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NETWORK BASICS

Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  


Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…

MEMORY

What Is RAM? Here's Everything You Need to Know About ItRAM (Random Access Memory) is the internal memory of the CPU for storing data, program and program result. It is read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased. Access time in RAM is independent of the address that is, each storage location inside the memory is as easy to reach as other locations and takes the same amount of time. Data in the RAM can be accessed randomly but it is very expensive. RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold. 

 Function of RAM: Random-access memory (RAM) is a type of computer data storage. A RAM device makes it possible to access data in random order, which makes it very fast to find a specific piece of information. Certain other types of storage are not random-access. For example, a hard disk drive and a CD will read and write data in a predetermined order. The mechanical design of these devices prescribes that data access is consecutive. This means that the time it takes to find a specific piece of information can vary greatly depending on where it is located on the disk. RAM devices are used in computer systems as the main memory. RAM is considered volatile memory, which means that the stored information is lost when there is no power. So, RAM is used by the central processing unit (CPU) when a computer is running to store information that needs to be used very quickly, but it does not store any information permanently. Present-day RAM devices use integrated circuits to store information. This is a relatively expensive form of storage and the cost per unit of storage is much higher than for devices like a hard drive. However, the time to access data is so much faster for RAM that speed outweighs cost. A computer therefore uses a certain amount of RAM for fast-access, temporary storage of information and a much larger amount of non-random, permanent mass storage, like a hard disk drive. For example, a typical computer system may have two to eight GB (gigabytes) of RAM, while the storage capacity of the hard disk drive can be several hundred GB or even one TB (terabyte). 
History of RAM: The earliest form of RAM goes back to the very first computers in the 1940s. Magnetic-core memory relied on an array of magnetized rings. Data could be stored by magnetizing each ring individually. Each ring was wired separately, which resulted in fairly large installations. A single ring could store a single bit of data and the direction of magnetization indicated zero or one. Technological advances resulted in smaller devices that could store more information but relied on the same principle. The memory unit in the photograph below is about 10 x 10 cm and could store 1,024 bits. That is very small by today's standards, but it was state-of-the-art in the 1960s. The real breakthrough for computer memory came in the 1970s with the invention of solid-state memory in integrated circuits. This uses very small transistors, making it possible to store a lot more information on a very small area. However, this increase in memory density came at the cost of volatility: a constant power supply is needed to maintain the state of each transistor. Today's RAM still relies on this same principle. 

Types of RAM

The following list gives you an idea of the vast array of memory types and sub-types:  

DRAM 
Asynchronous DRAM 
o FPM DRAM 
o EDO DRAM 
o BEDO DRAM 
Synchronous DRAM 
o SDR SDRAM  
o DDR SDRAM 
o DDR2 SDRAM  
o DDR3 SDRAM 
o DRDRAM 
SRAM 
ROM 

Static RAM (SRAM)

The word static indicates that the memory retains its contents as long as power is being supplied. However, data is lost when the power gets down due to volatile nature. SRAM chips use a matrix of 6 transistors and no capacitors. Transistors do not require power to prevent leakage, so SRAM need not have to be refreshed on a regular basis. Because of the extra space in the matrix, SRAM uses more chips than DRAM for the same amount of storage space, thus making the manufacturing costs higher. So SRAM is used as cache memory and has very fast access.

 Characteristic of the Static RAM 

  • It has long life 
  • There is no need to refresh 
  • Faster 
  • Used as cache memory 
  • Large size 
  • Expensive 
  • High power consumption 

Dynamic RAM (DRAM) 

DRAM, unlike SRAM, must be continually refreshed in order to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small. All DRAMs are made up of memory cells which are composed of one capacitor and one transistor. 

Characteristics of the Dynamic RAM 

  • It has short data lifetime 
  • Need to be refreshed continuously 
  • Slower as compared to SRAM 
  • Used as RAM 
  • Lesser in size 
  • Less expensive 
  • Less power consumption There are many different types of DRAMs that are available. Each different type of DRAM has slightly different properties and this makes it suitable for different applications. The choice of the correct type of DRAM is important to ensure that the system operates to its greatest efficiency. 

DRAM types 

The different types of DRAM are used for different applications as a result of their slightly varying properties. The different types are summarized below: 

  • Asynchronous DRAM: Asynchronous DRAM is the basic type of DRAM on which all other types are based. Asynchronous DRAMs have connections for power, address inputs, and bidirectional data lines.  

Although this type of DRAM is asynchronous, the system is run by a memory controller which is clocked, and this limits the speed of the system to multiples of the clock rate. Nevertheless the operation of the DRAM itself is not synchronous. There are various types of asynchronous DRAM within the overall family: 

RAS only Refresh, ROR: This is a classic asynchronous DRAM type and it is refreshed by opening each row in turn. The refresh cycles are spread across the overall refresh interval. An external counter is required to refresh the rows sequentially. 

CAS before RAS refresh, CBR:   To reduce the level of external circuitry the counter required for the refresh was incorporated into the main chip. This became the standard format for refresh of an asynchronous DRAM. (It is also the only form generally used with SDRAM). 

  • FPM DRAM: FPM DRAM or Fast Page Mode DRAM was designed to be faster than conventional types of DRAM. As such it was the main type of DRAM used in PCs, although it is now well out of date as it was only able to support memory bus speeds up to about 66 MHz. 
  • EDO DRAM: Extended Data out DRAM was a form of DRAM that provided a performance increase over FPM DRAM. Yet this type of DRAM was still only able to operate at speeds of up to about 66 MHz. EDO DRAM is sometimes referred to as Hyper Page Mode enabled DRAM because it is a development of FPM type of DRAM to which it bears many similarities. The EDO DRAM type has the additional feature that a new access cycle could be started while the data output from the previous cycle was still present. In some instances it was possible to carry out a memory transaction in one clock cycle, or provide an improvement from using three clock cycles to two dependent upon the scenario and memory used. This provided the opportunity to considerably increase the level of memory performance while also reducing costs. 
  • BEDO DRAM   The Burst EDO DRAM was a type of DRAM that gave improved performance of the straight EDO DRAM. The advantage of the BEDO DRAM type is that it could process four memory addresses in one burst saving three clock cycles when compared to EDO memory. This was done by adding an on-chip address counter count the next address. BEDO DRAM also added a pipe lined to enable the page-access cycle to be divided into two components: 
  • The first component accessed the data from the memory array to the output stage. 
  • The second component drove the data bus from this latch at the appropriate logic level. 

Since the data was already in the output buffer, a faster access time is achieved - up to 50% improvement when compared to conventional EDO DRAM. BEDO DRAM provided a significant improvement over previous types of DRAM, but by the time it was introduced, SDRAM had been launched and took the market. Therefore, BEDO DRAM was little used. 

  • SDRAM:   Synchronous DRAM is a type of DRAM that is much faster than previous, conventional forms of RAM and DRAM. It operates in a synchronous mode, synchronizing with the bus within the CPU. 
  • RD RAM:  This is Rambus DRAM - a type of DRAM that was developed by Rambus Inc., obviously taking its name from the company. It was a competitor to SDRAM and DDR SDRAM, and was able to operate at much faster speeds than previous versions of DRAM. 

DRAM IC package formats

DRAM is available in a wide variety of formats and packages to enable it to be used in a whole variety of applications. 
The DRAM formats and packages using in computers may be different to those found in other electronics equipment. 
Often the individual DRAM integrated circuits are mounted onto small printed circuit boards to form memory modules. This means that there are many different DRAM packages and formats that can be found. 
DRAM integrated circuits come in a variety of integrated circuit package formats including: 
  • Dual-in-line, DIL package - a traditional leaded package for integrated circuits 
  • SMT - DRAMs are available in a variety of surface mount packages 

Memory Packaging:

First of all, it should be noted that each motherboard supports memory based on the speed of the front side bus (or the CPU’s QPI) and the memory’s form factor.  For example, if the motherboard’s FSB is rated at a maximum speed of 1333MHz and you install memory that is rated at 1066MHz, the memory will operate at only 1066MHz, if it works at all, thus making the computer operate slower than it could. In their documentation, most mother- board manufacturers list which type(s) of memory they support as well as its maximum speeds and required pairings. The memory slots on a motherboard are designed for particular module form factors or styles. RAM historically  evolved from form factors no longer seen for such applications, such as dual inline package (DIP), single inline memory module (SIMM), and single inline pin package (SIPP). The most popular form factors for primary memory modules today are as follows: 

  • DIMM 
  • RIMM 
  • SODIMM 
  • MicroDIMM 
  • Single In-line Memory Module, SIMM:   This type of DRAM or memory package holds up to eight nine RAM chips (8 in Macs and 9 in PCs where the 9th chip is used for parity checking). Another important factor is the bus width, which for SIMMS is 32 bits.  With growing speed of processors and their increasing power has brought about an increase in the bus width. With later processors, e.g. after Intel Pentium, the 64-bit wide bus width requires SIMMs installed in matched pairs to match the data bus and so that the processor can access the two SIMMs simultaneously. 
  • Dual In-line Memory Module, DIMM:   With the increase in data bus width, DIMMs began to replace SIMMs as the predominant type of memory module.  
The main difference between a SIMM and a DIMM is that a DIMM has separate electrical contacts on each side of the module, while the contacts on a SIMM are on both sides are redundant. Standard SIMMs also have a 32-bit data bus, while standard DIMMs have a 64-bit data bus. 
  • Rambus In-line Memory Module, RIMM   This type of DRAM memory package is essentially the same as a DIMM but is referred to as RIMMs because of their manufacturer and proprietary slot required. 
  • Small outline DIMM, SO-DIMM This type of DRAM package is about half the size of the standard DIMM. Being smaller they are used in small footprint PCs including laptops, netbooks, etc., 
  • Small outline RIMM, SO-RIMM   This type of DRAM package is a small version of the RIMM. 

DRAM variations

Now we are going to discuss various variations of DRAM. 

SDRAM (Synchronous Dynamic Random Access Memory)

"Synchronous" tells about the behavior of the DRAM type. In late 1996, SDRAM began to appear in systems. Unlike previous technologies, SDRAM is designed to synchronize itself with the timing of the CPU. This enables the memory controller to know the exact clock cycle when the requested data will be ready, so the CPU no longer has to wait between memory accesses. For example, PC66 SDRAM runs at 66 MT/s, PC100 SDRAM runs at 100 MT/s, PC133 SDRAM runs at 133 MT/s, and so on.  SDRAM can stand for SDR SDRAM (Single Data Rate SDRAM), where the I/O, internal clock and bus clock are the same. For example, the I/O, internal clock and bus clock of PC133 are all 133 Mhz. Single Data Rate means that SDR SDRAM can only read/write one time in a clock cycle. SDRAM have to wait for the completion of the previous command to be able to do another read/write operation. 

DDR SDRAM (Double Data Rate SDRAM)

The next generation of SDRAM is DDR, which achieves greater bandwidth than the preceding single data rate SDRAM by transferring data on the rising and falling edges of the clock signal (double pumped). Effectively, it doubles the transfer rate without increasing the frequency of the clock. The transfer rate of DDR SDRAM is the double of SDR SDRAM without changing the internal clock. DDR SDRAM, as the first generation of DDR memory, the prefetch buffer is 2 bit, which is the double of SDR SDRAM. The transfer rate of DDR is between 266~400 MT/s. DDR266 and DDR400 are of this type. DDR SDRAM for desktops comes in 184-pin DIMMs. DDR SDRAM for laptops comes in either 200-pin SO-DIMMs or 172-pin microDIMMs. 

DDR2 SDRAM (Double Data Rate Two SDRAM)

Its primary benefit is the ability to operate the external data bus twice as fast as DDR SDRAM. This is achieved by improved bus signal. The prefetch buffer of DDR2 is 4 bit (double of DDR SDRAM). DDR2 memory is at the same internal clock speed (133~200MHz) as DDR, but the transfer rate of DDR2 can reach 533~800 MT/s with the improved I/O bus signal. DDR2 uses a 240-pin DIMM that’s not compatible with DDR. 

DDR3 SDRAM (Double Data Rate Three SDRAM)

DDR3 memory reduces 40% power consumption compared to current DDR2 modules, allowing for lower operating currents and voltages (1.5 V, compared to DDR2's 1.8 V or DDR's 2.5 V). The transfer rate of DDR3 is 800~1600 MT/s. DDR3's prefetch buffer width is 8 bit, whereas DDR2's is 4 bit, and DDR's is 2 bit. DDR3 also adds two functions, such as ASR (Automatic Self-Refresh) and SRT (Self-Refresh Temperature). They can make the memory control the refresh rate according to the temperature variation.

DDR4 SDRAM (Double Data Rate Fourth SDRAM)

DDR4 SDRAM provides the lower operating voltage (1.2V) and higher transfer rate. The transfer rate of DDR4 is 2133~3200 MT/s. DDR4 adds four new Bank Groups technology. Each bank group has the feature of single-handed operation. DDR4 can process 4 data within a clock cycle, so DDR4's efficiency is better than DDR3 obviously.  

DDR4 also adds some functions, such as DBI (Data Bus Inversion), CRC (Cyclic Redundancy Check) and CA parity. They can enhance DDR4 memory's signal integrity, and improve the stability of data transmission/access.  

DDR SDRAM
standard
Internal rate
(MHz)
BUS Clock
(MHz)
prefetch
Data rate (MT/S)
Transfer rate (GB/s)
Voltage
    (V)
SDRAM
100-166
100-166
     1n
100-166
    0.8-1.3
     3.3
DDR
133-200
133-200
     2n
266-400
    2.1-3.2
   2.5/2.6
DDR2
133-200
266-400
     4n
533-800
    4.2-6.4
     1.8
DDR3
133-200
533-800
     8n
1066-1600
    8.5-14.9
  1.35/1.5
DDR4
133-200
1066-1600
     8n
2133-3200
    17-21.3
      1.2

Essential  There are some concept in memory module which have discussed in this segment. Understanding Cache Cache Memory is fast memory that serves as a buffer between the processor and main memory. The cache holds data that was recently used by the processor and saves a trip all the way back to slower main memory. The memory structure of PCs is often thought of as just main memory, but it's really a five or six level structure: 
➣The first two levels of memory are contained in the processor itself, consisting of the processor's small internal memory, or registers, and L1 cache, which is the first level of cache, usually contained in the processor. 

➣The third level of memory is the L2 cache, usually contained on the motherboard. However, the Celeron chip from Intel actually contains 128K of L2 cache within the form factor of the chip. More and more chip makers are planning to put this cache on board the processor itself. The benefit is that it will then run at the same speed as the processor, and cost less to put on the chip than to set up a bus and logic externally from the processor. 

➣The fourth level, is being referred to as L3 cache. This cache used to be the L2 cache on the motherboard, but now that some processors include L1 and L2 cache on the chip, it becomes L3 cache. Usually, it runs slower than the processor, but faster than main memory. 

➣The fifth level (or fourth if you have no "L3 cache") of memory is the main memory itself. 

➣The sixth level is a piece of the hard disk used by the Operating System, usually called virtual memory. Most operating systems use this when they run out of main memory, but some use it in other ways as well. 

Memory Redundancy 

One important aspect to consider in memory is what level of redundancy you want. There are a few different levels of redundancy available in memory. Depending on your motherboard, it may support all or some of these types of memory: 
➣The cheapest and most prevalent level of redundancy is non-parity memory. When you have non-parity memory in your machine and it encounters a memory error, the operating system will have no way of knowing and will most likely crash, but could corrupt data as well with no way of telling the OS. This is the most common type of memory, and unless specified, that's what you're getting. 
➣The second level of redundancy is parity memory (also called true parity). Parity memory has extra chips that act as parity chips. Thus, the chip will be able to detect when a memory error has occurred and signal the operating system.  
➣The third level of redundancy is ECC (Error Checking and Correcting). This requires even more logic and is usually more expensive. Not only does it detect memory errors, but it also corrects 1-bit ECC errors. If you have a 2-bit error, you will still have some problems. Some motherboards enable you to have ECC memory. 

Latency RAM responds to electrical signals at varying rates. When the memory controller starts to grab a line of memory, for example, there’s a slight delay; think of it as the RAM getting off the couch. After the RAM sends out the requested line of memory, there’s another slight delay before the memory controller can ask for another line the RAM sat back down. The delay in RAM’s response time is called its latency. Latency numbers reflect how many clicks of the system clock it takes before the RAM responds. If you speed up the system clock, say from 166 MHz to 200 MHz, the same stick of RAM might take an extra click before it can respond. When you take RAM out of an older system and put it into a newer one, you might get a seemingly dead PC, even though the RAM fits in the DIMM slot. Many motherboards enable you to adjust the RAM timings manually. 

ECC Its stands for error correction code. ECC is a major advance in error checking on DRAM. First, ECC detects any time a single bit is incorrect. Second, ECC fixes these errors on the fly. The checking and fixing come at a price, as ECC RAM is always slower than non-ECC RAM. 

Troubleshooting
Symptoms of a RAM Problem
  • When you first turn on your 
  • Computer it runs fine, but as you go about your business you notice that its performance diminishes. By lunch time, websites take minutes to load and local programs run at a snail's pace. This type of gradual deterioration of PC performance, especially with memory-intensive programs, may be caused by a RAM problem. 
  • Your computer randomly restarts while you are in the middle of something or freeze sporadically. It may also reboot almost immediately upon opening the desktop. This could be a sign of faulty RAM. 
  • A blue screen with white text flashes before restarting. Blue-screen errors are annoying because you don't even have a chance to read the error message. Bad RAM is one thing that cause them. 
  • Files—particularly ones you frequently access and save—seem to be inexplicably corrupted. RAM issues can lead to this problem, which can worsen over time. The file structure of your hard drive may slowly degenerate and you will no longer be able to boot your machine. 
  • Your attempts to install a new program repeatedly fail for unknown reasons. You try to reinstall the operating system, but keep getting odd error messages. 

There are certainly a variety of problems that could cause the issues above, but faulty RAM is an often-overlooked root cause of inexplicable issues. If you have one or a combination of the above problems you are likely facing a memory issue. 


 What Causes Memory Damage?

  • Power surges can damage most computer components, including RAM. You should plug your computer and other expensive electronics into a surge protector. Make sure you know the difference between a surge protector and a power strip. 
  • Before you handle any parts in your computer, make sure you ground yourself by touching a piece of grounded metal to discharge static electricity. Electrostatic discharge can damage your computer. 
  • Excessive heat can cause RAM and other parts to wear out over time. Individual components can overheat, or heat from one component can cause damage to adjacent parts. 
  • If you have overclocked any part of your computer incorrectly, it may cause damage in the form of excess heat. 
  • Your memory module may have some fault that passed through quality control and worsened over time. This is the most likely cause behind a damaged RAM. 

It is also possible that the memory module is fine, but one or more memory slots on your motherboard are defective, hindering the RAM's performance. The defect may even be so bad that it damages the memory stick. 


Diagnosing the Problem
  • If the symptoms started after you recently added more memory, the new module could be faulty. This seems obvious, but any time a problem begins after making a change, first check to see if the change caused the problem. 
  • Your computer may produce multiple beeps or a continuous beep when you turn it on. These beeps can indicate many different problems, including being a symptom of bad RAM. Beep codes vary depending on the manufacturer of your BIOS. You can look up the beep codes for your specific computer to figure out its specific problem. 
  • If you are not comfortable fixing your computer yourself, I recommend taking your computer to a local repair store rather than a big retail store. Big stores are in the business of selling additional parts, not fixing problems. 
  • If you are an advanced user, there are diagnostic programs that can help you figure out if you have a memory problem. Your computer may have one pre-installed by the manufacturer or you can download a third-party program. The Windows Memory Diagnostic by Microsoft is also good if you suspect memory problems on a Windows PC. 
  • If the above programs indicate your memory sticks are functioning, but you still suspect a RAM problem or if you need to find which memory module is the problem, you can try removing the modules and placing them in different slots. 
Fixing the Problem by Removing Memory
  • To remove a RAM module you need to press the tiny levers on each end to release it. 
  • Test each of your memory modules one at a time to find which ones are bad. When you find the sticks that are bad you can order replacements. It is possible all the memory modules are damaged if your computer experienced a serious power surge. 
  • If all the memory modules appear bad, then the problem is likely with the memory slot itself. Try testing each memory module in each of the memory slots to find if one of the slots is faulty. To fix a faulty slot you would need to replace your motherboard. 
  • When replacing a module, make sure the indention on the bottom is lined up correctly with the slot and then press it in until you hear the levers snap. Note that some levers do not make a snapping sound. 

Other Possible Problems


RAM problems often go undiagnosed, but they also can be misdiagnosed. Some common problems that have nothing to do with memory can cause the symptoms listed above: 
  • Many of the symptoms listed above can actually be caused by a hard drive problem. You can run a utility called CHKDSK by pressing the Windows button and R at the same time, typing CHKDSK into the dialog box, and pressing enter. Alternatively, you can click on Computer, right-click the drive you intend to scan, click Properties, then the Tools tab, and click Check now. Note you will need to restart your computer. Also, be aware this process may take an extended amount of time to finish running. If you hear your hard drive making lots of noise during normal operation, it may be the cause of your problems. Defragmenting your drive every few months is a good idea as well. 
  • If the performance problems occur during graphic-intensive programs such as games or rendering, your graphics driver could be outdated. Be sure to always keep your drivers up to date. 
  • Excessive heat in your machine can cause a variety of problems. 
  • Random reboots can be cause by a failing power supply. 
  • Dirt and dust can also cause issues that look like memory problems. It is a good idea to clean dust out of your computer at least twice or thrice in a year, more often if you have furry pets. 
  • Your computer can also have a virus. Note that many people automatically assume any problem with their computer is caused by a virus and many large retail repair stores frequently misdiagnose hardware problems as viruses. Make sure you have a good anti-virus program and run scans on a regular basis. 


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