7.1 Introduction to DSL
Digital subscriber line (DSL) technology transmits broadband data over telephone lines. There are nearly a billion telephone lines connecting homes and businesses in the developed world, and most of these connect to network equipment with DSL capability. A telephone line consists of a dedicated twisted pair of wires that runs from one or more rooms in the customer premises to a telephone network serving node. In the past, the line ended at a local central office (CO, also known as an exchange). Today, a telephone line more commonly ends at a remote equipment cabinet containing a multiplexer serving a local neighborhood. As a result, the length of the line is reduced, and this enables the DSL technology to convey higher bit rates. With approximately 480 million DSL lines in service at the end of 2012, DSL technology is the most popular form of broadband access in the world.
Figure 7.1 is a high-level diagram of common DSL systems. The DSLAM (digital subscriber line access multiplexer) is the network end for each DSL. The DSLAM contains a DSL modem dedicated for each line, and it also multiplexes the traffic from all DSL lines into a high-speed trunk to the core network. Different types of DSL technology have different attributes:
- ADSL (asymmetric digital subscriber line) provides up to 8 Mbits/s downstream (towards the customer) and 1 Mbits/s upstream (from the customer) simultaneously with POTS (plain ...