| Packet
Data
The current packet
data technologies described in this section are used in North America
primarily to transmit short messages to "smart phones"
and PDA's (personal digital assistants).
To learn about the packet-data
technologies on the horizon for GSM networks, such as GPRS, click
here.
Circuit-switched
versus packet-switched
Current technologies are primarily
circuit-switched, meaning a continuous circuit transmission allows
the network to route continuous data to a single location. Circuit-switched
data requires a dedicated radio channel even when no data is being
sent.
Packet-switched data works quite differently
from circuit-switched data. With packet-switched data, the computer
that is connected to the cell phone can send bursts and receive
bursts of data. Each burst contains a sequence number allowing
the recreation of the blocks of data once all data is sent. A radio
channel is occupied only for the duration of the data transmission
instead of continuously.
Cellular
Digital Packet Data (CDPD)
CDPD is a packet-data wireless technology
developed in the US by AT&T based on Internet Protocol (IP)
networking that is deployed as an overlay to analog cellular networks.
It enables analog AMPS networks to carry packetized data alongside
voice, and operates on the 800 MHz frequency. It is primarily used
to transmit brief messages for Personal Digital Assistants and "smart
phones" for such purposes as merchant verification.
CDPD uses either idle voice channels
or dedicated data channels depending on network configuration. Service,
available in most major cities in the US and Canada, is provided
by cellular companies including AT&T Wireless Services, Ameritech,
Bell Atlantic/Nynex and GTE. In Canada, service is provided by BC
TEL Mobility and TELUS Mobility.
The current emphasis is on vertical
markets, but an increasing number of services are being targeted
for horizontal users, including both business users and consumers.
An example of this trend are smart phones such as the PocketNet*
phone from AT&T Wireless Services that include a CDPD modem
and specialized Web browser. These smart phones can also be connected
to mobile computers for general purpose mobile networking.
The following summarizes key aspects
of CDPD:
- Best Applications. Any IP
based application will work over CDPD, but due to service pricing
and performance, best applications are short and bursty in nature,
such as brief messages.
- Protocols. CDPD is IP based.
It connects seamlessly with the Internet. PPP or SLIP is used
between the mobile computer and the CDPD modem.
- Data Throughput and Round Trip
Delays. Raw throughput is 19,200 bps but actual throughput
after protocol overhead is 10,000 to 12,000 bps. Round trip delays
are typically less than one second. The 19,200 bps channel is
shared by all active users in that cell or cell sector.
- Modem Vendors. Wireless modems
vendors include INET, Motorola, Novatel, PCSI, Sierra Wireless
and Uniden. Smart phones are currently available from Samsung
and Mitsubishi.
- Service Pricing. Service
plans vary, but 10 cents per kilobyte is typical.
RAM
Mobile Data (Mobitex)
RAM Mobile Data, a wireless packet
data service owned by BellSouth, is based on Mobitex technology
which has been deployed in seventeen countries worldwide. RAM Mobile
Data offers coverage in most cities and towns in the US.
The following summarizes key aspects
of RAM Mobile Data:
- Best Applications. Applications
that send and receive short messages are best suited. Due to the
wireless specific Mobitex protocols and interfaces, specialized
software applications are generally required.
- Protocols. Mobitex defines
both wireless specific protocols as well as end-to-end networking
protocols.
- Data Throughput and Latency.
Raw throughput is 8,000 bps but actual throughput after protocol
overhead is about half this rate. Round trip delays usually range
from 4 to 8 seconds. The 8,000 bps channel is shared by all active
users in the operating area of the base station.
- Modem Vendors. Wireless modems
vendors include Ericsson, IBM, Motorola, Research in Motion and
3Com.
- Service Pricing. Typical
service plans are between 25 and 30 cents per kilobyte.
ARDIS
(DataTAC)
ARDIS is a wireless packet data network
based on technology originally developed by Motorola for IBM field
service engineers. ARDIS, now owned by American Mobile Satellite,
offers coverage in most major cities and towns in the US. The underlying
technology called DataTAC has also been used for wireless networks
in Europe and Asia.
The following summarizes key aspects
of the ARDIS network:
- Best Applications. Applications
that send and receive short messages are best suited. Due to the
wireless specific DataTAC protocols and interfaces, specialized
software applications are generally required.
- Protocols. DataTAC defines
both wireless specific protocols as well as end-to-end networking
protocols. Communications is session oriented using a host/terminal
approach. Two airlinks are available: MDC4800 and the newer RDLAP.
- Data Throughput and Latency.
Raw throughput is 19,200 bps with the RDLAP protocol and 4,800
bps with MDC4800. Actual throughputs after protocol overhead are
about half these rates. Round trip delays typically range from
4 to 8 seconds. The 19,200 bps or 4,800 bps channel is shared
by all active users in the operating area of the base station.
- Modem Vendors. Wireless modems
vendors include Motorola, IBM and Research in Motion.
- Service Pricing. Typical
service plans are about 30 cents per kilobyte.
Metricom
Ricochet
Ricochet is a packet data network operated
by Metricom in the unlicensed ISM 902 MHz to 928 MHz band. Service
is currently only available in the San Francisco Bay Area, Seattle
and Washington DC, as well as select corporate campuses and airports.
Service is oriented towards both horizontal business users and consumers.
The following summarizes key aspects
of Ricochet:
- Best Applications. Any Internet
application that works well at 14,400 bps or higher. Modem based
applications can access a modem gateway.
- Protocols. Ricochet uses
PPP between the mobile computer and the network for IP based communications
to the Internet as well as to corporate networks over private
connections.
- Data Throughput and Latency.
Data throughput varies between 14,400 bps and 28,800 bps depending
on a number of factors. Round trip delays are typically less than
one second. The radio channel is shared by all active users in
the operating area of the base station.
- Modem Vendors. Modems are
available only from Metricom.
- Service Pricing. Approximately
$30 per month for unlimited use.
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