The OSI Reference Model

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The OSI Reference Model

• ISO, OSI, ANSI

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ISO

• The International Organization for
  Standardization Geneva, Switzerland

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ISO

• An organization that sets international
  standards, founded in 1946.
• The U.S. member body is ANSI.

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ISO

• ISO deals with all fields except electrical and
  electronics.
• ISBN number system
• The Metric system
• Computing other communications protocols

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OSI Reference Model

• The Open Systems Interconnection Reference Model
  is a seven-layer model developed by the ISO
  (International Standardization Organization) to
  describe how to connect any combination of
  devices for purposes of communications.

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OSI Reference Model

• This model describes the task in terms of seven
  functional layers, and specifies the functions
  that must be available at each layer.
• The seven layers form a hierarchy from the
  applications at the top to the physical
  communications medium at the bottom.

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OSI Reference Model

• The functions and capabilities expected at each
  layer are specified in the reference model
  however, the model does not prescribe how this
  functionality must be implemented.

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OSI Reference Model

• The focus in this model is on the
  "interconnection" and on the information that can
  be passed over this connection.
• The OSI model does not concern itself with the
  internal operations of the systems involved.

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OSI Reference Model

• The OSI model merely describes what is placed on
  a network cable and when it will be placed there.
• It does not state how vendors must build their
  computers, only the kinds of behavior these
  systems may exhibit while performing certain
  communications operations.

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Protocols

• Rules governing communications between processes.

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Protocols

• A LAN protocol is a set of hardware/software
  rules for communicating between computers.
• Protocols govern format, timing, sequencing, and
  error control.
• Without these rules, the computer cannot make
  sense of the stream of incoming bits or voltage
  pulses.

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Protocols

• Computer communication, both within the computer
  and between computers, is broken down into
  steps.
• Each step has its own rules of operation and,
  consequently, its own protocol.

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Protocol Stack

• These steps must be executed in a certain order,
  from the top down on transmission (sending)
  between computers, and from the bottom up on
  reception (receiving.)

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Protocol Stack

• Because of this hierarchical arrangement, the
  term protocol stack is often used to describe
  these steps.
• A protocol stack, therefore, is a set of rules
  for communication, and each step in the sequence
  has its own subset of rules.

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Protocol Stacks

• There are a lot of protocols, and this often
  leads to confusion.
• Here are a few sets of different vendors rules
  for communicating - or their protocol stacks

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Protocol Stacks

• Microsoft NetBIOS/NetBEUI
• NetBIOS is the native Session layer networking
  protocol in DOS and Windows networks.
• NetBEUI the Transport layer protocol for NetBIOS

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Protocol Stacks

• Novell - IPX/SPX
• Internetwork Packet Exchange
• Sequenced Packet Exchange

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Protocol Stacks

• IBM SNA/SAA
• Systems Network Architecture mainframe
  networking standards.
• System Application Architecture - A set of
  interfaces designed to cross all IBM platforms
  from PC to mainframe.

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Protocol Stacks

• TCP/IP
• Transmission Control Protocol/Internet Protocol
• This de facto UNIX standard is the protocol of
  the Internet and has become the global standard
  for communications, and to internetwork
  dissimilar systems

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Protocol Stacks

• Since the transmitter and the receiver have to
  "speak" the same protocol, these four systems
  cannot talk directly to each other.

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OSI Reference Model

• In order to communicate, the following elements
  are needed
• 1 - At least two parties (functions) wishing to
  communicate.
• These can be the same or different programs on
  each machine, or they can be processes at two
  different layers on the same machine.

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OSI Reference Model

• 2 - A common language, or protocol, with which
  these parties (processes) can communicate.

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OSI Layers

• The OSI Reference Model uses seven functional
  layers to define the communication capabilities
  needed to enable any two hardware devices or
  software functions to communicate with each
  other.

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OSI Layers

• The seven layers range from the application layer
  at the top to the physical layer at the bottom.
• The top layer is where users and application
  programs communicate with a network.
• The bottom layer is where the actual
  transmissions take place.

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OSI Layers

• Services at one layer communicate with and make
  use of or provide services for processes at
  adjacent layers.

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OSI Reference Model

• Incorporates two communications models
• A vertical, service-based model by which layers
  on a single machine communicate.
• A horizontal, protocol-based model by which
  programs or processes on different machines
  communicate

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API

• Application Program Interface

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API

• A language and message format used by an
  application program to communicate with the
  operating system or some other control program
  such as a database management system (DBMS) or
  communications protocol.

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API

• The APIs define the available functions for a
  layer and provide the mechanisms for invoking
  these functions.

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API

• Understanding an API is a major part of what a
  programmer does.
• Except for writing the business logic that
  performs the actual data processing, all the rest
  of the programming is writing the code to
  communicate with the operating system and other
  software.

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PDUs (Protocol Data Units)

• Information is passed between layers in the form
  of packets, known as PDUs.
• The packet size and definition depends on the
  protocol suite involved in the horizontal
  communications.

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PDUs (Protocol Data Units)

• Packets are padded as they make their way down
  the layers on the sending machine, and are
  stripped as they make their way up the layers on
  the receiving machine.

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PCI (Protocol Control Information)

• Each sending layer gets material received from
  the layer above it, adds new material (which
  contains the layer's communication with its peer
  on the other machine), and passes this to the
  layer below for further processing.

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PCI (Protocol Control Information)

• At the sending station, this information is
  passed down until it reaches the physical layer.
• At this layer, the material is transmitted over
  an electrical or optical connection as a bit
  sequence.

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PCI (Protocol Control Information)

• At the receiving station, the bit sequence is
  converted to bytes and is passed up the layers.

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PCI (Protocol Control Information)

• As it reaches each layer on the receiving
  machine, the layer removes the material included
  for the layer, takes whatever actions are
  appropriate for the message and the request, and
  then passes the stripped packet on to the next
  higher layer.

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PCI (Protocol Control Information)

• The header information that is added and stripped
  is known as the PCI (protocol control
  information) component.
• Each layer adds its own PCI as the packet is
  passed down to the lower layer.

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PCI (Protocol Control Information)

• By the time a packet from an application reaches
  the data-link layer, it will have five PCIs
  attached from the application, presentation,
  session, transport, and network layers.

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PCI (Protocol Control Information)

• APCI data APDU
• PPCI APDU PPDU
• SPCI PPDU SPDU
• TPCI SPDU TPDU
• NPCI TPDU NPDU
• DPCI NPDU DPDU

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SAPs (Service Access Points)

• The actual interfaces between layers are through
  service access points (SAPs).
• These are unique addresses that the layers
  involved can use to exchange requests, replies,
  and data.

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SAPs (Service Access Points)

• Because multiple programs may be running at a
  given layer, each needs its own SAP for
  communicating with the layers above and below it.

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Port Numbers

• In a TCP/IP-based network such as the Internet,
  it is a logical number assigned to an application
  program running in the computer.
• The number is used to link the incoming data to
  the correct service or function.

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Port Numbers

• Well-known ports are standard port numbers used
  by everyone for example, port 80 is used for
  HTTP traffic (Web traffic).

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Socket

• A program identifies the program it wishes to
  communicate with by its socket, which is a
  combination of the server's IP address and the
  respective port for the program's requested
  function.

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Lets wrap this up

• ISO - International Standardization Organization
• OSI - Open Systems Interconnection
• API - Application Program Interface
• PDU - Protocol Data Unit
• PCI - Protocol Control Information
• SAP - Service Access Point
• Port Port Number
• Socket IP address Port number of process