I know they make special outdoor-rated network cable...but it's probably pretty expensive. Plus, will it stand up to the elements (like large hailstones) with a hard surface (the roof) beneath it? I would consider using normal network cable run through a electric PVC conduit (the grey stuff). I would also run a piece of string through the PVC conduit so you can easily pull a 2nd cable through if the first one fails. I am not an electrician though, and do not know where you live...so I would highly recommend that you consult an electrician who is knowledgeable in network cable run between buildings. If buildings share a different ground system, they will have a different potential, possibly resulting in a fire risk (worst-case) or equipment failure. You might have to end the network cable at each building (that will have networking equipment/connections), ground it, and then continue the run. Here is a link that you might want to read concerning grounding network cable between buildings:
http://www.ubnt.com/forum/showthread.php?t=26136http://itwlinx.com/wp-content/uploads/1Gb-CAT6-LAN_new.pdfThe 2nd link above is a product that you would mount inside the entry point of each building...it grounds the incoming cable for the potential differences that we discussed. I found this product for around $75/each online. You would need 2...one for the main building, and one for the building that the cable is terminating in. I don't think this device supports shielded network cable...so if shielded cable is required per your building code, you will need to get a device that supports shielded cable. ITW has other products, and I could be wrong about this one not supporting it...so call them (or similar company) to verify. You might not need to worry about this grounding issue at all if all of these buildings (on the left side of the alley) share the same electrical panel/ground. But, then again, you are talking about running cable outside on the roof...so talk to a pro to make sure.
Another thing to think about if you choose to go through the roof with the cable...where the cable transition from inside to outside, make a small dip in the wire (on the inside side) for 2 reasons...1) to avoid a 90 degree bend (not good for network cable) and 2) if a leak develops, the water will drip off of the lowest point in the dip as opposed to following the cable back down to your equipment. I would make the transition occur on a higher point in the roof so a lake isn't forming over the transition point.
Also, you need to avoid running the cables directly over/near any fluorescent lighting (which I'm guessing your buildings are full of). The EMF they throw off can disrupt data transmission through network cable.
As far as the wireless devices to bridge your connection across the alley, I'm not familiar with Apple's networking devices. As long as it supports "bridging," you should be ok. Just make sure to get N and not G wireless. The faster the better for this purpose. You also need to consider the environment that you will be mounting the devices. Obviously moisture, chemical fumes, etc, are going to result in corrosion and shorter device life. Plus, any equipment that consumes a large amount of electricity and/or has magnets in it, is probably going to give off a big EMF...wreaking havoc on wireless network connectivity. So try to mount the 2 devices as close as possible across the alley from each other and as far away as possible from any big equipment...and with as little metal/brick/etc as possible between the 2 devices. They also make point-to-point, line of sight microwave systems to bridge 2 sights, but they are pretty pricey. With an alley only 12-20 ft wide, you should do fine with 2 routers in bridge mode. Here is a router that I've used (briefly) in bridge mode:
http://www.amazon.com/RT-N13U-Wireless-N-Advance-multi-Media-Router/dp/B0045JCL22/ref=cm_cr_pr_product_topI've read about issues with the above router freezing up, but some reviews indicate it might be the firmware and not the hardware. You can actually replace the firmware with an open source system, but I'm not sure if those support bridging. This router also lets you change the signal strength. The stronger the signal, the more power consumed. The more power consumed, the more heat generated. The more heat generated, the more likely you are to have issues with the hardware locking up. Some people actually open the devices and attach a custom heatsink to replace the small default one. You also want to make sure it has plenty of ventilation so it doesn't overheat. Extreme cold can also cause problems with any device. Basically, you want to keep any devices (designed for home use) in a controlled climate with a normal indoor temperature range.
As far as your 2 cameras go, the one on the left will not be able to monitor the bay door (or that entire corner) in that red area. You'd probably want to move it to the opposite corner (closer to the bay door) of that building...or maybe even the opposite corner of the big building so you could see both bay doors. Anyway, you would want some network switches (not routers), which are cheap, located strategically in each building if there will be multiple cameras in that building. You'd want to place them where they will reduce the length of network cable required.
I'm guessing the project, if you run the cable and set everything up, could be done for around $1000. $75+ for 1000ft of network cable (bulk = much cheaper), $75/each for the grounders, $60+/each for 2 wireless routers capable of bridge mode, $15+/each for a few switches, a few dollars for the RJ45 connectors, $20 for a RJ45 crimper, $50+/each for the cameras, plus more for camera enclosures/etc. This is assuming, of course, that you already have a computer capable of monitoring the cameras 24/7 and a consistent high-speed Internet connection.