The purpose of network hardware and protocols

1  Layered network protocols

  • Network communication relies upon the interaction of many different protocols, which implement the rules and conventions governing how devices in a network communicate. These protocols are designed and implemented through a layered approach, which allows us to modify individual protocols to accommodate technological changes without impacting the operation of protocols in the layers above and below.
  • Using a layered network protocol reference model also makes it much easier to understand the underlying function of the individual protocols, and how they relate to protocols in other layers.
  • Remember that a reference model is a way of understanding the interrelation between protocols, and cannot support network communication. Network communication is supported by protocols that are compliant with the reference model – these are referred to as a suite of protocols. When an operating system is installed on a computing device, it will install a variety of Transmission Control Protocol (TCP)/User Datagram Protocol (UDP) protocols that are capable of supporting network communications.
  • You have seen in previous work that the programs on a computing device pass data to the TCP/IP protocols when the data needs to be transmitted across a network connection. The data is accepted by the application layer, before being passed to the transport, Internet and network access layers. Each layer adds additional information called a header in a process called encapsulation. The header provides additional information that allows the encapsulating protocol to perform its required functions. The addition of header information creates a Protocol Data Unit (PDU), and each layer has an identifiable PDU:
    • transport (TCP): segment
    • transport (UDP): datagram
    • Internet: packet
    • network access: frame.
  • Note that the two transport layer protocols have different names for their PDUs, indicating the slight differences in function between them.
  • Data that is received from the network media is passed back up through the TCP/IP layers. Each layer interprets the header information in the PDU that it is interested in, before passing the data to the layer above. Once the information within a header has been interpreted, the header is discarded, revealing the PDU for the next layer. This process is referred to as de-encapsulation.

2  Transport layer – TCP