ATM Cell Header Fields

The ATM cell header consists of the following fields: generic flow control (GFC), virtual path identifier (VPI), virtual channel identifier (VCI), payload type (PT), cell loss priority (CLP) and header error control (HEC). The header format is different at a B-ISDN user network interface (UNI) than it is in a B-ISDN network node interface (NNI), as illustrated in figure 3.

  

Figure 3: (a) UNI cell formats and (b) NNI cell format

The GFC field is present only in cells between a host and network. It is overwritten by the first switch it reaches, so it does not have end-to-end significance and is not delivered to the destination. The network ignores the GFC field.
The VPI field is a small integer selecting a particular virtual path (see Figure 4). Similarly, the VCI field selects a particular virtual circuit within the chosen Virtual path.

  

Figure 4: Virtual paths and virtual circuits in a transmission path.

The PTI field defines the type of payload the cell contains in accordance with the values given in Figure 5. Here the cell types are user supplied, but the congestion information is network supplied. In other words, a cell sent with PTI 000 might arrive with 010 to warn the destination of problems underway.

  

Figure 5: Values of the PTI fields

The CLP bit can be set by the host to differentiate between high-priority traffic and low-priority traffic. If congestion occurs and cells must be discarded, switches first attempt to discard cells with CLP set to 1 before throwing out any set to 0.

The HEC is the checksum over the header. The header comes with 48 bytes of payload.