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Although this is a designated traffic channel consisting of 26 frames, at most only 24 of these are used for TCH user data such as voice. The frame is also used to carry two logical control channels, the slow associated control channel (SACCH) and the fast associated control channel (FACCH). Time slots 12 and 25 are specifically used for SACCH. In fact, the SACCH may actually alternate between these on different multiframes and so there is only one SACCH channel transmitted per multiframe. During the frame that the mobile device is not transmitting or receiving on its dedicated TCH, it is constantly monitoring the strength of the received signals from the cell it is attached to as well as other cells in the area. The mobile station can actually monitor up to six surrounding cells. The SACCH is then used for sending these measurement results to the network. A SACCH message is 456 bits long (4 bursts×114 bits per burst); it therefore has a time interval of 480 ms before repeating itself. This information can be used to increase or decrease the transmitted power levels of both the mobile device and the network every 480 ms, or just a little over twice a second. The mobile device uses this information it receives but actually alters its power in steps every 60 ms. Enhanced circuit switched data (ECSD) can also use a fast power (FP) method which is sent within the data stream every 20 ms. Figure 3.12 illustrates the SACCH power control mechanisms: both the mobile device and the BTS send measurement reports to the BSC, which makes the decision to increase or decrease power.

The SACCH may also be used for sending SMS messages to and from the mobile device while a call is in progress. User data frames 0–11 and 13–24 also carry control information in the form of the FACCH channel, as described below. A single user voice

Base Station Subsystem BSC Abis Mobile Station SIM ME

Power Control and measurement reporting

Power Control and measurement reporting

Power Control Decision Power Control Decision

SACCH every 480ms

BTS

Figure 3.12 SACCH power control

call will be transmitted in frame 0 of the multiframe for a particular amount of time, followed by a particular amount of time in frame 1 etc. Eight such calls can be made on this carrier frequency. A traffic channel is only assigned when the mobile device is in dedicated mode, whereas in idle mode the mobile device does not have a traffic channel assigned to it. Figure 3.13 shows the relationship of the TDM frame within a traffic channel multiframe.

Each of the eight time slots in the TDM frame consists of 148 bits lasting 547µs and has the structure shown in Figure 3.14.

0 1 2 3 4 5 6 7 TDM frame 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 17 18 19 20 21 22 23 24 25 Control Frame Control Frame Multi-frame 32,500 bits in120msec

1250 bits in 4.615msec Figure 3.13 GSM multiframe 6 7 4 5 2 3 0 1

3 bits 57 bits 1 bit 26 bits 1 bit 57 bits 3 bits 8.25 bit guard band

148 bit data frame sent in 547us

000 Information F1 Training F1 Information 000

Actually each time slot (burst) consists of the GSM burst and the guard band and is thus 156.25 bits long and takes up 577µs, with each single bit taking approximately 3.69µs. Note that the guard band consisting of 8.25 bits is only used as a time inter- val (approximately 30µs) between time slots to separate one user transmission from the next.

Each TDM burst consists of 148 bits. The first 3 bits and last 3 bits are always zero and can be used for framing. The F1 bits are referred to as stealing bits and indicate whether the burst contains user data or control information. While a call is in progress, if it is decided that a handover to another cell should take place, this handover needs to be executed very quickly. The fastest way for the network to indicate to the mobile device that a handover should take place is via its dedicated traffic channel (TCH). When the traffic channel is used for this purpose, it is referred to as the fast associated control channel (FACCH). The stealing bits indicate to the subscriber that this burst is a control message and not user data. In the uplink, the mobile device will indicate control information to the network in the same manner. Using the traffic channel in such a way exposes the subscriber to a minimal amount of increased interference while the handover is executed. However, in most cases, the subscriber will not notice this extra interference. The training field is used to synchronize the transmitter and receiver. It is also used to ‘train’ the receiver to the particular characteristics of the channel being used. The receiver knows exactly what the transmitter has sent in this field since this is negotiated beforehand. By using this knowledge it can check for any distortion between the transmitted and received training field signal. This information can then be used to ‘equalize’ the information fields that are received, reducing the possibility of errors.