Page 104 - From GMS to LTE

90 From GSM to LTE-Advanced Pro and 5G

and its switching matrix are not suited to handling packet‐switched GPRS traffic.
Therefore, this task has been assigned to a new network component, the PCU. The PCU
is the packet‐switched counterpart of the BSC and fulfills the following tasks:
assignment of timeslots to subscribers in the uplink direction when requested by the
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mobile device via the RACH or the PRACH;
assignment of timeslots to subscribers in the downlink direction for data arriving
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from the core network;
flow control of data in the uplink and downlink directions and prioritization of traffic;
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error checking and retransmission of lost or faulty frames;
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subscriber paging;
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supervising entity for subscriber timing advance during data transmission.
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In order for the PCU to control the GPRS traffic, the BSC turns over control for some
of the timeslots to the PCU. This is done by redirecting timeslots in the BSC switching
matrix away from the MSC and Transcoding and Rate Adaptation Unit (TRAU) toward
the PCU. The BSC then simply forwards all data contained in these timeslots to and
from the PCU without any processing.
As GPRS uses GSM signaling channels like the RACH, PCH and AGCH to establish
the initial communication, a control connection has to exist between the PCU and the
BSC. When the mobile device requests GPRS resources from the network, the BSC
receives a Channel Request message for packet access. The BSC forwards such packet
access request messages straight to the PCU without further processing. It is then the
PCU’s task to assign uplink blocks on a PDTCH and return an immediate packet assignment
command, which contains a packet uplink assignment for the subscriber. The BSC just
forwards this return message from the PCU to the BTS without further processing.
Once GPRS uplink resources have been assigned to a user by the PCU, further signaling
will be handled by the PCU directly over the GPRS timeslots and no longer via the GSM
signaling channels.

Monitoring GSM and GPRS Signaling Messages
In GSM, it is quite easy to use a network tracer to monitor all signaling messages being
exchanged between the BSC, BTS and the mobile devices communicating over a BTS.
All messages use the same logical link access protocol (LAPD) channel that is transmit-
ted over dedicated LAPD timeslots on the Abis interface. The traffic channels used for
the voice data are transmitted on different timeslots. To monitor the signaling messages
it is only necessary to monitor the LAPD timeslots. Monitoring GPRS signaling mes-
sages is a far more complex task, as they can be sent over any GPRS timeslot and in
between user data blocks. Therefore, it is also necessary to trace user data and empty
packets on the Abis interface, which requires more processing power and memory on
the network tracer.

PCU Positioning
The GSM standards allow for a number of different positions for the PCU in the
network. The most common implementation is to have the PCU behind the BSC as
shown in Figure 2.15. As regards design of the PCU, some network suppliers deliver
it as one or more cards that can be plugged into the BSC. Others have chosen to base
the PCU on a more powerful computing architecture that is able to process the GPRS