Computer network devices, also known as communication devices, are the backbone of a data communication network. In this category we can find routers, switches, hubs, LAN cards, gateways, modems, hardware firewalls, CSU/DSU, ISDN terminals or transceivers. We are going to see a basic definition of the devices you can find in a corporate wired network, so you can understand basic IT engineering jargon.
The typical networking hardware for a local area network includes gateways, routers, network bridges, switches, hubs, and repeaters. Their classification depends on the layer of the OSI model at which the device works. As you can remember from a previous video, data communication in computer networks is logically structured in layers to simplify its implementation and managing. A device that works in a higher layer will be able to connect more different networks but it will be slower, as it will have to decode and re-encode the data units to a higher level. So, when designing a computer network we will have to select the device that makes the function we need working at the lowest level possible in the protocol stack. If we are going to use more than a transmission media we will also need devices that are able to convert the signal from one media into another (working at the level 1 of the OSI model). The generic term used for this kind of devices is transceiver, and they can be incorporated as expansion cards in some of the networking devices of higher levels. One special type of these transceivers are modems and CSU/DSUs, that are devices used to adapt the signal of the local network to be carried over a public network.
Modems are devices used to adapt the digital data of local networks or devices to be carried over public phone or cable networks. They work in the physical layer of the OSI model. Analog modems were extensively used to connect computers to the Internet, but nowadays the computers are connected to a local network and the modem function is usually embedded into another connection device connected to it (usually a router) that acts as a gateway. They are called Modems because they MODulate and DEModulate digital data.
A repeater is a device which amplifies or regenerates digital signals. It works in OSI layer 1. When the loss of signal strength due to distance is too much, repeaters are positioned along the path to maintain the signal in its allowed levels. They were used in Local Area Networks at first to extend the range of the networks, as the first Ethernet standards used a shared coaxial cable as a transmission medium, and this cable allowed for a maximum distance between connected computers. Nowadays the use of structured cabling systems has made its use unnecessary. They are still used in long distance transmission media, as fiber-optics submarine cables for example. It usually has two ports, and the word repeater refers specifically to the range extension function.
A hub is a kind of multiport repeater. It is a device that connects multiple network segments, making them act as a single segment with a shared media. Therefore, only one computer connected to the hub is able to transmit at a time. It works in OSI layer 1. You can find, for example, USB hubs to share your computer USB host. They used to be very popular for Ethernet local area networking when the first structured cabling systems were introduced, as there was a need to connect a lot of devices to a shared medium with individual cables. Nowadays their function has been superseded by Ethernet switches, very similar in outward appearance, but much more powerful, as they connect every device with full bandwidth instead of sharing the bandwidth between the connected devices. We will see an explanation of how switches work later.
A bridge is a device that connects multiple network segments at the data link layer level (working in OSI layer 2). That means that it can transfer data from one physical medium to another (like transceivers) but isolating one part of the network from the other, as it decodes the data up to level 2. In Ethernet they were used to separate different sectors of the local network at a low level.
A switch is an evolution of a bridge, as it can be seen as a multiport bridge. Nowadays most computers of an Ethernet local area network are connected to a switch. A switch has a high speed backbone and is able to assign the full Ethernet bandwidth to each of its connected devices, instead of sharing it between them (that is, for example if we have an 8 port 1 Gbit/sg, each of the 8 devices will be able to connect at 1 Gbit/sg, whereas in a hub they would have to share this 1 Gbit/sg). As OSI layer 2 devices they are also able to split network traffic, sending through a port only the traffic assigned to it.
A router is a specialised network device that, working in OSI layer 3, is able to determine the next network node to which a data packet has to be sent to reach its ultimate destination. They are the core switching device of the Internet as they perform the internetworking function between different networks (all based on the TCP/IP protocol stack). We will find them as the gateway of our local area network to the Internet. Sometimes they will have the necessary interfacing hardware incorporated (as in modem/routers) and in other cases they will be connected to an external device to interface with the external network.
A Gateway is a networking device that works at the highest levels of the protocol stack (up to application layer), what enables it to interconnect two networks working with a completely different set of protocols. It is the most versatile of the different network devices, but this implies that it s also the slower one, as it has to decode completely every packet of information and re-encode it in the new protocol set. With the current standardisation of network technologies, were most networks use Ethernet for the lower level protocols in LANs and TCP/IP as the network and transport protocols, most local traffic doesn t need any translation and most of the internetworking between different networks takes place at network level (level 3 in OSI protocol stack), that is much faster, so gateways are only found in very specific applications.
There are other specific hardware devices that can be found in a network to perform specialised functions, such as hardware firewalls to protect a network against external digital attacks or proxy servers to allow network clients to make external connections through them (for example to cache the most used web pages for speed).