wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Wireless Local Area Networks 48-11
First of all, an elimination of the 802.1x key exchange as described previously. Second, the four-way handshake
for session key establishment is integrated into the mandatory 802.11 open systems authentication.
And third, QoS resource requests are also exchanged before the reassociation with the new AP starts.
48.7.2 Mechanisms on the Client Side
The client-side mechanisms mostly optimize the search phase. It is desirable to know the set of possible new
APs a priori to get rid of the time-consuming scanning process during the handover. Background scanning
is a very promising solution to this problem. It eliminates the channel scanning delay completely because a
station actively probes available channels during the normal data exchange. That is, as long as no data has
to be transferred, the client continuously probes new channels for potential APs and gets back immediately
to the current one. If a handover is necessary, all information about neighboring cells is already present.
Moreover, an additional radio interface can also be used on the client side to both passively monitor
the channels and to gather information about neighboring cells. This approach is called the dual radio
client and has a second advantage. It also allows a seamless handover using both radio interfaces for
data communication. The data transfer is initiated with radio interface A in the first BSS. Whenever the
signal quality of radio interface A decreases, the second radio interface B starts to find a new AP and
gets associated to it. As soon as the signal quality of radio interface A is below a certain threshold, the
data communication will be directed to radio interface B, which is already connected to a new AP, and
no service interruption occurs.
48.8 Future Enhancements
Some of the latest additions to 802.11 is the IEEE 802.11n amendment [IEE08n], which aims at increasing
the throughput (from 20.Mbps up to approx. 200.Mbps data throughput), as well as extending the
range of reception (through reducing signal fading) in comparison to the legacy 802.11a/g standards.
One of the most important concepts to obtain these advantages is MIMO. Several MIMO features are
supported by the 802.11n amendment, including transmitter beamforming, spatial division multiplexing
(SDM), and space time block coding (STBC). Furthermore, using an advanced coding with low density
parity check codes (LDPC) and applying a channel bonding to increase the channel bandwidth to
40.MHz is possible. All characteristics mentioned above allow data rates up to 600.Mbps to be specified
in 802.11n, which is roughly 10 times more as compared to 802.11a/g.
Moreover, the Task Group v is currently working on a proposal for a wireless network management
framework, which is contained in the 802.11v draft [IEE08v]. Wireless network management mainly
deals with implementing system-wide functionalities, such as load management among different cells
and coordinating radio properties between neighboring APs. The load management algorithms basically
comprise load balancing and admission control approaches. The management can be centralized,
decentralized, or hybrid depending on the targeted architecture of the network. However, the centralized
architecture is most commonly deployed and also addressed by an Internet Engineering Task Force
(IETF) working group. The working group specifies an interoperable protocol called control and provisioning
of wireless access points (CAPWAP) [IET06]. It aims at reducing the complexity of managing a
large numbers of APs and mainly encompasses administrational tasks to ease operation, maintenance,
and configuration, and for centralizing client management.
References
[Bia00] G. Bianchi, Performance analysis of the IEEE 802.11 distributed coordination function, IEEE
Journal on Selected Areas in Communications, 18(3):535–547, March 2000.
[CLM07] C. Cicconetti, L. Lenzini, E. Mingozzi, and G. Stea, Design and performance analysis of the realtime
HCCA scheduler for IEEE 802.11e WLANs, ACM Computer Networks, 51(9):2311–2325, 2007.
© 2011 by Taylor and Francis Group, LLC