CS/ECE 438: Communication Networks for Computers Spring 2009
Problem Set 2 Due: start of class, Wednesday, February 11th
Direct Link Networks(Part I)
Assigned reading: Peterson and Dav ie: Chapter 2.1 – 2.4. All problems carry equal we ight.
All problems carry equal weight. To receive full cred it, show all of your work.
1. Encoding and Channel Capacity
a. Show the NRZ, NRZI, Manchester, and 4B/5B encoding signals (the resulting N RZI signal for 4B/5B),
using a diagram similar to tha t in Peterson and Davie, p. 77, Figure 2.7, for the data bit sequence 0101
1111 0001 1011. To be definite, assume the NRZI s ignals begin at low voltage.
b. What signal-to-noise ratio is requir ed to put a T1 carrier on a 20 Mhz line?
2. Noisy Channel Da ta Rates
The decibel is a measure of the ratio between two signal levels: Ndb = 10 log10 (P2/P1), where Ndb = the number of
decibels, P1 = the input power level and P2 = the output power level.
a. A telephone line is known to have a loss of 25db. Th e input signal power is measured as 0.6 watt and
the output noise is measured as 5 µwatt. Using this information, calculate th e output signal-to-noise
ratio in dB.
b. What is the capac ity of this phone line with a frequency range of 300 Hz – 3000 Hz ?
c. If the attenuation rate of this phone line is 4db /km, and the minimum output signal is 0.002 watt, given
the input signal from part a), how long can the phone line be before requiring a repeater?
3. Error Detection
a. A CRC is constructed to generate a 4-bit checksum for an 1 1-bit message. The generator polynomial
is x4 + x3 + 1. Encode the d ata bit sequence 10000110100. Now assume that bit 3 (counting from the
least significant bit) in the code word is in error and show how the error is detected .
b. The b it sequence 10010110011 corresponds to the polynomial x10+x7+x5+x4+x+1. Divide this
polynomial by the C RC generator polynomial x3+x+1 and report the remainder as a polynomial. Is the
bit sequence corre ctly encoded with the given generator ( i.e., is the remainder 0)?
c. A block of bits with n rows and k columns uses horizontal and vertical parity b its for error detection.
Suppose that exactly 4 bits are inverted due to transmission errors. Derive an expression for the
probability that the error will be undetected.
4. Framing
Consider the data bit sequence 0000 1010 1111 1111 0101 0000 0110 0000 1111 1111 0000 1110. In this problem,
you will frame these bits in three ways.
a. First, frame the bits with byte stuffing as us ed in the BISYNC protocol. You need show only the body
(including stuffed bytes) and the sentinel bits. DLE is 10 (decimal), STX is 2, and E TX is 3.
b. Second, frame the bits using b it stuffing as defined by the HDLC protocol. Again, you need show only
the (stuffed) data bits and the sentinel bits.
c. Third, fra me the bits into 8-bit R S-232 characters. Use “1” to represent s tart bits and “0” to represent
stop bits.
d. Now, counting only th e bits that you wrote, calculate the efficiency (as a percentage of real data per bit
sent) of your answer s to (a), (b), and (c).
5. Networking Utilities
Read the manual p ages for the Unix utility netstat on an eelnxxx.ews.uiuc.edu mach ine. Try running the command
