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].

Media Independent Handover (802.21) Reference Model

Media Independent Handover Reference Model (MIH) is a new layer that resides between the Network Layer (Layer 3 of OSI model) and lower layers: MAC and PHY (IEEE interfaces) or RRC and LAC ( 3GPP or 3GPP2). It provides asynchronous and synchronous services through Service Access Points (SAPs) for lower layers and upper layers. MIH Function (MIHF) helps Layer 3 Mobility Protocol in maintaining service continuity between different interfaces, adaptation to reach Quality of Service (QoS), link selection and network discovery.

The communication between MIHF on the network side is depends on MIHF location and type of scenario. When MIHF is in the access network such as Access Point (AP), Layer 2 or Layer 3 transport can be used over access. When network decides where and when to handover, the source MIHF can send messages to destination MIHF to inform destination MIHF to begin “preparation”. The preparation includes checking availability of appropriate resources including acceptable QoS levels at new PoA, procuring an IP address for the mobile station, etc. Another possible scenario is where the MIHF in an AP communicates with MIHF in an Access Router (AR). This type of communication includes events such as Link Up, Link Down, etc.
The IEEE 802.21 standard supports the Media Independent Event service, Media Independent Command service and Media Independent Information service. A management protocol is required for the exchange of information between MIH entities within a terminal and a network.

MIH Reference Model for Ethernet (802.3)
For 802.3 there are no peer management facilities in CSMA/CD. Thus, if the MIH services are supported over wired ethernet networks, in order to carry the payload over normal ethernet data frames, a L2 protocol with a new ethertype is necessary. The MIH ethertype is encoded in the SNAP header. At the same time 802 networks don’t support data frames in unauthenticated state. Hence the MIH Protocol cannot be used in this case. Only management frames can be used to transport information in the unauthenticated state.
The IEEE 802.21 standard should define the packet format and payloads in media independent manner in standard TLV format. Thereafter these packets can be encapsulated in a L2 MIH Protocol using MIH ethertype when the payload needs to be sent over normal data frames as in case of ethernet. In other cases the TLV based messages and payload can be directly encapsulated in media specific management frames.

MIH Reference Model for Wi-Fi (802.11)
The above figure shows the MIH functions for 802.11 stations and network PoA (APs). The MIH_MAC_SAP is the MIH interface to the data plane and can encapsulate MIH protocol packets in data packets. However since 802.11 does not currently support Class 1 data frames, traffic can be sent over the data plane only when the client is connected with the AP. The MIH_MGMT_SAP provides interface with the management plane (MLME) and allows MIH protocol packets to be stored in management frames.
The MIH_SAP shows the interface of MIH Function with other higher layer entities such as Transport, Handover policy, L3 Mobility protocol, etc. The MIH Function may interface with the OS or the system using the MIH_ME_SAP.

MIH Reference Model for WiMAX (802.16)
In 802.16 based system, the MIH_MGMT_SAP provides the MIH functionality over management plane and help with transporting MIH protocol messages across peer MIHFs. The MIH_SAP shows the interface of MIHF with other higher layer such as Transport, Handover policy, L3 Mobility protocol, etc. The MIH Function may interface with the OS or the system using the MIH_ME_SAP.

MIH Reference Model for 3rd Generation Mobile System (3GPP)
A potential realization of MIH functions for 3GPP enabled Mobile Stations is illustrated above. The MIH_RRC_SAP defines the MIH interface to the 3GPP Radio Resource Control (RRC) layer. The MIH_MGMT_SAP defines the MIH interface to the 3GPP GPRS Mobility Management or Global System (GMM) for Mobile Communication (GSM). Most of the MIH services may utilize the information that is already defined in the RRC layer and GMM/SM. The desired information from MIH Event Services, Command Services and Information Services will be provided to MIH_RRC_SAP and MIH_MGMT_SAP which will be further defined by 3GPP SDO. In the case of MIH in the Mobile Stations, the PHY and MAC SAPs communicate to the RRC as defined in the 3GPP standards. No new interfaces and primitives need to be defined for these SAPs.

Spanning Across Different Media
This figure shows how the MIH Function spans across different media specific technologies and provides a common abstraction of handover services to higher layers.

IEEE 802.21 Media Independent Handover [1]

The emerging IEEE 802.21 standard, Media Independent Handover, has the ability to seamlessly handover networks for both wired and wireless networks, which is addresses a prevalent problem when roaming. Interoperability between homogenous network types of 802 and non 802 networks is also a key feature of the 802.21 standard. This report consists of the understanding of 802.21 services and its functionality, specifically at the Data Link and Network Layers of the OSI Networking Model. Currently at the stage of proposal, the 802.21 working group hopes to finalize the standard’s document and begin implementation shortly thereafter. Future outlook of this standard is good when comparing with the existing proprietary UMA (Unlicensed Mobile Access), which is a similar technology, but fails to offer interoperability. The expectations for this standard are, but are not limited to: ability to roam between 802.11 and 3G networks, allowance for use by multiple vendors and users, application for both wired and wireless networks, and ability to engage in a teleconference.


Introduction
Wireless networking has provided us the ability to freely utilize our mobile devices (i.e. computer notebook, cellular phones, PDA, GPS) wherever and whenever a wireless access point is available. With the multitude of existing wireless standards (Wi-Fi, GSM, etc), a smooth transition from one network to another is utterly impossible with the current technology. Existing IEEE 802 standards go through a series of failures before acquiring the correct network access. For instance, as soon as a user unplugs the Ethernet cable from his or her computer, an error appears indicating that the user is no longer connected to a LAN network. If the computer has wireless capability (i.e. 802.11b), then the user may be able to connect to a wireless network, but network discovery would take some time to establish. An interruption of service is inevitable, thus, a network handover standard is necessary. This emerging standard is the IEEE 802.21, which working efforts debuted in March 2004 and implementation is still in progress [4].
IEEE 802.21 is a developing standard which enables handover and interoperability between heterogeneous network types including both 802 and non 802 networks [6]. The standard provides information to allow handing over to and from cellular and wireline, GSM, GPRS, WiFi, WiMAX, Personal Area Network (PAN), Bluetooth and 802.11 networks through different handover mechanisms [1].

Background
Initiated in March 2004, the working group for the 802.21 standard was chaired by Ajay Rajkumar from Lucent Technologies. The members involved include over 50 companies. These companies are the “largest vendors of end-to-end network infrastructure, device companies, chipset developers, and even service providers” [1]. One of the contributing companies, Intel Corporation, expressed enthusiasm towards providing users with the ability to move from a hotspot to a cellular connection without noticing. Envisioned is the idea of a future that unites network connectivity across a wide range of networks. Figure 1 shows a prototype mobile phone that has four different wireless networks: Bluetooth, GPS, GPRS, and Wi-Fi. With the implementation of 802.21, this device is expected to intellectually select available networks, with an ease of transition and without any interruption, when roaming [3].

standards: Bluetooth, GPS, GPRS, and Wi-Fi [3].

Objective
The objective is to enable an innovative fast Layer 2 and Layer 3 handoff algorithm between networks (for both wired and wireless networks). Network discovery and selection are factors that affect the quality of service during a handover decision. With this technology, end-user devices will have the ability to “automatically choose the best available network connection type and to seamlessly hand off sessions among networks during roaming without user involvement” [2]. 802.21 is expected to unite the multitude of existing networks.

Organization
The next immediate section describes the proposed 802.21 standard in detail. The specifics include the four Media Independent Handover services provided, packet format and information, and functionality in Layers 2 and 3 (Data Link and Network, respectively) of the OSI Networking Model. We will discuss the current progress, including accomplished milestones towards completing the 802.21. In addition to this standard’s expectations, we will also mention its future outlook. The report concludes with comparison of 802.21 and existing similar technologies.