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الموضوع في 'أرشيف المنتدى التعليمي' بواسطة king-0X, بتاريخ ‏20 مارس 2008.

  1. king-0X

    king-0X نجم المنتدى

    إنضم إلينا في:
    ‏21 فيفري 2008
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      20-03-2008 16:43
    :besmellah1:
    LAN COMMUNICATIONS
    1) OVERVIEW
    Many computer suppliers have developed their own proprietary methods of network communication.
    By definition, these methods do not allow dissimilar systems from multiple manufactures to communicate with each other.
    A) STANDARS
    Governments and industry soon realised that manufacturers of network products needed to comply with some sort of recognised, international communication standards.
    B) ISO AND IEEE
    A number of groups decided to work on developing standard networking protocols (or rules governing communications).
    These groups include the international standards organisation (ISO) and the Institute of Electrical and Electronic Engineers (IEEE).
    These two organisations have developed the most important sed of LAN standards, known as the Open Systems Interconnect (OSI) model and the 802 standards respectively.
    2) OSI
    OSI provides a reference of how products should work together. To date, most manufactures systems only comply with some parts of the standard.
    A) COMPLIANCE
    Compliance is voluntary, but with users demanding to comply.
    OSI is an architecture which consists of 7 layers .each layer is defined so that it works with the layer above and directly below it.
    • The OSI model
     Application
     Presentation
     Session
     Transport
     Network
     Data link
     Physical

    B) PHYSICAL LAYER
    This bottom layer describes electrical and physical characteristics of network cables, connectors, and the signals that flow through them.
    C) DATA LINK LAYER
    This second layer controls the rules of communications across the network.
    This includes error detection and putting the data into 'packets' for transmission ,controlling access to as hared network media (e.g.CSMA/CD or Token Passing).
    D) NETWORK LAYER
    The third layer controls the flow and routing of messages across networks
    E) TRANSPORT LAYER
    This fourth layer is concerned with data integrity .it breaks up large messages into smaller 'packets' that that the data link layer(layer 2) can them transmit .
    F) SESSION LAYER
    This fifth layer controls the interaction between the end user station and a resource on the network, from start to finish.
    G) PRESENTATION LAYER
    This sixth layer aids in the translation of messages, whether that includes text compression, data encryption or other format conversions.
    H) APPILICATION LAYER
    The seventh and final layer is accessed by applications, far example e-mail and file transfer software.
    3) IEEE
    A) IEEE STANDARDS
    IEEE 802 standards are concerned with the lowest two layers of the OSI model, the physical and Data Link Layers .
    • the OSI model
     Data link
     Physical

    B) IEEE TYPES
    The three most common IEEE standards are 802.3 ; 802.4 and 802.5 .
     IEEE 802.3
    802.3 describe a bus topology using CSMA/CD as the access method .It is based on the Ethernet standard.
    Several standards are included in this specification including 10BASET (where'10' represents the member of megabits per second, BASE indicates Base band, and T for twisted pair).
     IEEE 802.4
    802.4 describe a physical 'bus' using a token passing access method.
     IEEE 802.5
    802.5 describes a physical 'ring' using the a token passing access method
    4) OSI THECHNIQUES
    A) INTERCONNECT DEVICES
    The OSI model also specifies techniques for communication between different LANs and different protocols
    Users frequently need to interface with other LANs, for example when sending e-mail messages to offices overseas.
    Interconnection devices are needed to enable communication to occur.
    There are 4 types of interconnect devices available: repeaters, bridges, routers and gateways.
    B) REPEAER
    The simplest and cheapest LAN interconnect device.
    Hardware based, the repeater can only be used when communicating between two similar networks (e.g.two Ethernet networks).
    This is because a repeater does not convert the networks transmission, but simply repeats the electrical transmission so that it reaches the recipient network.
    The repeater only works at the physical layer level.
    C) BRIDGE
    A bridge is typically hardware based and like the repeater can only connect two similar LANs together .it does not perform any conversion of the network transmissions.
    The bridge uses the physical and Data Link layers of the OSI model.
    Most bridges filter the network traffic and only pass on messages destined for different networks.
    D) ROUTER
    A router, in contrast, can connect two or more LANs together and can send data to dissimilar networks.
    Since the router uses the bottom 3 levels of the OSI model, each network need only conform to these protocols.


    E) GATE WAY
    A gate way is the most complex of the 4device types and is expensive but is also rather low in functionality.
    However, agate way is critical in computer installations that have multiple networking techniques, because it is a comprehensive protocol conversion device.
    It has a full 7 layer OSI shack for each network it services.
    Put simply, this means that a gate way understands and plays by the communications 'rules' of a range of different networks.









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