# What is bit time in Ethernet CSMA/CD

"Propogation delay betwween two links is 200 bit times" I am little confused here, Propogation delay should be in seconds, milliseconds etc but here it is in bit times. I also want to know what is bit time and is it same as Propogation delay.

Bit time only calculates the ejection of one bit, it looks at how this bit transmits out of a network interface card at a determined speed. $$bit\_time = \frac{1}{NIC\_speed}$$ let NIC speed is $10Mbit/s$ then bit time = $$\frac{1}{(10 * 10^6)} = 10^{-7} = 100 * 10^{-9} = 100ns$$ Bit rate is different form propagation delay, as the propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver and bit rate is the number of bits that are conveyed or processed per unit of time.

• Ok but i have found one questio which says "The Propogation delay betwween A and B is 200 bit times" . but Propogation delay should be in seconds or millsec etc – JobLess Nov 2 '17 at 12:54
• propagation delay is $x$ bit times that means propagation delay equals to the amount of time to transmit $x$ bits. – Pragya Nov 3 '17 at 5:30

There are constraints on propagation delays expressed as "bit time" because the protocol, the collision detection, depends on the time each message lasts on the wires. Because the emitter must know if there is a collision to be able to re-transmit messages, having very long propagation delays, longer than message lengths, could hide collisions from the emitter.

This is not an absolute value as it is proportional to the data rate : Faster network need a smaller propagation delay than slower networks for CSMA/CD to work.

The automotive CAN protocol has similar constraints: the maximum data rate depends on the total length of the network for correct master arbitration.

You want values ? A 10Mbps Ethernet network has a nominal bit time of 100ns

A 1GHz Ethernet network has a nominal bit time of 1ns, but, because of encoding, there is no literally bits pushed serially at 1GHz : Sometimes it is 1.25GHz over optical fibres, sometimes it is a several bits per cycle at 125MHz over four twisted pairs, etc...

• I think you've misunderstood the question. As I see it, the question is "What does the term 'bit time' mean?" but you seem to be answering "How many seconds is 'one bit time'?" – David Richerby Aug 30 '18 at 9:27