An Introduction to Wireless Technology, Second Edition

3.2.2.1 Characteristics

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This method is quite simple and can be built in single chip hardware logic.

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The hardware is low in cost (compared to other techniques).

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It will operate at very high speeds.

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It provides sharing and channelization of the link; it does not take into

account the fact that telephone traffic is logically half-duplex (only one

person talks at once) and though a channel is provided in each direction,

only one is in use at any one time. Nor does it take advantage of “gaps” in

speech. There are intelligent multiplexing techniques (called statistical

multiplexors) which do this. For these reasons “good”, utilization for

telephone traffic is considered to be around 40%. This is a lot better than

the analog frequency division technique.

3.2.3 Packetization

This technique involves the breaking of incoming bit streams (voice or data) into

short “packets”. Different techniques use variable or fixed-length packets.

Packets have an identifier appended to the front of them which identifies the

circuit or channel to which they belong. In the TDM example in Figure 30 on

page 73, a time slot was allocated for a low-speed channel within every frame,

even if there was no data to be sent. In the packet technique, blocks are sent

only when a full block is available and “empty” packets are not sent. Thus,

utilization of the high-speed link can be dramatically improved.

In the TDM example, the 2 Mbps high-speed channel provided 32 subchannels of

64 Kbps. If each subchannel is only used on average for half of the time (as in

voice conversation) then perhaps 64 subchannels could be used with

packetization. However, now there will be statistical variations and there will be

a finite probability that all 64 channels will want the same direction at once. In

this case some data would be lost, but the probability of this occurring is very

small. However, the probability of 33 channels wanting to operate in the same

direction simultaneously is quite high. It is a matter for statisticians to decide

how many channels can be safely allocated without too much chance of losing

information (overrun). This will depend on the bandwidth available and the

number of channels. The larger the number of channels the smaller the

variation and the greater the safe utilization. A good starting assumption is that

the channel can be utilized to about 70% of its capacity safely (in a 2 Mbps

circuit).

3.2.4 Characteristics

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The equipment required for packetization is much more complex and

expensive.

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Operation at very high speeds increases the complexity of the required

equipment.

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Use of the packetization technique results in very much improved (optimal)

use of the trunk. This is because when there is silence a packet is not

transmitted. There are overheads inherent in the addressing technique

which must be used to route and to identify the packet but nevertheless, an

efficiency of perhaps 80% can be approached if the bandwidth is wide

enough to permit a large number of simultaneous calls.

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GSM and IS54 are good examples of TDMA wireless systems.

74 An Introduction to Wireless Technology