11
Signaling traffic in wireless
ATM networks
Handoff algorithms in terrestrial wireless networks focus on the connection rerouting
problem. Basically, there are three connection rerouting approaches: full connection estab-
lishment, partial connection reestablishment, and multicast connection reestablishment.
Full connection establishment algorithms calculate a new optimum route for the call as
for a new call request. The resulting route is always optimal; however, the call rerout-
ing delay and the signaling overheads are high. To alleviate these problems, a partial
connection reestablishment algorithm reestablishes certain parts of the connection route
while preserving the remaining route. This way the route update process involves only
local changes in the route and can be performed faster. However, the resulting route may
not be optimal. In the multicast connection reestablishment algorithm, a Virtual Con-
nection Tree (VCT) is created during the initial call admission process. The root of the
tree is a fixed switching node, while the leaves are the switching centers to serve the
user terminal in the future. By using the multicast connection reestablishment method,
when a call moves to a cell with a new switching center, connection rerouting is done
immediately owing to the already established routes. The disadvantage of this algorithm
is that network resources can be underutilized as a result of resources allocated in the
connection tree.
We define the Chain Routing Algorithm and implement it as a partial connection
reestablishment in the handoff scheme. This process is done during chain elongation.
This handoff scheme can be used in the Wireless ATM (WATM) model.
11.1 A MODEL OF WATM NETWORK
AgraphG(V , E) represents the topology of a WATM network. Graph G consists of two
sets: a finite set V of vertices and a finite set E of edges. Graph G is represented by two
Mobile Telecommunications Protocols For Data Networks. Anna Ha
´
c
Copyright


• Each edge e
i
in G
1
has a limited capacity to carry a number of calls; each of these calls
occupies one unit. The edges between ATM switching centers represent communication
channels. The number of links is the same in each channel. The capacity of each edge
is defined as C
1
(e
i
).
• The subtopology of the BSs and their related ATM switching centers are represented by
an undirected subgraph G
2
= (V
2
,E
2
), where each edge e ∈ E
2
represents the number
of channels between a base station and a switching center that are directly connected,
and each node in V
2
represents a base station connected to the ATM switching center.
G
2
⊂ G, V
2

2
). This call request
consists of six elements: s
1
and s
2
are the source ATM switching center and the source
BS, respectively; d
1
and d
2
are the destination switching center and the destination
BS, respectively; and h
1
and h
2
are the handoff switching center and the handoff BS,
respectively.
• When a call request is a general call without handoff, the call request is denoted by
r
i
= (s
1
,s
2
,d
1
,d
2
) and the handoff request options are h

) for all i,1≤ i ≤ number of links in a base station.
• For each edge e
i
, which is among switching centers, the total number of channels
allocated for a set of call requests R
1
(r
1
,r
2
,...,r
n
) that arrived in the switching center
from the BS cannot exceed the capacity of the edge between ATM switching centers.
That is, R
1
(r
i
) ≤= C
1
(e
i
) for all i,1≤ i ≤ number of links between a switching
center and its BSs.
• Let Idle r
1
(e
i
) denote the available number of channels e
i

i
)<R
2
(r
i
)).
• Any mobile host can access the network directly via a radio link to a base station that
is virtually connected.
CHAIN ROUTING ALGORITHM
199
11.2 CHAIN ROUTING ALGORITHM
Handoff procedures involve a set of protocols to notify all the related entities of a par-
ticular connection for which a handoff has been executed, and the connection has to be
redefined. During the process, conventional signaling and additional signaling for mobil-
ity requirements are needed. The mobile user is usually registered with a particular point
of attachment. In the voice networks, an idle mobile user selects a base station that is
serving the cell in which it is located. This is for the purpose of routing incoming data
packets or voice calls. When the mobile user moves and executes a handoff from one
point of attachment to another, the old serving point of attachment has to be informed
about the change. This is called dissociation. The mobile user will also have to reassociate
itself with the new point of access to the fixed network. Other network entities involve
routing data packets to the mobile user and switching voice calls that have to be aware
of the handoff in order to seamlessly continue the ongoing connection or call. Depending
on whether a new connection is created before breaking the old connection, handoffs are
classified into hard and seamless handoffs.
The Chaining scheme extends the connection route from the previous BS to the new
BS by provisioning some bandwidth using Virtual Channel (VC) or Virtual Path (VP)
reservations between neighboring BSs. Chaining can simplify the protocols and reduce
signaling traffic significantly and it can be accomplished quickly. However, chaining will
typically degrade the end-to-end performance of the connection and the connection route

connections by continuously elongating paths from original cells to the new cells will
cause the path to be inefficient.
When some parts of the route have a high occupancy rate, we propose two ways to
reroute the chain parts of the route:
From the last station on the chain after each elongation, we propose sending one bit
back through the chain and checking the ORP of each BS on the chain.
The path will be rerouted according to one of the following two schemes:
1. Select a route in which the length of the path is the shortest. If length of the route is
shorter, it is more likely to be selected.
2. Select the path in which the PVCs have lower occupancy rate. That is, a PVC between
an ATM switch and any BS in the elongation route can be set up in order to obtain
a low ORP. The number of options that are available is N,whereN is equal to the
length of the chain.
The chain has to be rerouted whenever there is a better chain route, and the speed of
elongation will be slowed down. The network efficiency can be improved significantly.
The path can be rerouted following the first scheme.
From the last station on the chain after each elongation, we send one bit back through
the chain and check the ORP of each BS on the chain. If the resultant ORP of a base
station is close to jam, we stop, move back one BS, and use this BS’s PVC to connect to
the ATM switch. If the BS at the end of the chain has a very high ORP or it is jammed,
we have to send a signal to the connection server to reroute the call.
If the speed of elongation is high, the signaling and calculation cost is reduced, and
the network efficiency is lower than in the Chain Routing.
We illustrate how the Chain Routing Algorithm operates by using an example. Referring
to Figure 11.1, let the BS in Cell 1 be denoted as BS
1
. When a mobile initiates a new call
in Cell 1, BS
1
will establish an SVC between itself and the ATM switch. We consider

1
2
3
4
5
6
BC
ATM switch ATM switch
Figure 11.2 Move with more than one ATM switch involved.
202
SIGNALING TRAFFIC IN WIRELESS ATM NETWORKS
the sent-back bit detects that it has arrived at the ATM switch in which the chain started,
it will begin applying the Chain Routing Algorithm.
Suppose the mobile roams into Cell 6 and one bit is sent back through the chain route.
When the sent-back bit sees the CASL, the Chain Routing Algorithm will be used.
In this case, we have three route options:
1. Cell6–Cell5–Cell4–Cell3–LinkC–ATMswitch.
2. Cell6–Cell5–Cell4–Cell3–Cell2–LinkB–ATMswitch
3. Cell6–Cell5–Cell4–Cell3–Cell2–Cell1–LinkA–ATMswitch.
If the BS of Cell 2 has a high ORP, for example, a new route 1 will be set up.
11.3 IMPLEMENTATION OF THE HANDOFF SCHEME
The Chain Routing Algorithm has to be implemented in the handoff scheme. The Chain
Routing Algorithm is added to the handoff scheme (chaining followed by make-break) in
Step 5 as follows:
1. The mobile host sends a handoff request message to the new BS identifying the old
BS and its connection server.
2. The new BS adds local translation table entries for its internal routing.
3. The new BS asks the old BS to forward packets pertaining to the mobile host.
4. The new BS sends back a handoff response message to the mobile host, instructing
the mobile host to transmit/receive through the new station.