Layer-7 ios

(1) Physical layer -- Physical
This is the lowest layer of the OSI reference model. Its task is to provide physical connections to the network. Therefore, the physical layer is built on physical media (rather than logical protocols and sessions) and provides mechanical and electrical interfaces. It mainly includes cables, physical ports, and ancillary equipment, such as twisted pair wires, coaxial cables, and wiring equipment (such as NICS) RJ-45 interface, serial port and parallel port in the network are working at this level.
Services provided by the physical layer include physical connections, data units of Physical services in sequence (the bits received by physical entities are received in the same order as those sent by physical entities), and data circuit identifiers.

(2) data link layer-DataLink
The data link layer transmits data in frame units based on the physical transmission capability. Its main task is to encapsulate data and establish data links. In the encapsulated data information, the address segment contains the addresses of the sending and receiving nodes. The control segment is used to indicate the type of the data connection frame. The data segment contains the actual data to be transmitted, the error control segment is used to detect frame errors during transmission.
Protocols available on the data link layer include SLIP, PPP, X.25, and frame relay. Common network devices of hubs and low-end switches work at this level, and dial-up devices such as Modem are also. Vswitches at this level are commonly known as Layer 2 vswitches ".
Specifically, the functions of the data link layer include: the establishment and release of data link connections, the construction of data link data units, the split, division and synchronization of data link connections, sequence and traffic control, and the detection and recovery of errors.

(3) Network layer-Network
The Network Layer belongs to a higher level in OSI. From its name, we can see that it solves the communication problem between the network and the network, that is, the Internet, rather than the internal problem within the same network segment. The main function of the network layer is to provide routes, that is, to select the optimal path to reach the target host and send data packets along the path. In addition, the network layer can also eliminate network congestion and have traffic control and congestion control capabilities. Vrouters in the network boundary work at this level. Currently, more advanced vswitches can work at this level. Therefore, they also provide the routing function, also known as "layer-3 vswitches ".
Features at the network layer include: establishing and removing network connections, selecting and relay routes, multiplexing of network connections, segmentation and block formation, service selection, and traffic control.

(4) Transport Layer-Transport
The transport layer solves the quality of data transmission between networks. The transport layer is used to improve the quality of service at the network layer and provide reliable end-to-end data transmission. QoS is the main service of this layer. This layer mainly involves the network transmission 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 division, establishment and release of transmission connection, segmentation and re-assembly, block and block.
Based on the main nature of the services provided by the transport layer, the transport layer services can be divided into the following three categories:
Class A: the network connection has an acceptable error rate and an acceptable fault notification rate (the rate of network connection disconnection and reset). Class A services are reliable network services, generally, it refers to the virtual circuit service.
Class C: the network connection has an unacceptable error rate, and class C has the worst quality of service. This is true for providing datagram services or radio group switching networks.
Class B: the network connection has an acceptable error rate and an unacceptable fault notification rate. Class B services are between Class A and Class C services, and Class B services are provided on the WAN and Internet.

The division of network service quality is based on user requirements. If the user requirements are relatively high, a network may belong to type C. Otherwise, a network may belong to type B or even type. For example, for an email system, the network that loses a group every week may be counted as a type, and the same network can only be counted as C type for the banking system.

(5) Session Layer-Senssion
The Session Layer uses the transport layer to provide the session service. A session may be a user logging on to a host over the network, or a session being established for transferring files.
The Session Layer provides the following functions: ing of Session connections to transmission connections, data transmission, restoration and release of Session connections, session management, Token management, and activity management.

(6) Presentation layer-Presentation
The presentation layer is used to represent data management, such as ASCII and EBCDIC for text files. It is used to represent the 1 s or 2 s complement representation of numbers. If the communication parties use different data representation methods, they cannot understand each other. The presentation layer is used to block such differences.
The presentation layer provides the following functions: Data syntax conversion, 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 highest level, that is, the problems in the application process. It directly faces the specific applications of users. The application layer includes the protocols and functions required for your applications to execute communication tasks, such as email and file transmission, in this layer, FTP, SMTP, POP, and other protocols in TCP/IP are fully applied.