CS601 Data Communication Lecture Wise Question and Answers. For final Term Exam Preparation By. Virtualians Social Network

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1 CS601 Data Communication Lecture Wise Question and Answers For final Term Exam Preparation By Virtualians Social Network CS601 Data Communication Lecture no.23 what is ITU-T stand for? ITU stands for International Telecommunication Union (ITU) and it coordinates the standards for telecommunications (T).That why it is called ITU-T. Is PCM is in the modem? PCM is used to convert analog signal to digital signal. As you know, modem is modulator and demodulator for the analog signal. So, modem uses PCM for analog to digital conversion. What is ISP and what is ISP stands for? ISP is short for Internet Service Provider. It refers to a company or a body that provides Internet services, including personal and business access to the Internet. For example, PTCL (Pakistan Telecommunication Company Limited) is serving as an ISP in Pakistan. What type of signal conversion required when we r talking to overseas friend? When two people communicate via a telephone line then this type of communication requires Analog to Analog conversion because human voice comes in Analog category of signals and typical telephone lines carry Analog signals. So, Analog to Analog conversion is required. What are the essential elements of Shannon s formula?

2 The essential elements of Shannon s formula are: 1. Proportionality to bandwidth W 2. Signal power S 3. Noise power P 4. A logarithmic function The channel bandwidth sets a limit to how fast symbols can be transmitted over the channel. The signal to noise ratio (P/N) determines how much information each symbol can represent. The signal and noise power levels are, of course, expected to be measured at the receiver end of the channel. Thus, the power level is a function both of transmitted power and the attenuation of the signal over the transmission medium (channel). CS601 Data Communication Lecture no.24 Is PCM is in the modem? PCM is used to convert analog signal to digital signal. As you know, modem is modulator and demodulator for the analog signal. So, modem uses PCM for analog to digital conversion. Explain why we are using modem if data is to be sent by digitizing it. Let me clear, from PC digital data is converted into analog using modem, then via local loop data is sent to Switching where it is converted into Digital using PCM. Then why did not PCM used directly from PC???? The local loop is basically provided by any telephone service provider and obviously it is analog. We use same local loop for computers which are digital. So it is necessary to convert our digital data into analog signal for transmitting the data through the local loop. That s why we use modem in our system to convert data from digital to analog conversion. What is FDM?

3 FDM stands for Frequency division multiplexing.it is the process by which the total bandwidth available to the system is divided. This division is into a series of non-overlapping frequencies bands that are then assigned to each communicating source and user pair. Define noise source? and give some example of it because it is outside the wire???? In communication, noise is an error or undesired random disturbance in a useful information signal, introduced before or after the detector. The noise is a sum of unwanted or disturbing energy from natural and sometimes man-made sources. Noise sources: Fluctuations of the electric current in an electrical conductor A signal or process with a frequency spectrum that falls off steadily into the higher frequencies etc What is male connector and what is female connector? A male connector is a connector that is attached to a wire, cable, or a piece of hardware, having one or more exposed, unshielded terminal and constructed in such a way that it can be inserted tightly into a holder (female connector) to ensure a reliable physical and electrical connection. The "female" connector is generally a receptacle that receives and holds the "male" connector. Can we calculate BW by physical measurement of Cables metal wire diameter? 2) What is the Role of X, Gamma, Cosmic Rays in Data Communication? Bandwidth is actually the capacity of the channel. The amount of data travels through cable in a unit time is referred to as bandwidth. The X-rays and Gamma rays have uncertain photon energy. Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilizing medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foods or preventing fruit and vegetables from sprouting to maintain freshness and flavor. The downloading via cable modem is 36Mbps and uploading is 12Mbps and it uses QAM and QPSK technique for uploading and downloading respectively; my question is same network card is used for uploading and downloading; then why not use the same modulation for upload and download, i mean to say on the network card there will be an electronic component which is

4 responsible to implement QAM for decoding the internet information received from the cable, and there would be another component on the network card which is implementing QPSK for upload information; why not use same technique for both upload and download.?? This term "Cable Modem" refers to a network interface card that communicates with the cable network, sending and receiving data in two slightly different ways. In the downstream direction, the digital data is modulated and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750MHz. Currently, 64 QAM (Quadrature Amplitude Modulation) is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals. In a two-way activated cable network, the upstream (also known as reverse path) is transmitted between 5 and 42 MHz. This tends to be a noisy environment with RF interface and impulse noise. Most cable companies use QPSK (Quadrature Phase Shift Keying) or a similar modulation scheme in the upstream direction, because QPSK is a more robust scheme than higher order modulation techniques in a noisy environment. The drawback is that QPSK is slower than QAM. For downstream and upstream different standars are used I.e Telephone cable and cable Modem that s why different techniques are used. DSL technology is using same telephone line which has restricted bandwidth of 4 Khz; then how using DSL modems we can have very good download speed as compared to dial-up internet connection? DSL, which stands for Digital Subscriber Line, is a broadband communication technology designed for use on regular phone lines. It has the ability to move data over the phone lines at speeds up to 140 times speedier than the fastest analog modems available today. Traditional phones do not utilize the full amount of the signals that are available and as a result the maximum speeds that you can get using a dial-up connection are only 128 Kbps. A DSL modem takes the analog signals and converts them into digital signals or bits.dsl provides always-on high-speed Internet access over a single dedicated telephone line; cable modems offer always-on high-speed Internet access over a shared cable television line, the bandwidth is shared among all users in a neighborhood, and will therefore vary, perhaps dramatically, as more users in a neighborhood get online at the same time. Cable modem upstream (from the home to the Internet) traffic will in many cases be slower than DSL, either because the particular cable modem is inherently slower, or because too many people in a neighborhood are trying to send or receive data at the same time. The bandwidth of telephone line is 4 KHz, due to which maximum data rate is restricted. what are those factors which restrict this bandwidth? on the other hand fiber optic has unlimited bandwidth...how?

5 Bandwidth is determined by the medium used and transmission speed of the networking equipment or Internet.bandwidth is used to view web pages and send data out from your computer (upload) or receive data into your computer (download).bandwidth is therefore also proportional to the complexity of data for a given level of system performance. so the complexity of data for given system restricts the bandwidth. In digital cable and fiber-optic systems, the demand for ever-increasing data speeds outweighs the need for bandwidth conservation. this is the reason for unlimited bandwidth. In the electromagnetic radiation spectrum, there is only so much available bandwidth to go around, but in hard-wired systems, available bandwidth can be constructed without limit simply by installing more and more cables. There are different types of TV cable available in the market as RG-7, RG-9, RG-11 and so on, is all these cables have same band width that is 750 MHz or each type has different band width? RG(Register jack) is the standard connector used in Ethernet standard cables, like twisted pair and coaxial cable. The bandwidth they carry is around 10 Mbps to 100 Mbps, regarldless of the RG numbers, such as RG11, RG7 or RG A LAN cable connecting a computer with HUB is example of UTP (Unshielded twisted Pair) cable or not? Yes commonly in network devices are attached to hub with UTP. Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks. The quality of UTP may vary from telephonegrade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. Mostly purchased Category 3 or Category 5. Category 5 cable is highly recommended. A disadvantage of UTP is that it may be liable to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference. What is Data compression? Data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an unencoded representation would use, through use of specific encoding schemes. What is Local loop? Traditionally, the local loop was wireline in nature from customer to central office, specifically in the form of an electrical circuit (i.e., loop) provisioned as a single twisted pair in support of voice communications. Where the number of local loops was restricted, different customers

6 could share the same loop, known as a party line. Modern implementations may include a digital loop carrier system segment or fiber optic transmission system known as fiber-in-the-loop. that who is the owner of internet? who runs it? how does it work? what are the resources for that? WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management over long distances. Plz explain the difference between Hub, Modem, Router, and Switches. Plz tell me the uses of it. How does its work? Where do we use it? Plz explain. If there is possible, i want to see them. You can explain with pictures also so that i can see them.??? A hub is used in a wired network to connect Ethernet cables from a number of devices together. The hub allows each device to talk to the others. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP?? Network. Routers are located at gateways, the places where two or more networks connect. A network switch or switching hub is a computer networking device that connects network segments. In networks, a device that filters and forwards packets between LAN segments. Short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms. what are the types of IP. define each.??? IP addresses are basically of two types: 1) Static IP address 2) Shared IP address An IP Address is a 32-bit number that identifies a computer on the Internet. Every web site on the internet is found not by its domain name but by its IP address. When someone types the address: it is translated into an IP address and then the computer is directed to that IP address which is the web site.

7 Now every single website has an IP address specifically allocated to it. If every site assigned a separate IP address then there could be a problem with running out of IP addresses so a lot of the sites on the server use a single IP address for multiple sites. Thus, using more than one site on an IP address is called a Shared IP address. If a site has its own IP address, and shares with no one else, it is called a Static IP address. You can always access a site which has a static IP address by using its IP address alone, but you cannot access a site using a shared IP address by typing in the IP address alone because when you type in a shared IP address you arrive at the server but the server does not know which site you are looking for as you have not told it which domain name you want. CS601 Data Communication Lecture no.25 What is thick Ethernet and thin Ethernet? and where is it use? Ethernet is the most widely-installed local area network (LAN) technology / architecture. Thick Ethernet also called 10 BASE 5 and Thin Ethernet also called 10 BASE 2 are the two implementations of Ethernet using Coaxial Cables as transmission medium What is mean by fragility?? Fragility means lacking physical or emotional strength. It also means easily breakable. It is used for the objects that are not made of strong material and the objects which require great care when handled. Fragility is one of the draw backs of Fiber Optics because Fiber Optics is made of glass which is a very fragile material. So, it cannot be used in extreme conditions where hardware portability is required. What is HUM? Hum can come from ground loops (cable won't help you if that's the case, but see our isolation transformer below) or, often, from EMI (electro-magnetic interference). High-energy, lowfrequency noise, like the 60-cycle hum from nearby power cords, fluorescent lights, and other miscellaneous sources, is the hardest type of interference to shield against, and the best defense here is a dense and highly-conductive braid shield. What are T connector and Terminators? T-connector: A T-shaped splitter (one connector to two connectors). For example the BNCT is a T-shaped splitter with one female BNC to two female BNCs (BNCT-1M is one male BNC to two female BNCs)

8 Terminator: A device attached to the end-points of a bus network or daisy-chain. The purpose of the terminator is to absorb signals so that they do not reflect back down the line. Ethernet networks require a terminator at both ends of the bus, and SCSI chains require a single terminator at the end of the chain. CS601 Data Communication Lecture no.26 Why only VLF (3KHz ~ 30Khz) frequency is used for surface propagation; why same frequency cannot be used for e.g. space propagation? Each frequency range is suited wart to specific layer of atmosphere and is most efficiently transmitted and received by technologies adapted to that layer. Frequency range for surface propagation (3KHz ~ 30Khz) cannot be used for extremely high frequency range of space propagation because radars and satellite communication will not take place with Low frequency band also VLF waves are propagated as surface waves through air so this frequency band is not enough for Extremely High Frequency (EHF)range of space propagation. The frequency of FM radio is usually 87 ~ 108 MHz, which falls in VHF frequency; and we have studied that in VHF band propagation is either satellite or LOS (line of sight) but FM radio stations are broadcasted; how it is achieved? Sorry sir, i am asking the question of lecture 26 in MDB of 25; Radio works on VHF (very high frequency) as the channels are operated in MHZ. We receive these channels on the basis of satellite signals with some impact of line of sight in it. It is broadcasted in a sense that all radio devices in the region able to receive it and listen it, as it is simplex mode of communication. W hat is the meaning of channels in frequency? Channel represents path of the specific network. Which are allocated different frequencies to represent different ranges of channels? If i have a high power Radar operating at 4Ghz, and it is facing upward, then does its transmission will pass through ionosphere or not? Will my radar be able to detect satellite in space? Sir one request, first of all i also try to search for answers to my question from Uncle GOOGLE, but once i don't get my precise answer i put it on MDB. I expect specific answers from the instructor in his own words. As far as your first query is concerned,for radar operating at a frequency of 4GHZ the transmission is Direct wave line of sight space propagation as this is the super high frequency

9 band. In your second query you have asked about radar detecting satellite in space,according to IEEE standards range specified for radar detection is 8.0 to 12.0Ghz if your radar is operating at 4Ghz it will not detect satellite in space. Dear student these MDB's are replied with best possible answers that are comprehensive to get understand by students. it is also advised to you that carefully read replies of your queries and ask one query per mdb. so that you can better understand one topic at a time. if still you are not satisfied then you can ask again and we are here to best facilitate you. Why VLF cannot pass through the ionosphere layer; whereas SHF (Super High Frequency) can pass through ionosphere layer? The ionosphere plays a very important role in the propagation of radio waves; the E-layer of the ionosphere reflects the radio waves and enables them to travel beyond the visible horizon. The physiochemical model of the ionosphere is very complicated with a multitude of photochemical reactions going on; ions forming and recombining with the availability or scarcity of energetic photons and particles. This already complicated model of the ionosphere is further complicated during a Sudden Ionosphere Disturbance (SID) which can be caused by a solar flare or a gamma burst. During a solar flare, the sun releases energy on the order of 1027 to 1032 ergs in the form of electromagnetic waves, energetic particle and mass motions. Although all of this energy obviously does not reach the earth, a fraction of that does reach the earth disrupts the normal balance of ion formation and recombination in the ionosphere. This disruption disturbs the propagation models of radio waves transmitted through the ionosphere. This disturbance is responsible for distorted waves which can be problematic for the receiver. During a solar flare, a VLF wave transmitted through the ionosphere shows a sharp rise in intensity. This intensity of the VLF wave rises because the highly ionized D-layer reflects the VLF wave instead of the E-layer. The VLF no longer has to travel through the partially ionized D-layer, while losing its energy, and travel up to the E layer to be reflected. Hat is the frequency band of mobile phone transmission? Is it a broadcast transmission; if i am in direct LOS(line of Sight) of mobile tower, then i will receive transmission directly; if i am not in LOS then the mobile transmission will be received after reflection from Ionosphere layer? am i correct? Mobile system which is termed as GSM uses different frequency bands i.e. 900, 1800, GSM-900 uses MHz to send information from the mobile station to the base station (uplink) and MHz for the other direction (downlink), providing 124 RF channels

10 (channel numbers 1 to 124) spaced at 200 khz. Duplex spacing of 45 MHz is used. Guard bands 100 khz wide are placed at either end of the range of frequencies.gsm-1800 uses MHz to send information from the mobile station to the base transceiver station (uplink) and MHz for the other direction (downlink), providing 374 channels (channel numbers 512 to 885). Duplex spacing is 95 MHz GSM-1800 is also called DCS (Digital Cellular Service) in the United Kingdom, while being called PCS in Hong Kong. Line of sight communication is in the VHF and higher frequencies of the RF spectrum where the wavelength is too short to pass over structures and hills, the transmitter and receiver antennas must be in line. In mobile communication there is only one way which is broadcast when messages regarding services from operators to multiple users are broadcasted. AM radio band is usually in VLF or LF band (in KHz) does that mean if i install my own highpowered radio station at Islamabad which operates in VLF or LF frequency band; then its transmission can be received by a person sitting in JAPAN; am i correct? Yes, it is receive anywhere around the globe, if frequency is permitted by that region. Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF). The frequency allocation is done by ITU. Can you kindly give us some daily life example of ionosphere propagation? The ionosphere affects modern technologies such as civilian and military communications, navigation systems and surveillance systems, etc. For many communication and navigation systems, this arises because the systems use signals transmitted to and from satellites, which must therefore pass through the ionosphere. For the most reliable communication and navigation it is necessary to correct the signals for effects imposed by the ionosphere. The frequency of FM radio is usually 87 ~ 108 MHz, which falls in VHF frequency; and we have studied that in VHF band propagation is either satellite or LOS (line of sight) but FM radio stations are broadcasted; how it is achieved? Sorry sir, i am asking the question of lecture 26 in MDB of 25; Radio works on VHF (very high frequency) as the channels are operated in MHZ. We receive these channels on the basis of satellite signals with some impact of line of sight in it. It is broadcasted in a sense that all radio devices in the region able to receive it and listen it, as it is simplex mode of communication.

11 The capability of modems to support 2 wire or 4 wire is it related to telephone network, does it mean that in case of 4-wires i need to hire 02 telephone lines; one to transmission and another for receiving for full duplex mode? Yes, In case of four-wire (4W) line, a pair of two-wire (2W) lines will be used, one for transmitting and one for receiving, in which the signals in the two directions are to be kept totally separate. What is MTSO? MTSO stands for Mobile Telephone Switching Office. The MTSO is the switching office that connects all of the individual cell towers to the Central Office (CO). The MTSO is responsible for monitoring the relative signal strength of your cellular phone as reported by each of the cell towers, and switching your conversation to the cell tower which will give you the best possible reception. What is BAND? A band is a small section of the spectrum of radio communication frequencies, in which channels are usually used or set aside for the same purpose. CS601 Data Communication Lecture no.27 What is NGN? Next-generation network (NGN) is a broad term used to describe key architectural evolutions in telecommunication core and access networks. As it will be more advance network, so it will use the guided and unguided media

12 What is frequency band? In telecommunication, a band sometimes called a frequency band is a specific range of frequencies in the radio frequency (RF) spectrum, which is divided among ranges from very low frequencies (elf) to extremely high frequencies (elf). Each band has a defined upper and lower frequency limit. How can calculate Attenuation in fiber optics? As we know that Attenuation means any reduction in the strength of a signal. So, imagine a signal travels through a transmission medium and its power is reduced to half. Suppose its original strength or power (P1) is 1 which reduces to its half 0.5, represented by P2. By putting these values in the formula we can calculate or measure Attenuation. db= 10 x Log 10 (P2/P1) db= 10 x Log 10 (0.5 / 1) db=10 x Log 10 (0.5) What is FOCUS? Every line parallel to the line of symmetry (line of sight) reflects off the curve at an angle such that they intersect in a common point called FOCUS. CS601 Data Communication Lecture no.28 What are Transmission impairments? Analog signal consist of varying a voltage with time to represent an information steam. If the transmission media were perfectly, the receiver could receive exactly the same signal that the transmitter sent. But communication lines are usually not perfect, so they receive signal is not the same as the transmitted signal. For digital data this difference can lead to errors. Transmission lines suffers from three major problems 1. Attenuation 2. Delay distortion 3. Noise

13 What is Los signal? Line of sight signal (los)los signal is the signal traveling directly following the straight path from transmitting antenna to the receiving antenna without any obstacle in its path. Los signal is also referred to as direct signal or direct wave. Explain the Concept of line of sight? Los (line of sight) means something aligned or in the same direction with something. If we talk about microwave's line of sight, it refers to the tower or a repeater which regenerates the signal because microwave needs to have some relay station, in order to avoid signal loss and it must be in line with the one relay station to the other, so to avoid signal loss. What is Attenuation? Attenuation is the type of impairment in which signal loses its energy while travelling. It happens when avoiding resistance, the energy of the signal is utilized in breaking the resistance and because of this, attenuation occurs. What is Distortion? Distortion is another type of impairment in which signal becomes spitted and lost. It happens in the composite signal, when two or more signals are combined to be sent on the common platform or channel, they combined in such a way that different frequency signals mix up to form distortion. CS601 Data communication Lecture no 29 What is FDM? FDM is an analog technique that can be applied when BW of link is greater than the combined bandwidth of the signal to be transmitted. Difference between MUX and Demur Multiplexing means combining multiple streams of information for transmission over a shared medium. De-multiplexing performs the reverse function: split a combined stream arriving from a shared medium into the original information streams. As it is clear from definitions they are inverse of each other.

14 What is Demultiplexing? Demultiplexing is to separate 2 or more channels that have been multiplexed. Signals are typically multiplexed or combined onto one higher speed channel to efficiently use the bandwidth. When different signals are sent from source to destination, all the signals have to be sent on singal channel from multichannel to save the channel resources, for that process signals have to be multiplexed on receiver side with the help of Multiplexer. Multiplexer combine the signals in one composite signal by modulating every signal on different carrier frequency. On reaching destination the receiver Demultiplex the composite signal for every receiver, this is done with the help of De-Multiplexer. De-multiplexer also demodulate the composite signal on different frequencies for each user. What is TDM? TDM is abbreviated as Short for time division multiplexing, It is a digital multiplexing technique that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. What is TX. System? Those systems which transmit data to destination are called Tx systems, The Term TX is widely used in Satellite Communication, Telecommunication system & data communication system. " TX " stands for Trans and "Rx" stands for Receive.For any Information or data to be communicated over a network there is a node that Generate or transmit that information, The sender node point which is transferring information is "TX" or Transmitter point & the Destination node point that has to receive that information is " Rx" or receiver point. What is g (t) and Tax. Stands for Those systems which transmits data to destination are called Tax systems, The Term TX is widely used in Satellite Communication, Telecommunication system & data communication system. " TX " stands for Trans and "Rx" stands for Receive. For any Information or data to be communicated over a network there is a node that Generate or transmit that information, The sender node point which is transferring information is "TX" or Transmitter point & the Destination node point that has to receive that information is " Rx" or receiver point. The term g'(t) used in different places for different purposes, as for as lecture number 29 and previous are concerned, I didn t find this term, kindly give reference of lecture or from where you have read this. Feel free to ask any question regarding lectures or the subject.

15 CS601 Data Communication Lecture no.30 What is Asynchronous? In general, asynchronous (pronounced ay-sihn-kro-nuhs, from Greek asyn-, meaning "not with," and chronos, meaning "time") is an adjective describing objects or events that are not coordinated in time. In information technology, the term has several different usages. In telecommunication signaling within a network or between networks, an asynchronous signal is one that is transmitted at a different clock rate than another signal. (plesiochronous signals are almost but not quite in synchronization - and a method is used to adjust them - and synchronous signals are those that run at the same clock rate. For example, a telephone conversation is asynchronous because both parties can talk whenever they like. If the communication were synchronous, each party would be required to wait a specified interval before speaking. What is Bit Stuffing? Bit stuffing is the insertion of one or more bits into a transmission unit as a way to provide signaling information to a receiver. The receiver knows how to detect and remove or disregard the stuffed bits. Example of bit stuffing, a standard HDLC packet begins and ends with To make sure this sequence doesn't appear again before the end of the packet, a 0 is inserted after every five consecutive 1s. Why do we need Inverse Multiplexing? An organization wants to send data, voice and video each of which requires a different data rate To send voice it needs 64Kbps, To send data, it needs 128 Kbps link To send video it may need Mbps link It can lease a Mbps line from a common carrier and only use it fully for sometime Or it can lease several separate channels of lower data rates Voice can be sent over any of these channels Data & Video can be broken into smaller portions using Inverse Multiplexing and TX What is a transceiver?

16 Transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry. CS601 Data Communication Lecture no.31 What is DSU in terms of digital service? DSU changes rate of digital data created by the subscriber device to 56kbps and encodes it in the format used by service provider. it used in dialing process.dsu is more expensive than modem it has better speed better quality and less susceptibility to noise What is FTTC? "Fiber to the curb" (FTTC) refers to the installation and use of optical fiber cable directly to the curbs near homes or any business environment as a replacement for "plain old telephone service" (POTS). Think of removing all the telephone lines you see in your neighborhood and replacing them with optical fiber lines. Such wiring would give us extremely high bandwidth and make possible movies-on-demand and online multimedia presentations arriving without noticeable delay. What is Bandwidth? Bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. In computer networks, bandwidth is often used as a synonym for data transfer rate - the amount of data that can be carried from one point to another in a given time period (usually a second). This kind of bandwidth is usually expressed in bits (of data) per second (bps). What are the telephone services provided? There are two main telephone services. 1. Analog Service 2. Digital Service Analog Services further has two important types of services. 1. Switched Analog Service

17 2. Leased Analog Service CS601 Data communication Lecture no 32 Difference of topology and multiplexing The physical arrangement of a network; the way in which cables are arranged in order to connect computers in a network is called the topology. In telecommunications and computer networks, multiplexing (also known as muxing) is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire. It originated in telegraphy, and is now widely applied in communications. What are T Line and E Line? T line and E line both are digital lines designed to transfer digital data. The concept behind T- line and E-line is same except that Europeans use a version of T-line called E-lines which have different capacity than T-lines What are the fractional T Lines? Many subscribers don t need the entire capacity of the T-line. A small business may need only one-fourth of the capacity of T-line. if four business of same size lie in the same building, they can share T-line.DSU/CSU allow the capacity of T-line to be interleaved in to four channels CS601 Data Communication Lecture no.33 What are the Types of Errors? There are two types of errors 1- Single bit error 2- Burst error Single Bit & Burst In a single-bit error, a 0 is changed to a 1 or a 1 to a 0

18 In a burst error multiple bits are changed For Example, a 0.01 second burst of impulse noise on a TX with a data rate of 1200 bps might change all or some of 12 bits of information Error Detection Even if we know what type of errors can occur, will we recognize one when we see it? If we have a copy of the intended TX for comparison, of course we will But what if we don t have a copy of the original Then we will have no way of knowing we have received an error until we have decoded the TX and failed to make sense of it What are the types of redundancy checks? There are four types of redundancy checks used in data communication; 1. Vertical Redundancy check (VCR) 2. Longitudinal Redundancy check(lrc) 3. Cyclic Redundancy check(crc) 4. Checksum What is CRC? Short for cyclic redundancy check, a common technique for detecting data transmission errors. Transmitted messages are divided into predetermined lengths that are divided by a fixed divisor. According to the calculation, the remainder number is appended onto and sent with the message. When the message is received, the computer recalculates the remainder and compares it to the transmitted remainder. If the numbers do not match, an error is detected. What is Checksum? A simple error detection scheme in which each transmitted message is accompanied by a numerical value based on the number of set bits in the message. The receiving station then applies the same formula to the message and checks to make sure the accompanying numerical value is the same. If not, the receiver can assume that the message has been garbled. CS601 Data Communication Lecture no.34 What is an Error?

19 An error is the change or the mismatching take place between the data unit sent by transmitter and the data unit received by the receiver e.g sent by sender received by receiver. Here is an error of 1 bit. Define Error Control. Error control refers to mechanisms to detect and correct errors that occur in the transmission of frames. The most common techniques for error control are based on some or all of the following: 1, Error detection 2. Positive acknowledgement 3. Retransmission after time-out 4. Negative acknowledgement and retransmission. These mechanisms are also referred as automatic repeat request (ARC)). Explain about error correction. Error correction is the mechanism by which we can make changes in the received erroneous data to make it free from error. The two most common error correction mechanisms are: (i) Error correction by Retransmission. (ii) Forward Error Correction. What are the different types of error? /How a single bit error does differ from a burst error? A single bit error is an isolated error condition that alters one bit but does not affect nearby bits. On the other hand A burst error is a contiguous sequence of bits in which the first and last bits and any number of intermediate bits are received in error. A single bit can occur in the preserve of while noise, when a slight random deterioration of single-to-noise ratio is sufficient to confuse the receiver s decision of a single bit. On the other hand burst errors are more common and more difficult to deal with. Burst error can be caused by impulse noise.

20 Discuss the concept of redundancy in error detection. It is a most common and powerful technique for the detection of errors. In this technique extra bits are added. But instead of repeating the entire data stream, a shorter group of bits may be appended to the end of each unit. The technique is called redundancy because the extra bits are redundant to the information. They are discarded as soon as the accuracy of transmission has been determined. Explain the bases of log base 2 and log base 10? In mathematics, a logarithm of a given number to a given base is the power to which you need to raise the base in order to get the number. Example for base 10 The logarithm of 1000 to the common base 10 = 3 or log(base 10) of 1000 = 3. Because 10 raised to a power of 3 is 1000 or 10^3 = 1000 Example for base 2 The Logarithm of 8 to common base 2 = 3 or log(base 2) of 8 = 3 Because 2 raised to a power of 3 is 8 or 2^3 = 8. CS601 Data Communication Lecture no.35 What is Single bit error? Single bit error is the error where only one bit of the transmitted data unit is changed from 0 to 1 and 1 to 0. While in burst error two or more bits are changed from 0 to 1 and from 1 to 0. The burst error is un detectable by checksum. What are timer and sequence number?

21 TIMER, if sender was not able to get acknowledgment in the particular time than, it sends the buffered data once again to receiver. When sender starts to send the data, it starts timer. SEQUENCE NUMBER, from this the sender sends the data with the specific sequence number so after receiving the data, receiver sends the data with that sequence number, and here at sender side it also expect the acknowledgment of the same sequence number. What is Hamming Code? It is a single bit error correction method using redundant bits. In this method redundant bits are included with the original data those bits are arrange such that different incorrect bits produce different error results, that incorrect bits can be identified. Once the bit is identified, the receiver can reverse its value and correct the error. Hamming code can be applied to any length of data unit and uses the relationships between the data and the redundancy bits. What is Flow Control? Flow Control is one important design issue for the Data Link Layer that controls the flow of data between sender and receiver. How does the CRC Checker work? A sequence of redundant bits called CRC or CRC remainder is appended to the end of the data unit, so that the resulting data unit becomes exactly divisible by a second predetermined binary number At its destination, the data unit is divided by the same number If at this step, there is no remainder, the incoming data unit is assumed to be intact and is therefore accepted A remainder indicates that a data unit has been damaged and therefore must be rejected

22 CS601 Data Communication Lecture no.36 What are Framing and Flow control? In computer networking, a frame is a data packet on the Layer 2 of the OSI model. A frame is "the unit of transmission in a link layer protocol, and consists of a link-layer header followed by a packet." Examples are Ethernet frames (maximum 1500 byte plus overhead), PPP frames and V.42 modem frames. In computer networking, flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with data from transmitting node Differentiate between Hamming Code and Hamming Distance? In telecommunication, a Hamming code is a linear error-correcting code named after its inventor, Richard Hamming. Hamming codes can detect up to two simultaneous bit errors, and correct single-bit errors; thus, reliable communication is possible when the Hamming distance between the transmitted and received bit patterns is less than or equal to one. By contrast, the simple parity code cannot correct errors, and can only detect an odd number of errors. The Hamming distance between two strings of equal length is the number of positions at which the corresponding symbols are different. Put another way, it measures the minimum number of substitutions required to change one string into the other, or the number of errors that transformed one string into the other CS601 Data Communication Lecture no.37 How we calculate the binary division of these no divided by 111. In binary division we will concatenate 0 in quotient left most side until divisor is less than from dividend, if dividend portion is greater than or equal to divisor then we will subtract divisor from dividend portion and will concatenate 1 in quotient left side and so on. For see your required binary example step by step please follows this link

23 Add your divisor and dividend in below filed of link and then click on start next statement for understand step by step. What is TTP? TTP stands for Time triggered protocol which is an open and modular control system platform technology that supports the design of upgradeable, reusable and easy-to-integrate systems As a fault-tolerant time-triggered protocol, TTP provides autonomous fault-tolerant message transport at known times and with minimal jitter by employing a TDMA (Time-Division Multiple Access) strategy on replicated communication channels. TTP offers fault-tolerant clock synchronization that establishes the global time base without relying on a central time server. TTP provides a membership service to inform every correct node about the consistency of data transmission. This mechanism can be viewed as a distributed acknowledgment service that informs the application promptly if an error in the communication system has occurred. If state consistency is lost, the application is notified immediately. Additionally, TTP includes the service of clique avoidance to detect faults outside the fault hypothesis, which cannot be tolerated at the protocol level. CS601 Data Communication Lecture no.38 Explain Sliding Window. Sliding windows, a technique also known as windowing, is used by the Internet's Transmission Control Protocol (TCP) as a method of controlling the flow of packets between two computers or network hosts method of for transferring data over network.tcp sends data across a network in packets and requires the receiving device to send an acknowledgement, or ACK, when it successfully has received the packet, which signals the sending device that another packet can be sent. Window size is the number of data packets that can be sent without waiting for an ACK. With the sliding window method, the receiving device can send a single acknowledgement message for multiple packets of data sent in one window. Within that acknowledgement message is information about the receiving device, which tells the sending device to increase or decrease the number of packets in the next transmission (this is where the "sliding" in the name comes in). What is the term STP? STP is Shielded Twisted Pair. It is the same as UTP but has a shield inside to protect from electrical interference. It should be used in Token Ring networks or in Ethernet networks where electrical interference may be a problem. STP cables have a conducting shield made of metallic foil encasing the twisted wire pairs, which blocks out electromagnetic interference, allowing it to carry data at a faster rate of speed.

24 What is EOT? EOT stands for end of transmission; this frame is sent to let the transmitter know that the session should be terminated. Explain Flow control. Flow control is the process of adjusting the flow of data from one device to another or between nodes on a network to ensure that the receiving device or node can handle all of the incoming data. This is particularly important where the sending device or node is capable of sending data much faster than the receiving device or node can receive it. Too much data arriving before a device or node can handle it causes data overflow, resulting in data loss and possible retransmission. What is Stop-and-Wait Flow Control? The simplest form of flow control is stop-and-wait and works as follows. A source entity transmits a frame. After reception, the destination entity indicates its willingness to accept another frame by sending back an acknowledgement to the frame just received. The source must wait until it receives the acknowledgement before sending the next frame. The destination can thus stop flow of data by just withholding the acknowledgement. This method fails if the message transmitted is very large. CS601 Data Communication Lecture no.39 What is Asynchronous? Asynchronous is a communications protocol that transmits one byte of information at a time at irregular time intervals. A start bit (one character) precedes the byte (eight characters) of information, and a stop bit (one character) follows the byte. The sending and receiving computers do not have to be synchronized Explain sliding Window ARQ A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP). Conceptually, each portion of the transmission (packets in most data link layers, but bytes in

25 TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to place received packets in the correct order, discarding duplicate packets and identifying missing ones. The problem with this is that there is no limit of the size of the sequence numbers that can be required. By placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence numbers. Explain XMODEM? XMODEM like most file transfer protocols, breaks up the original data into a series of Packet that are sent to the receiver, along with additional information allowing the receiver to determine whether that packet was correctly received. Files were transferred one packet at a time. When received, the packet's checksum was calculated by the receiver and compared to the one received from the sender at the end of the packet. If the two matched, the receiver sent an ACK (Acknowledgment) message back to the sender, which then sent the next packet in sequence. If there was a problem with the checksum, the receiver instead sent a NAK (Negative acknowledgment) If a NAK was received, the sender would re-send the packet, and continued to try several times, normally ten, before aborting the transfer. What is Go-Back-N ARQ? Retransmission begins with the last unacknowledged frame even if subsequent frames have arrived correctly.duplicate frames are discarded. Go-back-n ARQ -- Receiver must get Frames in correct order CS601 Data Communication Lecture no.40 How is CRC superior to the two dimensional parity? CRC is based on binary division. In CRC, instead of adding bits to achieve a desired parity, a sequence of redundant bits, called the CRC or the CRC remainder, is appended to the end of a data unit so that the resulting data unit becomes exactly divisible by a second, predetermined binary number. At its destination, the incoming data unit is divided by the same number. If at this step there is no remainder the data unit is assumed to be intact and is therefore accepted. A remainder indicates that the data unit has been damaged in transit and therefore must be rejected. The redundancy bits used by CRC are derived by dividing the data unit by a predetermined divisor; the remainder is the CRC. To be valid, a CRC must have two qualities: It must have exactly one less bit than the divisor, and appending into the end of the data string must make the resulting bit sequence exactly divisible by the divisor.

26 A CRC checker functions exactly as the generator does. After receiving the data appended with the CRC, it does the samemodulo-2 division. If the remainder is all 0 s, the CRC is dropped and the data are accepted: otherwise, the received stream of bits is discarded and data are resent. We assume that there is no error. The remainder is therefore all 0 s, and the data are accepted. What is Purpose of the hamming code? Hamming code is a method of error detection and correction in which every string of four bits is replaced with a string of seven bits. The last three added bits are parity checking bits that the receiving device uses to check for and correct any errors. Hamming code will detect any double errors but can only correct a single error. This method of error correction is best suited for situations in which randomly occurring errors are likely, not for errors that come in bursts. What is the difference between even parity and odd parity? "Parity" refers to the number of 1s in a given binary number. Odd parity means there are an odd number of 1s and even parity means that there is an even number of 1s. How can a parity bit detect a damaged data unit? In order to reduce errors parity bit is introduced (even parity or odd parity) Suppose If we want to transmit using even parity at transmitter then there are total five 1's so as we are using even parity to make the total no. of 1's even the Parity bit required is '1' if we are using odd parity bit required is 0.This parity bit appended to the data as ,and this is transmitted in place of (actual data),so as according to the parity used (even r odd) the receiver counts the no. of 1's in the received data. Process follows as at receiver if received data is the parity using here is even so on counting no. of 1's by receiver they are even so No error if odd No. of 1's appear then there is an error Using the Even Parity Using even parity, add the parity bit 1 and the set of data becomes: (8 bits of data including the parity bit), total number of 1s becomes even. Using the Odd Parity Using odd parity, total number is already odd, parity bit is set to 0 and the set of data becomes: (8 bits of data including the parity bit).