Chapter 17: The Parabolic Mesh Reflector Project

Troubleshooting the Network Setup

That is (I suppose I should say) you hope it works. Network configuration is closer to an art than a science, and troubleshooting is easier in a lab setting than up on a roof or a ladder. That's why I powerfully recommend setting up two simple networks in a lab to get the APs configured so that they bridge, long before you install anything in its final position up in the air.

Your first evidence that the bridge is functional will be the presence of the farside AP and the farside computers on the router's DHCP clients table. If any farside device is present on the nearside DHCP clients table, your bridge is working! If some devices are not present, check to make sure they're configured to request a dynamic IP via DHCP.

Note that it's possible to assign a local, non-routable IP address to an Ethernet device manually, without pulling it from a DHCP server, and some people prefer to work that way. If you're not using DHCP, try using the ping utility to get a response from one of the devices across the bridge.

If you can't ping any device across the bridge, log into the AP configuration pages and double-check these points, which I've arranged in the order that they're likely to be a problem:

1. The SSID must be the same on both APs.

2. The channel must be the same on both APs.

3. Bridge Mode must be selected on both APs.

4. Each AP must have the Ethernet MAC address of the other AP entered into its Remote Bridge MAC field. Double-check that you entered the MAC addresses correctly. Double-check that you entered the Ethernet, and not the wireless, MAC addresses.

5. Check that WEP is not enabled by mistake, either on one side or on both sides with different keys. WEP should ultimately be used, but enabling it is the last thing you should do before calling the bridge complete. Until everything is known to be working correctly, leave WEP disabled.

6. The bridging channel must not be one in use by any other nearby Wi-Fi device. Audit the area near both legs of the network with NetStumbler (See Chapter 18) to see if any other APs are operating on your bridging channel and possibly interfering. If either leg has additional access points, it's best to turn them off while troubleshooting your bridge.

7. Make sure you don't have two (or more) DHCP servers operating. Lots of Ethernet devices now contain DHCP servers, including the DWL-900AP+ access point itself. You should have one router-and one DHCP server-for the linked network. Make sure that any other DHCP servers connected to the network are disabled.

8. If you're using manually entered IP addresses, check for subnet conflicts between the two legs of the network. If you're using DHCP, subnet conflicts should not be an issue, since you're letting the router decide what addresses everything should use.

9. Check the firmware release levels of both APs to see if they're identical and up to date.

10. Finally, look for 'dumb stuff ' like detached antennas, yanked power cables, wall warts not plugged in, not-quite-inserted CAT 5 plugs, crossover cables accidentally used where a straight-through cable should be, and so on.

If all your troubleshooting fails, it may be time to call on expert help.

Aiming Your Antennas

Once you've gotten your two APs to bridge in a lab setting, the hard work is done. The only other really tricky thing is aligning your gain antennas across the space your bridge is to cross.

The best way to do this is to temporarily connect an access point in access point mode to one antenna, connect a laptop client adapter to the other antenna, and then use NetStumbler's field strength display to sense when the antennas are aimed optimally. You're looking for the best signal figure (the green portion of the line) and lowest noise figure (the red portion). The access point and client adapter used should have similar power output and receiver sensitivity to the APs used in the bridge link.

Some notes on aiming antennas:

• Have somebody on both ends, communicating via cellphone or FRS radio.

• Remember that gain antennas operate in three dimensions, and must be aligned up-and-down as well as side-to-side. The beam width is probably just as narrow vertically as horizontally.

• Be realistic about the distances involved. If NetStumbler can't 'hear' an AP across the gap you've chosen to bridge, your bridge APs are unlikely to hear one another either. Crossing distances measured in miles is quite a feat, and you're likely to need sizeable parabolic dish antennas to pull it off. Calculate your link budget first! (See Chapter 8.)

• Be careful if you're installing antennas in high places. It's easy to get distracted and forget that you're up on the roof or on a ladder. If you have a laptop with you, it's smart to rig a safety cord of some sort to keep it from making a oneway trip down to the pavement.

• Watch out for power lines. Power lines represent a hazard both to you on your ladder and to your AP/antenna setup. Ideally, you should be no closer than 50 feet to a power run.

Enclosures for Bridge APs

I've used two types of enclosures for bridging APs: Commercial plastic gasket boxes and military ammunition cans. Both are watertight and relatively inexpensive. I favor ammo cans because it's often hard to tell how well a plastic box will stand up to sun, wind, rain, and extremes in temperature. Ammo cans are steel and will stand up to almost anything, though they have the downside of being harder to drill. They have spring-loaded lids with rubber gaskets, and when the lid is snapped closed the box is not only watertight but airtight (see Figure 16.4).

Military surplus stores often have ammo cans, as do mail order surplus vendors. (See Appendix A for sources.) The cans come in various sizes, and you can't always count on finding any given type in any given store. Look around. The one I've used successfully is the 30 caliber can, which measures 10 1/2' x 4" x 7". (This is the type of can shown in Figure 16.4.) The can must be large enough to contain the access point, the power-over-Ethernet tap or other power supply, connecting wires, and some structure to keep everything in place. The 30 caliber size is the smallest ammo can I'm aware of, and it's more than large enough to hold the DWL-900AP+ access point, the power over Ethernet tap, and the various associated cables.

Figure 16.4: A Surplus 30: Caliber Ammo Can.

The cans are quite cheap; I've seen the 30 caliber size go for as little as $4 each, and usually between $5 and $8. Shipping and handling will be more than that!

Here are some tips if you decide to go the ammo can route for an AP enclosure:

• A 30 caliber ammo can will probably hold a Linksys WAP-11 access point, but it will be tight, and getting the AP, the PoE tap, and the wiring to coexist will be a challenge. If you have a long distance to span and some budget to spend, a Cisco Aironet 350 access point is a good choice for a 30 caliber can, as it's quite compact and puts out 100 mw. Keep in mind that there are larger ammo cans as well, and the 50 caliber size will fit most any access point without a struggle.

• If you're not familiar with ammo cans, take note that the can and the lid come apart nondestructively: Just swing them so that they're 90 to one another and slide the lid to one side. It's much easier to drill and work with the lid without the can flopping off of it.

• Make as few holes in the can assembly as possible, and make holes only in the lid. Mount the can on the pole or other support lid-down, using a bracket or 1/2' pipe floor flange attached to the lid rather than bolt holes through the can itself. This way, all holes will be on the underside of the enclosure, and water will be unlikely to enter the can via gravity.

• Use a double-ended bulkhead female N connector to bring your antenna feed into the can (see Figure 16.5). Your gain antenna cable plugs into the outside end of the connector, and the pigtail to the access point connects to the inside end. Note the rubber O-ring in the photo. The O-ring should be on the outside of the ammo can, and the hex hut on the inside. The O-ring is squeezed between the metal shoulder and the can, and provides a nice weather-tight seal.

  
  Figure 16.5: A Female Bulkhead N Connector.