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OSI MODEL

The 7 Layers of the OSI Model

The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. Use this handy guide to compare the different layers of the OSI model and understand how they interact with each other.

• Physical (Layer 1)
• Data Link (Layer 2)
• Network (Layer 3)
• Transport (Layer 4)
• Session (Layer 5)
• Presentation (Layer 6)
• Application (Layer 7)

Physical (Layer 1)


OSI Model, Layer 1 conveys the bit stream - electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.

Data Link (Layer 2)


At OSI Model, Layer 2, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.

Network (Layer 3)


Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Layer 3 Network examples include AppleTalk DDP, IP, IPX.
Transport (Layer 4)


OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Layer 4 Transport examples include SPX, TCP, UDP.
Session (Layer 5)


This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Layer 5 Session examples include NFS, NetBios names, RPC, SQL.
Presentation (Layer 6)


This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
Application (Layer 7)


OSI Model, Layer 7, supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

 Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP

Router (computing)

A router^[a] is a networking device that forwards data packets between computer networks. Routers perform the traffic directing functions on the Internet. A data packet is typically forwarded from one router to another router through the networks that constitute the internetwork until it reaches its destination node.^[2]

A router is connected to two or more data lines from different networks.^[b] When a data packet comes in on one of the lines, the router reads the address information in the packet to determine the ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey. This creates an overlay internetwork.

The most familiar type of routers are home and small office routers that simply pass IP packets between the home computers and the Internet. An example of a router would be the owner's cable or DSL router, which connects to the Internet through an Internet service provider (ISP). More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone. Though routers are typically dedicated hardware devices, software-based routers also exist.

Hub

•  When referring to a network, a hub is the most basic networking device that connects multiple computers or other network devices together. Unlike a network switch or router, a network hub has no routing tables or intelligence on where to send information and broadcasts all network data across each connection. Most hubs can detect basic network errors such as collisions, but having all information broadcast to multiple ports can be a security risk and cause bottlenecks. In the past, network hubs were popular because they were cheaper than a switch or router. Today, switches do not cost much more than a hub and are a much better solution for any network.
  

• 
• In general, a hub refers to a hardware device that enables multiple devices or connections to be connected to a computer. Another example besides the one given above is a USB hub, which allows multiple USB devices to be connected to one computer, even though that computer may only have a few USB connections. The picture to the right is an example of a USB hub.

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RAID

What is RAID?

RAID is an acronym for Redundant Array of Independent (or Inexpensive) Disks. In fact, RAID is the way of combining several independent and relatively small disks into a single storage of a large size. The disks included into the array are called array members. The disks can be combined into the array in different ways which are known as RAID levels. Each of RAID levels has its own characteristics of:

• Fault-tolerance which is the ability to survive of one or several disk failures.
• Performance which shows the change in the read and write speed of the entire array as compared to a single disk.
• The capacity of the array which is determined by the amount of user data that can be written to the array. The array capacity depends on the RAID level and does not always match the sum of the sizes of the RAID member disks. To calculate the capacity of the particular RAID type and a set of the member disks you can use a free online RAID calculator.

How RAID is organized?

Two independent aspects are clearly distinguished in the RAID organization.

1. The organization of data in the array (RAID storage techniques: striping, mirroring, parity, combination of them).
2. Implementation of each particular RAID installation - hardware or software.

RAID storage techniques

The main methods of storing data in the array are:

• Striping - splitting the flow of data into blocks of a certain size (called "block size") then writing of these blocks across the RAID one by one. This way of data storage affects on the performance.
• Mirroring is a storage technique in which the identical copies of data are stored on the RAID members simultaneously. This type of data placement affects the fault tolerance as well as the performance.
• Parity is a storage technique which is utilized striping and checksum methods. In parity technique, a certain parity function is calculated for the data blocks. If a drive fails, the missing block are recalculated from the checksum, providing the RAID fault tolerance.

All the existing RAID types are based on striping, mirroring, parity, or combination of these storage techniques.

RAID levels

• RAID 0 - based on striping. This RAID level doesn't provide fault tolerance but increases the system performance (high read and write speed).
• RAID 1 - utilizes mirroring technique, increases read speed in some cases, and provides fault tolerance in the loss of no more than one member disk.
• RAID 0+1 - based on the combination of striping and mirroring techniques. This RAID level inherits RAID 0 performance and RAID 1 fault tolerance.
• RAID1E - uses both striping and mirroring techniques, can survive a failure of one member disk or any number of nonadjacent disks. There are three subtypes of RAID 1E layout: near, interleaved, and far. More information and diagrams on the RAID 1E page.
• RAID 5 - utilizes both striping and parity techniques. Provides the read speed improvement as in RAID 0 approximately, survives the loss of one RAID member disk.
• RAID 5E - a variation of RAID 5 layout the only difference of which is an integrated spare space allowing to rebuild a failed array immediately in case of a disk failure. Read more on the RAID5E page.
• RAID 5 with delayed parity - pretty similar to basic RAID 5 layout, but uses nonstandard scheme of striping. More information about RAID5 with delayed parity.
• RAID 6 - similar to RAID 5 but uses two different parity functions. The read speed is the same as in RAID 5.

RAID stands for Redundant Array of Inexpensive (Independent) Disks.

On most situations you will be using one of the following four levels of RAIDs.

• RAID 0
• RAID 1
• RAID 5
• RAID 10 (also known as RAID 1+0)

This article explains the main difference between these raid levels along with an easy to understand diagram.

In all the diagrams mentioned below:

• A, B, C, D, E and F – represents blocks
• p1, p2, and p3 – represents parity

RAID LEVEL 0

Following are the key points to remember for RAID level 0.

• Minimum 2 disks.
• Excellent performance ( as blocks are striped ).
• No redundancy ( no mirror, no parity ).
• Don’t use this for any critical system.

RAID LEVEL 1

Following are the key points to remember for RAID level 1.

• Minimum 2 disks.
• Good performance ( no striping. no parity ).
• Excellent redundancy ( as blocks are mirrored ).

RAID LEVEL 5

Following are the key points to remember for RAID level 5.

• Minimum 3 disks.
• Good performance ( as blocks are striped ).
• Good redundancy ( distributed parity ).
• Best cost effective option providing both performance and redundancy. Use this for DB that is heavily read oriented. Write operations will be slow.

RAID LEVEL 10

Following are the key points to remember for RAID level 10.

• Minimum 4 disks.
• This is also called as “stripe of mirrors”
• Excellent redundancy ( as blocks are mirrored )
• Excellent performance ( as blocks are striped )
• If you can afford the dollar, this is the BEST option for any mission critical applications (especially databases).

 

RAID implementations

RAID can be created by two different ways:

• with the use of operating system drivers, so called software RAID;
• with the use of special hardware, so called hardware RAID.

Software RAID

Software RAID is one of the cheapest RAID solutions.

Nowadays, almost any of the operating systems has a built-in capability to create RAID, though not for all RAID levels. Thus, Windows home editions allow user to create only RAID 0, while RAID 1 and RAID 5 can be created only using Windows server editions. RAID layout created by means of Windows is inseparably linked with the host operating system and so its partitions cannot be used, for example, in dual boot.

Software RAID is created based on the user's computer and therefore it uses the host system CPU for implementation. It should be noted, that in case of RAID levels 0 and 1, CPU load is negligible, but for the RAID types based on parity, CPU load can vary from 1 to 5 percent depending on CPU power and the number of the disks, which is also negligible for practical purposes.

There are certain limitations on the use of the software RAID to boot the system. Only RAID 1 can contain boot partition, while system boot is impossible with a software RAID 5 and RAID 0.

Keep in mind that in most cases software RAID doesn't implement the hot-swapping and so it cannot be used where continuous availability is required.

Hardware RAID

Hardware RAID is created using separate hardware. Basically there are two options:

• inexpensive RAID chip possibly built into the motherboard,
• more expensive option with a complex standalone RAID controller. Such controllers can be equipped with their own CPU, battery-backed up cache memory, and they typically support hot-swapping.

A hardware RAID has some advantages over a software RAID, such as:

• doesn't use CPU of the host computer;
• allows user to create boot partitions;
• handles errors better, since communicates with the devices directly;
• supports hot-swapping.

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Ping vs Traceroute 

Ping

Ping (also written as PING or ping) is a utility that you use to determine whether or not a specific IP address is accessible. Ping works by sending a packet to a specified address and waiting for a reply. Ping is used primarily to troubleshoot Internet connections and there are many freeware and shareware Ping utilities available for download.

On a Windows PC you can run Ping using a command prompt. To do this, go to the Windows Start button, choose Programs, then MSDOS Prompt. When you get the C: prompt, type ping followed by the destination server name or an IP address, for example, ping google.com

Ping Localhost

When setting up a network you can use the ping command to make sure all of the computers are "alive" (at least in the TCP/IP sense). To do this, go to the Windows Start button, choose Programs, then MSDOS Prompt. When you get the C: prompt, type ping 127.0.0.1

If everything is OK, you should get the following response (or something similar):

Pinging 127.0.0.1 with 32 bytes of data

Reply from 127.0.0.1: bytes=32 time<10ms TTL=32
Reply from 127.0.0.1: bytes=32 time<10ms TTL=32
Reply from 127.0.0.1: bytes=32 time<10ms TTL=32
Reply from 127.0.0.1: bytes=32 time<10ms TTL=32

This means that TCP/IP is working on the machine that you are typing on. 127.0.0.1 is a special address that "loops back" to the machine you are pinging from. You can also type ping localhost and receive a similar response, since localhost and 127.0.0.1 mean the same thing.

Traceroute

Traceroute is a utility that traces a packet from your computer to an Internet host, but it will show you how many hops the packet requires to reach the host and how long each hop takes. If you're visiting a Web site and pages are appearing slowly, you can use traceroute to figure out where the longest delays are occurring. Traceroute utilities work by sending packets with low time-to-live (TTL) fields. The TTL value specifies how many hops the packet is allowed before it is returned. When a packet can't reach its destination because the TTL value is too low, the last host returns the packet and identifies itself. By sending a series of packets and incrementing the TTL value with each successive packet, traceroute finds out who all the intermediary hosts are.
The original traceroute is a UNIX utility, but nearly all platforms have something similar. Windows includes a traceroute utility called tracert. On a Windows PC you can run traceroute using a command prompt To do this, go to the Windows Start button, choose Programs, then MSDOS Prompt. When you get the C: prompt, type tracert followed by the destination server name or an IP address, for example: tracert google.com

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Ip Address

IP Address

IP address is short for Internet Protocol (IP) address. An IP address is an identifier for a computer or device on a TCP/IP network.

Static Versus Dynamic IP Addresses:

An IP address can be static or dynamic. A static IP address will never change and it is a permanent Internet address. A dynamic IP address is a temporary address that is assigned each time a computer or device accesses the Internet.

What is My IP Address?

To view your IP address you can use the ipconfig (IPCONFIG) command line tool.  Ipconfig displays all current TCP/IP network configuration values and refreshes Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) settings.

To launch the command prompt from a Windows-based computer click: Start > All Programs > Accessories > Command Prompt. Type ipconfig and press the Enter key.

• You can also use Google search to find your IP address. Type "what is my IP address" as a search query and Google will show the IP address of the computer from which the query was received as the top search result.

processor

processor

 A computer processor, also known as a microprocessor or central processing unit (CPU), is a component in a computer system that functions as the brains of a computer. It is mainly responsible for processing instructions of a computer program and carrying out computer operations
Intel Pentium Dual core CPU LogoIntel Pentium Dual Core Processors


The Intel Pentium processors with Intel dual-core technology deliver great desktop performance, low power enhancements, and multitasking for everyday computing.

Intel Dual Core i3 CPU  (Ivy Bridge)
Intel Core i3 dual core processors provide 4-way multitasking capability, runs at fixed speed ideal for typical tasks and media playback but not games.


Intel Dual Core i5 CPU  Processors
Intel i5 usually quad core but some dual processors deliver the next level of productivity. Mostly the same as i3 but with Intel Turbo Boost Technology, delivers extra speed when you need it. Like the i3 integrated graphics is included but is only ideal for normal use not for gaming.




Intel Dual Core i7 CPU Processors
Intel i7 processors dual or quad core for the most demanding applications with cache and faster clock speeds. Quad-core processors feature 8-way threading, four cores will run faster, and more L3 cache, but will consume more power. High-end use, video and gaming with dedicated video card.

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Difference Between RAM and ROM

• RAM is Random Access Memory, while ROM stands for Read Only Memory.

• RAM is volatile and is erased when the computer is switched off. ROM is non-volatile and generally cannot be written to.

• RAM is used for both read and write while ROM is used only for reading.

• RAM needs electricity to flow to retain information while ROM is permanent.

• RAM is analogous to a blackboard on which information can be written with a chalk and erased any number of times, while ROM is permanent and can only be read. One example is BIOS (basic input output system) that runs when computer is switched on and it prepares disk drives and processor to load OS from disk.

RAM(Random Access Memory) 

There are two basic types of RAM :

(i) Dynamic Ram
(ii) Static RAM
 RAM is the best known form of Computer Memory. The Read and write (R/W) memory of a computer is called RAM.  RAM holds data and processing instructions temporarily until the CPU needs it.
Dynamic RAM : loses its stored information in a very short time (for milli sec.) even when power supply is on. D-RAM’s are cheaper & lower.
Static RAM : It uses a completely different technology. S-RAM retains stored information only as long as the power supply is on. Static RAM’s are costlier and consume more power. They have higher speed than D-RAMs

History of Computers

The first substantial computer was the giant ENIAC machine by John W. Mauchly and J. Presper Eckert at the University of Pennsylvania. ENIAC (Electrical Numerical Integrator and Calculator) used a word of 10 decimal digits instead of binary ones like previous automated calculators/computers. ENIAC was also the first machine to use more than 2,000 vacuum tubes, using nearly 18,000 vacuum tubes. Storage of all those vacuum tubes and the machinery required to keep the cool took up over 167 square meters (1800 square feet) of floor space. Nonetheless, it had punched-card input and output and arithmetically had 1 multiplier, 1 divider-square rooter, and 20 adders employing decimal "ring counters," which served as adders and also as quick-access (0.0002 seconds) read-write register storage.
The executable instructions composing a program were embodied in the separate units of ENIAC, which were plugged together to form a route through the machine for the flow of computations. These connections had to be redone for each different problem, together with presetting function tables and switches. This "wire-your-own" instruction technique was inconvenient, and only with some license could ENIAC be considered programmable; it was, however, efficient in handling the particular programs for which it had been designed. ENIAC is generally acknowledged to be the first successful high-speed electronic digital computer (EDC) and was productively used from 1946 to 1955. A controversy developed in 1971, however, over the patentability of ENIAC's basic digital concepts, the claim being made that another U.S. physicist, John V. Atanasoff, had already used the same ideas in a simpler vacuum-tube device he built in the 1930s while at Iowa State College. In 1973, the court found in favor of the company using Atanasoff claim and Atanasoff received the acclaim he rightly deserved.

1.Vacuum Tubes (1950s) - one bit on the size of a thumb;
2.Transistors (1950s and 1960s) - one bit on the size of a fingernail;
3.Integrated Circuits (1960s and 70s) - thousands of bits on the size of a hand
4.Silicon computer chips (1970s and on) - millions of bits on the size of a finger nail.