Digital Subscriber Line:
Digital subscriber line (DSL) technology is one of the most promising for supporting high-speed digital communication over the existing local loops.
ADSL, like a 56K modem, provides higher speed (bit rate) in the downstream direction (from the Internet to the resident) than in the upstream direction (from the resident to the Internet). That is the reason it is called asymmetric.
Discrete Multitone Technique
The modulation technique that has become standard for ADSL is called the discrete multitone technique (DMT) which combines QAM and FDM. There is no set way that the bandwidth of a system is divided. Each system can decide on its bandwidth division. Typically, an available bandwidth of 1.104 MHz is divided into 256 channels. Each channel uses a bandwidth of 4.312 kHz.
Figure 1.56 shows how the bandwidth can be divided into the following:
· Voice. Channel 0 is reserved for voice communication.
· Idle. Channels 1 to 5 are not used and provide a gap between voice and datacommunication.
· Upstream data and control. Channels 6 to 30 (25 channels) are used for upstream datatransfer and control. One channel is for control, and 24 channels are for data transfer.
· Downstream data and control. Channels 31 to 255 (225 channels) are used fordownstream data transfer and control. One channel is for control, and 224 channels are for data.
Customer Site: ADSL Modem
An ADSL modem installed at a customer's site. The local loop connects to a splitter which separates voice and data communications. The ADSL modem modulates and demodulates the data, using DMT, and creates downstream and upstream channels.
Telephone Company Site: DSLAM
At the telephone company site, the situation is different. Instead of an ADSL modem, a device called a digital subscriber line access multiplexer (DSLAM) is installed that functions similarly. In addition, it packetizes the data to be sent to the Internet (ISP server).
2. ADSL Lite
The installation of splitters at the border of the premises and the new wiring for the data line can be expensive and impractical enough to dissuade most subscribers. A new version of ADSL technology called ADSL Lite (or Universal ADSL or splitterless ADSL) is available for these subscribers.
The high-bit-rate digital subscriber line (HDSL) was designed as an alternative to the T-l line (1.544 Mbps). The T-1line uses alternate mark inversion (AMI) encoding, which is very susceptible to attenuation at high frequencies. This limits the length of a T-l line to 3200 ft (1 km). For longer distances, a repeater is necessary, which means increased costs.
The symmetric digital subscriber line (SDSL) is a one twisted-pair version of HDSL. It provides full-duplex symmetric communication supporting up to 768 kbps in each direction. SDSL, which provides symmetric communication, can be considered an alternative to ADSL.
The very high-bit-rate digital subscriber line (VDSL), an alternative approach that is similar to ADSL, uses coaxial, fiber-optic, or twisted-pair cable for short distances. The modulating technique is DMT. It provides a range of bit rates (25 to 55 Mbps) for upstream communication at distances of 3000 to 10,000 ft. The downstream rate is normally 3.2 Mbps.