Internetworking Basics

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     Chapter 2: Networking Fundamentals


    Network Devices and OSI Layers

    These notes guide you through understanding the various networking devices and the OSI layers at which each one operates.

    Introduction
    Devices and their OSI Layer
    Network Interface Card
    Repeaters
    Hubs
    Bridges
    Switches
    Routers
    Summary

     
     

    Introduction

    On completion of these notes you should be

    • Describe the function of various network devices such as hubs, repeaters, switches, bridges and routers.
    • Describe the OSI layer each device operates at.
     

    Devices and their OSI Layer

    A network can contain many different types of devices. There are end-user devices such as workstations, printers and scanners. There are also network devices such as repeaters, hubs, switches, bridges and routers.

    Each of these devices plays a different role within a network and each one functions at a particular OSI layer.

     

    Network Interface Card

    For one host to communicate with another host on a LAN it would typically use a NIC (network interface card.)

    The picture to the right shows a typical internal PCI network card.

    Every network interface card in the world carries a unique MAC address, burned into the card by the manufacturer.

    A MAC address is used to find a host on a network.

      

    MAC stands for Media Access Control, which is also a sublayer of the OSI data link layer.

    Since NIC's carries a unique MAC addresses they are considered to operate at Layer 2 , the data link layer of the OSI reference model.

     

    Repeaters

    The purpose of a repeater is to amplify signals. When a signal leaves a source, it is initially strong and clear. However, as the signal travels over a wire it becomes weaker. The further it travels, the weaker it becomes. This loss of signal strength with distance is called attenuation.

    Signal attenuation puts a limit on how long a cable can be before a signal becomes unreadable. A repeater can be used to amplify and regenerate a signal.

    Originally, repeaters were single-port devices, one signal in - one signal out, but today multiport repeaters are also available.

      

    Repeaters make no decisions about the signals they receive, they operate purely on the physical level, amplifying and passing signals along. Thus, repeaters operate at the Layer 1, the physical layer of the OSI reference model.

     

    Hubs

    A hub is similar to a multiport repeater. The purpose of a hub, like that of a repeater is to retime, regenerate and amplify signals. Hubs are commonly used as network concentration devices.

    The picture to the right shows a typical hub.

    You can see the ports on the hub where you plug in the cables.

      

    A hub typically acts as a central device that connects many other devices together. The diagram below shows a typical star topology with a hub acting as the central concentration point. This arrangement is common for Ethernet 10BASE-T and 100BASE-TX networks.

    When a hub receives a signal from one devices, it automatically amplifies and copies the signal to all devices connected to it. Even when a signal, sent from one device is meant only for one particular device, the hub will copy the signal to all devices regardless.

    Sometimes, collisions can occur on the cable when more than one device transmits a signal at the same time. Since a hub copies signals to all cables attached to its ports, then it is possible for collisions to occur along any cable attached to the hub. This area where signals may collide is called the collision domain.

    A hub, like a repeater, makes no decisions regarding the data it receives, but just amplifies and passes the signals along. Thus, a hub operate at Layer 1, the physical layer of the OSI reference model.

     

    Bridges

    A bridge is a device that connects two or more LAN segments together. A bridge is different to a hub though because it does make decisions on the data it receives. When a device transmits a frame to another device, the MAC address of both the sending and receiving device are contained within the frame. You may recall that a MAC stands for Media Access Control and that every network interface card has a unique MAC address. Here is an example of a MAC address

    00-40-F3-2C-4C-9B

    When a bridge is first powered up, it reads the source and destination MAC address inside all frames that are transmitted on the LAN segments and builds a tables of MAC addresses. It uses this table to decide where to forward frames. When it receives a frame from a device on one LAN segment, it uses the MAC address to forward the data only to the segment which contains the receiving device. Unlike a hub, it does not send the data down all wires attached to its ports. If the sending are receiving device are on the same segment, it does not copy the data to any other segment. This selective forwarding of network data is called filtering.

    The advantage of a bridge over a hub is that a bridge filters network traffic. Since network traffic is not automatically sent to all network segments, just to the appropriate segment, then this effectively divides a collision domain. Whereas with a hub, all devices attached make up one big collision domain, a bridge creates two or more collision domains. The benefit of using a bridge is that there are less collisions on the wire.

    Sometimes, a device needs to send a message to all other devices. A generic message sent to all devices is called a broadcast. There may be various reasons for sending broadcast messages. As an example, perhaps the sending device needs to find the MAC address of another device. In such a case it would send a broadcast message. Both hubs and bridges forward broadcasts to all attached devices, a process called flooding. So, although a bridge splits up a collision domain, it does not divide a broadcast domain.

    Because a bridge makes forwarding decisions based on MAC addresses, it operate at Layer 2, the data link layer of the OSI reference model.

     

    Switches

    A switch is another devices that also creates a single concentration point, connecting many devices together. It is used for multiple bridge connections, connecting many devices together to form LAN segments.

    Just like bridges, switches keep MAC address tables and make forwarding decisions based on MAC addresses. A bridge usually has a single port in and a single port out that connect two segments, with many devices on each segment, whereas a switch has many ports.

    Every single port connection on a switch creates a separate segment between the port and the device attached at the other end.

      

    When the switch receives a signal from a sending device on one port, it will read the destination MAC address and unless it is a broadcast message, will relay the signal to the destination device only. When you consider a bridge, you consider the incoming and outgoing ports to be attached to two separate LAN segments. Since a switch has many ports, there will be many segments each - segment private and dedicated to creating a single private link to a single host device.

    This creation of dedicated links is called microsegmentation.  Moreover, with microsegmentation, because the link to each host device is dedicated to that host only, then in a full-duplex link, collisions do not occur and the host can make use of the full link bandwidth. The only time a switch relays traffic to more than one host is when it is relaying broadcast messages.

    Because a switch makes forwarding decisions based on MAC addresses, it operate at Layer 2, the data link layer of the OSI reference model.

     

    Routers

    Whereas switches and bridges make forwarding decisions base on Layer 2 MAC addresses, a router makes forwarding decisions based on Layer 3 addresses.

    Unlike a switch, a router is more concerned with routing network traffic along the most efficient path to the destination. The destination host can be on a different LAN segment or even a completely different network.

    Routers can also connect different LAN technologies together such as Ethernet, Token Ring, FDDI and ATM.

      

    Since routers deal with Layer 3 addresses and can route traffic, then traffic can be sent over the Internet. Routers are important devices on the Internet; the Internet as it is could not exist without routers. When the destination device is on a completely different network and communication needs to be routed over the Internet, routers use the IP address of the destination host to select the best pathway to the destination host.

    The diagram below shows a router acting as a gateway to the Internet.

    Because a router makes forwarding decisions based on Layer 3 addresses, then it operates at Layer 3, the network layer of the OSI reference model.

     

    Summary

    On completing these notes you should:-

    • be familiar with layer 1 devices - repeaters and hubs
    • be familiar with layer 2 devices - NICS, bridges and switches
    • be familiar with a layer 3 device - the router

    ~~Activity~~

     Activity A
    1. Try the flash card memory aid activity on Devices and OSI Layers
     
     

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