ระบบคอมพิวเตอร์ (Computer System)

ระบบ (System) คือกลุ่มขององค์ประกอบที่มีความสัมพันธ์กันและทำงานร่วมกัน ซึ่งระบบคอมพิวเตอร์จะมีองค์ประกอบที่สำคัญ 3 ส่วน คือ
ฮาร์ดแวร์ (Hardware)
ซอฟต์แวร์ (Software)
บุคลากร (Peopleware)

ฮาร์ดแวร์ (Hardware) หมายถึง อุปกรณ์ต่าง ๆ ที่เป็นตัวเครื่องคอมพิวเตอร์ แบ่งออกเป็นส่วนประกอบดังนี้
หน่วยรับข้อมูล หน่วยประมวลผล หน่วยแสดงผล

1. หน่วยรับข้อมูล (Input unit) เป็นอุปกรณ์รับเข้า ทำหน้าที่รับโปรแกรมและข้อมูลเข้าสู่เครื่องคอมพิวเตอร์ อุปกรณ์รับเข้าที่ใช้กันเป็นส่วนใหญ่ คือ แป้นพิมพ์ ( Keyboard ) และเมาส์ ( Mouse) นอกจากนี้ยังมีอุปกรณ์รับเข้าอื่น ๆ อีก ได้แก่ สแกนเนอร์ ( Scanner), วีดีโอคาเมรา (Video Camera), ไมโครโฟน (Microphone),ทัชสกรีน (Touch screen), แทร็คบอล (Trackball), ดิจิตเซอร์ เทเบิ้ล แอนด์ ครอสแชร์ (Digiter tablet and crosshair)

2. หน่วยประมวลผลกลาง (Central Processing Unit) หรือเรียกโดยทั่ว ๆ ไปว่า CPU ซึ่งถือว่าเป็นสมองของระบบคอมพิวเตอร์ มีส่วนประกอบที่สำคัญ 2 ส่วน คือ หน่วยควบคุม หน่วยคำนวณ

หน่วยควบคุม (Control Unit หรือ CU) ทำหน้าที่ควบคุมลำดับขั้นตอนการทำงานของหน่วยรับข้อมูล หน่วยแสดงผล หน่วยคำนวณและหน่วยตรรก หน่วยความจำและแปลคำสั่ง
หน่วยคำนวณและตรรก (Arithmetic and Logic Unit หรือ ALU) ทำหน้าที่ในการคำนวณหาตัวเลข เช่น การบวก ลบ การเปรียบเทียบ
หน่วยความจำ เป็นอุปกรณ์ใช้เก็บโปรแกรมและข้อมูลที่ใช้ในการประมวลผล

3. หน่วยความจำภายใน (Primary Storage Section หรือ Memory) เป็นหน่วยความจำที่อยู่ภายในเครื่องคอมพิวเตอร์ที่สามารถติดต่อกับหน่วยงานอื่น ๆ ได้โดยตรง แบ่งออกเป็น 2 ประเภท

หน่วยความจำภายใน
- หน่วยความจำแบบแรม (Random Access Memory หรือ Ram) เป็นหน่วยความจำชั่วคราว ที่ใช้สำหรับเก็บโปรแกรมที่กำลังใช้งานอยู่ขณะนั้น มีความจุของหน่วยเก็บข้อมูลไม่เกิน 640 KB คือผู้ใช้สามารถเขียนหรือลบไปได้ตลอดเวลา ถ้าหากปิดเครื่องคอมพิวเตอร์หรือไฟฟ้าดับ จะมีผลทำให้ข้อมูลต่าง ๆ ที่เก็บไว้สูญหายไปหมด และไม่สามารถเรียกกลับคืนมาได้

- หน่วยความจำแบบรอม (Read Only Memory หรือ Rom) เป็นหน่วยความจำถาวร ที่สามารถอ่านได้อย่างเดียว ไม่สามารถบันทึกข้อมูลได้ ถึงแม้ว่าจะปิดเครื่องหรือไฟฟ้าดับ ข้อมูลที่เก็บไว้จะยังคงอยู่

2. หน่วยความจำสำรอง ได้แก่ เทปแม่เหล็ก จานแม่เหล็ก แผ่นดิสก์ (Diskett) CD-ROM
แผ่นดิสก์หรือสเกต เป็นจานแม่เหล็กขนาดเล็ก ชนิดอ่อน จัดเก็บข้อมูลโดยใช้อำนาจแม่เหล็ก การใช้งานจะต้องมี Disk Drive เพื่อใช้เป็นอุปกรณ์ในการขับเคลื่อนแผ่นดิสก์ โดยแบ่งตำแหน่งพื้นผิวออกเป็น แทร็คและเซ็คเตอร์ แบ่งออกเป็น 3 ขนาด คือ

แผ่นดิสก์ขนาด 8 นิ้ว ปัจจุบันไม่นิยมใช้
แผ่นดิสก์ขนาด 5.25 นิ้ว แบ่งออกเป็น DD สามรถบันทึกข้อมูลได้ประมาณ 360 KB และ HD สามารถบันทึกข้อมูลได้ 1.2 MB
แผ่นดิสก์ขนาด 3.5 นิ้ว แบ่งออกเป็น DD สามารถบันทึกข้อมูลได้ประมาณ 720 KB และ HD สามารถบันทึกข้อมูลได้ 1.44 MB นิยมใช้กันมากในปัจจุบัน

ขนาด 1.44 MB

ขนาด 5.25 นิ้ว

หน่วยความจำต่ำสุด คือ บิต (BIT [Binary Digit]) โดยใช้บิตแทน 1 ตัวอักขระ หรือ 1 ไบต์ (Bite) หน่วยที่ใหญ่ขึ้นมาอีกหน่วย คือ กิโลไบต์ (Kilobyte) โดยที่ 1 กิโลไบต์ มีค่าเท่ากับ 2 10 ไบต์ หรือ 1,024 ไบต์ หน่วยความจำที่ใหญ่ขึ้นไปอีก เรียกว่า เมกะไบต์ กิกะไบต์ และเทระไบต์

ฮาร์ดดิสก์ ( Hard Disk ) เป็นจานแม่เหล็กชนิดแข็ง ชนิดติดแน่นไม่มีการเคลื่อนที่ สามารถบรรจุข้อมูลได้จำนวนมาก เป็น 2 ขนาด คือ
1. ขนาด 5.25 นิ้ว (ปัจจุบันเลิกใช้แล้ว)
2. ขนาด 3.5 นิ้ว
ทั้ง 2 ขนาดจะมีความจุ ตั้งแต่ 10,20,40,80,120,300,400 MB1 GB,2 GB ฯลฯ ปัจจุบันนิยมใช้ตั้งแต่ 10 GB ขึ้นไป

Hard disk

Data Rate หมายถึง ความเร็วในการอ่านข้อมูลจากดิสก์ไปสู่สมองของเครื่องคอมพิวเตอร์ (หรือมีความเร็วในการนำข้อมูลมาจากสมองเครื่องไปบันทึกลงบนดิสก์) มีหน่วยวัดเป็น จำนวนไบต์ต่อวินาที ( Bytes Per Second หรือ bps )

ซีดีรอม (CD-Rom ) เป็นจานแสงชนิดหนึ่ง ใช้เก็บข้อมูลที่มีความเร็วในการใช้งานสูง มี

คุณสมบัติดังนี้

• เป็นสือที่สามารถเก็บข้อมูลได้เป็นจำนวนมาก โดยจะมีความจุสูงถึง 2 GB (2 พันล้านไบต์)


• มีขนาดเล็ก สามารถเคลื่อนย้ายได้สะดวก


• ใช้เทคโนโลยีของแสงเลเซอร์ในการอ่านเขียนข้อมูล


• เป็นจานแสงชนิดอ่านได้อย่างเดียว ( Read Only Memory ) ไม่สามารถเขียนหรือลบข้อมูลได้

CD - ROM

หน่วยแสดงผล (Output Unit) ทำหน้าที่แสดงผลลัพธ์ที่ได้จากการประมวลผลของเครื่องคอมพิวเตอร์ หรือใช้เก็บผลลัพธ์เพื่อนำไปใช้ภายหลัง ได้แก่ จอภาพ (Monitor) เป็นอุปกรณ์ส่งออกมากที่สุด เครื่องพิมพ์ (Printer)
ซอฟแวร์ (Software) หมายถึง โปรแกรมชุดคำสั่งที่เขียนให้เครื่องคอมพิวเตอร์ปฏิบัติตาม ซึ่งมี 2ประเภท คือ

1. ซอฟแวร์ควบคุมระบบ (System Software) คือ ชุดคำสั่งหรือโปรแกรมที่ควบคุมการทำงานของคอมพิวเตอร์ เป็นสื่อกลางระหว่างโปรแกรมประยุกต์กับเครื่องคอมพิวเตอร์ เพื่อช่วยในการจัดการทรัพยากรของคอมพิวเตอร์ ได้แก่ โปรแกรมควบคุมเครื่อง ระบบปฏิบัติการ เช่น DOS, Windows, Os/2, Unix
2. ซอฟแวร์ประยุกต์ (Application Software) คือ ชุดคำสั่งหรือโปรแกรมที่เขียนขึ้นมาเพื่อให้เครื่องคอมพิวเตอร์ทำงานตามที่ผู้ใช้ต้องการ ได้แก่ โปรแกรมสำเร็จรูปต่าง ๆ
   

บุคลากร (Peopleware) หมายถึง บุคลากรทางคอมพิวเตอร์ที่ทำหน้าที่ในการใช้และดูแลเครื่องคอมพิวเตอร์ เช่น นักเขียนโปรแกรม (Programmer) นักวิเคราะห์ระบบ (System Analyst) เป็นต้น

Very Easy Home Network

Very Easy Home Network
By: Manbeer Singh

Ahhh yes, life’s pretty sweet with that high speed DSL line. But there is a glitch. The normal ISP dial up account can be used from any computer in the world. A DSL line connects to only one computer.

The obvious solution would be to install a network to carry the line to every computer in the house. I've steered clear of a having network in my home office. I once spent several days trying to hook up 2 computers using Windows 95, and finally gave up after only intermittently being able to make the computers communicate. I swore that my days on the floor with my computers were over and I ran my own "network" by transferring Zip disks from one computer to another.

A new technology is on the horizon, the Home PNA (Phone Networking Alliance) network. I remembered reading about this network that would connect through existing home phone lines. It was in its infancy - so I checked around.

HPNA has made some amazing strides. Now in version 2.0, it will run your home network at ethernet speed, 10Mbps (ten megabytes per second - the same speed as a real corporate network) and extend your high-speed internet connection to anywhere in your home or office. After checking out all the reviews, the same name kept coming up: Netgear.

Netgear offers a home PNA card that not only is plug and play, but also allows you to talk on the phone while using the network. Netgear's "do-not-disturb" feature causes it not to completely take over the phone line, nor will talking on the phone degrade the speed of the network connection.

The card was simple enough to install, just open the computer case and snap it into an available slot. Of course, this is usually the time that things go sour - when you start up the computer. Amazingly, "plug-and-play" was never easier. At first glance the instructions supplied on a single page fold-out sheet look like they’d never do the trick, but they do. Netgear includes a CD with all the software that you need. No need to search for the required drivers, their set-up program takes you step by step. Within a few minutes, you’ll be able to print a page on a remote computer’s printer or share files.

My teenage daughter installed the network on her own computer, and we can now share printers, files, a CD-burner and internet connections without using our "sneakernet" of the past.

My favorite part of the setup is the new Home Network USB adapter. I clipped the small unit to the back of my laptop, and literally within seconds, connected to my other computers!

If this sounds too easy, it probably is. I’m no techie and it worked for me on first try. One caveat though, when it comes to sharing a high speed internet connection over a Home Network line, there are a few more steps. You need to set up a TCP/IP connection. My advice? Don’t even attempt to do it unless you know what you’re doing. Call Netgear’s 24/7 tech support at 888-NETGEAR. They’ll patiently walk you through it, step by step.

I feel like I've really accomplished something, it's not "black-magic" any more. I have a flawless network that I never realized could be as useful as it is. I have a warp speed internet connection that goes beyond my wildest dreams. I passed the boundaries from novice to geek with the help of modern technology. Now my mornings are complete. I can stay that extra ½ hour in bed, watching Matt Lauer and Katie Couric on the Today show. No need to zoom into the office to check my auctions and email. I can just plug the laptop into the phone jack and fly like the wind.

Article Directory Source: http://www.1articleworld.com

Why Use Ethernet Routers

By: Benjamin Brook

Ethernet routers are a key component of your home computer system if you have many devices that need to be connected to your computer. For example, printers, modems, other computers and game consoles. Having Ethernet routers at home is a good way to connect all these devices seamlessly into one machine and the Internet.

Usually you can connect up to four devices with Ethernet routers. Depending on how many devices you need connected to your computer and the Internet you can find a router, which allows more devices. Any device that you wish to connect Ethernet routers must have an Ethernet network adaptor.

Adding On

If you have looked at Ethernet routers that have less Ethernet connections than you need but think that such models fit into your budget you can simply adjust them. Using a network switch will allow you to expand the number of devices that you can connect.

Having a wireless access port is a handy thing to have; it will take up on of the access ports but at the same time will allow many wireless devices to work. The drawback is that other Wifi computers using the network at the same time will slow down the system significantly.

Advantages

Ethernet routers have an advantage over simple Ethernet cables in that you can connect more devices to your home network system. Ethernet cables restrict you to only two devices or two computers. If you find yourself with an increasing amount of devices that need to be connected to one central computer then Ethernet routers are the way to go.

You may also be tempted to use an ad hoc Wifi system instead of an Ethernet router. While this does allow you greater flexibility in terms of distance from the central system and the number of devices, it does have drawbacks. It is a less secure system and is usually used on a temporary basis. Ethernet routers are more secure.

For an ad hoc system to work properly all devices must have a Wifi network adaptor. You may also have to configure the adaptors to ad hoc mode as usually they are configured for the typical infrastructure mode. This might take up some extra time to set up and is a lot more complex than simple Ethernet routers.

If you have multiple computers that need to be connected it may be worth it to use an Ethernet switch or hub. The only disadvantage being that one computer must be connected to the Internet. The other computers will access the Internet through this central computer.

Article Directory Source: http://www.1articleworld.com

Cisco CBAC – The Poor Mans Firewall

By: Nicholas Evra

CBAC Overview
The Cisco IOS Firewall Feature Set is a module that can be added to the existing IOS to provide firewall functionality without the need for hardware upgrades. There are two components to the Cisco IOS Firewall Feature Set in Intrusion Detection (which is an optional bolt-on) and Context-Based Access Control (CBAC). CBAC maintains a state table for all of the outbound connections on a Cisco router by inspecting tcp and udp connections at layer seven of the OSI model and populating the table accordingly. When return traffic is received on the external interface it is compared against the state table to see if the connection was originally established from within the internal network, and then either permitted or denied. Although basic this is a very effective mechanism to prevent unauthorized access to the internal network from external sources such as the internet.

CBAC Application-specific support

Cisco have also built in some additional functionality into CBAC in terms of application-specific inspection that enables the router to recognize and identify application specific data flows such as HTTP, SMTP, TFTP, and FTP. Understanding these applications and their data flows empowers the router to identify malformed packets or suspect application data flows and permit or deny accordingly. CBAC also provides the flexibility of downloading Java code from trusted sites, but it denying untrusted sites.

CBAC and Denial of Service (DOS) Attacks

Denial-Of-Service (DOS) attack protection is also in-built with real-time logging of alerts as well as pro-active responses to mitigate the threat. To do this CBAC can be configured to manage half-open TCP connections which are used in TCP SYN flood attacks to overload a targets resources resulting in a denial of service to legitimate users. To do this CBAC uses timeouts and thresholds, which are configurable, to determine how long state information for each connection should be kept for sessions and when to drop them. Note that UDP and ICMP require that an idle-timer limit is used to determine when a connection should be terminated. A very useful command to identify a DOS attack is ‘ip inspect audit-trail’ which logs all DOS connections including source and destination IP address and TCP or UDP ports allowing you to pin-point the exact source and destination of the attack.

Configuring CBAC

There are five steps to configuring CBAC on a Cisco router in order for it to function correctly. These are as follows:
1. Choose an interface to which inspection will be applied. This can be an internal or external interface as CBAC is only concerned with the direction of the first packet initiating the connection which is identified when applying CBAC to an interface.

2. Configure an IP access list in the correct direction on the selected interface to allow traffic through for CBAC to inspect.

3. Configure global timeouts and thresholds for established connections or sessions.

4. Define an inspection rule specifying exactly which protocols will be inspected by CBAC.

5. Apply the inspection rule to the interface in the correct direction.

Article Directory Source: http://www.1articleworld.com

Home Computer Networking

By: travis klein

Numbers of people these days have two computers in their homes; one is usually kept in the children's room and another in their personal room. So its important for you to know about home networking and its benefits that you can have from both systems in your home. But for this you should know certain things like how to set up home networking, what all you will need for it etc.

In order to make home network set-up easy and useful, you need to install a network friendly OS (operating system) like Windows ME or XP. But this again is not very important; it all depends upon your requirement. Further you will need many more items to make home networking successful. They are:

Two Network cards - If your computer is quite new, ask the shopkeeper for two PCI network cards of 10/100mbps specification.

RJ-45 crossover cable - This is the cable needed to join the machines together so ensure that the length is long enough.

Basic knowledge of using a Windows PC. We will discuss advantages of Home Networking now. Here are some of the benefits that you will get through home networking:

You can play various games across the network say with your kid in another room. Also, you will be able to share the pieces of hardware. For instance, if you have single printer but two computers, you could use the same printer for both computers.

If you have lots of files in any one computer and want both the systems to have access over it, it will be possible using a home network.

Steps for Home Networking

1. The first step is to take the cover/lid out of both the computers and then fit the Network cards into a free PCI slot. Screw them down into the case and close the lid.

2. After restarting the computer, Windows should reappear mentioning about the finding of a new hardware and will ask for a driver disk. Then you need to insert the CD into the drive and wait for the drivers to get installed. Do the same with second computer and thus computers are enabled to communicate after this step.

3. Now, click on Start, then Settings, Control Panel and Network Connections and finally click at Set up a home network. Then you will be asked several questions and the system will ask you to insert a floppy disk into the computer, repeat the same with second computer.

Your home network base is installed so start doing what you want. If you face major problems take help of professionals and for minor difficulties just click on Help option for assistance.

Article Directory Source: http://www.1articleworld.com

Networking (Computers)

By: Christoff Genviere

There are many kinds of networks, however this paper will be about networking computers. As we move further and further into the paperless society, the need for people to be connected and able to exchange data just as fast as they could by handing a paper to someone increases. This can be accomplished by having a group of computers connected by a network, so that as soon as data is entered into one computer, it can be immediately accessed by someone else on a connected computer, no matter how far away it may be (though usually it is in the same building). There is much work involved in this and it in includes a lot of math, from equations to basic problems. This report will be based around the mathematical aspects of setting up a network.

The first mathematical question in setting up a network is very basic. How many computers will be connected to this network and how many guest computers might come on at one time is the question. An example of a guest computer is if someone brought a laptop and connected it for a short while to download or access data. To find the answer to the question, simply count the desktop computers that will be connected and how many guest computers you expect to be connected at one time.

The second mathematical problem that occurs is best solved using an algebraic equation. Let x=the amount of desktop computers that will always be connected, y=the amount of guest computers that you expect to be connected at one time. So, the equation is: x+y+1. The one added on the end of the equation is another guest file just to make sure you don't fall short. So, this tells you how many files you need to create. The guest files will all be generically named so that all guests have the same access privileges, and all the permanent computers will have their own named file so they can have more personalized access privileges. These files are put on one main computer, the server. This controls all access privileges and any data put into a computer branching off from it in it's network can be accessed from this all-powerful server computer. The previously stated problems are a large part of networking, although I couldn't possibly tell about all the math involved without going on for another 3 or 4 pages. Those problems help with networking as far as setting up the network on the computer goes, but there is a whole nother side. The physical side.

The physical side of computer networking involves problems such as how many feet of cable are you going to need to connect the computers. Some large office buildings can have 1 mile of cable between their networked computers! If someone has 2 computers in their house, it may only involve 3 feet. The mathematical procedure is quite simple although it might take a while to complete. Just take out the old meter stick and start measuring. Don't measure direct lines between the computers unless you want the cable stretching in a straight line between them. Chances are you will want it to run along a wall or around another object. Once the measuring is done, just add up the cable length and you have the answer to the problem.


If you don't have a very tight budget, you can afford faster networks than cable networks. These are more sophisticated but I was lucky enough to get to try it this summer. It is called infrared data transfer (IDT). Instead of cables, you have an infrared connector hooked to your computer. Just aim the little infrared panel at the infrared panel on the other computer and it will trade information with infrared light. These panels are usually about 1 square inch in size.


This is much quicker and doesn't involve annoying cables. You still need to gauge distance because there is a distance limit on how far apart they can be and still work. When you install them, your computer will ask you questions such as how many lumens (measurement of brightness of light) you would like your panel to emit.

It is invisible to the naked eye but the amount of lumens it outputs is critical. If you have a fast computer, you might want more lumens so that your computer doesn't crash because of lagging. If you have a slower computer you will want less lumens because other wise you will be sending data too fast for your computer and there will end up being a lot of gibberish that will mess up the receiving computer.

The mathematical things that networking involves are almost endless depending on the situation. I couldn't adequately explain them if I had the time, because anybody who didn't understand quite a bit about computers wouldn't know what I was talking about, forcing me to explain many things that are off the subject of mathematics in networking. I hope I have given you an idea of what it involves, but if you want to know about all of the mathematics, you will just have to network some computers yourself.

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Why Use Ethernet Routers

By: Benjamin Brook

Ethernet routers are a key component of your home computer system if you have many devices that need to be connected to your computer. For example, printers, modems, other computers and game consoles. Having Ethernet routers at home is a good way to connect all these devices seamlessly into one machine and the Internet.

Usually you can connect up to four devices with Ethernet routers. Depending on how many devices you need connected to your computer and the Internet you can find a router, which allows more devices. Any device that you wish to connect Ethernet routers must have an Ethernet network adaptor.

Adding On
If you have looked at Ethernet routers that have less Ethernet connections than you need but think that such models fit into your budget you can simply adjust them. Using a network switch will allow you to expand the number of devices that you can connect.

Having a wireless access port is a handy thing to have; it will take up on of the access ports but at the same time will allow many wireless devices to work. The drawback is that other Wifi computers using the network at the same time will slow down the system significantly.

Advantages
Ethernet routers have an advantage over simple Ethernet cables in that you can connect more devices to your home network system. Ethernet cables restrict you to only two devices or two computers. If you find yourself with an increasing amount of devices that need to be connected to one central computer then Ethernet routers are the way to go.

You may also be tempted to use an ad hoc Wifi system instead of an Ethernet router. While this does allow you greater flexibility in terms of distance from the central system and the number of devices, it does have drawbacks. It is a less secure system and is usually used on a temporary basis. Ethernet routers are more secure.

For an ad hoc system to work properly all devices must have a Wifi network adaptor. You may also have to configure the adaptors to ad hoc mode as usually they are configured for the typical infrastructure mode. This might take up some extra time to set up and is a lot more complex than simple Ethernet routers.

If you have multiple computers that need to be connected it may be worth it to use an Ethernet switch or hub. The only disadvantage being that one computer must be connected to the Internet. The other computers will access the Internet through this central computer.

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