OSI Model (Open Systems Intercommunication)

7 Layers

· 7. Application Away

· 6. Presentation Pizza

· 5. Session Sausage

· 4. Transport Throw

· 3. Network Not

· 2. Data Link Do

· 1. Physical Please

7. Application

· Handles denial network access, flow control, & errors

· Allows application processes to access network services & manages communication b/t applications.

· Directly represents services that support user applications such as software that transfers information such as email & file transfers

6. Presentation

· Determines format used to exchange data (“Network translator”)

· Responsible for protocol conversion, encrypting data, & managing data compression (to reduce # of bits transmitted)

· Contains software called “Redirector Functions” that accepts I/O requests

5. Session

· Provides synchronization b/t user tasks by placing checkpoints in data stream

· Implements dialog control b/t communicating processes by regulating which side transmits, when, for how long, and so on.

· Allows 2 applications on different computers to establish, use, & end connection called a session

· Performs name recognition and functions, such as security, needed to allow 2 applications to communicate on the network

4. Transport

· Provides flow control error handling, and is involved in solving problems concerned with the transmission & reception of packets

· Ensures packets are delivered error free in sequence with no losses/duplications

· Un/Repackages messages split into multiple packets into one/multiple package(s)

· Typically sends acknowledgment of receipt.

3. Network

· Addresses messages & translates addresses from logical into physical addresses.

· Determines route (source to destination computer) – based on network conditions

· Manages traffic problems on network such as packet switching & routing

· If receiving computer can’t take data chunks as large as source computer is sending then network layer on source computer breaks down more

· (Receiving end) Reassembles data into the original large chunk

2. Data Link

· Sends data frames from Network layer to Physical layer

· (Receiving end) Packages raw bits into data frames (logical structure for data)

o Frame is often referred to as a packet, but there’s a difference

o Frame has beginning & ending delimiter, while Network layer packets & packets encapsulated @ higher levels have only headers

1. Physical

· Addresses transmission of unstructured raw bitstream over network cable

· Defines …

o how cable is attached to network adapter card

o transmission technique to be used

o how long each bit last

o how each bit is translated into appropriate elec. or optical impulse

Transmitting Data

· When data is sent, each layer adds a header with information then passes on

· Data Link layer also adds a trailer

· Physical layer (with all headers & trailer) passes to physical network

__Data____ _____+_____+_+__Data____

· When receiving, each layer removes appropriate header & trailer (if included), and reads info before sending data on to next layer

_____+_____+_+__Data____ __Data__

· This is repeated for every frame sent b/t the computers.