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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

COLLISION OR NO COLLISION

A station wishing to transmit a frame first listens to the coax, waiting for an idle period indicating no transmissions. After the coax is quiet for a time equal to (or longer than) the interframe gap, the station begins transmitting the frame, one bit at a time. The electronic signal representing each bit travels at a limited speed within the thick-wire coax, requiring 10.8 ýs worse case to travel the 2500 m from station A to station B or vice versa. This time is based on the speed coefficient of the cable (0.77 for thick-wire coax), the length of the cable, and the speed of light (300,000,000 m per second). The signal is absorbed at the end of each coaxial segment by the terminating resistor. There are no reflections to cause a collision, and no other stations begin transmitting during the frame. One or more stations receive and process the frame.

At some point during the transmission of one stationýs frame, a new station might begin transmitting its own frame. Their electronic signals eventually meet up with each other (a collision), causing signal and energy distortions, which are sensed by the Ethernet transceivers. All affected stations then output a jam sequence and begin a random waiting period before transmitting again. The random period is used to help prevent the same stations from re-colliding with each other and increases exponentially with successive collisions of the same frame.

It is important to note that the only way a station can detect a collision with its own frame is for the station to detect the collision while it is still transmitting. If the station finishes transmission and then a collision occurs, the station has no way of knowing if its frame was involved in the collision or if two other stations had frames colliding. Because the diameter of the 10Base5 network from Figure 4 is limited to 2500 m, the worst-case round trip time of a signal is 21.6 ýs (not including the delays associated with the four repeaters).

Now, letýs consider why the round-trip time is important to collision detection. Figure 5 shows an example time line of how station A detects a collision with a frame from station B. In Figure 5a, station A has listened to the coax, found it idle, and has begun transmitting its frame. Station B is also listening and finding the coax idle. The signal representing the station A frame travels towards station B, taking at least 10.8 ýs for the 2500-m trip, ignoring the delays of the four repeaters. Now, just as the signal is about to reach station B, station B begins transmitting its own frame (recall that station B also found the coax free of transmission). An instant later the collision occurs, indicated by the time T1 in Figure 5b. A distorted signal begins traveling back to station A, requiring an additional 10.8 ýs (ignoring repeater delays again) to travel another 2500 m. At time T2 in Figure 5c, station A detects the collision. The total time is 21.6 ýs for the round trip.

With each bit requiring 100 ns, this corresponds to 216 bits (27 bytes) transmitted during the round trip. So, an Ethernet frame must be at least 27 bytes long to detect a collision. But, remember that you ignored the delays associated with the four repeaters. The IEEE 802.3 Standard limits repeater delays to 8-bit times. This adds up to a total of 64 bits for the round trip, or another 8 bytes of length must be added to the Ethernet frame for collision detection. This gets us to 35 bytes for the minimum frame. Because an actual Ethernet frame is at least 72 bytes long, there is plenty of time for a round-trip collision signal to be detected.

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