7.2 Point-to-Point Links

From a radio perspective, point-to-point links are very straightforward to set up. You should always follow more or less the same steps when evaluating the possibility of a link:

  • Establish that you have line of sight from end to end.

  • Measure the distance between the points and calculate the path loss.

  • Add in the capabilities of your equipment to determine your link budget.

  • Go out and hook up your gear.

If you intend to make a long-distance point-to-point link, first find out the latitude, longitude, and altitude of each end point. You can find this by physically going to each site and marking the coordinates with a GPS, or you can estimate using topographical maps or software (see Chapter 6 for some examples of how to do this). With the coordinates and altitude of both sites, you can calculate a bearing and tilt angle, so you know roughly where to point the antennas on each end. A decent GPS can help here by giving you a bearing to and from each point. You should also check out the online wireless design CGIs at http://www.qsl.net/n9zia/wirelesspage09.html for help with many of the calculations you'll need to perform.

Obviously, if you can see the other point through binoculars or a telescope, this is a good first step. Ideally, there should be very little on the ground between the two points. The closer the path is to an actual valley, the better. Take a look at Chapter 6 for details about how to calculate the path loss and link budget for your link. I've mentioned it before, but here it is again: keep your antenna cable as short as possible! On a long-distance point-to-point link, every few decibels count.

Now that you're ready to hook up your gear, the question remains: what gear do you want to use? That depends on your (fiscal) budget and how you plan to use the link. As we saw in Chapter 5, it is very simple to set up a Linux gateway in IBSS or Host AP mode. This is a popular and flexible way to go, but setup can be a little complex if it's your first link. If you already have a hardware access point, you can use it for one end of the link, and have a computer using a client card on the other. Another alternative is to use an access point that will bridge over the air, such as the Linksys WAP11 (although there are varying reports of success and stability with that particular model). Keeping your WAP11 firmware up to date seems to be the best move you can make toward greater operating stability. Finally, it is also possible to use client hardware (such as the Orinoco Ethernet Converter or Linksys WET11) on one end of the link to talk to an access point on the other end. People have had different experiences with these devices, but, generally speaking, firmware updates seem to resolve most issues.

The farther apart your points are, the harder it will be to aim your antennas. At distances up to five miles or so, this is rarely a problem (as long as you have enough total gain to overcome the path loss). At greater distances, getting the antennas pointed directly at each other can be quite tricky. Here are some techniques that might help you get your dishes pointed in the right direction:

  • Use cell phones or radios to maintain communications between the two points while you're aiming the antennas. It helps to have at least two people at each end (one to manipulate the antenna, and another to coordinate with the other end).

  • Set up all your network settings ahead of time, so there aren't any variables once you get to the remote site. Check all gear, ping each box, and even transfer a file or two to be sure that your equipment works at close range. You don't want to question it later if you have problems getting the link going.

  • Use a tool such as the Lucent Link Test meter (which ships with the Windows driver for the Orinoco card) or any of the other tools mentioned earlier in this chapter to show the signal strength and noise readings in real time. This kind of tool is your best friend, short of an actual spectrum analyzer.

  • Work on one end of the link at a time, slowly changing one variable at a time until you see the maximum signal strength and lowest noise at each end of the link.

  • If you have one handy (and your link budget permits it), first try an omni or sector antenna on one end of the link. Once you find the other end of the link, replace it with your dish or yagi, and tune it in.

  • Sweep slowly, and don't be afraid to go beyond the best perceived signal. Most antennas have smaller side lobes that appear as a false positive. Keep moving until you find the main lobe. It should stand out significantly from the others, once you find it.

  • Do not touch the actual antenna when taking a reading. This is particularly easy to overlook when using tube yagis, like the Pringles can (see the later section). Resting your hand on the antenna tube will interfere with the radiation pattern and drain your signal very quickly. Take your readings with all hands clear of the equipment.

  • Don't forget to compare horizontal and vertical polarization. Try the antennas in both positions, and use the one that shows the lowest noise (see Section 7.5 later in this chapter).

  • Once your link is in place, consider using WEP to discourage others from attempting to connect to it. If you want to provide wireless access at either end-point, set up another gateway, preferably with caching services (such as caching DNS and a transparent web proxy, like Squid). This helps reduce the amount of traffic that goes over the long link, helps cut down on network collisions, and generally makes more efficient use of the link.

It can take all day to properly align antennas at a great distance, but it can also be a fun time with the right group of people. Just take your time, think about what you're doing, and be sure to leave time at the end of the day to celebrate!

There is something I must mention here: I know you're probably excited about getting your link up and running, but never neglect attention to safety. Mounting antennas on roofs or poles can be hazardous, particularly if you are preoccupied by thoughts of link budgets, pigtails, and signal strength meters. In February of 2002, I nearly lost my own life when I fell from a friend's roof while working on a point-to-point link. I had been on many, many roofs at that point, and carelessly went out on a roof after sunset. I remember thinking, "It's getting dark, but we're almost done. I'll just go out and finish up." In the next minute, I stepped off of the roofline and ended up in a hospital for the next week (and recovering over the next several months).

When working on a wireless project, take your time, make sure you have plenty of light, and always work with a friend if you're doing anything precarious. Pay attention to power lines. Whenever possible, you should wear a harness when working on a roof or other high place. Remember that the problem will always be there for you to solve . . . tomorrow. Building your own network is tremendously rewarding, but no link is worth risking your life.