What is Proxy means

In computer networks, a proxy server is a server, that could be a computer system or an application program that acts as a go-between for requests from clients seeking resources from other servers. In other word, proxy server called man in the midle between real server on internet and the client. Simple work from a proxy server, client connects to server, requesting some service, such as a file, connection, web page, or other resource, available from a different server that allocate on other network. The proxy server evaluates the request according to its filtering rules. For example, it may filter traffic by protocol, time access, or IP address. If the request is validated by the filter, the proxy provides the resource by connecting to the relevant server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server. In this case, it 'caches' responses from the remote server, and returns subsequent requests for the same content directly. With this use, our connection to internet will be more simple because we didn't need to contact the real server to have what we want. We can set our proxy server to update the data to synchronous with the real server.
A proxy server has two main purposes:

• To keep machines behind it anonymous. It's really help us to keep our privacy and network security.
  
• To speed up access to a resource (via caching). It is commonly used to cache web pages from a web server.

A proxy server that passes requests and replies unmodified is usually called a gateway or sometimes tunneling proxy. A proxy server can be placed in the user's local computer or at various points between the user and the destination servers or the Internet. A reverse proxy is a proxy used as a front-end to accelerate and cache in-demand resources (such as a web page).

Collaboration between Facebook with Twitter

We all know Facebook is a most growth social networking in internet. It's connect so many people so easily and make a distance has no meaning. Also with Twitter, this is a mini blog with high growing rate as well as Facebook does. Now we will make a collaboration between this two powerfull site. We will make an update from Facebook will be appear in our Twitter homepage and vice versa.

1. Facebook make an update to Twitter
   First, find your status RSS feed on Facebook. It is buried. To find it, we need to visit our profile and on our mini-feed select "See All". On the right you will see a list of items, select “status stories” and finally below this a feed link can be found. Finally add your newly found status RSS feed to the TwitterFeed service. In this step, we can set our update frequentcy, prefixed it with "From Facebook..." and am publishing just the title from the status feed.
2. Twitter make an update to Facebook
   The official Twitter Facebook app (found here: http://apps.facebook.com/twitter/) has an option after you install it to update your Facebook status with your latest tweet. We need to understand that there is a delay if you use twitter frequently. But overall it's really powerfull to make frequently update for both with one update in a side.
   

Both Facebook and Twitter have pretty extensive APIs that you can develop applications against. With these a lot is certainly possible, but it would take development time and effort to write the code necessary to accomplish whatever else you might want to do. Also keep in mind that "everything" isn't possible. The APIs are limited in certain respects, specially where privacy, technical, or other issues might be involved.

What is set associative mapping

set associative cache - A compromise between a direct mapped cache and a fully associative cache where each address is mapped to a certain set of cache locations. The address space is divided into blocks of 2^m bytes (the cache line size), discarding the bottom m address bits. An "n-way set associative" cache with S sets has n cache locations in each set. Block b is mapped to set "b mod S" and may be stored in any of the n locations in that set with its upper address bits as a tag. To determine whether block b is in the cache, set "b mod S" is searched associatively for the tag.A direct mapped cache could be described as "one-way set associative", i.e. one location in each set whereas a fully associative cache is N-way associative (where N is the total number of blocks in the cache). Performance studies have shown that it is generally more effective to increase the number of entries rather than associativity and that 2- to 16-way set associative caches perform almost as well as fully associative caches at little extra cost over direct mapping.

source : http://encyclopedia2.thefreedictionary.com/set+associative+cache

Technology And The Whistleblower

As the meeting comes to a close, your boss smirks with an self assured arrogance. He then says that you should seriously consider his offer, because he has it on good authority, that there may be a very localized downsizing in your department. Wouldn’t it be nice to smirk back and just say “no thanks”.

This scenario plays out more often then most people realize. With all of the recent business and government scandals there were a lot of people who knew that books were being cooked, laws being broken and the health of the public being put at risk, just to keep profits high and investors content. Many people felt that there was nothing they could do against powerful executives and corrupt bureaucrats. The credibility of a low level employee compared to that of a ‘successful’ corporate executive is a huge hurdle to overcome. Also, corporate lawyers are there to defend the company, even from internal hazards. So, once a person decides to speak up, they must be prepared to have their credibility brutally attacked and the onslaught of legal preceding from an all-star team of corporate lawyers.

Just the thought of legal proceedings can stop a person from making the ethical choice. It’s easy for people to be vilified by their employer, at which point the credibility of the person speaking out is destroyed. How is possible for a single person to fight a company or government with infinite resources? Many years ago, it was practically impossible. However, the “digital revolution” may have changed the balance of power. For a “whistleblower” to be effective they need proof of wrong doing and a method to communicate those issues to the public. The availability of compact electronic devices such as digital audio recorders and digital cameras allow an individual to document many of the issues and bring the power back to the whistleblower.

The first piece of technology that should be in every whistleblowers repertoire is a digital audio recorder. These devices are so small that they can be put in a shirt pocket without being noticed. Once you decide to “blow the whistle” you should start recording all of your conversations. Keep a log of the items discussed, so that you can refer to the appropriate recording when needed, otherwise you can spend hours going back over the content. The best part is that these devices are not expensive. A digital voice recorder costs about $100, or many times MP3 players have a microphone built into the device, so they can be used to record conversations.

Another device that is very handy is a voice modem with telephone recording software. This will allow you to, hook up your phone, and record any conversations that occur over the phone. If you get fired before you have all the evidence, sometimes telephone conversations, with the people in power, can help to document important facts, because people very rarely think they are being recorded and say things that they probably should not have.

Make sure you check your local laws regarding conversation recording. In most places it is legal to record with one-party consent. One-party consent means that if you are involved in the conversation, you do not need to notify the other parties that you are recording the conversation. In Canada, one-party consent is the law. In the United States it is a little more complicated, as some states have their own regulations. You can refer to this recording law chart (http://www.snakeoil.ca/docs/recordinglaw.html) to determine what laws affect you. If in doubt, speak with a lawyer.

The third device every whistleblower should have is a digital camera. Many times the violations are in manufacturing or packaging, which could be a public health concern. These can be very hard to document and prove, since the paper records may be manipulated. A picture says a thousand words though, and will help authorities uncover doctored documents. When taking pictures be very cautious as most companies prohibit camera’s on their premises. Many cell phones are now equipped with digital camera’s, so they are easier to conceal. Take as many pictures as you can and try to prove the date you took them. Some camera have a date stamp feature, even though this is easy to manipulate. The best option is to take a picture of the daily newspaper beside the offending subject. This is very hard for lawyers to argue against and disprove that the violations did occurred on that particular date.

The last piece of equipment needed is a computer. When the authorities begin investigating your complaint, you will need to produce your evidence such as documents pointing to the issues. Most corporations are moving towards a paperless environment, which means that these documents will only be accessible by a computer. The other important part your computer plays, is to backup all of those images and audio recordings. Make sure your computer has a CD writer or DVD writer. Take these backups and put them somewhere secure, like a bank deposit box.

One area of concern is the removal of corporate documents from the office. This can violate many legal agreements you may have signed. If you are worried about this, there is a very simple solution. Backup all of the evidence files at work and stash them somewhere in the office. In the ceiling or in duct work is a great place. Basically, anywhere nobody ever looks. When it is time to produce these documents, simply tell the authorities that you have stashed them in a safe place at the office. The evidence is kept safe and you haven’t violated any agreements, a win-win situation.

In the future, companies may begin banning digital devices from the workplace for the exact reasons being discussed here. However, if were are persistent, it may go the other way and corporate behaviour may change, which is the ultimate goal of being a whistleblower. The days of “plausible deniability” and “disgruntled employees” are disappearing quickly.

Writer : Darcy O'Neil
Source : 
http://www.articlecity.com/articles/computers_and_internet/article_1811.shtml

What is VPS??

A Virtual Private Server (VPS) is a hosting environment that combines the benefits of both shared hosting and dedicated hosting. It does this by creating a virtual server that runs inside a hardware server via a specially designed partition.

Each Virtual Private Server partition runs its own operating system in a secure and private environment and cannot be accessed or interrupted by its neighbours. This system gives you the same level of root access as a dedicated server whilst sharing the cost of the hardware. With a VPS you are virtually running your own server but at a fraction of the cost.
The flexibility of a dedicated server

Users share system resources, such as the CPU and memory, but unlike shared hosting (a virtual host on a shared hardware server ) the file system is fully partitioned. This means that up-time is more consistent; while root access to the server provides the flexibility to add and change modules and to install your own software.
As cost effective as shared hosting

Partitioning a single hardware server into multiple servers is nothing new. In fact, mainframe computers have long been partitioned into multiples. In 1996, Japanese giant Verio started to migrate the technique into hosting as a highly cost-effective storage solution.
How much technical knowledge do I need?

All our VPS options include the Plesk control panel, which makes it easier to administer your websites and services.

However, because a VPS gives you root-level access, allowing you to install whatever software you like, you will need some technical knowledge and understanding of how servers work to run a VPS effectively.
Why call it VPS 2.0?

We've called our new VPS range VPS 2.0 because of our next generation improvements to both performance and hardware. Our servers are 9th generation Dell machines with embedded Intel® Virtualization (VT®) optimisation in the Intel® boards. They are designed by Dell specifically to ramp up VPS performance.

The inclusion of the very latest range of software and control panels for Windows and Linux is accompanied by price reductions across the range so we're increasing performance, features and value at exactly the same time.
Powerful partnerships working for you

We've partnered with SWsoft to offer you a choice between Ubuntu Linux and Microsoft Windows virtual private server systems, making us the first UK host to deliver Windows SQL Server 2005 VPS hosting using the latest Plesk 8 and Virtuozzo Power Panel technology.

All VPS 2.0 systems are hosted on Dell PowerEdge 2950 servers with quad-core Intel® Xeon® processors for maximum multi-tasking performance and 15,000 RPM SCSI RAID 10 drives for data speed and integrity.

As we're a certified Intel Server Centre of Excellence using a Tier 1 Network with 50,000 Mbit Connectivity, you can also trust we set the access, speed and safety benchmarks you need.

source

Routing Versus Bridging

When Connecting to the Wireless, xDSL or Cable Network

Summary:

As wireless, cable and xDSL technologies emerge, a growing consideration is how to connect to the network. Although much can be said about different methods, it really boils down to two: bridging or routing. Now for those experienced in networking, they will know that this question has been around since routing and bridging technologies have been. However the new element here is that the discussions always centered on LAN’s and/or private WAN’s. When connecting to the Internet there are new considerations to keep in mind when making your decision. This technical document is geared for those who need assistance in the decision for choosing what equipment and how to connect to the cable or xDSL network.

Background:

Now before we can argue routing versus bridging (or vice versa for that matter) we need to be clear on what both devices are and how they behave. So let’s define each one:

Bridge:

A bridge is a device that connects two segments of the same network. The two networks being connected can be alike or dissimilar. Unlike routers, bridges are protocol-independent. They simply forward packets without analyzing and re-routing messages.

Router:

A router is a device that connects two distinct networks. Routers are similar to bridges, but provide additional functionality, such as the ability to filter messages and forward them to different places based on various criteria. The Internet uses routers extensively to forward packets from one host to another.

Based on these definitions we can see that key difference between a bridge and router is that a bridge does not look at protocols and a router does. A bridge does not look at traffic for the purpose of allowing or disallowing it, and it does not decide what to do with certain types of traffic; it simply moves data from one network to another. Whereas a router examines protocols and decides what to do with each packet based on defined criteria.

Router vs. Bridge in Cable & DSL Environments

When you order Internet access either via a cable or xDSL connection, they will offer you a bridge (although it may often be called a “modem,” it is a bridge). These are very low cost (usually around $200,) and will connect your machine to the ISP’s network. Now this is a cost-effective solution, however there are some very apparent reasons on why you should consider a router over a bridge. Let’s look at an example to illustrate this point

Security Risk: In this example, we have two different customers bridged to the Internet using the same ISP. You can see that both customers have IP addresses on the same IP network. This means that broadcast from Customer A’s network will propagate to Customer’s B network. For example if machines on both networks are utilizing file and print services through the Windows Network, it is perfectly plausible (and has happened) that machines from Customer A can show up in the Network Neighborhood of Machines on Customer B’s network. Now for security reasons alone this is a very undesirable situation.

Everyone is talking about firewalls, filtering, etc. Many people connecting to the Internet today do not realize how vulnerable they are to attacks from the Internet. When utilizing a dial up connection, the exposure is only for the time you are connected, however, along with the all the benefits of a permanent connection, there is a big drawback: your network is exposed to the Internet 24 hours a day, 7 days a week. When you are at home asleep, it could be quite possible someone out there is working away at your network in the hopes of destroying or stealing data. Although this is not a pleasant thought there are very simple means to prevent this happening.

The first thing is have a routed account. With a routed account you have a device examining all traffic before it enters your network. This means that traffic not destined for your network will not be routed to your LAN, broadcast storms, and other network related issues on other networks connecting to your ISP will not effect your network. Now the biggest gain from a routed account is security. A Router has the ability to filter Internet traffic. This means the router connecting you to the Internet can discriminate which traffic it will allow into your network, and which traffic it will not. A Router also has the ability to “hide” all of the IP addresses on your network and make them appear as if the were only one IP address out on the Internet. By doing this, you have just eliminated 90% of all security threats.

Other Benefits: Aside from security other benefits of using a Router over a bridge include:

· You will have a contiguous block of IP addresses, rather than sporadic address across a network.

· A Router can use DHCP to assign workstations addresses on the LAN, preventing specific configuration in each device.

· A router has the ability to use NAT (Network Address Translation), thus only requiring you to obtain 1 IP address from your ISP. Cost saving as well as security enhancement

For the above reasons it seems quite clear that a routed account has distinct advantages over a bridged connection. For wireless bridge connections an Ethernet broadband router can be placed between the bridge and the LAN, thus providing all the benefits of routing to cable, ADSL and other environments.

source

What is Hub

In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to frequent fliers who travel through airport "hubs" to make connecting flights from one point to another. In data communications, a hub is a place of convergence where data arrives from one or more directions and is forwarded out in one or more other directions. A hub usually includes a switch of some kind. (And a product that is called a "switch" could usually be considered a hub as well.) The distinction seems to be that the hub is the place where data comes together and the switch is what determines how and where data is forwarded from the place where data comes together. Regarded in its switching aspects, a hub can also include a router.

1) In describing network topologies, a hub topology consists of a backbone (main circuit) to which a number of outgoing lines can be attached ("dropped"), each providing one or more connection port for device to attach to. For Internet users not connected to a local area network, this is the general topology used by your access provider. Other common network topologies are the bus network and the ring network. (Either of these could possibly feed into a hub network, using a bridge.)

2) As a network product, a hub may include a group of modem cards for dial-in users, a gateway card for connections to a local area network (for example, an Ethernet or a Token Ring), and a connection to a line (the main line in this example).

What is bridge in network world

In telecommunication networks, a bridge is a product that connects a local area network (LAN) to another local area network that uses the same protocol (for example, Ethernet or Token Ring). You can envision a bridge as being a device that decides whether a message from you to someone else is going to the local area network in your building or to someone on the local area network in the building across the street. A bridge examines each message on a LAN, "passing" those known to be within the same LAN, and forwarding those known to be on the other interconnected LAN (or LANs).

In bridging networks, computer or node addresses have no specific relationship to location. For this reason, messages are sent out to every address on the network and accepted only by the intended destination node. Bridges learn which addresses are on which network and develop a learning table so that subsequent messages can be forwarded to the right network.

Bridging networks are generally always interconnected local area networks since broadcasting every message to all possible destinations would flood a larger network with unnecessary traffic. For this reason, router networks such as the Internet use a scheme that assigns addresses to nodes so that a message or packet can be forwarded only in one general direction rather than forwarded in all directions.

A bridge works at the data-link (physical network) level of a network, copying a data frame from one network to the next network along the communications path.

A bridge is sometimes combined with a router in a product called a brouter.

source

Switch is .....

In a telecommunications network, a switch is a device that channels incoming data from any of multiple input ports to the specific output port that will take the data toward its intended destination. In the traditional circuit-switched telephone network, one or more switches are used to set up a dedicated though temporary connection or circuit for an exchange between two or more parties. On an Ethernet local area network (LAN), a switch determines from the physical device (Media Access Control or MAC) address in each incoming message frame which output port to forward it to and out of. In a wide area packet-switched network such as the Internet, a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination.

In the Open Systems Interconnection (OSI) communications model, a switch performs the layer 2 or Data-Link layer function. That is, it simply looks at each packet or data unit and determines from a physical address (the "MAC address") which device a data unit is intended for and switches it out toward that device. However, in wide area networks such as the Internet, the destination address requires a look-up in a routing table by a device known as a router. Some newer switches also perform routing functions (layer 3 or the Network layer functions in OSI) and are sometimes called IP switches.

On larger networks, the trip from one switch point to another in the network is called a hop. The time a switch takes to figure out where to forward a data unit is called its latency. The price paid for having the flexibility that switches provide in a network is this latency. Switches are found at the backbone and gateway levels of a network where one network connects with another and at the subnetwork level where data is being forwarded close to its destination or origin. The former are often known as core switches and the latter as desktop switches.

In the simplest networks, a switch is not required for messages that are sent and received within the network. For example, a local area network may be organized in a Token Ring or bus arrangement in which each possible destination inspects each message and reads any message with its address.

Circuit-Switching version Packet-Switching

A network's paths can be used exclusively for a certain duration by two or more parties and then switched for use to another set of parties. This type of "switching" is known as circuit-switching and is really a dedicated and continuously connected path for its duration. Today, an ordinary voice phone call generally uses circuit-switching.

Most data today is sent, using digital signals, over networks that use packet-switching. Using packet-switching, all network users can share the same paths at the same time and the particular route a data unit travels can be varied as conditions change. In packet-switching, a message is divided into packets, which are units of a certain number of bytes. The network addresses of the sender and of the destination are added to the packet. Each network point looks at the packet to see where to send it next. Packets in the same message may travel different routes and may not arrive in the same order that they were sent. At the destination, the packets in a message are collected and reassembled into the original message.

source

Router is ......

In packet-switched networks such as the Internet, a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each point-of-presence on the Internet. A router is often included as part of a network switch.

A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. Typically, a packet may travel through a number of network points with routers before arriving at its destination. Routing is a function associated with the Network layer (layer 3) in the standard model of network programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switch that can perform routing functions.

An edge router is a router that interfaces with an asynchronous transfer mode (ATM) network. A brouter is a network bridge combined with a router.

For home and business computer users who have high-speed Internet connections such as cable, satellite, or DSL, a router can act as a hardware firewall. This is true even if the home or business has only one computer. Many engineers believe that the use of a router provides better protection against hacking than a software firewall, because no computer Internet Protocol address are directly exposed to the Internet. This makes port scans (a technique for exploring weaknesses) essentially impossible. In addition, a router does not consume computer resources as a software firewall does. Commercially manufactured routers are easy to install, reasonably priced, and available for hard-wired or wireless networks.

Why It’s Crucial To Have A Dedicated Server

by: Stu Pearson

A dedicated server is a single web server or computer on the internet that hosts websites and shows pages as viewers request. A dedicated server is within a network of computers, exclusively dedicated to one costumer or a large business, since it can meet many needs.

Dedicated servers are most commonly used in the web hosting industry; hundreds of sites are hosted under one dedicated server. A dedicated server is considered to be the next step on from shared hosting environments. Having your own dedicated server makes you free of worry about other websites slowing you down or crashing your server. Dedicated servers also give you total control, and allows for installing software on your website that opens doors for gaining extra performance.

The advantage of having a dedicated server is that the clients of the server can customize both hardware and software setup so they meet needs like faster data access and effortless accommodation of traffic on the site.

These dedicated servers come with good costumer service. The web host works with the client in making sure that the dedicated server meets the needs of the client. In cases of companies having several divisions like a chain of outlets, it is still crucial for each to have their own dedicated server because they can create many domains on a single server easily, which results in more efficiency rather than having to lease host space on different web servers for each division or outlet individually.

For large companies having websites for every dealership such as motorcycle manufacturers, the advantage of having a dedicated server is that the parent company can put all of the websites for each dealership under the same server.

Here’s how it works. Assume a company named “Taurus Cars Corp.” is running a series of dealerships, the parent company could host an individual website for every dealership. Each website may look like this; for the parent company “www.taurusmotorscorp.com”, for the dealership in Florida “www.taurusmotorsflorida.com “, for the dealership in Colorado “www.taurusmotorscolorado.com” and so on.

The advantage of this setup is the dealership in Florida is using the same online tools as the dealership in Colorado. Making the online presence of the company Taurus Motors Corp. very streamlined and cost-efficient, while making things significantly easier for the company’s administration and support, also known as customization and uniform of point of sale software. Considering the websites of each of the different divisions or dealerships residing on one dedicated server, makes this advantage beneficial in terms of increasing sales.

In some cases other business want to use dedicated servers for the sole purpose of customization, costumer service, and fast access. They host websites themselves or better yet sub-lease the extra space for interested companies to set up their websites and domains. The advantage derived by any business under a dedicated server is the ability to enhance security.

All these advantages imply that dedicated servers are the best option for most large companies or businesses. Hosting personal websites or small business websites in general doesn’t require a dedicated server, instead for this purpose you can lease from a standard web host.

source : articlecity

Cisco green plan looks beyond routers

Cisco Systems wants to turn the enterprise data network into an electricity meter.
InfoWorld InfoClipz


Using open standards, the company wants to get server and storage vendors to collect and share information about their equipment and send it to Cisco routers and switches. The data could include power consumption, operating temperature and more. It's becoming a critical job, and because the network touches all IT resources across the enterprise, data collection should happen there, according to Paul Marcoux, vice president of green engineering.

Marcoux joined Cisco from American Power Conversion only about six weeks ago after Cisco created the position to overlook energy issues across all parts of the company. Networking gear itself makes up a much smaller portion of IT power consumption than do servers or storage, but Cisco plans to go beyond just making its own products more efficient.

Power is a growing issue in datacenters as the cost of energy rises and concerns about global climate change increase. Being able to collect and analyze information about power usage is a big part of the battle and becoming more crucial in the age of virtualization, according to Marcoux. Distributing storage and processing cycles without regard for power issues is not just inefficient, it's dangerous, he said.

If virtualization software looks at a process that requires more computing power or storage space, then enlists servers or storage devices that are near to overheating or running out of power, it could send a rack of servers over the edge and shut it down, Marcoux said. For that reason, the virtualization system needs to know the power status of all the resources it may call upon, he said.

By the same token, consolidated datacenters typically serve many departments of an enterprise and consume a lot of power, but those groups generally don't have to pay for their part of the power. In fact, the electricity bill often bypasses even the IT department, going to building management instead, Marcoux said. Collecting data about the power consumed by each device, and eventually by individual transactions, would allow enterprises to bill each department for the power it uses, he said.

Software on routers and switches would collect the information and then take actions or forward it on to separate building management, energy management or virtualization control systems, Marcoux said. Given the large amount of energy data to be processed, Cisco may introduce daughtercards for its platforms to provide extra computing power, he said. He hopes the technology will be in place and collecting information in enterprises within three years.

Because datacenters contain gear from so many vendors, open standards are the only way to make such a system work, according to Cisco. Fortunately, there already are several available standards, Marcoux said. Having standards already in place will help speed up adoption, Marcoux said.

"We're not trying to reinvent the wheel, we're just trying now to utilize the wheel," Marcoux said.

Cisco's proposal would represent a whole new role for networks beyond communications, said Burton Group analyst Dave Passmore. Server vendors might go along with the plan, but Cisco can't count on smooth sailing, he said. Centralized power regulation would play a role in overall management of the datacenter, an area where Cisco is attempting to make inroads with other initiatives as well.

"Who controls virtualization in the data center is going to be the new battleground," Passmore said.


source : http://www.infoworld.com/article/07/12/24/Cisco-green-plan-looks-beyond-routers_1.html

System console

The system console, root console or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the init system and from the system logger.

On traditional minicomputers, the console was a serial console, an RS-232 serial link to a terminal such as a DEC VT100. This terminal was usually kept in a secured room since it could be used for certain privileged functions such as halting the system or selecting which media to boot from. Large midrange systems, e.g. those from Sun Microsystems, Hewlett-Packard and IBM, still use serial consoles. In larger installations, the console ports are attached to multiplexers or network-connected multiport serial servers that let an operator connect his terminal to any of the attached servers.

On PCs, the computer's attached keyboard and monitor have the equivalent function. Since the monitor cable carries video signals, it cannot be extended very far. Often, installations with many servers therefore use keyboard/video multiplexers (KVM switches) and possibly video amplifiers to centralize console access. In recent years, KVM/IP devices have become available that allow a remote computer to view the video output and send keyboard input via any TCP/IP network and therefore the Internet.

Some PC BIOSes, especially in servers, also support serial consoles, giving access to the BIOS through a serial port so that the simpler and cheaper serial console infrastructure can be used. Even where BIOS support is lacking, some operating systems, e.g. FreeBSD and Linux, can be configured for serial console operation either during bootup, or after startup.

It is usually possible to log in from the console. Depending on configuration, the operating system may treat a login session from the console as being more trustworthy than a login session from other sources.

Routers and Managed Switches (as well as other networking and telecoms equipment) may also have console ports in particular Cisco Systems routers and switches that use Cisco IOS are normally configured via their console ports.

From Wikipedia, the free encyclopedia

White screen of death

n Windows 95, Windows 98, Windows Me, Windows 2000, and (possibly) also Windows NT, a White Box of Doom or White screen of death will appear if an error occurs while loading a system-critical application or process, such as the system shell (e.g. Explorer.exe). It has a similar appearance to a General Protection Fault, or GPF.

Clicking "OK" will normally cause the computer to shut down, or it may result in another similar error. This error may also appear if, at startup, an illegal operation occurs during the startup process. Also it will make your computer freeze and you have to shut the computer down.

Some computers, such as Dell computers, have boot errors that result in a WSOD

This error may occasionally be ignored, as it may just have been a single occurrence of a failed startup sequence; however, if it should occur repeatedly, the user may then be required to reinstall Windows.

A way to work around this if you don't want to install Windows is to set the shell to Winfile.exe.

BSOD ( Blue Screen of Death)

Windows NT

In Windows NT, Windows 2000, Windows XP, Windows Server 2003, and Windows Vista, the blue screen of death occurs when the kernel or a driver running in kernel mode encounters an error from which it cannot recover. This is usually caused by an illegal operation being performed. The only safe action the operating system can take in this situation is to restart the computer. As a result, data may be lost, as users are not given an opportunity to save data that has not yet been saved to the hard drive.

Blue screens are known as "Stop errors" in the Windows Resource Kit documentation. They are referred to as "bug checks" in the Windows Software development kit and Driver development kit documentation.

Windows 2000 (can also be configured to display debug info like the Windows NT example)

The text on the error screen contains the code of the error as well as its symbolic name (e.g. 0x0000001E, KMODE_EXCEPTION_NOT_HANDLED) along with four error-dependent values in parentheses that are there to help software engineers with fixing the problem that occurred. Depending on the error code, it may display the address where the problem occurred, along with the driver which is loaded at that address. Under Windows NT and 2000, the second and third sections of the screen may contain information on all loaded drivers and a stack dump, respectively. The driver information is in three columns; the first lists the base address of the driver, the second lists the driver's creation date (as a Unix timestamp), and the third lists the name of the driver.

By default, Windows will create a memory dump file because a blue screen error occurs. Depending on the OS version, there may be several formats this can be saved in, ranging from a 64 KB "mini dump" to a "complete dump" which is effectively a copy of the entire contents of physical RAM. The resulting memory dump file may be debugged later, using a kernel debugger. A debugger is necessary to obtain a stack trace, and may be required to ascertain the true cause of the problem; as the information onscreen is limited and thus possibly misleading, it may hide the true source of the error.

Windows NT 3.5

Microsoft Windows can also be configured to send live debugging information to a kernel debugger running on a separate computer. (Windows XP also allows for kernel debugging from the machine that is running the OS.) If a blue screen error is encountered while a live kernel debugger is attached to the system, Windows will halt execution and cause the debugger to "break in", rather than displaying the BSOD. The debugger can then be used to examine the contents of memory and determine the source of the problem.

The Windows debugger is available as a free download from Microsoft.

Windows includes a feature that can be used to cause a blue screen manually. To enable it, the user must add a value to the Windows registry. After that, a BSOD will appear when the user presses the SCROLL LOCK key twice while holding the right CTRL key.^[3] This feature is primarily useful for obtaining a memory dump of the computer while it is in a given state. As such, it is generally used to aid in troubleshooting system hangs.

By default, Windows XP is configured to save only a 64K minidump when it encounters a blue screen, and then to automatically reboot the computer. Because this process happens very quickly, the blue screen may be seen only for an instant or not at all. Users have sometimes noted this as a random reboot rather than a traditional stop error, and are only aware of an issue after Windows reboots and displays a notification that it has recovered from a serious error.

A BSOD can also be caused by a critical boot loader error, where the operating system is unable to access the boot partition due to incorrect storage drivers or similar problems. The error code in this situation is STOP 0x0000007B (INACCESSIBLE_BOOT_DEVICE). In such cases, there is no memory dump saved. Since the system is unable to boot from the hard drive in this situation, correction of the problem often requires booting from the Microsoft Windows CD. After booting to the CD, it may be possible to correct the problem by performing a repair install or by using the Recovery Console (with CHKDSK).

The color blue was chosen because there was a version of Windows NT for the DEC Alpha platform and there the console colors could not be changed in an easy way. For consistency reasons blue became the color for Stop errors on all platforms (alpha/i386/mips/ppc).

ReactOS

ReactOS

ReactOS, an attempt at creating a free software/open source implementation of a Windows NT-compatible operating system, also features its own BSOD similar to the Windows NT/XP one.

Windows 9x

Windows 9x/Me

The blue screen of death also occurs in Microsoft's home desktop operating systems Windows 95, 98, and Me. Here it is less serious, but more common. In these operating systems, the BSOD is the main way for virtual device drivers to report errors to the user. It is internally referred to by the name of "_VWIN32_FaultPopup". A Windows 9x/Me BSOD gives the user the option either to restart or continue. However, VxDs do not display BSODs frivolously — they usually indicate a problem which cannot be fixed without restarting the computer, and hence after a BSOD is displayed the system is usually unstable or unresponsive.

Two of the most common reasons for BSODs are:

• Problems that occur with incompatible versions of DLLs. This cause is sometimes referred to as DLL hell. Windows loads these DLLs into memory when they are needed by application programs; if versions are changed, the next time an application loads the DLL it may be different from what the application expects. These incompatibilities increase over time as more new software is installed, and is one of the main reasons why a freshly-installed copy of Windows is more stable than an "old" one.

• Faulty or poorly written device drivers, hardware incompatibilities, or damaged hardware may also cause a BSOD. If you have just installed a new piece of hardware, updated a driver, or installed an Operating System update shortly before you see the BSOD, be sure to investigate these causes also.

In Windows 95 and 98, a BSOD occurred when the system attempted to access the file "c:\con\con" on the hard drive. This was often inserted on websites to crash users' machines. Microsoft has released a patch for this.

The BSOD can appear if a user ejects a removable medium while it is being read on 9x/ME. This is particularly common while using Microsoft Office: if a user simply wants to view a document, he might eject a floppy disk before exiting the program. Since Microsoft Office always creates a temporary file in the same directory, it will trigger a BSOD upon exiting because it will attempt to delete the file on the disk that is no longer in the drive.

This type of blue screen is no longer seen in Windows NT, 2000, and XP. In the case of these less serious software errors, the program may still crash, but it will not take down the entire operating system with it due to better memory management and decreased legacy support. In these systems, the "true" BSOD is seen only in cases where the entire operating system crashes.

Windows CE 5.0

Windows CE

The simplest version of the blue screen occurs in Windows CE except the versions for Pocket PC. The blue screen in Windows CE 3.0 is similar to the one in Windows 95 and 98.

Windows for Workgroups 3.11

Windows for Workgroups

Windows for Workgroups' Blue Screen of Death is very similair to the Windows 9x BSoD.

Xbox Error Message

Xbox

Although the Microsoft Xbox usually shows a Green Screen of Death when a critical error occurs, this model was seen showing a BSOD during the presentation of Forza Motorsport at the CeBIT computer fair in Hannover in March 2005.

Boot Loader Function

Boot loader

Most computer systems can only execute code found in the memory (ROM or RAM). Modern operating systems are stored on hard disks, or occasionally on LiveCDs, USB flash drives, or other non-volatile storage devices. When a computer is first powered on, it doesn't have an operating system in memory. The computer's hardware alone cannot perform complex actions such as loading a program from disk, so an apparent paradox exists: to load the operating system into memory, one appears to need to have an operating system already loaded.

The solution is to use a special small program, called a bootstrap loader, bootstrap or boot loader. This program's only job is to load other software for the operating system to start. Often, multiple-stage boot loaders are used, in which several small programs of increasing complexity summon each other, until the last of them loads the operating system. The name bootstrap loader comes from the image of one pulling oneself up by one's bootstraps (see bootstrapping). It derives from the very earliest days of computers and is possibly one of the oldest pieces of computer terminology in common use.

Early programmable computers had a row of toggle switches on the front panel to allow the operator to manually enter the binary boot instructions into memory before transferring control to the CPU. The boot loader would then read the operating system in from an outside storage medium such as paper tape, punched card, or a disk drive.

Pseudo-assembly code for the bootloader might be as simple as the following eight instructions:

0: set the P register to 8
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: if end of tape, jump to 8
5: store accumulator to address in P register
6: increment the P register
7: jump to 1

A related example is based on a loader for a 1970's Nicolet Instrument Corporation minicomputer. Note that the bytes of the second-stage loader are read from paper tape in reverse order.

0: set the P register to 106
1: check paper tape reader ready
2: if not ready, jump to 1
3: read a byte from paper tape reader to accumulator
4: store accumulator to address in P register
5: decrement the P register
6: jump to 1

The length of the second stage loader is such that the final byte overwrites location 6. After the instruction in location 5 executes, location 6 starts the second stage loader executing. The second stage loader then waits for the much longer tape containing the operating system to be placed in the tape reader. The difference between the boot loader and second stage loader is the addition of checking code to trap paper tape read errors, a frequent occurrence with the hardware of the time, which in this case was an ASR-33 teletype.

In modern computers the bootstrapping process begins with the CPU executing software contained in ROM (for example, the BIOS of an IBM PC) at a predefined address (the CPU is designed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section (most commonly the boot sector) of the most promising device. It is usually possible to configure the BIOS so that only a certain device can be booted from and/or to give priority to some devices over others (a CD or DVD drive is usually given priority over a hard disk, for instance).

Boot loaders may face peculiar constraints, especially in size; for instance, on the IBM PC and compatibles, the first stage of boot loaders located on hard drives must fit into the first 446 bytes of the Master Boot Record, in order to leave room for the 64-byte partition table and the 2-byte 0xAA55 'signature', which the BIOS requires for a proper boot loader.

Some operating systems, most notably pre-1995 Macintosh systems from Apple, are so closely interwoven with their hardware that it is impossible to natively boot an operating system other than the standard one. A common solution in such situations is to design a bootloader that works as a program belonging to the standard OS that hijacks the system and loads the alternative OS. This technique was used by Apple for its A/UX Unix implementation and copied by various freeware operating systems and BeOS Personal Edition 5.

What is msconfig

MSConfig, or System Configuration Utility, is a boot configuration utility bundled with all Microsoft Windows operating systems released after 1995 except Windows 2000. Windows 2000 users can download the utility separately, however. This tool modifies which programs run at startup, edits certain configuration files, and simplifies controls over Windows services. Part of the base Windows install, it can be accessed by running 'msconfig' on any system on which the user has administrator access.

Files that can be edited through MSConfig include AUTOEXEC.BAT, CONFIG.SYS, WIN.INI, SYSTEM.INI on Windows 9x systems, and BOOT.INI on Windows NT systems. The chief benefit to using MSCONFIG to edit these files is that it provides a simplified GUI to indirectly manipulate the sections of those files and the Windows registry tree pertaining to the Windows boot sequence.

The most recent version of MSConfig, released with Windows Vista, features a greatly simplified user interface and increased support for managing services.

Causes and cures of defragments action needed

Causes and cures

Fragmentation occurs when the operating system cannot or will not allocate enough contiguous space to store a complete file as a unit, but instead puts parts of it in gaps between other files (usually those gaps exist because they formerly held a file that the operating system has subsequently deleted or because the operating system allocated excess space for the file in the first place). As advances in technology bring larger disk drives, the performance loss due to fragmentation squares with each doubling of the size of the drive.^{[citation needed]} Larger files and greater numbers of files also contribute to fragmentation and consequent performance loss. Defragmentation attempts to alleviate these problems.

Consider the following scenario, as shown by the image on the right:

An otherwise blank disk has 5 files, A, B, C, D and E each using 10 blocks of space (for this section, a block is an allocation unit of that system, it could be 1K, 100K or 1 megabyte and is not any specific size). On a blank disk, all of these files will be allocated one after the other. (Example (1) on the image.) If file B is deleted, there are two options, leave the space for B empty and use it again later, or compress all the files after B so that the empty space follows it. This could be time consuming if there were hundreds or thousands of files which needed to be moved, so in general the empty space is simply left there, marked in a table as available for later use, then used again as needed.^[1] (Example (2) on the image.) Now, if a new file, F, is allocated 7 blocks of space, it can be placed into the first 7 blocks of the space formerly holding the file B and the 3 blocks following it will remain available. (Example (3) on the image.) If another new file, G is added, and needs only three blocks, it could then occupy the space after F and before C. (Example (4) on the image). Now, if subsequently F needs to be expanded, since the space immediately following it is no longer available, there are two options: (1) add a new block somewhere else and indicate that F has a second extent, or (2) move the file F to someplace else where it can be created as one contiguous file of the new, larger size. The latter operation may not be possible as the file may be larger than any one contiguous space available, or the file conceivably could be so large the operation would take an undesirably long period of time, thus the usual practice is simply to create an extent somewhere else and chain the new extent onto the old one. (Example (5) on the image.) Repeat this practice hundreds or thousands of times and eventually the file system has many free segments in many places and many files may be spread over many extents. If, as a result of free space fragmentation, a newly created file (or a file which has been extended) has to be placed in a large number of extents, access time for that file (or for all files) may become excessively long.

The process of creating new files, and of deleting and expanding existing files, may sometimes be colloquially referred to as churn, and can occur at both the level of the general root file system, but in subdirectories as well. Fragmentation not only occurs at the level of individual files, but also when different files in a directory (and maybe its subdirectories), that are often read in a sequence, start to "drift apart" as a result of "churn".

A defragmentation program must move files around within the free space available to undo fragmentation. This is a memory intensive operation and cannot be performed on a file system with no free space. The reorganization involved in defragmentation does not change logical location of the files (defined as their location within the directory structure).

Another common strategy to optimize defragmentation and to reduce the impact of fragmentation is to partition the hard disk(s) in a way that separates partitions of the file system that experience many more reads than writes from the more volatile zones where files are created and deleted frequently. In Microsoft Windows, the contents of directories such as "\Program Files" or "\Windows" are modified far less frequently than they are read. The directories that contain the users' profiles are modified constantly (especially with the Temp directory and Internet Explorer cache creating thousands of files that are deleted in a few days). If files from user profiles are held on a dedicated partition (as is commonly done on UNIX systems), the defragmenter runs better since it does not need to deal with all the static files from other directories. For partitions with relatively little write activity, defragmentation performance greatly improves after the first defragmentation, since the defragmenter will need to defrag only a small number of new files in the future.

Myths

Myths

Defragging the disk will not stop a system from malfunctioning or crashing because the filesystem is designed to work with fragmented files. [2] Since defrag cannot be run on a filesystem marked as dirty without first running chkdsk [3], a user who intends to run defrag "to fix a system acting strangely" often ends up running chkdsk, which repairs file system errors, the end result of which may mislead the user into thinking that defrag fixed the problem when it was actually fixed by chkdsk.

In fact, in a modern multi-user operating system, an ordinary user cannot defragment the system disks since superuser access is required to move system files. Additionally, modern file systems such as NTFS are designed to decrease the likelihood of fragmentation. [4] Improvements in modern hard drives such as RAM cache, faster platter rotation speed, and greater data density reduce the negative impact of fragmentation on system performance to some degree, though increases in commonly used data quantities offset those benefits. However, modern systems profit enormously from the huge disk capacities currently available, since partially filled disks fragment much less than full disks. [5] In any case, these limitations of defragmentation have led to design decisions in modern operating systems like Windows Vista to automatically defragment in a background process but not to attempt to defragment a volume 100% because doing so would only produce negligible performance gains. [6]

Kinds of File System in a Computer

Filesystems

• FAT DOS 6.x and Windows 9x-systems come with a defragmentation utility called Defrag. The DOS version is a limited version of Norton SpeedDisk.
• NTFS Windows 2000 and newer include an online defragmentation tool based on Diskeeper. NT 4 and below do not have built-in defragmentation utilities.
• ext2 uses an offline defragmenter called e2defrag, which does not work with its successor ext3, unless the ext3 filesystem is temporarily down-graded to ext2.
• JFS has a defragfs utility on IBM operating systems.
• HFS Plus in 1998 introduced a number of optimizations to the allocation algorithms in an attempt to defragment files while they're being accessed without a separate defragmenter.
• XFS provides an online defragmentation utility called xfs_fsr.