Asynchronous Transfer Mode (ATM) is a high-speed networking technology used mostly in backbones and wide area networks. ATM combines three types of data onto one network medium:
- Audio (voice telephony over a distance
- Video (including cable television)
- Data (computer communications within local area networks and wide area networks).
ATM uses CAT5 cabling or fiber-optic lines. ATM is a point-to-point (PTP) network, whereas Ethernet is a general broadcast medium. ATM uses switches to virtually connect to other nodes or devices.
Ethernet listens for silence before transmitting data, then hopes there was no data collision. ATM nodes are constantly transmitting. If they have no data, the switch filters out the excess empty data packages. If data is present, the switch sets up a virtual connection to the destination switch.
ATM works on cells, where Ethernet uses frames. Where Ethernet frames can vary in size, ATM cells are fixed in size. A cell is exactly 53 octets long, or fifty three 8-byte words.
As noted earlier, ATM makes virtual connection between nodes or devices to communicate. There are two types of virtual circuits:
- Permanent Virtual Circuit, or (PVC). A circuit or connection is planned out and coded into the switches. Example: A server in Denver has a PVC connection to the accounting server in San Diego in ACME Widgets, Inc.
- Switched Virtual Circuit, or (SVC). A circuit between devices is established to transfer data, then the virtual circuit is deleted. Example: Alexis in Denver downloads the latest sales figures from the San Diego server to her office workstation.
As with all networks, the better the connection medium, the higher the transfer rate. ATM transmission speeds are set by convention to be as follows:
Connection Medium Transfer Speed
T1 Carrier 1.544Mbps over copper phonelines
ATM-25 25Mbps over twisted pair
T3 Carrier 44.736Mbps over copper phonelines
OC1 SONET 51Mbps, fiber-optic cable
OC3 SONET 155Mbps, fiber-optic cable
OC12 SONET 622Mbps, fiber-optic cable
OC48 SONET 2.4Gbps, fiber-optic cable
As ATM is developed, more standards will be released.