Basics of COMPUTER NETWORKS

Basics of COMPUTER NETWORKS

Book in glance

Computer EngineeringFaculty of EngineeringKurdistan University

( Email : ghasem.mohammadi@gmail.com)

سرفصل هاي درس

Contact information

: مفاهيم اوليه1فصل •: اليه فيزيكي 2فصل •: اليه پيوند داده3فصل •: اليه شبكه4فصل •: اليه انتقال5فصل •: اليه كاربرد6فصل • : كابل كشي ساخت يافته7فصل •

What’s a Computer Network?

• Connecting two or more computers or other devices via a Communication media( Physical or wireless)

• Categorizing networks according to size:

– LAN (Local Area Network)– MAN (Metropolitan Area Network)– WAN (Wide Area Network)– Internet

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Types of Links

Point-to-Point Multiple Access

…

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Protocol Demultiplexing

FTP HTTP TFTPNV

TCP UDP

IP

NET1 NET2 NETn…

TCP/UDPIP

IPX

Port Number

Network

Protocol Field

Type Field

• Multiple choices at each layer

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OSI Layers and Locations

Switch RouterHost Host

Application

Transport

Network

Data Link

Presentation

Session

Physical

Hub

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Network Interface Card (NIC)

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Hub (layer 1 device)Just knows bits

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Switch (layer 2 device)Knows MAC addresses

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Router (layer 3 device)Knows Logical addresses (IP and IPX)

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Connector

Twisted Pair

UTP

STP

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Bad Good

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Connectors

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Connectors

Fiber Optic

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A Fiber Optic Ring with Active repeaters

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Electromagnetic Spectrum

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Types of Propagation

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Basic Modulation Types

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Compression:

Huffman Coding

Run-length Coding

Ziv Lempel compression

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FDM (Frequency Division Multiplexing)

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WDM (Wavelength Division Multiplexing)

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TDM (Time Division Multiplexing)

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Circuit Switching

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Time Division Switch

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Packet Switching

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Message Switching

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Framing Methods

Character Count

Byte Stuffing

Bit Stuffing

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Handling Errors

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Single bit Error

Multiple bit Error

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0110100

1011010

0010110

1110101

1001011

1

0

1

1

0

1000110 1

Parity bit

One Dimensional Parity

Two Dimensional Parity

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Cyclic Redundancy Check (CRC)

• Represent a (n+1)-bit message as an n-degree polynomial M(x)–E.g., 10101101 M(x) = x7 + x5 + x3 + x2 + x0

• Choose a divisor k-degree polynomial G(x)• Compute reminder R(x) of M(x)*xk / C(x), i.e., compute

A(x) such thatM(x)*xk = A(x)*G(x) + R(x), where degree(R(x)) < k

• Let T(x) = M(x)*xk – R(x) = A(x)*G(x)

• Then–T(x) is divisible by G(x)

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Hamming Code

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Categories of Flow Control

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Stop-and-Wait

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ReceiverReceiverSenderSender

Sliding WindowSender/Receiver State

… …

Sent & Acked Sent Not Acked

OK to Send Not Usable

… …

Max acceptable

Receiver window

Max ACK received Next seqnum

Received & Acked Acceptable Packet

Not Usable

Sender window

Next expected

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Example of Sliding Window

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Sliding WindowGo-Back-n, Damaged Frame

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Sliding windowSelective-Repeat, Damaged Frame

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Evolution of Contention Protocols

Developed in the 1970s for a packet radio network

Aloha

SlottedAloha

Improvement: Start transmission only at fixed times (slots)

CSMA

CSMA = Carrier Sense Multiple AccessImprovement: Start transmission only if no transmission is ongoing

CD = Collision Detection

Improvement: Stop ongoing transmission if a collision is detected (e.g. Ethernet)

CSMA/CD

Media Access Control

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The Aloha Protocol

•simple: if you have pkt to send, "just do it" •if pkt suffers collision, will try resending later

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Slotted Aloha

•synchronous system: time divided into slots •slot siz equals fixed packet transmission time •when pkt ready for transmission, wait until start of next slot •packets overlap completely or not at all

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Carrier Sensing Protocols

•Aloha is inefficient (and rude!): doesn't listen before talking! •Carrier Sense Multiple Access: CSMA

non-persistent CSMA: 1.sense (listen to) channel 2.if {channel sensed busy}

then wait random time; go to 1 else transmit packet

p-persistent CSMA: 1.sense (listen to) channel 2.when {channel sensed idle}

transmit with probability p else wait random time, go to 1

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CSMA/CD

•CSMA with collision detection(CD): •listen while talking! •stop transmitting when another pkt has collided with your pkt

•wait random time before attempting to resend •worst case time to detect a collision? •performance depends (as in CSMA) on channel length

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Collision free protocolsReservation

No collisions reservation is made before sending

average waiting time before transmission is N low load utilization: d/(d+N) – not good if N is large high-load utilization d/(d+1)

d bits

N bits

t

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Collision free protocols

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IEEE 802 Standards

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802.3 Frame structure

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10Base5

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10Base2

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10BaseT

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10BaseF

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IEEE 802.4Token Bus Network

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Token Ring IEEE 802.5

Data Token/Data

l1

l3 l2

l4

min/ TRTclPROPi i

TRT=Token Rotation Time

Listen:

Talk:

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Token Ring IEEE 802.5

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IEEE 802.11 Wireless LAN (WLAN)

802.11b

802.11 802.11a 802.11g

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ISM Bands(Unlicensed )

802.11b (11Mbps)

802.11g(54 Mbps)

802.11a(54 Mbps)

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Wireless Access PointWireless Network Interface Card

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Spread Spectrum Methods

FHSS DSSS

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FHSS

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DSSS

Original Data

Transmitted Data

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ESSExtended Service Set

BSS: Basic Service Set

Overlapped Area for roaming

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CSMA/CA

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Internetworking

Hop-by-hop

Network Layer

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Need of Network Layer

Host-to-host (End-to-end)

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Packet delivery in Network layer

Internet

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Routing Algorithms

• Calculate the shortest path (lowest cost)

• Two common methods– Distance vector routing (RIP , IGRP , … )– Link state routing (OSPF , IS-IS)

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The concept of distance vector routing

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Concept of link state routing

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Hierarchical Routing

Routing in Autonomous Systems (IGP)

EGP

IGP

Routing between Autonomous Systems (EGP)