iOS Seven layer protocol

iOS layer seven

(1) Physical layer--physical
This is the lowest layer of the entire OSI reference model, and its task is to provide a physical connection to the network. Therefore, the physical layer is built on the physical medium (not the logical protocol and session), which provides a mechanical and electrical interface. Mainly including cable, physical port and ancillary equipment, such as twisted pair, coaxial cable, wiring equipment (such as network card, etc.), RJ-45 interface, serial port and the same port in the network are working at this level.
The physical layer provides services such as physical connections, physical service data unit sequencing (receiving the order of bits received by physical entities, the same order of bits sent by physical entities), and data circuit identification.

(2) Data link layer--datalink
The data link layer is based on the physical transmission capability, transmits the data in the frame unit, its main task is to carry on the data encapsulation and the data link establishment. In the encapsulated data information, the address segment contains the address of the sending node and the receiving node, the control segment is used to represent the type of data connection frame, the data segment contains the actual data to be transferred, and the error control section is used to detect errors in the transmission frame.
The protocols that the data link layer can use are slip, PPP, X. +, and Frame relay. Common hubs and low-end switch network equipment are working at this level, modem and other dial-up devices are also. Switches that work at this level are commonly called "second-tier switches."
Specifically, the functions of the data link layer include: the establishment and release of data link connection, the formation of Data link data unit, Data Link connection splitting, delimitation and synchronization, sequence and flow control and error detection and recovery.

(3) Network layer--network
The network layer belongs to the higher level of OSI, from its name can be seen, it solves the network and network, that is, the communication between the Internet, rather than the same network segment inside the matter. The main function of the network layer is to provide routing, that is, to choose the best path to reach the target host and route the packets along that path. In addition, the network layer should be able to eliminate network congestion, with traffic control and congestion control capabilities. Routers in the network boundary work at this level, and now the higher-end switches can also work directly at this level, so they also provide routing capabilities, commonly known as "third-tier switches."
Network layer features include: Establish and dismantle network connections, path selection and trunking, network connectivity multiplexing, segmentation and block, service selection, and traffic control.

(4) Transport Layer--transport
The transport layer solves the problem of the transmission quality between the data and the network, which belongs to the higher level. The transport layer is used to improve the service quality of the network layer, provide reliable end-to-end data transmission, and the usual QoS is the primary service of this layer. The main concern of this layer is the network transport Protocol, which provides a set of network data transmission standards, such as the TCP protocol.
The functions of the transport layer include: image transfer address to network address, multiplexing and segmentation, the establishment and release of transport connections, fragmentation and reassembly, block and chunking.
Depending on the nature of the service provided by the transport layer, the Transport Layer service can be divided into the following three main categories:
Class A: Network connectivity has an acceptable error rate and acceptable failure notification rates (the ratio of network connection disconnects and resets), A-class service is a reliable network service, generally refers to virtual circuit services.
Class C: Network connectivity has an unacceptable error rate, Class C has the worst quality of service, and the provision of a datagram service or radio packet switching network belongs to this category.
Class B: Network connection has an acceptable error rate and unacceptable failure notification rates, Class B services between Class A and Class C, in the WAN and the Internet is more than the provision of Class B services.

The Division of Network Service quality is based on user's requirement. If the user requirements are relatively high, then a network may be attributed to the C type, conversely, a network may be attributed to type B or even a type. For example, for an e-mail system, a network that loses one packet per week may be counted as type A, while the same network can only count as type C for the banking system.

(5) Session Layer--senssion
The session layer uses the transport layer to provide the session service, which may be a user logging on to a host over the network, or a session being established for transferring files.
The functions of the session layer are: the mapping of the session connection to the transport connection, the data transfer, the recovery and release of Session connections, session management, token management, and activity management.

(6) Presentation layer--presentation
The presentation layer is used for presentation of data management, such as ASCII and EBCDIC for text files, to represent a 1S or 2S complement representation of a number. If the two sides of the communication use different data representation methods, they can not understand each other. The presentation layer is used to mask this difference.
The main functions of the presentation layer are: Data syntax translation, syntax representation, connection management, data encryption, and data compression.

(7) Application layer--application
This is the highest level of the OSI Reference Model, it solves the most high-level, that is, the application process of the problem, it directly facing the user's specific application. The application layer contains the protocols and functions that the user application needs to perform communication tasks, such as e-mail and file transfer, and the FTP, SMTP, and pop protocols in the TCP/IP protocol are fully applied in this layer.

iOS Seven layer protocol