Transcript طرح درس ‌‌‌‌[نام درس] شبکه های کامپیوتر

Basics of COMPUTER NETWORKS
Book in glance
Computer Engineering
Faculty of Engineering
Kurdistan University
( Email : [email protected])
‫‪Contact information‬‬
‫سرفصل‌هاي‌درس‬
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‫فصل ‪ :1‬مفاهيم اوليه‬
‫فصل ‪ :2‬اليه فيزيكي‬
‫فصل ‪ :3‬اليه پيوند داده‬
‫فصل ‪ :4‬اليه شبكه‬
‫فصل ‪ :5‬اليه انتقال‬
‫فصل ‪ :6‬اليه كاربرد‬
‫فصل ‪: 7‬كابل كشي ساخت يافته‬
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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Computer Network
Types of Links
Point-to-Point
Multiple Access
…
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Protocol Demultiplexing
• Multiple choices at each layer
FTP
HTTP NV
TCP
TFTP
UDP
Network IP
IPX
NET1
TCP/UDP
IP
NET2 …
NETn
Computer Network
Type
Field
Protocol
Field
Port Number
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OSI Layers and Locations
Application
Presentation
Session
Transport
Network
Data Link
Physical
Host
Hub
Switch
Computer Network
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
0010110
1
1110101
1
1001011
0
1000110
1
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 that
M(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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Sliding Window
Sender/Receiver State
Sender
Max ACK received
Receiver
Next expected
Next seqnum
…
…
Max acceptable
…
…
Sender window
Receiver window
Sent & Acked
Sent Not Acked
OK to Send
Not Usable
Received & Acked
Computer Network
Acceptable Packet
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
Slotted
Aloha
CSMA
CSMA/CD
Developed in the 1970s
for a packet radio
network
Improvement: Start transmission only at fixed
times (slots)
CSMA = Carrier Sense Multiple Access
Improvement: Start transmission only if no
transmission is ongoing
CD = Collision Detection
Improvement: Stop ongoing transmission if a
collision is detected (e.g. Ethernet)
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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 protocols
Reservation


No collisions
reservation is made before sending
t
d bits
N bits
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

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)
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Collision free protocols
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IEEE 802 Standards
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02.3 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
l4
l3
Token/Data
l1
l2
PROP  i li / c  TRTmin
TRT=Token Rotation Time
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Token Ring IEEE 802.5
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IEEE 802.11
Wireless LAN (WLAN)
802.11
802.11b
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
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