7-Application
6-Presentation
5-Session
4-Transport
3-Network
2-Data Link
1-Physical


The Application Layer

This layer is very closer to user than other layers.in this layer the user interface protocols like FTP,Telnet,SMTP and so on is working.in another word this layer take data from sender system interface and give it to receiver system interface in other side. the Aplication layer interfaces is Winsock And NetBios(microsoft base network only). the applicatio layer protocols often include the session and presentation layer functions.


The Presentation Layer

This layer purpose is to translate the different syntax of systems to the actual syntax of the user system. when the communicating system establish a connection at presentation layer, they exchange message containing information about the syntaxes they have in common, and together they choose the syntax they will use during the session. the sender system send the data in its own syntax and the presentation layer of the sender system translate it to the communication syntax that are chosen before and the receiver system presentation layer receive the data and translate it to its own system syntax and gives it to upper layer(application). another functions of this layer is Compression & Decompression, Encryption & Decryption.


The Session Layer

The session layer allows users on different machines to establish sessions between them One of the services of the session layer is to manage dialoguue control. Sessions can allow traffic to go in both directions at the same time, or in only one direction at a time. If traffic can only go one way at a time, the session layer can help keep track of whose turn it is. Another session service is synchronization. Consider the problems that might occur when trying to do a two-hour file transfer between two machines on a network with a 1 hour mean time between crashes. After each transfer was aborted, the whole transfer would have to start over again, and would probably fail again with the next network crash. To eliminate this problem, the session layer provides a way to insert checkpoints into the data stream, so that after a crash, only the data after the last checkpoint has to be repeated. the session layer provide 22 services but i described some important of that. there are no separate session layer protocols as there are at the lower layer. Session layer functions are instead integrated into other protocols that also included presentation and application layer functions.


The Transport Layer

The transport layer protocols provide services that compliment those provided by network layer.the difference between the protocols provided at the transport layer within a particular protocol suite(like TCP/IP, IPX/SPX) is that some are connection oriented and some are connectionless. A connection-oriented protocol is one in which the two communicating system exchange message to establish a connection before they transmit any application data. This ensure that the systems are both active and ready to exchange message. TCP; for example, is a connection-oriented protocol wen you use a web browser to to connect to an internet server, the browser and the server first perform what is known as three-way handshake to establish the connection. Only then dose the browser transmit the address of the desired web page to the server. when the data transmission is completed, the systems perform a similar handshake to break down connection. Connection-Oriented Protocols also provide additional services such as packet acknowledgment, data segmentation, flow control, and end-to-end error detection and correction.
Packet Acknowledgment:when a packet from a system sent to another system , the sender wait of the a acknowledgment packet from recipient that are says to sender system the packet are truly transmitted or the packet are damaged. if the packet damaged the sender system must to send the damaged packet again.
Data Segmentation:in the digital technology the main data is split to very smaller pieces that are called packet to transmit in any way.
Flow Control:this is the generation by a receiving of message that instruct the sending system to speed up or slow down its rate of transmission.
The Network Layer
the network layer protocol responsible to end-to-end communications, whereas data-link layer protocols function only on the local lan. the main function of the network layer is Routing. Routing is the process of directing a datagram from its source, through an internetwork, and to its ultimate destination using the most efficient path possible. The network layer determines how messages are routed within the network. It provides transport entities independence from routing and relay considerations, including the case where subnetworks (relay-only) nodes are used. All relay functions are operated within or below this layer. the other function of the network layer is to add a header that are contain the source and destination ip address. another function of network layer is fragmentation. for example, Token Ring frames can be as large as 4500 bytes, but Ethernet frames are limited to 1500 bytes. when a large datagram that originated on a Toke Ring network is routed to an Ethernet network, the network layer protocols must split it into pieces no larger than 1500 bytes each. This process is called fragmentation.


The Data Link Layer

The protocol at the data-link layer is the conduit between the computer's networking hardware and its networking software. Network layer protocols pass their outgoing data down to the data-link layer protocol, which packages it for transmission over the network. When the other systems on the network receive the transmitted data, their data-link layer protocols process it and pass it up to the network layer. When it comes to designing and building a LAN, the data-link layer protocol you choose is the single most important factor in determining what hardware you buy and how you install it. To implement a data-link layer protocol, you need the following hardware and software: -Network interface adapters (When an adapter is a discrete card plugged into a bus slot, it is referred to as a network interface card or NIC.) -Network adapter drivers -Network cables (or other media) and ancillary connecting hardware -Network hubs (in some cases) Network interface adapters and hubs are both designed for specific data-link layer protocols, and are not interchangeable with products for other protocols. Some network cables are protocol-specific, while others can be used with various protocols. By far the most popular data-link layer LAN protocol in use today (and throughout the history of the LAN) is Ethernet. Token Ring is a distant second, followed by other protocols such as the Fiber Distributed Data Interface (FDDI). Data-link layer protocol specifications typically include the following three basic elements: -A format for the frame (that is, the header and footer applied to the network layer data before transmission) -A mechanism for controlling access to the network medium One or more physical layer specifications for use with the protocol These three components are discussed in the following sections. Media Access Control The computers on a LAN usually share a common half-duplex network medium, which means that it is possible for two computers to transmit data at the same time. When this happens, a packet collision is said to occur, and the data in both packets is lost. One of the main functions of the data-link layer protocol on this type of network is to provide a mechanism that regulates access to the network medium. This mechanism, called a media access control (MAC) mechanism, provides each computer with an equal opportunity to transmit its data while minimizing the occurrence of packet collisions. The MAC mechanism is one of the primary defining characteristics of a data-link layer protocol. Ethernet uses a MAC mechanism called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Several other protocols, including Token Ring, use a scheme called token passing.


The Physical Layer

The physical layer, at the bottom of the OSI model, is, as the name implies, the layer that defines the nature of the network's hardware elements, such as what medium the network uses, how the network is installed, and the nature of the signals used to transmit binary data over the network. The physical layer also defines what kind of network interface adapter must be installed in each computer and what kind of hubs (if any) to use. Physical layer options include various types of copper or fiber optic cable, as well as many different wireless solutions. In the case of a LAN, the physical layer specifications are directly related to the data-link layer protocol used by the network. When you select a data-link layer protocol, you must use one of the physical layer specifications supported by that protocol. For example, Ethernet is a data-link layer protocol that supports several different physical layer options. You can use one of two types of coaxial cable with Ethernet, any one of several types of twisted pair cable, or fiber optic cable. The specifications for each of these options include a great deal of detailed information about the physical layer requirements, such as the exact type of cable and connectors to use, how long the cables can be, how many hubs you can have, and many other factors. These specific conditions are required for the protocol to function properly. A cable segment that is too long, for example, can prevent an Ethernet system from detecting packet collisions. When the system can't detect errors, it can't correct them, and data is lost. While some aspects of the physical layer are defined in the data-link layer protocol standard, others are defined in separate specifications. One of the most commonly used physical layer specifications is the "Commercial Building Telecommunications Cabling Standard," published jointly by the American National Standards Institute (ANSI), the Electronics Industry Association (EIA), and the Telecommunications Industry Association (TIA) as EIA/TIA 568A. This document includes detailed specifications for installing cables for data networks in a commercial environment, including the required distances from sources of electromagnetic interference and other general cabling policies. Nowadays, in most cases, large network cabling jobs are outsourced to specialized contractors, and any such contractor you hire for a LAN cabling job should be intimately familiar with EIA/TIA 568A and other such documents, including your city's building codes. The other communications element found at the physical layer is the particular type of signaling that is used to transmit data over the network medium. For copper-based cables, these signals are electrical charges. For fiber optic cables, the signals are pulses of light. Other types of network media can use radio frequencies, infrared pulses, and other types of signals. In addition to the physical nature of the signals, the physical layer dictates the signaling scheme that the computers use. The signaling scheme is the pattern of electrical charges or light pulses used to encode the binary data generated by the upper layers. Ethernet systems use a signaling scheme called Manchester encoding, and Token Ring systems use a scheme called Differential Manchester.

The OSI Reference Model Protocols

Application Layer
SMB, NCP, FTP,TFTP, NFS. SNMP,Telnet, SMTP

Presentation Layer
NCP, Telnet, FTP,TFTP, NFS, SNMP,SMTP

Session Layer
Telnet, FTP, TFTP,NFS, SNMP, SMTP

Transport Layer
SPX, TCP, UDP,NetBEUI

Network Layer
IPX, IP, ICMP, RIP,OSPF, EGP, IGMP,NetBEUI, DLC,DECNet

Data Link Layer
HDLC, ARP, RARP,NDIS, ODI, LLC, SAP, PPP, SLIP

Physical Layer
CSMA/CD & TokenPassing