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Monday, September 08, 2014

 One important piece of information to keep in mind is that data flows 2 ways in the OSI model, DOWN (data encapsulation) and UP (data decapsulation).




The computer in the above picture needs to send some data to another computer. The Application layer is where the user interface exists, here the user interacts with the application he or she is using, then this data is passed to the Presentation layer and then to the Session layer. These three layer add some extra information to the original data that came from the user and then passes it to the Transport layer. Here the data is broken into smaller pieces (one piece at a time transmitted) and the TCP header is a added. At this point, the data at the Transport layer is called a segment.
Each segment is sequenced so the data stream can be put back together on the receiving side exactly as transmitted. Each segment is then handed to the Network layer for network addressing (logical addressing) and routing through the internet network. At the Network layer, we call the data (which includes at this point the transport header and the upper layer information) a packet.
The Network layer add its IP header and then sends it off to the Datalink layer. Here we call the data (which includes the Network layer header, Transport layer header and upper layer information) a frame. The Datalink layer is responsible for taking packets from the Network layer and placing them on the network medium (cable). The Datalink layer encapsulates each packet in a frame which contains the hardware address (MAC) of the source and destination computer (host) and the LLC information which identifies to which protocol in the prevoius layer (Network layer) the packet should be passed when it arrives to its destination. Also, at the end, you will notice the FCS field which is the Frame Check Sequence. This is used for error checking and is also added at the end by the Datalink layer.
If the destination computer is on a remote network, then the frame is sent to the router or gateway to be routed to the desination. To put this frame on the network, it must be put into a digital signal. Since a frame is really a logical group of 1's and 0's, the Physical layer is responsible for encapsulating these digits into a digital signal which is read by devices on the same local network.
There are also a few 1's and 0's put at the begining of the frame, only so the receiving end can synchronize with the digital signal it will be receiving.
Below is a picture of what happens when the data is received at the destination computer.





The receiving computer will firstly synchronize with the digital signal by reading the few extra 1's and 0's as mentioned above. Once the synchonization is complete and it receives the whole frame and passes it to the layer above it which is the Datalink layer.
The Datalink layer will do a Cyclic Redundancy Check (CRC) on the frame. This is a computation which the comupter does and if the result it gets matches the value in the FCS field, then it assumes that the frame has been received without any errors. Once that's out of the way, the Datalink layer will strip off any information or header which was put on by the remote system's Datalink layer and pass the rest (now we are moving from the Datalink layer to the Network layer, so we call the data a packet) to the above layer which is the Network layer.
At the Network layer the IP address is checked and if it matches (with the machine's own IP address) then the Network layer header, or IP header if you like, is stripped off from the packet and the rest is passed to the above layer which is the Transport layer. Here the rest of the data is now called a segment.
The segment is processed at the Transport layer, which rebuilds the data stream (at this level on the sender's computer it was actually split into pieces so they can be transferred) and acknowledges to the transmitting computer that it received each piece. It is obvious that since we are sending an ACK back to the sender from this layer that we are using TCP and not UDP. Please refer to the Protocols section for more clarification. After all that, it then happily hands the data stream to the upper-layer application.
You will find that when analysing the way data travels from one computer to another most people never analyse in detail any layers above the Transport layer. This is because the whole process of getting data from one computer to another involves usually layers 1 to 4 (Physical to Transport) or layer 5 (Session) at the most, depending on the type of data.
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Thursday, August 28, 2014
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Albert Einstein
Albert Einstein's Biography
Early life and education
Albert Einstein was born in Ulm, in the Kingdom of Württemberg
in the German Empire on 14 March 1879.His father was Hermann Einstein,
a salesman and engineer. His mother was Pauline Einstein (née Koch).
In 1880, the family moved to Munich, where his father and his uncle
founded Elektrotechnische Fabrik J. Einstein & Cie, a company that
manufactured electrical equipment based on direct current.
Marriages and children
Einstein and Maric married in January 1903. In May 1904,
the couple's first son, Hans Albert Einstein,
was born in Bern, Switzerland. Their second son, Eduard, was
born in Zurich in July 1910. In 1914,Einstein moved to Berlin,
while his wife remained in Zurich with their sons.They divorced
on 14 February 1919,having lived apart for five years.Einstein
married Elsa Löwenthal on 2 June 1919, after having had a
relationship with her since 1912. She was his first cousin
maternallyand his second cousin paternally. In 1933, they
emigrated to the United States. In 1935,Elsa Einstein was
diagnosed with heart and kidney problems and died in December 1936
Scientific career
  • Thermodynamic fluctuations and statistical physics
  • General principles
  • Theory of relativity and E = mc²
  • Photons and energy quanta
  • Quantized atomic vibrations
  • Adiabatic principle and action-angle variables
  • Wave–particle duality
  • Theory of critical opalescence
  • Zero-point energy
  • General relativity and the equivalence principle
  • Modern quantum theory
  • Bose–Einstein statistics
  • Energy momentum pseudotensor
  • Unified field theory
  • Einstein–Cartan theory
  • Equations of motion
  • Cosmology
E=mc2 Awards and honors Einstein received numerous awards and honors, including the Nobel Prize in Physics

Expert in physics

Born: 14 March 1879, Ulm, Germany
Died: 18 April 1955, Princeton, NJ, USA

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Monday, August 11, 2014
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 This show that transition of 3G to 4G

4G sign
Anyone who’s been in the market for a new smartphone recently isn’t just spoilt for choice when it comes to awesome Android handsets, but there’s an ever growing range of data packages and network types to choose from as well.
Of course, you’re likely familiar with the older 3G standard, but newer handsets are often listed with a variety of “next generation” communication technologies, advertised as 4G, LTE, and sometimes 4G LTE. While that may make them appear virtually identical on the store shelves, there are actually some drastic differences between the technology advertised and the actual 4G mobile communication standard.

The 4G standard

All the way back in March 2008, the International Telecommunications Union-Radio (ITU-R) decided on a set of specifications for its new 4G standard. The ITU-R is the United Nation’s official agency for all manner of information and communication technologies, and aims to help promote and regulate various communication standards across nations.
The ITU-R decided upon a set of requirements for bandwidth, spectral efficiency, and a load of other technical points, for future 4G networks. But the most important point for us users is the peak download speeds, which are defined as 100 Mbit/s for high mobility devices, such as mobile data speeds on your smartphone while driving in car, and up to approximately 1 Gbit/s for low mobility local wireless access. To put that in some perspective, typical current download speeds are often in the range of 10Mbit/s, while 4G should offer 100 times faster downloads at a rate of 1Gbit/s.
It’s a long road, but faster data speeds are heading our way. True 4G could offer data speeds more than 10 times the current LTE download speeds.
That sounds great, but the ITU-R doesn’t have any control over implementation. As such, “first generation” 4G technologies, such as LTE or Mobile WiMAX, have been criticized for not matching up to the full specification. The reason for this is that other groups, such as 3GPP or IEEE, who work closely with the technology companies responsible for delivering the hardware, had already coordinated next-gen technologies with their members.

LTE-Advanced, the true 4G

It wasn’t until October 2010 that the ITU-R completed an assessment of six candidates to be considered true 4G technologies. After much deliberation, LTE-Advanced and WirelessMAN-Advanced were designated IMT-Advanced compliant technologies, and the age of real 4G began. LTE-Advanced is the technology that we are going to see in western markets, so here’s a little rundown of what makes LTE-A so special.
Firstly, it’s important to know that LTE-A isn’t just about handset download speeds, there’s a big push to make improvements to infrastructure in order to achieve these high download rates. LTE-A aims to improve data speeds by using a mix of traditional macro cells and vastly improved small cells. The aim is to offer better high speed coverage at the network’s edge and more bandwidth, but the transmitters will have to function on different frequency bands in order to avoid interference.
LTE carrier aggregation
The big buzzword with LTE-A is carrier aggregation, which will allow receiving handsets to make better use of these fragmented bands, in order to downloaded data faster. The LTE-A standard supports up to 5 carriers and up to 100MHz, which will enable download speeds of over 1Gbit/s. However, launch will only support the aggregation of two 10 MHz carriers, enabling peak data rates of 150 Mbit/s. The final important feature is the use of multi-antenna techniques (MIMO) and Coordinated Multi Point (CoMP) to provide more capacity and more consistent data rates across cell boundaries. In other words, you will be able to maintain a more consistant download rate as you move in and out of the range of transmitters.
However, under pressure from 3GPP and IEEE, HSPA+,WiMAX and LTE were also allowed to be labelled as 4G technologies despite not offering these features, as many companies had already begun investing in these networks during the two and a half year deliberation.

4G imposters

This has left the consumer market in a bit of a mess, allowing carriers to offer a variety of different “4G” plans for many years, despite none of them having a network which meets the official requirements. Let’s breakdown some of the technologies currently being offered as 4G, and why they don’t match up to the ITU-R standard.
4g technology table
A quick comparison of the theoretical and real world implementations of current and future network technologies. All “4G” products currently on the market are considered “pre-4G”. Source: Tech Spot
US consumers may remember that Sprint was the first to the “4G” market with its WiMAX technology. However this was only ever built to offer customers around 3-6Mbit/s download speeds and upload speeds of just 1 to 1.5Mbit/s, and was rightly shunned by disappointed consumers.
WiMAX fails to deliver even 1% of the theoretical peak download speed of the 4G standard created by the ITU-R. Fortunately, Sprint is phasing out WiMAX support in favour of its new LTE network.
HSPA+ is the pinnacle of current 3G technology, offering a theoretical 168Mbit/s downlink speed. In reality most HSPA+ coverage is only capable of 21Mbit/s, with some areas being upgraded to 42Mbit/s and occasionally even 84Mbit/s. But even the fastest implementation of HSPA+ is a long way behind the 1Gbit/s download speed required to be considered a real 4G network. But this hasn’t stopped a number of US carriers from advertising HSPA+ as 4G.
T-Mobile was one of the first companies to falsely advertise its HSPA+ network as 4G, and AT&T followed suit shortly after. Arguably AT&T is even worse, as its network started out with capabilities peaking at 14.4Mbit/s download and 5.8Mbit/s upload speeds, which is ridiculously slow for a 4G network. Users in the UK, and some other countries, will probably have noticed that some carriers actually offer HSPA+ as part of their 3G network packages. So it seems that US consumers are being sold and overcharged to use a slower network under the guise of a next generation technology.
HSPA+ vs. LTE here.
More recently, carriers have begun to offer LTE options that can theoretically offer a 100 Mbit/s download speed for mobile devices. Coverage currently varies depending on your carrier and real world data speeds are often nowhere this theoretical maximum, and are often reported to only be a tad faster than HSPA+.
US LTE coverage map
It takes a long time to upgrade an entire network, but LTE is slowing making its way across the US. Source: US Cellular
To be a little more specific about the other shortcomings of LTE, other than the lack of download speed, it’s also lacking in uplink spectral efficiency and speed, and it falls short of the true 4G capacity of 3.7 bps/Hz/cell, mainly due to the lack of carrier aggregation and multi-antenna techniques, which will enable higher speeds. LTE is a stopgap solution before the real 4G experience reaches us with the rollout of LTE-A.

Carrier marketing

If all these different technologies weren’t confusing enough, carriers have been all too keen to exploit the 4G marketing term to consumers who are seeking faster data speeds. Other than offering network types which aren’t capable of true 4G speeds, carriers are beginning to offer compromised 4G packages as they transfer their existing networks over to LTE. Take a look at AT&T’s current 4G marketing, which claims to offer users “fast 4G speeds on both the HSPA+ network and on the LTE network”.
ATT 4G marketing
Based on this advertisement, you’d assume that LTE was 2.5 times as fast as HSPA+, but the reality is often disappointing.
We already know that HSPA+ is not fast enough to be considered true 4G, and the LTE coverage areas are also surprisingly small. Despite the PR talk, this “smarter” network essentially means that you’ll be dropping down to the slightly older HSPA+ network when out of range of the LTE network, and the small print even states that 4G speeds aren’t available everywhere either. That doesn’t sound like anything promised by true 4G or LTE-A.
Fortunately companies openly offer information on their network coverage, so it’s always worth checking out exactly what coverage and speed you’ll receive in your area before deciding on a 4G contract.

When can we use “true 4G”?

The bottom line is that technically, no company yet offers a true 4G experience for consumers, but LTE-A and WM-A aren’t too far away. Having said that, current LTE data plans are a step above the older 3G networks and the pre-4G connections, and is definitely what you should look out for if you’re planning to get a device with a fast, premium mobile data experience.
The technology is only going to improve with time and, because LTE and LTE-A are fully compatible, you won’t lose out when companies finish upgrading their networks.
If you’re planning on waiting for a full 4G experience, you will need a compatible handset to achieve the full speed. A good example of this is the Korean version of the Galaxy Note 3, which makes use of the built in LTE-A modem in the Snapdragon 800 chip. LTE-A is scheduled to start rolling out in the US at some point this year.
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Wednesday, July 17, 2013
Friday, June 28, 2013