Asynchronous Transfer Mode (ATM) networking is the newest topology available at this time. unlike others, it can carry both voice and data over network wire or fiber. ATM transmits all packets as 53-byte cells, that have a variety of identifiers on them to determine such things as quality of service.

Quality of Service is similar to the quality of service in regular mail. in the regular mail, you have a choice of service namely speed post, air, mail, etc. when you send a speed post, it receives priority over other types of mail so it gets to its destination first.

The first few bits in a packet of data indicate the quality of service required for that packet. when the quality of service required for that packet. when the quality of service feature is implemented you can send packets based on their need for the bandwidth, for example, e-mail is of relatively low priority and might be given lower-class service. video or audio content that has to run constantly gets a higher priority.

ATM is capable of extremely high-speed routing. At the lowest, it runs at 25 megabits per second. At the fastest, it can run up to 622 megabits per second. In addition to its speed, ATM is more complex than either Ethernet or Token Ring. Presently, Fore Systems and IBM have both invested very heavily amount in ATM-to-the-desktop technology. which means, they use ATMs to link servers and workstations and are banking on the need for multimedia networks over the next several years.

ATM can provide for simultaneous data, video, and voice transmission. It can be used for WAN, LAN, and MAN. it can reach speeds of up to 2.488 gigabits per second.


How ATM works?


ATM communicates with cell rather than transmitting frames. instead of specifying the source and destination addresses of the stations communicating, an ATM cell indicates the path the data will flow through. Small cells all of the same size are used to make it easy for devices to process a cell, so intermediate devices (called switches) can maintain a very high data rate.

On an ATM network, every station is always transmitting. however, most of the cells transmitted are empty cells that can be discarded at the switch. when a cell that is not empty, enters the switch, the addresses are read to determine where the cell will go next. the cell is then sent out in the next available slot, according to the type of cell it is.

At each switching node, the ATM header identifies a virtual circuit that will route the cell's message to the destination computer. the cell header enables the switch to forward the call and data to the next neighboring link in the total circuit. the virtual circuit is set up through the appropriate ATM switches when two endpoints wish to communicate. this scheme allows ATM switching to be implemented in hardware that is fast enough to support transmission rates up to 2.488 gigabits per second.

ATM is compatible with current communication systems and widely used for cable media such as twisted pair, coaxial and fiber-optic as well as a great deal of LAN, WAN, and MAN technologies. However, some cable media lack sufficient bandwidth to fully realize ATM's potential. To date, there are ATM standards for transmitting at 25 Mbps, 45 Mbps, cells can be encapsulated in other protocols including those of FDDI, SONET, etc.


Role of ATM in the internetwork


ATM has emerged as one of the technologies for integrating LANs and WANs. ATM can support any traffic type in separate or mixed streams, delay-sensitive traffic, and non-delay-sensitive traffics as shown in the give below image.

What is Asynchronous Transfer Mode (ATM)
ATM support of various traffic types

ATM can also scale from low to high speeds. it has been adopted by all the industry's equipment vendors, from LAN to private branch exchange (PBX). with ATM, network designers can integrate LANs and WANs to support emerging applications with the economy in the enterprise.

Network designers are deploying ATM technology to migrate from Time Division Multiplexing (TDM) networks for the following reasons:
  1. To increase WAN bandwidth
  2. To improve performance
  3. To reduce downtime