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
Unlike in a circuit-switched cellular system, handoff in wireless ATM net- works introduces ATM cell loss. To avoid the cell loss, buffering is required
during handoff, especially for loss-sensitive traffic. For uplink traffic from a mobile terminal to the fixed ATM network via a
base station, buffering is performed at a mobile terminal until a connection is reestablished. Since we can expect that all ATM cells which left the mobile
terminal before the mobile terminal starts to buffer will be sent to a COS during the buffering time, we assume that uplink traffic does not suffer a cell
out-of-sequence problem.
For downlink traffic, buffering is required at a COS until the new connection subpath between a COS and a new base station is established by
a handoff procedure. Handoff is usually initiated by a mobile terminal. After new base station is chosen, the next step in a handoff procedure is to
determine a COS. Once a COS is determined, the switch stops transmitting ATM cells
4
destined for the mobile terminal and starts buffering them. After
3
In the path extension scheme, the old base station can be considered a COS. In the virtual w
x connection tree of 17 the root node becomes a COS.
4
Since a mobile’s handoff request implies degradation of wireless link quality, transmission through the current wireless link has to be avoided as much as possible to reduce cell loss.
HANDOFF IN WIRELESS ATM
351
the new connection subpath is established, these buffered ATM cells are forwarded to the mobile terminal via the newly established subpath. To
guarantee in-sequence cell delivery, these buffered cells should be transmit- ted before newly arriving cells.
12.4.3 Cell Routing in a COS
As mentioned in Section 12.4.1, a COS is an ATM switch that acts like an anchor in a rerouting procedure, and in which buffering is performed during
handoff. Figure 12.6 illustrates ATM cell flow within a COS. We assume that the
original connection path between the mobile terminal and its peer party is routed through input port 1 and output port 1 of the COS. Before handoff
Ž .
occurs, all downlink cells shaded cells in the figure departed from the peer party are multiplexed with other ATM cells and arrive at the input port 1.
These downlink cells are routed to output port 1 and transmitted to the mobile terminal via a current base station.
When the mobile terminal moves away from the coverage area of the old base station, handoff is initiated. Here we assume that the new base station is
routed from output port j of the COS. Once a signaling message for handoff request is sent from a mobile terminal and a new base station is determined,
the ATM network will select a COS. If it is identified as a COS, the switch
Fig. 12.6 Cell routing within a crossover switch.
WIRELESS ATM SWITCHES
352
will start buffering
5
the downlink cells until a new connection subpath is established between the COS and the new base station. After the handoff, all
downlink cells, including the buffered cells, will be routed to output port j and transmitted to the mobile terminal through the new base station.
In an ATM switch, all cells arriving at the switch are stored in a switch memory. Their output ports are determined based on their input VCI values.
Even though a COS starts buffering newly arriving cells destined for a mobile terminal, the cells for the mobile terminal that are already stored in the
switch will be transmitted through the old path.
For example, in Figure 12.6, when the COS starts buffering the newly arriving downlink cells, we can notice that some downlink cells are already
stored in the output buffer of port 1. These cells will be lost unless the switch prevents them from being transmitted through output port 1, since they will
be transmitted to the previous base station after the mobile terminal leaves the base station.
To avoid this cell loss, a COS has to stop transmitting not only newly arriving downlink cells but also the cells that are already stored in the switch
Ž .
memory the shaded cells in the output buffer of port 1 in Fig. 12.6 . If the COS is an output-buffered ATM switch, to avoid cell loss these cells have to
be physically moved to other memory, which will be used for buffering cells during handoff. Moving cells from one buffer to the other also increases the
end-to-end delay and wastes memory bandwidth. These buffered cells and physically moved cells from the output buffer will be routed to a different
Ž .
output port output port j in the example of Fig. 12.6 after a new connection setup is completed between the switch and the new base station. In addition,
these cells should be transmitted in sequence.
6
To realize these functions Ž
. with a regular ATM switch is not straightforward, since normal nonmobile
ATM traffic does not change its route during the lifetime of a connection.
12.5 MOBILITY-SUPPORT ATM SWITCH
