DSL Digital Subscriber Line (DSL) provides a dedicated digital circuit between a user’s premises and the Internet through the telephone company’s central ofﬁ ce via ordinary copper telephone wires. The two primary forms of DSL are Asymmetric Digital Subscriber Line (ADSL) and Symmetric Digital Subscriber Line (SDSL). ADSL has a higher download speed (1.544 to 6.1 Mbps downstream) and a lower upload speed (16 Kbps to 1.5 Mbps). SDSL’s download and upload speeds (1.544 Mbps) are equal. SDSL does not provide voice capabilities. ADSL – which is more widely used and available – must be within 18,000 feet of the central ofﬁ ce while SDSL users must be within 12,000 feet. Some companies, however, have begun to use new technologies such as ﬁ ber lines and/or repeaters to extend DSL capabilities up to 25,000 feet.
An ADSL modem has a “plain old telephone service” POTS) splitter and a channel separator. The POTS splitter divides the phone line into two channels (voice and data) and the channel separator divides the data channel into two sections (downstream and upstream). Data are transported to another ADSL modem in the central ofﬁ ce. This modem sends the voice calls to the public switched telephone network (PSTN) and sends the data to the digital subscriber line access multiplexer DSLAM). The DSLAM connects many ADSL lines to a single asynchronus transfer mode (ATM) line or switch. This ATM line acts as both a trafﬁ c aggregator and as a multiservice switch that is capable of forwarding trafﬁ c in different ways, depending on needs. The ATM line then sends the data over the Internet.