I am reading a document:

1.Transmission delay: (the time required for the host or router to send data frames, formula: transmission delay = data frame length / transmission rate).

2.Propagation delay: (The time it takes for electromagnetic waves to propagate a certain distance in the channel, formula: propagation delay: channel length / propagation rate of electromagnetic waves on the channel).

High-speed network link: The increase is the data transmission rate rather than the bit propagation rate on the link. Increasing the data transmission rate only reduces the data transmission delay.

I can not understand the High-speed network link increasing the data transmission rate only reduces the data transmission delay. in the 1 and 2 definition, the reduces should be Propagation delay.

2 Answers 2


"Bit propagation rate" is a bit misleading here as it jumbles up the different aspects.

When the data rate is increased, a transmitting interface can pump out the data faster and a data packet of a given size finishes outgoing transmission earlier.

The electromagnetic wave carrying the information travels (propagates) always at the same speed, depending on the medium but independent from the data rate. For most media (copper wire, fiber), this is roughly 2/3 of the speed of light c0. A medium's velocity factor tells you the fraction of c0 that is achieved. Of course, wireless communication through air is almost at 1.0; in a vacuum it's exactly 1.0.

The effective total delay is the transmission delay (also serialization delay) plus the propagation delay. The beginning of transmission overlaps with propagation, so it only counts once.

  • If a rephrasing helps at all, I usually describe the propagation delay by thinking of the beginning of the first bit of the frame: the propagation delay is the time between this first bit starting to leave the transmitter and it starting to arrive at the receiver: ~1.3 sec from earth to moon through (almost) vacuum; or 100 ms north pole to south pole on earth through fibre. The bits then take one bit time each to finish arriving.
    – jonathanjo
    Sep 8, 2018 at 10:25
  • @jonathanjo Exactly, well put.
    – Zac67
    Sep 8, 2018 at 12:02

Strange document your read, with strange terminology. Network delay consists of 3 parts:

  1. Queueing delay - the time needed for a packet to go through all buffers - OS buffer queue, network interface queues buffer. There always some queue because network speed is usually much lower than system bus speed.

  2. Serialization delay - time needed to code packet bits into line. Thats because telecom lines send data bit by bit from start of packet to end and line clocking frequency if fixed, like 100mbps line obviuosly can send hundred million bits per second no more no less.

Ser-delay = packet-bit-lenght / line-bit-speed

1500bytes * 8bits in byte / 64kbps = 1500 milliseconds.

It takes 0.0015sec to put 1500 bytes packet into 64kbps line

  1. Propagation delay - time needed for electrical or optical or electromagnetic wave signal to propagate over line. =Line_length / speed-of-light_in_line_medium

so if you increase the link bit speed, like it was 1G ethernet and you upgraded it to 10G ethernet you obviously will decrease part 2 - serialization delay, cause you have significantly increased the divider of formula in 2. Also it's usually can decrease queueing delay cause networks adapters with faster speeds has much faster CPUs or ASICs which flushes queues faster.

For short links like 10 meter of fiber optics between switch and server the most parts are 1 and 2. For WAN links between two cities, like 1500km, 3rd part plays role.

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