今天听说一个说法，觉得很神奇“网线太短会有问题？”

自自在在

某物太短确实不行!
但貌似不是网线啊！:blink:

吃饭睡觉打东东

最大的问题就是浪费水晶头吧～？！

我学网络的时候从来没听说太短不行呀

zhdick

我的千兆交换机和路由器是叠在一起的，自己做了5CM长的网线连接。没任何问题

lws009

应该太短不会有问题的

woami

阻抗匹配的问题。太短信号会反射。

家用一般没事，服务器高负载场合影响网络质量。

clawhammer

的确不可以太短

agrantleung

教科书上说理论上最长100m，没有说最短哦~
理论上最短是0，相当于两个水晶头的触点直接连着，应该没问题吧~

xpressure7

对的，不能太短
http://cisco.ccxx.net/cisco/tech/buxian/2008/1016/17235.html
这里倒不是说网线长度，是指链路长度，就是2台设备间的网线总长，短了也不是不能用，只是有些参数过不了关。

BCM5788

阻抗匹配跟长短会有关系？不知道各位的传输线理论是哪儿学的？

clawhammer

100BaseT - Also known as fast ethernet. Uses RJ-45 connectors. Topologyis star. Uses CSMA/CD media access. Minimum length between nodes is 2.5meters. Maximum number of connected segments is 1024. Maximum number ofnodes per segment is 1 (star topology). IEEE802.3 specification.
IEEE802.3规定的，有意见吗？
http://www.comptechdoc.org/independent/networking/cert/nethwethernet.html

clawhammer

Various networking technologies specify that stations must be separatedby some minimum distance. Occasionally a customer is surprised aboutthe lack of a minimum length requirement for their CAT 5 cable thatconnects one Industrial Ethernet device to another.

Networkcable minimum length specifications arise because of an impedancemismatch between elements in the signal path. When electrical energy istransmitted through a cable, some of it is not absorbed by thereceiving station and is instead "reflected" back to the source. Thisreflected energy disturbs the purity of the signal -- and if the effectis of sufficient magnitude, the signal can be degraded to the extentthat its information is corrupted or even useless. If source anddestination transceiver impedances match the impedance of the cableitself, there is no reflected energy and signal energy is deliveredcompletely.

Reflected signal is always an issue in a bustopology because only the end devices have transceiver impedance valuesthat match the impedance of the cable. Mid-bus device transceivers musthave higher impedance values to avoid "loading" the cable (preventingproper signal delivery to devices further along the bus). Thus, amid-bus device transceiver presents an impedance mismatch to the cableand inevitably creates some degree of reflected energy. To keep thereflected energy to acceptable levels, a bus protocol will specify aminimum length of cable that separates two bus devices.

IndustrialEthernet uses a star topology in which devices exist only at each endof a cable. With this topology, both transceiver impedances match thecable impedance and no appreciable signal reflection occurs. This lackof signal reflection is true whether the devices are separated by 100 mor 100 mm. Therefore, there is no need to specify a minimum cablelength.
这里头说总线型才要注意最短长度，星形topology就不用管了

紫夜星辰

应该不会有太大问题吧

sonicxz

AD貓和路由的橋接呢？
神的马甲 发表于 2009-3-23 17:49

这里头说总线型才要注意最短长度，星形topology就不用管了
上面某人说了，跳线类算星形吧

你看机房，交换机上跳线都是20CM左右的就知道了

znai9

太短不行  家用的话一般不会有问题

sonicxz

太短不行  家用的话一般不会有问题
znai9 发表于 2009-3-23 20:36

我那40CM左右连路由的线
用螃蟹卡检测出来6M= =

clawhammer

桥接线可能算是总线型啊……
神的马甲 发表于 2009-3-24 03:04

星形，以前那种很古老的很多电脑接一条线上才是总线型