Assignment 3 CIS436 – Data Communication Due: March 24, 2005
2.
Marks: 2 points for each question (i.e., 2 points for 1a), 2 points for 1 b) etc.). Answers must be typewritten.
1. a) What is the main family of WLAN standards today? b) Why are access points needed in WLANs? c) How can a firm create a large WLAN? d) What are hot spots? e) Contrast 802.11 and Bluetooth WLANs. a) What are the three characteristics of radio waves?
3. b) What is a hertz? c) How are two radio characteristics related? d) When would you use frequency to describe propagation? e) When would you use wavelength to describe propagation? a) Distinguish between omnidirectional and dish antennas in terms of operation.
Omnidirectional antennas transmit signals in all directions and receive incoming signals equally well from all directions.
Dish antennas point in a particular direction, allowing them to send stronger outgoing signals in that direction for the same power and to receive weaker incoming signals from that direction. b) Under what circumstances would you use an omnidirectional antenna?
You would use an omnidirectional antenna for short distances (WLANs), and when users don't know the location of the nearest access point. c) Under what circumstances would you use a dish antenna?
You would use a dish antenna for long distances (Satellites) because of their focusing ability. d) What type of antenna usually is used in WLANs?
WLANs typically use omnidirectional antennas. e) If the signal strength from an omnidirectional radio source is 8 milliwatts at 30 meters, how strong will it be at 60 meters?
S(2) = S(1)/2^2 = 8/4 = 2 milliwatts f) At 120 meters?
S(3) = S(2)/2^2 = 2/4 = 0.5 milliwatts

4.
5. a) Which offers more reliable transmission characteristics—UTP and optical fiber transmission or radio transmission?
UTP and fiber transmission have more reliable transmission characteristics because they have known medium characteristics. b) Why is attenuation especially bad in radio transmission compared to wire transmission?
Radio transmission attenuates according to an inverse square law. c) How are shadow zones (dead spots) created?
If there is a thick wall or a large, dense object blocking the direct path between the sender and receiver, the receiver may be in a shadow zone (dead spot where it cannot receive the signal). d) Why is multipath interference very sensitive to location?
Multipath interference is very sensitive to location because a receiver may receive two or more signals; a direct signal and one or more reflected signals. Different signals will travel different distances and so may be out of phase when they reach the receiver. e) List some sources of EMI.
Many devices produce EMI such as fluorescent lamps, cordless phones, microwaves, and WLAN devices. f) What propagation problems become worse as frequency increases?
Higher-frequency waves attenuate more rapidly than lower-frequency waves because they are absorbed more rapidly by moisture in the air, and other water-bearing obstacles. a) Does a signal travel at a single frequency or over a range of frequencies?
Most signals travel over a range of frequencies. b) What is channel bandwidth?
Channel bandwidth is the difference between the highest frequency and the lowest frequency of a channel. c) Why is large channel bandwidth desirable?
Large channel bandwidth is desirable because it can result in higher transmission/signaling speeds. d) What do we call a system whose channels have large bandwidth?

7.
Channels with large bandwidth are called broadband channels. e) What is the Shannon equation?
An equation to determine the maximum possible transmission speed, given a channel bandwidth, and signal-to-noise ratio.
C = B * log(base2) * (1 + S/N), where;
C = Transmission speed,
B = Channel bandwidth, and
S/N = Signal to Noise ratio f) What happens to the maximum possible propagation speed in a channel if the bandwidth is tripled?
The maximum possible propagation speed is also tripled. g) What is the dividing line between narrowband and broadband speeds?
Narrowband < 100kbps < Broadband
6.
a) In normal radio operation, how does channel bandwidth usually relate to the bandwidth required to transmit a data stream of a given speed?
If you want to transmit at a given speed, you will need a channel whose bandwidth is sufficiently wide. b) How does this change in spread spectrum transmission?
Spread spectrum transmission takes the original signal, and spreads the signal energy over a much broader channel. c) What is the benefit of spread spectrum transmission for business communication?
Spread spectrum transmission reduces frequency-specific propagation problems. d) Is spread spectrum transmission done for security reasons in commercial WLANs?
No. a) Describe FHSS.
In FHSS (Frequency Hopping Spread Spectrum), the signal uses only the bandwidth required by the signal but hops frequently within the spread spectrum channel. b) What is its limitation?

It is only useful for relatively low speeds because the transmitter must be retuned with each frequency hop. c) Describe DSSS.
In DSSS (Direct Sequence Spread Spectrum), a signal is spread over the entire bandwidth of a channel. d) For what WLAN standard is it used?
DSSS is used in 802.11b WLANs. e) What spread spectrum transmission method is used for 54 Mbps WLANs?
Orthogonal Frequency Division Multiplexing (OFDM). f) Describe it.
In OFDM, each broadband channel is divided into many smaller sub-channels.
Parts of each frame are transmitted in each sub-carrier. OFDM can be used at very high speeds because it is easier to send many signals in many small subcarriers than it is to send one signal very rapidly over a very wide bandwidth.
8. a) List the elements in a typical 802.11 LAN today. b) Why is a wired LAN still needed if you have a wireless LAN? c) What two things do access points do? d) Why must the access point remove an arriving packet from the frame in which the packet arrives and place it in a different frame when it sends the packet back out? e) If someone says, “wireless Ethernet,” what do they probably mean?
9. f) What is a handoff in 802.11? g) Are handoffs standardized? What are the implications of your answer? a) Why can’t wireless networking use CSMA/CD? b) Describe CSMA/CA+ACK. c) Why is CSMA/CA+ACK inefficient? d) Do you think CSMA/CA+ACK governs transmission by NICs, access points, or both?
Explain your reasoning. e) Describe RTS/CTS. f) Is CSMA/CA+ACK required or optional? g) Is RTS/CTS required or optional?
10. a) Distinguish between rated speed, shared throughput, and individual throughput.
Rated speeds are the maximum speeds according to the standard specification.
Shared throughput is the accumulative speed of all stations sharing an access point.
Individual throughput is the speed of any one station using an access point at a given time.

b) What is the most widely used 802.11 LAN standard today?
802.11b is the most widely used 802.11 LAN standard today. c) How many nonoverlapping channels does it support?
802.11b supports three nonoverlapping channels in the 2.4GHz band. d) Why is the number of nonoverlapping channels that can be used important?
This is important because access points that are near each other will interfere with each other if they use the same nonoverlapping channels. e) What advantages does upgrading from 802.11b to 802.11g instead of 802.11a bring?
802.11g operates in the 2.4GHz band, whereas 802.11a operates in the 5GHz band. Therefore, 802.11g throughput falls off more slowly with distance than
802.11a throughput. Further, 802.11b and 802.11g can operate on the same access point.
11. a) Contrast how 802.11 and Bluetooth are likely to be used in organizations.
802.11 is good for fairly large wireless LANs, (e.g. Internet access). Bluetooth is meant for personal area networks (PANs), for replacing cables between devices, (e.g. wireless printers/mice/keyboards). b) What is a PAN?
PAN stands for ‘personal area network’. c) What are the speeds of Bluetooth transmission?
Bluetooth currently offers a speed of 722kbps (with a back channel of 56kbps). d) What is the normal maximum propagation distance for Bluetooth?
The normal maximum propagation distance for Bluetooth is 10 meters. e) What problem may occur if both 802.11b and Bluetooth are used in the same office?
Both wireless technologies operate on the same 2.4GHz band, so they might interfere with each other.
12. a) What are the four technical elements in the PSTN?
Customer premises equipment, access, transport, and signaling. b) What is customer premises equipment?

Equipment owned by the customer, including PBXs, internal wiring, and telephone handsets. c) What are the two parts of the access system?
The access system includes both access lines and termination equipment in the end office at the edge of the transport core. d) What is the local loop?
The local loop is another term for access lines. e) What is the transport core?
The transport core is the medium through which transmission from one subscriber is delivered to another. f) What are the two elements of the transport core?
The transport core consists of trunk lines and switches. g) Which is changing more rapidly—the access system or the transport core?
Changes have been much more rapid in the transport core technologies. h) Explain why.
Changes in the transport core save carriers a great deal of money because it is much cheaper than changing the access line to every home connected to the transport core. i) What is signaling?
Signaling means the controlling of calling, including setting up a path for a conversation through the transport core, maintaining and terminating the conversation path, collecting billing information, and handling other supervisory functions. j) In telephony, distinguish between transport and signaling.
Transport is the transmission of voice communication.
Signaling is the process of supervising voice communication sessions.
13. a) What is circuit switching?
A traditional telephone system whose capacity for a voice conversation is reserved on every switch and trunk line end-to-end between the two subscribers.

b) Explain frames and slots in time division multiplexing (TDM).
Time is divided into brief periods called frames.
Each frame is divided into even briefer periods, called slots. c) How is a circuit allocated capacity on a TDM line?
A given circuit is allocated the same slot in every frame. d) What is the advantage of TDM?
TDM provides the reserved capacity required for circuit switching. e) What is the disadvantage?
TDM wastes unused capacity.
14. a) Why does circuit switching make sense for voice communication?
Circuit switching works well for voice, because of its consistency. b) What does it mean that data transmission is bursty?
Data traffic is bursty, with short, high-speed bursts separated by long silences. c) Why is burstiness bad for circuit switching?
Reserved capacity in circuit switching is extremely wasteful for data transmission which typically uses only 5% capacity.
15. a) What are the differences between dial-up and private line circuits?
Dial-up circuits operate on a separate circuit for each call, support up to
56kbps, and only support one voice call at a time.
Private line circuits are permanent circuits that are always on, support between
56kbps up to gigabit speeds, and support several voice calls at a given time due to multiplexing. b) What is multiplexing in the context of telephone calls and private lines?
Multiplexing means mixing together multiple voice circuits to and from the corporate premises.
16. a) What is an end office switch?
The nearest switch of the telephone company to the customer premises. b) Why is it called a Class 5 switch?

The telephone network traditionally has used a five-class hierarchy, and end office switches are the lowest switches in the hierarchy.
17. a) Distinguish between analog and digital signals.
An analog signal rises and falls in intensity smoothly, with no clock cycles and no limited numbers of states as in digital signaling. b) What parts of the telephone system are largely digital today?
The PSTN transport core is almost entirely digital today. c) What parts of the telephone system are largely analog today?
The local (customer)loop is mainly analog. d) What is the role of the codec in the end office switch?
To convert between the analog local loops signals and the digital signals of the end office. e) Explain why bandpass filtering is done.
To filter out all signals between 300Hz and about 3400Hz. f) Explain how the ADC generates 64 kbps of data for voice calls when it uses PCM.
The codec ADC samples the bandpass-filtered voice signal 8000 times per second. g) Why do we need DACs?
DACs are needed for converting transmissions from the digital telephone network’s core to signals on the analog local loop.
18. a) What is the worldwide signaling system for telephony?
Signaling System 7 is the worldwide standard for supervisory signaling. b) Distinguish between SS7 and C7.
C7 is the European version of SS7. c) Does having two versions of the standard cause major problems?
No, simple gateways can convert between them.
19. a) What is IP telephony?

IP telephony is transmitting telephone signals over IP internets instead of over circuit-switched networks. b) What is VoIP?
Voice over IP. c) What is an IP telephone?
A client which has the electronics to encode voice for digital transmission and to send and receive packets over an IP internet. d) What is a media gateway?
It connects IP telephones to ordinary telephones on the public switched telephone network.
20. a) What do speech codecs do?
Speech codecs convert analog voice signals into digital bit streams. b) What is the tradeoff to consider when selecting a speech codec standard for use in an
IP telephony system?
Some speech codecs compress voice more, reducing traffic transmission requirements and, therefore, cost. c) What is the drawback of having many speech codecs from which to select?
This can create problems for interoperability unless planning and vendor selection is done carefully.
21. a) What two signaling protocols are used in IP telephony?
H.323 signaling protocol and Session Initiation Protocol (SIP) b) Why is SIP likely to be dominant in the future?
Because of its simplicity.