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84 From GSM to LTE-Advanced Pro and 5G

GPRS Packet Control Unit (PCU), described below, of the incoming SMS or call. If an
active data transfer is ongoing, the PCU will send the Paging message during the data
transfer to the mobile device. The data transfer can then be interrupted, and the mobile
device can respond to the Paging message from the MSC.
To inform mobile devices which of the two GPRS network modes is used, GPRS uses
the GSM BCCH channel and the SysInfo 13 message.

2.3.7 GPRS Logical Channels on the Air Interface
GPRS uses a number of logical channels on the air interface in addition to those which
are shared with GSM. They are used for transmitting user data and signaling data in the
uplink and downlink direction. The following logical channels, which are shown in
Figure 2.10, are mandatory for the operation of GPRS:

The Packet Data Traffic Channel (PDTCH): This is a bidirectional channel, which
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means it exists in the uplink and downlink direction. It is used to send user data
across the air interface. The PDTCH is carried over timeslots that are dedicated for
GPRS in a 52‐multiframe structure, which was introduced in Section 2.3.1.
The Packet-Associated Control Channel (PACCH): This channel is also bidirec-
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tional and is used to send control messages. These are necessary to acknowledge
packets that are transported over the PDTCH. When a mobile device receives data
packets from the network via a downlink PDTCH, it has to acknowledge them via the
uplink PACCH. Similar to the PDTCH, the PACCH is also carried over the GPRS‐
dedicated timeslots in blocks of the 52‐multiframe structure introduced earlier. In

GPRS channels

PDTCH PACCH PTCCH

Packet data traffic channel Packet timing advance control channel
Packet associated control channel

Random access channel Paging channel
Broadcast channel Access grant channel

BCCH RACH AGCH PCH

GSM channels for GPRS

Figure 2.10 PDTCH and PACCH are sent on the same timeslot.