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                      91.564 Data Communications II

                            Homework 1

1. Find the frequency band allocations for the following systems. AM radio, FM
radio, Broadcast TV, AMPS, ISM band, IEEE 802.11b, IEEE 802.11a, Bluetooth.

Ans: The frequency allocations are


          System                     Frequency Band Allocations

                                     535-1605 KHz
           AM radio

           FM radio                  88-108 MHz


            Broadcast TV             54MHz – 806MHz

                                     800-900MHz
               AMPS
                                     ISM Band I (Cordless phones, 1G WLANs)
              ISM band               902-928 MHz
                                     ISM Band II (Bluetooth, 802.11b WLANs)
                                     2.4-2.4835 GHz
                                     ISM Band III (Wireless PBX) 5.725-5.85
                                     GHz

                                     ISM Band II (Bluetooth, 802.11b WLANs)
            IEEE 802.11b             2.4-2.4835 GHz

                                     NII Band I (Indoor systems, 802.11a
            IEEE 802.11a             WLANs) 5.15-5.25 GHz

                                     ISM Band II (Bluetooth, 802.11b WLANs)
            Bluetooth                2.4-2.4835 GHz




2. If you have a cell phone (if not, ask a friend), try to find the following
information: What is the provider (AT&T, Verizon, etc)? What is the underlying
multiplexing technique (TDMA, CDMA, etc) ? What is the frequency band in use?
Does the company offer data service? If so, what is the standard (CDPD, GPRS,
etc)? and what is the promised data rate?

Ans: Provider: T-mobile
     Multiplexing technique: GSM
     Frequency band: 1900MHz
     Company offers data service, using standard GPRS, average data rate 56kbps.

     Or:
     Provider: Cingular
     Multiplexing technique: TDMA
     Frequency band: 850/1900MHz
     Company offers data service, using standard GPRS, data rate peak at 70-135 Kbps.

    Or:
    Provider: Verizon
    Multiplexing technique: CDMA
    Frequency band: 800 / 1900MHz
    Company offers data service, using standard CDPD, promised data rate 144bps with
    an average rate of 40 – 60kbps

   Or:
    Provider: Sprint
    Multiplexing technique: CDMA
    Frequency band: 1900MHz
    Company offers data service, using standard CDPD, data rate up to 144 kbps,
    50-70kbps average



3. What is the layered architecture of the Internet? In a wireless network, describe
how each layer would be different from a wired network.

Ans: 5- Layer architecture of the Internet:

                        Application (Layer 5)
                        Transport (Layer 4)
                        Network (Layer 3)
                        Data Link (Layer 2)
                        Physical (Layer 1)

     Differences between wireless network and wired network:

     (1) Wired network uses wires (e.g., twisted pair, coax, fiber optic) as transmission
media in physical layer. Wireless network uses air waves, which requires different
coding, modulation schemes.
      (2) Interference and collision is much more severe in wireless networks. Data link
layer in wireless network uses different techniques to avoid collision and make sure data
was received.
      (3) Wireless network needs to deal with device location and handover in network
layer. In wireless ad hoc networks, routing can be different than wired networks.
      (4) No big difference between wireless and wired network in transport layer.
      (5) Application layer is unaware of the physical aspects of the network. Application
layer in wired network supports services like FTP, SMTP etc. One can also take the
physical layer characteristics into account when writing an application.

4. As storage capacity increases, we can store larger and larger amounts of data on
smaller and smaller storage devices. Indeed, we can envision microscopic computer
chips storing teraflops of data. Suppose this data is to be transferred over some
distance. Discuss the pros and cons of putting a larger number of these storage
devices in a truck and driving them to their destination rather than sending the data
electronically. (from Goldsmith Wireless Communications book).

Ans:
For delivering devices in truck:
 Pros: Reliable, no error, no interference
        Guaranteed Data delivery
        “The positive side of shipping by truck is that it keeps thousands of people
         dressed in brown employed”
 Cons: Slow, expensive, inconvenient,
        “Lots of human and other factors come into consideration in transportation”


5. What is the advantage of using ISM band? What is the disadvantage? And how
does the FCC mitigate the problem? Name a few systems that use the ISM-band.

Ans: The advantage is that an FCC license is not required to operate in these bands.
However, since many other systems operate in these bands for the same reason, the issues
of signal interference and coexistent are serious among communications in ISM bands.
The FCC mitigates the interference problem by setting a limit on the power per unit
bandwidth for ISM-band systems.


6. How many CDPD data lines are needed to achieve the same data rate as the
average rate of Wi-Max (from Goldsmith Wireless Communications book) ?

Ans:
CPPD’s transmission rate is 19.2kbps.
Wi-Max’s average data rate is 70Mbps
So, we need 70,000 / 19.2 = 3464 CDPD data lines to achieve the same rate.
7. Describe three disadvantages of using a wireless LAN instead of a wired LAN.
For what applications will these disadvantages be outweighed by the benefits of
wireless mobility. For what applications will the disadvantages override the
advantages. (from Goldsmith’s Wireless Communications book)

Ans:
Three disadvantages of wireless LAN:
 (1) may exhibit poor performance, low throughput, lower QoS, high error rates due to
     interference
 (2) not secure
 (3) Lower power, limited capacity, high setup cost, small coverage

For example, the disadvantages be outweighed by the benefits of wireless mobility in
following applications:
   “voice application, short message application, remote sensor”
   “not bandwidth-intensive application, like email, web browsing”

The disadvantages override the advantages in the following application:
  “e-commerce, video application”
  “high security, defense networks”
  “multimedia application”
  “stock trading application”

8. What are the main problems of signal propagation? Why do radio waves not
always follow a straight line? Why is reflection both useful and harmful?

Ans: Wireless signal propagation is subject to signal strength attenuation, interference,
shadowing, and other propagation effects, e.g., reflection, refraction, scattering, and
diffraction. These can cause intermittent connectivity and high bit error rates in wireless
communications. Radio waves do not always follow a straight line because of the
reflection, refraction, scattering, and diffraction effects. Reflection is useful because radio
waves can now reach places without line of sight. Reflection is harmful since it can lead
to multipath propagation, and hence time dispersion and distortion in received signals.

9. Why, typically, is digital modulation not enough for radio transmission? What
are general goals for digital modulation? What are typical schemes for digital
modulation?

Ans: In addition to the digital modulation which converts the bits to baseband signals,
analog modulation is usually applied to shift the center frequency of baseband signal up
to the radio carrier. The main reasons are (1) smaller antennas (e.g., /4) can be used (2)
Frequency Division Multiplexing can be implemented (3) the signal can be adapted to
the specific medium characteristics (path loss, reflection etc depend on medium, long
wave for submarine, very short wave for direct microwave). The general goal for digital
modulation is that digital data is translated into an analog signal for transmission
(baseband). Main digital modulation schemes include ASK, FSK, PSK.
10. What are the main benefits of a spread spectrum system? How can spreading be
achieved? Why is DSSS system resistant to multi-path fading effects?

Ans: The main benefits include
           resistant to frequency-selective fading and multi-path effect
           coexistence of several signals without dynamic coordination
           a characteristics like background noise
           tap-proof

Spreading can be achieved by Direct Sequence or Frequency Hopping spread spectrum
mechanisms.
DSSS system spreads the narrow band signal into a broad band signal using a special
code, making the signal resistant to multiple-path fading.

				
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