Exam 1 Flashcards

Question

10. Explain how a message is transmitted from one computer to another using layers.

Answer 1

The application layer is the application software used by the network user. The transport layer is responsible for obtaining the address of the end user (if needed), breaking a large data transmission into smaller packets (if needed), ensuring that all the packets have been received, eliminating duplicate packets, and performing flow control to ensure that no computer is overwhelmed by the number of messages it receives. The network layer takes the message generated by the application layer and if necessary, breaks it into several smaller messages. It then addresses the message(s) and determines their route through the network, and records message accounting information before passing it to the data link layer. The data link layer formats the message to indicate where it starts and ends, decides when to transmit it over the physical media, and detects and corrects any errors that occur in transmission. The physical layer is the physical connection between the sender and receiver, including the hardware devices (e.g., computers, terminals, and modems) and physical media (e.g., cables, and satellites).

Answer 2

The formal standardization process has three stages: specification, identification of choices, and acceptance. The specification stage consists of developing a nomenclature and identifying the problems to be addressed. In the identification of choices stage, those working on the standard identify the various solutions and choose the optimum solution from among the alternatives. Acceptance, which is the most difficult stage, consists of defining the solution and getting recognized industry leaders to agree on a single, uniform solution. ISO standards development is pursued at the national and international levels. Authorized national technical committees can be designated as Technical Advisory Groups (TAGs) to international subcommittees or workgroups.

Answer 3

Standards Designation Name of National Standards Body (ISO Member) ANSI American National Standards Institute SCC Standards Council of Canada DGN Dirección General de Normas BSI British Standards Institution JISC Japanese Industrial Standards Committee AFNOR Association française de normalisation BIS Bureau of Indian Standards CSBTS China State Bureau of Quality and Technical Supervision

Answer 4

The Internet Engineering Task Force (IETF; www.ietf.org) sets the standards that govern how much of the Internet will operate. Developing a standard usually takes 1-2 years. Usually, a standard begins as a protocol developed by a vendor. When a protocol is proposed for standardization, IETF forms a working group of technical experts to study it. The working group examines the protocol to identify potential problems and possible extensions and improvements, and then issues a report to IETF. If the report is favorable, the IETF issues a Request for Comment (RFC) that describes the proposed standard and solicits comments from the entire world. Once no additional changes have been identified, it becomes a Proposed Standard. Once at least two vendors have developed software based on it, and it has proven successful in operation, the Proposed Standard is changed to a Draft Standard. This is usually the final specification, although some protocols have been elevated to Internet Standards, which usually signifies a mature standard not likely to change. There is a correlation of IETF RFCs to ISO standards.

Answer 5

The International Organization for Standardization (ISO) makes technical recommendations about data communication interfaces. The Telecommunications group (ITU-T) is the technical standards-setting organization of the United Nations International Telecommunications Union (ITU). Postal Telephone and Telegraphs (PTTs) are telephone companies outside of the United States. ITU-T establishes recommendations for use by PTTs, other common carriers, and hardware and software vendors. Although a complicated series of acronyms, it is useful to point out that the ISO created the OSI model! Information technology standards contribute to data communications. In the USA, the National Committee for Information Technology Standards (NCITS) has responsibility (under ANSI) for multimedia (MPEG/JPEG), intercommunication among computing devices and information systems (including the Information Infrastructure, SCSI-2 interfaces, Geographic Information Systems), storage media (hard drives, removable cartridges), database (including SQL3), security, and programming languages (such as C++). The NCITS T3 committee on Open Distributed Processing (ODP) is the US Technical Advisory Group (TAG) to JTC 1/SC 6/WG 7 (Subcommittee 6, Workgroup 7). JTC 1 is the ISO/IEC Joint Technical Committee 1 on Information Technology. Among NCITS/T3's current projects are: Abstract Syntax Notation One (ASN.1), the OSI Directory Services (and protocols), routing information exchange protocols, multicasting (all of considerable interest to the telecommunications industry.) T3 has US TAG responsibility for codes and character sets. IEEE plays an important standards role for data communications, particularly in LAN technology protocols. Note that the HTML specifications state that HTML uses the ISO 8859-1 (Latin 1) character set.

Answer 6

The use of standards makes it much easier to develop software and hardware that link different networks because software and hardware can be developed one layer at a time. The software or hardware defined by the standard at one network layer can be easily updated, as long as the interface between that layer and the ones around it remains unchanged.

Answer 7

Two of the largest inter-exchange carriers (IXCs) in North America are AT&T and Sprint, and the formerly large MCI was acquired in a post bankruptcy merger with Verizon.

Answer 8

First, pervasive networking will change how and where we work and with whom we do business. Pervasive networking means that we will have high speed communications networks everywhere, and that virtually any device will be able to communicate with any other device in the world. Prices for these networks will drop and the globalization of world economies will continue to accelerate. Second, the integration of voice, video, and data onto the same networks will greatly simplify networks and enable anyone to access any media at any point. Third, the rise in these pervasive, integrated networks will mean a significant increase the availability of information and new information services. It is likely that application service providers will evolve that act as information utilities.

Answer 9

The Internet model is simpler (effectively collapsing the top three layers of the OSI model into a single model) and easier to remember and understand. Further, the ISO OSI Reference Model is the result of a formal standardization process and is technical in its presentation. By contrast, the Internet model is appropriate for those within the networking community with practical needs related to implementing the Internet and networking. However, only a few years ago the Internet model was commonly understood to have only four layers. Today, the transport layer is now separately identified in the Internet model, yielding an important, fifth layer for comprehension. This evolution in presentation may show that at least one technical distinction from the OSI model is now considered practical as the scope, volume of traffic, and complexity of networking (and of the Internet) grows.

Answer 10

Today there is convergence around the non-proprietary use of TCP/IP as the protocol of choice for all networks. For the most part, network software is designed to interface with networks using this protocol. By non-proprietary, this means that TCP/IP is an interoperable protocol portable to any manufacturer's hardware. All manufacturers are developing their products to use TCP/IP as their protocol of choice. This is of great benefit for those operating networks because they do not have to deal with the incompatibilities of various proprietary networks. In the past, network equipment such as IBM’s SNA and Novell’s Netware products had retained proprietary protocols that did not interface with as much ease as today’s more compatible and TCP/IP based products. The decline of the number of competing protocols is related to the emergence of TCP/IP as the universal connector, along with the rise in competition and subsequently better price availability from those vendors who market to this protocol, thus ensuring the viability of this standard for a long time to come for network managers.

Answer 11

The biggest reason that there are more standardized protocols at the application layer is related to the predominant use of the Web and its standardized graphic interface (HTTP, DHCP, for example). In a way, many new protocols ride on top of TCP/IP networks, and some of these new protocols have been developed to enable the retrofitting of new technologies on top of an older networking architecture. On the other hand some proprietary protocols connected with such models as IBM's SNA and DECNet have declined in significance while the importance of Internet-related protocols has grown,

Answer 12

First, some assumptions must be made. Assume each word averages seven letters and there is one space between each word. Next assume we are using 8-bit ASCII. . Multiply 350 words by 8 bytes (7 letters plus a space) to get 2,800 bytes per page. Multiply 2,800 by 10 pages to get 28,000 Multiply 28,000 bytes by 8 bits per byte to get 224,000 bits

Answer 13

Host-based (all processing done on host system and all data on host with terminals providing access), client-based (with processing done on client and all data stored on server), and client-server (balanced processing; usually host provides data access and storage while the client provides application and presentation logic).

Answer 14

Data storage, data access logic, application logic, and presentation logic.

Answer 15

Advantages: Centralized security Integrated architecture from single vendor Simpler, centralized installation Disadvantages: Having all processing on host may lead to overload Cost of software and upgrades; expensive infrastructure Terminal totally dependent on server

Answer 16

Advantages: Balanced processing demands Lower cost; inexpensive infrastructure Can use software and hardware from different vendors Scalability Disadvantages: Problems with using software and/or hardware from different vendors More complex installation or updating (although automated installation software helps greatly in this area).

Answer 17

Middleware manages client-server message transfer and shields application software from impacts of hardware changes. Middleware provides standard communication between products of different vendors through translation.

Answer 18

Infrastructure supporting cabling hardware and software will need to be redesigned to support the client-server approach to the architecture. Someone would need to be designated to manage what would now become the local area network, so there may be a personnel impact. Security would be one area of concern, since processing can be done on individual workstations. There may be somewhat greater complexity of upgrades, although newer software is reducing the impact of this kind of problem.

Answer 19

Client-server networks are less expensive because in a competitive market involving multiple vendors, software and hardware upgrades cost substantially less. Upgrades for host-based networks are generally very expensive, and occur in what is generally termed a “step function,” meaning requiring large, discrete steps in expenditure. LANs have the ability to be deployed with a smoother cost curve in less severe increments.

Answer 20

Two-tiered architectures have only clients and servers. Three-tiered architectures typical separate (1) presentation logic, (2) application logic, and (3) and data access logic and storage. In n-tiered architecture more than one tier may be used to support application logic, typically due to a Web server tier being included. Three-tiered or n-tiered architectures place a greater load on the network, but balances server load better and is more scalable.

Answer 21

Thick clients support all or most application logic while thin clients support little or no application logic. Development and maintenance costs for more complex thick-client environments can be higher than for thin clients.

Answer 22

Benefits include gaining access to experts to manage the cloud, potentially lower costs, scalability, and pay-as-you-g0.

Answer 23

See Figure 2-7

Answer 24

A network computer supports Internet access but has no hard disk local storage.

Answer 25

The standard protocol for communication between a Web browser and a Web server is Hypertext Transfer Protocol (HTTP). An HTTP request from a Web browser to a Web server has three parts. Only the first part is required; the other two are optional. • the request line, which starts with a command (e.g., GET), provides the URL, and ends with the HTTP version number that the browser understands. • the request header, which contains a variety of optional information such as the Web browser being used (e.g., Internet Explorer), the date, and a userid and password for use if the Web page is password-protected. • the request body, which contains information sent to the server, such as information from a form. The format of an HTTP response from the server to the browser is very similar to the browser request. It has three parts, but only the last part is required; the first two are optional: • the response status, which contains the HTTP version number the server has used, a status code (e.g., 200 means OK, 404 means page not found), and reason phrase (a text description of the status code) • the response header, which contains a variety of optional information such as the Web server being used (e.g., Apache), the date, the exact URL of the page in the response body, and the format used for the body (e.g., HTML) • the response body, which is the Web page itself.

Answer 26

HTML is the language in which web pages are created. The response body of an HTTP response can be in any format, such as text, Microsoft Word, Adobe PDF, or a host of other formats, but the most commonly used format is HTML. The major parts of HTML are the heading (denoted by the tag) and the body (denoted by the tag) of the response.

Answer 27

In order to get a page from the Web, the user must type the Internet Uniform Resource Locator (URL) for the page he or she wants, or click on a link that provides the URL. The URL specifies the Internet address of the Web server and the directory and name of the specific page wanted. In order for the requests from the Web browser to be understood by the Web server, they must use the same standard protocol. The standard protocol for communication between a Web browser and a Web server is Hypertext Transfer Protocol (HTTP).

Answer 28

Yes. With e-mail, users with the two-tier architecture will use the user agent software to interface with their email server, which will send out web based, SMTP packets to the far end receiver’s server computer with mail server software. The server at the far end will issue an IMAP or SMTP packet to the receiver’s server computer, which will then arrive at the receiver when they ask for the email with an HTTP request to the web based email application. Thus, a 2-tiered system easily interfaces with a three-tiered architecture over the internet using the appropriate protocols.

Answer 29

The sender of an e-mail uses a user agent (an application layer software package) to write the e-mail message. The user agent sends the message to a mail server that runs a special application layer software package called a message transfer agent. These agents read the envelope and then send the message through the network (possibly through dozens of mail transfer agents) until the message arrives at the receiver’s mail server. The mail transfer agent on this server then stores the message in the receiver’s mailbox on the server. When the receiver next accesses his or her e-mail, the user agent on his or her client computer contacts the mail transfer agent on the mail server and asks for the contents of the user’s mailbox. The mail transfer agent sends the e-mail message to the client computer, which the user reads with the user agent.

Answer 30

SMTP defines how message transfer agents operate and how they format messages sent to other message transfer agents. The SMTP standard covers message transmission between message transfer agents (i.e., mail server to mail server). A different standard called Post Office Protocol (POP) defines how user agents operate and how messages to and from mail transfer agents are formatted. POP is gradually being replaced by a newer standard called Internet Mail Access Protocol (IMAP). While there are several important technical differences between POP and IMAP, the most noticeable difference is that before a user can read a mail message with a POP user agent, the e-mail message must be copied to the client computer’s hard disk and deleted from the mail server. With IMAP, e-mail messages can remain stored on the mail server after they are read.

Answer 31

The major parts of an e-mail message are: • the header, which lists source and destination e-mail addresses (possibly in text form (e.g., “Susan Smith”) as well as the address itself (e.g., smiths@robert-morris.edu)), date, subject, and so on • the body, which is the message itself.

Answer 32

A virtual server is one computer that acts as several servers. Using special software like Microsoft Virtual PC, WMWare, or VirtualBox, several operating systems are installed on the same physical computer so that one computer appears as several different ones on the network.

Answer 33

Telnet enables users on one computer to login into other computers on the Internet. Once Telnet makes the connection from the client to the server, a user can login into the server or host computer in the same way as that person would if they dialed in with a modem; the user must know the account name and password of an authorized user. Telnet enables a person to connect to a remote computer without incurring long distance telephone charges. Telnet can be useful because it enables access to servers or host computers without sitting at the dedicated computer’s keyboard. Most network managers use Telnet to work on their organization’s servers, rather than physically sitting in front of them and using the keyboards.

Answer 34

With cloud computing, a company contracts with another firm to provide software services over the Internet, rather than installing the software on its own servers. The company no longer buys and manages its own servers and software, but instead pays a monthly subscription fee or a fee based on how much they use the application.

Answer 35

An instant messaging (client) communicates with an IM server application. Once a user is online, the server application can monitor connections so that multiple pre-identified clients can be notified and decide to participate in real-time messaging. IM may include video or audio. Video exchange, of course, requires cameras. Underlying this application requires a full-duplex connection between destination and host.

Answer 36

Videoconferencing must deliver real-time services demanding high capacity data transfer for both image and voice transmission. Specialized hardware (and even rooms) may be required. E-mail messages (typically without large attachments) are relatively small by comparison, can be received by any Internet-capable computer, and do not have to be consumed in real time.

Answer 37

The best architecture for email can depend on how one wants to use e-mail. If a person wants to be able to access their e-mail from anywhere, then Web-based is best. If the person wants professional backup and storage within an organization, then two-tier client-server is best. If the person wants storage of e-mail strictly under their control and they also want to be able to access their e-mail files off-line when there is a network service interruption, then host-based is best. Employers may choose to use client-server architecture for email access within the organization and Web-based architecture for access to the same system for those times when employees are outside the company (at home, at another business, or on travel).

Answer 38

While thin client have substantially less application logic than thick client, they have sufficient application logic (as, for example, a Web browser possibly with Java applets) to participate in a client-server relationship. The older host-based terminals did not even have this much application logic. While thin-client use today reflects some level of return to a more centralized approach, the client is likely served by multiple servers (and even multiple tiers), rather than a single large host server as in the past. Thus, the two approaches are similar, but not exact, from a technological design perspective.

Answer 39

A point-to-point configuration is so named because it goes from one point to another (e.g., one computer to another computer). These circuits sometimes are called dedicated circuits because they are dedicated to the use of these two computers. In a multipoint configuration (also called a shared circuit), many computers are connected on the same circuit. This means that each must share the circuit with the others, much like a party line in telephone communications. The disadvantage is that only one computer can use the circuit at a time. Multipoint configurations are cheaper than point-to-point configurations.

Answer 40

The three types of data flows are simplex, half-duplex and full duplex. Simplex is one-way transmission, such as that in radio or TV transmission. Half duplex is two-way transmission, but you can transmit in only one direction at a time. A half duplex communication link is similar to a walkie-talkie link; only one computer can transmit at a time. With full duplex transmission, you can transmit in both directions simultaneously, with no turnaround time.

Answer 41

Guided media are those in which the message flows through a physical media such as a twisted pair wire, coaxial cable, or fiber optic cable; the media “guides” the signal. One of the most commonly used types of guided media is twisted pair wire, insulated or unshielded twisted pairs (UTP) of wires that can be packed quite close together. Bundles of several thousand wire pairs are placed under city streets and in large buildings. Twisted pair wire is usually twisted to minimize the electromagnetic interference between one pair and any other pair in the bundle. Coaxial cable is another type of commonly used guided media. 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. Coaxial cables have very little distortion and are less prone to interference, they tend to have low error rates. Fiber optics, is becoming much more widely used for many applications, and its use is continuing to expand. Instead of carrying telecommunication signals in the traditional electrical form, this technology utilizes high-speed streams of light pulses from lasers or LEDs (light emitting diodes) that carry information inside hair-thin strands of glass or plastic called optical fibers.

Answer 42

Wireless media are those in which the message is broadcast through the air, such as radio, infrared, microwave, or satellite. One of the most commonly used forms of wireless media is radio. Radio data transmission uses the same basic principles as standard 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 up to 500 feet and are often built into portable or handheld computers. Infrared transmission uses low frequency light waves (below the visible spectrum) to carry the data through the air on a direct line-of-sight path between two points. This technology is similar to the technology used in infrared TV remote controls. It is prone to interference, particularly from heavy rain, smoke, and fog that obscure the light transmission. Infrared is not very common, but it is sometimes used to transmit data from building to building. A microwave is an 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. Microwave radio transmissions perform the same functions as cables. Similar to visible light waves, microwave signals can be focused into narrow, powerful beams that can be projected over long distances. Transmission via satellite is similar to transmission via microwave except, instead of transmitting to another nearby microwave dish antenna, it transmits to a satellite 22,300 miles in space.

Answer 43

Computers produce digital data that are binary, either on or off. In contrast, telephones produce analog data whose electrical signals are shaped like the sound waves they transfer. Analog data are signals that vary continuously within a range of values (e.g., temperature is analog).

Answer 44

Data can be transmitted through a circuit in the same form they are produced. Most computers, for example, transmit their data through digital circuits to printers and other attached devices. Likewise, analog voice data can be transmitted through telephone networks in analog form. In general, networks designed primarily to transmit digital computer data tend to use digital transmission, and networks designed primarily to transmit analog voice data tend to use analog transmission (at least for some parts of the transmission).

Answer 45

Most telephone company circuits are now digital because digital transmission is “better” than analog transmission. Specifically, digital transmission offers several key benefits over analog transmission: • Digital transmission produces fewer errors than analog transmission. Because the transmitted data is binary (only two distinct values), it is easier to detect and correct errors. • Digital transmission is more efficient. Time division multiplexing (TDM) is more efficient than frequency division multiplexing (FDM) because TDM requires no guardbands. TDM is commonly used for digital transmission, while FDM is used for analog transmission. • Digital transmission permits higher maximum transmission rates. Fiber optic cable, for example, is designed for digital transmission. • Digital transmission is more secure because it is easier to encrypt. • Digital transmission is less expensive than analog in many instances. Finally, and most importantly, integrating voice, video and data on the same circuit is far simpler with digital transmission.

Answer 46

Coding is the representation of one set of symbols by another set of symbols. In data communications, this coding is a specific arrangement of binary 0s and 1s used to represent letters, numbers, and other symbols that have meaning.

Answer 47

There are three predominant coding schemes in use today. United States of America Standard Code for Information Interchange (USASCII), or more commonly ASCII, is the most popular code for data communications and is the standard code on most terminals and 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. Extended Binary Coded Decimal Interchange Code (EBCDIC) is IBM's standard information code. This code has 8 bits, giving 256 valid character combinations.

Answer 48

Parallel mode is 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 eight 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.

Answer 49

Serial mode is distinguished from parallel mode by the time cycle in which the bits are transmitted. Parallel implies that all bits of a character are transmitted, followed by a time delay, and then all bits of the next character are transmitted, followed by a time delay. Serial implies that characters are sent one bit at a time, with each bit followed by a time delay. Put another way, parallel is character-by-character and serial is bit-by-bit.

Answer 50

With unipolar signaling, the voltage is always positive or negative (like a dc current). In bipolar signaling, the 1’s and 0’s vary from a plus voltage to a minus voltage (like an ac current). In general, bipolar signaling experiences fewer errors than unipolar signaling because the signals are more distinct. Manchester encoding is a special type of unipolar signaling in which the signal is changed from a 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 1 (or a 0), while the opposite (a change low to high) is used to represent a 0 (or a 1). Manchester encoding is less susceptible to having errors go undetected, because if there is no transition in mid-signal the receive knows that an error must have occurred.

Answer 51

Sounds waves have three important characteristics. The first is the height of the wave, called amplitude. Our ears detect amplitude as the loudness or volume of sound. The second characteristic is the length of the wave, usually expressed as the number of waves per second or frequency. Frequency is expressed in hertz (Hz). Our ears detect frequency as the pitch of the sound. Human hearing ranges from about 20 hertz to about 14,000 hertz, although some people can hear up to 20,000 hertz. The third characteristic is the phase, refers to the direction in which the wave begins.

Answer 52

Bandwidth refers to a range of frequencies. It is the difference between the highest and the lowest frequencies in a band; thus the bandwidth of human voice is from 20 Hz to 14,000 Hz or 13,880 Hz. The bandwidth of a voice grade telephone circuit is from 0 to 4000 Hz, or 4000 Hz; however, not all of this is available for use by telephone or data communications equipment. To start, there is a 300-hertz guardband at the bottom of the bandwidth and a 700-hertz guardband at the top. These prevent data transmissions from interfering with other transmissions when these circuits are multiplexed using frequency division multiplexing. This leaves the bandwidth from 300 to 3300 hertz or a total of 3000 Hz for voice or data transmission.

Answer 53

With amplitude modulation (AM) (also called amplitude shift keying (ASK)), the amplitude or height of the wave is changed. One amplitude is defined to be zero, and another amplitude is defined to be a one.

Answer 54

Frequency modulation (FM) (also called frequency shift keying (FSK)), is a 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 defined to be a one, and a different frequency (a different number of waves per second) is defined to be a one.

Answer 55

Phase modulation (PM) (also called phase shift keying (PSK)), is the most difficult to understand. 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 0º phase wave). Waves can also start down and to the right. This is called a phase of 180º. With phase modulation, one phase is defined to be a zero and the other phase is defined to be a one.

Answer 56

It is possible to use amplitude modulation, frequency modulation, and phase modulation techniques on the same circuit. For example, we could combine amplitude modulation with four defined amplitudes (capable of sending two bits) with frequency modulation with four defined frequencies (capable of sending two bits) to enable us to send four bits on the same symbol.

Answer 57

The terms bit rate (i.e., the number bits per second transmitted) and baud rate are used incorrectly much of the time. They often are used interchangeably, but they are not the same. In reality, the network designer or network user is interested in bits per second because it is the bits that are assembled into characters, characters into words and, thus, business information. Because of the confusion over the term baud rate among the general public, ITU-T now recommends the term baud rate be replaced by the term symbol rate. The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. For example, if we use amplitude modulation with two amplitudes, we send one bit on one symbol. Here the bit rate equals the symbol rate. However, if we use QAM, we can send four bits on every symbol; the bit rate would be four times the symbol rate.

Answer 58

Modem is an acronym for MOdulator/DEModulator. A modem takes the digital electrical pulses received from a computer, terminal, or microcomputer and converts them into a continuous analog signal that is needed for transmission over an analog voice grade circuit. Modems are either internal (i.e., inside the computer) or external (i.e., connected to the computer by a cable).

Answer 59

One popular technique is quadrature amplitude modulation (QAM). QAM involves splitting the symbol into eight different phases (three bits) and two different amplitudes (one bit), for a total of 16 different possible values. Thus, one symbol in QAM can represent four bits.

Answer 60

If we use 64-QAM, the bit rate is six times the symbol rate. A circuit with a 10 MHz bandwidth using 64-QAM can provide up to 60 Mbps.

Answer 61

The factors that affect the transmission speed are the number of bits per signal sample and the number of samples per second.

Answer 62

For voice digitization, one typically samples at twice the highest frequency transmitted, or a minimum of 8,000 times a second. Sampling more frequently than this will improve signal quality. For example, CDs sample at 44,100 times a second and use 16 bits per sample to produce almost error-free music.

Answer 63

Data compression can increase throughput of data over a communication link literally by compressing the data. A 2:1 compression ratio means that for every two characters in the original signal, only one is needed in the compressed signal (e.g., if the original signal contained 1000 bytes, only 500 would needed in the compressed signal). In 1996, ITU-T revised the V.34 standard to include a higher data rate 33.6 Kbps. This revision is popularly known as V.34+. The faster data rate is accomplished by using a new form of TCM that averages 9.8 bits per symbol (symbol rate remains at 3429).

Answer 64

V.42bis, the ISO standard for data compression, uses 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. Any time the same character pattern reoccurs in the message, the index to the dictionary entry is transmitted rather than sending the actual data. V.42bis compression can be added to almost any modem standard; thus a V.32 modem providing a data rate of 14,400 bps, could provide a data rate of 57,600 bps when upgraded to use V.42bis.

Answer 65

Analog voice data must be translated into a series of binary digits before they can be transmitted. With PAM-based methods, the amplitude of the sound wave is sampled at regular intervals, and translated into a binary number. The most commonly used type of PAM is Pulse Code Modulation (PCM). With PCM, the input voice signal is sampled 8000 times per second. Each time the input voice signal is sampled, eight bits are generated. Therefore, the transmission speed on the digital circuit must be 64,000 bits per second (8 bits per sample x 8,000 samples per second) in order to transmit a voice signal when it is in digital form.

Answer 66

Quantizing error is the difference between the replicated analog signal and its original form, shown with jagged “steps” rather than the original, smooth flow. Voice transmissions using digitized signals that have a great deal of quantizing error will sound metallic or machinelike to the human ear.

Answer 67

Multiplexing is the term used to describe the placing of two or more signals on a single circuit.

Answer 68

A multiplexer puts two or more simultaneous transmissions on a single communication circuit. Multiplexing a voice telephone call means that two or more separate conversations are sent simultaneously over one communication circuit between two different cities. Multiplexing a data communication network means that two or more messages are sent simultaneously over one communication circuit. In general, no person or device is aware of the multiplexer; it is “transparent.”

Answer 69

DSL services are quite new, and not all common carriers offer them. In general, DSL services have advanced more quickly in the Canada (and Europe, Australia and Asia) than in the United States due to their newer telephone networks from the end offices to the customer. Unlike other services that operate through the telephone network end-to-end from the sender to the receiver, DSL only operates in the local loop from the carrier's end office to the customer's telephone. DSL uses the existing local loop cable, but places one DSL network interface device (called customer premises equipment (CPE)) in the home or business and another one in the common carrier's end office. The end office DSL device is then connected to a high speed digital line from the end office to elsewhere in the carrier's network (often an Internet service provider) using some other service (e.g., T carrier, SMDS).

Answer 70

a. Frequency division multiplexing (FDM)? Frequency division multiplexing can be described as 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. b. Time division multiplexing (TDM)? Time division multiplexing shares a communication circuit among two or more terminals by having them take turns, dividing the circuit “vertically.” In TDM, one character is taken from each terminal in turn, transmitted down the circuit, and delivered to the appropriate device at the far end. Time on the circuit is allocated even when data are not be transmitted, so that some capacity is wasted when terminals are idle. c. Statistical time division multiplexing (STDM)? Statistical time division multiplexing is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines. STDM allows more terminals or computers to be connected to a circuit than FDM or TDM. STDM is called statistical because selecting the transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed. STDM is like TDM, except that each frame has a terminal address and no blanks are sent. d. Wavelength division multiplexing (WDM)? Wavelength division multiplexing is a version of FDM used in fiber optic cables. WDM works by using lasers to transmit different frequencies of light (i.e., colors) through the same fiber optic cable; each channel is assigned a different frequency so that the light generated by one laser does not interfere with the light produced by another. WDM permits up to 40 simultaneous circuits each transmitting up to 10 Gbps, giving a total network capacity in one fiber optic cable of 400 Gbps (i.e., 400 billion bits per second).

Answer 71

Inverse multiplexing (IMUX) combines several low speed circuits to make them appear as one high-speed circuit to the user.

Answer 72

If buying a multiplexer, you would choose TDM over FDM. In general, TDM is preferred to FDM, because it provides higher data transmission speeds and because TDM multiplexers are cheaper. Time division multiplexing generally is more efficient than frequency division multiplexing, because it does not need guardbands. Guardbands use “space” on the circuit that otherwise could be used to transmit data. It is not uncommon to have time division multiplexers that share a line among 32 different low speed terminals. It is easy to change the number of channels in a time division multiplexer. Time division multiplexers generally are less costly to maintain.

Answer 73

The traditional context of MODEM no doubt has become obsolete. We no longer can consider a MODEM a device that connects two computers by merely modulating and demodulating transmission signals over the Public Switched Network at speeds ranging up to 56Kbps. This type of MODEM now must be evaluated in light of newer technologies such as xDSL MODEMs and Cable MODEMs each of which generates, propagates and transmits signals at far greater speeds. In addition we must evaluate the traditional MODEMs as well in light of newer protocols and compression techniques which have greatly improved overall bandwidth and throughput with the traditional types of MODEMs. In short, though many new forms of MODEMs and MODEM supported technology have come in to play, the traditional MODEM continues to be a cost-effective and flexible means of networking if your bandwidth requirements remain under the 56Kbps threshold.

Answer 74

Under perfect circumstances, the maximum symbol rate is about 4,000 symbols per second. If we were to use QAM (4 bits per symbol), the maximum data rate would be 4 bits per symbol X 4,000 symbols per second = 16,000 bps.

Answer 75

A circuit with a 10 MHz bandwidth using 64-QAM can provide up to 60 Mbps. A V.44 modem can provide as much as 6:1 compression ratio, depending on the type of data sent. Thus, the maximum data rate of 64-QAM with compression is 360 Mbps.

Answer 76

B = s \* n B = (10 MHz) \* 1 bit per symbol Capacity is 10 Mbps

Answer 77

B = s \* n 100 Mbps = s \* 1 bit per symbol Symbol rate is 100 Mhz

Answer 78

Voice over IP is commonly used to transmit phone conversations over the digital networks. VoIP uses digital phones with built-in codecs to convert analog to digital.

Answer 79

The data link layer controls the way messages are sent on the physical media. The data link layer handles three functions: media access control, message delineation, and error control. The data link layer accepts messages from the network layer and controls the hardware that actually transmits them. The data link layer is responsible for getting a message from one computer to another without errors. The data link layer also accepts streams of bits from the physical layer and organizes them into coherent messages that it passes to the network layer.

Answer 80

Media access control handles when the message gets sent. Media access control becomes important when several computers share the same communication circuit, such as a point-to-point configuration with a half duplex line that requires computers to take turns, or a multipoint configuration in which several computers share the same circuit. Here, it is critical to ensure that no two computers attempt to transmit data at the same time -- or if they do, there must be a way to recover from the problem. Media access control is critical in local area networks.

Answer 81

With point-to-point full duplex configurations, media access control is unnecessary because there are only two computers on the circuit and full duplex permits either computer to transmit at anytime. There is no media access control.

Answer 82

With roll call polling, the front end processor works consecutively through a list of clients, first polling terminal 1, then terminal 2, and so on, until all are polled. Roll call polling can be modified to select clients in priority so that some get polled more often than others. For example, one could increase the priority of terminal 1 by using a polling sequence such as 1, 2, 3, 1, 4, 5, 1, 6, 7, 1, 8, 9. Hub polling is often used in LAN multipoint configurations (i.e., token ring) that do not have a central host computer. One computer starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so on, until it reaches the first computer, which restarts the process again. Contention is the opposite of controlled access. Computers wait until the circuit is free (i.e., no other computers are transmitting), and then transmit whenever they have data to send. Contention is commonly used in Ethernet local area networks.

Answer 83

The key consideration for which is better is throughput -- which approach will permit the largest amount of user data to be transmitted through the network. In most of the 1990s, contention approaches worked better than controlled approaches for small networks that have low usage. In this case, each computer can transmit when necessary, without waiting for permission. In high volume networks, where many computers want to transmit at the same time, the well-controlled circuit originally prevented collisions and delivered better throughput in such networks. Today contention-based systems have been improved to the point where they deliver substantially better throughput and are competitive because of hardware cost considerations.

Answer 84

There are two fundamental types of errors: human errors and network errors. Human errors, such as a mistake in typing a number, usually are controlled through the application program. Network errors, such as those that occur during transmission, are controlled by the network hardware and software. There are two categories of network errors: corrupted data (data that have been changed) and lost data.

Answer 85

Errors normally appear in bursts, which is when more than one data bit is changed by the error-causing condition.

Answer 86

Yes, normally lower speed lines have higher error rates because (1) leased lines can be conditioned to prevent noise, but dial-up lines can not and (2) dial-up lines have less stable transmission parameters.

Answer 87

Noise consists of undesirable electrical signals, or, in the instance of fiber optic cable, undesirable light. Noise is typically introduced by equipment or natural disturbances, and it can seriously degrade the performance of a communication circuit. Noise manifests itself as extra bits, missing bits, or bits that have been "flipped," (i.e., changed from 1 to 0 or vice versa).

Answer 88

The following list summarizes the major sources of error. The first six are the most important; the last three are more common in analog rather that digital circuits. Line outages are a catastrophic cause of errors and incomplete transmission. Occasionally, a communication circuit fails for a brief period. This type of failure may be caused by faulty telephone end office equipment, storms, loss of the carrier signal, and any other failure that causes a short circuit. When constructing and designing redundant networks that are fault survivable, this is usually called designing for the “farmer with a back hoe” problem. White noise or gaussian noise (the familiar background hiss or static on radios and telephones) is caused by the thermal agitation of electrons and therefore is inescapable. Even if the equipment was perfect and the wires were perfectly insulated from any and all external interference, there still would be some white noise. White noise usually is not a problem unless it becomes so strong that it obliterates the transmission. In this case, the strength of the electrical signal is increased so it overpowers the white noise; in technical terms, we increase the signal to noise ratio. Impulse noise (sometimes called spikes) is the primary source of errors in data communications. Some of the sources of impulse noise are voltage changes in adjacent lines, lightning flashes during thunderstorms, fluorescent lights, and poor connections in circuits. Cross-talk occurs when one circuit picks up signals in another. It occurs between pairs of wires that are carrying separate signals, in multiplexed links carrying many discrete signals, or in microwave links in which one antenna picks up a minute reflection from another antenna. Cross-talk between lines increases with increased communication distance, increased proximity of the two wires, increased signal strength, and higher frequency signals. Wet or damp weather can also increase cross-talk. Like white noise, cross-talk has such a low signal strength that it normally is not bothersome. Echoes can cause errors. Echoes are caused by poor connections that cause the signal to reflect back to the transmitting equipment. If the strength of the echo is strong enough to be detected, it causes errors. Echoes, like cross-talk and white noise, have such a low signal strength that they normally are not bothersome. In networks, echo suppressors are devices that reduce the potential for this type of error. Echoes can also occur in fiber optic cables when connections between cables are not properly aligned. Attenuation is the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer. Some power is absorbed by the medium or is lost before it reaches the receiver. This power loss is a function of the transmission method and circuit medium. High frequencies lose power more rapidly than low frequencies during transmission, so the received signal can thus be distorted by unequal loss of its component frequencies. Attenuation increases as frequency increases or as the diameter of the wire decreases, or as the distance of the transmission increases. Repeaters can be used in a digital environment to correct for attenuation due to distance, where amplifiers can be used to boost diminishing or attenuating analog signals over longer distances. A repeater will perfectly replicate the incoming, distorted digital signal and send it on deeper into the network as if new. An amplifier will boost an attenuating analog signal, but also boost the error noise in the signal as it does so. Fewer repeaters are necessary as compared to amplifiers to correct for attenuation, thus helping to make digital more cost effective when compared to analog transmission in controlling for noise. Intermodulation noise is a special type of cross-talk. The signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal. On a multiplexed line, many different signals are amplified together, and slight variations in the adjustment of the equipment can cause intermodulation noise. A maladjusted modem may transmit a strong frequency tone when not transmitting data, thus producing this type of noise. Jitter may affect the accuracy of the data being transmitted because minute variations in amplitude, phase, and frequency always occur. The generation of a pure carrier signal in an analog circuit is impossible. The signal may be impaired by continuous and rapid gain and/or phase changes. This jitter may be random or periodic. Harmonic distortion usually is caused by an amplifier on a circuit that does not correctly represent its output with what was delivered to it on the input side. Phase hits are short-term shifts "out of phase," with the possibility of a shift back into phase.

Answer 89

An amplifier takes the incoming signal, increases its strength, and retransmits it on the next section of the circuit. They are typically used on analog circuits such as the telephone company’s voice circuits. On analog circuits, it is important to recognize that the noise and distortion are also amplified, along with the signal. Repeaters are commonly used on digital circuits. A repeater receives the incoming signal, translates it into a digital message, and retransmits the message. Because the message is re-created at each repeater, noise and distortion from the previous circuit are not amplified.

Answer 90

Shielding (protecting wires by covering them with an insulating coating) is one of the best ways to prevent impulse noise, cross-talk and intermodulation noise. Moving cables away from sources of noise (especially power sources) can also reduce impulse noise cross-talk and intermodulation noise. For impulse noise, this means avoiding lights and heavy machinery. Locating communication cables away from power cables is always a good idea. For cross-talk, this means physically separating the cables from other communication cables. Cross-talk and intermodulation noise is often caused by improper multiplexing. Changing multiplexing techniques (e.g., from FDM to TDM), or changing the frequencies or size of the guardbands in frequency division multiplexing can help. Many types of noise (e.g., echoes, white noise, jitter, harmonic distortion) can be caused by poorly maintained equipment or poor connections and splices among cables. The solution here is obvious: tune the transmission equipment and redo the connections. To avoid attenuation, telephone circuits have repeaters or amplifiers spaced throughout their length.

Answer 91

Three common error detection methods are parity checking, longitudinal redundancy checking, and polynomial checking (particularly checksum and cyclic redundancy checking). One of the oldest and simplest error detection methods is parity. With this technique, one additional bit is added to each byte in the message. The value of this additional parity bit is based on the number of 1’s in each byte transmitted. This parity bit is set to make the total number of ones in the byte (including the parity bit) either an even number or an odd number. Any single error (a switch of a 1 to a 0 or vice versa) will be detected by parity, but it cannot determine which bit was in error. But, if two bits are switched, the parity check will not detect any error. Parity can detect errors only when an odd number of bits have been switched; any even number of errors cancel each other out. Therefore, the probability of detecting an error, given that one has occurred, is only about 50 percent. Many networks today do not use parity because of its low error detection rate. Polynomial checking adds a character or series of characters to the end of the message based on a mathematical algorithm. With the checksum technique, a checksum (typically one byte) is added to the end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum. The receiver calculates its own checksum in the same way and compares it with the transmitted checksum. If the two values are equal, the message is presumed to contain no errors. Use of checksum detects close to 95 percent of the errors for multiple bit burst errors. The most popular polynomial error checking scheme is cyclical redundancy check (see the answer to # 16 below for more discussion). The probability of detecting an error is nearly 100% or, in some cases, 100%.

Answer 92

Even parity is when the seven bits of an ASCII character have an even (2, 4, or 6) number of 1s, and therefore a 0 is placed in the eighth parity position. Odd parity is when the seven bits of an ASCII character have an odd (1, 3, 5, or 7) number of 1s, and therefore a 1 is placed in the eighth parity position.

Answer 93

Checksum error checking adds a checksum (typically 1 byte) is added to the end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and then using the remainder as the checksum. The same approach is used at the receiving end. If the receiver gets the same result, the block has been received correctly.

Answer 94

Cyclical redundancy check (CRC) adds 8, 16, 24 or 32 bits to the message. With CRC, a message is treated as one long binary number, P. Before transmission, the data link layer (or hardware device) divides P by a fixed binary number, G, resulting in a whole number, Q, and a remainder, R/G. So, P/G = Q + R/G. For example, if P = 58 and G = 8, then Q = 7 and R = 2. G is chosen so that the remainder R will be either 8 bits, 16 bits, 24 bits, or 32 bits. The remainder, R, is appended to the message as the error checking characters before transmission. The receiving hardware divides the received message by the same G, which generates an R. The receiving hardware checks to ascertain whether the received R agrees with the locally generated R. If it does not, the message is assumed to be in error.

Answer 95

Forward error correction uses codes containing sufficient redundancy to prevent errors by detecting and correcting them at the receiving end without retransmission of the original message. The redundancy, or extra bits required, varies with different schemes. It ranges from a small percentage of extra bits to 100 percent redundancy, with the number of error detecting bits roughly equaling the number of data bits.

Answer 96

Forward error correction is commonly used in satellite transmission. A round trip from the Earth station to the satellite and back includes a significant delay. Error rates can fluctuate depending on the condition of equipment, sun spots, or the weather. Indeed, some weather conditions make it impossible to transmit without some errors, making forward error correction essential. Compared to satellite equipment costs, the additional cost of forward error correction is insignificant.

Answer 97

With stop-and-wait ARQ, the sender stops and waits for a response from the receiver after eWith continuous ARQ, the sender does not wait for an acknowledgment after sending a message; it immediately sends the next one. While the messages are being transmitted, the sender examines the stream of returning acknowledgments. If it receives an NAK, the sender retransmits the needed messages. Continuous ARQ is by definition a full duplex transmission technique, because both the sender and the receiver are transmitting simultaneously (the sender is sending messages, and the receiver is sending ACKs and NAKs). ach message or data packet. After receiving a packet, the receiver sends either an acknowledgment (ACK) if the message was received without error, or a negative acknowledgment (NAK) if the message contained an error. If it is an NAK, the sender resends the previous message. If it is an ACK, the sender continues with the next message. Stop-and-wait ARQ is by definition, a half duplex transmission technique.

Answer 98

An argument could be made for SDLC, HDLC, or PPP. Each of these are similar in many ways.

Answer 99

Each SDLC frame begins and ends with a special bit pattern, known as the flag. The address field identifies the destination. The length of the address field is usually 8 bits but can be set at 16 bits; all computers on the same network must use the same length. The control field identifies the kind of frame that is being transmitted, either information or supervisory. An information frame is used for the transfer and reception of messages, frame numbering of contiguous frames, and the like. A supervisory frame is used to transmit acknowledgments (ACKs and NAKs). The message field is of variable length and is the user's message. The frame check sequence field is a 16-bit or 32-bit cyclical redundancy checking (CRC) code. For a typical Ethernet packet, the destination address specifies the receiver, while the source address specifies the sender. The length indicates the length in 8-bit bytes of the message portion of the packet. The LLC control and SNAP control are used to pass control information between the sender and receiver. These are often used to indicate the type of network layer protocol the packet contains (e.g., TCP/IP or IPX/SPX as described in Chapter 6). The maximum length of the message is 1492 bytes. The packet ends with a CRC-32 frame check sequence used for error detection. The PPP frame is similar to the SDLC frame. The frame starts with a flag and has a one-byte address. It also contains a control field which is rarely used. The protocol field indicates what type of data is contained. The message portion is variable in length and may be up to 1,500 bytes long. The frame check sequence is either CRC-16 or -32. The frame ends with a flag.

Answer 100

Transmission efficiency is defined as the total number of information bits (i.e., bits in the message sent by the user) divided by the total bits in transmission (i.e., information bits plus overhead bits).

Answer 101

Information bits are those used to convey the user’s meaning. Overhead bits are used for purposes such as error checking, and marking the start and end of characters and packets.

Answer 102

Web Browser

Answer 103

data access logic

Answer 104

Mail transfer agent

Answer 105

response status, response header, response body

Answer 106

is generally more “scalable” than a three-tiered architecture

Answer 107

Videoconferencing

Answer 108

all of these

Answer 109

wavelength division multiplexer

Answer 110

* Data, data, data! * Modern organizations rely on the efficient transmission of data * Enables distributed systems, real-time communication, electronic commerce, social media, and the Web

Answer 111

* the movement of computer information from one point to another by means of electrical or optical transmission systems

Answer 112

* a broader term and includes the transmission of voice and video (images and graphics) as well as data and usually implies longer distances

Answer 113

* •user device to access network and receive data from server –e.g., desktops, laptops, tablets, cell phones, etc.

Answer 114

* a device that stores and transmits data to a client –e.g., Web server, mail server, file server

Answer 115

* a pathway or connection between client and server –e.g., copper wire, fiber optic cable, wireless

Answer 116

* in terms of geographic scope

Answer 117

1. Local Area Networks (LANs) 2. Backbone Networks (BNs) 3. Wide Area Networks (WANs)

Answer 118

–Covers a small, clearly defined area –Might contain a single floor or work area or single building –When LANs use wireless circuits, they are called Wireless Local Area Networks (WLAN)

Answer 119

–High-speed networks connecting other networks together –May span hundreds of feet to several miles

Answer 120

–Largest geographic scope –Often composed of leased circuits –May spans hundreds or thousands of miles

Answer 121

1. Intranet 2. Extranet

Answer 122

–A network (often a LAN) that uses the Internet technologies to share information within an organization –Open only those inside the organization –e.g., employees accessing budgets, calendars, and payroll information available through the organization’s intranet

Answer 123

–A network that uses the Internet technologies to share information between organizations –Open only those invited users outside the organization –Accessible through the Internet –e.g., suppliers and customers accessing the inventory information of a company over an extranet

Answer 124

* divide communication functions into layers

Answer 125

1. –Open Systems Interconnection Reference Model (OSI model) 2. –Internet Model (or TCP/IP model)

Answer 126

1. Internet model

Answer 127

- defines the language of transmission –It specifies the rules, functionality, and messages for communication at the layer

Answer 128

•contains layer-specific information necessary for a message to be transmitted through a network

Answer 129

* each layer adds a PDU

Answer 130

* PDUs act like nested envelopes

Answer 131

* occurs when a higher level PDU is placed inside of a lower level PDU

Answer 132

1. Steps move from Sender to Receiver. Each step gets place inside the next step and the data is unwrapped in when it gets to receiver's last step 2. Steps Sender: * Application (HTTP Request) then goes to * Transport (TCP) then goes to * Network (IP) then goes to * Data link (Ethernet) * Physical (Data turned into electrical signal and sent to receiver) 3. Steps Reciever * Physical then goes to * Data link (Ethernet) then goes to * Network (IP) then goes to * Transport (TCP) then goes to * Application (HTTP request)

Answer 133

–Networking functionality is modular and the software/hardware at any layer can be more easily substituted •E.g., substitute wired for wireless at the physical layer –Easier to troubleshoot or make changes to one layer at a time –Application developers only need to worry about the application layer in their programs

Answer 134

–Inefficient because the encapsulation/de-encapsulation at each layer requires processing –Inefficient because encapsulation in a PDU increases overhead at each layer

Answer 135

* Standards ensure that hardware and software from different vendors work together and “speak the same language

Answer 136

* De jure standards * De facto standards

Answer 137

–Formalized by an industry or government body –e.g. HTTP, IEEE 802.3, 802.11n

Answer 138

–Widely accepted, but not formalized –e.g. Microsoft Windows –Often become de jure standards eventually

Answer 139

1. specification 2. identification of choices 3. acceptance

Answer 140

* HTTP * HTML (web) * MPEG, H.323 (audio/visual) * SMTP, IMAP, POP (email)

Answer 141

* TCP (internet and LANs)

Answer 142

* IP (Internet and LANs)

Answer 143

* Ethernet (LAN) * Frame relay (WAN) * T1 (MAN and WAN)

Answer 144

* RS-232C cable (LAN) * Category 5 cable (LAN) * V.92 (56 Kbps modem)

Answer 145

1. Bring your own device (BYOD) 2. Huge demand for employees to connect their personal smartphones, tablets, and other devices to organizational networks

Answer 146

•The Web of Things –Everything connects to the network! •e.g., cars, refrigerators, thermostats, shoes, doors, etc. –Networks need to support the increased demands of these devices

Answer 147

* The software that enables users to interact with the network and accomplish tasks

Answer 148

1. Presentation logic 2. Application logic 3. Data access logic 4. Data Storage

Answer 149

* The way the functions of the application layer are spread out across the client and server

Answer 150

1. Server contains all components (“server-based”) 2. Common in the 1960s with mainframes and terminals

Answer 151

•Advantages –Very simple –Single point of control

Answer 152

–Host (server) can become a bottleneck –Upgrades typically expensive

Answer 153

1. Client contains presentation, application, and data access logic 2. Most common in the 1980s

Answer 154

–Hardware and applications less expensive –Simple architecture

Answer 155

–Data must travel back and forth between server and client

Answer 156

* client-server architecture

Answer 157

1. thick-client architecture 2. thin-client architecture

Answer 158

* thick clients have more functionality

Answer 159

* Thin clients are easier to manage

Answer 160

–More efficient because of distributed processing –Allows hardware/software from different vendors to be used together –Less bandwidth required

Answer 161

–May be challenges in configuring hardware/software from different vendors to work together –In many cases, middleware is required

Answer 162

* software acts as an intermediary by “sitting between” client and server applications

Answer 163

* Provides a standard way of translating between software from different vendors * Manages message transfers * Insulates network changes from the clients (e.g., adding a new server)

Answer 164

–Load balancing –More scalable

Answer 165

–Each tier increases network load –More complex and difficult to develop applications

Answer 166

* An older architecture that became popular again with Napster, BitTorrent, etc. * All devices can act as client and server

Answer 167

–Resilient to failure –Data can be stored anywhere on network –Distributes bandwidth requirements

Answer 168

–Finding the stored data –Security

Answer 169

* the general term for enabling access to computing services over the network (most commonly the Internet)

Answer 170

* Models of cloud computing define who manages each application function and associated hardware/software

Answer 171

1. Software as a Service (SaaS) 2. Platform as a Service (PaaS) 3. Infrastructure as a Service (IaaS)

Answer 172

* All application components and associated hardware/software outsourced * Based on multitenancy * e.g. Salesforce.com

Answer 173

* Application logic and data are managed internally * e.g., Microsoft Azure

Answer 174

•All hardware is outsourced

Answer 175

•Infrastructure –Cost of servers, clients, and circuits –Reliability •Development Costs –Software; cheaper on host-based architectures •Scalability –Ability to increase (or decrease) in computing capacity as network demand changes –Easier in client-server architectures

Answer 176

* Hypertext Transfer Protocol (HTTP) * HTTP Request and Response

Answer 177

* Mail Transfer Agent (MTA) * Mail User Agent (MUA) * Simple Mail Transfer Protocol (SMTP) * Internet Message Access Protocol (IMAP) or Post Office Protocol (POP) * American Standard Code for Information Interchange (ASCII)

Answer 178

•Mail Transfer Agent (MTA) –Formal name for mail server software –e.g., Sendmail, Postfix,

Answer 179

•Mail User Agent (MUA) –Formal name for mail client software –e.g., Outlook, Apple Mail, Thunderbird

Answer 180

•Simple Mail Transfer Protocol (SMTP) –Protocol used to send a message to a MTA –Originally only handled text files

Answer 181

•Internet Message Access Protocol (IMAP) or Post Office Protocol (POP) –Protocols used by a MUA to retrieve messages from an MTA

Answer 182

•American Standard Code for Information Interchange (ASCII) –A standard for encoding text characters (a-z, A-Z, 0-9, a few symbols)

Answer 183

2-tier, thick client

Answer 184

1`. Sending Client → Sender’s Mail Server (SMTP) 2. Sender’s Mail Server → Receiver’s Mail Server (SMTP) 3. Message waits on Receiver’s Mail Server in “mailbox” 4. Receiving Client → Receiver’s Mail Server (IMAP or POP) 5. Receiver’s Mail Server → Receiving Client (IMAP or POP)

Answer 185

1. Sending Client → Sender’s Web Server (HTTP) 2. Sender’s Web Server → Sender’s Mail Server (SMTP) 3. Sender’s Mail Server → Receiver’s Mail Server (SMTP) 4. Mail waits on Receiver’s Mail Server in “mailbox” 5. Receiving Client → Receiver’s Web Server (HTTP) 6. Receiver’s Web Server → Receiver’s Mail Server (IMAP or POP) 7. Receiver’s Mail Server → Receiver’s Web Server (IMAP or POP) 8. Receiver’s Web Server → Receiving Client (HTTP)

Answer 186

•SMTP Message Format –RFC 822: Standard for text message format

Answer 187

1. Header 2. Body

Answer 188

–Contain information about the message such as 1. To, 2. From, 3. Subject

Answer 189

–Contains the “content of the message –Begins with the “DATA” keyword –Only uses ASCII characters

Answer 190

•Inside an SMTP packet

Answer 191

1. •SMTP is a simple protocol to send plain text 2. •Not designed to send images or attachments

Answer 192

•Multipurpose Internet Mail Extension (MIME)

Answer 193

–A standard to extend support for attachments and non-ASCII characters in email –Used by sender to convert (encode) any non-ASCII content into ASCII –Receivers then convert (decode) the ASCII back to its original format

Answer 194

1. File Transfer Protocol (FTP) 2. Telnet, Secure Shell (SSH) 3. Internet Relay Chat (IRC), Extensible Messaging and Presence Protocol (XMPP) 4. Domain Name System (DNS)

Answer 195

•for moving files between clients and servers

Answer 196

* for executing commands on a remote system

Answer 197

* real-time text chat (instant messaging)

Answer 198

* for mapping domain names to IP addresses

Answer 199

•Primary purpose of networks is to provide environment for applications

Answer 200

* OSI model

Answer 201

1. First layer 2. Focus on transmission over circuits

Answer 202

* Physical * Logical

Answer 203

* Physical circuits connect devices & include wires

Answer 204

–Logical circuits refer to the transmission characteristics of the circuit

Answer 205

–Point-to-Point –Multipoint

Answer 206

1. Point-to-Point circuits include most wired connections today 2. Multipoint circuits are most commonly used in wireless today

Answer 207

1. Shared circuits (multipoint) 2. are less expensive

Answer 208

1. client and server 2. 3 3. Ways 1. simplex 2. half-duplex 3. full duplex

Answer 209

•Data flows in one direction

Answer 210

•Data flows both directions, but only one at a time

Answer 211

•Data flows simultaneously in both directions

Answer 212

1. Divide high-speed circuit into several slower (logical) circuits 2. Main advantage is cost

Answer 213

1. categories * Frequency/Wavelength * Time 2. circuits

Answer 214

* FrequencyDivision Multiplexing (FDM) * Wavelength Division Multiplexing (WDM) * Time Division Multiplexing (TDM) * Statistical Time Division Multiplexing (STDM) * Inverse multiplexing

Answer 215

–Creates “channels” from larger frequency band

Answer 216

1. Guardbands separate channels to prevent interference 2. Multiplexing

Answer 217

–A variant of FDM used in fiber optic circuits –Makes use of multiple light wavelengths (colors) todivide circuit into channels –Dense WDM can divide circuit into more than 100 channels per fiber each transmitting at 10 Gbps

Answer 218

–Circuitis divided by devices taking turns –In traditional TDM, all have equal turns –More efficient than FDM, but may have idle time slots

Answer 219

* A variation of TDM * Designed to reduce idle time slots by allocating slots based on statistical network usage

Answer 220

–Potential time delays when actual usage does not match statistically allocated time slots –Additional logical addressing requirements

Answer 221

–Combines many low-speed circuits into one high-speed circuit –e.g., two T-1 lines multiplexed (creating acapacityof 2 x 1.544Mbps= 3.088 Mbps)

Answer 222

•Physical matter used to carry voice or data transmissions

Answer 223

* Guided media * Wireless (Radiated) media

Answer 224

•Guided media – transmission flows along physical medium

Answer 225

•Wireless (Radiated) media - transmission flows through the air

Answer 226

* Twisted-pair (TP) cable * Coax cable * Fiber optic cable

Answer 227

1. Insulated pairs of wires bundled together 2. Wires twisted to reduce electromagnetic interference 3. Commonly used for telephones, LANs 4. Characteristics * Price – inexpensive * Distance – typically up to 100m * Use - Telephones,LANs

Answer 228

1. Has a single copper core, plus outer insulation, shielding, and inner insulation 2. Less prone to interference 3. Characteristics * Price - inexpensive (but more costly than TP) * Distance - up to 2 km (1.2 miles) * Use: Cable TV / Internet

Answer 229

1. Opticalcore made of glass or plastic 2. Advantages * Resistant to interference and corrosion * Extremely fast data rates 3. Characteristics * Price:Expensive * Distance:500m – 100km 4. Use: Trunk line / Backbone, long distancecircuits (e.g., undersea cables)

Answer 230

* radio * microwave * satellite

Answer 231

- Factors to consider in media selection * Type of network * Cost * Transmission distance * Security * Error rates * Transmission speeds

Answer 232

1. Digital 2. Analog

Answer 233

1. Digital transmission involves discrete binary values (i.e., 0 or 1) 2. Analog transmission involves continuous waves

Answer 234

1. Digital cabling tends to be least expensive and most reliable 2. Data and voice networks continue to converge 3. Wired networks remain more secure and reliable than wireless

Answer 235

1. Coding scheme needed to ensure sender and receiver understand messages (e.g., ASCII, Unicode, etc.) 2. A character is represented by a group of bits

Answer 236

* Set of symbols * Symbol rate

Answer 237

* Unipolar * Bipolar NRZ * Bipolar AMI * Bipolar RZ * Manchester

Answer 238

1. Telephone system built for analog data 2. Need a modem(modulator/demodulator) to convert from analog to digital and vice versa

Answer 239

* sound is converted to electrical signals

Answer 240

1. Amplitude:height of wave (decibels) 2. Frequency:waves per second (hertz) •Wavelengthis the inverse of frequency 3.Phase:wave direction (degrees) or the point at which the wave begins

Answer 241

* Wavelength is the inverse of frequency

Answer 242

•Carrier wave is basic wave transmitted through a circuit

Answer 243

•Modulation is the modification of a carrier wave’s fundamental characteristics in order to encode information

Answer 244

1. AM 2. PM 3. FM

Answer 245

1.Amplitude Modulation (AM) or Amplitude Shift Keying (ASK)

Answer 246

1.Frequency Modulation (FM) or Frequency Shift Keying (FSK)

Answer 247

1.Phase Modulation (PM) or Phase Shift Keying (PSK)

Answer 248

1. Data rate (or bit rate) is the number of bits transmitted per second 2. bit rate 3. •Symbol rate: number of symbols transmitted per second

Answer 249

Data rate = symbol rate × (# bits/symbol) •Example * Symbol rate = 16,000 symbols/sec * #bits/symbol = 4 bits/symbol * Data rate = 16,000 symbols/sec × 4 bits/symbol = 64,000 bits/sec = 64Kbps

Answer 250

is a device or software that converts an analog signal (e.g., voice) into digital form and the reverse

Answer 251

converts analog to digital

Answer 252

## Footnote 1. Sampling the analog signal at regular intervals 2. Measuring the amplitude of each sample 3. Encoding (quantizing) the amplitude as binary data

Answer 253

•Quantizing Error is the difference between the original analog signal and the approximated, digital signal

Answer 254

–Sampling more frequently –Using more levels of amplitude in encoding

Answer 255

1. Layer 2 in the Internet model 2. Responsible for moving messages from one device to another 3. Controls the way messages are sent on media

Answer 256

–Media Access Control –Error Control –Message Delineation

Answer 257

1. Controls which device transmits and when 2. Important on: * Multipoint (shared) circuits * Half-duplex point-to-point circuits

Answer 258

1. Contention access 2. Controlled access

Answer 259

1. Transmit whenever circuit is available with no centralized control 2. Common in Ethernet LANs 3. When devices transmit at the same time, a collision occurs

Answer 260

–Devices must be “polite” and follow these steps: 1. “Listen” for traffic 2. If another device is transmitting, wait to transmit 3. Otherwise, transmit (and keep listening) 4. If another device begins to transmit, stop and wait

Answer 261

1. Common in wireless LANs 2. –Access Request 3. types 1. Roll call polling 2. Hub polling

Answer 262

•Each device must get “permission” to transmit, similar to raising a hand

Answer 263

1. car polling: * Central device (controller) determines which devices can transmit * Each client is checked periodically to see if it needs to transmit 2. –One device begins the poll and then passes it to another device until it reaches them all 3. token passing

Answer 264

–Types * Corrupted data * Lost data

Answer 265

–Caused by problems in transmission (not humans)

Answer 266

•Networks should be designed with: –Error prevention –Error detection –Error correction

Answer 267

•Line noise and distortion –Major reason for errors and caused by several sources –More likely on electrical media –Undesirable electrical signal –Degrades performance of a circuit –Manifestation

Answer 268

* Extra bits * “Flipped” bits * Missing bits

Answer 269

* Receivers need to know when the data transmitted is not correct * Add “check value” (error detection value) to message * Check value produced by mathematical formula * Both sender and receiver calculate check value * Sender tests whether the check values match

Answer 270

* parity check * checksum * cyclic redundancy check (CRC)

Answer 271

1. 95% 2. 50% 3. two types: * CRC-16 (~99.998% error detection rate) * CRC-32 (\>99.99999% error detection rate)

Answer 272

1. Once detected, errors must be corrected 2. types: 1. retransmission 2. forward error correction

Answer 273

* type of error correction technique * provides flow control by limiting number of messages sent

Answer 274

* type of error correction technique * Receiving device can correct messages without retransmission

Answer 275

1. Used only when retransmission is impossible, very costly, or time consuming (e.g., satellite connections) 2. Includes a certain level of redundancy in transmitted data so that receiving device can correct errors

Answer 276

1. On wired connections, errors are quite rare 2. no 3. only detect them and discard frames with errors 4. Error correction must then be done at a higher layer (Transport)

Answer 277

1. they are two types * Asynchronous transmission * Synchronous transmission 2. differ by * Message delineation * Frame length * Frame field structure

Answer 278

–Old protocol (e.g., used in teletype) –Transmits one character at a time –Delineation indicated by start and stop bits

Answer 279

–Data sent in a large block called a frame –Includes addressing information –Includes synchronization characters to let the receiver know when data transmission begins

Answer 280

* SDLC * HDLC * Ethernet * PPP

Answer 281

* Synchronous Data Link Control (SDLC) * Synchronous bit-oriented protocol developed by IBM * Uses bit stuffing (zero insertion) to overcome transparency problem

Answer 282

•Ethernet –IEEE 802.3 standard and Ethernet II –Most widely used LAN protocol –Uses contention media access control – –Ethernet II Frame

Answer 283

•Point-to-point protocol (PPP) –Common WAN protocol

Answer 284

1. increase by: * Protocol design contributes to network efficiency\ * increase data bits and decrease overhead bits 2. formula:

Answer 285

•More complicated than simple protocol efficiency because it depends on the retransmission rate, transmission rate, and delay

Answer 286

* Transmission Rate of Information Bits (TRIB) * it has a formula

Answer 287

is a measure of the effective # of bits transmitted in a unit of time

Answer 288

* Industry has seen a rapid consolidation in the number of data link layer protocols * Use fewer, well-established protocols for simpler network management, greater access to trained experts, and less expensive equipment

Exam 1 Flashcards

refers to chapters 1-4