Data Acquisition

there is no mechanism for the mixed analog signals to be ‘unscrambled’. The transceiverquickly detects this collision, since it is monitoring both its input and output andrecognizes the difference. The node now goes into the third state of contention. The nodewill continue to transmit the jam signal for a short time, to ensure that the othertransmitting node detects the contention, and then performs a back-off algorithm todetermine when it should again attempt to transmit its waiting frames.When a frame is to be transmitted, the medium-access-control monitors the bus anddefers to any passing traffic. After a period of 96 bit times, known as the inter-frame gap,to allow the passing frame to be received and processed by the destination node, thetransmission process commences. Since there is a finite time for this transmission topropagate to the ends of the bus cable, and thus ensure that all nodes recognize that themedium is busy, the transceiver turns on a collision detect circuit whilst the transmissiontakes place. In fact, once a certain number of bits (576 bits in a 10 Mbps system) havebeen transmitted, provided that the network cable segment specifications have beencomplied with, the collision detection circuitry can be disabled. If a collision should takeplace after this, it will be the responsibility of higher protocols to request retransmission –a far slower process than the hardware collision detection process. Here is a good reasonto comply with cable segment specifications! This initial ‘danger’ period is known as thecollision window, and is effectively twice the time interval for the first bit of anytransmission, to propagate to all parts of the network. The slot time for the network isthen defined as the worst-case time delay that a node must wait, before it can reliablyknow that a collision has occurred. It is defined as:Slot time = 2 * (transmission path delay) + safety marginFor a 10 Mbps system, the slot time is FIXED as 512 bits or 64 octets.The transceiver of each node is constantly monitoring the bus for a transmission signal.As soon as one is recognized, the NIC activates a carrier sense signal to indicate thattransmissions cannot be made. The first bits of the MAC frame are a preamble andconsist of 56 bits of 1010 etc. On recognizing these, the receiver synchronizes its clock,and converts the Manchester encoded signal back into binary form. The eighth octet is astart of frame delimiter, and this is used to indicate to the receiver that it should strip offthe first eight octets and commence determining whether this frame is for its node byreading the destination address. If the address is recognized, the data is loaded into aframe buffer within the NIC. Further processing then takes place, including thecalculation and comparison of the frame CRC with the transmitted CRC, checking thatthe frame contains an integral number of octets and is neither too short nor too long.Provided all is correct, the frame is passed to the LLC layer for further processing.Collisions are a normal part of a CSMA/CD network. The monitoring and detection ofcollisions is the method by which a node ensures unique access to the shared medium. Itis only a problem when there are excessive collisions. This reduces the availablebandwidth of the cable and slows the system down while re-transmission attempts occur.There are many reasons for excessive collisions and these will be investigated shortly.The principle of collision cause and detection is shown in the diagram below.