Network topologies

• The different arrangements of computer connections and devices are called topologies.
• In the early days of computer networking, the way that a network was arranged and cabled was directly linked to the way that a network behaved. In today's networks, the physical layout often conceals the logical network topology.
• The bus topology has one cable that connects every device on the network, which is called the backbone. At each end of the main cable there is a small device called a terminator, which absorbs the signals when they reach the end, to prevent interference. Each device is connected to the main cable to join the network.
• The bus topology was the first implementation of an Ethernet network. In this type of network, network signals, representing messages between devices, are sent through the main cable to reach all connected devices. Each message contains a destination address that identifies its intended recipient. The MAC address that is hardcoded into the Network Interface Controller (NIC) of each device, is used for this purpose. The intended receiving device picks up the communication and all other devices ignore it.
• In a bus topology, if two (or more) devices try to transmit at the same time, it will cause a collision, i.e. the signals will interfere with each other. If a collision occurs, the devices must stop and try again later. Each device waits a different random amount of time before trying again. At times when the network is in heavy use, lots of computers will be transmitting data, leading to a high number of collisions. This causes the overall performance of the network to deteriorate.
• In a bus topology, the fact that signals are transitted across the whole network can be a security issue because every device attached to the bus can "read" every message, unless the messages are encrypted.
• In a bus topology, a failure of the main bus cable will bring the whole network to a stop. However, if there is a break or failure in the cable that connects a single device to the bus, it will affect just this device; the rest of the network will carry on working.
• In a star topology, each device on the network is connected to a central communication device, such as a Hub or a Switch.
• In a star topology, if the central device is a hub, network signals that reach the hub are sent out to all connected devices. The hub does not process the signals that pass through it, but just repeats them on all of its connections.
• However, a switch uses a table that maps each device to the port it is connected to. In this way, the switch is able to direct messages to the correct recipient (receiving device).
• The use of a switch improves the security and speed of a star network. Messages are transmitted along a specific path from source to destination, so they do not pass through every cable in the network.
• In a star network, the switch or hub is a central point of failure. If this component fails, none of the connected devices will be able to communicate or access network resources. However, a failure of any of the individual cables or workstations will not affect the network. If one workstation fails, the rest of the network can continue to operate.