Bandwidth Utilization: Multiplexing and Spreading

Definition of Terms

• Bandwidth utilization is the use of available bandwidth to achieve specific goals. 
• Efficiency can be achieved by using multiplexing; privacy and antijamming can be achieved by using spreading.
• Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. 
• In a multiplexed system, n lines share the bandwidth of one link. The word link refers to the physical path. The word channel refers to the portion of a link that carries a transmission.
• Frequency-division multiplexing (FDM) and wave-division multiplexing (WDM) are techniques for analog signals, while time-division multiplexing (TDM) is for digital signals. 
• Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted.
• Telephone companies use FDM to combine voice channels into successively larger groups for more efficient transmission. 
• Wavelength-division multiplexing (WDM) is designed to use the high bandwidth capability of fiber-optic cable. WDM is an analog multiplexing technique to combine optical signals.
• Time-division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a link. TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.
• Framing bits allow the TDM multiplexer to synchronize properly.  
• Interleaving - process of sending a unit in the multiplexing and receiving on the demultiplexing side.
• Digital signal (DS) is a hierarchy of TDM signals. 
• T lines (T-1 to T-4) are the implementation of DS services. A T-1 line consists of 24 voice channels. 
• T lines are used in North America. The European standard defines a variation called E lines. 
• Inverse multiplexing splits a data stream from one high-speed line onto multiple lower-speed lines. 
• In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth.
• Spread spectrum is designed to be used in wireless applications in which stations must be able to share the medium without interception by an eavesdropper and without being subject to jamming from a malicious intruder. To achieve these goals, spread spectrum techniques add redundancy.
• The frequency hopping spread spectrum (FHSS) technique uses M different carrier frequencies that are modulated by the source signal. At one moment, the signal modulates one carrier frequency; at the next moment, the signal modulates another carrier frequency.
• The direct sequence spread spectrum (DSSS) technique expands the bandwidth of a signal by replacing each data bit with n bits using a spreading code. In other words, each bit is assigned a code of n bits, called chips.

Three basic Multiplexing Techniques

• Frequency-division multiplexing (FDM) - each signal modulates a different carrier frequency. The modulated carriers are combined to form a new signal that is then sent across the link. In FDM, multiplexers modulate and combine signals while demultiplexers decompose and demodulate. Also in FDM, guard bands keep the modulated signals from overlapping and interfering with one another.
• Wavelength-division multiplexing (WDM) - similar in concept to FDM, however, the signals being multiplexed are light waves.
• Time-division multiplexing (TDM) - digital signals from n devices are interleaved with one another, forming a frame of data (bits, bytes, or any other data unit).

Two different schemes of TDM

• Synchronous TDM - each input connection has an allotment in the output even if it is not sending data. The data rate of the link is n times faster, and the unit duration is n times shorter.
• Statistical TDM - slots are dynamically allocated to improve bandwidth efficiency.

Digital hierarchy

DS and T line rates

E line rates

Note: You can proceed to take the multiple choice exam regarding this topic. Bandwidth Utilization: Multiplexing and Spreading - Set 1 MCQs

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credit: Behrouz A. Forouzan©2013 www.FroydWess.com

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