Difference Between a Serial Signal and an Ethernet Signal

Different encoding techniques.

Serial signaling and Ethernet signaling are different techniques for transmitting bits using electrical signals on wires. Three basic characteristics to consider are: clock recovery, bit density and DC, or direct current, balance. Ethernet signaling uses Manchester encoding. Serial signals use one of three types of signaling: asynchronous, synchronous or Alternate Mark Inversion, referred to as AMI.

Serial Signaling

Serial signals usually use 9 pin connectors.

Synchronous and Asynchronous transmission both use NRZ, or Non Return to Zero, encoding to represent the bits. This uses two voltage levels. A bit value of one is represented by a high voltage, and a bit value of zero is represented by a low voltage. The highest transition density is sequence of ones and zeros, or an upward transition for one bit followed by a downward transition for the next bit. Each cycle, or hertz, contains two bits. Non Return to Zero coding is characterized by a bit density of two bits per hertz. The minimum bit density is a long string of similar bits and there would be no transitions. The transmit clock and receive clock might wander away from each other. Non Return to Zero encoding does not provide clock recovery.

Synchronous signaling

Synchronous signals must be coordinated.

Synchronous signals transmit bits at regular intervals all the time. One special bit pattern, the flag, separates the information into frames. If there is no information to transmit, flags are transmitted continuously. The bits must be synchronized from the transmitter to the receiver. Since NRZ does not provide clock recovery, a separate line must be provided to give clock signaling. Synchronous communication is normally used on serial lines between routers. Synchronous communications typically run at speeds of between 64 Kbps and 2 Mbps.

Asynchronous Communications

Asyncronous signals transmit whenever ready.

When there is nothing to send on an asynchronous line, the line voltage is held at the low level. When one end wants to transmit some information, it will begin the transmission with one or two start bits. This allows the receiver to synchronize the clock in order to correctly interpret the remaining bits. The data bits are then sent, usually eight bits, followed by a stop bit. Each eight bits represents one character. There is no need to synchronize transmission and so the term asynchronous applies. Personal computers use asynchronous communications by default. Asynchronous lines run much slower. Speeds from 9.6 Kbps to 115.2 Kbps are common.

Manchester Encoding

Manchester encoding provides clock recovery.

Ethernet signaling uses Manchester Encoding. Manchester encoding always has a transition in the middle of the bit time. This is an upward transition for a zero and a downward transition for a one. A transition might be required between bits. The highest transition density is a series of similar bits and requires one hertz per bit. Manchester encoding requires more bandwidth to transmit the same amount of data. Because Manchester encoding guarantees one transition in the middle of the bit time, it is possible to recover the clock from the signal and a separate clocking signal is not needed.

Serial T1 Lines

With AMI every second bit is upside down.

Serial T1 lines encode bits with Alternate Mark Inversion or AMI. This is a three level return to zero code where the zero bit, or space, is represented by a zero voltage level. The one bit, or mark, is represented by a positive or negative voltage. Each mark is inverted. So a sequence of one, zero, one, one, zero would be +5V, 0V, -5V, +5V, 0V. The highest transition density would be a sequence of ones, but because the ones are opposite polarity, the bit density is still two bits per hertz. AMI provides good bit density and is DC balanced. AMI does not provide clock recovery.

Binary Eight Zero Suppression

B8ZS provides clock recovery.

To provide clock recovery we need to guarantee a minimum transition density. Because AMI has coding has rules we can use regular violations of the rules to encode long strings of zeros. A sequence of eight zeros is encoded as "1 0 V 0 0 1 0 V." Here V represents a violation. These are two mark bits, or ones encoded with the same polarity. B8ZS allows clock recovery. T1 lines can be used as serial lines between routers, but they always run at exactly 1.544 Mbps.

References

• University of Aberdeen: Non Return to Zero Coding
• University of Aberdeen: Manchester Encoding
• Granite Island Group: Robbed Big Signalling.