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Old 23-09-2008, 11:22 PM
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User Introduction Multiplexing techniques



•To transmit two or more signals at the same time or on the same carrier frequency.
• To combine two or more electrical signals into a single, composite signal and transmitting them over a single transmission link
•To combine multiple signals from possibly separate sources, in order to transmit them over a single path.

•It is the name of technique which allows a communication circuit to carry more than one channel
•Demulitplexing is the reverse process of multiplexing which separates the multiplexed signals at the receiver end of the transmission link.

Why Multiplexing?

Cost-effectiveness : cost/kbps declines with increasing data rate
Efficiency : most devices require modest data-rate support (compared to available transmission facility)

Types of Multiplexing

•Space Division Multiplexing (SDM)

•Frequency Division Multiplexing (FDM)

•Time Division Multiplexing (TDM)

•Wavelength Division Multiplexing (WDM)

Space Division Multiplexing

•It is simply the combining of physically separate signals into a bundled cable
•Subscribers loop and trunk circuits are bundled.
•Since large space is usually not available, other multiplexing techniques are used with SDM.

Frequency Division Multiplexing

FDM utilizes a channel’s available bandwidth to send multiple signals simultaneously.
Each signal is modulated onto a carrier frequency, then combined into a single transmission medium
Channels are separated by guard bands to minimize interference

Time Division Multiplexing

The management of multiple signals on one channel by alternately sending portions of each signal and assigning each portion to particular block of time.

•A digital technique in contrast to FDM being an analog technique
•TDM involves the distribution of multiple signals in time domain.

Wavelength Division Multiplexing (WDM)

•A variation of FDM for fiber optic channels

•Bandwidth of a single fiber is around 25,000 GHz, so great potential for multiplexing many channels together

•A technology that uses optical signals on different wavelengths to increase the capacity of fiber optic networks in order to handle a number of services simultaneously.
•A technique by which two or more optical signals having different wavelengths may be simultaneously transmitted in the same direction over one fiber, and then be separated by wavelength at the distant end.

•In 1997, Bell Labs showed a optic fiber carrying 100 beams, each offering a bit rate of 10 Gbps for a total of 1 Tbps (terabits per sec).
•Today, commercial equipment can carry 160 channels each at 10 Gbps.
•Alcatel had demonstrated 259 channels at 39.8 Gbps for a total of 10.1 Tbps over a 100 km span.
•WDM/DWDM increases the capacity of embedded fiber by first assigning incoming optical signals to specific frequencies (wavelength, lambda) within a designated frequency band and then multiplexing the resulting signals out onto one fiber.
•Combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity. Each signal carried can be at a different rate.

The main difference between WDM and DWDM is that the distance between the signals is significantly shorter in dense WDM technology and that the number of signals in a single fiber is even more.
According to the ITU-T standard for DWDM the distance is given to 100 GHz (0.8 nm), then one can send 50 independent signals in the 1530-to-1560 nm band in a fiber. Today the upper limit is at about 80 wavelengths with a data rate at up to 40 Gbit/s per channel

Waqas Ahmed

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