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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

SystemFrequency Band Allocations

AM radio 535-1605 KHz

FM radio88-108 MHz

Broadcast TV54MHz 806MHz

AMPS800-900MHz

ISM bandISM Band I (Cordless phones, 1G WLANs) 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

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

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

BluetoothISM Band II (Bluetooth, 802.11b WLANs) 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:

CPPDs transmission rate is 19.2kbps.

Wi-Maxs 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 Goldsmiths 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 coverageFor 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-proofSpreading 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.