Data Communications Essay Example for Free

Data Communications Essay Differentiate between an analog and a digital electromagnetic signal. What are three important characteristics of a periodic signal. What is the relationship between the wavelength and frequency of a sine wave. Define fundamental frequency. What is the relationship between a signals spectrum and its bandwidth. What is attenuation? Define channel capacity. What key factors affect channel capacity? Problems: Figure 1 shows the frequency domain function for a single square pulse. The signal pulse could represent a digital 1 in a communications system. Note that an infinite number of higher frequencies of decreasing magnitudes is needed to represent the single pulse. What implication does that have for a real digital transmission system? Suppose that a digitised TV picture is to be transmitted from a source that uses a matrix of 480 x 500 picture elements (pixels), where each pixel can take on one of 32 intensity values. Assume that 30 pictures are sent per second. (This digital source is is roughly equivalent to broadcast TV standards that have been adopted.) Find the source rate R (bps) Assume that the TV picture is to be transmitted over a channel with 4.5Mhz bandwidth and a 35dB signal-to -noise ratio. Find the capacity of the channel (bps). Discuss how the parameters given in part (a.) could be modified to allow transmission of colour TV signals without increasing the required value for R. What is the channel capacity for a teleprinter channel with a 300Hz bandwidth and a signal-to-noise ratio of 3dB, where the noise is white thermal noise? A digital signalling system is required to operate at 9600bps. If a signal element encodes a 4-bit word, what is the minimum required bandwidth of the channel? Repeat part (a .) for the case of 8-bit words. Given the narrow (usable) audio bandwidth of a telephone transmission facility, a nominal SNR of 56dB (400,000), and a certain level of distortion, What is the theoretical maximum channel capacity (kbps) of traditional telephone lines? What can we say about the actual maximum channel capacity? Given a channel with an intended capacity of 20Mpbs, the bandwidth of the channel is 3MHz. Assuming white thermal noise, what signal-to-noise ratio is required to achieve this capacity? If an amplifier has a 30dB voltage gain, what voltage ratio does the gain represent? An amplifier has an output of 20W. What is the output in dBW? Answers: Short Questions With guided media, the electromagnetic waves are guided along an enclosed physical path whereas unguided media provide a means for transmitting electromagnetic waves but do not guide them. A continuous or analog signal is one in which the signal intensity varies in a smooth fashion over time while a discrete or digital signal is one in which the signal intensity maintains one of a finite number of constant levels for some period of time and then changes to another constant level. Amplitude, frequency, and phase are three important characteristics of a periodic signal. The relationship is ÃŽ »f = v, where ÃŽ » is the wavelength, f is the frequency, and v is the speed at which the signal is traveling. The fundamental frequency is the lowest frequency component in the Fourier representation of a periodic quantity. The spectrum of a signal is the frequencies it contains while the bandwidth of a signal is the width of the spectrum. Attenuation is the gradual weakening of a signal over dis tance. The rate at which data can be transmitted over a given communication path, or channel, under given conditions, is referred to as the channel capacity. Bandwidth, noise, and error rate.

The Three Little Users and the Big Bad Wolf Essay examples -- Creative

The Three Little Users and the Big Bad Wolf Once upon a time, there were three little computer users of three different professions and the time came for them to leave home and seek their fortunes. The first little user with the screen name LiLestPiggy, the youngest, bought his computer at the local chain computer store CompUSA, and had just recently installed a modem so he could surf the net. Email being the primary reason for the use of his computer, LiLestPiggy purchased a computer with the most basic set up. He was planning to surf the web and play games online, pursuing a job as an English teacher at the nearby elementary school. It was the most basic set up for a beginner. The second little user with the screen name LiLPiggy, the middle of the three, bought a laptop, installed Norton Anti-Virus, and installed a cable modem to insure a fast and safe internet connection. He was planning to pursue a business job so he needed a fast and safe computer. The third little user with the screen name of IllestPiggy, the eldest of three, built his own computer, installing several Anti-Virus programs and setting up a firewall and top-notch secret government security devices. A firewall protects your network from unwanted Internet traffic. The primary functions of a firewall are to let good traffic pass through while ‘bad’ traffic is blocked. The most important part of a firewall is its access control features that distinguish between good and bad traffic. One night, a hacker with the screen name BigBadWolf, who dearly loved to ruin little pigs lives, was out and about. The BigBadWolf instant messaged (IM’ed) LiLestPiggy who has the most basic internet connection. The BigBadWolf said, â€Å"Hello, I was wondering if ... ...estions brought up was whether the government should take action and patrol cyberspace for possible dangers. Some of the blame of the extensive abuse of the First Amendment can be pointed at the government. However, cyberspace is a place for people to act as they wish and speak freely, unless it would be breaking the first amendment of freedom of speech. If the government took action, there would most likely be conflict and it would be a big deal. In conclusion, hackers have tremendous power to cause trouble due to the unlimited resources and possibilities of the internet. Although they are capable of trouble, most hackers create viruses because of the purposes of it being challenging and entertaining. In this case, the government should step in and regulate cyberspace to a certain degree. We cannot let people commit crime and let them get away with it.

The Secret of Ella and Micha Chapter 11

Ella The next morning Lila is feeling much better. It's like last night never happened, but I wonder if it's an act. â€Å"I have a feeling that today is going to be full of rainbows and sunshine,† Lila says cheerily as she applies her lipstick, using the mirror on the closet door. Against my protest, she took down some of my sketches, so she could see her reflection. â€Å"See what, though?† I'd asked and she laughed, totally confused. â€Å"Are you high?† I tease as I twist my hair onto the back of my head and secure it with a clip. She pauses, looking at me over her shoulder. â€Å"Why do you always ask questions like that?† I slip on my boots and tie up the laces. â€Å"What kinds of questions?† She blots her lips. â€Å"Every time I'm happy, you always ask if I'm drunk or on something. People can be happy without substances.† I clasp a watch around my wrist. â€Å"Most people can, but not all.† Lila clips a diamond earring in. â€Å"You look really nice today.† I glance down at the black and purple dress I have on and the boots on my feet. â€Å"I forgot to do laundry so I had to wear some of my old clothes, which don't match any of my new shoes.† â€Å"Well you look nice.† She gives an elongated pause. â€Å"So what's on the agenda for today?† â€Å"It depends on what you're doing?† I ask. â€Å"Are you†¦ where are you planning on staying?† She shuts off her phone and then discards it onto the bed. â€Å"I'd like to stick around with you for a while, if you don't mind. We could hang out. I don't have anything scheduled for the summer and I'm not going back home.† â€Å"Do you want to tell me what happened?† â€Å"No, not really.† â€Å"Okay†¦ well, I need to find a job,† I say. â€Å"I have to save up for the rest of my tuition since it doesn't look like I'll be getting that internship.† She puts a headband in her hair. â€Å"The one at that art museum.† â€Å"That's the one and it doesn't start until mid-June,† I explain. â€Å"But that's only five weeks away, so I figured they'd have notified me if I got it.† â€Å"You never know. Sometimes things like that move slow.† She folds up a shirt and packs it into her bag, then ties a ribbon on the back of her shirt. â€Å"Although, if you get it that'd mean you'd have to go back to Vegas, right?† Nodding, I head for the door. Two weeks ago the idea of returning to the desert would make me happy, but something's changed. I still want to go, though leaving will be a little bit harder. I collect my phone off the dresser, noting the flashing voicemail on the screen – Micha's unread message. My finger hovers above the button as I step into the hall. He told me I wasn't ready for what was on it? But am I ready for it now? â€Å"I don't know why you think it's so bad here.† Lila follows me. â€Å"Yeah, people are a little rough, but they're not all bad and everywhere has bad stuff. You can't hide from it.† â€Å"That's very insightful.† I close my phone and put it away. â€Å"Bad comes in different forms,† Lila continues. â€Å"Whether it's drug dealers on the corner or if it's corrupt rich people or just your run-of-the-mill douche bag.† I don't know much about Lila, other than she's rich, her dad works as a lawyer and her mom stays home. She likes clothes, is great with numbers and was the only reason I passed pre-calculus. My brother's door is open and he walks out as we're passing by. He has a black and red polo shirt on and a pair of cargo pants. There's some kind of gel in his hair and it looks shiny. â€Å"Hey, have you seen dad?† he asks, giving an acknowledging glance at Lila. I point at the shut door at the end of the hall. â€Å"I thought I heard him come in late last night and go into his room.† â€Å"He did, but he got up this morning.† He leans against the doorframe and crosses his arms. â€Å"I heard him stumbling around in that bathroom and crying all night, but now I can't find him and I didn't hear him leave. His work called the house, saying he didn't show up, so he's not there.† My fists clench so that my nails dig into my palms. â€Å"Did you check in the bathroom?† Dean's eyes travel down the hall to the bathroom door and he shakes his head. â€Å"I haven't and I don't want to.† â€Å"Hi, I'm Lila,† she introduces herself and offers her hand. â€Å"You must be Ella's brother, Dean.† Dean is vaguely amused and shakes her hand. â€Å"Yeah†¦ how do you know Ella?† â€Å"I was her roommate,† she responds, pressing her hand to her chest, faking being offended. â€Å"Didn't she ever mention me?† â€Å"We don't talk that much.† I eye the bathroom door again and my stomach twists. â€Å"We need to find Dad.† â€Å"I'm not looking in that bathroom, Ella, but if you want to, go ahead.† With legs flimsier than wet noodles, I walk down the dark hallway and stop in front of the door, having a flashback of the day my mom died. The door was closed and the house was soundless, except for the running of water. My hands tremble as I open the door. The room is bare, the tub empty, and the tile floor is clean, except for a small stain. There are no towels on the hooks and the mirror on the wall across from me shows my reflection. My auburn hair is curled perfectly in place, my lips are lined with gloss, and my green eyes are immense and reveal everything. â€Å"Dad isn't in here,† I tell him, unable to look away from the mirror. â€Å"Are you sure you didn't hear him leave the house?† â€Å"He could have left and I just didn't hear him,† he answers. â€Å"But when has he ever left the house quietly before?† I quickly slam the bathroom door, like I'm trying to put out a fire, and race back down the hall. â€Å"Someone needs to find him. Did you try and call him?† â€Å"Of course. I'm not a moron.† He rolls his eyes and nods. â€Å"And he didn't answer.† Lila shifts her weight and forces the uncomfortable conversation elsewhere. â€Å"So you play the drums, Dean?† He motions to his drum set in the middle of his small room with dark blue walls. The floor and bed are cluttered with boxes and the curtain is pulled back, letting the sunlight spill in. â€Å"I used to, but I don't much anymore. I have work and a fiance.† â€Å"Fiance?† Lila and I say simultaneously. â€Å"Yeah, as in we're engaged.† Dean rolls his eyes and goes back into his room. â€Å"It's what happens when two people date for a really long time.† â€Å"Why didn't you tell me?† I ask, following him into his room. He picks up a small box and drops it onto the floor. â€Å"Do you really care that I am?† I carefully nudge the box out of the way with my foot. â€Å"You're my brother. Of course I care.† â€Å"But it's not like we've ever really gotten along,† he points out. â€Å"I haven't even talked to you for a year. God, I didn't even know you went to college until a week ago.† He's right, which is sad. I barely know him, he barely knows me, and I'm starting to think I barely know me, too. â€Å"Does Dad know you're engaged?† I ask. â€Å"Were you at least planning on telling him?† â€Å"Even if I told him, he'd just forget the next day.† He empties a dresser drawer into a large open box and then aligns the drawer back into place. â€Å"You know how he is. Christ, I don't even think half the time he knows that you and I don't live here anymore.† â€Å"He still deserves to be told,† I say. â€Å"He's not a bad guy and you know it. He just has problems.† â€Å"Problems that fucked up our childhood.† He kicks a box out of the way with force and it crashes into the wall. â€Å"You do realize that how we grew up wasn't normal. God, even Micha had it easier and his dad bailed out on him, but at least he had a stable mom to take care of him.† â€Å"Umm†¦Ã¢â‚¬  Lila pokes her head in the room. â€Å"I think I'm going to wait outside for you, Ella.† God, I'd forgotten she was even there and she just heard all of that. â€Å"Okay, I'll be down in a second,† I tell her and she leaves readily. I wander around Dean's room, taking in the photos he has up. â€Å"I think we may have just scared her to death.† Dean picks up his drumsticks and places them into a large duffel bag. â€Å"Okay, I have to ask. How did you end up being friends with her?† â€Å"She was my roommate and we just sort of bonded.† I shrug, picking up a photo of Dean and his friends on a sunny beach. It was taken during his Senior Field Trip and he looks happy. â€Å"You bonded,† he accuses. â€Å"The girl looks like a spoiled princess.† I eye his preppy clothes. â€Å"So do you.† â€Å"First off, I'm not a princess and I've earned what I have,† he says. â€Å"It wasn't just handed to me.† â€Å"Maybe she did, too.† â€Å"Did she?† I hate to give him the benefit of being right. â€Å"No, her parents are pretty well off.† He looks at me with that stupid arrogant expression he gets when I admit he's right. â€Å"Well, there you have it then.† â€Å"She's nice,† I protest. â€Å"And she doesn't ask a lot of questions.† â€Å"It may seem like you need to keep things to yourself,† he says, putting a blanket into a box. â€Å"But it's not healthy. You need to find someone you can let it all out to. Otherwise you're going to lose it.† My eyes roam to the window where the edge of Micha's house is visible. â€Å"I think I already did.† Dean's forehead creases as he drops a handful of guitar picks into a trunk. â€Å"Lose it? Or talk to someone about it?† â€Å"Both.† I back toward the door. â€Å"When are you heading back to Chicago?† â€Å"Hopefully by tonight. No offense or anything, but this place brings back way too many unpleasant memories.† â€Å"Try to say good-bye before you leave.† He doesn't respond and I don't wait around for an answer. That was probably the longest conversation that we've ever had and I have a feeling it may be our last for a very long time.

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