sfi

NDP

ENTERPRISE IRELAND

NCNRC

Communications Network Research Institute

CNRI Wireless Mesh Testbed

Karol Kowalik

We have recently built a WMN (Wireless Mesh Network) testbed. The goal of this project is to build large and scalable WMN to conduct various networking studies. The CNRI WMN consists of 17 IEEE 802.11b/g based mesh routers, spread around FOCAS building. Various statistical information about our mesh testbed can be obtained from http://mesh.cnri.dit.ie. As a base platform we have used Soekris net4521 boards. And currently we are evaluating feasibility of our approach.

Map
Map of our testbed (Click on image for larger view)

Goal

How our testbed is different from already deployed testbeds? - The goal of our project is to enhance 802.11e QoS support features by selecting paths which provide enough available bandwidth. Our approach uses novel method of measuring available bandwidth (click here for more information about our WLAN Resource Monitor). When selecting paths it takes into account the signal strength (because the propagation characteristic determine the feasible transmission rates). It considers also contention with other nodes to select paths which do require competing with other nodes for bandwidth.

Testbed observations

Observations of links quality of our testbed confirm observations of other researchers that wireless links often exhibit quite different propagation conditions in one direction than in the other. Broadcast packets may be successfully sent from node to another but not in the opposite direction. This is known as link asymmetry. The links in our static 17 node indoor WLAN mesh network also exhibit this link asymmetry. We demonstrate this by using the signal-strength symmetry (SSS) parameter first defined in [1].

Histogram-Pairs
Figure 1

Figure 1 shows the SSS distribution measured on our static 17 node indoor WLAN mesh network where the signal strength values were collected over a 24 hour period. This diagram demonstrates that in our mesh network most links were asymmetric. Only one link was perfectly symmetric (SSS = 1), 14 links were completely asymmetric (SSS = 0), and most links exhibiting significant asymmetry (SSS < 0:7). Furthermore, from Figure 2, it can be observed that there is no correlation between the forward and reverse signal strengths. Therefore, this lack of correlation suggests that a successful broadcast of a packet from one node to another does not imply successful transmission in the opposite direction.

Correlation
Figure 2

The routing protocols for IEEE 802.11 WLAN mesh networks need to be aware of this link asymmetry. This is because HELLO messages which are exchanged between nodes to discover their neighbors are transmitted as broadcast messages without acknowledgements. As such, they may get through to recipients by utilizing highly asymmetric links. Data frames on the other hand require acknowledgements which require bidirectional communication links. This creates the possibility that links which cannot be used to transfer data frames may be erroneously defined as valid links in the routing table.

In our testbed we employ one of the methods described in [2] for dealing with link asymmetry, namely the link hand-shaking. This method extends HELLO messages so they include a nodes neighbor set. Such modification of HELLO messages allows the other nodes to detect if the link is bidirectional or not. Therefore, by using the method of hand shaking the routing protocol can detect if the link can be used for data transfers and can therefore incorporate the link into the routing table.

References

[1] D. Kotz, C. Newport, and C. Elliott. The mistaken axioms of wireless network research. Technical report TR2003-647, Dartmouth CS Department, July 2003.

[2] Henrik Lundgren, Erik Nordstrm, and Christian Tschudin. Coping with communication gray zones in IEEE 802.11b based ad hoc networks. In 5th ACM international workshop on Wireless mobile multimedia (WoWMoM 2002, pages 49-55. ACM Press, 2002. Available at http://pdos.csail.mit.edu/decouto/papers/lundgren02.pdf

If you have any comments or suggestions please feel free to contact us at cnri@cnri.dit.ie .