Handover 841 Overview

Handover is the transfer of an existing voice connection to a new base station. There are different reasons for the handover to become necessary. In GSM, a handover decision is made by the network, not the mobile station, and it is based on BSS criteria (received signal level, channel quality, distance between MS and BTS) and on network operation criteria (e.g. current traffic load of the cell and ongoing maintenance work).

The functions for preparation of handover are part of the Radio Subsystem Link Control. Above all, this includes the measurement of the channel. Periodically, a mobile station checks the signal field strength of its current downlinks as well as those of the neighboring base stations, including their BSICs. The MS sends measurement reports to its current base station (quality monitoring); see Section 5.5.1. On the network side, the signal quality of the uplink is monitored, the measurement reports are evaluated, and handover decisions are made.

As a matter of principle, handovers are only performed between base stations of the same PLMN. Handovers between BSS in different networks are not allowed. Two kinds of handover are distinguished (Figure 8.13):

• Intracell Handover: for administrative reasons or because of channel quality (channel-selective interferences), the mobile station is assigned a new channel within the same cell. This decision is made locally by the Radio Resource Management (RR) of the BSS and is also executed within the BSS.

• Intercell Handover: the connection to an MS is transferred over the cell boundary to a new BTS. The decision about the time of handover is made by the RR protocol module of the network based on measurement data from MS and BSS. The MSC, however, can participate in the selection of the new cell or BTS. The intercell handover occurs most often when it is recognized from weak signal field strength and bad channel quality (high bit error ratio) that a mobile station is moving near the cell boundary. However, an intercell handover can also occur due to administrative reasons, say for traffic load balancing. The decision about such a network-directed handover is made by the MSC, which instructs the BSS to select candidates for such a handover.

Two cases need to be distinguished with regard to participation of network components in the handover, depending on whether the signaling sequences of a handover execution also

Error Indication Figure 8.12: Forwarding short messages in a PLMN

involve an MSC. Since the RR module of the network resides in the BSC (see Figure 7.11), the BSS can perform the handover without participation of the MSC. Such handovers occur between cells which are controlled by the same BSC and are called internal handover. They can be performed independently by the BSS; the MSC is only informed about the successful execution of internal handovers. All other handovers require participation of at least one MSC, or their BSSMAP and MAP parts, respectively. These handovers are known as external handovers.

Participating MSCs can act in the role of MSC-A or MSC-B. MSC-A is the MSC which

What The Role The Handovers

performed the initial connection setup, and it keeps the MSC-A role and complete control (anchor MSC) for the entire life of the connection. A handover is therefore in general the extension of the connection from the anchor MSC-A to another MSC (MSC-B). In this case, the mobile connection is passed from MSC-A to MSC-B with MSC-A keeping the ultimate control over the connection. An example is presented in Figure 8.14. A mobile station occupies an active connection via BTS1 and moves into the next cell. This cell of BTS2 is controlled by the same BSC so that an internal handover is indicated. The connection is now carried from MSC-A over the BSC and the BTS2 to the mobile station; the connections of BTS1 (radio channel and ISDN channel between BTS and BSC) were taken down. As the mobile station moves on to the cell handled by BTS3, it enters a new BSS which requires an external handover. Besides, this BSS belongs to another MSC, which now has to play the role of MSC-B. Logically, the connection is extended from MSC-A to MSC-B and carried over the BSS to the mobile station. At the next change of the MSC, the connection element between MSC-A and MSC-B is taken down, and a

Connection route

Connection route

Mobile Controlled Handoff
Figure 8.14: Internal and external handover

connection to the new MSC from MSC-A is set up. Then the new MSC takes over the role of MSC-B.

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  • ANNA
    What is the role of the MS in handovers?
    8 years ago

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