MOBILITY AND HANDOFF
MANAGEMENT IN NEXT GENERATION
WIRELESS NETWORK
Seminar ID: 1578
A Technical Seminar Report
submitted for fulfilment of
the requirements for the
Degree of Bachelor of Technology
Under Biju Pattnaik University of Technology
Submitted By
B DHANA SAGAR
Roll No.# CSE 201010148
April - 2013
Under the guidance of
Prof. Jaydip Sen
NATIONAL INSTITUTE OF SCIENCE & TECHNOLOGY
PALUR HILLS, BERHAMPUR, ODISHA – 761008, INDIA
ABSTRACT
For efficient delivery of services to the mobile users, the next-generation wireless
networks require new mechanisms of mobility management where the location of
every user is proactively determined before the service is delivered. Moreover, for
designing an
adaptive
communication
protocol,
various
existing mobility
management schemes are to be seamlessly integrated.
In this seminar, the design issues of a number of mobility management schemes will
be studied. Each of these schemes utilizes IP-based technologies to enable efficient
roaming in heterogeneous network. Since efficient handoff mechanisms are essential
for ensuring seamless connectivity and uninterrupted service delivery, a number of
handoff schemes for heterogeneous networking environment will also be studied and
presented.
i
ACKNOWLEDGEMENT
My sincere thanks to Prof. Jaydip Sen, Faculty Advisor, for giving his valuable time
and help during preparation of my seminar report.
I owe my special thanks to Mr. Ranjit Kumar Behera, Technical Seminar In-charge,
for giving me an opportunity to present this topic in the seminar.
And finally thanks to Prof. Sangram Mudali, Director & Prof. Geetika Mudali,
Placement Director, NIST for their continued drive for better quality in everything
that happens at NIST. This report is a small contribution towards that greater goal.
Last but not the least I would like to thank my family and friends without whom this
project would not be completed.
April 10, 2013
Berhampur, Odisha, India
B DHANA SAGAR
Roll # 201010148
B.TECH - CSE
Semester – VI
ii
TABLE OF CONTENT
ABSTRACT ................................................................................................................... i
ACKNOWLEDGEMENT ...........................................................................................ii
TABLE OF CONTENT..............................................................................................iii
LIST OF FIGURES .................................................................................................... iv
1. INTRODUCTION.................................................................................................... 1
2. MOBILITY MANAGEMENT ............................................................................... 3
2.1 Location Management .......................................................................................... 3
2.2 Handoff Management ........................................................................................... 4
2.3 Mobility Management at Different Layers........................................................... 5
3. NETWORK LAYER MOBILITY MANAGEMENT MECHANISMS ............. 6
3.1 Macro-mobility Protocols .................................................................................... 7
3.2 Micro-mobility Protocols ................................................................................... 10
4. LINK LAYER MOBILITY MANAGEMENT MECHANISMS ...................... 15
4.1 Location Management Protocols........................................................................ 15
5. HANDOFF MANAGEMENT PROTOCOLS .................................................... 18
5.1 Taxonomy of Handoff Mechanisms................................................................... 18
5.2 Delays in Handoff .............................................................................................. 19
5.4 Cross-layer Handoff Mechanisms ...................................................................... 20
6. IEEE 802.21- MEDIA INDEPENDENT HANDOVER SERVICES ................ 24
6.1 Mobility using IEEE 802.21 in a Heterogeneous IMT-Advanced (4G)
Network............................................................................................................... 25
7. SECURITY IN HANDOFF PROCEDURES ...................................................... 26
8. SOME OPEN ISSUES IN MOBILITY & HANDOVER MANAGEMENT .... 28
9. CONCLUSION ...................................................................................................... 30
REFERENCES........................................................................................................... 31
iii
LIST OF FIGURES
Fig. 3.1. Mobile IP Architecture [4] .............................................................................. 6
Fig. 3.2. Architecture of MMUSE [33]........................................................................ 10
Fig. 3.3. Architecture of Cellular IP [03] ..................................................................... 13
Fig. 3.4. Architecture of HAWAII Protocol ................................................................ 14
Fig. 4.1. The Boundary Location Register Protocol .................................................... 17
Fig. 5.1. Handoff Scenario in Forward Direction ........................................................ 21
Fig. 5.2. Architecture of the Wireless Mesh Network [39] ......................................... 22
Fig. 5.3. Macro-Mobility and Micro-Mobility Scenario [39] ...................................... 23
iv
1. INTRODUCTION
With the increasing demands for new data and real-time services, wireless networks
should support calls with different traffic characteristics and different Quality of
Service (QoS)satisfy different needs and requirements of mobile users. Since these
different wireless networks act as complementary to each other in terms of their
capabilities and suitability for different applications, integration of these networks
will enable the mobile users to be always connected to the best available access
network depending on their requirements.
This integration of heterogeneous networks will, however, lead to heterogeneities in
access technologies and network protocols. To meet the requirements of mobile users
under this heterogeneous environment, a common infrastructure to interconnect
multiple access networks will be needed. Although IP has been recognized to be the
de facto protocol for next-generation integrated wireless, for inter-operation between
different communication protocols, an adaptive protocol stack is also required to be
developed that will adapt itself to the different characteristics and properties of the
networks.[2].
Finally, adaptive and intelligent terminal devices and smart base stations (BSs) with
multiple air interfaces will enable users to seamlessly switch between different access
technologies . For efficient delivery of services to the mobile users, the nextgeneration wireless networks require new mechanisms of mobility management where
the location of every user is proactively determined before the service is delivered.
Moreover, for designing an adaptive communication protocol, various existing
mobility management schemes are to be seamlessly integrated. In this chapter, the
design issues of a number of mobility management schemes have been presented.
Each of these schemes utilizes IP-based technologies to enable efficient roaming in
heterogeneous network [10]. Efficient handoff mechanisms are essential for ensuring
seamless connectivity and uninterrupted service delivery. A number of handoff
schemes in a heterogeneous networking environment are also presented in this
chapter. The chapter is organized as follows. Section 2 introduces the concept of
mobility management and its two important components- location management and
1
handoff management. Section 3 presents various network layer protocols for macromobility and micro-mobility. Section 4 discusses various link layer protocols for
location management. Section 5 introduces the concept of handoff.
Different types of handoff mechanisms are Classified, and the delays associated with
a handoff procedure are identified. Some Important cross-layer handoff mechanisms
are discussed in detail. Section 6 presents media independent handover (MIH)
services as proposed in IEEE 802.21 standards. It also discusses how MIH services
can be utilized for designing seamless mobility protocols in next generation
heterogeneous wireless networks. Section 7 discusses security issues in handover
protocols. Section 8 identifies some open areas of research in mobility management.
Section9 concludes the chapter.
2
2. MOBILITY MANAGEMENT
With the convergence of the Internet and wireless mobile communications and with
the rapid growth in the number of mobile subscribers, mobility management emerges
as one of the most important and challenging problems for wireless mobile
communication over the Internet. Mobility management enables the serving networks
to locate a mobile subscriber’s point of attachment for delivering data packets (i.e.
location management), and maintain a mobile subscriber’s connection as it continues
to change its point of attachment (i.e. handoff management). The issues and
functionalities of these activities are discussed in this section.
2.1 Location Management
Location management enables the networks to track the locations of mobile nodes.
Location management has two major sub-tasks: (i) location registration, and (ii) call
delivery or paging. In location registration procedure, the mobile node periodically
sends specific signals to inform the network of its current location so that the location
database is kept updated. The call delivery procedure is invoked after the completion
of the location registration. Based on the information that has been registered in the
network during the location registration, the cal delivery procedure queries the
network about the exact location of the mobile device so that a call may be delivered
successfully. The design of a location management scheme must address the
following issues: (i) minimization of signaling overhead and latency in the service
delivery, (ii) meeting the guaranteed quality of service (QoS) of applications, and
(iii)in a fully overlapping area where several wireless networks co-exist, an efficient
and robust algorithm must be designed so as to select the network through which a
mobile devices would perform registration, deciding on where and how frequently the
location information should be stored, and how to determine the exact location of a
mobile device with in a specific time frame.
3
2.2 Handoff Management
Handoff management is the process by which a mobile node keeps its connection
active when it moves from one access point to another. There are three stages in a
handoff process .First, the initiation of handoff is triggered by either the mobile
device, or a network agent, or the changing network conditions. The second stage is
for a new connection generation, where the network must find new resources for the
handoff connection and perform any additional routing operations. Finally, data-flow
control needs to maintain the delivery of the data from the old connection path to the
new connection path according to the agreed upon QoS guarantees. Depending on the
movement of the mobile device, it may undergo Mobility and Handoff Management
in Wireless Networks 3various types of handoff.
In a broad sense, handoffs may be of two types:
(i)
intra-system handoff (horizontal handoff)
(ii)
inter-system handoff (vertical handoff).
Handoffs inhomogeneous networks are referred to as intra-system handoffs. This type
of handoff occurs when the signal strength of the serving BS goes below a certain
threshold value.[25]
An inter-system handoff between heterogeneous networks may arise in the following
scenarios
(i)
when a user moves out of the serving network and enters an overlying network
(ii)
when a user connected to a network chooses to handoff to an underlying or
overlaid network for his/her service requirements
(iii)
When the overall load on the network is required to be distributed among
different systems.
The design of handoff management techniques in all-IP based next-generation
wireless networks must address the following issues:
(i)
signalling overhead and power requirement for processing handoff messages
should be minimized
(ii)
QoS guarantees must be made
4
(iii)
Network resources should be efficiently used
(iv)
The handoff mechanism should be scalable, reliable and robust.
2.3 Mobility Management at Different Layers
A number of mobility management mechanisms in homogeneous networks have been
presented and discussed in [1]. Mobility management in heterogeneous networks is a
much more complex issue and usually involves different layers of the TCP/IP
protocol stack. Several mobility management protocols have been proposed in the
literature for next-generation all-IP wireless networks. Depending on the layers of
communication protocol they primarily use, these mechanisms can be classified into
three categories –protocols at the networks layer, protocols at the link layer and the
cross-layer protocols. Network layer mobility protocols use messages at the IP layer,
and are agnostic of the underlying wireless access technologies [24]. Link layer
mobility mechanisms provide mobility-related features in the underlying radio
systems. Additional gateways are usually required to be deployed to handle the interoperating issues when roaming across heterogeneous access networks. In link layer
protocols, handoff signals are transmitted through wireless links, and therefore, these
protocols are tightly-coupled with specific wireless technologies. Mobility supported
at the link layer is also called access mobility or link layer mobility [10]. The crosslayer protocols are more common for handoff management. These protocols aim to
achieve network layer handoff with the help of communication and signaling from the
link layer. By receiving and analyzing, in advance ,the signal strength reports and the
information regarding the direction of movement of the mobile node from the link
layer, the system gets ready for a network layer handoff so that packet loss is
minimized and latency is reduced.
5
3. NETWORK LAYER MOBILITY MANAGEMENT
MECHANISMS
Over the past several years, a number of IP mobility management protocols have been
proposed. Different mobility management frameworks can be broadly distinguished
into two categories - device mobility management protocol for localized or micromobility and protocols for inter-domain or macro mobility. The movement of a mobile
node (MN) between two subnets within one domain is referred to as micro-mobility.
For example, the movement of MN from subnet B to subnet C in Figure 3.1 is an
example of micro-mobility. An example of micro-mobility in UMTS [14] Terrestrial
Radio Access Networks (UTRAN) is movement of an MN from one BS to another,
both BSs belonging to the same random access network RAN),while in WLAN it is a
node movement between two access points (APs). The ovement of devices between
two network domains is referred to as macro-mobility. For example, the movement of
MN from domain 1 to domain 2 in Figure 3.1 is an example of macro-mobility .A
domain represents an administrative body, which may include different access
networks, such as WLAN, second-generation (2G), and third-generation (3G)
networks [4]. Next-generation all-IP wireless network will include various
heterogeneous networks, each of them using possibly different access technologies.
Therefore, satisfactory macro-mobility solution supporting all these technologies is
needed.
Fig. 3.1. Mobile IP Architecture [4]
6
3.1 Macro-mobility Protocols
Mobile IP is the most widely used protocol for macro-mobility management. In
addition to Mobile IP, three macro-mobility architectures are discussed in the section.
These protocols are: Session Initiation Protocol (SIP)-based mobility management,
multi-tier hybrid SIP and Mobile IP protocol, and network inter-working agent-based
mobility protocol.; all packets sent to the MN are delivered to the CoA. Mobile IP
protocol has three steps: (i) agent discovery, (ii) registration, and (iii) routing and
tunnelling .Agent discovery: An MN is able to detect whether it has moved into a new
subnet by two methods – agent advertisement and agent solicitation. In the agent
advertisement method, FAs and HAs advertise their presence periodically using agent
advertisement messages
*Registration: After the MN receives its CoA, it registers it with the HA. The main
objective of the registration is to inform the HA about the current location of MN. The
registration maybe done in two ways depending on the location of the CoA. If the
CoA is the FA, the MN sends its registration request to the FA which in turn forwards
it to the HA. If the CoA is located, the MN may send the request directly to the HA.
*Routing and tunnelling: When a CN sends an IP packet to the MN, the packet is
intercepted bythe HA. The HA encapsulates the packet and tunnels it to the MN’s
CoA. With FA CoA, the encapsulated packet reaches the FA serving the MN. The FA
decapsulates the packet and forwards it to the MN. With co-located CoA, the
encapsulated packets reach the MN, which decapsulates them. In Figure 1, the
tunnelling (step b) ends at the MN instead of at the FA.
*Paging Extension for Mobile IP: For saving battery power at MNs, IP paging
mechanism has been proposed [18]. Paging typically includes transmitting a request
for an MN to a set of locations, in one of which the MN is expected to be. The set of
locations is called a paging area and it consists of a set of neighbouring base stations.
A network that supports paging allows the MNs to operate in two different states – an
active state and a standby state. In an active state, the MN is tracked at the finest
granularity such as its current base station (resulting in no need for paging). In the
standby state, the MN is tracked at a much coarser granularity such as a paging area.
7
The MN updates the networks frequently in stand by mode (every paging area
change) than in active state (every base station). The cost of paging, however, is the
complexity of the algorithms and the protocols required to implement the procedures,
and the delay incurred for locating an MN .Drawbacks of Mobile IP: The Mobile IP
has the following shortcomings:
•
The packets sent from a CN to an MN are received by the HA before being
tunnelled to the MN. However, packets from the MN are sent directly to the
CN. This inefficient mechanism of non-optimized Mobile IP is called
triangular routing. It results in longer routes and more delay in packet
delivery.
•
When an MN moves across two different subnets, the new CoA cannot inform
the old CoA about MN’s current location. Packets tunneled to the old CoA are
lost.
•
Mobile IP is not an efficient mechanism in a highly mobile scenario as it
requires an MN to send a location update to the HA whenever it changes its
subnet. The signaling cost for location updates and the associated delay may
be very high if the distance between the visited network and the home network
is large.
Optimization in Mobile IP: In [28], an optimization technique has been proposed to
solve the problem of triangular routing. The idea is to inform the CN about the current
location of the MN so as to bypass the HA. The CN can learn the location of the
CoAs of the MN by caching them in a binding cache in the CN. When a CN sends
packets to an MN, it first checks if it has a binding cache entry for the MN. If there is
an entry, the CN tunnels the packets directly to the CoA. If no binding cache entry is
available, the CN sends the packets to the HA, which in turn tunnels them to the CoA.
In optimized Mobile IP, the packets tunneled by the HA to the old CoA are not lost in
transit. When an MN registers with a new FA, it requests the new FA to notify the
previous FA about its movement. is the old FA now knows the location of the current
FA, it can forward the packets to the few FA.
SIP-Based Mobility Management: In [33] a Session Initiation Protocol (SIP)-based
solution, called mobility management using SIP extension (MMUSE), has been
proposed that supports vertical handoffs in next-generation wireless networks. SIP has
8
been chosen by the Third Generation Partnership Project (3GPP) as the signaling
protocol to set up and control real-time multimedia sessions. In MMUSE, a mobile
host (MH) is assumed to be equipped with multiple network interfaces; each of them
is assigned a separate IP address when connected to different access networks (ANs).
The MH uses the SIP protocol to set up multimedia sessions. The architecture of the
scheme is depicted in Figure 3.2. The session border controller (SBC) is a device that
is typically located at the border of an IP network, and manages all the sessions for
that network. A new entity, called the mobility management server (MMS) resides
within the SBC. The MMS cooperates with another entity – mobility management
client (MMC) that resides in each MH. Both the SIP user agents (UAs) in the MH and
on the corresponding host (CH) remain unaware of all the handoff procedures, which
are handled by the MMC and the MMS. On the MH, the UA sees only the MMC as
its out bound proxy and forwards the normal SIP signaling and media flows to it.
MMC relays the packets to the MMS/SBC. From there on, the packets follow the path
determined by the usual SIP routing procedure. Every time the MH moves across two
ANs, a location update SIP message is sent to the MMS. This is done over the new
network so that the procedure can be completed even if the old network is suddenly
not available. If the MMS receives a call addressed to one of its served MHs, it
forwards it to the correct interface. When the MH changes its AN while it is engaged
in a call, the procedure is almost identical. However, in this case, the MMC sends to
the MMS an SIP message that contains the additional information required to identify
the call to be shifted to new interface. To minimize the handoff duration, the real-time
transport protocol (RTP) flow coming from the MH during the handoff is duplicated
using the MMC. When the MMC starts the handoff procedures, it sends the handover
request to the MMS and at the same time, it starts duplicating the RTP packets over
both interfaces. As soon as the MMS receives the handover message, the packets
coming from the new interface are already available. The MMS performs the
switching and sends the reply back to the MMC. When the MMC receives the reply
message, it stops duplicating the packets.
9
Fig. 3.2. Architecture of MMUSE [33]
Multi-Layer Mobility Management using Hybrid SIP and Mobile IP:
In [31] two mobility management architectures based on SIP and Mobile IP are
presented. The two approaches provide mobility in two different layers: application
and network layers respectively. The scheme is therefore called multi-layer mobility
management scheme. The SIP-based protocol uses SIP in combination with IP
encapsulation Mechanisms on CHs to support mobility for all types of traffic from/to
the MH. The second approach performs separation of traffic and employs SIP in
combination with network address translation (NAT) mechanisms to support mobility
for real-time traffic over UDP. The mobility for non-real-time traffic (mainly TCPbased applications) is supported by MobileIP. In the SIP-based approach, if the MH
moves during a session, the SIP UA sends a SIP re-INVITE request message to each
of its CHs. If a CH runs a TCP session, IP encapsulation isused to forward packets to
MH. However, if a CH runs a UDP session, the packets are sent directly to the MH’s
new address. The MH completes the handoff by sending a SIPREGSITER message to
the SIP server. For the hybrid SIP/Mobile IP scheme, the inter domain mobility is
based on the synergy of SIP with Mobile IP. Traffic from/to an MH is separated on
the domain edge routers. SIP signalling is used to support inter-domain mobility for
real-time (RTP over UDP) traffic, while Mobile IP supports non-real-time traffic.
3.2 Micro-mobility Protocols
Over the past several years a number of IP micro-mobility protocols have been
proposed, designed and implemented that complement the base Mobile IP [8] by
providing fast, seamless and local handoff control. IP micro-mobility protocols are
10
designed for environments where MHs changes their point of attachment to the
network so frequently that the base Mobile IP mechanism introduces significant
network overhead in terms of increased delay, packet loss and signaling. For example,
many real-time wireless applications, e.g. VOIP, would experience noticeable
degradation of service with frequent handoff. Establishment of new tunnels can
introduce additional delays in the handoff process, causing packet loss and delayed
delivery of data to applications. This delay is inherent in the round-trip incurred by
the Mobile IP as the registration request is sent to the HA and the response sent back
to the FA. Route optimization [28]can improve service quality but it cannot eliminate
poor performance when an MH moves while communicating with a distant CH.
Micro-mobility protocols aim to handle local movement(e.g., within a domain) of
MHs without interaction with the Mobile IP-enabled Internet. This reduces delay and
packet loss during handoff and eliminates registration between MHs and possibly
distant HAs when MHs remain inside their local coverage areas. Eliminating
registration in this manner also reduces the signalling load experienced by the
network .The micro-mobility management schemes can be broadly divided into two
groups: (i)tunnel-based schemes and (ii) routing-based schemes.
Mobile IP Regional Registration:
In Mobile IP, an MN registers with its HA each time it changes its CoA. If the
distance between the visited network and the home network of the MN is large, the
signalling delay for these registrations may be long. MIP-RR [15] attempts to
minimize the number of signalling messages to the home network and reduce the
signaling delay by performing registrations locally in a regional network. His reduces
the load on the home network, and speeds up the process of handover.
Intra-Domain Mobility Management Protocol:
Intra-domain mobility management protocol (IDMP) [24] is a two-level hierarchical
multi-CoA intra-domain mobility management protocol. The first level of the
hierarchy consists of different mobility domains. The second level consists of IP
subnets within each domain. This hierarchical approach localizes the scope of intradomain location update messages and thereby reduces both the global signaling load
and update latency. The two-level hierarchical architecture defined by IDMP is shown
11
in Figure 4. IDMP consists of two types of entities: (i) mobility agent(MA) and (ii)
subnet agent (SA). The MA provides a domain-wide stable access point for an MN.
An SA handles the mobility of MNs within a subnet. Similar to HMIP, each MN can
get two CoAs - global CoA (GCoA) and local CoA (LCoA). The GCoA specifies the
domain to which the MN is currently attached. The LCoA identifies the MN’s present
subnet. The packets destined to an MN are first received by the HA. The HA tunnels
the packets to the MA using the MN’s GCoA. The MA first decapsulates the packets,
determines the current LCoA of the MN using its internal table, and tunnels them to
the LCoA. The encapsulated packets are received by the SA. Finally, the SA
decapsulates the packets and forwards them to the MN.
Cellular IP:
Cellular IP [07] is a mobility management protocol provides access to a Mobile IPenabled Internet for fast moving MHs. The architecture of Cellular IP is shown in
Figure 3.3. It consists of three major components: (i) cellular IP node or the base
station (BS), (ii) cellular IP gateway (GW), and (iii) cellular IP mobile host (MH). A
Cellular IP network consists of interconnected BSs. The BSs route IP packets inside
the cellular network and communicate with MHs via wireless interface. The GW is a
cellular IP node that is connected to a regular IP network by at least one of its
interfaces. The BSs periodically emit beacon signals. MHs use these beacon signals to
locate the nearest BSs. All IP packets transmitted by an MH are routed from the BS to
the GW by hop-by-hop shortest path routing, regardless of the destination address.
The BSs maintain route cache. Packets transmitted by the MH create and update
entries in BS’s cache. An entry maps the MH’s IP address to the neighbour from
which the packet arrived to the host. The chain of cached mappings referring to an
MH constitutes a reverse path for downlink packets for the MH. To prevent timing
out of these mappings, an MH periodically transmits control packets. MHs that are
not actively transmitting or receiving data themselves may still remain reachable by
maintaining paging caches. MHs listen to the beacons transmitted by Ss and initiate
handoff based on signal strength. To perform a handoff, an MH tunes its radio to the
new BS and sends a route update packet. This creates routing cache mappings on
route to the new BS. Handoff latency is the time that elapses between the handoff and
12
the arrival of the Mobility and Handoff Management in Wireless Networks 11first
packet through the new route.
Fig. 3.3. Architecture of Cellular IP [03]
Handoff Aware Wireless Access Internet Infrastructure: Handoff-Aware Wireless
Access Internet Infrastructure (HAWAII) [32] is a domain-based approach for
supporting mobility. The network architecture of HAWAII is shown in Figure 3.4.
Mobility management within a domain is handled by a gateway called a domain root
router (DRR).Each MH is assumed to have an IP address and a home domain. While
moving in its home domain, the MN retains its IP address. The packets destined to the
MH reach the DRR based on the subnet address of the domain and are then forwarded
to the MH. The paths to MH are established dynamically. When the MH is in a
foreign domain, packets for the MH are intercepted by its HA. The HA tunnels the
packets to the DRR of the MH. The DRR routes the packets to the MH using the hostbased routing entries. If the MH moves across different subnets in the same domain,
the route from the DRR to the BS serving the MN is modified, while the other paths
remain unchanged. This causes a reduction in signaling message and handoff latency
during intra-domain handoff. In traditional Mobile IP, the MH is directly attached
either to the HA (i.e. the home domain router) or the FA (i.e. the foreign domain
router). Thus, every handoff causes a change in the IP address for the MH, resulting in
lack of scalability. HAWAII also supports IP paging. It uses IP multicasting to page
13
idle MHs when packets destined to an MH arrive at the domain root router and no
recent routing information is available.
Fig. 3.4. Architecture of HAWAII Protocol
14
4. LINK LAYER MOBILITY MANAGEMENT
MECHANISMS
Link layer mobility management mechanisms deal with issues related to inter-system
roaming between heterogeneous access networks with different radio technologies
and network management protocols. Two important considerations for designing
inter-system roaming standards are: (i) the protocols for air interface and (ii) the
mobile application part(MAP). In situations where a mobile node enters one wireless
access network from an other that support the same air interface protocols and MAP,
the services are seamlessly migrated .Mobility and Handoff Management in Wireless
Networks 13However, when the MAPs are different for the two networks, additional
network entities need to be placed and signaling traffic are to be transmitted for interworking. Since each network has its own mobility management protocols, the new
inter-working entities should not replace existing systems. Rather, the entities should
coexist and inter-work.
4.1 Location Management Protocols
For next-generation heterogeneous wireless networks, the inter-working and interoperating function is suggested to accommodate roaming between dissimilar networks
[26]. For existing practical systems, several solutions are proposed for some specific
pairs of inter-working systems. In these schemes, the inter-operating function is
implemented in either some additional inter-working unit with the help of dual-mode
handsets [30], or a dual-mode home location register (HLR) [16] to take care of the
transformation of signaling formats, authentication, and retrieval of user profiles.
Recent research efforts attempt to design general location management mechanisms
for the integration and inter-working of heterogeneous networks. The research
activities can be grouped into two categories: location management for adjacent
dissimilar systems with partially overlapping coverage at the boundaries [36] and
location management in multi-tier sys terms where service areas of heterogeneous
networks are fully overlapped [22] . All these solutions propose additional entities
that take care of inter-working issues.
15
Location Management for Adjacent Networks: Researchers have addressed the
issues of location management in two adjacent networks with overlapping areas
[36],[14] Some of the protocols are discussed briefly.
Gateway Location Register Protocol: To enable inter-system roaming, a new level
has been introduced in the hierarchy of location management entities for UMTS/
IMT-2000 networks. The new level consists of a gateway location register (GLR)
[14] The GLR is a gateway that enables inter-working between two networks by
suitably converting signaling and data formats. It is located between the visitor
location register (VLR) and the serving GPRS supportnode (SGSN) and the home
location register (HLR). When a subscriber roams, the GLR plays the role of the HLR
toward the VLR and SGSN in a visited public land mobile network(VPLMN), and the
role of the VLR and SGSN to the HLR in a home public land mobile
network(HPLMN). The GLR protocol assists the operators in lowering costs and
optimizing roaming traffic. However, the protocol is not designed for ongoing call
connection during intersystem roaming. The incoming calls are routed to the home
network even when the MN is roaming. This makes the protocol inefficient.
Boundary Location Register Protocol: In a location management mechanism has
been proposed for heterogeneous network environment. It involves a mechanism for
inter-system location updates and paging. Inter-system location update is
implemented by using the concept of a boundary location area (BLA) existing at the
boundary between two systems - X and Y in Figure 7. The BLA is controlled by a
boundary interworking unit (BIU), which is connected to the mobile switching centers
(MSCs) in both the systems. The BIU queries the user’s service information, converts
the message formats, checks the compatibility of the air interfaces and performs
authentication of mobile users.
When an MN is inside its BLA, it sends a location registration request to the new
system. A distance-based location update mechanism reports MN’s location when its
distance from the boundary is less than a pre-defined threshold. An entity called a
boundary location register (BLR) is used for inter-system paging. The
BLR
maintains in its cache the location information of the MN and its roaming information
16
when it crosses an intersystem boundary. During the intersystem paging process, only
one system (X or Y) is searched. The associated MAP protocol is designed for mobile
nodes with ongoing connections during inter-system roaming. Instead of performing
location registration after a mobile node arrives at the new system, the BLR protocol
enables the node to update its location and user information actively before it enters
the new system. In this way, the incoming calls to the MN during its inter-system
roaming are delivered to the node.
Fig. 4.1. The Boundary Location Register Protocol
Location Management in Heterogeneous Networks: An MN is reachable via
multiple networks when their service areas are fully overlapped. Since heterogeneous
networks use different signalling formats, authentication procedures, and registration
messages, it is difficult to merge heterogeneous HLRs into a single HLR. A multi-tier
HLR (MHLR) is proposed in where a tier manager is connected to all the HLRs. Two
types of location registration are possible: (i) single registration (SR) and (ii) multiple
registrations (MR). Under SR scheme, an MN associates with the lowest tier of the
MHLR, and receives services at low cost and high bandwidth.
17
5. HANDOFF MANAGEMENT PROTOCOLS
Handoff or handover is a process by which an MN moves from one point of network
attachment to another. Handovers can be classified as either homogeneous or
heterogeneous. A heterogeneous handover occurs when an MN either moves between
networks with different access technologies, or between different domains. As the
diversity of available networks increases, it is important that mobility technologies
become agnostic to link layer technologies, and can operate in an optimized and
secure fashion without incurring unreasonable delay and complexity [11] Supporting
handovers across heterogeneous access networks, such as IEEE 802.11 (Wi-Fi),
global system for mobile communications (GSM), code-division multiple access
(CDMA), and world wide inter operability for microwave access (WiMAX) is a
challenge, as each has different quality of service (QoS), security, and bandwidth
characteristics. Similarly, movement between different administrative domains poses
a challenge since MNs need to perform access authentication and authorization in the
new domain. Thus, it is desirable to devise a mobility optimization technique that can
reduce these delays and is not tightly coupled to a specific mobility protocol. In this
section, we describe different types of handovers and investigate the components that
contribute to a handover delay. Some inter-technology and media-independent
handover frameworks are then described.
5.1 Taxonomy of Handoff Mechanisms
Different types of handovers may be classified based on three parameters as follows:
(i) subnets, (ii) administrative domains, and (iii) access technologies [11]. Intertechnology: this type of handover is possible with an MN that is equipped with
multiple interfaces supporting different technologies. An inter-technology handover
occurs when the two points of attachment use different access technologies. During
the handoff, the MN may move out of the range of one network (e.g., Wi-Fi) into that
of a different one (e.g., CDMA).This is also known as vertical handover. Intratechnology: this type of handoff occurs when an MN moves between points of
attachments supporting the same access technology, such as between two Wi-Fi
access points. An intra-technology handover may happen due to intra-subnet or inter18
subnet movement and thus may involve the layer 3 trigger. Inter-domain: when the
points of attachment of an MN belong to different domains, this type of handoff takes
place. A domain is defined as a set of network resources managed by a single
administrative entity that authenticates and authorizes access for the MNs. An
administrative entity may be a service provider or an enterprise. An inter-domain
handover possibly involves an inter-subnet handover also.
Intra-domain: handovers of this type occurs when the movement of an MN is
confined within an administrative domain. Intra-domain movement may also involve
intra-subnet, inter-subnet, intra-technology, and/or inter-technology handovers as
well. Inter-subnet: an inter-subnet handover occurs when the two points of attachment
belong to different subnets. The MN acquires a new IP address and possibly
undergoes a new security procedure. A handover of this type may occur along with
either an inter- or an intra-domain handover and also with either an inter- or an intratechnology handover.
Intra-subnet: an intra-subnet handover occurs when the two points of attachment
belong to the same subnet. This is typically a link layer handover between two access
points in a WLAN networks, or between different cell sectors in cellular networks. It
is administered by the radio network and requires no additional authentication and
security procedures.
5.2 Delays in Handoff
All the layers in the communication protocol stack contribute to the delay in a
handoff.
Link layer delay: depending on the access technology, an MN may go through several
steps with each step adding its contribution to the overall delay before a new link is
established. For example, a Wi-Fi link goes through the process of scanning,
authentication, and association before being attached to a new access point. For intrasubnet handovers, where network layer configurations are necessary, link layer
contributes the maximum to the overall delay.
19
Network layer delay: after completion of the link layer procedures, it may be
necessary to initiate a network layer transition. A network layer transition may
involve steps such as acquiring a new IP address, detecting a duplicate address,
address resolution protocol(ARP) update, and subnet-level authentication.
Application layer delay: the delay of this type is due to reestablishment and
modification of the application layer properties such as IP address while using session
initiation protocol (SIP). The authentication and authorization procedure such as
extensible authentication protocol(EAP) includes several round-trip messages
between the MN and the authentication authorization and accounting (AAA) server
causing delay in handoff.
5.4 Cross-layer Handoff Mechanisms
The cross-layer protocols for mobility management are mainly applied for handoff.
Most of these mechanisms use link layer information to make an efficient network
layer handoff. The utilization of link layer information reduces the delay in movement
detection of the MN so that the overall handoff delay is minimized. In [37] a lowlatency handoff algorithm for a WLAN has been proposed that uses access points and
a dedicated medium access control (MAC) bridge. A seamless handoff architecture
for Mobile IP, called S-MIP is presented in [17] that combines a location tracking
scheme with the HMIP handoff. A vertical handoff mechanism between IEEE 802.11
(WLAN) and IEEE 802.16e (Mobile WiMAX) networks in a wireless mesh backbone
is proposed in [38] In [11]a media-independent pre authentication scheme has been
proposed.
Link Layer-Assisted Fast Handoff over WLAN: In the Mobile IP protocol, the MN
movement can be detected from advertisements of the FAs that differ from the
previously received advertisement. The new CoA is registered with the HA. However,
data packets are not forwarded to the new FA before the registration is complete. This
interruption may degrade the QoS especially in real-time applications. To tackle this
issue, a handoff mechanism is proposed in which APs in a WLAN and a dedicated
MAC bridge are jointly used to eliminate packet loss [38]. The authors have noted
that the delay in Mobile IP handoff is contributed by two elements: (i) the delay in
20
movement detection of the MN, and (ii) delay due to signaling for registration. The
proposed mechanism reduces the movement detection delay. It has two parts: (i)
handoff for the forward direction (i.e., mobile-terminated data) and (ii) handoff for the
reverse direction (i.e. mobile-originated data). The APs in the WLAN have the
capability to notify the MAC address of an MN that moves into their coverage areas.
The MAC bridge is configured in a way that it sends only those MAC frames whose
destination addresses are registered in the filtering database (DB).
Fig. 5.1. Handoff Scenario in Forward Direction
The handoff in the forward direction happens as follows. In Figure 5.1, the MN
establishes an association with an access point- AP1, and registers the CoA with HA.
The packets destined to the MN are encapsulated by the HA and tunnelled to FA1- the
FA of the MN. FA1encapsulates the packets and sends them directly to the MN.
When the signal strength of the channel of communication between AP1 and the MN
falls below a threshold, MN attempts to find a new AP. The MN establishes
association with a new AP- AP2. AP2 places the MAC address of the MN in a MAC
address registration request message and broadcasts it on the local segment. The
MAC bridge receives the address registration request. It then makes an entry of the
MAC address contained in the message and the port on which the message was
received into the filtering DB. When the MAC bridge receives a MAC frame on a
21
port, it refers to the filtering DB to see if the destination MAC address is registered. If
the address is registered, the MAC bridge sends it out to the corresponding port. The
latency due to Mobile IP handoff has been found to be equal to that of a link layer
handoff [37]
A Vertical Handoff Scheme between WLAN and Mobile WiMAX Networks:
In [39], a vertical handoff scheme has been proposed between 802.11(WLAN) and
802.16e (Mobile WiMAX) networks. The framework has been discussed with a
wireless mesh network (WMN) that provides high speed, scalable and ubiquitous
wireless Internet services. A wireless mesh router (WMR) is a gateway that has
routing capabilities to support mesh networking. Each WMR is assumed to have
802.11e functions, 802.16e BS functions with point-to-multi-point mode (PMP),
routing capabilities, and 802.16e subscriber station (SS) functions with mesh mode.
Fig. 5.2. Architecture of the Wireless Mesh Network [39]
In Figure 5.3, two domains are served by service providers A and B respectively. The
WMRs in the same and different domains are called intra-mesh routers and intermesh routers respectively. If the CN is in the same domain as the MN, the IP packets
are routed through the intra-mesh routers only. When the MN moves to another
domain, the packets from the CN are routed via the HA. Four scenarios are considered
for MN mobility.
22
Scenario 1: the MN is connected to the WLAN. It moves out of WLAN and connects
to the WiMAX. The movements 1a, 1b, 1c depict this situation. The WMR does not
change, only the medium access interface changes in case of 1a. The handoff occurs
between intra-mesh routers in 1c and between inter-mesh routers in case 1b.
Scenario 2: the MN is currently connected to the WiMAX. It moves into the WLAN
and either connects to the WLAN or continues with the WiMAX connection
depending on the network conditions, user preference, or application QoS
requirements.
Scenario 3: the MN is located in the double-coverage area (i.e. area covered by
WLAN and WiMAX) and is currently stationary. If the WLAN is congested, the MN
can switch to the WiMAX if it can provide more bandwidth for the MN to transmit its
data packets.
Scenario 4: A horizontal handoff occurs when the MN moves in 2a and 2b. In [21] a
scheme called last packet marking (LPM) has been proposed for case 2a. The
MIPSHOP (Mobility for IP: Performance, Signaling and Handoff Optimization)
working group of the Internet Engineering Task Force (IETF) has developed Mobile
IPv6 fast handoff over 802.16e networks for case 2b [9]
Fig. 5.3. Macro-Mobility and Micro-Mobility Scenario [39]
23
6. IEEE 802.21- MEDIA INDEPENDENT
HANDOVER SERVICES
A novel solution that ensures interoperability between several types of wireless access
network is given by the developing IEEE 802.21 standard (Eastwood et al., 2008).
The work on the standard began in 2004 and is expected to be finalized around 2010.
The IEEE 802.21is focused on handover facilitation between different wireless
networks in heterogeneous environments. The standard names this type of vertical
handover as Media Independent handover (MIH). In MIH, the handover procedures
can use the information gathered from both the mobile terminals and the network
infrastructure. At the same time, several factors may determine the handover decision,
e.g., service continuity, application class and QoS, negotiation of QoS, security,
power management, handover policy etc. IEEE 802.21facilitates, speeds, and thereby
increases the success rate of inter-technology handover decision making and other
pre-execution processes. These processes include inter technology candidate network
discovery, target network selection, target network preparation, and handover
execution timing and initiation. IEEE 802.21 defines three services to facilitate intertechnology handovers: (i) media independent information service(MIIS), (ii) media
independent command service (MICS), and (iii) media independent event
service(MIES). MIIS provides information about the neighboring networks, their
capabilities and available services. MICS allows effective management and control of
different link interfaces on multimodal device and enables both mobile- and networkinitiated handovers. It supports querying of target networks about the status of the
rapidly changing resources. Some MICS commands are part of the signaling between
inter-radio access technology (RAT) gateways. MIES provides events triggered by
changes in the link characteristics and status. This interface provides service
primitives to the upper layers that are independent of the access technology One of the
most important aspects of MIH is the fact that it allows for network controlled
handovers and user controlled handovers. The advantages of the network controlled
handover lies in the lower user battery consumption since the monitoring of various
network conditions is done by the networks themselves. However, it incurs a huge
signalling overhead and a high processing load in the network elements. In user
24
controlled handover, the user collects necessary data and initiates the appropriate
actions. The disadvantage of this approach is the high battery power consumption..
6.1 Mobility using IEEE 802.21 in a Heterogeneous IMTAdvanced (4G) Network
The telecommunication industry is defining a new generation of mobile wireless
technologies, called fourth generation (4G). In this regard, the International
Telecommunications Union- Radio Standardization Sector (ITU-R) has defined the
concept of IMT-Advanced that targets peak data rates of about 100 Mb/s for highly
mobile access (at speeds of up to 250 km/hr), and 1 Gb/s for low mobility (pedestrian
speeds or fixed) access.
The IEEE is developing extensions to both IEEE 802.11 and 802.16 to meet IMTAdvanced requirements. The evolving standard of IEEE 802.16m aims to achieve a
data rate of 100Mb/s in a highly mobile (25 km/hr) scenario. These data rate and
mobility capabilities make 802.16m a candidate for the high mobility portion of the
IMT-Advanced standard requirements. Another working group of IEEE 802.11n is
working towards designing a very high throughput (VHT) radio capable of data rates
up to 1 Gb/s at stationary or pedestrian speeds. Together, 802.16m and 802.11n will
satisfy both the low-mobility and fully mobile user velocity vs. data rate requirements
for IMT-Advanced systems. If IEEE proposes a combination of 802.11m and 802.11n
for IMT-Advanced standard, an interworking mechanism must be designed for tying
up these two systems. In [4] the authors have proposed a mobility management
approach in 4G using IEEE 802.21 Media Independent Handover (MIH) services.
25
7. SECURITY IN HANDOFF PROCEDURES
Whenever an MN connects to a point of network access, it establishes a security
context with the service provider. During the handover process, some or all the
network entities involved in the security mechanism may change. Thus the current
security context changes as well. The MN and the network have to ensure that they
still communicate with each other and they agree upon the keys to protect their
communication.
However, during handovers in networks like GSM/GPRS and UMTS no
authentication is used. This makes the handover procedures vulnerable to a hijacking
attack. An attacker can masquerade as an authentic mobile station (MS) just by
sending message at the right frequency and time slot during handover. As long as the
attacker does not know the encryption and/or integrity keys currently being used, he
cannot insert valid traffic into the channel. However, if an attacker can gain access to
the key(s) (e.g. because of a missing protection on the backbone network), he can
impersonate the MS.
In fact, in GSM/GPRS, UMTS and WLAN networks, no standard protection
mechanism in the backbone network has been specified. Many GSM operators do not
protect the radio link between their fixed networks and the BSs. In UMTS, during a
handover, the keys used to protect the traffic between the MS and the previous BS are
reused in communication with the next BS. While the keys are being transmitted, they
can be intercepted by an adversary, if the wireless link is not protected.
Usually an authentication process happens before location updates and call setups.
The same mechanisms cannot however, be applied in establishing connection during a
handover process because of the stringent time constraint. In GSM, for example, the
time between the handover command and the handover complete or handover failure
message is restricted to0.5- 1.5 s. The generation of an authentication response,
however, takes about 0.5 s at the MS side.
26
Thus an authentication overhead will cause connection disruption. As we have seen
earlier in this chapter, efficient cell prediction mechanisms can reduce the signalling
overhead between the MS and the old BS. The free time slots may be used to forward
authentication traffic between the MS, the old BS and the new BS. The MS can
recomputed an authentication challenge and the encryption and integrity protection
keys before the actual change of channel. When the MS and the new BS establish
connection, the MS sends the pre-computed authentication response for the new BS to
check. If the checking yields positive results, a handover complete message is sent
and the old BS releases its resources. Otherwise, a handover failure happens and the
MS falls back to the old channel.
27
8. SOME OPEN ISSUES IN MOBILITY AND
HANDOVER MANAGEMENT
Future wireless networks will be based on all-IP framework and heterogeneous access
technologies. Design of efficient mobility management mechanisms will be playing
ever important role in providing seamless services. Following issues will play
dominant roles.
•
QoS issues – next-generation all-IP wireless networks will have to provide
guaranteed QoSto mobile terminals. QoS provisioning in a heterogeneous
wireless and mobile networks will bring in new problems to mobility
management, such as location management for efficient access and timely
service delivery, QoS negotiation during intersystem handoff, etc.
•
User terminals – the design of a single user terminal that is able to
autonomously operate indifferent heterogeneous access networks will be
another important factor. This terminal will have to exploit various
surrounding information (e.g., communication with localization systems,
cross-layering with network entities etc.) in order to provide richer user
services (e.g. location/situation/context–aware multimedia services). This will
also put strong emphasis on the concept of cognitive radio and cognitive
algorithms for terminal reconfigurability.
•
Location and handoff management in wireless overlay networks – future
wireless
•
networks will be inherently hierarchical where access networks have different
coverage areas. Mobility management in wireless overlay networks will be a
very important issue.
•
Mobile services – sophisticated 4G service discovery mechanisms will
combine the
•
location/situation information and context-awareness in order to deliver users’
services in a best possible manner. Additionally, future mobile services will
require more complex personal and session mobility management to provision
personalized services through different personalized operating environments
to a single user terminal address. Whether SIP should be the core 4G protocol,
28
and whether the service delivering framework be the network layer-based or
application layer-based is still an open question.
•
Cross-Layer optimization – design of efficient cross-layer-based approaches
will play a key role is developing new mobility management schemes.
•
Other issues – fault-tolerance, availability of network services, enhanced
security, intelligent packet and call routing, intelligent gateway discovery and
selection procedures and design of a unified protocol stack and vertical
protocol integration mechanisms are some of the other important issues in
next-generation heterogeneous networks
29
9. CONCLUSION
In this chapter, a comprehensive discussion has been made on mobility management
in next-generation wireless networks. Issues in location registration and handoff
management have been identified and several existing mechanisms have been
presented. Since global roaming will be an increasing trend in future, attention has
been paid on mechanisms which are applicable in heterogeneous networks. Media
Independent Handover Services of IEEE802.21 standard as an enabler for handover
has also been presented. Security and authentication issues in next-generation
heterogeneous networks are discussed briefly. Finally, the chapter concludes by
highlighting some open areas of research in mobility management.
30
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