Site hosted by Angelfire.com: Build your free website today!
    Principles
    Home

    Essentials
    Home

      Concepts and Models

      Packets and Protocols

      In contrast to circuit centric local phone networks, data networks tend to be packet centric.

      A packet is a unit of information transmitted as a whole from one device to another on a network.

      Protocols define the methods that keep order on the media and make packet networks possible.

      Packets

      When transmitting, network layers add appropriate information to each packet.
      When receiving, each corresponding layer removes that information.
      Arriving at the receiver's application layer, the information is in its original form.
      Protocols A set of rules or standards that enable computing devices to connect and exchange information.
      May function on a single ISO layer or multiple layers.
      A complete set of methods and rules that a computer needs is called a protocol stack or protocol suite.
        Well Known Protocols
        Protocol Description
        IPX/SPX Novel's protocol suite
        NWLink Microsoft's 32 bit version of IPX/SPX
        TCP/IP The Internet protocol suite
        NetBEUI Microsoft's Transport protocol
        NetBIOS Enhanced User Interface (NetBEUI)
        AppleTalk Macintosh protocol suite

      Networking Models Models have contributed significantly to open standards based networking.
      The two most important open models are the ISO/OSI and IEEE Project 802 models.
      Layered Networking Models Functionality is divided among separate layers.
      Each task or layer represents a particular aspect of the process.
      Boundaries between layers, called interfaces, are well defined.
      Only well defined products can pass between layers.
      Each layer provides well defined services exclusively with and to adjacent layers.
      Each layer is shielded from the details of adjacent layers.
      Layers Promote Independence All requests pass exclusively from one layer through the interface to the adjacent layer.
      Each layer, except for the physical layer, thinks that it is communicating exclusively with its associated (peer) layer on the other computer.
      OSI and 802 Models International Standards Organization's Open System Interconnect (OSI) Model Seven Layers IEEE 802.3 Lower two OSI Layers
        OSI Model
        Layer Function
        seven Application
        six Presentation
        five Session
        four Transport
        three Network
        two Data link
        one Physical
        Created 1978, revised 1984

      Layer Seven Application

      Provides interface specifications so that applications can obtain access to networked services.
      Handles general network access, flow control, and error recovery.
      FTP, HTTP, Telenet, Gopher, and Mail
      Layer Six Presentation Network's translator
      Determines data format
      Protocol conversion
      Encryption
      Data compression
      The redirector operates at this level.
      Redirects I/O operations to resources on a server.

      Layer Five Session Layer
        Allows two applications on different computers to establish, use, and end a connection.
        Provides synchronization between user tasks by placing checkpoints in the data stream.
        Conceptually, facilitates a conversation between two computers.

      Layer Four Transport Layer Sequences packets.
      Packages messages.
      Breaks long messages into shorter packets, groups smaller messages into a single packet.
      Provides error handling.
      Requests packet retransmission when appropriate.
      Layer Three Network Layer Addressing messages
      Translates logical addresses and names into physical addresses.
      Determines the route from the source to destination.
      Can be based on network conditions, priority of service, or length of path.
      Manages traffic problems, such as packet switching, routing, and congestion.
      Layer Two Data Link Packages raw bits from the physical level into data frames.
      Responsible for providing error free Physical level transfer of frames.
      Adds CRC trailer to frame.
      Responsible for MAC Addressing.
      Layer One Physical layer Transmits unstructured raw bit stream over a physical medium.
      Defines how the cable is attached to the network adapter card.
      Layers one and two governed by the IEEE 802 Project Model.
      802 Project Model In Feb. 1980, IEEE started project 802 (1980, second month).
      Defines network standards for physical network components i.e. the NIC, the cabling, and related connectivity components.
      802 Model Defines Network adapter cards
      Wide area network components
      Components used to create twisted pair and coaxial cable networks
      The way network adapter cards access and transfer data over physical media.
      Data Link Sublayers Logical Link Layer (LLC) LLC sublayers manages data link communications
      Defines logical interface points called service address points (SAPs).             Media Access Control (MAC) MAC Access control sublayer provides shared access for the computer's network adapter cards to the Physical layer.