Describe how communication over networks is broken down into different layers.


Teaching Note:

Awareness of the OSI seven layer model is required, but an understanding of the functioning of each layer is not.


Sample Question:


JSR Notes



This assessment statement points strongly to the OSI (Open Systems Interconnection) model. But the really important thing to realize about this model is that ***it is a model*** only. In fact the modifier word ***conceptual*** is crucial to its understanding. It is a conceptual model. It is not something that can be applied directly to every network. You cannot necessarily definitively break down any particular network into the seven layers of the OSI model.

But what the 7 layer OSI model does do is speak to the fact that there are indeed many layers to a network and network activity, from the absolute basics of electricity flowing through wires, on up to the network software that we use via the operating system of our computer.

Below is a diagram of the 7 OSI layers, with a general description of what they are (Adjusted from wikipedia).

In terms of how you can be "aware of the model", without having to understand the functioning of each layer, I think a good approach is to dip in and try to understand four of them; the most "lower level", the most "higher level", and the two in the middle that are most closely associated with the TCP/IP protocol, which is the main set of rules for Internet network activity. My notes and analogies are in italics.


OSI Model
Data unit Layer Function
Data 7. Application


Network process to application

(So, for you, this might be you using your Browser to "download" a web page, or using your Bluetooth app to send a picture from your phone to your computer - the applications being used are your browser and the Mac OS operating system, and your Bluetooth app.

The postal service analogy doesn't work so well here, since you don't need an app to read a paper and pen letter - the "app" is you, your eyes, and your brain.)


6. Presentation


Data representation, encryption and decryption, convert machine dependent data to machine independent data


5. Session


Inter-host communication, managing sessions between applications


Segments 4. Transport


Reliable delivery of packets between points on a network.

(This is everything between when you, for example, click Send to an e-mail, and it arrives at the Mail server of your recipient - the actual transport of the data.

In a postal service example, everything that happens to your letter between dropping it off at the post office, and it arriving at its destination.)

(The "TCP" - Transmission Control Protocol - part of the TCP/IP Internet protocol resides primarily in this layer).


Packet 3. Network


Addressing, routing and (not necessarily reliable) delivery of packets between points on a network.

(This is how the information being sent is put together - i.e. how each packet is put together; what's in the header, such as source IP address, destination IP address, and time to live.

In a postal service analogy, this is the addressing of your letter, including the return address, going to the post office putting proper postage on it etc., and dropping it off there.)

(The "IP" - Internet Protocol - part of the TCP/IP Internet protocol resides primarily in this layer.)


Bit/Frame 2. Data link


A reliable direct point-to-point data connection.


Bit 1. Physical


A (not necessarily reliable) direct point-to-point data connection.

(Here we're talking about the physical bits - the 0s and 1s - of network activity moving along a wire, or through the air.

In a postal service example, we could say the analogy would be the individual 'a', 'b', 'c' letters of your letter to your Grandma.)


So regarding the structure of a question about this assessment statement, it would go something like this:

Communication across networks is conceptually seen as being divided up into 7 layers as seen by the OSI model, which include low level descriptions of data physically going back and forth..., up to layers, such as the network and transport layers (in which TCP and IP protocols exist...), on up to the actual application layer which users see, such as...sdfsad


JSR Notes -FORMER CURRICULUM - 6.4.4 Outline the need for protocols in packet switching.

Refer to the FORMER CURRICULUM notes AT THE BOTTOM OF 3.1.7 for more on PACKET SWITCHING and TCP/IP.



Make three columns and the third can be connecting via Microsoft Remote Desktopasdfasdfasdfasdfasdf asdfasdfasdf

  Postal Analogy

Real Example # 1:

E-mail Actual

Real Example # 2:

Microsoft Remote Desktop Actual

Application Somehow read the letter; glasses or another person may be needed to interpre it. The browser (Firefox) and the OS and the networking software The application "Microsoft Remote Desktop" which I use to connect to my server in the server room around the corner by the bathrooms, to work on these johnrayworth.info pages live in class.
Presentation Opening of envelop and being able to read becuause know the language Unencrypting and reading of the e-mail message. So just bits and bytes translated to words.

With this, what needs to be presented is not just words, but what my server is "presenting" to what would be a monitor hooked up to it; i.e. my Windows desktop and Dreamweaver etc.

And in this case likely no need for unencrypting since just transfer of data across our internal LAN.

Session Going to post office or mailbox. The actual checking of mailbox. The time of the sdfadsfasd The time between the initial handshaking between a sender and a receiver and everything that happens between them in the transfer of information. (same as other real example, only likely slightly different protocol)

Letters put in mail boxes, and picked up according to a certain schedule

Tracking of mail and packages


Protocol which controls the transmission; like error handling, time to live etc. how re-assembled

(same as other real example, since using TCP/IP across our school LAN)
Network Postal code and other rules for addressing envelopes and packages (including return address requirment) - and how you have to package things (envelopes etc.)

(IP of TCP/IP)

Protocol for stipulation Addressing of packets

For example is an IP address

(same as other real example, since using TCP/IP across our school LAN)
Data Link System of mailmen/women and trucks and planes Wires Wires (Ethernet, though when I was unplugged, via wireless)
Physical Paper & Lead of Pencil Electricity as bits Electricity as bits