A spin-off of the
project, a project of the Stanford
Operating Systems and
- 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
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
- 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
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
- 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
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
- 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:
- 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
- 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.
- 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.
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.
You can obtain MetriTalk by anonymous ftp from
Please let me know if you have any problems.
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