Computer networks can have a wide assortment of devices, each of which performs a specific job. These devices also work at different layers of the OSI model. We will briefly cover some of these devices and what they do.
Repeaters
We start with repeaters since they operate at the first layer of the OSI model. Repeaters are devices that receive a digital signal on one interface and retransmit the same signal out another interface. By definition of a layer 1 device, a repeater is a non-intelligent device that has no knowledge of the information the signal contains and so just simply “repeats” the signal. Repeaters are used to overcome the problem of attenuation (weakening of a signal) as the signal travels across a medium. For example, an Ethernet network using Category 5e cable has a maximum distance of 100 meters without repeating. To extend the range beyond this limitation, an active device such as a repeater would have to be used to re-time and regenerate the signal (however, in most cases a switch would be preferred).
As the price of fiber optic cable has become cheaper over the years, its popularity has increased as a way to achieve longer distances since it isn”t as susceptible to signal attenuation like copper cabling is. Media converters are a form of repeater that not only regenerate the signal, but also translate between different mediums. Shown below is a media converter that accepts SC multi-mode fiber (100Base-FX) and RJ-45 UTP copper (10/100Base-TX).
Hubs
A hub is essentially nothing more than a multi-port repeater. Hubs used to be much more common than they are today. A hub, as its name suggests, creates the “hub” or center of a physical star topology as multiple devices are connected to it. A signal introduced to an interface is repeated out of every other interface except for the original incoming. Although a hub can look identical to a switch, they operate very differently from each other. Hubs have no intelligence. If you have hubs in your network, get rid of them! Hubs are a horrible performance bottleneck and have largely been replaced by switches.
Bridges
A bridge is an OSI layer 2 device that makes data forwarding decisions based on media access control (MAC) addresses. Typically, the intelligence of a bridge resides in software and the underlying hardware supports two interfaces (think special purpose PC). Each interface of a bridge connects separate network segments. The job of a bridge is to prevent unnecessary traffic from crossing over to the other segment. Over time, a bridge learns which MAC addresses belong on which side of the network. If a frame destined for a particular host is on the same network segment as the source host, the frame is not forwarded by the bridge. Likewise, If the source and destination MAC addresses are on different interfaces, the bridge will forward the frame. This behavior creates two separate broadcast domains and helps to optimize congested networks.
Switches
A switch is effectively a multi-port bridge that also works at layer 2. However, a switches logic is built into its hardware so that it is able to process frames faster than its software-based ancestor. Smaller workgroup switches can be purchased from $10 all the way up to over $150,000 or more for an enterprise-class switch.
Higher-end switches typically provide other features such as VLANs, 802.1X authentication, or Power over Ethernet (PoE) to power various network devices such as IP phones and wireless access points. Below is a small Netgear workgroup switch that costs around $20.00. It easily fits in my hand.
And here we have a larger module-based HP ProCurve switch hidden behind all of the cables. The bottom row of modules support fiber interfaces.
Routers
A router is an OSI layer 3 device that makes data forwarding decisions based on IP addresses. Routers connect multiple networks together, such as connecting LANs with WANs. The Internet itself is made possible with routers. Routers tend to have fewer interfaces than switches, and commonly have different physical types of network connections. Like switches, routers can also radically differ in cost. Higher-end routers typically support more network services other than simply “routing” packets.
Firewalls
Firewalls can work at layer 4 all the way up to layer 7 of the OSI model. Firewalls permit or deny network communications based upon a set of rules created by an administrator. Firewalls are used to secure networks by preventing unauthorized access while allowing legitimate traffic to pass through. While usually implemented as a standalone network appliance, a firewall can also exist as a piece of software that protects the host OS.
Network Interface Cards (NIC)
A network interface card, or NIC, is the physical interface between a computer and a network. NICs operate at layers 1 and 2 of the OSI model. In the past, a NIC was usually an add-in to a computer in the form of a PCI or PCMCIA card. Nowadays, most modern computers have NICs built into the motherboard. The de facto standard is a 10/100 or 10/100/1000Mbps RJ-45 interface. NICs can also interface with wireless networks, and therefore are often referred to as wireless NICs (WNIC). You would be hard-pressed to find a modern laptop or tablet PC that doesn’t already have a built-in WNIC.
Wireless Access Points (WAP)
A wireless access point, or WAP is a an OSI layer 2 device that allows wireless devices such as laptops, tablet PCs, or smartphones to connect to a wired network. The WAP “translates” between wireless 802.11 communications and 802.3 Ethernet (wired) communications. Many SOHO routers on the market today have a built-in WAP for convenience.
Modems
A Modem (modulator-demodulator) is a device that modulates/demodulates an analog signal in order to encode/decode digital information. Modems can interface with all sorts of analog carriers including the Public Switched Telephone Network (PSTN) for DSL service, or cable companies. Most residential-grade Internet connections require some form of DSL or cable modem. Modems used to be common onboard devices with computers which allow for older (and slower) 56K dial-up over the PSTN. Since the proliferation of broadband, a 56K modem is almost always an add-on. Although some may disagree, a modem operates at layer 1 of the OSI model.
This list of networking devices is not intended to be an exhaustive reference. New devices are being created all the time. Additionally, many devices you will encounter are actually a combination of two or more of the above devices – all rolled into one single unit. When I purchased DSL service for my home years ago, my ISP sent me what they called a “router”. However, besides performing router functions, the single unit also contains a small 4-port switch, DSL modem, firewall, and WAP!. Enterprise networks also contain devices that perform multiple functions, such as layer 3 switches which include some routing functionality.
