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OSI - Layers Transcription

Welcome to our OSI model layers module. They data link layer, or layer two of the OSI model receives data from higher layers and prepares it to be transmitted on the physical layer. The data link layer is concerned with rules of accessing the media. And the protocols that work at this layer are Ethernet, token ring and WiFi.

The physical device that we see being used at the data link layer is a switch. At the data link layer we use MAC addresses or media access control addresses, which are hardware addresses for our network interface cards that allow them to be identified on the network by the switches.

The physical layer or layer one is where the data is converted into bits to be prepared to transmit over some type of physical media. It can either be transmitted as pulses of light or as voltage. Over coaxial twisted pair, or fiber optic cabling. The network layer or layer three of the OSI model is responsible for internet protocol addressing.

This layer is responsible for routing packets between systems, or between different networks. At this layer we can provide confidentiality authentication and integrity. Routers will examine the headers of packets to select the best route for a packet to take, in order for it to get to its destination as efficiently as possible.

Routers are our network devices that operate at the third layer, or the network layer of the OSI model. At the transport layer, or Layer 4, we have our Transmission Control Protocol, and User Datagram Protocol. These protocols are concerned with transferring packets from end to end, or computer to computer, using either a reliable connection-oriented protocol, such as TCP, or a connectionless service UDP.

At this layer they segment the data to correct packet sizes and when receiving data on the other side it will be reassembled in order to form a data-stream. At this layer we see sequence numbers being used to identify the order that packets are transmitted and should be received in, error control, port numbers, and flags in the headers.

TCP and UDP are the protocols that will actually carry the data to the destination. We have things like Office documents, Outlook emails, and Internet Explorer web browser data that is being transmitted from the application layer and moved down to the transport layer to make sure that it can be delivered correctly to its intended destination.

It is important to understand the difference between TCP and UDP. TCP is very reliable. It is a connection oriented protocol that is concerned with making sure that data gets to the other side correctly without any errors, and making sure that the other system is receiving it properly. It uses flow control to make sure that it is flowing the correct amount of data, and it is commonly used on the Internet. TCP transmits data in a sequential series of numbered packets. TCP also uses a three way handshake. This begins with the sending computer sending a SYN or synchronized packet To the recipient's computer. The recipient responds with a synac, or a synchronization acknowledgement. And then the sending computer responds with an acknowledgement letting the receiving computer know that it received it's acknowledgement of the synchronize, and this three-way handshake is used to ensure reliability.

To make sure that the receiving computer is online and is receiving data properly. TCP will send packets and confirm that the recipient received the packet correctly. And if it did not, it will retransmit the packet until it knows that the recipient received it. UDP is not as reliable as TCP but it is much faster.

It is known as a connectionless protocol because it is not concerned with the reliability of the data. There is no handshake performed. And this is a fire-and-forget protocol where it basically makes its best effort to send the data over to the receiving computer. It does not confirm whether the data was received at the other end.

And if the data was not received, it will not attempt to resend it. It is commonly used for Voice Over IP and streaming technologies where data needs to get to the recipient quickly. And a few missed packets will not cause a significant problem, because the majority of the data reached the destination.

For the CISSP examination you should remember that TCP is much more reliable. But is slower than UDP and UDP is much faster, but is not as reliable as TCP. The session layer or layer five will allow applications to organize and synchronize how they can transmit data to each other.

It is involved in the opening, managing, and closing of communication sessions. And most communication is point to point between two systems, and bi-directional or full duplex, meaning that data can be sent and received simultaneously. Layers six or the presentation layer is concerned with translating data into standard format such as JPEG which is a common picture format.

It is responsible for negotiating supported formatting and encoding protocols that are compatible with both systems. File level encryption and file level compression are both done at the presentation layer. At the bottom we can see data from the top layer, or application layer, of the OSI model. It's forwarded down to the presentation layer where compression, encryption or conversion to comp formats is done before the data is moved down to the session layer.

The application layer, or layer seven, is where we have application programming interfaces, or APIs, that are able to send and receive data to network aware applications. There are many popular protocols at this layer including SMTP, used for transferring email messages. HTTP for transferring websites. FTP and TFTP which are used for transferring files.

And Telnet which is used for remote connections, and control of devices. There are several different services and protocols that can be required for applications to work correctly, such as file transferring, access control services, email, file management, gateways and proxy servers. Web browsers and non repudiation, so a person cannot deny the activities they performed on a system.

This concludes our OSI model, layers model. Thank you for watching