Physical topologies of a network

following: Exploring the functions of a network

Physical topologies:

  • Bus topology: in a bus topology a cable proceeds from one computer to the next. The main cable segment must end with a terminator to absorb the signal when it reaches the end of the line or wire. If there is no terminator, the electrical signal representing the data bounces back at the end of the wire, causing errors in the network.
  • Star and Extended-Star topologies: the Star topology is the most common physical topology in Ethernet LANs. It is made up of a central connection point that is a device, such as a hub, switch, or router, where all the cabling segments actually meet. Each device on the network is connected to the central device with its own cable.
    Although a physical star topology costs more to implement than the physical bus topology, the advantages of a star topology make it worth the additional cost. Each device is connected to the central device with its own wire, so that if that cable has a problem, only that one device is affected, and the rest of the network remains operational. This benefit is extremely important and is the reason why almost every newly designed Ethernet LAN has a physical star topology.
    When a star network is expanded to include an additional network device that is connected to the main network devices, the topology is referred to as an extended-star topology. The problem with the pure extended-star topology is that if the central node point fails, large portions of the network can become isolated.
  • Ring and Dual-Ring topologies: unlike the physical bus topology, a ring type of topology has no beginning or end that needs to be terminated. Data is transmitted in a way that is very different from the logical bus topology. In one implementation, a “token” travels around the ring, stopping at each device. If a device wants to transmit data, it adds that data and the destination address to the token. The token then continues around the ring until it finds the destination device, which takes the data out of the token. The advantage of using this type of method is that there are no collisions of data packets, later I will explain what collisions in a network are.
    In a single ring topology, all the devices on the network share a single cable, and the data travels in one direction only. Each device waits its turn to send data over the network. The single ring, however, is susceptible to a single failure, stopping the entire ring from functioning.
    In a dual ring topology, two rings allow data to be sent in both directions. This setup creates redundancy (fault tolerance), meaning that if one ring fails, data can be transmitted on the other ring.
  • Mesh and Partial-Mesh topologies: another type of topologies that is similar to the bus topology is mesh topology (لو كانت مش طبولوجي تبقى ايه :P). The mesh topology connects all devices to one another for redundancy and fault tolerance. Implementing a full mesh topology is expensive and difficult. This method is the most resistant to failures, because any single link failing will not affect reach ability, why? You will understand that when I explain switching and interconnections.
    In a partial-mesh topology, at least one device maintains multiple connections to all other devices, without being fully meshed. This method reduces the cost of meshing all devices by allowing the network designer to choose which nodes are the most critical and appropriately interconnect them.

How do we connect to the Internet?

There are 3 common methods of connecting the small office to the Internet. DSL uses the existing telephone lines. Cable uses the cable television (CATV) infrastructure. Serial links uses the classical digital local loops.

In the case of DSL and Cable, the incoming lines are terminated into a modem that converts the incoming digital encoding into Ethernet format. In the case of serial, this is done by CSU/DSU (I will explain later). In all three cases the Ethernet output is sent to the router that is part of the customer premises equipment (CPE). Without a router, you simply can not connect to the Internet (and that’s the case of Ethernet connections).


  • A network is a connected collection of devices that can communicate with each other. Networks carry data in many kinds of environments, including homes, small businesses and large enterprises.
  • There are 4 major categories of physical components in a computer network, the computer, interconnections, switches and routers.
  • Networks are depicted graphically using a set of standard icons.
  • The major resources that are shared in a computer network include data and applications, peripherals, storage devices and backup devices.
  • The most common network user applications include e-mail, web browsers, instant messaging, collaboration and databases.
  • User applications affect the network by consuming network resources.
  • The ways in which networks can be described include characterstics that address network performance and structure: speed, cost, security, availability, scalability, reliability and topology.
  • A physical topology describes the layout for wiring the physical devices, while a logical topology describes how information flows through a network.
  • In a physical bus topology, a single cable effectively connects all the devices.
  • In a physical star topology, each device in the network is connected to the central device with its own cable.
  • When a star network is expanded to include additional networking devices that are connected to the main networking device, it is called extended-star topology.
  • In a ring topology, all the hosts are connected in the form of a ring or circle. In a dual-ring topology, there are two rings to provide redundancy in the network.
  • A full mesh topology connects all devices to each other, in a partial mesh topology; at least one device has multiple connections to all other devices.
  • There are three common methods of connection the small office to the Internet, DSL using the existing telephone lines, cable using the CATV infra, and serial links using the classic digital local loops.

Hope you have enjoyed this breif introduction to networks. Next will be an expansion into the OSI models 😉

2 Responses to “Physical topologies of a network”

  1. i think you are good

  2. Realy good i like this article.Verry interesting information for me.

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