Ebook Network warrior (Second edition): Part 2

Ebook Network warrior (Second edition): Part 2 include of the following contents: Chapter 20: Telecom Nomenclature, Chapter 21: T1, Chapter 22: DS3, Chapter 23: Frame Relay, Chapter 24: MPLS, Chapter 25: Access Lists, Chapter 26: Authentication in Cisco Devices, Chapter 27: Basic Firewall Theory, Chapter 28: ASA Firewall Configuration, Chapter 29: Wireless, Chapter 30: VoIP, Chapter 31: Introduction to QoS, Chapter 32: Designing QoS, Chapter 33: The Congested Network, Chapter 34: The Converged Network, Chapter 35: Designing Networks, Chapter 36: IP Design, Chapter 37: IPv6, Chapter 38: Network Time Protocol, Chapter 39: Failures, Chapter 40: GAD’s Maxims, Chapter 41: Avoiding Frustration.
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Ebook Network warrior (Second edition): Part 2 CHAPTER 20 Telecom Nomenclature The telecom world is a bit different from the data world, as endless telecom engineers will no doubt tell you. For example, a lot of the telecom infrastructure that exists today is the way it is because of standards that have been in place for upward of 100 years. Samuel Morse invented the telegraph in 1835. Alexander Graham Bell invented the telephone in 1876. In 1961, Bell Labs invented the T1 as a way to aggregate links be- tween the central offices (COs) of the phone companies. It took almost 100 years to get from the first telephone to the invention of the T1. In contrast, consider the data world: the ARPANET was started in 1969, Robert Met- calfe and David Boggs built the first Ethernet in 1973, and Vint Cerf and Bob Kahn published the original TCP/IP standard in 1974. Hayes introduced the first modem in 1977 (300 bps, baby!), and 3Com shipped the first 10 Mbps Ethernet card in 1981. The first commercial router was sold in 1983. Let’s think about that for a moment—the first commercial router was sold in 1983. Ask anyone around you if she can remember a time when there weren’t phones. The telecom world is built on standards that work and have worked for a very long time. How often does your phone stop working? The telecom infrastructure is so reli- able that we expect reliable phone service even more than we expect reliable electrical service (cellular service is a whole different ball game, and does not apply to this dis- cussion). As with any technology, the engineers in the trenches (and their bosses behind the desks) like to sling the lingo. If you’ve spent your professional life around data equip- ment, telecom lingo might seem pretty foreign to you. To help bridge the gap between the data and telecom worlds, I’ve put together a list of terms that you might hear when dealing with telecom technologies. 341 Most telecom words and phrases have standard meanings defined in Federal Standard 1037C, titled “Telecommunications: Glossary of Tel- ecommunication Terms.” These definitions are often very simple and don’t go into a lot of detail. The terms I’ll cover here are the ones most often encountered in the life of a network administrator or engineer. If you need to know how to refer to circuit noise voltage measured with a psophometer that includes a CCIF weighting network, Federal Standard 1037C is a good place to look. Another excellent source that should be on the bookshelf of anyone in the telecom world is Newton’s Telecom Dictionary, by Harry Newton (CMP Books). The meanings of many widely used telecom terms have changed over time. Through regulation over the years, the functions of entities like IXCs and LECs (both defined in the following glossary) have changed. I will cover the original intended meanings in this text. Telecom Glossary ACD ACD stands for Automatic Call Distributor. An ACD is usually found in a call center, where calls may come in from anywhere and need to be directed to the next available operator or queued until one is available. Add/drop The term add/drop is used in telecom to describe the capability of peeling off channels from a circuit and routing them elsewhere. An add/drop CSU/DSU can separate ranges of channels to discrete data ports, thus allowing a T1 to be split into two partial T1s. One can be used for voice and the other for data, or both can be used for either function and routed differently. You can make an add/drop device function like a non-add/drop device simply by assigning all of the channels to a single data port. However, add/drop adds cost to devices, so it should only be considered if the extra functionality is required. Analog and digital Would you like to have some fun? Ask someone in the computer field to define the word “analog.” You might be surprised at some of the answers you receive. Analog means, literally, the same. When one item is analogous to another, it is the same as the other item. In the telecom and data worlds, “analog” refers to a signal that is continuous in amplitude and time. An analog signal is not composed of discrete values: any small fluctuation of the signal is important. Radio waves are analog, as are power waves. Sound is also analog. When you speak, you create waves of air that hit people’s eardrums. The sound waves are an analog signal. 342 | Chapter 20: Telecom Nomenclature Digital refers to a signal that has discrete values. If you analyze a sound wave, and then assign a value to each sample of the wave at specific time intervals, you will create a digital representation of the analog wave. Because digital involves discrete values and analog does not, converting analog signals to digital will always result in loss of information. While increasing the rate at which the signal is sampled (among other things) can increase the quality of the final reproduction, technically, the signal cannot be reproduced exactly the same way. Bandwidth Bandwidth is one of those terms that is thrown around a lot by people who don’t really know what it means. A range of frequencies is called a band. The width of the band is referred to as bandwidth. For those of you who aren’t old enough to remember FM radios with analog dials, I’ve drawn one in Figure 20-1. Figure 20-1. True bandwidth example An FM radio dial displays the range of frequencies allocated by the US government for stereo radio broadcasts. The exact range of frequencies is 87.9 ...