ATM Cell Header Fields

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


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 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 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.
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