Data Acquisition

of a cell as well, but these are ignored by the receiver. The transitions in every cell allowthe clock to be extracted, and synchronized with the transmitter.Figure 9.7Manchester encodingThe voltage swings were from –0.225 to –1.825 volts in the original Ethernetspecification. In the 802.3 standard, voltages on coax cables are specified to swingbetween 0 and –2.05 volts with a rise and fall time of 25 ns at 10 Mbps.9.3 Medium access controlEssentially, the method used is one of contention. As was described in the first section onthis architecture, each node has a connection via a transceiver to the common bus. As atransceiver, it can both transmit and receive at the same time. Each node can be in anyone of three states at any time. These states are:• Idle, or listen• Transmit• ContentionIn the idle state, the node merely listens to the bus, monitoring all traffic that passes. Ifa node then wishes to transmit information, it will defer while there is any activity on thebus, since this is the ‘carrier sense’ component of the architecture. At some stage, the buswill become silent, and the node, sensing this, will then commence its transmission. It isnow in the transmit mode, and will both transmit and listen at the same time. This isbecause there is no guarantee that another node at some other point on the bus has notalso started transmitting having recognized the absence of traffic. After a short delay asthe two signals propagate towards each other on the cable, there will be a collision ofsignals. Quite obviously, the two transmissions cannot coexist on the common bus, since