Protocol layers and service models

The vertical view of the Internet considers a different problem of communication:

• How is an email is eventually translated into bits and sent to the network media?
• How can we guarantee that the email is received, even if the communication channel is noisy and prone to errors?
• Can a huge piece of information (for example, a streaming video) be sent in just a single packet or should it be fragmented to prevent the link from collapsing?

Figure 4 Human language in comparison to a network communication protocol

However, just a single protocol might be too complex to translate an email into bits. For this reason, the design is divided into protocol layers:

• Communication functions are divided into a number of modules or layers
• Network design is simplified:
  • “Big problem” is fragmented in simpler problems
    • → "divide and conquer”
  • Layered design makes things easier
  • The OSI model defines 7 layers
  • The Internet (practical) model reduces it to 5 layers
    • → The TCP/IP stack

Figure 5 Protocol layers and services model diagram
Layered Communication in OSI-Model by Runtux (Own work) from Wikipedia Public Domain