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Network Layers

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Network Layers

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    1. Network Layers The OSI and Internet Models

    2. What is a layered model? Models help us to visualize different aspects of complex abstract systems

    3. What is a layered model?

    4. What is a layered model? Example (Benkler’s layers in action…) My brother in Sweden wants the recipe for my famous 7-layer dip What will we need to negotiate to communicate?

    5. What is a layered model? What language will we use? Swedish or English?

    6. What is a layered model?

    7. What is a layered model? Terminology Service Performance of a specific communication function Layer Self contained set of related services Interface Defines which operations and services are offered between layers, from lower to the next layer up Protocol An agreement between communicating parties on how the communication is to proceed (i.e., “handshake”) Stack List of protocols used by a particular system

    8. What is a layered model? OSI 7-Layer Model International Standards Organization’s Open Systems Interconnection model TCP/IP Model Developed by the Department of Defense

    9. The OSI Layered Model OSI – Open System Interconnection Layered Approach Allows better interoperability between software and hardware Allows design of elaborate but highly reliable protocol stacks The OSI Model is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. A.K.A – the OSI Seven Layer Model Layered Approach - Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. Protocol Stack - A system that implements protocol behaviour consisting of a series of these layers. Protocol stacks can be implemented either in hardware or software, or a mixture of both. The OSI Model is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. A.K.A – the OSI Seven Layer Model Layered Approach - Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. Protocol Stack - A system that implements protocol behaviour consisting of a series of these layers. Protocol stacks can be implemented either in hardware or software, or a mixture of both.

    10. OSI Layers - What does each layer do? The Physical Layer Defines all electrical and physical specifications for devices. Major Functions Establishment & Termination of Connections Connection Resolution & Flow Control of Communication Resources Modulation & Conversion between Digital Data Example – radio, SCSI (Small Computer System Interface) The physical layer defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices. establishment and termination of a connection to a communications medium. participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. This is signals operating over the physical cabling -- copper and fibre optic, for example. SCSI operates at this level. The physical layer defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices. establishment and termination of a connection to a communications medium. participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. This is signals operating over the physical cabling -- copper and fibre optic, for example. SCSI operates at this level.

    11. OSI Layers - What does each layer do? The Data Link Layer Controls data transfer between network entities Performs error detection & correction Uses physical/flat Addressing Scheme Example - Ethernet The Data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. the layer at which bridges and switches operate The Data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. the layer at which bridges and switches operate

    12. OSI Layers - What does each layer do? The Network Layer Performs network routing, flow control, segmentation, and error control functions The router operates at this layer Uses local addressing scheme Example – IP, token ring - The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. Error Control = Error Detection & Correction logical addressing scheme - values are chosen by the network engineer. - The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. Error Control = Error Detection & Correction logical addressing scheme - values are chosen by the network engineer.

    13. OSI Layers - What does each layer do? The Transport Layer Provide transparent transfer of data between end users Controls reliability of a given link Some protocols are stateful and connection oriented (cookies) Example – TCP / UDP stateful and connection oriented means that the session layer can keep track of the packets and retransmit those that fail. stateful and connection oriented means that the session layer can keep track of the packets and retransmit those that fail.

    14. OSI Layers - What does each layer do? The Session Layer Provides mechanism for managing the dialogue between end-user application processes Provides for either duplex or half-duplex operation Responsible for setting up and tearing down TCP/IP sessions Example – NetBIOS A simplex channel is a one-way channel. A half-duplex channel is one that can carry information in both directions, but not at the same time. A full-duplex channel is a channel which can carry information in both directions at once NetBIOS is an acronym for Network Basic Input/Output System. It generally refers to a programming API for local network communication. NetBIOS over TokenRing or Ethernet is now referred to as NetBEUI. A simplex channel is a one-way channel. A half-duplex channel is one that can carry information in both directions, but not at the same time. A full-duplex channel is a channel which can carry information in both directions at once NetBIOS is an acronym for Network Basic Input/Output System. It generally refers to a programming API for local network communication. NetBIOS over TokenRing or Ethernet is now referred to as NetBEUI.

    15. OSI Layers - What does each layer do? The Presentation Layer Little to do with PowerPoint Controls syntactical differences in data representation within end-user systems MIME encoding is done at this layer Example - XML Relieves the Application layer of concern regarding syntactical differences in data representation within the end-user systems Relieves the Application layer of concern regarding syntactical differences in data representation within the end-user systems

    16. OSI Layers - What does each layer do? The Application Layer Provide semantic conversion between associated application processes Interfaces directly to and performs common application services for the application processes Example – Telnet, Virtual Terminal

    17. TCP/IP layered network model Transmission Control Protocol and Internet Protocol TCP/IP is a suite of protocols, also known as the Internet Protocol Suite It was originally developed for the US Department of Defense Advanced Research Project Agency (DARPA) network, but it is now the basis for the Internet

    18. TCP/IP network model layers

    19. TCP/IP Layers - What does each layer do? As with the OSI model, the TCP/IP suite uses a layered model. TCP/IP model has four or five - depending on who you talk to and which books you read! Some people call it a four layer suite - Application, Transport, Internet and Network Access, others split the Network Access layer into its Physical and Datalink components.

    20. TCP/IP Layers - What does each layer do? The combination of datalink and physical layers deals with pure hardware (wires, satellite links, network interface cards, etc.) Access methods such as CSMA/CD (carrier sensed multiple access with collision detection) Ethernet exists at the network access layer - its hardware operates at the physical layer and its medium access control method (CSMA/CD) operates at the datalink layer.

    21. This layer is responsible for the routing and delivery of data across networks. It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes. IP (Internet Protocol) and ARP (Address Resolution Protocol) are both to be found at the Internet layer. TCP/IP Layers - What does each layer do?

    22. The transport layer is similar to the OSI transport model, but with elements of the OSI session layer functionality. The two protocols found at the transport layer are: TCP (Transmission Control Protocol): reliable, connection-oriented protocol that provides error checking and flow control through a virtual link that it establishes and finally terminates. Examples include FTP and Email UDP (User Datagram Protocol): unreliable, connectionless protocol that not error check or offer any flow control. Examples include SNMP TCP/IP Layers - What does each layer do?

    23. This layer is broadly equivalent to the application, presentation and session layers of the OSI model. It gives an application access to the communication environment. Examples: Telnet HTTP (Hyper Text Transfer Protocol) SMTP (Simple Mail Transfer Protocol) TCP/IP Layers - What does each layer do?

    24. OSI & TCP/IP: How do they compare? Similarities Based on a stack of independent protocols Layers have roughly same functionality Transport layer and below provide network-independent transport services Layers above transport are application-oriented Why is this important? Easier to blend, use what works best

    25. OSI & TCP/IP: How do they compare? OSI: General model before protocols Model was conceptual, designers didn’t know what functionality to put in the layers Model is general, easier to replace protocols Model had to adjust when networks didn’t match the service specifications (wireless networks, internetworking) TCP/IP: model describes existing protocols Model only describes TCP/IP – not useful for describing any other networks (such as telephone networks) Why does this matter? Knowing which model to use for your context

    26. OSI & TCP/IP: How do they compare? Number of layers OSI has 7, TCP/IP has 4 Why does this matter? Real world vs. conceptual

    27. OSI & TCP/IP: How do they compare? Connectionless vs. connection-oriented OSI Network layer supports both Transport layers supports only connection-oriented TCP/IP Network layer supports only connectionless Transport layers supports both Why does this matter? What do you need for your situation?

    28. OSI & TCP/IP: How do they compare? OSI Flaws Bad Timing TCP/IP already well-established in academia Bad Technology Complicated, controversial model Unbalanced layers Repeating functions Designed for communications, not computing

    29. OSI & TCP/IP: How do they compare? OSI Flaws (cont’d) Bad Implementations Complicated to understand and implement Bad Politics Seen as biased toward European telecom, European Community and U.S. government Why does this matter? Knowing which model to use for your context

    30. OSI & TCP/IP: How do they compare? TCP/IP Flaws Blurred lines Doesn’t clearly distinguish between services (what a layer does), interfaces (how the layer communicates) and protocols (how the layer does what it does). Too specific Model is only suited to describing TCP/IP, not other networks Protocols can be very specific, inflexible

    31. OSI & TCP/IP: How do they compare? TCP/IP Flaws (cont’d) No distinction between physical and data link layers No description of transmission media, nor frame delimiters Why does this matter? Model is too specific, not specific enough

    32. Conclusion Layered models are useful in describing complex communication systems Allows developers to focus on layers independently Applies to conceptualization as well as implementation Models vs. protocols OSI model is useful in describing networks, but protocols are too general TCP/IP model is weak, but protocols are specific and widely used

    33. Questions?

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