The ATM cell is 53 bytes in length. The first 5 bytes are used for the header. The payload portion of the cell is 48 bytes. Keep in mind as we discuss the protocol that the payload is also used for the AAL overhead and any other overhead from upper layers.

There are two formats for the ATM header. One header is used for the UNI, while the other is used for the NNI. The difference lies in the Generic Flow Control (GFC) parameter found in the UNI header. The GFC is not supported in the public network, nor was it intended to be.

The header is placed in front of the payload (it arrives first). There is no trailer used in ATM. Figure 9.4 shows the two-header formats. In the UNI header, the GFC can be used to throttle traffic from a specific destination. The GFC values are not currently defined, but the intent is that the GFC could be used to provide flow control from the user to the network (and vice versa). This parameter is not used out in the public network and is overwritten by network nodes.

Two modes are defined for GFC, controlled and uncontrolled. Controlled GFC allows subscriber equipment to control flow of ATM cells, however, the values for this have not been defined. They are of local significance only, which means they are related to a link connection at a switch and used to communicate with the remote end of a link. Uncontrolled GFC simply means that GFC is not supported, and the parameter is set to all zeroes.

The VPI is used to identify a group of virtual channels within the same endpoint. This is the form of addressing supported in ATM. Rather than identifying millions of unique nodes, ATM addressing identifies a connection. A virtual channel is used for a communication link. Each virtual channel is identified by the VCI.

The meta signaling channel is a dedicated channel used to establish virtual channel connections between two endpoints. The virtual paths are predetermined at the time of installation (either through administration or by the service provider). Virtual paths can be negotiated by the user or the network using meta signaling. This means the establishment or release of virtual paths can be controlled by the user or the network.

NEXT PAGE: ATM Header and Payload (cont.)

Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.