ATM - Asynchronous Transfer Mode


ATM.  ATM stands for Asynchronous Transfer Mode.  ATM is actually specified as a Data Link Layer technology on layer 2 within the OSI Reference Model.  ATM sends 53-byte cells instead of variable length packets.  Using this fixed length results in certain advantages.

  • cell based networks run better in point-to-point mode in which the receiving station is ready to actively receive and process the cells
  • speed is improved because hardware knows exactly where the header ends and data starts in every cell
  • quality of service can be more readily controlled and traffic prioritized

ATM is a switching technology in which virtual circuits are set up before a transmission starts.  A virtual circuit is built over a path of various hubs, switches and routers.  Each end of the virtual circuit must agree to the path before the transmission can happen.  This differs from Ethernet and Token Ring which transmit without prior notification or routing determination.  ATM's effective yield is well above even Token Ring's 75%.

ATM is designed to run over fiber optic cable operating the SONET - Synchronous Optical Network specification.  SONET specifications are set up for various cable speeds or OC - optical carrier levels.  The following table displays the standard speeds.





52 Mbps fiber optic cable

OC-3 155 Mbps
OC-12 622 Mbps
OC-24 1.2 Gbps
OC-48 2.5 Gbps


Most ATM backbone LANs run OC-3 or OC-12.  Most intercity links run OC-12, although some major backbone providers are now installing OC-48.

ATM is heavily identified with multicasting of transmissions that have latency and/or priority sensitivities.  IN general, its fixed cell format and virtual circuits are much better at dealing with these kinds of issues.

As we have already stated, ATM is primarily used for backbone LANs.  But at some point, the backbone LAN needs to interact with an access LAN which is almost always running Ethernet or Token Ring.  Because ATM deals in cells rather than packets it uses an encapsulation technique called LANE - LAN Emulation to traverse these incompatibilities.  LANE is used to break down Ethernet or Token Ring packets into ATM cells on one end of the backbone and reassemble them at the other.

While ATM's cost per port is relatively high, its inherent predictability and speed are making it the technology of choice, especially for multimedia applications.

Gigabit Ethernet.  Gigabit Ethernet is a 1000 Mbps enhancement of the Ethernet standard largely to be used for backbones.  The push for it is largely due to its inherent compatibility with Ethernet access LANs.  Gigabit Ethernet is the main competition for ATM.  It can also scale to WANs.