Asynchronous Transfer Mode


ATM is a cell-switching and multiplexing technology that combines the benefits of circuit switching (guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency for intermittent traffic). It provides scalable bandwidth from a few megabits per second (Mbps) to many gigabits per second (Gbps). Because of its asynchronous nature, ATM is more efficient than synchronous technologies, such as time-division multiplexing (TDM).

With TDM, each user is assigned to a time slot, and no other station can send in that time slot. If a station has a lot of data to send, it can send only when its time slot comes up, even if all other time slots are empty. If, however, a station has nothing to transmit when its time slot comes up, the time slot is sent empty and is wasted. Because ATM is asynchronous, time slots are available on demand with information identifying the source of the transmission contained in the header of each ATM cell.

ATM Cell Format

Asynchronous Transfer Mode works with very short, fixed-length units called cells. ATM uses 53 byte cells, consisting of a 5 byte header and a 48 byte payload. Because ATM is connection-oriented, the cells can have a short adress space and the cells are not used for establishing the circuit and maintaining it. Once a circuit is set up the bandwidth can be used entirely for data transport. After the circuit is set up, ATM associates each cell with the virtual connection between origin and destination. This can be a virtual channel or path. The 40 bit header holds 8 bits for the virtual path (256 max), and 16 bits for the virtual channel (65536 max). Having both virtual paths and channels make it easy for the switch to handle many connections with the same origin and destination.

The proces that segments a longer entity of data into 53 byte cells is called 'segmentation and reassembly' (SAR). The data that goes into these cells comes from different native mode protocols, such as TCP/IP. The ATM Adaptation Layer (AAL) takes care of the differences between the different sources. The AAL adapts the protocols to an ATM intermediate format. It uses socalled 'classes' to do so. AAL type 3 and 4 handle transmissions of connectionless data, AAL type 5 is intended for connection-oriented services.

ATM Circuits

Three types of ATM services exist: permanent virtual circuits (PVC), switched virtual circuits (SVC), and connectionless service (which is similar to SMDS).

A PVC allows direct connectivity between sites. In this way, a PVC is similar to a leased line. Among its advantages, a PVC guarantees availability of a connection and does not require call setup procedures between switches. Disadvantages of PVCs include static connectivity and manual setup.

An SVC is created and released dynamically and remains in use only as long as data is being transferred. In this sense, it is similar to a telephone call. Dynamic call control requires a signaling protocol between the ATM endpoint and the ATM switch. The advantages of SVCs include connection flexibility and call setup that can be handled automatically by a networking device. Disadvantages include the extra time and overhead required to set up the connection.

ATM Virtual Connections

ATM networks are fundamentally connection oriented, which means that a virtual channel (VC) must be set up across the ATM network prior to any data transfer. (A virtual channel is roughly equivalent to a virtual circuit.)

Two types of ATM connections exist: virtual paths, which are identified by virtual path identifiers, and virtual channels, which are identified by the combination of a VPI and a virtual channel identifier (VCI).

A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM network on the basis of the common VPI. All VCIs and VPIs, however, have only local significance across a particular link and are remapped, as appropriate, at each switch.

A transmission path is a bundle of VPs as shown below:

ATM Service Types

ATM relies on different classes of service to accomodate different applications (voice, video, data). They define the bits and bytes that are actually transmitted, as well as the required bandwidth, allowable error rates, and so forth. Class A and B, have timing compensation, for applications that cannot tolerate variable delays. Class C and D, no timing compensation, for data applications like LAN interconnect. Class D also simulates connectionless communicaations, comonly found on LANs.

Class A B C D
Timing yes yesno no
Bit rate constant variable variable variable
Mode * ** *** ****
AAL Type 1 Type 2 Type 3/4 Type 3/4
        Type 5
* Connection-oriented, circuit emulation
** Connection-oriented, variablee bit-rate video
*** Connection-oriented, connection-oriented data
**** Connectionless, connectionless data
AAL 1:
for isochronous, constaant bit-rate services, such as audio and video. This adaption layer corresponds to fractional and full T1 and T3, but with a greater range of choices for data rates.
AAL 2:
for isochronous variale bit-rate services, such as compressed video.
AAL 3/4:
for variable bi-rate data, such as LAN applications. Originally designed as two different layers, one for connetion-oriented services (like frame relay) and one for connectionles services (like SMDS). both can be done by the same AAL though.
AAL 5:
for vriable bit-rate data that must be formatted into 53-byte cells. Similar to AAL 3/4, easier to implement, less features.

The service-specific convergence sublayer (SSCS) maps (converts) the data to the ATM layer. The convergence sublayer (CS) then compensates for the various interfaces (copper and fiber) that may be used on an ATM network. The ATM network can use Sonet, T1, E1, T3, E3, E4, FDDI, pure cells, Sonet SDH, block-encoded fiber, etc.

Further Reading

ATM Quick Reference (PDF)