Network+ Certification Exam Tutorial:

The Bus Topology, Collisions, CSMA/CD, And Switches

By Chris Bryant, CCIE #12933

In a previous CompTIA Network+ certification exam tutorial on the bus topology, we discussed the pros and cons of bus topologies. We also briefly touched on a major issue with bus topologies, and that is the possibility of data colliding when two hosts on the shared segment simultaneously transmit data. If Host A and Host E on the following network segment send data at the same time, the data from both hosts will collide and therefore become unusable.

Token Ring networks allow us to give hosts priority over others when it comes to traffic, but Ethernet does not. Ethernet does have a set of standards and rules that help to lessen the number of collisions - Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

We say "CSMA/CD" so often that we forget the full name tells us exactly what's going on!

The first rule of CSMA/CD is this: Any host on the segment that wants to send data must check the segment to see if another host is already sending data. This is called "listening to the wire", and this is the Carrier Sense part of CSMA/CD.

All the hosts on the segment will do this - that's the Multiple Access part.

Since a host will quickly be able to detect any collision that its data is involved in... well, that's Collision Detection. If Host A and Host E listen to the wire at the exact same time, determine that no one else is sending data, and then send their data onto the segment at the same time, a collision will occur.

Collisions cause what we refer to as "noise", but is really a change in the voltage of the signals on the wire. The sending hosts detect this voltage change and basically say "Uh oh, I just sent data and now there's a collision. It must have been my data. I'll tell the other stations not to send data." The hosts that send the now-useless data will send a jam signal out to tell the other stations not to send data at this time due to the collision.

Both Host A and Host E need to retransmit their data, but the one thing we don't want at this point is for them to send their data simultaneously again! To avoid this, both hosts will start a random timer that determines how long it will be until they begin the entire CSMA/CD process again by listening to the wire. Since this random timer is set in milliseconds, the chance of both hosts setting this timer to the exact same value is remote at best.

This shared segment is one big collision domain - an area where collisions can occur.

As I mentioned in the bus topology tutorial, we don't see too many bus topologies anymore -- it's much more efficient to have a switch separate hosts. One of the many benefits of switching is achieved when each host is connected to its own dedicated switch port. Each switch port is a collision domain, so when hosts are all connected to their own port, collisions no longer occur. In the following network diagram, we have five hosts and five different collision domains. Collisions literally cannot occur.

Switches help break up collision domains, but they don't do the same for broadcast domains. We'll look at broadcast domains, the dangers they pose to our network, and how to break them up in a future CompTIA Network+ certification exam tutorial. See you then!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com