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Chapter 3. Physical Layer

Business Data Communications and Networking Fitzgerald and Dennis,

7th EditionCopyright © 2002 John Wiley & Sons, Inc.

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Copyright John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that named in Section 117 of the United States Copyright Act without the express written consent of the copyright owner is unlawful. Requests for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. Adopters of the textbook are granted permission to make back-up copies for their own use only, to make copies for distribution to students of the course the textbook is used in, and to modify this material to best suit their instructional needs. Under no circumstances can copies be made for resale. The Publisher assumes no responsibility for errors, omissions, or damages, caused by the use of these programs or from the use of the information contained herein.

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Chapter 3. Learning Objectives

• Be familiar with different types of network circuits and media

• Understand digital transmission of digital data

• Understand analog transmission of digital data

• Understand digital transmission of analog data

• Be familiar with analog/digital modems

• Be familiar with multiplexing

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Chapter 3. Outline• Introduction• Circuits

– Circuit Configuration, Data Flow, Communication Media, Media Selection

• Digital Transmission of Digital Data– Coding, Transmission Modes, Dig. Trans, Ethernet Data Transmission

• Analog Transmission of Digital Data– Modulation, Voice Circuit Capacity, How Modems Transmit Data

• Digital Transmission of Analog Data– Pulse Amplitude Modulation, How Telephones Transmit Voice Data,

How Instant Messenger Transmits Voice Data

• Analog/Digital Modems• Multiplexing

– FDM, TDM, STDM, WDM, Inverse Multiplexing, DSL Transmission

• Summary

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Introduction

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The Physical Layer• The physical layer includes network hardware and

circuits.• Network circuits include physical media (e.g., cables)

and special purposes devices (e.g., routers and hubs). Networks are made of both physical and logical circuits.– Physical circuits connect devices & include actual wires.– Logical circuits refer to the transmission characteristics of

the circuit, such as a T-1 connection.

• Sometimes the physical and logical circuits are the same, but they can be different. For example, in multiplexing, one wire carries several logical circuits.

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Special Purpose Devices

HubRouter

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Analog and Digital Data

• Another fundamental physical layer distinction is between digital and analog forms of data.

• Sounds waves, which vary continuously over time are analog data.

• Computers produce digital data that is in binary form, that is, it is represented as a series of ones and zeros.

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Analog vs. Digital Transmission• Transmissions can also be either analog or digital.

– Analog transmissions, like analog data, vary continuously. Examples of analog data being sent using analog transmissions are broadcast TV and radio.

– Digital transmissions are made of square waves with a clear beginning and ending. Computer networks send digital data using digital transmissions.

• Data can be converted between analog and digital formats. – When digital data is sent as an analog transmission modem

(modulator/demodulator) is used. – When analog data is sent as a digital transmission, a codec

(coder/decoder) is used.

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Converters

Modem

Codec

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Data Type vs. Transmission Type

Analog

Transmission

Digital

Transmission

Analog

Data

Radio, Broadcast TV

Pulse Code Modulation & Video standards using codecs

Digital Data Modem-based

Communications

LAN Cable Standards

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Advantages of Digital Transmission

• Digital transmission:– produces fewer errors than analog transmission. Because

the transmitted data is binary (1s and 0s), it is easier to detect and correct errors.

– permits higher transmission rates. Optical fiber, for example, is designed for digital transmission.

– is more efficient. It’s possible to send more data through a given circuit using digital rather than analog transmission.

– is more secure since it is easier to encrypt.

• Integrating voice, video and data on the same circuit is also far simpler with digital transmission since signals made up of digital data are easier to combine.

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Circuits

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Circuit Configuration

• Two basic circuit configurations: • Point-to-point connects just one sender and

receiver together (Figure 3-1)• Multipoint (also called a shared circuit)

connects a number of senders and receivers together (Figure 3-2)– The advantage of multipoint is that it is cheaper

and simpler to wire.– The disadvantage is that only one computer can

use the circuit at a time.

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Figure 3-1 Point-to-point configuration

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Figure 3-2 Multipoint configuration

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Data Flow (Figure 3-3)

• Data can move in one direction or both directions. • Simplex data flows move in one direction only,

such as radio or cable television broadcasts.• Half Duplex data flows both ways, but only one

direction at a time. As with CB radio, some kind of control information must also be included so that sender and receiver don’t send at the same time.

• Full Duplex data flows in both directions at the same time.

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Figure 3-3 Simplex, half-duplex and full duplex transmission

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Communications Media

• Medium: the physical matter that carries the transmission. Two basic categories of media:

• With Guided media the transmission flows along a physical guide. The three main types of guided media: twisted pair wiring, coaxial cable and optical fiber cable.

• With Wireless media there is no wave guide and the transmission just flows through the air (or space). The main forms of wireless communications are radio, infrared, microwave, and satellite communications.

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Guided Media: Twisted Pair Wires• Twisted pair wire cables are commonly used for

telephones and local area networks.• Twisting two wires together reduces

electromagnetic interference. • TP cables have a number of pairs of wires.

– Telephone lines have two pairs (4 wires, usually only one pair is used by the telephone)

– LAN cables have 4 pairs (8 wires)

• Shielded twisted pair (STP) also exists, but is more expensive.

• TP cables are also used in telephone trunk lines and can have up to several thousand pairs.

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Guided Media: Coaxial Cable

• Formerly common on LANs, but now disappearing (but still used on other comm. equipment, e.g., CATV – cable television).

• More expensive than twisted pair, but coax is shielded, so it’s less prone to interference than twisted pair.

• Coaxial Cable Structure (Figure 3-5):– Inner conductor– Insulator– Wire mesh ground– Outer protective jacket or shell

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Figure 3-5 Coaxial Cable

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Guided Media: Fiber Optic Cable• Widely used and has extremely high capacity.• Light created by an LED (light-emitting diode) or

laser is sent down a thin glass or plastic fiber.• Ideal for broadband. Most observers feel that fiber

will be used more and more extensively in the future.

• Fiber optic cable structure (from center):– Core (v. small, 5-50 microns, ~ the size of a

single hair)– Cladding, which reflects the signal– Protective outer jacket

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Optical Fiber

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Guided Media: Fiber Optic Cable• Types of Optical Fiber:

– Multimode is cheap, but the signal spreads out over short distances (up to ~500m). Multimode: light can reflect inside the cable at many angles.

– Graded index multimode reduces the spreading problem by changing the refractive properties of the fiber to refocus the signal can be used over distances of up to about 1000 meters.

– Single mode is expensive because difficult to manufacture, but signal can be sent over many kilometers without spreading.

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Figure 3-6 Optical Fiber

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Wireless Media• Wireless media signals are becoming popular for

LAN use. The main forms are:– Radio: wireless transmission of electrical waves.

Includes AM and FM radio bands. Microwave is also a form of radio transmission.

– Infrared: “invisible” light waves whose frequency is below that of red light. Requires line of sight and are generally subject to interference from heavy rain. Used in remote control units (e.g., TV).

– Microwave: high frequency form of radio with extremely short wavelength (1 cm to 1 m). Often used for long distance, terrestrial transmissions and cellular telephones. Requires line-of-sight.

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Guided Media: Satellite Communications

• Satellite communications are a special form of microwave communications.

• Instead of transmitting from one terrestrial microwave dish to another, satellite communications are sent from the ground to a satellite, usually in geosynchronous orbit, about 23,000 miles above the earth. The satellite then relays the signal to its destination ground station.

• Even with signals traveling at light speed, the great distance between ground station and satellite means a relatively long propagation delay occurs between sending and receiving a signal.

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Figure 3-8 Satellites in operation

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Media Selection depends on many factors including:

• Type of network

• Cost

• Transmission distance

• Security

• Error rates

• Transmission speeds

• See Figure 3-9.

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Digital Transmission of Digital Data

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Coding• Any written language uses symbols, but computers

send signals in 1s and 0s (bits). • Each written character needs a bit code in order to be

used by a computer. A set of these codes for a language is called a coding scheme.

• The two main character codes in use in North America are ASCII and EBCDIC.– ASCII: American Standard Code for Information

Interchange, originally used a 7-bit code (128 combinations), but an 8-bit version is now in use.

– EBCDIC: Extended Binary Coded Decimal Interchange Code, an 8-bit code developed by IBM.

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Transmission Modes• Data can be sent either in serial or in parallel• Parallel mode (Figure 3-10): uses several wires,

each wire sending one bit at the same time as the others. – A parallel printer cable sends 8 bits together. – Your computer’s processor and motherboard also use

parallel busses to move data around.

• Serial Mode (Figure 3-11): sends bit by bit over a single line. Serial mode is slower than parallel, but can be used over longer distances because the bits stay in the order they were sent, while bits sent in parallel mode tend to spread out over long distances.

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Figure 3-10 Parallel transmission

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Figure 3-11 Serial transmission

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Digital Transmission (Figure 3-12)• Digital signals are sent as a series of “square waves”

of either positive or negative voltage. Voltages vary between +3/-3 and +24/-24 depending on the circuit.

• Each digital transmission standard defines what voltage levels correspond to a bit value of 0 or 1.

• Unipolar signal voltages either vary between 0 and a positive value or between 0 and some negative value.

• Figure 3-12 shows a unipolar signal for which 5 volts mean a binary 1, and 0 volts mean a binary 0.

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Digital Transmission (cont.)• With bipolar signals, signals are sent using both

positive and negative voltages.• A second digital transmission factor, called return

to zero (RZ) means the signal returns to the 0 voltage level after sending a bit. In non return to zero (NRZ), the signals maintains its voltage at the end of a bit.

• Ethernet uses Manchester encoding in which the bit value is defined by a mid-bit transition. A high to low voltage transition is a binary 0 and a low-high mid-bit transition defines a binary 1.

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Figure 3-12 Digital transmission types

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Analog Transmission of Digital Data

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The Telephone Network

• Originally designed for analog communications only.

• Today, standard analog telephone service is called POTS (Plain Old Telephone Service).

• Modem communications use the telephone network to send digital data that has been converted into an analog format.

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Carrier Waves• Modems use carrier waves to send information

(Figure 3-13). • Each wave has three fundamental characteristics:

– Amplitude, meaning the height (intensity) of the wave– Frequency, which is the number of waves that pass in

a single second and is measured in Hertz (cycles/second) (wavelength, the length of the wave from crest to crest, is related to frequency.).

– Phase is a third characteristic that describes the point in the wave’s cycle at which a wave begins and is measured in degrees. (From example, changing a wave’s cycle from crest to trough corresponds to a 180 degree phase shift).

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Figure 3-13 A Carrier Wave

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Modulation

• Modulation is the modification of a carrier wave’s fundamental characteristics in order to encode information.

• There are three basic ways to modulate a carrier wave:– Amplitude Modulation – Frequency Modulation– Phase Modulation

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Amplitude Modulation

• Amplitude Modulation (AM), also called Amplitude Shift Keying (ASK), means changing the height of the wave to encode data.

• The AM dial on the radio uses amplitude modulation to encode analog information.

• Figure 3-14 shows a simple case of amplitude modulation in which one bit is encoded for each carrier wave change.– A high amplitude means a bit value of 1

– Zero amplitude means a bit value of 0

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Figure 3-14 Amplitude Modulation

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Frequency Modulation• Frequency Modulation (FM), also called

Frequency Shift Keying (FSK), means changing the frequency of the carrier wave to encode data.

• The FM dial on the radio uses frequency modulation to encode analog information.

• Figure 3-15 shows a simple case of frequency modulation in which one bit is encoded for each carrier wave change.– Changing the carrier wave to a higher frequency

encodes a bit value of 1– No change in the carrier wave frequency means a bit

value of 0

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Figure 3-15 Frequency Modulation

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Phase Modulation• Phase refers to the point in each wave cycle at

which the wave begins.• Phase Modulation (PM), also called Phase Shift

Keying (PSK) means changing the phase of the carrier wave to encode data.

• Figure 3-16 shows a simple case of phase modulation in which one bit is encoded for each carrier wave change.– Changing the carrier wave’s phase by 180o corresponds

to a bit value of 1– No change in the carrier wave’s phase means a bit

value of 0

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Figure 3-16 Phase Modulation

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Sending Multiple Bits Simultaneously

• Each modification of the carrier wave to encode information is called a symbol.

• By using a more complicated information coding system, it is possible to encode more than 1 bit/symbol.

• Figure 3-17 gives an example of amplitude modulation using 4 amplitude levels, corresponding to 2 bits/symbol.

• Increasing the possible number of symbols from 4 to 8 corresponds with encoding 3 bits/symbol, 16 levels to 4 bits, and so on.

• Likewise, multiple bits per symbol might be encoded using phase modulation, say using phase shifts of 0o, 90o, 180o, and 270o.

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Fig. 3-17 Two-bit Amplitude Modulation

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Quadrature Amplitude Modulation (QAM)

• QAM is a family of encoding schemes that are widely used for encoding multiple bits per symbol that combine Amplitude and Phase Modulation.

• 16QAM is a common form of that uses 8 different phase shifts and 2 different amplitude levels. Since there are 16 possible symbols, each symbol encodes 4 bits.

• QAM and related techniques are commonly used for voice modems with a data rate of up to about 28 kilobits/second.

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Bit Rate vs. Baud Rate (Symbol Rate)

• Bit rate (or data rate) is the number of bits transmitted per second.

• Baud rate (same as symbol rate) refers to the number of symbols transmitted per second.

• As we have just seen, since multiple bits can be encoded per symbol, the two terms are not the same.

• For example, in Figure 3-17, the bit rate is twice the baud rate. The general formula is:

Data Rate (bits/second)=

Symbol Rate (symbols/sec.) x No. of bits/symbol

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Capacity of a Voice Circuit• The capacity of the telephone network is

constrained by the limitations of the telephone lines and equipment used to transmit voice.

• Human hearing has a freq. range of 20 Hz to 14-20 kHz, but the voice circuit range is 0-4000 Hz.

• QAM using 6 bits per symbol and the maximum voice channel carrier wave frequency, corresponds to a data rate of 6 * 4000 = 24 kbps.

• Phone lines have a far higher transmission capacity: 1 MHz for lines up to 2 miles (3 km) from a telephone exchange and 300 kHz for lines 2-3 miles (3 km) away.

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How Modems Transmit Data• Modem means modulator/demodulator. It is the device that

encodes and decodes data by manipulating the carrier wave.• The V-series of modem standards are those approved by the

ITU-T standards group.– V.22, an early standard, had a 2400 bps bit rate– V.34, one of the robust V standards, includes multiple data

rates (up to 28.8 kbps) and a handshaking sequence that tests the circuit and determines the optimum data rate. V.34+ increases the max. to 33.6 kbps

• Modems also use data compression which looks for more efficient ways to encode redundant data strings.

• For example, Lempel-Ziv encoding, compresses data by creating a dictionary of character patterns and then transmitting compressed versions of those patterns.

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Digital Transmission of Analog Data

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Pulse Amplitude Modulation (PAM)

• An analog voice signal can be converted into digital form using a device called a codec (coder/decoder) which also converts it back to analog data at the receiving end.

• The codecs used by the phone system use Pulse Amplitude Modulation. PAM involves 3 steps (see Figures 3-19a-c):– Measuring the signal– Taking samples of the signal– Encoding the signal as a binary data sample

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Figure 3-19a Pulse Amplitude Modulation

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Figure 3-19b Pulse Amplitude Modulation

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Figure 3-19c Pulse Amplitude Modulation

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How Telephones Transmit Voice• The line from your phone to the first phone switch

(called the local loop), still uses the analog techniques developed over a century ago by Bell.

• Today’s switches are now almost completely digital and convert these analog signals to digital data using a technique called Pulse Code Modulation (PCM).

• PCM (similar to PAM) specifies a sample rate of 8000 samples/second and 8 bits/sample. The basic digital communications unit used by the phone network, the DS-0, has a data rate of 64 kbps, corresponding to 1 digital voice signal.

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How Instant Messenger Transmits Voice

• Instead of PCM, Instant Messaging uses an alternative technique called ADPCM, adaptive differential pulse code modulation.

• ADPCM encodes the differences between samples. Instead of 8 bits/sample, ADPCM uses only 4 bits/sample, generally at 8000 samples/second. This allows a voice signal to be sent at 32 kbps, which makes it possible for IM to send voice signals as digital signals using POTS-based analog phone lines.

• ADPCM can also use lower sampling rates, at 8 or 16 kbps, but these produce lower quality voice signals.

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Analog/Digital Modems

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56k Modems• 56k modems, the fastest possible on voice grade

lines, are based on the V.90 and V.92 standards.• Downstream transmissions (from phone switch to

the user’s computer) use a technique based on recognizing PCM’s 8-bit digital symbols.

• With the V.90 standard, upstream transmissions are still based on the V.34+ standard. The V.92 standard uses this PCM symbol recognition technique for both up and downstream channels.

• The technique is very sensitive to noise and both V.90 and V.92 modems often must use lower data rates. The max. V.92 upstream rate is 48 kbps.

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Multiplexing

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Multiplexing

• Multiplexing means breaking up a higher speed circuit into several slower circuits.

• The main advantage of multiplexing is cost; multiplexing is cheaper because fewer network circuits are needed.

• There are four categories of multiplexing:– Frequency division multiplexing (FDM)

– Time division multiplexing (TDM)

– Statistical time division multiplexing (STDM)

– Wavelength division multiplexing (WDM)

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Frequency Division Multiplexing (FDM)

• FDM works by making a number of smaller channels from a larger frequency band (Figure 3-21). FDM is sometimes referred to as dividing the circuit “horizontally”.

• In order to prevent interference between channels, unused frequency bands called guardbands are used to separate the channels. Because of the guardbands, there is some wasted capacity on an FDM circuit.

• CATV uses FDM. FDM was also commonly used to multiplex telephone signals before digital transmission became common and is still used on some older transmission lines.

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Figure 3-21 Frequency Division Multiplexing

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Time Division Multiplexing (TDM)

• TDM allows multiple channels to be used by allowing the channels to send data by taking turns. TDM is sometimes referred to as dividing the circuit “vertically”

• Figure 3-22 shows an example of 4 terminals sharing a circuit, with each terminal sending one character at a time.

• With TDM, time on the circuit is shared equally with each channel getting a specified time slot, whether or not it has any data to send.

• TDM is more efficient than FDM, since TDM doesn’t use guardbands, so the entire capacity can be divided up between the data channels.

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Figure 3-22 Time Division Multiplexing

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Statistical Time Division Multiplexing (STDM)

• STDM is designed to make use of the idle time created when terminals are not using the multiplexed circuit.

• Like regular TDM, STDM uses time slots, but the time slots are not fixed. Instead, they are used as needed by the different terminals on the multiplexed circuit.

• Since the source of a data sample is not identified by the time slot it occupies, additional addressing information must be added to each sample.

• If all terminals try to use the multiplexed circuit intensively, response time delays can occur. The multiplexer also needs to contain memory to store data in case more data samples come in than its outgoing circuit capacity can handle.

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Wavelength Division Multiplexing (WDM)

• Optical fiber uses lasers or LEDs which previously transmitted at only a single frequency, with a typical transmission rate being around 622 Mbps.

• With WDM, data is transmitted at several different frequencies over the same fiber.

• The data transmission capacity of optical continues to increase dramatically. A new version of WDM, Dense WDM or DWDM promises data rates in the terabits, with over a hundred channels per fiber, each transmitting at a rate of 10 Gbps, making aggregate data rates in the low terabit range possible.

• Future versions of DWDM will make petabit aggregate transmission rates possible as per channel data rates and the total number of channels both continue to rise.

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Inverse Multiplexing (Figure 3-24)

• Instead of using a single line, an inverse multiplexer (IMUX) shares the load by sending multiplexed data over two or more lines.

• For example, two T-1 lines can be used to send data, creating a combined multiplexed capacity of 2 x 1.544 = 3.088 Mbps.

• A recent IMUX standard, the Bandwidth ON Demand Network Interoperability Group (BONDING) standard, is often used for videoconferencing applications. With the BONDING standard, six 64 kbps lines can be combined to create an aggregate line of 384 kbps for transmitting video.

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Figure 3-24. Inverse Multiplexing

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How DSL Transmits Data• Digital Subscriber Line is becoming popular as a way to

increase data rates in the local loop.

• Instead of using the 0-4000 KHz voice channel, DSL uses the physical capacity of the copper phone lines of up to 1 MHz.

• The 1 MHz capacity is split into: 1) a 4 KHz voice channel, 2) an upstream channel and 3) a downstream channel.

• There are several versions of DSL, with the main differences being how much of the bandwidth is allocated between the upstream and downstream channels.

• One form of DSL, G.Lite provides a 4 Khz voice channel, 384 kbps upstream and 1.5 Mbps downstream (provided line conditions are optimal).

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Next Day Air Service Case Study

77Figure 3-25 NDAS Case Study

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End of Chapter 3