Download Part 2

Chapter 2
Communications
Systems Modeling
Postacademic Interuniversity Course in Information Technology – Module C1
p1
Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
Postacademic Interuniversity Course in Information Technology – Module C1
p2
Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
Postacademic Interuniversity Course in Information Technology – Module C1
p3
Layered Network Models
• The problem:
Complexity of networked systems
• The solution:
Decomposition into independent subsystems
• Layered network model
– Introduced by IBM (SNA, 1971)
– Standardized by ITU (X200) and ISO (7498)
– Method universally adopted
– Standards not really obeyed in practice
Postacademic Interuniversity Course in Information Technology – Module C1
p4
Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
Postacademic Interuniversity Course in Information Technology – Module C1
p5
A diplomatic negotiation
Political arguments
Service Access Points
Translation
Cryptography
Transmission
Postacademic Interuniversity Course in Information Technology – Module C1
p6
A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Cryptography
Transmission
Postacademic Interuniversity Course in Information Technology – Module C1
p7
A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Encrypted messages
Transmission
Postacademic Interuniversity Course in Information Technology – Module C1
p8
A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Encrypted messages
Electromagnetic waves
Postacademic Interuniversity Course in Information Technology – Module C1
p9
Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
Postacademic Interuniversity Course in Information Technology – Module C1
p10
Layered Model Definitions
Level (n+1)
entity
Virtual level n+1 communication path
using a level n+1 protocol
Level (n+1)
entity
Physical Communication Path
Level n
Service Access Points
Level (n)
entity
Virtual level n communication path
using a level n protocol
Level (n)
entity
Level n Addresses
Level (n-1) Service Provider
Postacademic Interuniversity Course in Information Technology – Module C1
p11
Layered Model Definitions
Example: The Telephone Service
Virtual Interpersonal communication path
Physical Communication Path
Telephone
Service Access Points
Telephone Numbers
Telephone Network
Postacademic Interuniversity Course in Information Technology – Module C1
p12
Layered Model Definitions
Protocol Data Units
Level (n+1)
entity
(n+1)PDU
Level (n+1)
entity
PDU encapsulation
Level (n)
entity
(n)PDU
Level (n)
entity
Level (n-1) Service Provider
Postacademic Interuniversity Course in Information Technology – Module C1
p13
Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
Postacademic Interuniversity Course in Information Technology – Module C1
p14
Connection Oriented
Communications
Typical example:
A telephone conversation
3 step process:
1. Establish connection
using the signaling protocol
2. Use connection
using an informal protocol
being billed for duration of usage
3. Terminate connection
using the signaling protocol
Postacademic Interuniversity Course in Information Technology – Module C1
p15
Connection Oriented
Communications
A temporary communication channel
is built through the telephone network
Postacademic Interuniversity Course in Information Technology – Module C1
p16
Connection Oriented
Communications
• Network has a state : the list of active connections
• Network can guarantee Quality of Service
– Reservation of transmission capacity at connection
establishment
– Monitoring of data transfer and correction of
transmission errors by retransmission
• Network is fragile : state can be lost.
• Application domain :
– Very efficient for transferring high volumes of data
– Mandatory if stringent Quality of Service requirements
– Poor initial response times due to connection setup
– Inapplicable if nodes have high failure rate.
Postacademic Interuniversity Course in Information Technology – Module C1
p17
Connectionless
Communications
Typical example:
The postal service
Letters, with an address and a stamp,
are individually carried from a
Post office box to a personal mailbox
Postacademic Interuniversity Course in Information Technology – Module C1
p18
Connectionless
Communications
No connections are needed to carry mail
Postacademic Interuniversity Course in Information Technology – Module C1
p19
Connectionless
Communications
• The network is stateless.
• No guaranteed Quality of Service.
– No reservation of transmission capacity
– No error correction by retransmission
• Network is robust :
– No state stored in any node
– Each node operates almost independently from
others
• Application domain :
– Whenever single message response times are
critical
– Whenever nodes are unreliable
Postacademic Interuniversity Course in Information Technology – Module C1
p20
Connection Policy
Layer independance
Level (n+1)
entity
Connectionless protocol
Level (n+1)
entity
Connection oriented protocol
Level (n)
entity
Level (n)
entity
Level (n-1) Service Provider
Postacademic Interuniversity Course in Information Technology – Module C1
p21
Contents
•
•
•
•
•
Introduction
Example: a diplomatic negotiation
Principles of a layered model
Connection oriented and connectionless communications
Communication networks according to a layered model
Postacademic Interuniversity Course in Information Technology – Module C1
p22
A two layers model.
Networks Layer
Postacademic Interuniversity Course in Information Technology – Module C1
Interoperability
Connectivity
Applications Layer
p23
A three layers model.
Networks Layer
Postacademic Interuniversity Course in Information Technology – Module C1
Connectivity
Internet & Transport Layer
Interoperability
Applications Layer
p24
Internet & Transport Layers
Applications Layer
Transport Layer
Internet Layer
Networks Layer
Postacademic Interuniversity Course in Information Technology – Module C1
p25
Internet & Transport Layers
Example
WWW
Transport Layer
TCP
TCP
Internet Layer
IP
SKYNET
IP
UUNET
Networks Layer
Postacademic Interuniversity Course in Information Technology – Module C1
p26
Introduced concepts
• Layered communications system
– protocol = horizontal convention
– Service access point & address
• Connection Oriented vs. Connectionless Communications
• Applications and networks layers
• Connectivity :
– service offered by lower layers
• Interoperability :
– full communications services
• Internet & Transport layers
Postacademic Interuniversity Course in Information Technology – Module C1
p27
Bibliography
To know More about network modeling
Fred Halsall
Data Communications,
Computer Networks
and Open Systems
Fourth Edition
Addison-Wesley 1995.
ISBN 0-201-42293-X
Recommended for this chapter
Postacademic Interuniversity Course in Information Technology – Module C1
p28