By James H. Green

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There are two principal reasons for using short fixed-length cells instead of the long variable-length packets that TCP/IP employs. The first reason is that cells can be switched in hardware, which is faster than routing. The fixed cell length makes it easier to maintain a high degree of utilization. The second reason is that it is easier to control latency and jitter, which are critical quality of service (QoS) elements in real-time applications such as voice and video. If a cell is lost in transmission, it has a minimal impact on quality because the amount of voice or video information contained in one cell is negligible.

Unlike other DSL versions, HDSL does not support regular telephone service on a cable pair. SDSL SDSL (also known as symmetrical DSL) serves the same market as HDSL, with two important functional differences. Instead of dual cable pairs, it provides T1/E1 service on a single cable pair and it also derives a POTS line under the data signal. It transmits and receives in the same band of frequencies using an echo-canceling protocol, making it susceptible to NEXT. SDSL uses 2B1Q line coding that is adopted from basic rate ISDN.

In the real world, many things happen to alter the impedance of lines. In twisted-pair cable, gauge changes, irregular splices, and moisture are examples of things that can cause impedance changes. The effects of some of these may be 42 Access Technologies: DSL and Cable d In Frequency e nc ita ac ap C e nc ta uc Reactance Figure 3-2 Reactive Effects of Capacitance and Inductance moderate, but an unterminated cable causes a severe irregularity, the effects of which are difficult to predict. When an electrical wave reaches the end of an unterminated line, some of the energy is reflected back to the source.

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