Title: Basics of COMPUTER NETWORKS
1Basics of COMPUTER NETWORKS
Computer Engineering Faculty of
Engineering Kurdistan University ( Email
ghasem.mohammadi_at_gmail.com)
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Contact information
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3Whats 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
4Types of Links
Point-to-Point
Multiple Access
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7Protocol 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
8OSI Layers and Locations
Application
Presentation
Session
Transport
Network
Data Link
Physical
Switch
Hub
Host
Router
Host
9Network Interface Card (NIC)
10Hub (layer 1 device) Just knows bits
11Switch (layer 2 device) Knows MAC addresses
12Router (layer 3 device) Knows Logical addresses
(IP and IPX)
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14Twisted Pair
UTP
Connector
STP
15Bad Good
16Connectors
17Fiber Optic
Connectors
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19A Fiber Optic Ring with Active repeaters
20Electromagnetic Spectrum
21Types of Propagation
22Basic Modulation Types
23Compression
- Huffman Coding
- Run-length Coding
- Ziv Lempel compression
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25FDM (Frequency Division Multiplexing)
26WDM (Wavelength Division Multiplexing)
27TDM (Time Division Multiplexing)
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29Circuit Switching
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31Time Division Switch
32Packet Switching
33Message Switching
34Framing Methods
Character Count
Byte Stuffing
Bit Stuffing
35Handling Errors
36Single bit Error
Multiple bit Error
37Parity bit
0110100
1
1011010
0
One Dimensional Parity
0010110
1
1110101
1
1001011
0
1000110
1
Two Dimensional Parity
38Cyclic 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)
39Hamming Code
40Categories of Flow Control
41Stop-and-Wait
42 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
43Example of Sliding Window
44Sliding Window Go-Back-n, Damaged Frame
45Sliding window Selective-Repeat, Damaged Frame
46Media 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
47- The Aloha Protocol
- simple if you have pkt to send, "just do it"
- if pkt suffers collision, will try resending
later - Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
       Â
48- 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
- Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â
         Â
49- 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
50- 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 -
51Collision 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)
t
52Collision free protocols
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54IEEE 802 Standards
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56802.3 Frame structure
5710Base5
5810Base2
5910BaseT
6010BaseF
61IEEE 802.4 Token Bus Network
62Token Ring IEEE 802.5
Listen
Talk
Data
Token/Data
l1
l4
l3
l2
TRTToken Rotation Time
63Token Ring IEEE 802.5
64IEEE 802.11 Wireless LAN (WLAN)
802.11b
802.11
802.11a
802.11g
65 ISM Bands (Unlicensed )
802.11b (11Mbps)
802.11a (54 Mbps)
802.11g (54 Mbps)
66Wireless Network Interface Card
Wireless Access Point
67Spread Spectrum Methods
DSSS
FHSS
68FHSS
69DSSS
Original Data
Transmitted Data
70ESS Extended Service Set
Overlapped Area for roaming
BSS Basic Service Set
71CSMA/CA
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75Network Layer
Internetworking
Hop-by-hop
76Need of Network Layer
Host-to-host (End-to-end)
77Packet delivery in Network layer
Internet
78Routing Algorithms
- Calculate the shortest path (lowest cost)
- Two common methods
- Distance vector routing (RIP , IGRP , )
- Link state routing (OSPF , IS-IS)
79The concept of distance vector routing
80Concept of link state routing
81Hierarchical Routing Routing in Autonomous
Systems (IGP)
Routing between Autonomous Systems (EGP)
EGP
IGP