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3.1.3

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:

sdfsdfsf

JSR Notes

********** SAVE THIS FOR AFTER GOING THROUGH PACKET SWITCHING 3.1.11 **************

 

This assessment statement points, in the Teaching Note, to the OSI (Open Systems Interconnection) model. Though, an important thing to note from the beginning is that it is indeed a ***model***, and a conceptual model at that. It cannot necessarily definitively break down a particular network into the seven distinct layers. But generally, the OSI model does reflect the different layers of networking technology.

And those seven layers go all the way from the absolute basics of electricity flowing through wires, 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.

OSI Model
Layer Function
Host
layers
7. Application

Provides the ability for user applications, like Chrome for example, to interact with the network. Or your Bluetooth app on your phone which allows you to interact with the network to send a picture from your phone to your computer.

6. Presentation

 

Preparation of appropriate data representation (like a certain ASCII set), and possibly encryption and/or decryption. Thereby machine dependent data is converted into either machine independent data, or data in a correct form for its destination. (So a file readable on a Mac is converted to a Windows PC compatible file, for example.)

So put another way, the preparation of the data, either for the network or the application, depending on the direction it is going.

 

5. Session

 

Inter-host communication management of a particular session between applications. So management of the particular interaction between the receiving/sending component and the network - beginning the session, managing it, closing it etc.

 

4. Transport

 

Reliable delivery of packets between points on a network. In particular control of sequencing, and error checking.

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

 

Media
layers
3. Network

 

Mainly routing. "Routing" is the determination of the path through the network the data will go. This is done by algorithms which will aim to pick the best available path. And the basic - not necessarily reliable - delivery of packets between points on the network.

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

 

2. Data link

 

Establishment of a reliable direct point-to-point data connection - by following, for example, Ethernet protocol, and using associated Ethernet cables etc.

Reliability is assured by the protocol through its management of frames, in which the data is organized. Along with the data itself, the frames will contain information about frame type, address, along with error control information. Timing of the sending of the frames is also controlled at this level.

 

1. Physical

A direct point-to-point data connection via hardware - cables, connectors and so on.
(Note that at this level of the model, the connection may not necessarily be reliable.)


Data Encapsulation in the Layers of the OSI Model

Note how, with this model, each layer is "wrapped" by the next, like the layers of an onion - each encapsulates the data in a better, more useful form for the next layer to work with - yet another example of abstraction.

TCP/IP

Note that one of the most commonly used set of networking protocols is TCP/IP; we use it on our laptops for connecting to various network resources such as the Internet. It is fundamentally a "packet switching" protocol which resides in the Network and Transport layers of the OSI model. But since it's mainly dealing with packet switching, when covering 3.1.11, do make a point to connect it back to the OSI model.

IP Addresses

An important part of IP protocol is the way devices on a network are addressed. In version 4 of IP (IPv4), the "IP address" is, a 32-bit number, usually expressed in four decimal numbers separated by dots. So, for example:

192.45.7.201
23.67.123.255

(Since each number is 8 binary bits, in decimal, that makes the range of each number to be from 0 to 255.)

IPv4 had a limit of 2 ^ 32, or ~4 billion addresses. And since that wasn't enough for all the networked devices in the world, a new version, IPv6, was developed, in which each address is 28 bits. So in IPv6, there are 2^28 possible addresses, which is a bizzarely large number, and IPv6 will have no problem to deal with the "Internet of Things" era that we are entering into.

There's lots to play around with vis-a-vis TCP-IP in both the Mac Network System Preference, and the Mac Networking Utility. I'll include a couple of examples below:

With this "Advanced" pane from the Network System Preferences of a Mac
you can see the IPv4 address of my computer (192.168.2.11) - though it is a "Dynamic" DHCP address
which only applies within the school network.

Mac Network Utility - Traceroute

From the Mac Network Utility, one neat thing you can do is see all of the nodes network activity goes through to get to a particular website, with Traceroute.

The particular Traceroute, above, showed me the good news that the CDN ("Content Delivery Network") I pay $24.95 USD a month is actually working. Becuase to get to my website, which is hosted by GoDaddy some place cold in America, it only needs to go, via Honk Kong ("hk") to some place in Japan ("jp") where a copy of my website is stored. That makes the access here in Bangkok faster than having to go all the way to America each time.

 

In terms of the details of the OSI model, check out this interesting PowerPoint found by Mr Laornual giving you and idea of the complexity of Networking.

 

And regarding the structure of a question about this assessment statement, it could 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. Describe some of the higher/lower level things that a network has to attend to in order for communication to proceed without error.

 

 

 

 

 

 

 

FORMER CURRICULUM ONLY

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.

 

  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 interpret 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 because 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)
Transportation

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

Tracking of mail and packages

(TCP of TCP/IP)

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 requirement) - and how you have to package things (envelopes etc.)

(IP of TCP/IP)

Protocol for stipulation Addressing of packets

For example 213.123.1.1 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