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PA-A2 ATM CES Port Adapter Installation and Configuration
OL-3460-01
Chapter 1 Overview: PA-A2 ATM CES Port Adapter
Circuit Emulation Services Overview
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Without the optional CAS feature enabled (the default state).
In this case, the ATM CES port adapter does not sense the CAS information (carried as so-called
“ABCD” bits in the CBR bit stream) and provides no support for CAS functions.
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With the optional CAS feature enabled, but without the optional (Cisco-proprietary) “on-hook
detection” feature enabled.
In this case, in addition to packaging incoming CBR data into ATM AAL1 cells in the usual manner
for transport through the network, the ATM CES port adapter in the ingress node senses the ABCD
bit patterns in the incoming data, incorporates these patterns in the ATM cell stream, and propagates
the cells to the next node in the network. The ATM cells are transported across the network from
link to link until the egress node is reached.
At the egress node, the ATM CES port adapter strips off the ABCD bit patterns carried by the ATM
cells, reassembles the CAS ABCD bits and the user’s CBR data into the original form, and passes
the frames out of the ATM network in the proper DS0 time slot.
All these processes occur transparently without user intervention.
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With both the optional CAS and on-hook detection features enabled.
In this case, the CAS and on-hook detection features work together to enable an ingress node in an
ATM network to monitor on-hook and off-hook conditions for a specified 1 x 64 structured CES
circuit. As implied by the notation “1 x 64,” the on-hook detection (or bandwidth-release) feature is
supported only in a structured CES circuit that involves a single DS0 time slot at each end of the
connection.
The DS0 time slot configured for the structured CES circuit at the ingress node (time slot 2) can be
different from the DS0 time slot configured at the egress node (time slot 4). Only one such time slot
can be configured at each end of the circuit when the on-hook detection feature is used.
When you invoke this feature, the ingress ATM CES port adapter monitors the ABCD bits in the
incoming CBR bit stream to detect on-hook and off-hook conditions in the circuit. In an “off-hook”
condition, all the bandwidth provisioned for the specified CES circuit is used for transporting ATM
AAL1 cells across the network from the ingress node to the egress node.
In an on-hook condition, the network periodically sends dummy ATM cells from the ingress node
to the egress node to maintain the connection. However, these dummy cells consume only a fraction
of the circuit’s reserved bandwidth, leaving the remainder of the bandwidth available for use by
other network traffic. This bandwidth-release feature enables the network to make more efficient
use of its resources.
When the CAS feature is enabled for a CES circuit, the bandwidth of the DS0 channel is limited to 56
kbps for user data, because CAS functions consume 8 kbps of channel bandwidth for transporting the
ABCD signaling bits. These signaling bits are passed transparently from the ingress node to the egress
node as part of the ATM AAL1 cell stream.
In summary, when the optional CAS and on-hook detection features are enabled, the following
conditions apply:
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The permanent virtual connection (PVC) provisioned for the CES circuit always exists.
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The bandwidth for the CES circuit is always reserved.
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During an on-hook state, most of the bandwidth reserved for the CES circuit is not in use. (Dummy
cells are sent from the ingress node to the egress node to maintain the connection.) Therefore, this
bandwidth becomes available for use by other network traffic, such as available bit rate (ABR)
traffic.
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During an off-hook state, all the bandwidth reserved for the CES circuit is dedicated to that circuit.
