Overview- Basic

Chong-kwon Kim

Topics of Today’s Lecture Networking Architecture Protocol

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Communication & Network Communication is exchange of information between

users (stations, nodes) at a distance

Network– A system consists of devices (often referred to as nodes) and

links for transportation of entities– Example: roads, railroads, water

Two types of communication network– Voice– Computer networks

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Connectivity Impossible to connect (large) number of users directly

Share resources (links)– Network is a mechanism to make the connectivity easy by sharing

resources

Sharing mechanisms– Multiplexing– Access control

Requires - O(N^2) links- O(N) accesses/user

s1

sn

sis3

s2

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Simplex/Duplex

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Link Types

7

Architecture Divide & Conquer

– To solve a large & complex problem, first partition the problem into small pieces

– Solve each partial problem– Combine sub-solutions into a whole solution

Architecture– A set of sub-functions that comprise a larger function

Abstraction– Shield internal implementation details and show only interfaces

Example– Program modules

M2M1

M4M3

M5

8

Layered Architecture Layered architecture

– Keep the interaction simple

Layer 1

Layer N

Layer n+1

Layer n

Layer n-1

Raw

Abstract

Layer n uses serviceprovided by layer n-1, addsits own functions andprovide more abstract service to layer n+1

Q: What functions layer 1provides? And Layer N?

Overview- Protocol

Chong-kwon Kim

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Protocol Communications involve with two or more devices Suppose A and B communicate each other

– Should A and B use the same program?– If A and B use Windows and Linux OS, respectively, how they

communicate?

Note that communication is exchanges of messages Protocol

– Rules that communicating entities should abide to understand and properly process messages received

– Protocol specifies the meaning (semantics) and syntax of messages

– And timing of messages

A B

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Protocol – Example 1 Error detection

– Communication links are not 100 % reliable– Errors may change, add, delete bits in the original message– An Internet bank user C requests to transfer $100 from account

A1 to A2– If the first bit is changed to 1, then you transfer $228

How do you detect errors?– There are many solutions

• Parity bit• One’s complement addition• CRC

01100100 11100100

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Protocol – Example 2 We need to agree

– Use the same method (Algorithm)– How to apply the method– How to represent additional data

Assume we agreed to 1. Even parity bit2. Apply parity to every bytes3. Attach parity bits to the end of the original message as a

byte stream

01001100 11100111 10101100 00010010

0000100001001100 11100111 10101100 00010010

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Protocol – Example 3 Suppose layer n performs error detection

Layer n

Layer n+1

Layer n-1

Layer n

Layer n-1

Layer n+1

Sender Receiver

01001100 11100111

01001100 11100111 00000010

Protocol- Syntax- Semantics- Timing

Today’s News Supplement of Oct. 2 class

– Sept. 11 at 7:00PM

Revised note and problem set will be posted on the class homepage by Sept. 7 noon

Topics– Comm., Networks, Architecture, Protocol(Sept. 4) – PD: 1.3

– Multiplexing, Queueing (Sept. 6, Sept. 11) – PD: 1.2, 1.5– Layer 2

• Data Link (Sept. 11 supp.) – PD: 2.1• Error control (Sept. 13, Sept. 18) – PD: 2.5

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Protocol Principle - 1

Good protocols abide the protocol principle Why is the protocol principle important in designing

protocols?

PROTOCOL PRINCIPLEMessage that layer n generates at the sender

Message that layer n receives at the receiver

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Protocol Principle - 2

Layer n

Layer n+1

Layer n-1

Layer n

Layer n-1

Layer n+1

Sender Receiver

01001100 11100111

01001100 11100111 00000010

01001100 11100111

01001100 11100111 00000010

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Virtual Communication Two layer entities at the same level think they

communicate directly In fact, a message goes down to lower layers at the

sender and then goes up from lower layers at the receiver

Layer n

Layer n+1

Layer n-1

Layer n

Layer n-1

Layer n+1Msg Msg

MsgCtl MsgCtl

Video

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Standard Protocols To communicate, should use the same protocol Proprietary protocols

– Created (usually) by one or more companies– Closed protocol

• Protocol is hidden or the owner may claim IPR

Open protocol– Specifications are open to the public and everyone can use them

free

Standard protocol– Open protocol that many agree to use

Examples of standard protocols– ISO OSI– IEEE LAN, WLAN, …– TCP/IP– …

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TCP/IP

Typical Network Configuration

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Multiplexing Multiplexing

– Set of techniques allowing simultaneous transmission of multiple signals across a single data link

– Channel: portion of a link that carries a transmission between a given pair of sender & receiver

• A link with n channels supports n simultaneous communications

Demultiplexing– Separation of combined signals

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Categories of MuxTechniques

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Frequency-Division Multiplexing (FDM)

Analog communication technique Medium (link) bandwidth (or spectrum) is much larger than a

single station requires– A co-axial cable supports a few Mhz while a voice BW is 20 KHz

Signals generated by each sender modulate different carrier frequency

Guard Band– Unused bandwidth separating channels

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FDM Multiplexing Example

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FDM Demultiplexing Example

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Synchronous TDM (STDM) Digital transmission system Frame

– Time is partitioned into small, equal time duration called (time) slots each of which is dedicated to one sending device

– Frame: One complete cycle of time slots – Multiplexer allocates the time slots of the same position to a

sender– The slots are dedicated to the sender regardless of actual use/idle

Demultiplexing– Based on slot position

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STDM Examle: T-1 Line for Telephony

How to detect the start(or end) of a frame?

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Synchronous TDM Disadvantage of Synchronous TDM

– Waste transmission resources

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Asynchronous TDM On-demand resource allocation

– Dynamically allocate resources to a station that needs the resources

Demultiplexing is the problem– Should specify

• The start and end of each frame• Destinations of frames

Addressing and Overhead– Framing bits– Specify the destination ID (address, port, ..) in a head– In addition, layer 2/3 header usually contains

• Message size• Source address• …

Basic Queueing Theory

Chong-kwon Kim

Ref: Gallager’s book pp.149-170

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Queueing System & Theory Queueing system

– Any systems that are shared by multiple users– Example: Bus, bank, …

Server

DepartureArrival

Customer(Job)

Queue

Queueing theory analyzes the performance of queueing systems

Performance = Delay, Blocking prob.

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Queueing Theory

Parameters– Arrival Rate,

• The number of customers arrived at the system per time unit

• Inter-arrival time– Service Rate,

• The number of service completions given that the server is busy at all times

• Service time– Memoryless property

• The next (Arrival, departure) events is independent of the previous events

• Exponential distribution– Utilization Factor,

• Measure of how busy is the system

/

Little’s Theorem (N=λT)

Queue

Server

A1 A2

A1

A3

A2 A3

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Queueing Theory M/M/1 Queueing System

– Exponential (Memoryless process) Inter-arrival and Service time

– One server– p : Steady-state prob. that n customers are in the

system

– Balance equations• , n = 0,1,2,...

Pnn ( )1

Pii

0

1

P Pn n 1

n

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Queueing Theory

M/M/1 Queueing System– N : Average number of customers in the system

– T : Average system time experienced by a customer– Little’s Theorem

T 1/( )

N T

N n Pn /( )1

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Queueing Theory Two different systems

– System 1 : Three slow servers each of which is dedicated to one source

– System 2 : A fast server that is three times faster than a slow server. Arrivals from three sources are aggregated and use one queue

Server 1

Server 2

Server 3

Source 1

Source 2

Source 3

System 1

System 2

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Queueing Theory Comparison of FDM (TDM), ATDM

– System 1 : FDM that separates a link into three channels– System 2 : ATDM that is shared dynamically by three users

2 1

2 1

1 2

3

3

3

T T

Performance Performance of a transmission link

– How long does it take to transfer a message from one end to the other end?

Depends on three factors– Bandwidth (or data rate) of a link– Message size– Length of a link

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Bandwidth & Transmit Time Bandwidth (Data Rate)

– Bit transmission (receiving) rate• How fast a transmitter feeds bits into the medium• bps (bits per second)

Transmit time– Time to completely transmitting a message– S/B where S is message size and B is bandwidth

1 Mbps

2 Mbps

1 μs

0.5 μs

ExampleHow many seconds will it take to transmit a 12 Kbit TCP segment over a network of 1 Mbps bandwidth?

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Propagation Delay (Time) Propagation delay

– Time required for a signal travels from a transmitter to areceiver

• The speed of EM signal is about 65~80% of the speed of light

– d/L where• d: distance between the transmitter to the receiver• L: Speed of EM signal

Example– What is the propagation time if the distance between the

two points is 12,000 km? Assume the propagation speed to be 2.4 × 10 m/s in cable

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Latency Time gap between the start of first bit transmission

from a sender to the reception of the last bit at a receiver

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First bit

Last bit

Perceived Latency

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Large message Bandwidth is importantSmall message Propagation is important

RTT & Throughput Usually, a message is sent after a request or a receiver

sent an Acknowledgement (ACK) when it receives a message successfully

RTT (Round Trip Time)– Two times of

propagation time

Transfer time= RTT + Transmit time

Throughput= Transfer size / Transfer time

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First bit

Last bit

Delay-Bandwidth Product Consider a link as a pipe

Delay is the length of the pipe and Bandwidth is the width of the pipe

Delay-bandwidth product– Maximum number of bits in the pipe– Example: Delay = 50 ms, Bandwidth = 45 Mbps

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High Speed Network Network with large bandwidth

– Decrease transmission delay– Not propagation speed

Multimedia communications & High Speed Network

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