What is 8B10B Line Encoding

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http://www.fiberoptics4sale.com/wordp... • • Hello everyone, this is Colin from Fiber Optics For Sale. • • In this video, I will explain what is 8b/10b encoding. • • 8b/10b encoding is used by Fiber Optic Gigabit Ethernet. In this coding, each 8 bit data is mapped to a 10 bit data, and then transmitted to the receiving side. Let's see why we need to go through this extra step. • • If we want to understand the 8b/10b mapping process, we have to understand the problems of directly transmitting baseband data. • • Here we can see an example of raw baseband digital data. We have a long sequence of 0s. You may also get a long sequence of 1s. They both present the same problem, there is not enough transitions, which means the jumping from 0 to 1, and from 1 to 0. Transitions are very important for the receiver to correctly recover the data clock. Without a correct clock, you will get a very high error rate. • • Another problem is that the baseband data is not optimized for the transmission medium. The transmission medium may be UTP copper cable or fiber optic cable. • • So that means we should not directly transmit the baseband data to the line. That is when line codes come to the rescue. • • What is exactly line code? Line code is developed to solve the problem we just talked about. • • Line code is basically a digital data coding process. In this process, a line code is chosen for a particular transmission system. This line code is optimized for the physical properties of the medium. A line code will typically reflect technical requirements of the transmission medium, such as optical fiber or twisted pair copper cable. • • The line code should make it easy to recover the clock from the data, it should have no DC part and small low frequency part and have high transmission efficiency. • • There are many types of line code used for different purposes. 8b/10b is one commonly used for fiber optic communication systems such as optical gigabit Ethernet. Other line codes include RZ, NRZ, AMI, Manchester Code, and more. • • Now let's take a closer look at 8b/10b line code. • • 8b/10b line code is based on 5b/6b and 3b/4b codes. Now let's see how a 8 bit byte is encoded. • • In the left figure, the hexadecimal word 45 is encoded as a 8 bit byte. 4 is coded as 0100, it occupies position EFGH. 5 is coded as 0101, it occupies position ABCD. The 8 bit byte is then split to a 5 bit ABCDE and a 3 bit FGH group. • • The positions of these two groups are then switched in the transmission process. ABCDE is moved to the front, and FGH is moved to the back. • • Then comes the 8b/10b coding process as shown in the right side picture. A i bit joins the 5 bit ABCDE group. And a j bit joins the FGH group. • • Here we have to understand the concept of Running Disparity. Running disparity means the difference between the number of 1 bits and the number of 0 bits in the data. • • The value of i bit and j bit is set on the fly depending on the current running disparity. We will see clearly how this works in the next slide. • • Here is the mapping table of 5b/6b and 3b/4b. • • In the 5b/6b table, if the raw input data has ABCDE bit of 00000, it is mapped to either 100111 or 011000. Which one is chosen depends on the running disparity. • • If the previous running disparity is -1, which means there was one less 1 bit than 0 bit, then 100111 is chosen, so in this code it has 2 more 1s than 0s. -1 plus 2 equals 1, so the running disparity becomes +1. • • If the previous running disparity is +1, which means there is one more 1 bit than 0 bit, then 011000 is chosen, so in this code it has 2 less 1s than 0s. 1 plus minus 2 equals -1, the running disparity becomes -1. • • In the 3b/4b mapping table, it follows the same procedure. • • This running disparity process makes sure the difference between the number of 1s and 0s is always between 1 and -1. And the 5b/6b and 3b/4b mapping process makes sure there is frequent transitions from 1 to 0 and from 0 to 1, so the clock can be easily recovered on the receiver side. • • 8b/10b line coding has many benefits than directly transmitting baseband data. Here is a few listed. • • 8b/10b coding provides frequent transitions for easy clock recovery on the receiver side. • • It provides a balance between the number of one bits and zero bits transmitted. This is called DC balance. • • DC balance makes sure the optical laser is maintained at around 50% duty cycle for best performance and power usage. • • DC balance puts a limit on the DC offset on the receiver side which helps maintain a good level detect thresholds. • • So there you have it. Please leave your comment below if you'd like to see other topics. • • Don't forget to visit http://www.fiberoptics4sale.com for more free fiber optic tutorials. • • I will see you in the next video!

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