Chapter 3 Spread spectrum technology

15. A DSSS channel is more susceptible to narrowband interference than a FHSS channel because of which of the following? Choose all that apply. A. The DSSS channel is much smaller (22 MHz wide instead of the 79 MHz wide band used by FHSS) B. The information is transmitted along the entire band simultaneously instead of one frequency at a time C. FHSS systems simply avoid the frequency on which the narrowband interference is located D. FHSS systems only use one frequency at a time, so the narrowband interference must be on the same exact frequency at the same time

A, B, D. Frequency hopping systems use the entire useable range of the 2.4 GHz ISM band with a range of 83.5 MHz whereas direct sequence systems use only a 22 MHz portion of the same frequency band. For this reason, the same narrowband signal will disrupt the DSSS system more. Additionally, if the narrowband signal is either intermittent or changes frequencies, the chance of it affecting FHSS is slim.

6. Which of the following are advantages of 802.11b DSSS over 802.11 FHSS? Choose all that apply. A. Cost B. Throughput C. Security D. Resistance to narrowband interference

A, B. The 802.11b standard specifies rates up to 11 Mbps whereas the highest data rate specified by the 802.11 standard is 2 Mbps. Therefore, the DSSS technology specified in 802.11b is significantly faster than that of 802.11 FHSS. 802.11b compliant hardware is often as little as 1

11. You have been hired on as a consultant to increase the capacity of an existing wireless LAN based on FHSS technology. After your research is completed you recommend that a replacement system based on DSSS would be better. Which of the following could be your arguments to defend your position? Choose all that apply. A. The DSSS devices will cost less and have more throughput B. The DSSS devices will cost more but have more throughput C. Additional new FHSS devices may not be compatible with the older devices D. DSSS is more secure than FHSS

A, C. Implementations of DSSS hardware meeting the 802.11b standard are both fast and inexpensive. One problem that might be encountered with FHSS systems is compatibility. There are two standards manufacturers can choose to use for creating their hardware - 802.11 and OpenAir - but both are losing popularity. They are incompatible standards and there is no organization performing OpenAir and 802.11 FHSS testing for interoperability. Buying two OpenAir FHSS systems is no guarantee of interoperability.

Increasing the dwell time for an FHSS system will increase the throughput. A. This statement is always true B. This statement is always false C. It depends on the manufacturer of the equipment

A. The dwell time is the time spent by a transmitter on a certain frequency actually transmitting data. The longer a transmitter stays on a given frequency, the higher the throughput of the system will be. Hopping between frequencies takes time and takes away from the system throughput.

16. The noise floor is defined by which one of the following? A. The general level of RF noise in the environment around the wireless LAN B. The noise that is generated as a result of foot traffic C. A fixed level of -100 dBm D. The level of noise at which a wireless LAN starts working

A. The noise floor is a mixture of all of the background RF radiation found in the environment surrounding the system in use. The noise floor is generally in the -100 dBm area, but can be much higher or much lower depending on the environment. RF signals must be higher than the noise floor to be detectible as a valid, useful signal by a receiver. Spread spectrum systems are much closer to the noise floor than are narrowband signals.

An 802.11b compliant wireless LAN configuration using DSSS can have a maximum of ___ non-overlapping, co-located access points. A. 3 B. 15 C. 26 D. 79

A. Using Direct Sequence technology within the 2.4 GHz ISM allows only three non-overlapping channels within the 83.5 MHz allotted by the FCC. These channels are 1, 6, & 11.

7. If having compatible equipment from different manufactures were an important factor when purchasing wireless LAN equipment, which of the following spread spectrum technologies would be the best choice? A. FHSS B. DSSS

B. DSSS technologies have become amazingly popular with 802.11b Wi-Fi compliant devices. FHSS devices have taken a backseat with 802.11 and OpenAir standards losing popularity in comparison to IEEE's 802.11b and the Wi-Fi Alliance's Wi-Fi standards. Wi-Fi has become the de-facto standard in DSSS wireless LAN equipment interoperability.

19. Some 2.4 GHz FHSS systems operate at 3 Mbps or more. Which of the following is true regarding these systems? A. They are always IEEE 802.11 compliant B. They may not interoperate with other FHSS systems C. They are always OpenAir compliant D. They are backwards compatible with 900 MHz systems

B. FHSS systems that can transmit data at 3 Mbps may or may not have the ability to slow their data rate to 2 or 1 Mbps, which is specified by the 802.11 standard. These systems could comply with the IEEE standard if they could transmit at 2 & 1 Mbps or the OpenAir standard if they could transmit at 1.6 Mbps and 800 kbps. While they are transmitting at other than these speeds, they are NOT compliant with either standard

Consider the following two wireless LAN configurations: System 1. IEEE 802.11 compliant FHSS system, 6 co-located access points running at maximum data rate. System 2. IEEE 802.11b compliant DSSS system, 3 co-located access points running at 50% of maximum data rate. Which one of the following statements is true? A. System 1 will have more throughput B. System 2 will have more throughput C. System 1 and System 2 will have the same throughput

B. Six IEEE 802.11 compliant access points, synchronized to have absolutely no collisions, would yield a maximum data rate of 12 Mbps because each system has a maximum data rate of 2 Mbps. Three 802.11b compliant access points on non-overlapping channels would yield a maximum data rate of 16.5 Mbps because each system has a data rate of 5.5 Mbps (half of the maximum 11 Mbps). Since throughput on each of these systems is approximately 50% of the data rate due to overhead with the CSMA

20. How many different types of implementations of spread spectrum technology does the FCC specify for the 2.4 GHz ISM band? A. 1 B. 2 C. 3 D. 4

B. The FCC specifies use of two types of spread spectrum technology in the 2.4 GHz ISM band. These types are Frequency Hopping and Direct Sequence.

14. In Frequency Hopping wireless LAN systems, the term hopping refers to which one of the following? A. Switching between throughput speeds from 11 Mbps to 5.5 Mbps B. What happens when the carrier frequency is changed C. The change that occurs as a result of the RF signal getting weaker D. Changing technologies from FHSS to DSSS

B. When a FHSS system hops, both the transmitter and receiver systems change the carrier frequency in a synchronized fashion.

9. The latest published FCC rules regarding power output for FHSS states a maximum output of which one of the following? A. 100 mW B. 125 mW C. 200 mW D. 1 W

B. With the FCC rules regarding FHSS equipment, a manufacturer could either implement a piece of equipment to meet the old rules or the new rules. Under the old rules, the maximum power output was 1 watt, but under the new rules, due to a significant increase in carrier bandwidth, the power limit was changed to 125 mW.

17. Which one of the following is not described by the IEEE and OpenAir standards regarding FHSS systems? A. What frequency bands may be used B. Hop sequences C. Allowable levels of interference D. Dwell times E. Data rates

C. Both the OpenAir and 802.11 standards define all parameters necessary for functioning of the equipment. These specifications include dwell times, data rates, data rate fallback functionality, frequency hop patterns (channels), and the frequency bands to be used. This is not a comprehensive list.

5. Channels on direct sequence systems for 802.11b equipment are ___ MHz wide. A. 5 B. 20 C. 22 D. 83

C. Channels on 802.11b compliant DSSS systems in the 2.4 GHz band are 22 MHz wide. This is in contrast to the OFDM channels used by the 802.11a standard using the 5 GHz UNII bands, which are each 20 MHz wide.

2001

C. On 8

18. An RF signal is considered spread spectrum when which of the following are true? Choose all that apply. A. The system sending the signal is using infrared technology B. The power required to send the information is significantly greater than is necessary C. The bandwidth used is much wider than what is required to send the information D. The bandwidth used is much less than what is used to send the information

C. Spread spectrum transmitters spread a data signal out over a wide band of frequencies using many symbols per bit for redundancy and using very low power.

12. The statement, "802.11b wireless LAN devices are backward compatible with 802.11 wireless LAN devices" is: A. Always true B. Always false C. Sometimes true

C. The 802.11b standard supports DSSS devices only whereas the 802.11 standard supports DSSS and FHSS. This being the case, an administrator could have a situation where there are 802.11 FHSS nodes and access points in place. Trying to add 802.11b devices into the network will not work for clients that are using FHSS PCMCIA cards. This would only work if the existing WLAN consists of 802.11 DSSS clients and access points.

Which one of the following dwell times will result in the greatest throughput in a FHSS system and will still be within FCC regulations? A. 100 ms B. 200 ms C. 300 ms D. 400 ms

D. 400 ms is the legal limit on dwell time per the FCC. This dwell time is also the most advantageous for systems with the goal of maximized throughput.

10. The FCC specifies how many channels in the 2.4 GHz ISM band that can be used for DSSS in the United States? A. 3 B. 6 C. 9 D. 11

D. There are 14 channels specified for use by the FCC and IEEE for spread spectrum technologies in the 2.4 GHz ISM band. Of these, only 11 may be used in the United States.

13. What is considered to be the maximum number of co-located FHSS access points in a wireless LAN, if non-synchronized radios are to be used? A. 3 B. 16 C. 20 D. 26

D. When synchronized radios are used, a maximum of 12 radios in a system is currently available (no vendor currently has the ability to synchronize more than 12 radios). With unsynchronized radios, a recommended maximum of 15 radios should be used for the best performance, but up to 26 radios can be used before collisions hinder performance more than the throughput gain of adding another access point.