10
Handoff in mobile
and wireless networks
Wireless data services use small-coverage high-bandwidth data networks such as IEEE
802.11 whenever they are available and switch to an overlay service such as the Gen-
eral Packet Radio Service (GPRS) network with low bandwidth when the coverage of a
Wireless Local Area Network (WLAN) is not available.
From the service point of view, Asynchronous Transfer Mode (ATM) combines both
the data and multimedia information into the wired networks while scaling well from
backbones to the customer premises networks. In Wireless ATM (WATM) networks, end
user devices are connected to switches via wired or wireless channels. The switch is
responsible for establishing connections with the fixed infrastructure network component,
either through a wired or a wireless channel. A mobile end user establishes a Virtual
Circuit (VC) to communicate with another end user (either mobile or ATM end user).
When the mobile end user moves from one Access Point (AP) to another AP, a handoff
is required. To minimize the interruption of cell transport, an efficient switching of the
active VCs from the old data path to the new data path is needed. Also, the switching
should be fast enough to make the new VCs available to the mobile users.
When the handoff occurs, the current QoS may not be supported by the new data path.
In this case, a negotiation is required to set up new QoS. Since a mobile user may be in
the access range of several APs, it will select the AP that provides the best QoS.
During the handoff, an old path is released and then a new path is established. For
the mobility feature of a mobile ATM, routing of signaling is slightly different from that
of the wired ATM network. First, mapping of Mobile Terminal (MT) routing identifiers
to paths in the network is necessary. Also, rerouting is needed to reestablish connection
when the mobiles move around. It is one of the most important challenges to reroute
ongoing connections to/from mobile users as those users move among Base Stations
(BSs). Connection rerouting schemes must exhibit low handoff latency, maintain efficient
routes, and limit disruption to continuous media traffic while minimizing reroute updates
to the network switches.
Mobile Telecommunications Protocols For Data Networks. Anna Ha

nection from the old to the new BS in the form of a chain. Chaining results in increased
end-to-end delay and less efficient routing of the connection.
Chaining, followed by the make-break scheme, which involved a real-time handoff
using the chaining scheme and, if necessary, a non-real-time rerouting using the make-
break scheme, shows good performance in connection rerouting, because the separation
of the real-time nature of handoffs and efficient route identification in this scheme allows
it to perform handoffs quickly, and, at the same time, maintains efficient routes in the
fixed part of the network.
The main development in shaping up the future high-speed (gigabit) networking is
the emergence of Broadband ISDN (B-ISDN) and ATM. With its cell switching and
the support of Virtual Path (VP) and Virtual Circuit (VC), ATM can provide a wide
variety of traffic and diverse services, including real-time multimedia (data, voice, and
video) applications. Because of its efficiency and flexibility, ATM is considered the most
promising transfer technique for the implementation of B-ISDN, and for the future of
high-speed wide and local area networks.
Handoff is important in any mobile network because of the default cellular architecture
employed to maximize spectrum utilization. When a Mobile Terminal moves away from
a BS, the signal level degrades, and there is a need to switch communications to another
BS. Handoff is the mechanism by which an ongoing connection between an MT or host
(MH) and a correspondent terminal or host (CH) is transferred from one point of access to
the fixed network, and to another. In cellular voice telephony and mobile data networks,
such points of attachment are referred to as base stations and in WLANs they are called
access points. In either case, such a point of attachment serves a coverage area called
HANDOFF IN MOBILE AND WIRELESS NETWORKS
183
a cell. Handoff, in the case of cellular telephony, involves the transfer of voice call
from one BS to another. In the case of WLANs, it involves transferring the connection
from one AP to another. In hybrid networks, it will involve the transfer of a connection
from one BS to another, from an AP to another, between a BS and an AP, or vice versa.
WATM networks are typically inter-networked with a wired network (an ATM net-

CSU
Figure 10.1 Configuration of WATM network.
184
HANDOFF IN MOBILE AND WIRELESS NETWORKS
Performance measures such as call blocking and call dropping are applicable only to
real-time traffic and may not be suitable for the bursty traffic that exists in client-server
applications. When a voice call is in progress, allowed latency is very limited, resource
allocation has to be guaranteed, and, while occasionally some packets may be dropped and
moderate error rates are permissible, retransmissions are not possible, and connectivity has
to be maintained continuously. On the other hand, bursty data traffic by definition needs
only intermittent connectivity, and it can tolerate greater latencies and employ retransmis-
sion of lost packets. In such networks, handoff is warranted only when the terminal moves
out of coverage of the current point of attachment, or the traffic load is so high that a
handoff may result in greater throughput and utilization.
10.1 SIGNALING HANDOFF PROTOCOL IN WATM
NETWORKS
Signaling is a problem area in WATM networks. Apart from the conventional signal-
ing solutions encountered in wired networks, additional signaling is needed to cover the
mobility requirements of terminals. Wired ATM networks, which are enjoying commer-
cial growth, do not support mobility of user terminal equipment. A possible solution
to this problem is the integration of the required mobility extensions with the standard
signaling protocols.
Protocol stacks in WATM are shown in Figure 10.2. This protocol includes mobility
function for handoff. In Figure 10.2, we have the following components:
• MMC : Mobility Management and Control
• RRM : Radio Resource Manager
• SAAL: Signaling ATM Adaptation Layer
• CCS : Call Control and Signaling
• UNI : User-Network Interface
U-plane

• M-channels: PVCs intended for mobility signaling
• U-plane:Userplane.
In Figure 10.2, the standard signaling is left unaffected. To support mobility functions,
the only modifications added to the existing infrastructure are the new interfaces with
the controlling entities of standard signaling (i.e., CCS, resource manager). In terms of
module-entity instances, there is a one-to-one mapping between the RRM and the BSs
(each BS has an RRM instance). There is also a one-to-one relationship between active
MTs and MMC instances residing within the MMC in the CCS entity. In each MT, only
one MMC and one CCS instance are needed.
The CS
MMC module is responsible for handling all mobility-related procedures (i.e.,
handover, registration, and location update) on the network side. Specifically, the CS
MMC
deals with the following tasks:
• the establishment of the M-channel through which the mobility-related messages are
exchanged;
• the coordination of wireless and fixed resources, during the execution of mobility and
standard signaling procedures;
• the switching of signaling and data connections whenever an MT crosses the boundaries
of a cell;
• the updating of the location of an MT in the CSU-hosted DB.
The basic steps involved in handoff occur in an application scenario, involving Mobile
Multi-User Platforms (MMUPs) equipped with (onboard) private ATM networks.
Connection handoff is the procedure of rerouting an existing connection from the
previous AP to the next when a mobile moves across a cell boundary. Success rate of
handoffs and their smooth completion are crucial to providing satisfactory quality of
service to mobile users. A handoff is successful if the connection is reestablished with
the MMUP in the new cell. A handoff is smooth if the connection suffers no or minimum
perceivable disruption during the transfer. Smoothness of handoffs depends on the number
of connections requiring handoff, and the time between initiation of a handoff and loss of

• S1, S2, S3, and S4, which are the cellular network switches
• COS, which is the crossover switch
• AP1, and AP2, which are the access points.
The selection of a particular switch as a COS for a connection depends on several
factors, including
• switch capability
• selection policy