ATM Layer. ATM Layer Functions

The ATM layer lies atop the physical layer and specifies the functions required for the switching and flow control of ATM cells (1).

There are two interfaces in an ATM network: the user-network interface (UNI) between the ATM endpoint and the ATM switch, and the network-network interface (NNI) between two ATM switches. Although a 48-octet cell payload is used at both interfaces, the 5-octet cell header differs slightly at these interfaces.

Figure 3 shows the cell header structures used at the UNI and NNI (1). At the UNI, the header contains a 4-bit generic flow control (GFC) field, a 24-bit label field containing virtual path identifier (VPI) and virtual channel identifier (VCI) subfields (8 bits for the VPI and 16 bits for the VCI), a 2-bit payload type (PT) field, a 1-bit cell loss priority (CLP) field, and an 8-bit header error check (HEC) field. The cell header for an NNI cell is identical to that for the UNI cell, except that it lacks the GFC field; these four bits are used for an additional 4 VPI bits in the NNI cell header.

Figure 3. ATM cell header structure

The VCI and VPI fields are identifier values for virtual channel (VC) and virtual path (VP), respectively. A virtual channel connects two ATM communication endpoints. A virtual path connects two ATM devices, which can be switches or endpoints, and several virtual channels may be multiplexed onto the same virtual path. The 2-bit PT field identifies whether the cell payload contains data or control information. The CLP bit is used by the user for explicit indication of cell loss priority.

If the value of the CLP is 1, then the cell is subject to discarding in case of congestion. The HEC field is an 8-bit CRC that protects the contents of the cell header. The GFC field, which appears only at the UNI, is used to assist the customer premises network in controlling the traffic flow. At the time of writing, the exact procedures for use of this field have not been agreed upon.

ATM Layer Functions. The primary function of the ATM layer is VPI/VCI translation. As ATM cells arrive at ATM switches, the VPI and VCI values contained in their headers are examined by the switch to determine which outport port should be used to forward the cell. In the process, the switch translates the cell’s original VPI and VCI values into new outgoing VPI and VCI values, which are used in turn by the next ATM switch to send the cell toward its intended destination. The table used to perform this translation is initialized during the establishment of the call.

An ATM switch may either be a VP switch, in which case it translates only the VPI values contained in cell headers, or it may be a VP/VC switch, in which case it translates the incoming VPI/VCI value into an outgoing VPI/VCI pair. Because VPI and VCI values do not represent a unique end- to-end virtual connection, they can be reused at different switches through the network. This is important because the VPI and VCI fields are limited in length and would be quickly exhausted if they were used simply as destination addresses.

The ATM layer supports two types of virtual connections: switched virtual connections (SVC) and permanent, or semipermanent, virtual connections (PVC). Switched virtual connections are established and torn down dynamically by an ATM signaling procedure. That is, they exist only for the duration of a single call. Permanent virtual connections, on the other hand, are established by network administrators and continue to exist as long as the administrator leaves them up, even if they are not used to transmit data.

Other important functions of the ATM layer include cell multiplexing and demultiplexing, cell header creation and extraction, and generic flow control. Cell multiplexing is the merging of cells from several calls onto a single transmission path, cell header creation is the attachment of a 5-octet cell header to each 48-octet block of user payload, and generic flow control is used at the UNI to prevent shortterm overload conditions from occurring within the network.