Page 111 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 97

block. If they are not equal, the mobile device discards the received block. Once the
PCU has sent all data for the subscriber currently in its queue, it will set the ‘final block
indicator’ bit in the last block it sends to the mobile device. Subsequently, the mobile
device stops listening on the assigned timeslots and the TFI can be reused for another
subscriber. To improve performance, the network can also choose to keep the downlink
TBF established for several seconds so that no TBF establishment is necessary if further
data for the user arrives.
To acknowledge blocks received from the network, the mobile device has to send
control information via the logical PACCH. For sending control information to the net-
work, it is not necessary to assign an uplink TBF. The network informs the mobile
device in the header of downlink blocks which uplink blocks it can use to send control
information.

Timing Advance Control
The farther a mobile device is away from a BTS, the sooner it has to start sending its
data bursts to the network in order for them to arrive at the BTS at the correct time. As
the position of the user can change during the data exchange, it is necessary for the
network to constantly monitor how far away the user is from the serving base station. If
the user moves closer to the BTS, the network has to inform the mobile device to delay
sending its data compared to the current timing. If the user moves farther away, it has
to start sending its bursts earlier. This process is called timing advance control.
As we have seen in the previous paragraph, the assignment of uplink and downlink
resources is independent of each other. When a user is downloading a large web page,
for example, it might happen that a downlink TBF is assigned while no uplink TBF is
established because the mobile device has no data to send.
Even though no uplink TBF is established, it is necessary from time to time to send
layer 2 acknowledgment messages to the network for the data that has been received in
the downlink. To send these messages quickly, no uplink TBF has to be established. In
this case, the PCU informs the mobile device in the downlink TBF from time to time
which block to use to send the acknowledgment. As this only happens infrequently, the
network cannot utilize the previous acknowledgment bursts for the timing advance
calculation for the following bursts. Hence, a number of methods have been standard-
ized to measure and update the timing advance value while the mobile device is engaged
in exchanging GPRS data.

The Continuous Timing Advance Update Procedure
In a GPRS 52‐multiframe, frames 12 and 38 are dedicated to the logical PTCCH uplink
and downlink. The PTCCH is further divided into 16 subchannels. When the PCU
assigns a TBF to a mobile device, the assignment message also contains an information
element that instructs the mobile device to send access bursts on one of the 16 subchan-
nels in the uplink with a timing advance 0. These bursts can be sent without a timing
advance because they are much shorter than a normal burst. For more information
about the access burst, see Chapter 1. The BTS monitors frames 12 and 38 for access
bursts and calculates the timing advance value for every subchannel. The result of the
calculation is sent on the PTCCH in the following downlink block. As the PTCCH is
divided into 16 subchannels, the mobile device sends an access burst on the PTCCH
and receives an updated value every 1.92 seconds.