Special Issue: Scalability Issues in Wireless Networks - Architectures, Protocols and Services

Scalability of computer-communication networks has been an important topic for quite some time. Internet community examined the services and protocols for scalability, and switching and routing researchers looked at the scalability of the switches and routers. Service providers studied the application architectures of the services for the scalable performance that they need to provide to subscribers and, the maintenance folks paid attention to scalability of service provisioning algorithms, protocols and software. Similarly, the wireless networking community has also started to look at the scalability issues that are relevant to wireless cellular, wireless local, as well as wireless personal area networks besides the emerging area of infrastructure-less networks such as ad-hoc and sensor networks. Surprisingly there is significant scope for research in the wireless scalability area as it relates to the ad hoc and sensor networks and this is one of the reasons why this topic was chosen for a special issue. A wireless sensor network is likely to have a large number of nodes perhaps in thousands to millions and that may create, potentially, numerous scaling problems in the networking protocols, addressing schemes, hardware architectures etc. The key questions that people are asking in this area are: whether wireless networks are scalable? And if they are, what are the appropriate conditions under which the scalability properties are achievable? One of the pioneering works on the capacity of wireless networks was published by Gupta and Kumar [1]. This paper presented a theoretical framework to analyze the capacity of peer-to-peer wireless networks. Two node placement models were analyzed in this work. The first is called an ‘arbitrary model’ that consists of network with static nodes that are arbitrarily located within the domain area of 1 m. In this model, there is no restriction on traffic patterns, range or power levels for transmission, routing protocol and transmission scheduling policies. Each node is capable of maintaining at most one transmission or one reception at a given time. The second model is the ‘random network’ model, which assumes an independent and uniform distribution of node locations, a random traffic pattern and a fixed level of transmission power that is selected to ensure the connectivity of the network as N grows large (where N is the number of nodes in the network). This paper concluded that the per-node throughput approaches zero as the number of nodes in such networks grows to infinity. This is a very powerful result and its publication received a lot of attention within the research community stimulating a significant activity with the appearance of several newer results. As more results were published, the need to have a standard framework for defining and comparing scalability of a method, an algorithm or protocol was realized. IETF came up with such a framework document recently [3]. According to Reference [3], the scalability of a method in a network is measured with respect to the triplet (environment E, independent parameter P, primary metric M). A method is absolutely scalable if the primary metric(s) considered does not become arbitrarily large when the independent parameter tends to be infinity in a particular environment defined by a certain set of environmental variables. This document defines independent parameters for a network as the parameters that can be varied freely such as, number of nodes, traffic load, node density etc.; as well as environmental parameters such as mobility patterns and the radio propagation path loss etc.; both of which can affect and change the scalability evaluation of a method. In addition, Reference [3] precisely defines the notions of relative scalability of two methods and the optimal scalability. A method A1 is termed relatively more scalable than another method A2 with respect to a given triplet [E, P, M] if in the