Networking Chapter 3

Control Signals

Used by computers to negotiate which will send and which will receive data.

Fiber-Optic Cable

Uses high-speed streams of light pulses from lasers or LEDs (light-emitting diodes) that carry information inside hair-thin strands of glass called optical fibers.

Manchester Encoding

A special type of bipolar signaling in which the signal is changed from high to low or from low to high in the middle of the signal. A change from high to low is used to represent a 0, whereas the opposite (a change from low to high) is used to represent a 1. Manchester encoding is less susceptible to having errors go undetected, because if there is no transition in midsignal the receiver knows that an error must have occurred.

Serial Transmission

A stream of data is sent over a communication circuit sequentially in a bit-by-bit fashion. In this case, there is only one physical wire inside the bundle and all data must be transmitted over that one physical wire. The transmitting device sends one bit, then a second bit, and so on, until all the bits are transmitted. It takes n iterations or cycles to transmit n bits. Thus, serial transmission is considerably slower than parallel transmission

Analog Data

Electrical signals are shaped like the sound waves they transfer; they can take on any value in a wide range of possibilities, not just 0 or 1.

Codec

Two codecs, one at the sender's end and one at the receiver's end. Used to translate analog voice data into digital form for transmission over digital computer circuits

Half-Duplex Transmission

Two-way transmission, but you can transmit in only one direction at a time.

Coaxial Cable

Type of guided media that is quickly disappearing. Coaxial cable has a copper core (the inner conductor) with an outer cylindrical shell for insulation. The outer shield, just under the shell, is the second conductor. Because they have additional shielding provided by their multiple layers of material, coaxial cables are less prone to interference and errors than basic low-cost twisted pair wires. Coaxial cables cost about three times as much as twisted pair wires but offer few additional benefits other than better shielding.

2 types of data that can flow through a circuit

1. Analog 2. Digital

FSK

Frequency Shift Keying (another term for FM)

bit rate

(Also known as Data Rate) The number of bits per second transmitted.

Data Rate

(Also known as bit rate) Calculated by multiplying the number of bits sent on each symbol by the maximum symbol rate.

local loop

(Sometimes called the last mile), the wires that run from your home or business to the telephone switch that connects your local loop into the telephone network.

Digital Data

Binary, either on or off, 0 or 1.

Modulation

Shape Changes

Three important characteristics of sound waves transmitted through the voice circuit

1. Amplitude 2. Frequency 3. Phase

5 Benefits of Digital over Analog transmission

1. Digital transmission produces fewer errors than analog transmission. Because the transmitted data is binary, it is easier to detect and correct errors 2. Digital transmission permits higher maximum transmission rates. Fiber-optic cable is designed for digital transmission 3. Digital transmission is more efficient. It is possible to send more data through a circuit using digital transmission 4. Digital transmission is more secure because it is easier to encrypt 5. Integrating voice, video, and data on the same circuit is far simpler with digital transmission

Two types of Media

1. Guided Media 2. Wireless Media

Transmission modes - 2

1. Parallel Transmission 2. Serial Transmission

Circuit - 2 kinds

1. Physical 2. Logical

Modem

A modem at the sender's computer translates the computer's digital data into analog data that can be transmitted through the voice communication circuits, and a second modem at the receiver's end translates the analog transmission back into digital data for use by the receiver's computer.

ADPCM

Adaptive Differential Pulse Code Modulation - The alternative used by IM and many other applications that provide voice services over lower-speed digital circuits. ADPCM works in much the same way as PCM. It samples incoming voice signal 8,000 times per second and calculates the same 8-bit amplitude value as PCM. However, instead of transmitting the 8-bit value, it transmits the difference between the 8-bit value in the last time interval and the current 8-bit value (i.e., how the amplitude has changed from one time period to another). Because analog voice signals change slowly, these changes can be adequately represented by using only 4 bits. This means that ADPCM can be used on digital circuits that provide only 32 Kbps (4 bits per sample × 8,000 samples per second = 32,000 bps).

Multipoint Circuit

Also called a shared circuit. In this configuration, many computers are connected on the same circuit. This means that each must share the circuit with the others. The disadvantage is that only one computer can use the circuit at a time. The advantage of multipoint circuits is that they reduce the amount of cable required and typically use the available communication circuit more efficiently.

AM

Amplitude Modulation- The amplitude or height of the wave is changed. One amplitude is the symbol defined to be 0, and another amplitude is the symbol defined to be a 1.

ASK

Amplitude Shift Keying (another term for AM)

Lempel-Ziv Encoding

As a message is being transmitted, Lempel-Ziv encoding builds a dictionary of two-, three-, and four-character combinations that occur in the message. Anytime the same character pattern reoccurs in the message, the index to the dictionary entry is transmitted rather than sending the actual data. The reduction provided by V.44 compression depends on the actual data sent but usually averages about 6:1 (i.e., almost six times as much data can be sent per second using V.44 as without it).

BONDING

Bandwidth On Demand Interoperability Networking Group - Splits outgoing messages from one client or host across several low-speed telephone lines and combines incoming messages from several telephone lines into one circuit so that the client or host "thinks" it has a faster circuit.

baud rate

Being replaced by the term Symbol Rate.

bps

Bits per second.

Data Compression

Can increase throughput of data over a communication link by literally compressing the data.

Full-Duplex Transmission

Can transmit in both directions simultaneously, with no turnaround time

Channel

Circuits can be separated into channels. Each channel is a separate logical circuit, and the devices connected to them are unaware that their circuit is multiplexed.

ISO 8859

Commonly used coding scheme. ISO 8859 is an 8-bit code that includes the ASCII codes plus non-English letters used by many European languages (e.g., letters with accents).

Unicode

Commonly used coding scheme. There are many different versions of Unicode. UTF-8 is an 8-bit version which is very similar to ASCII. UTF-16, which uses 16 bits per character (i.e., two bytes, called a "word"), is used by Windows. By using more bits, UTF-16 can represent many more characters beyond the usual English or Latin characters, such as Cyrillic or Chinese.

switch

Contains a codec that converts the analog signal from your phone into a digital signal. This digital signal is then sent through the telephone network until it hits the switch for local loop for the person you are calling. This switch uses its codec to convert the digital signal used inside the phone network back into the analog signal needed by that person's local loop and telephone.

CPE

Customer Premises Equipment - A DSL Modem installed in the customer's home or office and another DSL modem is installed at the telephone company switch closest to the customer's home or office.

DSL

Digital Subscriber Line - Family of techniques that combines analog transmission and FDM to provide a set of voice and data circuits.

Quantizing Error

Digitized signal is only a rough approximation of the original signal. The original signal had a smooth flow, but the digitized signal has jagged "steps." The difference between the two signals is called quantizing error. Voice transmissions using digitized signals that have a great deal of quantizing error sound metallic or machinelike to the ear.

Microwave Transmission

Extremely high-frequency radio communication beam that is transmitted over a direct line-of-sight path between any two points. As its name implies, a microwave signal is an extremely short wavelength, thus the word micro-wave. Microwave radio transmissions perform the same functions as cables.

FDM

Frequency Division Multiplexing - Dividing the circuit "horizontally" so that many signals can travel a single communication circuit simultaneously. The circuit is divided into a series of separate channels, each transmitting on a different frequency, much like series of different radio or TV stations. All signals exist in the media at the same time, but because they are on different frequencies, they do not interfere with each other.

FM

Frequency Modulation- Modulation technique whereby each 0 or 1 is represented by a number of waves per second (i.e., a different frequency). In this case, the amplitude does not vary. One frequency (i.e., a certain number of waves per second) is the symbol defined to be a 1, and a different frequency (a different number of waves per second) is the symbol defined to be a 0.

Hz

Hertz - Symbols per second (Cycles per Second).

Symbol Rate

How many symbols will be sent over the circuit per second

Twisted Pair Cable

Insulated pairs of wires that can be packed quite close together. The wires usually are twisted to minimize the electromagnetic interference between one pair and any other pair in the bundle.

IMUX

Inverse Multiplexing - Combines several low-speed circuits to make them appear as one high-speed circuit to the user.

Analog Transmission

Occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern much like the human voice. Modems translate the digital binary data produced by computers into the analog signals required by voice transmission circuits. One modem is used by the transmitter to produce the analog signals and a second by the receiver to translate the analog signals back into digital signals.

Simplex Transmission

One-way transmission, such as that with radios and TVs.

PM

Phase Modulation - Phase refers to the direction in which the wave begins. Until now, the waves we have shown start by moving up and to the right (this is called a 0◦ phase wave). Waves can also start down and to the right. This is called a phase of 180◦. With phase modulation, one phase symbol is defined to be a 0 and the other phase symbol is defined to be a 1.

PSK

Phase Shift Keying (another term for PM)

POTS

Plain Old Telephone Service - Enables voice communication between any two telephones within its network. The telephone converts the sound waves produced by the human voice at the sending end into electrical signals for the telephone network. These electrical signals travel through the network until they reach the other telephone and are converted back into sound waves.

PCM

Pulse Code Modulation - The input voice signal is sampled 8,000 times per second. Each time the input voice signal is sampled, 8 bits are generated. Therefore, the transmission speed on the digital circuit must be 64,000 bps (8 bits per sample × 8,000 samples per second) to transmit a voice signal when it is in digital form. Thus, the North American telephone network is built using millions of 64 Kbps digital circuits that connect via codecs to the millions of miles of analog local loop circuits into the users' residences and businesses.

QAM

Quadrature Amplitude Modulation - involves splitting the symbol into eight different phases (3 bits) and two different amplitudes (1 bit), for a total of 16 different possible values. Thus, one symbol in QAM can represent 4 bits, while 256-QAM sends 8 bits per symbol. 64-QAM and 256-QAM are commonly used in digital TV services and cable modem Internet services.

Point-to-Point Circuit

Sometimes called dedicated circuits because they are dedicated to the use of two computers. This type of configuration is used when the computers generate enough data to fill the capacity of the communication circuit.

STDM

Statistical Time Division Multiplexing - Allows more terminals or computers to be connected to a circuit than does FDM or TDM.

Character

Symbol that has a common, constant meaning. A character might be the letter A or B, or it might be a number such as 1 or 2. Characters also may be special symbols such as ? or &. Characters in data communications, as in computer systems, are represented by groups of bits that are binary zeros (0) and ones (1).

Physical Circuit

The actual wire used to connect two devices

Turnaround Time

The amount of time half-duplex communication takes to switch between sending and receiving. Also called retrain time or reclocking time

Circuit Configuration - 2 kinds

The basic physical layout of the circuit. 1. Point-to-Point 2. Multipoint

V.44

The ISO standard for data compression, uses Lempel-Ziv encoding.

bandwidth

The difference between the highest and the lowest frequencies in a band or set of frequencies.

Phase

The direction in which the wave begins. Phase is measured in the number of degrees (◦).

Amplitude

The height of the wave.

Coding Scheme

The language that computers use to represent data.

Wireless Media

The message is broadcast through the air, such as microwave or satellite.

Guided Media

The message flows through a physical media such as a twisted pair wire, coaxial cable, or fiber-optic cable; the media "guides" the signal.

Frequency

The number of waves per second.

Bipolar Signaling

The ones and zeros vary from a plus voltage to a minus voltage (like an AC current).

Medium (Media if plural)

The physical matter or substance that carries the voice or data transmission.

Logical Circuit

The transmission characteristics of the connection, such as when we say a company has a T1 connection into the Internet. In this case, T1 refers not to the physical media (i.e., what type of wire is used) but rather to how fast data can be sent through the connection.

Guardband

The unused portions of the circuit that separate these frequencies from each other.

Unipolar signaling

The voltage is always positive or negative (like a DC current).

Parallel Transmission

The way the internal transfer of binary data takes place inside a computer. If the internal structure of the computer is 8 bit, then all 8 bits of the data element are transferred between main memory and the central processing unit simultaneously on 8 separate connections. The same is true of computers that use a 32-bit structure; all 32 bits are transferred simultaneously on 32 connections.

TDM

Time Division Multiplexing - Shares a communication circuit among two or more computers by having them take turns, dividing the circuit vertically, so to speak.

Multiplexing

To break one high-speed physical communication circuit into several lower-speed logical circuits so that many different devices can simultaneously use it but still "think" that they have their own separate circuits (the multiplexer is "transparent").

baud

Unit of signaling speed used to indicate the number of times per second the signal on the communication circuit changes.

USASCII

United States of America Standard Code for Information Interchange (More commonly known as ASCII) - the most popular code for data communications and is the standard code on most microcomputers. There are two types of ASCII; one is a 7-bit code that has 128 valid character combinations, and the other is an 8-bit code that has 256 combinations.

VoIP

Voice Over Internet Protocol - Commonly used to transmit phone conversations over digital networks. VoIP is a relatively new standard that uses digital telephones with built-in codecs to convert analog voice data into digital data. Because the codec is built into the telephone, the telephone transmits digital data and therefore can be connected directly into a local area network, in much the same manner as a typical computer. Because VoIP phones operate on the same networks as computers, we can reduce the amount of wiring needed; with VoIP, we need to operate and maintain only one network throughout our offices, rather than two separate networks—one for voice and one for data. However, this also means that data networks with VoIP phones must be designed to operate in emergencies (to enable 911 calls) even when the power fails; they must have uninterruptable power supplies (UPS) for all network circuits.

WDM

Wavelength Division Multiplexing - A version of FDM used in fiber-optic cables.

Carrier Wave

When we transmit data through the telephone lines, we use the shape of the sound waves we transmit (in terms of amplitude, frequency, and phase) to represent different data values. We do this by transmitting a simple sound wave through the circuit (called the carrier wave) and then changing its shape in different ways to represent a 1 or a 0.

Radio Transmission

When you connect your laptop into the network wirelessly, you are using radio transmission. Each device or computer on the network has a radio receiver/transmitter that uses a specific frequency range that does not interfere with commercial radio stations. The transmitters are very low power, designed to transmit a signal only a short distance, and are often built into portable computers or handheld devices such as phones and personal digital assistants.

Digital Transmission

he transmission of binary electrical or light pulses in that it only has two possible states, a 1 or a 0. The most commonly encountered voltage levels range from a low of +3/−3 to a high of +24/−24 volts. Digital signals are usually sent over wire of no more than a few thousand feet in length.

Satellite Transmission

similar to microwave transmission except instead of transmission involving another nearby microwave dish antenna, it involves a satellite many miles up in space.

bit

unit of information