Asynchronous Transfer Mode ATM

Asynchronous Transfer Mode (ATM) is a telecommunications standard used for transferring voice, data, and video traffic. It is a cell-based switching technology that uses fixed-length packets, called cells, to carry data. The cells have a size of 53 bytes, consisting of a 5-byte header and a 48-byte payload.

ATM networks use virtual circuits (VCs) to establish a path between two end systems. There are two types of virtual circuits: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). PVCs are preconfigured and provide a fixed path between two devices, while SVCs are established on demand and are torn down when no longer needed.

ATM supports a variety of traffic types, including voice, data, and video. It offers several benefits, such as high bandwidth, low latency, and quality of service (QoS) capabilities. However, it is not widely used today and has largely been replaced by newer technologies like Ethernet and IP-based networks.

Asynchronous Transfer Mode (ATM) can be used as a packet-switching service (like frame relay), or as a switching technology. Routers connect to ATM service by an access link to an ATM switch inside the service provider’s network. For multiple sites, each router would need a single access link to the ATM network, with a virtual circuit (VC) between sites as needed. ATM can use permanent VCs (PVC) like Frame Relay. ATM typically supports much higher speed physical links, especially those using a specification called Synchronous Optical Network (SONET). ATM does not forward fames – it forwards cells.

A cell is just like a packet or frame, but is a string of bits sent over some network. The difference is that while packets and frames can vary in size, ATM cells are always a fixed 53 bytes in length. ATM cells contain 48 bytes of payload (data) and a 5‐byte header. The header contains two fields that together act like the data-link connection identifier (DLCI) for Frame Relay by identifying each VC. The two fields are named Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI). Just like Frame Relay switches forward frames based on DLCI, devices called ATM switches, resident in the service provider network, forward cells based on the VPI/VCI pair.