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Segment Number
- Current segment: 0 (0h)
N-PDU Number
- UNACK mode: 2449 (991h)
Data Segment
IP DATAGRAM
Source address : 192.168.1.2
Destination address : 10.1.2.123


Figure 4.19 LLC trace. Reproduced by permission of NetHawk Oyj


the blocks. The sending side will transmit the blocks within a preset window size and
will receive ACK/NACK messages when required. The ACK/NACK will acknowl-
edge all blocks up to an indicated BSN. If there are any blocks missing these can
be indicated within a bitmap ¬eld and selective retransmission of these blocks can
take place. The acknowledgement procedures for the LLC (mobile device to SGSN)
and RLC/MAC (mobile device to BSC) layers should be seen as independent of
each other.
• Unacknowledged mode: the transfer of blocks in the unacknowledged mode is con-
trolled by block sequence numbering, and ACK/NACK messages are sent by the
receiver. However, this mode of operation does not include retransmissions and thus
the ACK/NACK information can only be used for assessing the quality of the link. The
length of the user ¬eld is preserved by inserting dummy information.
4.6 GPRS PROTOCOLS 109


Input Direction Input Direction



ciphering ciphering
Kc Kc
algorithm algorithm



Unciphered Ciphered Deciphered
frame frame frame


Figure 4.20 Ciphering algorithm

RLC/MAC data blocks are used to transport LLC PDUs containing both user data and
upper layer signalling. RLC/MAC control blocks are used to transport control messages
related to TBF management, paging and system information, etc. These RLC/MAC control
blocks have a higher priority than the data blocks.


4.6.4.1 Temporary block ¬‚ow (TBF)
The temporary block ¬‚ow mechanism is a unidirectional physical connection to support
the transfer of LLC frames. It enables a number of mobile devices to share either a single
time slot or occupy a number of time slots in both the uplink and downlink directions. The
uplink and downlink are independently assigned, giving the opportunity for asymmetric
transfer. This is more suitable for data traf¬c, where in general there is a greater volume
of traf¬c in the downlink. Each of the mobile devices is assigned a single or multiple
radio blocks at a time, therefore there could be a number of mobile devices sharing the
same 9.05 kbps channel. This would give a very slow data rate but would, as previously
discussed, be suf¬cient to keep a TCP/IP connection from timing out. A mobile device is
given an assignment consisting of the channel, time slot and radio block on which they can
transmit. This is required as a means of contention control since there may be a number
of mobile devices wishing to transmit on the uplink at the same time. This situation does
not occur in the downlink since there is only one base station. There are three techniques
of working this system: the dynamic, extended dynamic and ¬xed allocation methods.
Both the dynamic and ¬xed allocation methods are required to be supported by all GPRS
networks whereas the extended dynamic allocation is an optional feature.

• Dynamic allocation: an uplink state ¬‚ag (USF) is transmitted on the downlink data
channel to allow the multiplexing of a number of mobile devices to send data on the
uplink in the correct radio blocks. The USF comprises three bits. Each mobile device
is allocated an individual number and thus a maximum of 8 mobile devices can be
multiplexed at any one time even though there are 12 radio blocks.
• Extended dynamic allocation: this method is similar to the above but eliminates the
need to receive a USF on each of the time slots. In this case when a mobile device
receives a USF on a PDCH downlink channel time slot it will assume that it can send
data not only on this time slot but on all the others that have been allocated to it. For
110 GENERAL PACKET RADIO SERVICE


example, if a mobile device has been allocated blocks on time slots 0, 2 and 3 and it
receives a USF on time slot 0, it will transmit on time slots 0, 2 and 3, not just on
time slot 0.
• Fixed allocation: a packet uplink assignment message is used to communicate to the
mobile device which resources (radio blocks) it has been allocated. The ¬xed allocation
method does not include the USF and the mobile device is free to transmit on the uplink
without monitoring the downlink for the USF.

Each TBF is allocated a temporary ¬‚ow indicator (TFI) which is assigned by the net-
work. The TFI is a 5-bit ¬eld contained within the RLC header and is unique within the
cell. It is used in both uplink and downlink to ensure that the received radio blocks are
associated with the correct LLC frame and SGSN/mobile device. Figure 4.21, illustrates
how four subscribers can share a single time slot using the dynamic allocation method.
The 52-frame multiframe is split into 12 blocks each consisting of four GSM bursts.
Mobile device 1 has been assigned USF = 1. It monitors the base station downlink sig-
nal and notices that blocks 2 and 3 have USF = 1 in their header information, it knows
therefore that it has been allocated these blocks to send information. In the uplink to
the base station. Notice that the mobile device which has been assigned USF = 2 has
been assigned a larger number of blocks. This may be because the subscriber has paid a
premium for higher QoS.

4.6.4.2 Establishment of TBF
The mobile station can establish a TBF for the transfer of LLC packets in the uplink
(both signalling3 and data packets) by sending the packet channel request message on the

USF=0 USF=1 USF=2 USF=2 USF=2 USF=3



Block Block Block Block Block Block Block Block Block Block Block Block
T I T I
0 1 2 3 4 5 6 7 8 9 10 11


52 TDMA multi-frame downlink data

1
on blocks 0 and
Mobile transmits
2 and 3
its on blocks
Mobile transm
, 8 and 9
7
cks 4, 5, 6,
smits on blo
Mobile tran
and 11
blocks 10
smits on
obile tran
USF=0 M
BTS

USF=1

USF=2

USF=3

Figure 4.21 GPRS users sharing time slots

3
When signalling data is sent this is transferred using the RLC acknowledged mode of operation.
4.6 GPRS PROTOCOLS 111


(P)RACH channel to the network. The mobile device may use a one- or two-phase access
method. It is possible under certain circumstances, highlighted below, that the mobile
device will use channels other than the PRACH.

• If a TBF has already been established in the downlink direction from the network to
the mobile device then the PACCH can be used for the initial access.
• If the mobile device is operating in dual-transfer mode4 (DTM) and already has
a dedicated mode connection established then the DCCH should be used for the
TBF initiation.

In the one- and two-phase access methods the network will respond with a packet
uplink assignment message on the (P)AGCH. If the mobile station does not receive a
packet queuing noti¬cation or a packet uplink assignment message from the network it
will wait for a certain time period before resending the message again. The packet queuing
noti¬cation message may be sent if there are more packet channel requests than can be
handled. This message indicates to the mobile device that its packet channel request
message has been received correctly and a packet uplink assignment message will be
forthcoming. For the single-phase access, reservation of resources is accomplished with
use of the information within the packet channel request message. On the RACH there are
only two cause values available for GPRS which can be used, to request limited resources
or two-phase access. The PRACH channel (if available), which is designed for GPRS,
can contain more adequate information about the requested resources than the RACH.
In the two-phase access method the packet uplink assignment message sent on the
(P)AGCH will reserve the limited resources required for the packet resource request
message (not for data transfer) which is sent on the PACCH. This packet resource request
message carries a complete description of the requested resources for the data transfer. The
network will respond on the PACCH with a packet uplink assignment message and will
reserve the resources required and de¬ne the actual parameters for the data transfer. These
parameters may include the power control information, number of blocks allocated, TFI
to be used, USF issued and channel coding type (e.g. CS-2). This procedure is highlighted
in Figure 4.22 and data transfer would follow this procedure. The resources are usually
released by the mobile device as it counts down (see countdown value in uplink frame
format; Section 4.6.4.3) the last few blocks it wishes to send. The network is then free
to reallocate the USF to some other user. To initiate downlink data transfer to a mobile
device in standby state the network would page the device as illustrated in Figure 4.23.
As can be seen, the mobile device sends a packet channel request message in the same
way as before.
This procedure enables the mobile device to send a packet paging response to the
network. Once this is received, the mobile device will be in ready mode, where it can send
and receive data (see later). The network can now initiate a packet downlink assignment
message. If there is already an uplink packet transfer in progress this message will be sent
on the PACCH; otherwise it will be sent on the PCCCH. If there is no PCCCH allocated

4
The mobile device can be simultaneously connected to the network in dedicated mode and packet
transfer mode. DTM is a subset of the class A mode of operation.
112 GENERAL PACKET RADIO SERVICE


Mobile
Network
Device

Packet Channel Request (P)RACH

Packet Uplink Assignment (P)AGCH

Packet Resource Request PACCH
optional

Packet Uplink Assignment PACCH
optional

Figure 4.22 Packet uplink assignment


Mobile
Network
Device

Packet Paging Request (P)PCH

Packet Channel Request (P)RACH

Packet Uplink Assignment (P)CCCH

Packet Resource Request PACCH
optional

Packet Uplink Assignment PACCH
optional
Packet Paging Response PDTCH



Figure 4.23 Paging for downlink packet transfer


in the cell then an immediate assign message will be sent on the CCCH. This message
will include the PDCHs that will be used for the data transfer.
There are two message formats that can be used for the packet channel request message,
containing 8 bits or 11 bits of information. The correct one to use for the cell for a
particular purpose is broadcast on the (P)BCCH. The 11-bit format includes two bits
which de¬ne four levels of priority. Both formats have the same access types speci¬ed
and these are shown in Table 4.6. The one-phase access request has ¬ve bits set aside to
indicate the multislot capability of the mobile device.

• If the mobile device wishes to establish the TBF for user data using the RLC unac-
knowledged mode the two-phase access method will be used.
• If acknowledged mode and the amount of data consists of eight or fewer RLC/MAC
blocks then the short access method will be used.
• If acknowledged mode (>eight blocks) is to be transferred then either the single or
two-stage access can be used.
4.6 GPRS PROTOCOLS 113


Table 4.6 Packet channel request access
types
Access type
One-phase access request
Short access request
Two-phase access request
Paging response
Cell update
Mobility management procedure
Single block without TBF establishment


• If the purpose of the packet access procedure is to respond to a page then page response
will be used.
• If the purpose of the packet access procedure is to send a cell update then the cell
update will be used.
• If the purpose of the packet access procedure is for any mobility management procedure

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