MetriTalk

A spin-off of the MosquitoNet project, a project of the Stanford Operating Systems and Networking Group

About MetriTalk

• What is MetriTalk?
  MetriTalk is a protocol for carrying AppleTalk packets over the "Starmode" interface of Metricom's 900MHz packet radios. MetriTalk gives you a "wireless AppleTalk segment", similar to a conventional Ethernet or LocalTalk network, except slower (and without the wires).

• Why would I use MetriTalk?
  Metricom's radios operate in two modes, Starmode and modem emulation mode. A collection of Macs using Metricom's radios in Starmode can send packets to each other directly, like a collection of Macs on an Ethernet LAN can. If one of those Macs also has a connection to your wired AppleTalk network, you can run AppleTalk Internet Router on that Mac and make it be the router for all the other Macs, just as you would on Ethernet. The alternative to MetriTalk is to use the radios in Metricom's modem emulation mode, and send AppleTalk packets using ARA over the modem emulation layer emulating a telephone call over the underlying packet service. Obviously Starmode (sending packets using a packet service) doesn't have quite so many layers of inefficiency. Modem emulation also has the drawback that the mobile computers cannot send data directly to each other any more -- everything has to be sent to the ARA server because a modem can only 'dial' one telephone number at a time. It also has the disadvantage that instead of an AppleTalk router with one radio on it, you now need a whole "modem bank" of Metricom "modems" for all of your wireless Macs to "dial" into. You also have to deal with dialing scripts, modem initialization strings, etc. The biggest concern for us though, was that in Starmode you can power-cycle the radio, change the battery, etc. without losing any of your network connections. In modem emulation mode, if you have to change the battery, then the "call" gets "hung up" and you lose all your connections, just like with a real modem. The reason that Metricom provides the modem emulation is that you don't need any special driver software to use it -- you can just use your existing PPP or SLIP or ARA software -- but when you use the radio that way you lose all the benefits of having an underlying connectionless packet service.

About Metricom Radios

• How do Metricom radios work?
  They use the 902-928MHz band set aside by the FCC's Part 15 rules for free unlicensed low-power use. To comply with the FCC rules, they must not transmit at more than 1W, and they must not use the same frequency for more than 400ms, which is why they hop around in a pseudo-random order between 162 different channels in that frequency range. As well as allowing them to comply with the FCC rules, this channel hopping makes the radios relatively immune to interference -- if there is interference on one channel, they automatically switch to another one.

• How much does a Metricom radio cost?
  The price for a Metricom radio ranges from about $200 to about $600, depending on how many you buy, whether you are a student or not, whether you commit to subscribing to their wide-area packet forwarding service (see below) for a year, etc. For more details see Metricom's Web site.

• How fast is a Metricom radio?
  The raw over-the-air rate is 100kbit/sec, although with packet header overhead, collisions, and congestion, etc. you typically only get about 30-40kbit/sec real end-user throughput. Still, that's faster than a 28.8 modem.

• What is the range of a Metricom radio?
  The outdoor range is typically about a mile, in our experience. In poor conditions it can be as low as 1/4 mile, and in ideal conditions (like clear line-of-sight across the San Francisco Bay) it can be over ten miles.

• What if I'm out of range with the radio I want to communicate with?
  In the San Francisco Bay area, and near the University of Michigan, and in a few other locations, Metricom have set up pole-top repeaters which will forward packets for you when you are out of range of direct communication. This packet forwarding service is called "The Richochet Network".

• How much does Ricochet cost?
  The subscription charge is $30 per month, for unlimited usage. There are no per-packet or per-minute connection time charges.

• What about congestion?
  Many people mistakenly assume that that Metricom's service is like one big shared Ethernet running at 100kb/sec, and that each user gets only a tiny fraction of that. In fact, each radio has the choice of 162 different channels (of 100kb/sec each) for communicating with its neighbours, so there is reasonable capacity. The point where congestion is more likely to occur is at shared resources -- for example, in our research group, we have five laptops running STRIP, and one desktop machine acting as the IP router for our wireless subnet. That desktop machine with it's single Metricom radio is more likely to become the bottleneck than is lack of free radio channels. With five users this has not yet caused us any perceptible degradation of service, but when we have more users it probably will, so we are currenly investigating ideas for load-sharing between multiple radios on the gateway machine (or between multiple gateway machines).

• When will Metricom have coverage in my area?
  Many people ask this question, and it's not really important. Did the early adopters of Ethernet ask "When will Xerox have Ethernet coverage in my area?" Of course not. The question doesn't make any sense. If you connect two ethernet cards they will talk to each other, even though Xerox hasn't deployed any kind of nationwide Ethernet 'coverage' service and never will. In the same way, If you have two (or more) Metricom radios they will communicate with each other. These are not like celluar telephones that only work when there is a base station nearby. We've used our radios in Colorado at the SOSP conference and in San Diego at the Usenix conference, where Metricom hasn't even begun to install any of their wide-area pole-top repeaters. If you're a late adopter who wants one-stop shopping and won't buy anything until it is shrink-wrapped in Radio Shack, then you probably want to stay away from bleeding edge technology, but if you're an early adopter who wants to experiment with new ways to do computer networking (and why else are you reading our web page?) then you can take our packet drivers and use Metricom radios anywhere in the world (subject to local laws about radio devices of course). If you need to extend the range beyond a single hop then Metricom's pole-top repeaters are also very cheap -- roughly $1000 -- so it wouldn't be very expensive to install a few where they are needed.

• How long does the battery last?
  In our experience, the radio battery lasts about six hours -- much longer than the laptop computer's battery.

• What does a Metricom Radio look like?
  Here's a picture of a Metricom Radio, and a picture of a PowerBook Duo with a Metricom Radio attached underneath the legs using Velcro.

  
  
  

• What do you look like?
  Here's a picture of me, hard at work at the office :-)

  

To summarise, there are four ways that you can use Metricom radios:

1. Peer-to-peer, local area
   You can buy a set of Metricom radios and make a little 'wireless network segment' for your Powerbooks. You do this exactly the way you would if you were setting up a little LocalTalk segment. Put one radio on each laptop, and one on a desktop machine that has a wired connection to your AppleTalk network. Run AppleTalk Internet Router on that Mac, and it will forward your wireless packets onto the wired AppleTalk network.

2. Peer-to-peer, with a few pole-top repeaters installed
   As (1), but if you want to increase the range, you can contact Metricom about having some pole-tops installed in your area.

3. Peer-to-peer, with Metricom's wide-area forwarding
   As (1), but if you're in an area that already has Ricochet, then you can simply pay $30 a month for the packet forwarding.

4. Client-to-Server
   If you really want to experience the full joy of modem configuration problems, and you'd rather have to connect a whole rack of Metricom radios to your gateway machine instead of just one, and you'd like to be able to experience your connection unexpectedly 'hanging-up' on you at inconvenient times, then you could use Metricom's modem emulation service and 'dial-in' to a PPP server instead of using Starmode. As described above, making a computer network emulate a modem so that you can run a computer network over it is hardly ideal, but some people might prefer it.

More musings on Metricom Radios.

Getting MetriTalk

You can obtain MetriTalk by anonymous ftp from Bolo.Stanford.EDU

Please let me know if you have any problems.