Review Questions

R1. What is the difference between a transmission link and a circuit?

The term linkis used to designate the segment between two neighboring nodes. This means that the link doesn't include transit switching and multiplexing devices.

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A circuitis a complex path between network end nodes. Thus, circuits comprise individual links and internal connections within switches.

 

R2. Can circuits include links? Can links include circuits?

A circuit normally includes several links, but link cant include circuits.

 

R3. Can a digital channel transmit analog data?

It can if they are digitized.

R4. What are the functions of DTE and DCE? To which type of devices do network adapters belong?

A network adapter combines functionality of DTE and DCE.

R5. To what type of communications link characteristics do the following belong: noise level, bandwidth, and linear capacitance?

R6. What measures can be taken to increase the information rate of the link? Choose one:

A. Decrease the cable length - correct

B. Choose a cable with less resistance

. Choose a cable with wider bandwidth - correct

D. Use an encoding method with a narrower spectrum - correct

R7. Why is it not always possible to increase the channel capacity by increasing the number of information signal states?

Because a signal spectrum can exceed bandwidth limits of a channel, so that signal gets distort and a receiver wont be able to recognize the information sent.

R8. What mechanism helps to suppress noise in UTP cables?

Wires twisting

R9. What cable ensures a higher quality of signal transmission: the one with a larger value for the NEXT parameter or the one with a smaller NEXT value?

With a greater absolute value of NEXT (which is always negative). For example, a cable with -24 value of NEXT (absolute value is 24) is better than a cable with -20 value of NEXT (absolute value is 20).

R10. What is the spectrum width of an ideal pulse?

An ideal pulse has an infinitive spectrum

R11. List the types of fibre-optic cable.

R12. What would happen if a UTP cable was replaced with a STP cable?

A. The share of distorted frames would be reduced, since external noise would be suppressed more efficiently.

B. Nothing would change.

C. The share of distorted frames would increase, since the output resistance of the transmitters would not coincide with the cable impedance. - Correct

R13. List the main advantages of the structured cabling system.

R14. What types of cables are used for the horizontal subsystem of the SCS?

R15. What are the problems related to using fiber-optic cables in horizontal subsystems?

Problems

Q1. The following values are given:

q    Minimum transmitter power out (dBm)

q    Cable attenuation A (dB/km)

q    Receiver sensitivity threshold in (dBm)

Find the maximum possible length of the communications link at which signals will be transmitted adequately.

To provide a stable functioning of a receiver the following equation should take place:

out   -  A x Lmax = in , where Lmax is a maximum cable length. This gives that Lmax = (out -  in) / A.

 

Q2. What will be the theoretical limit of transmission rate (bps) using the channel with a bandwidth equal to 20 kHz if the power of transmitter is 0.01 mW and the noise level in the channel is 0.0001 mW?

C = F log2 (1 + Pc/P) = 20000 x log2 (1 + 0.01/0.0001) = 20000 x log2 (101) = 20000 x 6.68 = 133600 (bits/s)

 

Q3. Determine the capacity of the duplex communications link for each direction provided that its bandwidth equals 600 kHz and the encoding method uses 10 states of the information signal.

The bandwidth of the link should be divided in two as in duplex mode two flows of equal speed exist, one flow for each direction. So C = (2F log2 M ) / 2 = 600000 x log2 10 = 600000 x 3.32 = 1993356 bits/s = 1,99 Mbits/s. 

Q4. Calculate the signal propagation delay and data transmission delay for transmitting a 128 bytes packet (the signal propagation speed is considered to be light propagation speed in a vacuum i.e., 300,000 km/sec):

q    Using a 100 m twisted pair cable at a transmission speed equal to 100 Mbps
0.33 μs and 10.24 μs

q    Using a 2 km coaxial cable at a transmission rate equal to 10 Mbps.
6.6 μs and 102.4 μs

q    Using a 72,000 km satellite channel at a transmission rate equal to 128 Kbps

q    0.26 s and 0.008 s

Q5. Calculate the rate of the channel, given that the transmitter clock frequency is 125 MHz and the signal has five states.

On every time slot transmitter sends log2 5 = 2.32 bits, hence a transmission rate is equal to 125 2.32 = 290.24 Mbps.  

Q6. Transmitters and receivers of the network adapter are connected to adjacent pairs of the UTP cable. What is the power of induced noise at the receiver input, provided that the transmitter has the power of 30 dBm and the NEXT parameter of the cable is 20 dB?

The power of induced noise is equal to 30 20 = 10 (dBm)

Q7. Provided that a modem transmits data in duplex mode at a rate of 33.6 Kbps, calculate how many states the signal has if the channel bandwidth is 3.43 kHz.

For each of two directions the bandwidth used is equal to 3.43/2 = 1.715 KHz.

Therefore we have equality:

2 1715 log2 M =   33600,

from which we have:

M = 2 9.8 = 888.9