
Configuring ATM Services
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NULL Encapsulation (VC-Based Multiplexing)
In NULL encapsulation, the carried network protocol is identified implicitly by
the virtual circuit connecting the two ATM stations. Because each protocol must
travel over a separate virtual circuit, there is no need to include explicit
multiplexing information in the payload of the PDU. This means that the
bandwidth requirements and processing overhead remain minimal.
You can either manually configure the carried protocol or let the signaling
procedures negotiate it dynamically during call establishment.
NULL encapsulation cells do not receive a header in a routed environment. In a
bridged environment, the content of the PDU itself includes the necessary
information for bridging the multiplexed protocols.
Encapsulation Rules for PVCs
When assigning a data encapsulation type to a PVC or group of PVCs, keep the
following in mind:
• The ATM Service Records List and ATM Virtual Channel Link windows
initially contain a default data encapsulation type of LLC/SNAP.
• When you add a PVC, it reads and uses the data encapsulation type specified
in its ATM service record.
• You can globally assign a data encapsulation type to all nonhybrid PVCs in a
particular service record, or you can assign a data encapsulation type to
individual group PVCs using the ATM Virtual Channel Link window.
Caution: Changing the data encapsulation type in the ATM Service Records
List window changes the data encapsulation type for all group access mode
PVCs on that service record. For example, if you change the ATM service
record data encapsulation type from LLC/SNAP to NULL, the data
encapsulation type for all PVCs (except hybrid PVCs) on that service record
changes to NULL. Site Manager does not ask you to verify this change.
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