Qa

1. What do ISDN, BRI, and PRI stand for?

Answer: ISDN stands for Integrated Services Digital Network. BRI stands for Basic Rate Interface. PRI stands for Primary Rate Interface.

2. How many bearer channels are in a BRI? What about a PRI in North America? What about a PRI in Europe?

Answer: BRI uses two bearer channels and one signaling channel (2B+D). PRI uses 23B+D in North America and 30B+D in Europe. The signaling channel on BRI is a 16-kbps channel; on PRI, it is a 64-kbps channel.

3. Define what a voice codec does, and explain why a PCM codec needs 64 kbps for a single voice call.

Answer: Voice codecs code and decode voice signals, converting from analog to digital, and digital to analog. A PCM codec samples the analog signal 8000 times per second, generating an 8-bit code to represent each sample. So, 64,000 bits are needed for a single second of voice.

4. Two terms were shortened and combined to first create the word modem. Identify those two words and describe what each word means.

Answer: The term modem is formed as a combination of the words modulation and demodulation. Modulation means to vary or change a wave form to encode information. A modem varies an analog electrical signal to encode information, representing binary digits, onto an analog signal. Modulation refers to the creation of the analog signal based on a string of bits, and demodulation simply refers to a modem performing the reverse process upon receiving the analog signal.

5. Define what the terms symmetric and asymmetric mean in relation to modem specifications. Also explain why asymmetric might be a better option.

Answer: Symmetric means that the speed in each direction of flow is the same, whereas asymmetric means that the speed in one direction is faster than the other. Asymmetric speeds might be a good choice because typical traffic flows require a much greater amount of data to flow in one direction, typically from a server to a client. Asymmetric speeds allow the speed in one direction to be faster than it could be with symmetric speeds, accommodating the need for more bandwidth in one direction.

6. Compare the V.90 and V.92 modem specifications.

Answer: Based on this chapter, V.92 is an improvement over the V.90 standard. V.92 supports symmetric and asymmetric speeds, whereas V.90 supports only asymmetric speeds. The upstream speed has been increased from 33 kbps up to 48 kbps. It supports modem-on-hold, which allows the user to accept a voice call in response to a call-waiting signal, putting the modem connection on hold. It also senses the correct operational speed more quickly than V.90.

7. Compare analog modems, ISDN BRIs, DSL, and cable modems in terms of concurrent support for voice and data.

Answer: Analog modems do not support concurrent voice and data transmission. ISDN and DSL both support simultaneous voice and data over the same local loop (local phone line). Cable allows simultaneous data, voice, and TV reception.

8. Compare analog modems, ISDN BRIs, DSL, and cable modems in terms of whether the data service is always on.

Answer: Analog modems and ISDN BRIs must signal to set up a circuit, so any data capabilities, such as Internet connectivity, are not "always on." DSL and cable do not require any signaling to set up a circuit—in fact, no circuit is needed in the PSTN to support these technologies, so these services are "always on."

9. List some of the pros and cons regarding the use of analog modems for remote access.

Answer: Modems have the great advantage of being the most pervasively available remote access technology. The history of modems is long, with modems growing to be a very reliable choice for remote access. Speeds have improved over the years, with compression technology increasing the effective throughput to beyond 100 kbps. The biggest negatives about using modems include their relatively low speed and the fact that you cannot use the phone at the same time as you send data.

10. List some of the pros and cons regarding the use of ISDN for remote access.

Answer: ISDN's advantages include the capability to support voice calls concurrently with a data call. Also, ISDN can be used over the local telco loop, with no significant distance limitations. And it provides more bandwidth than do modems, particularly with both B channels dialed to the same remote site. ISDN does have a few disadvantages, with the biggest disadvantage being the lower speeds than DSL or cable.

11. List some of the pros and cons regarding the use of DSL for remote access.

Answer: DSL provides high-speed Internet access to the home, exceeding downstream speeds of 1 Mbps. It supports concurrent voice and data, with the data service always being turned on—no dialing is required. And the service speed does not degrade when more users are added to the network. However, DSL simply will not be available to some people, based on the distance to the local CO or the availability of DSL services from the local telco. Also, even when the home is close enough to the CO, sites farther from the CO might run slower than sites closer to the CO.

12. Define what the acronym DSLAM stands for, and explain the concept behind how a DSLAM allows voice and data to flow over the same local loop phone line.

Answer: DSLAM stands for DSL access multiplexer, with DSL meaning digital subscriber line. The DSLAM is connected to the local loop, splitting off the voice frequencies (0 to 4000 Hz) for the voice switch in the CO. It also interprets the higher frequencies as encoded digital signals, receiving the ATM cells sent over that digital signal, and forwards those ATM cells to the appropriate router.

13. Which of the DSL standards is the most common in the United States today? What is the range of upstream and downstream speeds for that type of DSL, as well as the maximum distance of the local loop?

Answer: ADSL, meaning asynchronous DSL, is the most popular. The downstream speeds range from 1.5 to 8 Mbps, with upstream speeds from 64 to 800 kbps. The maximum distance is 18,000 feet (approximately 5500 meters).

14. What protocols are used by DSL at the data link layer? Answer: ATM, Ethernet, and PPP.

15. Imagine that Andy and Barney are neighbors, and they both use cable modems. Describe the type of traffic that they could generate that could cause collisions, and tell what is done to help prevent those collisions.

Answer: Downstream data never can cause a collision with upstream data because the two are sent in different frequency ranges. Because only the head end sends downstream data, no collisions can occur. Upstream data from all subscribers uses the same frequency range, so data sent to the head end by Andy and Barney could collide. Cable standards use a feature called time-division multiple access (TDMA) to assign time slots to each subscriber. This prevents most collisions so no collisions should occur.

16. Name the four different Layer 1 encoding methods defined for use by cable modems. For each one, list whether it is used for upstream data, downstream data, or both.

Answer: QAM-64 and QAM-256 are both available for use as downstream encoding methods. QAM-16 and QPSK are both available for upstream encoding.

17. Which of the four different remote access technologies support IP, TCP, UDP, and the rest of the higher-layer TCP/IP protocols?

Answer: All of them!

18. Compare and contrast the cabling used by an analog modem and a DSL router/modem when connecting to the local phone company line. Identify the purpose of each pin on the connector.

Answer: Both use a cable with two wires, using an RJ-11 connector. Pin 3 is used for transmit; pin 4 is used for receive.

19. Compare and contrast the cabling used by an ISDN modem and a cable modem when connecting to the local phone company line or cable drop line. Identify the purpose of each pin on the connector.

Answer: ISDN uses a four-wire cable using an RJ-45 connector. The pinouts: pins 3 and 6 for transmit, and pins 4 and 5 for receive. Cable modems use coaxial cable with a single conductor, so there are no pins. The round connector on the end of the cable is called an f-connector.

20. List four standards bodies that have been involved in the development of DSL standards. Answer: ANSI, IEEE, ETSI, ITU.

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