• the community understands the needs and motivation for the existing system, so variations on it require less communication and education;
• achieving community buy-in, which is fundamental to sustainability, is easier; and
• it is rarely clear to outsiders why the current system is the way it is, which means larger changes might fail due to unforeseen circumstances.
• increased component robustness,
• easy-to-use management tools for local staff, and
• tools for remote management by experts.
• more extensive eye care than the eye-camp approach because new patients get treatment every month, and
• higher-quality eye care with increased patient follow-ups.
Advantages. Wi-Fi reduces both deployment and operational costs in many ways. First, it does not incur frequency licensing fees. In addition, unlicensed frequency gives the hospital the operational freedom to put up links whenever and wherever needed, improving sustainability. Previously, Aravind could not add clinics in areas carriers considered unprofitable. Moreover, Wi-Fi equipment is low cost and low power.
One tradeoff we make is to provide islands of coverage using point-to-point links rather than blanket coverage for the whole region. This approach is clearly cheaper and seems to be sufficient, as the VCs are spread out and there is no need to support mobility. The blanket model also eliminates the ability to reuse spectrum, which reduces the effective local bandwidth.
Our solution comprises inexpensive, off-the-shelf hardware components. We use commodity Wi-Fi network cards, high-gain directional antennas and cheap, low-power single-board computers as Linux routers. The cost of a long-distance Wi-Fi link, excluding tower costs, is less than $800.
Limitations. Wi-Fi is designed to work in high-density, short-range zones such as hotspots. Standard Wi-Fi makes little sense in point-to-point long-distance settings in rural areas with sparsely distributed populations. There are two main reasons for poor performance: protocol- and channel-induced losses. 11,12
IEEE 802.11 relies on a simple stop-and-wait link-recovery mechanism. As the link distance increases, the propagation delay increases as well, and the sender waits for a longer time for the ACK to return. This decreases channel utilization. Beyond the ACK-timeout limit, the sender may retransmit unnecessarily and waste bandwidth.
The protocol uses carrier sense multiple access with collision avoidance (CSMA/CA) technology, in which all stations listen to the medium before transmitting and send only when the channel is idle. Over long links, the state of the medium at the sender does not reflect the receiver's state. Thus, in the presence of bidirectional traffic, the sender and receiver cannot correctly assess the channel, leading to collisions. With increasing link distance, the probability that two end-hosts begin transmission within a window defined by correspondingly increasing propagation delay also increases, causing more collisions.
Another problem is that point-to-point links emerging from multiple radios colocated at the same wireless router interfere with one another when the directional antennas' high-energy side lobes overlap.
Along with these protocol shortcomings are channel-induced losses due mainly to external Wi-Fi interference. These losses can be highly variable and asymmetric. In addition to constant residual losses between 1 and 10 percent, there are bursty losses of varying magnitude and duration.
Limited local expertise. Most grassroots wireless network deployments are run by small groups with limited IT experience, which can lead to misconfiguration, limited diagnostic capabilities, and inadvertent equipment misuse.
Hardware and software failures. Poor-quality power causes various problems to low-cost equipment. Frequent power surges can damage the routers, Ethernet hubs, or power over Ethernet (PoE) injectors, while frequent power cycling can corrupt the software on the router's CompactFlash (CF) storage.
Lack of connectivity. Many problems can be investigated and solved by remotely logging in to the router. However, this assumes the network is working. When links failed, a frequent occurrence, we had to physically go on-site.
Lack of physical access. Traveling all day or night to get on-site is not uncommon. Worse, towers and relay points tend to be in uninhabited locations and might require hiking in. During many of our early deployments, we were forced to personally go into these remote places, and it took us days and considerable cost to deploy or repair equipment.