Author Topic: Cellular routers  (Read 4331 times)

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Offline SLOweather

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Cellular routers
« on: March 13, 2011, 10:35:24 AM »
I copied part of Johnd's WeatherLink IP post over here to kick off this new board. reply below quote...



Quote
Who's 3G router are you using?

The one I'm using is a MultiTech Multimodem GPRS (not sure if this is a current model any longer with an Ethernet interface - it is 3-4 years old now.) And yes I know plain GPRS isn't very fast, but it's fine just for passing the small per-minute amount of WLIP data.

Actually, for all the talk about minimising power drain from an additional hub/switch, it is the GPRS/3G router that is the real power hog. But I guess if it's going to be radiating up to 2W of RF power then several different makes/models of router are going to be wanting much the same power overall of 3-5W depending on frequency band, because it's the RF transmission requirement that dominates the power consumption and not the rest of the circuitry. Two things I wonder about:

1. How much it might be possible to reduce average router power draw by using a high-gain yagi antenna for the cellular signal. Aren't 3G routers supposed to be clever enough to limit Tx power according to the strength of the signal from the cellular base station?

2. In the context of eg the WLIP, which only needs to transmit data every minute, is this long enough for the router to send its cellular Tx circuitry to sleep and then to set up the connection again when data needs to be passed. So is it worth looking for a router with an auto-sleep function (assuming that they do exist) and would this work OK with a WLIP logger?

BTW I don't know whether Admin is likely to read this post, but I'd be really pleased to see a forum/subforum set up dealing specifically with remote wx sensing - ie just the sort of topic we're dealing with here - it's a topic that's not really covered much elsewhere and can be tricky territory for anyone interested in setting up such an installation who is not already an M2M professional.

I have a Bluetree Wireless EVDO router on a solar powered WeatherElement site. While it was on the bench for antenna replacement (to a Yagi, BTW), I learned how to use their configuration and monitoring program to watch how the unit is operating. The same parameters are also available via telnet and AT commands.

I can see the RSSI, noise, TX power, cellular channel and a bunch of other stuff. The problem is that, since the unit is DHCPed, the IP changes. I set the WeatherElement data uploading script to log the IP address on every upload. I can look it up and put it into the program, connect to the router, and monitor it from afar.

Do you have access to your unit's IP address and is there a way to log into it remotely?

Offline SLOweather

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Re: Cellular routers
« Reply #1 on: March 13, 2011, 04:52:00 PM »
Quote

1. How much it might be possible to reduce average router power draw by using a high-gain yagi antenna for the cellular signal. Aren't 3G routers supposed to be clever enough to limit Tx power according to the strength of the signal from the cellular base station?

Yes they are. When logged into my router and watching the RSSI and Tx power numbers, it clearly increases power when the RSSI drops.

But I don't believe there's a lot of power saving to be had. I measured my BlueTree on the bench before deployment, and, while the power draw does go up when transmitting, it just isn't transmitting for very long.

Quote
2. In the context of eg the WLIP, which only needs to transmit data every minute, is this long enough for the router to send its cellular Tx circuitry to sleep and then to set up the connection again when data needs to be passed. So is it worth looking for a router with an auto-sleep function (assuming that they do exist) and would this work OK with a WLIP logger?

In my experience, the EVDO router does shut off the transmitter when not actively needed (based on current draw readings during operation). Kinda like push-to-talk. That reduces power draw, and frees up the cell channel for someone else if needed. On busy days, I've seen mine switch channels and probably even cell sites quite often.

Does your router have LEDs on it? If so, is there a way to disable them, either in firmware or with a hardware jumper? That will save a few milliamps of current draw.

Offline linuxfreak

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Re: Cellular routers
« Reply #2 on: March 14, 2011, 03:11:07 AM »
How about using DynDns to give a pseudo domain name to your changing public IP address. I know most routers have the option built in. I don't run a cellular enabled router, but the factory firmware and also dd-wrt both had the feature in the software setup screens to enter your login for DynDns. I use the feature so I can remotely log into the server I use for development here.

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Online johnd

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Re: Cellular routers
« Reply #3 on: March 14, 2011, 04:12:25 AM »
How about using DynDns to give a pseudo domain name to your changing public IP address.

There seems to be a general problem about using dyndns with cellular routers. To what extent this is country-specific I don't know, so circumstances in the US, for example, may be different.

My picture of the cellular IP network is that it works like a giant LAN, but I can imagine that technically this isn't an accurate description. In the UK (and other parts of Europe too I think) what seems to happen is that all cellular IP addresses are effectively dynamic private IP addresses and therefore routable only in the context of the internal network (internal in this sense meaning the network's entire cellular network or at least a limb of it). So when a cellular router accesses the Internet, it effectively does so via some process like NAT at the telco central gateway.

What this means is that while a device behind a cellular router can readily initiate a link to a remote server, the converse is not true - you cannot initiate the connection from outside to a device behind a cellular router even using dyndns (because the routing would need more information than the single IP address broadcast by the dyndns feature of the cellular router). This isn't a problem using a wx data source that only needs to pass data outbound, ie like a WeatherlinkIP working in default mode, but is not good news if you need to contact a logger from the outside eg to request a data download.

There is a way round this by acquiring a SIM with a static IP address - network connections then just work as normal, though there issues like extra latency to worry about. Certainly in the UK these SIMS are available, although not always easy to source - there seems to be just one company that specialises in providing this sort of service on a one-off basis. The monthly plan for the SIM typically works differently to a general cellular SIM in that you just get a modest monthly data bandwidth allowance and no call or text allowance. I think we're currently getting 200-300MB/month bandwidth for around $20.

NB I'm really not sure that the description of cellular network architecture is accurate, although the ultimate effect of not being able to initiate contact with a device behind a cellular router from the outside is what seems to happen in practice. Feel free to correct this description if you know better - this is another of those areas where getting solid information isn't always easy.
« Last Edit: March 14, 2011, 04:30:43 AM by johnd »
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Offline SLOweather

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Re: Cellular routers
« Reply #4 on: March 14, 2011, 11:50:15 AM »
In the US, at least on Verizon, the cellular router gets a real, live public IP address. Static IPs are available, but most are DHCPed.

I know this because I log the IP address of my WeatherElement Verizon EVDO router on Condor Lookout every time it uploads data to the server. Using that info, I can log into the router directly with telnet, or look at the simple web page my data hub provides. I have port 80 forwarded through the router.

You can see it at http://75.208.128.251/INDEX.HTM today until whenever Verizon rolls the IP address.

This router does have some sort of dynamic DNS ability, but it seems to be proprietary.

Online johnd

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Re: Cellular routers
« Reply #5 on: March 14, 2011, 12:30:45 PM »
In the US, at least on Verizon, the cellular router gets a real, live public IP address.

Hmm, interesting. I think the same may be true in Australia/NZ.

Actually and to be more specific, the issue in the UK is not so much that it's impossible to get a (dynamic) public IP on a cellular router but that it depends on so many things that the only reliable solution is to get a service with a SIM that has a static IP. 'Things' include:

Which network you're using;

Whether you're interested in 100 SIMs (in which case you'll probably be talking to a more clueful corporate helpline) or just one (where you'll be lucky to hit a service agent who even understands a question about 'can it have a public IP address', let alone be able to give a useful answer). Actually this is the number one problem.

What network speed you're connecting to (ie is it GPRS/EDGE/3G) etc. With some networks it seems that the faster speeds connect to a more modern network architecture, ie 3G may give you a public IP, but if the network has fallen back to GPRS then it may be a NAT connection. (And bearing in mind that remote sites will often be in more rural areas with poorer cellular coverage) The unpredictability here obviously isn't helpful.

Which APN you have set. I have seen it said that if you specify one APN on a network (with a choice of APNs) then you'll get a public IP but otherwise it will be NAT.

And so on...

So you end up going the static IP route just to get some predictability of connection type. Hopefully it might be the case that as and when IPv6 penetrates the cellular networks then there will be public IP addresses for everyone, maybe even fixed IP addresses. But I guess that won't happen for a year or two yet.
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