Saturday, 1 November 2014

2. Understanding networks

Understanding networks

1.what is network?
 A network is a group of two or more computer systems linked together. There are many types of computer networks, including:

  • local-area networks (LANs) : The computers are geographically close together (that is, in the same building).
  • wide-area networks (WANs) : The computers are farther apart and are connected by telephone lines or radio waves.
  • campus-area networks (CANs): The computers are within a limited geographic area, such as a campus or military base.
  • metropolitan-area networks MANs): A data network designed for a town or city.
  • home-area networks (HANs): A network contained within a user's home that connects a person's digital devices.  

  • 2. Network topologyIn communication networks, a topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology.
    The physical topology of a network is the actual geometric layout of workstations. There are several common physical topologies, as described below and as shown in the illustration.
    In the bus network topology, every workstation is connected to a main cable called the bus. Therefore, in effect, each workstation is directly connected to every other workstation in the network.
    In the star network topology, there is a central computer or server to which all the workstations are directly connected. Every workstation is indirectly connected to every other through the central computer.
    In the ring network topology, the workstations are connected in a closed loop configuration. Adjacent pairs of workstations are directly connected. Other pairs of workstations are indirectly connected, the data passing through one or more intermediate nodes.
    If a Token Ring protocol is used in a star or ring topology, the signal travels in only one direction, carried by a so-called token from node to node.
    The mesh network topology employs either of two schemes, called full mesh and partial mesh. In the full mesh topology, each workstation is connected directly to each of the others. In the partial mesh topology, some workstations are connected to all the others, and some are connected only to those other nodes with which they exchange the most data.
    The tree network topology uses two or more star networks connected together. The central computers of the star networks are connected to a main bus. Thus, a tree network is a bus network of star networks.
    Logical (or signal) topology refers to the nature of the paths the signals follow from node to node. In many instances, the logical topology is the same as the physical topology. But this is not always the case. For example, some networks are physically laid out in a star configuration, but they operate logically as bus or ring networks.

    3. Network Devices & Cables

     Networking Devices


    Networks using a Star topology require a central point for the devices to connect. Originally this device was called a concentrator since it consolidated the cable runs from all network devices. The basic form of concentrator is the hub.
    hub stackable hub
    As shown in Figure; the hub is a hardware device that contains multiple, independent ports that match the cable type of the network. Most common hubs interconnect Category 3 or 5 twisted-pair cable with RJ-45 ends, although Coax BNC and Fiber Optic BNC hubs also exist. The hub is considered the least common denominator in device concentrators. Hubs offer an inexpensive option for transporting data between devices, but hubs don't offer any form of intelligence. Hubs can be active or passive.
    An active hub strengthens and regenerates the incoming signals before sending the data on to its destination.
    Passive hubs do nothing with the signal.

    Ethernet Hubs

    An Ethernet hub is also called a multiport repeater. A repeater is a device that amplifies a signal as it passes through it, to counteract the effects of attenuation. If, for example, you have a thin Ethernet network with a cable segment longer than the prescribed maximum of 185 meters, you can install a repeater at some point in the segment to strengthen the signals and increase the maximum segment length. This type of repeater only has two BNC connectors, and is rarely seen these days.
    ethernet hub switch
    8 Port mini Ethernet Hub
    The hubs used on UTP Ethernet networks are repeaters as well, but they can have many RJ45 ports instead of just two BNC connectors. When data enters the hub through any of its ports, the hub amplifies the signal and transmits it out through all of the other ports. This enables a star network to have a shared medium, even though each computer has its own separate cable. The hub relays every packet transmitted by any computer on the network to all of the other computers, and also amplifies the signals.
    The maximum segment length for a UTP cable on an Ethernet network is 100 meters. A segment is defined as the distance between two communicating computers. However, because the hub also functions as a repeater, each of the cables connecting a computer to a hub port can be up to 100 meters long, allowing a segment length of up to 200 meters when one hub is inserted in the network.

    Multistation Access Unit

    Multistation Access Unit (MAU) is a special type of hub used for token ring networks. The word "hub" is used most often in relation to Ethernet networks, and MAU only refers to token ring networks. On the outside, the MAU looks like a hub. It connects to multiple network devices, each with a separate cable.
    Unlike a hub that uses a logical bus topology over a physical star, the MAU uses a logical ring topology over a physical star.
    When the MAU detects a problem with a connection, the ring will beacon. Because it uses a physical star topology, the MAU can easily detect which port the problem exists on and close the port, or "wrap" it. The MAU does actively regenerate signals as it transmits data around the ring.

    Types of Cables (Media)

    Choosing the right cable necessary to make a successful Local Area Network (LAN) or Wide Area Networking (WAN) connection requires deliberation of the different media or cable types. For the benefit of beginners, there are many different Physical layer implementations that support multiple cable types:
    UTP (Category 5, 5e, 6, and 7)
    Each cable type has its advantages and disadvantages. Some of the factors to consider are:
    Cable length – how far will the cables have to go around the room or building?
    Cost - Does the budget allow for using a more expensive media type?
    Bandwidth – this is one of the criteria to focus your thoughts on. Does the technology used with the Cable provide adequate bandwidth?
    Susceptible to Electromagnetic interference also called radio frequency interference (EMI/RFI) - Is the local environment going to interfere with the signal?
    UTP cabling connections are specified by the Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA).


    The RJ-45 connector is the male part crimped on the end of the cable. When viewed from the front, the pins are numbered from 8 to 1. When viewed from above with the opening gate facing you, the pins are numbered 1 through 8, from left to right. The standard lengths for this cable are 1.83m (6ft) and 3.05m(10ft). This orientation is important to remember when identifying a cable.
    RJ45 Top View 
                                                    A                                       B          

                                                                     RJ45 Cable Orientation

    PinPinFunctionWire Color
    41No usedBlue
    51Not usedWhite/Blue
    74Not usedWhite/Brown
    84No usedBrown
    PinPinFunctionWire Color
    41Not usedBlue
    51Not usedWhite/Blue
    74Not usedWhite/Brown
    84Not usedBrown

    No comments:

    Post a Comment