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| 2.1 |  | Using a Layered Architectural Model to Describe Data Flow |
| 2.1.2 |  | Bits on the physical medium |

The Ethernet receiver derives the clock rate from the incoming data stream. Using a direct signal encoding of 0 volts for a logic 0 value and 5 volts for a logic 1 value could lead to timing problems. Specifically, a long string of 1s or 0s could cause the receiver to lose synchronization with the data. Further, the recipient would be unable to determine the difference between an idle sender (0 voltage) and a string of 0s (again 0 voltage).

The solution for this dilemma is found in the Ethernet encoding scheme. Rather than transmitting the logic level directly, Manchester encoding is used. With this technique, one transition is guaranteed for each bit cycle:

With a Manchester encoded signal, a binary 1 is represented by a change of amplitude from a low to a high during the middle of a bit-time. Conversely, a binary 0 is represented by a change of amplitude from a high to a low during the middle of a bit-time.

However, the trade-off for this synchronization technique is that twice the signaling bandwidth is required, since there must be two pulses for every bit transmitted. As a result, 10-Mbps Ethernet actually works with a 20 MHz serial data signal.

Data moving through the physical layer medium from the source to the destination is the end product of the encapsulation process.