求帮忙~~~~~~~~~

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wzlwin
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求帮忙~~~~~~~~~

#1

帖子 wzlwin » 2010-08-10 23:59

能告诉我这图怎么理解吗?
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sunjacker
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Re: 求帮忙~~~~~~~~~

#2

帖子 sunjacker » 2010-08-11 0:02

:em20 不是明明白白码
1的 十进制 是 1

1的 二进制 是 1(0001)

啥啥 4b 5b 也是那样
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Hello World!
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Re: 求帮忙~~~~~~~~~

#3

帖子 Hello World! » 2010-08-11 0:24

以前学过,现在都忘了。应该和容错有关。
http://noie.name 网站改版中。
wzlwin
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Re: 求帮忙~~~~~~~~~

#4

帖子 wzlwin » 2010-08-11 13:26

sunjacker 写了::em20 不是明明白白码
1的 十进制 是 1

1的 二进制 是 1(0001)

啥啥 4b 5b 也是那样
我是不明白那个4B/5B码那栏怎么对映前面的,到底有什么规律呀
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Re: 求帮忙~~~~~~~~~

#5

帖子 sunjacker » 2010-08-11 13:31

4B/5B编码是一种冗余编码体制,用5比特的码组来编码4比特的输入数据。这种编码方式要求每个5比特码组中不含多于3个“0”,或者不会少于2个“1”。将5比特码组转换成电信号的波形采用了NRZI,即不归零制,且在每个比特“1”的持续期间的开始处都有电平跳变.该编码方法已广泛用于100Mbps以太网和FDDI环形网.

:em20

好复杂阿 不懂

In telecommunication, 4B5B is a form of data communications line code. 4B5B maps groups of four bits onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock transitions for the receiver. For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits. These 5 bit words are pre-determined in a dictionary and they are chosen to ensure that there will be at least two transitions per block of bits.

A collateral effect of the code is that more bits are needed to send the same information than with 4 bits. An alternate to using 4B5B coding is to use a scrambler. Depending on the standard or specification of interest, there may be several 4b5b characters left unused. The presence of any of the "unused" characters in the data stream can be used as an indication that there is a fault somewhere in the link. Therefore, the unused characters can actually be used to detect errors in the data stream.

4B5B was popularized by Fiber distributed data interface (FDDI) in the mid-1980s, and was later adopted by

100BASE-TX standard defined by IEEE 802.3u in 1995.
AES10-2003 MADI [1] (Multichannel Audio Digital Interface)
The name "4B5B" is generally taken to mean the FDDI version. Other 4-to-5-bit codes have been used for magnetic recording are known as group code recording (GCR).

On optical fiber, the 4B5B output is NRZI-encoded. FDDI over copper (CDDI) uses MLT-3 encoding instead, as does 100BASE-TX.
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