Download OSI Model: Physical Layer Data Link Network

7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical
Varna Free University
COMPUTER NETWORKS
OSI MODEL:
Physical Layer
Data Link
Network
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2 Data Link
1 Physical
Source
1. Computer Networks, Andrew S.
Tanenbaum
2. www.cisco.com
3. www.novell.com
4. www.rad.com
5. www.3com.com
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INTRODUCTION
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NETWORK GOALS
The two main benefits of networking computers are…
Communications
Information can be distributed very quickly, such as
email and video conferencing.
Saving Money
Resources such as information, software, and
hardware can be shared.
CPUs and hard disks can be pooled together to
create a more powerful machine.
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APPLICATIONS
A lot of things we take for granted are the result of
computer networks.
• Email
• Chat
• Web sites
• Sharing of documents and pictures
• Accessing a centralized database of information
• Mobile workers
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NETWORK STRUCTURE
The subnet interconnects hosts.
4 Transport
Subnet
Carries messages from host to host. It is made up
of telecommunication lines (i.e. circuits, channels,
trunks) and switching elements (i.e. IMPs, routers).
3 Network
Hosts
End user machines or computers.
2 Data Link
Q: Is the host part of the subnet?
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1 Physical
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NETWORK ARCHITECTURES
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A set of layers and protocols is called the network
architecture.
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1. Protocol Hierarchies
4 Transport
Networks are organized as layers to reduce design
complexity. Each layer offers services to the higher
layers. Between adjacent layers is an interface.
3 Network
Services – connection oriented and
connectionless.
2 Data Link
Interface – defines which primitives and services
the lower layer will offer to the upper layer.
1 Physical
Primitives – operations such as request, indicate,
response, confirm.
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NETWORK ARCHITECTURES
2. Design Issues for the Layers
• Mechanism for connection establishment
• Rules for data transfer
• Error control
• Fast sender swamping a slow receiver
• Inability of processes to accept long messages
• Routing in the case of multiple paths
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OSI REFERENCE MODEL
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The Open Systems Interconnection is the model
developed by the International Standards Organization.
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Benefits
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• Interconnection of different systems (open)
• Not limited to a single vendor solution
Negative Aspect
• Systems might be less secure
• Systems might be less stable
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OSI REFERENCE MODEL
1. Physical Layer
a) Convert the logical 1’s and 0’s coming from
layer 2 into electrical signals.
b) Transmission of the electrical signals over a
communication channel.
Main topics:
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2 Data Link
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• Transmission mediums
• Encoding
• Modulation
• RS232 and RS422 standards
• Repeaters
• Hubs (multi-port repeater)
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OSI REFERENCE MODEL
2. Data Link Layer
a) Error control to compensate for the
imperfections of the physical layer.
b) Flow control to keep a fast sender from
swamping a slow receiver.
Main topics:
3 Network
2 Data Link
1 Physical
• Framing methods
• Error detection and correction methods
• Flow control
• Frame format
• IEEE LAN standards
• Bridges
• Switches (multi-port bridges)
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OSI REFERENCE MODEL
3. Network Layer
a) Controls the operation of the subnet.
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b) Routing packets from source to destination.
c) Logical addressing.
4 Transport
Main topics:
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2 Data Link
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• Internetworking
• Routing algorithms
• Internet Protocol (IP) addressing
• Routers
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OSI REFERENCE MODEL
4. Transport Layer
a) Provides additional Quality of Service.
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b) Heart of the OSI model.
Main topics:
• Connection-oriented and connectionless services
• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)
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OSI REFERENCE MODEL
5. Session Layer
a) Allows users on different machines to establish
sessions between them.
b) One of the services is managing dialogue
control.
c) Token management.
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d) Synchronization.
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OSI REFERENCE MODEL
6. Presentation Layer
a) Concerned with the syntax and semantics of the
information.
b) Preserves the meaning of the information.
4 Transport
c) Data compression.
d) Data encryption.
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OSI REFERENCE MODEL
7. Application Layer
a) Provides protocols that are commonly needed.
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Main topics:
• File Transfer Protocol (FTP)
• HyperText Transfer Protocol (HTTP)
• Simple Mail Transfer Protocol (SMTP)
• Simple Network Management Protocol (SNMP)
• Network File System (NFS)
• Telnet
SERVICES
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Each layer provides services to the layer above it.
1. Terminologies
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Entities – active elements in each layer (e.g.
process, intelligent I/O chip).
4 Transport
Peer Entities – entities in the same layer on
different machines.
3 Network
Service Provider – Layer N.
Service User – Layer N + 1.
2 Data Link
1 Physical
Service Access Points – places where layer N + 1
can access services offered by layer N.
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SERVICES
2. Connection-Oriented and Connectionless
Connection-Oriented – before data is sent, the
service from the sending computer must establish
a connection with the receiving computer.
Connectionless – data can be sent at any time by
the service from the sending computer.
3 Network
2 Data Link
Q: Is downloading a music file from the Internet
connection-oriented or connectionless?
Q: Is email connection-oriented or connectionless?
1 Physical
SERVICES
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3. Service Primitives
Request – entity wants the service to do some
work
Indicate – entity is to be informed about an event
Response – entity responds to an event
Confirm – entity is to be informed about its request
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Sending Computer
4 Transport
1. request
4. confirm
3 Network
Receiving Computer
4 Transport
2. indicate
3. response
3 Network
BANDWIDTH
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The capacity of the medium to transmit data.
Analog Bandwidth
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4 Transport
• Measurement is in Hertz (Hz) or cycles/sec.
Digital Bandwidth
• Measurement is in bits per second (bps).
3 Network
Q: Is 100MHz = 100Mbps?
2 Data Link
1 Physical
Q: Is 100Mbps = 100MBps?
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DH
Bits
DT
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PHYSICAL LAYER
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OVERVIEW
1. Signals
• Fourier analysis
• Maximum data rate of a channel
2. Transmission Media
• Guided and Unguided
3. Analog Transmission
• Modulation
• Modems
• RS-232, RS-422
4. Digital Transmission
• Encoding schemes
• Repeaters and hubs
5. Transmission and Switching
• Multiplexing (FDM and TDM)
• Circuit vs. packet switching
SIGNALS
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1. Fourier Analysis
a) All signals can be represented mathematically.
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b) A periodic function can be constructed by adding
a number of sine and cosine functions.
4 Transport
Fundamental frequency – where f = 1/T
3 Network
Harmonics – integer multiples of the fundamental
frequency
Baud – number of signal level changes per second
2 Data Link
Q: Is baud and data rate different terms?
1 Physical
Q: Is 1 baud equal to 1bps?
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SIGNALS
2. Maximum Data Rate of a Channel
Nyquist
Maximum data rate = 2H log2V (bits/sec)
H = line bandwidth
V = a signal with V discrete levels
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Example:
A noiseless 3kHz channel cannot transmit binary (2
level) signals at a rate faster than 6000bps
2(3k) log22 = 6000bps
logAV = (1 / ln A) ln V
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SIGNALS
Shannon
Maximum data rate (bits/sec) = H log2(1+ PS/PN)
H = line bandwidth
PS = signal strength in watts
PN = noise strength in watts
Example:
A 3kHz channel with a noise ratio of 30dB
(PS/PN = 1000) cannot transmit at a rate faster
than 30,000bps
(3k) log2(1001) = 30,000bps
Note: SNR = 10log10(PS/PN)
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SIGNALS
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3. Attenuation vs. Amplification
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Attenuation
The signal received is weaker than the signal sent.
Attenuation (dB) = 10log10(P1/P2)
4 Transport
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Amplification
The signal received is stronger than the signal
sent.
Amplification (dB) = 10log10(P2/P1)
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Note:
P1 = transmitted signal power in watts
P2 = received signal power in watts
Q: If the result of the attenuation formula is negative, what
happened to the signal?
TRANSMISSION MEDIA
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1. Guided
Data is sent via a wire or optical cable.
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Twisted Pair
Two copper wires are twisted together to reduce
the effect of crosstalk noise. (e.g. Cat5, UTP, STP)
Baseband Coaxial Cable
A 50-ohm cable used for digital transmission. Used
in 10Base2 and 10Base5.
Broadband Coaxial Cable
A 75-ohm cable used for analog transmission such
as Cable TV.
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TRANSMISSION MEDIA
Fiber Optic Cables
Two general types are multimode and single mode.
5 Session
In multimode, light is reflected internally. Light
source is an LED.
4 Transport
3 Network
2 Data Link
In single mode, the light propagates in a straight
line. Light source come from expensive laser
diodes. Faster and longer distances as compared
to multimode.
1 Physical
* Fiber optic cables are difficult to tap (higher security)
and are normally used for backbone cabling.
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TRANSMISSION MEDIA
2. Unguided
Data is sent through the air.
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4 Transport
3 Network
Line-of-sight
Transmitter and receiver must “see” each other,
such as a terrestrial microwave system.
Communication Satellites
A big microwave repeater in the sky. Data is
broadcasted, and can be “pirated.”
2 Data Link
1 Physical
Radio
Term used to include all frequency bands, such as
FM, UHF, and VHF television.
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ANALOG TRANSMISSION
1. Modulation
Modulating a sine wave carrier to convey data.
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4 Transport
3 Network
Amplitude Modulation (AM)
Amplitude is increased/decreased while frequency
remains constant.
Frequency Modulation (FM)
Frequency is increased/decreased while amplitude
remains constant.
2 Data Link
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Phase Modulation
Wave is shifted, while amplitude and frequency
remains constant.
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ANALOG TRANSMISSION
2. Modems
A device that accepts digital signals and outputs a
modulated carrier wave, and vice versa.
4 Transport
It is used to interconnect the digital computer to the
analog telephone network.
3 Network
* Modems for PC’s can be external or internal.
* Nokia makes modems for leased line connections.
2 Data Link
1 Physical
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ANALOG TRANSMISSION
3. RS-232 and RS-449
Two well known physical layer standards.
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4 Transport
RS-232
• 20 kbps
• Cables up to 15 meters
• Unbalanced transmission (common ground)
3 Network
RS-422
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• 2 Mbps at 60 meters
• 1 Mbps at 100 meters
• Balanced transmission (a pair of wires for Tx, Rx)
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DIGITAL TRANSMISSION
1. Encoding Schemes
Converting logical data into electrical signals
suitable for transmission.
Manchester
3 Network
• Mid bit transition for clock synchronization and
data
• Logic 0 = high to low transition
• Logic 1 = low to high transition
2 Data Link
Differential Manchester
4 Transport
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• Mid bit transition for clock synchronization only
• Logic 0 = transition at the beginning of each bit
period
• Logic 1 = no transition at the beginning of each
bit period
DIGITAL TRANSMISSION
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2. Repeaters and Hubs
These are physical layer devices.
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Repeaters
4 Transport
• Restores the strength of an attenuated signal.
• Used to increase the transmission distance.
• Does not filter data traffic.
3 Network
Hubs
2 Data Link
• Multi-port repeater.
• Interconnects several computers.
• Does not filter data traffic.
1 Physical
* Picture from 3com.com
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NETWORK LAYER
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OVERVIEW
1. Routing Algorithms
• Shortest Path
• Flooding
• Flow-based
• Distance Vector
• Link State
• Hierarchical
• Broadcast
• Multicast
• Routing for Mobile Hosts
2. Congestion control
3. IP Addressing
4. Routers
ROUTING ALGORITHMS
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1. Shortest Path
C(B,3)
B(A,2)
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B
C
1
2
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2
3
A(-,-)
2
A
E(A,2)
1
E
1 Physical
F(E,4)
D
2
3
D(E,3)
A–E–D–F
A – E – F is the answer.
1
2
F
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ROUTING ALGORITHMS
2. Flooding
Packet
IMP
B
Packet to IMP C
Packet to IMP D
Packet to IMP E
To prevent packets from circulating indefinitely, a
packet has a hop counter. Every time a packet arrives
at an IMP, the hop counter is decrease by 1. Once the
hop counter of a packet reaches 0, the packet is
discarded.
IP ADDRESSING
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Format
xxxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
where x is either 0 or 1
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Example 1:
4 Transport
11111111. 11111111.00000000.00000000
255.255.0.0
3 Network
Example 2:
2 Data Link
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11111111. 11111111.10000000.00000000
255.255.192.0
IP ADDRESSING
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Network Address
Example 1:
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IP address of computer 180.100.7.1
Mask
255.255.0.0
Network address
180.100.0.0
Example 2:
3 Network
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IP address of computer 180.100.7.1
Mask
255.255.255.0
Network address
180.100.7.0
Example 3:
IP address of computer 180.100.7.2
Mask
255.255.192.0
Network address
180.100.0.0
IP ADDRESSING
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Mask
Valid mask are contiguous 1’s from left to right.
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Examples:
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Valid
255.0.0.0
255.255.0.0
255.255.255.0
Invalid
255.1.0.0
255.0.255.0
255.255.64.0
200.255.0.0
IP ADDRESSING
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Subnets
The Internet is running out of IP address. One solution
is to subnet a network address.
This is done by borrowing host bits to be used as
network bits.
Example:
Class B mask 255.255.0.0
Borrowing 1 bit gives a subnet mask of 255.255.128.0
Borrowing 2 bits gives a subnet mask of 255.255.192.0
Borrowing 3 bits gives a subnet mask of 255.255.224.0
Borrowing 4 bits gives a subnet mask of 255.255.240.0
IP ADDRESSING
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Example:
Given an IP address of 180.200.0.0, subnet by
borrowing 4 bits.
Subnet mask = 255.255.240.0
The 4 bits borrowed are value 128, 64, 32, 16. This will
create 16 sub networks, where the first and last will be
unusable.
Sub network address:
180.200.0.0
180.200.16.0
180.200.32.0
180.200.48.0
180.200.64.0
etc…
IP ADDRESSING
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The first 3 usable sub networks are:
180.200.16.0
180.200.32.0
180.200.48.0
4 Transport
For sub network 180.200.16.0, the valid IP address
are:
3 Network
180.200.16.1 to 180.200.31.254
2 Data Link
Directed broadcast address is:
180.200.31.255
1 Physical
ROUTERS
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A layer 3 device that is used to interconnect 2 or more
logical networks.
Can filter broadcast traffic, preventing broadcast traffic
from one network from reaching another network.
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180.200.0.0
202.5.3.0