The ATM Cell:

The ATM cell is the basic unit of information transfer in the B-ISDN ATM communication. The cell is comprised of 53 bytes. Five of the bytes make up the header field and the remaining 48 bytes form the user information field. The following is the structure of the Network Node Interface (NNI) ATM Cell Header:

The following is the structure of the User Node Interface (UNI) ATM Cell Header:

The header field is divided into GFC , VPI , VCI , PT, CLP and HEC fields. The associated bit sizes differ slightly at the NNI and the UNI. The bit sizes are as follows.

Generic Flow Control (GFC):

 Although the primary function of this header is the physical access control, it is often used to reduce cell jitters in CBR services, assign fair capacity for VBR services, and to control traffic for VBR flows. Such functionality requires the power to control any UNI structure, be it a ring, a star, some bus configuration, or any combination of these.

Virtual Path Identifier / Virtual Channel Identifier (VPI/VCI):

The role of the VPI/VCI fields is to indicate Virtual Path or Virtual Channel identification numbers, so that the cells belonging to the same connection can be distinguished. A unique and separate VPI/VCI identifier is assigned in advance to indicate which type of cell is following, unassigned cells, physical layer OAM cells, metasignalling channel or a generic broadcast signaling channel.

Payload Type (PT) / Cell Loss Priority (CLP) / Header Error Control (HEC):

When user information is present or the ATM cell has suffered traffic congestion then the PT field will yield this information.

The CLP bit is used to tell the system whether the corresponding byte is to be discarded during network congestion . ATM cells with CLP=0 have a priority in regard to cell loss than ATM cells with CLP=1. Therefore, during resource congestions, CLP=1 cells are dropped before any CLP=0 cell is dropped.

HEC is a CRC byte for the cell header field and is used for sensing and correcting cell errors and in delineating the cell header.