Concept of Handover:
The process of transforming an active call session from one BTS’s (base transceiver station) frequency to another BTS’s frequency is called Handover.
There are number of BTS’s configured for different specific areas having specific range of frequencies. When you are travelling from one place to another, your channel changes from one BTS to the other. This process happens so fast that it cannot be easily noticed.
Peshawar university has one BTS and Agriculture University has their own. So, when you travel from Agriculture university to Peshawar university and during that you call someone.
when you are in Agriculture university you are using the frequency of Agriculture university BTS but as you cross its range and enter in the range of Peshawar university your signals changes or Handover to Peshawar university BTS without losing your live call.
Sometimes, when signals change from one BTS to another the call drops, it happens due to less frequency of the BTS recently connected or due to the lower range of frequency.
The call might disconnect due to the specific the specific frequency range.
If the range of frequency is fulfilled, then also new user cannot be connected to this network directly. He will have to wait to connect until the number of users connected to that BTS finishes their period of frequency gaining. As one or more users finishes their cycle, you are then able then to connect to that network.
There are two types of handoff/handover.
1. Hard Handoff:
In Hard Handoff when you want to switch to another BTS, the connection with the previous BTS should be broken first only then the new connection can be established.
You are currently on one job and want to search for another so, first you have to resign to the current job and after that you will start searching new job and that’s Hard Handoff.
The call can disconnect in hard handoff when your connection is broken with one BTS. There may be a case that the new BTS with which you are going to connect is busy or unable to provide frequency at the time.
In this case your call will drop, and you will lose the connection. Then after a few seconds as the new BTS becomes free and provides frequency, you will be able to call again.
Soft handoff is type where you don’t need to break your connection with previous BTS for connecting to the another BTS. In this case your line will not disconnect, and call will not be dropped.
It enables you to switch to the next BTS without breaking the connection to the previous BTS. When your connection establishes with the new BTS then your connection to the previous BTS would finish automatically.
Vertical Handover Process:
The process of Vertical handover is categorized into 4 different stages:
- Handover Initialization stage.
- System discovery stage.
- Decision making stage.
- Handover execution stage
1.Handover Initialization Stage:
- The process is initiated by one or more than one network selection parameters.
- Checks for link quality or decrease in signal strength is done in this stage.
2. System Discovery Stage:
- In system discovery stage the mobile terminal collects all the information about the available networks for selecting network for handover.
- QoS (Quality of Service) is used for the sharing of information between the node(mobile) and the network.
3.Decision Making Stage:
- In the stage of decision making, Vertical handover have to decide whether it should continue the connection with the current network or to switch to the previous one.
- The selection and the decision depend on the bandwidth, availability, frequency etc. of the network.
4.Handover Execution Stage:
- In this stage after decision making, the connections are re-routed from the source network to the target network.
- It involves the transfer of signals along with authorization and authentication.
Vertical Handover Classification:
Vertical handover is further classified into four (4) categories:
- Depending on direction
- Depending on process
- Depending on decision
- Depending on control
1.Depending on direction: (Upward & Downward)
The directions here can be either upward or downward and the handovers can be
- Upward handover
- Downward handover
When a mobile user switches from a network with low area coverage to the network with high area coverage, then the handover is called Upward handover.
when the mobile user switches from network with high area coverage to the network with lower area coverage, that process is called Downward handover.
2.Depending on process: (Hard & Soft)
When the mobile user breaks the connection with the current network before establishing connection with the new network is known to be hard handover.
When the mobile user connects with the new network after that breaks connection with the previous network, this type of handover is called soft handover.
(Both of them are discussed in detail above)
3. Depending on decision: (Imperative & Alternative)
When the value of RSS (Received Signal Strength) decreases the value of threshold, the handover is called imperative handover.
Where as in alternative several handover parameters are taken into consideration such as bandwidth etc.
4. Depending on control: (Mobile & Network)
Simply it can be defined, when handover is controlled by the mobile node it is called mobile controlled.
In network-controlled, a central entity is used to control the handover.