What is HDB3?
It is a kind of line coding .for synchronizing the trans and receive.
With in an exchange, you can run a separate clock wire between trans and receive.
But exterrnally, that is , when the distance between trans and receive is more, you cannot run a separate wire. Instead, the data being transmitted is modified so that the receiver will extract the clock signal from the data itself. To modify this data, different methods are employed. Out of them hdb3 is one of them.
Where HDB3 is used?
For example, in E1 streams , that is 2mbps links, hdb3 is used as line coding.
HDB3 stands for?
It stands for High Density Bipolar 3. It is bipolar, that means, positive and negative will be there in the signal form. (if only the positive and zero are there, it is not bipolar). The number of continuous zero is restricted to 3.
Can you explain hdb3?
It is an improved for AMI. AMI stands for alternate mark inversion. here, mark means ONES (space means ZEROS). In AMI if you want to transmit
Why they use the word MARK AND SPACE?
In the olden days,to transmit ONE, they will press a key and at the receiving end, a stylus will come down and mark a line on a continuously moving paper. When there is zero, there will be space on the paper.
How i will transmit 1111 using ami?
+ - + -
How 1010 will be transmitted in ami?
+ 0 - 0
How 0000 is transmitted in ami?
0 0 0 0
What is the drawbacks of ami, why we should go for hdb3?
In ami, continuous zeros are possible, for example, if you transmit 000000, then in the medium, you will be trasmitting 0volt continuously for 6 times. If such thing happens, then synchronization will be lost.
In case of hdb3, the number of continuous zeros are restricted to just 3.
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In case of Digital modems, I was seeing LOCAL ALARM glowing at times. What is the meaning of LOCAL ALARM?
Local alarm need NOT be local problem. The local sync is lost. That is sync with the other end is lost. The reasons may be ; medium is faulty or remote modem may be faulty.
But how to confirm, my end is ok?
In case of RAD modem, if you activate local analog loop-back and if the alarm goes out, then your end is ok.
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