Framing and Schedule Dissemination for Multi-hop TDMA-based Wireless Networks

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Introduction
Wireless mesh networks (WMN) have become increasingly popular in the last couple
of years due to advantages like low setup cost, easy network maintenance, robustness
and reliable service coverage and technology-independence [1]. In WMNs, each node
acts both as a host and a router, forwarding packets on behalf of other nodes, provided
that the nodes are in range of one another. This property can be exploited to extend
wireless connectivity in hard-to-reach locations [2].
1.1 Motivation
Outdoor IEE 802.11-based [3] mesh networks are increasingly becoming a viable op-
tion for rural connectivity [2, 4]. This is because, the use of o -the-shelf 802.11-based
hardware provides a low-cost option for the platform to be used. Additionally, rural
regions in India are characterized by intermittent power supply, varying population
density and low-income levels. Due to these reasons, it is not a pro table option for
the service providers/operators to roll out communication technologies like GSM and
WiMAX. Taking the above factors into consideration, the solutions being developed
for rural connectivity should be characterized by low service-cost as well as low power
consumption.
Chebrolu et al present a fresh perspective of providing connectivity to rural
regions with the use of commodity 802.11 hardware [5]. The network proposed by
the authors is a combination of short-distance (up to few hundred metres) and long-
distance (up to few tens of kilometres) links. The short-distance links form the Local
Access Networks (LAcN) and the long-distance links form the Long Distance Network
(LDN) as show in Figure 1.1. The root in the LDN is the landline node, which has
wired connectivity. All the other nodes in the LDN extend connectivity from the
landline to a particular point in each village and are, therefore, referred to as the
local gateway of the village. Connectivity from the local gateway may be extended
by the means of local access links to multiple points within the village. A network
with the above mentioned characteristics is termed by the authors as a FRACTEL1
network.
Tra c ranging from web to real-time audio/video are expected to be supported
1wiFi based Rural data ACcess and TELephony

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Framing and Schedule Dissemination for Multi-hop TDMA-based Wireless Networks

TDMA-based protocols with a connection setup mechanism can be used to provide QoS guarantees in a network. Wireless Mesh Networks (WMNs) are multi-hop in nature and supporting QoS intensive application on them requires multi-hop TDMA-based protocol. However, most designs of TDMA-based protocols are limited to single-hop based settings.

In this work, we have designed, implemented and evaluated a centrally-controlled TDMA-based MAC protocol for multi-hop wireless networks. To accommodate the changing traffic requirements of the network, the protocol makes use of dynamic scheduling, and therefore, has a mechanism for schedule dissemination. This makes it suitable for real-time voice/video applications.

Our design is not limited to any particular wireless technology. The implementation is carried out on tmote-sky. The achieved throughput is upto 90 percent of the optimal throughput, which is good enough to support 1-2 GSM-quality voice calls simultaneously over three hops, and even more, over lesser number of hops. We have also conducted feasibility studies for 802.11-based platforms and concluded that an implementation could provide throughput greater than that of the existing CSMA/CA MAC.