3Com 7600 manual Virtual Channels

Virtual Channels 12-3

Virtual Channels ATM is a connection-oriented transport service, much like a telephone. As opposed to Ethernet, where data is sent out on a common bus, ATM requires that a virtual channel connecting the source station to the destination station be established before data transmission can begin. A virtual channel does not necessarily correspond to a single physical line and, in fact, may pass through many different network devices and cables before reaching its destination.

Virtual Channel ATM seeks to establish an optimum path for a cell to travel through the Connection ATM network from source to destination. This path is called a virtual

channel connection (VCC). Its specification is coded into a VCC identifier which is then placed in each cell header. Using this information, high-speed buffer management and control schemes route ATM cells between ports along the path. To access the ATM network, a transmitting station requests a VCC between itself and a destination station, using the signalling protocol to the ATM switch. At connection time, a VCC is established between the stations and a VCC identifier is assigned to the connection.

Virtual Path In addition to the VCC, another type of virtual connection called a virtual Connection path connection (VPC) can exist on the same physical link. The VPC is a

higher level “container” connection which includes one or more VCCs.

A VCC is uniquely identified by the combination of Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) values and a VPC is uniquely identified by the VPI value.

VPCs and VCCs can be set up either dynamically via signalling, in which case it is called a switched virtual circuit (SVC), or permanently by management where it is called a permanent virtual circuit (PVC).

Permanent Virtual A PVC connection is set up manually by management (for example, Channels SNMP). A PVC connection, once established, is not automatically released

and is not dynamically checked and reinitiated.

The characteristics of the PVC connection include:

QConnections initiated by network administrator

QConnections established and released manually

QLong-term duration connections