First, a confession:

I didn't have to buy a laptop computer to use Ricochet's modem radio. The reality is that these devices work quite nicely on desk top systems.

I read of these modems in the local free computer newspaper, but since I was quite happy with my 56 Kbit/sec frame relay connection, I didn't give them much consideration. Then one day my connection failed and I realized I had no backup for Internet access. After a quick trip to a local computer store and a few minutes configuring a PPP connect script, I was back in action. (The frame relay was fixed shortly thereafter, but I still have my Ricochet modem. I use it when visiting client sites, when testing firewall configurations, and I've loaned it out to friends suffering from phone noise and ISP mis-management.)

How do you use it?

Plug it into your serial port (It has two connections--one for external power and one to connect to your computer.) and the computer sees it as a Hays compatible modem which happens to have a few additional AT commands.

I mentioned I had to fiddle with the PPP connection scripts. The changes are small, but crucial. The ppp-on-dialer script needs to send the strings 

      ATZ
      ATE0X4Q0V1&C1&D2&K3
to the modem to ensure the configuration is correct. The ppp-on script needed a line 
      route del default
because my system is also on a local LAN and normally gateways through the frame relay. The phone number, 777**PPP, looks unusual. It says "connect to the regular Internet service using the PPP protocol". The script doesn't need a user name or password because the modem's serial number is used for authentication. As long as your account is current, your connection is established with a dynamically allocated IP address.

How does is it work?

First, it's not a cell-phone modem. It is based on an infrastructure which is independent of the various telephone systems. Metricom has been in the wireless communications business for many years--providing remote-access monitoring of meters to public utilities. The Ricochet division was started in order to offer wireless Internet connectivity to the public. They're currently offering service in the Seattle, San Francisco Bay, and Washington, D.C. areas. Ricochet plans to extend coverage to the Los Angeles area by the end of this year and will then work on other cities on the U.S. east coast.

If you pay attention, you'll see cell phone antenna towers or roof-top clusters appearing everywhere. The Ricochet system is quite a bit harder to spot. There are three elements: the "modem radio" attached to your computer, a shoebox sized "microcell radio" attached to streetlamp poles at quarter mile intervals, and "wired access points" to serve thirty or so microcell radios. For instance, in this map of a Seattle neighborhood the small dots are the microcell radios, the red stars are wired access points, and the blue star is the home of the Linux Journal.

Just as the Internet is based on a store-and-forward model to get data from one place to another, Ricochet packets are forwarded from pole to pole from the modem radio to the wired access point, at which point they enter the conventional Internet routing.

What's the future?

Since I got my modem last fall, Ricochet has offered two new products: a modem (without battery) for fixed stations and a newer, lighter weight version of the portable modem. In addition, Ricochet has distributed a firmware upgrade which allows a roaming user to stay connected while moving from one wired access point coverage area to another.

The current technology operates in the 908-926 MHz band, dividing it into 162 channels, each 160 kHz wide. Each connection hops among these channels using spread spectrum technology. Modems provide a connection with performance comparable to a 28.8 kbit/s modem. Future models will use more channels and other bands to provide higher speeds and better coverage.

What if you're not in one of the "blessed cities"?

In addition to the cities mentioned above, Ricochet has established service areas at several universities including Austin College, Cal Poly, George Washington University, Oregon State University, San Francisco State, Stanford University, UC Berkeley, UC Santa Cruz, University of Miami, and University of Oregon. (Think of these as small cities.)

Remember the strange phone number? You can also enter the serial number of another modem. If it's nearby, and not currently engaged, it will see a "RING" and the other user's program can answer the phone. This is the way in which a peer-to-peer service is established between two modems.

There's another mode of operation as well--called Star. In normal mode, the modem acts like a Hayes compatible modem; in Star mode, the modem delivers packets into space (the original ether) and other modems see these packets and can deal with them much as they do Ethernet packets. This mode only works in modem-to-modem communication, but the greater bit capacity makes it worth exploring. If you look in the kernel configuration, you'll find support for this mode is already available. With this compiled in, you and your co-workers and friends can have your own wireless LAN whereever you go. Very convenient for conferences!

About the laptop...

I finally broke down and bought one. Naturally it runs Linux and when I'm out of the office I unplug the PCMCIA Ethernet card and turn on the Ricochet modem.

When the microcell radios are installed, they're configured with their latitude and longitude. One of the extra "AT" commands causes the modem to report the location and signal strength of up to ten nearby microcell radios. I've wandered around Seattle getting these reports and looking for microcell radios. That's how I built the map.

Needless to say, I had to check out the roaming feature once I get my laptop. As I drove around town, I issued a ping command directed at my home system. Sometimes the latency was quite high, but it also seems that this went down when a second ping was issued. I suspect that routing tables are updated often enough that the system quickly finds the best route.