Multiplexing and Transmission Media full report
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31-03-2010, 08:22 PM

Multiplexing and Transmission Media


Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link

can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals

across a single data link. As data and telecommunications use increases, so does traffic.

Frequency Division Multiplexing
¢ Each signal is modulated to a different carrier frequency
¢ Carrier frequencies separated so signals do not overlap (guard bands)
¢ Channel allocated even if no data
¢ each channel occupies a fraction of the bandwidth of the link
¢ a channel is defined by its center frequency, and its bandwidth.
¢ Example: radio and television signal transmission

Wavelength Division Multiplexing
¢ Multiple beams of light at different frequency
¢ Carried by optical fiber
¢ A form of FDM
¢ Each color of light (wavelength) carries separate data channel
¢ 1997 Bell Labs
“ 100 beams
“ Each at 10 Gbps
“ Giving 1 terabit per second (Tbps)

WDM Operation
¢ Same general architecture as other FDM
¢ Number of sources generating laser beams at different frequencies
¢ Multiplexer consolidates sources for transmission over single fiber
¢ Optical amplifiers amplify all wavelengths
“ Typically tens of km apart
¢ Demux separates channels at the destination

Transmission Media

A transmission medium can be broadly defined as anything that can carry information from a source to a destination.
The transmission medium is usually free space, metallic cable or fiber “ optic cable.

Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable,

coaxial cable, and fiber-optic cable. Out of these twisted-pair cable, coaxial cable transport signals in the form

of electric signals and fiber-optic cable transport signals in the form of light.

Coaxial cable

Coaxial cable carries signals of higher frequency ranges than those in twisted-pair cable, in part because the two

media are constructed quite differently.

Fiber-Optic Cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. Light travels in a

straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one

substance suddenly enters another substance, the ray changes direction.

Fiber-optic cable connectors

Subscriber Channel (SC) Connector is used for cable TV. It uses a push/pull locking system
Straight “ tip (ST) Connector is used for connecting cable to networking devices. It uses a bayonet locking system

and is more reliable than SC.
MT-RJ is a connector that is the same size as RJ45.


Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is

often referred to as wireless communication.


A circuit-switched network consists of a set of switches connected by physical links. A connection between two

stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on

each link. Each link is normally divided into n channels by using FDM or TDM.


In data communications, we need to send messages from one end system to another. If the message is going to pass

through a packet-switched network, it needs to be divided into packets of fixed or variable size. The size of the

packet is determined by the network and the governing protocol.

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12-01-2011, 11:07 AM

Under the simplest conditions, a medium can carry only one signal at any moment in time.
For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth.
The current techniques that can accomplish this include
frequency division multiplexing (FDM)
time division multiplexing (TDM)
Synchronous vs statistical
wavelength division multiplexing (WDM)
code division multiplexing (CDM)


Multiplexor (MUX)
Demultiplexor (DEMUX)
Sometimes just called a MUX
Two or more simultaneous transmissions on a single circuit.
Transparent to end user.
Multiplexing costs less.

Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to each “user” or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies.
A multiplexor accepts inputs and assigns frequencies to each device.
The multiplexor is attached to a high-speed communications line.
A corresponding multiplexor, or demultiplexor, is on the end of the high-speed line and separates the multiplexed signals.
Analog signaling is used to transmits the signals.
Broadcast radio and television, cable television, and the AMPS cellular phone systems use frequency division multiplexing.
This technique is the oldest multiplexing technique.
Since it involves analog signaling, it is more susceptible to noise.

Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users.
Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
1. Synchronous time division multiplexing, and
2. Statistical, or asynchronous time division multiplexing.

Synchronous Time Division Multiplexing
The original time division multiplexing.
The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern.
T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing.

If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream.
If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream.

So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream.

Synchronous TDM

Very popular
Line will require as much bandwidth as all the bandwidths of the sources

Statistical Time Division Multiplexing
A statistical multiplexor transmits only the data from active workstations (or why work when you don’t have to).
If a workstation is not active, no space is wasted on the multiplexed stream.
A statistical multiplexor accepts the incoming data streams and creates a frame containing only the data to be transmitted.
A statistical multiplexor does not require a line over as high a speed line as synchronous time division multiplexing since STDM does not assume all sources will transmit all of the time!
Good for low bandwidth lines (used for LANs)
Much more efficient use of bandwidth!

Wavelength Division Multiplexing (WDM)
Give each message a different wavelength (frequency)
Easy to do with fiber optics and optical sources

Dense Wavelength Division Multiplexing (DWDM)
Dense wavelength division multiplexing is often called just wavelength division multiplexing
Dense wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line.
Different wavelength lasers (called lambdas) transmit the multiple signals.
Each signal carried on the fiber can be transmitted at a different rate from the other signals.
Dense wavelength division multiplexing combines many (30, 40, 50, 60, more?) onto one fiber.

Code Division Multiplexing (CDM)
Old but now new method
Also known as code division multiple access (CDMA)
An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes
Used for mobile communications

An advanced technique that allows multiple devices to transmit on the same frequencies at the same time.
Each mobile device is assigned a unique 64-bit code (chip spreading code)
To send a binary 1, mobile device transmits the unique code
To send a binary 0, mobile device transmits the inverse of code

Receiver gets summed signal, multiplies it by receiver code, adds up the resulting values
Interprets as a binary 1 if sum is near +64
Interprets as a binary 0 if sum is near –64

Business Multiplexing In Action
XYZ Corporation has two buildings separated by a distance of 300 meters.
A 3-inch diameter tunnel extends underground between the two buildings.
Building A has a mainframe computer and Building B has 66 terminals.
List some efficient techniques to link the two buildings.

Possible Solutions
Connect each terminal to the mainframe computer using separate point-to-point lines.
Connect all the terminals to the mainframe computer using one multipoint line.
Connect all the terminal outputs and use microwave transmissions to send the data to the mainframe.
Collect all the terminal outputs using multiplexing and send the data to the mainframe computer using a conducted line.

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