Network Models

The idea of network models is to conceptualise the many parts of a network. There are two main network models used today, the Open Systems Interconnection (OSI) Seven Layer model and the Transmission Control Protocol/Internet Protocol (TCP/IP) model. Both models split a network up into layers. Understanding these models helps to determine at what layer certain problems can occur and therefore aid in finding a resolution.

Open Systems Interconnection (OSI) Seven Layer Model

The OSI model is the most popular of the two. It splits a network up into seven layers, as shown below.

Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data Link
Layer 1 Physical

Each layer of the model defines an important function in computer networking, and the protocols that operate at that layer offer solutions to those functions. Protocols are sets of clearly defined rules, regulations, standards, and procedures that enable hardware and software developers to make devices and applications that function properly at a particular layer. Below are some of the commonly used protocols that operate at each layer.

Layer Commonly Used Protocols
Application HTTP(S), SNMP, SMTP, FTP(S), Telnet, SSH, DNS
Presentation XDR, SSL, TLS, IMAP, SSH
Session NetBIOS, RTP, PPTP
Transport TCP, UDP, SCTP
Network IP, IPSec, ICMP, IGMP, RIP, OSPF
Data Link IEEE 802.3 (Ethernet), IEEE 802.5 (Token Ring), ARP, RARP, PPP, SLIP
Physical IEEE 802.3 (Ethernet), IEEE 802.5 (Token Ring) hardware, USB, Bluetooth,, IEEE 802.11

Application Layer (Layer 7)

The application layer is the interface to the applications being used and responsible for initiating the request for communication. This is the layer that represents the users access to the system and the network. Applications such as web browsers operate at this layer, using HTTP and HTTPS to move data between systems.

Presentation Layer (Layer 6)

This layer is responsible for preparing data for different interfaces on different types of terminal or display so that the application doesn’t have to deal with this task. Functions such as data compression, character set translation and encryption are carried out at this layer.

Session Layer (Layer 5)

The session layer is primarily responsible for managing communication sessions between machines, which includes functions such as initiating, maintaining and terminating sessions. Each session can have its own parameters, this layer is responsible for setting them up, including things like security and encryption.

This layer communicates exclusively with the presentation layer above it and the transport layer below it.

Transport Layer (Layer 4)

The transport layer is responsible for dealing with the end-to-end transport of data across the network connection, handling data that enters and leaves the network through logical connections. It can add and use address specific information, such as ports, to accomplish this task. A port is an address specific extension that enables multiple simultaneous communications between machines. If the data is too large to be transmitted in a single chunk, or packet, as it is known, then this layer is responsible for breaking up the data into more manageable chunks, as well as reassembling it at the other end. It ensures that all packets are sent and received, including requesting any lost packets and eliminating duplicates.

This layer communicates exclusively with the session layer above it and the network layer below it.

Network Layer (Layer 3)

The network layer is responsible for routing packets across the network. It determines the next best destination for a packet and will determine the full address of the target computer if necessary.

This layer communicates exclusively with the transport layer above it and the data link layer below it.

Data Link Layer (Layer 2)

The data link layer is responsible for the delivery and receipt of data from the hardware in the physical layer. The physical layer only manipulates a stream of bits, so the data link layer must convert the packets from the network layer into bit streams in a form that can be understood by the physical layer.

To ensure accurate transmission, the data link layer adds end-of-message markers onto each packet and also manages error detection, correction and retransmission functions, as well as performing media access functions, deciding when to send and receive data based on network traffic.

The data link layer communicates exclusively with the network layer above it and the physical layer below it.

Physical Layer (Layer 1)

The physical layer defines the method of moving data between computers and is the point of connection to the outside world. Anything that moves data from one system to another, such as copper cabling, fibre optics and even radio waves, are part of the physical layer.

The physical layer communicates with the data link layer above it, as well as the physical layer on other devices.

Transmission Control Protocol/Internet Protocol (TCP/IP) Model

The TCP/IP model splits a network up into five layers.

Layer 5 Application
Layer 4 Transport
Layer 3 Network
Layer 2 Data Link
Layer 1 Physical

As can be seen, the TCP/IP model doesn’t have the presentation and session layers that are present in the OSI model. The functions in these layers of the OSI model are combined into the application layer of the TCP/IP model.