Gateways and Capacity in Ad Hoc Networks Erlend Larsen erl@unik.no Q2S NTNU Norway Vinh Pham vph@unik.no Q2S NTNU Norway Paal Engelstad paal.engelstad@telenor.com Telenor Norway Øivind Kure okure@unik.no Q2S NTNU Norway Abstract Connectivity with external networks is an essential fea- ture of many ad hoc networks, and such connectivity is en- abled by gateway nodes. This paper investigates how the gateways affect the throughput in the ad hoc network. The throughput’s dependency on gateway positions, number of gateways and handover properties is sought uncovered by simulations. The results show that the relative positions of gateways in ad hoc networks may have defining impact on the performance of the network. The average path length and the gateways’ shared interference coverage are param- eters that affect the performance. 1 Introduction In recent years, ad hoc networking research is mainly focusing on ad hoc networks that are connected to exter- nal networks, enabling communication with other networks or even with the Internet. Connectivity with external net- works is a relevant feature e.g. in emergency operations. In such a scenario, a connection to the headquarters (HQ) would enhance the usefulness of the ad hoc communication infrastructure. Through the connection to the HQ, updated information regarding the ongoing operation could be com- municated both ways. The access to external networks is enabled by introduc- ing gateways (GWs) between the ad hoc network and other networks. However, there are few works studying the im- pact that the gateways have on the ad hoc network. The gateways are in the transmission range of other nodes in the ad hoc network, and they affect the performance of the ad hoc network, depending on their position, not only as forwarders of traffic into the ad hoc network, but also as receivers of traffic from the ad hoc nodes. Positioning the gateways strategically could improve the gateway coverage and reduce the path length for traffic traveling the ad hoc network There are several different ways the gateways could be positioned in the ad hoc network: • Static, placed independently of the ongoing scenario • Semi-mobile, moved only when necessary • Mobile, but collocated with a regular ad hoc node • Mobile and independent of the ad hoc nodes Several issues may limit the possibilities for moving a gateway: the access network availability (wired or wire- less), available power sources, the physical gateway size and type of terrain. In addition to the gateways’ positions, their number also affects the performance of the ad hoc network. Operating with only one gateway, for example, the connectivity with external networks could be vulnerable, both in terms of re- liability and coverage. Introducing more gateways provides several important benefits. First, the probability of having a working connection to the access network increases. If one gateway fails, nodes can send traffic via another gate- way. Second, with several gateways the average path length for traffic traveling between the ad hoc network and exter- nal networks is reduced. Third, the risk of partitioning and disconnection from the access network in case of drifting is reduced. Finally, load balancing may be employed by routing traffic in such a manner that the capacity of both or all gateways is better utilized than the capacity provided by only one gateway. To control the number of gateways, it is assumed that the gateways can be activated or deactivated as necessary. In most cases the gateways will not be optimally posi- tioned relative to the ad hoc nodes. Thus, we need to under- stand how this affects the capacity and, in the event that the gateways’ positions can be changed or their number con- trolled (e.g. by turning gateways on or off), be able to ad- vice the best action. The intention of this paper is to investigate the potential for different strategies and algorithms concerning the po- sitioning, activation and mobility of gateways. The main reason is not to identify optimal gateway positions or the 1