Digital Subscriber Line (DSL)
The other promising candidate for a home broadband transmission medium is the telephone network. Of course, the conventional telephone modem already uses the phone network, but telephone companies thought they could get better performance if they used a different approach. The result of this effort is a communications form known as Digital Subscriber Line (DSL).
In fact, the twisted pair cabling used in telephone networks has much more capacity than is typically used for voice communication. The DSL transceiver, which acts as an interface from the local network to the telephone network, operates in a frequency range that doesn’t interfere with voice communication over the line. Consequently, DSL can operate continually without tying up the line or interfering with phone service.
Like a cable network, a DSL network requires a device at the other end of the line that receives the signal and interfaces with the Internet through the provider’s network. A device known as a Digital Service Line Access Multiplexer (DSLAM) serves as the other end point for the DSL connection (see Figure 9.8). Unlike on a cable network, where the medium is essentially shard by users on the segment, each DSL customer has a dedicated line from the transceiver to the DSLAM, which means that performance is less susceptible to degradation with increased traffic.
Figure 9.8. Connecting to the Internet with DSL.
DSL comes in several forms, including ADSL (Asynchronous DSL—the most popular variant for small office and homes), HDSL (High bit-rate DSL), VDSL (Very High bit-rate DSL), SDSL (Symmetric DSL, in which the upstream and downstream bandwidths are equal), and IDSL (ISDN over DSL). The view of DSL from the protocol level varies depending on the equipment and implementation. Some DSL devices are integrated with switches or routers. Other devices act as bridges (similar to a cable modem), filtering traffic at the Network Access layer by physical (MAC) address. DSL devices often encapsulate data in a point-to-point protocol such as PPP. The so-called PPP over Ethernet protocol (PPPoE), for instance, is a popular option for DSL.