On Seamless Connectivity over Multi-Radio andMulti-Channel Wireless Mesh Networks
Sho Fujita

Tadashi Yasumoto

Hiroshi Esaki

The University of Tokyo

The University of Tokyo

The University of Tokyofujisho@hongo.wide.ad.jp tadashi@hongo.wide.ad.jp

11bg

1. INTRODUCTION
In IEEE802.11 wireless mesh networks (WMN), we
had suﬀered from performance degradation caused bycontention and interference. To avoid the issue with areasonable cost, we have been working on the conﬁguration where nodes are equipped with multiple commodity IEEE802.11 interfaces and antennas. Although theWMNs of the conﬁguration were segmented by multichannelized links, traditional applications needed it to
be a single-tier and easy-to-use network. In this paper,
we present a scheme introducing a virtual link layer thatallows mobile nodes to move around without changing
its identiﬁer and losing its connectivity to such kind of
WMNs. We have implemented a prototype system of
our scheme using the click modular router framework[2]. We have conﬁrmed that existing applications, suchas DHCP and mDNS, run on the multi-radio and multichannel WMN without any modiﬁcation.

2. AD-HOC WIRELESS MESH NETWORK
Some IEEE802.11 based large-scale WMNs have been
deployed so far. These WMNs were carefully plannedalong with the population distribution, and were composed of specialized hardware, such as high-gain and/or
directional antennas. It needs still large investment
while it might be paid oﬀ for a long time. Hence, it
is diﬃcult to apply them to the situations where thenetwork is needed only for a short time. For example,meetings or conferences held where network infrastructure does not exist and disaster-stricken areas whereexisting network infrastructure become unavailable.
Therefore, we have been studying on ad-hoc WMN,
which is deployed and withdrawn on demand. So as

Permission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copies are
not made or distributed for proﬁt or commercial advantage and that copies
bear this notice and the full citation on the ﬁrst page. To copy otherwise, torepublish, to post on servers or to redistribute to lists, requires prior speciﬁc
permission and/or a fee.
ACM CoNEXT 2008, December 9-12, 2008, Madrid, SPAIN
Copyright 2008 ACM 978-1-60558-264-1/08/0012 ...$5.00.

hiroshi@wide.ad.jp

11aFigure 1: An example conﬁguration of ourmulti-radio and multi-channel WMNs
to make it perform well with a reasonable cost, wehave developed a node equipped with multiple commodity IEEE 802.11 WLAN interfaces and omni-directional
dipole antennas. These WLAN interfaces use IBSS mode.
Figure 1 depicts an example conﬁguration of the WMN
consisting of the nodes. Each node assigns diﬀerent
channels to two interfaces. Note that we assigned channels from diﬀerent frequency bands to two interfaces ofa node, as Draves et al. pointed out in [1]. Otherwise,
we observed interference between common IEEE802.11
devices even if we assigned the most distant channelsof the same frequency band. In this conﬁguration ofWMNs, we can achieve high throughput since we canreduce interference among channels. On the other hand,
it is diﬃcult for mobile nodes to maintain connectivity
because they have relatively small cells with diﬀerent
channels and do not have any shared channel like cellular networks.

3.

SEAMLESS CONNECTIVITY

We consider host-based routing on a set of stationary nodes conﬁgured like the example in the last section
and a set of mobile nodes. However, we can not simply apply ad-hoc routing protocols to them since mostof ad-hoc routing protocols assume on operation under the single-channel environment. We have to assign
the same channel as the neighboring nodes before getting connectivity to them. Consequently, routes, which
mobile nodes search for, largely depends on channel assignments. We introduce a virtual link layer to decouple
the handover and routing process and then utilize existing ad-hoc routing protocols so as to provide seamlessconnectivity to mobile nodes.

routing daemon
virtualiface

scanphysical
iface0(working)

physical
iface1(backup)switchconfigswitchhandover daemon

3.1 Routing over Virtual Link
First, we introduce a virtual network interface that is
independent of physical network interfaces. It is always
active no matter if physical ones are active or not. Its
conﬁguration is invariant even if physical ones are replaced. Hence, if we use the MAC address of the virtual
network interface as routing identiﬁcation, we can conduct the routing process independently of the handoverprocess.
At this stage, assuming that access points have a simple tree topology, we use the simple forwarding scheme,
which is bridging with loop prevention based on a duplicated packet detection. Each node maintains a sequencenumber for the duplicated packet detection. The framestransmitted from the virtual network interfaces are encapsulated by a header that includes this sequence number. This mechanism, which is similar to that of Simpliﬁed Multicast Flooding [3], conﬁrms uniqueness ofpacket. While packets are forwarded to all the physicalnetwork interfaces in the initial state, each node learns
the connectivity to other nodes then forwards packets
with the speciﬁc destination MAC address only to thespeciﬁc physical network interface.

3.2 Interaction with Handover
We have to select the channel and BSSID to handoverin IEEE802.11 IBSS mode. As Velayos et al. analyzed
handover time in [4], detection phase, where decrease inlink quality is detected, and scan phase, where new access points are scanned, are also dominant in the IBSS
mode. As an example conﬁguration of handover process, we explain a handover technique using two physical network interfaces at the same time. Figure 2 depicts the interaction between the routing and the handover daemon in this conﬁguration. In this technique,the switch maintains which network interface is communicating. The interface in use, which is called working
interface, is connected to the virtual network interface
without doing any process that might degrade the linkquality, such as scanning. On the other hand, the interface not in use, which is called backup interface, proactively searches for neighboring nodes and evaluates thequalities of them. The handover process compares thequalities of the working and backup interfaces and thenswaps their roles if the backup interface is superior. We

Figure 2: Interaction between the handover androuting
can achieve seamless handover that does not include
the detect and scan phases since at least one network
interface is connected to the APs.

4.

EARLY RESULTS AND CONCLUSION

We have constructed a commodity-based IEEE802.11
WLAN multi-radio and multi-channel WMN and addressed its practical conﬁguration. In order to achieve
seamless connectivity over the WMN in the conﬁguration, we introduced a scheme that introduces a virtual
link layer decoupling the routing and handover processes. We have implemented a prototype system of
the scheme on the click modular router framework [2].
We combined the simpliﬁed routing process, which isbridging with loop prevention, with the handover process, which uses two physical network interfaces at thesame time. Although we have to modify the system
software of the mobile nodes, the virtual network interface hides the conﬁguration changes of the physicalnetwork interfaces. We have conﬁrmed that existing applications, such as DHCP and mDNS, can run on theWMN without any modiﬁcation.

5.

REFERENCES[1] R. Draves, J. Padhye, and B. Zill. Routing in
Multi-Radio, Multi-Hop Wireless Mesh Networks.
In Mobicom, 2004.
[2] E. Kohler, R. Morris, B. Chen, J. Jannotti, andM. F. Kaashoek. The Click Modular Router. In
ACM Transactions on Computer Systems, 2000.
[3] J. Macker and S. D. Team. Simpliﬁed MulticastForwarding for MANET. Work in Progress,
Feburary 2008.
[4] H. Velayos and G. Karlsson. Techniques to Reduce
IEEE 802.11b MAC Layer Handover Time.
Technical report, KTH, Royal Institute ofTechnology, 2003.