Data Communications II, Autumn 2004

Problem set 5 (27.10.2004) On Wednedsay 8-10 in room???

  1. The new fast Ethernets (100 Mbps, 1 Gbps, 10 Gbps) do not use the same Manchester coding as the classical 10 Mbps Ethernet does. Instead they first encode data with encodings called 4B/5B, 8B/6T or 8B/10B and then send the bits using NRZ-I (non-return-to-zero, invert-on-one) coding. Why is this done? Find out also what is meant by these encodings.

  2. Nyquist and Shannon
    1. What is the content of the theorems of Nyquist and Shannon? How do these theorems relate to each other?
    2. A binary signal (getting only values 0 and 1) is sent over a 3 kHz channel whose noise-to signal ratio is 20dB, what is the maximum achievable data rate?

  3. If a T1 carrier system slips and loses track where it is, it tries to resynchronize using the 1st bit in each frame. This has the bit pattern 01010101010101 ... in the frames following each other. How many frames will have to be inspected on the average to resynchronize with a probability of 0.001 of being wrong?

  4. SONET clocks are pretty exact and their timings are drifting apart only about 3 seconds in 100 years (a year = 365 days). How long does it take before the drift between two SONET clocks is equal to the width of 1 bit? What are the implications of this calculation in practice?

  5. A modem modulates digital bits first into analog form so that they can be transmitted in the analog local loop and at the receivers the modem demodulates the analog signal back to the digital bits. A codec samples the analog signal and encodes the samples into digital bits and at the other end the codec decodes the digital signals back to analog. How does the coding side of codec differ from the demodulation side of the modem or is the no difference at all? Both transform an analog signal to a digital one.

  6. ATM
    1. Digitally encoded data is first packed into packets and then sent packet by packet forward. The time required to fill a packet is the packetization delay. Packetization delays greater than 20 ms can cause noticeable and unpleasant echo. A typical encoding and packetizing rate is 64 Kbps. How long is the packetizing delay for a maximum Ethernet frame of 1500 bytes? How long is the packetizing delay for a packet of 48 bytes.
    2. Why is ATM using small fixed lenght packets?