Whew, it's been a long day and all I've done is bike around MIT and respond to emails.
This post is an overview of the challenges we are facing at this point in the project.
1. We have not yet been able to establish a stable link over a distance of more than 0.2 km. (By stable link, I mean the Bullet repeater modems on both sides register some received signal power that does not spontaneously flicker on and off.) We were only able to achieve transient connections over the longest distances we have tested at (0.4 km at Wellesley College, and 0.8 km between the top floor of ADPhi fraternity house and a 3rd floor apartment in Cambridge). We think that this is due to obstructions such as tall trees and buildings.
We will proceed by finding better testing sites with actual line of sight connections between the two ends of the link. Hopefully this will allow us to decouple the signal-attenuating effects of long distance and obstructions, which we should not have much of in Tanzania (although we can't know for sure). After a scouting trip around MIT by bike and a stunningly accurate fly-through of campus on Google Earth, a few locations look particularly promising, although none of them are more than 0.6 km:
McCormick to the Tang (grad dorm) side of the field- 600 m
Tang to McCormick- 600 m
Tang to MIT West parking garage- 600 m
We are also planning to upgrade from the 19 dBi directional antennas we have been using to 24 dBi antennas, which are only around $15 more expensive, which we should have gotten in the first place. (To get from 24 dBi to the 27-30 dBi range is apparently another story! The two I found at 27 and 30 dBi were huge dishes 6 and 8 feet in diameter, respectively.) And if the link still isn't strong enough, we need add the third repeater modem at a midpoint, and reconfigure the Bullet modems to obey the new network structure (put in new IP addresses and make them recognize different receiving and broadcasting machines).
2. We do not know if it is possible to use 3G routers to access the Internet in Monduli. In the current system, a 3G modem provides Internet access to a 3G router, which in turn provides it to the Bullet modem through one of its Ethernet ports. It is possible that none of the ISPs that provide service to Monduli use 3G modems that are compatible with any 3G routers, whether it is the TP-Link MR3220 that we are using for testing, or any 3G routers available in Tanzania. I also think it is possible that the networks in Tanzania are not configured to allow 3G routers to access them, although I am not sure how they would do this.
The solution might be to use a computer as a router; all models of 3G modem must be compatible with computers in order to function. The mobile broadband connection can then be configured on the computer to be shareable with other computers on the network, where the "other computer" can be a Bullet modem. I have so far been able to share a mobile broadband connection (the prepaid Verizon service I am currently using to test) using a Windows 7 laptop and a MacBook, but encountered difficulties on my Windows XP netbook; even with the same configuration as in the Windows 7, it will not share Internet access. I'm not sure what's going on there. The good news is that there are several old MacBooks at the Orkeeswa school, so if they are all in working order, I should be able to use one of them as a router.
-E
This post is an overview of the challenges we are facing at this point in the project.
1. We have not yet been able to establish a stable link over a distance of more than 0.2 km. (By stable link, I mean the Bullet repeater modems on both sides register some received signal power that does not spontaneously flicker on and off.) We were only able to achieve transient connections over the longest distances we have tested at (0.4 km at Wellesley College, and 0.8 km between the top floor of ADPhi fraternity house and a 3rd floor apartment in Cambridge). We think that this is due to obstructions such as tall trees and buildings.
We will proceed by finding better testing sites with actual line of sight connections between the two ends of the link. Hopefully this will allow us to decouple the signal-attenuating effects of long distance and obstructions, which we should not have much of in Tanzania (although we can't know for sure). After a scouting trip around MIT by bike and a stunningly accurate fly-through of campus on Google Earth, a few locations look particularly promising, although none of them are more than 0.6 km:
McCormick to the Tang (grad dorm) side of the field- 600 m
Tang to McCormick- 600 m
Tang to MIT West parking garage- 600 m
 |
| View across the MIT rugby field, taken from the Tang side. The tall building to the right is McGregor dorm. The tan building a bit further away (directly to the right of the white bulge) is McCormick dorm. |
2. We do not know if it is possible to use 3G routers to access the Internet in Monduli. In the current system, a 3G modem provides Internet access to a 3G router, which in turn provides it to the Bullet modem through one of its Ethernet ports. It is possible that none of the ISPs that provide service to Monduli use 3G modems that are compatible with any 3G routers, whether it is the TP-Link MR3220 that we are using for testing, or any 3G routers available in Tanzania. I also think it is possible that the networks in Tanzania are not configured to allow 3G routers to access them, although I am not sure how they would do this.
The solution might be to use a computer as a router; all models of 3G modem must be compatible with computers in order to function. The mobile broadband connection can then be configured on the computer to be shareable with other computers on the network, where the "other computer" can be a Bullet modem. I have so far been able to share a mobile broadband connection (the prepaid Verizon service I am currently using to test) using a Windows 7 laptop and a MacBook, but encountered difficulties on my Windows XP netbook; even with the same configuration as in the Windows 7, it will not share Internet access. I'm not sure what's going on there. The good news is that there are several old MacBooks at the Orkeeswa school, so if they are all in working order, I should be able to use one of them as a router.
-E
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