Page 88 - DCAP406_DCAP_207_Computer Networks

Unit 6: Multiplexing

Notes
Figure 6.5: Time Slots in TDM

The main disadvantage of this scheme is that a precise synchronization between different senders
is necessary to avoid co-channel interference.

Statistical Time Division Multiplexing (STDM)

In case of TDM, time slots are allocated to channels, even if they have no information to transmit.
This is just wastage of the bandwidth and to overcome this inefficiency of standard TDM, a
techniques known as STDM is implemented in which time is allocated to lines only on demand.
This is achieved with the uses of intelligent devices that are capable of identifying when a
terminal is idle and statistically compensates for normal idle time so that more lines can be
connected to a transmission medium. During the peak traffic period a buffer memory temporarily
stores the data so high-speed line time can be effectively utilized with active channels. It adopts
a methodology where each transmission has identification information (a channel identifier).
This increases the overheads, which are handled by grouping a number of characters for each
channel together for transmission. It is also referred as “Intelligent” TDM. In this case, data rate
capacity is well below the sum of connected capacity of each channel because it utilizes the idle
time very effectively. It is digital only and requires more complex framing of data. It is widely
used for remote communications with multiple terminals. The additional services such as data
compression, line priority, mixed speed lines, host ports sharing, network port control, automatic
speed detection etc are available with STDM techniques.

6.2.3 Code Division Multiplexing (CDM)/Spread Spectrum

CDM is widely used in so-called second-generation (2G) and third-generation 3G wireless
communications. The technology is used in ultra-high-frequency (UHF) cellular telephone systems
in the 800-MHz and 1.9-GHz bands. This is a combination of analog-to-digital conversion and
spread spectrum technology. CDM may be defined as a form of multiplexing where the
transmitter encodes the signal using a pseudo random sequence. CDM involves the original
digital signal with a spreading code. This has the effect of spreading the spectrum of the signal
greatly and reducing the power over any one part of the spectrum. On the other hand, the
receiver knows about the code generated and transmitted by the transmitter and therefore can
decode the received signal. Each different random sequence corresponds to a different
communication channel from multiple stations.

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