1-2-Multiplexing Sharing a Medium-2011

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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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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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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1-2-Multiplexing Sharing a Medium-2011