week 6: NETWORK TOPOLOGY~

Topology Network:

  Think of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home LAN may be arranged in a circle in a family room, but it would be highly unlikely to find a ring topology there. 

·        Network topologies are categorized into the following basic types:

·         bus

·         ring

·         star

·         tree

·         mesh

More complex networks can be built as hybrids of two or more of the above basic topologies 

BUS TOPOLOGY

     Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

      Advantages

·        Easy to connect a computer or peripheral to a linear bus.

·        Requires less cable length than a star topology.

     Disadvantages

·        Entire network shuts down if there is a break in the main cable.

·        A faulty cable or workstation will take the entire LAN down

·        Terminators are required at both ends of the backbone cable.

·        Difficult to identify the problem if the entire network shuts down.

·        Not meant to be used as a stand-alone solution in a large building.

RING TOPOLOGY

     In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.

To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.

STAR TOPOLOGY

     Many home networks use the star topology. A star network features a central connection point called a "hub" that may be a hub, switch or router. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.

Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)

      Advantages

·        Easy to install, and wire.

·        Easy to add new workstations

·        No disruptions to the network when connecting or removing devices.

·        Any non- centralised failure will have very little effect on the network

·        Easy to detect faults and to remove parts.

·        Centralized control

·        Centralized network/hub monitoring

     Disadvantages

·        Requires more cable length than a linear topology.

·        If the hub or concentrator fails, nodes attached are disabled.

·        More expensive than linear bus topologies because of the cost of the concentrators.

TREE TOPOLOGY

     Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the "root" of a tree of devices. This bus/star hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.

   Advantage

·        Point-to-point wiring for individual segments

     Disadvantages

·        Overall length of each segment is limited by the type of cabling used.

·        If the backbone line breaks, the entire segment goes down.

·        More difficult to configure and wire than other topologies.

MESH TOPOLOGY

     Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.

A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partial mesh networks also exist in which some devices connect only indirectly to others.

ETHERNET

      Ethernet is the most widely-installed local area network ( LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox from an earlier specification called Alohanet (for the Palo Alto Research Center Aloha network) and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless

Ethernet Cables :

LOCALTALK

    LocalTalk is a particular implementation of the physical layer of the AppleTalk networking system from Apple Computer. LocalTalk specifies a system of shielded twisted pair cabling, plugged into self-terminating transceivers, running at a rate of 230.4 kbit/s.CSMA/CA was implemented as a random multiple access method.