ISO / OSI network architecture
Computer networks are very complex systems, two computer systems communicate with each other must be highly coordinated to work properly. To design such a complex computer network, a method has been proposed in the network hierarchy. Layered may convert large and complex problem into smaller local problem treated, so that simplification of the problem.
International Organization for Standardization was established in 1997 a sub-committee specializing in architectural issues of network communication, and proposed the Open Systems Interconnection reference model, which is a heterogeneous computer connects to a standard frame structure definition. OSI provides the foundation for connecting distributed applications "open" system. The so-called "open" refers to any system as long as the two follow the reference model and related standards can be interconnected. OSI uses a hierarchical structure construction techniques.
A, ISO / OSI reference model
ISO / OSI seven layer reference model, as shown in FIG. From the bottom to the top were a physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer.
The OSI reference model has the following characteristics.
- It is a system of interconnected heterogeneous hierarchy.
- It provides a standard framework for the interconnected system interaction rules.
- It defines an abstract structure, rather than the specific implementation described.
- Different entities of the same layer on the peer entity of the system worked.
- Communication between the protocol is managed by the peer entity of the layer.
- Between adjacent layers interface defines the primitive operations and services provided to the lower level.
- Public services provided by connection-oriented or connectionless data services.
- Direct data transfer is only achieved at the lowest level.
- Each completion defined editing this layer does not affect other layers.
OSI / 1 ~ 3 RM layer mainly responsible for the communication function, generally referred to as communication sub-layer. Three belong to the scope of the functionality resources subnets, subnet called resource layer. Since the transmission convergence layer, under the action of three layers. Description of each layer follows.
The physical layer (Physical Layer)
The physical layer provides for the establishment, maintenance and removal of mechanical characteristics, electrical, functional and physical links required procedures; bitstream and provide a physical transmission link fault detection indication unstructured over a transmission medium.
User information to be transmitted must use some physical media, such as twisted pair, coaxial cable, etc., but not in the specific physical media within the OSI 7 layers, it was the physical medium as a level 0, the task is the physical layer it is on a physical connection, and their mechanical, electrical, functional and procedural characteristics of a layer. For example, using a predetermined type of cables and connectors, voltage or the like of the transmission signal. In this layer, data is not organized, as only the original bitstream voltage or electrical treatment, in bits.
Data Link Layer (Data Link Layer)
The data link layer is responsible for transmitting two bits of data in units of frames on the line between nodes without error, and adjacent flow control. Each frame comprises a number of data and control information necessary. And similar physical layer, data link layer is responsible for establishing, maintaining and releasing a data link layer connection. When transmitting data, if the reception point is detected in transmitted data has an error, the sender should retransmit the notice frame.
Network layer (Network Layer)
Transport layer, network layer entities bit-end switching function of the data network, so that out of the transport layer routing, congestion control, and exchange network transmission details; can create, maintain, and removal of one or more communication paths as a transmission layer entity; for unrecoverable error in the transmission network be reported.
May go through a lot of data link communications between two computers in a computer network, it may go through a lot of subnetworks. The task is to choose the appropriate network layer routing and inter-network switching nodes, to ensure the timely delivery of data. Frames the packet network layer provides data link layer, the network layer packets are encapsulated in header containing logical address information, i.e., address of the source network site and the destination site.
Transport Layer (Transport Layer)
Session layer provides transport layer is a transparent solid, reliable data transmission services to ensure end to end data integrity; the network layer to provide optimum services; establishing, maintaining and removing transport connection. The optimal use of the transport layer characteristics of the communication sub-network resources to provide established between two end systems (i.e. a source station and a destination station) session layer, and a cancel function to maintain transport connections, reliable and economical the transmission of data. In this layer, the transmission unit of information is the message.
The session layer (Session Layer)
Presentation Layer Session Layer entities collaborate with each other to provide for the establishment, maintenance and end of the session function; correspondence between the logical name and the physical names of the completion of the communication process; providing session management services.
This layer may also be referred to the session layer or session layer, the session layer and above level, the unit of data transmission no additional names, referred to as packets. Session layer does not participate in a particular transmission, it provides a mechanism for communication between the establishment and maintenance of applications including access, including authentication and session management. Trying to validate user login server is completed by the session layer.
Presentation Layer (Presentation Layer)
The presentation layer provides for the application layer process can exchange information interprets the meaning of a set of services, the data is about to be exchanged for converting abstract syntax from a user is adapted to transmit an internal syntax OSI system; providing representation and convert formatted data service. (Data compression, decompression, encryption and decryption by the presentation layer is responsible for work).
The application layer (Application Layer)
OSI application layer provides user services, that determine the nature of the communication between processes, in order to meet user needs and provides the interface between the network service and user application software, for example, transaction processing programs, email and network management procedures.
Second, the reference model of the flow of information
, User B transmission data of the user would like the system A system is provided as shown below. A system user data is first sent to the application layer, which additional control information to it after the AH (header), into the presentation layer. Layer represents the necessary data conversion and added into the session layer header PH after. The session layer is also added into the transport layer header SH. The length of the transport layer packet is segmented and added to the network layer header TH. The network layer packet information into packets, and add NH group number to send the data link layer. The Data Link layer information with a header and trailer (DH and DT) into the frame, to the counterpart bit physically layer (B system). After receiving the information system B, system A in accordance with the opposite action, strip the layers of control information, and then transmits the data to the original system B user. Be seen, the system has only two real physical communication layer, the other layers are virtual communication. Thus, there is only a physical connection between the physical layer of the two figures, the other layers have no connection.
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