 Cell- 53 octets Octet 53 Octet 1 Figure 20.4 An ATM cell stream illustrating the basic makeup of a cell. Bit 8 Bit 8 Octet Octet GFC Generic flow control VPI Virtual path identifier VCI Virtual channel identifier PTI Payload type identifier CLP Cell loss priority HEC Header error control Figure 20.5 Basic ATM header structures. (a) UNI cell-header structure; (b) NNI header structure. ATM designer team was driven to shorten the payload as much as possible. The issue in this case was what is called packetization delay. This is the amount of time required to fill a cell at a rate of 64 kbps-that is, the rate required to fill the cell with digitized (PCM) voice samples. According to Ref. 8, the design team was torn between efficiency and packetization delay. One school of thought fought for a 64-octet cell, and another argued for a 32-octet cell size. Thus the ITU-T organization opted for a fixed-length 53-octet compromise. Now let's return to the discussion of the ATM cell and its headers. The left-hand side of Figure 20.5 shows the structure of a UNI header, whereas the right-hand side illustrates the NNI header. The only difference is the presence of the GFC field in the UNI header. The following paragraphs define each header field. By removing the GFC field, the NNI has four additional bits for addressing. 20.4.1.1 GFC - Generic Flow Control. The GFC field contains 4 bits. When the GFC function is not used, the value of this field is 0000. This field has local significance only and can be used to provide standardized local flow control functions on the customer side. In fact, the value encoded in the GFC is not carried end-to-end and will be overwritten by ATM switches (i.e., the NNI interface). ASYNCHRONOUS TRANSFER MODE Two modes of operation have been defined for operation of the GFC field. These are uncontrolled access and controlled access. The uncontrolled access mode of operation is used in the early ATM environment. This mode has no impact on the traffic which a host generates. Each host transmits the GF