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OSI data transmission

Source: Shangpin China | Type: website encyclopedia | Time: June 23, 2014
1. Data encapsulation technology

Data encapsulation refers to the packaging of data to be transmitted. In OSI's seven layer reference model, data encapsulation is the process in which the upper layer PDU, as the transmission data of this layer, is encapsulated between the protocol header and protocol tail of this layer or behind the protocol header of this layer.

Protocol header, transmission data and protocol tail are three relative concepts. For example, the transport layer protocol header (TH) contains information that can only be seen by the peer transport layer, while the network layer below the transport layer will transmit the transport layer protocol header as the data part of the network layer. At the network layer, a PDU is composed of the network layer protocol header (NH) and the PDU transmitted from the transport layer; At the data link layer, a PDU consists of the data link layer protocol header, the PDU passed from the network layer, and the data link layer protocol tail.

2. Data transmission process

[Example] The user passes a Beijing website production The application program generates a data stream and sends it to host B. The entire data transmission process - data encapsulation and data unpacking is shown in Figure 3-3.
The whole data transmission process - data encapsulation and data unpacking

(1) Add the control information (AH) necessary for communication between the application layer of host A and the application layer of host B to the header of the transmission data DATA to form an application layer message sent to the presentation layer.

(2) Host A presentation layer receives the upper layer message data, and adds the control information (PH) required for communication with host B presentation layer in front of it to form the message data of this layer and send it to the session layer.

(3) When the host A session layer receives the upper layer message data, the control information (SH) required for communication with the host B session layer is added in front of it to form the message data of this layer and send it to the transmission layer.

(4) When the host A transport layer receives the upper layer message data, it first groups the data according to the "maximum transmission unit" of the network, and then adds the control information (TH) required for communication with the host B transport layer before the data after the packet to form the packet data to be submitted to the network layer.

(5) Host A network layer receives the upper layer packet datagram, and adds the control information (NH) required for communication with host B network layer in front of it to form a packet sent to the data link layer.

(6) After the data link layer of host A receives the upper layer data packet, the control information (DH and DT) required for communication with the data link layer of host B (when B and A are on the same link) or the next service node (when B and A are not on the same link) is added before and after the upper layer data packet to form a data frame sent to the physical layer.

(7) The physical layer transmits the data transmitted from the data link layer as a bit stream signal through the communication medium.

(8) The physical layer of host B receives the bit stream signal and transmits it to the data link layer of host B.

(9) Host B unpacks the data layer by layer from bottom to top in reverse according to the process of host A encapsulating the data. After removing the protocol header of this layer (the data link layer has protocol tails), it submits the data to the upper layer until the user of host B sees the data DATA originally sent at the application layer.

The data unit encapsulation process of each layer seems complicated, but it is very important and necessary in the whole network architecture. It makes the network communication modular and easy to manage.
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