In the CCSDS AOS protocol the bitstream service may be used to transmit variable bit rate video. The protocol data unit structure for the CCSDS AOS bit stream service is shown in Figure 8.
Figure 8: Bitstream Packet Format
The spare field is currently undefined and is set to 00. An insufficient number of bits may have been received before it is necessary to release a B_PDU. Therefore the bitstream data pointer is used to indicated the location of the last valid user data bit within the B_PDU bitstream data zone. The corresponding ATM service is provided by AAL-2. In AAL-2 the sequence number SN is used to detect lost or misinserted cells. The information type field (IT) is used to indicate that video is being transmitted. The LI field provides a length indicator. In addition a CRC field is provided.