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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.