7.7 RLP

ETSI and T-1 standards bodies defined the GPRS and 3G standards to carry packet data traffic in addition to voice in wireless access links. A new protocol layer, Radio Link Protocol (RLP), is defined on top of the MAC layer to improve bit error rate performance by using an ARQ retransmission technique. In addition to retransmission, GPRS RLC (Radio Link Control) performs block segmentation, reassembly, and buffering. As GPRS provides limited data capability, 3G standards are defined that extend the data capability of radio frequency. The main operation principles of GPRS and 3G RLP standards are very similar, although 3G provides some additional QoS management capability in RLP management at the MAC layer. In our analysis, we will use GPRS and 3G RLP capabilities interchangeably without significant impact on the results. cdma2000 ^[28] is one of several 3G air-interface standards under consideration by standards bodies such as the 3GPP/3GPP2 (3G Partnership Project). The MAC layer of cdma2000 provides two important functions:

Best-effort delivery using the retransmission mechanism of RLP that provides reliability
Multiplexing and QoS management by mediating conflicting service requests

In addition, voice packets are directly given to the multiplex sublayer that bypasses the RLP function. Many transport channels have been defined for cdma2000 to provide services from physical to higher layers. These channels are unidirectional and either common (shared between multiple users) or fully dedicated to a user for the duration of the service. cdma2000 has defined many channels for its operation, the following transport channels are of interest:

Forward common control channel (F-CCCH): Communication from base station to mobile station for layer 3 and MAC messages
Forward supplemental channel (F-SCH): Operated in two modes, blind mode for data rate not exceeding 14.4 kbps, and explicit mode, where data rate information is explicitly provided, individual F-SCH target frame error rates can be different than other F-SCHs
Forward fundamental channel (F-FCH): Transmits at variable data rates, as specified in TIA/EIA-95-B
Reverse access channel (R-ACH) and reverse common control channel (R-CCCH): Used by a mobile station to communicate layer 3 and MAC messages
R-CCCH supports the low latency access procedure required for efficient operation of packet data suspend state
Reverse supplemental channel (R-SCH): Operates on two modes, blind and explicit
Reverse fundamental channel (F-SCH) supports 5- and 20-millisecond frames, the 20-millisecond frame structures provide rates derived from the TIA/EIA-95-B Rate Set-1 or Rate Set-2

The RLC and MAC layers are responsible for efficient data transfer of both real-time and nonreal-time services. The transfer of nonreal-time data includes the ARQ for low-level data to provide reliable transfers at higher levels. The network layer data PDUs (N-PDUs) are first segmented into smaller packets and transformed into link access control PDUs. The link access control overhead includes a service access point identifier, a sequence number for higher-level ARQ, and other data fields. The link access control PDUs are then transferred to SRBP (Signaling Radio Burst Protocol), a connectionless protocol for signaling messages. The data PDUs are segmented into smaller packet RLC PDUs corresponding to the physical layer transport blocks. Each RLC PDU contains a sequence number for lower-level ARQ and CRC fields for error detection. CRC is calculated and appended by the physical layer. When RLP at the receiving end finds a frame in error or missing, it sends back a NAK (negative acknowledgment) request for retransmission of this frame and starts a retransmission timer. When the timer expires for the first attempt, the RLP resets the timer and sends back a NAK request. This NAK triggers a retransmission of the requested frame from the sender. In this way, the number of attempts per retransmission increases with every retransmission trial. As noted in Bao, ^[29] the number of trials is usually less than four.

The GPRS structures of different radio channels and MAC/RLC are very similar to cdma2000. GPRS uses the same TDMA/FDMA structure as GSM to form the physical channels. For the uplink and downlink direction, many frequency channels with a bandwidth of 200 kHz are defined. These channels are further subdivided into the length of 4.615 milliseconds. Each TDMA frame is further split into eight time slots of equal size. As an extension to GSM, GPRS uses the same frequency bands as GSM and both share the same physical channels. Each time slot can be assigned to either GSM or GPRS. Time slots used by the GPRS are known as packet data channels (PDCH). The basic transmission unit of a PDCH is called a radio block. To transmit a radio block, four consecutive TDMA frames are utilized. Depending on the message type transmitted in one radio block, a sequence of radio blocks forms a logical channel.

PRACH (packet random access channel, uplink): This common channel is used by the mobile stations to initiate the transfer in the uplink direction.
PPCH (packet paging channel, uplink): The base station controllers (BSC) uses this channel to page the mobile station prior to downlink data transmission.
PAGCH (packet access grant channel, downlink): Resource assignment on the uplink and downlink channel is sent on this channel.
PBCCH (packet broadcast control channel): GPRS specific information is broadcast on this channel.
PDTCH (packet data transfer channel) : Data packets are sent on this channel. A mobile station can use one or several PDTCHs at the same time.
PACCH (packet associated control channel): This channel conveys signaling information related to a given mobile station and the corresponding PDTCHs.

^[28]http://3GPP2.org.

^[29]Bao, G., Performance evaluation of TCP/RLP protocol stack over CDMA wireless link, Wireless Networks, 3 (2), 229–237, 1996.