When a host receives a data transmission from another host on the network, that data is processed at each of
the OSI layers to the next higher layer, in order to render the data transmission useful to the end-user. To
facilitate this processing, headers and trailers are created by the sending host's software or hardware, that are
placed before or after the data given to the next higher layer. Thus, each layer has a header and trailer,
typically in each data packet that comprises the data flow. The sequence of processing at each OSI layer, i.e.,
the processing between adjacent OSUI layers, is as follows:
• The Physical Layer (Layer 1) ensures bit synchronization and places the received binary pattern into a
buffer. It notifies the Data Link Layer (Layer 2) that a frame has been received after decoding the
incoming signal into a bit stream. Thus, Layer 1 provides delivery of a stream of bits across the medium.
• The Data Link Layer (Layer 2) examines the frame check sequence (FCS) in the trailer to determine
whether errors occurred in transmission, providing error detection. If an error has occurred, the frame is
discarded. The current host examines data link address is examined to determine if the data is addressed
to it 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 handed over to the Network Layer (Layer 3) software. Thus, the data link layer
delivers data across the link.
• The Network Layer (Layer 3) examines the destination address. If the address is the current host's
address, processing continues and the data after the Layer 3 header is handed over to the Transport Layer
(Layer 4) software. Thus, Layer 3 provides end-to-end delivery.
• If error recovery was an option chosen for the Transport Layer (Layer 4), the counters identifying this
piece of data are encoded in the Layer 4 header along with acknowledgment information, which is called
error recovery. After error recovery and reordering of the incoming data, the data is given to the
Session Layer (Layer 5).
• The Session Layer (Layer 5) ensures that a series of messages is completed. The Layer 5 header
includes fields signifying sequence of the packet in the data stream, indicating the position of the data
packet in the flow. After the session layer ensures that all flows are completed, it passes the data after the
Layer 5 header to the Presentation Layer (Layer 6) software.
• The Presentation Layer (Layer 6) defines and manipulates the data format of the data transmission. It
converts the data to the proper format specified in the Layer 6 header. Typically, this header is included
only for initialization flows, not with every data packet being transmitted. After the data formats have
been converted, the data after the Layer 6 header is passed to the Application Layer (Layer 7) software.
• The Application Layer (Layer 7) processes the final header and examines the end-user data. This header
signifies agreement to operating parameters by the applications on the two hosts. The headers are used to
signal the values for all parameters; therefore, the header typically is sent and received at application
initialization time only.
In addition to processing between adjacent OSI layers, the various layers must also interact with the same
layer on another computer to successfully implement its functions. To interact with the same layer on
another computer, each layer defines additional data bits in the header and, in some cases, trailer that is
created by the sending host's software or hardware. The layer on the receiving host interprets the headers
and trailers created by the corresponding layer on the sending host to determine how that layer's processing
is being defined, and how to interact within that framework.