• ATM is a high-speed switching network architecture
• ATM can be used to carry data, voice, and video
– separately or simultaneously over same network path
• ATM has a robust quality of service (QoS)
– can provide seamless interconnectivity between LAN’s and WAN’s
– supports a wide range of data rates:
• 25 to 155 Mbps over copper
• 100 to 622 Mbps and higher over fiber
• common implementation is 155-Mbps ATM
• ATM is specified via a three-layer reference model:
• Physical layer (OSIs physical layer)
• ATM layer (generally OSIs data link layer)
• ATM adaptation layer (AAL) (generally OSIs higher-level layers (transport,
session, and application)
• Physical layer (2 sublayers)
– Physical medium PM (lower sublayer)
• definition for the medium
• the bit-timing capabilities.
– Transmission convergence (TC) (upper sublayer)
• makes sure that valid cells are being created and transmitted
• involves breaking off individual cells from the data stream of the
higher layer (the ATM layer)
• checking the cell’s header
• Encoding the bit values
• ATM layer
– service-independent layer
– creates cell headers and trailers
– defines virtual channels and paths and gives them unique identifiers
– cells are multiplexed or demultiplexed.
– ATM layer creates the cells and uses the physical layer to transmit them.
• ATM adaptation layer (AAL) (2 sublayers)
– Segmentation and reassembly SAR (lower sublayer)
• packages variable size packets into fixed-size cells at the
transmitting end
• repackages the cells at the receiving end
• responsible for finding and dealing with cells that are out of order
or lost
– convergence sublayer CS (upper sublayer)
• provides the interface for the various services (e.g. data, voice, and
• users connect to CS through service access points
• (SAPs).
• ATM cells are always 53 bytes long
• partitioned into
– 5 byte header è contains addressing information
– 48 byte payload è contains user data
• ATM virtual connections consist of either
– permanent or switched virtual circuits
– that logically connect source and destination sites
– Virtual circuits are identified by specific virtual channel identifiers (VCIs).
• A collection of virtual channels that all have the same endpoints is called a virtual
path connection (VPC)
• VPCs are specified by virtual path identifiers (VPIs)
• Virtual connections established
– VCI and VPI assignments are made dynamically by ATM end nodes and
switches at the time data are to be transmitted
– VCI is not of interest to e.g. public switches they would only use the VPI
• Local area network emulation (LANE) interface
– can provide a service interface for the network layer that functions exactly
as the same as Ethernet/802.3 and token ring
– LANs with this interface – Emulated LANs (ELAN)
• involve special client/server processes that enables MAC-to-ATM address resolution

• support connectionless nature of local area networks