communications between the ran 20 and the mss 12 u2014 is depicted in fig2 at the upper layers the pcn 22 provides packet data service and the is 41 network 24 provides voice service and circuit switched data as described above the is 2000 layer 3 also called the signaling layer comprises base station and mobile station interoperability procedures and associated signaling that allow the end user to receive service the link access control lac layer provides transport services over logical channels for layer 3 signaling the logical channels may be dedicated or common dedicated channels are mapped to physical channels assigned to a single user such as for transmitting voice communications common channels are mapped to physical channels shared by multiple users and may carry system overhead traffic paging messages and the like the lac encapsulates layer 3 messages into lac protocol data units pdu that are subject to fragmentation and reassembly to make them suitable for transport by the lower layers the media access control mac layer maps the logical channels to physical channels and coordinates the use of physical resources the mac also enforces the negotiated quality of service qos level the mac implements a multiplexing and demultiplexing function to allow access to the medium to both pdus received from the lac and data units received from user applications the mac ensures reliable transmission of data u2014 nearly error free and in order u2014 through the radio link protocol rlp and other protocols the rlp is a specialized form of selective repeat automatic repeat request arq protocol defined in the is 707 standard which is.