Basics of COMPUTER NETWORKS

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Basics of COMPUTER NETWORKS

• Book in glance

Computer Engineering Faculty of
Engineering Kurdistan University ( Email
ghasem.mohammadi_at_gmail.com)
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Contact information

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Whats 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

• Multiple choices at each layer

FTP
HTTP
TFTP
NV
TCP
UDP
TCP/UDP
IP
Network
IP
IPX
Port Number
Protocol Field
Type Field
NET1
NET2
NETn

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OSI Layers and Locations
Application
Presentation
Session
Transport
Network
Data Link
Physical
Switch
Hub
Host
Router
Host
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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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Twisted Pair
UTP
Connector
STP
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Bad Good
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Connectors
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Fiber Optic
Connectors
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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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Parity bit
0110100
1
1011010
0
One Dimensional Parity
0010110
1
1110101
1
1001011
0
1000110
1
Two Dimensional Parity
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Cyclic Redundancy Check (CRC)

• Represent a (n1)-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 that
• M(x)xk A(x)G(x) R(x), where degree(R(x)) lt
  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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Sliding Window Sender/Receiver State
Receiver
Sender
Max acceptable
Next expected
Max ACK received
Next seqnum




Receiver window
Sender window
Sent Acked
Sent Not Acked
Received Acked
Acceptable Packet
OK to Send
Not Usable
Not Usable
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Example of Sliding Window
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Sliding Window Go-Back-n, Damaged Frame
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Sliding window Selective-Repeat, Damaged Frame
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Media Access Control
Evolution of Contention Protocols
Aloha
Developed in the 1970s for a packet radio network
SlottedAloha
Improvement Start transmission only at fixed
times (slots)
CSMA Carrier Sense Multiple Access Improvement
Start transmission only if no transmission is
ongoing
CSMA
CD Collision Detection Improvement Stop
ongoing transmission if a collision is detected
(e.g. Ethernet)
CSMA/CD
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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
• sense (listen to) channel
• if channel sensed busy
• then wait random time go to 1
• else transmit packet
• p-persistent CSMA
• sense (listen to) channel
• 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 protocols
Reservation

• No collisions
• reservation is made before sending
• average waiting time before transmission is N
• low load utilization d/(dN) not good if N is
  large
• high-load utilization d/(d1)

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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 802.4 Token Bus Network
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Token Ring IEEE 802.5
Listen
Talk
Data
Token/Data
l1
l4
l3
l2
TRTToken Rotation Time
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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)
802.11a (54 Mbps)
802.11g (54 Mbps)
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Wireless Network Interface Card
Wireless Access Point
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Spread Spectrum Methods
DSSS
FHSS
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FHSS
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DSSS
Original Data
Transmitted Data
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ESS Extended Service Set
Overlapped Area for roaming
BSS Basic Service Set
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CSMA/CA
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Network Layer
Internetworking
Hop-by-hop
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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)
Routing between Autonomous Systems (EGP)
EGP
IGP