Protocol Fundamentals (Cambridge (CIE) A Level Computer Science): Revision Note

Need for protocols

What is a protocol?

• A protocol is a set of rules that define how computers communicate over a network

• They ensure that data is sent, received, and understood correctly by all devices, regardless of their manufacturer or operating system

• Without protocols:

  • Devices would not know how to structure or interpret data

  • Data could be lost, corrupted, or misrouted

  • Communication between different hardware and software systems would break down

• Protocols are essential to guarantee:

  • Reliable data transfer

  • Consistent formatting and addressing

  • Error checking and correction

  • Security during transmission

• Examples of protocols include:

  • TCP, IP, HTTP, FTP, SMTP

Protocol stacks

• The protocol stack is a way of organising network protocols into layers, where each layer is responsible for a specific function

  • Each layer handles its own task independently

  • Layers communicate with the layers directly above and below

  • The stacked model makes the network easier to design, update, and troubleshoot

• Makes communication modular and efficient

• Allows different technologies to work together using standardised rules

• An example of a protocol stack is TCP/IP

TCP/IP

What is TCP/IP?

• TCP/IP, or the Transmission Control Protocol/Internet Protocol, is a suite of communication protocols used to interconnect network devices on the internet

• This model splits the various protocols into four layers:

  • Application

  • Transport

  • Internet

  • Link

Application layer

• This is the layer where the communication process begins

• The application layer interacts directly with software applications, such as web browsers and email clients

• The application layer prepares data for transmission over the network by converting it into a format that can be sent and received over the network (known as encapsulation)

Transport layer

• The transport layer receives data from the application layer

• The transport layer is responsible for end-to-end communication between the source and destination

• The transport layer breaks the data it receives down into smaller units called packets

• Each packet is assigned a port number (so the data can be reassembled in the correct order at the destination)

• Each packet is also labelled with a header containing information (e.g. the packet number) 

Internet layer

• The internet layer receives packets from the transport layer

• It adds a header to each packet, including the sender's IP address and the receiver's IP address

• The internet layer is responsible for routing each packet across the network using the IP addresses in the headers

Link layer

• Also known as the network interface layer

• The link layer receives packets from the Internet layer and prepares them for transmission over the physical network

• The link layer translates the digital packets into an electrical, optical, or wireless signal that can be sent over the network

• Once the signal reaches the receiving end, the network layer translates it back into digital packets