The Basics of DSL

DSL is the commonly used acronym for digital subscriber line, but there are many different DSL technologies on the market.  Most forms of DSL are referred to as xDSL, where the letter x is replaced by another letter that is part of a larger acronym that describes that particular form of DSL.  For example, ADSL (asynchronous digital subscriber lines) are easily the most prevalent form of DSL on the market, but VDSL (very high speed digital subscriber line) networks are becoming far more prevalent.  Despite the wide discrepancies between the dozen or so basic DSL types and technologies, the principals behind all DSL systems is fairly similar: data is compressed by hardware devices on a per-packet level and transmitted over metal wiring, often the same copper wiring installed by telecoms for voice transmission years or even decades before DSL became available, at frequencies outside the human hearing range.

The fact that DSL transmissions can take place over existing infrastructure helps reduce the overhead of providers in many cases, which in turn makes DSL services more affordable for consumers.  Of course, not all public switched telephone networks (PSTN) are 100% compatible with DSL technologies, so some upgrades are typically required, but the very high-band nature of DSL is designed to be highly compatible with existing telecom infrastructures.

A Tale of Two Frequencies

The result is that data and voice communications can both use the same wiring at the same time without interruption, but there is a catch: most phone lines will need to be fitted with a filter that limits static and other noises that would interfere with the data transmission portion of the spectrum.  The ability to simultaneously transmit data and voice communications requires separating these frequencies, or the use of a phone may interrupt the transmission of data.

The use of higher frequencies also permits more data to be transmitted than data transmitted by standard analog modems that used portions of the frequency spectrum that overlapped those of the human hearing range.  Simply put, higher frequencies mean that more data can be transmitted in any given period of time.  The higher the frequency that a DSL system uses, the better the data transmission rates are likely to be.

Distance and DSLAMs

When a consumer turns on a typical DSL modem, the device will usually run a self-diagnosis routine before attempting to use the upper frequencies to establish a connection to the nearest DSLAM (Digital Subscribe Line Access Multiplexer) device deployed by the local telecom.  Once connected, the DSL modem will attempt to synchronize with the DSLAM, after which data can be sent and received.  A DSLAM is an extremely expensive networking device which essentially acts like a remote sub-station, or a tremendously powerful network router; data transmitted to and from local DSL users come to and from the DSLAM which in turn handles traffic to and from the local telecom’s backbone.  In most cases, the DSLAM uses a technology other than DSL to connect to and communicate with the actual telecom backbone.

The length of wiring between the DSL modem and the local DSLAM plays a major role in determining the overall speed and quality of a DSL connection.  In cases where the consumer is located near the outer limits of the effective range of a local DSLAM, they will only be able to effectively send and receive data at limited speeds due to the degradation of electrical signals traveling through metal wiring.  Higher frequency signals degrade and lose fidelity much faster than lower frequency signals, which is why basic telephone service may be available far away from a DSLAM or local substation while DSL services may not be.

The Many Flavors of DSL

As previously mentioned, there are many different kinds of DSL available, the most common of which is ADSL.  ADSL networks communicate asynchronously, meaning that data speeds for uploads and downloads will not be the same.  In almost all cases, ADSL services offer greater download speeds than upload speeds, which makes ADSL ideal for a wide variety of the most common tasks, such as: web surfing, video streaming, downloading files, and gaming.  ADSL may not be ideal for hosting web sites via dedicated IP addresses, and other tasks that require a great deal of upstream bandwidth.

SDSL, or synchronous digital subscriber line, offers consumers equal upload and download speeds.  This approach means that SDSL has no technological weaknesses or flaws that can be exposed by certain tasks, but SDSL is not as widely deployed as ADSL and often has greater restrictions in terms of distance from a DSLAM and pricing plans that are considerably higher than comparable ADSL packages.

IDSL is a type of DSL that runs over ISDN (Integrated Services Digital Network) lines, but has only seen limited deployment due to ISDN’s lack of market penetration.  HSDL (High Speed Digital Subscriber Lines) are also available in some markets, but it truly seems that the aforementioned VDSL is the standard that will eventually supersede ADSL and become the de facto standard people refer to when using the generic term DSL.

VDSL, It Is Not Just For Data Anymore

VDSL is currently in its second generation, and while tech sites may claim that the theoretical performance of the newest generation of VDSL is in the hundreds of Mbps, the fact is that those numbers are only attainable by those within a few hundred feet of the DSL.  This distance limitation means that such incredible speeds are not typically part of the offerings of companies that provide VDSL services such as AT&T’s U-verse, but speeds in excess of 30 Mbps are readily available to consumers within a kilometer or so of a DSLAM.  Not all of this bandwidth is necessarily dedicated to broadband data access, however, as carriers such as AT&T offer a variety of other services with a portion of the bandwidth.  AT&T’s U-verse is an example of VDSL that is used to provide consumers with high-speed broadband access as well as feature-rich digital television and digital telephone services.