DSL - Digital Subscriber Line

Now even the average Joe on the street knows at least a bit about DSL.  Yeah, it's that technology that allows you to connect at high-speed to the Internet.  Well, it started out with a bang, and now is in a bit of a retreat, due to the onslaught of it's main competitor . . . the cable companies.  Nevertheless, it is the Internet access solution for millions of homes and small businesses, and it has the advantage of being able to use your phone line - while at the same time allowing you to place calls on the same line !!

This is actually a family of technologies that have begun to transform the narrow band copper access network into broadband network is the xDSL family of technologies. The term DSL, or digital subscriber line, refers to the modem which when connected at either ends of a normal twisted wire pair line, converts it into a digital line capable of handling data rates well into broad band. By using higher frequencies, DSL technologies enable much higher speeds over the twisted pair lines. Speeds up to 2 Mbps are readily achievable -- 35 times faster than today's fastest analog modems. Much higher speeds, up to about 52 Mbps, are today possible. This means that customers can have applications that need these higher speeds even if their towns or villages do not have optical fibre in the local loop.

The letter "x" indicates that there are many variants of DSL technology. Some of these are HDSL, SDSL, ADSL and VDSL. By far, the most common is ADSL, with SDSL running a distant second.  Although the transmission varies greatly among these technologies, there are some distinct similarities. They are all bi-directional, digital signals that run on non-loaded, copper loop. Notwithstanding these similarities each type of DSL technology has its own characteristics. A brief discussion is given below

3.1 High data rate digital subscriber line (HDSL) is a technology that delivers 2 Mbps to the customers. It is in effect a better way of transmitting E1 over a twisted pair of wires. HDSL requires 2 non- loaded copper pairs for bi-directional transmission. The loop length limitation is 4 to 5 kms on 0.5mm cable. Typical applications include PBX network connections, cellular base stations, Internet servers, WAN/LAN access. In some applications it is likely to give way to ADSL and SDSL in near future.

3.2 Single line digital subscriber line (SDSL) is the single line version of HDSL, transmitting E1/T1 signals over a single twisted pair and in most cases operating over POTS so a single line can support POTS and E1/T1 simultaneously. Most subscriber premises are equipped with a single pair and SDSL has an edge under such circumstances. Since SDSL works on a single pair, it commonly provides high- speed connections for work-at-home and Internet access applications over residential local loops. It should however be noted that SDSL has a limitation of about 3 to 4 km, a distance over which ADSL gives much higher rates.

3.3 Asymmetric digital subscriber line (ADSL) transmits an asymmetric data stream, much more going downstream to the subscriber and much less coming back. This is not a serious limitation because the most of the target applications for digital subscriber services are asymmetric. Fast internet access, video on demand, home shopping, remote LAN access, multimedia access, specialized PC services all feature high data rate demands downstream but relatively low data rate demands upstream. ADSL requires only one pair and delivers upto 2-8 Mbps. For 2Mbps the distance is about 5.5 km and for 8 Mbps it is about 3 km. ADSL enables POTS to be delivered over the same copper pair.

3.4 Very high Data rate Digital Subscriber Line (VDSL) is a new technology not expected to be in use in public networks for some years. VDSL transmit asymmetric streams at data rates higher than ADSL but over shorter lines. It is expected to provide speeds as high as 52 Mbps downstream and between 1.5 and 2.3 Mbps upstream. The distance at the upper end at 52 Mbps would be about 0.3kms and more at lower speeds (1.3 km at 13 Mbps). These could work in conjunction with fibre to the curb or building to deliver high bandwidth to the homes. VDSL can take care of all ADSL applications and additionally high definition TV.

 

Fig 2. General Model of DSL systems

Comparison of various DSL technologies is given below:

 
Name Meaning Data Rate Connection Type Distance to exchange Applications
DSL Digital subscriber Line 160kb/s Symmetrical ~5 km ISDN series, voice and data comm.
HDSL High Data Rate Digital Subscriber Line 2Mb/s

 

Symmetrical 4-5km No POTS, E1 LAN/WAN, Service access feeder plant.
SDSL Single Line Digital Subscriber Line 2Mb/s Symmetrical 3-4km Same as HDSL + POTS
ADSL Asymmetric Digital Subscriber Line 1.5 to 8Mb/s Down 128kb-768kb Up Asymmetrical 3-6km Interest access, video on demand, simplex video, remote LAN access, interactive multimedia.
VDSL Very High Data Rate Digital Subscriber Line 13-52Mbp/s Down 1.5-2.3Mbp/s Up Asymmetrical 0.3-1.5km Same as ADSL plus HDTV