On any LAN or WAN you’re likely to come across a variety of different hardware devices. Examples include hubs, switches, and routers to name but a few. When considering any given piece of network equipment, it’s important to understand both the role it plays in the communication process, as well as how it relates to the OSI model. Furthermore, you should be able to explain why you might choose one piece of hardware over another. For example, you should be able to outline the reasons why a switch provides performance advantages over a hub. The equipment that we’re going to look at in this section includes repeaters, hubs, bridges, switches and routers.
Although you’re not likely to run into too many traditional LAN repeaters any more, they once served an important purpose. A repeater is a powered network device that exists for the purpose of regenerating a signal as it travels along the network, allowing longer distances to be spanned. As data travels over a network, signal strength tends to degrade as it moves farther along its path, a concept referred to as attenuation. When signals enter a repeater, the device boosts their strength, but doesn’t actually modify the data. Most commonly found on Ethernet bus networks using coaxial cable, repeaters are considered to exist at the Physical Layer of the OSI model.
A hub is a network connectivity device that you’re almost certainly familiar with. In the most basic sense, a hub is really nothing more than a multiport repeater. As signals are sent between systems connected to a hub, they are not only regenerated, but also forwarded out each port. In this way, all devices plugged into a hub are exposed to all traffic passing between systems, regardless of whether that traffic is actually relevant to them. Like repeaters, hubs are also considered to exist at the Physical Layer of the OSI model – they neither modify the data stream nor make any decisions. Instead, they act as a conduit by which systems can interconnect and communicate.
The limitations of a hub become evident as a network grows. While providing basic connectivity, a hub does nothing to prevent network collisions or broadcasts from reaching all connected systems. For this reason, all devices connected to a hub are considered to be part of the same broadcast domain and the same collision domain – concepts we’ll look at shortly. In order to be able to control traffic to a greater degree, you would need to implement devices capable of making forwarding decisions based on source or destination addresses. Examples of such devices include switches, bridges, and routers.
While you’re probably familiar with Ethernet hubs, there are a variety of hubs that exist for different network technologies, including Token Ring. In a Token Ring environment, systems are connected to something that looks similar to a hub, but is properly referred to as a Multi-Station Access Unit (MSAU). We’ll discuss the details of how an MSAU works in the Token Ring section. For now it is enough to understand that it is also a Physical Layer device.
Tip: Hubs and repeaters exist at the Physical Layer of the OSI model.