1. Networks Layering
Chapter 7

2. OCR GCSE Computer Science
Chapter 7: Networks – layering
Large and complex items are best dealt with in smaller more manageable parts.

3. OCR GCSE Computer Science
In a car, the parts are manufactured by a range of suppliers.
Each separate part may fit several makes and models of car.
Providing the parts fit with those around them, they can be modified to improve the individual part.
Developing parts that will interconnect with those around them is ‘layering’.

4. OCR GCSE Computer Science
In computer science programmers often use library routines.
These library routines have fixed entry and exit points and formats so they can simply be inserted into another program and work.
This means that several programmers can cooperate or contribute to a complex problem.

5. OCR GCSE Computer Science
The LINUX kernel is the basis for a range of LINUX-based operating systems:
Debian
GNU
Ubuntu
Mint
fedora
and so on all have the same OS kernel.

6. OCR GCSE Computer Science
A computer system uses interchangeable hardware components and we are free to add whatever operating system, utilities, applications and peripherals we choose. See the Chapter 7 animation: A layered overview of a computer system.

7. OCR GCSE Computer Science
Networks use layered applications, for example communications protocols.
TCP/IP (transfer control protocol / internet protocol) enables computer systems to communicate over the internet.

8. OCR GCSE Computer Science
TCP/IP is a complete set of rules (protocols) that covers the movement of data and includes rules about how:
the data should be formatted
to address the data
to route the data
to receive the data.

9. OCR GCSE Computer Science
Layer 1: Application This layer makes sure the data produced is in a form that is acceptable to the software that receives it.

10. OCR GCSE Computer Science
Layer 2: Transport
This layer establishes connections across the network and communicates with the receiving device to agree on:
communication protocols
the size of packet to use
the speed data can be sent and received
whether or not receipt of data packets is to be acknowledged.

11. OCR GCSE Computer Science
Layer 3: Internet
This layer is responsible for transmitting the data across different networks; it:
identifies the destination IP
establishes a path across the network via routers to the destination.

12. OCR GCSE Computer Science
Layer 4: Network
This layer passes data to the local physical network and converts the data into electrical signals that can be:
understood by the network hardware
transmitted over any transmission media, e.g. copper wire, optical fibre and wirelessly.

13. OCR GCSE Computer Science
Advantages of layering
Individual layers make a complex system easier to develop.
A range of developers can work on the layers independently.
Since a range of developers can work on separate layers, development time is reduced.
It creates standardised interfaces.

14. OCR GCSE Computer Science
Important points
In order for layers to work there must be defined rules or protocols.
The layers must be interoperable (they must work together).