Thursday, October 8, 2009

Medium Access Control Protocol Data Units (MAC PDUs)

Medium access control protocol data units are a package of data (group of data bits) that contain header, connection address and data protocol information that is used to control and transfer information across a type of medium (such as a radio channel). The WiMAX system MAC PDUs contain a header, which holds the connection identifier along with control information. MAC PDUs may also have payload of data and error checking bits (CRC) bits after the header (e.g. user data). A MAC PDU header contains a header type, encryption control field, payload type and error checking (CRC) code.

A header type is a data field within the packet header that indicates the type of the header. The header type typically indicates the field format of the header and/or sub-headers that are part of the data packet.

Encryption control is the transferring parameters or sending of signaling messages in a data field within the header of a data packet that indicates encryption is used and possibly the type of encryption that is used. An encryption control field in a header typically indicates the payload of the data packet is encrypted.

Payload type is a data field within a packet header that indicates the format of the payload of data including any sub-headers

A cyclic redundancy check indicator is a data field within a packet header that indicates if and how a CRC error check code is used for the packet of data.

Encryption key sequence is an index value that is used to identify the location of a data packet within a sequence of packets to enable the decryption of the packet.

A length field is a data field within a packet header that holds a number or value that indicates the length of a data packet or a block of data.

A connection identifier is a unique name or number that is used to identify a specific logical connection path in a communication system. For the WiMAX system, the connection identifier is a 16 bit code.

A header check sequence is a calculated code that is used to check if the bits within a header have been received correctly during transmission.

Figure 1 shows the typical construction of generic WiMAX medium access control packet. This diagram shows that generic WiMAX MAC data packets contain addressing and controlling information and the payload can have variable length. This diagram shows that the MPDU header is 6 bytes long and that the header type indicator is used to determine which, if any, subheaders may be used.

Figure 1: WiMax MAC PDU
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