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Saturday, September 23 2017 @ 02:23 AM CDT

Distant Internet

Previous: Alternate Power Sources - Next: Automation and Remote Control

While power is the biggest determining factor for many remote projects, the internet connection is almost as big, and in some cases might even be bigger since it can involve supplying power at multiple remote locations.


If you can't connect via wire, then the best scenario is one where you have line-of-sight from site to some place where internet is available inexpensively. In this case it is likely that a pair of industrial WiFi radios will provide you with more than enough connectivity for several cameras, or a couple of streams (low and high rate from each) for two or three cameras.

The worst-case is where you have to provide some sort of satellite link as these tend to be very expensive per Gigabyte over a fairly minimal amount included in the account. We’ve priced this out at as much as $10,000/month for some services.

It is also possible to use fiber-optic cable in some instances. Less expensive, multi-mode fiber cable can be used for distances out to a couple of kilometers. Past that you’ll need the more expensive single-mode cable. See this web page for some basic information on the different types of cable.

One of the more popular fiber media converter suppliers lists multi-mode specs to 2km and single-mode to 15km (different converters) – and note that the multi-mode requires 2 fibers where the single mode only requires one.

Like Ethernet cable, fiber-optic cable can be buried or run aerial.

Wireless Links

Most people are familiar with WiFi. The difference between the typical home WiFi setup and an industrial-strength system is mostly in the antenna, but also in the quality and sensitivity of the radios, as well as the options available for setting things up optimally.

In addition, there are other frequencies (than the typical 2.4GHz unlicensed band used by WiFi) that may be used in certain circumstances. Some require a license from your local communications agency (Dept. of Communications (DOC) in Canada or Federal Communications Commission (FCC) in the US) and some are much higher bandwidth but more limited in distance than long-haul WiFi gear might be, but might be useful in some cases. These include things like 60GHz microwave and “free-air optical” systems (aka lasers with broad beams) that are coming down in price from when they were first introduced.

There are also systems in the 800 and 900 MHz range that are better able to deal with things like trees in the line of sight. Use of these systems is very much location dependent.

Multi-hop Wireless

The typical WiFi system is not very useful where more than one or two links is in line between a camera and the rest of the internet. This is due mostly to the latency (time a packet needs to transit the links) and congestion (interference from other uses of the link and from other radios on the same frequency as the link)

The reason for this is that the typical consumer equipment is neither fast enough (to pass packets quickly) nor good enough at discriminating (and protecting) its link from interference. Putting more than a couple of these systems back-to-back to get longer distances, even with directional antennas instead of the typical omni-directional “whip” antennas, simply doesn’t work well.

The location to the left is the mid-point of a 2-hop WiFi link for the Goldstream Estuary link. The bottom radio linked 2 miles to the Goldstream center. The top one linked about 1/4 mile to a home on the other side of the estuary. The box I'm working on connected the two and contained the power supplies for them.

With commercial-quality equipment designed for back haul it is possible to string together several links in a multi-hop line without fear of degrading the link or getting unacceptable latency. In addition, the radios have more flexibility and configurability in how they handle power and things like how long beacon transmissions last – all things that make setting up the link a bit more complex, but which in the long run make it run much better.

For links up to 3-4 hops, using single-radio (half-duplex) should work fine. For links with more hops it is most useful to use systems with two radios for full-duplex, where packets can flow in both directions at the same time instead of being held at a point while the radio “turns around” the direction from receive to send. Commercial systems with full-duplex capability are available for just this situation.

 

 

Fiber Optic and Mixed Systems

It is possible to mix and match any/all types of transmission of packets over any given distance, provided the link latency works for your particular system. We’ve seen systems where radio links are themselves liked together by twisted pair for a few hundred feet, and in once case by a fiber-optic link of several thousand feet. The electronics at each point where the physical link changes from one to the other needs to be fast (i.e. not consumer, industrial) but aside from the need for power at each point this can make a link almost any length.

The major concerns with any technology that extends Ethernet over a distance are:

  • latency – the time it takes for a packet to traverse the network from the camera to the server it needs to talk to, plus the time it takes for an acknowledgment packet to get back to the camera saying the packet got through without problem
  • The “bit error rate” (BER) of the total length of the link. This affects how often a packet is likely to have to be re-sent. In many cases it will only have to be re-transmitted over the section that the error occurred on, but in some cases it will cause a complete loss of the packet which will cause a time-out at the sending end due to not receiving the acknowledgment packet (does not matter whether it was the sent packet or the ack packet that actually disappeared)

Latency includes the hardware at each end, not just the link itself. If a server is extremely busy (doing other things) then its software may not ack the incoming packet fast enough. Similarly, a slow camera may not ack an incoming command packet from the server fast enough if it is working too hard doing encoding and/or serving up multiple streams.

BER can be affected by things like “rain fade” due to moisture in the air which absorbs the radio signal in the 2.4GHz range, or kinks/bends in a fiber-optic cable from wind moving a hanging cable (or animals stepping on one on the ground) or from oxidation of metal connectors, noise from power supplies, lightning, etc. A link may work perfectly for months then go bad during a heavy rain storm – and get better when it dries. It also may work perfectly for years, then get worse and worse until it fails completely for no obvious reason. Jiggling a single connector can bring it back to health for a time – then it fails again, all because the connectors were not gold-plated or otherwise properly protected from the elements in the first place.

If you’re going to the trouble of putting in a long link, do it right the first time, even if it costs extra. In the long run it will be worth it.

 

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