Where does a collision domain exist in a switched network?

Answer 1

In a LAN packet-switched environment, such as with an Ethernet network, the transmission of the data packets relies on packet switches, routers, and LAN cables. In a LAN, the switch establishes a connection between two segments only long enough to send the current packet. Incoming packets are saved to a temporary memory area or buffer in memory. In an Ethernet-based LAN, an Ethernet frame contains the payload or data portion of the packet and a special header that includes the media access control (MAC) address information for the source and destination of the packet. When the packets arrive at their destination, they are put back in order by a packet assembler. A packet assembler is needed because of the different routes that the packets may take. Bottom Line is a router breaks up a number of computers into segments. So the collision domain is all the computers in that segment. The router prevents data traveling to the other segment when it is not addressed to that segment.

Answer 2

Through that long answer, Rtrahan is correct: the collision domain is the segment to which the computers in question are attached. What causes a collision is when two computers try to transmit at the same time in an "uncontrolled" environment. Since most networks nowadays use switches, collisions are very rare.

In the old days when networks used concentrators instead of switches, the various computers would listen to the cable plant to see if anyone was transmitting. If not, then the computer would transmit. If two (or more) computers listened at the same time and heard nothing and then started transmitting at the same time, a collision would occur and the concentrator would sense this. It would then respond by generating a "jam signal" which would cause all machines to reset their delay clocks to a random interval and after that interval expired for one computer, it would listen to see whether anyone else was transmitting and then would proceed with its communication. On and on the process went. As you can probably guess, with more and more computers attached to the network, the slower and slower communication became because of the frequency of collisions. This type of network design is referred to as Carrier Sense Multiple Access/Collision Detect (CSMA/CD) and is very inefficient for active networks. This is why the network switch gained so much popularity.

In contrast to the above, the network switch doesn't allow stations to broadcast their transmissions across all ports the way that concentrators did because it keeps track of which machines are available through which ports. Because of this, it can set up virtual isolated circuits between the two stations' ports so that only those two stations that need to communicate are privy to those transmissions. Since this effectively isolates each machine from the others when each is transmitting, there is no longer an issue with collisions unless, of course, something is amiss with the network switch or configuration of the network.

The introduction of network switches increased the average throughput of networks dramatically. In my professional experience, I have seen speeds increase as much as tenfold by getting rid of concentrators.

If you have other questions about collision domains or other network design issues, feel free to drop me a line.