7-4 Chapter 7

Logical channels differentiate the virtual circuits supported by the packet level. Each packet is associated with a logical channel which is mapped to the appropriate destination of the virtual circuit. Multiple connections are provided simultaneously by multiplexing virtual circuits over the access line. This enables a more effective use of the access circuit for the available bandwidth and the traffic density for each logical channel.

Note: Only one PVC or SVC can be established at a time on each logical channel.

Logical Channel Numbers

The logical channel number is embedded in a 12 bit field within the packet. For SVCs, the association between a logical channel identifier and a particular virtual circuit is accomplished at call setup time when the identifier is chosen from a pool of unused identifiers. The logical channel number is determined by the X.25 service provider for that particular network subscription.

At network subscription time, the user specifies the number of logical channels needed for both SVCs and PVCs by designating ranges of logical channel numbers. Emulex X.25 configuration uses the decimal equivalent of the complete 12-bit Logical Channel Identifier in two subfields: the Logical Channel Group (4 bits) and the Logical Channel Number (8 bits).

Within any of the following categories, the range of available logical channels must be continuous. Each successive range of logical channel numbers must be numerically higher than the previous range.

In increasing order, the logical channel ranges are:

1)PVCs

2)SVCs supporting only calls from the network (inbound SVC)

3)SVCs supporting calls from and to the network (two-way SVC)

4)SVCs supporting only calls to the network (outbound SVC)

NOTE: The total number of logical channel on all ports on an Emulex card should not exceed 255.

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Emulex DCP_link manual Logical Channel Numbers