Although WiFi technology was designed for local area networks, its impact in developing countries is more dramatic in long-distance applications.
In developed countries, fiber optics cables offering large bandwidths have been installed satisfying the communication needs of most cities. The penetration of optical fiber in the developing world is not enough to cover the needs, and the cost of its expansion often does not meet the ROI (Return on Investment) goals of telcos within a reasonable period of time. Wireless technologies, on the other hand, have been successful in developing countries.
Telcos have installed traditional microwave radio links in most countries. This is a mature technology that offers high reliability and availability reaching 99.999%. However, these systems cost many thousand dollars and require specially trained personnel for its installation.
On the other hand, satellite systems have proved adequate for broadcast traffic and certain applications. However, satellite solutions are still expensive for bidirectional traffic, especially where other alternatives are not available which is the case in most developing countries.
Two hurdles have to be overcome when applying WiFi to long distance: Power budget limitations and timing limitations.
The major limitations for using WiFi over long distances are the requirement for line of sight between the endpoints and the vulnerability to interference in the unlicensed band. The first limitation can often be addressed by taking advantage of the terrain elevations, or by using towers to overcome obstacles such as the curvature of the earth and to provide Fresnel zone clearance.
For indoor applications, line of sight is not as important since the equipment tends to be very close together. But for long distance applications, line of sight is absolutely critical.
The second limitation is less pronounced in rural areas and can be alleviated by migrating to the less crowded 5 GHz band.
The third limitations has to do with the media access techniques. WiFi uses a random access method to share the communications medium. This makes it subject to collisions , therefore the transmitter relies on receiving an acknowledgment for every successfully received frame. If, after a specified amount of time, called the "ACKtimeout", the acknowledge frame is not received, the transmitter will resend the frame.
Since the transmitter will not send a new frame until the ACK for the previous one has been received, the ACKtimeout must be kept short.
This works well in the original scenario intended for WiFi, in which the propagation time of 33.3 microseconds per kilometer is negligible, but breaks down for links over a few kilometers. Although many WiFi devices do not have provisions for modifying the ACKtimeout, newer equipment meant for outdoor applications (or third party firmware like Open WRT) will give you this possibility, often by means of a distance field in the GUI (Graphical User Interface). Changing this parameter will allow for a reasonable throughput, which will anyway decrease proportionally to the distance.
Other manufacturers have chosen to move from random access to a Time Division Multiple Access (TDMA) instead. TDMA divides access to a given channel into multiple time slots, and assigns these slots to each node on the network. Each mode transmits only in its assigned slot, thereby avoiding collisions. In a point to point link this provides a great advantage since ACKs are not needed because each stations takes turns at transmitting and receiving. While this method is much more efficient, it is not compliant with the WiFi standard, so several manufactures offer it as an optional proprietary protocol, besides the standard WiFi. The contention window slot-time need also to be increased to adapt to longer distances.
Proprietary protocols (such as WiMAX, Mikrotik Nstreme, or Ubiquiti Networks AirMAX) use TDMA to avoid these ACK timing issues.
The 802.11 standard defines the receiver sensitivity as the received signal level required to guarantee a BER < 10−5 (Bit Error Rate). This specifies the amount of energy per bit required to overcome the noise. As the number of bits/second transmitted increases, more receiver power will be needed to provide the same energy per bit. So the receiver sensitivity decreases as the transmitter rate increases, so to maintain the same signal/noise ratio as the distance increases the throughput diminishes, or, in other words, for longer distances one should choose lower data rates to compensate for the reduction of the signal strength with distance.
What is needed for a long distance link?
There are four aspects that need to be considered to adapt WiFi devices to long distance: increase the radio dynamic range; increase the antenna gain; decrease the antenna cable loss; and take provisions to account for the the signal propagation time.
*****Figure 1: Power in dBm vs distance in a radio link (Power budget).
This graph shows the power level at each point in a wireless link.
The transmitter provides some amount of power. A small amount is lost in attenuation between the transmitter and the antenna. The antenna then focuses the power, providing a gain. At this point, the power is at the maximum possible value for the link.
Then there are free space and environmental losses, which increase with the distance between the link endpoints. The receiving antenna provides some additional gain. Then there is a small amount of loss between the receiving antenna and the receiving radio.
If the received amount of energy at the far end is greater than the receive sensitivity of the radio, then the link is possible.
Increasing the transmission power can lead to violations of the regulatory framework of the country.
Increasing the antenna gain is by far the most effective way to improve range. Make sure that the radio to be employed has connectors for an external antenna (some devices have an embedded or otherwise non removable antenna).
Decreasing loss in antenna cables is still an important issue, and the most radical way to attain it is to place the radio outside, directly attached to the antenna, employing a weatherproof box, and maybe, power the radio using PoE (Power over Ethernet) techniques.
Improving the receiver sensitivity implies choosing a model with better perfomance, or settle for lower transmission speeds where sensitivity is higher. Although high gain antennas can be expensive, in many countries one can find satellite antennas that are no longer being used and can be modified for the WiFi bands.
In a perfect world, we would use the highest gain antennas with the loudest and most sensitive radios possible. But a number of practical considerations make this impossible.
Amplifiers introduce an additional point of failure, might violate maximum power permitted by local regulations and add noise in reception, so they should be avoided. High power transmitters are available from many manufacturers that offer up to 1 W of output power which could be used instead of amplifiers.
In general, it is better to use high gain antennas than high power output. A greater antenna gain will help both in transmission and reception making a greater impact in the link budget.
Using high gain antennas at long distance links requires special alignment techniques. In the following section we will describe them.
fig 2: profile of a 280 km path over which standard WiFi gear with open WRT firmware which allows for the ACKtimeout increase was used to transfer files at about 65 kb/s in April 2006 between Pico del Aguila and El Baul in Venezuela.
Notice that the earth curvature is quite apparent, and was overcome because one of the stations was at 4200 m altitude and the other at 200 m. Frequency was 2412 MHz, output power 100 mW, antenna gain around 34 dBi. Streaming video was succesfully transmitted despite the limited bandwidth.
A year later the experiment was repeated with the same WiFi gear but with commercial 32 dBi antennas at both ends and similar results were obtained. Then, another type of firmware developed by the TIER group of Berkeley University that implements TDD (Time Division Duplexing) was tried which showed a remarkable bidirectional throughput of 6 Mbit/s with standard 802.11b hardware
fig 3 Profile of a 382 km test at 2.4 GHz performed in April and August 2007, Venezuela.
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