ze ROHC as the header compression scheme for all types of traffic. In PHS, the repetitive portion of the payload headers of the higher layers is suppressed in the MAC SDU by the sending entity and restored by the receiving entity. In the uplink, the sending entity is the MS and the receiving entity is the BS. In the DL, the sending entity is the BS and the receiving entity is the MS. If PHS is enabled for a MAC connection, each MAC SDU is prefixed with a PHS Index (PHSI), which references the PHS Field (PHSF), i.e., a string of bytes representing the header portion of a PDU in which one or more bytes are suppressed. The sending entity uses classification rules to map packets to a service flow. The classification rule uniquely maps packets to its associated PHS rule. The receiving entity uses the CID or STID+FID and the PHSI to restore the PHSF. Once a PHSF has been assigned to a PHSI, it will not change. To change the value of a PHSF on a service flow, a new PHS rule must be defined; the old rule is removed from the service flow, and the new rule is added. When all classification rules associated with the PHS rule are deleted, then the PHS rule can also be deleted. The PHS scheme has a PHS Valid (PHSV) option to verify the payload header before sup- pressing it. The PHS scheme has also a PHS Mask (PHSM) option to allow certain bytes not to be suppressed. The PHSM facilitates suppression of header fields that remain static, while enabling transmission of fields that change from packet to packet. The BS assigns the PHSI values in the same way that it assigns the CID or STID+FID values. Either the sending or the receiving entity specifies the PHSF and the PHS Size (PHSS). This provision allows for preconfigured headers or for higher level signaling protocols outside the scope of the standard to establish cache entries. It is the responsibility of the higher layer service entity to generate a PHS rule that uniquely identifies the suppressed header within the service flow. It is also the responsibility of the higher l