This diagram shows that the application layer is the interface to the user that permits the user to request delivery of their email. The application layer presents this request to the transport layer as a data file. The data file is divided up into smaller blocks of data and presented to the session layer. The session layer determines a new session is required (communication link)
between the client and the server and this session information is passed on
to the transport layer that will oversee the transfer of data during the session. The transport layer sends the destination address of the email server
to the network layer. The network layer sends this information to the data
link layer that establishes and maintains a data link connection to the network. The data link layer sends information to the physical layer that converts to data signals to either radio, electrical, or optical formats suitable for
transmission.
Audio digitization is the conversion of analog audio signals into digital form.
To convert an analog audio signal to digital form, the analog signal is digitized by using an analog-to-digital (pronounced A to D) converter. The A/D
converter periodically senses (samples) the level of the analog signal and
creates a binary number or series of digital pulses that represent the level
of the signal.
Analog to digital conversion is a process that changes a continuously varying signal (analog) into a digital values. A typical conversion process
includes an initial filtering process to remove extremely high and low frequencies that could confuse the digital converter. A periodic sampling section that at fixed intervals locks in the instantaneous analog signal voltage,