• This completes our discussion of common physical links. Perhaps the main takeaway point is that transmitting bits over any distance is not quite as simple as it may appear; simple NRZ transmission is not effective.
• ## Exercises

Exercises are given fractional (floating point) numbers, to allow for interpolation of future exercises. Exercise 3.5 is distinct, for example, from exercises 3.0 and 4.0. Exercises marked with a ♢ have solutions or hints at 24.4 Solutions for Links.

1.0. What is encoded by the following NRZI signal? The first two bits are shown.

    ┌───┐   ┌───────────────────┐       ┌───┐   ┌───────┐   ┌───────
│   │   │                   │       │   │   │       │   │
───┘   └───┘                   └───────┘   └───┘       └───┘
0   1


2.0. Argue that sending 4 0-bits via NRZI requires a clock accurate to within 1 part in 8. Assume that the receiver resynchronizes its clock whenever a 1-bit transition is received, but that otherwise it attempts to sample a bit in the middle of the bit’s timeslot.

3.0.(a) What bits are encoded by the following Manchester-encoded sequence?

 ┌─┐ ┌─┐ ┌─┐ ┌───┐ ┌─┐   ┌───┐   ┌─┐ ┌───┐ ┌─┐   ┌───┐   ┌───┐   ┌───┐
│ │ │ │ │ │ │   │ │ │   │   │   │ │ │   │ │ │   │   │   │   │   │   │
─┘ └─┘ └─┘ └─┘   └─┘ └───┘   └───┘ └─┘   └─┘ └───┘   └───┘   └───┘   └─

(b). Why is there no ambiguity as to whether the first transition is a clock transition or a data (1-bit) transition?
(c). Give an example of a signal pattern consisting of an NRZI encoding of 0-bits and 1-bits that does not contain two consecutive 0-bits and which is not a valid Manchester encoding of data. Such a pattern could thus could be used as a special non-data marker.

3.5.♢ What is the 4B/5B encoding for the 3-byte string “Net”? (Hint: ‘N’ is 0100 1110.)

4.0. What three ASCII letters (bytes) are encoded by the following 4B/5B pattern? (Be careful about upper-case vs lower-case.)

010110101001110101010111111110

5.0.(a) Suppose a device is forwarding SONET STS-1 frames. How much clock drift, as a percentage, on the incoming line would mean that the output payload envelopes must slip backwards by one byte per three physical frames? | (b). In 4.2.2 SONET it was claimed that sending 250 0-bits required a clock accurate to within 1 part in 500. Describe how a SONET clock might meet the requirement of part (a) above, and yet fail at this second requirement. (Hint: in part (a) the requirement is a long-term average).