Overview of the OSI Model

The OSI Model is a seven layered concept that describe how computer networks operate.  Layer seven is what the computer user interfaces with.  Layer 1 is the data bits that computers and devices understand.  The OSI model describes how data moves from using an application that humans use down to code that machines use, how data moves from one machine to the next, and how that data gets rebuild back into an application that humans use.  Sounds complicated?  Let’s take a look at all the layers again and a brief description at what they do.

7. Application

The application layer is what a computer user interfaces with.  Some sources I have read divides this layer into two categories: computer applications and network applications.  An example of a computer application would be Microsoft Outlook while e-mail is a network application.  Other sources describe this layer as any software that is outside the OSI Model.  I understand this layer as programs and software.  It is the layer where users get their work done.

6. Presentation

The presentation layer is used to make sure that the format of the data can be used between the communicating devices.  I like to think of this layer as the file extension layer.  This layer tells how the data should be presented to the application layer.  Is it a picture?  Is it text, video or sound?  The presentation layer is the reason why I can create this page on a computer running on Windows XP and can be read on computers running Linux or Mac OS X.  Examples of presentation formats are JPEG, MIDI and TIFF.

5. Session

Just as the name applies, this layer is responsible for managing sessions between the communicating devices.  When you are trying to send data over the network or Internet, the session layer gets a connection between the devices and then passes the data between them.  The session layer provides communication between presentation layers of the communicating devices as well as error handling of the presentation and application layers.  Examples of Session layer protocols are NFS (Network file System,) X-Window System and ASP (AppleTalk Session Protocol.)

4. Transport

The transport layer is responsible for breaking up the data into manageable chunks when sending the data and recreating the data when receiving it.  A great way of understanding this layer is comparing it to moving.  Instead of getting one huge box to put everything you own into it, you would use a lot of little boxes.  The data chunks are marked in a sequential order when sent so that the receiver can rebuild the data regardless whether or not the data chunks were received in order.  Depending on the protocol, layer 4 also provides the transmission reliability between the communicating equipment.  A reason why reliability may not be used is if speed is more important than reliability (such as video streaming) or if one of the higher layers provides reliability (such as VPN.)  Examples of layer 4 protocols are TCP (Transmission Control Protocol,) UDP (User Datagram Protocol) and SPX (Sequenced Packet Exchange.)

3. Network

The network layer is responsible for the communicating devices to find each other.  This includes logical addressing and layer 3 devices to find and/or choose the best route between devices.  Examples of layer 3 protocols are IP (Internet Protocol,) Internetwork Packet Exchange (IPX,) and AppleTalk.  Routers and gateways are examples of layer 3 devices.

2. Data Link

The purpose of the data link layer is to provide reliable data transmission over a physical medium.  It is a glue between the logical software/data world and the physical world.  The data link layer consists of a physical address (as opposed to a logical one,) topology, error notification and flow control.  An example of a physical address is the MAC (Media Access Control) address.  Switches, NICs (Network Interface Cards,) and wireless cards are examples of layer 2 devices.

1. Physical

The physical layer describes how the data is physically sent from one device to another.  Is it wireless?  If it is, are you using radio frequencies, microwaves or infrared?  This layer describes the medium that is being used (the wires themselves.)  It also describes the connectors as well as other devices that connects the wires together (such as hubs) or devices that amplifies the signal for longer distance transmissions (such as repeaters.)

I challenge you: read the layer descriptions a few more times.  Is it starting to make sense?  The user interfaces with layer 7.  If sending data to another machine is required, that data starts at layer 7, moves down layer 6, to layer 5… and down the ladder to layer 1.  The receiving machine receives the data at layer 1, passes it to layer 2, then layer 3… all the way back up to layer 7.  How exactly doe this work?  What if there is a device between the sending and receiving machines such as a router or switch?  I will be discussing this in future posts.