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518 Chapter 14: SNA Topologies
·
Transaction rate is important. A typical transaction rate is one transaction per logical unit
per minute. By doing some arithmetic, you can determine the number of logical units per
physical units on average, which can give you an average approximation. To determine
router utilization requirements, you must understand transaction rates and requirements.
NOTE
Transaction message size is about 40 bytes inbound and 1000 bytes outbound.
·
The type of encapsulation is important. Each type of encapsulation impacts router
performance differently. If you choose an encapsulation that provides for local
acknowledgment, that puts more overhead on the processor. Of the four types of
encapsulation used with DLSw+, two provide for local acknowledgment (TCP and
LLC2). The other two types, FST and Direct encapsulation, can run in pass-through mode
over fast, reliable networks and require very little router-processing overhead. However,
more bandwidth is consumed across the links.
WARNING
FST encapsulation can be used only on Token Ring ports; there is no support for Ethernet.
NOTE
If there is a lot of unused bandwidth between sites, use DLSw in pass-through mode, which
does not require local acknowledgments. This decreases the router overhead, as shown in
Figure 14-6. In the figure, the two end stations send acknowledgments that must traverse the
WAN.
Figure 14-6
Passthru
Token Ring
5
IBM compatible
Token Ring
8
Station A
Router A
Frame Relay
Router B
Station B
Virtual Ring 10
Plenty of bandwidth
available
No local acknowledgment
Local acknowledgments
No local acknowledgment
87200333.book Page 518 Wednesday, August 22, 2001 1:41 PM