The Open System Interconnection Reference Model

OSI reference model is a unified model or standard that helps to stick devices manufactured by different companies work together.

OSI reference model consists of 7 layers, that define functionality & limitations. Network devices can operate on a single or multiple layers based on a device capability or network configuration.

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

OSI Layer 1 – Physical

Devices that operate on a Physical layer:

• Analog modem connects a network segment by wire to higher network layers using a modulator (digital signals to telephone line signals)/demodulator(opposite) hardware. Every modem in a network takes a single accessible network spot (for example, a unique phone number in the telephone system – POTS)
• Hub is a device that replicates an incoming electrical signal to multiple connected ports, no matter how many devices are connected to the hardware

Physical devices have minimal or no software, as these network elements merely modify a plain electrical signal. The devices aren’t able to manipulate the data or perform any logical action.

OSI Layer 2

Devices that operate on a data link layer:

• A switch is a more intelligent version of a hub with ASIC chip and software. Despite doing basically the same job – connecting a single port to multiple ports – the switch is able to control data flow by utilizing a unique identifier of the ports it communicates with – MAC (media access control) address. This approach guarantees that the data requested from the network by one of the connected computers is sent to the same machine
• WAP (Wireless access point) is a hardware/software bridge between wireless devices and an actual wired network (e.g. connects 802.11 Wi-Fi with 802.3 Ethernet)

802.11 is a family of IEEE local area network standards that specifies how hardware and software should work in wireless devices such as Wi-Fi routers. 802.3 is another family of IEEE standards that do the same for wired devices, Ethernet protocols, connections via cable.

OSI Layer 3

Devices that operate on a network layer

• A router is an even more intelligent version of a switch and usually a very simplistic full-fledged computer that controls the network using software rather than a specific chip (as in switch), that can control traffic from different network (e.g. wired to/from wireless, local/non-local networks), direct it to the specific machine or port, limit types of supported data and many other functions depends on the software installed. Unlike switches, routers can use IP addresses (not just MACs) for routing (including local IPs which in turn creates a local area network – LAN)
• MLS or multilayer switch (usually L3 switch) is also capable of creating a LAN but is limited by ASIC chip functionality, for instance, most MLS support a small routing table and only one set of VLANs

Nowadays the difference between L3 Switches and routers is very abstract as well as functionality included. The L3 switch is usually a network policy controller, but the router can shape the network traffic in addition.