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

傳播時延(propagation delay)這個概念，是指電磁信號或者光信號在傳輸介質中傳輸的時延，而在光纖或者銅線中，光信號和電磁信號的傳播速度都在20萬公里/秒以上，在傳輸介質中傳輸的是電磁信號或者光信號，而不是數據！

發送時延(transmission delay)是指結點在發送數據時使數據塊從結點進入到傳輸媒體所需的時間，也就是從數據塊的第一個比特開始發送算起，到最后一個比特發送完畢所需的時間。發送時延又稱為傳輸時延

而帶寬，也稱媒體接入速率，即數據從結點進入到媒體的速率。

傳播時延和要發送的數據大小沒有關系，發送數據的大小只與發送時延有關，因為發送時延=數據/帶寬，而傳播時延=發送距離/傳播速率，在光纖，雙絞線，電纜等傳輸媒體中，傳播速率是固定的，因此，傳播時延主要是與傳輸距離有關！而與發送的數據大小無關！

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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.

(more…)

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