As we saw in the post on the architecture of a mobile communication system, a handoff, otherwise known as a handover, is a technique employed to maintain connectivity even when a user moves from one location to another, across cells, which could pose problems as each cell operates at a different frequency. It is the process of automatically transferring the call from one radio frequency to the other, without interrupting the user’s conversation.
In this post, we will be having a more detailed look at handoffs, its need, and its different types.
The need for Handoff
- One of the building blocks of cellular communication is mobility, which refers to providing users with the freedom of movement while they still are connected to the network.
- Handoffs play a major role in allowing users to move across cells without the fear of being disconnected.
- It is also to be noted that a handoff may also be triggered when the number of subscribers in a particular cell has already reached the cell’s maximum limit, keeping the network safe from the threat of being congested and overloaded.
- It can be assumed to be an example of “make before break” as a standby connection is supposed to be present before the switch is done.
Types of Handoffs
There are various types of handoffs as listed below, each of which are used in different scenarios.
- Soft handoff
- Hard handoff
- Forced handoff
- Delayed handoff
- Mobile-Assisted handoff
- Intersystem handoff
- Intercell handoff
- Intracell handoff
Next, we will have a look at the above types of handoffs, their advantages and disadvantages, and where they are used. First, we will see inter-MSC handoffs, which are the ones that occur when the user moves from one MSC to another.
Soft Handoff is a feature where a cellular device gets connected to two or more cell BTS (or cell sectors) at the same time. If all the sectors to which the MS is connected to are from the same cell, then it is referred to as a Softer Handoff.
- It provides better Quality Assurance as a channel is always on stand by in case of power loss in any other channel.
- More than one repeater can send and receive signals to transmit signals to and from mobiles, increasing transfer speed.
- Delay is very low
- Soft Handoffs lead to an increase in the signal to interference ratio, without performance loss. This is known as the Soft Handover Gain.
- Only supported for phones that employ CDMA/ WCDMA, and cannot be implemented in LTE or GSM.
- Costlier to implement than Hard Handoff as a channel is always wasted by being the backup.
Soft Handoffs are generally used in MS that employ Code Division Multiple Access (CDMA) or Wideband CDMA (WCDMA), and its associated services, and also in applications that require a continuous connection throughout, possibly for security purposes.
Hard Handoff is a technique that requires the user’s connection to be broken before connecting to another while switching between two BTS and hence is equivalent to “breaking before making”. It is generally implemented in Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) based devices and is implemented when the subscriber/user is being connected to a base station with a different radio frequency than the current base station. The following image shows the difference between Soft and Hard Handovers.
- Hard Handovers are cheaper as they require only one channel to be active at a time.
- Hard Handoffs are generally implemented more than Soft Handoffs thanks to their efficiency.
- A delay is often experienced while switching, but it generally is quite small such that the user does not experience it.
Apart from being implemented in FDMA and TDMA devices, Hard Handoffs are also used in applications that can afford slight delays, such as VoIP, Internet, and WiMAX.
A Forced Handoff can occur in two ways, either by forcing a handoff to occur or forcibly preventing a handoff that was supposed to happen from not happening. This could be done when the number of users exceeds the capacity of the network.
- Helps in mobility management by controlling which devices are to remain connected.
- Prevents the network from the threat of being down due to congestion.
- Increased chances of high latency and dropped calls.
Forced Handoffs are generally used for security and surveillance purposes, and also to manage network traffic better.
A delayed handoff occurs when no BTS is available to accept the transfer. In such a case, the call generally continues until the signal strength reaches a certain threshold. If it goes below the threshold, the call is dropped.
- Help in handling the call more adequately.
- The quality of the call could go for a toss.
- Chances of calls being dropped are very high.
Delayed handoffs are generally used when users are somewhere where dead spots (areas where there is no network coverage) are quite common.
A mobile-assisted handoff (MAHO) is a process used specifically in GSM networks where a mobile phone assists or helps the BTS to transfer a call to another BTS, with stronger signal strength and improved connectivity.
- Reduced handoff time as the device is responsible for facilitating the handoff.
- Reduced traffic at the BSC/ MSC as the decision to switch is taken by the MS.
- Suitable when handoffs are frequent.
- Added load on the processor of the MS in comparing signal strength of nearby BTS.
They are primarily used in GSM devices, which is based on the TDMA technique.
Intersystem Handoffs come into play when the user moves from one network that is under the jurisdiction of an MSC to another. During this handoff, the call is transferred to the destination MSC, which further tries to allocate a specific bandwidth at the cell site where the user is at present.
- Facilitates roaming and preserves the mobility promise of cellular networks.
- This is only possible if the two MSCs are compatible and have the necessary software that can facilitate these handoffs.
Intersystem Handoffs are used when the user moves from one system to another.
The handoffs that we just saw are those that happen between two MSCs. Handoffs also happen between two BTSs or cells as well, which we will have a look at in this section.
An intercell handoff occurs when an MS is transferred from one BTS to the other, mostly to balance the load on the network. It is generally the BSC that takes control here and acts as the switching agent.
Intracell Handoffs are those that occur between two physical channels within the same cell. The frequency of the channel is changed generally due to interference or similar reasons. Here, the MS remains connected to the same BTS throughout the process.
Even though we mentioned certain disadvantages for each type of handoff, their advantages overweigh them and hence play a major role in cellular systems.
Image Credits: Prof. W.-G. Teng, “Wireless & Mobile Networks”, NCKU ES