for him or her and then goes ahead and reverses part of
the process you went through, removing the paper from
the envelope and reading the message. The message is
successfully delivered from source to destination in both
cases—on a network and via snail mail—but it takes a
fraction of a second for data to get anywhere on planet
Earth on a network, whereas it might take days or weeks
for snail mail to reach a recipient, depending on how far
the recipient is from the sender.
Data transmission protocols have been developed at each
layer of the OSI model to perform the functionality
required by that layer. Each layer is supposed to perform
specific tasks and provide specific functionality to the
layers above it.
The physical layer is tasked with the transmission and
reception of raw bit streams. At this layer, specifications
about the voltage levels, pinouts for the connectors,
physical transmission data rates, modulation schemes,
and so on are defined. This is where the proverbial
rubber meets the road, and data to be transmitted is
converted into electrical, optical, or radio signals.
Protocols such as Ethernet and Bluetooth have
specifications at the physical layer.
The data link layer provides device-to-device data
transfer on a local-area network. It defines how data is
formatted for transmission and how access to the
physical medium is controlled. Errors that might have
happened in transit on the physical layer are corrected at
the data link layer. Bridges and switches are devices that
operate at the data link layer. The IEEE has several
protocols defined for the data link layer, organized into
the IEEE 802 family of protocols. According to the
specifications in these protocols, the data link layer is
further subdivided into two sublayers:
The Media Access Control (MAC) layer provides specifications on how
devices gain access to the transmission medium.