At 08:21 PM 10/22/01, xie rootstock wrote: >why 2 repeaters only gains five meters of distance?simply because it >increases the number of ports avaiable in the system > >from lan switching
It sounds like you're reading a book on switching when you should be reading a book on basic Ethernet. Look for books by Charles Spurgeon. They are terrific. Also his Web site here is great: http://www.ots.utexas.edu/ethernet/ Also I wrote a paper on Ethernet for http://www.certificationzone.com, which I will quote from below. As far as 2 repeaters and 5 meters, I guess you must be referring to the typical example used for 100BaseT with two class II repeaters. The examples usually say that there can be only 5 meters between the repeaters. This assumes that the length of cabling from the repeaters to end stations is 100 meters. The max collision domain for 100BaseT with copper cabling is 205 meters. You could use more than 5 meters between the repeaters if you weren't already using 100 meters for each node (or if you used fiber-optic cabling). The reason the length of cabling between the repeaters is short is that "the round-trip propagation delay in one collision domain must not exceed the time it takes a sender to transmit 512 bits, which is 51.2 5s for 10-Mbps Ethernet, but only 5.12 5s for 100-Mbps Ethernet. A single collision domain must be limited in size so that a station sending a minimum-sized frame (64 bytes or 512 bits) can detect a collision reflecting back from the opposite side of the network, while the station is still sending the frame. Otherwise, the station would be finished sending and not listening for a collision, thus losing the efficiency of Ethernet to detect a collision and quickly retransmit the frame. For a higher layer to notice that the frame needs retransmitting takes much more time. Ethernet retransmissions occur within nanoseconds. With some exceptions, 100-Mbps Ethernet is simply standard Ethernet, just ten times faster. In most cases, delay parameters for 100-Mbps Ethernet are the same as 10-Mbps, just ten times smaller. As mentioned before, the round-trip propagation delay in one Ethernet collision domain must not exceed the time it takes a sender to transmit 512 bits, which is only 5.12 5s on 100-Mbps Ethernet. To make 100-Mbps Ethernet work, there are more severe distance limitations than those required for 10-Mbps Ethernet. Distance limitations for shared 100-Mbps Ethernet depend on the type of repeaters (hubs) that are used. In the IEEE 100BaseT specification, two types of repeaters are defined: Class I repeaters have a latency of 0.7 5s or less. Only one repeater hop is allowed. Class II repeaters have a latency of 0.46 5s or less. One or two repeater hops are allowed. Many Ethernet topologies are possible as long as the round-trip propagation delay does not exceed 512 bit times." If this is a problem, then break up your collision domain (use switches). Priscilla >please help me again ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=23827&t=23824 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
