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

Server

s2 sns1

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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

computer architecture is a set of rules and methods

that describe the functionality, organization, and

implementation of computer systems

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 service

provided by layer n-1, adds

its own functions and

provide more abstract

service to layer n+1

Q: What functions layer 1

provides? 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 bit

2. Apply parity to every bytes

3. 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

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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+1

Msg 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

Resource Sharing - Multiplexing

Recall network and resource sharing

The most important principle in building efficient

networks

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Typical Network Configuration

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21

Multiplexing Multiplexing

– Techniques allowing transmission of multiple signals over a single

(data) link

– Channel: portion of a link that carries signals (information) 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

Slot & Frame structure

– Time is partitioned into small, same-length durations called (time)

slots each of which is dedicated to one sending device

– Frame: One complete cycle of time slots

Slot allocation

– Mux allocates time slots of the same position to a sender

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

Demultiplexing?

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STDM Example: 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

Link Performance

Bandwidth (Data rate / bit rate)

Transmission time and propagation time

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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 linkSNU SCONE lab. 31

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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 at the sender

– L/B where L is message size and B is bandwidth

1 Mbps

2 Mbps

1 μs

0.5 μs

Example:

How many seconds will it take to transmit a 12 Kbit message

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 a

receiver

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

– d/V where

• d: distance between the transmitter to the receiver

• V: 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 cable8

Latency

Time gap between the transmission of the first bit from

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

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𝒕𝟏

𝒕𝟑

𝒕𝟐

𝒕𝟒

First bit

Last bit

Perceived Latency

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Large message Bandwidth is important

Small message Propagation is important

RTT & Throughput Usually, a message is sent after a request from a user

RTT (Round Trip Time)

– Two times of

propagation time

Transfer time

= RTT + Transmit time

Throughput

= Message size / Transfer time

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𝒕𝟏

𝒕𝟑

𝒕𝟐

𝒕𝟒

First bit

Last bit

𝒕𝟎

PLT: Page Loading TimeTime for a complete loading of a webpage after request

Delay-Bandwidth Product

Consider a link as a pipe

Delay is the length of the pipe and Bandwidth is the

width of the pipe

DBP: 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

Read Cisco VNI Data

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Queueing System & Theory Recall resource sharing

Queueing system

– Any systems that are shared by multiple users

– Example: Bus, bank, …

Server

Departure Arrival

Customer

(Job)

Queue

Queueing theory analyzes

the performance of

queueing systems

Performance =

Delay, Blocking prob.

Ref: Gallager’s book pp.149-170

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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

• Poisson process, Exponential distribution

– Utilization Factor,

• Measure of how busy is the system

/

Little’s Theorem (N = λT)

Queue

Server

A1 A2

D1

A3

D2 D3

Little’s Theorem

Suppose we observe the system for [0, τ >>0]

– 𝑇𝑖: System time of i-th customer

– n(τ ): Number of customers arrived during [0, τ]

– K(t): Number of customers in the system at time t

– σ𝑖=1𝑛(τ)

𝑇𝑖 ≅ 0τ𝐾 𝑡 𝑑𝑡

– 𝑛(τ)/τ ≅ 42

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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

Exercises

1. Google “ietf rfc index” and retrieve the RFC index. Also, retrieve the

protocol specifications for TCP, IP, and UDP.

2. Calculate the total time required to transfer a 1000-KB file in the following

cases, assuming an RTT of 50 ms, a packet size of 1 KB data, and an initial

2×RTT of “handshaking” before data is sent:

(a) The bandwidth is 1.5 Mbps, and data packets can be sent continuously.

(b) The bandwidth is 1.5 Mbps, but after we finish sending each data packet

we must wait one RTT before sending the next.

(c) The bandwidth is “infinite,” meaning that we take transmit time to be

zero, and up to 20 packets can be sent per RTT.

(d) The bandwidth is infinite, and during the first RTT we can send one packet

(2^1−1), during the second RTT we can send two packets (2^2−1), during the

third we can send four (2^3−1), and so on. (A justification for such an

exponential increase will be given in Chapter 6.)

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Exercises5. Consider a point-to-point link 4 km in length. At what bandwidth would

propagation delay (at a speed of 2×10^8m/s) equal transmit delay for 100-byte

packets? What about 512-byte packets?

10. What differences in traffic patterns account for the fact that STDM is a cost-

effective form of multiplexing for a voice telephone network and FDM is a cost-

effective form of multiplexing for television and radio networks, yet we reject

both as not being cost effective for a general-purpose computer network?

11. How “wide” is a bit on a 10-Gbps link? How long is a bit in copper wire,

where the speed of propagation is 2.3×108 m/s?

15. For each of the following operations on a remote file server, discuss

whether they are more likely to be delay sensitive or bandwidth sensitive:

(a) Open a file.

(b) Read the contents of a file.

(c) List the contents of a directory.

(d) Display the attributes of a file. 48

Selected Problems (1, 2, 5, 10, 11, 15, 20)

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