ATM Overview

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ATM is a connection-oriented technique where end-to-end connections are established before information can be sent. Because all data is transmitted over the same path, all cells arrive in order at the receiver. This is an advantage when transmitting MPEG streams, because MPEG frames need information from other frames in order to be decoded correctly. The main disadvantage of using ATM is the cost. The infrastructure of and ATM network is much more expensive than corresponding infrastructure of an Ethernet.

The ATM cell

In ATM, information is transferred in fixed-size cells, 53 bytes long, consisting of a 48 byte information field and a 5 byte header. Figure 15 shows the structure of the 5 byte long ATM cell header at User-Network interface (UNI). The Generic flow control is used to control the flow of traffic for different Qualities of Service (QoS). Virtual path identifier and Virtual channel identifier are the unique numbers identifying the Virtual Path (VP) and Virtual Channel (VC). Payload type is a three bit field identifying the type of a cell (e.g., user data cell, management information cell). Cell loss priority is used to indicate if the cell should be discarded in case of congestion or not. The Header Error Control (HEC) value over the first four bytes is calculated and inserted into the fifth byte of the cell header called Header Error Control field.


Figure 15 - ATM cell header (User-Network Interface (UNI))

Connection set-up

A connection between a sender and a receiver is set up using Virtual Channel Connections (VCC) and Virtual Path Connections (VPC). A VCC is an end-to-end connection which is defined by the concatenation of the Virtual Channel (VC) links. VC links are defined by the routing table entities of two nodes connected by a point-to-point physical link. [12]

The use of VCCs create a large amount of processing overhead because of the large routing tables needed. Virtual Paths (VPs) are used to bundle VCs to reduce this burden (see Figure 16). The Virtual Path Identifier (VPI) in the cell header is a unique number identifying the Virtual Path the cell will travel to its destination.


Figure 16 - A physical link containing two virtual paths, each containing two virtual channels.

VCI/VPI values only have local significance and are translated at VP/VC switches (see Figure 17). A VP switch translates incoming VPIs to outgoing VPIs. A VC switch terminates both VC links and VP links and translates both VCIs and VPIs.


Figure 17 - VP/VC switching.