Communications Systems Modeling

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Chapter 2

• Communications Systems Modeling

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Contents

• Introduction
• Example a diplomatic negotiation
• Principles of a layered model
• Connection oriented and connectionless
  communications
• Communication networks according to a layered
  model

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Contents

• Introduction
• Example a diplomatic negotiation
• Principles of a layered model
• Connection oriented and connectionless
  communications
• Communication networks according to a layered
  model

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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

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Contents

• Introduction
• Example a diplomatic negotiation
• Principles of a layered model
• Connection oriented and connectionless
  communications
• Communication networks according to a layered
  model

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A diplomatic negotiation
Political arguments
Service Access Points
Translation Cryptography Transmission
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A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Cryptography Transmission
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A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Encrypted messages
Transmission
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A diplomatic negotiation
Political arguments
Service Access Points
Single language messages
Encrypted messages
Electromagnetic waves
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Contents

• Introduction
• Example a diplomatic negotiation
• Principles of a layered model
• Connection oriented and connectionless
  communications
• Communication networks according to a layered
  model

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Layered Model Definitions
Level (n1) entity
Level (n1) entity
Virtual level n1 communication path using a
level n1 protocol
Physical Communication Path
Level n Service Access Points
Level (n) entity
Level (n) entity
Virtual level n communication path using a level
n protocol
Level n Addresses
Level (n-1) Service Provider
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Layered Model DefinitionsExample The Telephone
Service
Virtual Interpersonal communication path
Physical Communication Path
Telephone Service Access Points
Telephone Numbers
Telephone Network
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Layered Model DefinitionsProtocol Data Units
(n1)PDU
Level (n1) entity
Level (n1) entity
PDU encapsulation
(n)PDU
Level (n) entity
Level (n) entity
Level (n-1) Service Provider
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Contents

• Introduction
• Example a diplomatic negotiation
• Principles of a layered model
• Connection oriented and connectionless
  communications
• Communication networks according to a layered
  model

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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
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Connection Oriented Communications
A temporary communication channel is built
through the telephone network
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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.

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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
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Connectionless Communications
No connections are needed to carry mail
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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

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Connection PolicyLayer independance
Connectionless protocol
Level (n1) entity
Level (n1) entity
Connection oriented protocol
Level (n) entity
Level (n) entity
Level (n-1) Service Provider
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Contents

• Introduction
• Example a diplomatic negotiation
• Principles of a layered model
• Connection oriented and connectionless
  communications
• Communication networks according to a layered
  model

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A two layers model.
Applications Layer
Connectivity
Networks Layer
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A three layers model.
Applications Layer
Internet Transport Layer
Connectivity
Networks Layer
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Internet Transport Layers
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Internet Transport LayersExample
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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

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BibliographyTo know More about network modeling

• Fred Halsall
• Data Communications,
• Computer Networks
• and Open Systems
• Fourth Edition
• Addison-Wesley 1995.
• ISBN 0-201-42293-X

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