ISO OSI 7-Layer Model

Question 1

Application Layer

Answer 1

The Application layer, the top most layer of the OSI model, serves as the interface to the user’s application. This Application layer, despite its name, is not referring to ‘Microsoft Word’ or any other user software. It refers to application services on a device used for file transfers, emails, network printing, telnet, network file browsing, and other network-related services.

Question 2

Presentation Layer

Answer 2

The sixth layer, Presentation layer, is responsible for ensuring data is encoded in a fashion that can be understood at the other end. It converts data from one data format into another so that two devices can communication. This is analogous to a person translating a book written in English into Russian - the story (information) is the same, but it is ‘presented’ in a format that a Russian-speaking person can understand. In computer, such a translation would be ASCII to EBCDIC, and visa versa.

Question 3

Session Layer

Answer 3

The Session layer, layer five of the OSI model, handles the creation, management, and termination of connection between applications. Sessions ensure data for one application is isolated from data for another.

Question 4

Transport Layer

Answer 4

This fourth layer is responsible for data flow control and error-handling/re-transmission to ensure reliable delivery of segments. In other words, the Transport layers makes sure data are delivered intact - no errors, no losses or duplicates, and in sequence.
TCP/IP port numbers are used at the Transport layer to ensure data is delivered to the correct application for handling. For example, port 21 is used by FTP (file transfer protocol) and port 53 is used by DNS (domain name system).

Question 5

Network Layer

Answer 5

This third layer, Network layer, is where routing is handled. Routing takes place when data for a device is on a network that is different than the sending device. This is where network routers operate - handling the transmission of data between different networks. Because of this, routers are often called layer 3 devices. If data is being sent from and received by devices that are on the same network (subnet), no routing needs to take place.

Question 6

Data Link Layer

Answer 6

Data link is the second layer of the OSI model and is responsible for ensuring successful transmission of the data from one device to another on the same network. It accomplishes this by managing flow control and data errors. This layer is divided into two sub-layers:

Media Access Control (MAC) Layer - Determines the physical (aka hardware) address (the 6-byte ‘burned-in’ MAC address of a network device) of hosts.
Logical Link Control (LLC) Layer - Controls frame synchronization, flow control, and error-checking (such as using Common Redundancy Check (CRC)).
This layer makes use of a network device’s MAC address to receive and forward data. Because most network switches use MAC addresses, to move data to the correct destination, they are often called layer-2 switches.

Question 7

Physical Layer

Answer 7

This first layer of the OSI model is where physical circuits are made. This layer involves the network components such as network cards, repeaters, hubs, and any component that takes data (in the form of zeros and ones) and converts them into electrical, optical, or radio signal, and visa versa.

The network cables (e.g., twisted pair, coaxial cables, fiber optic cables) or any other transmission medium used to transport the signal (data) is considered to be at layer 0, which is included in the 7-layer OSI model. However, many include the transmission medium as part of the Physical layer, layer 1, of the OSI model.