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  • Computer Networks UNISA - Chap 8 &ndash; Wireless Networking

    - by MarkPearl
    After reading this section you should be able to Explain how nodes exchange wireless signals Identify potential obstacles to successful transmission and their repercussions, such as interference and reflection Understand WLAN architecture Specify the characteristics of popular WLAN transmission methods including 802.11 a/b/g/n Install and configure wireless access points and their clients Describe wireless MAN and WAN technologies, including 802.16 and satellite communications The Wireless Spectrum All wireless signals are carried through the air by electromagnetic waves. The wireless spectrum is a continuum of the electromagnetic waves used for data and voice communication. The wireless spectrum falls between 9KHZ and 300 GHZ. Characteristics of Wireless Transmission Antennas Each type of wireless service requires an antenna specifically designed for that service. The service’s specification determine the antenna’s power output, frequency, and radiation pattern. A directional antenna issues wireless signals along a single direction. An omnidirectional antenna issues and receives wireless signals with equal strength and clarity in all directions The geographical area that an antenna or wireless system can reach is known as its range Signal Propagation LOS (line of sight) uses the least amount of energy and results in the reception of the clearest possible signal. When there is an obstacle in the way, the signal may… pass through the object or be obsrobed by the object or may be subject to reflection, diffraction or scattering. Reflection – waves encounter an object and bounces off it. Diffraction – signal splits into secondary waves when it encounters an obstruction Scattering – is the diffusion or the reflection in multiple different directions of a signal Signal Degradation Fading occurs as a signal hits various objects. Because of fading, the strength of the signal that reaches the receiver is lower than the transmitted signal strength. The further a signal moves from its source, the weaker it gets (this is called attenuation) Signals are also affected by noise – the electromagnetic interference) Interference can distort and weaken a wireless signal in the same way that noise distorts and weakens a wired signal. Frequency Ranges Older wireless devices used the 2.4 GHZ band to send and receive signals. This had 11 communication channels that are unlicensed. Newer wireless devices can also use the 5 GHZ band which has 24 unlicensed bands Narrowband, Broadband, and Spread Spectrum Signals Narrowband – a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies Broadband – uses a relatively wide band of the wireless spectrum and offers higher throughputs than narrowband technologies The use of multiple frequencies to transmit a signal is known as spread-spectrum technology. In other words a signal never stays continuously within one frequency range during its transmission. One specific implementation of spread spectrum is FHSS (frequency hoping spread spectrum). Another type is known as DSS (direct sequence spread spectrum) Fixed vs. Mobile Each type of wireless communication falls into one of two categories Fixed – the location of the transmitted and receiver do not move (results in energy saved because weaker signal strength is possible with directional antennas) Mobile – the location can change WLAN (Wireless LAN) Architecture There are two main types of arrangements Adhoc – data is sent directly between devices – good for small local devices Infrastructure mode – a wireless access point is placed centrally, that all devices connect with 802.11 WLANs The most popular wireless standards used on contemporary LANs are those developed by IEEE’s 802.11 committee. Over the years several distinct standards related to wireless networking have been released. Four of the best known standards are also referred to as Wi-Fi. They are…. 802.11b 802.11a 802.11g 802.11n These four standards share many characteristics. i.e. All 4 use half duplex signalling Follow the same access method Access Method 802.11 standards specify the use of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to access a shared medium. Using CSMA/CA before a station begins to send data on an 802.11 network, it checks for existing wireless transmissions. If the source node detects no transmission activity on the network, it waits a brief period of time and then sends its transmission. If the source does detect activity, it waits a brief period of time before checking again. The destination node receives the transmission and, after verifying its accuracy, issues an acknowledgement (ACT) packet to the source. If the source receives the ACK it assumes the transmission was successful, – if it does not receive an ACK it assumes the transmission failed and sends it again. Association Two types of scanning… Active – station transmits a special frame, known as a prove, on all available channels within its frequency range. When an access point finds the probe frame, it issues a probe response. Passive – wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access point – the beacon frame contains information necessary to connect to the point. Re-association occurs when a mobile user moves out of one access point’s range and into the range of another. Frames Read page 378 – 381 about frames and specific 802.11 protocols Bluetooth Networks Sony Ericson originally invented the Bluetooth technology in the early 1990s. In 1998 other manufacturers joined Ericsson in the Special Interest Group (SIG) whose aim was to refine and standardize the technology. Bluetooth was designed to be used on small networks composed of personal communications devices. It has become popular wireless technology for communicating among cellular telephones, phone headsets, etc. Wireless WANs and Internet Access Refer to pages 396 – 402 of the textbook for details.

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  • How to make a deb install only when the dependency is installed, or the conflicting packages aren't?

    - by adam
    I'm making a package for Cydia, but since it uses APT/dpkg, I thought I'd ask this question here. I want my package to only install if gsc.wildcat (represents an iPad) is installed, or if gsc.front-facing-camera (represents an iPhone 4/4S) is not installed. This setup would let the package only install on the 3GS or an iPad, but there's no package that represents a 3GS that I know of. It also requires iOS 5 (firmware (>= 5.0)). Here's an excerpt from my control file: Depends: gsc.wildcat, firmware (>= 5.0) Conflicts: gsc.front-facing-camera How can I do this with only Depends/Conflicts? (Please note that this question is about dpkg and not Cydia.)

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  • TestDrive equivalent for Ubuntu Server

    - by Marius Gedminas
    Every now and then I'd like to play with a fresh minimal install of Ubuntu (to test sysadminish scripts, application install instructions, package dependency lists etc.). I'd like to have a tool as simple to use as testdrive: pick a version (say, 'maverick'), run a command, get a shell in a new virtual machine. I'd like that shell to be in the current terminal, rather than a new GUI window that testdrive uses. Setting up the new VM to accept SSH logins with my ssh public key is fine. I'd like the VM to have network access out of the box; NAT to a virtual network interface is fine. Why a VM? Chroots don't really cut it: installing, say, Apache in a chroot would fail because it would try to listen on port 80, which is already taken. Containers might work, though, if there are any that are supported by standard Ubuntu kernels.

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  • Slow gvfs Samba Performance

    - by Wolfgang
    if I use/acces a Samba Share using Nautilus or manually using "gvfs-mount smb://SERVER-IP/Share" I get a poor Performance, only about 7 mb/s on my 100 Mbit Network. If I access the same Share on Windows I get Fullspeed 100 Mbit (About 11 mb/s), also if I mount using sudo mount -t cifs //SERVER-IP/Share /mountpount which uses cifs instead of gvfs I get fullspeed too, so can anyone tell me if there is a Performance Problem/Bug in gvfs or how gvfs is using SMB Shares differerently ? I tested read Perfomance (From my Network Samba Share) with multiples files and always, the cifs-Version is fullspeed and the GVFS-Version has some mb/s less. After some research I found some tips to optimize the Samba Settings of my Ubuntu Installation and some network tuning tips, but as the CIFS mounted share gets the full Network Speed I don't believe its that kind of problem.

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  • Wireless speed drops over time and doesn't get up if not reconnected

    - by Vili Lehto
    I am using Ubuntu 12.10 64bit and I have a Asus PCE-N15 wireless card(in PCI-E slot). The problem is that first when I connect to my WiFi network the speed is just fine, and actually my link speed never drops, it shows a solid 150mb/s+ link speed and good signal. But the download and upload speeds drop dramatically after couple minutes of use and I have to reconnect to fix this. Network is type IEEE 802.11bgn and uses WPA2/AES encryption. I don't have similar problems on the windows side, and the network is working fine on every other device. iwconfig shows this: wlan0 IEEE 802.11bgn ESSID:"WLAN-AP" Mode:Managed Frequency:2.412 GHz Access Point: 00:1E:AB:05:EF:31 Bit Rate=300 Mb/s Tx-Power=20 dBm Retry long limit:7 RTS thr=2347 B Fragment thr:off Encryption key:off Power Management:off Link Quality=61/70 Signal level=-49 dBm Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0 Tx excessive retries:0 Invalid misc:2 Missed beacon:0 So the question is: Is there a way to fix this? Thanks in advance.

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  • How to estimate freight / shipping costs ??

    - by Vani
    Hi, I am working on a PHP web application and want to know the best way to estimate freight costs in USA. The site deals with construction materials that uses LTL or Truck loads. I found a few sites like www.freightCenter.com that provide quotes using webservice. Two drawbacks, its paid service and the other, my site response time is slow if I use the webservice. Is there a open source tool/logic avaliable for estimating shipping / freight costs?? Or a way to determine the rate per mile per pound for different freight classes? Thank you, Vani

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  • How to make openJDK automatically update-alternatives?

    - by cgc512
    Well, I've made a fresh Ubuntu 11.10 install and I installed openJDK 7. It updated alternatives just after installing, so it was all good. Then, I compiled a source and executed it and it didn't worked, then I noticed that java was using the version 1.6, while the javac was 1.7. I thought I had installed it wrong, so I uninstalled it and installed it again. But now, it does not update alternatives automatically (it does it only for javaws). Is there a way to "restart" it so the apt-get does the update-alternatives by itself like the first time? Is it normal that it uses the java 1.6 instead of the java 1.7 when installing openJDK 7? Thank you very much!

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  • Why is there a "new" in Go?

    - by dystroy
    I'm still puzzled as why we have new in Go. When you want to instantiate a struct, you do t := Thing{} and, obviously, you can get a pointer to a new instance by doing t := &Thing{} But there's also this possibility : t := new(Thing) This last one seems a little alien to me. &Thing{} is as clear and concise as new(Thing) and it uses only constructs you often use elsewhere. It's also more extensible as you might change it to &Thing{3} or &Thing{Feets:7}. In my opinion, having a supplementary keyword is costly, it makes the language more complex and adds to what you must know. And it might mask to newcomers what's behind instantiating a struct. It also makes one more reserved word. So what's the reasoning behind new ? Is it something useful ? Should we use it ?

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  • When Googlebot sees a link, will it click it or navigate to it?

    - by FakeRainBrigand
    My site uses pushState and JSON data to display content. So, for example, this might appear on my page: <a href="/some/page">some page</a> The JavaScript then prevents the default action (following the link), and instead renders a view (using a different api, such as /getjson?some_page). $('[href]').click(function(){ history.pushState(...); handleURL(...); }); Assume my server will respond to requests at /some/page with a pre-rendered version. My questions are: will Googlebot receive the prerendered version, or allow JavaScript to instead invoke pushState, etc. if it doesn't make the direct request, will it wait for AJAX content to be loaded? does Googlebot implement pushState, so it will show the proper URL in search results?

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  • ubuntu 12.04 ambiance dark side bar issue when clicking folder on desktop

    - by Lou Crittenden
    ubuntu 12.04 gnome: concerning an updated custom ambiance theme: why is there no dark side bar when I click on a folder, like the home folder icon on the desktop, but the theme works as planned when I type "nautilus" in the terminal to open the home folder or when opening a folder up as root? Permissions issue perhaps? note: i am using cinnamon instead of unity and i noticed it uses the nemo file manager instead of nautilus and i suspected that it was causing me the grief. so i uninstalled it and now am using nautilus only as the file manager. i found this out when i typed: "sudo killall nemo" and the problem went away. i'll see how this goes... (and hopefully cinnamon doesn't care about it...) has anyone else had any issues with this?

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  • How do I edit my resolv.conf file?

    - by Ahatius
    I have the problem that my ubuntu machine uses the wrong dns server. For some reason he queries localhost for dns records. I have added the dns server in the network settings gui, but /etc/resolv.conf still contains 127.0.0.1 as dns server. Now I tought I could just edit the file by myself, but it explicitly says I should not edit the file by hand. Now, since the network settings GUI didn't generate the file with the right settings, how do I generate a new resolv.conf file by myself?

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  • Joystick acts as mouse; won't stop

    - by Shazzner
    Joystick acts as a mouse, even when I'm playing a game that uses a joystick so I get random mouse events going on. I plugged a joystick in to play Spiral Knights, also installed joystick and jcalibrate. Everything is working good, except by default the joystick moves the mouse around and the button activate mouse keys. Now normally this would be good behavior if I'm on a Myth-box or something, unfortunately when I play Spiral Knights with joystick input I see my mouse cursor moving in the back ground and when I hit a button it thinks I'm pressing right-click so it minimizes everything. Also it creates folders and probably deletes stuff. So, basically how the heck do I stop it from acting as a mouse?

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  • Super-quick MIDI generator with nonrestrictive license?

    - by Ricket
    I'm working on my Ludum Dare entry and trying to figure out how in the world I'm ever going to get background music. I found WolframTones, but the license is too restrictive: Unless otherwise specified, this Site and content presented on this Site are for your personal and noncommercial use. You may not modify, copy, distribute, transmit, display, perform, reproduce, publish, license, create derivative works from, transfer, or sell any information or content obtained from this Site. For commercial and other uses, contact us. But I really like the interface! It's a lot like sfxr - click a genre and download a song. That's so cool. Is there another program that does this same sort of thing but without a restrictive license, so that I can generate a bgm and use it in my game?

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  • .pam_environment in kerberized nfs4 home directory

    - by Paul Stoever
    How can I get pam_env to read the user's .pam_environment file, if the user's file is located in a kerberized NFS4 mount? The file and directory permissions for the .pam_environment file are set in a way, that allows the local root to read the file. Reading .pam_environment only fails on the first login. Subsequent logins successfully read the file. The client uses Ubuntu 12.04 Desktop, NFS/Kerberos server is 12.04 Server. The Kerberos/NFS4 stuff works with exception of this. From /var/log/auth for first login: ... lightdm: pam_krb5(lightdm:auth): user USERNAME authenticated as USERNAME@REALM lightdm: pam_unix(lightdm:session): session closed for user lightdm lightdm: pam_env(lightdm:setcred): Unable to open config file: USERHOME/.pam_environment: Permission denied lightdm: pam_env(lightdm:setcred): Unable to open config file: USERHOME/.pam_environment: Permission denied lightdm: pam_unix(lightdm:session): session opened for user USERNAME by (uid=0) ...

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  • How can i run my .LÖVE game directly from the lua interpreter?

    - by jonathan
    I've just started with LOVE and LUA , i'm interested in LOVE because i want to play around with something different from my dayjob(i'm a webdeveloper) and since it uses LUA and is interpreted , i though it would be a great way to try out the API. but i couldn't find how to run my .LÖVE game directly from the lua interpreter? i'm finding it bothersome to package the game each time i make a little test with the API. since i couldn't find the answer i'm asking, but maybe i'm serching for the wrong terms, if this it is a simple matter like "import the library" or set the global, i'll gladly remove my question.

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  • How to profile LINQ to Entities queries in your asp.net applications - part 1

    - by nikolaosk
    I have been teaching ASP.Net and EF in one of my classes and I have been asked on the various ways we can profile database activity. Everyone that I know that uses EF as its data access layer has the same question. "How can I see the T-SQL code that the LINQ to Entities engine generates on the fly?" I know a lot of people use VS studio built-in visualisers but that is not enough. A lot of developers use SQL Server Profiler. That is also a good solution since we can see the queries(generated from...(read more)

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  • Developing Salesforce apps in Ruby on Rails

    - by Robert S.
    I want to build a web app that uses Salesforce.com data, and I want to build it fast. I'm a .NET developer (WPF, C#, ASP.NET MVC). I understand Ruby and RoR fairly well, but I haven't delivered any Rails apps. I'm wondering, is Ruby on Rails a suitable tool for rapidly building Salesforce apps, or is it better for the "traditional" web2.0 stuff like Groupon and Twitter? In other words, would using RoR help me achieve my fast (e.g., three months) goal over using .NET, which I already know?

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  • Connected to wifi, but quits loading pages after a random amount of time

    - by Collin
    I'm using Ubuntu 12.10, on a TOSHIBA Satellite L755 laptop, and using a Cradlepoint MBR-1000 wireless router. The wifi supposedly works fine on my sisters' and mom's laptops, and my dad uses a wired connection through the router. Nobody else has the problem. It shows that I'm connected and everything. The network manager GUI shows that everything is going just dandy... but it's not. I'll be able to load a page or two, then I have to disconnect, and reconnect to the router to get it to work again for a couple minutes. I noticed when I was downstairs, close to the router, this problem seemed to go away (at least, I was connected for a couple of hours without running into this problem). When I run ping -n 8.8.8.8 it shows this while the connection is still running fine 64 bytes from 8.8.8.8: icmp_req=1187 ttl=47 time=665 ms and whe

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  • What user-friendly term should I use for a view that lives under a tab in a tab bar app?

    - by Emile Cormier
    My app uses a tab bar controller. In the user documentation, I'm not sure what name to use for a view that lives under a tab. For example, the app has a Settings tab. In the user documentation, I have a sentence that goes something like this: This threshold can be adjusted in the Settings tab. "Settings tab" is not terribly user-friendly. What would be a better term than "tab"? I've looked though Apple's Human Interface Guideline, but I can't find what would be the official user-friendly term for "view that lives under a tab".

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  • How can I upgrade to Lubuntu 14.04.1 withot a PAE kernel?

    - by Richard
    On my Dell Latitude D800 laptop, which has an old Pentium M without PAE support, I was able to successfully install Lubuntu 14.04 from the CD. When I try to upgrade, I get the error: PAE not enabled Your system uses a CPU that does not have PAE enabled. Ubuntu only supports non-PAE systems up to Ubuntu 12.04. To upgrade to a later version of Ubuntu, you must enable PAE (if this is possible) see: http://help.ubuntu.com/community/EnablingPAE I have seen the Communit Wiki page for PAE. The suggestion is to add forcepae to the command line options. When I do this, I get the same message. Moreover, dmesg does not indicate PAE was ever enabled. Is there anything else I can ry to get Lubuntu to upgrade correctly?

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  • What is the best way to find a python google app engine coach?

    - by David Haddad
    i'm a software engineer and have been building Google App Engine apps with Python for about a year. I have a pretty good familiarity with the main concepts: web app framework, modeling, queues, memcache, django templates, etc. Where I think I'm lacking is in methodology. Architecting the app, using git for versioning, designing an writing unit tests. I'm totally convinced to incorporate these practices in my development style, and have started reading up on them. However I've learned that I'm a much faster learner when I have someone experienced to ask questions to and interact with. IRC channels and forums like stack overflow are great. But sometimes you want something more dynamic that produces results faster. So my question is how can a person find an experienced engineer that is familiar with the technologies he uses and that is willing to give them a couple of hours of Skype coaching sessions per week in return for an hourly fee...

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  • What skills does a web developer need to have/learn?

    - by Victor
    I've been I've asked around, and here's what I gathered so far in no particular order: Knowledge Web server management (IIS, Apache, etc.) Shell scripting Security (E.g. ethical hacking knowledge?) Regular Expression HTML and CSS HTTP Web programming language (PHP, Ruby, etc.) SQL (command based, not GUI, since most server environment uses terminal only) Javascript and library (jQuery) Versioning (SVN, Git) Unit and functional test Tools Build tools (Ant, NAnt, Maven) Debugging tools (Firebug, Fiddler) Mastering the above makes you a good web developer. Any comments?

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  • Compilable modern alternatives to C/C++

    - by Jeremy French
    I am considering writing a new software product. Performance will be critical, so I am wary of using an interpreted or language or one that uses a emulation layer (read java). Which leads me to thinking of using C (or C++) however these are both rather long in the tooth. I haven't used either for a long time. I figure in the last 20 years someone should have created something which is reasonably popular and is nice to code in and is complied. What more modern alternatives are there to C for writing high performance code compiled code? edit in response to comments If C++ is a different beast than it was 15 years ago, I would consider it, I guess I had an assumption that it had some inherent problems. Parallelisation would be important, but probably not across multiple machines.

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  • What algorithm(s) can be used to achieve reasonably good next word prediction?

    - by yati sagade
    What is a good way of implementing "next-word prediction"? For example, the user types "I am" and the system suggests "a" and "not" (or possibly others) as the next word. I am aware of a method that uses Markov Chains and some training text(obviously) to more or less achieve this. But I read somewhere that this method is very restrictive and applies to very simple cases. I understand basics of neural networks and genetic algorithms(though have never used them in a serious project) and maybe they could be of some help. I wonder if there are any algorithms that, given appropriate training text(e.g., newspaper articles, and the user's own typing) can come up with reasonably appropriate suggestions for the next word. If not (links to)algorithms, general high-level methods to attack this problem are welcome.

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  • wordpress woocommerce php variable usage %1$s

    - by tech
    I am using wordpress with woocommerce and I am trying to manipulate a copy of myaccount.php The default code uses some variables of some sort that I am not familiar with nor have I been able to find documentation on. The variables in question are %1$s, %2$s and %s <p class="myaccount_user"> <?php printf( __( 'Hello <strong>%1$s</strong> (not %1$s? <a href="%2$s">Sign out</a>).', 'woocommerce' ) . ' ', $current_user->display_name, wp_logout_url( get_permalink( wc_get_page_id( 'myaccount' ) ) ) ); ?> <?php printf( __( 'From this page you can view your recent orders, manage your shipping and billing addresses and <a href="%s">edit your password and account details</a>.', 'woocommerce' ), wc_customer_edit_account_url() ); ?> </p> How can I identify the variables, what they represent and how to use them? Thank you.

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