2.10 ATM Connections and Cell Switching


2.10 ATM Connections and Cell Switching

Having a basic understanding of the ATM cell header, to include the virtual path and virtual channel identifiers, we can turn our attention to the methods used to establish connections between endpoints as well as how connection identifiers are used for cell switching to route cells to their destination.

2.10.1 Connections

In comparison to most LANs that are connectionless, ATM is primarily a connection-oriented communications technology. This means that a connection must be established between two ATM endpoints prior to actual data being transmitted between the endpoints. The actual ATM connection can be established as a permanent virtual circuit (PVC) or as a switched virtual circuit (SVC).

A PVC can be considered similar to a leased line, with routing established for long- term use. Once a PVC is established, no further network intervention is required any time a user wishes to transfer data between endpoints connected via a PVC. In comparison, an SVC can be considered as being similar to a telephone call made on the switched telephone network. That is, the SVC requires network intervention to establish the path linking endpoints each time a SVC occurs.

Both PVCs and SVCs obtain the "V" as they represent virtual rather than permanent or dedicated connections. This means that through statistical multiplexing, an endpoint can receive calls from one or more distant endpoints.

2.10.2 Cell Switching

As previously mentioned, signaling packets are transmitted on the well-known virtual channel VPI = 0, VCI = 5 to set up a connection. That connection results in each switch allocating a VPI and VCI to route data between switches or from a switch to an endpoint. Because a switch can support numerous simultaneous connections as cells arrive , the switch examines the VIP and VCI fields in the cell header to determine the output port for relaying or transferring a cell. To determine the output port, the ATM switch first reads the incoming VPI, VCI, or both fields with the field read dependent upon the location of the switch in the network. Next, the switch will use the connection identifier information to perform a table lookup operation. That operation uses the current connection identifier as a match criterion to determine the output port the cell will be routed onto as well as a new connection identifier to be placed into the cell header. The new connection identifier is then used for routing between the next pair of switches or from a switch to an endpoint.




Enhancing LAN Performance
Enhancing LAN Performance
ISBN: 0849319420
EAN: 2147483647
Year: 2003
Pages: 111
Authors: Gilbert Held

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