Data Communications Networking Devices - 4th Ed.pdf

844 ______________________________________________ CSMA /CD NETWORK PERFORMANCEFigure E.1IEEE 802.3 Ethernet) frame formatdelimiter ®eld were combined into a common eight-byte preamble ®eld for simplicity.In actuality, the preamble ®eld used by Ethernet is an eight-byte sequence ofalternating 1's and 0's, while the IEEE 802.3 frame format uses a seven-bytepreamble ®eld of alternating 1's and 0's. The start of frame delimiter one-byte ®eldused in IEEE 802.3 frames follows a seven-byte preamble ®eld and the sequence ofalternating 1's and 0's but ends with two set bits instead of the 1 and 0 used in theEthernet preamble ®eld. Since computations required to estimate Ethernet networkperformance are based upon frame length and not frame composition, the use of acommon eight-byte preamble ®eld, while not technically correct on a framecomposition basis, does not affect our computations. As indicated bythe tabulationof frame ®eld lengths in the lower portion of Figure E.1, the frame size can varyfrom a minimum of 72 to a maximum of 1526 bytes.Under Ethernet and 802.3 standards there is a dead time of 9.6 ms betweenframes. Using the frame size and dead time between frames you can compute themaximum number of frames per second that can ¯ow on an Ethernet network. Forour example, let us assume we have a 10 Mbps LAN, such as a 10BASE 2,10BASE 5, or 10BASE T network. Here the bit time then becomes 1/10 7 secondsor 100 ns.Now let us assume all frames are at the maximum length of 1526 bytes. Then, thetime per frame becomes:9:6s ‡1526 bytes 8 bits 100 nsbyte bitˆ 1:23 msSince one 1526-byte frame requires 1.23 ms then in one second 1000 ms) there canbe 1000/1.23 or approximately812 maximum-sized frames. Thus, the maximumtransmission rate on an Ethernet network is 812 frames per second wheninformation is transferred in 1500-byte units within a sequence of frames. Oneexample of a situation in which data would be transferred in 1500-byte units is