ENVS 1200 Chapter Notes - Chapter 13: Ethernet Frame, 4B5B, Non-Return-To-Zero
13 May 2018
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ENVS 1200 Chapter 13 Notes – Summary
Introduction
• For practice, you might wish to create the waveform for the original example shown the
case of twelve zeros.
• An alternative encoding methodology is called block coding.
• Block coding adds additional bits to small blocks of data
• It then converts each block to a different block of data that supplies the required self-
clocking.
• At the receiving end, the blocks are converted back to the original data.
• Block coding is often used to compensate for shortcomings of other methods.
• A simple example will clarify the idea.
• 4B/5B is a block coding algorithm that compensates for a shortcoming of a method
called NRZ-I that loses synchronization if the data contains a lot of zeros.
• 4B/5B converts 4-bit groups into 5 bits for transmission using the conversion table
shown
• Shows the resulting encoding
• Block encoding has the additional advantage that it can detect certain errors.
• There are sixteen unused 5-bit blocks.
• If any of these blocks appear at the receiver, the system knows that there is an error.
• There are a number of different block codes in use.
• The block coding shown in this example is used for most 100 Mbps Ethernet
transmissions.
• In addition to self-synchronization, there must be a means to synchronize the data so
that the receiver knows the boundaries of each byte.
• Ethernet frames use an 8-byte preamble for this purpose.
• The pattern 10 is repeated twenty-eight times, followed by a start frame delimiter with
the pattern 10101011.
• As you know, digital signals can also be used to represent analog waveforms.
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