Chapter 1:
OSI Model
Abdullah Konak
School of Information Sciences and Technology
Penn State Berks Lehigh Valley
Learning Objectives
„ Define the term protocol architecture and
explain the need and benefits of a
communication architecture.
A. Konak
IST 220/Ch1: Introduction
2
1
Challenge?
Professor A
Professor B
I like
rabbits
Urdu & English
Translator
L: Dutch
Ik vind
konijnen
leuk
Secretary
Fax:#
L: Dutch
A. Konak
Ik vind
konijnen
leuk
J’aime
bien les
lapins
Use Translator
Information
for the
remote
translator
Chinese & French
Translator
Ik vind
konijnen
leuk
use Dutch
Information
for the
remote
secretary
L: Dutch
Secretary
use fax
IST 220/Ch1: Introduction
Fax:#
L: Dutch
Ik vind
konijnen
leuk
3
What is a Protocol?
„ Shared conventions and rules for
communicating information are called
protocols
„ Allows entities (i.e. application programs)
from different systems to communicate.
„ Includes syntax, semantics, and timing.
„ A protocol architecture consists of a layered
set of protocols.
A. Konak
IST 220/Ch1: Introduction
4
2
Why Use Layered Protocol
Architecture?
„ Breaks complex tasks into subtasks
„ Each module handles specific subset of tasks
„ Communication occurs
between different modules on the same
system
„ between similar modules on different systems
„
„ Easier application development
„ Network can change without all programs
being modified
A. Konak
IST 220/Ch1: Introduction
5
Standardized Protocol Architectures
„ Vendors like standards because they make
their products more marketable
„ Customers like standards because they
enable products from different vendors to
interoperate
„ Two protocol standards are well-known:
TCP/IP: widely implemented
„ OSI: well-known, less used, still useful for
modeling/conceptualizing
„
A. Konak
IST 220/Ch1: Introduction
6
3
The OSI and TCP/IP (Internet)
Protocol Architecture
„ Open Systems Interconnection (OSI) Reference Model: Still an
excellent model for conceptualizing and understanding protocol
architectures
„ Transmission Control Protocol/Internet Protocol (TCP/IP)
A. Konak
IST 220/Ch1: Introduction
7
The lower levels
The upper levels
The OSI Layers
Application
Presentation
Session
• Transfer data into standard format
before transmission
• Keeps data different applications’ data
separate
• Control the data exchange
Transport
• End-to-end data error free data
transmission
Network
• Logical addressing for data packets
Routing and error handling
Data Link
• NIC software function
• How data encapsulation
• Error detection and collision prevention
Physical
A. Konak
• Provides user interface
• Initiates services
• Moves bits between devices
• Specifies voltages, cables, and cables
IST 220/Ch1: Introduction
8
4
OSI Protocol Architecture
Server
Application:
Web Server
Application
Transport
Client
Application:
Web browser
Session
Presentation
Session
A. Konak
Network
Data Link
IST 220/Ch1: Introduction
9
Physical
Use A starts a chat program, an application layer entity
1
Application
2
IST 220 is fun!
HOST A
The user types “IST 220 is fun!”
The message is coded into an
internationally known code (ASCII)
3
Encoding
10100101001010000011111 plaintext
Encryption
0101101011010111111000000 ciphertext
Presentation
If necessary, the message is
encrypted.
4
Compression
5
01011010110101111110
Session
01011010110101111110
A communication between hosts A and B is
established. A session ID is assigned (S1).
6
S1
If necessary, the message is compressed.
7
The transport layer protocols breaks the bit
stream into smaller segments. Each segment
is attached with a sequence number
Transport
01011
Network
8
S1
9
+
-
S1
2
01111110
S1
3
segment
segment
S1
1
0101101
B
S1
2
B
01111110
S1
3
B packet
At the data link layer, each packet is framed according to specifications of the network. Packets
are now called frames.
01011
A. Konak
Physical
0101101
The network layer protocols make a decision about how to route segments through the network.
The address of host B is added to segments, and they are now called packets.
01011
Data Link
1
S1
1
B
H
0101101
S1
2
B
H
01111110 S1
IST 220/Ch1: Introduction
3
B
H
10
10 At the physical layer, frames are converted into electrical pulses.
5
HOST B
Application
IST 220 is fun!
The message is decoded into text.
8
decoding
10100101001010000011111 plaintext
decryption
0101101011010111111000000 ciphertext
Presentation
01011010110101111110
Using the session ID, the session layer
sends the bit stream to the correct
application
5
S1
Using the sequence numbers to segments are
assembled into bit streams at the receiving host.
If segments are lost on the way, transport layer
protocols acknowledge the sender.
4
Transport
01011
Network
S1
01011
2
S1
2
01111110
S1
3
S1
1
0101101
B
S1
2
B
01111110
S1
3
B packet
At the data link layer, each packet is framed according to the specifications of the network.
Packets are now called frames. Each frame carries a header with point-to-point physical
addresses.
01011
A. Konak
Physical
0101101
The network layer does not have many functions here.
3
Data Link
1
the message is decrypted.
the message is decompressed.
6
decompression
01011010110101111110
Session
7
S1
1
B
H
+
0101101
S1
2
B
H
01111110 S1
IST 220/Ch1: Introduction
-
1
3
B
H
11
At the physical layer reads the electrical pulses.
Application Layer
„ The application layer gives services to the
user of the network
Directory
Services
FTP
Telnet
Web browser
Directory
Services
Telnet
Application Layer
Application Layer
Application Layer Data
Application Layer Data
To presentation layer
A. Konak
FTP
Web browser
From presentation layer
IST 220/Ch1: Introduction
12
6
Presentation layer
„ The presentation layer is responsible for
translation, translating data from the
computer's encoding system to some
international code.
To Application Layer
From Application Layer
Application Layer Data
Application Layer Data
Presentation layer
Presentation layer
Encoded, encrypted, and compressed data H6
A. Konak
To Session Layer
Decoded, decrypted, and decompressed data H6
IST 220/Ch1: Introduction
From Session Layer
13
Session Layer
„ The session layer provides two-way communication
between hosts by establishing, synchronizing, and
terminating connection.
To Presentation Layer
From Presentation Layer
Presentation Layer Data
Presentation Layer Data
Session layer
Session layer
SYN
A. Konak To
SYN
Transport Layer
SYN
H5
IST 220/Ch1: Introduction
SYN
SYN
SYN
From Transport Layer
H5
14
7
Transport Layer
„ Prepares data for transmission and provides
a reliable mechanism for exchange of data
between hosts.
Session Layer Data
Session Layer Data
From Session Layer
To Session Layer
Transport layer
Transport layer
H4
A. Konak
H4
H4
H4
To Network Layer
IST 220/Ch1: Introduction
H4
H4
From Network Layer
15
Network Layer
„ Network layer provides addressing and
routing services across different types of
networks.
To Transport Layer
From Transport Layer
Transport Layer Data
Transport Layer Data
Network layer
Network layer
H3
A. Konak
To Data Link Layer
H3
IST 220/Ch1: Introduction
From Data Link Layer
16
8
Data Link Layer
„ Provides error correction and control services to the
higher layers.
„ Framing makes sure that packets can travel different
networks.
From Network Layer
Network Layer Data
Network Layer Data
Data Link Layer
Data Link Layer
T2
A. Konak
To Network Layer
H2
To Physical Layer
T2
IST 220/Ch1: Introduction
H2
From Physical Layer
17
Physical Layer
„ Transmits bits over a medium; provides
electrical and mechanical specifications.
From Data Link Layer
To Data Link Layer
Data Link Layer Data
Data Link Layer Data
Physical Layer
Physical Layer
01010101010101
A. Konak
01010101010101
IST 220/Ch1: Introduction
18
9