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The Unique Demands Of Networked Video Traffic

packet-lossWhen we talk about business class video networking, we definitely don’t mean Skype and its many cousins. Of course Skype is perfectly usable, most of the time, but its also all too often flaky, drops out unexpectedly or even just refuses to work when its having a bad day. The beauty of Skype is its value for money. Its free therefore VFM = ∞. However this only goes to show that value for money is not everything. It clearly isn’t. When dealing with customers, potential or existing, talking internally within an organisation, perhaps to a room full of people, or even just delivering training videos to the consumer, the requirement begins at the statement that it just needs to work.

Lets look at the reasons why network video traffic is so different.

Networked video generally exists in two flavours. Real time and Non real time. Non real time video is usually stored on a server and is compressed. Depending on the complexity of the compression algorithm, a tradeoff between quality versus transfer speed ensures that the file is transferred at a speed which is (usually) greater than the speed at which the video is being watched. Realtime video however is a very different animal and requires very specific and different network conditions.

Consider exactly what is happening across the network. At a location somewhere on the network video is being encoded into a data stream and fed into the network. That data stream must then cross the network with minimal delay and be reconstituted so that it can be decoded and ultimately viewed on a screen. Delay is the key here although there are other major considerations. Consistent delay can be dealt with albeit is not great on a video conference. The system will buffer the necessary data in order to overcome the delay and from there things pretty much work. When the delay is inconsistent and unpredictable we then see the real challenge. In these conditions, certain parts of the data may be late because the network dropped them and a retransmit was requested. For the most part however the drop simply results in glitches in sound and picture. Artefacts on the screen as the clever video engineers like to euphemistically call them.

video-landing-cros-sell-room-com-222x157-v2-enusSo, next time you’re sitting watching someone failing miserably to conduct an interview from home over their Skype console on the national news. Consider for a moment exactly what is not present in their network connection and conversely, when you’re watching a high quality video conference consider perhaps exactly how good the network in between must be.

Its not all about the network however, important as it undoubtedly is. The quality of the equipment in use at the endpoints of networked video connections play a major part in the overall experience. To take a look at the equipment that we at Rustyice sanction, sell and support click here.

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Spread Spectrum Modulation Techniques

Wireless Local Area Networking technology today exploits a technology which was thitherto mostly hidden inside the shadowy domain of military communications and radar. This technology comprises a collection of ideas which are termed Spread Spectrum Techniques (SST). Spread Spectrum techniques have some powerful properties which make them an excellent candidate for networking applications. To better understand why, we will take a closer look at this fascinating area, and its implications for networking.

Spreading the Spectrum

The first major application of Spread Spectrum Techniques (SST) arose during the mid-sixties, when NASA employed the method to precisely measure the range to deep space probes. In the following years, the US military became a fan of SST due to its ability to withstand jamming (ie intentional interference), and its ability to resist eavesdropping.

Today this technology forms the basis for the ubiquitous Global Positioning System (GPS), the not so ubiquitous NMIDS (Nato Multifunction Information Distribution System/AWACS) datalink (used between aircraft, ships and land vehicles), and last but not least, the virtually undetectable bombing and navigation radar on the bat-winged B-2 bomber. If you ever get asked what technology your home shares with a stealth bomber (excluding astronomical cost), you can state without fear of contradiction that it uses the same class of modulation algorithm.

How is this black magic achieved? The starting point is Claude Shannon’s information theory, a topic beloved by diehard communications engineers. Shannon’s formula for channel capacity is a relationship between achievable bit rate, signal bandwidth and signal to noise ratio.

Shannons theory states that channel capacity is proportional to bandwidth and the logarithm to the base of two of one plus the signal to noise ratio, or:

Capacity = Bandwidth*log2 (1 + SNR).

What this means is that the more bandwidth and the better the signal to noise ratio, the more bits per second you can push through a channel. This is indeed common sense. However, let us consider a situation where the signal is weaker than the noise which is trashing it. Under these conditions this relationship becomes much simpler, and can be approximated by a ratio of Capacity/Bandwidth = 1.44* SNR.

What this says is that we can trade signal to noise ratio for bandwidth, or vice versa. If we can find a way of encoding our data into a large signal bandwidth, then we can get error free transmission under conditions where the noise is much more powerful than the signal we are using. This very simple idea is the secret behind spread spectrum techniques.

Consider the example of a 3 kHz voice signal which we wish to send through a channel with a noise level 100 times as powerful as the signal. Manipulating the preceding equation, we soon find that we require a bandwidth of 208 kHz, which is about 70 times greater than the voice signal we wish to carry. Readers with a knowledge of radio will note here that this idea of spreading is a central part of FM radio and the reason why it produces better sound quality compared to the simpler AM scheme.

Other than punching through large levels of background noise, why would we otherwise consider using spread spectrum techniques ? There are a number of good practical reasons why spread spectrum modulation is technically superior to the intuitively more obvious techniques such as AM and FM, and all of the hybrids which lie in between.

  • The Ability to Selectively Address. If we are clever about how we spread the signal, and use the proper encoding method, then the signal can only be decoded by a receiver which knows the transmitter’s code. Therefore by setting the transmitter’s code, we can target a specific receiver in a group, or vice versa. This is termed Code Division Multiple Access. or CDMA
  • Bandwidth Sharing. If we are clever about selecting our modulation codes, it is entirely feasible to have multiple pairs of receivers and transmitters occupying the same bandwidth. This would be equivalent to having say ten TV channels all operating at the same frequency. In a world where the radio spectrum is being busily carved up for commercial broadcast users, the ability to share bandwidth is a valuable capability.
  • Security from Eavesdropping. If an eavesdropper does not know the modulation code of a spread spectrum transmission, all the eavesdropper will see is random electrical noise rather than something to eavesdrop. If done properly, this can provide almost perfect immunity to interception.
  • Immunity to Interference. If an external radio signal interferes with a spread spectrum transmission, it will be rejected by the demodulation mechanism in a fashion similar to noise. Therefore we return to the starting point of this discussion, which is that spread spectrum methods can provide excellent error rates even with very faint signals.
  • Difficulty in Detection. Because a spread spectrum link puts out much less power per bandwidth than a conventional radio, this means that they can coexist with other more conventional signals without causing catastrophic interference to narrowband links.

These characteristics endeared spread spectrum comms to the military community, who are understandably paranoid about being eavesdropped and jammed. However, the same properties are no less useful for local area networking over radio links. Indeed these are the reasons why the current IEEE draft specification for radio LANs is written around spread spectrum modulations. To better understand the inner workings of this fascinating area, we will now more closely examine the various choices we have for spread spectrum designs. The two basic methods are indeed both used in LAN equipment.

Direct Sequence Systems 

Direct Sequence (DS) methods are the most frequently used spread spectrum technique, and also the conceptually simplest to understand. DS modulation is achieved by modulating the carrier wave with a digital code sequence which has a bit rate much higher than that of the message to be sent. This code sequence is typically a pseudorandom binary code (often termed “pseudo-noise” or PN), specifically chosen for desirable statistical properties. In effect we are transmitting a wideband noise like signal which contains embedded message data. The time period of a single bit in the PN code is termed a chip, and the bit rate of the PN code is termed the chip rate.

A wide range of pseudorandom codes exist which can be applied to this task. These codes should ideally be balanced, with an equal number of ones and zeroes over the length of the sequence (also termed the code run), as well as being cryptographically secure. This is necessary because a spread spectrum system which uses a cryptographically insecure code will still possess the properties previously discussed, but if an eavesdropper can synchronise on to the signal they will eventually be able to crack it and extract the data. Using a secure code prevents this. The mechanics of generating pseudorandom codes is a fascinating area within itself. The most commonly used approach for producing a wide range of code types is the use of a tapped register with feedback as well as a modulo 2 adder. These are very simple to implement in hardware.

A PN code generator of this type uses a register with taps between selected stages. These taps are logically ORed and then fed back in to the input stage of the register. The state machine produced in this fashion will periodically cycle through the same PN sequence as the clock is applied.

Significantly, code sequence lengths of up to thousands of bits in length can be produced with about a dozen register stages. With modern VLSI techniques it is feasible to build generators with clock speeds up to hundreds of MHz on any die, moreover recent high speed Emitter Coupled Logic (ECL) devices allow the creation of generators with clock speeds into the GHz region.

Having produced a black box which generates a PN code with the required characteristics, the process of combining the PN modulation with the data to be transmitted, and modulating this upon a carrier is not technically difficult at all. The simplest technique, one of many, is to invert the PN code when a ‘0’ bit of message data is to be sent, and to transmit the PN code unchanged when a ‘1’ bit of message data is to be sent. This technique is termed Bit Inversion Modulation. The result is a PN code with an embedded data message.

The simplest form of carrier modulation which can be used is AM, however in practice one or another form of Phase Shift Keying (PSK) is usually employed. PSK schemes are commonly used in modems, and involve the modulation of the carrier phase with the data signal. In a DS transmitter using Binary PSK, the carrier wave is phase shifted back and forth 180 degrees with each 1 or 0 in the PN code chip stream being sent. The process of modulating the carrier with the PN code is often termed spreading.

The internals of a DS receiver are somewhat more complex than those of the transmitter, but not vastly so. The central idea in all SST receivers is the use of the correlation operation.

Correlation, a favourite method of our friends in the statistics community, is a mathematical operation which determines a measure of likeness or similarity between two sets of data or two time processes. In an SST receiver, the correlation operation is use to measure the similarity of a received PN code sequence to an internally generated PN code sequence. Ideally, if these PN sequences are the same, a high correlation will be detected, whereas if the codes are different, a low correlation is detected.

Mathematically the correlation operation, in its simplest form, is the integral of the product of two time varying functions. In a DS receiver of the simplest kind, the hardware maps directly onto the basic maths. The correlator is built by combining a multiplier with a low pass filter (ie integrator in a control engineer’s language).

One of the two time varying functions is the received PN modulated signal, the other is the PN sequence produced by a PN generator internal to the receiver. In the simplest situation, the receiver’s PN generator is a clone of the PN generator in the transmitter.

The multiplier can be one of many designs, importantly it multiplies in effect two single numbers and is therefore trivially simple. Classical textbooks cite the analogue doubly balanced mixer as the standard multiplier. The output from the multiplier is a time varying measure of the similarity between the two codes, blended with the remnants of uncorrelated (ie real) noise and interfering signals.

The integration operation disposes of the latter, and we are then left with the data which we intended to extract. This series of operations is often termed despreading. In practice, we often need to synchronise our receiver’s PN generator to the incoming SST signal, therefore there is often much additional complexity required to produce an internal reference PN sequence in proper sync with the incoming message PN sequence.

At this point it is worth reflecting upon what we have. We can generate either cryptographically secure or insecure codes. We can embed a digital data stream in one or another fashion into the code stream. All of this can be performed with pure digital logic. Once we have a combined data/code stream, we can use a very simple analogue modulation to put the message upon a carrier.

The resulting radio signal looks like white noise to a third party who doesn’t know the code. Our receiver shares similar hardware design with our transmitter. It uses a trivial demodulation scheme, and extracts digital data from the incoming PN data/code stream. Other radio signals occupying our bandwidth are largely ignored. Whilst an SST transmitter-receiver pair may be conceptually more complex to understand than most classical analogue schemes, it is well suited to implementation in digital logic because most of the smarts at either end of the link are purely digital. This means that such hardware can be made much more compact than many classical narrowband analogue schemes, which often require a lot of analogue hardware which may or may not be easy to squeeze into Silicon.

Consider a narrowband 16 or 64 level QAM scheme, which is not only vulnerable to interference and noise, but also requires a digital signal processing chip to demodulate. For those readers with a bent toward radio engineering, the spectral envelope of a DS system is typically a sine function, with suppressed outer sidebands beyond the first null, and often a suppressed carrier. A parameter which radio types will appreciate is process gain, a measure of signal to noise ratio improvement achieved by despreading the received signal. For a DS system it is typically about twice the ratio of RF bandwidth to message bandwidth. Therefore to improve your ability to reject interference by 20 dB, you need to increase your chip rate by a factor of 100.

Frequency Hopping Systems 

Frequency Hoppers (FH) are a more sophisticated and arguably better family of spread spectrum techniques than the simpler DS systems. However, performance comes with a price tag here, and FH systems are significantly more complex than DS systems. The central idea behind a FH system is to retune the transmitter RF carrier frequency to a pseudorandomly determined frequency value. In this fashion the carrier keeps popping up a different frequencies, in a pseudorandom pattern. The carrier itself can be modulated directly with the data using one of many possible schemes. The available radio spectrum is thus split up into a discrete number of frequency channels, which are occupied by the RF carrier pseudorandomly in time.

Unless you know the PN code used, you have no idea where the carrier wave is likely to pop up next, therefore eavesdropping will be quite difficult. Frequency hoppers are typically divided into fast and slow hoppers. A slow frequency hopper will change carrier frequency pseudorandomly at a frequency which is much slower than the data bit rate on the carrier. A fast frequency hopper will do so at a frequency which is faster than that of the data message.

Hybrid (FH/DS) Systems

If we are really paranoid about being eavesdropped, we can take further steps to make our signal difficult to find. A commonly used example is that of a hybrid spread spectrum system using both FH and DS techniques. Such schemes will typically employ frequency hopping of the carrier wave, while concurrently using a DS modulation technique to modulate the data upon the carrier.

In this way an essentially DS modulated message is hopped about the spectrum. To successfully intercept such a signal you must first crack the FH code, and then crack the DS code. If you want to be even more secure, you encrypt your data stream with a very secure crypto code before you feed it into your DS modulator, and employ cryptographically secure PN codes for the DS and FH operations. Your eavesdropper then has to chew his way through three levels of encoding. Such a scheme is used in the NMIDS (Nato Multifunction Information Distribution System/AWACS) datalink.

Summary 

Spread Spectrum techniques are technologically superior to conventional narrowband modulation techniques in a number of important areas. They form the datalink layer of todays WLANs in operation in most households in the UK as well as in most offices. Their ubiquity belies their complexity and without SST the modern day advantages of mobile telephony as well as wireless LAN networking would not be possible. If your organisation needs assistance with its radio communications in the field of wireless networking, give us a call free today on 0800 012 1090. We look forward to your call.

Tablet Operating Systems and Features / A Comparison

Wedded to the intuitive Apple iOS? Or keen to stick with the Google Android OS you’ve become used to on your smartphone? The choice of operating system for your tablet is as likely to be a ‘brand’ choice as it is one about features and function. So how do the two market leaders square up?

With so much hype, it’s easy to believe that there are just two operating systems available for tablets. But take a closer look: the growing demand for business tablets means that other top tech brand names are also offering their own operating systems. These have a number of plus points for certain user groups.

Apple iOS

Familiarity keeps users happy..

Ease of use is viewed as one of the biggest factors in the success of Apple’s mobile operating system iOS. There is Little difference to the user interface experience whether an Apple® Pad, Phone or Pod Touch is used. And for Apple® aficionados, this makes the choice of tablet an easy one: it has to be the Apple® Pad.
The iOS offers Pad users a touch controlled interface, familiar and intuitive usability and easy-to-grasp layout. With the launch of iOS 5 imminent, users are promised some 200 new features, including a new messaging service; iMessage that will let the user easily send text messages, photos and videos between all iOS devices.

Android

Like bees to the honeypot…

The latest Google mobile technology platform, codenamed Honeycomb during its development, is proving popular with those already using the mobile Android OS on their smartphones. A key difference between the earlier OS and ‘Honeycomb’ is that Android 3 has been designed with the larger screen of a tablet in mind. It features a brand new, truly virtual and ‘holographic’ user interface design and a redesigned keyboard helping to make entering text fast and accurate on larger screen sizes. You can also get the full web experience on your device with Adobe® Flash Player® 10.2 which can be downloaded from Android Market.
One of the key attractions of the new Android 3 is its customisation capability. This offers up to five customisable home screens to which you can add widgets and group app icons, such as Google Books, YouTube and Gmail anywhere you want.

Windows7 OS

Microsoft has unsurprisingly entered the fray with tablet-friendly versions of its Windows7 Operating System. With the majority of PC users already familiar with Windows0 OS, Microsoft is counting on the ability for tablets operating on Windows7 to seamlessly synchronise with home and office computers. Windows7 offers several improvements for tablet users in the Professional, Ultimate and Enterprise editions and Microsoft has worked with manufacturers to enhance technologies, such as touch control and user interface.
Windows7 OS refines some of the improvements offered in Windows Vista, such as the quick and easy way Windows searches your PC to find what you want. Popular features include HomeGroup file sharing and Jump Lists that enable you to quickly access your files. The new AppLocker feature available on Windows7 Ultimate is ideal for enterprise tablet users. It enables you to Lock down certain applications for specified users to provide access control for applications.

BlackBerry Tablet OS

Research in Motions BlackBerry Tablet OS is the perfect choice for users of the BlackBerry Playbook tablet. It is designed to work with multi-core devices and comes with enhanced multi-tasking, as well as support for Adobe PDF Reader, Youtube, email, browsing and more.
There is a sense that it is targeted more at the enterprise than the consumer, being great for handling and creating documents, presentations and spreadsheets. It is compatible with Adobe Air, Adobe Flash 10.1, WebKit, Java, OpenGL, and POSIX. All apps can be downloaded via the BlackBerry App World and the BlackBerry Bridge allows users to connect the PlayBook tablet to their BlackBerry smartphones, with BlackBerry OS 5 or above.

Wi=Fi for the wireless generation

Today’s tablets offer you the opportunity to connect to the internet and communicate with colleagues, friends and family from just about anywhere. All tablets support Wi-Fi connectivity allowing them, or other wireless devices, within range of an office or home Hot Spot to be used to access the internet. One of the benefits of Wi-Fi is that so many places now offer an open Hot Spot. For example, cafes, airports, libraries and exhibition halls invariably offer Wi-Fi internet access.

For the enterprise user, such as a road warrior, a field-based engineer, or remote healthcare worker, the ability to tap into office applications from a local Hot Spot and to communicate directly with colleagues offers huge advantage in terms of both customer satisfaction and efficiency. Instant and remote access to calendar, email and even patient histories or maintenance and sales records can speed up and improve service delivery.

For non-commercial tablet users, the benefits are equally clear. Imagine being able to check flight details or to find the location of an event from a Hot Spot while you’re already in transit. School children can be given access to secure school intranets to support their studies, while social networking groups of all ages can communicate almost anywhere, at any time.

3G anywhere, any time

With Wi-Fi connectivity available just about anywhere, why would you need a 3G-enabled tablet? The debate about whether to opt for 3G or not rumbles on, but for dedicated 3G users there is no question about its benefits.

Although Wi-Fi is ideal if you have a wireless high speed router in your home or are likely to be using your tablet near a wireless Hot Spot, third generation (3G) internet access doesn’t depend on the availability of a wireless Hot Spot for its connectivity. Instead, connectivity is provided from anywhere that 30 coverage is available.

Mobile operators maintain a network of mobile base stations that users (without any interruption to service) are switched between as they move from location to location. This means that they can continue to use their tablet to access the internet while they’re on the move, without the need to find a Hot Spot.

3G is offered as a service purchased from a mobile provider, usually on a monthly subscription basis. Users can stay permanently connected to the internet and only pay for the amount of information they receive or transmit. Advances in this third generation of internet technology make it ideal for tablet users likely to need advanced multimedia access, high-speed transmission or global roaming.

There is great audio and video streaming and what might take several hours to download on a non-3G tablet, such as songs or film trailers, takes just minutes with 3G.

3G is all about staying connected on the move. In our fast-paced business and social world, it is the ‘only’ option for many tablet users.