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ETHERNET TECHNOLOGY

Part 1: Frames, Collisions, and 10 Mbps LANs

by James Antonakos

Start ý Ethernet Frame Format ý The Interframe Gap ý Collision Or No Collision ý Detecting Errors ý Random Waiting Period ý 10-Mbps Ethernet ý 10BaseFý Errors In Ethernet LANS ý More To Come ý Sources and PDF

THE INTERFRAME GAP

The interframe gap is a self-imposed quiet time appended to the end of every frame. This idle time gives the network media a chance to stabilize, and other network components time to process the frame. Figure 2 shows a sequence of frames separated by the fixed-size interframe gap. For 10-Mbps Ethernet, the interframe gap is 9.6 ýs.

This corresponds to 96 bit times (divide 9.6 ýs by 100 ns per bit). Thus, the 576 bits of a minimum-length Ethernet frame are followed by 96 bit times of silence. Dividing 10 Mbps by 672 bits for each frame plus the interframe gap gives a frame rate of 14,880 frames per second (minimum sized frames). Because each frame is followed by 96 bits of silence, there are a total of 14,880 ý 96, or 1,428,480, bits of the 10-Mbps bandwidth (14.28%) lost as a result of the interframe gap. These calculations are shown in Figure 3.

When the frame size is larger, more of the available bandwidth is used (see Table 2). Note that as the frame size increases, the 96 bits of the interframe gap become less significant. The actual frame lengths in a real network will be constantly fluctuating, as shown in Figure 2, so the bandwidth utilization will be constantly changing.

Carrier sense multiple access with collision detection (CSMA/CD) is the technique used to share access to the available bandwidth. To describe the basic operation of CSMA/CD Iýll use the 10Base5 network shown in Figure 4. This network consists of five 500-m coaxial segments connected by four repeaters. This is the largest network possible using 10Base5 technology.

Two stations, A and B, are located at the farthest ends of the network. There may be many more stations connected to each segment, all competing for bandwidth using the same CSMA/CD method. Collectively, these stations operate in a collision domain. A collision domain is a portion of a LAN (or the entire LAN) where two or more stations transmitting at the same time will interfere with each other. For example, in Figure 4, the entire 2500-m network is a single collision domain, because of the use of repeaters between each segment that simply broadcast frames between segments. In general, all ports on repeaters or hubs are in the same collision domain. Ports on switches and routers operate in their own, individual collision domains. Letýs examine what occurs during good and bad frame transmissions in a collision domain.

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