Network Topologies

• Author: Dr. Jim Marquardson (jimarqua@nmu.edu)
• Updated 2024-06-13

Networks can be designed using different topologies. The physical network topology describes how a signal travels over cables (or the air, in the case of wireless) to reach its destination. A logical topology describes the route packets take to reach their destinations. Each topology has its pros and cons.

Learning Objectives

In this exercise, you will learn to:

• describe different network topologies, and
• list the pros and cons of different topologies.

Materials Required

In groups of 5, the following are needed:

• 10 short lengths of rope (~2.5 feet) with loops on each end.
• 1 longer length of rope (~8 feet) to serve as the backbone for a bus topology.

Topologies

Network topologies describe how networks are designed to move packets from a sender to a receiver. Topologies can be physical or logical. A physical topology focuses on how the electrical signal flows through a network. A logical topology is more abstract and defines the routes taken from sender to receiver.

Ring Topology

In a ring topology, hosts are connected in a circular path. In the diagram below, if person A wants to send a message to person C, person A has to send that message to person B who then sends the message to person C. Hopefully, we trust that person B is not going to snoop on the message content because person B is not the intended recipient. As ring topologies grow, the number of extra intermediaries also grows which could negatively impact performance.

In the early days of computer networking, ring topologies were useful because they did not require a central hub or switch to connect multiple devices. A computer network could be composed entirely of computers and network cables.

Practice communicating using a ring topology.

• Form a circle.
• Each person should hold a cable going to each person to your left and right.
• One at a time, say the name of the person you want to send a message to, then tug on the cable a couple of times. The number of tugs represents your message. Intermediaries should pass on the message until the recipient receives the correct number of tugs on the cable.

Star Topology

In a star topology, hosts connect to a central device--either a hub or a switch. Hubs work by receiving a signal from one port and sending that signal out to all other ports. Hubs suffered from network congestion and data privacy concerns. Though they might look similar, switches provide much better performance and security. Switches work by learning the hardware addresses of the devices connected to each port. When a switch receives a signal from one port, the switch sends that signal only to the intended recipient.

Unlike the ring topology, star topologies require a dedicated switch. The introduction of a central switch allows the network to scale without suffering performance problems that would occur with a ring topology. But the switch becomes a single point of failure. If the switch goes down, everybody connected to that switch loses access to the network.

In the diagram below, one of the hosts has become a central switch that facilitates communication.

The image below shows actual Cisco switches (source: MWiner/stock.adobe.com).

Practice communicating using a star topology.

• Have one person become the "switch".
• Loop the network cables onto the switch's fingers.
• One at a time, the "hosts" should say the name of the person they want to send a message to, then tug the cable a couple of times.
• The switch will receive the message on one finger (i.e., a port), and then only send that message by moving the finger that is connected to the intended recipient.

Notice that people should only receive messages from the switch if they are the intended recipient. Also, note that the switch can get very busy. If the person performing the switch role stops doing their job, communication stops.

Mesh Topology

In a mesh topology, every host is directly connected to every other host. Mesh networks can perform well because packets do not need to travel through any intermediate hosts or other devices to reach their destination. But, the number of connections grows dramatically as the number of hosts increases. A mesh network with 5 hosts requires 10 cables. Adding a 6th host requires an extra 5 cables. Mesh networks are often implemented using wireless technologies, such as wireless access points communicating with each other to increase coverage.

Practice communicating using a mesh topology.

• Everybody needs enough cables to directly connect to every other person.
• Put each cable on a separate finger
• Send messages by tugging on the cable connected to the intended recipient.
• Verbally confirm when you receive a message.

If you closed your eyes, would you still know who sent you a message?

Bus Topology

In a bus topology, hosts connect to a single cable. The cable is shared by all hosts that connect to it. Essentially, the cables become one big piece of copper. When a host sends a message on a bus network, every host receives a copy of that message. Bus networks are no longer used for building local area networks for computers in homes or offices. However, the bus network is used in cars, trucks, and boats. In cars, the infotainment system might be connected to brake sensors, tire pressure sensors, backup sensors, and other electronic control units (ECUs) using a bus topology. The bus topology minimizes the amount of cable that cars need to be manufactured and therefore makes them lighter and more fuel efficient.

Practice communicating using a bus topology.

• Create loops in your main bus cable using slip knots. Here is a great video to learn to tie the slip knot.
• Loop your cable through the main bus cable loops. This simulates creating a physical connection to the main bus cable.

• Everybody should be connected to the shared main bus cable.
• Say a peer's name, and tug the rope to send a message. Who receives that message?

Practice again, but this time close your eyes.

• Say a peer's name and tug the cable.
• Periodically, become a malicious actor and tug the cable when somebody else is sending the message. Can the recipient tell who actually sent the message?

Reflection

• Why would vehicles implement bus topologies?
• Which topology would give you the most privacy?
• Which topology might give you the best performance?