This article first appeared in the August 1997 issue of Monitoring Times.

PACKET-SWITCHED NETWORKS

The Internet has popularized the idea of the packet switched network. Instead of using a dedicated, direct connection between the source and the destination to move information, like a telephone call uses, a packet switched network moves chunks of information (packets) from one point in the network to another, allowing the communications channel to be shared among a number of users.

This month we'll take a look at three companies offering wireless packet data communication service, each of which allows mobile users to access and send information anytime from (almost) anywhere. A typical subscriber has custom software running on a laptop, notebook, or palmtop computer that is connected to a wireless modem. The most common application at this point in time seems to be electronic mail, although inventory management, sales and marketing information, and even web browsing are becoming popular.

Before cellular carriers began to market data communication solutions (see the March 1997 PCS Front Line), two major wireless providers were offering nationwide radio data communications service. Both services are similar in a number of ways, and offer the following features:

  • Transparent nationwide roaming, allowing a user to travel from city to city and be able to use the service without a complicated registration process.
  • Store and forward capability, holding messages in the network until the recipient is available.
  • Group messaging, allowing a single message to be "broadcast" to a number of subscribers at once.
  • Robust error detection and correction, indentifying and fixing packets that may have been corrupted during transmission.
  • Per use charges. Instead of paying by the minute, customers pay according to the amount of data they transfer, typically measured by the kilobyte (about a thousand characters).

ARDIS

Advanced Radio Data Information Services (ARDIS) started out in 1990 as a joint venture between IBM and Motorola, primarily to serve IBM field technicians. A few years later Motorola bought IBMís 50% share for about $35 million. A network operations center (NOC) is maintained in Lexington, Kentucky, with a backup near Chicago, Illinois.

ARDIS claims to serve more than 44,000 customers in at least 400 of the top U.S. metropolitan areas. That figure includes many large corporations, and makes it the largest installed base of mobile data network users in the United States. ARDIS supports sending and receiving wireless messages from the Internet, corporate and personal computer systems, and other ARDIS subscribers.

Every ARDIS service area supports, at a minimum, data rates of 4800 bits per second (bps) using a Motorola transmission protocol called MDC-4800. In a handful of major metropolitan areas another proprietary Motorola protocol, RD-LAP, is used to support speeds of up to 19,200 bps. Regardless of speed, each protocol makes use of error detection and correction information sent along with the data, as well as allowing badly garbled packets to be retransmitted. A single packet can hold up to 240 characters, although the average is about half that size.

More than 1300 base stations provide service to ARDIS users, transmitting at an average power of 45 watts. Mobile units transmit using three to four watts of power, and all radio communication takes place in the 800 MHz Specialized Mobile Radio (SMR) band.

The most recent pricing information I have shows a "DataPak 20", the least expensive plan, at $19.95 a month for 20 kilobytes of data.

RAM Mobile Data

RAM Mobile Data is the second largest wireless data communications provider in the United States. Founded in 1989 by RAM Broadcasting Corporation, BellSouth spent more than $300 million to buy 49% of the company in 1989. RAM Mobile maintains a Network Control Center (NCC) at their corporate headquarters in New Jersey, and a backup NCC in Dallas, Texas.

RAM Mobile Data claims to serve at least 40,000 customers in more than 260 of the 316 metropolitan statistical areas (MSAs) in the United States. This service area represents more than 90% of their desired customer base, the urban business population. In each MSA, no fewer than 10, and as many as 30 radio frequency pairs are available at numerous base stations with overlapping coverage areas.

RAM Mobile Data uses an open, non-proprietary protocol called Mobitex, which is used in at least 16 other countries for two-way wireless data communications. Mobitex follows the Open System Interconnection (OSI) model, and supports wireless Internet Protocol (IP) connectivity.

Like ARDIS, RAM Mobile Data uses frequencies in the Specialized Mobile Radio band. The mobile units transmit bertween 896 MHz and 901 MHz, and the base stations transmit 39 MHz higher, between 935 MHz and 940 MHz. Data is transmitted at 8,000 bits per second (bps) and uses automatic repeat and forward error correction to combat noise and fading typical in a mobile radio environment.

Messages may be sent to an individual mobile unit or to a group. If a subscriber is not reachable, because the modem is turned off or the unit is out of a coverage area, the message may be stored in the network for up to 72 hours. When the modem is turned on or re-enters a coverage area, stored messages are automatically delivered. As an option, the sender can receive a confirmation notice when the original message is delivered to the recipient.

Mobile units stay in contact with the radio network through the roaming capabilities of the system, in many ways similar to a cellular telephone network. Mobile units monitor signal strength from nearby base stations and determine if and when a transfer to another base station is necessary.

Modems are available from major equipment suppliers such as Ericsson, GE, and Motorola. Pricing starts at $25 per month for up to 100 kilobytes worth of messages, with each additional kilobyte costing 35 cents.

RAM Mobile Data claims the security is high on their system, and provides several mechanisms to discourage eavesdroppers. First, the mobile radio unit determines the frequency to use, and can hop through each frequency supported in the service area. As the mobile unit travels, it will inform the closest base station of the channel on which data will be communicated. Since packets from different modems will all be sent on the same radio channel, and each modem only uses the channel for short intervals, RAM Mobile believes that this interleaving of packets "discourages the isolation of messages within RAM's network" and makes the eavesdropper's task more difficult.

Second, each terminal in the network contains an electronic serial number (ESN) and is assigned a mobile access number (MAN). When a unit attempts to access the network the ESN and the MAN must match. If they donít, or if fraud is suspected, the Mobitex network has the capability of remotely disabling the modem. In addition, users may also have a password-protected subscription that can be accessed from any terminal.

Metricom

One of the winners in the recent Wireless Communications Service (WCS) was Metricom, Inc. (for more information on the auction, see the July 1997 PCS Front Line column). New spectrum in the 2.3 GHz range will help them expand an innovative service they currently offer in a few cities, university campuses, and airports across the country.

The Ricochet data network provides wireless access to the Internet and other data applications at speeds approaching 28,800 bits per second at an affordable price. For a flat monthy charge of about $30, customers can move unlimited amounts of data using a 600 milliwatt paperback book-sized modem attached to the back of a personal computer. Many world wide web addicts have subscribed to Ricochet instead of ordering a second telephone line. As Metricom admits, "Our competitor is the wireline modem."

Ricochet transmits information in the unlicensed 902 to 928 MHz band with a technique called frequency hopping spread spectrum (FHSS). The 26 MHz of bandwidth is divided into 162 channels, each 160 kHz wide. A Ricochet modem selects a sequence of 55 channels and "hops" from channel to channel, spending only about 100 milliseconds on any one channel before moving to the next one in the list. The sequence may gradually change as channels with high interference levels are dropped and replaced with new channels, causing the connection quality to improve over time. This technique also allows many users to share the spectrum without interfering with each other, and makes eavesdropping rather difficult.

Packets average 500 bytes in size, and are transmitted to and from one watt transceivers mounted on nearby lightpoles. Rather than building a tower and installing large, expensive, and complicated equipment, Metricom contracts with a local utility to mount a small rectangular box on a lightpole or similar structure, spacing them a quarter to half a mile apart. Each "pole unit" receives packets from local modems, adds an identifier and forwards it to either another pole unit or a wireless access point (WAP), where it is placed on a leased line and sent to a network interconnection point, eventually reaching the Internet.

At the end of 1996 Metricom had more than 9,000 Ricochet customers, but as great as it is, the service has one major drawback - availability. Metricom has networks in Seattle, San Francisco, and most recently Washington, D.C., as well as a dozen or so colleges and corporate campuses. If it's available in your area and you like to surf the web, this is the way to go.

 

ARDIS

RAM Mobile Data

Metricom Ricochet

Coverage

nationwide

nationwide

selected areas

Data rate

(in bits per second)

4800 (19,200 in limited areas)

8,000

typical throughput is about 28,800

Packet size (in bytes)

240

512

500

Mobile transmit power (watts)

3

2 to 10

0.6

Response time (seconds)

2 to 7

4 to 8

1 to 3

Transmit Frequencies

(in MHz)

800 (SMR band)

Mobile: 896 to 901

Base: 935 to 941

902 to 928

Channels

600

200

162

Channel spacing

25 kHz

12 kHz

160 kHz

Pricing (representative)

$20 / 20 kilobytes

$30 / 100 kilobytes

$30 / unlimited

There's more information on these services at the PCS Front Line website at http://www.grove.net/~dan, and I can be reached by electronic mail at dan@decode.com. Until next month, happy monitoring!


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Updated May 1, 2003