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Introduction

A quick look at network fundamentals

F. M. Marchetti, Ph.D.CSE / Rm 353

fmm@engr.smu.edu

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Introduction

Transmission Fundamentals • Factors that determine the best

way to connect – Cost of connection – Speed – Immunity to interference – Security – Logistics

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Purpose of a Network

To deliver data from one entity to another

Factors to consider: – Need for reliable transmission – Quality of Service – Need for transmission of real-time data

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Types of Network

Maximum intercomputer distance

 Computer located in same …

 Example

1 m System Multicomputer

10 m / 100 m / 1 km Room / building / campus

Local area network (LAN)

10 km City Metropolitan area network

100 km / 1000 km Country / Continent Wide area netwok (WAN)

10000 km Planet The Internet

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LANs

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WANs

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The Internet

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Forms of Data Transmission

Data must be converted to a physical signal for transmission

• Analog transmission (such as speech over telephone lines)– Suffers from degradation which cannot

be reconstructed

• Digital transmission (such as VoIP)– Suffers from all-or-none degradation

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Digital vs. Analog

• Digital less susceptible to distortion and interference compared to Analog

• Digital signals can be regenerated to extend the length of the cable

• Extremely low error rate (BER)• Cheaper

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Data Encoding Techniques Analog Signals Digital

Data • Amplitude shift keying

(ASK)

• Frequency shift keying (FSK)

• Phase shift keying

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Digital Data, Digital Signals

– Non-return-to-Zero Level (NRZ-L) – NRZ - I – Manchester Encoding – Differential Manchester encoding

Disadvantages of NRZ – Synchronization is difficult– DC component

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Digital Data, Digital Signal (cont’d)

• Advantages of Differential encoding– Easy detection – Can keep track of the polarity

• Advantages of bi-phase encoding – Synchronization – No dc component – Error detection

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Analog Data, Digital Signal

• Based on the sampling theorem – Nyquist Limit

• Pulse code modulation (PCM)– ADPCM (more compressed)

• Delta modulation – Only the change of information is sent

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Parallel vs. Serial Transmission

Parallel – Dedicated functions to the wires – Higher speed for short distance

interconnections – Not feasible for long distances

(more than 100m )• Reduction in performance• Cross talk in long cables• Cost

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Parallel vs. Serial

Serial – Serialize the data – Add control characters– Format the data into frames– Two types of transmission:

• asynchronous: transmitter and receiver clocks are independent

• synchronous: transmitter and receiver are synchronized

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Direction of Transmission

• Simplex – Transmission in one direction only

• Half-Duplex – Transmission in one direction at a

time

• Full-Duplex– Transmission in both directions

screen

keyboard

cpu

outputport

inputport

echo

Serial Link

screen

keyboard

cpu

outputport

inputport

Serial Link

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Synchronous vs. Asynchronous

Asynchronous – One character at a time – One start bit – one or more stop bits – No clock – receiver resynchronizes after each stop code – Cheap but inefficient – large overhead (20% or more)– Relatively low data rates (up to 115.2 kbps, in practice

38.4 kbps)– Uses:

• suitable for data transmitted at random intervals (e.g. keyboard to computer)

• simplicity and availability: UART and RS232 are present in any PC

• used in the great majority of dial-up connections

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Synchronous vs. Asynchronous

Synchronous– Arrival time of each bit is predictable – To prevent timing drift the receiver and

transmitter clock are synchronized – Preamble and Postamble SYNC characters– Character Oriented – Clock signal transmitted either:

• over a separate line (see V.35, RS232 lines)

• or encoded into the data (Manchester, differential Manchester encoding) to allow a single line for both data and clock

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Transmission Media

• Guided Media – Twisted Pair of Cables – Coaxial Cables – Optical Fibers

• Unguided Media – Radio – Microwave – Satellite

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Multiplexing

Link Sharing– For cost-effective transmission– The medium carries multiple signals

simultaneously – Commonly used techniques:

• FDM - Frequency Division Mux • TDM - Time Division Mux (STDM) • WDM - Wavelength Division Mux • CDM - Code Division Mux (frequency hopping and

spread spectrum techniques)

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Statistical MUX

• Link is shared over time (like STDM)

• Scan the buffer and create a variable-size frame

• Transmission-on-demand • Also called Concentrator

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Communication Switching Techniques

Spectrum of Switching Techniques

– Circuit switching

– Multi-rate circuit switching

– Cell relay

– Frame relay

– Packet switching

Dedicated

Virtual

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Circuit Switching

• Dedicated communication path between two stations

• Three Phases – Check also whether the destination is

ready to accept the request – Data transfer

• Could be either digital or analog signaling

• Generally full duplex

– Circuit disconnect

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Packet Switching

• Greater line efficiency • Data rate conversion • Connection request is always

accepted irrespective of the traffic

• Dynamic routing and priority assignment possible

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Packet Switching - Approaches

• Datagram

• Virtual Circuit

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Packet Switching - Approaches

Datagram– Each packet is treated independently – The packets may be received out of

sequence – Some packets may be lost in the event of

some node crashes

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Packet Switching - Approaches

Virtual Circuit– A preplanned route is established before the

data is sent – At any time, each station can have more

than one VC to any other station and can have VCs to more than one station

– Provides sequencing, error control, and flow control

– If an intermediate node fails, all virtual circuits going through that node may be lost - less reliable when compared to datagrams

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Frame relay • Too much overhead built into

packet switching • Frame relays take advantage of

the low error rates of networking facilities

• Cell Relay – ATM – Fixed length packets

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OSI Layers

• Application – provides electronic mail, file transfers etc.

• Presentation – Translates data format, encrypts and

decrypts data

• Session – Synchronizes communicating users,

recovers from errors

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OSI Layers (cont’d)

• Transport – Determines network, may assemble and

reassemble packets

• Network – Determines routes, manages billing

information

• Data Link – Detects or correct errors, defines frames

• Physical – Transmits physical data

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End of Class 1