Page 99 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 85

Header Payload Payload type =
Control block

Data Data PACCH Data

Data frame consisting of 4 bursts of a timeslot
Payload type = every 20 milliseconds. Number of user data
Data, (PDTCH) bits depending on coding scheme.
Figure 2.11 GPRS logical channels.

addition, the PACCH is used for signaling messages that assign uplink and downlink
resources. For the mobile device and the network to distinguish between PDTCH and
PACCH that are carried over the same physical resource, the header of each block
contains a logical channel information field as shown in Figure 2.11.
The Packet Timing Advance Control Channel (PTCCH): This channel is used for
●
timing advance estimation and control of active mobile devices. To calculate the tim-
ing advance, the network can instruct an active mobile device to send a short burst at
regular intervals on the PTCCH. The network then calculates the timing advance and
sends the result back in the downlink direction of the PTCCH.
In addition, GPRS shares a number of channels with GSM to initially request for the
assignment of resources. Figure 2.12 shows how some of these channels are used and
how data is transferred once uplink and downlink resources have been assigned.

Figure 2.12 Packet resources: requests and
assignments. MS BSS

Channel request (packet)
RACH

Immediate assignment uplink
AGCH
UL user data + TLLI
PACCH 400 – 600 ms
Packet timeslot reconfig
PACCH
PDTCH block with USF flag
PDTCH / DL
User data
PDTCH / UL
User data
PDTCH / UL + DL