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Asynchronous Transfer Mode, or ATM, is a relatively new high-speed transfer technology. ATM is a physical-layer protocol in which a standard 53-byte packet (known as a cell!) is used to simultaneously transmit voice/data over the same cable. The cells contain ident(ification) information, which allows an ATM-switch to separate data types and re-assemble them in the right order.

(so sue me, I know about data communications/computers, but not about Olympic mascots!!)

Additional information:

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:

  1. 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.
  2. 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.

ATM is rarely used to connect actual workstations because the hardware is very expensive, and Ethernet is generally good enough for the end user. However, its very powerful capabilities to support service level agreements directly through the hardware makes it popular for backbone bandwidth providers.

ATM routing is done through virtual channels and paths. Each cell contains a VPI (virtual path identifier) and VCI virtual channel identifier, and a router only has to know that (e.g.) incoming cells from port A with VPI 0 and VCI 17 have to be sent to VPI 1 and VCI 25 on port C. This can be done very quickly and efficiently in hardware.

Of course, this routing rules have to be set up first, which is done through a separate, complex signaling protocol.

What's the difference between a virtual path and a virtual channel? Well, paths can be used to "group together" channels, so that heavy-duty backbone routers can do their routing by looking only at the VPI and ignore the VCI. This makes their routing tables much smaller so that they are quicker and can handle much higher data rates.

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