A computer network, often simply referred to as a network, is a collection of computers and devices interconnected by communications channels that facilitate communications among users and allows users to share resources. Networks may be classified according to a wide variety of characteristics.
Those are defined according to the geographical location and capacity.
1.LAN (Local Area Network)
2.MAN (Metropolitan Area Network)
3.WAN (Wide Area Network)
We generally communicate in the network through three mediums:
a. Messaging (Email, SMS etc)
b. Chatting (Gtalk, Yahoo Messanger etc.)
c. Voice Communication
We only work in the interface layer, but we don't know , what is actually happening at the back end. How the data is transferring from one machine to another machine, one phone to another phone.
The transferring process is handle by the OSI network layers which are explained below:
There are seven OSI layers :
- Application
- Presentation
- Session
- Transport
- Network
- Data Link
- Physical
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| OSI Layers From Top to Bottom |
The Application Layer
At the very top of the OSI Reference Model stack of layers, we find layer 7, the application layer. Continuing the trend that we saw in layers 5 and 6, this one too is named very appropriately: the application layer is the one that is used by network applications. These programs are what actually implement the functions performed by users to accomplish various tasks over the network.
It's important to understand that what the OSI model calls an “application” is not exactly the same as what we normally think of as an “application”. In the OSI model, the application layer provides services for user applications to employ. For example, when you use your Web browser, that actual software is an application running on your PC
. It doesn't really “reside” at the application layer. Rather, it makes use of the services offered by a protocol that operates at the application layer, which is called the Hypertext Transfer Protocol (HTTP). The distinction between the browser and HTTP is subtle, but important.
The reason for pointing this out is because not all user applications use the application layer of the network in the same way. Sure, your Web browser does, and so does your e-mail client and your Usenet news reader. But if you use a text editor to open a file on another machine on your network, that editor is not using the application layer. In fact, it has no clue that the file you are using is on the network: it just sees a file addressed with a name that has been mapped to a network somewhere else. The operating system takes care of redirecting what the editor does, over the network.
Similarly, not all uses of the application layer are by applications. The operating system itself can (and does) use services directly at the application layer.
That caveat aside, under normal circumstances, whenever you interact with a program on your computer that is designed specifically for use on a network, you are dealing directly with the application layer. For example, sending an e-mail, firing up a Web browser, or using an IRC chat program—all of these involve protocols that reside at the application layer.
There are dozens of different application layer protocols that enable various functions at this layer. Some of the most popular ones include HTTP, FTP, SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP and IRC. Lots of alphabet soup, sorry. J I describe all of these and more in the chapter on higher-layer protocols and applications.
As the “top of the stack” layer, the application layer is the only one that does not provide any services to the layer above it in the stack—there isn't one! Instead, it provides services to programs that want to use the network, and to you, the user. So the responsibilities at this layer are simply to implement the functions that are needed by users of the network. And, of course, to issue the appropriate commands to make use of the services provided by the lower layers.
The Presentation Layer
The presentation layer is the sixth layer of the OSI Reference Model protocol stack, and second from the top. It is different from the other layers in two key respects. First, it has a much more limited and specific function than the other layers; it's actually somewhat easy to describe, hurray! Second, it is used much less often than the other layers; in many types of connections it is not required.
The name of this layer suggests its main function as well: it deals with the presentation of data. More specifically, the presentation layer is charged with taking care of any issues that might arise where data sent from one system needs to be viewed in a different way by the other system. It also takes care of any special processing that must be done to data from the time an application tries to send it until the time it is sent over the network.
Presentation Layer Functions
Here are some of the specific types of data handling issues that the presentation layer handles:
* Translation: Networks can connect very different types of computers together: PCs, Macintoshes, UNIX systems, AS/400 servers and mainframes
can all exist on the same network. These systems have many distinct characteristics and represent data in different ways; they may use different character sets for example. The presentation layer handles the job of hiding these differences between machines.
* Compression: Compression (and decompression) may be done at the presentation layer to improve the throughput of data. (There are some who believe this is not, strictly speaking, a function of the presentation layer.)
* Encryption: Some types of encryption (and decryption) are performed at the presentation layer. This ensures the security of the data as it travels down the protocol stack. For example, one of the most popular encryption schemes that is usually associated with the presentation layer is the Secure Sockets Layer (SSL) protocol. Not all encryption is done at layer 6, however; some encryption is often done at lower layers in the protocol stack, in technologies such as IPSec.
Presentation Layer Role in the OSI Model
The reason that the presentation layer is not always used in network communications is that the jobs mentioned above are simply not always needed. Compression and encryption are usually considered “optional”, and translation features are also only needed in certain circumstances. Another reason why the presentation layer is sometimes not mentioned is that its functions may be performed as part of the application layer.
The fact that the translation job done by the presentation layer isn't always needed means that it is common for it to be “skipped” by actual protocol stack implementations. This means that protocols at layer seven may talk directly with those at layer five. Once again, this is part of the reason why all of the functions of layers five through seven may be included together in the same software package, as described in the overview of layers and layer groupings.
The Session Layer
The fifth layer in the OSI Reference Model is the session layer. As we proceed up the OSI layer stack from the bottom, the session layer is the first one where pretty much all practical matters related to the addressing, packaging and delivery of data are left behind—they are functions of layers four and below. It is the lowest of the three upper layers, which collectively are concerned mainly with software application
issues and not with the details of network and internet implementation.
The name of this layer tells you much about what it is designed to do: to allow devices to establish and manage sessions. In general terms, a session is a persistent logical linking of two software application processes, to allow them to exchange data over a prolonged period of time. In some discussions, these sessions are called dialogs; they are roughly analogous to a telephone call made between two people.
Application Program Interfaces (APIs)
The primary job of session layer protocols is to provide the means necessary to set up, manage, and end sessions. In fact, in some ways, session layer software products are more sets of tools than specific protocols. These session-layer tools are normally provided to higher layer protocols through command sets often called application program interfaces or APIs.
Common APIs include NetBIOS, TCP/IP Sockets and Remote Procedure Calls (RPCs). They allow an application to accomplish certain high-level communications over the network easily, by using a standardized set of services. Most of these session-layer tools are of primary interest to the developers of application software. The programmers use the APIs to write software that is able to communicate using TCP/IP without having to know the implementation details of how TCP/IP works.
For example, the Sockets interface lies conceptually at layer five and is used by TCP/IP application programmers to create sessions between software programs over the Internet on the UNIX operating system. Windows Sockets similarly lets programmers create Windows software that is Internet-capable and able to interact easily with other software that uses that interface. (Strictly speaking, Sockets is not a protocol, but rather a programming method.)
Transport Layer
In theory, the transport layer and network layer are distinct, but in practice, they are often very closely related to each other. You can see this easily just by looking at the names of common protocol stacks—they are often named after the layer three and four protocols in the suite, implying their close relationship. For example, the name “TCP/IP” comes from the suite’s most commonly used transport layer protocol (TCP) and network layer protocol (IP). Similarly, the Novell NetWare suite is often called “IPX/SPX” for its layer three (IPX) and layer four (SPX) protocols. Typically, specific transport layer protocols use the network layers in the same family. You won't often find a network using the transport layer protocol from one suite and the network layer protocol from another.
The most commonly used transport layer protocols are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) in the TCP/IP suite, the Sequenced Packet Exchange (SPX) protocol in the NetWare protocol suite, and the NetBEUI protocol in the NetBIOS/NetBEUI/NBF suite (though NetBEUI is more difficult to categorize.)
The third-lowest layer of the OSI Reference Model is the network layer. If the data link layer is the one that basically defines the boundaries of what is considered a network, the network layer is the one that defines how internetworks (interconnected networks) function. The network layer is the lowest one in the OSI model that is concerned with actually getting data from one computer to another even if it is on a remote network; in contrast, the data link layer only deals with devices that are local to each other.
While all of layers 2 through 6 in the OSI Reference Model serve to act as “fences” between the layers below them and the layers above them, the network layer is particularly important in this regard. It is at this layer that the transition really begins from the more abstract functions of the higher layers—which don't concern themselves as much with data delivery—into the specific tasks required to get data to its destination. The transport layer, which is related to the network layer in a number of ways, continues this “abstraction transition” as you go up the OSI protocol stack.
Network Layer Functions
Some of the specific jobs normally performed by the network layer include:
* Logical Addressing: Every device
that communicates over a network has associated with it a logical address, sometimes called a layer three address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. Note that addressing is done at the data link layer as well, but those addresses refer to local physical devices. In contrast, logical addresses are independent of particular hardware and must be unique across an entire internetwork.
* Routing: Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go. I discuss routing in the OSI model more completely in this topic on the topic on indirect device connection, and show how it works by way of an OSI model analogy.
* Datagram Encapsulation: The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.
* Fragmentation and Reassembly: The network layer must send messages down to the data link layer for transmission. Some data link layer technologies have limits on the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up, send each piece to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination machine. A good example is how this is done by the Internet Protocol.
* Error Handling and Diagnostics: Special protocols are used at the network layer to allow devices that are logically connected, or that are trying to route traffic, to exchange information about the status of hosts on the network or the devices themselves.
The Datalink Layer
The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer, often abbreviated “DLL” (though that abbreviation has other meanings as well in the computer world). The data link layer, also sometimes just called the link layer, is where many wired and wireless local area networking (LAN) technologies primarily function. For example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all sometimes called “data link layer technologies”. The set of devices connected at the data link layer is what is commonly considered a simple “network”, as opposed to an internetwork.
Data Link Layer Sublayers: Logical Link Control (LLC) and Media Access Control (MAC)
The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.
Data Link Layer Functions
The following are the key tasks performed at the data link layer:
* Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.
* Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.
* Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.
* Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.
* Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.
The Physical Layer
The lowest layer of the OSI Reference Model is layer 1, the physical layer; it is commonly abbreviated “PHY”. The physical layer is special compared to the other layers of the model, because it is the only one where data is physically moved across the network interface. All of the other layers perform useful functions to create messages to be sent, but they must all be transmitted down the protocol stack to the physical layer, where they are actually sent out over the network.
Note: The physical layer is also “special” in that it is the only layer that really does not apply specifically to TCP/IP. Even in studying TCP/IP, however, it is still important to understand its significance and role in relation to the other layers where TCP/IP protocols reside.
Understanding the Role of the Physical Layer
The name “physical layer” can be a bit problematic. Because of that name, and because of what I just said about the physical layer actually transmitting data, many people who study networking get the impression that the physical layer is only about actual network hardware. Some people may say the physical layer is “the network interface cards and cables”. This is not actually the case, however. The physical layer defines a number of network functions, not just hardware cables and cards.
A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't strictly accurate. All hardware must have some relation to the physical layer in order to send data over the network, but hardware devices generally implement multiple layers of the OSI model, including the physical layer but also others. For example, an Ethernet network interface card performs functions at both the physical layer and the data link layer.
Physical Layer Functions
The following are the main responsibilities of the physical layer in the OSI Reference Model:
* Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer (although also partially the data link layer; see below).
* Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.
* Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it. Note that this applies equally to wired and wireless networks, even if there is no tangible cable in a wireless network!
* Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology.
In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output. Devices like switches and routers operate at higher layers and look at the data they receive as being more than voltage or light pulses that represent one or zero.
The below given diagram will give the clear idea about how data transfers from one layer to another layer:
At the physical layer data are transmitted in the bit format.
When we are composing some message in our email client , and clicking the send button then the actual network process starts.
We generally works at the application layer. When we send the message ,it goes to presentation layer for well presenting the message so that message will transfer to the below layer in a reliable manner. In this layer some headers are added and compression, encryption are done , then it transfers to the session layer. The session layer keeps track of which packets and data belong to which file and keeps track of where they go. Then the message goes to physical layer.
Physical layer is responsible for process to process delivery of entire message. In this layer, error detection, flow control and end-to-end error recovery takes place. The message is goes in the form of packets. The packets are then transmitted to the lower layer i.e. Network layer. The network layer route the packets to the destination host by the help of destination IP address or we can say it as the logical address. This layer also provide congestion control mechanism. Then the packets comes to the data link layer, in this layer framing of packets takes place. This layer is responsible for flow regulation, error detection and correction, and framing of bits for transmission. The network data frame is made up of checksum, source address, destination address, and the data itself. The largest frame size that can be sent is known as the maximum transmission Unit (MTU). In this layer physical addressing works. Physical address is nothing but the hardware address or MAC(Media Access Control) address , which is very helpful for recognizing the particular machine/host in the world. In the destination field of the frame the MAC address of the receiver would be there. After the frame transferred, it goes to the Physical layer, which works on the bit level. In this layer the bits are in the digital format, so the digital data are converted to the analog signal and transmitted through the physical wire. And at the receiver side again the analog signal is converted to digital data through analog to digital converter. Then at the receiver side reverse process takes place for retrieving the message.
Hope you have understood the stuff.
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