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ATM Layer: Asynchronous Transfer Mode LayerThe ATM layer, the layer 2 in the ATM reference model, provides an interface between the ATM adaption layer (AAL ) and the physical layer. This layer is responsible for relaying cells from the AAL to the physical layer, such as SONET , for transmission and from the physical layer to the AAL for use at the end systems, it determines where the incoming cells should be forwarded to, resets the corresponding connection identifiers and forwards the cells to the next link, as well as buffers incoming and outgoing cells, and handles various traffic management functions such as cell loss priority marking, congestion indication, and generic flow control access. It also monitors the transmission rate and conformance to the service contract (traffic policing).
The fields in the ATM header define the functionality of the ATM layer. The format of the header for ATM cells has two different forms, one for use at the user-to-network interface (UNI ) and the other for use internal to the network, the network-to-node interface (NNI ).
Protocol Structure - ATM Layer: Asynchronous Transfer Mode Layer
UNI Structure:
| 8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
|
GFC |
VPI |
|
VPI |
VCI |
|
VCI |
|
VCI |
PT (3 Bit) |
CLP |
|
HEC |
|
Information (48 Bytes) |
-
GFC - generic flow control, use to limit amount of data entering network during periods of congestion.
- VPI - Virtual Path Identifier.
- VCI - Virtual Channel Identifier. The VPI and the virtual channel identifier (VCI) together form the routing field, also called VPCI, which associates each cell with a particular channel or circuit. The VCI is a single-channel identifier; the VPI allows grouping of VCs with different VCIs and allows the group to be switched together as an entity. However, the VPIs and VCIs have significance only on the local link; the contents of the routing field will generally change as the cell traverses from link to link. These fields, in UNI, can support up to 16 million desk (user) to network sessions.
- PT - Payload Type.
- CLP - Cell Loss Priority.
- HEC - Header Error Control.
NNI Structure:
| 8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
|
VPI |
|
VPI |
VCI |
|
VCI |
|
VCI |
PT (3 Bit) |
CLP |
|
HEC |
|
Information (48 Bytes) |
-
VPI - Virtual Path Identifier.
- VCI - Virtual Channel Identifier. See above for VPCI detail definition. These fields, which allows 268 millions NNI sessions, represent the network-to-node routing information within the ATM cell.
- PT - Payload Type.
- CLP - Cell Loss Priority.
- HEC - Header Error Control
Related Protocols
ATM , SONET , AAL0-AAL5 , LAN Emulation (LANE ), CES, UNI , NNI and Q.2931
Sponsor Source
The ATM protocols is based on standards developed by the ITU.
http://www.atmforum.com/standards/approved.html#uni ATM User-Network Interface Specification
Reference
ITU-T Recommendation I.363, B-ISDN ATM Adaptation Layer (AAL) Specification
http://www.atmforum.com/standards/approved.html : ATM Forum approved specifications
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm : ATM Overview |
