Networking Basics seven principles
1. What is the seven principles of network infrastructure
II. Originated seven layer model
III. Principles and protocols seven layer model
IV. What does the seven-layer model
1. The seven-layer model, also known as the OSI (Open System Interconnection). Reference Model International Organization for Standardization (ISO) to develop a standard system for communication between computers or interconnected system, commonly referred to as OSI Reference Model or seven-layer model.
It is a seven-layer model abstract body, including not only a series of abstract terms and concepts, including the specific protocol.
2.OSI most of the design work is really just Honeywell Information System
The company completed a team, team technical director is Charlie Bachman. In the mid-1970s, this group is mainly to develop some prototype system was established, the design focuses on database systems. In the 1970s, in order to support access to the database system, distributed communication system architecture requires a structured.
The team then studied a number of existing solutions, including IBM's SNA (System Network Architecture), ARPANET (Internet 's predecessor) protocol, as well as standardized database is studying some of the presentation services (presentation services) of concepts, in 1977, proposed a seven-story structure model system, they call internal distributed systems architecture (DSA).
At the same time, in 1977 the British Standards Institute to propose to the International Organization for Standardization (ISO), in order to define the distribution of communications infrastructure between the treatment requires a standard architecture. The results, ISO on the Open Systems Interconnection (OSI) issues set up a special committee (TC 97, Subcomittee 16), specified by the American National Standards Institute (ANSI) to develop a draft standard, submitted before the first official meeting of the special committee .
Bachman [1] ANSI attended early meetings, and submitted his seven-layer model, this model became the only ISO submit a draft special committee.
March 1978, at a meeting of the ISO OSI special committee convened in Washington, experts soon reached a consensus that a layered architecture that can meet the needs of the majority of open systems, but also has the ability to be expanded to meet the new demand.
Then, in 1978, issued a temporary version of this, a little refinement in 1979, became the final version. So, OSI model and the 1977 DSA model is basically the same.
3. The principles and detailed description 
1. application layer (Application Layer)
The application layer is the OSI layer closest to the user. This layer is the user's application (such as e-mail, file transfer and terminal emulation) to provide network services. .
There are protocols: HTTP FTP TFTP SMTP SNMP DNS TELNET HTTPS POP3 DHCP and so on.
The application layer is also referred to as an application entity (AE), which consists of a plurality of specific application service element (the SASE) and one or more common application service elements (CASE) composition. Each SASE provide specific application services, such as file access and transport management (FTAM), electronic document electrical treatment (MHS), virtual terminal protocol (VAP) and so on. CASE provides a common set of application services, such as association control service element (the ACSE), reliable transport service element (the RTSE) and Remote Operations Service Element (ROSE) and the like. Primarily responsible for providing an interface to the software so that programs can use network services. The term "application layer" does not refer to a particular service application, the application layer running on the network include information processing file transfer, file management, and e-mail.
Presentation Layer (Presentation Layer)
represent data, security, compression. Information system ensures that a transmitted application layer may be read by the application layer of another system.
Format: JPEG, ASCll, DECOIC, encrypted format.
Translator between the application and the network, the presentation layer, the data will be formatted according to the network can understand programs; this format is also used depending on the type of network varies.
And decrypting the encrypted management data presentation layer, the processing system passwords. For example: check your bank account on the Internet, that is, using a secure connection. Your account data is encrypted before transmission, the other end of the network, the data will be received presentation layer decryption. In addition, the presentation layer protocol and also the picture file format information to decode and encode.
Session layer (Session Layer)
to build, manage, terminate sessions, corresponding to host process, means the local host and the remote host ongoing conversation.
Establishing the data transmission path: through the transport layer (transmission and receive ports of port number). Mainly between your system initiates a session or accept the session request (need to recognize each other device can be IP or MAC can also be a host name).
In charge of the network between the two nodes to establish, maintain and terminate communication. Session layer functions include: establishing a communication link, communication links remain open session, simultaneous dialogue between two nodes, decide whether the communication is interrupted and decide where to send again when communication is interrupted.
You may often hear the session layer called "traffic cop" network communications. When a request to connect to the Internet, your ISP (Internet Service Provider) through dial-up, session layer on the ISP server negotiate a connection with the session layer on your PC clients to you. If your telephone line accidentally detached from the wall jack, the session layer on your terminal will detect the interrupt and re-initiate the connection is connected. Session layer is provided by the length of the communication period and the communication priority decision node of the communication time.
Transport Layer (Transport Layer)
definition of transmission of data protocol port, and flow control and error checking.
There protocols: TCP UDP and the like, i.e. packets entering the network card upon leaving the transport layer.
Defines the protocol and port number (WWW port 80, etc.) for transmitting data, such as: TCP (Transmission Control Protocol, low transmission efficiency, reliability, high transmission reliability for the data amount of data), the UDP ( user datagram protocol, TCP characteristics and on the contrary, for the less demanding transmission reliability, a small data amount of data, such as QQ chat data is transmitted in this way). Segment is primarily from the lower layer receiving and transmitting data to the destination address after restructuring. Often this layer is called a data segment.
OSI model is the most important one. At the same time transport protocol flow control or predetermined based on how fast data is received before receiving an appropriate transmission rate. In addition, the transport layer according to the maximum size of the network can process packets longer forced division. For example, can not receive Ethernet greater than 1500 byte packets. When the transport layer of the sending node data is divided into smaller pieces of data, while the data for each slice to arrange a sequence number, so that data arrives at the transport layer of the receiving node can be recombined to correct order. The sorting process is to be known. In one work service transport layer is a TCP / IP protocol suite TCP (Transmission Control Protocol), another transport layer service is SPX IPX / SPX protocol suite (SEQ packet switching).
Network layer (Network Layer)
for addressing the logical address, to achieve routing between different networks.
There protocol: ICMP IGMP IP (IPV4 IPV6) ARP RARP and so on.
And provides a connection path selection between two host systems are located geographically in the network. Development of Internet allows users to access information from the site of the world greatly increased, and the network layer is the management of this layer connections.
The third layer of the OSI model, its main function is to translate a network address into a corresponding physical address, and to determine how to route data from a sender to a recipient.
By considering the network layer takes priority transmission, the network congestion level, quality of service and routing alternative is determined from one network node to another network node A path B is the best. Because the network layer processing, and guide smart data transfer network segments connected to the router, so router that belongs to the network layer. In the network, "routing" is transmitted addressing scheme used to direct mode, and data accessibility based.
The network layer is responsible for establishing the route they use between the source machine and the target machine. This layer does not have any error detection and correction mechanism, therefore, it must rely on the reliable network layer transport services provided by end to end between the DLL.
A computer system on the network layer of the local LAN segment to establish a communication, it can do this, because the routing address of its own structure, which machine address and the second layer are separate and independent. This protocol may be referred to as a route or a routing protocol. Routing protocols including IP, N ovell's IPX protocol and A pple Ta lk.
The network layer is optional, it is only used when the computer system is in two different division network by the router to open the case, or when a communication application requires a certain service network layer or transport layer provides the characteristics or ability . For example, when the two hosts are in the same LAN segment directly connected to this case, the communication between the LAN communication mechanisms are used only on it (i.e. a Layer of the OSI reference model).
The data link layer (Datalink Layer)
establishing a logical connection, hardware address addressing, error checking and other functions. (Defined by the underlying network protocol)
the bits into bytes and then combined into frames, using the MAC address access media, an error is found but not corrected.
Representative data link layer protocol comprises: SDLC, HDLC, PPP, STP , FR.
It defines how to format data for transmission, and how to control access to the physical medium. This layer usually provide error detection and correction, to ensure reliable data transmission.
The second layer of the OSI model, which controls communication between the network layer and the physical layer. Its main function is to how reliable delivery of data over unreliable physical line. In order to ensure transmission, the received data from the network layer is divided into a particular frame may be transmitted on the physical layer. The frame structure is used to move the data packet, which includes not only the original data, further comprises a physical address of the sender and receiver as well as error detection and error control information. Wherein determining the address of where to send the frame, and the error correction and control information to ensure error-free frame arrival. If the transmission of data, the reception point is detected in transmitted data has an error, the sender should retransmit the notice frame.
The data link layer is independent of the network and its nodes and type of physical layer used, it does not care whether running Wo rd, E xcel or I nternet. Some connecting devices, such as switches, since they are to be decoded and the frame to transmit data to the correct recipient using the frame information, so they work in the data link layer.
Data link layer (DataLinkLayer): the physical layer bitstream based on the service, establishing a data link between adjacent nodes, there is provided a data frame (Frame) in an error-free transmission through the channel error control, and operation on each series circuit.
The data link layer provides reliable transmission over unreliable physical media. This layer serves comprising: addressing a physical address, data framing, flow control, error detection data, such as retransmission.
The physical layer (Physical Layer)
to establish, maintain, is physically disconnected. (Defined by the underlying network protocol)
primary standard defines a physical device, such as a cable interface types, the interface of the optical fiber type, transmission rate and other transmission media. Its main role is the transmission bit stream (1,0 is made into the current strength to be transmitted, the destination after conversion to 1,0, that is, we often say that the digital to analog conversion and analog to digital conversion). This layer is called the data bits.
A first lowest layer or layers of the OSI model, the physical layer includes a networking media, such as wired cable connector. Physical layer protocol and generates a detection voltage to transmit and receive data carrying signals. Into the network interface card on your desktop PC, you can build a foundation connected computer. In other words, you provide a physical layer. Although the physical layer does not provide error correction service, but it can be set to monitor the data transmission rate and the data error rate. Network physical problems, such as wire disconnection, will influence the physical layer. User information to be transmitted must use some physical media, such as twisted pair, coaxial cable, etc., but the medium is not specific physical layers in the OSI 7, it was the physical medium as a level 0, the task is to the physical layer it provides a layer of a physical connection, and their mechanical, electrical, functional and procedural characteristics. The use of predetermined types 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 units of bits bit.
4. The protocol of each layer and how data is received
5. Application Layer protocols: HTTP Hypertext Transfer Protocol, FTP File Transfer Protocol, TFTP: Trivial File Transfer Protocol, IP address, domain name parsing protocol
6. Transport Layer: TCP: Port Protocol security inefficient, UDP: port protocol, efficient insecure
7. network layer: ICMP: packet data control protocol, IGMP: control user datagram protocol, IP protocol, ARP: IP address to a MAC address resolution protocol, RARP: MAC address resolve the MAC address. 
8. seven models use
a device in which a layer of working, the key information header data which layer it uses during operation. When the bridge work, based on the MAC header to determine the forwarding port, the device is therefore evident that it is the data link layer.
Specifically:
a physical layer: LAN, cable, hub, repeater, a modem
data link layer: a bridge, switch
network layer: router
gateway program in the fourth layer is the transport layer and above.
Hubs are physical-layer device, in the form of broadcast information to transmit.
Switch the machine is used for packet switching. Multi-link layer device (switcher), address learning can be performed using the store and forward message exchange form ..
The role of a router is different communication network and the other role is to select the information transmission lines. Select smooth and efficient shortcut, can greatly increase the communication speed, to reduce network traffic load, saving network resources and enhance the network flow rate.