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HI (Handover Interface)

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The Handover Interface (HI) is a critical component of cellular networks that enables seamless transfer of an ongoing call or data session from one base station to another as the mobile device moves out of the coverage area of one base station and into the coverage area of another. The HI is a standardized protocol that enables communication between the mobile device, the serving base station, and the target base station during the handover process. This process is also commonly referred to as handoff or handover.

In this article, we will discuss the Handover Interface in detail, including its purpose, architecture, protocol stack, and the different types of handovers supported by the HI.

Purpose of Handover Interface

The primary purpose of the Handover Interface is to provide uninterrupted voice and data services to the mobile user while he/she is on the move. As a mobile user moves out of the coverage area of one base station, the quality of service (QoS) may degrade due to a decrease in signal strength, interference, or congestion. To maintain the QoS, the serving base station must initiate a handover process, and transfer the call or data session to a neighboring base station with a stronger signal or better QoS.

The Handover Interface ensures that the handover process is seamless and transparent to the user, without any noticeable interruption or delay. The HI provides a standardized protocol that enables communication between the serving base station and the target base station, to exchange the necessary information required for the handover process. The HI also enables the mobile device to switch to the new frequency and timeslot allocated by the target base station, and update its location and routing information in the network.

Architecture of Handover Interface

The Handover Interface consists of two main components: the Serving Mobile Switching Center (SMSC) and the Target Mobile Switching Center (TMSC). The SMSC is the current serving base station that is handling the call or data session of the mobile user. The TMSC is the target base station that will take over the call or data session after the handover.

The SMSC and TMSC are connected through the Base Station System (BSS) and the Mobile Switching Center (MSC) of the cellular network. The BSS is responsible for the radio communication between the mobile device and the base station. The MSC is responsible for the call routing and management within the network.

The Handover Interface is implemented as a set of protocols that define the communication between the SMSC and TMSC. The protocols are implemented as layers in the protocol stack, as shown in the figure below.

The protocol stack consists of three main layers: the Radio Resource Control (RRC) layer, the Mobility Management (MM) layer, and the Session Management (SM) layer.

The RRC layer is responsible for the radio resource allocation and management between the mobile device and the base station. The RRC layer is implemented in the BSS and the mobile device.

The MM layer is responsible for the mobility management of the mobile device, including location updating, handover, and authentication. The MM layer is implemented in the MSC and the mobile device.

The SM layer is responsible for the session management of the call or data session, including call setup, teardown, and transfer. The SM layer is implemented in the MSC and the mobile device.

Protocol Stack of Handover Interface

The Handover Interface protocol stack is shown in the figure below.

The RRC layer consists of the following protocols:

  1. Radio Resource Control (RRC) protocol: This protocol is responsible for the allocation and management of radio resources between the mobile device and the base station. The RRC protocol is implemented in the BSS and the mobile device.
  2. Medium Access Control (MAC) protocol: This protocol is responsible for the medium access control of the radio resources allocated by the RRC protocol, including the allocation of timeslots and frequency channels. The MAC protocol is implemented in the BSS and the mobile device.
  3. Physical Layer (PHY) protocol: This protocol is responsible for the transmission and reception of the radio signals between the mobile device and the base station. The PHY protocol is implemented in the BSS and the mobile device.

The MM layer consists of the following protocols:

  1. Mobility Management (MM) protocol: This protocol is responsible for the mobility management of the mobile device, including location updating, handover, and authentication. The MM protocol is implemented in the MSC and the mobile device.
  2. Authentication and Key Agreement (AKA) protocol: This protocol is responsible for the authentication and key agreement between the mobile device and the network. The AKA protocol is implemented in the MSC and the mobile device.
  3. Location Management (LM) protocol: This protocol is responsible for the management of the location information of the mobile device in the network. The LM protocol is implemented in the MSC.
  4. Subscriber Identity Module (SIM) protocol: This protocol is responsible for the management of the SIM card information of the mobile device. The SIM protocol is implemented in the mobile device.

The SM layer consists of the following protocols:

  1. Call Control (CC) protocol: This protocol is responsible for the setup, teardown, and transfer of the call or data session between the mobile device and the network. The CC protocol is implemented in the MSC and the mobile device.
  2. Session Management (SM) protocol: This protocol is responsible for the management of the call or data session between the mobile device and the network, including flow control and error recovery. The SM protocol is implemented in the MSC and the mobile device.

Types of Handovers supported by Handover Interface

The Handover Interface supports several types of handovers, depending on the nature of the handover and the network topology. The different types of handovers supported by the HI are as follows:

  1. Intra-cell Handover: This type of handover occurs when the mobile device moves within the coverage area of the same base station, but the signal strength or QoS degrades. In this case, the serving base station initiates the handover process, and transfers the call or data session to a different frequency or timeslot within the same cell. This type of handover is also known as an intra-frequency handover or an intra-BTS handover.
  2. Inter-cell Handover: This type of handover occurs when the mobile device moves from the coverage area of one base station to the coverage area of another base station, but within the same MSC. In this case, the serving base station initiates the handover process, and transfers the call or data session to a different base station within the same MSC. This type of handover is also known as an inter-frequency handover or an inter-BTS handover.
  3. Inter-MSC Handover: This type of handover occurs when the mobile device moves from the coverage area of one MSC to the coverage area of another MSC, but within the same network. In this case, the serving MSC initiates the handover process, and transfers the call or data session to a different MSC within the same network. This type of handover is also known as an inter-system handover.
  4. Inter-technology Handover: This type of handover occurs when the mobile device moves from the coverage area of one network technology to the coverage area of another network technology, such as from a 2G network to a 3G network or from a 3G network to a 4G network. In this case, the serving network initiates the handover process, and transfers the call or data session to a different network technology.

Handover Procedure in Handover Interface

The handover procedure in the Handover Interface involves the following steps:

  1. Measurement: The mobile device measures the signal strength and other quality parameters of the neighboring cells and sends the measurement report to the serving base station.
  2. Decision: Based on the measurement report and other criteria, such as QoS requirements, the serving base station decides whether to initiate a handover or not.
  3. Handover Request: If the serving base station decides to initiate a handover, it sends a handover request message to the target base station.
  4. Handover Response: The target base station responds with a handover response message, indicating whether it can accept the handover or not.
  5. Handover Command: If the handover request is accepted, the serving base station sends a handover command message to the mobile device, instructing it to switch to the target base station.
  6. Handover Complete: After the mobile device switches to the target base station, the target base station sends a handover complete message to the serving base station, indicating that the handover is complete.

Benefits of Handover Interface

The Handover Interface provides several benefits to the mobile network operator and the mobile device users, including the following:

  1. Seamless Connectivity: The Handover Interface allows the mobile device to maintain a continuous connection to the network, even when it moves between different base stations or network technologies. This ensures that the mobile device users do not experience any interruption or loss of connectivity during the handover process.
  2. Optimal Resource Utilization: The Handover Interface enables the mobile network operator to optimize the utilization of network resources, such as frequency channels and timeslots, by dynamically allocating them based on the traffic demand and the location of the mobile devices.
  3. Improved QoS: The Handover Interface helps to maintain the QoS requirements of the mobile device users by ensuring that the handover is triggered when the signal strength or other quality parameters degrade below a certain threshold.
  4. Reduced Call Drops: The Handover Interface reduces the number of call drops, which occur when the mobile device moves out of the coverage area of the serving base station, by ensuring that the handover is initiated before the signal strength or other quality parameters degrade below a certain threshold.

Conclusion

The Handover Interface is a critical component of the GSM and other mobile communication networks, which enables the seamless handover of call and data sessions between different base stations and network technologies. The Handover Interface consists of several protocols and procedures, including the Radio Resource Control (RRC), Medium Access Control (MAC), Physical Layer (PHY), Mobility Management (MM), Authentication and Key Agreement (AKA), Location Management (LM), Subscriber Identity Module (SIM), Call Control (CC), and Session Management (SM) protocols. The Handover Interface supports several types of handovers, including intra-cell handover, inter-cell handover, inter-MSC handover, and inter-technology handover, depending on the nature of the handover and the network topology. The Handover Interface provides several benefits to the mobile network operator and the mobile device users, including seamless connectivity, optimal resource utilization, improved QoS, and reduced call drops.