At 12:50 PM 4/11/02, Jeffrey Reed wrote: >In theory, doesn't electricity travel at the speed of light? 186,000 feet >per second, I think.
Miles per seconds! ;-) As Wes said, "The rough calculation for the propagation of an electromagnetic signal in wire is 66% of the speed of light in a vacuum. Or 186000 x 2/3 = 124000 miles/sec." Here's what we'll be saying in our new book. (It is copyrighted, so don't steal the exact wording please, although the concepts are certainly universal! ;-) "A signal in a cable propagates at roughly 2/3 the speed of light in a vacuum. The value 0.59c is used for twisted-pair cabling, where c stands for the speed of light in a vacuum, which is 300,000 km/s, or 186,282 mi/s. To determine how far a signal can travel in 1 second, use the following equation: 300,000 km/sec * 0.59 = 177,000 km/sec In 1 second, a signal can propagate 177,000 km. In that same second, there can be 10 million bits on a 10-Mbps Ethernet network. Hence, the electrical energy associated with a single bit stretches out 0.0177 km or 17.7 meters. A single bit is a very long thing on a cable. The mental image that many of us have of bits looking like numerous little box cars in a railroad train, moving along a cable, is not very accurate. The 100-meter cable between a workstation and a wiring closet, for example, contains only about five discrete bits." (By the way, this has some real-world relevance in Ethernet CSMA/CD. You know how you always hear that a single collision domain must not exceed the round-trip propagation delay of a minimum sized-frame, 512 bits? The info above can help you figure out if your network exceeds that or not.) Priscilla >I once read theory that electricity flows through copper more efficiently >than light flows through fiber, so the electrical bits would arrive first in >a race. > >I have both interfaces here in the lab... I'll time it this afternoon... >(just kidding). > >Jeffrey Reed >Classic Networking, Inc. > >-----Original Message----- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of >Matthew Tayler >Sent: Thursday, April 11, 2002 12:01 PM >To: [EMAIL PROTECTED] >Subject: How fast do bits travel ? [7:41192] > >Ok I have spent ages trying to find an answer to this question, and probably >only added to my confusion. You know how it is you spend ages looking at >something and become snow blind or get tunnel vision or whatever, but I >cannot see the answer to the following: > >How far does a bit travel in say 1 second or put another way how long does a >bit take to travel a certain distance ? > >I understand, or think I do that if the line is say 128kbps then I can, in >theory at least, expect 128,000 (approx) bits start down that line every >second. > >But how long do they take to reach the other end, assuming a point to point >link and both ends being the same speed, obviously. > >There has to be a nice simple formula for this somewhere, you know the sort >of thing x= line speed, y = distance z = time etc > >Any ideas or poitners would be appreciated > >Thanks ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=41216&t=41192 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
