An Introduction to Wireless Technology, Second Edition

synchronized regular time at which a station transmits and the data packet may

take different routes through the network at different times. This may cause

packets to arrive at their destination out of sequence and the network must be

able to re-assemble them into a meaningful data stream. Mobile Data Networks

such as ARDIS and RAM use packetization. See 3.2.3, “Packetization” on

page 74 for further information.

Code Division Multiple Access (CDMA) technology is used by Qualcomm in the

US and a service has started in Korea. Apart from these two, there are no other

CDMA networks known to the author at the time of writing. 2.3.3, “US Digital

Cellular (Qualcomm)” on page 47 and 5.2, “Spread Spectrum and Code Division

Multiple Access (CDMA)” on page 97 for further information.

6.2 Differences in Packet and Circuit Switched Networks

There has been much discussion concerning the relative merits of

packet-switched wireless data network architectures such as Mobitex and

RD-LAP and using circuit-switched networks such as AMPS and GSM for the

transmission of data. Both types of networks have significant advantages for

certain kinds of applications; other factors such as coverage, cost and

availability can also influence choice. It is not our intention to discuss the merits

of network operators in these pages, but rather to relate technology to

applications.

One key factor in determining whether to use packet-switched or circuit-switched

networks is the need for voice communications. Although voice support was

available on the early VHF Mobitex system in Sweden, it is no longer used, and

any requirement to have voice communications in the same terminal device as

data communications will require the use of circuit-switched operation. The only

exception to this is CDPD where the data service is packetized and overlayed on

a circuit-switched network.

If the data application is required to be interactive in real time, then it is more

likely that a circuit-switched network will be the best suited. Network delays on

packet-switched networks can be of the order of several seconds and users, and

sometimes software applications cannot tolerate this.

If the application involves sending large amounts of data across the link (file

transfer), again a circuit switched connection is best. There are two reasons for

this. First, a packet-switched network will take much longer for the transfer to

complete. Secondly, packet-switched networks normally have tariffs which

charge the user by the amount of data sent and not by connection time as in

circuit-switched networks. The exception to this may be GSM where some

operators may offer a data-only tariff which is charged by the byte.

Packet-switched networks operate in what is known as “disconnected mode”,

which means that the network may hold on to the packets of data and send them

to the mobile terminal at some later time. There may be a number of reasons

for doing this, but one of the most common is when a mobile terminal is not in

communication with the radio network. In this case the network will hold the

data in a store and forward location until the mobile terminal is recognized as

being in radio contact again. This can occur either when the host system is

sending information to the mobile. If the data transfer is from mobile to host,

then the mobile terminal will retain the data until it is again in contact with the

network and then transmit the information without any intervention on the part of

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