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
OAM Cell Structure
Generic Identifier Transport IE Used by signaling
LANE Data Frame

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-Node Interface (NNI) header format is defined by the NNI specification.

The UNI specification defines communications between ATM endpoints (such as workstations and routers) and ATM switch routers in private ATM networks. The format of the UNI cell header is shown in Figure B-1.

Figure B-1: UNI Header Format

The UNI header consists of the following fields:

GFC---4 bits of generic flow control that are used to provide local functions, such as identifying multiple stations that share a single ATM interface. The GFC field is typically not used and is set to a default value.
VPI---8 bits of virtual path identifier that is used, in conjunction with the VCI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.
VCI---16 bits of virtual channel identifier that is used, in conjunction with the VPI, to identify the next destination of a cell as it passes through a series of ATM switch routers on its way to its destination.
PT---3 bits of payload type. The first bit indicates whether the cell contains user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.
CLP---1 bit of congestion loss priority that indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network.
HEC---8 bits of header error control that are a checksum calculated only on the header itself.

The NNI specification defines communications between ATM switch routers. The format of the NNI header is shown in Figure B-2.

Figure B-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 ATM 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.

OAM Cell Structure

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:

The entire switch router
A specific ATM interface
Each ATM connection

Figure B-3 shows the format of the OAM loopback cell.

Figure B-3: OAM Cell Structure

The OAM cell structure has the following features:

OAM cell type is coded as 0001.
OAM function type is coded as 0010.
350 bits that are specific to the OAM type are divided into the following elements:
- Loopback indicator---A bit that is set to 1 before the cell is looped back. The loopback node then sets the bit to 0, indicating it has been looped back.
- Correlation tag---Identifies (correlates) related OAM cells within the same connection.
- Loopback location ID---An optional field that identifies the site that is to loopback the cell.
- Source ID---An optional field that identifies the site generating the cell.

Generic Identifier Transport IE Used by Signaling

The generic identifier transport information element (IE) is used by signaling to carry an identifier between two users.

Figure B-4 shows the format of the generic signaling IE.

Figure B-4: Generic Identifier Transport IE Used by signaling

The generic identifier transport IE used by signaling has the following fields:

Generic identifier transport information IE.
Ext.
Coding standard.
Flag.
Reserved.
IE instruction action Indication.
Length of generic indentifier transport IE.
Identifier related standard/application---Each application requiring a different set or structure of identifiers (coded in octet 6 and possibly in subsequent octet groups) should use a different value of octet 5.
Identifier type---This value is independent of the identifier related standard/application field, octet 5. The maximum length is 20 octets.
Identifier length---A binary number indicating the length in octets of the identifier code in the subsequent octets of the octet group.
Identifier value---Value of an identifier according to the recommendation or the standard identifier in octet 5.

LANE Data Frame

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 B-5 shows the format of the LANE data frame.

Figure B-5: LANE Data Frame Format for IEEE 802.3/Ethernet

The LANE data frame has the following fields:

LE header---Contains either the LAN emulation client identifier value, the sending client, or X'0000'.
Destination address.
Source address.
Type information---Logical link control (LLC) data frames whose total length, including the LLC field and data, but not including padding required to meet minimum data frame length, is less than 1536 (X"0600"). It must be encoded by placing the length value in the type/length field. LLC data frames longer than the maximum must be encoded by placing the value 0 in the type/length field.
Information---Encapsulated Ethernet data.

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