual computer. Once installed, the DSL router provides the customer site with continuous connection to the Internet and use of the telephone at the same time.

DSL Benefits

• Always-On Service
• Phone/Internet Simultaneously
• Up to 25x Times Faster Than Dial-up Modem
• Cost Effective
• No More Busy Signals
• No More Dropped Connections
• Faster Downloads
• Faster Games
• Multiple Computers on Single DSL Line
• Dedicated Connection & Speed

Here are some advantages of DSL:

• You can leave your Internet connection open and still use the phone line for voice calls.
• The speed is much higher than a regular modem (1.5 Mps vs. 56 kps)
• DSL doesn't necessarily require new wiring; it can use the phone line you already have.
• The company that offers DSL will usually provide the modem as part of the installation.

• A DSL connection works better when you are closer to the provider's central office.
• The connection is faster for receiving data than it is for sending data over the Internet.
• The service is not available everywhere.

Voice and Data

Other Types of DSL Very high bit-rate DSL (VDSL) -- This is a fast connection, but works only over a short distance. Symmetric DSL (SDSL) -- This connection, used mainly by small businesses, doesn't allow you to use the phone at the same time, but the speed of receiving and sending data is the same. Rate-Adaptive DSL (RADSL) -- This is a variation of ADSL, but the modem can adjust the speed of the connection, depending on the length and quality of the line.

Most home and small business users are connected to an Asymmetrical DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is 3-4 times faster than the connection from the user back to the Internet, then the user will see the most benefit, most of the time.

Precisely how much benefit you see will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection's length increases, the signal quality decreases, and the connection speed goes down. The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have the potential for seeing very high speeds in the future. For example, ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64-640 kbps.

You might wonder, if distance is a limitation for DSL, why it's not also a limitation for voice telephone calls. The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:

• The presence of "bridge taps." These are extensions, between you and the central office, that extend service to other customers. While you wouldn't notice these bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
• Fiber-optic cables. ADSL signals can't pass through the conversion from analog to digital and back to analog that occurs if a portion of your telephone circuit comes through fiber-optic cables.
• Distance. Even if you know where your central office is (don't be surprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal must travel between your house and the office.

Bandwidth aggregation

I'm going to talk about one of the keys to broadband, and why, despite all the manipulation, the telcos will never be able to monopolize Internet service on Broadband.  DSL users make a basic tradeoff: They give up slow-but-guaranteed data delivery, for fast-but-uncertain data delivery. Since most end users are technically illiterate, all they see is that "Wow, this DSL stuff runs pretty darn fast!" Few DSL users understand that the telcos aren't stupid; they have a very long term master plan, and ultimately the fantastic speed is going to be whittled away. This is done through bandwidth aggregation

To understand DSL, we need to understand the basic principle of a packet-switched network. While packet-switched networks come in many shapes and sizes, ultimately they all boil down to one of two types: Single Access or Multiple Access.

A Single Access switched network is represented by two endpoints - the user and the user's destination. This is often referred to as a circuit-based network. Since DSL networks and other broadband technologies don't operate on Single-Access networks, for space reasons I won't discuss them further.

With a Multiple Access network, all nodes on the network can place packets on the network whenever they please, without regard for what any other node is doing. Common examples are Ethernet, DSL, Cable, Frame Relay, and so on. All of these use a technology where data are grouped into packets.

Multiple Access networks operate on the assumption that some packets are going to be trashed: Any node may disrupt a packet by attempting to inject another. Some technologies, such as Ethernet, add a Carrier Sense that at least waits until the network is clear of packets before injecting one. This fails if two nodes attempt to inject at exactly the same time, because then both packets will be trashed. Also, network propagation delays render Carrier Sensing worthless over distances of a few hundred feet on copper, and a few kilometers on fiber.

While it may seem that Multiple Access cannot work, the beauty of the system is that statistically most nodes won't transmit at the same time, and packets are so fast that most of them will get through. As more nodes transmit, and more packets get trashed, though, congestion occurs, and different technologies have different ways of dealing with such congestion.

Since packet loss can occur on a Multiple Access network, the transmitting and receiving electronics must have some mechanism for detecting missing packets and retransmitting them. Usually these networks, like TCP/IP over DSL, are designed so that the user's protocol stack handles retransmits.

Telcos love Multiple Access networks because most of the time users on the network are not transmitting packets, and the telco does not have to devote any resources to them. So, DSL networks were organized on Multiple Access packet switching technologies; in Portland, the US West network uses ATM, and the GTE network uses frame relay. Thus, on DSL networks, telcos simply continue adding users until congestion rises to the point that the network becomes unusable.

DSL Packet Loss

In a DSL network, packet loss does not occur by crudely smashing packets together and dealing with the results (like it does on Ethernet). Instead, packet loss is created either by the network equipment itself through aggregation, or by random line noise on that ruins packets.

To see where packet loss is taking place I'll sketch the Portland US West DSL ATM network. In the Portland network, there is a core network of intermeshed ATM switches. (Intermeshing is where every ATM switch in the group is connected to every other ATM switch) ISPs plug into this mesh with their routers. Radiating out from the mesh are high-capacity point-to-point circuits that are run to every single Central Office in the city, and are plugged into multiple DSL Access Multiplexers (DSLAMs) in that office. Each DSLAM in a Central Office is one of the Cisco 61xx series of DSLAMS, some carrying up to 200 ports per unit. Radiating out from each DSLAM like spokes on a wheel are the individual subscriber DSL lines.

So, when a packet originates at a Customer Premise Equipment (CPE) DSL device, it first passes over the subscriber phone line to the DSLAM. It can be destroyed on this line as a result of line noise, and a certain percentage of packets are lost this way. The next point of loss is the DSLAM itself. For example, a Cisco 6260 with 256 DSL lines running at 256K represents 65Mbt of data -- but the 6260 only has a DS3 interface running at 45Mbts. If all lines are running full, then 30% of the packets are thrown away. The next loss point is at the main core ATM switches, and while we don't know how many of these switches exist and what their pipe connections are, another 50% aggregation loss at full-bore wouldn't be unreasonable. The next loss point is from the core to the ISP -- and this depends on how many subscribers the ISP handles per circuit.

Consider that US West charges a minimum of about $1200 for 3Mbt of access for an ISPs MegaCentral DSL connection. An Internet feed of true 3MB of unaggregated data from the backbone (not crummy aggregated DSL) is going to cost at least $1500. If the ISP already has a POP in the US West service area and is selling DSL at $20 per 256K connection, to break even they have to load 135 customers on the 3Mbt of bandwidth. Thus, if all customers run flat out, the ISP is effectively shoving 34Mbt of bandwidth into the 3Mbt they purchase from US West. This is 92% packet loss: During periods of high congestion, your TCP/IP packet has an 8% chance of even making it through the ISPs intertie into US West, and probably another 8% chance of making it through the telco network after that!

Now you see what they can afford to sell DSL at $20 per month!

Of course, it's easy to see that it would be exceedingly rare for the DSL network to be running at 100% utilization. But it's also easy to see that as the US West DSL network approaches 100% utilization, then packet loss is going to approach 100% loss!

This is what the telcos really have in store for the DSL network. When the Portland DSL network was first deployed, no users were on it. As the first users got on it, there was a situation where a small handful of users had access to these giant high-capacity pipes. It's great service when your 256K CPE is plugged into a DSLAM that has a 45Mbt DS3 coming off of it, and there are only ten people on it. But the telco is running at a financial loss when only ten people are on it: They want to stack ten to thirty times that number of people on it. Ultimately the master plan is going to call for about 90% deliberate packet loss during peak periods of high congestion, such as at 10:00am during business hours, or 6:00pm-9:00pm evenings (depending on your ISP)

Now, I mentioned earlier that Bandwidth Aggregation was the key to preventing the telcos from monopolizing Internet service. At first glance, it seems that the ISPs are always going to have the worse end of the stick: Their aggregation is always going to be higher than the telcos' because the ISPs have to pay for their circuits and the telcos own the circuits.

But this is the key: The ISPs have control of what I refer to as the Aggregation Factor.

ISPs, in general, are unregulated companies. Telcos -- at least the circuit portion of them -- are regulated. If Joe User decides to get onto the US West DSL network, then no matter what he does on the network, no matter how much bandwidth he sucks down, US West is powerless to throw him off without risking a lawsuit. After all, their own tariffs state that a DSL circuit is unguaranteed bandwidth; the Achilles heel is that there's no part of the tariff that says the bandwidth is more unguaranteed for some people than others!

Users on the telco DSL network don't have to fear that the telco is killing their bandwidth. The Telco isn't -- it's the other users! The situation is exactly analogous to rush hour on the Sunset Highway: You can't complain to the State that the highway is overcrowded, because it's not the State that is overcrowding the highway, it's the other drivers!

ISPs, on the other hand, don't have Big Brother Government and the PUC breathing down their necks. If Joe User buys a residential 256K account from Random ISP and sets up an illegal MP3/Porno site that is slammed 24 hours a day, then Random ISP can toss Joe off the service without penalty.

ISPs that understand this have teeth like bandwidth download limits in their contracts, and provisions against abuse of their DSL networks. Of course, from a technical perspective, there's nothing wrong with Joe User's MP3/Porno site as long as Joe pays $200 per month instead of $20 per month, so that Random ISP doesn't have to put 60 people on the circuit to get the same revenue.

This is where giant ISPs like US West net fall on their faces, and the smaller ISPs really shine. A big ISP doesn't have time to police their giant DSL networks: They simply have too many subscribers. They also have a very strong financial incentive not to police their networks: US West in particular makes far more money selling frame relay and other expensive high-capacity business services. If they policed their DSL network, then the businesses would throw out their expensive frame connections and switch to DSL.

Smaller ISPs have fewer connections and can keep a much closer eye on their DSL nets. They can raise the prices for users who run illegal MP3/Porno sites that are getting slammed. They have an incentive to do this: ISPs make no money from the circuit portion of an Internet connection. To understand this, consider the following scenario. Random ISP delivers 256K of guaranteed bandwidth on DSL that costs $100 per month. The DSL circuit portion paid to US West costs $30 per month. Random ISP gets $100. Random ISP also sells the same 256K of guaranteed Internet service on frame relay at $100 per month. In this case, the circuit portion costs $300 per month to US West for a 256K Frame circuit. Random ISP still gets $100 per month.

By contrast, US West proposes to deliver 256K of guaranteed bandwidth on DSL that costs $100 per month. The DSL circuit portion paid to them costs $30 per month. So, US West gets $130 per month for this. But US West also sells 256K of guaranteed bandwidth on Frame Relay at $100 per month. In this case, the circuit portion costs $300 per month. US West has just gotten $400 per month on this sale! So, the incentive is there to deliver wildly varying bandwidth on DSL from the telco, so that the business customer continues to buy the more expensive Frame Relay.

A telco ISP has no incentive to deliver guaranteed, or even decent, bandwidth on DSL. In the previous scenario, if US West delivers guaranteed 256K on DSL, instead of frame relay, they lose $170 per month! Thus, as long as a telco is acting as an ISP, it is never going to have the same level of DSL bandwidth service that a non-telco ISP can afford. Think about this the next time you consider purchasing a unified ISP/Telephone bill from US West or GTE, or for that matter any telco/ISP combination company. There's a fundamental conflict of interest that may result in you paying more -- a lot more -- than you might otherwise pay.

As interesting as the business politics have been, I'm going to turn away from them next month and tackle the next critical component of the DSL equation: the Customer Premise Equipment, or CPE.