Computer Networking

1. De-Facto

A format, language, or protocol that has become a standard not because it has been approved by a standards organization but because it is widely used and recognized by the industry as being standard. Some examples of de facto standards include:

· Hayes command set for controlling modems

· Kermit Communications Protocol

· Xmodem Communications Protocol

· Hewlett-Packard Printer Control Language (PCL) for laser printers

· PostScript page description language for laser printers

· The QWERTY system for typewriters and keyboards

1. Immunity to Electromagnetic Interference

Although fiber optics can solve data communications problems, they are not needed everywhere. Most computer data goes over ordinary wires. Most data is sent over short distances at low speed. In ordinary environments, it is not practical to use fiber optics to transmit data between personal computers and printers as it's too costly. Electromagnetic Interference is a common type of noise that originates with one of the basic properties of electromagnetism. Magnetic field lines generate an electrical current as they cut across conductors. The flow of electrons in a conductor generates a magnetic field that changes with the current flow. Electromagnetic Interference does occur in coaxial cables, since current does cut across the conductor. Fiber optics are immune to this EMI since signals are transmitted as light instead of current. Thus, they can carry signals through places where EMI would block transmission.

2. Data Security

Magnetic fields and current induction work in two ways. They don't just generate noise in signal carrying conductors; they also let the information on the conductor to be leaked out. Fluctuations in the induced magnetic field outside a conductor carry the same information as the current passing through the conductor. Shielding the wire, as in coaxial cables can reduce the problem, but sometimes shielding can allow enough signal leak to allow tapping, which is exactly what we wouldn't want.

There are no radiated magnetic fields around optical fibers; the electromagnetic fields are confined within the fiber. That makes it impossible to tap the signal being transmitted through a fiber without cutting into the fiber. Since fiber optics do not radiate electromagnetic energy, emissions cannot be intercepted and physically tapping the fiber takes great skill to do undetected. Thus, the fiber is the most secure medium available for carrying sensitive data.

3. Non Conductive Cables

Metal cables can encounter other signal transmission problems because of subtle variations in electrical potential. Electronic designers assume that ground is a uniform potential. That is reasonable if ground is a single metal chassis, and it's not too bad if ground is a good conductor that extends through a small building. However, the nominal ground potential can differ by several volts if cables run between different buildings or sometimes even different parts of the same building.

Signal levels in semiconductor circuits are just a few volts, creating a problem known as ground loop. When the difference in ground potential at two ends of a wire gets comparable to the signal level, stray currents begin to cause noise. If the differences grow large enough, they can even damage components. Electric utilities have the biggest problems because their switching stations and power plants may have large potential differences.

A serious concern with outdoor cables in certain computer networks is that they can be hit by lightning, causing destruction to wires and other cables that are involved in the network. Certain computer companies are aware of this problem and trying to solve it by having protective devices for wire circuits to block current and voltage surges.

Any conductive cables can carry power surges or ground loops. Fiber optic cables can be made non-conductive by avoiding metal in their design. These kinds of cables are economical and standard for many indoor applications. Outdoor versions are more expensive since they require special strength members, but they can still be valuable in eliminating ground loops and protecting electronic equipment from surge damage.

4. Eliminating Spark Hazards

In some cases, transmitting signals electrically can be extremely dangerous. Most electric potentials create small sparks. The sparks ordinarily pose no danger, but can be really bad in a chemical plant or oil refinery where the air is contaminated with potentially explosive vapours. One tiny spark can create a big explosion. potential spark hazards seriously hinder data and communication in such facilities. Fiber optic cables do not produce sparks since they do not carry current.

5. Ease Of Installation

Increasing transmission capacity of wire cables generally makes them thicker and more rigid. Such thick cables can be difficult to install in existing buildings where they must go through walls and cable ducts. Fiber cables are easier to install since they are smaller and more flexible. They can also run along the same routes as electric cables without picking up excessive noise.

One way to simplify installation in existing buildings is to run cables through ventilation ducts. However, fire codes require that such plenum cables be made of costly fire retardant materials that emit little smoke. The advantage of fiber types is that they are smaller and hence require less of the costly fire retardant materials. The small size, lightweight and flexibility of fiber optic cables also make them easier to be used in temporary or portable installations.

6. High Bandwidth Over Long Distances

Fiber optics have a large capacity to carry high speed signals over longer distances without repeaters than other types of cables. The information carrying capacity increases with frequency. This however, doesn't mean that optical fiber has infinit bandwidth, but it's certainly greater than coaxial cables. Generally, coaxial cables have a bandwidth parameter of a few MHz/km, where else the fiber optic cable has a bandwidth of 400MHz/km. (These figures are just approximations and do vary from cable to cable!) This is an important factor that leads to the choice of fiber for data communications. Fiber can be added to a wire network so it can reach terminals outside its normal range.
The three main advantages are:

Speed - Fibre-optic cable is capable of transmitting data at a MUCH faster rate than copper cable.

Signal - Copper cable loses signal readily with length, and has to be boosted at regular intervals. The signal in Fibre-optic cable doesn't deteriorate nearly as much over distance and requires less boosting.

Capacity - Copper cable (using analogue signals) can only carry ONE signal at a time. Fibre-optic cable (using digital signals) can carry many data 'streams' as you just encode each stream so it gets to the right destination.

Link refers to the physical path and channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many (n) channels.

The permanent link includes only that portion of the cabling installation that is "permanent" (think punched down). It includes jack to rack -or- wall outlet to patch panel. One connector is allowed at each end but any patch cables must be accounted for and removed from the test results. This generally means you must use the cables that came with your test equipment so the equipment knows exactly what to subtract.

The channel link includes two or more patch cables and may include multiple patch panels - possibly in multiple closets.

By: ISRAFIL CSE

The sublayer below it is Media Access Control (MAC). Sometimes this refers to the sublayer that determines who is allowed to access the media at any one time (usually CSMA/CD). Other times it refers to a frame structure with MAC addresses inside.

There are generally two forms of media access control: distributed and centralized. Both of these may be compared to communication between people. In a network made up of people speaking, i.e. a conversation, we look for clues from our fellow talkers to see if any of them appear to be about to speak. If two people speak at the same time, they will back off and begin a long and elaborate game of saying "no, you first".

The Media Access Control sublayer also determines where one frame of data ends and the next one starts -- frame synchronization. There are four means of frame synchronization: time based, character counting, byte stuffing and bit stuffing.

• The time based approach simply puts a specified amount of time between frames. The major drawback of this is that new gaps can be introduced or old gaps can be lost due to external influences.
• Character counting simply notes the count of remaining characters in the frame's header. This method, however, is easily disturbed if this field gets faulty in some way, thus making it hard to keep up synchronization.
• Byte stuffing precedes the frame with a special byte sequence such as DLE STX and succeeds it with DLE ETX. Appearances of DLE (byte value 0x10) has to be escaped with another DLE. The start and stop marks are detected at the receiver and removed as well as the inserted DLE characters.
• Similarly, bit stuffing replaces these start and end marks with flag consisting of a special bit pattern (e.g. a 0, six 1 bits and a 0). Occurrences of this bit pattern in the data to be transmitted is avoided by inserting a bit. To use the example where the flag is 01111110, a 0 is inserted after 5 consecutive 1's in the data stream. The flags and the inserted 0's are removed at the receiving end. This makes for arbitrary long frames and easy synchronization for the recipient. Note that this stuffed bit is added even if the following data bit is 0, which could not be mistaken for a sync sequence, so that the receiver can unambiguously distinguish stuffed bits from normal bits.

I copied this from someone else's question.

Go to Start > Run.

In the Run window type CMD and click Ok.

In the command prompt enter the command ipconfig your IP address will be listed under your local connection (your Internet connection), this is not always accurate if you use a router, for better results try ipchicken.com
Use this website

http://geotool.flagfox.net/

EDIT: There are two ways on Windows XP and Windows Vista:

XP

1. Start>Run
2. Type cmd
3. Once it has opened with a black window type 'ipconfig' (Without the ' marks)

1. Start> On the little search bar type 'cmd' (Without the ' marks)
2. Once it has opened with a black window type 'ipconfig' (Without the ' marks)