The control information is carried in the four octet header of the cell. This consists of two octets of VCI and two octets of FAS. The user payload of the cell carries forty eight octets of data.
The VCI is allocated by the receiving entity except in the cases of promiscuous associations. Each entity may be prepared to allocate as many or as few VCIs as it wishes. For this reason promiscuous associations ought to be allocated a VCI which is likely to be available for reception on all entities.
The VCI consists of two octets. In the remainder of this section they shall be referred to as a pair where the first element of the pair is the one that is transmitted first on the network.
VCIs of 0x00,0x00 and 0xff,0xff are reserved for special purposes by
entities and may not be used for communication. They shall never be
allocated or transmitted on.
The FAS, otherwise known as user control information has two purposes. One purpose is to indicate framing information to allow for optimisations at the FDL level for example when blocking cells together or when interrupting a host. There are two bits of framing information. Bit 7 of the second octet is defined to be set on the first cell of a block. Bit 6 of the second octet is defined to be set on the last cell of a block.
The other six bits in the second octet and all of the first octet are defined to be opaque at the FDL level. FDL networks use a variant of SAR1a which is fully described in the ``Segmentation and Reassembly'' document found elsewhere in this collection. This variant is to permit the temporary interworking of this protocol with newer standard conformant equipment.
The cell header is shown in table 1
