Propagation Delay vs Transmission delay

Because they're measuring different things.

Propagation delay is how long it takes one bit to travel from one end of the "wire" to the other (it's proportional to the length of the wire, crudely).

Transmission delay is how long it takes to get all the bits into the wire in the first place (it's packet_length/data_rate).

Obviously , packet length * propagation delay = trasmission delay is wrong.

Let us assume that you have a packet which has 4 bits 1010.You have to send it from A to B.

For this scenario,Transmission delay is the time taken by the sender to place the packet on the link(Transmission medium).Because the bits(1010) has to be converted in to signals.So it takes some time.Note that here only the packet is placed.It is not moving to receiver.

Propagation delay is the time taken by a bit(Mostly MSB ,Here 1) to reach from sender(A) to receiver(B).

Transmission Delay : Amount of time required to pump all bits/packets into the electric wire/optic fibre.

Propagation delay : It's the amount of time needed for a packet to reach the destination.

If propagation delay is very high than transmission delay the chance of losing the packet is high.

The transmission delay is the amount of time required for the router to push out the packet.

The propagation delay, is the time it takes a bit to propagate from one router to the next.

the transmission and propagation delay are completely different! if denote the length of the packet by L bits, and denote the transmission rate of the link from first router to second router by R bits/sec. then transmission delay will be L/R. and this is depended to transmission rate of link and the length of packet.

then if denote the distance between two routers d and denote the propagation speed s, the propagation delay will be d/s. it is a function of the Distance between the two routers, but has no dependence to the packet's length or the transmission rate of the link.