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Question:
Published on: 29 May, 2024

What is hand-off? What are the different hand-off strategies? Discuss the merits and demerits of each such strategy?

Answer:

A handoff refers to the process of transferring an active call or data session from one cell in a cellular network to another or from one channel in a cell to another. A well-implemented handoff is important for delivering uninterrupted service to a caller or data session user.

There are two types of hand-off strategies:

  • Hard handoff: in this type of hand-off the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to the source is broken before the connection to the target is made,so such handovers are also known as break-before-make.
    • Merits:
      • at any moment in time one call uses only one channel.
      • the phone's hardware does not need to be capable of receiving two or more channels in parallel, which makes it cheaper and simpler.
    • Demerits:
      • if a handover fails the call may be temporarily disrupted or even terminated abnormally.

 

  • Soft handoff: in this type of hand-off the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken, hence this handover is called make-before-break. 
    • Merits:
      • the connection to the source cell is broken only when a reliable connection to the target cell has been established and therefore the chances that the call will be terminated abnormally due to failed handovers are lower.
      • simultaneously channels in multiple cells are maintained and the call could only fail if all of the channels are interfered or fade at the same time, so the reliability is higher.
    • Demerits:
      • more complex hardware in the phone, which must be capable of processing several channels in parallel.
      • use of several channels in the network to support just a single call which reduces the number of remaining free channels and thus reduces the capacity of the network.
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