# Multiplexer, Stuff Bits

Suppose a multiplexer has two input streams, each at a nominal rate of 1 Mbps. To accommodate deviations from the nominal rate, the multiplexer transmits at a rate of 2.2 Mbps as follows. Each group of 22 bits in the output of the multiplexer contains 18 positions that always carry information bits, nine from each input. The remaining four positions consist of two flag bits and two data bits. Each flag bit indicates whether the corresponding data bit carries user information or a stuff bit because user information was not available at the input.

Suppose that the two input lines operate at exactly 1 Mbps. How frequently are the stuff bits used? Explain why.

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#### Solution Preview

In each group of 22 bits in the output of the multiplexer, 18 positions always carry information bits, nine from each input. This means that we always have 18 information bits in each group of 22 bits.

Remaining four positions consist of two flag bits and two (you can say, optional) data bits. Whether data bits are carrying actual information or stuff bits is determined by the flag bits. So the additional information (apart from 18 information bits) that can be carried in each group of 22 bits would be either 0 bits, or 1 bit, or 2 bits. Which means that each group of ...

#### Solution Summary

To achieve the information output rate of 2 Mbps from the multiplexer, it would need to utilize maximum bits available for transmitting information in each group of 22 bits.