Asynchronous Transfer Mode (ATM)

What is Asynchronous Transfer Mode (ATM)?

Introduction:
ATM stands for Asynchronous Transfer Mode. ATM technology uses ATM cells for data transmission between source and destination. ATM cell uses fixed size of 53 bytes which consists of header (5 bytes) and data (48 bytes). It is behind the success of B-ISDN used for voice/data/video transportation.
ATM transmits cells only when there is data to be transmitted unlike STM (Synchronous Transfer Mode) where bandwidth is assigned periodically like TDM. ATM is connection oriented which uses virtual packet switching. Multiple logical connections are multiplexed over single physical connection.

ATM network Architecture, ATM cell size, ATM addressing

This ATM architecture tutorial describes ATM network Architecture, ATM cell size, ATM addressing and ATM network interfaces etc. ATM Cell size and ATM cell types viz. UNI cell and NNI cell and ATM addressing formats viz. DCC, ICD and E.164 are explained.
In the year 1980, ITU-T has initiated efforts to develop a network which can transport voice, video and data simultaneously from source to destination. As a result B-ISDN (Broadband Integrated Services Digital Network) has been developed. The B-ISDN combines telephony network, data network and cable TV network in a single network. ATM as well as SONET/SDH are behind the success of B-ISDN.

The same is shown in the figure-1. Unlike Synchronous Transfer Mode where in each source will get periodic assignment of bandwidth similar to TDM, Source in ATM packetizes data into smaller fixed size cells and cells are sent only if there is data to be transmitted. In ATM each of the cells will have its own header as described later.

Following are the silent features of ATM (Asynchronous Transfer Mode):
• ATM uses Virtual Circuit Packet Switching. It reserves capacity for the virtual circuit.
• Packets are known as cells in ATM. The smaller packets are good for voice as well as video transmissions. One ATM cell consists of header (5 bytes) and data payload (48 bytes).
• It allows multiple logical connections to be multiplexed over single physical interface.
• It is a connection oriented technology.
• As mentioned it provides dynamic allocation of bandwidth for efficient traffic management.
• Supports minimum flow control and error control capabilities.
• Supports LAN, CAN and WAN
• ATM can be deployed in private, public or hybrid networks.

• In ATM (Asynchronous Transfer Mode) logical connections are known as virtual channel connections or VCCs. The VCC is similar to virtual circuit (VC) in packet switched network. It is unit of switching used in ATM network.
• Initially VCC is established between end users in the network.
• After the VCC connection is established variable rate and fixed size cells are exchanged.
• Here VPC i.e. Virtual Path Connection is combination of VCCs having same end points or destinations. Hence all the ATM cells travelling through VCCs in one common VPC are switched together.
• Virtual path is identified by VPI and Virtual Channel is identified by VCI as mentioned below.
➨VPI (Virtual Path Identifier) identifies virtual path (8/12 bits in size).
➨VCI (Virtual Channel Identifier) identifies virtual channel in a virtual path (16 bits).