network technology cse3020 - 2006 1 network technology cse3020 week 4

Network Technology CSE3020 - 2006

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Network Technology CSE3020

Week 4


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Transmission Media• Guided Transmission:

– Twisted Pair - Unshielded Twisted Pair (UTP) - Shielded Twisted Pair (STP)

– Coaxial Cable– Optical Fiber

• Unguided (wireless) Transmission:– Terrestrial Microwave– Satellite Microwave– Broadcast Radio– Infrared


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Transmission Media

• Guided (wire) and Unguided (wireless) use electromagnetic waves.

• Characteristics and quality determined by medium and the transmitted signal.– For guided, the medium is more important.

– For unguided, the bandwidth of signal produced by the antenna is more important.

• Key concerns are data rate and distance.


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Transmission Media• Design Factors:

– Bandwidth - Higher bandwidth gives higher data rate.

– Transmission impairments - Attenuation limits the distance.

– Interference (Noise) - Overlapping frequency bands can distort or wipe out

a signal

- Emanations from nearby cables

– Number of receivers- More receivers (multi-point), each attachment introduces some attenuation in guided media.


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Interference (Noise)

Additional signals inserted between transmitter and receiver.

Crosstalk: signal from one line is picked up by another.

Thermal: - Due to thermal agitation of electrons.- Uniformly distributed.- White noise.

Intermodulation: signals that are the sum and difference of original frequencies sharing a medium.

Impulse: - Irregular pulses or spikes- e.g. External electromagnetic interference- short duration & high amplitude


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Electromagnetic Spectrum


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Twisted Pair

• Two separately insulated copper wires twisted together form a pair.

• A wire pair acts as a single communication link• Twisting reduces crosstalk between adjacent pairs• Applications:

– Common medium for both analog and digital.– Telephone network - Between house and local exchange (local

loop) – 56kbps modem, ADSL modem, voice.– Within buildings - Private branch exchange (PBX).– For local area networks (LAN) - 10Mbps,100Mbps, 1Gbps


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Twisted Pair

• Two categories – Unshielded Twisted Pair (UTP)

– Shield Twisted Pair (STP)


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UTP – TIA/EIA 568A• Cat 1

• Ordinary home telephone cable

• Cat 3

– Bandwidth 16 MHz.

– Voice grade cable found in most offices.

– Twist length of 7.5 cm to 10 cm.

• Cat 5

– Bandwidth 100 MHz.

– Commonly pre-installed in new office buildings

– 100BaseTx – max 100m, then need repeater

– Twist length 0.6 cm to 0.85 cm

– Cat 6 – 250MHz

– Cat 7 – 600MHz


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STP• Metal braid or sheathing that reduces interference.

• More expensive.

• Harder to handle and work with

Near End Crosstalk

• Coupling of signal from one pair to another.• Coupling takes place when transmit signal entering the link

couples back to receiving pair.• i.e. near transmitted signal is picked up by near receiving pair.


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Coaxial Cable

• A coax cable consists of the following:

- A center conductor – usually copper.

- A metalic outer conductor serves as a ground.

- An insulator covering the center conductor.

- A plastic jacket.

• Two types: Thinnet and Thicknet.


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Coaxial Cable• Applications:

– Television distribution - Cable TV– Long distance telephone transmission:

- Can carry 10,000 voice calls simultaneously using FDM

- Being replaced by fiber optic.– Short distance computer systems links.

• Transmission Characteristics:– Higher frequency characteristics than twisted pair.

– Analog (Up to 500MHz): Amplifiers every few km, closer if higher frequency.

– Digital: Repeater every 1km, closer for higher data rates.


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Optical Fiber

• Greater capacity - Data rates of hundreds of Gbps.• Smaller size & lighter weight.• Lower attenuation.• Electromagnetic isolation.• Greater repeater spacing - tens of km at least.• Transmits a signal-encoded beam of light by means of total

internal reflection. • Act as wave guide for 1014 to 1015 Hz.

– Portions of infrared and visible spectrum.


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Optical Fiber

• TIR – Total Internal Reflection– 100% of light that strikes a surface is reflected– By comparison a mirror reflects 90%

• Optical fibers should guide light waves with minimal loss

• Angle of Incidence I – the angle the ray makes with the line perpendicular (90o) to the interface between two substances

• Critical angle• Refraction - I <= critical angle• Reflection – I > critical angle


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Optical Fiber - Applications• Long-haul trunks

- about 1500 km in length & 20,000-60,000 voice channels. - undersea optical fiber

• Metropolitan trunks. - about 12 km in length & 100,000 voice channels. - underground conduits joining telephone exchanges.

• Rural exchange trunks. - about 40 – 160 km in length & less than 5000 voice channels.

• Subscriber loops. - handling voice, data, image and video.

• LANs. - Capacity of 100 Mbps to 1 Gbps.


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Optical Fiber - Transmission

Characteristics• Three types of transmission modes.– Step-index multimode

• Rays are reflected, absorbed and propagated along the fiber.

• Light pulses spread out in time.– Single mode

• Single transmission path.• Long distance application (telephone & cable TV).

– Graded-index multimode• Intermediate between single mode and step-index

multimode.• Used in LAN.


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Optical Fiber Transmission Characteristics


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Optical Fiber - Transmission Characteristics

• Light sources: semiconductor devices that emit a beam of light when a voltage is applied.

– Light Emitting Diode (LED):

• Cheaper.

• Wider operating temperature range.

• Last longer.

– Injection Laser Diode (ILD):

• More efficient.

• Greater data rate.

• Wavelength Division Multiplexing (FDM)

– multiple beams of light at different frequencies (or wavelengths) are transmitted on the same fiber.

– Commercial systems – 80 channels of 10 Gbps


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Wireless Transmission• Unguided media (Transmission and reception via antenna).• Directional (Higher Frequencies):

– Focused beam.• Omnidirectional (Lower Frequencies):

– Signal spreads in all directions.

• Propagation in free space always like light (straight line).• Receiving power proportional to 1/d².

(d = distance between sender and receiver).• Signal can take many different paths between sender

and receiver due to reflection, scattering, diffraction.


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• Channel characteristics change over time and location.• Influences on the received signal power:

– Fading: variation of signal strength with time.– shadowing– reflection at large obstacles– scattering at small obstacles– diffraction at edges

reflection scatteringshadowing

Wireless Transmission

diffractiondiffraction


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signal at sendersignal at receiver

X X X

DelayTransmitted

signal

DelayDelay

Received signal

• Multipath channel:


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• Frequencies:– 2GHz to 40GHz

• Microwave.• Highly directional beam.• Point-to-point transmission.• Satellite.

– 30MHz to 1GHz• Broadcast radio. • Omnidirectional transmission.

– 3 x 1011 to 2 x 1014

• Infrared.• Local point-to-point and multipoint application.

• To reduce channel effects:– Error correction codes: remove bit errors introduced in transmission.– Equalizations: remove channel attenuation & delay (multipath effects).– Diversity techniques: use of more than one signal.


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Terrestrial Microwave• Parabolic dish antenna (typical size of 3 m in diameter).• Focused beam to achieve line of sight transmission.• Located at substantial heights above ground level.• Microwave relay towers used to achieve long distance.• Applications:

– Long haul telecommunications service.– Short point-to-point links between buildings.

• Higher frequencies give higher data rates.• Attenuation and interference.

– Attenuation is increased with rainfall.– With growing applications, transmission areas overlap and

resulting in interference.


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Satellite Microwave• Satellite is a microwave relay station.• Satellite receives on one frequency band (uplink) and amplifies or

repeats signal and transmits on another frequency band (downlink).

• May requires geo-stationary orbit:– To maintain line of sight.– Height of 35,784km.

• Applications.– Television distribution.– Long-distance telephone transmission.– Private business networks.– VSAT systems.

• High propagation delay (about 0.25 seconds).


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Broadcast Radio• Omnidirectional.• Does not require dish-shaped antenna.• Applications:

– FM radio– UHF and VHF television

• Limited by line of sight.• Less sensitive to attenuation from rainfall.• Suffers from multipath interference (reflections).


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Infrared• Modulate noncoherent infrared light.• Transceivers (transmitters/receivers) must be within the line of

sight.• Does not penetrate walls.• E.g.: TV remote control.• No frequency allocation & no licensing.

Required Reading

• W. Stallings, “Data and Computer Communications (6th edition),” Prentice-Hall, 2000.

>> Chapter 4


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Asynchronous and SynchronousTransmission• Transmission requires cooperation and agreement between the

two sides.• Fundamental requirement is Synchronization: Receiver must

know the beginning and end of a bit/rate at which bits are received.– Asynchronous transmission– Synchronous transmission

• Asynchronous transmission:– Each character treated independently and begins with a start bit.– Not good for long block of data.

• Synchronous transmission:– Block of data is formatted as a frame with a starting and an ending flag.– Good for block of data.


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Asynchronous Transmission• Avoid sending long, uninterrupted streams of bits.

• Data transmitted on character at a time (5 to 8 bits).• Timing only needs to be maintained within each character.• Receiver can resynchronize with each new character.

• Operation:• In idle state (binary 1), receiver looks for transition from 1

to 0 (start bit).• Then samples next seven intervals (char length).• May add a parity bit (odd or even).• The final signal element is the stop bit (binary 1). • The stop bit is same as idle state.• Then looks for next 1 to 0 transition for next character.

• Advantages: Simple and Cheap.• Disadvantage: Overhead of 2 or 3 bits per char (~20%).


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Asynchronous Transmission


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Synchronous Transmission• Block of data transmitted without start or stop bits.

• Clocks must be synchronized.

• Can use separate clock line.

– Good over short distances.

– Subject to impairments.

• Embed clock signal in data.

– Manchester encoding (digital).

– Carrier frequency (analog)


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Synchronous Transmission• Block Level:

– Need to indicate start and end of block

– Use preamble and postamble.

• e.g. series of SYN (hex 16) characters

• e.g. block of 11111111 patterns ending in 11111110

– More efficient (lower overhead) than asynchronous.


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Line Configuration

• Topology: physical arrangement of stations on a

transmission medium. – Point to point– Multi point.

• Half duplex– Only one station at a time.– Requires one data path.

• Full duplex– Simultaneous transmission.– Requires two data paths.


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Interfacing• Data processing devices or data terminal equipments (DTE) do

not (usually) include data transmission facilities.

• Need an interface called data circuit terminating equipment (DCE).


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Interfacing• DCE transmits bits on medium.• DCE communicates data and control information with DTE.

– Done over interchange circuits.– Clear interface standards required.

• Characteristics of Interface: – Mechanical:

• Connection plugs. – Electrical:

• Voltage levels, timing of voltage changes, encoding• Determines the data rate and the distance.

– Functional:• Data, control, timing, grounding.

– Procedural:• Sequence of events for transmitting data.


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V.24/EIA-232-F• Most widely used interface (e.g.: PC serial port)

• ITU-T standard: V.24

– Only specifies functional and procedural aspects.

– References other standards for electrical and mechanical.

• Virtually identical standard EIA-232-F (USA) covers all aspects.

– Mechanical: ISO 2110

– Electrical: V.28

– Functional: V.24

– Procedural: V.24

• EIA-232 was first issued by Electronic Industries Alliance in 1962, as RS-232.

• Used in modem and other interconnection applications.


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Mechanical Specification


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Electrical Specification

• Digital signals on all interchange circuits.• Values interpreted as data or control, depending on circuit.• Less than -3v is binary 1, more than +3v is binary 0 (NRZ-L).• Signal rate < 20kbps. • Distance <15m.• For control, less than -3v is off, and more than +3v is on.

Functional Specification• Refer table 6.1 in Stallings chapter 6 for functional specifications.

• Loopback control is a useful fault isolation tool.

- Make use of functional specifications.


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Local and Remote Loopback


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Procedural Specification

• Defines the sequence in which the various circuits are used.

• E.g. Asynchronous private line modem:– When turned on and ready, modem (DCE) asserts DCE ready.

– When DTE ready to send data, it asserts Request to Send.

– Modem responds when ready by asserting Clear to send.

– DTE sends data.

– When data arrives, local modem asserts Receive Line Signal Detector and delivers data.


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Dial Up Operation


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Null Modem


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ISDN Physical Interface

• Connection between terminal equipment (correspond to DTE) and network terminating equipment (correspond to DCE).

• Defined in ISO 8877.

• Cables terminate in matching connectors with 8 contacts.

• Transmit/receive carry both data and control


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ISDN Electrical Specification

• Balanced transmission:– Carried on two lines. e.g., twisted pair

– Signals as currents down one conductor and up the other.

– Differential signaling

– Value depends on direction of voltage.

– Tolerates more noise and generates less (Unbalanced, e.g. RS-232 uses single signal line and ground).

– Data encoding depends on data rate.

– Basic rate 192kbps uses pseudoternary.

– Primary rate uses alternative mark inversion (AMI) and B8ZS or HDB3.


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Required Reading

• W. Stallings, “Data and Computer Communications (6th edition),” Prentice-Hall, 2000.

>> Chapter 6

network technology cse3020 - 2006 1 network technology cse3020 week 4

Documents

telephone network

gbps slide

bandwidth of signal

wire pair acts

transmitted signal

transmission media guided

guided media

bandwidth higher bandwidth