OSI Reference Model
What is the OSI Reference Model
In order to promote research and development of the Internet, the International Organization for Standardization ISO has developed a seven-layer reference model framework of network interconnection, known as the OSI reference model, referred to as the OSI / RM (Open System Internetwork Reference Model) . OSI reference model is a layer 7 having the Open Systems Interconnection protocol structure, in the early 1980s, the development of a universal set of specifications by the International Organization for Standardization, so that the computer can be a global open communications.
OSI reference model is a system having a seven-layer model. Content and the corresponding device transmitting and receiving information related to an entity referred to. Each layer of the OSI contains a plurality of entities, at the same level entity called peer entities.
OSI reference model technique also uses a hierarchical structure, the network system is divided into a plurality of layers, each layer to achieve different functions, the function of each layer are in the form of formal description of the protocol, the protocol defines a layer with the same distance one pair other set of rules and conventions used in communications layer. Each layer provides a defined set of services to the adjacent upper and lower adjacent using the service provided. Conceptually, each layer communication with a remote peer layer, but in fact the protocol information unit is generated by means of a layer adjacent to the lower layer transport services are provided. Thus, the communication between the other layer referred to as virtual communication.
Hierarchical structure and function editing
| The number of layers | OSI Reference Model |
|---|---|
| 7 | Application layer |
| 6 | Presentation layer |
| 5 | Session Layer |
| 4 | Transport Layer |
| 3 | Network layer |
| 2 | data link layer |
| 1 | Physical Layer |
Physical Layer
- The physical layer is the lowest layer reference model, and is responsible for transmitting an optical / electrical signal . Such as: voltage, physical data rate, the maximum transmission distance, physical coupling and other similar characteristics. The main function of the physical layer is the transmission medium using a data link layer provides a physical connection, is responsible for the physical transmission of the data stream is work. The basic unit is the physical layer transmission bit stream, i.e., 0 and 1, which is the most basic electrical or optical signal, is the most basic physical transmission characteristics.
data link layer
- The data link layer entity is established between the data communications link coupled to the basic unit of transmission "frame", and provide error control and flow control services for the network layer, and transmits the identification data frames between devices responsible for the 0 , a sequence into meaningful data frame to the opposite end . A data link layer MAC (Media Access Control sublayer) and LLC (Logical Link Control sublayer) composition. The main task of media access control sublayer is specified how a transmission frame on a physical line. LLC sublayer of the communication between devices on the same network link in a managed. Identification of different types of data link control protocol sublayer is responsible for logical, and subjected to encapsulation. That data link control sublayer of the network protocol to accept data packets and packet data control information to add more, thereby the packet to its destination device.
Network layer
- Creating a logical network layer data transfer links between the nodes by routing algorithms to select the best path for the packet, to implement congestion control, networking and other functions mainly. The network layer is critical for network layer router overlooking the highest node, which is responsible for the transmission of packets from the source network to the destination network routing work, transfer the data to the target address, addressing and routing . The Internet is a collection of a plurality of networks composed together, it is by means of a network layer routing path selection function, in order to make the coupling between the flow to a plurality of networks, information can be shared. With a service network layer provides services for the two kinds of coupled and non-coupled oriented. Connection-oriented service is reliable connection service, data connection must be established before the exchange, and then transmit the data to establish the connection after termination of service before the end. As to the network layer service virtual circuit connection-oriented service implementation. No connection-oriented service is an unreliable service, it can not prevent the loss of packets, retransmission or disorder. No advantage for service in its service coupled flexible manner, and very quickly. Network layer datagram service as to achieve the coupling faces free service.
Transport Layer
- Transport layer is a network architecture of the interface between a high and low interfacial layer. Transporting layer is not only a single layer structure, but the core of the whole system, protocol analysis, acts reliable transmission, a communication processing between both nodes, the router does not need treatment . The main transport layer to provide users with End-to-End (end) service, transmission problems datagram processing errors, packet ordering the like. Transport layer is a critical computer communication architecture layer, which shields the details of the underlying data communication to the high-level, so that users need not consider the physical layer, data link layer and the details of the work of the network layer. The transport layer using a network connection service provided by the network layer, using a service-oriented system according to the coupling requirements may be selected when the data transfer or connectionless oriented services.
Session Layer
- The main function of the session layer is responsible for establishing and disconnecting a session connection, and the divided data like data related to the transmission management . Session layer establish in the application process, manage and terminate sessions. Session layer may also be used to determine which communication, full duplex communication or half duplex communication by using session control. Coordination session layer request and responses through its own protocol.
Presentation layer
- The presentation layer provides services for the representation of information transmitted between the application process. The presentation layer is mainly to complete the following layers from the source end to the destination end reliable data transmission, and the presentation layer is more concerned with the syntax and semantics of the data transmitted. The main function of the presentation layer representation of information exchange processing in the two communication systems, including changes in the data format, data encryption and decryption, data compression and decompression and the like . Under certain preconditions bandwidth data compression smaller the faster the transmission rate, the data indicating the compression and decompression layer is considered a key factor in control of transport rate. The presentation layer provides data encryption service is an important network security features, which ensure the secure transmission of data, but also the various security services of the most important key. The presentation layer to the application layer services offered include: syntax conversion, grammar and select Connection Manager.
Application layer
- The application layer is the highest layer in the OSI model is a direct user-oriented layer, the content of the user's communication to be solved by the application process, which requires the application layer using different application protocols to solve different types of application requirements, and to ensure that these different low-level communication protocol used by the application type is the same . The application layer contains a number of generic service independent of user protocol module, a dedicated network for communication between the user program service. Note that the application layer is not the application, but to provide services for applications.
Control protocol layers
About common protocol
| protocol | Features |
|---|---|
| IP protocol | The end of the design principles, the host provides a connectionless and unreliable, best-effort datagram delivery service. |
| ICMP protocol | Is based on network-layer connectionless messages, Internet Control Message Protocol, the role of the backhaul network is smooth, whether the host is reachable, such as message routing information is available. |
| ARP protocol | ARP, is to obtain a physical address of the TCP / IP protocol, the IP address |
| PARP agreement | Reverse Address Resolution Protocol, according to the physical address, IP address mapping |
| UDP protocol | User data transfer protocol, connectionless unreliable datagram-oriented |
| TCP protocol | User transmission protocol, there are links, reliable transmission, the byte stream oriented |
| FTP protocol | File transfer protocol, application layer protocol |
| Telent agreement | A protocol for remote login network computers |
| DNS protocol | Domain Name Service system, a protocol on the internet domain names and IP addresses associated with each other. |
| SMTP protocol | Simple Mail Transfer Protocol, based on application-layer protocol TCP / IP protocol suite |
| POP3 protocol | Post Office Protocol version 3, is mainly used to support the use of e-mail client remote management on the server. |
| HTTP protocol | Hypertext Transfer Protocol, used to transmit hypermedia documents, such as HTML, for communication between web browsers and web servers designed |
| SNMP protocol | A standard protocol designed specifically for managing IP network nodes (servers, workstations, routers, switches) |
| TFTP protocol | It is the TCP / IP protocol suite designed for a simple file transfer protocol between servers and clients, providing less complex, less overhead file transfer service |
Layers of physical devices
Physical Layer : repeaters, hubs
data link layer : a bridge, switch
network layer : Routers
