OSI Model Flashcards
Define OSI
Open Systems Interconnection, a reference model for network protocols. 7 layers.
7) Application
6) Presentation
5) Session
4) Transport
3) Network
2) Data Link
1) Physical
Programs talk to the 7th layer. Below the 1st layer is the network physical media/cabling.
Each layer adds (when computer sends data) and removes (when computer receives data) control information.
Group the 7 OSI layers into main groups.
We can list out 3:
1) Application (5-7). High level layers that put data into the format for the program.
2) Transport (4): In charge of getting data received from the network and transforming them in a format nearer to the data format understood by the program. If computer is transmitting data, this layer gets the data and divides it into several packets to be transmitted over the network. If receiving, this layer gets the received packets and puts them back together.
3) Network (1-3). Low level layers that deal with transmission and reception of the data over the network.
Layer 7 - Application
It makes the interface between the program that is sending or is receiving data and the protocol stack.
Layer 6 - Presentation
Also called the translation layer. It converts the data format received by the application layer to a common format used by the protocol stack. This layer can compress data and add encryption. Data compression increases network speed, as less data is sent to the layers below. If encryption is used, the data is encrypted between layers 5 and 1, and they will only be decrypted by layer 6 of the computer at the other end.
Layer 5 - Session
Allows 2 programs in different computers to establish a communication session. In this session, the 2 programs can define how data transmission is done. For example, adding progress markers to transmitted data.
Layer 4 - Transport
This layer separates the Application layers (5-7) from the Network layers (1-3). Network layers care about how data packets are transmitted; Application layers care about the data itself.
From a transmission viewpoint, the Transport Layer must get data from the Session Layer and split it into packets. From a receiving viewpoint, it puts the packets in order (flow control), and checks data integrity (sends control signal called acknowledge or ack to confirm packet has arrived and is intact)
Layer 3 - Network
In charge of packet addressing, converting logical addresses into physical addresses, making it possible for data packets to arrive at destination. It sets the route the packets use to arrive based on traffic and priorities.
Layer 2 - Data Link
If transmitting, the layer converts data packets from the network layer into frames. Adds on physical address of source and destination, control data, checksum data onto the frames. The created frame is sent to physical layer, where it is converted into an electric signal (electromagnetic if on a wireless network).
On the receiving side, the checksum is recalculated and it it matches the value sent, the receiving computer sends an ack to transmitter.
Layer 1 - Physical
Layer receives frames from data link and converts into signals compatible with the transmission media.
If receiving data, it takes the signal received and converts into 0s/1s and sends to data link layer, which puts the frame back together and checks integrity.
