Media Independent Handover Function and Protocol

MIHF provides asynchronous and synchronous services through SAPs for lower layers and upper layers. For a system with multiple network interfaces of arbitrary type, we can use the Event service, Command service and Information service provided by MIH to manage, determine, and control the state of the underlying interfaces.
These services helps the L3MP and other protocols in maintaining service continuity, service adaptation to varying quality of service, battery life conservation, and network discovery and link selection. In a system containing heterogeneous network interfaces of 802 types and cellular 3GPP, 3GPP2 types, the Media Independent Handover Function can help the L3MP to implement effective procedures to couple services across heterogeneous network interfaces.
The MIH Function provides the exchanging functionality of information between the network and host entities of the same media type. However, if information exchange mechanism is already exists in a given type of media (such as some cellular media types), the MIH Function will use of the existing mechanism whenever possible.

Media Independent Event Service
The event service will typically be used to facilitate handover detection within the L3MP. Events defined include Link Up, Link Down, Link Parameters Change, Link Going Down, L2SDU Transmission Status, Link Event Rollback, Pre trigger (L2 Handoff Imminent), etc.

Media Independent Command Service
The command service refers to the commands sent from the higher layers to the lower layers in the reference model. It includes the commands from upper layer to MIH (e.g. upper layer mobility protocol to MIH, or policy engine to MIH, etc), and from MIH to lower layer (e.g. MIH to MAC, or MIH to PHY). Commands can also be sent from a local MIH entity to a peer MIH entity. These commands mainly carry the upper layer decisions to the lower layers on local device entity or at remote entity, and thus control the behavior of lower layers.

Media Independent Information Service
Media Independent Information Service (MIIS) provides a framework and corresponding mechanisms by which a MIHF (Media Independent Handover Function) entity can discover and obtain network information existing within a geographical area to facilitate the handovers. MIIS primarily provides a set of information elements (IEs), the information structure and its representation and a query/response type of mechanism for information transfer. This contrasts with the asynchronous push model of information transfer for the event service. The information may be stored within the MIH functional (MIHF) entity or maybe present in some information server from where the MIH in the station can access it. The definition of the information server and the mechanism to access it are out of scope.

Media Independent Handover Protocol
The Media Independent Handover protocol defines frame formats for exchanging messages between peer MIH Function entities. These messages are based on the primitives which are part of Media Independent Event service, Media Independent Command service and Media Independent Information service. IEEE 802.21 supports Media Independent Handover Function in mobile terminal, and network. The MIHF Protocol allows peer MIH Function entities to interact with each other.
The Media Independent Handover Protocol provides the following services:
 MIH capability discovery: MIHF in mobile terminal or MIHF in the network discovers which entity supports MIHF. Thereafter the peer MIH Functions negotiate/discover an optimum transport for communication. The MIH Function entities also discover list of supported events and commands. The MIH Function can also query the information schema for list of supported information elements.
 MIH remote registration: Remote MIHF in different entities can register with each other to receive Media Independent Handover Messages including remote MIES. No registration is required for command services.
 MIH message exchange: MIHF can exchange MIH messages using MIH payload and MIH protocol over a suitable transport. As part of message exchange the peer MIH Function entities can use the MIES, MICS and MIIS for effective handovers.

MIH Protocol Transport
The table below shows the various transport options for different media types.

No Media Type Preferred
Transport L2 Transport L3 Transport
1 Ethernet L2 Data Frames IP based
2 802.11 L2 Data Frames, Management Frames IP based
3 802.16 L2 Data Frames, Management Frames IP based
4 3GPP L3 Requires protocol stack changes IP based
5 3GPP2 L3 Requires protocol stack changes IP based
Table 1: MIH Protocol Transport [5].



MIH General Packet Format
MIH messages can be transported over L2 using data frames or media specific management frames. MIH messages can also be sent over L3 using a suitable L3 transport protocol.


The description for each of the fields in the MIH general packet format is detailed in Table 2.
Name of the Field Size Description
Protocol Version 1 octet Version of the MIH protocol. Default: 0x01
MIH Service ID 1 octet MIH Service Identifier
1:Event Service
2:Command Service
3: Information Service
MIH Opcode 1 octet Operation to be performed
1: Request
2: Response
3: Indication
Transaction ID 1 octet Transaction Identifier used to match requests and responses wherever applicable.
Fr 1 bit Fragmentation flag.
1: Specifies current packet has more than 1 fragment
0: Current packet is not fragmented
Fragment No 7 bit Fragment Number of this packet
Message Length 2 octets Total length of whole message in bytes (header + payload)
MIH Function Identifier Length (MIHFL) 4 octets Length of each of Source and Destination MIH Identifier fields
Source MIH Function Identifier MIHFL octets This is the Source MIHF identifier. This can be the L2 hardware address of the interface at the source node. This can also be IP based for L3 transports. This can also be something generated.
Destination MIH Function Identifier MIHFL octets This is the Destination MIHF identifier. This can be the L2 hardware address of the interface at the destination node. This can also be IP based for L3 transports. This can also be something generated during MIHF discovery and connectivity phase.
MIH Message ID 4 octet Actual MIH Message identifier for the service identified above.
e.g. Link_Going_Down Event Indication for Event Service
MIH Message Data Variable MIH Service specific data of variable length
Table 2: Description for each field in the MIH Packet [5].