Chapter 2 Fundamental of Data Communications
differentiate between bit rates and baud rates
define the data codes used in data communications
describe the purpose of a modem
define the voice communication network
describe how the telephone network operates
describe the difference between a dialed and dedicated circuit
Data communication is the transmission of data from one point to another over a network. Data must be in a form that can be transmitted quickly and efficiently. The sender and receiver must be able to use the form. Any errors that occur during transmission also must be detected and corrected. In this chapter we will talk about the format of data as it is transmitted over a network and the various forms that data can take. We will also examine the phone system.
Signal Representation
When data is sent over a network it is transmitted by means of a signal.
A signal can contain only the data being sent to it or it can contain control
info needed by the network to transmit the data correctly. This signal
must be in a particular form, depending on the type of network being used.
A bit is the smallest unit of information used in computer. It is represented by a 0(off) or 1(on). The use of two digits to represents bits is referred to as binary notation.
Bits are used together to create characters within the computer. The different ways of representing these characters are known as data codes. Characters can be represented by 5, 6, 7, or 8 bits, depending on the computer and which data codes is used. The string of bits used to represent a character is referred to as a byte.
Bit rate - the speed at which data is sent over a network, usually
expressed in bits per second (bps). Today's communications networks
transmit in the Mbps or Gbps range.
Baud - a change in signal and thus, the baud rate is the number of times a signal changes per second. The change that a signal takes refers to the voltage level of the signal and the direction of the signal. P.2-3
The up and down reversal of the wave (current) produces what is called and alternating current (AC). Alternates from 0 to +(max) to 0 to -(min) to 0. This entire process is called a cycle.
The alternative to AC is direct current (DC) (Battery). Current flows from + to -. Current only flows in one direction where with AC it flows in both directions.
Bit and baud rate sometimes used interchangeably, not always correct.
If the number of bits per signal is 1, then they are the same. If
there are more than 1 bit per signal, then the rates are different.
frequency - refers to the number of times that a current passes
through a complete cycle. Measurement is a hertz(Hz), one cycle per
second. Range from 0 Hz to 300 gigahertz.
Kilo(K) - multiple of 1,000 mega(m) - multiple of 1,000,000
giga(G) -multiple of 1,000,000,000
Radio and television transmit over specific frequencies as determined by the FCC. Short-wave radio and microwave transmissions also make use of specific groups of frequencies. Lowest frequencies (15Hz to 30KHz) called audio frequencies. Table 2-1, p2-4
bandwidth - defined as a range of frequencies (Hz) = higher frequency
- lower frequency
2500 Hz - 100 Hz = 2400 Hz bandwidth
- important because it indicates how fast data can be transmitted
over a specific channel
Digital Signals Versus Analog Signals
Digital data is composed of items that are distinct from one another (text, integers, and Morse code). Computers manipulate this type of data. Even if info is entered into the computer in a non-digital form (voice or video), the computer must convert it to a digital form (0 and 1's) before it can process it.
Analog data is continuous data - no individual element can be identified from any other element (light, voice, video)
Difference on p 2-6, figure 2-4
Modems
A device used to convert a digital signal into an analog signal, and conversely, from and analog signal to a digital signal is called a Modulator/DEModulator (modem). It is needed when data generated by a computer(digital) is to be transmitted over the telephone(analog) network. Two types of modems - internal(printed circuit board called a card) and external. Modem speeds vary from 300 bps (very obsolete) to 56,000 bps. Common use is to dial into bulletin board system or Internet service providers to send e-mail or browse the WWW. Bulletin board system (BBS) is a computerized service provided by either an individual or a company on which others may leave messages or retrieve message left by others. Newsgroups (Usenet). Internet service provider (ISP) is an organization that has a permanent connection to the Internet and provides temporary connections to other for a fee.
Modem eliminator or null modem - device that can be used for short-distance communication between two devices. High-speed communication at a very low price for distances of 50ft or less; transmission speeds range from 1,200 bps to 38,400 bps.
Short haul modems - permit communication between two devices that are up to 20 miles apart. As distance increases, transmission speeds get slower.
Fax modem - modems that also have the capability to provide facsimile(fax) services.
Modems are also being built with the capability of voice mail.
A computer connected to the telephone system with a modem can be used as
an answering service.
Data Codes
As we talked about before, computers store data as strings of bits that are combined into bytes, which represent characters. In order to represent these characters, data codes have been developed. The codes are necessary because computers must have rules on how to convert the individuals bits into characters. The use of data codes ensures that computers that use the same codes can communicate. Common data codes p 2-11, fig 2-10
Morse code - precursor to modern data codes, uses a series of dots and dashes to represent characters, it does not use a consistent number of dots and dashes for each character
Baudot - patterned after the type of codes used by the telegraph industry , uses two shirt characters to create 64 characters, no longer in use today
American Standard Code for Information Interchange (ACSII) -
one of the most widely used data transmission codes, most computer store
data in the ACSII format
- made up of a string of seven bits which provides the possibility
of creating 128 unique combinations of bits
Extended ASCII - uses an eight bits, the eight bit (known as the high-order bit) is used to create an additional 128 characters know as high code.
Extended Binary-Coded Decimal Interchange Code (EBCDIC) - eight-bit
code used by IBM mainframe computers. Allows 256 characters to be
represented
Telecommunications - Voice Communications
The telephone system is made up of many devices, some of which are located in the home or business of the customer and some of which are located at the telephone. To use the network, a telephone, lines, switches, and an electrical signal are needed. We will talk about parts of the network and how they interact.
Telephone
- primary purpose is to take the human voice and
convert it into a signal that can transmitted over a line or other
communication channel. Another telephone converts it back to voice
on the receiving end.
-contains devices called converters that change
human speech into an analog electrical signal. Each telephone has
two
converters: one is in the portion of the telephone handset used for
speaking(transmitter) and the other is in the part of
the telephone handset used for hearing (receiver). Telephone requires
electricity to create a telephone signal, it is
provided by the telephone company. P.2-17
Telephone network - made up of telephones in homes in businesses, the wires that connect the homes and businesses, one or more telephone switching offices, and local or long-distance telephone circuits.
Touch-tone phones use electrical pulses for dialing, called dual tone multifrequency (DTMF) signaling, two different frequency tones are generated each time a button on the telephone is pressed. That signal is transmitted, and the computers at the telephone company's office recognize the generated tone.
Caller-ID, call forwarding, call waiting, and voice mail. Companies and banks are using the telephone to conduct business through use of touch-tone.
Fig 2-20 on p 2-18 , Typical telephone network. Telephones are connected by wires to a telephone company end office. End office (central office(CO), exchange office(EO), or local exchange carrier(LEC)) is where call switching takes place. Switching refers to the routing of the telephone call onto the correct set of wires in order to connect it to the proper receiver. The wires that connect a home or business to the end office are called a local loop. Any communication system requires a continuous circuit(dedicated connection). The time during which the call is connected is called a session.
Telephone calls
When a telephone number is dialed, the switching equip. in the local end office uses the number as an address to connect to another end office. A call placed within local end offices is considered a local call. If the first three digits (prefix) of the called number is the same as the calling number, it is an intraoffice call. If a customer makes a call to a phone not connected to the local end office - an interoffice call - the call must be placed through another end office. These end offices are connected by means of trunks, the telephone company's term for telephone line that connects telephone company offices.
Calls made within the same area code, but for which the caller must dial a 1 before the number are routed through tandem offices. Tandem offices has no lines connected directly to customers and thus is well-equipped to handle a large volume of calls between exchanges. Calls routed through a tandem office are called unit calls, they incur an additional charge.
A call placed to a different area code is a long-distance call.
They are routed through tolls offices, which are connected to long-distance
switching offices that route the calls to the appropriate number.
Toll offices are connected by toll trunks which are long-distance telephone
lines (microwave, copper wires, fiber-optic wires). Figure 2-22,
p 2-21
Telephone lines
have many levels of quality and are used for different purposes
voice-grade circuits - used for voice telephone calls and dial-up
lines are the lowest grade, often used for data transmission
multiplexing - use to efficiently transmit both data and voice signals over the same line, it combines several signals on a single medium thus allowing a single communications link to carry many signals simultaneously.
Dedicated telephone circuit - a single, continuous circuit is available between two locations 24 hours a day, 7 days a week; it allows uses the same path
Telephone signals
p2-22, table 2-6
Telephone Numbering System
Direct Distance Dialing (DDD) - customer dials a telephone number with no assistance from an operator
Area Codes - used to facilitate long-distance telephone calls, NC
Exchange Codes - within each area code, different exchange codes, simply called exchanges are established because no two telephones can have the same telephone number. It is the first three digits (after the area code) of a telephone number
Country Codes - to make and international long-distance call, the caller must dial a country code with the number because the area code in a country outside of the US may be the same as one in the US. Country codes are not a consistent length.
800 Service/Wide Area Telephone Services (WATS) - telephone calls are made to numbers with a 800 area code and the calls are toll free to the caller. The called party pays for the call.
900 Service - telephone calls are made to numbers with a 900 area code. 7,000 calls can be handled simultaneously on a single telephone number. Viewers can dial and express views and opinions, provided recorded or live messages, dating and sexually oriented services
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