Asynchronous Transfer Mode
Asynchronous Transfer Mode is a telephony and digital telecommunications architecture, making use of optical transmission and digital control using Signaling System 7 (SS7), which promised to revolutionize the efficient transfer of digital information, both voice and data. It is a variant on circuit switching, in which all information was chopped into 53-byte units called cells, fixed-length units being optimal for the switching hardware of the time. It is still in significant use in telephone networks, although it is being replaced there gradually, and in data transmission almost completely, by Internet Protocol (IP) routed packet technology, rather than switched cells. SS7 remains a key internal management and control part of telephone networks, although it increasingly runs over internal IP networks rather than ATM, and may, in some applications, be replaced by telephony applications based on the Session Initiation Protocol (SIP).
Closely associated with the ATM deployment was the Integrated Services Digital Network (ISDN) for principally on-demand digital streams and limited low-speed packet transfer, and frame relay (FR) for medium-speed dedicated access to what was often called the Broadband ISDN (B-ISDN) system. B-ISDN is really the combination of ATM, SONET/SDH, and SS7; it has no existence other than those components. While the components have roles, B-ISDN, as a fundamental architecture, is largely abandoned. There are some newer pure optical architectures that, to some extent, replace it as a backbone technology.
ISDN is also declining in use, but FR became a useful technology independent of B-ISDN. Multiprotocol label switching (MPLS) is actually closer to a replacement of ATM, and is often called "ATM without cells". MPLS runs as an overlay over a network defined by Internet Protocol routing. Both ATM and MPLS can work as the virtual transmission path within the SS7 architecture. There is a trend for MPLS to replace frame relay.
ATM ran on top of synchronous optical transmission systems such as the Synchronous Optical Network (North America and Japan) and the Synchronous Digital Hierarchy (SDH) in the rest of the world. SONET/SDH actually do allow for some elasticity in the merging of time division multiplexed digital streams, but ATM is asynchronous in that its virtual circuits do not need to be locked into a strict timing system.
The 53-byte cell had a 5-byte header specifically used by ATM switches, with addressing information that was link-local between switches. The next 48 bytes were considered payload, although some of its bytes were typically used for AAL services. Why 48 bytes? It was a compromise. Digital voice engineers wanted a short cell length, preferably no longer than 32 bytes of payload, to maximize interleaving and avoiding certain types of delay. Data engineers wanted longer, preferably at least 64 bytes of payload, to amortize the 5-byte headers over more data, increasing throughput by improving the ratio between overhead and payload.
Eventually, the head of the standards committee made a Solomonic compromise, and averaged the payload between the 32 byte maximum desired for voice, and the 64 byte minimum desired for data. The resulting average was 48, so that the cell payload length was optimal for no application.
ATM, internally, has several reference points:
- F5: Virtual channel (VC), between endpoints of the ATM network. An individual telephone call might be a VC
- F4: Virtual path (VP), or aggregates of VCs, such as all the telephone calls between two carriers
- F3: Transmission path', between elements that carry out ATM Adaptation Layer (AAL) functions
- F2: Digital section between frame synchronization points
- F1: Regenerator section between hardware regenerator.
F1 through F3 correspond closely to the three topological levels of SONET and SDH.
VCs and VPs can be set up as point-to-point (P2P) or point-to-multipoint (P2MP). They are bidirectional, as opposed to MPLS unidirectional paths.
 ATM services and adaptation layers
Above the ATM cells proper are ATM Adaptation Layers (AAL). AAL are of three main types:
- AAL1: called Constant Bit Rate (CBR), strictly controlled in delay and intended for time-division multiplexed digitized voice
- AAL2: known as Variable Bit Rate (VBR), allows more variation in delay, which is tolerable for services such as digitized video and some digital voice
- AAL3/4: intended for connection-oriented data services such as X.25, but never widely used
- AAL5: connectionless, used for packet data applications
 Virtual call setup
 ATM endpoint addressing
 Q.2931 call setup
 ATM routing
 Operations, administration and maintenance
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