legacy generic MAC header [1] CHAPTER 6 The IEEE 802. 16m MAC CPS (Part I) Encryption Key Sequence (EKS) is the index of the traffic encryption key and initialization vector used to encrypt the payload. This field is only meaningful if the EC field is set to 1. Extended Sub-header Field (ESF) indicates whether the extended sub-header is present. If set to 1, the extended sub-header is present and follows the generic MAC header immediately. The ESF is applicable to downlink and in uplink. Header Checksum Sequence (HCS) is an 8-bit field that is used to detect errors in the header. The transmitter calculates the HCS value for the first five bytes of the MAC header and inserts the result into the HCS field. The HCS is the remainder of the modulo-2 division by the generator polynomial g(D) = D8 + D2 + D + 1 of the polynomial D8 multiplied by the content of the header excluding the HCS field. The ESF bit in the generic legacy MAC header is an indication of the presence of extended sub- headers. Using this field, a number of additional sub-headers can be used within a MAC PDU. The extended sub-header always appears immediately after the generic MAC header and before all other sub-headers. Unlike other sub-headers, extended sub-headers are not considered part of the MAC PDU payload and are not encrypted. When an IEEE 802.16-2009 standard entity transmits a MAC PDU without a payload, it sets the EC bit in the generic MAC header to 0, even if the connection on which it transmits the MAC PDU is associated with data encryption. When an entity receives a MAC PDU that does not contain a payload, it processes the MAC PDU, if the EC bit is set to 0; otherwise, it should discard this MAC PDU. Five types of MAC sub-headers may be present in a legacy MAC PDU in conjunction with the generic MAC header; four per-PDU sub-header types and one per-SDU sub-header type [1]. The per- PDU sub-headers, i.e., extended sub-headers, Fragmentation Sub-Header (FSH), Fast-Feedback Allocation Sub-Header (FFSH), and Grant Management Sub-Header (GMS