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Handoff Procedure for Heterogeneous Wireless Networks
M. Ylianttila, R. Pichna
1
, J. Vallström
1
, J. Mäkelä,
Centre for Wireless Communications
Department of Electrical Engineering
University of Oulu
Finland
A. Zahedi2, P. Krishnamurthy, and K. Pahlavan
Center for Wireless Information Network Studies
Department of Electrical and Computer Engineering
Worcester Polytechnic Institute
Worcester, MA 01609 USA
1 Currently with Nokia Ltd.
2 Currently with 3-COM
ABSTRACT
Handoff is an important issue when considering mobility
in heterogeneous telecommunication networks. During
the past ten years, telecommunication networks have
emerged as a central strategic component in various
fields. Today, added flexibility of the heterogeneous
wireless networks in the form of robust inter-technology
mobility management schemes and sophisticated
algorithms is becoming more important. Also data
application and IP protocols have become more
important players when designing future architechtures.
This paper gives a novel approach to procedures,
algorithms and metrics involved in handoff in
heterogeneous wireless networks. A network layer Mobile
IP based handoff procedure between WLAN and GPRS is
presented as a case study example.
1. INTRODUCTION
During the past ten years, telecommunication networks
have emerged as a central strategic component in various
fields. They are needed to form the worldwide
infrastructure needed to support educational purposes,
economic development, scientific research, and social
interaction between people in various fields. The
proliferation of high-speed local data networking and new
multimedia services has driven the broadband
telecommunication networks to the focus of research,
development, and standard activities worldwide. One
accelaring factor in the IT markets is and will be the
concept of mobility [1]. Mobility can be defined as a
possibility for a nomadic user to use his network
resources freely in any place and in any time. He can
access remote databases and mail-boxes by using
lightweight GUI devices, e.g., laptops or palm pilot,
anywhere and any time. Incorporating mobility into
broadband systems requires many considerations in every
layer of the communication: power control in the physical
layer, traffic management in the data link layer, mobility
management in the network layer and communication
optimizations in the transport and application layer, just to
mention few examples. Combining the servives of
telecommunication and data networks brings value
addition for both of them by increasing the usability and
scalability. Both operators and software industry are
looking for solutions which can be implemented with
minimal changes to existing infrastructures. Despite of the
hard work of standardization organizations and
committees, there are many competing standards in the
area of wireless communications. This means, that in
order the nomadic user can take a full advantage of the
available network services, he needs to roam between
different technologies.
Figure 1: Inter-technology mobility between two wireless
technologies (GSM and WLAN).
There are several possible levels to solve this problem:
above TCP/IP level, transport level (TCP), network level
(IP) and underlying protocol level [4]. A special inter-
technology roaming protocol layer can be inserted
between the application and the transport protocol layer.
Representatives of this approach are the project On The
Move [5], and Mobile TCP/IP[6]. There is another
category of approaches that offers modifications of
existing protocols sitting above the TCP/IP stack. One of
them is the work on the X-interface mobility [7]. Mobility
gateway can be used to provide mobility in the transport
MH
AP
Subnet 130.231.25
130.231.25.10 (eth0, WaveLAN )
130.231.25.14
130.231.25.20
CH
130.231.25.16
RT
130.231.25.15
HA
194.142.3.x
(ppp0, GSM Data)
2,4GHz
890-960 MHz
BSC
MSC
BTS
PSTN
PM
10631061
194.197.68.15
Internet
FW
Subnet 194.142.3
level. Good examples of this approach are Indirect TCP
[8] and MSOCKS [9]. Solutions to inter-technology
handoff can be found also in layers below IP [4]. In the
network layer most widespread approach is the IETF
Mobile IP. Network layer implementations have been
done in projects like Monarch [10], MosquitoNet [11] and
Daedalus/Barwan [12]). Here we focus on network layer
handoff strategies, and we show how Fuzzy Logic and
Neural Networks class of algorithms can be used to
control Inter-technology handoff procedure. WLAN and
GPRS are used as a case study example. Section 2
presents different scenarios for interconnecting the
services of WLAN and GPRS. In section 3, a general
level handoff procedure for inter-tech roaming is
presented. In section 4 potential metrics for handoff
decision are discussed, and summary and conclusions are
presented in section 5.
2. INTER-TECH MOBILITY SCENARIOS
As presented in [12], there can be seen many different
scenarios to implement inter-tech roaming. Figure 2
shows five different scenarios that enable the
implementation of inter-technology (inter-tech) roaming
between WLAN and GPRS.
Internet
GGSN
SGSN
GSM BSS LAN
AP VAP
3
1
2
4
MG 5
Figure 2: Overview of different scenarios for Inter-
technology roaming between GPRS and WLAN.
The first two scenarios consider implementation of the
WLAN as a base station in the GPRS network. In these
architectures GPRS is a master network and WLAN plays
the slave network. The third scenario considers
exploiting the Mobile-IP for implementation of inter-tech
roaming. The fourth scenario considers implementation
of the GPRS network as an access point in the WLAN.
The fifth scenario considers using a mobility gateway to
interconnect the two networks. In the fourth scenario
WLAN is the master network and GPRS is a slave for the
inter-technology roaming. In the third and the fifth
scenario GPRS and WLAN are interconnected as peers in
a larger network. It is preferable to reduce, as far as
possible, major changes to existing networks and
technologies especially at the lower layers such as MAC
and physical layers. This will ensure that existing
networks will continue to function as before. Scenarios 3
and 5 were preferred, since they treat GPRS and WLAN
as peer networks, without any modification to GPRS or
WLAN specific protocols.
Application
TCP
IP
SNDCP
LLC
RLC
MAC
PHY
RLC
MAC
PHY
LLC
BSSGP
Fr. Rel
L1 bis L1 bis
Fr. Rel
BSSGP
LLC
SNDCP
IP
GTP
UDP
IP
BB L2
BB L1
IP
GTP
UDP
IP
BB L2
BB L1 PDN L1
PDN
L2
802 LLC
802.11 MAC
802.11 PHY
802 LLC
802.11 MAC 802.3 MAC
802.11 PHY 802.3 PHY 802.3 PHY
802.3 MAC 802.3 MAC
802.3 PHY
802 LLC 802 LLC
IP
IPIP
802 LLC
802.3 MAC
802.3 PHY
PDN
L2
PDN L1
Mobile
Base Station
Access Point
SGSN GGSN
on the LAN
Home or
Foreign
Agent
Figure 3: Interconnecting WLAN and GPRS by using
Mobile IP (Scenario 3).
The service scenario was that when a wideband local area
service is available, the terminal will use it. When it is
not available the terminal roams to a lower speed
underlay wireless data service. In our test case the user
has an application running on the terminal and moves out
of the range of the local wireless network. The mobile
data service should notice this and fall back from the
wireless LAN mode to an underlay low speed mobile data
service (GPRS or CDPD). A daemon program is needed
in the terminal to monitor the network resources. When
one network is not available it should adjust the routing
table and trigger the M-IP registration procedure.
Application
TCP
IP
SNDCP
PHY
MAC
PHY
Fr. Rel
PVC
IP
PVC PHY
802 LLC
802.11 MAC
802.11 PHY
802 LLC
802.11 MAC 802.3 MAC
802.11 PHY 802.3 PHY 802.3 PHY
802.3 MAC 802.3 MAC
802.3 PHY
802 LLC 802 LLC
IP
IPIP
802 LLC
802.3 MAC
802.3 PHY
PDN
L2
PDN L1
Mobile
MDBS
Access Point
MD-IS
on the LAN
Mobility
Gateway
MDLP
MAC
MDLP
PVC PVC
Frame Relay
PVC
Fr. Rel
MDLP
SNDCP 802 LLC
802.3 MAC
Figure 4: Interconnecting WLAN and CDPD by Mobility
Gateway (Scenario 5).
Figures 3 and 4 illustrate the protocols involved in
selected scenarios.
3. HANDOFF PROCEDURE
Figure 5 shows the block diagram of the handoff
procedure for inter-tech mobility between WLAN and
GPRS. The handoff is two-fold: handoff from WLAN to
GPRS and handoff from GPRS to WLAN. Figure 7
describes the interactions among the algorithm, metrics,
and the outcomes of executing the algorithm. There are
three items in need of detailed explanation: the HO
architecture (step-by-step description of the procedure to
relate the elements of the network), HO metrics, and the
HO decision mechanism and algorithm. These issues are
addressed in the following.
Figure 5: Handoff procedure for inter-tech mobility.
The general architecture describes the functions of
elements of the network when we have a HO from
WLAN to GPRS or the other way around. First we
describe the step-by-step procedure to go from the WLAN
connection to the GPRS connection. As shown in Figure 6
the following stages occur while the mobile moves away
from the coverage of the WLAN within the GPRS
coverage:
1. Signal received from the AP in the WLAN is strong
2. Signal from the AP becomes weak
3. HO algorithm at the mobile host decides to handoff
from WLAN to GPRS
4. Start/update Mobile IP: Foreign Agent in the MH
gets activated using the Mobile-IP protocol and the
mobile host uses the new IP address.
5. The Home Agent in the WLAN is informed about
the new IP address through the Mobile-IP protocol .
In the reverse situation when the MH is connected to the
GPRS and it realizes that a WLAN is available the
following stages will occur:
1. Signal from the WLAN does not exist.
2. MH detects a signal from the AP of a WLAN
3. HO algorithm decides on the HO.
4. Update Mobile IP: FA in the MH is deactivated by
the Mobile-IP protocol and the regular IP address is
used.
5. HA in the WLAN is informed by the MH through
the M-IP protocol.
Bea con p erio dical ly
1. S tron g sig nal
AP
1
Ho me A gent
LA N
2. W eak sign al
Rou ter
INTERN ET
GPR S
Net work
BTS
3. H and off P roce dure
4. A ctiv ate F orei gn A gent
5. I nform Ho me A gen t
Figure 6: Stages in Inter-tech handoff.
Figure 7 shows the HO mechanism. The overall block
diagram shows the mechanism. The most important item
is the algorithm. We have studied Fuzzy Logic (FL) and
Neural Network (NN) class of algorithms [13,14].
Figure 7: Handoff mechanism.
HO algorithm (Figure 7) gives one of the three possible
action outputs: A, B and C. For a mobile host working
for the underlay network (WLAN) the output A (Relax)
indicates that the link quality of the underlay network is
satisfactory and there is no handoff imminent. Output B
Working in WLAN Working in GPRS
Prepare Ready
for
GPRS
Prepare
Handover
to GPRS
Near
WLAN?
Prepare
Wait
N(sec) Wait
M<N(sec)
Handover
to WLAN
ABC
Yes
Yes
No
No
CB A
HO Algorithm
HO Algorithm
Measurements
Monitoring
Stable
WLAN Unstable
WLAN Poor
WLAN
A B C
Algorithm
(Alert) indicates that there is an increased possibility of
handoff in the near future and gives the system a chance
to prepare for a handoff procedure. The C (Handoff)
output indicates that a handoff is needed and the handoff
procedure to the overlay network (GPRS) is invoked. For
simplicity reasons, this diagram does not show the
possibility of failure, as there was an assumption made
that the overlay network is always available.
A mobile host working for the overlay network (GPRS) is
regularly checking if it is close to the underlay network
(WLAN). The optimum would be to compare
geographical coordinates first. Whatever is the resolution
of the checking, if MH realizes that the underlay network
could be available, it regularly checks for the presence of
WLAN pilot and invokes the fuzzy or neural net engine to
evaluate the quality of the connection. The output of this
engine is the same as in the previous case. The difference
in action from the previous case is that if the connection
indicator is unstable (B), no handoff preparations are
made. MH waits till the link is stable (A) and then a
handoff to the underlay network is performed. Action
output C indicates that no handoff should be performed.
The difference between these is given by the fact that a
user working for the overlay network does not have to be
worried about loosing the connection. Intra-overlay
handoff (within GPRS) is guaranteed by the lower GPRS
layers. Therefore, the user working for the overlay layer
just occasionally checks for the availability of the
underlay network hopefully aided by the location
information to avoid needless search if the MH is, e.g., in
a different country. The finer the location information
becomes the more a useless search for the underlay
network can be avoided. If the location information can
be given by geographical coordinates, MH can store the
underlay network coordinates, e.g., at the time when it left
it last time and start searching for it as soon as it
approaches this area. Also a user-controlled handoff
management is possible, i.e., the user clicks a button in
the screen of laptop when he arrives to his office
environment and otherwise the mobility management
works automatically.
The WLAN to GPRS handoff triggering algorithm is
more crucial for the reliable operation of the system, since
a MH moving away from the underlay network coverage
may suddenly experience a severe degradation of the
service and has to handoff very fast to maintain the higher
layer connection. To facilitate features, such as power
saving by powering down unused interface cards, loading
of interface drivers on demand, the proposed algorithm
allows for an alert, or possibly several levels of alert,
which would enable the system to prepare for an
upcoming handoff. For these reasons the triggering
algorithm is decomposed into two parts, overlay (WLAN)
and underlay (GPRS). To enable a reuse of the code, both
parts use the same HO engine; just the frequency of its
invocation and acting on its output is different.
4. METRICS
There are several handoff metrics that are being employed
in various voice and data mobile networks. Clearly, the
received signal strength is the most popular handoff
metric. However, using only the RSS need not be
optimum as discussed in [13]. A combination of RSS and
some parameters like hysteresis margin, traffic, velocity
have been used with benefits and disadvantages in a
variety of handoff algorithms. We are primarily
concerned with handoff between heterogeneous systems
such as GPRS and IEEE 802.11 networks. A priority type
handoff is required and the RSS is only one metric that
may be employed for handoff decision. Handoff metrics
are closely related to the type of algorithm that may be
ultimately employed to make a handoff decision. We
suggest studying a broad class of handoff metrics
including the RSS, beacon packets, SNR (or noise
indicator), BER, and packet error rates (or
retransmissions) with parameters such as hysteresis
margin and traffic for our project. A second reason for
suggesting a broad class of metrics is the fact that the
algorithms that we are studying, based on neural networks
and fuzzy logic systems can make highly sensitive
decisions based on a large number of inputs. The decision
is made on a pattern match between inputs and previously
observed situations. Consequently, the pattern match may
be made better by employing particular subsets of handoff
metrics as the input. Since the handoff that is being
considered is for data networks, the sensitivity of
performance to the metrics and algorithms will also have
to be investigated. Future work is to concentrate on
investigating the performance and sensitivity of NN and
FL algorithms to different HO metrics.
5. SUMMARY
In this paper we have discussed about procedures,
algorithms and metrics involved in handoff in
heterogeneous wireless networks. A general level
procedure was given for inter-technology roaming
between WLAN and GPRS. However, this procedure can
be applied with any two wireless systems, i.e., WLAN
and UMTS. The main objective of the Wireless LANs for
UMTS (WiLU) project was to explore wideband
technologies for UMTS and develop a network
demonstrator for that purpose. The particular emphasis
was on incorporating WLAN technology into the UMTS.
Since the wideband wireless services will not have a
comprehensive coverage we envision that WLANs should
be embedded into wide area wireless data networks. In
another words, when a wideband wireless data service is
available it will be used by the terminal and when it is not
available the terminal should seamlessly switch to a lower
speed wireless data service offered by UMTS. The focus
for implementation was to incorporate new triggers,
entities or protocols that operate at the management layer,
network or higher layers to enable inter-tech roaming that
will be transparent to the mobile user to the extent
possible. A general level procedure was shown and
potential metcis was discussed for handoff in a
heterogeneous environment.
ACKNOWLEDGMENTS
The acknowledgments are due to the project team of
WiLU (Wireless LAN for UMTS) at the CWC (Centre for
Wireless Communications) in University of Oulu, Finland
and at CWINS (Center for Wireless Information Network
Studies) in Worchester Polytechnic Institute, USA. The
authors would like to thank TEKES, Nokia and Sonera
Ltd. for supporting financially this project.
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