18-02-2013, 09:44 AM
A Low-Cost Efficient Wireless Architecture for Rural Network Connectivity
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Introduction
Many rural regions around the world, especially in developing regions, do not have good connectivity
solutions which are economically viable. As a result, many of these regions remain disconnected
from both the rest of the world and from progress in general. In this proposal, I will describe
the design of WiFi-based Rural Extensions (WiRE), a new wireless network architecture that can
provide connectivity to rural regions at extremely low costs. The WiRE architecture is tailored for
the typical rural landscape in several developing regions, in which the population is spread across
small but scattered rural regions (less than 1-2 sq kms) within 100-200 kms of the city. WiRE
is designed to be a wireless distribution network that extends connectivity from the city to each
village.
The WiRE architecture has largely been inspired by my prior work on WiFi-based Long
Distance (WiLD) Networks [42, 62, 35, 54, 64, 34], a low cost point-to-point network connectivity
solution that provides very high bandwidth (typically 6−10 Mbps) over very long-distances. While
prior work on WiLD networks [48, 5, 42, 62, 35] has made significant progress in the design of high-
performance MAC layer solutions, we still lack a vision of how to design a comprehensive, low-cost,
rural connectivity architecture that can efficiently support a wide-range of applications. It is this
goal that I wish to achieve in the WiRE network architecture design. To realize this architectural
vision, we need to address several challenges at various protocol layers including the MAC, network,
transport and the application layers. We will first motivate the need for low-cost connectivity before
we outline these challenges in greater detail.
Motivation: Need for Low-Cost Rural Connectivity
As of Internet World Stats 2007 [28], the Internet penetration in North America is 69.7% of the
population compared to 10.7% in Asia and 3.6% in Africa primarily restricted to urban areas. The
fundamental problem in connecting rural regions is economics [34, 9, 8]. None of the traditional
wire-line connectivity solutions (fiber, broadband and dial-up) are economically viable for such
regions over at least the next decade due to low user densities [34, 15, 9]. Satellite networks provide
great rural coverage but at very high costs: the ISP rate for 1 Mb/s of satellite connectivity in
Africa exceeds $3000/month [3].
In recent years, many developing countries have undergone a cellular revolution with a sig-
nificant penetration of cellular networks in rural areas [26, 27, 23]. Commercial wireless broadband
networks based on GPRS [55], WiMax [70, 22] and CDMA [36] technologies are also being widely
deployed [36, 27, 26]. While a sizable fraction of the rural population owns cellphones for telephony
services in Africa and Asia [46, 26, 71] the network usage is limited due to exorbitantly high usage
costs, roughly ranging from 10 cents to $1/min [2, 24, 23, 25]. Given that a large majority in rural
areas earns less than a few dollars/day, these costs are unaffordable.
Research Agenda
WiRE uses a network structure (illustrated in Figure 2) that is significantly different from the
traditional cellular, WiMAX, WiLD, and wireless mesh network models. For comparison, an ex-
ample WiLD network is illustrated in Figure 1. Unlike the cellular network philosophy of providing
broad network coverage, WiRE provides focused coverage within rural regions with little coverage
outside. The network structure of a WiRE deployment is optimized based on the topography and
the spread of rural regions. To efficiently reach out to sparsely spread out rural regions, WiRE uses
a combinational network structure with four important components: (a) point-to-point network
links; (b) point-to-multipoint network links; © local distribution mesh networks; (d) cellphones
as end-devices in addition to PCs and kiosks. Given that land and tower costs are expensive, this
network structure explicitly attempts to achieve maximum distribution with a small set of towers.
While point-to-point links with highly directional antennas provide a high bandwidth backhaul
that can cover long distances, point-to-multipoint links with sector antennas provide efficient dis-
tribution capabilities within shorter regions and mesh networks with omni-directional antennas are
primarily used in small localities to provide coverage.
WiRE Network Architecture
In this section, we describe the WiRE network architecture and discuss important real-world chal-
lenges in deploying rural wireless networks based on our experiences. Figure 2 describes the basic
WiRE network architecture. Unlike the traditional cellular model of providing broad coverage,
the design philosophy of WiRE is to provide focused coverage within specific rural regions where
connectivity is most required. The WiRE architecture has six important network components:
1. wireless nodes which are low-power single board computers that have the capability to support
multiple wireless cards for different network links.
2. point-to-point links using highly directional antennas to provide network connectivity over
long distances in the range of 50 − 100 kms.
3. point-to-multipoint links using sector antennas to distribute connectivity to multiple endpoints
within relatively short distance lasting a few kilometers.
4. multi-radio mesh links using omni-directional links to extend wireless coverage within small
local regions.
5. cellphones or low cost computing devices with WiFi-enabled interfaces that can act as end-
devices.
6. large local storage of at least a few GB at each local wireless node to perform in-network
optimizations for applications as well as store-and-forward intermittent operations in the
event of a network outage.
MAC Layer Challenges
The overarching MAC challenge in WiRE is to develop a unified MAC protocol that is configurable
to different network settings and which can provide high throughput and predictive performance
in multi-hop settings. While there have been several advances in these individual networks [6, 14,
38, 50, 4, 7, 49, 5, 48, 42, 62, 69, 1, 29, 51, 43], a unified approach has not been expored. Achieving
high throughput in WiRE is a challenging problem due to a variety of factors:
Variable network characteristics: WiRE operates in three different types of network settings (point-
to-point, point-to-multipoint, omni-directional) with completely varied physical and MAC layer
characteristics. In addition, given the limited number of non-overlapping channels in 802.11b and
inherent limitations in using the 802.11a frequency band over long distances, interference across
these network links within WiRE is unavoidable.
Towards a Unified, Adaptive and Auto-configurable MAC Layer
While one may envision designing specific MAC protocols for specific environments, in practice, we
require a unified MAC that can be installed as a single software in all the nodes that can adapt
and be configured to specific environments. Part of the challenge is that, operators who install
networks in rural areas are not sophisticated enough to properly configure these networks. In large
scale deployments, the network configuration and management becomes a much harder challenge
as we have experienced in prior deployments [42, 21, 37]. The preliminary design of our unified
MAC layer borrows ideas from several existing MAC layer protocols [50, 42, 35, 48, 69, 10, 51, 43]
including WiLDMAC and JazzyMAC.
Robust Network Design Challenges
The design and operation of rural wireless networks raises many challenges which cannot be solved
by just using high-performance equipment [64]. The key challenges we want to address are: (1)
optimal design of network topology to decrease deployment cost, (2) increased component failure
due to low quality power, (3) difficulty in doing fault diagnosis because of non-expert local staff
and limited connectivity for remote experts, and (4) difficulty of frequent maintenance because of
remoteness of node locations. All of these problems can be fixed by having higher operating budgets
that can afford highly trained staff, stable power sources, and robust high-end equipment. But the
real challenge is to find solutions that are sustainable and low-cost at all levels of the system.
Network Design
The key network design challenge is: given a topography and the location of rural areas in a region,
how do we design an optimal network topology that minimizes the number of towers and achieves
a certain minimum level of network redundancy? In addition, we need to consider the line of sight
as an important issue since point-to-point links require line of sight for operation; this usually
implies towers of a minimum height at each end. A variant of this problem was studied by Sen and
Raman [52] for long-distance WiFi networks.
Robust Power Solutions
An important challenge to robust wireless network design is the lack of reliable power. From our
past experience in the Aravind [21] and the AirJaldi [37] networks, we found out that lack of stable
and quality power has greatly contributed to a substantial decrease in the robustness of system
components that would otherwise work quite reliably. Although issues such as frequent power
outages in rural areas are well known, we were surprised by the degree of power quality problems in
rural villages even when power is available. Our measurements of the grid power supply in India
showed that power spikes above 500V, often with reversed polarity, and some even reaching 1000V
are common, and so are extended sags below 70V and swells above 350V.
The key to understand the power problem is that the real cost of power in rural areas is not
the cost of grid power supply, but of cleaning it using power controllers, batteries and solar-power
backup solutions. Also, due to short lifetime of batteries and ineffective UPSs, power cleaning is
a recurring cost [64, 39]. Solar power, although still expensive, turns out to be more competitive
than expected as it produces clean power directly.