
Networking Speeds
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This table normalizes.
Kbps = Kilo bits per second
WDM = Wave Division Multiplexing
To calculate how long it would take to download a 100KB file, multiply your file's size by 8 to get the number of bits, then divide by the speed in kilobytes. Example: 100Kb file * 8 / 56Kbs = 14 seconds. To avoid excessive Propagration delays on 802.3 networks, observe the 543 Rule: No more than 5 segments of cable, connected by 4 nonfiltering repeaters, where only 3 of the segments have computers attached. A network's physical characteristics are described using these values:
d = distance of network in meters (10 m, 100 m, 100 km) V = propagation velocity of signal: (about 2 ´ 10m for copper wire) L = length of frame in bits (for instance, 1000 b, 5000 b) (R * d) / V defines the number of bits that can be on the wire at any time. For a 10 Mbps Ethernet segment that is 500 m long, the bit length is 25. Since a sending device will want to put its entire frame on the wire, a useful way to think about the length of the network in bits is to put it in relation to the size of the frame that is being transmitted on the wire. The variable a represents the length of the medium in relation to the frame size:
If a = 1, then the length of the physical medium is equal to the length of one frame. Usually, a is much less than 1 for LANs, however. This means the frame is much longer than the length of the medium in bits. In the Ethernet example used above, the length of the wire was 25 bits, but frames are typically much larger than this on LANsthousands of bits, thus a = 0.01 to 0.1.

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