ATM Cell and Packet Structures
Table of ContentsATM Cell Structures
Formats of the ATM Cell Header
OAM Cell Structure
Generic Identifier Transport IE Used by Signalling
LANE Data Frame
This appendix describes the various ATM cell types and their configurations and includes the following sections:
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
The NNI specification defines communications between switch routers. The format of the NNI header is shown in Figure C-2.
Figure C-2: NNI Header Format
The GFC field is not present in the format of the NNI header. Instead, the VPI field occupies the first 12 bits, which allows switch routers to assign larger VPI values. With that exception, the format of the NNI header is identical to the format of the UNI header.
Operation, Administration, and Maintenance (OAM) performs standard loopback (end-to-end or segment) and fault detection and notification (alarm indication signal [AIS] and remote defect identification [RDI]) for each connection. It also maintains a group of timers for the OAM functions. When there is an OAM state change such as loopback failure, OAM software notifies the connection management software. You can enable or disable OAM operation for the following switch router components:
Figure C-3 shows the format of the OAM loopback cell.
Figure C-3: OAM Cell Structure
The generic identifier transport information element (IE) is used by signalling to carry an identifier between two users.
Figure C-4 shows the format of the generic signalling IE.
Figure C-4: Generic Identifier Transport IE Used by Signalling
The LAN emulation data frame for Ethernet is based on ISO 8802.3/CSMA-CD (IEEE 802.3) and is used to provide connectivity between ATM attached end systems and LAN attached stations.
Figure C-5 shows the format of the LANE data frame.
Figure C-5: LANE Data Frame Format for IEEE 802.3/Ethernet