Mobile Cellular-Based Ad Hoc Networks in Hybrid Infrastracture/Ad Hoc Modes

Recent advances in Wireless LAN (WLAN) technologies, such as the IEEE 802.11 standard, have provided new opportunities to develop an experimental platform to design and assess ad hoc networks for transmission of multimedia information in realistic environments. This standard defines the carrier sense multiple access protocol combined with collision avoidance (CSMA/CA). The protocol supports WLAN in two different modes: infrastructure and ad hoc. For a peer-to-peer multihop ad hoc operation, the standard specifies a basic access protocol called Distributed Coordination Function (DCF). In the infrastructure mode, mobile nodes communicate with each other via an access point (AP) . In addition to the DCF, the infrastructure mode can support another access protocol known as the Point Coordination Function (PCF), which is suitable for time-bounded services . The PCF is a pooling-based protocol where nodes access channels in a round robin fashion. Based on performance tradeoffs between the centralized and the distributed schemes, we have recently developed a multihop cellular network structure whose basic network configuration is shown in Fig.1. Nodes in this network are distinctly divided into two types: Mobile Base Node (MBN) and Mobile Slave Node (MSN). The mobile base node (MBN) consists of a combination of an AP that operates in the infrastructure mode, LAN interface, and WLAN card.

Figure 1: Multihop cellular network for group based operation.

This combination allows protocol conversion from the infrastructure mode to the ad hoc mode and vice versa. The MSN, which uses a WLAN card, can only communicate with other nodes via its associated MBN. For example, communication between slave nodes located in different cells is accomplished through their MBN's in a similar fashion as conventional cellular networks (see Fig. 1). A distinct difference in this case is that MBN's themselves can also function as active mobile nodes (e.g., group leaders). These nodes, under group oriented operations, should be capable of initiating communications not only with their slave nodes, but also with other MBN's and/or their associated slave nodes. As shown in Fig. 1, if packets are initiated at an MSN but destined to another MSN attached to a different network (cell), they have to undergo a tunneling process. In other words, they are first encapsulated at the source MBN and then de-capsulated at the destination MBN before being forwarded to their final destination. Naturally, the tunneling process won't be necessary if IP packets are initiated and terminated at MBN's.

DARPA SensIT Demo Presentation