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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, [email protected] **School Of Engineering and Design, Brunel University, [email protected]
*** School Of Engineering and Design, Brunel University, [email protected]
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.
REFERENCES
[1] D. Johnson et. al.,”Mobility Support in IPv6”. RFC
3755, IETF June 2004
[2] X. Pérez, “A MIPv6, FMIPv6 and HMIPv6 handover
latency study: Analytical approach”, March 2006
available at: http://www.ist-mobydick.org/publications
[3] Koodli et al, “Fast Handovers for Mobile IPv6”, RFC
4608, IETF, July 2005
[4] S. Sreemanthula et al, “A Problem Statement for
Event Services and Command Services for Media
Independent Handovers”, Internet Draft (works in
progress), IETF March 2006
[5] S. Faccin et al, “Some Requirement for Handover
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[8] R.Droms ,“Dynamic Host Control Protocol” RFC
2131, IETF
March 1997
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[10]E. Hepworth et al, “Media Independent Handovers:
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