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Circuit Emulation Services Overview
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Network designers and administrators calculate a maximum allowable CDV value for the network in
order to establish network cell delay tolerance limits. Thus, to some degree, the network's maximum
allowable CDV value is a measure of the network's expected performance.
When a CDV threshold for the network is established, appropriate buffer sizing can be derived for the
network devices involved in any given CES circuit. This helps to ensure that the network will operate
as expected.
In the case of an ATM CES port adapter, for example, the maximum allowable CDV value for the
network is used to determine an appropriate size (depth) for the segmentation and reassembly (SAR)
buffer built into the board. This sizing of the SAR buffer prevents buffer overflow or underflow
conditions. An overflow condition can cause a loss of frames, and an underflow condition can cause
frames to be repeated.
The actual CDV value for a circuit varies according to the particular data path used for the circuit.
Consequently, the depth of the SAR buffer may increase or decrease in proportion to the CDV value for
the CES circuit being set up.
You can issue the show ces circuit interface command in an unstructured (clear channel) circuit to
measure the current CDV value. See
more information on verifying a configured hard PVC.
For an unstructured hard PVC, the CDV value for the circuit (including all hops) should not exceed a
maximum allowable CDV value. The procedure for setting up a hard PVC is described in
Chapter 4, "Configuring the PA-A2 ATM CES Port Adapter."
For an unstructured hard PVC, the network automatically determines the best data path through the
network and handles the routing of CBR traffic. The network accomplishes this dynamically by means
of the ATM connection admission control (CAC) mechanism. The CAC determines the best path
through the network by executing a routing algorithm that consults local routing tables in network
devices.
If the requested data path is equal to or less than the maximum allowable CDV value established by the
network administrator, the connection request is granted. If the requested CES circuit exceeds the
maximum allowable CDV value, the connection request is denied. These connection admission control
processes occur "on the fly" as network connection requests are initiated.
For example, when a user requests a connection from source node A at one edge of the network to
destination node B at the opposite edge of the network, the CAC mechanism takes into account the CDV
value for each hop in the requested connection to determine a suitable path through the network that does
not exceed the network's maximum allowable CDV value.
PA-A2 ATM CES Port Adapter Installation and Configuration
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The number of intermediate hops through which the CBR data must pass in traversing the
network from source to destination. The network designer/administrator calculates a CDV value
for each hop in the data path in order to establish a maximum allowable CDV value for the
network at large.
The type and speed of the trunk lines interconnecting the ATM networks.
The volume of traffic being handled by the trunk lines at any given time, that is, the degree to
which the network may be experiencing congestion conditions.
Chapter 1
Chapter 4, "Configuring the PA-A2 ATM CES Port Adapter,"
Overview: PA-A2 ATM CES Port Adapter
OL-3460-01
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