原文：
http://wzw19191.blog.sohu.com/135110823.html

传播时延(propagation delay)这个概念，是指电磁信号或者光信号在传输介质中传输的时延，而在光纤或者铜线中，光信号和电磁信号的传播速度都在20万公里/秒以上，在传输介质中传输的是电磁信号或者光信号，而不是数据！

发送时延(transmission delay)是指结点在发送数据时使数据块从结点进入到传输媒体所需的时间，也就是从数据块的第一个比特开始发送算起，到最后一个比特发送完毕所需的时间。发送时延又称为传输时延

而带宽，也称媒体接入速率，即数据从结点进入到媒体的速率。

传播时延和要发送的数据大小没有关系，发送数据的大小只与发送时延有关，因为发送时延=数据/带宽，而传播时延=发送距离/传播速率，在光纤，双绞线，电缆等传输媒体中，传播速率是固定的，因此，传播时延主要是与传输距离有关！而与发送的数据大小无关！

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http://answers.yahoo.com/question/index?qid=20090218195707AA8qyoK

transmission delay

In a network based on packet switching, transmission delay (or store-and-forward delay) is the amount of time required to push all of the packet’s bits into the wire. In other words, this is the delay caused by the data-rate of the link.

Transmission delay is a function of the packet’s length and has nothing to do with the distance between the two nodes. This delay is proportional to the packet’s length in bits,

It is given by the following formula:

DT = N / R
where

DT is the transmission delay
N is the number of bits, and
R is the rate of transmission (say in bits per second)
Most packet switched networks use store-and-forward transmission at the input of the link. A switch using store-and-forward transmission will receive (save) the entire packet to the buffer and check it for CRC errors or other problems before sending the first bit of the packet into the outbound link. Thus store-and-forward packet switches introduce a store-and-forward delay at the input to each link along the packet’s route.

http://www.answers.com/transmission%20de…

propagation delay

The time required for a signal to pass through a given complete operating circuit; it is generally of the order of nanoseconds, and is of extreme importance in computer circuits.

The time it takes to transmit a signal from one place to another. Propagation delay is dependent solely on distance and two thirds the speed of light. Signals going through a wire or fiber generally travel at two thirds the speed of light.

Networking
Propagation delay is defined as the amount of time it takes for a certain number of bytes to be transferred over a medium. Propagation delay is the distance between the two routers divided by the propagation speed.

Propagation delay = d/s where d is the distance and s is the speed.

In electronics, digital circuits and digital electronics, the propagation delay, or gate delay, is the length of time starting from when the input to a logic gate becomes stable and valid, to the time that the output of that logic gate is stable and valid. Often this refers to the time required for the output to reach 50% of its final output level when the input changes. Reducing gate delays in digital circuits allows them to process data at a faster rate and improve overall performance.

The difference in propagation delays of logic elements is the major contributor to glitches in asynchronous circuits as a result of race conditions.

The principle of logical effort utilizes propagation delays to compare designs implementing the same logical statement.

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