Troubleshooting guide

C-1
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
APPENDIX C
ATM Cell Structures
This appendix describes the various ATM cell types and their configurations and includes the following
sections:
• Formats of the ATM Cell Header, page C-1
• OAM Cell Structure, page C-3
• Generic Identifier Transport IE Used by Signalling, page C-4
• LANE Data Frame, page C-5
Formats of the ATM Cell Header
The ATM standards groups have defined two header formats. The User-Network Interface (UNI) header
format is defined by the UNI specification, and the Network-to-Network Interface (NNI) header format
is defined by the NNI specification.
The UNI specification defines communications between ATM endpoints (such as workstations and
routers) and switch routers in private ATM networks. The format of the UNI cell header is shown in
Figure C-1.
Figure C-1 UNI Header Format
GFC
GFC VPI
VPI VCI PT HEC
C
L
P
VPI VCI
VCI
VCI PT CLP
HEC
Field length
in bits
48 16 31 8
40 bits
10409
840
1
2
3
4
5
Bits
Octets