Signal threshold adaptation for vertical handoff in heterogeneous wireless networks.

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Signal Threshold Adaptation for Vertical
Handoff in Heterogeneous Wireless Networks
Ahmed H. Zahran
Department of Electrical and Computer Engineering
University of Toronto
10 King’s College Road
Toronto, Ontario, M5S 3G4, Canada
Email: zahran@comm.utoronto.ca
Tel: 1-416-978-4829, Fax: 1-416-978-4425
Ben Liang
Department of Electrical and Computer Engineering
University of Toronto
10 King’s College Road
Toronto, Ontario, M5S 3G4, Canada
Email: liang@comm.utoronto.ca
Tel: 1-416-946-8614, Fax: 1-416-978-4425
Aladin Saleh
Bell Canada/ Wireless Technology
Floor 5N - 5099 Creekbank Road
Mississauga, Ontario, L4W 5N2, Canada
Email: aladdin.saleh@bell.ca
Tel: 1-905-282-3264, Fax: 1-905-282-3337
*1
1To appear in ACM/Springer Mobile Networks and Applications (MONET) journal, this article
is the extended version of a paper presented in IFIP Networking 2005.
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Abstract
Theconvergenceofheterogeneouswirelessaccesstechnologieshasbeen
envisioned to characterize the next generation wireless networks. In such
converged systems, the seamless and efficient handoff between different
access technologies (vertical handoff) is essential and remains a challeng-
ing problem. The heterogeneous co-existence of access technologies with
largely different characteristics results in handoff asymmetry that differs
from the traditional intra-network handoff (horizontal handoff) problem. In
the case where one network is preferred, the vertical handoff decision should
be carefully executed, based on the wireless channel state, network layer
characteristics, as well as application requirements. In this paper, we study
the performance of vertical handoff using the integration of 3G cellular and
wireless local area networks as an example. In particular, we investigate
the effect of an application-based signal strength threshold on an adaptive
preferred-network lifetime-based handoff strategy, in terms of the signalling
load, available bandwidth, and packet delay for an inter-network roaming
mobile. We present an analytical framework to evaluate the converged sys-
tem performance, which is validated by computer simulation. We show how
the proposed analytical model can be used to provide design guidelines for
the optimization of vertical handoff in the next generation integrated wire-
less networks.
Keywords: heterogeneous wireless networks, seamless integration, vertical
handoff, application signal strength threshold, 3G cellular, wireless LAN
1Introduction
Wireless technologies are evolving toward broadband information access across
multiple networking platforms, in order to provide ubiquitous availability of mul-
timedia applications. Recent trends indicate that wide-area cellular networks
based on the 3G standards and wireless Local Area Networks (WLANs) will co-
exist to offer multimedia services to end users. These two wireless access tech-
nologies have characteristics that perfectly complement each other. By strategi-
cally combining these technologies, a converged system can provide both univer-
sal coverage and broadband access. Therefore, the integration of heterogeneous
networks is expected to become a main focus in the development toward the next
generation wireless networks [1–3].
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MO
MI
AP
Horizontal Handoff
Vertical Handoff
AP
AP
Base Station
Access Point
BS
BS
Figure 1: Mobile handoff in heterogeneous wireless system
Mobility management is a main challenge in the converged network. It ad-
dresses two main problems: location management and handoff management [4,5].
Location management tracks the Mobile Terminals (MT) for successful informa-
tion delivery. For this purpose, Mobile IP (MIP) enables seamless roaming and
is expected to be the main engine for location management in the next generation
networks. Handoffmanagementmaintainstheactiveconnectionsforroamingmo-
bile terminals as they change their point of attachment to the network. Handoff
management is the main concern of this paper.
In the converged network, both intra-technology handoff and inter-technology
handoff take place as illustrated in Figure 1. Intra-technology handoff is the tradi-
tional Horizontal Handoff (HHO) process in which the mobile terminal hands-off
between two Access Points (AP) or Base Stations (BS) using the same access
technology. On the other hand, inter-technology handoff, or Vertical Handoff
(VHO), occurs when the MT roams between different access technologies. The
main distinction between VHO and HHO is symmetry. While HHO is a sym-
metric process, VHO is an asymmetric process in which the MT moves between
two different networks with different characteristics. This introduces the concept
of a preferred network, which is usually the underlay WLAN that provides better
throughput performance at lower cost, even if both networks are available and in
good condition for the user.
There are two main scenarios in VHO: moving out of the preferred network
(MO) and moving into the preferred network (MI) [6]. In the converged model,
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it is highly desirable to associate the MT with the preferred network, as long as
the preferred network satisfies the user application. This can improve the resource
utilization of both access networks, as well as improving the user perceived QoS.
Furthermore, this handoff should be seamless with minimum user intervention,
while dynamically adapting to the wireless channel state, network layer charac-
teristics, and application requirements.
In this work, we present an adaptive lifetime-based VHO (ALIVE-HO) algo-
rithm which takes into consideration the wireless signal strength, handoff latency,
and application QoS and delay tolerance. It can satisfy the system handoff sig-
nalling load, as well as different application requirements by the tuning of an
application-based signal strength threshold (ASST). We further propose an ana-
lytical model to evaluate the performance of adaptive VHO. This analytical frame-
work is then applied to show how the VHO decision and the ASST choice can be
optimized based on multiple conflicting criteria including vertical handoff signal-
ing, user available bandwidth, and encountered packet delay. Hence, the optimal
ASST value is determined for different QoS requirements.
The rest of this paper is organized as follows. Section 2 provides an overview
for the handoff algorithms in wireless heterogeneous networks and the related
literature work. In Section 3, we present a vertical handoff algorithm that incor-
porates cross-layer adaptation to terminal mobility, channel state, and application
demand. In Section 4, we propose an analytical framework to study the effect
of cross-layer adaptation. Numerical and simulation results are provided in Sec-
tion 5, where we show how the ASST can be tuned to optimize VHO decision.
Concluding remarks are presented in Section 6.
2 Related Work
The traditional HHO problem has been studied extensively in the past. Several
approaches have been considered in cellular networks using the Received Sig-
nal Strength (RSS) as an indicator for service availability from a certain point of
attachment. Additionally, several handoff initiation strategies have been defined
based on the comparison between the current attachment point RSS and that of
the candidate attachment points as shown in [7]:
• RSS: handoff takes place if the candidate attachment point RSS is higher
than the current attachment point RSS (RSSnew> RSScur).
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• RSS plus threshold: handoff takes place if the candidate attachment point
RSS is higher than the current attachment point RSS and the current attach-
ment point RSS is less than a pre-defined threshold T (RSSnew> RSScur
and RSScur< T).
• RSS plus hysteresis: handoff takes place if the candidate attachment point
RSS is higher than the current attachment point RSS with a pre-defined
hysteresis margin H. (RSSnew> RSScur+ H).
• A dwell timer can be added to any of the above algorithms. In this case,
the timer is started when one of the above conditions is satisfied, and the
MT performs a handoff if the condition is satisfied for the entire dwell timer
interval.
In VHO, the RSSs are incomparable due to VHO’s asymmetrical nature. How-
ever, they can be used to determine the availability as well as the condition of
different networks. If the MI decision is based only on the preferred network
availability, the MT should start the MI process as it discovers the WLAN. In ad-
dition, if more than one WLAN APs are available, the MT should associate itself
with the one having the strongest RSS as it does in HHO2. When the MT is asso-
ciated with the preferred network, it enjoys all the preferred network advantages
before moving out. Therefore, in the ideal MO scenario, the MT performs no
more than one handoff at the WLAN edge when the network is expected to be
unavailable. This ideal MO decision usually cannot be achieved. Thus, the main
design requirements of a VHO algorithm are
• minimizing the number of unnecessary handoffs to avoid overloading the
network with signaling traffic,
• maximizing the underlay network utilization,
• providing active application with the required degree of QoS,
• prioritizing handoff to the underlay network over MO to the overlay net-
work,
• avoiding MI to a congested network, and
2If other criteria such as available bandwidth are considered, the MT may not move instanta-
neously to a WLAN, but may consider other factors such as QoS, user preference, cost, and power
consumption.
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