DSL (Digital Subscriber Line) evolved from the necessity to transfer digital information for the last few thousand feet of ‘local loop’. There are several different DSL technologies, the earliest being HDSL (High Bit Rate Digital Subscriber Line), but HDSL really only replaced existing digital technologies with another. The newer DSL technologies allow for simultaneous analogue and digital transmission, with the digital portion at much higher data rates. The technology behind DSL systems continues to evolve. Although there are standards, various proprietary systems have been developed for DSL systems that deviate from the standards. The result is that some DSL equipment is not compatible with other DSL equipment of the same classification.
The first Digital Subscriber Line system was HDSL, which increased the distance that high speed digital signals could be transmitted without the use of a repeater/amplifier. The HDSL system did require 2 (or 3) pairs of copper wires to allow simultaneous send and receive of up to 2Mbps. To conserve the number of copper pairs for data transmissions, synchronous digital subscriber line technology was developed. Although SDSL (Symmetric Digital Subscriber Line) systems offered lower data rates than HDSL, only 2 wire pairs were required. Since SDSL was developed, the HDSL system has evolved to a 2nd generation (HDSL2) that allows the use of 2 wire pairs for duplex transmission with reduced emissions (lower egress). New efficient modulation technology used by ADSL systems dramatically increased the data transmission rates from the Central Office to the customer to over 6Mbps (some ADSL systems to 8Mbps). To take advantage of ISDN equipment and efficiency, an offshoot of ISDN technology that was adapted for the local loop developed called IDSL. ADSL systems evolved to rate adaptive digital subscriber line (RADSL) that allows the data rate to be automatically or manually changed by the service provider. To simplify the installation of consumer-based DSL equipment, a low data transmission offshoot of ADSL developed, called ADSL-lite. Using similar technology to ADSL, another system called VDSL (Very High Bit Rate Digital Subscriber Line) evolved that can provide data rates up to 52 Mbps transferred over shorter distances.
The key to xDSL technology is a more efficient use ot the 1Mhz bandwidth afforded on a single pair of copper wire telephone lines. An XDSL system consists of compatible modems on each end of the local loop. For some systems, the xDSL system allows for multiple types of transmission on a single copper wire. This includes analogue or ISDN digital telephone (POTS) and digital communications (ADSL or VDSL). The above diagram shows that there are basic tradeoffs for a DSL system. Generally, the longer the distance of the copper line, the lower the data rate. Distances of less than 1000 feet can achieve data rates of over 50 Mbps.
ADSL (Asymmetric Digital Subscriber Line) characterized by different downstream and upstream data rates; typically 10 times higher in downstream and maximum rate is dependant on length and condition of loop determined in initialization or on maximum provisioned rate.
HDSL (High Bit Rate Digital Subscriber Line) is a symmetric technology, the same speed in each direction; HSDL uses 2B1Q modulation and usually requires two separate twisted pairs both of which must be full duplex. HDSL2 is a follow-on technology currently undergoing standardization.
IDSL (ISDN Digital Subscriber Line) uses same 2B1Q line code technology as ISDN; it is a symmetric service.
RADSL (Rate Adaptive ADSL) provides automatic selection of optimum operating rate for a twisted pair; name seldom used as ADSL is usually assumed to be rate adaptive.
SDSL (Symmetric Digital Subscriber Line) otherwise known as G.SHDSL, data streams are symmetric in upstream and downstream directions; typical rates are 384 Kbps and 768 Kbps.
VDSL (Very High Speed Digital Subscriber Line) can run symmetric or asymmetric rates; highest rate currently proposed is 26Mbps in each direction, other rates proposed include 52 Mbps downstream/6.4 Mbps upstream and 26 Mbps downstream/3.2 Mbps upstream. Maximum loop length is less than possible for ADSL.. Potential applications of VDSL, while currently unclear, could be as the last link in a full-service multi-media network delivering voice, video and data services.
CDSL (Consumer DSL) or ADSL.Lite are approaches to getting lower cost installation and deployment of ADSL.
DSL modems improve over the rates of traditional modems by two means:
- They restrict the length of the connection between the modems
- They use higher frequencies on the wire than a normal telephone system allows.
ADSL can run over the majority of existing copper loops in place today. This covers over 400 million telephone lines. This can be increased if repeaters are installed to reduce the longest un -amplified length of wire down to around 12000 feet (3.6 km). Manufacturers of ADSL equipment are clearly targeting this length as an area for improvement.
Note that different technologies in the DSL family have different needs as far as distance and the number of twisted pairs needed are concerned.
An ADSL transmission unit can simultaneously convey all of the following: a number of downstream frame bearers, a number of upstream frame bearers, a baseband POTS/ISDN duplex channel, and ADSL line overhead for framing, error control, operations, and maintenance. Systems support a net data rate ranging up to a minimum of 8 Mbit/s downstream and 800 kbit/s upstream. Support of net data rates above 8 Mbit/s downstream and support of net data rates above 800 kbit/s upstream are optional.
Remember that ADSL is a distance-limited technology – the longer the wire, the slower it runs. As a direct consequence of this the less data rate you need then the further from the exchange the user can be and still receive the service.
This tends to measured in kilofeet To translate this into more normal units.
10 kilofeet is 3 km or about 1.9 miles for those who think in such units.