xDSL - what is it and what types are there
Digital Subscriber Line (DSL) is a technology that allows for high speed data transmission over a single copper wire pair. It's original purpose was to deliver this service over the already existent copper infrastructure connecting the telephone lines of businesses and residences.
DSL is a local loop technology, which means it provides connectivity between the customer premises and the telco central office. The CPE consists of a DSL modem/router which terminates the copper pair. On the telco side, the circuit terminates on a Digital Subscriber Line Access Multiplexer (DSLAM) which terminates multiple DSL circuits.
Conventional analog PSTN lines use frequencies between 300 Hz and 4000 Hz to convey the human voice. The use of different frequencies allows for both voice and data to be sent over the same wire pair. DSL exploits higher frequencies of 25 kHz and above to transmit data.
Even though it uses the word digital in its name, the signaling sent on the copper wires is actually analog. It is modulated at the DSL modem, and then demodulated when it reaches the DSLAM. Thus, the CPE is actually a modem, which modulates and demodulates the signaling. Even so, DSL CPEs usually incorporate additional functionality including a modem, router, switch, and Wi-Fi access point all in a single device.
There are various types of DSL, which is why the term xDSL is often used, where "x" is replaced with a letter indicating the "flavor" of the technology being employed. Many of the most common are listed below:
- Asymmetric Digital Subscriber Line (ADSL)
- Symmetric Digital Subscriber Line (SDSL)
- Very-high-bit-rate Digital Subscriber Line (VDSL)
Actual performance depends a great deal upon the wire-distance from the CPE to the DSLAM. This distance should be on the order of several kilometers. As distance increases, attainable speeds decrease. One solution to this problem is to bring the DSLAM closer to the subscriber by placing it within curb-side housings.