What is Data Encapsulation?

Data is encapsulated with protocol information at each layer when it is transmitted across a network. As data moves through the layers, communication occurs within the peer layer before moving to the next layer. Each layer uses Protocol Data Units (PDUs) to communicate and share information. PDUs contain the data's control information at each layer of the OSI model. Each PDU has a specific name and is encapsulated to the data at each layer. The peer layer that receives the data reads the PDU information before stripping it off and sending the data to the next layer.

Upper-layer data is converted for transmission and the data stream moves down to the Transport layer, resulting in a virtual circuit. From there, the data stream is segmented and a new PDU is created and attached to the data field's header. This new data segment is aptly named a "segment." The individual segments are sequenced for later reassembly on the receiving device.

The segments are handed to the Network layer where network addressing and routing take place. The segments arrive to the appropriate network because of logical addressing. The segments receive a control header from the Network layer protocol resulting in a packet or datagram.

The Data Link layer takes these packets and encapsulates them into a frame with the hardware addresses stored in the frame’s header. When a destination device is located on a remote network, the frame goes to a router for routing. After arriving at the network, a new frame transports the packet to the destination host.

The Physical layer encodes the frame's digits of 1s and 0s into a digital signal. This signal is then read and decoded by the receiving device. From there, the device builds the frames based on the encoded signal and verifies the integrity of the frame by running a CRC and cross-referencing the answer in the frame’s FCS field. If the answer is correct, the packet is extracted and the remainder of the frame discarded. After de-encapsulation, the packet goes to the Network layer. After verifying the address, the segment is extracted the packet and the remainder of the packet discarded. The process continues with the segment going through the Transport layer, the Transport layer rebuilding the data stream, and the Transport layer sending verification that each piece arrived safely. The Transport layer then sends the data stream to the upper-layer application.

After a data receives header information from the Transport layer, the data becomes a segment which is handed, along with the destination IP address, to the Network layer. From there, the Network Layer adds header and IP address information to each segment. A protocol field of a packet details its origin and allows the Network layer to use the proper protocol. Using Address Resolution Protocol (ARP), the Network layer finds the destination address. If the address is a local network, an ARP request is issued for the local host's hardware address. If not, IP looks for the IP address of the router. From there, the packet and its destination address goes to the Data Link layer which adds a header. The data is now a frame. Cycle redundancy checks are run before the frame is sent to the Physical layer. The Physical layer encodes the data. After being encoded into a digital signal, the data can then be decoded. The devices on the network will decode the data, rebuild the frame, and check the destination address.

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