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Copyright © 1996 by Virtual Publishing Co.

Technology Review

Metricom's Ricochet Packet Radio Network

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Metricom, Inc., operates a low cost, high performance packet radio data network in Silicon Valley (South San Francisco Bay area) and other parts of the Bay area in California, USA. For $29.95 a month plus the cost of a $299.95 radio modem, network users get unlimited wireless Internet access, at data throughputs typically running from 9600 bps to about 30,000 bps. For an additional service charge, users can access outgoing modems to initiate data calls into the telephone network.

How It Works

The Ricochet network was inspired by Amateur packet networks in the late 1980s. Today, the network bears little resemblance to AX.25 networks but an understanding of ham radio packet networks will help you come up to speed quickly in understanding the basic concepts of the Ricochet network.

To the end user, Ricochet consists of a modem, about the size of a cellular phone, with a short three inch long antenna. The modem speaks popular "AT" commands used by telephone modems so it works with most existing data communications software applications. The modem, though, provides "packet assembly and disassembly" functions (sometimes called a PAD) - combining groups of characters into packets and sending the packets in a burst to the nearest Ricochet network repeater. So while the modem looks like a circuit-switched landline connection, the link is actually a packet radio link.

In the Ricochet network, a repeater is a digipeater in the traditional amateur radio sense, receiving a packet and forwarding the packet on into the network. A Ricochet digipeater is about the size of a shoe box with a mobile cellular look-a-like antenna mounted on the top or bottom. Digipeaters are typically mounted on of light standards (a.k.a. street lights) or occassionally buildings. The digipeater unit is completely self-contained, except for power which is drawn from the street light. Photo 1 shows a picture of the inexpensive pole top radio.

Photo 1. The poletop mounted digipeater.
Ricochet Digipeater
Digital image provided by Metricom, Inc.

The handheld-sized modem (see Photo 2) transmits to and from the nearest digipeater. The handheld features a six inch or eight inch antenna, depending on desired gain (photo shows an earlier three inch antenna). The digipeater, in turn retransmits signals to the nearest wired network access point. Because "pole top" digipeaters can see another pole top radio up to five miles away or so, your modem communicates typically through one digipeater hop to the nearest network access point, where your signal is received and interconnected to the Internet.

Photo 2. The Ricochet modem is about the size of a cellular phone.
Ricochet modem
Photo by Virtual Publishing Co.

Radio Technology

The Ricochet network operates as unlicensed devices in the 902-928 Mhz band.

The radio modem uses frequency hopping spread spectrum technology and dynamically adjusts the transmitter power up to a maximum of 0.6 watts. The raw over-the-air transmission data rate is 100 k bits per second. The actual throughput depends on a variety of factors, including error correction codes packaged with your data, your signal strength into the network, other users and your distance from the nearest network access point. Typical user throughput is in the 20,000 to 30,000 bits per second range making Ricochet Internet access a wireless equivalent of a landline modem connection.

Using Ricochet

Use of the Ricochet system is straightforward since it was designed to be compatible with existing communications software. The Ricochet modem uses the popular "AT" command set used in telephone modems. Using the AT command set, standard software can dial into Internet access or other types of network access. This enables most all existing communications software to work over the Ricochet network. We've used the Microsoft Windows 95 Remote Access Services to dial into the wireless Internet connection, and then run both Netscape and Internet Explorer to surf the net. We've also used the network to access America Online both through an outgoing modem pool and through AOL's tcp/ip interface.

On a business trip to the Bay area, we used Ricochet from several locations in the Bay area, including hotels, restaurants and homes. Overall, the system was easily setup, configured and put to use on our notebook PC. To access the network, you turn on the modem. The radio beeps at power up and then beeps twice when the network has been acquired and the radio has registered with the network. Network registration takes about 10 seconds. Connecting to the Internet access took about 15 seconds more. Then we could run any of our Internet client software.

Metricom also works well inside buildings. Radio signals run a gauntlet of obstacles inside buildings facing up to 22 db of path loss and deep fades caused by interior multipath. There are several ways to attack the multipath problem - one is to use higher power transmitters; the others vary one of the properties of the radio environment - space, time or frequency. Adding more antennas on the receiver changes the spacial domain; retransmiting a signal multiple times on the theory that the receiver will eventually move to a better location uses time domain changes; and the last is to adjust the radio frequency. When a radio changes frequency, so does the wavelength. A different wavelength produces a different multipath scenario inside the building. What was a deep null will then become a solid radio signal. Since Ricochet uses frequency hopping spread spectrum, the wavelength is constantly changing, enabling Ricochet

signals to work effectively inside buildings compared to other fixed channel systems.

Probably one of the coolest things was using Ricochet while sitting at San Jose International Airport waiting for a flight. I checked my email while sitting in the lounge, then surfed the net and read the online version of Dilbert cartoons. The thought of using all this high tech productivity equipment - designed to make us more productive and efficient - to electronically look at Dilbert cartoons struck me as ironic. I then proceded to fire off a note - wirelessly of course - to Scott Adams, creator of Dilbert musing on the absurdity of using all this high tech gear to read Dilbert cartoons while wirelessly connected to the Internet. He wrote back that he couldn't think of anything better to do than read Dilbert cartoons in such a situation ....


Metricom offers several subscription packages. The typical user pays a $45 sign up fee and either buys or rents a radio modem. The modem sells for $299.95 and then you pay a $29.95 per month service charge for unlimited use of the wireless network and access to the Internet. If you rent your modem, you pay $39.95 per month for service and radio rental. The latter option makes it an easy purchase decision to try out the system before making a larger investment.


The major limitation to the network is its limited deployment (San Francisco Bay area, Santa Cruz and Capitola, all in California, plus a dozen or so university campuses), although network deployment will be expanding to new regions throughout the United States soon. Figure 1 shows approximate Bay area coverage at the end of 1995. The system is not intended for nor deisgned to work while mobile. It is designed for the portable computer or PDA user.

Figure 1. Ricochet network coverage in the S.F. Bay area at the end of 1995.
Coverage map
Map provided by Metricom, Inc.

If you have a second phone line dedicated to your computer, Ricochet makes a reasonable substitute. If you have a notebook computer, Ricochet is a great addition since it enables you to surf the Net from your easy chair at home, while eating your lunch in the cafeteria, while keeping an eye on the kids at the park, or while waiting for a meeting to get underway while stuck in a conference room at the office.

Overall, Ricochet is the only consumer priced, mass market, high speed, wireless data network that enables you to send unlimited data [1]. Ricochet is appropriate for the ordinary user, unlike other wireless services and provides true, high speed wireless Internet access.

From an Amateur radio perspective, Metricom proves that high speed (100 kbps) data links are possible. Amateur radio operators need to move much more quickly to higher packet radio speeds. In the early 1980's 1200 bps VHF packet radio links were fast compared to the then common 300 bps landline modem. But Amateur packet networks have been nearly frozen in time with relatively few packet radio operations moving to 9600 bps or higher speeds. 9600 bps equipment is now commonly available. As Amateurs, we should make the "96" in 1996 mean "9600 bps in 96". 56 kbps modems are not far behind.

Footnote [1]: ARDIS is a commercial wireless data network operated by Motorola. ARDIS is targetted at specific industrial applications and is high in cost on a per byte basis. RAM Mobile Data operates a nationwide wireless data network. RAM has reasonable network capacity but slow link speeds (2400 to 4800 bps typical user throughput) with long latency waits making it unsuitable for surfing the Internet. RAM also focuses on providing corporate business solutions. Cellular Digital Packet Data or CDPD operates within the cellular phone system and provides user throughputs of about 9600 bps but at high costs - both the radio modem and the air time charges are inappropriate for use in general Internet browsing.

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Copyright © 1996 Virtual Publishing Company. All Rights Reserved.