Digcom_104

DIgital Communication ECE 422L

2013

IV. Bandwidth Utilization

•Bandwidth is the precious commodity in communication

•Bandwidth utilization is the wise use of available bandwidth to achieve

specific goals.

Bandwidth utilization

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Efficiency can be achieved by multiplexing;

privacy and anti-jamming can be achieved by spreading.

Bandwidth utilization

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MULTIPLEXINGMULTIPLEXING

• Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared.

• 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.

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Dividing a link into channels

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Categories of Multiplexing

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Frequency-division multiplexing

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FDM is an analog multiplexing technique that combines narrow band analog

signals.


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FDM process

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FDM demultiplexing example

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Applications:• Broadcasting• Telephone and data

communication system• Cable television• Data distribution networks

Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.

Example 1

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SolutionWe shift (modulate) each of the three voice channels to a different bandwidth, as shown in next slide. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them

Example 1

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Example 1

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Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference?

Example 2

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SolutionFor five channels, we need at least four guard bands. This means that the required bandwidth is at least

5 × 100 + 4 × 10 = 540 kHz, as shown in next slide

Example 2

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Example 2

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Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration, using FDM.

Example 3

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Solution The satellite channel limited to 1MHz. We divide this among the four data channel that is each channel is allocated with 250 kHz bandwidth. Since the system is using FDM (analog) and the input signal is in digital, we are going to used digital to analog modulation. One solution is 16-QAM modulation.

Example 3

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Example 3

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Analog hierarchy (AT&T)

Message Channel

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The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously?

Example 4

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SolutionEach band is 25 MHz. If we divide 25 MHz by 30 kHz, we get 833.33. In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users.

Example 4

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• The first commercial use of WDM was to use different frequencies of light for transmitters on each end of one fiber.

• The velocity of propagation is equal to the product of the wavelength and the frequency– vp = λ * f

– In a vacuum, the speed of light, C, is 3x108 m/s

Wavelength Division Multiplexing


• Also called Dense wavelength division multiplexing

• The development of the erbium-doped fiber amplifier (EDFA) made DWDM possible.

• Easy to do with fiber optics and optical sources

• Give each message a different wavelength (frequency)

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• multiplexes multiple data streams onto a single fiber optic line.(4, 8, 16, 32, and 180 lasers in the transmitter)

• C band EDFAs operate from 1530 nm to 1560 nm.• The bandwidth of a C Band system is 4 trillion Hz.• Frequencies used chosen from the ITU Grid.


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ITU Grid

• Each signal carried on the fiber can be transmitted at a different rate from the other signals.

• Different wavelength in a light pulse travels through an optical fiber at different speeds

– Blue light is slower than red light

• Each wavelength takes a different transmission path


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MeritsEnhanced capacity - >100 gbpsFull dup;ex transmissionEasier to configurereliable

Demeritsclose wavelength may cause interference(0.8nm)physical limitation of the fiberlimited to two point circuit or a combination of many

two point circuitrequires mod/demoulator as many as the propagating

wavelenght

is a digital multiplexing technique for combining several low-rate

channels into one high-rate one.

Time Division Multiplexing

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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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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.

Synchronous TDM

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Synchronous TDM

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In synchronous TDM, the data rate of the link is n times faster, and the unit

duration is n times shorter.

Synchronous TDM

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• 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.

Synchronous TDM

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Synchronous TDM

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Synchronous TDM

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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

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• One eight-bit PCM code from each channel is called a TDM frame and the time it takes to transmit one TDM frame is called frame time and it is equal to reciprocal of sample rate.

Synchronous TDM

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Three types popular today:

•T-1 multiplexing (the classic)

•ISDN multiplexing

•SONET (Synchronous Optical NETwork)

Synchronous TDM

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• North American Telephone standards

• Recognized by ITU-T - Recommendation G.733

• The T1 (1.54 Mbps) multiplexor stream is a continuous series of frames of both digitized data and voice channels.

• Each channel contains an 8 bit PCM code and is sampled at 8000 times per second

T1 Digital Carrier System

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A digital carrier system is a communications system that uses digital pulse rather than analog signals to encode information.

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T-1 line for multiplexing telephone lines

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T-1 frame structure

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• The multiplexer has 24 independent inputs and one time-division multiplexed output. The 24 PCM output signals are sequentially selected and connected through the multiplexer to the transmission line.

• A transmitting portion of a Channel Bank digitally encodes the 24 analog channels, adds signaling information into each channel, and multiplexes the digital stream onto the transmission medium.


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The line speed is calculated as:

Each of the 24 channels contains an eight-bit PCM code and is sampled 8000 times a second. Each channel is sampled at the same rate, but may not be at the same time.


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• Later, an additional bit called the framing bit is added to each frame. The framing bit occurs once per frame and is recovered at the receiver and its main purpose is to maintain frame and sample synchronization between TDM transmitter and receiver.


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• As a result of this extra bit, each frame now contains 193 bits and the line speed for a T1 digital carrier system is 1.544 Mbps. { 193 bits × 8000 frames = 1.544 Mbps} .


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Superframe TDM Format

• The 8-kbps signaling rate used with the early digital channel banks was excessive for signaling on standard telephone voice circuits.

• Therefore, with modern channel banks, a signaling bit is substituted only into the least-significant bit (LSB) of every sixth frame.

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• Within each superframe are 12 consecutively numbered frames (1 to12). The signaling bits are substituted in frames 6 and 12, the MSB into frame 6, and the LSB into frame 12. Frames 1 to 6 are called the A highway, with frame 6 designated the A channel signaling frame. Frames 7 to 12 are called the B high way, with frame 12 designated the B channel signaling frame.


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• To identify frames 6 and 12, a different framing bit sequence is used for the odd- and even-numbered frames. The odd frames (frames 1, 3, 5, 7, 9, and 11) have an alternating 1/0 pattern, and the even frames (frames 2, 4, 6, 8, 10, and 12) have a 00 1110repetitive pattern.

• As a result, the combined framing bit pattern is

1000 11011100


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• D4 channel banks time-division multiplex 48 voice-band telephone channels and operate at a transmission rate of 3.152 Mbps, which is slightly more than twice the line speed for 24-channel D1, D2, or D3 channel banks because with D4 channel banks, rather than transmitting a single framing bit with each frame, a 10-bit frame synchronization pattern is used.


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• Line speed is calculated as: total no of bits is 8 bits/channel × 48 channels = 384 bits/frame An additional 10 bits are added for frame: so 394 bits/frame. Therefore, line speed of DS-1C system is 394×8000 = 3.152 Mbps.


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Extended Superframe

Fractional T Carrier Service

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Fractional T Carrier Service

• Fractional T1 systems distribute the channels (i.e., bits) in a standard T1 system among more than one user, allowing several subscribers to share one T1line.

• The above figure shows four subscribers combining their transmissions in a special unit called a data service unit/channel service unit (DSU/CSU). A DSU/CSU is a digital interface that provides the physical connection to a digital carrier network. User 1 is allocated 128 kbps, user 2 - 256 kbps, user 3 - 384 kbps, and user 4 - 768 kbps for a total of 1.536 kbps (8 kbps is reserved for the framing bit).

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North American Digital Multiplexing Hierarchy

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North American DS System• The basic building block for digital

transmission standards begins with the DS0 signal level.– One voice equivalent– 8 bits/sample x 8,000 samples/second

• The DS1 signal has 24 voice equivalents– 193 bits per frame

• 24 x 8 bits per channel

• 1 framing bit

D-Type Channel Banks • D type Channel Bank refers to the terms used in T1

technology.

• Channel Bank defines the type of formatting that is required for transmission on T1 trunk.

• The purpose of a Channel Bank in the telephone company is to form the foundation of multiplexing and de multiplexing the 24 voice channels (DS0).

• D type Channel Bank is one of the types of Channel Bank which is used for digital signals.

• There are five kinds of Channel Banks that are used in the System: D1, D2, D3, D4, and DCT (Digital Carrier Trunk).

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North American Digital Multiplexing hierarchy

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• A special device called muldem (multiplexers/

demultiplexer) is used to upgrade from one level in the hierarchy to the next-higher level.

• They handle bit-rate conversions in both directions and are designated as M12, M23 etc. which identifies the respective input and output digital signals.

• As shown, an M12 muldem interfaces DS-1 and DS-2 digital signals.


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• As shown, an M12 muldem interfaces DS-1 and DS-2 digital signals.

• Digital signals are routed at central locations called digital cross-connects (DSX), which are convenient for making patchable interconnections and routine maintenance and troubleshooting. Each digital signal (i.e. DS-1, DS-2, etc) has its own digital switch (DSX-1, DSX-2/).


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One Frame of a DS1 Signal

DS and T line rates

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T1 Carrier System

• T1 carrier systems were designed to combine PCM and TDM techniques for the transmission of 24 64-kbps channels with each channel capable of carrying digitally encoded voice band telephone signals or data. The transmission bit rate (line speed) for a T1 carrier is 1.544 Mbps.

• All 24 DS-0 channels combined has a data rate of 1.544 Mbps; this digital signal level is called DS-1. Therefore T1 lines are sometimes referred to as DS-1 lines.

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T2 Carrier System

• T2 carriers time-division multiplex 96 64-kbps voice or data channels into a single 6.312 Mbps data signal for transmission over twisted-pair copper wire upto 500 miles over a special LOCAP (low capacitance) metallic cable.

• Higher transmission rates make clock synchronization even more critical.

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T3 Carrier System

• T3 carriers time-division multiplex 672 64-kbps voice or data channels for transmission over a single 3A-RDS coaxial cable. The transmission bit rate is 44.736 Mbps and coding technique used with T3 carriers is binary three zero substitution (B3ZS).

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T4M Carrier System

• T4M carriers time division multiplex 4032 64-kbps voice or data channels for transmitting over a single T4M coaxial cable upto 500 miles. The transmission rate is very high (274.16kbps) making substituting patterns impractical. They transmit scrambled unipolar NRZ digital signals.

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T5 Carrier System

• T5 carriers time-division multiplex 8064 64-kbps voice or data channels and transmits them at 560.16 Mbps over a single coaxial cable.

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• In Europe, a different version of T carrier lines is used called E lines.

• With the basic E1 system, a 125µs frame is divided into 32 equal time slots.

European TDM 30 + 2 System

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• The European TDM system multiplexes 32 DS0 channels together.– Channel 0 is used for synchronizing (framing) and

signaling.

– Channels 1-15 and 17-31 are used for voice.

– Channel 16 is reserved for future use as a signaling channel.

– Time slot 17 is used for a common signaling channel (CSC).

• The total signal rate is 2.048 Mbps (64 kbps * 32 channels)

• The signalling for all 32 voice-band channels is accomplished on the common signalling channel. Consequently, 32 voice-band channels are time-division multiplexed into each E1 frame.

And the line speed can be given as 256 bits/frame × 8000 frames/second = 2.408 Mbps


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•Is a telephone system which provide digital telephone and data services.

•designed to provide access to voice and data services simultaneously

•there is no digital to analog conversion.

•Developed by CCITT (Comate Consultative International Telephonique Telegraphs) to limitation of POTS (Plane old Telephone system).

Integrated Service Digital Network

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The ISDN multiplexor stream is also a continuous stream of frames. Each frame contains various control and sync info.

ISDN frame

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1. B Channel• Carries voice, data, video

etc.• functions at a constant 64

kbps. • can be used for packet and

circuit switching applications.

Channels of ISDN

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2. D Channel (Denial)•is used to convey user signaling massages.•used out of band signaling .This means that network related signals are carried on a separate channel than used data.

Channels of ISDN

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3. H Channel•Have a considerably higher transfer rate than B channels. •sustains rates of approximately 1920 mbps.effectively meet the needs of real time video conferencing, digital qualityaudio and other services requiring a much higher bandwidth.

Channels of ISDN

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• Electronic library Inter connection.

• Electronic resources accessing.• Images, sound and video

retrieval.• Video conferencing.• Call center.• Internet Access.

Use of ISDN

SONET was developed by ANSI;SDH was developed by ITU-T.

Synchronous Optical Network

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SONET Standards

• Fiber optics use synchronous optical network (SONET) standards.

• The initial SONET standard is OC-1, this level is known as synchronous transport level 1 (STS-1).– It has a synchronous frame structure at a speed

of 51.840 Mbps.– OC-1 is an envelope containing a DS3 signal

(28 DS1 signals or 672 channels).

SONET

• Used in massive data rates

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SONET/SDH rates

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SONET defines four layers:path, line, section, and photonic.

SONET

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SONET

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STS Circuits

SONET FRAMESSONET FRAMES

Each synchronous transfer signal STS-n is composed of Each synchronous transfer signal STS-n is composed of 8000 frames. Each frame is a two-dimensional matrix of 8000 frames. Each frame is a two-dimensional matrix of bytes with 9 rows by 90 × n columns.bytes with 9 rows by 90 × n columns.

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An STS-1 and an STS-n frame

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A SONET STS-nsignal is transmitted at

8000 frames per second.

Each byte in a SONET frame can carry a digitized voice channel.

SONETSONET

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TDM Comparisons

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• improves the efficiency of a TDM system.– Channel units do not have reserved time slots.– Time slots are dynamically assigned.

• Also called stat muxs, intelligent multiplexers, and asynchronous multiplexers.

Statistical TDM

• 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.

Statistical TDM

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Statistical TDM

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To identify each piece of data, an address is included.

Statistical TDM

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If the data is of variable size, a length is also included.

Statistical TDM

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More precisely, the transmitted frame contains a collection of data groups.

Statistical TDM

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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!

Statistical TDM

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TDM slot comparison

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• The data rate for each one of the 3 input connection is 1 kbps. If 1 bit at a time is multiplexed (a unit is 1 bit), What is the duration of: (a) each input slot, (b) each output slot, (c) each frame?

Example 5

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Example 5

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Solution a. The data rate of each input connection is 1

kbps. This means that the bit duration is 1/1000 s or 1 ms. The duration of the input time slot is 1 ms (same as bit duration).

Example 5

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(3signals * 1 kbps)/ 3 channels = 1 kbps

1sec / 1000 bits

b. The duration of each output time slot is one-third of the input time slot. This means that the duration of the output time slot is 1/3 ms.

Example 5 (continued)

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1/3 ms per channel

1 ms

c. Each frame carries three output time slots. So the duration of a frame is 3 × 1/3 ms, or 1 ms.

“The duration of a frame is the same as the duration of an input unit.“


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1 ms

Consider a synchronous TDM with a data stream for each input and one data stream for the output. The unit of data is 1 bit. Find :(a) the input bit duration, (b) the output bit duration, (c) the output bit rate, (d) the output frame rate.

Example 6

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Example 6

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Solution

a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs.

b. The output bit duration ( is one-fourth of the input bit duration, or ¼ μs.

Example 6

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c. The output bit rate is the inverse of the output bit duration or 1/(4μs) or 4 Mbps.

This can also be deduced from the fact that the output rate is 4 times as fast as any input rate;

so the output rate = 4 × 1 Mbps = 4 Mbps.

d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000 frames per second.


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Four 1-kbps connections are multiplexed together. A unit is 1 bit. Find: (a) the duration of 1 bit before multiplexing,(b) the transmission rate of the link, (c) the duration of a time slot (d) the duration of a frame.

Example 7

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SolutionWe can answer the questions as follows:a. The duration of 1 bit before multiplexing is: 1 / 1 kbps, or 0.001 s (1 ms).

b. The rate of the link is 4 times the rate of a connection, or 4 kbps.

Example 7

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c. The duration of each time slot is one-fourth of the duration of each bit before multiplexing, or 1/4 ms or 250 μs.

Note that we can also calculate this from the data rate of the link, 4 kbps. The bit duration is the inverse of the data rate, or 1/4 kbps or 250 μs.


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d. The duration of a frame is always the same as the duration of a unit before multiplexing, or 1 ms.

another way:Each frame in this case has four time slots. So the

duration of a frame is 4 times 250 μs, or 1 ms.


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Interleaving

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Four channels are multiplexed using TDM. If each channel sends 100 bytes /s and we multiplex 1 byte per channel, •show the frame traveling on the link, •the size of the frame, •the duration of a frame, •the frame rate, •the bit rate for the link.

Example 8

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SolutionEach frame carries 1 byte from each channel; •the size of each frame = 4 bytes, or 32 bits.•each channel = 100 bytes/sec and a frame = 1 byte •frame rate must be 100 frames / sec.•The bit rate is 100 × 32, or 3200 bps.

Example 8

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Example 8

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A multiplexer combines four 100-kbps channels using a time slot of 2 bits. •What is the frame rate? •What is the frame duration? •What is the bit rate? •What is the bit duration?

Example 9

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Solution•The frame rate is : (4 channels * 100 kbps)/ (2 bits/channel* 4 channel) = 50,000 frames per second. •The frame duration is: 1sec/50,000 frames = 20 μs. • the bit rate is: 50,000 frames/s × 8bit/frame = 400,000 bits/s. •The bit duration is: 1sec /400,000 bits = 2.5 μs.

Example 9

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Example 9

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Empty slots

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Multilevel multiplexing

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Multiple-slot multiplexing

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Pulse stuffing

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Synchronization

• To ensure that the receiver correctly reads the incoming bits, i.e., knows the incoming bit boundaries to interpret a “1” and a “0”, a known bit pattern is used between the frames.

• The receiver looks for the anticipated bit and starts counting bits till the end of the frame.

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Synchronization

• Then it starts over again with the reception of another known bit.

• These bits (or bit patterns) are called synchronization bit(s).

• They are part of the overhead of transmission.

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Framing bits

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We have four sources, each creating 250 8-bit characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (a) the data rate of each source, (b) the duration of each character in each source, (c) the frame rate, (d) the duration of each frame, (e) the number of bits in each frame, and (f) the data rate of the link.

Example 10

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Solution

a. The data rate of each source is 250 × 8 = 2000 bps = 2 kbps.

b. Each source sends 250 characters per second; therefore, the duration of a character is 1/250 s, or 4 ms.

Example 10

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c. Each frame has one character from each source, which means the link needs to send 250 frames per second to keep the transmission rate of each source.

d. The duration of each frame is 1/250 s, or 4 ms. Note that the duration of each frame is the same as the duration of each character coming from each source.

e. Each frame carries 4 characters and 1 extra synchronizing bit. This means that each frame is 4 × 8 + 1 = 33 bits.


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Two channels, one with a bit rate of 100 kbps and another with a bit rate of 200 kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link?

Example 11

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SolutionWe can allocate one slot to the first channel and two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second because it carries 1 bit from the first channel.

The bit rate is 100,000 frames/s × 3 bits per frame, or 300 kbps.

Example 11

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Code Division Multiplexing

• 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

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• 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

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• 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


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Summary

• 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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SPREAD SPECTRUM

In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.

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History of Spread Spectrum

Spread Spectrum was actually invented by 1940s Hollywood actress Hedy Lamarr(1913-2000).

An Austrian refugee, in 1940 at the age of 26, she devised together with music composer George Antheil a system to stop enemy detection and jamming of radio controlled torpedoes by hopping around a set of frequencies in a random fashion. 143

History of Spread Spectrum

She was granted a patent in 1942 (US pat. 2292387) but considered it her contribution to the war effort and never profited.

Techniques known since 1940s and used in military communication systems since 1950s.

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“Spread” radio signal over a wide frequency range

Several magnitudes higher than minimum requirement

Gained popularity by the needs of military communication

Proved resistant against hostile jammers

Ratio of information bandwidth and spreading bandwidth is identified as spreading gain or processing gain

Processing gain does not combat white Noise

Introduction to Spread Spectrum

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applications:

• able to deal with multi-path

• multiple access due to different spreading sequences

• spreading sequence design is very important for performance

• low probability of interception

• privacy

• anti-jam capabilities

SPREAD SPECTRUM

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Spread Spectrum Applications

InterferenceB̶Prevents interference at specific frequenciesB̶E.g. other radio users, electrical systems

MilitaryB̶Prevents signal jammingB̶Scrambling of ‘secret’ messagesB̶gps

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Spread Spectrum Applications

Wireless LAN securityB̶Prevents ‘eavesdropping’ of wireless linksB̶Prevents ‘hacking’ into wireless LANs

CDMA (Code Division Multiple Access)B̶Multiple separate channels in same medium using

different spreading codes

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Spread Spectrum Criteria

A communication system is considered a spread spectrum system

if it satisfies the following two criteria:Bandwidth of the spread spectrum signal has to be greater than the information bandwidth. (This is also true for frequency and pulse code modulation!)

The spreading sequence has to be independent from the information. Thus, no possibility to calculate the information if the sequence is known and vice versa.

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Spread Spectrum Classification

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Spread Spectrum Classification

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Direct Sequence Spread Spectrum

Information signal is directly modulated (multiplicated) by a spreading sequences

• Commonly used with digital modulation schemes• The idea is to modulate the transmitter with a bit

stream consisting of pseudo-random noise (PN) that has a much higher rate than the actual data to be communicated

• Near/far effectRequire continuous bandwidth

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Direct Sequence Spread Spectrum

• The use of the high-speed PN sequence results in an increase in the bandwidth of the signal, regardless of what modulation scheme is used to encode the bits into the signal

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DSSS example

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The information signal is transmitted on different frequencies

Time is divided in slotsEach slot the frequency is changed

Frequency Hopping Spread Spectrum

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The change of the frequency is referred to as slow if more than one bit is transmitted on one frequency, and as fast if one bit is transmitted over multiple frequenciesThe frequencies are chosen based on the spreading sequences

Frequency Hopping Spread Spectrum

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FHSS

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Frequency selection in FHSS

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FHSS cycles

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Bandwidth sharing

Time Hopping Spread Spectrum

Time divided into frames; each TF longEach frame is divided in slotsEach wireless terminal send in exactly one of these slots per frame regarding the spreading sequenceNo near far effect

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Time Hopping Spread Spectrum

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Comparison of different Spread SpectrumTechniques

SS Technique advantage disadvantage

Direct Sequence • best behavior in multi path rejection

• no synchronization• simple implementation• difficult to detect

• near far effect• coherent bandwidth

FrequencyHopper

• no need for coherent• bandwidth• less affected by the near

far effect

• complex hardware• error correction • needed

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SS Technique advantage disadvantage

Time Hopper• high bandwidth

efficiency • less complex

hardware• less affected by the

near far effect

• error correction needed

Comparison of different Spread SpectrumTechniques

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Code Division Multiple Access

• analog communication systems used frequency division multiplexing

• digital systems have employed time division multiplexing

• to combine many information signals into a single transmission channel from different sources, these two methods become frequency division multiple access (FDMA) and time-division multiple access (TDMA), respectively.

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• Spread-spectrum communication allows a third method for multiplexing signals from different sources:

code-division multiple access

• allow multiple users to use the same frequency using separate PN codes and a spread-spectrum modulation scheme

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Code Division Multiple Access

Reference

• Digital Communication – by Sanjay Sharma

• Advance Electronic Communication – by Robert Tomasi

• World Wide Web

• Slideshare.net

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Education

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