ADSL system basics overview
In this page we will cover ADSL system basics overview along with typical ADSL setup, ADSL types and ADSL standards.
Applications such as video streaming, VoIP, online video gaming, online shopping and TV programs need high speed internet for effective operation. Existing systems such as coaxial cable, ISDN and analog modem are not able to provide high data rate. This increase in demand for the high speed internet applications pioneered development of high speed internet services such as ADSL.
ADSL uses existing unshielded copper lines. Hence it can be used over existing millions of telephone lines already laid across the earth. The development of low cost DSP chips due to advancement of VLSI has made this technology more popular.
The fig-1 depicts typical ADSL setup. It consists of various subsystems such as
DSLAM(Digital Subscriber Line Access Multiplexer), voice switch, frequency selector switch,
main distribution panel, DP Box, Splitter and ADSL modem. Let us understand these systems.
As mentioned in order to provide ADSL connection to subscribers existing telephone
network is utilized. DSLAM at telephone exchange or CO(central office) and ADSL modem at
subscriber premise is installed.
• DSLAM: It is interfaced with internet backbone and takes care of routing of data traffic. It works similar to concentrator and provides multiple DSL connections to access the internet bandwidth. The connection to subscriber is given via copper lines.
• Frequency selector Switch: It is installed at telephone exchange. It is basically a filter which filters incoming ADSL signal and diverts voice traffic to voice switch and data traffic to DSLAM.
• Voice Switch:It is connected with PSTN and provides analog voice connections.
• Main Distribution Panel(MDP):DSLAM pairs are terminated at MDP from where connections to subscribers are provided using twisted pair of copper lines.
• Splitter: The twisted pair terminate at DP box housed in subscriber premise which is connected first with splitter. The splitter is basically a Low Pass filter which removes low frequency POTS voice signals within the band 300Hz to 3400Hz. It passes high frequency ADSL signals to the ADSL modem.
• ADSL Modem: Finally connection to user computer is provided using ethernet cable connected to ethernet port of the ADSL modem. The physical layer portion in the ADSL modem converts signal to the format understood by the computer.
There is another variant of ADSL known as ADSL-Lite. It is based on G.992.2 or G.Lite. This version does not need POTS splitter. ADSL-Lite delivers downstream speed of 1.5Mbps and upstream speed of 512Kbps.
ADSL is a full duplex communication system based either using TDD or FDD or ECD. FDD uses two separate frequency bands one for downstream and another for upstream. ECD stands for Echo Cancelling Duplex. Refer basic difference between TDD and FDD.
ADSL uses modulation techniques such as OFDM and DMT. Refer OFDM vs DMT for difference between OFDM and DMT techniques.
DSL family is collectively referred as xDSL. There are various different types such as IDSL, HDSL, SHDSL, RADSL, UDSL, VDSL, GDSL as outlined with their data rates, bandwidth allocations and maximum coverage distances. The most popular ADSL (Asymmetric ADSL) provides downstream data rate of about 8Mbps and upstream data rate of about 500Kbps.
The IDSL is the short form of Integrated DSL. It delivers data rate of 144Kbps for maximum distance of about 5.4 Km.
The HDSL is the short form of High bit rate DSL. It is symmetric technology. It delivers data rate of 2.048Mbps for maximum distance of about 4.6 Km. It uses pair of copper wires.
The SHDSL is the short form of single pair high speed DSL. It delivers data rate of about 384Kbps to 4.608Mbps. The step size is about 128Kbps.
The RADSL is the short form of Rate adoptive DSL. This version of DSL adjusts its downstream data rate and upstream data rate as per line noise and distance required to be covered.
The UDSL is the short form of Uni-DSL or Universal DSL. It delivers downstream and upstream data rate of about 200Mbps.
The VDSL is the short form of Very High Speed DSL. It is symmetric and delivers downstream data rate of about 13 to 52Mbps and upstream data rate of about 1.5Mbps to 2.3 Mbps using single copper line. It covers distance of about 300meters to 1.4Km from exchange.
The GDSL is the short form of Gigabit DSL. It is based on MIMO concept. It delivers data rate of about 1Gbps over distance of about 300 meter.
Ethernet local loop incorporates features of both ethernet and DSL. It delivers data rate speed of about 6Mbps over distance of 6.5Km.
ADSL Standards or versions
Various ADSL standards or versions with their respective standard, upstream data rate and downstream data rate is mentioned in the following table.
|ADSL Version||ADSL Standard||Upstream data rate||Downstream data rate|
|ADSL||ANSI T1.413-1998 issue 2||1Mbps||8Mbps|
|ADSL G.DMT||ITU G.992.1||1.3Mbps||12Mbps|
|ADSL over POTS||ITU G.992.1 annex A||1.3Mbps||12Mbps|
|ADSL over ISDN||ITU G.992.1 annex B||1.8Mbps||12Mbps|
|ADSL Lite||ITU G.992.2||512Kbps||1.5Mbps|
|ADSL2||ITU G.992.3 annex J||3.5Mbps||12Mbps|
|RE-ADSL2||ITU G.992.3 annex L||800Kbps||5Mbps|
|Splitter less ADSL2||ITU G.992.4||500Kbps||1.5Mbps|
|ADSL2+M||ITU G.992.5 annex M||3.5Mbps||24Mbps|