Wednesday, October 21, 2009

Network Crash

In network environment lots of equipments take to make the connectivity between client and servers. Those equipments are transferring data each and every where in side the network. That data is transferring in data packet way.

In a huge network environment there is lots of equipment transferring lots of data packet at once. Some times some data packets are transferring very long distance inside the network and some times lot of data packet traveling inside one cable.

The summery of two paragraphs, the major point is considered about the data packet. We were preparing network Because of transferring data easily between the computers. We must considered about data packets and protect it. If we lost data mean network crash inside the network, in huge network this can be happen,

If we taking this example 1 picture there is a centralize HUB in that network. But the reason is with using HUB we can’t manage a network because it is not providing any traffic management in side the network.

And another point for the cables, if we take coaxial cable for this does this network it is not possible because inside the network environment magnetic affect, long distances can happen if we use coaxial for the ca balling some time data me be lost of that reason.

If the distances is higher we can’t transferring data because if range is high data might be lost by traveling. NIC is another major point is does the network. Because some NIC’s are not speed that mean it is not transferring large capacity of data packet that is a reason for lost the data.

Gigabit Ethernet

Before inception Xerox Corporation in 1970s, Ethernet has been the dominant networking protocol. If we take all protocols Ethernet has, by far, the highest number of installed ports and provides the greatest cost performance relative to token Ring, Fiber Distributed Data Interface (FDDI), and ATM for desktop connectivity. And those Ethernet provide their connectivity from 10 to 100 megabit per second (Mbps).


Gigabits Ethernet build on top of the Ethernet protocol that mean it provide it service speed increases by tenfold over. It is a logical choice for low-cost, high-performance network upgrade, providing unparalleled speed and ease of management. The Gigabit Ethernet provides speed from 1000 megabit per second or 1 gigabit per second (Gaps). This protocol performed high-speed connectivity like lightning; it is gaining more and more presence in the server connectivity market. Network administrators require higher network performance to cope with the increasingly data intensive applications, backup, and software distributions or desktop, educational system. Which is the one success point of the network connection; customers will be able to leverage their existing knowledge base to manage and maintain gigabit networks. In
future it will be more than one gigabit.

Friday, October 16, 2009

windows vista wireless

How to connect a Vista computer to a wireless network using WLAN AutoConfig This document will serve as a guide for Microsoft Vista clients in joining a wireless network using WLAN AutoConfig service.

What is WZC or WLAN AutoConfig?


Wireless Zero Configuration (WZC) or WLAN AutoConfig is a service included with modern versions of Microsoft Windows that dynamically selects a wireless network to connect to base on a user's preferences and various settings. This can be used instead (or in the absence) of a wireless network utility from the manufacturer of a computer's wireless networking device.

To join a wireless network:


1. Open the Start menu and click Control Panel. 2. In the Control Panel Home window, click View network status and tasks.






2.If you are using Classic View, double- click Network and Sharing Center.





3. Click Connect to a network.


4. Select your network from the list and click Connect.


5.If security is enabled on the network, you will be asked to type your network security key or passphrase for your router. After typing security key, click Connect.



6.After typing the correct security key, you should be connected to your router. Click Close.


7.You can select a location profile for your network or skip it by clicking Cancel.



8.You are now ready to connect to the Internet.

Thursday, October 15, 2009

Hybrid topologies


Different topologies may be interconnected to form subnets to a main network. It is possible, for example, to connect a star, a hub and a ring together beneath another star topology network.

Tree


A tree topology is a variation on a star. In this case the central hub might connect to a number of secondary hubs forming a sort of hierarchy of stars. Secondary hubs may be passive or active devices. An active hub acts as a repeater and amplifier for the signals in each direction. This is ideal for networks with remote nodes as link distances can be effectively lengthened without attenuation. A passive hub supplies connections only between the next hub and the nodes below.


Advantages:

  • Secondary hubs increase the number of devices that may be part of the network
  • Hubs can be used to prioritize and isolate traffic on different parts of the network.
  • Other advantages as per star topology

Mesh Topology


A mesh topology is a network topology in which each communicating node is directly connected to all other nodes.
Or
Mesh topology provides each device with a point-to-point connection to every other device in the network. These are most commonly used in WAN's, which connect networks over telecommunication links. Mesh topologies use routers to determine the best path.
Mesh networks provide redundancy, in the event of a link failure; meshed networks enable data to be routed through any other site connected to the network. Because each device has a point-to-point connection to every other device, mesh topologies are the most expensive and difficult to maintain.

Advantages:

  • Dedicated links eliminate congestion due to excessive traffic.
  • Robust as one damaged link will not cripple the network.
  • Point-to-point connections make fault isolation and identification simple.

Disadvantages:
  • High number of connections requires large number of network ports and large amounts of cabling.
  • Wiring bulk may exceed space available.
  • Duplication of hardware becomes expensive

Monday, October 12, 2009

Route

Introduction to Routing

Routing is an extremely important function of IP. It is the process of choosing a path over which to send packets. The device that performs this task is called a router, which forwards packets from one physical network to another. Your knowledge of IP will enable you to see the correlation between IP and routing.
The Internet layer, or OSI/RM network layer (Layer 3), performs the routing function. A packet, or datagram, carries sufficient information for routing from the originating host to the destination host using the IP address. Packets may traverse several networks before reaching their destination host.
Packets are routed transparently, and not necessarily reliably, to the destination host. The term "transparent," when applied to routing, means that after the routing hardware and software are installed, changes are undetectable by users because the routing process is largely automated. The complexity of routing is not visible to the user. The transport or application layer is responsible for reliability, which ensures that the data arrives at the other end.


Routing can be summarized as:

  • The process that determines the path that packets will travel across networks.
  • One of the most important IP functions.
Routing can be divided into two general classifications: direct and indirect.

Bluetooth

Named after a tenth-century Danish king, Bluetooth is a specification for a small form-factor, low-cost radio solution providing links between mobile computers, mobile phones and other portable handheld devices, and connectivity to the internet which it is hoped will revolutionise mobile computing and communications by providing freedom from wired connections. The initiative is backed by leaders in the telecommunications, computing, and network industries - including 3Com, Ericsson, IBM, Intel, Lucent, Motorola, Nokia and Toshiba - and more than 1300 adopter companies. Microsoft's decision to join the group driving the standard at the end of 1999 significantly increased hopes that one of the problems that had afflicted Bluetooth hitherto - namely that the standard had yet to extend beyond hardware compatibility to encompass the software that runs across it - would not be allowed to undermine the standard to the extent that it had its forerunner, infrared.

Bluetooth's chief advantage over infrared is that Bluetooth does not require line of sight. Its backers hope the component costs will be so low that it will ultimately replace infrared. Others believe that, although there are clear areas of overlap, IrDA and Bluetooth are complementary with either being the most appropriate for certain applications and intended usage models.

Bluetooth operates in the unlicensed Industrial Scientific and Medical (ISM) 2.4GHz band, ranging from 2.4 to 2.4835 GHz in the US, Japan and Europe. Parts of this band are also available in France and Spain. By using the 2.4 GHz band, Bluetooth Wireless Technology promises to be a universal wireless solution. Essentially, it is the same kind of microwave radio technology that has given us wireless door chimes and automatic garage door openers. It draws heavily on existing wireless LAN technology since it is based around the IEEE's 802.11 - the existing standard for wireless Ethernet. The main differences are that in order to consume less power, Bluetooth is initially restricted to an operating distance of just 10 metres and a speed of approximately 1 Mbit/s.

Saturday, October 10, 2009

Network Topology

The term network topology is, refers to the way a network is laid out, either physically or logically. The topology of a network is the geometric representation of the relationship between the links and linking devices (called nodes). There are five basic topologies possible: Combining these topologies, a more complex hybrid network topology can be created.


The topology of a particular network can be chosen depending upon the relative status of each device in the network. A peer-to-peer relationship, where each node has equal importance and make equal use of each link lends itself to a ring, mesh or bus topology. Where there is a 'master-slave' or primary-secondary relationship between one device and the rest then the star, tree or bus topologies best fit.

Wednesday, October 7, 2009

Protect Network

The threat of email viruses & Trojans

The widespread use of email has provided hackers and crackers with an easy way to distribute harmful content to the internal network. Hackers can easily circumvent the protection offered by a firewall by tunneling through the email protocol, since it does not analyze email content.

CNN reported says that the MyDoom virus cost companies about US$250 million in lost productivity and tech support expenses, while NetworkWorld (September 2003) cited studies that placed the cost of fighting Blaster, SoBig.F, Wechia and other email viruses at US$3.5 billion for US companies alone.

Furthermore, email is also used to install Trojans, targeted specifically at your organization to obtain confidential information or gain control of your servers. Described as "instructive viruses" or "spy viruses" by computer security experts, these can be potent tools in industrial espionage. A case in point is the email attack on Microsoft's network in October 2000, which a Microsoft Corp. spokesman described as "an act of industrial espionage pure and simple". According to reports, Microsoft's network was hacked by means of a backdoor Trojan virus maliciously emailed to a network user.

Saturday, October 3, 2009

Wednesday, September 9, 2009

Wireless Internet


The IMT-2000 wideband standard is driven by skyrocketing demand for wireless Internet applications. According to Cahners In-Stat Group research, in the United States alone more than 85 million people subscribe to wireless services, and consumers in 46 million U.S. households have Internet access. It was only a matter of time before people would want to have access to the Internet from their wireless handsets. Cahners In-Stat Group projects that more that nearly 25 million people in the U.S. will use mobile wireless Internet services by 2003. In Japan, demand for the much publicized i-Mode service is primarily driven by the fact that the vast majority of Japanese people do not have wireline Internet access. Within less than six months, more than 5 million Japanese people subscribed to this service, and phenomenal future growth is inevitable.

Internet users are becoming wireless users, and expect applications to converge and fit smaller, untethered form factors. Cahners In- Stat Group expects that consumers, whether mobile or at their home PCs, will exhibit insatiable appetites for Web downloads and messaging; streaming media, color graphics, gaming, Java-enabled location-based applets, Internet browsing, and access to corporate intranets through secure firewalls. (Moreover, there are bandwidth-intensive applications, as yet unknown, that will unfold within the next few years.)

Thursday, September 3, 2009

Windows NT Networks

In the summer of 1993 Microsoft released their new Network Operating System, NT for workstations. The NT server followed in the autumn of 1994. The network protocols used in NT are NetBEUI, TCP/IP, IPX and AppleTalk, which gives NT the opportunity to communicate with different types of network platforms. Windows NT is a multisession operating system. NT can run on different types of processors like Intel, Alpha and PowerPC. NT has also a good support for remote dial-in communication, called RAS (Remote Access Service).

NT is normally used on top of Ethernet, but it is also possible to use in Token Ring.

Novell Netware


Novell Netware has for a long time been a market leader of NOS. Netware has been on the market since 1983 and it runs only on Intel processors.

Novell developed NetWare, which is a powerful multisession system. This means that NetWare can handle different parallel sessions as previously only found in larger types of computers.

NetWare is sold in many versions corresponding to the number of users, types of connections and level of security.

The client could be a personal computer with or without its own hard disc and with an operating system such as MS-DOS, OS/2, MacOS or UNIX.

Netware handles both IPX and TCP/IP traffic over Ethernet or Token Ring.

Network operating systems


After the physical network the NOS, Network Operating System, is the most important part in a PC network. The network operating system is responsible for the levels 3 to 7 in the OSI model. This means that NOS is independent of the type of network. It works in the same manner on an Ethernet as a Token Ring.

There are two types of NOS. The first type is called "peer to peer” which means that no dedicated server is needed. The other type is server based and is used for larger, more demanding networks.

There are many vendors with their own types of NOS on the market.

The leading systems for "peer to peer” based NOS are NetWare Lite and Windows for Workgroups.

The leading systems for server based NOS are Novell NetWare, Windows NT and OS/2 LAN Server.

Networks Shared

One of the main reasons for using local networks is to share common resources, such as printers, files, e-mail and devices for communication.

In the picture you see a shared printer and a file server which is used to share files. Routers and gateways are used for communication to other LANs and remote devices such as computers and faxes.

Networks Components


A PC network is used to share resources such as files and printers. A PC network is normally based on Ethernet or Token Ring and consists of a number of components:


  1. Cabling is used to physically connect the components together. A common type of cable used in both Ethernet and Token Ring is twisted pair.
  2. Clients such as PCs or Macintoshes can be used together on the same network. One often distinguishes between the hardware and the software components of a client. One important hardware component is the NIC, which stands for Network Interface Card, inside the client. The software component responsible for communication consists of two parts: the operating system and communication software. These two together are called Network Operating System. An example of a Network Operating System on a client is DOS and IPX software.
  3. Servers are used to share documents and programs. Servers are often dedicated powerful computers. There are different types of Network Operating Systems for a server. Examples are Novell Netware, Windows NT and IBM LANServer.
  4. Printers are common components found in a PC network.
  5. Gateways, not seen in this picture, are used to translate between two applications or protocols. Gateways are used for communication between different protocols.

Tuesday, September 1, 2009

spyware programs

• Bonzai Buddy
• Comet Cursor
• Download Accelerator
• Go!Zilla
• Gator
• Hotbar
• Huntfly
• Lop.com
• Web3000 programs
• Xupiter Toolbar
- Spyware components can also be found in many popular file-sharing programs
such as KaZaa, BearShare, LimeWire, iMesh and Grokster.

Adware

Adware is advertising-supported software that displays pop-up advertisements
whenever the program is running. The software is usually available via free download from the Internet, and it is the advertisements that create revenue for the company. Although seemingly harmless (aside from intrusiveness and annoyance of pop-up ads), adware can install components onto your computer that track personal information (including your age, gender, location, buying preferences, surfing habits, etc.). Most advertising supported software doesn't inform you that it installs adware on your system, other than through a buried reference in a license agreement. In many cases the software will not function without the adware component. Some Adware will install itself on your computer even if you decline the offer.

Dialers

Dialers are a type of software typically used by vendors serving pornography via the Internet. Once dialer software is installed, the user is disconnected from their usual Internet service provider and then redirected by the dialer program to call into another phone number where the user is billed per minute. Dialers do not "spy" on their intended victims, but these malevolent programs can rack up significant long distance phone charges, costing victims time and money.

System Monitors

System monitors are applications designed to monitor computer activity to
various degrees. These programs can capture virtually everything you do on your computer including recording all keystrokes, emails, chat room dialogue, web sites visited, and programs run. System monitors usually run in the background so that you do not know that you are being monitored. The information gathered by the system monitor is stored on your computer in an encrypted log file for later retrieval. Some programs are capable of emailing the log files to another location/person. System monitors can be installed by someone that shares your computer, or come disguised as email attachments or "freeware" software products.

Trojan Horses

As mentioned previously, Trojans are malicious programs that appear as harmless or desirable applications. Trojans are often designed to cause loss or theft of computer data. Some Trojans called RATs (Remote Administration Tools) allow an attacker to gain unrestricted access of your computer whenever you are online. The attacker can perform activities such as file transfers, adding/deleting files or programs, and controlling your mouse and keyboard. Trojans are generally distributed as a desirable program or file, in email attachments or bundled with another software program.

3G Vs WiFi

One of the key distinctions between 3G and WiFi that we have only touched upon lightly thus far is that 3G and other mobile technologies use licensed spectrum, while WiFi uses unlicensed shared spectrum. This has important implications for (1) Cost of service; (2) Quality of Service (QoS) and Congestion Management; and (3) Industry structure.

First, the upfront cost of acquiring a spectrum license represents a substantial share of the capital costs of deploying 3G services. This cost is not faced by WiFi which uses the shared 2.4Ghz unlicensed, shared spectrum. The cost of a spectrum license represents a substantial entry barrier that makes it less likely that 3G services (or other services requiring licensed spectrum) could emerge in a decentralized fashion. Of course, with increased flexibility in spectrum licensing rules and with the emergence ofsecondary markets that are being facilitated by these rules, it is possible that the upfront costs of obtaining a spectrum license could be shared to allow decentralized infrastructure deployment to proceed. Under the traditional licensing approach, the licensing of the spectrum, the construction of the network infrastructure, and the management/operation of the service were all undertaken by a single firm. Moreover, rigid licensing rules(motivated in part by interference concerns, but also in part, by interest group politic) limited the ability of spectrum license holders to flexibly innovate with respect to the technologies used, the services offered, or their mode of operation. In the face of rapid technical progress, changing supply and demand dynamics, this lack of flexibility increased the costs and reduced the efficiency of spectrum utilization. High value spectrum trapped in low value uses could not be readily redeployed. With the emergence of secondary markets, it would be possible for spectrum brokers to emerge or service integrators that could help distribute the spectrum cost to enable decentralized infrastructure investment for licensed spectrum.


Second, while licensed spectrum is expensive, it does have the advantage of facilitating QoS management. With licensed spectrum, the licensee is protected from interference from other service providers. This means that the licensee can enforce centralized allocation of scarce frequencies to adopt the congestion management strategy that is most appropriate. In contrast, the unlicensed spectrum used by WiFi imposes strict power limits on users (i.e., responsibility not to interfere with other users) and forces users to accept interference from others. This makes it easier for a 3G provider to market a service with a predictable level of service and to support delay-sensitive services such as real-time telephony. In contrast, while a WiFi network can address the problem of congestion associated with users on the WiFi network, it cannot control potential interference from other WiFi service providers or other RF sources that are sharing the unlicensed spectrum (both of which will appear as elevated background noise). This represents a serious challenge to supporting delay-sensitive services and to scaling service in the face of increasing competition from multiple and overlapping multiple service providers. A number of researchers have started thinking about how to facilitate more efficient resource allocation of unlicensed spectrum, including research on possible protocols that would enable QoS to be managed more effectively. Third, the different spectrum regimes have direct implications for industry structure. For example, the FreeNet movement is not easily conceivable in the 3G world of licensed spectrum. Alternatively, it seems that the current licensing regime favors incumbency and, because it raises entry barriers, may make wireless- facilities-based competition less feasible.

Adware Cookies

Cookies are pieces of information that are generated by a web server and stored on your computer for future access. Cookies were originally implemented to allow you to customize your web experience, and continue to serve a useful purpose in enabling a personalized web experience. However, some web sites now issue “adware” cookies, which allow multiple web sites to store and access cookies that may contain personal information (including surfing habits, user names and passwords, areas of interest, etc.), and then simultaneously share the information they contain with other web sites. This sharing of information allows marketing firms to create a user profile based on your personal information, which they then sell it other firms. Adware cookies are almost always installed and accessed without your knowledge or consent.

Monday, August 31, 2009

AntiVirus software






These are stand-alone programs that will protect your computer from viruses, worms and some trojans, and allow you to clean it should you become infected.

• Norton Antivirus (http://www.symantec.com/) Trial / Buy
• Mcafee Virus Scan (http://www.mcafee.com/) Trial / Buy
• PC-Cillin (http://www.trendmicro.com/) Trial / Buy
• Anti-Viral Toolkit Pro (http://www.kaspersky.com/) Trial / Buy
• AVG Anti-Virus (http://www.grisoft.com/) Free / Buy

Note that all of these websites (except grisoft.com) have areas that will conduct free online virus scans on your machine; however, only Trend Micro’s “House Call” page (housecall.trendmicro.com) will actually clean your machine if it finds any. In any case, these online scans are only band-aid solutions for disinfecting your machine after the fact – you should still be purchasing a realtime Anti-Virus software package (such as those listed above) to prevent viruses from getting into your system in the first place.


  • Update your virus/trojan definitions on a regular basis. Definition files are the
    database that your anti-virus/trojan software draw from to detect, identify and
    remove new viruses, worms and trojans. With new ones constantly emerging, new
    definition files are constantly being compiled and released by the software
    companies to keep up. Make sure you download these definitions regularly
    (weekly) from your company’s website to keep your software up-to-date for your
    own protection (ie, for Norton Anti-Virus you can use their “live update” feature).
  • Scan all downloaded files/attachments with Anti-Virus/Anti-Trojan software,
    before you open them.
  • Backup your data. No matter how safe you think you are, unless you back up
    your data prior to being infected, you may lose it during the infection/payload
    process (viruses/worms/trojans may delete or scramble files) or anti-virus/trojan
    cleaning process (files may need to be deleted to clean the machine). If there is
    important data that you need, make sure you back it up on a regular basis (either
    on diskette, CD-ROM or uploaded to a website).

Viruses, Worms, Trojans Preventative Solutions

  • Use common sense - don’t visit, or download programs from, people/websites you don’t know or trust.
  • Don’t open “spam” or unknown emails or attachments – delete them instead.
  • Check with your friend over the phone if you receive an email from them with
    an attachment you weren’t expecting – verify they did intend to send you the
    email/attachment, to make sure it wasn’t a worm sending it instead (many
    viruses/worms/trojans are sent from people unaware they’ve been infected).
  • Obtain the latest patches for Windows, Internet Explorer and Outlook Express
    to plug the many security holes and vulnerable exploits found in these programs.
  • Un-hide file extensions. This addresses the problem where Windows by default
    likes to hide the extensions of filenames; ie "susie.jpg" shows up as just "susie".
    The danger is that "susie.jpg.exe", which is an executable program, would be
    shown as "susie.jpg" which many would mistake for just a picture. This is
    potentially very dangerous and confusing.

In Windows 95 / 98:
- Open Explorer
- Under View menu, select Options
- Check "show all files"
- UNcheck "hide MSDOS file extensions that are registered"
- Click OK to finish
In Windows ME / 2000 / XP:
- select Start Settings Control Panels Folder Options
- select the View tab
- check "show hidden files and folders"
- UNcheck "hide file extensions for known file types"
- Click OK to finish

…or follow this illustrated guide: www.granneman.com/techinfo/windows/showexte

IMPORTANT EXCEPTION: Even after you unhide the extensions using the
above steps, you still cannot see certain hidden extensions for files ending with
.shs, .pif, and .lnk (blame Microsoft for its infinite lack of wisdom).
Unfortunately these files are executable, and are rapidly becoming the most
popular choices for many trojan horses, worms and viruses, such as
"Movie.avi.pif" which will look like "Movie.avi", and
"LIFE_STAGES.TXT.SHS" which will look like "LIFE_STAGES.TXT". Instead
of being a movie and text file, respectively, they are both dangerous programs.
Again, unless you know where the file you are getting comes from or who sent it,
do not download/accept, or open it.

Virus Suspicious

Computer Virus Suspicious File Types. Be wary if any of these show up in your email

•.ade: Microsoft Access projectextension
• .adp: Microsoft Access project
• .bas: Microsoft Visual Basic class module
• .bat: Batch file
• .chm: Compiled HTML Help file
• .cmd: Microsoft Windows NT Command script
• .com: Microsoft MS-DOS program
• .cpl: Control Panel extension
• .crt: Security certificate
• .exe: Program
• .hlp: Help file
• .hta: HTML program
• .inf: Setup Information
• .ins: Internet Naming Service
• .isp: Internet Communication settings
• .js: JScript file
• .jse: Jscript Encoded Script file
• .lnk: Shortcut
• .mdb: Microsoft Access program
• .mde: Microsoft Access MDE database
• .msc: Microsoft Common Console document
• .msi: Microsoft Windows Installer package
• .msp: Microsoft Windows Installer patch
• .mst: Microsoft Visual Test source files
• .pcd: Photo CD image, Microsoft Visual compiled script
• .pif: Shortcut to MS-DOS program
• .reg: Registration entries
• .scr: Screen saver
• .sct: Windows Script Component
• .shs: Shell Scrap object
• .shb: Shell Scrap object
• .url: Internet shortcut
• .vb: VBScript file
• .vbe: VBScript Encoded script file
• .vbs: VBScript file
• .wsc: Windows Script Component
• .wsf: Windows Script file
• .wsh: Windows Script Host Settings file

Computer Virus How you get infected?

In each case, viruses/worms/trojans can all be spread in the guise of literally ANYTHING people find desirable, such as a free game, movie, song, etc. Victims typically downloaded the file from a website, via file sharing programs such as KaZaa, over instant messaging (such as MSN), or by just carelessly opening some email attachment without thinking. Although these programs can also be exchanged on floppy disks, today the Internet has taken precedent as the distribution medium of choice.

• Viruses, worms and trojans often masquerade under files with various extensions, the most common of which are .exe (executable program – beware!), .com, .vbs, .pif, and .js. Be very suspicious if files with the following extensions arrive in your email without you expecting them, as they could contain a virus, worm or trojan.

• Macro viruses may arrive in the form of Microsoft Word (.doc), Microsoft Excel (.xls), and Microsoft Powerpoint (.ppt) files. Also, be wary of .htm and .html files; because they can access the Internet, they may direct you to a website that will forcibly attempt your machine to download unsafe files or exploit an un-patched security hole in Windows or Internet Explorer.


• In the case of worms, it is also possible to get one that activates just by reading an email, even when there is no attachment. A recent worm spread by taking advantage of a security hole in Microsoft Outlook Express that allowed it to run, even though there was no attachment in the email.

Trojan Types


Again, just as with viruses and worms, there are many types of Trojan programs - the most popular being “Back Orifice”, “Netbus” and “SubSeven”. Depending on the type of trojan installed and the motives of someone who gains access to an infected machine, the results can be disastrous: trojans can allow someone to see what you are seeing (on their screen), to transfer files from your computer to theirs, to delete your files and crash your computer, to use keyloggers to track (log) what you type in order to steal passwords/bank account/credit card info, to open and close CD-ROM drives, take control of your mouse, turn the machine and monitor on/off, and much more.

Almost all trojans will attempt to open a “port” (metaphorically, an open port is like an open door to a house) to broadcast the presence of an infected machine to “port scanners” (people looking for open ports on infected machines, to break into). Some trojans are also programmed to establish a direct connection to a specific person/computer, or to commit illegal DoS (Denial of Service) attacks on specific websites. Firewalls (see futher in this document) are particularly useful to block Trojans from trying to access the Internet, and from people trying to gain access to your machine.

What is computer Trojan



In today's computer world, a Trojan horse is often defined as a program that seemingly does one thing, but its true function is hidden in order to fool you. For example, you download what appears to be a movie or music file, but when you click to open it, you unleash a program that could do any number of things. Trojans usually operate silently, in the background - the most common purpose for them is that they can allow someone to gain access and control your computer over the Internet, and use it for whatever purposes they wish, often without your knowledge.

Computer Worm Types

There are many types of worm programs, many of which are quite insidious - they can compromise the security of an infected machine and leave it vulnerable to future attacks, collect passwords and other confidential information (which can then be automatically emailed to other people), or be programmed to delete files or deface websites.


Worms are particularly well-known for scanning through a person’s computer for email addresses, and then propagating themselves to the addresses found. Some worms will also send a file from your computer to every person they propagate to, which could be disastrous if the attached file is confidential/personal information.

Computer Worms


Worms are programs that once run, take advantage of a computer’s ability to send and receive information. They use this ability to propagate themselves automatically (usually through email) over a network such as the Internet, and cause massive congestion (slow response time, server overloads) in the process. They can also do more malicious acts, and slow down your machine.

Computer Virus types

  • Boot Sector Virus: replaces or implants itself in the boot sector - an area of the hard drive (or any other disk) accessed when you first turn on your computer. This kind of virus can prevent you from being able to boot your hard disk/computer.
  • File Virus: infects applications. These executables then spread the virus by infecting associated documents and other applications whenever they're opened or run.
  • Macro Virus: Written using a simplified macro programming language, these virusesaffect Microsoft Office applications, such as Word (.doc) and Excel (.xls). A document infected with a macro virus generally modifies a pre-existing, commonly used command (such as Save) to trigger its payload upon execution of that command.
  • Multipartite Virus: infects both files and the boot sector - a double whammy that can
    reinfect your system dozens of times before it's caught.
  • Polymorphic Virus: changes code whenever it passes to another machine; in theory these
    viruses should be more difficult for antivirus scanners to detect, but in practice they're
    usually not that well written.
  • Stealth Virus: hides its presence by making an infected file not appear infected, but
    doesn't usually stand up to antivirus software.

Depending on the virus, some will perform more malicious deeds than others. Examples include deleting and renaming of files, scrambling contents of the entire hard drive (so you can’t access your data), or not letting the machine boot into Windows. Some viruses also slow down your machine, disable certain functions, or cause erratic system behavior and crashes.

Viruses, Worms, Trojans What are they?

Virus:
Simply put, viruses are (primarily) destructive computer programs created by someone that once run, attempt to destroy the data (files) on your computer. A virus spreads when an infected program is run (executing the virus code), which in turn infects more files on the same machine. This usually happens silently and without your knowledge until its too late. In general, viruses have 1) an infection phase where they reproduce widely, 2) an attack/trigger phase (such as a certain date or time) which causesthem to 3) deliver their “payload”, and do whatever damage they have been programmed to do (if any).

Spyware What is it?


Spyware is any application that collects information about your computer activities and then sends that information to another individual or company without your knowledge or permission. Spyware often arrives bundled with freeware (free) or shareware (trial) programs, through email or instant messenger, as an Active X install, or by someone with access to your computer. Once on your drive, spyware secretly installs itself and goes to work. Spyware can be difficult to detect, and difficult (if not impossible) for the average user to remove.

Spyware can:
• Track your online surfing habits, profile your shopping preferences, gather personal information (age, sex, etc, possibly credit card info, PIN numbers)

• Send your email address to the company/person that made the spyware; that company/person can now send spam to your email account.

• Decrease your connection speed/hog your internet connection by sending information about you and your computer to the company/person that made the spyware

• Hijack your web browser’s start page, bombard you with pop-up advertisement boxes
• Run in the background and slow your computer down, alter important system files, make your computer unstable and crash

Spyware comes in many flavors including:

• Trojan Horses

As mentioned previously, Trojans are malicious programs that appear asharmless or desirable applications. Trojans are often designed to cause loss or theft of computer data. Some Trojans called RATs (Remote Administration Tools) allow an attacker to gain unrestricted access of your computer whenever you are online. The attacker can perform activities such as file transfers, adding/deleting files or programs, and controlling your mouse and keyboard. Trojans are generally distributed as a desirable program or file, in email attachments or bundled with another software program.

System Monitors/Keyloggers

System monitors are applications designed to monitor computer activity to various degrees. These programs can capture virtually everything you do on your computer including recording all keystrokes, emails, chat room dialogue, web sites visited, and programs run. System monitors usually run in the background so that you do not know that you are being monitored. The
information gathered by the system monitor is stored on your computer in an encrypted log file for later retrieval. Some programs are capable of emailing the log files to another location/person. System monitors can be installed by someone that shares your computer, or come disguised as email attachments or "freeware" software products.

Dialers

Dialers are a type of software typically used by vendors serving pornography via the Internet. Once dialer software is installed, the user is disconnected from their usual Internet service provider and then redirected by the dialer program to call into another phone number where the user is billed per minute. Dialers do not "spy" on their intended victims, but these malevolent programs can rack up significant long distance phone charges, costing victims time and money.

Adware

Adware is advertising-supported software that displays pop-up advertisements whenever the program is running. The software is usually available via free download from the Internet, and it is the advertisements that create revenue for the company. Although seemingly harmless (aside from intrusiveness and annoyance of pop-up ads), adware can install components onto your computer that track personal information (including your age, gender, location, buying preferences, surfing habits, etc.). Most advertising supported software doesn't inform you that it installs adware on your system, other than through a buried reference in a license agreement. In many cases the software will not function without the adware component. Some Adware will install itself on your computer even if you decline the offer.

Adware Cookies

Cookies are pieces of information that are generated by a web server and stored on your computer for future access. Cookies were originally implemented to allow you to customize your web experience, and continue to serve a useful purpose in enabling a personalized web experience. However, some web sites now issue “adware” cookies, which allow multiple web sites to store and access cookies that may contain personal information (including surfing habits, user names and passwords, areas of interest, etc.), and then simultaneously share the information they contain with other web sites. This sharing of information allows marketing firms to create a user profile based on your personal information, which they then sell it other firms. Adware cookies are almost always installed and accessed without your knowledge or consent.

A list of some common spyware programs:
• Bonzai Buddy
• Comet Cursor
• Download Accelerator
• Go!Zilla
• Gator
• Hotbar
• Huntfly
• Lop.com
• Web3000 programs
• Xupiter Toolbar
- Spyware components can also be found in many popular file-sharing programs such as KaZaa, BearShare, LimeWire, iMesh and Grokster.

Sunday, August 30, 2009

VIRUS Announcement

You should be alert during the next few days. Do not open any message with an attachment entitled 'POSTCARD FROM BEJING', regardless of who sent it to you. It is a virus which opens A POSTCARD IMAGE, which 'burns' the whole hard disc C of your computer. This virus will be received from someone who has your e-mail address in his/her contact list. This is the reason why you need to send this e-mail to all your contacts.. It is better to receive this

message 25 times than to receive the virus and open it..

If you receive a mail called 'POSTCARD FROM BEJING,' even though sent to you by a friend, do not open it! Shut down your computer immediately.

This is the worst virus announced by CNN. It has been classified by Microsoft as the most destructive virus ever. This virus was discovered by McAfee yesterday, and there is no repair yet for this kind of virus. This virus simply destroys the Zero Sector of the Hard Disc, where the vital information is kept.More :www.mcafee.com

Free WiFi

While commercial services attempt to move existing business models to Wi-Fi, many
groups, communities, cities, and individuals have set up free Wi-Fi networks, often adopting a common peering agreement in order that networks can openly share with each other. Free wireless mesh networks are often considered the future of the Internet.

Many municipalities have joined with local community groups to help expand free Wi-Fi networks (see Mu-Fi). Some community groups have built their Wi-Fi networks entirely based on volunteer efforts and donations.

For more information, see wireless community network, where there is also a list of the free Wi-Fi networks one can find around the globe. OLSR is one of the protocols used to set up free networks. Some networks use static routing; others rely completely on OSPF. Wireless Leiden developed their own routing software under the name LVrouteD for community wi-fi networks that consist of a completely wireless backbone. Most networks rely heavily on open source software, or even publish their setup under an open source license.

Some smaller countries and municipalities already provide free Wi-Fi hotspots and residential Wi-Fi internet access to everyone. Examples include Estonia which have already a large number of free Wi-Fi hotspots throughout their countries.

In Paris, France, OzoneParis offers free Internet access for life to anybody who contributes to the Pervasive Network’s development by making their rooftop available for the Wi-Fi Network.

Annapolis, Maryland is in the early phases (as of April 2006) of a pilot program to provide free, advertisement-financed Wi-Fi to all its residents. A private company, Annapolis Wireless Internet, will administrate the network. Users will only see local advertisements upon accessing the network.

Many universities provide free Wi-Fi internet access to their students, visitors, and anyone on campus. Similarly, some commercial entities such as Panera Bread and Culver's offer free Wi-Fi access to patrons. McDonald's Corporation also offers Wi-Fi access, often branded 'McInternet'. This was launched at their flagship restaurant in Oak Brook, Illinois, USA, and is also available in many branches in London, UK.

However, there is also a third subcategory of networks set up by certain communities such as universities where the service is provided free to members and guests of the community such as students, yet used to make money by letting the service out to companies and individuals outside. An example of such a service is Sparknet in Finland.

Sparknet also supports OpenSpark, a project where people can share their own wireless access point and become as a part of Sparknet and OpenSpark community in return for certain benefits.

Recently commercial Wi-Fi providers have built free Wi-Fi hotspots and hotzones. These providers hope that free Wi-Fi access would equate to more users and significant return
on investment.

Wifi Universal

Another business model seems to be making its way into the news. The idea is that users
will share their bandwidth through their personal wireless routers, which are supplied with specific software. An example is FON, a Spanish start-up created in November 2005. It aims to become the largest network of hotspots in the world by the end of 2006 with 70000 access points. The users are divided into three categories: linus share Internet access for free; bills sell their personal bandwidth; and aliens buy access from bills. Thus the system can be described as a peer-to-peer sharing service, which we usually relate to software.

Although FON has received some financial support by companies like Google and Skype, it remains to be seen whether the idea can actually work. There are three main challenges for this service at the moment. The first is that it needs much media and community attention first in order to get through the phase of "early adoption" and into the mainstream. Then comes the fact that sharing your Internet connection is often against the terms of use of your ISP. This means that in the next few months we can see ISPs trying to defend their interests in the same way music companies united against free MP3 distribution. And third, the FON software is still in Beta-version and it remains to be seen if it presents a good solution of the imminent security issues.

WiFi Commercial

Commercial Wi-Fi services are available in places such as Internet cafes, coffee houses, hotels and airports around the world (commonly called Wi-Fi-cafés), although coverage is patchy in comparison with cellular.

IN USA

• T-Mobile provides HotSpots in many partner retail locations including many Starbucks, Borders Books, and a variety of hotels and airports.

• a Columbia Rural Electric Association subsidiary offers 2.4 GHz Wi-Fi service across a 3,700 mi² (9,500 km²) region within Walla Walla and Columbia counties in Washington and Umatilla County, Oregon.

• WiSE Technologies provides commercial hotspots for airports, universities, and independent cafes in the US;

• Boingo Wireless has over 45,000 hotspots worldwide, including most major airports in the U.S.

• restaurant chain Panera Bread provides free Wi-Fi access at its restaurants.

• Other large hotspot providers include Wayport, iPass, and iBahn.

• There are also a number of aggregators of Wi-Fi, the main one being BOZII, they allow users access to over 250 networks including BT Openzone and Orange France, all with one username and password for a flat fee and no roaming charges.


IN UK

• T-Mobile provides hotspots in many Starbucks and Airports in the UK too.

• BT Openzone provides many hotspots across the United Kingdom and Ireland, notably in most McDonalds restaurants, and have roaming agreements with TMobile UK and ReadyToSurf. Their customers are also able to access hotspots managed by The Cloud.


In France:

• Ozone and OzoneParis In France, in September 2003, Ozone started deploying the OzoneParis network across the City of Lights. The objective: to construct a wireless metropolitan network with full Wi-Fi coverage of Paris. Ozone is also deploying its network in Brussels (Belgium) and other cities in France like Rennes. Ozone Pervasive Network philosophy is based on a nationwide scale.

• als@tis One of the largest Wireless Internet Service Provider for rural areas in France.

In other places

• GlobeQUEST, under Globe Telecom, provides for prepaid Wi-Fi services for nearly all cafes in the Philippines
• Pacific Century Cyberworks provides hotspots in Pacific Coffee shops in Hong Kong;
• Vex offers a big network of hotspots spread over Brazil. Telefónica Speedy Wi-Fi has started its services in a new and growing network distributed over the state of São Paulo.
• Netstop provides hotspots in New Zealand;
• FatPort is Canada's oldest independent Wi-Fi HotSpot operator with coverage from coast to coast.

WiFi cellular

Some argue that Wi-Fi and related consumer technologies hold the key to replacing
cellular telephone networks such as GSM. Some obstacles to this happening in the near future are missing roaming and authentication features (see 802.1x, SIM cards and RADIUS), the narrowness of the available spectrum and the limited range of Wi-Fi. It is more likely that WiMax will compete with other cellular phone protocols such as GSM, UMTS or CDMA. However, Wi-Fi is ideal for VoIP applications e.g. in a corporate LAN or SOHO environment. Early adopters were already available in the late '90s, though not until 2005 did the market explode. Companies such as Zyxel, UT Starcomm, Sony, Samsung, Hitachi and many more are offering VoIP Wi-Fi phones for reasonable prices.

In 2005, low-latency broadband ISPs started offering VoIP services to their customers. Since calling via VoIP is free or low-cost, VoIP enabled ISPs have the potential to open up the VoIP market. GSM phones with integrated Wi-Fi & VoIP capabilities are being introduced into the market and have the potential to replace land line telephone services.

Currently it seems unlikely that Wi-Fi will directly compete against cellular in areas that have only sparse Wi-Fi coverage. Wi-Fi-only phones have a very limited range, so setting up a covering network would be too expensive. Additionally, cellular technology allows the user to travel while connected, bouncing the connection from tower to tower (or "cells") as proximity changes, all the while maintaining one solid connection to the user. Many current Wi-Fi devices and drivers do not support roaming yet and connect to only one access point at a time. In this case, once you are out of range of one "hotspot", the connection will drop and will need to be re-connected to the next one each time.

For these reasons, Wi-Fi phones are still best suited for local use such as corporate or home networks. However, devices capable of multiple standards, called converged devices, (using SIP or UMA) may well compete in the market. Top-tier handset manufacturers have announced converged dual-radio handsets. Converged handsets present several compelling advantages to mobile carriers:

• Efficient spectrum allocation, as more data-intensive services come online and bandwidth demands increase
• Improved in-building coverage in markets such as the US, where dropped calls
are still a major cause of customer dissatisfaction
• Opportunities for mobile operators to offer differentiated pricing and services.

Range Extender

A wireless range extender (or wireless repeater) can increase the range of an existing
wireless network by being strategically placed in locations where a wireless signal is sufficiently strong and near by locations that have poor to no signal strength. An example location would be at the corner of an L shaped corridor, where the access point is at the end of one leg and a strong signal is desired at the end of the other leg. Another example would be 75% of the way between the access point and the edge of its useable signal. This would effectively increase the range by 75%.

Wireless ethernet Bridge

A wireless ethernet bridge connects a wired network to a wireless network. This is different from an access point in the sense that an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes.

WiFi Devices

Wireless Access Point (WAP)

A wireless access point (AP) connects a group of wireless stations to an adjacent wired
local area network (LAN). An access point is similar to an ethernet hub, but instead of
relaying LAN data only to other LAN stations, an access point can relay wireless data to
all other compatible wireless devices as well as to a single (usually) connected LAN
device, in most cases an ethernet hub or switch, allowing wireless devices to
communicate with any other device on the LAN.



WiFi works

The typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, "Network name") via packets that are called beacons, which are broadcast every 100 ms. The beacons are transmitted at 1 Mbit/s, and are of relatively short duration and therefore do not have a significant influence on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client who receives the beacon can communicate at least 1 Mbit/s. Based on the settings (e.g. the SSID), the client may decide whether to connect to an AP. Also the firmware running on the client Wi-Fi card is of influence. Say two APs of the same SSID are in range of the client, the firmware may decide based on signal strength to which of the two APs it will connect. The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than the other. Since Wi-Fi transmits in the air, it has the same properties as a non-switched ethernet network. Even collisions can therefore appear like in non-switched ethernet LAN's.

Channels


Except for 802.11a, which operates at 5 GHz, Wi-Fi uses the spectrum near 2.4 GHz, which is standardized and unlicensed by international agreement, although the exact frequency allocations vary slightly in different parts of the world, as does maximum permitted power. However, channel numbers are standardized by frequency throughout the world, so authorized frequencies can be identified by channel numbers.

The frequencies for 802.11 b/g span 2.400 GHz to 2.487 GHz. Each channel is 22 MHz wide and 5 MHz spacers between the channels are required. With the required spacers, only 3 channels (1,6, and 11) can be used simultaneously without interference.

Wifi Origin

Despite the similarity between the terms "Wi-Fi" and "Hi-Fi", statements reportedly [3] made by Phil Belanger of the Wi-Fi Alliance contradict the popular conclusion that "Wi- Fi" stands for "Wireless Fidelity".

According to Mr. Belanger, the Interbrand Corporation developed the brand "Wi-Fi" for the Wi-Fi Alliance to use to describe WLAN products that are based on the IEEE 802.11 standards. In Mr. Belanger's words, "Wi-Fi and the yin yang style logo were invented by Interbrand. We (the founding members of the Wireless Ethernet Compatibility Alliance, now called the Wi-Fi Alliance) hired Interbrand to come up with the name and logo that we could use for our interoperability seal and marketing efforts. We needed something that was a little catchier than 'IEEE 802.11b Direct Sequence'."

The Wi-Fi Alliance themselves invoked the term "Wireless Fidelity" with the marketing of a tag line, "The Standard for Wireless Fidelity," but later removed the tag from their marketing. The Wi-Fi Alliance now seems to discourage propagation of the notion that "Wi-Fi" stands for "Wireless Fidelity" but includes it in their knowledge base:

To understand the value of Wi-Fi Certification, you need to know that Wi-Fi is short for "Wireless Fidelity," and it is the popular name for 802.11-based technologies that have passed Wi-FI certification testing. This includes IEEE 802.11a, 802.11b, 802.11g and upcoming 802.11n technologies.

WiFi History

Wi-Fi uses both single carrier direct-sequence spread spectrum radio technology, part of the larger family of spread spectrum systems and multi-carrier OFDM (Orthogonal Frequency Division Multiplexing) radio technology. Unlicensed spread spectrum was first authorized by the Federal Communications Commission in 1985 and these FCC regulations were later copied with some changes in many other countries enabling use of this technology in all major countries. These regulations then enabled the development of Wi-Fi, its onetime competitor HomeRF, and Bluetooth.

The precursor to Wi-Fi was invented in 1991 by NCR Corporation/AT&T (later Lucent & Agere Systems) in Nieuwegein, the Netherlands. It was initially intended for cashier systems; the first wireless products were brought on the market under the name Wave LAN with speeds of 1 Mbit/s to 2 Mbit/s. Vic Hayes, who was the primary inventor of Wi-Fi and has been named the 'father of Wi-Fi,' was involved in designing standards such as IEEE 802.11b, 802.11a and 802.11g. In 2003, Vic retired from Agere Systems. Agere Systems suffered from strong competition in the market even though their products were high quality, as many opted for cheaper Wi-Fi solutions. Agere's 802.11a/b/g all-inone chipset (code named: WARP) never made it to market, and Agere Systems decided to quit the Wi-Fi market in late 2004.

About Wifi

Wi-Fi, also, WiFi, Wi-fi or wifi, is a brand originally licensed by the Wi-Fi Alliance to describe the underlying technology of wireless local area networks (WLAN) based on the IEEE 802.11 specifications.

Wi-Fi was developed to be used for mobile computing devices, such as laptops, in LANs, but is now increasingly used for more applications, including Internet and VoIP phone access, gaming, and basic connectivity of consumer electronics such as televisions and DVD players, or digital cameras. There are even more standards in development that will allow Wi-Fi to be used by cars in highways in support of an Intelligent Transportation System to increase safety, gather statistics, and enable mobile commerce IEEE 802.11p.

A person with a Wi-Fi device, such as a computer, telephone, or personal digital assistant (PDA) can connect to the Internet when in proximity of an access point. The region covered by one or several access points is called a hotspot. Hotspots can range from a single room to many square miles of overlapping hotspots. Wi-Fi can also be used to create a Wireless mesh network. Both architectures are used in Wireless community network, municipal wireless networks like Wireless Philadelphia [1], and metro-scale networks like M-Taipei [2].

Wi-Fi also allows connectivity in peer-to-peer mode, which enables devices to connect directly with each other. This connectivity mode is useful in consumer electronics and gaming applications.

When the technology was first commercialized there were many problems because consumers could not be sure that products from different vendors would work together. The Wi-Fi Alliance began as a community to solve this issue so as to address the needs of the end user and allow the technology to mature. The Alliance created another brand "Wi-Fi CERTIFIED" to denote products are interoperable with other products displaying the "Wi-Fi CERTIFIED" brand.

User Base

In the future there will be other VLAN solutions. One promissing solution, that many poeple wait for, is user based VLAN. When a user logs on a particular host, the user identity is anylized by the switch and then the host becomes a part of a particular VLAN. For example, the user Bob logs on the host P in the picture with his own user ID and password. The switch A decides that Bob belongs to VLAN 1.

Protocol based


Protocol based VLAN means that a host belongs to a particular VLAN based on which
protocol it uses for communication. For example, the host P in the picture is a Netware
client which normally uses IPX protocol, which means that it belongs to IPX VLAN.

Mac based

Mac based VLAN means that a host belongs to a particular VLAN based on which MAC
address the host has. MAC based VLAN is independent of which physical switch port the
host is connected to. For example, the host P in the picture has the MAC address 00-10-4-
B-62-1E-A4, which means that host P belongs to VLAN 1, as can be seen in the left table.
As you can see the same MAC address of the host P is also in the table for switch B. This
means that if we connect host P to any port of switch B, the host P will still belong to
VLAN 1.

Port based

There are a number of different sollutions to create VLAN. Port based solution means that a
host belongs to a particular VLAN based on which physical port in the switch the host is
connected to. For example, the host P in the picture is connected to port 4 of switch A,
which means that host P belongs to VLAN 1, as can be seen in the left table.

VLAN


A switch makes it possible to configure something called VLAN. A VLAN, which stands for Virtual Local Area Network, is a logical LAN consisting of a group of hosts. One physical LAN can be divided into several VLANs. A VLAN can be configured by one or several switches, which makes it possible to be geographically distributed but having a logical presence. Users of the same VLAN can communicate with each other at LAN speeds and with no router latency.

There are different solutions for communication between VLANs, but the most common way is to use a router. The router is sometimes integrated in the switch.

LAN Switching

Fast Forward
Fast forward or cut-through switching is the fastest way of forwarding packets thorough a
switch. The switch forwards the packets as soon as the switch is able to determine the
destination MAC address. Although this generally reduces network latency, fast forward
switching doesn't verify the checksum and consequently allows bad packets to pass, which
can reduce the available bandwidth. In fast forward switching the sending direction is never
established which means that two hosts can send to each other simultaneously which will
lead to a collision.

Store and Forward
In Store and forward switching the switch waits until the entire packet is received before
sending it to the destination. This lets the switch verify the packet's checksum and eliminate
the possibility of forwarding bad packets. While the packet is stored in the buffer of the
switch, the transmission direction is established, which means that no collisions can occur.
A disadvantage with store and forward switching is that a delay occurs because the switch
needs time to buffer and analyze the packet.

Fragment Free
The fragment free switch works just like fast forward, but it buffers 64 bytes of every
packet in order to avoid collisions.

LAN Components

A hub is a commonly used device for connecting hosts to each other, using bus or ring topology. Each host is attached to a hub via a port. When a hub receives a signal on one port it transmits that signal to all other ports. Many hubs also regenerate and amplify weak signals before re-transmitting them.

A switch is a multiport device that handles routing between different hosts based on their MAC addresses. A switch ”learns” MAC addresses from the hosts that are connected to the switch, and stores them in an internal table. When two hosts communicate with each other, the switch creates a temporary connection path between them. This means that only two hosts will hear each other and not like the hub where everyone hears everything. For example if host A and host B have a conversation with each other, then host C and D can also communicate at the same time without any disturbance from host A or B. There is also possibility for one host to broadcast, which means that the packets will be transmitted on all ports in the switch.

Switches improve the performance of a LAN in two ways. First they increase the available bandwidth for each host, since the collisions are avoided.The second improvement is the security. A user on a host connected via a hub, can by using a sniffer software, hear other conversations. This is not possible in a switched network.

Saturday, August 29, 2009

ATM


ATM, which stands for Asynchronous Transfer Mode is a ”de facto standard” developed
by the ATM Forum and is a switching method of communication, which can be used in both
LANs and WANs.

ATM specifications are being written to ensure that ATM smoothly integrates numerous
existing network technologies.

Today, in many instances, separate networks are used to carry voice, data and video
information, mostly because these traffic types have different characteristics. For instance,
data traffic tends to be "bursty" while voice and video tend to be more "continuous".
With ATM, separate networks will not be required. ATM is the only technology which
from the beginning, was designed to accommodate the simultaneous transmission of data,
voice and video.

ATM is available at various speeds but the most commonly used are 25, 155 and 622 Mbps.

Fiber Data


FDDI stands for Fiber Distributed Data Interface. FDDI standard was developed by ANSI,
the American National Standards Institute. It is based on the use of double optical fiber cable
and provides for a token-passing ring configuration, operating at 100 Mbps.
FDDI is being developed to deal with the requirements of high-speed LANs, MANs and
backbone networks. Since FDDI consists of two fiber rings, primary and secondary ring,
there is good redundancy and high availability. Normally traffic only flows on the primary
ring, but if the primary ring is broken then the secondary ring is used.

Token Ring

Token Ring is mainly used to connect equipment from IBM and Novell.
In this picture you have two environments, IBM and Novell, co-existing on a single Token
Ring. Although these two environments cannot communicate with each other in this
configuration, they can still use the same Token Ring.

Token Ring Novell

A normal way for an organization to go from mainframes to more modern computers is to
use the existing Token Ring network but to change the earlier IBM devices to personal
computers.

Novell was one of the first to see this market and they are using Token Ring to connect their
servers and clients together.

In the picture you can see a typical configuration with different types of personal computers
working as Novell clients and servers.

IBM implementation

In this picture we see an IBM implementation of Token Ring. An IBM mainframe 3090
cannot directly communicate with the Token Ring. To do that it needs an NCP which is a
dedicated computer that only handles the communication between the mainframe and the
Token Ring network.

The users sitting on terminals can access the data from the mainframe through a terminal
server. There can be several thousand terminals connected to a mainframe.

Another possibility is to use mini computers such as AS/400. These mini computers can be
accessed by directly connected terminals as in the picture, or from a terminal server.

Token ring Network

Token Ring was introduced by IBM in 1987 and became their main architecture. The
standard for Token Ring from IEEE came in 1989.

Token Ring is physical star and logical ring topology. This means that you connect the
computers physically in a star configuration to the hub, but the computers still pass the
access rights with help of a token in a ring.

The bandwidths used in Token Ring are 4 or 16 Mbps.

Ethernet

Ethernet can be used to connect equipment from different vendors. Different protocols can
also be used at the same time on Ethernet. For example Novell’s IPX/SPX can be used
together with TCP/IP. Almost all modern computers, printers and network components can
connect to Ethernet.

In this picture you have three environments, Novell, SUN and Digital, co-existing at the
same time on a single Ethernet. Although these three environments cannot communicate with
each other in this configuration, they can still use the same Ethernet.

Sun microsystems

Sun microsystems was one of the earliest manufacturers of UNIX workstations. Sun had an
early vision that ”The network is the computer”. SUN is using Ethernet and TCP/IP as a
strategic platform. Since every UNIX workstation and UNIX server comes with an Ethernet
card and TCP/IP software, it is ready for direct connection to the network.
For the PC market, SUN has developed PCNFS software, so that a PC can communicate
with SUN equipment.

Ethernet Digital


Ethernet, as defined in IEEE 802.3 standard, can use both star and bus topology with bandwidths between 10 and 100 Mbps. Ethernet is today the most common technique used
in Local Area Networks.

Digital uses Ethernet for communication between their products. This picture represents an
early implementation by Digital. You can see that Vax computers can be accessed by VT220
terminals, through a terminal server.

LAN Technologies

This diagram shows what has happened to the development of the two most used LAN technologies today, Ethernet and Token ring. 10 Megabits Ethernet exists in two versions. Version two as specified by Digital. Intel and Xerox, is the most commonly used version and IEEE standard 802.3 which is not so commonly used. These two versions are not compatible, because the frame format differs.

Fast Ethernet, which is specified in IEEE 802.3u, offers 100 Mbps. Fast Ethernet is a modern version of Ethernet and is often used in LAN backbone networks today (that is 1999), but is still not so commonly used for clients.

Gigabit Ethernet over fiber, is specified in IEEE 802.3z, offers 1000 Mbps. Gigabit Ethernet is not so common today (that is 1999). Gigabit Ethernet is only used in LAN backbone networks because it is expensive and there is not any need today for so high bandwidth to clients.


Gigabit Ethernet over twisted pair cable, is specified in IEEE 802.3ab, offers 1000 Mbps. This standard is not fully specified today, that is in the spring 1999. Gigabit Ethernet is the future of LAN development, because Ethernet is simple, reliable and will become cheap.

Token ring as specified in IEEE 802.5 offers 4 and 16 Mbps. The use of token ring technology is diminishing even though a new standard, called high speed token ring, offering 100 Mbps, has been specified.

CSMA/CD


CSMA/CD stands for "Carrier Sense Multiple Access with Collision Detect”. CSMA/CD
is a random control access method.

The CSMA/CD access method is used as the access control method in Ethernet and is defined in a standard from IEEE. The CSMA/CD algorithm is quite simple and the efficiency for an ordinary Ethernet is about 65%. This means that the effective bandwidth for a 10 Mbps Ethernet is about 6.5 Mbps. The rest is lost, mainly due to collisions.


Before one host will transmit it must ”listen” on the medium whether or not another host is
transmitting. If the medium is ”quiet” the host can send its data. The term "Carrier Sense" indicates that a host listens before it transmits.

"Multiple Access" means that many hosts can be connected to the network and all hosts
have the same right to transmit.

With CSMA/CD, it occasionally happens that two hosts send their packets at the same
time. This will make a collision on the network. The information about the collision is
detected by all the other hosts on the network. This is called "Collision Detect". If a host
detects a collision it will wait a random period of time before it tries to transmit again.

Access Methods


A characteristic common to all Local Area Networks is that multiple hosts have to share access to a single physical transmission medium. Several methods can be employed to control the sharing of access to the transmission medium. The various access control methods can be characterized by where in the network the transmission control function is performed. An access method can use following forms of transmission control:


1. Random control
With random control any host can transmit and permission is not required. A host may
check the medium to see if it is free before beginning to transmit.

2. Distributed control
With distributed control only one host at a time has the right to transmit and that right is
passed from host to host. This is usually done by passing on a small piece of data called a
token. The host that has the token, is the one that has the right to transmit.

Tag

Assignment Lanka Tag Cloud
Computer Networks The History of Local Area Networks, LAN, The Topologies of a Networks, LANs describe different types of transmission Medias, Local Area Networks Access Methods, Carrier Sense Multiple Access with Collision Detect, Development of LAN Technologies. LAN -Token Ring, LAN Ethernet Digital, LAN - Ethernet Sun microsystems, LAN - Ethernet Mixed Environment, LAN - Token Ring was introduced by IBM LAN - IBM implementation of Token Ring, Token Ring Novell, LAN Token Ring - in a mixed environment, LAN - Fiber Distributed Data Interface, LAN - ATM, LAN Components, LAN Switching Methods, Virtual Local Area Network, Port based VLAN, Mac based VLAN, Protocol based VLAN, User Base VLAN, PC networks Components, PC networks Shared resources, PC Network operating systems, PC networks Novell Netware, PC networks Windows NT, PC networks IBM LAN Server Computer Programming Languages HTML Language, The Generations of Programming Languages, Different types of High Level Languages, Different types of High Level Languages Disadvantages
Computer Networks - IBM LAN Server, Windows NT Networks, Novell Netware, Network operating systems, Networks Shared, Networks Components, User Base, Protocol based, Mac based, Port based, VLAN, LAN Switching, LAN Components, ATM, Fiber Data, Token Ring, Token Ring Novell, IBM implementation, Ethernet, Sun microsystems, Ethernet Digital, Token passing, LAN Technologies, CSMA/CD, Access Methods, Transmission, Networks, The History of Local Area Networks, LAN