The radio link layer is composed of different sublayers. The sublayers of interest for IP services are link access control (LAC) and media access control (MAC). This section describes some of the important functions of LAC and MAC. 10.5.1 Link Access ControlThe purpose of the LAC sublayer is to process cdma2000 layer 3 signaling according to the radio characteristics of the physical channel without the need for the upper layers to be aware of them. LAC does not process IP-level signaling (e.g., MIP registration request). The IP level signaling is processed as a user data packet by the MAC sublayer. The processing within LAC is done sequentially, with processing entities passing the partially formed LAC protocol data unit (PDU) to each other in an established order. The following are the functions performed by the LAC sublayer (Figure 10-7): Figure 10-7. LAC sublayer functions. Reproduced under written permission from Telecommunications Industry Association.
10.5.2 MACMAC provides access control by managing physical layer resources. It is also responsible for providing the QoS level requested by a user data service. Data services use two important MAC functions: radio link protocol (RLP) and multiplex /QoS. RADIO LINK PROTOCOLThe RLP sublayer divides the PPP packets into radio frames for transmission. RLP provides ARQ, FEC, and flow control functions. These functions improve performance in the radio segment of the data connection. RLP was already present in IS-95 and it is enhanced in cdma2000 to accommodate higher data rates. MULTIPLEX AND QOS SUBLAYERThe multiplex sublayer multiplexes signaling, voice, and data blocks received through LAC, voice services, and RLP (Figure 10-8). It forms physical layer service data unit (SDUs) for transmission. There are two types of physical layer channels available for transferring IP traffic: fundamental channel (FCH) and supplemental channel (SCH). On the SCH only data blocks can be multiplexed and CRC is added for protection against errors. This combination of data and CRC is called the link transmission unit (LTU). In the reverse direction, the multiplex sublayer de-multiplexes the information contained in the physical layer SDUs and directs the information to the correct entity (i.e., upper-layer signaling, data, or voice service). Figure 10-8. Multiplex sublayer function. Reproduced under written permission from Telecommunications Industry Association.
QoS for over-the-air transmission is specified in terms of these parameters: assured/nonassured, user priority, minimum data rate, maximum data loss rate, and maximum delay. The multiplex sublayer may use QoS in delivering data over the air. The exact manner for using the QoS to deliver data over the air interface is not specified in the standard and is left for implementers to decide. |