The situation ....
shared bus, 10Mbps
A B C D
10Mbps 10Mbps 10Mbps 10Mbps
A logical bus topology with station B transmitting a frame to station D.
Your queries...
1) Data is framed by the data link layer & handed over to the NIC at the physical layer. Assuming it's OK for the station to transmit, what is the job of the NIC when it's transmitting >
The answer...
When it's OK to transmit , the NIC send out the frame onto the bus and also puts a copy of the frame on it's loopback interface, in order for it to detect any collisions that might occur.
2) How does a signal travel in a logical bus topology ?
The answer...
Assuming that you're using a 'baseband' logical bus topology, the signals will be sent out in bidirectional way i.e. in both directions from the workstation transmitting the signal. (Broadband is unidirectional.)
3) All stations constantly "listen" for a signal. This is called "Carrier Sensing" What is a NIC supposed to do when it senses a signal on the wire? Differentiate the actions taken by Station D & station A & C.
The answer...
When the NIC senses a signal on the wire, it should not transmit and hold back till there is no signal on the wire. This is done in order to avoid collisions.
Station D would look into the Destination Address of the frame being transmitted by Station B and see that it is its own address. Therefore, Station D would copy the frame off the wire and process it accordingly by removing the Ethernet hearder etc. Stations A & C would also see the frame and copy it off the wire but after looking into the Destination Adress, both A & C would drop the frame since it would not match with their addresses. (Address = MAC address)
4) Let's say that station C has something to transmit simultaneously with station B but B has already begun transmitting. The first step for sharing the medium is for a station to "listen". What is purpose of listening first? When you listen and somebody else is transmitting, what do you do ?
The answer...
The purpose of listening is to avoid collisions. When you listen before transmitting, you can check whether the medium (wire) is free of signals or not.
When somebody else is transmitting, you hold back and do not transmit until the medium is free of signals.
5) Let's say that both B & C had listened and nobody was transmitting and had simultaneously begun to transmit. What is this condition called ? How is it resolved? Why wouldn't this problem ever happen on a Token Ring network? How does Ehternet switching avoid this problem ?
The answer....
When both B & C transmit simultaneously, a condition called 'collision' would occur. A collision can be resloved by listening for signals on a medium.
Token Rings work on a collision prevention principle. A token is used to control access to the medium. Only the node having possession of the token is allowed to transmit. The token constantly circulates throughout the ring. A Station which wants to transmit must first seize the token. Once it has successfully done this, it may transmit data for a period of time after which it must put the token back onto the ring.
An Ethernet switch divides a network into 'multiple collision domains' and a 'single broadcast domain'. A switch, 'switches' the signal directly to the destination node by looking at the destination address in the frame (this is done by maintaining a mapping of each MAC addresses to the port from which the MAC address was learnt) and so the switch does not has to broadcast the frame to every single node.
