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<h2>GPRS Data Transfer (protocols, scenarios)</h2>

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<p>The GSM RF is the normal GSM physical radio layer. The
Radio Link Control (RLC) function offers a reliable radio link to the upper
layers. The Medium Access Control (MAC) function handles the channel allocation
and the multiplexing, that is, the use of physical layer functions. The RLC
and the MAC together form the OSI Layer 2 protocol for the Um interface. The
Logical Link Control (LLC) layer offers a secure and reliable logical link
between the MS and the SGSN to upper layers and is independent of the lower
layers. The LLC layer has two transfer modes, the acknowledged and unacknowledged.
The LLC conveys signalling, SMS, and SNDCP packets. The Subnetwork Dependent
Convergence Protocol (SNDCP) is a mapping and compression function between
the network layer and lower layers. It also performs segmentation, re-assembly,
and multiplexing.</p>


<p>The Base Station System GPRS Protocol (BSSGP) transfers
control information and data between a BSS and a SGSN. The Network Services
relays the BSSGP packets over the Gb interface and has load sharing and redundancy
on top of Frame Relay. The L1bis is a vendor-dependent OSI Layer 1 protocol.
The Relay function relays LLC PDUs (Protocol Data Units) between the LLC and
BSSGP.</p>


<p>The PCU (Packet Control Unit, see chapter 3.2.9) is responsible
for the following GPRS MAC and RLC layer functions as defined in GSM 03.64:</p>


<p>LLC layer PDU segmentation into RLC blocks for downlink
transmission</p>


<p>LLC layer PDU re-assembly from RLC blocks for uplink transmission</p>


<p>PDCH scheduling functions for the uplink and downlink data
transfers</p>


<p>PDCH uplink ARQ functions, including RLC block ack/nack</p>


<p>PDCH downlink ARQ function, including buffering and retransmission
of RLC blocks</p>


<p>Channel access control functions, for example, access requests
and grants</p>


<p>Radio channel management functions, for example, power
control, congestion control, broadcast control information, etc.</p>


<p>The functions inside the Channel Codec Unit (CCU) are:</p>


<p>Channel coding functions, including FEC and interleaving</p>


<p>Radio channel measurement functions, including received
quality level, received signal level, and information related to timing advance
measurements.</p>


<p>The Network Protocol Data Units (N-PDU) are segmented into
the Subnetwork Protocol Data Units (SN-PDU) by the Subnetwork Dependent Convergence
(SNDC) protocol, and SN-PDUs are encapsulated into one or several LLC frames.
LLC frames are of variable length. The maximum size of the LLC frame is 1600
octets minus GP protocol control information. See GSM 03.60 for information
on SNDC and LLC. The details on SNDC can be found in GSM 04.65 and the details
on LLC in GSM 04.64. LLC frames are segmented into RLC Data Blocks. In the
RLC/MAC layer, a selective ARQ protocol (including block numbering) between
the MS and the network provides retransmission of erroneous RLC Data Blocks.
When a complete LLC frame is successfully transferred across the RLC layer,
it is forwarded to the LLC layer.</p>


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