ENEL616 Telecommunications Network Services

Assignment 1
1. Determine the minimum signal-to-noise (SNR) required to achieve a transmission rate of 28
kbps over a telephone channel carrying voice signals of frequencies ranging between 30 Hz and
3.13 kHz. Express your answer in decibels (dB). (5 marks)
2. For the bit stream 1 1 1 0 0 1 0 0, sketch the waveforms for NRZ-L, NRZI, Manchester, and
Differential Manchester encoding. For NRZI, it may be assumed that the signal level for the
preceding bit was high. (5 marks)
3. Given generator polynomial function g(x) = x3

+ x + 1, find the 7-bit codeword corresponding
to the information sequence 1101. Furthermore, if this codeword is received with an error in the
most significant bit (MSB), determine what the receiver will obtain when it conducts an error
check on this codeword. (5 marks)
4. In Figure 1, frames are generated at node A and sent to node C through node B. Determine the
minimum transmission rate (in bits per second) required between B and C so that buffer of B is
not flooded, with the following assumptions: (5 marks)
? Data rate between A and B is 80 kbps.
? Both lines are full duplex lines having identical propagation delay of 4 ?s/km.
? All data frames are 800 bits; ACK frames are separate frames of negligible length.
? Between A and B, a sliding-window flow protocol with a window size of 3 is used.
? Between B and C, stop-and-wait flow control is used.
? There are no errors.
Hint: In order not to flood the buffer of B, the average number of frames entering and leaving B
must be the same over a long interval.

Figure 1

5. Suppose that a transmission line connecting two computers has a transmission speed of 3 Mbps,
transmission bit-error-rate (BER) of 2?10-5, and a one-way propagation time of 5 ms. If a frame
of 1200 bytes is transmitted: (5 marks)
(a) Find the transmission efficiency if Stop-and-Wait ARQ is used.
(b) Repeat for Go-Back-N ARQ that uses a three-bit sequence numbering.
Assume the number of bits for header, CRC, and ACK are negligible compared to the number of
information bits in the frame. Further assume the processing time is negligible.
A B C
5000 km 1250 km

6. For the LAN configuration shown in Figure 2, complete the entries of the forwarding table of the
bridges 101 and 106. (5 marks)