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Appendix BProtocol stack and packet formats pB.2 Packet formatsIn the upstream channel, a 64-byte slot format is used for the transmission of user dataor MAC messages, as shown in Figure B.2a. A Unique Word (UW) of 4-bytes providesa burst mode acquisition method for synchronisation purposes. The payload area is of53-bytes and contains a single ATM cell for user data or MAC control transmissions.Then, a Reed-Solomon (RS) parity field of 6-bytes provides 3-bytes of RS protectionover the payload area. The Guard Band of 1-byte provides spacing between adjacentpackets.In the case where minislots are used, one contention-based upstream slot is divided intothree independent minislots of 21-bytes long as illustrated in Figure B.2b. Each minislotconsists of 4-byte UW, a single byte Start Field (ST), a 16-byte payload and a singlebyte Guard Band (GB), and can only be used to carry only a shortened MAC reservationrequest message when the splitting tree algorithm (introduced in Section 3.4.3) isselected.a) Upstream Slot Formatb) Minislot Format4-bytesUW53-bytes6-bytes1-bytePayload Area (ATM Cell) RS GB5-bytesATMHeader48-bytesInformation Payload64-byte Upstream Slot21-bytes 21-bytes 21-bytes 1-byteMinislot Minislot Minislot GBBursty Data TrafficMAC Control MessageUWSF3-bytes 1-byteMssg. Conf.TypePayloadMACAddress16-bytesMAC Info.ElementGB1-byteFigure B.2 – Upstream slot and minislot frame structure of the DVB/DAVIC protocol.A-vii