Cisco BPX 8600 Series, Руководство по установке и настройке

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C-5
Cisco BPX 8600 Series Installation and Configuration
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001
Appendix C ATM: An Overview
A 3-bit Payload Type Indicator (PTI) field indicates the type of data being carried in the payload. The
high-order bit is a “0” if the payload contains user information and is a “1” if it carries connection
management information. The second bit indicates if the cell experienced congestion over a path. If the
payload is user information, the third bit indicates if the information is from Customer Premises
Equipment. The PTI field is identical for UNI/NNI/STI.
In the STI header (see ), the Payload Class is used to indicate various classes of service and BPX switch
queues, such as Opticlass, the enhanced class of service feature of the BPX switch. The ForeSight
Forward Congestion Indication, the F bit, is used by ForeSight for congestion status.
The Cell Loss Priority (CLP) bit follows the PTI bits in all header types. When set, it indicates that the
cell is subject to discard if congestion is encountered in the network. For Frame Relay connections,
depending on mapping considerations, the frame Discard Eligibility status is carried by the CLP bit in
the ATM Cell. The CLP bit is also set at the ingress to the network for all cells carrying user data
transmitted above the minimum rate guaranteed to the user.
ATM Cell Addressing
Each ATM cell contains a two-part address, VPI/VCI, in the cell header. This address uniquely
identifies an individual ATM virtual connection on a physical interface. VCI bits are used to identify
the individual circuit or connection. Multiple virtual circuits that traverse the same physical layer
connection between nodes are grouped together in a virtual path. The virtual path address is given by
the VPI bits. The Virtual Path can be viewed as a trunk that carries multiple circuits all routed the same
between switches
The VPI and VCI addresses may be translated at each ATM switch in the network connection route.
They are unique only for a given physical link. Therefore, they may be reused in other parts of the
network as long as care is taken to avoid conflicts.
The VCI field is 16 bits wide with UNI and NNI header types described earlier. This allows for a total
possible 65, 535 unique circuit numbers. The UNI header reserves 8 bits for VPI (256 unique paths)
while the NNI reserves 12 bits (4,096 unique paths) as it is likely that more virtual paths will be routed
between networks than between a user and the network. The STI header reserves 8 bits for VCI and 10
bits for VPI addresses.
ATM Adaptation Layer
The purpose of the ATM Adaptation Layer (AAL) is to receive the data from the various sources or
applications and convert, or adapt, it to 48-byte segments that will fit into the payload of an ATM cell.
Since ATM benefits from its ability to accommodate data from various sources with differing
characteristics, the Adaptation Layer must be flexible.
Traffic from the various sources have been categorized by the standards committees into four general
classifications, Class A through Class D, as indicated in Table C-1. This categorization is somewhat
preliminary and initial developments have indicated that it may be desirable to have more than these
initial four classes of service.
Table C-1 Classes of Traffic and Associated AAL Layers
Traffic Class Class A Class B Class C Class D
Adaptation Layer
(AAL) AAL-1 AAL-2 AAL-3/4
AAL-5
AAL-3/4
Connection
Mode Connection-orien
ted Connection-orien
ted Connection-orien
ted Connectionle
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