Media Independent Handover Function

Published Date: 02 Nov 2017

Introduction

A research study report by ARC Chart estimated that Heterogeneous Networks (Hetnets) would push the global mobile infrastructure market to be worth $57 Billion by 2017 [1]. Hetnets revolve around the idea of coexistence of different wireless technologies. The foremost of these wireless technologies include the cellular technologies of 3G and 4G, IEEE 802.11 Wireless Fidelity (Wi-Fi), IEEE 802.16 Worldwide Interoperability of Microwave access or WiMAX and IEEE 802.20 Wireless Broadband Mobile Access. With the growth of cellular technologies, many telecommunication companies are looking to offload their data usage to Wi-Fi and small cell networks. WMBA promises to provide seamless connectivity for vehicular environments and thus is sought as a tool for high speed vehicular communication in terms of data transfers. WiMAX, often termed as ‘last-mile communication’ promised to provide high speed high speed internet communication over vast areas. Many other wireless technologies are being developed for efficient use of spectrum and the white spaces in the spectrum. This creates a complex picture of heterogeneous wireless technologies working in the same environment.

The main challenge in this environment is to maintain connectivity and true mobility between these heterogeneous networks. To achieve this goal, handovers between different nodes of the same access technology is not enough. True vertical handover is necessary. So if a personal digital assistant (or PDA) can connect to cellular technologies and Wi-Fi, the user might want to switch between the two technologies without disrupting the current session. Achieving this would enable true seamless mobility between different access technologies. In this context, IEEE developed a standard to enable handovers between different heterogeneous technologies called IEEE 802.21 Media Independent Handover Services (MIH) for Local and Metropolitan Area Networks in 2008. The main resolution for the standard is to improve user experience for data connections between different IEEE 802 technologies and cellular technologies by empowering handovers between these heterogeneous wireless access technologies.

Section II gives an overview of the working of IEEE 802.21. In order to standardize the handover procedure the IEEE 802.21 defines the aims and objectives for its development and deployment. To enable the handover the standard defines a set of Media-Independent Service Access Points (or SAPs) to provide a generic link-layer intelligence. Thereafter, each access technology will just have to boost their access technology-specific SAPs to satisfy the functionality of the abstraction layer. IEEE 802.21also recommends suitable amendments to the medium access control (MAC) layer and the physical (PHY) layer of the access technologies to meet the requirements of the standard-defined generic link layer intelligence.

802.21 – Overview

An IEEE work group as 802.21 was created first in 2004 to develop a standard for enabling handovers between heterogeneous networks. The working group came up with an initial draft in 2005. IEEE-SA accepted the latest version of the standard in 2008. And the standard was published in early 2009. Then, it was anticipated that the standard would come under actual deployment by 2010. [2]

For handover between different access technologies, one approach would be to define a set of rules for each pair of access technology at a time. In other words a set of rules can be defined for handover between Wi-Fi and 3GPP standard and then a set of different rules for handovers between Wi-Fi and WiMAX and a new set of rules would have to be laid off for handovers between WiMAX and 3GPP technologies. Now even though this approach works if the access technologies being limited to these three, if the number of access technologies considered for this increase. Mathematically, N technologies taken into consideration would require N x (N-1) set of rules which just makes this approach more complex over time. Another approach is that of the IEEE 802.21 specification in which it defines an abstraction layer which provides handovers independent of the access technology used in lower layers. The upper layers can now in effect interact with the lower layers with their different functionalities related to the different access technologies used; in a homogenous manner. [3]

This abstraction layer not only standardizes the inter-access technology handovers but also allows any new standard that might be developed in the future to be easily managed under the same framework. It also allows the upper layers to communicate with the lower layers without having to deal with the functionalities of the access technology used in the lower layers. [4]

In order to achieve this IEEE 802.21 standard defines three main objectives namely: (i) MIH protocol stack to enable a framework to achieve handovers without disrupting the session continuity; (ii) A set of new SAPs which represent the access technologies considered in the standard and are mapped to each of their specific functionalities; and (iii) Media-Independent Handover function (MIHF) to enable upper layer perform the handovers via the above-defined 802.21 framework. [3] Apart from these, the standard also defines the following goals:

The handover functionality should enable session continuity when switching from one access technology to another.

Handover functions should be able to maintain the same QoS for a given session from switching to a different access technology

The IEEE standard must provide functions to different applications within its framework so as to allow the application in participating in the handover decisions

Functions related to network discovery and network selection must be provided within the framework. The scope of IEEE 802.21 standard is to manage the QoS criteria based potential neighbors for handover and a standard to perform the handover. However handover decisions like when to go through with the handover are left for the upper layers and are out of the scope of the standard

User can benefit from the knowledge of neighboring networks and thus provide Power Management. [3]

Media Independent Handover Function

The abstraction layer introduced by the IEEE 802.21 as discussed above is called Media Independent Handover Function (MIHF). Figure 1 shows the 802.21 general architecture as defined by the standard. [5]

Figure . General Architecture For IEEE 802.21 Framework Implemented For 802 Network and Cellular Network

Figure 1 shows that all Mobile Nodes (MN) and other network interfaces have logical connections to an abstraction layer called Media Independent Handover Function (MIHF). MIHF enables communication between upper layers of MN with the underlying access technology in the lower layers. To facilitate this communication, the standard defines the following SAPs:

MIH_SAP enables uniform interface for communication between MIHF and upper layer protocols or MIH users (generally L3 and above).

MIH_LINK_SAP interface enables communication between MIHF and access technologies in the lower layer. This interface is mapped to each individual access technology to facilitate particular functions of that technology. This interface allows for the ‘independence’ feature of the MIHF layer above.

MIH_NET_SAP interface enables communication between MIHF of the MN and other remote MIHF. It mainly communicates the handover information between two or more MIHF over the same data plane. [3]

IEEE 802.21 standard also defines the following services for efficient functioning of MIHF:

Media Independent Event Services (MIES) provides events alerts which originate in the MAC layer, MIHF layer or remote MIHF layers to the upper layers. The events include changes in the link layer; like LINK_UP and LINK_DOWN; which affect handover decisions for the upper layers. The upper layers have to subscribe to each of such events in order to get notified when they are generated.

Media Independent Command Services (MICS) enables upper layers to get updates of the link state and also to control it. These upper layers can be local to MN or can be from the other network MIH entities. The upper layers use these commands to initiate handover decisions thus enabling both mobile or network initiated handovers. The standard defines a set of such commands in IEEE 802.21.

Media Independent Information Service (MIIS) enables local MIHF in the MN to acquire information about different heterogeneous network in its geographical area. This service enables the Network Discovery function of the standard. The information can be about type of access technology in use, location of the network, QoS criteria, connecting cost, roming agreements, channel parameters, etc. [3] [5] [6]

Media Independent Handover Procedure

Figure 2 shows an example of handover procedure. The MN is currently on 3G network and handover is initiated for a WLAN network [5]

The standard defines some network entities for ease like: (i) MIH Point of Service (POS) which one or more entities the MN exchanges network information with; (ii) MIH non-POS which those entities which do not exchange network with MN; and (iii) MIH Point of Attachment (POA) which are the endpoint entities connected in L2 with MN. [5]

Following is the procedure for seamless handover in IEEE 802.21:

Handoff may be triggered by some QoS threshold or power threshold. The first step in the procedure is initiated by the upper layers (MIH User or MU) of the MN. Information is requested from an Information server also known as MIIS server. This server will have information on all the networks in the given geographical location.

Upon receiving the request from upper layers, MIHF passes the query to the MIIS server.

MIIS server responds with a list of candidate networks with their specifics and functionalities.

MIHF forwards this response to the MU. MU selects one or more candidate networks to go ahead with the handover. Accordingly it turns the respective interfaces on. In this case since handover is performed to a WLAN network MU turns on the WLAN interface card.

In case, the candidate networks have different access technologies then scanning of the network is done by switching on one interface card at a time.

The 802.11 network card will listen the beacon from the access point (AP). Upon receiving a beacon frame it sends a link detection indicator to MIHF

MIHF forwards the indicator to the MU

As soon as the MU gets the indicator, the MU starts the handoff procedure by sending a query request to MIHF for each of the candidate networks

MIHF forwards these queries request to the serving POS’s non-POA

Upon reception of these queries, the serving POS starts querying the each candidate POS for resources such as QoS, resource availability, etc. A number of queries are exchanged between the POS and each candidate POS

All these information is then relayed to on to MIHF in the MN

MIHF relays this information forward to the MU. MU now decides amongst the candidate POS to choose one to complete the handover

Once MU has made the decision, it requests MIHF to switch the network to WLAN

Upon receiving the request, MIHF triggers the connection establishment with the WLAN network and updates the MU about it through LINK_UP indicator

Upon successfully completing the connecting the MAC layer of WLAN updates the MIHF indicating the completion of link handover.

MIHF sends a confirmation about the network switch to the MU.

Upon reception of message 15, MU starts an upper layer handover with the help of a mobility managing protocol like MIP or FMIPv6 which completes the binding process and enables reception of packets on the new link. Upon completion of the upper layer handover, MU sends a MIH handover complete message to the MIHF.

MIHF relays this handover complete message to the new serving POS, WLAN network in this case.

The new serving POS notifies the old serving POS and all other related network entities about the handover completion. The old serving POS may use this message to free up the resources being used by the MU earlier.

After completing all the necessary notifications, the new serving POS sends a response message to the MIHF of the MN

MIHF forwards this message to the MU thus completing the handover procedure successfully. [5]

Conclusion

A number of simulation experiments have been performed to test the efficiency of the IEEE 802.21 standard in different scenarios and using different access technologies.