An Introduction to Wireless Technology, Second Edition

2.5 Packet Network Methodologies

With the exception of CDPD, the cellular networks discussed so far have been

designed primarily for use with speech information. Radio packet data networks

are designed for data only.

Note: Some early Mobitex services in Scandinavia offered voice services, but

these are being phased out.

Refer to 1.3.4, “Mobile Data Services” on page 14 for an overview of radio

packet data networks.

2.5.1 Mobitex

The description which follows applies to the newer UHF Mobitex systems in

North America and Europe. The original VHF systems which operated in

Scandinavia are being replaced by UHF systems with much higher data rates.

Mobitex in the US transmits from the base station on channels in the range of

frequencies 935-940 MHz, and receives in the range of 896-901 MHz. The

transmit and receive frequencies for any channel are always 39 MHz apart.

Mobitex in Europe has different frequency allocations for each country but they

are all in the range of 410-459 MHz. Apart from this difference in radio carrier

frequency, the European and North American systems are the same.

The radio channels are 12.5 KHz wide and each Base Radio Station (BRS) can

have up to sixteen different channel frequencies. The physical BRS can be used

as a number of logical base stations using directional antennae. The radio

channel is modulated using a modified type of Gaussian Minimum Shift Keying

(GMSK). GMSK is described in 3.1.6.2, “Shift Keying (ASK, FSK, PSK...)” on

page 63. The Mobitex network is cellular in structure, but because the system

uses packets of data instead of continuous connection, there is no requirement

for dynamic handoff as in a cellular telephone network.

Chapter 2. Wireless Methodologies 51

Figure 17. Mobitex Network Structure

The data rate of the Mobitex network is 9.6 Kbps, which gives a maximum

end-to-end throughput of 8.0 Kbps. One of the key factors when dealing with a

packet data network is the network delay. The delay can vary according to the

load on the network and may be a few seconds. Network operators are careful

to ensure that no single application will load the network in such a way that

other users are affected. Legacy host applications may need to be modified to

work properly with packet networks, which may be a key factor in choosing a

suitable network.

Access to the Mobitex network from external host systems is provided by a

number of different gateways. The availability and definition of these gateways

will vary from network to network, but will normally include:

•

X.25

− MTP/1 transport layer is optional

•

TCP/IP

− Host connection - X.25 or SLIP (Serial Line Internet Protocol)

52 An Introduction to Wireless Technology