A popular way to remember the OSI model is to use a mnemonic device. A mnemonic device is where the first letter of each word in a sentence stands in for the first letter of whatever list it is that one is attempting to memorize.
From top to bottom:
All People Seem To Need Data Proccesing.
From bottom to top:
Please Do Not Throw Sausage Pizza Away.
When a computer has data to send to another computer it formats that request and sends it to the network protocol at the top of the layer, the application layer. The application layer performs an operation on the request and passes it down the layer protocol stack and into whichever physical media is being used to connect the two computers. At the receiving end the data works its' way back up the stack; from the physical layer to the application layer.
This layer is responsible for controlling the functioning of the physical interface and includes transmission technique, pin layout, and connector type.
On the receiving
end the data link layer takes raw data handed up to it by the physical layer
and determines which computer sent the data, the validity of the bytes sent,
and where it should be sent. On the sending end after data is sent the data
link layer waits for a positive ACK (acknowledgment signal). If no ACK is received,
or if the data frame is damaged, another frame is sent. This layer also controls
functions of logical network topologies, physical addressing, data transmission,
synchronization, and connection.
3. The Network Layer
This layer performs
logical addressing and the translation of logical names into physical addresses.
The network layer also carries out QoS (Quality of Service) prioritization that
determines the order of various data transmission processes that may be underway.
Congestion control, data routing, and the building and tearing down of packets
is also performed at this level.
4. The Transport layer
This layer is responsible
for making sure that data is error free. It divides long messages into shorter
ones for transmission and, in the reverse, takes short messages and recombines
them into one long one. This layer also handles logical address/name resolution
and the sending of ACK (acknowledgment) messages that it got the data packet.
5. The Session layer
The session layer
defines how networked computers establish, maintain, and end a session. Any
communications that require checkpoints or a "did you receive the data
I sent?" questions are done in this layer. If an error in data transmission
occurs retransmission is only necessary from the last checkpoint rather then
from the begining.
6. The Presentation layer
This layer is responsible
for formatting data. Character sets are converted, data is encrypted. Data can
also be compressed or redirected at this layer.
7. The application layer
This doesn't refer to applications but, rather,
to a set of tools that an application can use to accomplish a task in a network.
File, print, messaging, and sometimes error recovery are examples of some of
these tools.
Back to the Computer Area
Back to The Nest
On to the next lesson