| In cellular wireless networks, it is very | | | | often used in conjunction with MAHO. Rather |
| important to deal with Mobile station (MS) | | | | than immediately terminating the connection |
| handoff between cells so that they can | | | | between a MS and a BS. In the course of |
| maintain a continuous and QoS-guaranteed | | | | handoff, a new connection is established |
| service. There are four basic types of | | | | first between the MS and a new BS, while |
| handoff protocols; network-controlled handoff | | | | keeping the old connection between the MS and |
| (NCHO), mobile-assisted handoff (MAHO), soft | | | | the old station. Only after the new |
| handoff (SHO), and mobile-controlled handoff | | | | connection can stably transmit data, the old |
| (MCHO). From NCHO to MCHO, the control | | | | connection is released. Thus, SHO is a "make |
| protocols tend to decentralize the decision | | | | before break" mechanism. This mechanism helps |
| making process, which help shorten handoff | | | | ensure the service continuity, which is |
| delays; however, meanwhile, the | | | | however at the cost of more capacity resource |
| decentralization makes the measurement | | | | consumption during the handoff (as two |
| information available to make a handoff | | | | connections are established simultaneously). |
| decision also decreased. Next, we briefly | | | | Mobile-controlled handoff In contrast to |
| introduce these four types of handoff | | | | NCHO, it is MS that totally control and make |
| mechanisms. Network-controlled handoff NCHO | | | | decisions on handoff in the MCHO approach. A |
| is a centralized handoff protocol, in which | | | | MS keeps on measuring signal strength from |
| it is network that makes handoff decision | | | | all the surround base station (BS)s. If the |
| based on measurements of the signal quality | | | | MS find that there is a new BS who has a |
| of mobile station (MS) at a number of based | | | | stronger signal than that of an old BS, it |
| stations (BS). Specifically, if the MS is | | | | may consider to handoff from the old BS to |
| measured to have a weaker signal in its old | | | | the new BS given a certain signal threshold |
| cell, while a stronger signal in a | | | | is reached. MCHO is the highest degree of |
| neighbouring cell, then a handoff decision | | | | handoff decentralization, thereby enabling it |
| could be made by the network to switch BS | | | | to have a very fast handoff speed, typically |
| from the old cell to the new cell. Such a | | | | on the order of 0.1 s. Summary We have |
| type of handoff in general takes 100-200 ms | | | | introduced four types of handoff mechanisms |
| and produces a noticeable "interruption" in | | | | that are widely used in cellular wireless |
| the conversation. However, overall delay of | | | | networks. From centralization to |
| such a type of handoff is in general in the | | | | decentralization, network-controlled handoff |
| range of 5-10 s. Thus, this type of handoff | | | | (NCHO) shows the highest centralization with |
| is not suitable to a rapid changing | | | | the network totally controlling and making |
| environment and a high density of users due | | | | decisions on handoff, while mobile-controlled |
| to the associated delay. NCHO is used in the | | | | handoff (MCHO) gives full flexibility to MS |
| first-generation analogue systems such as | | | | to allow it to make decisions on handoff. As |
| AMPS. Mobile-assisted handoff MAHO | | | | a result, decentralized handoff mechanisms |
| distributes the handoff decision process. It | | | | show advantages of very fast handoff speed, |
| is MS makes measurements, and the Mobile | | | | while centralized mechanisms generally take |
| switch centre (MSC) makes decisions on | | | | much longer time. In modern cellular wireless |
| handoff. Compared to NCHO, this mechanism has | | | | networks, decentralized handoff mechanisms |
| more distributed control, thereby helping to | | | | are widely applied due to their good |
| improve the overall handoff delay, typically | | | | scalability and fast handoff speeds. |
| in the range of 1 s. Soft handoff SHO is | | | | |