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Network Software

Sub-topic Outline:

• Layered Software
• Services and Protocols
• Network Software Models:
  • The OSI Reference Model
  • The TCP/IP Reference Model
• Network Standards Organisations

Layered Software

Fig 1-9 Tanenbaum Textbook p17

The Reason for Having Layered Software

• Networks have become very complex - needs some structure to design the software
• Divide and Conquer:
  • break up the requirements into different layers
  • each layer only worry about meeting their own requirements - assumes other layers does their job

The Components of Layered Software

• Services - a set of operations done in software or hardware, that other parts of software and hardware can make use of.
• Protocol - a set of rules and conventions defining how a service is to be provided. Software and hardware implements the protocols
• Layer - a set of services grouped together logically.
• Peers - layers on multiple machines at the same level.
• Interface - services offered by one layer to the layer above it - never below!
• Network Architecture - The collection of layers plus the a set of protocols for those layers. The particular set of protocols for a particular collection of layers is called a protocol stack.

How Layered Software Works

Fig 1-11 Tanenbaum Textbook p20

Design Issues for Layered Software

What do the protocols in a layer have to define?

• How to identify sender and receiver.
• Rules for data transfer: simplex, half-duplex, full-duplex
• Error Control
• Sequencing of messages
• Disassembling and Reassembling messages
• Avoid flooding a receiver
• Routing

Most layers only do a subset of the above.

Reference Models for Network Software

Reference models define how a network architecture (the services and protocols) is to be designed, or analysed. It DOESN'T define what the actual architecture is.

Specific networks are designed or analysed following a reference model.

Two very important reference models today are:

1. The ISO OSI Reference Model
2. The TCP/IP Reference Model - used by the Internet (by definition)

The OSI Reference Model

Fig 1.16 Tanenbaum textbook p29

Summary of Services in the OSI Layers

• Application Layer - to allow access to network resources
• Presentation Layer - to translate, encrypt and compress data
• Session Layer - to establish, manage and terminate sessions
• Transport Layer - to provide reliable end-to-end message delivery
• Network Layer - to route packets from source to destination
• Data Link Layer - to put bits into frames and deliver to next node
• Physical Layer - to transmit bits over a medium

The TCP/IP Reference Model

Fig 1.19 Tanenbaum textbook p37

Comparison of OSI to TCP/IP

Fig 1.18 Tanenbaum textbook p36

Similarities:

• Protocols are independent
• The services in the layers are roughly similar
• Layers above transport are applications and user oriented

Differences:

• OSI makes clear distinction between Services, Interfaces and Protocols
• Number of layers

Who's Who in Network Standardization

Telecommunications:

• ITU (International Telecommunication Union)
• ITU-R - Radiocommunications Sector
• ITU-T - Telecommunications Standardization Sector
• ITU-D - Development Sector

International Standards:

• ISO (International Standards Organization)
• ANSI (American National Standards Institute)
• NIST (National Institute of Standards and Technology)
• IEEE (Institute of Electrical and Electronics Engineers)

Internet Standards:

• ISOC (Internet Society)
• IAB (Internet Achitecture Board)
• IRTF (Internet Research Task Force)
• IETF (Internet Engineering Task Force)
• ISTF (Internet Society Task Force)

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