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

There are 2 main types of model:

TCP/IP model (4 Layers)
OSI Model (7 Layers)

TCP/IP Model

“TCP/IP is a model that revolves around the Internet.

In Summary

“TCP and IP are two of the network standards that define the internet. IP defines how computers can get data to each other over a routed, interconnected set of networks. TCP defines how applications can create reliable channels of communication across a network. IP basically defines addressing and routing, while TCP defines how to have a conversation across the link without garbling or losing data. “

Application Layer

Application layer is the top most layer of four layer TCP/IP model. Application layer is present on the top of the Transport layer. Application layer defines TCP/IP application protocols and how host programs interface with Transport layer services to use the network.

Application layer includes all the higher-level protocols like DNS (Domain Naming System), HTTP (Hypertext Transfer Protocol), Telnet, SSH, FTP (File Transfer Protocol), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), SMTP (Simple Mail Transfer Protocol) , DHCP (Dynamic Host Configuration Protocol), X Windows, RDP (Remote Desktop Protocol) etc.

Transport Layer

The position of the Transport layer is between Application layer and Internet layer. The purpose of Transport layer is to permit devices on the source and destination hosts to carry on a conversation. Transport layer defines the level of service and status of the connection used when transporting data.

The main protocols included at Transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Internet Layer

The position of Internet layer is between Network Access Layer and Transport layer. Internet layer pack data into data packets known as IP datagrams, which contain source and destination address (logical address or IP address) information that is used to forward the datagrams between hosts and across networks. The Internet layer is also responsible for routing of IP datagrams.

Packet switching network depends upon a connectionless internetwork layer. This layer is known as Internet layer. Its job is to allow hosts to insert packets into any network and have them to deliver independently to the destination. At the destination side data packets may appear in a different order than they were sent. It is the job of the higher layers to rearrange them in order to deliver them to proper network applications operating at the Application layer.

Network Access Layer

Network Access Layer defines details of how data is physically sent through the network, including how bits are electrically or optically signaled by hardware devices that interface directly with a network medium, such as coaxial cable, optical fiber, or twisted pair copper wire.

The protocols included in Network Access Layer are Ethernet, Token Ring, FDDI, X.25, Frame Relay etc.

The most popular LAN architecture among those listed above is Ethernet. Ethernet uses an Access Method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection) to access the media, when Ethernet operates in a shared media. An Access Method determines how a host will place data on the medium.

In base terms, TCP/IP allows one computer to talk to another computer via the Internet through compiling packets of data and sending them to right location.

TCP

The top layer, TCP, is responsible for taking large amounts of data, compiling it into packets and sending them on their way to be received by a fellow TCP layer, which turns the packets into useful information/data.

IP

The bottom layer, IP, is the locational aspect of the pair allowing the packets of information to be sent and received to the correct location.

In summary, TCP is the data. IP is the Internet location GPS.

OSI Model

OSI is a theoretical model.

The 7 layers – Please do not teach students pointless acronyms!

The Open Systems Interconnection, or OSI, model is a standard reference model created by the International Organization for Standardization to describe how the different software and hardware components involved in a network communication should divide labor and interact with one another.

  • The physical layer is the lowest layer of the model and is concerned with the transmission and collection of raw data over a physical medium. 
  • The data link layer ensures error free transmission of data over the physical layer. 
  • The network layer decides the route that the data takes through a series of network nodes. 
  • The transport layer ensures that packets are delivered error free and in sequence.
  • The session layer establishes the establishes the session between the end points in the process. 
  • The presentation layer can be considered as a translator between the application and the network and deals with such things as encryption and compression.
  • The application layer serves the window for the user and other application processes that access network services such as email remote file access.