Why Install Linux?
Download Linux and Create a Bootable DriveDownload any Linux distribution for free on their websites. For first time users, I recommend trying Ubuntu where although there are many user friendly Linux distributions suited for beginners Ubuntu is the most popular general users Linux distribution today. If the information regarding to the problems you are facing are not already available online, then you can just ask the community and they usually respond quickly. After downloading the Linux disk image is to create a bootable drive. The popular bootable drive today is a bootable USB where we the Linux into a USB and boot it in startup. I am guessing that most of you are Windows user and my favourite software to create a bootable drive is Rufus. If you are on MacOS or rarely you bought a laptop with Linux installed and want to install another Linux, my favourite is Balena Etcher. Ofcourse there are other utilities such as Win32 Disk Imager and Unetbootin, my favourites for now are Rufus and Etcher but feel free to try other utilities but becareful of malware. Today's utility provides protection as it does not read internal and large external hard disks preventing us from messing our existing system. We can show them by clicking a checkbox and will notify us to proceed with caution. Well the only risk is that we may forget to backup the contents of our flash drive and procceed with the flash erasing the contents in the process. The main process from the video are:
By the way, if you prefer the old way of using a CD drive, you can try downloading any CD burning utilities such as Nero and Daemon Tools. Installing Linux UbuntuOn Virtual MachineIf you want to try Ubuntu or any other operating systems (OS) but not yet confident in installing directly to the computer with reasons such as afraid of messing up the current system, then you can try virtual machines. Virtual machines runs your OS on top of existing one just like other software and applications so you do not need to worry about messing your current system. Another reason is if you have a high spec computer that is powerful enough to run 2 OS at the same time which may make you feel lazy to install two seperate OS in one PC (dualboot) and just install the OS on top of existing one. If you are confident enough or do need this step, just skip it. If you can afford, you can try VMware. If not, you can try VirtualBox. Detail discussion is not necessary here as you can just install and explore and delete and try again if you fail as there are no risk. However, from the video, here are some highlighted steps:
On Internal Hard Drive (Default Way)Unfortunately, I do not have a video of installing alongside them but the recording should not be much different from install fresh. I do have a video of the "something else" option on the next subsection. Ofcourse, if you are an expert, you would probably prefer "something else" and make your own customization. The "install alongside ..." is just a user friendly option for general users. On External Hard DriveInstalling on external hard drive is what something people rarely do but people do it because:
The installation method is similar but with few difference as follow after choosing the "Choose something else":
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NoteThis is my undergraduate assignment that I translated to English in the Electric Field Theory course where my task is to write an essay on the Amperes Law and Magnetic Fields. Apart from me this group consists of Vencisisca Jeane R, Rr. Sanita Eling Cipta Nesa, Putu Arie Pratama, Fajar Purnama, Muhammad Audy Bazly, Muhammad Nordiansyah. This assignment has never been published anywhere and we as the authors and the copyright holders license this task customized CC-BY-SA where anyone can share, copy, republish and sell on condition to state our name as the author and inform that the original and open version available here. Chapter 1 Introduction1.1 BackgroundIn everyday life, many applications of several physical laws occur, one of which is Ampere's law and magnetic fields. Where in general the relationship between Ampere's law and magnetic fields, namely the Ampere's law states that the magnetic field can be generated in two ways, namely through an electric current (the initial formulation of the Ampere law), and by changing the electric field (Maxwell's addition). Maxwell's correction of the Ampere law is quite important, thus this law states that changes in the electric field can give rise to a magnetic field, and vice versa. Three genius scientists from France, Andre Marie Ampere (1775-1863), Jean Baptista Biot (1774-1862) and Victor Savart (1803-1862) stated that the force will be generated by an electric current flowing in a conductor that is between the magnetic fields. This is also the reverse of Faraday's law, in which Faraday predicts that an induced voltage will arise in a moving conductor and cut across the magnetic field. This law is applied to electric machines, and the image below will explain this phenomenon. 1.2 ProblemThe problems raised in this assignment are as follows:
1.3 ObjectiveThe main purpose of writing the assignment "Ampere Law and Magnetic Field" this time is to know and understand in detail about Ampere's law and magnetic fields. The other objectives of this paper are:
1.4 BenefitThe benefits obtained from the paper "Ampere Law and Magnetic Field" are as follows:
1.5 Scope of MaterialIn this assignment, the scope discussed by the author is limited to the issues discussed, namely the Ampere Law and Magnetic Fields. Chapter 2 Discussion2.1 Biot-Savart LawThe magnetic field differential, dH is the result of the current element differential I dH. The field is inversely proportional to the distance (r2), is a free medium that is around it and has a direction which is the cross product of I dI and aR. This relationship is known as the Biot-Savart Law, where the equation can be, seen as below: dH = (I dI x aR)/(4ÏR2) (A/m) The direction of R must come from the current element at the point where dH has been determined, as in the following figure: Current elements must not be separated. All elements complement the current filament contribution to H and the current must be involved. The integral form in Biot-Savart's Law is as follows: H = â®(I dI x aR)/(4ÏR2) A simple closed integral requires all the current elements involved in obtaining the H. Example Problem 1: Current I is long and straight along the z-axis of the cylindrical coordinates as shown in the figure below. Without losing any of its general properties, choose a point in the z = 0 plane. Then in its differential form, it is as follows: dH = (I dz ax X (rar X zaz) / (4Ï(r2+z2)<</sup>sup>(3â2)) = (I dz raâ ) / (4Ï(r2+z2)(3â2)) H = [â«-ââ(Ir dz) / (4Ï(r2+z2)(3â2))] aâ = aâ /2Ïr The magnetic field by surface currents and volume is also given by the Biot-Savart law of integral form, where I dI is replaced by K dS or Jdv respectively, and where the integral is taken over the entire surface, or volume, concerned. An important example is the current in an infinite plane with a constant density K. Where the equation can be seen as follows: H = K/2 X an 2.2 Ampere LawThe line integral of the tangential component H along the closed path is equal to the magnitude of the current in the vicinity: â®H.dI = Ienc This is Ampere's law. At first sight, we might think that the application of this law is to determine flow by an integration. In fact, usually the current is known and this law will give us a way of determining H. So the application of the law is very similar to using Gauss's law to determine D in a given charge distribution. To be able to take advantage of Ampere's law in determining H there must be a sufficiently high degree of symmetry to the problem being discussed. The two conditions that must be met are as follows:
The Biot-Savart law can be used to determine which path meets these conditions. But in most cases, the path is immediately visible. Example Problem 2: Use the amperes law to get H by a long, straight current I. The Biot-Savart law shows that H is tangential and with the same magnitude along the circle in Figure 2.3, then the equation is as follows: â®H.dI=H(2Ïr)=I H = aÏ/2Ïr 2.3 CurlThe curl of the vector field A is another vector field. Point P in Figure 2.3 is located in the area âS which is bounded by closed curve C. In the integration that defines curl, C is the area traversed by the curl so that the closed area is on the left. Normal units, determined using the rule of thumb, are as shown in the figure. Thus, the curl component of A in the direction an is defined as follows: (curl A).an = lim(âSâ0)(â®A.dI/âS) In the coordinate system, curl A is the fully specified value defined by the component along the three vector units. For example, the x component of Cartesian coordinates is defined by taking the contour line C of the square in the plane x = conts, via the point P, as shown in Figure 2.5: (curl A).an = lim(ây âzâ0) â¡(â®A.dI / ây âz) If, A = Axax + Ayay + Azaz, at the angle from closed âS to the center (point 1), then: â® = â«12 + â«23 + â«34 + â«41 =Ayây + (Az+(âAz/âyây))âz + (Ay+(âAy/âzâz))(-ây) + Az(-âz) =((âAz/ây)-(âAy/âz)) âyâz (curl A).ax = (âAz/ây)-(âAz/âz) The y and z components can be determined in the same way. The combination of the three components can be seen in the equation below: curl A = ((âAz/ây)-(âAz/âz)) ax+((âAx)/âz-(âAz)/âx) ay+((âAy)/âx-(âAx)/âx) az (cartesian) The third-order determinant can be written like the equation below, the expansion of which gives the Cartesian curl of A. curl A=|(ax&ay&az @ â/âx&â/ây&â/âz @ Ax&Ay&Az)| The elements of the second line are components of the del operation. This shows that â à A can be written for curl A. As with other expressions of a vector analysis, this notation is used for curl A in other coordinate systems, even though à A is defined in Cartesian coordinates only. The expressions for curl A at the coordinates of the cylinder and the sphere can be derived in the same way as above. Although, with a higher difficulty. curl A = (1/r (âAz)/(ââ ) - (âAâ /âz)) ar + ((âAr/âz) - (âAz/âr)) aâ + ((â(rAâ ))/âr - (âAr/ââ )) az (cylinder) curl A = 1/(r sinâ¡ ) [(â(Aâ sinâ¡Î¸))/âθ - (âθ/ââ )] ar+1/r [1/sinâ¡Î¸ (âAr/ââ )-(â(rAâ )/âr)] aâ + 1/r [(â(rAâ ))/âr-(âAr)/(ââ )] az (spherical) The frequency of use of the two properties of the curl operator can be explained as follows:
2.4 The relationship of J and HIn the law of amperes, the equation definition of (curl H) x can be written as follows: (curl H).ax=lim(ây âzâ0)â¡Ix/(ây âz)- Jx Where Jx = dlxâdS is the area of density in x direction of current. So that the x component of curl H and direction of current J are the same at every point. Likewise for the x and y components, it can be seen as follows. âÃH=J This is one of Maxwell's equations in the static electric field. if H is known for the entire region, then â à H = J will produce J for the region. Sample Problem 3: I usually do not make short posts. However, as I see the narrative of this scam and just received the email now, I evaluated that it maybe a potential threat which is why it is best to spread this as soon as possible. Here is the content of the email: The reason why I considered this as a threat because I almost believed and planned to send my ETH to this address. However, I remembered my first time of being scammed which seems similar and thankfully I stopped. Why I almost fallen because ETH 2.0 is a hot topic and people are eager to give it a try for example whether the fees got smaller and here the email tells us to convert our ETH to ETH 2.0 by sending it to an address to enjoy the features and don't worry, the ETH will return automatically as it is a smart contract it said. What to do?
Final Warning! Please verify whether this conversion is true or not. Send your ETH to the suspicious address above at your own risk!CatatanIni merupakan tugas S1 saya di mata kuliah Teori Medan Listrik dimana tugasnya adalah menulis essai mengenai hukum ampere dan medan magnetik. Selain saya kelompok ini terdiri dari Vencisisca Jeane R, Rr. Sanita Eling Cipta Nesa, Putu Arie Pratama, Fajar Purnama, Muhammad Audy Bazly, Muhamad Nordiansyah. Tugas ini tidak pernah dipublikasi dimanapun dan kami sebagai penulis dan pemegang hak cipta melisensi tugas ini customized CC-BY-SA dimana siapa saja boleh membagi, menyalin, mempublikasi ulang, dan menjualnya dengan syarat mencatumkan nama kami sebagai penulis dan memberitahu bahwa versi asli dan terbuka tersedia disini. BAB 1 Pendahuluan1.1 Latar BelakangDalam kehidupan sehari-hari banyak penerapan dari beberapa hukum-hukum fisika yang terjadi, salah satunya yaitu hukum ampere dan medan magnetik. Dimana secara umum hubungan antara hukum ampere dan medan magnetik, yaitu hukum ampere menyatakan bahwa medan magnet dapat ditimbulkan melalui dua cara yaitu lewat arus listrik (perumusan awal hukum ampere), dan dengan mengubah medan listrik (tambahan Maxwell). Koreksi Maxwell terhadap hukum ampere cukup penting, dengan demikian hukum ini menyatakan bahwa perubahan medan listrik dapat menimbulkan medan magnet, dan sebaliknya. Tiga orang ilmuwan jenius dari perancis, Andre Marie Ampere (1775-1863), Jean Baptista Biot (1774-1862) dan Victor Savart (1803-1862) menyatakan bahwa gaya akan dihasilkan oleh arus listrik yang mengalir pada suatu penghantar yang berada diantara medan magnetik. Hal ini juga merupakan kebalikan dari hukum Faraday, dimana Faraday memprediksikan bahwa tegangan induksi akan timbul pada penghantar yang bergerak dan memotong medan magnetik. Hukum ini diaplikasikan pada mesin-mesin listrik, dan gambar dibawah ini akan menjelaskan mengenai fenomena tersebut. 1.2 Rumusan MasalahAdapun permasalahan yang diangkat dalam makalah ini adalah sebagai berikut:
1.3 TujuanTujuan utama dari penulisan makalah “ Hukum Ampere dan Medan Magnetik “ kali ini adalah untuk mengetahui dan memahami secara rinci mengenai hukum ampere dan medan magnetik. Adapun tujuan lain dari pembuatan makalah ini adalah:
1.4 ManfaatManfaat yang diperoleh dari makalah “ Hukum Ampere dan Medan Magnetik “ ini adalah sebagai berikut:
1.5 Ruang Lingkup MateriDalam makalah ini, ruang lingkup yang dibahas oleh penulis hanya sebatas mengenai permasalahan yang di bahas, yaitu sebatas Hukum Ampere dan Medan Magnetik. BAB 2 Pembahasan2.1 Hukum Biot-SavartDifferensial medan magnetik, dH merupakan hasil dari differensial elemen arus I dH. Medan berbanding terbalik dengan jarak (r2), merupakan media bebas yang ada sekitarnya dan memiliki arah yang merupakan cross product dari I dI dan aR .Hubungan ini diketahui sebagai Hukum Biot-Savart, dimana dapat persamaan tersebut, dilihat seperi dibawah ini : dH = (I dI x aR)/(4πR2) (A/m) Arah dari R harus berasal dari elemen arus pada titik dimana dH sudah ditentukan, seperti pada gambar berikut: Elemen arus tidak boleh dipisahkan. Semua elemen melengkapi kontribusi filament arus ke H dan arus harus terlibat. Bentuk integral dalam Hukum Biot-Savart adalah sebagai berikut: H = ∮(I dI x aR)/(4πR2) Integral tertutup sederhana membutuhkan semua elemen arus yang dilibatkan dalam memperoleh nilai H. Contoh Soal 1: Arus I yang panjang dan lurus sepanjang sumbu z dari koordinat silindris adalah seperti tampak pada gambar dibawah ini. Tanpa kehilangan sesuatu sifat umumnya, pilih suatu titik dalam bidang z = 0. Maka dalam bentuk differensialnya, adalah sebagai berikut: dH = (I dz ax X (rar X zaz) / (4π(r2+z2)<</sup>sup>(3⁄2)) = (I dz ra∅) / (4π(r2+z2)(3⁄2)) H = [∫-∞∞(Ir dz) / (4π(r2+z2)(3⁄2))] a∅ = a∅/2πr Medan magnetik oleh arus permukaan dan volume juga diberikan oleh hukum Biot-Savart bentuk integral, dengan I dI digantikan oleh K dS atau Jdv masing-masingnya, dan dimana integral itu diambil diseluruh permukaan, atau volume, yang bersangkutan. Contoh penting adalah arus pada bidang datar tak berhingga dengan kerapatan K yang konstan. Dimana persamaannya dapat dilihat sebagai berikut ini: H = K/2 X an 2.2 Hukum AmpereIntegral garis dari komponen tangensial H sepanjang lintasan tertutup adalah sama dengan besarnya arus yang berada di sekitar lintasan itu: ∮H.dI = Ienc Ini merupakan hukum Ampere. Pada pengamatan pertama, kita barangkali akan mengira bahwa penerapan hukum ini adalah untuk menentukan arus dengan suatu integrasi. Padahal, biasanya arusnya telah dikenal dan hukum ini akan memberi kita cara menentukan H. Jadi penerapan hukum tadi sangat serupa dengan penggunaan hukum Gauss untuk menentukan D dalam distribusi muatan yang diberikan. Untuk dapat memanfaatkan hukum Ampere dalam menentukan H haruslah ada simetri bertaraf cukup tinggi pada masalah yang dibahas. Dua syarat yang harus dipenuhi, adalah sebagai berikut:
Hukum Biot-Savart dapat digunakan untuk menentukan dalam memilih lintasan yang memenuhi syarat-syarat itu. Tapi dalam banyak hal, lintasan tersebut akan segera terlihat. Contoh Soal 2: Gunakan hukum ampere untuk memperoleh H oleh arus I yang panjang dan lurus. Hukum Biot-Savart menunjukkan H adalah tangensial dan dengan besar yang sama sepanjang lingkaran pada gambar 2.3, Maka persamaannya adalah sebagai berikut: ∮H.dI=H(2πr)=I H = aϕ/2πr 2.3 CurlCurl dari medan vektor A merupakan medan vektor yang lain. Titik P pada gambar 2.3 terletak di area ∆S yang dibatasi oleh kuva tertutup C. Dalam integrasi yang mendefinisikan curl, C merupakan area yang dilalui oleh curl sehingga area yang tertutup berada disebelah kiri. Unit normal an , ditentukan dengan menggunakan aturan kaedah tangan kanan, seperti yang ditunjukkan pada gambar. Maka, komponen curl dari A dalam arah an akan didefinisikan sebagai berikut: (curl A).an = lim(∆S→0)(∮A.dI/∆S) Dalam sistem koordinat, curl A merupakan nilai spesifikasi sepenuhnya yang ditentukan oleh komponen di sepanjang tiga unit vektor tersebut. Sebagai contoh, komponen x pada koordinat kartesian didefinisikan dengan cara mengambil garis kontur C persegi pada bidang x = conts, melalui titik P, seperti yang ditunjukkan pada gambar 2.5: (curl A).an = lim(∆y ∆z→0) (∮A.dI / ∆y ∆z) Jika, A = Axax + Ayay + Azaz , pada sudut dari ∆S tertutup ke titik pusat (titik 1), kemudian: ∮ = ∫12 + ∫23 + ∫34 + ∫41 =Ay∆y + (Az+(∂Az/∂y∆y))∆z + (Ay+(∂Ay/∂z∆z))(-∆y) + Az(-∆z) =((∂Az/∂y)-(∂Ay/∂z)) ∆y∆z (curl A).ax = (∂Az/∂y)-(∂Az/∂z) Untuk komponen y dan z dapat ditentukan dengan cara yang sama. Kombinasi ketiga komponen, dapat dilihat pada persamaan di bawah ini: curl A = ((∂Az/∂y)-(∂Az/∂z)) ax+((∂Ax)/∂z-(∂Az)/∂x) ay+((∂Ay)/∂x-(∂Ax)/∂x) az (cartesian) Penentu urutan ketiga dapat ditulis seperti persamaan di bawah ini, ekspansi yang memberikan curl Cartesian dari A. curl A=|(ax&ay&az @ ∂/∂x&∂/∂y&∂/∂z @ Ax&Ay&Az)| Elemen dari baris kedua merupakan komponen dari operasi del. Ini menunjukkan bahwa ∇×A dapat ditulis untuk curl A. Seperti pada ekspresi lain dari suatu analisis vektor, notasi ini digunakan untuk curl A dalam sistem koordinat lain, meskipun∇×A hanya didefinisikan dalam koordinat Cartesian saja. Ekspresi untuk curl A pada koordinat silinder dan bola dapat diturunkan dengan cara yang sama seperti di atas. Meskipun, dengan kesulitan yang lebih tinggi. curl A = (1/r (∂Az)/(∂∅) - (∂A∅/∂z)) ar + ((∂Ar/∂z) - (∂Az/∂r)) a∅ + ((∂(rA∅))/∂r - (∂Ar/∂∅)) az (silinder) curl A = 1/(r sin ) [(∂(A∅ sinθ))/∂θ - (∂θ/∂∅)] ar+1/r [1/sinθ (∂Ar/∂∅)-(∂(rA∅)/∂r)] a∅ + 1/r [(∂(rA∅))/∂r-(∂Ar)/(∂∅)] az (sperikal) Frekuensi penggunaan dari dua sifat operator curl, dapat dijelaskan sebagai berikut:
2.4 Hubungan dari J dan HPada hukum ampere, definisi persamaan dari (curl H)x dapat ditulis sebagai berikut: (curl H).ax=lim(∆y ∆z→0)Ix/(∆y ∆z)- Jx Dimana Jx=dlx⁄dS merupakan daerah kerapatan dari x arah arus. Sehingga komponen x dari curl H dan arah arus J sama pada setiap titik. Demikian pula pada komponen x dan y, dapat dilihat sebagai berikut. ∇×H=J Ini merupakan salah satu persamaan dari Maxwell yang terdapat pada medan listrik statis. jika H diketahui pada seluruh daerah, maka ∇×H=J akan menghasilkan J untuk daerah tersebut. Contoh Soal 3: NoteThis is my undergraduate assignment already in English in the Environmental Studies and Occupational Health & Safety course and I remembered that I was asked to write in Indonesian next time due to that time, English language was not yet supported. Here, I chose a topic about the server room in Global Development Learning Network (GDLN) Udayana University (UNUD) because I have human connections and was able to access there. GDLN back then in 2012 was a place to house the main computer and telecommunication network of the university and a place to perform any kind of distant communication activities such as video conferences and distant learnings. This task has never been published anywhere and I as the author and copyright holders license this task customized CC-BY-SA where anyone can share, copy, republish, and sell it on condition that to state our name as the author and notify that the original and open version available here. AbstractA good server room is a server room that last long or last forever. There are lots of things to consider before planning a server room. Even though we have the things necessary to create a server room but it will only be good for that time. We also must guard the condition of the server room as time pass. Here we will discuss about the physical and virtual safety and security of server room and data center from unwanted threats. Keywords: Server Room, Data Center, Safety, Security, Threat, Physical, Virtual. 1. IntroductionNowadays electronic media are use by almost all people in the world, whether they are for studying, working, entertainment and many other things. These electronic media help civilization to quickly and efficiently finish their jobs. Before, people rely on books, papers, drawing pad and manual equipments to work. These methods took much time and much energy which is very low efficient compare to modern civilization. With human instinct to always want to make life much easier, efficient and flashier, technology will always grow. Thus with new technology being discovered through the continue flow of time there’s always a possibility a new threat or negative impact exist with the new discovered invention. Even though we were able to create a new technology ready to use like server and data center but it doesn’t stop there. After it is useable we must also learn how to take care of this technology. There’s no point a technology created ready to use if it doesn’t last long. So in this assignment we will discuss considerations needed in a server room or data center. In information technology, server is defined as a hardware system to fulfill the request of clients the services could be database server, file server, mail server, print server, web servers, or other. The server here we’ll be discussing is a physical computer. A data center is a facility to house computer systems and associated components, telecommunication and storage. A server room is a place to house servers. Small server can cause large noise and cause great heating to a large room. A large server is even worse. So a standard room and air conditioning isn’t enough to satisfy the demands of storage, for servers. Other than that the safety and security must be considered. Another additional thing that must be considered is the size of the server room. A fixed space is rather risky because it will provide insufficient space to store backups, maintenance and it will be unavailable for upgrading. If there are damage parts replacing it will be difficult due to small or none extra space. So a large extra space is recommended. An ideal server room should have cooling from air conditioner, ventilation for free air cooling, a UPS system to guard the server from instant shutdown, a diesel powered generator in order to keep it alive longer incase the electricity is down, a fire suppressor to minimize the risk of fire, ladders or raised floors in order to access the systems, a false ceiling to place cables and air conditioner, lighting system to provide good vision, environmental monitoring is a strong mean to protect the server room, security construction for the protection, in advance a CCTV and alarms, access control systems is the next thing after security, data cabinet to simplify cable structure, cool aisle containment for extra cooling, cabling is also affecting, and intelligent patching for the server itself. An ideal server room can be said if those things available. In this article we will discuss about what are the cause of server failure, what endanger the servers and how we respond to them. The dangers can either come from inside or outside the room itself, the cause could be by humans or environmental. A server can also be damage due to faulty installation. There are variety of things that could be done and the tools are developing as time pass. So we will discuss them in general. 2. Safety and Security Measures2.1 Threats2.1.1 Environment Factor2.1.1.1 Inner ThreatsInner threats are threats that are potential from inside the room for housing the server itself (from room server). The threats we could mention here are potential of fire, faulty electrical system, faulty room temperature or temperature produced by hardware, short circuit (unstructured cable), faulty room keeping, and faulty room architecture. 2.1.1.2 Outer ThreatsOuther threats are threats that are potential from outside the room for housing the server itself (outside room server). The threats we could mention here are fire from outside, pests, bugs, floods, moisture, outside temperature, thunder, and earthquake. Thunder flood and earthquake can be label as natural disaster. 2.1.2 Human FactorHumans also plays a major role, since servers are sensitive even the smallest faulty can cause great damage. The simple damage that humans can cause is due by clumsiness. We must always be careful in a server room. The other one is insufficient knowledge about the server and the server room itself. 2.1.3 Virtual FactorVirtual factor can either be data overload, faulty in the program, virus and malware, and hacking. Another threat is the loss of data. 2.2 Server Failure (Impacts from Threats)2.2.1 Small Damage – Severe DamageThis kind of damage can be short time malfunction or a repairable damage. A high temperature can cause start failure, hardware malfunction which can be fix if cooled down but a continual damage can cause permanent damage. It’s basic computer knowledge. Another additional damage is hard disks are made of metal. It’s not the heat that is the main problem. It’s because the effect of the heat that’s causing servers or any CPU to stop instantly. Stopping the work of CPU without proper procedure is the cause of severe damage. It can be picture as a human who’s shock. 2.2.2 Fatal DamageFatal damage here means the breakdown of a server. A high voltage can cause a server to explode. It can be expected for most computers. A high voltage can either be from the source itself or from thunders. A faulty electrical system can cause short-circuit or SPOF (single point of failure). Earthquake and faulty room architecture could cause physical damage to servers due to physical impacts. This could permanently damage a server. 2.3 Physical Security and Safety2.3.1 Building2.3.1.1 LocationNullify disasters, like unsusceptible to fire, strong against tremor, earthquake, vibration. set a current/voltage stabilizer to prevent unstable current/voltage from thunder or the source itself. 2.3.1.2 Inner SecurityInside the buildings must be secure. CCTV is recommended for security, fire sensor to detect fire, access by people must be recorded and only those permitted can access, instead of using wall use glass so it’s transparent and easy to watch. 2.3.1.3 Outer SecurityThe surrounding environment around ± 10 meter of the building must be ground zero, empty. Place CCTV outside, anti terrorist motive like a bomb detector. Beyond the empty ground place a park, lots of trees so the building is isolated and easy to monitor. 2.3.2 Fire PotentialTo plan protection from fire potential, we must first examine the cause of the risk of fire. With different type of cause of fire, the chemicals used to put them out are different each type. Fire sensors and alarms are crucial, instant water spray as soon as fire detected is advisable. A fire panel and a fire extinguisher is essential within the room. 2.3.3 TemperatureBoth the server and the room temperature must be considered. Other than keeping the server cooled using hardware fans, we must approximate the heat produce by each server. By knowing the average heat produced we estimate the maximum amount of servers allowed in a room. To control the room temperature, ventilation and air conductors are advised, air conditioner is an excellent choice not only to control the room temperature in normal terms but to control the room temperature when problem occurs where heat suddenly raise up. The room temperature should be 72°F (±2°F) and humidity 45% (±5%). 2.3.4 ElectricitySince servers are sensitive to electricity, a power supply must be chose carefully. Set a current/voltage stabilizer to prevent unstable current/voltage from thunder or the source itself. Calculating the total amount of power needed is the next thing. To prevent damage from black out (electricity go out) a UPS is recommended. Cables should be well isolated so a short circuit does not occur. Between servers and other machines (AC, mechanical) is recommended for the power supply to be separated. The power must not exceed 300 watts per square foot.
2.3.5 Environmental IssueThe danger of environmental issue depends on the construction and the location of the building. Preventing damage from natural disaster is not too recommended on this time. The technology we bare is still insufficient, instead we rely on backups. 2.3.6 Hardware CareHardware care is an important issue to keep the hardware in a good condition for long time. Simply frequently clean the hardware and check often. There are special methods to take care of different hardware. 2.4 Virtual Security and Safety2.4.1 Data SafetyTo guard the data from unwanted user is to place security on the access. The basic security is to place a username and password. It’s best to change the username and password overtime to make sure it’s secure. In modern days fingertip sensors are used to tighten security. Virus and malware protection, firewall, and so-on is recommended to protect the server from virtual threats. To create a tight security, seek a professional. 2.4.2 BackupA regular backup is recommended to minimalize the data loss due to unwanted problems. While a disaster can destroy an entire data, with backup the data is copied and instead of starting from a scratch, we use the backup to retrieve the loss data. 3. Server Room in GDLN (Global Developing Learning Network) at Udayana University, Sudirman, Denpasar, Bali.The location of the building is isolated around 10 meters. It’s a strategic from the disaster of floods. Still, the surrounding the area is crowded. The location of the server room is on the 2nd floor of GDLN building. The access to the server room is secured by a pin lock and normal lock. There are 2 doors to enter the room secured by a lock. Half of the walls are brick walls and half of them are glasses. There are no windows for safety reason. Within the room, the size of the room is about 6.5 x 5 meters wide. 5.5 x 5 meters area is use for the server but there are still spaces left in that area. There is a free space about 1 x 5 meters wide. The height of the room is 3 meters. The ISP (internet server provider) used is Tellabs, Tellabs is the brand of the machine. It provides data rate (people usually say bandwidth) of metro gigabytes (higher than one gigabytes). Some server that are used are Sun Fire x 4106, Aten, HP, some CPU modified into server and maybe others that can’t be mention. There is one terminal to control the entire server. There is also a TV antenna VHF (very high frequency) and UHF (ultra high frequency) for video streaming on the web. There is a wireless router for HOTSPOT UNUD. The switch that is use is G First Base TX. There are also some routers to connect on south GDLN, library, Pasca Sarjana building and others. There is also a rack to organize cables which is called wall mount with the brand fortuna. The cables then go behind the walls or above the ceiling to connect to other places that are necessary. For the safety, security and care of the room will be explain in following. There are 4 AC (air conditioner in the room) 1 big one and 3 normal ones use in normal room, at home for example. With that the room temperature is maintained 170C. There is 1 phone, 1 IP camera with a brand of Sony, 1 fire panel and 1 UPS. The UPS is used in case of a black out (electricity go out). The UPS is SENDON model: SX-530K, 30KVA 3 Phase in 1 Phase out, uninterruptible Power Supply, also a diesel type powered generator if extra power is needed. The lighting of the room is standard (the same as normal room). Within the free space or other place of the buildings there is also other equipment such as ladders to satisfy the needs of the server room. 4. ConclusionThe consideration needed in a server room is to know the risk, things that could be harmful the server, how the server work and its limits. After that then we can plan the security and safety whether it is in physical or virtual manner. A good server is not a server that is good at that time but a server that last long. How we take care of them affects the life span of a server and the storage itself such as how the rooms must be set. An ideal server room should have cooling (AC), ventilation, a UPS, a diesel powered generator, a fire system, ladders or raised floors, a false ceiling, lighting system, environmental monitoring, security construction, in advance a CCTV and alarms, access control systems, data cabinet, cool aisle containment, cabling, and intelligent patching for the server itself. Judging from the criteria above GDLN server room is an ideal room server. The only problem for GDLN server room is the place, it is not isolated, crowded outside, most of the security is still manual but tight (manual locks). It’s not an issue but the security is still moderate not as advance as it is provided with fingertip scanner, face scanner, ID card scanner and so-on. Otherwise everything is complete. 5. Attachment5.1 UCSD (University of California, San Diego) Server Room StandardsThe following standards provide campus units with information on how to design or retrofit small to medium size computer server rooms. Following these standards will help to ensure that critical computational resources are maintained in an environment that protects them both during normal operation, as well as in the event of power failures, fires, floods, and other emergency conditions. For more information, or for questions regarding these standards, contact [email protected]. 5.1.1 General Space Characteristics5.1.1.1 Room specifications
5.1.1.2 Equipment
5.1.1.3 Fire prevention
5.1.2 CoolingAn under floor air distribution system is preferred, although ducted systems are acceptable. In either system, racks should be arranged in a hot isle/cold isle configuration. If under floor, minimum height should be 24" and the raised floor must be designed to accommodate the weight of fully loaded server racks, as well as any lifting and transportation devices used in the movement of racks, computational equipment, and ancillary support systems. 5.1.3 Mechanical Systems5.1.3.1 Air conditioning (AC)
5.1.3.2 Future Planning
5.1.4 Electrical Systems5.1.4.1 Capacity and quality
5.1.4.2 Emergency planning
5.1.5 Alarms and Security5.1.5.1 Alarm systems
5.1.5.2 SecurityAll entrances to the room should be properly secured and alarmed where appropriate. 6. Reference
1. Siapa Saja Timnya?Ada dua hal pertama yang setiap orang harus lakukan tetapi dapat dilakukan ketika mempertimbangkan proyek apa pun (bukan hanya kripto) dan itu adalah (1) Naratif dan (2) Fundamental. Namun, ada lebih banyak hal yang perlu diketahui jika kita ingin berinvestasi dengan aman dalam proyek seperti (3) sistem. Namun, apakah kita memiliki kemampuan untuk memeriksa kode sumber (source code) dan melakukan analisis rantai (chain analysis)? Bahkan jika Anda melakukannya, apakah Anda benar-benar punya waktu? Bagi yang tidak bisa, hanya dapat mengandalkan dua hal lagi yaitu tim dan komunitas. Siapa tim di Torum? 2. Dimana Markas Torum?3. Hampir Tidak Ada Resiko Bergabung Dengan Torum4. Siapa Yang Untuk Diikuti Torum?4.1 TimnyaTidak diragukan lagi, yang pertama harus Anda ikuti adalah tim. Saya menyediakan hyperlink di sini dan yang harus Anda lakukan hanyalah klik kanan padanya dan buka tab baru:
4.2 Para Duta BesarMenurut saya, yang kedua diikuti adalah duta besar yang merupakan pendarat di luar tim internal. Kelompok pertama dipilih oleh @jayson dan tim karena mereka secara positif aktif di Torum, pemberi pengaruh kripto populer, pembuat konten, dan hal lain yang memberikan kualitas, itulah mengapa saya yakin untuk mengumumkan bahwa mereka layak untuk diikuti. Berikut adalah daftar terbaru dari 1 Desember 2020:
4.3 Orang lain yang Anda gemari
1. Who are the teams?There are two first things that everyone must but can do when considering any projects (not just cryptos) and that is (1) Narrative and (2) Fundamental. However, there are more things to know if we want to be safe investing in the project such as (3) the system. However, do you have the abilities to inspect the source codes and perform chain analysis? Even if you do, do you really have the time? For those who cannot, there are only two more things to rely on and those are the team and the community. Who are the teams in Torum? 2. Where is Torum's headquater?3. Almost no risk in joining Torum4. Who to Follow Torum?4.1 The TeamNo doubt the first ones you have to follow are the teams. I provided the hyperlink here and all you have to do is right click on them and open new tab:
4.2 The AmbassadorsIn my opinion, the second ones to follow are the ambassadors who are landers outside of the internal team. The first batch were chosen by @jayson and the team because they are positively active in Torum, popular crypto influencers, content creators, and anything else that bring quality which is why I am confident to announce that they are worthy to follow. Here are the latest list from December 1st, 2020:
4.3 Anyone else you enjoy
Mirrors
NoteThis is my undergraduate assignment that I translated to English myself in the Data Communication course where my assignment is to write an essay on flow control, error detection and control, and selective repeat ARQ. Apart from me, this group consists of I Gede Ariana, I Made Dwi Angga Pratama, I Putu Arie Pratama, Yulianti Murprayana, I Wayan Alit Wigunawan, I Gusti Made Widiarsana, I Nym Apriana Arta Putra, Muhammad Audy Bazly, I Kadek Agus Riki Gunawan, Muhamad Nordiansyah. This task has never been published anywhere and we as the authors and the copyright holders license this task customized CC-BY-SA where anyone can share, copy, republish, and sell on condition to state our name as the author and notify that the original and open version available here. Chapter 1 Introduction1.1 BackgroundThe development of computer technology is increasing very rapidly, this can be seen in the 80s that computer networks are still a puzzle that academics want to answer, and in 1988 computer networks began to be used in universities, companies, and now. Entering this millennium era, especially WWW (World Wide Web) has become the daily reality of millions of people on this earth. In addition, network hardware and software have really changed, at the beginning of its development almost all networks were built from coaxial cables, now many of them are built from fiber optics (fiber optics) or wireless communication. With the development of computer and communication technology, a single computer model that serves all the computing tasks of an organization has now been replaced by a collection of computers that are separate but interconnected in carrying out their duties, such a system is called a computer network. Two computers are said to be interconnected if they can exchange information. The form of connection does not have to go through copper wire alone but can use optical fiber, microwave, or communication satellites. In a data communication, data packets are sent through a certain medium, there will always be a difference between the sent signal and the received signal. This difference can result in an error or error in the data packet sent so that the data cannot be read properly by the recipient. To avoid errors or errors in the data packets sent, error detection (error checking), error correction (error correction) and error control (error control) are required. In error control, there are three methods that are often used in data communication, namely stop and wait requests, go back N requests, and selective requests. Error control includes the following mechanisms: ACK/NAK, timeout, and sequence number. In error control, the following processes occur: error detection, acknowledgment, transmission after timeout, and negative acknowledgment. 1.2 ProblemThe formulations of the problems raised in this paper are as follows:
1.3 ObjectiveThe objectives of this paper are as follows:
1.4 Scope of MaterialIn this assignment, the scope discussed by the author is only limited to the issues discussed, which is limited to Selective Repeat ARQ in a data communication network, and also to provide an overview to readers about the process of controlling errors in data communication. Chapter 2 DiscussionThe purpose of error control is to deliver error-free frames in the correct order to the network layer. The technique commonly used for error control is based on two functions, namely:
Meanwhile, error control is divided into three, namely:
2.1 Flow ControlFlow control is a technique for ensuring that a sending station does not flood the receiving station with data. The receiving station will typically provide a data buffer of a specified length. When the data is received, it has to do some work before it can clear the buffer and prepare for the next data reception. A simple form of flow control known as stop and wait, it works as follows. The recipient indicates that it is ready to receive data by sending a poll or responding with select. The sender then sends the data. This flow control is regulated/managed by Data Link Control (DLC) or commonly referred to as Line Protocol so that the sending and receiving of thousands of messages can occur in the shortest possible time. The DLC must move data in efficient traffic. Communication lines should be used as level as possible, so that no station is idle while the other stations are saturated with excess traffic. So flow control is a very critical part of a network. The following shows the Time Flow Control Diagram when communication occurs in conditions without errors and errors. Flow control mechanisms that are commonly used are Stop and Wait and Sliding window. 2.2 Error Detection and CorrectionAs a result, the physical processes that cause errors in some media (for example, radio) tend to occur in bursts rather than one by one. Error bursting like that has both the advantage and the disadvantage of isolated single-bit errors. On the plus side, computer data is always sent in blocks of bits. Assume the block size is 1000 bits, and the error rate is 0.001 per bit. If the errors are independent, then most blocks will contain errors. If an error occurs with 100 bursts, then only one or two blocks in 100 blocks will be affected, on average. The disadvantage of burst errors is that they are more difficult to detect and correct than isolated errors. 2.2.1 Error Correction CodesThe network designers have devised two basic strategies regarding errors. The first way is to involve sufficient redundant information together with each block of data transmitted to allow the recipient to draw conclusions about what the transmitted character should be. Another way is to involve only enough redundancy to draw the conclusion that an error has occurred, and to allow it to request retransmission. The first strategy uses error-correcting codes, while the second strategy uses error-detecting codes. In order to understand error handling, it is necessary to take a close look at what errors are. Typically, a frame consists of m bits of data (i.e. messages) and redundant r, or check bits. Take the total length of n (that is, n = m + r). An n-bit unit of data and a checkbit is often associated with an n-bit codeword. Two codewords are specified: 10001001 and 10110001. Here we can determine how many different corresponding bits are. In this case, there are 3 different bits. To determine it, simply perform an EXCLUSIVE OR operation on both codewords, and count the number of bits 1 in the result of the operation. The number of bit positions where the two codewords differ is called the Hamming distance (Hamming, 1950). The thing to note is that if two codewords are separated by a Hamming distance d, it will take a single bit error d to convert from one to the other. In most data transmission applications, all 2m of data messages are legal data. However, due to the way the check bit is calculated, not all 2n are used. If an algorithm is specified for calculating the check bits, it is possible to generate a complete list of legal codewords. From this list two codewords with the minimum Hamming distance can be found. This distance is the Hamming distance for the complete code. The error detection and error correction properties of a code depend on the Hamming distance. To detect d errors, you need a code with a distance of d + 1 because with such a code it is impossible that a single bit error d can change one valid codeword to another valid codeword. When the receiver sees an invalid codeword, it can say that an error has occurred during transmission. Likewise, to fix error d, you need a code that is 2d + 1 apart because it says legal codewords can be separated even with a change to d, the original codeword will be closer than other codewords, so error fixes can be determined uniquely. As a simple example of error detection code, take a code where a single parity bit is appended to the data. The parity bit is chosen so that the number of 1 bits in the codeword is even (or odd). For example, if 10110101 is sent in even parity by adding a bit at the end, the data becomes 101101011, whereas with even parity 10110001 becomes 101100010. A code with a single parity bit has a distance of 2, because any single bit error results in a codeword with parity. wrong. This method can be used to detect a single error. As a simple example of error correction code, take a code that has only four valid codewords: 0000000000, 0000011111, 1111100000 and 1111111111. This code has a space of 5, which means that it can fix multiple errors. When the codeword 0000011111 arrives, the recipient will know that the original data should be 0000011111. However, if the triple error changes 0000000000 to 0000000111, the error cannot be corrected. Imagine that we are going to design code with m message bits and r check bits that will allow all single errors to be fixed. Each of the 2m legal messages requires an n + 1 bit pattern. Since the total number of bit patterns is 2 n, we must have (n + 1) 2 m ≤ 2 n. Using n = m + r, this requirement becomes (m + r + 1) ≤ 2 r. When m is specified, it places a lower limit on the number of check bits required to correct a single error. In fact, this theoretical lower limit can be obtained using the Hamming method. The codeword bits are numbered consecutively, starting with bit 1 on the far left. Bits that are powers of 2 (1,2,4,8,16 and so on) are check bits. The rest (3,5,6,7,9 and so on) are inserted with m bits of data. Each check bit forces the parity of a portion of the bit set, including itself, to be even (or odd). A bit can be included in multiple parity computations. To find the check bit to which the data bit at position k contributes, rewrite k as a sum to the powers of 2.For example, 11 = 1 + 2 + 8 and 29 = 1 + 4 + 8 + 16. A bit is checked by the check bit that occurs in its expansion. (for example, bit 11 is checked by bits 1, 2 and 8). When a codeword arrives, the receiver initializes the counter to zero. Then the codeword checks each check bit, k (k = 1, 2, 4, 8, ...) to see if it has the correct parity. If not, the codeword will add k to the counter. If the counter equals zero after all the check bits have been tested (that is, if all the check bits are true), the codeword will be accepted as valid. If the counter is not zero, the message contains an invalid number of bits. For example, if the check bits 1, 2, and 8 have an error, then the inversion bit is 11, because that's the only one checked by bits 1, 2, and 8. Figure 2.2 illustrates some of the 7-bit encoded ASCII characters as an 11 bit codeword using Hamming code. Note that the data is in the bit positions 3, 5, 6, 7, 9, 10, 11. Hamming code can only fix a single error. However, there is a trick that can be used to allow Hamming's code to fix popping errors. A consecutive k number of codewords are arranged as a matrix, one codeword per row. Typically, data will be transmitted one codeword line at a time, from left to right. To correct popped errors, data must be transmitted one column at a time, starting with the leftmost column. When all the k bits have been sent, the second column begins sending, and so on. When the frame arrives at the receiver, the matrix is reconstructed, one column per unit time. If an error burst occurs, at most 1 bit per k codeword will be affected. However Hamming code can fix one error per codeword, so the entire block can be fixed. This method uses the kr check bit to make km of data bits immune to single error bursts of length k or less. 2.3 Selective Repeat ARQSelective Repeat or also known as Selective-reject. With selective reject automatic repeat request (ARQ), the frames that will be sent are only those frames that receive a negative reply, in this case called SREJ or frames whose time has expired. The method of selective-reject ARQ in correcting errors is by at first the transmitter part sends a continuous stream of data from a continuous frame, on the receiving side the data is stored and a cyclic redundancy check (CRC) process is carried out. But this CRC process takes place on a continuous stream of data from the received frame. When an error frame is encountered, the receiver sends the information to the sender via the "return channel". The sending part then takes the frame from the data storage area and puts it in the transmission queue. There are several important points for frame readers, the first is that there must be a way to identify the frame. The second is that there has to be a good way to know if the frame is getting a positive reply or a negative one. Both of these important points can be solved by using sequential numbers sent and received. The sequential number that is sent is at the sender, as well as the sequential number that is received at the receiver. Received sequential numbers that are sent back on the sender's side are the sequential numbers sent by the sender side and get a positive reply from the receiving side. When the receiving side detects an error in the frame, the receiving side will send a sequential number that is sent along with the frame that is considered an error. Of course, in the end the receiving side has to rearrange the frames in the same order as the sending side before the data is passed on to the end user. Selective-reject ARQ is more efficient against Go-Back-N as it reduces the number of retransmissions. However, selective-reject requires a larger data storage capacity for both the sender and receiver side to store the frame until the damaged frame is retransmitted. In addition, on the receiver side there must also be software that can send frames out of sequence so that data can arrive in sequence to the end user. Selective repeat can be implemented in two ways:
From these two events, the frame event is received out of sequence, S holds this frame on the receiving channel until the next incoming frame sequence is received. In the case of implicit retransmission, assuming all ACK frames are received correctly, this can be explained as follows (Figure 2.3):
Whereas implicit retransmission assuming that all I-frames are received correctly except ACK frame N can be explained as follows.
The above operation relies on receiving the ACK frame associated with subsequent frames to initiate retransmission of the previous damaged frame. Another attempt is to use explicit negative acknowledgment frames to request retransmit the error frames. The negative acknowledgment is known as selective reject. On explicit requests, the assumption that acknowledgments are lost on the way can be explained as follows (see Figure 2.5 below).
Chapter 3 Closing3.1 ConclusionThe conclusions that can be drawn from this paper are as follows:
3.2 SuggestionAfter the authors have made this assignment, the suggestions the authors can suggest are, in sending data packets over a certain medium, there will always be a difference between the sent signal and the received signal. This difference can result in an error or error in the data packet sent so that the data cannot be read properly by the recipient. To avoid errors or errors in the data packet sent, error detection, error correction, and error control are required. So the reader should be able to recognize and know the errors that often occur in a data communication. Bibliography
Mirror
CatatanIni merupakan tugas S1 saya di mata kuliah Komunikasi Data dimana tugasnya adalah menulis essai mengenai flow control, error detection & control, dan selective repeat ARQ. Selain saya kelompok ini terdiri dari I Gede Ariana, I Made Dwi Angga Pratama, I Putu Arie Pratama, Yulianti Murprayana, I Wayan Alit Wigunawan, I Gusti Made Widiarsana , I Nym Apriana Arta Putra, Muhammad Audy Bazly, I Kadek Agus Riki Gunawan, Muhamad Nordiansyah. Tugas ini tidak pernah dipublikasi dimanapun dan kami sebagai penulis dan pemegang hak cipta melisensi tugas ini customized CC-BY-SA dimana siapa saja boleh membagi, menyalin, mempublikasi ulang, dan menjualnya dengan syarat mencatumkan nama kami sebagai penulis dan memberitahu bahwa versi asli dan terbuka tersedia disini. BAB 1 Pendahuluan1.1 Latar BelakangPerkembangan teknologi komputer meningkat dengan sangat cepat, hal ini terlihat pada era tahun 80-an jaringan komputer masih merupakan teka-teki yang ingin dijawab oleh kalangan akademisi, dan pada tahun 1988 jaringan komputer mulai digunakan di universitas-universitas, perusahaan-perusahaan, dan sekarang memasuki era milenium ini terutama WWW (World Wide Web) telah menjadi realitas sehari-hari jutaan manusia di muka bumi ini. Selain itu, perangkat keras dan perangkat lunak jaringan telah benar-benar berubah, di awal perkembangannya hampir seluruh jaringan dibangun dari kabel koaxial, kini banyak telah diantaranya dibangun dari serat optik (fiber optics) atau komunikasi tanpa kabel. Dengan berkembangnya teknologi komputer dan komunikasi suatu model komputer tunggal yang melayani seluruh tugas-tugas komputasi suatu organisasi kini telah diganti dengan sekumpulan komputer yang terpisah-pisah akan tetapi saling berhubungan dalam melaksanakan tugasnya, sistem seperti ini disebut jaringan komputer (computer network). Dua buah komputer dikatakan terinterkoneksi bila keduanya dapat saling bertukar informasi. Betuk koneksinya tidak harus melalui kawat tembaga saja melainkan dapat menggunakan serat optik, gelombang mikro, atau satelit komunikasi. Dalam suatu komunikasi data, pastinya dilakukan pengiriman paket data melalui suatu medium tertentu, akan selalu terjadi adanya perbedaan antara sinyal yang dikirim dengan sinyal yang diterima. Perbedaan tersebut dapat mengakibatkan adanya error atau kesalahan pada paket data yang dikirim tersebut sehingga data tersebut tidak dapat dibaca dengan baik oleh penerima. Untuk menghindari terjadinya error atau kesalahan pada paket data yang dikirim maka diperlukan adanya deteksi kesalahan (checking error), perbaikan kesalahan (correction error), dan pengendalian kesalahan (control error). Pada error control terdapat tiga metode yang sering dipakai dalam komunikasi data, yaitu stop and wait request, go back N request, dan selective request. Error control meliputi mekanisme-mekanisme sebagai berikut : ACK/NAK, timeout, dan sequence number. Pada error control terjadi prosesproses sebagai berikut: error detection, acknowledgment, etransmission after timeout, dan negative acknowledgment. 1.2 Rumusan MasalahAdapun rumusan masalah yang diangkat dalam makalah ini, adalah sebagai berikut :
1.3 TujuanAdapun tujuan dari pembuatan makalah ini adalah, sebagai berikut:
1.4 Ruang Lingkup MateriDalam makalah ini, ruang lingkup yang dibahas oleh penulis hanya sebatas mengenai permasalahan yang di bahas, yaitu sebatas Selective Repeat ARQ dalam suatu jaringan komunikasi data, dan juga untuk memberikan gambaran kepada para pembaca mengenai proses pengendalian kesalahan dalam komunikasi data. BAB 2 PembahasanTujuan dilakukan pengontrolan terhadap error adalah untuk menyampaikan frame-frame tanpa error dalam urutan yang tepat ke lapisan jaringan. Teknik yang umum digunakan untuk error control berbasis pada dua fungsi, yaitu:
Sedangkan, error control dibagi menjadi tiga yaitu:
2.1 Kontrol Aliran (Flow Control)Flow control adalah suatu teknik untuk menjamin bahwa sebuah stasiun pengirim tidak membanjiri stasiun penerima dengan data. Stasiun penerima secara khas akan menyediakan suatu buffer data dengan panjang tertentu. Ketika data diterima, dia harus mengerjakan beberapa poses sebelum dia dapat membersihkan buffer dan mempersiapkan penerimaan data berikutnya. Bentuk sederhana dari kontrol aliran dikenal sebagai stop and wait, dia bekerja sebagai berikut. Penerima mengindikasikan bahwa dia siap untuk menerima data dengan mengirim sebual poll atau menjawab dengan select. Pengirim kemudian mengirimkan data. Flow control ini diatur/dikelola oleh Data Link Control (DLC) atau biasa disebut sebagai Line Protocol sehingga pengiriman maupun penerimaan ribuan message dapat terjadi dalam kurun waktu sesingkat mungkin. DLC harus memindahkan data dalam lalu lintas yang efisien. Jalur komunikasi harus digunakan sedatar mungkin, sehingga tidak ada stasiun yang berada dalam kadaan idle sementara stasiun yang lain saturasi dengan lalu lintas yang berkelebihan. Jadi flow control merupakan bagian yang sangat kritis dari suatu jaringan. Berikut ini ditampilkan Time Diagram Flow Control saat komunikasi terjadi pada kondisi tanpa error dan ada error. Mekanisme Flow control yang sudah umum digunakan adalah Stop and Wait dan Sliding window. 2.2 Deteksi dan Koreksi ErrorSebagai akibat proses-proses fisika yang menyebabkannya terjadinya error pada beberapa media (misalnya, radio) cenderung timbul secara meletup (burst) bukannya satu demi satu. Error yang meletup seperti itu memiliki baik keuntungan maupun kerugian pada error bit tunggal yang terisolasi. Sisi keuntungannya, data komputer selalu dikirim dalam bentuk blok-blok bit. Anggap ukuran blok sama dengan 1000 bit, dan laju error adalah 0,001 per bit. Bila error-errornya independen, maka sebagian besar blok akan mengandung error. Bila error terjadi dengan letupan 100, maka hanya satu atau dua blok dalam 100 blok yang akan terpengaruh, secara rata-ratanya. Kerugian error letupan adalah bahwa error seperti itu lebih sulit untuk dideteksi dan dikoreksi dibanding dengan error yang terisolasi. 2.2.1 Kode – Kode Pengkoreksian ErrorPara perancang jaringan telah membuat dua strategi dasar yang berkenaan dengan error. Cara pertama adalah dengan melibatkan informasi redundan secukupnya bersama-sama dengan setiap blok data yang dikirimkan untuk memungkinkan penerima menarik kesimpulan tentang apa karakter yang ditransmisikan yang seharusnya ada. Cara lainnya adalah dengan hanya melibatkan redundansi secukupnya untuk menarik kesimpulan bahwa suatu error telah terjadi, dan membiarkannya untuk meminta pengiriman ulang. Strategi pertama menggunakan kode-kode pengkoreksian error (error-correcting codes), sedangkan strategi kedua menggunakan kode-kode pendeteksian error (error-detecting codes). Untuk bisa mengerti tentang penanganan error, maka perlu melihat dari dekat tentang apa yang disebut error itu. Biasanya, sebuah frame terdiri dari m bit data (yaitu pesan) dan r redundan, atau check bits. Ambil panjang total sebesar n (yaitu, n = m + r). Sebuah satuan n-bit yang berisi data dan checkbit sering kali dikaitkan sebagai codeword n-bit. Ditentukan dua buah codeword: 10001001 dan 10110001. Disini kita dapat menentukan berapa banyak bit yang berkaitan berbeda. Dalam hal ini, terdapat 3 bit yang berlainan. Untuk menentukannya cukup melakukan operasi EXCLUSIVE OR pada kedua codeword, dan menghitung jumlah bit 1 pada hasil operasi. Jumlah posisi bit dimana dua codeword berbeda disebut jarak Hamming (Hamming, 1950). Hal yang perlu diperhatikan adalah bahwa bila dua codeword terpisah dengan jarak Hamming d, maka akan diperlukan error bit tunggal d untuk mengkonversi dari yang satu menjadi yang lainnya. Pada sebagian besar aplikasi transmisi data, seluruh 2m pesan data merupakan data yang legal. Tetapi sehubungan dengan cara penghitungan check bit, tidak semua 2n digunakan. Bila ditentukan algoritma untuk menghitung check bit, maka akan dimungkinkan untuk membuat daftar lengkap codeword yang legal. Dari daftar ini dapat dicari dua codeword yang jarak Hamming-nya minimum. Jarak ini merupakan jarak Hamming bagi kode yang lengkap. Sifat-sifat pendeteksian error dan perbaikan error suatu kode tergantung pada jarak Hamming-nya. Untuk mendeteksi d error, anda membutuhkan kode dengan jarak d + 1 karena dengan kode seperti itu tidak mungkin bahwa error bit tunggal d dapat mengubah sebuah codeword yang valid menjadi codeword valid lainnya. Ketika penerima melihat codeword yang tidak valid, maka penerima dapat berkata bahwa telah terjadi error pada transmisi. Demikian juga, untuk memperbaiki error d, anda memerlukan kode yang berjarak 2d + 1 karena hal itu menyatakan codeword legal dapat terpisah bahkan dengan perubahan d, codeword orisinil akan lebih dekat dibanding codeword lainnya, maka perbaikan error dapat ditentukan secara unik. Sebagai sebuah contoh sederhana bagi kode pendeteksian error, ambil sebuah kode dimana parity bit tunggal ditambahkan ke data. Parity bit dipilih supaya jumlah bit-bit 1 dalam codeword menjadi genap (atau ganjil). Misalnya, bila 10110101 dikirimkan dalam parity genap dengan menambahkan sebuah bit pada bagian ujungnya, maka data itu menjadi 101101011, sedangkan dengan parity genap 10110001 menjadi 101100010. Sebuah kode dengan parity bit tunggal mempunyai jarak 2, karena sembarang error bit tunggal menghasilkan sebuah codeword dengan parity yang salah. Cara ini dapat digunakan untuk mendeteksi erro-error tunggal. Sebagai contoh sederhana dari kode perbaikan error, ambil sebuah kode yang hanya memiliki empat buah codeword valid: 0000000000, 0000011111, 1111100000 dan 1111111111. Kode ini mempunyai jarak 5, yang berarti bahwa code tersebut dapat memperbaiki error ganda. Bila codeword 0000011111 tiba, maka penerima akan tahun bahwa data orisinil seharusnya adalah 0000011111. Akan tetapi bila error tripel mengubah 0000000000 menjadi 0000000111, maka error tidak akan dapat diperbaiki. Bayangkan bahwa kita akan merancang kode dengan m bit pesan dan r bit check yang akan memungkinkan semua error tunggal bisa diperbaiki. Masing-masing dari 2m pesan yang legal membutuhkan pola bit n + 1. Karena jumlah total pola bit adalah 2n, kita harus memiliki (n + 1)2m ≤ 2n. Dengan memakai n = m + r, persyaratan ini menjadi (m + r + 1) ≤ 2r. Bila m ditentukan, maka ini akan meletakkan batas bawah pada jumlah bit check yang diperlukan untuk mengkoreksi error tunggal. Dalam kenyataannya, batas bawah teoritis ini dapat diperoleh dengan menggunakan metoda Hamming. Bit-bit codeword dinomori secara berurutan, diawali dengan bit 1 pada sisi paling kiri. Bit bit yang merupakan pangkat 2 (1,2,4,8,16 dan seterusnya) adalah bit check. Sisanya (3,5,6,7,9 dan seterusnya) disisipi dengan m bit data. Setiap bit check memaksa parity sebagian kumpulan bit, termasuk dirinya sendiri, menjadi genap (atau ganjil). Sebuah bit dapat dimasukkan dalam beberapa komputasi parity. Untuk mengetahui bit check dimana bit data pada posisi k berkontribusi, tulis ulang k sebagai jumlahan pangkat 2. Misalnya, 11 = 1 + 2 + 8 dan 29 = 1 + 4 + 8 + 16. Sebuah bit dicek oleh bit check yang terjadi pada ekspansinya (misalnya, bit 11 dicek oleh bit 1, 2 dan 8). Ketika sebuah codeword tiba, penerima menginisialisasi counter ke nol. Kemudian codeword memeriksa setiap bit check, k (k= 1, 2, 4, 8,....) untuk melihat apakah bit check tersebut mempunyai parity yang benar. Bila tidak, codeword akan menambahkan k ke counter. Bila counter sama dengan nol setelah semua bit check diuji (yaitu, bila semua bit checknya benar), codeword akan diterima sebagai valid. Bila counter tidak sama dengan nol, maka pesan mengandung sejumlah bit yang tidak benar. Misalnya bila bit check 1,2, dan 8 mengalami kesalahan (error), maka bit inversinya adalah 11, karena itu hanya satu-satunya yang diperiksa oleh bit 1,2, dan 8. Gambar 2.2 menggambarkan beberapa karakter ASCII 7-bit yang diencode sebagai codeword 11 bit dengan menggunakan kode Hamming. Perlu diingat bahwa data terdapat pada posisi bit 3, 5, 6, 7, 9, 10, 11. Kode Hamming hanya bisa memperbaiki error tunggal. Akan tetapi, ada trick yang dapat digunakan untuk memungkinkan kode Hamming dapat memperbaiki error yang meletup. Sejumlah k buah codeword yang berurutan disusun sebagai sebuah matriks, satu codeword per baris. Biasanya, data akan ditransmisikan satu baris codeword sekali, dari kiri ke kanan. Untuk mengkoreksi error yang meletup, data harus ditransmisikan satu kolom sekali, diawali dengan kolom yang paling kiri. Ketika seluruh k bit telah dikirimkan, kolom kedua mulai dikirimkan, dan seterusnya. Pada saat frame tiba pada penerima, matriks direkonstruksi, satu kolom per satuan waktu. Bila suatu error yang meletup terjadi, paling banyak 1 bit pada setiap k codeword akan terpengaruh. Akan tetapi kode Hamming dapat memperbaiki satu error per codeword, sehingga seluruh blok dapat diperbaiki. Metode ini memakai kr bit check untuk membuat km bit data dapat immune terhadap error tunggal yang meletup dengan panjang k atau kurang. 2.3 Selective Repeat ARQSelective Repeat atau dikenal juga sebagai Selective-reject. Dengan selective reject ARQ, frame-frame yang akan dikirimkan hanya frame-frame yang mendapat balasan negatif, dalam hal ini disebut SREJ atau frame-frame yang waktunya sudah habis. Cara selective-reject ARQ dalam memperbaiki kesalahan adalah dengan cara, mulanya bagian transmitter mengirim stream data kontinyu dari frame yang kontinyu, pada sisi penerima data tersebut disimpan dan dilakukan proses CRC. Tapi proses CRC ini berlangsung pada stream data yang kontinyu dari frame yang diterima. Ketika dijumpai frame yang error, penerima mengirim informasi pada sisi pengirim melalui “return channel”. Bagian pengirim kemudian mengambil frame tersebut dari tempat penyimpanan data, lalu dimasukkan dalam antrian transmisi. Ada beberapa poin penting bagi pembaca frame, yang pertama adalah harus ada cara untuk mengidentifikasi frame. Yang kedua adalah harus ada cara yang baik untuk mengetahui frame mendapat balasan positif atau balasan negatif. Kedua poin penting tersebut bisa di atasi dengan menggunakan angka sekuensial yang dikirim dan yang diterima. Angka sekuensial yang dikirim ada pada pengirim, begitu juga dengan angka sekuensial yang diterima berada pada penerima. Angka sekuensial yang diterima yang dikirim kembali pada sisi pengirim adalah angka sekuensial yang dikirim oleh sisi pengirim dan mendapat balasan positif dari sisi penerima. Ketika sisi penerima mendeteksi kesalahan pada frame, maka sisi penerima akan mengirimkan angka sekuensial yang dikirim bersama frame yang dianggap error. Tentu daja pada akhirnya sisi penerima harus menata kembali frame sesuai urutannya seperti pada sisi pengirim sebelum data diteruskan ke pengguna akhir. Selective-reject ARQ lebih efisien terhadap Go-Back-N karena mengurangi jumlah retransmisi. Namun selective-reject memerlukan kapasitas penyimpanan data yang lebih besar baik disisi pengirim dan penerima untuk menyimpan frame sampai frame yang rusak diretransmisi. Selain itu disisi penerima juga harus terdapat software yang dapat mengirim frame diluar urutan agar data bisa sampai secara urut pada pengguna akhir. Selective repeat dapat diimplementasikan dengan dua cara:
Dari kedua event tersebut kejadian frame diterima di luar sequence, S menahan frame ini di saluran penerima sampai sequence frame masuk berikutnya diterima. Pada kasus implicit retransmission dengan asumsi semua frame ACK diterima dengan benar dapat dijelaskan sebagai berikut (Gambar 2.3):
Sedangkan pada implicit retransmission dengan asumsi bahwa semua I-frame diterima dengan benar kecuali ACK frame N dapat dijelaskan sebagai berikut.
Operasi di atas bergantung pada penerimaan frame ACK yang berhubungan dengan frame-frame berikutnya untuk menginisiasi retransmisi frame sebelumnya yang rusak. Usaha yang lain adalah dengan menggunakan explicit negative acknowledgment frame untuk meminta trasnmisi ulang frame yang error. Negative acknowledgment tersebut dikenal dengan selective reject. Pada explicit request dengan asumsi bahwa acknowledgments hilang dalam perjalanan dapat dijelaskan sebagai berikut (lihat Gambar. 2.5 di bawah ini).
BAB 3 Penutup3.1 KesimpulanAdapun kesimpulan yang dapat ditarik dari makalah ini, adalah sebagai berikut:
3.2 Usul dan SaranSetelah penulis membuat makalah ini, maka usul dan saran yang dapat penulis sarankan. Dalam pengiriman paket data melalui suatu medium tertentu, akan selalu terjadi adanya perbedaan antara sinyal yang dikirim dengan sinyal yang diterima. Perbedaan tersebut dapat mengakibatkan adanya error atau kesalahan pada paket data yang dikirim tersebut sehingga data tersebut tidak dapat dibaca dengan baik oleh penerima. Untuk menghindari terjadinya error atau kesalahan pada paket data yang dikirim maka diperlukan adanya deteksi kesalahan (checking error), perbaikan kesalahan (correction error), dan pengendalian kesalahan (control error). Maka hendaknya pembaca mampu mengenali serta mengetahui kesalahan-kesalahan yang sering terjadi dalam suatu komunikasi data. Daftar Pustaka
I was wreck $600 eventhough I have bought after the big dump because it turned out that the dumped had not finished. Slowly, it was still dumping a half price more and for once in my life, I sold at the bottom. Second to worst than buying high selling low which was buying middle selling low. This is my story: Another lesson of this article is to always take responsibility when recommending an investment. Even if you are simply telling your story and not pushing other's to invest, it wise to make a disclaimer "this is not financial advice" as you would not want to be responsible for the loss of your listener. Mirrors
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