- Open Systems Interconnection
- Theoretical model in understanding how data is transfered between devices on a network.
7) Application Layer
provides an interface between an application running on the system and the rest of the network.
6) Presentation Layer
is the formatting layer. Compression, encryption, and syntax are dealt with here. SSL encrypts data at the presentation layer.
5) Session Layer
is responsible for connecting clients into a session. It IDs every session so that information is sent and received by the correct clients. It also terminates sessions.
4) Transport Layer
- is responsible for delivering data on a network. It segments, divides, the data received by the upper layers and IDs each segment with a number. It delivers data in segments and the segments number is used to reassemble the data in the right order.
- It is also responsible for controlling how fast or slow data is sent.
- Protocols for information transfer
- TCP vs UDP
- TCP represents a slower, reliable transfer that makes sure all the information is sent and received. Connection based
- UDP represents speed over content. Information is sent but it doesn't care if it was all received. connection-less
Port Numbers are used to identify what the data intended for so data can be sent to the correct service. Port 80 for HTTP or 25 for SMTP(mail).
3) Network Layer
is responsible for moving data between systems through the internetwork and routing occurs here. Routing takes a message from a device sends it to router to router until it reaches the destination device. Routing protocols identify the destination and the path to get there.
The network layer assigns the source and destination ip addresses to the segments of data and they become packets.
2) Data-Link Layer
- Data at this layer is called a frame. A frame is the network layer packet with the MAC address of the source and destination devices added.
- Also included in a frame is a Cyclic Redundancy Check, or CRC. This is a mathematical value that helps the receiving device identify any errors that may have occurred during transmission.
- The data link layer also defines the logical topology of the network, or how devices access the media.
1) Physical Layer
The physical layer is where we work with physical hardware.The physical layer also includes protocols that identify the cables, connectors, and devices that can be used on the network. For example, we could use the CAT6 protocol to define the type of cables used in the network and the RJ-45 protocol to define the connectors to use.
At this point, the data that comes down to the physical layer is just a series of bits(1s and 0s); it becomes electrical impulses, or light pulses, or some kind of a radio signal depending on the physical medium used.