Telecom OnLine: Lecture Mail

Welcome to Lesson 12, Day 1- Chapter 11 - Enterprise and Distributed Networks

Course Notes

Lesson 12 Notes

Modems in Network Communications

Modem stands for MOdulator/DEModulator. Modems convert digital signals to analog signals (modulation) for transmission over phone lines, and upon receipt convert analog signals to digital signals (demodulation).

Modems can be either internal or external. This is important as modem speeds increase. A serial port (RS-232) on a computer can only communicate at 115 Kbps. ISDN, which is discussed in detail in Chapter 12, has a maximum speed of 128 Kbps. For this reason, it is often best to have an internal ISDN adapter.

Modem Speed

The International Telecommunications Union has developed standards to define modem speed. These V-series standards measure modem speed in the number of bits per second (bps). Modem speed really does affect communication. A V.22bis modem will transmit a 1,000 word document in 25 seconds, whereas a V.42bis modem with compression will transmit it in one second. To see the history of the v-series modem standards, review Table 11-1 on page 401 of your text.

Originally, baud and bits per second were interchangeable. A modem that had 2,400 oscillations per second could transmit 2,400 bits per second. However, this has changed dramatically and now a modem transmitting 28,800 bps may only be using 9,600 oscillations per second or be transmitting at 9600 baud.

Modem Types

There are two types of modems used today: asynchronous and synchronous. The type of modem used depends on the type of phone lines and the networks involved in the communication.

• Asynchronous communication: asynchronous modems delineate each byte in a transmission. Async separate each byte with a start and stop bit. To ensure data integrity, a parity bit is added to each byte in an asynchronous communication stream. This parity bits works much the same way a CRC works in Ethernet.
  
• To increase communication throughput, many modems use compression such as Microcom’s MNP 5.
  
• Synchronous communication: Sync modems, as opposed to async modems, rely on timing to ensure that data frames are correctly delineated. Because of this, sync modems must be “in sync” to communicate. Sync modems are significantly faster than async modems.
  T here are three synchronous communications protocols, SDLC, HDLC, and bisync. Synchronous modems cannot operate over regular phone lines the way async modems can. Synchronous modems generally require a leased-line environment, which will be discussed in the next chapter. This increases the costs of using synchronous modems.
  
• Digital “modems”: These are not modems, in fact, but rather interfaces to connect computers to digital networks such as ISDN. Cable modems are also considered digital modems and are used to connect computer networks to CATV networks.
  
• Digital Subscriber Lines (DSL): DSL’s primary disadvantage is its distance limitation between the end user’s location and the nearest Central Office.
  
• Both cable modems and DSL connections are “always on”. This provides instant access to the Internet for end users, but also leaves connected computers vulnerable to attack. Many users with cable and DSL connections use software to protect their systems or hardware firewalls that prevent unauthorized access.

Carriers

When deciding the type of carrier (telephone line or communication mode) to use for your network, three general considerations will be the primary focus of your decision:

• Throughput
• Distance
• Cost

Public Switched Telephone Network (PSTN)

• Dial-up - the standard for home use and provides a temporary connection.
• ISDN - provides a dial-up solution for transmitting voice and data over a digital phone line - significantly more bandwidth
• DSL- represent an all-digital service that local carriers provide as a lower-cost, medium bandwidth alternative to conventional modems and dedicated leased lines.
• Leased Lines - provide continuous connections between two sites. More expensive, but higher speed. Speeds above T1 remain the strongest in competition with the other forms available.

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Lesson 12, Day 1 - Case Question

You have been tasked with suggesting a telecommuting alternative for employees at your company. The employees affected all live in areas that offer CATV service, but some live as far as six miles from the nearest cable central office. What kind of connectivity solution would you suggest that would use the same technology for all the involved users? List the positives and negatives of your suggested solution.

Remember to keep up with your on-line lessons and quizzes!

mdm