Layered Network Approach

Learning Outcomes  Student Understanding of the following : Network Layering, Protocols and packet switching

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

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

3.1.11 Explain how data is transmitted by packet switching.

A simple diagram that explains the basics of a TCP/IP network packet:

The “Level” labels above refer to the different layers of the OSI model. It is helpful to think about a network packet as a present, wrapped in multiple boxes. The outermost box corresponds to level 2 in the OSI model (Data Link layer) and it is the one containing the physical addresses of the source and the destination machines. A typical level 2 device is a network switch.

Inside this big box is the level 3 (Network layer) box that contains the IP address of the computer sending the packet and the one that it is directed to. An example of a level 3 device is a router.

The next box is the one that identifies what port the packet originated from and the number of the port that the destination computer should receive the data on. The port number ultimately determines what application is responsible for processing the data. For example, a packet coming on port 21 means that it is an ftp request and that the ftp daemon on the machine should process it. This box corresponds to level 4 on the OSI model – the Transport layer. A PIX firewall is a typical level 4 network device.

And the innermost box of the packet is the “present” itself – the data. This is what was needed to be delivered from one computer to another on the first place, and the rest of the boxes were just the means to accomplish this.

The different network devices and computers open the different boxes and read the information they carry as needed. For example, it is helpful to remember that routers strips the Data Link layer to find out the destination (the IP) address of the packet. Then they reconstruct that layer with a new destination physical address – the MAC address of the next hop on the network and with their own MAC address as the source address. This process repeats until the packet reaches its destination.

Beginners are often confused about the difference between the network and transport layers , previous picture makes the distinction clear. The network layer carries on a conversation with its peer network layer on the next hop, whereas the transport layer carries on a conversation with its peer layer on the final destination. To put it another way, the transport layer behaves as if it were directly connected to its peer layer and is unaware of the actual path that its data takes through the network. The network layer, on the other hand, is concerned with choosing the path that the data takes and, as such, is involved with processing at each hop. Note from the figure that routers do not necessarily even have transport layers.


Encapsulation is an fundamental term in network architecture , as data travels down the stack on its way to the network cable or other media, each layer adds a header and, possibly, a trailer to the data. We say that each layer encapsulates the data from the previous layer.The picture below  illustrates how data for a TCP session moves  through the stack. The data that, say, the user types in at the console is encapsulated by the application layer, which adds an application header. When the encapsulated application data enters the transport layer, it is encapsulated into a TCP segment by the addition of a TCP header. Similarly, when the TCP segment arrives at the network layer, IP adds its own header, encapsulating the TCP segment into an IP datagram. Finally, when the IP datagram gets to the interface layer, the Ethernet driver encapsulates it in an Ethernet frame by adding a header and trailer.

3.1.11 Explain how data is transmitted by packet switching.


Packet: A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packet-switched network.

Packet switching

Packet-switching describes the type of network in which relatively small units of data called packets are routed through a network based on the destination address contained within each packet.

Packets & its structure

Network communication is broken down into packets. Every packet has a header,(footer / trailer) and data. The header of a packet contains (among other information) the source and the destination IP address of the packet. The packet is ROUTED to it's destination by a collection of routers, all using an agreed-upon protocol of communication. Packets are reassembled at the destination and de-encapsulated (unzipped) into a network request your operating system (and application) can understand. Any messages exceeding the maximum limit will be broken into smaller units, and that smaller unit are considered as packets.

Email Example

A packet is constructed as a unit of data routed from the origin in the packet-switched network to its destination. For Ex: A user sends an email to the company's customer support. An email can contain an attached image or PDF etc. The email is sent through the network as a packet-based transfer. The protocol in the network layer ie TCP/IP breaks the data into smaller chunks as packets and routed towards the destination. Each packet will be numbered and routed to different routes; when it has arrived at the destination, the packets are assembled to the original format. This reassembling is done by TCP at the destination.

3.1.7  Explain why protocols are necessary.  

Teacher Note Including data integrity, flow control, deadlock, congestion, error checking.

Network Congestion - Traffic Flow -  Data Integrity  

Protocols are needed in computer networks primarily because the networks are made up of devices and software made by many different companies. The only way to ensure compatibility among everything is to have common documents, i.e. protocol specifications, that stipulate things such as the format of the data to be sent, and the mechanics of how it is to be sent and received.

Protocols are necessary so as to assure data integrity, manage the flow of data, prevent congestion and deadlock, and supply an agreed upon way of error checking".

Data Integrity

The "correctness" of information over its entire life-cycle, meaning what is sent is what is received. In fact, this is part of what error checking is for. Refer to the error checking section below - but the point is that the protocol needs to have ***some*** way of assuring data integrity. (When playing "telephone" as elementary students, there was not good data integrity with the passing on of the message by whispers from student to student!)

Flow Control

Protocols dictate the ways servers are able control the flow of traffic through a network, particularly the speed of transmission. This helps prevent a fast sender from overwhelming a slow receiver.

The good analogy here for controlling flow would be how traffic is controlled though a city. Some of the rules:

  • stop at stop signs
  • keep to the speed limit
  • stick to your lane
  • emergency vehicles take priority
  • Congestion

    As Internet can be considered as a Queue of packets, where transmitting nodes are constantly adding packets and some of them (receiving nodes) are removing packets from the queue. So, consider a situation where too many packets are present in this queue (or internet or a part of internet), such that constantly transmitting nodes are pouring packets at a higher rate than receiving nodes are removing them. This degrades the performance, and such a situation is termed as Congestion. 

    when everything in a network slows down due to the amount of traffic going through particular paths. Typical effects include queuing delay, packet loss or the blocking of new connections.

    What can Cause Congestion?

    • Slow router cpu
    • Lines with low bandwidth

    Fix Congestion?

    • Congestion can also be relieved simply by re-directing network traffic to alternative routes which are not the shortest but which are relatively uncontested
    • Install Lines with higher bandwidth

    Traffic ANALOGY Re-routing of truck/lorrie traffic around a city on a "ring road" is often done to relieve inner city congestion.


    Road network Analogy  "Gridlock" in the automotive traffic analogy which refers to a situation in which there is such a high level of traffic congestion that no car can move.

    Error Checking

    Error Checking the protocol will dictate the use of some sort of error checking algorithm to help assure that what was sent is what was received. Common error checking algorithms include parity checking and check sums.

    Parity Checking

    Parity checking is a system in which the number of binary 0s or the number of binary 1s in a message ("message", in the case of network activity is a packet) are calculated before the message is sent, and after it is received. That number should be exactly the same if no errors occurred during the transmission. If it is not the same, it means there (was at least one) error. So re-transmission is requested.

    In the case where a particular protocol uses Even Number of Zeros Parity Checking, the number of 0s in a packet are counted up, and if that number is odd, another 0 is added as the "parity bit" to make the total number of 0s even. And if the number of 0s is even, then it is kept even by adding a 1 as the parity bit. When the packet arrives at its destination, the number of 0s is added up, and if it's still even, then no error is assumed to have occurred, if it's odd, then an error must have happened during transmission, and re-transmission is demanded by the protocol.

    Wi Fi Security

    In some places you can be held liable for crimes committed on your WIFI connections

    • WEP  ( wired Equivalent Protocol) " Not secure can be broken easily with downloaded software : incredibly weak can be hacked in minutes with BRUTE FORCE 
    • WPA introduced 2006 with a key that changes , but still can be hacked : Much longer encryption and constantly changing keys.
    • WPA2- PSK  ( Pre Shared keys ) Now Mandatory use of Hackers can crack weak Pre-Shared Keys by using brute-force cracking tools Make your Pre-Shared Key over 25 characters long and make it random 

    Class Activity

    • 1
      Describe key functions of the Web Browser including how Caching Works 
    • 2
      Create Page / Section network Traffic - Add in section describing Congestion / Deadlock  / Data Integrity and  Flow Control
    • 3
      Using an online compression tool compress and image  example using 
    • 4
      Create Page Security Cyber Crimes -  Give examples of   Man in the Middle Attack /    Denial of Service / Distributed denial of Service and Brute Force ( to break  encryption) .  Describe the difference between the these 4 cyber crimes
    • 5
      Create Table for Presentation listing advantages and disadvantages of Wi Fi in Comparison to a wired network


    Upload your responses to google classroom for grading please

    1) Describe with illustrations how communication over networks is broken down into different layers. ( maxim word count 500)

    2) Describe with illustrations how data is transmitted by packet switching ( maxim word count 500)


    • Question

    • IB Answer


    • Question

    • IB Answer


    What is Default Gateway ?

    Name a protocol that resides at the network layer

    Name a protocol that resides at the transport layer

    Name a protocol that resides at the application  layer

    Highlight the key differences between UDP and TCP

    Give an example of a device at each layer of the 5 layer model (application,Transport, Network, Data link, Physical )

    What is function of Router?

    VPN ( virtual private networks )

    3.1.4 Identify the technologies required to provide a VPN.

    3.1.5 Evaluate the use of a VPN.

    Networks and Compression

    3.1.8 Explain why the speed of data transmission across a network can vary.

    3.1.9 Explain why compression of data is often necessary when transmitting across a network.

    3.1.10 Outline the characteristics of different transmission media

    Past Paper Question Practice

    Identify two different types of file which, if compressed, could make use of a lossy compression algorithm.

    Evaluate lossy compression and lossless compression when used to download files. [4]

    Discuss two factors that would affect the decision to use either lossless or lossy compression when transferring across the Internet.

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