Atm Layer; Figure C-2 Atm Cell Format - Cisco BPX-BXM-155-8DX Installation And Configuration Manual

•
•

ATM Layer

The ATM layer processes ATM cells. The ATM cell consists of a 5-byte header and a 48-byte payload.
The header contains the ATM cell address and other management information as shown in Figure C-2.
Figure C-2
ATM cell
ATM Cell Headers
Two basic header types are defined by the standards committees, a UNI header and a NNI header; both
are quite similar. Cisco has expanded on these header types to provide additional features beyond those
proposed for basic ATM service.
•
•
•
Cisco BPX 8600 Series Installation and Configuration
C-2
The Physical Medium sub-layer (PMD)
This handles processing specific to a particular physical layer, such as transmission rate, clock
extractions, and so on.
The Transmission Convergence sub-layer (TC)
This extracts the information content from the physical layer data format. This includes HEC
generation and checking, extraction of cells from the data stream, processing of idle cells, and so
on.
ATM Cell Format
5 bytes
header
The UNI header (see Figure C-3) must be specified for each User-to-Network Interface. A UNI is
any interface between a user device, such as an ATM router, and an ATM network.
The NNI header (see Figure C-4) must be specified for each Network-to-Network Interface. This is
used, for example, at the interface between a user's private ATM network and a service provider's
public ATM network.
The STI header (see Figure C-5) is an extension of these two header types and is used between
Cisco switching nodes to provide advanced network features, including ForeSight, that improve
performance, efficiency, and congestion control.
53 byte cell
48 Byte payload
Information payload
Release 9.3.0, Part Number 78-10674-01 Rev. D0, July 2001
Appendix C ATM: An Overview