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Digital Subscriber Line

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The topic is Digital Subscriber Line. A monument to humanity's stubborn refusal to replace old things when a perfectly good, if tortured, workaround exists. Let's get this over with.

"DSL" redirects here. If you were looking for something else, your search terms were as imprecise as your request. See DSL (disambiguation).

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• v • t • e

Digital subscriber line (DSL; originally conceived as digital subscriber loop) represents a family of technologies united by a single, audacious goal: to transmit digital data over the ancient copper telephone lines that were designed for your grandmother's analog chatter. [1] In the lexicon of telecommunications marketing—a field not known for its subtlety—the term DSL is almost universally understood to mean asymmetric digital subscriber line (ADSL). It is the most commonly inflicted DSL technology, a begrudgingly effective method for delivering Internet access.

The "asymmetric" in ADSL isn't just a technical descriptor; it's a commentary on user behavior. Data throughput in the upstream direction—that is, from you to the service provider—is deliberately throttled to be lower than the downstream flow. This design choice caters to a consumption-heavy model of internet use. Conversely, in the less common symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are treated as equals, a quaint notion in today's media-saturated world.

One of DSL's more clever tricks is its ability to coexist with a wired telephone service on the exact same line. This is achieved by hijacking the higher, unused frequency bands for data, leaving the lower frequencies for voice. To prevent your phone calls from sounding like a modem screaming into the void, a simple DSL filter is installed on each telephone, a small but crucial gatekeeper maintaining the peace between old and new technology.

The performance you can expect from this arrangement varies wildly. Consumer ADSL services typically crawl along at speeds from 256 kbit/s up to a more respectable 25 Mbit/s. The later iteration, VDSL+, pushes this envelope, delivering between 16 Mbit/s and 250 Mbit/s in the downstream direction, with a more generous upstream of up to 40 Mbit/s. Of course, these are optimistic figures. The actual performance is a messy function of the specific technology, the decrepitude of your phone line, and the service-level agreement you've signed. In a fit of engineering brilliance, researchers at Bell Labs have demonstrated SDSL speeds exceeding 1 Gbit/s on traditional copper lines, a feat of technical wizardry that, predictably, has yet to escape the lab and find its way to end customers. [2] [3] [4]

History

There was a time, born of limited imagination, when it was believed that ordinary phone lines were good for little more than modest data speeds, typically topping out below 9600 bits per second. This assumption ignored the evidence right in front of them. In the 1950s, the same ordinary twisted-pair telephone cable was routinely used to carry 4 MHz television signals between studios, a clear indication that these lines were capable of transmitting many megabits per second. One such circuit in the United Kingdom stretched for about 10 miles (16 km) between the BBC studios in Newcastle-upon-Tyne and the Pontop Pike transmitting station. The problem wasn't the copper itself, but the host of other impairments beyond simple Gaussian noise that made such high rates impractical in the field. The 1980s, however, saw the development of sophisticated techniques for broadband communications that allowed this theoretical limit to be shattered. A patent was filed in 1979 for a system using existing telephone wires for both voice and data terminals connected to a remote computer, a prescient glimpse of the future. [5]

The primary motivation for digital subscriber line technology was the grand, and somewhat bureaucratic, vision of the Integrated Services Digital Network (ISDN). This specification was proposed in 1984 by the CCITT (now the ITU-T) as part of Recommendation I.120, and its principles were later repurposed for what became known as ISDN digital subscriber line (IDSL). Employees at Bellcore (now Telcordia Technologies) then developed asymmetric digital subscriber line (ADSL). Their key innovation was to place wide-band digital signals at frequencies far above the existing baseband analog voice signal, which was carried on the conventional twisted pair cabling between telephone exchanges and customers. [6] AT&T Bell Labs filed a patent on this foundational DSL concept in 1988. [7]

The contribution of Joseph W. Lechleider to DSL was his critical insight that an asymmetric arrangement could offer more than double the bandwidth capacity of a symmetric one. [8] This was not just a technical tweak; it was a fundamental shift that recognized the reality of internet usage. It allowed Internet service providers to offer a service that felt fast to consumers, who were far more interested in downloading large amounts of data than they were in uploading comparable amounts. ADSL supports two transport modes: a fast channel, preferred for streaming multimedia where an occasional lost bit is a small price to pay for low latency, and an interleaved channel, which is better for file transfers where data integrity is paramount, and the time delay from retransmitting packets is acceptable.

Consumer-focused ADSL was engineered to operate on existing lines that had already been conditioned for Basic Rate Interface ISDN services. Concurrently, engineers developed high-speed DSL facilities like high bit rate digital subscriber line (HDSL) and symmetric digital subscriber line (SDSL) to provision traditional Digital Signal 1 (DS1) services over standard copper infrastructure.

Early ADSL standards managed to deliver 8 Mbit/s to the customer over approximately 2 km (1.2 mi) of unshielded twisted-pair copper wire. Newer versions improved on these rates, but the physics remained unforgiving. Distances greater than 2 km dramatically reduce the usable bandwidth, crippling the data rate. To combat this, ADSL loop extenders were developed, acting as repeaters for the signal and allowing the local exchange carrier (LEC) to deliver DSL speeds over greater distances. [9]

DSL SoC

Until the late 1990s, the cost of the digital signal processors required for DSL was prohibitively high. All forms of DSL rely on intensely complex digital signal processing algorithms to overcome the inherent flaws and limitations of existing twisted pair wires. It was the relentless advance of very-large-scale integration (VLSI) technology that finally made DSL economically viable. The cost of the two main pieces of equipment—a digital subscriber line access multiplexer (DSLAM) at the exchange and a DSL modem at the customer's end—plummeted.

Setting up a DSL connection over an existing cable became vastly cheaper than the monumental expense of installing new, high-bandwidth fiber-optic cable over the same route. This economic reality, true for both ADSL and SDSL, is why DSL persists. Its continued use is a testament to advancements in electronics that have squeezed every last drop of performance from an aging infrastructure, offsetting the high cost of laying new physical lines.

These advantages made ADSL a far more attractive proposition for customers needing Internet access than metered dial-up, with the added benefit of allowing voice calls and data connections simultaneously. Telephone companies, feeling the heat from cable companies armed with DOCSIS cable modem technology, were pressured to adopt ADSL. The rising demand for high-bandwidth applications like video streaming and file sharing further cemented its popularity. Some of the first field trials for DSL were conducted in 1996. [10]

Initially, DSL service required a dedicated dry loop. However, a regulatory shift occurred when the U.S. Federal Communications Commission (FCC) mandated that incumbent local exchange carriers (ILECs) must lease their lines to competing DSL service providers. This gave rise to shared-line DSL, also known as DSL over an unbundled network element. This unbundling allowed a single subscriber to receive two distinct services from two different providers on a single copper pair. The competitor's equipment is co-located in the same telephone exchange as the ILEC's. The subscriber's circuit is then rewired to interface with hardware that combines the DSL frequencies and the plain old telephone service (POTS) signals onto that one line.

Since 1999, some ISPs have been offering microfilters. These small, user-installable devices perform the same function as the larger, outdoor-deployed DSL splitters: they separate the frequencies for ADSL from those used for POTS phone calls. [11] [12] The push for these filters came from a desire to make DSL a self-installation product, eliminating the need for a technician to install the earlier outdoor splitters near the demarcation point between the customer's property and the ISP's network. [13]

By 2012, the landscape was shifting again. Some carriers in the United States reported that DSL remote terminals, connected with fiber backhaul, were beginning to replace the older, purely copper-based ADSL systems, a quiet admission that copper's days were numbered. [14]

Operation

The connection between your telephone and the telephone exchange is made via a local loop—a physical pair of copper wires. This loop was designed for one thing: transmitting human speech, a task which occupies a narrow audio frequency range of 300 to 3400 hertz (the commercial bandwidth). But as long-distance trunks were upgraded from analog to digital, the idea of forcing data through the local loop by using frequencies above the voiceband gained traction, eventually culminating in DSL.

The local loop has far more capacity than POTS requires. Depending on its length and quality, it can carry frequencies well into the tens of megahertz. DSL exploits this vast, unused bandwidth by creating thousands of 4312.5 Hz wide channels, starting somewhere between 10 and 100 kHz. The allocation of these channels continues up to higher frequencies—as high as 1.1 MHz for ADSL—until the signal degrades to the point of being unusable. Each of these tiny channels is continuously evaluated for usability, much like an analog modem probes a POTS line. The more usable channels, the more available bandwidth. This is why distance and line quality are the tyrannical dictators of DSL performance; the higher frequencies essential for high speeds degrade rapidly over distance.

This pool of usable channels is then bisected into two frequency bands, one for upstream and one for downstream traffic, based on a preconfigured ratio. This segregation is a simple but effective way to reduce interference. Once these channel groups are established, the individual channels are bonded into a pair of virtual circuits, one for each direction. Like analog modems, DSL transceivers are paranoid monitors, constantly checking the quality of each channel and adding or removing them from service as conditions change. With upstream and downstream circuits established, a subscriber can finally connect to a service, be it an Internet service provider or a corporate MPLS network.

The underlying technology is a form of modulation of high-frequency carrier waves, an analog signal transmission. At each end of the circuit, a modem modulates patterns of bits into high-frequency impulses for the journey to the opposing modem. The received signals are then demodulated to reconstruct the original bit pattern, which is passed in digital form to a computer, router, or switch.

Unlike old dial-up modems, which operated within the 300–3400 Hz audio baseband, DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This strict frequency separation is what allows DSL and plain old telephone service (POTS) to coexist on the same voice-grade cables. [15] At the subscriber's end, inline DSL filters are installed on every telephone. These are simple low-pass filters that pass voice frequencies but block the high-frequency DSL signals, which would otherwise manifest as an unpleasant hiss. They also prevent non-linear elements in the phone from creating audible intermodulation that could corrupt the data signal. Conversely, DSL modems incorporate high-pass filters to block the low-frequency voice signals.

Because DSL operates far above the 3.4 kHz voice limit, it is fatally incompatible with a loading coil. A loading coil is an inductor, periodically placed in longer POTS lines to counteract the signal loss caused by capacitance between the two wires. Voice service simply cannot be maintained over long distances without them. This means some areas, while technically within range for DSL, are disqualified because of these coils. Consequently, phone companies are often forced to remove them from copper loops that can function without them. For even longer lines, the only solution is often to replace them with fiber to the neighborhood or node (FTTN).

Most residential and small-office DSL setups reserve the lowest frequencies for POTS, ensuring that existing voice service, including fax machines and even dial-up modems, can continue to operate alongside the DSL service. It's important to note that only one DSL modem can use the subscriber line at a time. The standard method for sharing a single DSL connection among multiple computers involves a router, which connects the DSL modem to a local Ethernet, powerline, or Wi-Fi network.

The theoretical underpinnings of DSL can be traced back to Claude Shannon's foundational 1948 paper, "A Mathematical Theory of Communication". While higher bit rates generally require wider frequency bands, the relationship between bit rate, symbol rate, and bandwidth is not linear, thanks to significant innovations in digital signal processing and digital modulation methods.

Naked DSL

Naked DSL is the practice of providing only DSL services over a local loop, without an accompanying voice service. It's for customers who have no need for traditional telephony, either because they use a voice service on top of the DSL (like VoIP) or rely on another network entirely (such as mobile telephony). This service is also known as an unbundled network element (UNE) in the United States. In Australia, it's called an unconditioned local loop (ULL); [16] in Belgium, "raw copper"; and in the UK, Single Order GEA (SoGEA). [17]

This more logical approach began to gain traction in the United States around 2004, when Qwest started offering it, soon followed by Speakeasy. Following major mergers, like AT&T with SBC [18] and Verizon with MCI, [19] these giant telephone companies were obligated to offer naked DSL to consumers.

Typical setup

On the customer's side of the equation, a DSL modem is connected to a phone line. The telephone company connects the other end of that line to a DSLAM (digital subscriber line access multiplexer), a device that concentrates a large number of individual DSL connections into a single, more manageable unit. The DSLAM must be located relatively close to the customer due to the signal attenuation that occurs between it and the user's modem. It's common for a single DSLAM to serve a few residential blocks.

The schematic above illustrates a simple DSL connection. The right side depicts the DSLAM, typically housed in the telephone company's exchange. The left side shows the customer premises equipment, often including a router that manages a local area network for PCs and other devices. Many customers opt for an integrated modem that combines the modem, router, and wireless access point into a single box, simplifying the setup.

Exchange equipment

At the telephone exchange, a DSLAM terminates the incoming DSL circuits and aggregates them before they are handed off to other network transports, such as a PON network or Ethernet. The DSLAM's job is to terminate all these connections and recover the original digital information. For ADSL, the voice component is also separated at this stage, either by a filter or splitter built into the DSLAM or by specialized equipment installed before it. [20] Any load coils present in the phone lines, which only allow frequencies up to 4000 Hz, must be removed for DSL to function. [21] [22] [23]

Customer equipment

The customer's end of the connection is anchored by a DSL modem. This device is a translator, converting data between the digital signals used by computers and the analog voltage signal in the appropriate frequency range that is sent over the phone line.

In some DSL variants, like HDSL, the modem connects directly to a computer via a serial interface, using protocols such as Ethernet or V.35. In most consumer cases, particularly with ADSL, the customer equipment is an integrated device with higher-level functions like routing and firewalling. In this context, the equipment is more accurately called a gateway.

Most DSL technologies mandate the installation of DSL filters at the customer's premises to separate the high-frequency DSL signal from the low-frequency voice signal. This separation can occur at the main demarcation point or with individual filters installed at each telephone outlet. Some DSL gateways integrate the filter, allowing telephones to be connected directly through the gateway itself.

Modern DSL gateways are complex devices. Upon booting, the system first synchronizes the DSL connection and then establishes the internet IP services and connection between the local network and the service provider, using protocols like DHCP or PPPoE.

Protocols and configurations

Many DSL technologies implement an Asynchronous Transfer Mode (ATM) layer over the low-level bitstream. This layer of abstraction enables the adaptation of numerous different technologies over the same physical link.

DSL implementations can create either bridged or routed networks. In a bridged configuration, the subscriber's computers effectively connect into a single large subnetwork. The earliest implementations used DHCP to assign an IP address to the subscriber's equipment, with authentication handled via MAC address or an assigned hostname. Later, implementations shifted to using the Point-to-Point Protocol (PPP) to authenticate with a user ID and password, a more robust and manageable system.

Transmission modulation methods

The methods used for transmission vary by market, region, carrier, and equipment. They are all, fundamentally, different ways to shout data down a copper wire and hope it arrives intact.

DSL technologies

The sprawling family of DSL technologies, sometimes unhelpfully summarized as xDSL, is a testament to the endless effort to squeeze more performance from aging infrastructure.

Full name Abbreviation ITU-T standard Date
Asymmetric digital subscriber line ADSL G.992.1 (G.dmt) 1999
ADSL2 ADSL2 G.992.3 (G.dmt.bis) 2002
ADSL2plus ADSL2+ G.992.5 2003
Asymmetric digital subscriber line-Reach Extended ADSL2-RE G.992.3 2003
Single-pair high-speed digital subscriber line SHDSL G.991.2 2003
Very-high-bit-rate digital subscriber line VDSL G.993.1 2004
VDSL2 -12 MHz long reach VDSL2 G.993.2 2005
VDSL2 -30 MHz short reach VDSL2 G.993.2 2005

The list of technologies includes:

The severe line-length limitations impose a hard ceiling on data transmission rates. Technologies like VDSL provide very high speeds but only over very short ranges, making them suitable for delivering triple play services in network architectures like fiber to the curb.

Terabit DSL is a theoretical proposal that suggests using the space between the dielectrics on copper twisted pair lines as waveguides for 300 GHz signals. This could offer speeds of up to 1 terabit per second at 100 meters, 100 gigabits per second at 300 meters, and 10 gigabits per second at 500 meters. [35] [36] The first experiments were conducted with parallel, untwisted copper lines inside a metal pipe to simulate the armoring in large telephone cables. [37] [38] Whether this ever escapes the lab is anyone's guess.

See also