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Introduction and Review

MET CS-635 Unit 1

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The Network Pipe

• Most basic model of the network– Phone call– Tin cans and string– Two computers connected to one another

• Goal : Move data from one end to other

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Class Exercise

The Living Network Exercise #1

Two individuals are issued cards and are challenged to communicate the contents of the cards to each other using only verbal commands. Participants sit with their backs to each other. Objective is to observe the process that takes place, and draw conclusions about the considerations that must be taken into account in any networking situation.

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Standards

• Stations sharing the network media must use a common set of agreed upon rules to cooperate– Protocols– Standards

• Sources of LAN standards– IEEE– IETF– ATM Forum

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LAN vs. WAN

• Local Area Network– Privately owned and operated– All data belongs to your company– Limited geographic extent– High Speed– Building or Campus

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WAN

• Wide Area Network– You pay somebody to move your data– You share capacity with other companies– Wide geographic extent– Global

• Phone Company• Leased Lines• Interconnect LANs

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Basic Network Requirements

• Reliability• Get data across network securely and error free• Know when an error has occurred, and handle it

– Corrupted data– Cable break

• Speed• Get data across network as fast as possible

• Scalability• Be able to grow the network• Be able to migrate to new designs and protocols

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Requirements Continued

• Efficiency• Divide network capacity among multiple users in

an equitable manner• MANY approaches to this very fundamental

problem• All users are equal, but some are more equal than

others...

• Cost Effectiveness• Meet all requirements as inexpensively as

possible

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Additional Requirements

• From Interconnections by Perlman• Scope

• Network should solve as general a problem as possible

• Autoconfigurability• Plug and play networks• Auto assignment of addressing• Auto discovery of topological information

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Data Sources

• Digital Data– Files, GIF Images, Web Pages, etc.– Data must still be properly framed for

transmission over the LAN

• Analog Data– Audio, Video– Data must be converted first using a codec

and then framed for transmission over the LAN

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Data on LANs

• The LANs we will study all use digital signaling and digital transmission

• All of the data is converted to 1’s and 0’s by the time it gets to the network

• The network just moves the 1’s and 0’s

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Signal Encoding

• Moving the 1’s and 0’s– Need to be able to move digital data over

analog media like copper wire and fiber optic cable

– In fiber, we have presence or absence of light

– In copper, we have a range of voltage levels

• We’ll consider NRZ and Biphase encoding– NRZ Non-Return to Zero

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NRZ Encoding

• Differential Encoding– Uses positive and negative voltages as

opposed to absolute voltage levels– Can reverse wires transmitting differential

signal and it still works

• Easily implemented• Problems with long strings of 1’s and 0’s

– Clock synchronization gets lost– So how come FDDI and 100BASE-T use this?

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NRZ-L

• Non-Return to Zero Level– Constant negative voltage represents 1– Constant positive voltage represents 0

1 1 0 1 1 0 1 0

0

-V

+V

t

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NRZI

• Non-Return to Zero, Invert on Ones– Transition up or down at start of bit time

means 1– No Transition means 0

• Users: FDDI and 100BASE-T1 1 0 1 1 0 1 0

0

-V

+V

t

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Biphase Encoding

• Solve the NRZ synchronization problem by providing a predictable transition during each clock phase

• The additional clock transitions double the bandwidth of the signal– This puts higher demands on the cabling

used

• You can still use differential encoding

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Manchester Encoding

• Transition in middle of the phase– Low to High is 1, High to Low is 0– Always a mid-phase transition, so you never

lose the clock– Users: 10BASE-T and 10BASE-2

1 1 0 1 1 0 1 0

0

-V

+V

t

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Differential Manchester

• Transition in middle of phase again– Transition just provides clocking– Transition at start of phase is 0– No Transition at start of phase is 1

1 1 0 1 1 0 1 0

0

-V

+V

t

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Recovering the Clock

• LANs are, for the most part, asynchronous networks

• Clocks are locally generated and recovered

• Need a data encoding technique that allows the clock to be recovered

• Problems with synchronous networks and clock skew

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Nyquist and PCM

• PCM = Pulse Coded Modulation– Sample signal at regular intervals– Send the numbers over the network

• Nyquist’s result:– max data rate = 2H log2 V bits/second

– H is the bandwidth of the signal in Hertz– V is the number of sampling levels used

• This fundamental limit discovered in 19241924 drives all digital communication today

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Multiplexing

• Sharing the communications channel– Need to share with LANs and WANs– Cost effective use of the capacity

• TDM– A time slice just for you– Most LANs– Phone trunk lines

• FDM– A frequency just for you– CATV

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Switching

• Process of establishing a path through the network

• Need a way to get from here to there• Many ways to accomplish this• We will examine:

– Circuit Switching– Packet Switching– Cell Switching

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Circuit Switching

• Connection Oriented• Phone call• Dedicated circuit established

– Fixed path through network– Predictable performance and delay– Wasted bandwidth when quiet

• Call setup and teardown

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Packet Switching

• Connectionless• Most LANs• No dedicated circuit established

– Random path through network– No predictable performance or delay– Bandwidth only used when needed

• Store and forward network

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Cell Switching

• Hybrid technique used in ATM networks• Makes efficient use of media like packet

switched network• Has predictable characteristics of circuit

switched network

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Metrics

• We will use the following performance metrics throughout the course– Bandwidth

– A range of frequencies in Hertz– For signals: Delta between the maximum and

minimum frequency components– For media: Maximum frequency component that

may be carried by the media

– Data Rate– Speed at which data is communicated in bits per

second

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Metrics Continued

– Capacity– Maximum Data Rate sustainable by a channel

allowing for Gaussian (thermal) noise– Shannon’s Equation: C = H log2 (1 + S/N)

– Latency– Delay in seconds from the time a transmission is

initiated until it is received

– Throughput– Also called effective data rate– Rate in bits per second or bytes per second in

which actual application data is moved, in its entirety, across a medium

– Allows for latency and protocol overhead

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OSI Reference Model

Physical

Data Link

Network

Transport

Session

Presentation

Application

1

2

3

Most of our time

Rest of our time

As needed

Course Emphasis OSI Stack

Layer

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Using the stack to communicate

• Systems that wish to communicate use stacks that implement the same standards

• Different vendors may produce different implementation but implementations must both be compliant

• Each layer in a station must interoperate with corresponding layer on the remote station

• Need interoperability testing– Note: This is how the trade show “Interop”

started

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Interconnecting two hosts

Physical

Data Link

Network

Transport

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2

3

Physical

Data Link

Network

Transport

1

2

3

Host A Host B

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Making the connection

• Corresponding layers in each system establish a logical connection

• Data is sent from the application ‘down’ the stack

• Each layer encapsulates data from next higher layer

• Encapsulations stripped on way ‘up’ the receiving stack

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Layer Encapsulation

PHY

DLL

Net

Trans

PHY

DLL

Net

Trans

data

datadata

This is whatgoes on the

wire