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).