1.1 Network Topology

1.1 Network Topology

Topology is the name given to the way a network is physically laid out and also to the way it behaves logically. There are many network topologies. The three main topologies are bus, ring and star.

A network’s physical topology is how the cable and devices are laid out whereas the logical topology is determined by how the network behaves or works. A network’s behaviour is determined by the Network Interface Card (NIC) installed. In order for a network to perform properly all devices on it must conform to the same logical topology.

A network’s physical topology works at layer 1 (Physical) of the OSI Model, while a network’s logical topology works at layer 2 (Data Link).

In this course when the term topology is used it will refer to physical topology unless otherwise indicated.

Physical Topology

A network’s physical topology is the arrangement or layout of computers, cables, and other components on the network.

The physical topology of a network directly affects its capabilities. The choice of one topology over another will have an impact on the following:

  • Type of equipment the network needs
  • Capabilities of the equipment
  • Growth of the network
  • The way the network is managed

Before computers can either share resources or perform other communications tasks, they must be connected. Most networks use cable to connect one computer to another. It is not as simple as just plugging a computer into a cable connecting other computers. There are many different types of cable, network cards, network operating systems, and other components, all of which require different types of arrangements.

To work properly, a network topology takes a lot of careful planning, determining not only the type of cable to be used but also how it runs through floors, ceilings, and walls.

Logical Topology

A network’s logical topology can also be referred to as its signal topology. This type of topology is not interested in how devices on the network are connected but how they communicate with each other. Logical topologies are created by the network protocols on NICs that determine the movement of data on the physical topology. Examples of these are:

  • Ethernet – logical bus topology
  • LocalTalk – logical bus or star topology
  • IBM Token Ring – logical ring.

To add to this seeming confusion it may be the case that a network’s logical topology is not always the same as its physical topology.

Example 1: UTP Ethernet – A UTP Ethernet network operates logically as a bus but is physically organised by the devices connecting to a hub in a physical star

Example 2: IBM’s Token Ring – IBM’s Token Ring operates as a logical ring but the devices are all connected to a hub-like device called a Multi-Station Access Unit (MSAU) which makes its physical topology a star.

Media Access Control

A key function of a network interface card (NIC) is to determine how the network media is accessed. Each NIC has a unique physical hardware address burned into it. This address is known as a Media Access Control (MAC)physical address and is also referred to as a Burned In Address (BIA).

The MAC address is a 48 bit hexadecimal address that is programmed into the card by the manufacturer and it cannot be changed. It consists of two parts: the manufacturer’s id and a unique address generated by the manufacturer.

MAC Example: 00-0C-76-19-6D-42

The term media access refers to how the communications media is accessed and is topology and media dependant, eg: Ethernet media access is different from token ring access.