Asynchronous Transfer Mode
It can handle both traditional high-throughput data traffic and real-time, low-latency content such as telephony (voice) and video.
ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the data exchange begins.
[5] These virtual circuits may be either permanent (dedicated connections that are usually preconfigured by the service provider), or switched (set up on a per-call basis using signaling and disconnected when the call is terminated).
A queuing delay induced by several such data packets might exceed the figure of 7.8 ms several times over.
Cells were introduced to provide short queuing delays while continuing to support datagram traffic.
ATM broke up all data packets and voice streams into 48-byte pieces, adding a 5-byte routing header to each one so that they could be reassembled later.
The United States, due to its larger size, already had echo cancellers widely deployed.
Most of the European parties eventually came around to the arguments made by the Americans, but France and a few others held out for a shorter cell length.
48 bytes was chosen as a compromise, despite having all the disadvantages of both proposals and the additional inconvenience of not being a power of two in size.
[10] 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information.
A UNI cell reserves the GFC field for a local flow control and sub-multiplexing system between users.
ATM provides a useful ability to carry multiple logical circuits on a single physical or virtual medium, although other techniques exist, such as Multi-link PPP, Ethernet VLANs, VxLAN, MPLS, and multi-protocol support over SONET.
Call admission is then performed by the network to confirm that the requested resources are available and that a route exists for the connection.
As these cells traverse an ATM network, switching takes place by changing the VPI/VCI values (label swapping).
[14] ATM switches use the VPI/VCI fields to identify the virtual channel link (VCL) of the next network that a cell needs to transit on its way to its final destination.
Another advantage of the use of virtual circuits comes with the ability to use them as a multiplexing layer, allowing different services (such as voice, Frame Relay, IP).
ATM networks create and remove switched virtual circuits (SVCs) on demand when requested by an end station.
PNNI also includes a very powerful route summarization mechanism to allow construction of very large networks, as well as a call admission control (CAC) algorithm which determines the availability of sufficient bandwidth on a proposed route through a network in order to satisfy the service requirements of a VC or VP.
ATM traffic contracts form part of the mechanism by which quality of service (QoS) is ensured.
Most traffic classes also introduce the concept of cell-delay variation tolerance (CDVT), which defines the clumping of cells in time.
EPD and PPD work with AAL5 connections as they use the end of packet marker: the ATM user-to-ATM user (AUU) indication bit in the payload-type field of the header, which is set in the last cell of a SAR-SDU.
Traffic shaping usually takes place in the network interface controller (NIC) in user equipment, and attempts to ensure that the cell flow on a VC will meet its traffic contract, i.e. cells will not be dropped or reduced in priority at the UNI.
Since the reference model given for traffic policing in the network is the GCRA, this algorithm is normally used for shaping as well, and single and dual leaky bucket implementations may be used as appropriate.
However, even by the end of the decade, the better price–performance ratio of Internet Protocol-based products was competing with ATM technology for integrating real-time and bursty network traffic.
