3.1 ATM and xDSL

3.1 ATM and xDSL

Asynchronous Transfer Mode (ATM) is a data link and network layer circuit switched protocol which is connection based, ie: a logical connection must be made between the two end points before data communication can take place.

ATM divides data into small fixed sized cells, each consisting of 53 bytes. The cells are composed of:

  • 48 Bytes for Data
  • 5 Bytes for Header Information

It is this use of fixed sized cells that differentiates ATM from packet switched networks, such as IP that use variable length packets.


The ‘x’ in Digital Subscriber Line (xDSL) means that there is a range of different DSL technologies available. DSL provides digital data services using the wires of local telephone networks.

DSL is a popular alternative to leased lines and ISDN because it is cheaper than leased lines and faster than ISDN. DSL has become a common form of broadband connection for home users. Unlike an analogue modem, a DSL modem transmits digital data without conversion thus allowing for a greater bandwidth for download and uploads.

The most popular version of DSL is Asymmetric DSL (ADSL) which allows downloads at speeds of up to 1.544 Mbps and uploads at speeds of 128 kbps.