dancy Check (CRC). The CRC consists of four octets, based on IEEE 802.3, and is calculated on the entire MAC PDU including the header. The size of MAC header (six octets) is considered a significant overhead for certain applications such as VoIP interactive gaming, which consist of frequent bursts of small payloads. As an example, the size of Real-time Transport Protocol (RTP) payloads corresponding to VoIP application, depending on the type of the voice codec is typically less than 40 octets; therefore, each VoIP packet encapsulated in a MAC PDU would have more than 25% overhead (typically 50% with smaller packets, e.g., 20 octets for ITU-T G.729) overhead due to the size of the MAC header and the trailing CRC. Most of the fields in the legacy MAC header are not usually used in VoIP and interactive gaming. The structure of a legacy MAC PDU is illustrated in Figure 6-2. Each PDU begins with a fixed- length MAC header that may be followed by a payload. If the payload is present, it may consist of zero or more sub-headers and zero or more MAC SDUs and/or fragments of MAC SDUs. The payload information may vary in length; therefore, the MAC PDU may have variable size. This allows the MAC to tunnel various higher layer traffic types without knowledge of the formats or bit patterns of those messages. All reserved fields are set to zero on transmission and ignored on reception. The generic MAC header is used with all MAC SDUs and MAC management messages. The information fields of the generic MAC header are as follows: Header Type (HT) is an indicator of whether the MAC PDU contains any payload. If set to 1, the MAC PDU does not contain any payload and the MAC header is a signaling header. CRC Indicator (CI) shows whether a CRC is suffixed to the MAC PDU. Encryption Control (EC) indicates if the payload is encrypted. The header is never encrypted. MAC Header Payload MSB of Length (6 Bits) (3 Bits) LSB of Length MSB of CID (8 bits) (8 Bits) LSB of CID Header Checksum Sequence (8 Bits) (8 Bits) FIGURE 6-2 Structure of the 