WIRELESS ATM SWITCHES
348
and part of it is replaced with a new subpath for continuous support of the ongoing call. Buffering is also necessary for in-sequence and loss-free deliv-
ery of the ATM cell containing user data in order to guarantee the QoS on the connection.
12.4.1 Connection Rerouting
In a cell-structured WATM system, the radio coverage area of a base station is limited to its cell size. A mobile terminal cannot communicate with a base
station if the terminal is beyond its coverage area. To provide seamless service to a mobile user crossing a cell boundary, handoff is needed. When
the radio link quality between a mobile terminal and its current base station deteriorates, the mobile terminal has to terminate communication with the
base station and communicate with an adjacent base station through a new radio link, which is established by a handoff procedure.
In a fixed ATM network, handoff means connection rerouting during a call. As a result of handoff, the original connection path between two end
points is broken, and part of it is replaced with a new subpath for continuous support of the ongoing call.
Figure 12.5 shows a typical handoff procedure in a WATM system. Assume that the mobile terminal communicates with the other end party
through BS1. When the mobile terminal moves away from BS1 and goes into the cell of BS2, handoff occurs. That is, the mobile terminal switches its
access point from BS1 to BS2 and uses the newly established radio link between the mobile and BS2. In the fixed ATM network of the figure, as a
result of handoff, the path between SW3 and BS1 is released, and the path between SW3 and BS2 is added to the original connection path.
Ž .
Virtual channel VC routes need to be continually modified whenever a mobile terminal switches its network access point during the lifetime of
a connection. One possible rerouting scheme is to rebuild a total end-to-end VC
whenever handoff occurs. This introduces large handoff latency. Handoff should be done locally without involving the other end party to reduce this
latency.
2
Many connection rerouting methods for handoff have been proposed in w
x the literature 6, 17, 27, 28, 29, 30, 31, 32 . Most of them rely on partial con-
nection rerouting. w
x In 17 , the virtual connection tree concept was proposed. When a mobile
terminal establishes a connection to the ATM networks, a connection tree is Ž
. formed from a root an ATM switch to a set of base stations to which a
mobile terminal may move. Whenever the mobile terminal moves into one of
2
Handoff should be performed quickly, since it usually introduces temporary interruption of communication. Due to this service interruption, the QoS may degenerate below an acceptable
level.
HANDOFF IN WIRELESS ATM
349
Fig. 12.5
Handoff in a wireless ATM system.
those base stations, it uses one of a set of preestablished virtual channels from the root node to the corresponding base station. The goal of a virtual
connection tree is to reduce handoff latency by removing connection setup time for the new connection subpath. However, this method uses preestab-
lished multiple virtual channels, even though only one of them is used at a
Ž time. This requires more network resources VC space, bandwidth, and
. memory for routing information than other schemes.
Ž .
The path or VC extension scheme extends an original connection path and adds a new connection subpath from an old base station to a new base
w x
station that a mobile terminal is going to visit 28, 29 . This method is simple, and makes it easy to maintain the ATM cell sequence during handoff.
However, the extended connection path will increase end-to-end latency, and if base stations and ATM switches have a hierarchical structure as in Figure
12.5, this scheme will produce poor routing efficiency. In addition, a base station needs an ATM switching function to forward ATM cells between base
stations.
Ž .
w x
In the path or VC rerouting scheme of 28, 29 , an original connection path between the mobile’s peer party and an old base station is torn down
WIRELESS ATM SWITCHES
350
and part of it is replaced with a new connection segment between a new base station and an intermediate node on the original connection path during
handoff. The intermediate node from which a new connection segment is Ž
. w x
3
Ž established is known as a crossover switch COS
27 . In the handoff
. example of Figure 12.5, SW3 is used as a COS. The important issue related
to WATM handoff has been the discovery of a COS in the network. The location of a COS is determined by considering the tradeoff between handoff
latency and routing efficiency after the handoff.
w x
In a handoff procedure, a COS can be fixed 6, 17 or can be determined w
x dynamically 27, 30, 31 . The algorithms for selection of a COS determine the
tradeoff between end-to-end routing efficiency and handoff latency. For an optimal end-to-end route, a complicated algorithm is needed for selection of
w x
a COS. This will increase handoff latency 27 . A fast but less optimal scheme is to select a common ancestor node of two handoff-related base stations as a
w x
COS 30, 31 . Although all rerouting schemes have their pros and cons as described
above, they are based on the following three basic steps:
䢇
Ž Select a crossover switch. This is omitted in the path extension scheme
. and the fixed COS scheme.
䢇
Ž Set up a new subpath between a COS and a new base station. If the
. preestablished-multiple-VC scheme is used, this step is unnecessary.
䢇
Release an old subpath between a COS and an old base station.
12.4.2 Buffering
