According to the IEEE802.11b standard the wireless channel provides transfer
rates of 1, 2, 5 Mbps and up to 11 Mbps depending on the signal-to-interference
(SIR) ratio. In the following, we use the term signal-to-noise (SNR) ratio
synonymously if there are no dominating interference effects of other disturbing
sources. The access points are connected to our local Intranet by IEEE802.3
compatible Ethernet segments with a transfer rate of 10 Mbps that constitute
the distribution system (DS) of the IEEE802.11 architecture (cf.
[3]). This Intranet
comprises FDDI and fast Ethernet segments providing a transfer rate of
100 Mbps, an access to a Gigabit-Ethernet by a fast Ethernet switch as
well as the access to an external 155 Mbps ATM research network by a corresponding
ATM switch. Reconfiguring our access network in an appropriate manner,
we are able to evaluate the performance and interworking of our WLAN protocols
with IPSec, the resource-reservation policy and the flow- and packet-level
controls, and to investigate their impact on the data transfer by TCP or
UDP and on the adaptive applications.
The protocol context of our testbed comprises the physical and data
link layers of an IEEE802.11 compatible WaveLAN supporting channels of
1 up to 11 Mbps over the air-interface. Due to the overhead of the MAC
layer the maximal throughput at the network layer is limited, e.g., to
1.810 Mbps for 2 Mbps and Ethernet frames with a maximal size of 1526 byte
and according to our experience to approximately 5.5 Mbps on average for
the 11 Mbps environment. In accordance with Bing's [7]
detailed throughput analysis, in our experiments both the UDP and TCP flows
never exceeded these bounds significantly for a persistent period under
stationary or roaming conditions (see Figs. 18,
19
- see Sec. 3.2).
As network layer offering mobility-management functions we have used
the Mobile IP version 4 (MIP) protocol provided by the Dynamics implementation
(version 0.6.2, 11/2000) of the Helsinki University of Technology (HUT)
(cf. [16], [25]).
The latter provides the standard functionality of MIP to create and manage
the interworking of a home agent (HA), foreign agents (FAs) and the mobile
nodes (MNs) and some additional optimization options such as a hierarchical
structure of foreign agents and the notification of the MIP layer by events
of the MAC to improve handoffs. It guarantees a seamless data transfer
by TCP or UDP when the MNs are traversing the microcellular structure and
have to change their corresponding APs belonging to different subnetworks
(see Fig. 9).
Figure 9:Resource reservation
under roaming conditions.
Using the possibilities of a network re-configuration described subsequently,
we have analyzed in a systematic manner the impact of the terminal mobility
on the data flows and the quality of service of the adaptive applications
as well as the interworking of the corresponding protocol functions. To
study the performance of the data transfer at the network and transport
layers and to monitor the status of the wireless infrastructure, we have
used a family of different measurement tools including the Distributed
Benchmarking System (DBS) developed at the Nara Institute of Science
and Technology in Japan (cf. [23])
and the developed enhanced delay-monitoring tool WVPing as well as the
status monitor YASM (see Fig. 10
- cf. [22]).
