12
Two-phase combined QoS-based
handoff scheme
Wireless Personal Communication Services (PCS) and broadband networking for deliver-
ing multimedia information represent two well-established trends in telecommunications.
While technologies for PCS and broadband communications have historically been devel-
oped independently, harmonization into a single architectural framework is motivated by
an emerging need to extend multimedia services to portable terminals. With the grow-
ing acceptance of Asynchronous Transfer Mode (ATM) as the standard for broadband
networking, it has become appropriate to consider the feasibility of standard ATM ser-
vices into next-generation microcellular wireless and PCS scenarios. The use of ATM
protocols in both fixed and wireless networks promises the important benefit of seam-
less multimedia services with end-to-end Quality-of-Service (QoS) control. The wireless
ATM (WATM) specification provides an option to existing ATM networks that wish to
support terminal mobility and radio access while still retaining backward compatibility
with ATM equipments.
The current developments on WATM are mainly based on ATM as the backbone
network with a wireless last-hop extension to the mobile host. Mobility functions are
implemented into the ATM switches and the Base Stations (BSs). WATM helps to bring
multimedia to mobile computers. Compared with the wireless LANs, which have a lim-
itation of bandwidth to support multimedia traffic and slow handoff, the bandwidth of
existing mobile phone systems is sufficient for data and voice, but it is still insufficient for
real-time multimedia traffic. ATM has more efficient networking technology for integrating
services, flexible bandwidth allocation, and service type selection for a range of applica-
tions. The current interest and research efforts are intense enough to claim that WATM
will continue to be pursued as a research and development topic in the next few years.
There are two major components in WATM networks:
1. A radio access layer providing high-bandwidth wireless transmission with appropriate
Medium Access Control (MAC), Data Link Control (DLC), and so on.
Mobile Telecommunications Protocols For Data Networks. Anna Ha
´

phase interswitch handoff scheme for WATM networks. We use path extension for each
interswitch handoff, and invoke path optimization when the handoff path exceeds the
delay constraint or maximum path extension hops constraint. We study three types of
path optimization schemes: combined QoS-based, delay-based and hop-based path rerout-
ing schemes.
We use QoS combined path optimization scheme for WATM network. We focus on the
problems related to the support of mobility in the WATM network. This scheme determines
when to trigger path optimization for the two-phase handoff and how to minimize the
service disruption during path optimization.
12.1 WIRELESS ATM ARCHITECTURE
A WATM network is intended to support integrated broadband services to MTs through an
ATM User Network Interface (UNI). Figure 12.1 shows a network diagram that illustrates
various network entities and the functions that are required to support mobility in such
an ATM network.
In this architecture, the MT is an ATM end system that can support multimedia appli-
cations. The wireless link between the MT and BS provides the desired ATM transport
services to the MT. A mobility-enhanced signaling protocol based on the ITU recommen-
dation Q.2931 is used by the MT, BS, and Mobility Support Switches (MSS) to support
handoff-related functions.
WIRELESS ATM ARCHITECTURE
215
Wireless control
Wireless control
Wireless control
Mobile
support
ATM switch
Mobile
support
ATM switch

sonably high data rates. A MAC protocol in WATM is needed to meet the following
requirements:
• It should be able to work with the upper-layer protocol seamlessly.
• The MAC layer should be designed to use bandwidth efficiently to accommodate a
reasonably large number of users.
• The MAC protocol should guarantee a certain QoS to the user for various services,
such as Constant Bit Rate (CBR), Variable Bit Rate (VBR), Available Bit Rate (ABR),
and Unspecified Bit Rate (UBR).
216
TWO-PHASE COMBINED QoS-BASED HANDOFF SCHEME
Wireline ATM protocol stack Wireless ATM protocol stack
User plane
(transport)
Control plane
(signaling)
ATM adaptation layer
ATM layer
Physical layer
User
plane
Control plane
ATM adaptation layer
ATM layer
Data link control
Medium access control
Wireless physical layer
Radio
access
control
Mobility control

B Connection oriented, VBR-RT, needs to transmit timing information
over the ATM cells, e.g., multimedia service with VBR video
and audio.
C Connection oriented, VBR-NRT, ABR, UBR, does not need to
transmit timing information over the ATM cells, e.g., traditional
data traffic such as X.25.
D Connectionless, VBR-NRT, ABR, UBR, does not need to transmit
timing information over the ATM cells, e.g., e-mail service.
The mobility control sublayer immediately above the MAC layer performs control
functions related to the physical radio channel control and metasignaling between the MT
and BSs (e.g., terminal initialization, handoff, and power control).
12.2 MOBILITY SUPPORT IN WIRELESS ATM
A key feature of any wireless network is the capability to support handoff. Handoff is an
action of switching a call in progress in order to maintain continuity and the required QoS
of the call when a MT moves from one cell to another. In a mobile ATM network, an MT
can have several active links with different QoS requirements. These Virtual Channels
(VCs) with different QoS introduce challenges to the handoff protocol. In general, the
handoff with multirate ATM connections must be supported with low cell loss, latency,
and control overhead. The QoS constraints for each individual connection should be
maintained during the MT migration.
There are several types of handoff. We can classify the types of handoff on the basis of
the number of active connections and the direction of the handoff signaling. We describe
these types of handoff as follows:
On the basis of the number of active connections
The handoffs can be classified on the basis of the number of connections that an MT
maintains during the handoff procedure. There are two types of handoffs based on this
classification: hard handoff and soft handoff.
In hard handoff, the MT switches the communication from the old link to the new
link. Thus, there is only one active connection from the MT at any time. There is a short
interruption in the transmission. This interruption should be minimized in order to make

station
ATM switch
Base
station
Base
station
D
D
S − source
D − destination
Path before handoff
Path after handoff
Figure 12.3 Handoff using full reestablishment.
MOBILITY SUPPORT IN WIRELESS ATM
219
Handoff using multicasting
Multicasting is used to support handoffs in both the connection oriented and connectionless
scenarios. In the case of WATM environment, multicasting is used to establish links to
all BSs that are neighboring the BS that is currently controlling a MT. Subsequently, in
whichever direction the MT moves, a handoff path has already been established. Also,
since the data is being multicast, it continues to flow without any interruption. This scheme
ensures a lossless and seamless handoff. However, since data is being multicast to the
entire set of nodes, most of which is unused, bandwidth is being utilized very inefficiently.
Also, if an MT is at the edge of two cells, it is very likely that it might get two copies
of the data packets. This leads to other complications like BS synchronization. Thus, this
scheme is not recommended. Figure 12.4 shows the handoff using multicasting.
Handoff using connection extension
The basic idea of this scheme is that the local paths are more affordable than the global
paths. When an MT migrates from one BS to another, the old BS extends the connection to
the new BS. The obvious disadvantage of this method is that the new path to the MT is not

station
Base
station
D
D
S − source
D − destination
Path before handoff
Path after handoff
Figure 12.5 Handoff using connection extension.
S
Base
station
ATM switch
Base
station
Base
station
D
D
S − source
D − destination
Path before handoff
Path after handoff
Figure 12.6 Handoff using partial reestablishment.
connection, by opening a connection to the COS. This way, it attempts to reuse as much
of the existing connection as possible. The old partial path is then torn down and the
resources are released. Buffering is done at the COS, which ensures in-order delivery of
the cells. Figure 12.6 shows the handoff using partial reestablishment.
Handoff using two-phase protocol