The OSI Model

Who designed the OSI Model?

ISO (International Organization for Standards) The reason that ISO is used instead of IOS is that ISO is not supposed to be an acronym for International Organization for Standards. In fact ISO is the Greek word for "equal".

Frame

A frame is a structured package for moving data similar to how a train is configured to move cargo. Like the train that has the Engine at the front, cargo in the middle, and the caboose at the end, a data frame also has different components.

Packets

Packets are formatted blocks of data carried over a network. Packets vary in length and can be smaller or larger than the frames they are carried in. For this reason packets can span more than one frame

APPLICATION LAYER

The Application Layer also referred to as Layer 7 is where the interaction between certain programs or the operating system. This is not where there is interaction between the computer and humans but where on request a file is transferred over the network or mail is read. An example of how the Application Layer works: If you are running Microsoft Word on a network and choose to open a file, your request for that data is transferred to the network by the Application Layer.

THE DATA LINK LAYER

The Data Link Layer also referred to as Layer 2 is the layer responsible for breaking raw data into smaller chunks called frames. Frames from particular nodes are usually the same size and used for timing purposes just like frames in a film. Layer 2/Data Link Layer is responsible for making sure the data reaches the desired destination. Layer 2/Data Link Layer is also responsible for making sure data flows at the right speed.

MAC Addresses

The Data Link Layer uses the Destination and Source MAC Addresses A MAC Address (Media Access Control) is the unique identifier (address) that every NIC (Network Interface Card) in the world has There are 248 or 281,474,976,710,656 different combinations.

What is the OSI Model?

The OSI Model is a universal set of specifications that enable computer platforms across the world to communicate openly with each other.

THE PHYSICAL LAYER

The Physical Layer also referred to as Layer 1 is where everything connects together and comprises of devices that do not translate the information flowing on the network, but only pass it on.

Network Layer: Routing

The other important job of the Network Layer is figure out the right way for data packets to proceed through the network to get to their destination known as Routing. Routers work at the Network Layer and are devices that store tables known as Routing Tables. These tables have the directions to every device connected to that Router.

These layers are from the top to the bottom:

- Application - Presentation - Session - Transport - Network - Data Link - Physical Each layer has its own set of functions (chores that it is responsible for) and interacts with the layers directly above or below.

Network Layer: Segmentation

Another task that the Network Layer is responsible for is either reducing or increasing the size of Data units (called Segmentation) As different segments of a network can move data at different speeds the Network Layer segments the data to make maximum use of the network.

The Data Link Layer consists of things like:

As mentioned before The Data Link layer interacts with both the Layer below it (The Physical Layer) and the Layer above it (The Network Layer) The Data Link Layer provides the control information (information that it can use to make sure the whole frame is transmitted) The Data Link Layer breaks the Source Data into transmittable frames The Data Link Layer provides the error checking information (information that network can use to make sure the whole frame was delivered) One way that this is done is by size. The Data Link Layer knows that a full Data Frame has 500 (fictitious size) bits of information. If the receiving device reads 500 bits then it assumes all the Frame is present, otherwise it will respond back to the sending device to please resend.

IP

IP stands for Internet Protocol and is a core protocol of the TCP/IP protocol. A Protocol is the set of rules used to transfer data.

What does OSI stand for?

OSI stands for Open System Interconnection It is used to divide networking architecture into seven different layers. (the word architecture is used to describe a networks infrastructure because like a building, networks are comprised of many integrated elements: cabling, servers, applications, NIC's and many other elements) .

TCP or Transmission Control Protocol

TCP is a protocol whose rules govern how Data is broken up and reassembled. As Data is broken into smaller packets for transport over the network TCP places a sequencer on the data to denote where in the data stream it should be placed when received. This is very important since there are many ways for data to travel over a network some packets could be received out of sequence or be lost on the network. With this sequencer in place the receiving node can reassemble the data correctly. Also if a particular packet does not make it to the receiving computer the receiving computer would be able to request that particular packet be resent instead of asking for all of the data to be resent. TCP also sends a reply saying that the information was received. This reply is called an (ACK) or acknowledgement. This ACK is very important, if the sending PC did not receive an ACK then its Transport Layer would know that the receiving PC either did not receive the Data or could not understand the Data. After a certain amount of time the sending PC would resend the Data. Waiting for an acknowledgement of receipt of data takes extra time but is critical in making sure that all the information makes it to its destination. There are other Protocols that don't require acknowledgement (UDP) but the transmission of data can not be guaranteed. This guarantee of transmission is thought to be worth the extra time needed to transmit.

So who is the ISO and what do they do?

The ISO is a collection of Standards organizations that represent 130 countries. Its headquarters are in Geneva Switzerland ISO's goal is to establish international technological standards to facilitate global exchange of information and barrier-free trade.

THE NETWORK LAYER

The Network Layer also referred to as Layer 3 is primarily responsible for getting packets from the source node (computer) to the destination node using the quickest know route. It does this by using the IP address (Layer 3 protocol) together with the physical MAC address (Layer 2) of both nodes

PRESENTATION LAYER

The Presentation Layer also referred to as Layer 6 serves as the translator between the Application Layer and the Network Layer The Presentation Layer compiles data from the Network Layer into a format that the Application Layer can understand. The Presentation Layer compiles data from the Application Layer into a format that the Network Layer can understand for transport over the network. It is at the Presentation Layer that data is either encrypted or decrypted.

SESSION LAYER

The Session Layer also referred to as Layer 5 is primarily responsible for maintaining communication between two nodes on the network. The Session Layers also sets the terms of communication by deciding which node will communicate first and how long a node can communicate similar to a judge at a debate competition.

TRANSPORT LAYER

The Transport Layer also referred to as Layer 4 is the layer primarily responsible for ensuring that the data is transferred from point A to point B As the Data being transferred might be of a very large size another protocol is used at this layer to make sure all of the information is put back together in the right order when it is received by the receiving node. One of the most popular protocols used on the Transport Layer is TCP.

The Physical Layer consists of things like:

The wires in the network, which could be Ethernet cables or fiber optic cables Other technology for sending signals such as infrared or micro wave The connectors at the ends of the wires (used to join devices together) Repeaters (devices that amplify any signal coming in without checking the signal) Hubs (devices used to join multiple computers together) Devices that control the speed at which information is sent so that most of the information makes it to its destination. These devices know the speed of the networks they are connected to as in 10 Mbps or 100 Mbps and slow down or speed up the packets on the network. Without these network devices it would be like pouring water in a funnel. If you pour too fast it will overflow and the water that overflows does not make it to the desired location) Basically the Physical Layer provides the hardware means of sending and receiving data on a carrier.