When a host receives a data transmission from another host on the network, that data is processed at each level of the OSI to the next higher layer, in order to present a useful data transmission to the end-user. Part of this processing entails the creation of headers and trailers by the sending host’s software or hardware that are placed before or after the data is sent to the next higher layer. Each layer is tagged with a header and trailer, usually in each data packet that comprises the data flow. The following is a break down of the sequence of processing at each OSI layer:
- The Physical Layer (1st Layer) ensures bit synchronization and places the received binary pattern into a buffer. It alerts the Data Link Layer (Layer 2) a frame has been received after decoding the incoming signal into a bit stream. Layer 1 performs as a delivery system for a stream of bits across the medium.
- The Data-Link Layer (2nd Layer) looks at the frame check sequence (FCS) in the trailer to check if errors occurred in transmission. If an error is discovered, the frame is discarded. The current host inspects the data link address to determine if it’s intended to receive the data or whether to process the data further. If the data is addressed to the host, the data between the Layer 2 header and trailer is passed on to the Network Layer (Layer 3) software.
- The Network Layer (3rd Layer) checks the destination address. If the address is the current host’s address, processing continues and data after the Layer 3 header is passed to the Transport Layer (Layer 4) software. In this case Layer 3 provides end-to-end delivery.
- The Transport Layer (4th Layer) performs error recovery. To detect these errors, identifying pieces of data are encoded in the Layer 4 header along with acknowledgment of information. After error recovery and reordering of the incoming data, it is then sent to the Session Layer (Layer 5).
- The Session Layer (5th Layer) verifies that a series of messages is completed. The Layer 5 header includes fields indicating sequence of the packet in the data stream, and the position of the data packet in the flow. After this layer has confirmed that all flows are completed, it sends the data after the Layer 5 header to the Presentation Layer (Layer 6).
- The Presentation Layer (6th Layer) defines and controls the data format of the data transmission. It places the data in the proper format specified in the Layer 6 header. Typically, this header is included exclusively for initialization flows, not with every data packet that’s sent. After the data formats have been converted, the data after the Layer 6 header is forwarded to the Application Layer (Layer 7).
- The Application Layer (7th Layer) works with the final header and inspects the end-user data. The agreement of operating standards is signaled by the header by the applications on the two hosts. The headers indicate the values for all parameters so the header is usually transmitted and received at application initialization time only. Not only is this phase responsible for processing between adjacent OSI layers, all layers must interact with their corresponding layers on another computer to successfully utilize its functions.
For authentic communication with the same layer on another computer, each layer identifies additional data bits in the header as well as the trailer in some instances that’s generated by the sending host’s software or hardware. The recipient host reads the headers and trailers generated by all corresponding layers of the sending host in order to decipher processes of each layer and how to interact within their structure.