Page 47 - From GMS to LTE

Global System for Mobile Communications (GSM) 33

Mobile device BTS BSC
RACH: Channel request
Channel required

Channel activation (SDCCH x)
Channel activation ack.

Immediate assignment (SDCCH x)
AGCH: Immediate assignment
(SDCCH x)

SDCCH x is used for further signaling

Figure 1.27 Establishment of a signaling connection.

The Access Grant Channel (AGCH): If a subscriber sends a Channel Request message
●
on the RACH, the network allocates an SDCCH or, in exceptional cases, a TCH, and
notifies the subscriber on the AGCH via an Immediate Assignment message. The
message contains information about which SDCCH or TCH the subscriber is allowed
to use.
Figure 1.27 shows how PCH, AGCH and SDCCH are used during the establishment
of a signaling link between the mobile device and the network. The base station control-
ler (BSC), which is responsible for assigning SDCCH and TCH of a base station, is fur-
ther described in Section 1.7.4.
As can also be seen from Figure 1.25, not all bursts on timeslots 2–7 are used for
TCHs. Every 12th burst of a timeslot is used for the SACCH. Furthermore, the 25th
burst is also not used for carrying user data. This gap is used to enable the mobile device
to perform signal strength measurements of neighboring cells on other frequencies.
This is necessary so that the network can redirect the connection to a different cell
(handover) to maintain the call while the user is moving.
The GSM standard offers two possibilities to use the available frequencies. The sim-
plest case, which has been described already, is the use of a constant carrier frequency
(ARFCN) for each channel. To improve the transmission quality, it is also possible to
use alternating frequencies for a single channel of a cell. This concept is known as
frequency hopping, and it changes the carrier frequency for every burst during a trans-
mission. This increases the probability that only few bits are lost if one carrier frequency
experiences a lot of interference from other sources like neighboring cells. In the worst
case, only a single burst is affected because the next burst is already sent on a different