Packet (including payload header) PHSI#0 Header Compressed IP Packet FIGURE 5-7 IP CS PDU format with and without header suppression [1] 5.4 Generic packet convergence sub-layer correspondence between an ROHC compressor instance and an ROHC channel, where the ROHC compressor is located at the input end of the ROHC channel. Similarly, an ROHC decompressor instance is a logical entity that performs header decompression according to one or several ROHC profiles. There is a one-to-one correspondence between an ROHC decompressor instance and an ROHC channel, where the ROHC decompressor is located at the output end of the ROHC channel. A channel is essentially a logical point-to-point connection between the IP interfaces of two communicating network elements. By that definition, the channel characterizes a logical connection that is needed to make header compression generally applicable, and further the channel properties control whether compression can operate in a uni-directional or a bi-directional manner. An ROHC channel has the same properties as a channel, with the difference that an ROHC channel is always uni- directional. An ROHC channel therefore has one single input endpoint, connected to a single ROHC compressor instance, and one single output endpoint, connected to a single ROHC decompressor instance. An ROHC channel must be logically dedicated to one ROHC compressor and one ROHC decompressor, also referred to as ROHC peers, creating a one-to-one mapping between an ROHC channel and two ROHC compressor/decompressor peers. 5.4 GENERIC PACKET CONVERGENCE SUB-LAYER The Generic Packet CS (GPCS) is a protocol-independent packet convergence sub-layer that can support multiple network-layer protocols (e.g., IPv4, IPv6, or IEEE 802.3 Ethernet) over the IEEE 802.16 air interface. The GPCS provides a generic packet convergence layer. This layer uses the MAC SAP and exposes a SAP to GPCS applications. It does not redefine or replace other convergence sub- layers. Instead, it provides a SAP that is not protocol specific