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8/3/2019 1-2-Multiplexing Sharing a Medium-2011
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Introduction
Under the simplest conditions, a medium can carry only onesignal 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)
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Multiplexing
Multiplexor (MUX)
Demultiplexor (DEMUX)
Sometimes just called a MUX
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Multiplexing
Two or more simultaneous transmissions
on a single circuit.
Transparent to end user.
Multiplexing costs less.
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Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to each
user or signal on a medium. Thus, all signals aretransmitted 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.
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Frequency Division Multiplexing
Analog signaling is used to transmits the signals.
Broadcast radio and television, cable television.
This technique is the oldest multiplexing technique.
Since it involves analog signaling, it is more susceptible to
noise.
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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.
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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.
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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.
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Synchronous time division multiplexing
So that the receiver may stay synchronized with the incomingdata stream, the transmitting multiplexor can insert alternating
1s and 0s into the data stream.
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Synchronous Time Division
Multiplexing
Three types popular today:
T-1 multiplexing (the classic)
ISDN multiplexing
SONET (Synchronous Optical NETwork)
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The T1 (1.54 Mbps) multiplexor stream is a continuousseries
of frames of both digitized data and voice channels.
24 separate 64Kbps channels
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The ISDN multiplexor stream is also a continuous stream offrames. Each frame contains various control and sync info.
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SONETmassive data rates
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Synchronous TDM
Very popular
Line will require as much bandwidth as all the
bandwidths of the sources
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Statistical Time Division Multiplexing
A statistical multiplexor transmits only the data from active
workstations (or why work when you dont have to).
If a workstation is not active, no space is wasted on the
multiplexed stream.
A statistical multiplexor accepts the incoming data streamsand creates a frame containing only the data to be transmitted.
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To identify each piece of data, an address is included.
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If the data is of variable size, a length is also included.
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More precisely, the transmitted frame contains a collectionof data groups.
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Statistical Time Division Multiplexing
A statistical multiplexor does not require a line over as high a
speed line as synchronous time division multiplexing sinceSTDM 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!
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Wavelength Division Multiplexing
(WDM)
Give each message a different wavelength (frequency)
Easy to do with fiber optics and optical sources
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Dense Wavelength Division
Multiplexing (DWDM)Dense wavelength division multiplexing is often called just
wavelength division multiplexing
Dense wavelength division multiplexing multiplexes multipledata 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 adifferent rate from the other signals.
Dense wavelength division multiplexing combines many (30,
40, 50, 60, more?) onto one fiber.
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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 samefrequencies at the sametime using
different codesUsed for mobile communications
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Code Division Multiplexing
An advanced technique that allows multiple devices to
transmit on the samefrequencies at the sametime.
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
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Code Division Multiplexing
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 near64
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What did we cover
Multiplexing
Types of multiplexing
TDM Synchronous TDM (T-1, ISDN, optical fiber)
Statistical TDM (LANs)
FDM (cable, cell phones, broadband)
WDM (optical fiber) CDM (cell phones)
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CSMA/CD
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CSMA/CD
3 1 7 Noise
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3.1.7 Noise
Noise is unwanted electrical, electromagnetic, or radio frequency energy that can degrade and distort the qualityof signals and communications of all types. Noise occurs in digital and analog systems. In the case of analogsignals, the signal becomes noisy and scratchy. For instance, a telephone conversation can be interrupted bynoises in the background. In digital systems, bits can sometimes merge, becoming indistinguishable to thedestination computer. The result is an increase in the bit-error rate, which is the number of bits that weredistorted so much that the destination computer reads the bit incorrectly. A clearly defined digital signal does notalways reach its destination without some change. Electrical noise can occur on the line. When the two signalsmeet, they can merge into a new signal . The original clear signal can be interpreted incorrectly by the receivingdevice .
Also, signals that are external to the cables, such as the emissions of radio transmitters and radars, or the
electrical fields that emanate from electric motors and florescent light fixtures, can interfere with the signals thatare traveling down cables. This noise is called Electromagnetic Interference (EMI) when it occurs from electricalsources, and Radio Frequency Interference (RFI) when it occurs from radio, radar, or microwave sources. Noise canalso be conducted from AC lines and lightning.
Each wire in a cable can act like an antenna. When this happens, the wire actually absorbs electrical signals fromother wires in the cable and from electrical sources outside the cable. If the resulting electrical noise reaches ahigh enough level, it can become difficult for the computer to discriminate the noise from the data signal.
Optical and wireless systems experience some of these forms of noise but are immune to others. For example,optical fiber is immune to most forms of crosstalk (interference from adjacent cables) and noises associated withAC power, and ground reference problems. Radio waves and microwaves are immune as well, but these can be
affected by simultaneous transmissions on adjacent radio frequencies. For copper links, external noise is picked up from appliances in the vicinity, electrical transformers, the
atmosphere, and even outer space. During severe thunderstorms or in locations where many electrical appliancesare in use, external noise can affect communications. The biggest source of signal distortion for copper wire occurswhen signals inadvertently pass out of one wire within the cable and onto an adjacent one. This is called crosstalk
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