Transcript Chapter 3 OSI Model

COMPUTER NETWORKS
(ECS 601)
MAHUA S. MAITY
CSE DEPTT.
UNIT 1
Introduction Concepts: Goals and Applications of
Networks, Network structure and architecture, The
OSI reference model, services, Network Topology
Design - Delay Analysis, Back Bone Design, Local
Access Network Design. Physical Layer Transmission
Media, Switching methods, ISDN, Terminal Handling.
Introduction
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Data Communication
Networks
Protocols and Standards
Standard Organizations
Uses of Computer Networks
1. Network Goals & Application
• Resource Sharing
• High Reliability.
• Saving Money.
• Powerful Communication Medium
1. LAN
2. MAN
3. WAN
2. Networks for Companies (1-tier, 2-tier, 3-tier)
Client-server model
CONTT.
2. Networks for Companies
Client-server model
Concurrent server vs. Iterative server
Stateful server vs. Stateless server
3.Networks for People
•Person-to-person communication
Electronic mail, ICQ (I seek you), Videoconference
•Interactive entertainment
Video-on-Demand, Games
•Access to remote information
World Wide Web
4. Social Issues
Privacy
Copyright
Pornography
Anonymity
freedom of speech vs. censorship
responsibility of the service providers
Data Communication System Components
Basic Concepts
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Line Configuration
Topology
Transmission Mode
Categories of Networks
Internetworks
Point-to-Point Line Configuration
continued
Point-to-Point Line Configuration
continued
Point-to-Point Line Configuration
Multipoint Line Configuration
Mesh Topology
Star Topology
Tree Topology
Bus Topology
Ring Topology
Hybrid Topology
Simplex
Half-Duplex
Full-Duplex
OSI Model
(Open systems interconnection references model)
• The model
• Functions of the layers
OSI Model
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Application (Network Services like email, file
transfer)
Presentation (formatting, encryption and
compression)
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Session (setup and management of end-to-end
conversion )
4
Transport (end - to – end delivery of messages )
3
Network (end - to – end transmission of packets)
2
Data Link (transmission of packets on one given link)
1
Physical (transmission of bits)
Design Issues for the Layers
•A mechanism for identifying senders and receivers (naming and addressing)
•rules of transfer (simplex, half-duplex, full-duplex)
•error control (error correction and error detection)
•ordering and sequencing
•flow control, congestion control
•message or packet size (disassembling and reassembling)
•multiplexing and de-multiplexing
•routing
•security
OSI Layers
(The interaction between layers in the OSI model )
The OSI Reference Model (Encapsulation)
data
7
application
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presentation
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session
4
3
transport
2
data link
1
network
physical
H: header
T: trail
Each may be empty.
AH data
PH
AH data
SH
PH
AH data
TH SH
PH
AH data
NH TH SH
PH
AH data
DH NH TH SH
PH
bit streams
AH data DT
An Exchange Using the OSI Model
Physical Layer
Physical Layer
The physical layer is also concerned with the following:
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Physical characteristics of interfaces and medium
Representation of bits.
Data rate.
Synchronization of bits.
Line configuration.
Physical topology.
Transmission mode.
Figure 3-5
Data Link Layer
Data Link Layer
The Data Link Layer is also concerned with the following:
• Framing
• Physical addressing.
• Flow control.
• Error control.
Data Link Layer Example
Network Layer
Network Layer
The Network Layer is also concerned with the following:
• Logical Address
• Routing
Network Layer Example
continued
Network Layer Example
Transport Layer
Transport Layer
The transport layer include the following:
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Service-point addressing.
Segmentation and reassembly.
Connection control
Flow control
Transport Layer Example
continued
Transport Layer Example
Session Layer
Session Layer
The session layer include the following:
• Dialog control(half-duplex or full duplex)
• Synchronization
Presentation Layer
Presentation Layer
Specific responsibilities of the presentation layer include
the following:
• Translation.
• Encryption.
• Compression.
Application Layer
Reference Models
The TCP/IP Reference Model
(Transmission Control
Protocol/Internet Protocol
The TCP/IP Reference Model
A Comparison of the OSI and TCP/IP
Reference Model
SIMILARITIES
The main similarities between the two models include the following:
• They share similar architecture. - Both of the models share a similar
architecture. This can be illustrated by the fact that both of them are
constructed with layers.
• They share a common application layer.- Both of the models share a
common "application layer". However in practice this layer includes
different services depending upon each model.
• Both models have comparable transport and network layers.- This can
be illustrated by the fact that whatever functions are performed
between the presentation and network layer of the OSI model similar
functions are performed at the Transport layer of the TCP/IP model.
•Knowledge of both models is required by networking
professionals.- According to article obtained from the internet
networking professionals "need to know both models".
•Both models assume that packets are switched.- Basically this
means that individual packets may take differing paths in order to
reach the same destination.
DIFFERENCES
•The main differences between the two models are as follows:
TCP/IP Protocols are considered to be standards around which the
internet has developed. The OSI model however is a "generic,
protocol- independent standard.”
•TCP/IP combines the presentation and session layer issues into its
application layer.
•TCP/IP combines the OSI data link and physical layers into the
network access layer.
•TCP/IP appears to be a more simpler model and this is mainly due to the
fact that it has fewer layers.
•TCP/IP is considered to be a more credible model- This is mainly due to
the fact because TCP/IP protocols are the standards around which the
internet was developed therefore it mainly gains creditability due to this
reason. Where as in contrast networks are not usually built around the
OSI model as it is merely used as a guidance tool.
•The OSI model consists of 7 architectural layers whereas the TCP/IP only
has 4 layers
A Critique of the OSI Model and Protocols
1. Bad timing
2. Bad technology
3. Bad implementation
4. Bad politics
A Critique of the OSI Model and Protocols
Bad
timing
Local Area Networks
Ethernet
Token
Ring
Local Area Networks
Standardization Body
IEEE (Institute of Electric and Electronic Engineers) 802 group
For example:
802.3: CSMA/CD (Carrier Sense Multiple Access with
Collision Detection) (Ethernet is one of them.)
802.4: Token Bus
802.5: Token Ring
Local Area Network
continued
Local Area Network
Metropolitan Area Networks
DQDB: Distributed Queue Dual Bus (IEEE 802.6 standard)
Wide Area Networks
Wide Area Networks
Network topologies
Metropolitan Area Network (Example)
Wide Area Networks
store-and-forward network
B
A
A sends a message to C through B.
C
B must store this message until B is sure that C has received it.
Store first, then forward. But when to start forwarding?
Wide Area Networks
store-and-forward network
B
A
A sends a message to C through B.
C
When to starting forwarding?
1. After the message is completely received
2. Start forwarding after a fixed amount of information(bits) received
3. Start forwarding immediately after receiving data (cut-through)
Contt.
B
A
A sends a message to C through B.
C
If a message takes 1 minute to travel a link:
A B C
0
m1
0.25 m2 m1
0.5 m3 m2 m1
0.75 m4 m3 m2
1.0
m4 m3
1.25
m4
(1) A to B, then B to A: 2 minutes
(2) message is decomposed into 4 parts: 1.25 minutes
(each part is called a packet)
Wireless Networks
The fast-growing segment of the industry:
•notebook computers
•personal digital assistants
•cellular phones
Before long, we would have:
•palmtop computers
•wristwatch computers
Wireless Networks
Wide Area Network
Figure 2-19
WCB/McGraw-Hill
Internetwork
(Internet)
 The McGraw-Hill Companies, Inc., 1998
Wide Area Networks
Switching
B
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A
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H
C
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(1) circuit switching (in telephone)
(2) packet switching
(3) message switching
Wide Area Networks:
Dod: ARPANET in 1960s
IBM: SNA in 1974
DEC: DECNET in 1975
CCITT X.25 in 1970s
Current network practice: store-and-forward packet switching
Switching Networks
• Long distance transmission is typically done over a
network of switched nodes
• Nodes not concerned with content of data
• End devices are stations
– Computer, terminal, phone, etc.
• A collection of nodes and connections is a communications
network
• Data routed by being switched from node to node
Simple Switched Network
Circuit Switching
• Dedicated communication path between two
stations
• Three phases
– Establish
– Transfer
– Disconnect
• Must have switching capacity and channel
capacity to establish connection
• Must have intelligence to work out routing
Circuit Switching - Applications
• Inefficient
– Channel capacity dedicated for duration of connection
– If no data, capacity wasted
• Set up (connection) takes time
• Once connected, transfer is transparent
• Developed for voice traffic (phone)
Public Circuit Switched Network
Telecomms Components
• Subscriber
– Devices attached to network
• Subscriber line
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Local Loop
Subscriber loop
Connection to network
Few km up to few tens of km
• Exchange
– Switching centers
– End office - supports subscribers
• Trunks
– Branches between exchanges
– Multiplexed
Circuit Establishment
Circuit Switching Concepts
• Digital Switch
– Provide transparent signal path between devices
• Network Interface
• Control Unit
– Establish connections
• Generally on demand
• Handle and acknowledge requests
• Determine if destination is free
• construct path
– Maintain connection
– Disconnect
Packet Switching Principles
• Circuit switching designed for voice
– Resources dedicated to a particular call
– Much of the time a data connection is idle
– Data rate is fixed
• Both ends must operate at the same rate
Basic Operation
• Data transmitted in small packets
– Typically 1000 octets
– Longer messages split into series of packets
– Each packet contains a portion of user data plus some
control info
• Control info
– Routing (addressing) info
• Packets are received, stored briefly (buffered) and past on
to the next node
– Store and forward
Use of Packets
Advantages
• Line efficiency
– Single node to node link can be shared by many packets over time
– Packets queued and transmitted as fast as possible
• Data rate conversion
– Each station connects to the local node at its own speed
– Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy
– Delivery may slow down
• Priorities can be used
Disadvantages
Disadvantages:
• Protocols for packet switching are typically more complex.
• It can add some initial costs in implementation.
• If packet is lost, sender needs to retransmit the data.
• Another disadvantage is that packet-switched systems still
can’t deliver the same quality as dedicated circuits in
applications requiring very little delay - like voice
conversations or moving images.
Message Switching
• In message switching there is no need to establish a dedicated
path between two stations.
• When a station sends a message, the destination address is
appended to the message.
• The message is then transmitted through the network, in its
entirety, from node to node.
• Each node receives the entire message, stores it in its entirety
on disk, and then transmits the message to the next node.
• This type of network is called a store-and-forward network.
Message Switching
Advantages:
• Channel efficiency can be greater compared to circuitswitched systems, because more devices are sharing the
channel.
• Traffic congestion can be reduced, because messages may
be temporarily stored in route.
• Message priorities can be established due to store-andforward technique.
• Message broadcasting can be achieved with the use of
broadcast address appended in the message.
Message Switching
Disadvantages:
• Message switching is not compatible with interactive
applications.
• Store-and-forward devices are expensive, because they
must have large disks to hold potentially long messages.
Transmission Media(Overview)
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Guided - wire
Unguided - wireless
Characteristics and quality determined by
medium and signal
For guided, the medium is more important
For unguided, the bandwidth produced by the
antenna is more important
Key concerns are data rate and distance
Design Factors
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Bandwidth
Higher bandwidth gives higher data rate
Transmission impairments
Attenuation
Order of losses: Twisted pair, coaxial then fibre
Interference
Overlapping of frequencies in unguided medium
Emanations from adjacent cables in guided. (Use screening)
Number of receivers
In guided media
More receivers (multi-point) introduce more attenuation
Guided Transmission Media
• Twisted Pair
• Coaxial cable
• Optical fiber
Twisted Pair
Twisted Pair - Applications
• Most common medium
• Telephone network
Between house and local exchange (subscriber loop)
• Within buildings
To private branch exchange (PBX)
• For local area networks (LAN)
10Mbps or 100Mbps
Twisted Pair - Pros and Cons
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Cheap
Easy to work with
Low data rate
Short range
Twisted Pair - Transmission
Characteristics
• Analog
Amplifiers every 5km to 6km
• Digital
Use either analog or digital signals
repeater every 2km or 3km
• Limited distance
• Limited bandwidth (1MHz)
• Limited data rate (100MHz)
1 Ghz at short distances & new encoding schemes
• Susceptible to interference and noise
UTP(Unshielded Twisted Pair ) Categories
• Cat 3
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up to 16MHz
Voice grade found in most offices
Twist length of 7.5 cm to 10 cm
• Cat 4
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up to 20 MHz
• Cat 5
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up to 100MHz (1 GHz using 4 pair & compression)
Data grade cable
Commonly pre-installed in new office buildings
Twist length 0.6 cm to 0.85 cm
• Cat 6
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200 MHz to 250MHz
1 Ghz uncompressed: 4 x 250 Mhz
Coaxial Cable
Coaxial Cable Applications
• Most versatile medium
• Television distribution
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Ariel to TV
Cable TV
• Long distance telephone transmission
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Can carry 10,000 voice calls simultaneously
Being replaced by fiber optic
• Short distance computer systems links
• Local area networks
Coaxial Cable - Transmission
Characteristics
• Analog
– Amplifiers every few km
– Closer if higher frequency
– Usuable spectrum up to 500MHz
• Digital
– Repeater every 1km
– Closer for higher data rates
Optical Fiber - Benefits
• Greater capacity
– Data rates of hundreds of Gbps
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Smaller size & weight
Lower attenuation
Electromagnetic isolation
Greater repeater spacing
– 10s of km at least
Optical Fiber - Applications
• Long-haul trunks
– 1500km, 20 – 60k voice channels
• Metropolitan trunks
– 12 km, 100k channels
• Rural exchange trunks
– 40 – 160Km, 5k voice channels
• Subscriber loops
– Voice data cables leased by corporate clients
• LANs
– 100Mbps – 1 Ghz
Optical Fiber - Applications
• Long-haul trunks
– 1500km, 20 – 60k voice channels
• Metropolitan trunks
– 12 km, 100k channels
• Rural exchange trunks
– 40 – 160Km, 5k voice channels
• Subscriber loops
– Voice data cables leased by corporate clients
• LANs
– 100Mbps – 1 Ghz
Delay Analysis
1. Processing Delay(header)
2. Queuing Delay
3. Transmission Delay(1st come 1st
serve)
4. Propagation Delay(physical
medium)
Integrated Services Digital
Network
Networking Devices
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NIC
Hub
Bridge
Switch
Router
Gateway
NIC
Hub
(Figure :4-port Ethernet Hub)
Switch
Router
Gateway