he same number and if the remainder is nonzero errors have occurred.] If errors are detected, the frame is discarded by the MAC layer and it is left up to the upper protocol layers to recover this situation. Note that if a frame is in error we cannot be sure that the destination address is correct and we may have received a frame that does not belong to us. An IEEE 802.3 MAC frame (type/length 0000-05DC) data field does not give any information about the network layer protocol. However, it indi- cates that the data link layer contains an upper sublayer, LLC, on the top of MAC, carried in the data field. Network layer protocol is identified in the LLC PDU (in the MAC data field), which contains destination service access Introduction to Telecommunications Network Engineering point (DSAP) and source service access point (SSAP) numbers that define net- work layer protocol in the source and the destination machine. 6.5.5 CSMA/CD Collision Detection Suppose that two stations both begin to transmit at the same time to the same cable. The minimum time needed to detect collision is the signal propagation time from one station to the other. However, in the worst-case scenario, the station cannot be sure that it has seized the cable until after two times the propagation delay. This is the case because the far-end station may transmit just before receiving the signal from the distant station. Then it takes another end-to-end propagation delay until this transmission is detected at the distant transmitting station. As a conclusion, a station can be sure that it has seized the cable and transmitted successfully after two times the worst-case propagation delay. As a consequence, to detect collision (before the transmission is finalized), the shortest frame has to be longer than two times the worst-case propagation delay. In the case of 10-Mbps coaxial network, the minimum frame length is 64 bytes and correspondingly the maximum length of the network is 2.5 km. The propagation speed in coaxial cable is approximately 70% of the 