IEEE802.21 Assisted Network Layer Mobility Support

Qazi Bouland Mussabbir*, Wenbing Yao

** and John Cosmas

***

*School Of Engineering and Design, Brunel University

Uxbridge, London, UB83PH, UK, qazi.mussabbir@brunel.ac.uk **School Of Engineering and Design, Brunel University, eesrwwy@brunel.ac.uk

*** School Of Engineering and Design, Brunel University, eestjpc@brunel.ac.uk

ABSTRACT

The emerging IEEE802.21 standard defines a Media

Independent Handover Function (MIHF) that would assist

mobile devices to seamlessly roam across heterogeneous

access networks. The aim of this paper is to provide a

survey of the IEEE 802.21 Media Independent Handover

(MIH) Services and present how these services can be used

to assist the MIPv6 based network layer mobility. The three

primary functional components of the MIHF which

includes Event service (ES), Command Service (CS) and

Information service (IS) are extensively reviewed. The

paper focuses on presenting a scheme using MIHF services

to facilitate the network layer handover between 802.11,

802.16 and GPRS access networks, and assist signalling

protocols in network layer mobility support. We will show

how to get network neighbourhood information through the

802.21 Information Service to aid intelligent handover

decisions, and how the use of the link layer indications,

well known as triggers or events, to assist the network layer

handover initiation.

Keywords: Media Independent Handover Function (MIHF),

MIES, MICS, MIIS, Fast Mobile IPv6 (FMIPv6)

1 INTRODUCTION

In the vision of the 4G wireless communications, it is

requisite to provide seamless mobility support across

heterogeneous access networks using wireless technologies

such as 802.11 (WiFi), 802.16 (WiMax), CDMA, and wired

access technologies like LAN, xDSL. Among many

proposed mobility management solutions, Mobile IPv6

(MIPv6) [1] has been widely accepted in the academic

world and the industry as the front runner for tackling this

challenge.

Handover performance is a vital part in the end-to-end

delay and packet loss control for the QoS provisioning of

real time services in heterogeneous networks. In MIPv6,

when a handover process is initiated, the Mobile Node

(MN) will acquire a new address, called Care-of-Address

(CoA), and use Binding Update messages (BUs) to register

the CoA with its Home Agent (HA) and Correspondent

Node (CN) which will then communicate with the MN

directly through the CoA. Handover delay will occur due to

the processes of neighbour network discovery, CoA

configuration, mobility binding updates, and sometimes

through the network-specific authentication and

authorization. Various other extension to the Mobile IP

protocol within the IETF have been proposed such as

hierarchical mobility (HMIPv6) [2] and fast handovers

(FMIPv6) [3] to provide signalling and handover

optimizations.

In order to quickly detect any Layer3 movement (i.e.

loss of attachment with default router and discovery of new

router), link-layer indication in the form of event might be

beneficial. Link layer information of the current and

neighbouring access networks, which may use the same or

different access technologies, is extremely useful for

reducing the handover latency. The link characteristics of

these networks may help to select which neighbour network

the MN should handover to, and the information of certain

link-layer events at either the MN side or the access

network side will assist to decide when and how to initiate

the MN handover process. In MIPv6, for instance, the

MIHF Event Service could drastically reduce the handover

latency by providing generic link layer indications.

Moreover, MIHF Information Service would allow

intelligent handover decisions through prior neighbouring

network knowledge.

The IEEE802.21 - Media Independent Handover (MIH)

Service WG [6], which was formed in 2003, is developing a

draft standard to enable handover and interoperability

between heterogeneous networks including both 802 and

non 802 networks. Within IETF’s MIPSHOP Working

Group, a few drafts have proposed usage model and

scenarios in which the 802.21 framework could facilitate

handover across heterogeneous access networks. The

intention of this paper is to provide a survey of the work in

the IEEE 802.21 WG and present a scheme which uses the

MIH services to assist the network layer mobility support

based on existing works. The paper takes into account these

scenarios in [4] and [5] and extends them into a detailed

discussion on how the 802.21 functional components

enhance the overall handover process. The overall scenario

given in [4] [5] is elaborated in this paper by describing

MIH capability discovery, the Event Service registration

process and Information Service discovery mechanisms.

The rest of this paper will be organized as follow: In

section 2, we outline some of the existing work and

research related to IEEE802.21 standard. We will outline

the 802.21 MIHF functions in section 3, and discuss the

three primary functional components: The Event Service,

The Command Service and the Information service. In

section 4, we will present a scheme to use MIH services to

facilitate the MIPv6 handover procedure between

heterogeneous access networks, and assist signalling

protocols in network layer mobility support. We will

discuss the conclusion in section 5.

2 RELATED WORKS

There are several initiatives to optimize mobility across

heterogeneous networks. The MIPSHOP Working Group

within the IETF and the IEEE802.21 standard Working

Group have been working to develop a framework in which

the mobility management protocols would use the 802.21 to

enhance the handover process. Reference [10] describes the

transport and security requirements for the MIH signalling

in order to aid IP handover mechanisms. References [4]

and [5] outline few usage models of Event, Command and

Information services. They also discuss security

considerations for these services. In reference [7], 802.21

assisted SIP based mobility Test-bed across heterogeneous

access network was implemented. The IEEE802.21

Working Group is addressing various scenarios in detail

and is in the process of standardizing a Media Independent

Handover Framework. This paper presents this framework

and describes a usage scenario in which IP layer handover

is optimized using the functional component of the 802.21

framework.

3 MEDIA INDEPENDENT HANDOVER

FUNCTION

In the mobility management protocol stack of both

mobile node and network element, the Media Independent

Handover Function (MIHF) is logically defined as a shim

layer between the L2 data link layer and L3 network layer

[6]. The upper layers are provided services by the MIH

function through a unified interface. The services exposed

by the unified interface are independent of access

technologies. This unified interface is known as Service

Access Point (SAP). The lower layer protocols

communicate with the MIHF via media dependent SAP.

Figure 1 illustrates the IEEE802.21 MIH Handover

Framework

MIHF defines three main services that facilitate

handovers between heterogeneous networks: Media

Independent Event Service (MIES), Media Independent

Command Service (MICS) and Media Independent

Information Service (MIIS). Detailed discussions of each of

the services are given below.

Figure 1

3.1 Media Independent Event Service

Media Independent Event Services (MIES) provide event

reporting, event filtering and event classification

corresponding to the dynamic changes in link

characteristics, link quality and link status. The MIES

report both local and remote events to the upper layers. The

upper layers perform registration to receive events from the

MIHF using a request/response primitive. Some of the

events that have been specified by IEEE 802.21 are “Link

Up”, “Link Down”, “Link Detect”,” “Link Parameter

Reports” and “Link Going Down”.

3.2 Media Independent Command Service

Media Independent Command Service (MICS) use the

MIHF primitives to send commands from higher layers to

lower layers. The MICS command are utilized to determine

the status of the connected links and also to execute mobile

and connectivity decisions of the higher layers to the lower

layers. MIH Commands are identified as either being local

or remote. Local MIH commands flows from upper layers

to the MIH function, and then to lower layers in the local

stack. Remote commands, messages propagate from upper

layer to the MIHF in one stack to the MIHF in a peer stack

(with usage of the MIH protocol). Messages are further

propagated to lower layer.

3.3 Media Independent Information Service

Media Independent Information Service (MIIS)

provides a framework and mechanism for an MIHF entity

to discover available neighbouring network information

within a geographical area to facilitate the handover process.

The primary idea is for the MIIS to provide a set of

information elements, the information structure and its

representation and a query/response type mechanism for

information transfer. Both static and dynamic information

is provided by the MIIS. Examples of Static information

would include the names and service providers of the

mobile terminal’s exiting network neighbourhood.

Dynamic information would include link layer parameters

such as channel information, MAC addresses, security

Figure 2: Network Selection Procedure

information, and other higher layer service information to

make intelligent handover decision. The information could

be made available through lower layers as well as higher

layers. In cases where layer 2 information is not available

or sufficient to make efficient handover decisions, then

higher layer information services may be required. In order

to represent the information across different technologies,

the MIIS specifies a common way of representing this

information by using a standardized format such as XML or

ASN.1.

4 NETWORK CONTROLLED LAYER 3

802.21 ASSISTED HANDOVER

In this section we present a survey on Network initiated

IP layer handover scheme using the functional components

of the IEEE802l.21 framework based on the existing

IETF’s MIPSHOP’s working group drafts [4] [5]. We

consider the Fast Mobile IPv6(FMIPv6) scheme here. It is

however extensible to other handover signalling schemes

such as Hierarchical Mobile IPv6(HMIPv6), HIP, SIP etc

The motivation behind choosing a network initiated

handover is due to the fact that service providers, including

those with multiple access technologies, under any given

instance would not like to see that any specific part of their

network is operating under heavy loads and prefer to

balance the traffic across all the available network paths for

optimization of paths. In other words, the network wishes to

exercise control over the mobile nodes to make use of a

certain network path that would mutually benefit the users

and service providers themselves.

The handover decision will be made by network using

the 802.21 services in two steps: Step1) Network Selection

and Step2) Handover Control.

4.1 Network Selection

The process of selecting a favourable network for a mobile

node to transfer or handover the ongoing services to the

selected network is known as Network Selection [4]. The

network that is selected maybe a different link access

technology from the previous one. It is possible that the

mobile node, after handover will not experience the same

level of QoS when compared to the current link due to the

Network Selection. The selection process in general is

meant to provide some user benefit in one way or another,

such as, cost savings, higher bandwidth etc [4].

Figure 2 illustrates a Network Selection procedure with

the help of the mobile node. In the scenario remote Event

Service (ES) and remote Information Service (IS) are

shown to play an integral part in the Network Selection

procedure. The Mobility Management Entity (MME) is

assumed to be a core network element, that is, beyond

Layer2. The MME functionality utilizes the MIHF (Media

Independent Handover Function). The MME implements

network selection handover algorithms and utilizes mobility

signalling protocols (Fast Mobile IPv6 in this case) and aid

mobility functions. The following subsections describe the

details of how MIES and MIIS of 802.21 framework are

used in the Network Selection Procedure for Layer 3

handover optimizations.

4.1.1 Discovery, Registration and Indications

of 802.21 MIES

In this scenario, the mobile node initially performs a

registration or attachment to the network on any link, e.g.

3GPP network in this case. The MME and Mobile node will

also need to discover each others MIH capabilities before

any service related information could be passed between

the two entities. In this case, the MIH function in MME and

mobile node could exchange message by a request/response

mechanism to determine each others MIH capabilities of

the link layers. IEEE802.21 defines the semantics of these

service primitives and includes source of the requesting

entity, the destination identifier the request/response of

local or remote MIH function, and the list of supported

events and commands.

The MME then registers to remote “Link Detect” event

services from the MIHF in the mobile node. The MIH user

(upper layer) of the MME would initially send MIH event

request message using the MIH_SAP and associated

primitives inside its local stack. The request message would

then further propagate from the local MIHF to the peer

MIHF in the mobile node. The request message in our case

would contain the set of events it would like to receive

indications for (“Link Detect” in our case) with appropriate

filter information. The triggers or indications of the events

from the link layer will be checked and scoped based on the

filter rules set by the MIH user in the MME. This feature

would ensure that the protocol does not result in excessive

load on either the network or mobile node processing the

event notifications/indications from multiple event

generating nodes. Figure 3 shows the MIH

registration/deregistration flow model.

Figure 3: MIH Event Registration and flow

It is shown in the scenario in figure 2, that 802.16

broadcast is received by the mobile node and a “link detect”

event indication is sent by the 802.16 MAC layer to the

MIHF Event Service (ES). The MIHF (ES) translates the

indication to an ES “Link Detect” message and sends it to

the MIHF (ES) in the network (collocated in the MME)

with all the basic information received from the 802.16

broadcast.

4.1.2 Usage of Information Services

After the MME in the figure 2 receives the ES “Link

Detect”, it requests via the MIIS query mechanism to an

Information Server (IS) to check the suitability of the

detected network (802.16) based on the roaming

agreements between the two networks [4]. As with various

deployment scenarios, the system would need to provide

discovery mechanisms, security association (SA) bootstrap,

and transport of information services over IP. For the

information services, it is possible the network information

may be either centrally stored in a server or distributed in

each individual access network. In order to identify or

discover a valid information server, a layer 2 or layer 3

mechanisms is required. At the time of writing, DHCP

(Dynamic Host Control Protocol)[8] was decided as

candidate discovery mechanism within IEEE802.21 MIIS

specification. Figure 4 shows the three phases in relation to

our MIIS usage scenario: Discovery, SA bootstrap,

request/response.

Figure 4: Information Service Message Exchange

The MME in our scenario initially uses DHCP to acquire

the location of Information Server in terms of IS server IP

address, IS server FDQN (Fully Qualified Domain Name)

and URI(Uniform Resource Identifier). Before the MME

can exchange any messages with the IS server, a set of

Security associations (SA) are established. Authentication

and encryption must be provided by each SA for the

purpose of mobile device anonymity from eavesdroppers.

The SA negotiation mechanism depends on the transport

layer used, and security services required [9]. For Instance,

TLS will be advisable if upper layer protocols use TCP,

while ESP using IPSec/ IKE will work in most

Figure 5: Network Initiated Handover

situations without regard of the upper layer protocol, so

long as the IS protocol identifiers are handled by IKE [9]

After the discovery and SA phase, the MME sends a

request message to the IS server to check the suitability of

the 802.16 detected network. The IS server responds with a

response message containing the Information Elements (IE)

requested the by MME .The Handover module in the MME

in our scenario decides that this particular 802.16 is not a

favourable one and takes no action. This decision could be

based on static or dynamic information such as roaming

agreements, QoS, channel information and higher mobility

management service supported by the network.

During a later time, the mobile receives beacon

information from a 802.11 Access Point (AP). The MAC

layer of the mobile node informs the MIH Event service

along with the SSID information. The MIHF (ES) scopes

and filters the this link layer information against the rules

set by the MIH user (upper layer) of the MME. The MIHF

(ES) processes another “ Link Detect” event indication

message along with SSID information sends it the peer

MIHF of the MME. The MME performs an IS query and

upon receiving a response determines that the SSID belongs

to a favourable network. Thereby, the network selection is

complete

4.2 Handover Control

Handover Control procedure follows a Network

Selection process described in the previous section. The

following scenario portrayed in figure 5 shows a network

controlled handover process with Fast Mobile IP signalling

mechanism. The MME here uses the MIH Control Service

(CS) and generates a “Link Switch” command. This

command is in the form a request message is transported to

the MIHF in the mobile node. The included parameters in

the message may include a make-before-break mechanism

to be performed with target link. In our case, the target link

is 802.11 network as shown from the Network Selection

process earlier. The MIHF (CS) sends an indication to the

Mobile IP function of the awaiting link switch along with

new link information. If the Mobile IP function does not

have valid Access Router Tuple-Info, for instance, then it

sends a Proxy Router Solicitation (PrRtrSol) with necessary

link layer information, such as, MAC address of the AP.

The Proxy Router Advertisement (PrRtrAdv) provides the

relevant layer 3 information for the new link. Upon

execution of the MIFH (CS), necessary layer 2 association

and authentication procedures by sending an “associate”

request to the target 802.11 MAC.

After the Layer 2 association, the MIHF (ES) send a

“Link Up” indication to the Mobile IP function. The Mobile

Function performs a Fast Binding Update (FBU) with old

foreign agent (FA) over the old link. The mobile node

receives packets from the new FA that are tunnelled from

the old FA. Later, the Mobile IP function in the mobile

node performs update procedure to register the new binding

with the HA and reroute the tunnel to the new FA in the

corresponding 802.11 network link. As the traffic start to

use the link, the MIHF (CS) sends a request to that MAC

layer release the old link (3GPP radio link). A MIHF (CS)

“Link Switch” response is sent back to the MME with the

termination of the command.

5 CONCLUSION

In this paper we have presented the three primary

functional components defined by the IEEE802.21 standard.

We have shown how these services interact with both the

upper and lower layers of the mobility protocol stack

through generic SAPs to optimize the handover process.

The paper presents a scenario in which Network Controlled

IP layer handover process is optimized through the usage of

the 802.21 framework. The presented scenario outlines

network discovery, network selection, pre-configuration

and pre-authentication to facilitate a pro-active handover

using the MIES, MICS and MIIS. MIH Capability

discovery, Event Registration, Information Server

discovery mechanisms have also been extensively

discussed.

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