India starts blocking file storage websites in a move against piracy

India starts blocking file storage websites in a move against piracy

File storage websites such as Megaupload, Putlocker, Rapidshare are being blocked in India.

A thread started on a popular online forum focused on developments in Indian broadband has furious and surprised users sharing a list of websites blocked by their Internet service providers. Going through the thread and my Twitter interactions, India’s Department of Telecommunication is behind the block.

Some users say they are not able to browse the file uploading websites, others say they can access and download content. A screenshot shared by Sunny Chahal on Twitter confirms that the block is in effect. Some other users are seeing the same notice. The error message:

Users of popular Internet Service Providers MTNL, BSNL and Airtel have confirmed seeing the same error while visiting some or all of the following websites:

Megaupload.com
Mediafire.com
Megavideo.com
VideoBB.com
Novamov.com
Movshare.net
Rapidshare.com
Putlocker.com
Hotfile.com
Fileserve.com
Filesonic.com
Filesonic.in
Depositfiles.com
Wupload.com

The decision behind banning the websites looks like piracy. Most of these websites are widely used to share content production houses and record labels would want users to pay for. Users are comparing the ban to Internet rules in China. The Department of Homeland Security in the US also made definitive moves against piracy by seizing domains of websites hosting pirated content.

As of now there is no official word from the DoT, the story will be updated as we get more information.

Update: Reliance BIG Pictures obtained a court order to get the sites banned. Users are reporting that most of these websites are now working.

Boost Weak Cell Service at Home: Your Options

Boost Weak Cell Service at Home: Your Options

Even the best wireless networks often can't deliver a consistent signal inside your house. Femtocells and repeaters will usually help, but they'll cost you.

The problem is familiar to many cell phone users: When you're indoors, even within your carrier's coverage area, the network's signal sometimes simply isn't strong enough to support voice calls or data services. With so many consumers ditching their landlines for cell phones, poor reception at home is no longer acceptable. Fortunately, a slew of products now address the problem, though they don't come cheap.

Ssignal-boosting products generally fall into one of two categories: femtocells or signal repeaters. Both types of equipment can deliver strong signals within buildings, but they work in fundamentally different ways.

Femtocells act as miniature cell phone towers that connect to a home network router and use your wired broadband connection to move voice calls and data services to and from your carrier's network. Femtocells don't rely on the carrier's towers at all, but because they use technology that can direct data and calls back to the carrier networks over the Internet, most of them are sold by the carriers themselves.

Repeaters and boosters, on the other hand, amplify and rebroadcast cell tower signals. Since they don't require carrier support, they tend to be manufactured and sold by third parties such as Wi-Ex (under the brand name ZBoost) and Wilson Electronics.
Understanding Femtocells

While repeaters have been around for several years, femtocells are relative newcomers to the mobile scene. Of the four major U.S. carriers, two--Sprint and Verizon--have been selling femtocells (under the brand names Airave and Network Extender, respectively) for the past few years; AT&T introduced its 3G MicroCell femtocell last year.

The first Sprint and Verizon Wireless femtocells supported only voice calls, but current models support high-speed data as well as voice. Sprint charges $130 for its Airave access point (with a two-year service plan); there's no additional charge for using Airave to enhance coverage with an existing plan, but Sprint does offer a $25-a-month unlimited calling plan for Airave calls--which might appeal to anyone who is considering dropping landline voice service. Current Airave access points support EvDO Rev. A 3G data speeds and up to six simultaneous calls.

Verizon Wireless charges $250 for its Network Extender, which also supports 3G data, and the carrier imposes no monthly service fee. AT&T sells the MicroCell for $150, also with no mandatory service fee; it also offer an optional $20-a-month unlimited calling plan, plus price breaks for customers who sign up for the company's DSL service.

The Beauty of the Femtocell

AT&T's 3G MicroCell, made by Cisco.

Femtocells appeal to consumers and carriers on several levels. Consumers like them because they provide immediate five-bar coverage across an area of up to 2500 square feet, regardless of how far you are from a cell phone tower. A strong signal typically means better voice quality, fewer dropped calls, and faster data speeds. Good signals also improve a phone's battery life, since its radios don't have to work as hard.

Femtocells are low-power wireless access points that operate within a licensed spectrum to connect standard mobile devices to a mobile operator’s network via residential DSL or cable broadband connections.

Another benefit: You can configure the femtocell to recognize and prioritize the phones you authorize, so neighbors can't mooch off your service at your expense. Interestingly, Verizon Wireless's FAQ for its Network Extender says that, though you can prioritize up to 50 Verizon Wireless numbers, the device will remain available to all Verizon Wireless subscribers within range when priority numbers aren't using it.

This brings us to a big reason why wireless providers like femtocells: They offload some of the expense that the carriers might otherwise incur in order to expand network capacity and/or coverage. Femtocells also help retain customers in locations where carriers can't significantly improve reception on their own (such as basements, suburbs that don't want new cell towers, locales with difficult topography such as steep hills and valleys, and skyscrapers with thick walls). In fact, forum posts indicate that Sprint has occasionally given customers Airave units to keep them from switching to another carrier.


Femtocell Weaknesses

Price aside, femtocells have some limitations. For one thing, you can use a femtocell only at the address where it is registered. To enforce that restriction, the FCC stipulates that all femtocells must be equipped with GPS receivers, and that they either must be placed close enough to a window to receive GPS signals directly or must be connected to a GPS antenna that can pick up the signals.

David Nowicki, marketing vice president for femtocell vendor Airvana and for the Femto Forum trade association, says that the FCC imposed the location restriction to enable carriers to track the femtocell's position and disable its operation in areas where the carrier doesn't own the frequencies that the femtocell supports and where, consequently, using the femtocell might interfere with another carrier's network. Even so, forum posts suggest that femtocells sometimes hurt neighbors' regular network reception when deployed in close proximity.

Then again, it’s not necessarily a bad thing that femtocells are aware of their location. That, coupled with the device's ability to recognize when an authorized phone is nearby, could enable carriers to offer customers additional services that go beyond improving reception, including 911 services.

Network handoff issues are another concern. You can start a call on femtocell service and then hand it off to the regular network as you leave the femtocell's coverage area (assuming that the nearest cell tower is close enough). But calls initiated on the regular network can't be handed off to a femtocell.

And finally, critics complain about having to pay for a service that essentially deliver what the wireless network is supposed to have provided all along. "It's a fair point," Nowicki concedes. But carrier spokespeople say that no carrier can provide coverage in all locations of its service area. Verizon Wireless spokesperson Thomas Pica describes the Network Extender as a "niche product," given that the vast majority of Verizon customers don't need one because of the carrier's robust network.

What about the broadband ISP that now gets to carry the wireless carrier's traffic? Nowicki says that the bandwidth consumption of mobile devices is tiny compared to that of most home and office networks. For that reason, femtocells don't impose a significant burden on the ISP, which in some cases is run by the same company as the mobile carrier anyway.
T-Mobile’s Approach

T-Mobile has taken a different approach to improving weak cell signals. Phones that support Wi-Fi Calling use UMA (Unified Mobile Access) technology to route calls over Wi-Fi hotspots. T-Mobile notes that this helps with calls made through any hotspot that its phones can access, and not just at a single location (which is the case with femtocells). However, T-Mobile's Android phones can't hand off calls made using Wi-Fi hotspots to T-Mobile's cellular network; other phones that support Wi-Fi Calling can. T-Mobile has no announced plans to introduce femtocells, but a spokesperson says that the carrier continues to review customer equipment options for improving coverage.


Repeaters and Boosters

Repeaters (also called "boosters")--the other class of products designed to improve poor cell reception--depend on at least a weak cell tower signal to amplify and rebroadcast. They work with specific frequencies, independent of carrier; some support more than one frequency. On the frequencies they support, repeaters improve both voice calls and data speeds.




Many repeaters have multiple parts, starting with an antenna that you place as close as possible to the strongest cell tower signal--typically, near a window or even outdoors. The antenna transmits signals over a cable connected to an amplifier, which boosts the signal and retransmits it indoors. In some cases, the amplifier and indoor antenna are integrated; in others, the amplifier hooks up to a separate, centrally located indoor antenna in the home or office. The area of coverage depends on the strength of the amplifier and of the signal being amplified.

Unlike femtocells, repeaters can't be configured to recognize specific phones. Consequently, the amplified signals benefit any mobile device within range that operates on the supported frequencies--though obviously the owner's devices are more likely to benefit than a neighbor's devices located farther away from the indoor antenna. It's up to you (with the help of your reseller) to ensure that your repeater kit amplifies the frequency or frequencies that your carrier uses in your area. You can get help in choosing the right repeater kit from specialty retailers such as wpsantennas.com or RepeaterStore.
Repeaters Don’t Come Cheap

Repeater kits can be more expensive than femtocells. Pricing depends on how large an area you want to cover. Wi-Ex's ZBoost line includes SoHo products that range in price from $169 to $399, and you can spend up to $600 for kits that cover areas larger than 2500 feet--the maximum handled by femtocells. On the other hand, Wi-Ex marketing vice president Sharon Cupett notes, there are no monthly fees for repeaters--and of course, no broadband service is involved.

Though repeaters rebroadcast over carrier-owned frequencies, they are legal if they don't interfere with cell tower signals (which is why repeater signals have limited strength). One way to avoid problems is to look for FCC-certified repeaters; some are still awaiting approval, however. Check with the reseller.
Repeater vs. Femtocell

Should you get a repeater or a femtocell? Generally, femtocells are a good match for cell phone customers with broadband service who don't want to pay $300 or more for equivalent repeater coverage and who want to avoid many of the antenna-positioning hassles that repeaters entail. You do need a GPS signal, but those are available pretty much everywhere. Cell tower sites with signals that repeaters can work with are probably less common. But if your carrier doesn't offer a femtocell, a repeater may be the only way to improve your indoor coverage.

Airtel 3G Tariff Plans Explained

Airtel 3G Tariff Plans Explained

Airtel finally launched their 3G services yesterday after months of waiting. While the Airtel representatives did explain the company’s 3G tariff plans in brief at the press conference, we received another communication from Airtel that shed new light on the tariffs.

So here are Airtel’s 3G tariff plans explained with the entire table to conclude the article:

1.Default Plan: By default if you have enabled 3G services on your handset, you will continue with the same charges, data limit and validity as you had when you were using GPRS services.

2.Sachet Plan (Rs.9): Opting for the Rs. 9 sachet plan will give you a data limit of 10MB and validity till midnight of the same day. Going over the data limit will cost you 30p/20kb.

3.Sachet Plan (Rs. 45): The Rs. 45 sachet plan is time-based and will last you 30 minutes of usage.

4.Sachet Plan (Rs. 60): This plan has a data limit of 65MB with a validity of 3 days. After exceeding data limit usage you will be charged at the same rate per kb as the default plan.

Standard Plans

In all Standard Plans, after crossing the data limit, your extra charges will revert back to the default charges (30p/20KB or Rs. 15/Mb) and you get 30 days of validity.

1.Standard Plan (Rs. 103); data limit- 100MB.

2.Standard Plan (Rs. 200); data limit- 250MB

3.Standard Plan (Rs. 450); data limit- 600MB

4.Standard Plan (Rs. 750); data limit- 2GB

The final plan is what Airtel refers to as the FlexiShield Plan, meant for people who don’t want to pay more than Rs. 2,000 per month. In this plan, you pay Rs. 675 for a month and get a data limit of 1.25GB. After crossing this data limit you will be charged 1p for every 100kb you use (or 80p for every 1MB of usage) till your bill value reaches Rs. 2,000. After that point you will still be able to use Internet on your mobile device but your speed will sharply drop to 20kbps.


So what does it mean for me?

(Note: Although, I’ve tried my level best to keep my calculations accurate, do keep in mind that all figures and calculations are approximations at best). Although it might not hit you at first glance, keep in mind that in any plan apart from the FlexiShield plan will not cap your bill amount. So you will have to keep an eye out on how much data you are using. I consider my mobile Internet usage slightly above average with about 50 e-mails (approximately) received and sent, about 25 social media posts, some app downloads among others daily. The Airtel calculator estimated my monthly Internet usage to be 2.06GB. So (ignoring the sachet plans) let’s take a look at what you would have to pay if you opt for the fourth Standard Plan or the FlexiShield Plan if your mobile Internet usage is similar to mine:



1.Standard Plan (Rs. 750): Total monthly bill- Rs. 1,650. I had to recheck my calculations twice because I couldn’t believe it the first time. You are paying Rs. 750 for the first 2GB and then you are paying an extra Rs. 900 for the additional 60MB! This looks like all sorts of wrong. I still can’t believe my calculations but if I were you, I certainly wouldn’t opt for this plan.

2.FlexiShield Plan: If I average out my monthly usage on a daily basis, it would give me a rough estimate of 69MB of data use every day. In this plan I will have used up my data limit within 18 days. My monthly bill at the end of the month will then amount to Rs. 1,323 (Rs. 675 for the first 1.25GB and then Rs. 648 for the remaining 810MB of data). If I was on Airtel and opting for 3G, I would opt for this plan.

Now, if you don’t plan to use as much data as me and will be content with about 500MB of data usage every month, let’s see what it means for you (please see the below image to get a very rough idea of what all you will be able to do with 500MB monthly):




1.Standard Plan (Rs. 103): Total monthly bill- Rs. 6,103 (Rs. 103 for the first 100MB and a whopping Rs. 6,000 for the remaining 400MB).

2.Standard Plan (Rs. 200): Total monthly bill- Rs. 3,950 (Rs. 200 for the first 250MB and Rs. 3,750 for the remaining 250MB).

If you opt for any of the rest of the plans, your data usage will be within the data limit and therefore you will only have to pay the minimum amount designated for that plan (for e.g. Rs. 450 for 600MB).

What this means is that unless you want to get a shocker of a bill at the end of the month, you should pick your plan very carefully according what you think your usage will be. Online reports are claiming that an Airtel 3.6Mbps card was offering download speeds of about 300KBps, so it will be quite easy for you to use up your data limits pretty quickly.

Firefox plug-in Linux problems

Firefox plug-in Linux problems

Q I have an AMD64 PC with a 64bit Linux system installed. Everything runs fine except I can’t seem to get the Flash plug-in working in Firefox.
David Fensom

A The problem here is the mixture between 32 and 64bit software. A 64bit system can run 32bit programs in addition to 64bit, but it cannot mix code in the same program – a single application must be entirely 32bit or entirely 64bit.

Plug-ins in your browser extend the browser’s functionality by adding in extra code, as the name suggests.

Your version of Firefox is 64bit, but unfortunately the Flash plug-in from Macromedia is currently only available as 32bit.

Until a 64bit version is released, the easiest way round this is to download the 32bit Firefox binary from Mozilla.org and use this version instead of the 64bit version supplied with your distribution.

Source : PCW.co.uk

Changing names with shared Linux and Windows networks

Changing names with shared Linux and Windows networks

Q On my Linux system I have a few folders shared for Windows networking. I can access these, but the shares show up in ‘mshome’ instead of ‘workgroup’. How can I change this?
Olivia Reynolds

A Distributions usually have configuration tools for setting the domain or workgroup on the Samba server. You can edit the configuration file directly. It’s usually at /etc/samba/smb.conf. Edit the file and look for the line ‘workgroup = mshome’ and make the change there.

Source : PCW.co.uk

Smart dust

Smart dust

Advance in hardware technology and engineering design have led to dramatic reduction in size, power consumption and cost for digital circuiting, wireless communication and micro electro mechanics sensors (MEMS). This has enabled very compact, autonomous and mobile nodes each containing of one or more sensors. This millimeter scale nodes each called as SMARTDUST. It was discovered at Berkeley at University of California by a team led by Prof.Kristofer.S.J. Pister This device is around size of grain of sand and contains sensors, computing ability, bi-directional wireless communication and a power supply.As tiny as dust particles, smart dust motes can be spread throughout buildings or into the atmosphere to collect and monitor data. Thus smart dust as small as a grain of rice is able to sense, think, talk and listen. Smart dust devices have applications in everything from military to meteorological to medical fields.
From light to the vibrations can be recognized using small wireless micro electromechanical sensors (MEMS), which are broadly classified as Smart Dust devices. As innovative ideas in silicon and fabrication followed, the smart dust devices, which carries out the communication, computation, and sensing into an all-in-one package, has been able to reduce its size to that of a sand grain.

These motes would collect the data compute and then pass the information using the two-way band radio between motes at distances approaching 1,000 feet. Some of the uses of these smart dust devices are identifying the manufacturing defects using vibrations, tracking patient movements in hospitals etc.

Open RAN

Open RAN

The vision of the OpenRAN architecture is to design radio access network architecture with the following characteristics:

pen,Flexible,Distributed,Scalable.

Such architecture would be open because it defines open, standardized interfaces at key points that in past architectures were closed and proprietary. It would be flexible because it admits of several implementations, depending on the wired network resources available in the deployment situation. It would be distributed because monolithic network elements in architectures would have been broken down into their respective functional entities, and the functional entities would have been grouped into network elements that can be realized as a distributed system.

The architecture would define an interface with the core network that allows the core network to be designed independently from the RAN, preserving access network independence in the core. Finally, the architecture would not require changes in radio link protocols; in particular, a radio link protocol based on IP would not be necessary.

This document presents the first steps in developing the OpenRAN vision. In its first phase, the subject of this document, the OpenRAN architecture is purely concerned with distributing RAN functions to facilitate achieving open interfaces and flexible deployment. The transport substrate for implementing the architecture is assumed to be IP but no attempts is made to optimize the use of IP protocols, nor are specific interfaces designated as open. The architecture could as well be implemented on top of existing functional architectures that maintain a strict isolation between the transport layer and radio network layer, by splitting an existing radio network layer into control and bearer parts.

In addition, interoperation with existing core and RAN networks is supported via interworking functions. Chapters 7 through 11 in this report are exclusively concerned with this first phase of the architecture, and it is possible that the architecture may change as the actual implementation of the OpenRAN is considered and For Further Study items are resolved

NETWORK CARD

NETWORK CARD

A network card, network adapter, network interface card or NIC is a piece of computer hardware designed to allow computers to communicate over a computer network. It is an OSI model layer 2 item because it has a MAC address.

Every network card has a unique 48-bit serial number called a MAC address, which is written to ROM carried on the card. Every computer on a network must have a card with a unique MAC address. The IEEE is responsible for assigning MAC addresses to the vendors of network interface cards. No two cards ever manufactured should share the same address.

Whereas network cards used to be expansion cards to plug into a computer bus, most newer computers have a network interface built into the motherboard. A separate network card is not required unless multiple interfaces are needed or some other type of network is used.

The card implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as ethernet or token ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP.

There are four techniques used for transfer of data, the NIC may use one or more of these techniques.

• Polling is where the microprocessor examines the status of the peripheral under program control.
• Programmed I/O is where the microprocessor alerts the designated peripheral by applying its address to the system's address bus.
• Interrupt-driven I/O is where the peripheral alerts the microprocessor that it's ready to transfer data.
• DMA is where the intelligent peripheral assumes control of the system bus to access memory directly. This removes load from the CPU but requires a separate processor on the card.

SYNCHRONOUS OPTICAL NETWORKING

SYNCHRONOUS OPTICAL NETWORKING

The synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors. The more recent synchronous digital hierarchy (SDH) standard developed by the International Telecommunication Union (ITU) is built on experience in the development of SONET. It is documented in standard G.707 and its extension G.708. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.

SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is inaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.

OPTICAL FIBER

OPTICAL FIBER

A bundle of optical fibers. Theoretically, using advanced techniques such as DWDM, the modest number of fibers seen here could have sufficient bandwidth to easily carry the sum of all types of current data transmission needs for the entire planet. (~100 terabits per second per fiber )

An optical fiber or fibre is a thin, transparent fiber, usually made of glass or plastic, for transmitting light. Fiber optics is the branch of applied science and engineering concerned with such optical fibers.

Optical fibers are commonly used in telecommunication systems, as well as in illumination, sensors, and imaging optics.

Principle of operation

An optical fiber (American spelling) or fibre (British spelling) is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a denser core surrounded by a cladding layer. For total internal reflection to confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.

A diagram which illustrates the propagation of light through a multi-mode optical fiber.

Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis (see below). In a step-index fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary) are completely reflected. The minimum angle for total internal reflection is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, where they are not useful for conveying light along the fiber. In this way, the minimum angle for total internal reflection determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture makes it easier to efficiently couple a transmitter or receiver to the fiber. However, by allowing light to propagate down the fiber in rays both close to the axis and at various angles, a high numerical aperture also increases the amount of multi-path spreading, or dispersion, that affects light pulses in the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflect abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Fiber with a core diameter narrower than a few wavelengths of the light carried, is analyzed as an electromagnetic structure, by solution of Maxwell's equations, as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along its axis. Fiber supporting only one mode is called single-mode or mono-mode fiber, while fiber that supports more than one mode is called multi-mode fiber. By the waveguide analysis, it is seen that the light energy in the fiber is not completely confined in the core, but, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.

A typical single-mode optical fiber, showing diameters of the component layers.

Optical fiber communication

The optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. Although fibers can be made out of either transparent plastic or glass, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers, single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

The light used is typically infrared light, at wavelengths near to the minimum absorption wavelength of the fiber in use. The fiber absorption is minimal for 1550 nm light and dispersion is minimal at 1310 nm making these the optimal wavelength regions for data transmission. A local minimum of absorption is found near 850 nm, a wavelength for which low cost transmitters and receivers can be designed, and this wavelength is often used for short distance applications. Fibers are generally used in pairs, with one fiber of the pair carrying a signal in each direction.


SYNCHRONOUS OPTICAL NETWORKING

The synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors. The more recent synchronous digital hierarchy (SDH) standard developed by the International Telecommunication Union (ITU) is built on experience in the development of SONET. It is documented in standard G.707 and its extension G.708. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.

SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized across the entire network, made possible by atomic clocks. This synchronization system allows entire inter-country networks to operate synchronously, greatly reducing the amount of buffering required between each element in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is inaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.

Statistical Time-division Multiplexing (STDM)

Statistical Time-division Multiplexing (STDM)

STDM is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. Using STDM allows bandwidth to be split over 1 line. Many college and corporate campuses use this type of TDM to logically distribute bandwidth.

If there is one 10MBit line coming into the building, STDM can be used to provide 178 terminals with a dedicated 56k connection (178 * 56k = 9.96Mb). A more common use however is to only grant the bandwidth when that much is needed. STDM does NOT reserve a time slot for each terminal, rather assign a slot when the terminal is requiring data to be sent/received.

Synchronous Digital Hierarchy (SDH)

Synchronous Digital Hierarchy (SDH)

SDH has become the primary transmission protocol in most PSTN networks. It was developed to allow streams 1.544 Mbit/s and above to be multiplexed, so as to create larger SDH frames known as Synchronous Transport Modules (STM) . The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52 Mbit/s frame . SDH can also multiplex packet based frames such as Ethernet, PPP and ATM .

While SDH is considered to be a transmission protocol (Layer 1 in the OSI Reference Model), it also performs some switching functions, as stated in the third bullet point requirement listed above . The most common SDH Networking functions are as follows:

• SDH Crossconnect – The SDH Crossconnect is the SDH version of a Time-Space-Time crosspoint switch. It connects any channel on any of its inputs to any channel on any of its outputs. The SDH Crossconnect is used in Transit Exchanges, where all inputs and outputs are connected to other exchanges .
• SDH Add-Drop Multiplexer – The SDH Add-Drop Multiplexer (ADM) can add or remove any multiplexed frame down to 1.544Mb. Below this level, standard TDM can be performed. SDH ADMs can also perform the task of an SDH Crossconnect and are used in End Exchanges where the channels from subscribers are connected to the core PSTN network .

SDH Network functions are connected using high-speed Optic Fibre. Optic Fibre uses light pulses to transmit data and is therefore extremely fast . Modern optic fibre transmission makes use of Wavelength Division Multiplexing (WDM) where signals transmitted across the fibre are transmitted at different wavelengths, creating additional channels for transmission . This increases the speed and capacity of the link, which in turn reduces both unit and total costs .

TIME-DIVISION MULTIPLEXING

TIME-DIVISION MULTIPLEXING


Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e., bit stream, by interleaving pulses representing bits from different channels.

In some TDM systems, successive pulses represent bits from successive channels. In other systems different channels take turns using the channels for a group of successive pulse-times (a so-called "time slot") eg voice channels in E1/T1 systems.

What distinguishes coarse time-division multiplexing from packet switching is that the time-slots are pre-allocated to the channels, rather than arbitrated on a per-time slot basis.

Uses of time-division multiplexing:

• The RIFF (WAV) audio standard interleaves channel data on a per-sample basis
• The PDH and SDH network transmission standards
• The GSM telephone system
• The left-right channel splitting in use for Stereoscopic Liquid Crystle shutter glasses
• Multichannel digital audio processing sub-systems, notably the Motorola 56000 based hardware acceleration used in Pro Tools

Time Division Multiplexing (TDM) is the means by which multiple digital signals (or analogue signals carrying digital data) can be carried on a single transmission path by interleaving portions of each signal in time. Interleaving can be done at bits or blocks of bytes. This enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched network. This article consists of two sections, namely, Transmission using TDM and Synchronous Digital Hierarchy (SDH). The first section examines the basic principles underlying TDM, while the second section discusses how SDH is used to switch TDM frames.

TDM was a synchronous technique invented during World War II to encrypt transatlantic radio conversation between Churchill and Roosevelt. By the early '60s, engineers from Bell Labs had developed the first T1 Channel Banks, which combined 24 digitised voice calls over a 4-wire copper trunk between Bell central office analogue switches. A channel bank sliced a 1.544 Mbit/s digital signal into 8,000 separate frames, each composed of 24 contiguous bytes. Each byte represented a single telephone call encoded into a constant bit rate signal of 64 Kbit/s. Channel banks used a byte's fixed position (temporal alignment) in the frame to determine which call it belonged to .


Transmission using Time Division Multiplexing (TDM)

In circuit switched networks such as the Public Switched Telephone Network (PSTN) there exists the need to transmit multiple subscribers’ calls along the same transmission medium . To accomplish this, network designers make use of TDM. TDM allows switches to create channels, also known as tributaries, within a transmission stream . A standard voice signal has a bandwidth of 64 kbit/s, determined using Nyquist’s Sampling Criterion . TDM takes frames of the voice signals and multiplexes them into a TDM frame which runs at a higher bandwidth. So if the TDM frame consists of n voice frames, the bandwidth will be n*64 kbit/s .

Each voice frame in the TDM frame is called a channel or tributary. In European systems, TDM frames contain 30 digital voice frames and in American systems, TDM frames contain 24 digital voice frames . Both of the standards also contain extra space for signalling and synchronisation data .

PUBLIC SWITCHED TELEPHONE NETWORK

PUBLIC SWITCHED TELEPHONE NETWORK

The public switched telephone network (PSTN) is the concentration of the world's public circuit-switched telephone networks, in much the same way that the Internet is the concentration of the world's public IP-based packet-switched networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and now includes mobile as well as fixed telephones.

Architecture and context

The PSTN was the earliest example of traffic engineering to deliver Quality of Service guarantees. A.K. Erlang (1878-1929) is credited with establishing the mathematical foundations of methods required to determine the amount and configuration of equipment and personnel required to deliver a specific level of service.

Only the very oldest and most backward parts of the telephone network still use analogue technology for anything other than the last mile loop to the end user, and in recent years digital services have been increasingly rolled out to end users using services such as DSL, ISDN and Cable Systems.

MODEM

A modem (a portmanteau constructed from modulate and demodulate) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio. Experiments have even been performed in the use of modems over the medium of two cans connected by a string.

The most familiar example of a modem turns the digital '1s and 0s' of a personal computer into sounds that can be transmitted over the telephone lines of Plain Old Telephone Systems (POTS), and once received on the other side, converts those sounds back into 1s and 0s. Modems are generally classified by the amount of data they can send in a given time, normally measured in bits per second, or "bps".

Far more exotic modems are used by Internet users every day, notably cable modems and ADSL modems. In telecommunications, "radio modems" transmit repeating frames of data at very high data rates over microwave radio links. Some microwave modems transmit more than a hundred million bits per second. Optical modems transmit data over optic fibers. Most intercontinental data links now use optic modems transmitting over undersea optical fibers. Optical modems routinely have data rates in excess of a billion (1x109) bits per second.

Dedicated line
In computer networks and telecommunications, a dedicated line is a communications cable dedicated to a specific application, in contrast with a shared resource such as the telephone network or the Internet.
In practice, such services may not be provided by a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guaranteed for more public systems. Such properties add a considerable premium to the price charged.
As more general-purpose systems have improved, dedicated lines have been steadily replaced by intranets and the public Internet, but they are still useful for time-critical, high-bandwidth applications such as video transmission.

Leased line
A leased line is a symmetric telecommunications line connecting two locations together. Unlike traditional PSTN lines they do not have a telephone number, each side of the line being permanently connected to the other. They can be used for telephone, data or Internet services.

WIRED TRANSMISSION

WIRED TRANSMISSION

Wire

A wire is a single, usually cylindrical, elongated strand of drawn metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Standard sizes are determined by various wire gauges. The term wire is also used more loosely to refer to a bundle of such strands, as in 'multistranded wire', which is more correctly termed a cable.

Wire has many uses. It forms the raw material of many important manufacturers, such as the wire-net industry, wire-cloth making and wire-rope spinning, in which it occupies a place analogous to a textile fibre. Wire-cloth of all degrees of strength and fineness of mesh is used for sifting and screening machinery, for draining paper pulp, for window screens, and for many other purposes. Vast quantities of aluminium, copper, nickel and steel wire are employed for telephone and data wires and cables, and as conductors in electric power transmission, and heating. It is in no less demand for fencing, and much is consumed in the construction of suspension bridges, and cages, etc. In the manufacture of stringed musical instruments and scientific instruments wire is again largely used. Among its other sources of consumption it is sufficient to mention pin and hair-pin making, the needle and fish-hook industries, nail, peg and rivet making, and carding machinery; indeed there are few industries into which it does not enter.

DATA TRANSMISSION

DATA TRANSMISSION

Data transmission is the conveyance of any kind of information from one space to another. Historically this could be done by courier, a chain of bonfires or semaphores, and later by Morse code over copper wires.

In recent computer terms, it means sending a stream of bits or bytes from one location to another using any number of technologies, such as copper wire, optical fibre, laser, radio, infra-red light or even a so called Bluetooth. Practical examples include moving data from one storage device to another and accessing a website, which involves data transfer from web servers to a user's browser.

A related concept to data transmission is the data transmission protocol used to make the data transfer legible. Current protocols favour packet based communication.

Types of data transmission

Serial Transmission bits are sent over a single wire individually. Whilst only one bit is sent at a time high transfer are possible. This can be used over longer distances as a check digit or Parity bit can be sent along it easily.

Parallel Transmission Multiple wires are used and transmit bits simultaneously and is much faster than Serial transmission as one byte can be sent rather than one bit. This method is used internally within the computer, for example the internal buses, and sometimes externally for such things as printers, however this method of transmission is only available over short distances as the signal will degrade and become unreadable, as there is more interferance between many wires than between one, so it degrades.

Speed of transmission

The speed of data transmission is governed by the Baud rate and bit rate. In a summary the bit rate is the rate of data transmission serially, measured in bits per second. The baud and bit rates are usually the same at lower speeds, however the baud rate can be lower but still giving a high bit rate. The baud rate is the number of bits that can be encoded into a single signal per unit of time, e.g.: the volts on a line, to increase bit rate you can encode 0,1,2 or 3 volts rather than 0 and 1 volts.

0 = 00
1 = 01
2 = 10
3 = 11
So the baud rate could be 28kbps but the bit rate would be double at 56kbps.

Asynchronous and synchronous data transmission

Asynchronous transmission uses start and stop bits to signify the beginning and end of a transmission. This means that an 8 bit ASCII character would actually be transmitted using 10 bits e.g.: A would become 1 0100 0001 0. The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the following example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognise when the second packet of information is being sent.

Synchronous transmission uses no start and stop bits but instead synchronises transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component. A continual stream of data is then sent between the two nodes. Due to there being no start and stop bits the data transfer rate is quicker although more errors will occur, as the clocks will eventually get out of sync, and the receiving device would have the wrong time that had been agreed in protocol (computing) for sending/receiving data, so some bytes could become corrupted (by losing bits). Ways to get around this problem include re-synchronisation of the clocks and use of check digits to ensure the byte is correctly interpreted and received.

Interfaces

Interfaces

In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below (usually operating-system–specific). For example, Microsoft Windows's Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).

OSI MODEL

OSI MODEL

OSI Model
7 Application layer

6 Presentation layer

5 Session layer

4 Transport layer

3 Network layer

2 Data link layer

1 Physical layer

The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative. It is also called the OSI seven layer model.

The OSI model divides the functions of a protocol into a series of layers. Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. A system that implements protocol behavior consisting of a series of these layers is known as a 'protocol stack' or 'stack'. Protocol stacks can be implemented either in hardware or software, or a mixture of both. Typically, only the lower layers are implemented in hardware, with the higher layers being implemented in software.

This OSI model is roughly adhered to in the computing and networking industry. Its main feature is in the interface between layers which dictates the specifications on how one layer interacts with another. This means that a layer written by one manufacturer can operate with a layer from another (assuming that the specification is interpreted correctly). These specifications are typically known as Requests for Comments or "RFC"s in the TCP/IP community. They are ISO standards in the OSI community.

The OSI reference model is a hierarchical structure of seven layers that defines the requirements for communications between two computers. The model was defined by the International Organization for Standardization in the ISO standard 7498-1. It was conceived to allow interoperability across the various platforms offered by vendors. The model allows all network elements to operate together, regardless of who built them. By the late 1980's, ISO was recommending the implementation of the OSI model as a networking standard.

Description of OSI layers

OSI Model
	Data unit	Layer	Function
Host layers	Data	Application	Network process to application
		Presentation	Data representation and encryption
		Session	Interhost communication
	Segments	Transport	End-to-end connections and reliability
Media layers	Packets	Network	Path determination and logical addressing (IP)
	Frames	Data link	Physical addressing (MAC & LLC)
	Bits	Physical	Media, signal and binary transmission

Layer 7: Application Layer
The Application layer is closest to the end user. It provides a means for the user to access information on the network through an application. This layer is the main interface for the user(s) to interact with the application and therefore the network. Some examples of application layer protocols include Telnet, File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP). A Web Browser is a good example of an entity that works at the application layer.

Layer 6: Presentation Layer
The Presentation layer transforms data to provide a standard interface for the Application layer. MIME encoding, data compression, data encryption and similar manipulation of the presentation is done at this layer to present the data as a service or protocol developer sees fit. Examples: converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML.

Layer 5: Session Layer
The Session layer controls the dialogues (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocol suite.

Layer 4: Transport Layer
The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable and cost-effective data transfer. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control . Some protocols are state and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). It is the layer that converts messages into TCP, User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets.

Layer 3: Network Layer
The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform segmentation/desegmentation, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible (there also exist layer 3 (or IP) switches). This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol (IP).

Layer 2: Data Link Layer
The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The addressing scheme is physical which means that the addresses (MAC address) are hard-coded into the network cards at the time of manufacture. The addressing scheme is flat. Note: The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks. On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer.
This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes. However, there's a reasonable argument to be made that these really belong at "layer 2.5" rather than strictly at layer 2.

Layer 1: Physical Layer
The Physical layer defines all the electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are:
• Establishment and termination of a connection to a communications medium.
• Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
• Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and fiber optic) or over a radio link.