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

傳播時(shí)延(propagation delay)這個(gè)概念，是指電磁信號(hào)或者光信號(hào)在傳輸介質(zhì)中傳輸?shù)臅r(shí)延，而在光纖或者銅線中，光信號(hào)和電磁信號(hào)的傳播速度都在20萬(wàn)公里/秒以上，在傳輸介質(zhì)中傳輸?shù)氖请姶判盘?hào)或者光信號(hào)，而不是數(shù)據(jù)！

發(fā)送時(shí)延(transmission delay)是指結(jié)點(diǎn)在發(fā)送數(shù)據(jù)時(shí)使數(shù)據(jù)塊從結(jié)點(diǎn)進(jìn)入到傳輸媒體所需的時(shí)間，也就是從數(shù)據(jù)塊的第一個(gè)比特開(kāi)始發(fā)送算起，到最后一個(gè)比特發(fā)送完畢所需的時(shí)間。發(fā)送時(shí)延又稱(chēng)為傳輸時(shí)延

而帶寬，也稱(chēng)媒體接入速率，即數(shù)據(jù)從結(jié)點(diǎn)進(jìn)入到媒體的速率。

傳播時(shí)延和要發(fā)送的數(shù)據(jù)大小沒(méi)有關(guān)系，發(fā)送數(shù)據(jù)的大小只與發(fā)送時(shí)延有關(guān)，因?yàn)榘l(fā)送時(shí)延=數(shù)據(jù)/帶寬，而傳播時(shí)延=發(fā)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(shí)延主要是與傳輸距離有關(guān)！而與發(fā)送的數(shù)據(jù)大小無(wú)關(guān)！

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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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