ns.uoregon.edu/~jremy/strip.html
STRIP is a protocol for carrying IP packets over the "Starmode" interface of Metricom's 900MHz packet radios. STRIP gives you a "wireless IP subnet", similar to a conventional Ethernet network, except slower (and without the wires). Metricom's radios operate in two modes, Starmode and modem emulation mode. A collection of hosts using Metricom's radios in Starmode can send packets to each other directly, like a collection of hosts on an Ethernet LAN can. If one of those hosts also has a connection to the rest of the Internet, you can make it be the router for all the other hosts, just as you would on Ethernet. The alternative to STRIP is to use the radios in Metricom's modem emulation mode, and send IP packets using PPP 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 PPP server because a modem can only 'dial' one telephone number at a time. It also has the disadvantage that instead of an IP router with one radio on it, you now need a whole "modem bank" of Metricom "modems" for all of your wireless hosts 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. 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 software -- but when you use the radio that way you lose all the benefits of having an underlying connectionless packet service.
The raw over-the-air rate is 100kbit/sec, although with packet header overhead, collisions, and congestion, etc. 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. 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". The subscription charge is $30 per month, for unlimited usage. There are no per-packet or per-minute connection time charges. 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 group, we have five laptops, 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). We will almost certainly leverage off our work with Mobile IP, which allows a host to change its point of attachment to the Internet without changing its IP address. Many people ask this question, and it's not really important. 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 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. In our experience, the radio battery lasts about six hours -- much longer than the laptop computer's battery. Here's a picture of a Metricom Radio, and a picture of one of our Gateway 2000 Handbook computers with a Metricom Radio attached to the back of the screen using Velcro.
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