Practical TCP/IP and Ethernet Networking- P8: One of the great protocols that has been inherited from the Internet is TCP/IP and this is being used as the open standard today for all network and communications systems. The reasons for this popularity are not hard to find. | 52 Practical TCP IP and Ethernet Networking integral number of octets and is either too short or too long. Provided all is correct the frame is passed to the LLC layer for further processing. Collisions You should recognize that collisions are a normal part of a CSMA CD network. The monitoring and detection of collisions is the method by which a node ensures unique access to the shared medium. It is only a problem when there are excessive collisions. This reduces the available bandwidth of the cable and slows the system down while retransmission attempts occur. There are many reasons for excessive collisions and you will investigate these shortly. The principle of collision cause and detection is shown in the following diagram. Figure CSMA CD collisions Assume that both node 1 and node 2 are in listen mode and node 1 has frames queued to transmit. All previous traffic on the medium has ceased . there is no carrier and the interframe gap from the last transmission has timed out. Node 1 now commences to transmit its preamble signal which immediately commences to propagate both left and right on the cable. At the left end the termination resistance absorbs the transmission but the signal continues to propagate to the right. However the MAC sub layer in node 2 has also been given a frame to transmit from the LLC sub layer and since the node sees a free cable it too commences to transmit its preamble. Again the signals propagate on to Ethernet networks 53 the cable and some short time later they collide . Almost immediately node 2 s transceiver recognizes that the signals on the cable are corrupted and the logic incorporated on the NIC asserts a collision detect signal. This causes node 2 to send a jam signal of 32 bits of random data and then stop transmitting. In fact the standard allows any data to be sent as long as by design it is not the value of the CRC field of the frame. It appears that most nodes will send the next 32 bits of the data frame as a jam
