6.3. Transfer service. in GSM network

BTS, located approximately in the centers of the cells, serve all MS within their cells. If the MS moves within cell A (Fig. 6.2), then the BTS-a base station serves all MSs located in cell A.

Fig. 6.2. Transfer service within two cells A and B

When an MS moves from cell A to cell B, its service is transferred from BTS-a to BTS-b. This process is called handover.

It should be noted that the transfer of service is implemented only in the case when the MS crosses the border of the cells during a communication session and the communication (telephone conversation) is not interrupted. If the MS is in standby mode, but moves from one cell to another, then the MS simply tracks these movements in the cellular network information transmitted via the control channels and is re-tuned to a stronger signal at another BTS at the right time.

The handover procedure is technically done as follows: when the MS is moved, the need for a handover occurs when the quality of the communication channel, assessed either by signal level or by the frequency of the BER bit error, falls below the acceptable limit.

In the GSM standard, these parameters are constantly measured in MS for both its cell and for a number of adjacent cells (up to 16 cells), and the measurement results are transmitted to the base station. According to the results of these measurements, the switching center selects a cell to which the service can be transferred. It should be noted that the organization of the handover is based on measurements performed on the MS in time slots free from transmission and reception of information. In addition, the results of measurements performed on the BSS can be used. This is reflected in the Mobile Assisted Hand Over procedure - handover using the help of MS itself. A prerequisite for the transfer of service is a higher quality of the communication channel in the cell, compared to the cell where the quality of the channel is lower and from which the MS has already moved.

This suggests that the service is transferred from the cell with the worst quality of the communication channel to the cell with the best quality, and the difference should be no less than a certain predetermined value. This is important especially when moving MS along the borders of neighboring cells, because if you do not specify an excess amount, the cellular system will implement multiple handoffs from one cell to another and back, which will increase the load of the entire system with inefficient work and reduce the quality of communication.

So, the MSC switching center, deciding on the transfer of service and choosing a new cell, informs the BTS of the new cell about this, and the MS issues the necessary commands indicating the new frequency range, the number of the working slot, etc. through the BTS of the old cell. MS is rebuilt to a new channel and is configured to work together with the new BTS, performing approximately the same steps as preparing a communication session, after which communication continues through the new base station in the new cell. A break in a telephone conversation with such switchings does not exceed a second and remains almost unnoticed by subscribers.