Basics of Data Transmission Across a Wire

In the figure, PC2 generates some electricity on the wire. In this case, PC2 wants to send the binary value 0101. So, it sends a 5-volt signal, then 10 volts, then 5 volts, and then 10, because the imaginary encoding scheme in this example states that 5 volts means 0, and 10 volts means 1. PC1, on the other end of the wire, senses the incoming electrical signal and interprets the electricity, using that same set of encoding rules to mean 0101, exactly as PC2 intended.

Note that the graph shown in Figure 4-3 shows a discrete, or constant, voltage. Because the X-axis (horizontal axis) represents time, when the voltage changes, it changes immediately to the next value. The use of discrete, constant values, which are then instantly changed to other possible discrete values (as in Figure 4-3), is called digital transmission. To transmit binary numbers, or binary digits, it is useful to transmit the data using digital transmission.

For the digital transmission of data to work correctly, not only must the sender and receiver agree to what electrical characteristics mean a binary 0 or 1, but they also must agree to the rate at which the bits are transmitted over the wire. In Figure 4-3, the receiver (PC1) must think about the electrical signal at different points in time, on a regular interval. Likewise, the sender (PC2) must use this same regular time interval to decide when it should change the digital electrical signal. For instance, if PC2 varied the voltage to mean either 0 or 1 every .1 seconds, and PC1 sampled the incoming electrical signal every .1 seconds, they could transfer 10 bits in a second. The speed of this network connection would be 10 bits per second.

If the two PCs did not agree on the transmission speed, the devices couldn't transfer the binary information. For instance, imagine that PC2 thought the speed was 10 bits per second, meaning it should encode a new bit every 1/10 of a second. If PC1 thought that it should be receiving a bit 20 times per second, it would sample the incoming electrical signal every 1/20 of a second. PC1 would think it was sending 10 bits each second, and PC2 would think it received 20 bits.

The term bps (short for bits per second) often refers to the speed of networking connections. Note that the unit is bits, not bytes. In real life, LANs typically run at much higher speeds, with a slow LAN transmitting at 10 million bits per second (Mbps, also called megabits per second).

Notice that Figure 4-3 represents electricity as a square waveform, with positive and negative voltages. You don't really need to worry about the electrical details, but as you progress through learning about networking, you will see other drawings like this one. The networking cards use an alternating current, or AC. The positive voltage means the current is in one direction, and the negative current means the current runs in the opposite direction.