Common Network Connectivity Devices

Now that you are familiar with the various types of network media and connections, you should learn about some devices commonly found on today’s networks. Because these devices connect network entities, they are known as connectivity devices. These devices include:

• The network interface card (NIC)

• The hub

• The switch

• The bridge

• The router

• The gateway

• Other devices

NIC

The network interface card (NIC), as its name suggests, is the expansion card you install in your computer to connect, or interface, your computer to the network. This device provides the physical, electrical, and electronic connections to the network media. NICs are either an expansion card (the most popular implementation) or built in to the motherboard of the computer. In most cases, a NIC connects to the computer through expansion slots. An expansion slot connects expansion cards that are plugged in to a slot in the main computer assembly through a deceptively simple-looking connector, which is known as a bus. In some notebook computers, NIC adapters can be connected to the printer port or through a PC card slot.

Hub

As you learned earlier, in a star topology Ethernet network, a hub is the device that connects all the segments of that network together. Every device in the network connects directly to the hub through a single cable. Any transmission received on one port will be rebroadcast to all the other ports in the hub. So, if one station sends it, all the others receive it, but only the intended recipient listens to it.

Switch

Like a hub, a switch connects multiple segments of a network together, with one important difference. Whereas a hub rebroadcasts anything it receives on one port to all the others, a switch makes a direct link between the transmitting device and receiving device. Any party not involved in that communication will not receive the transmission. The benefit of a switch over a hub is that the switch increases performance because it doesn’t suffer from the wasted bandwidth of the extra transmissions.

Note 

Both hubs and switches will be discussed in more detail in Chapter 2, “The OSI Model.”

Bridge

A bridge is a network device that connects two similar network segments together. The primary function of a bridge is to keep traffic separated on both sides of the bridge. Traffic is allowed to pass through the bridge only if the transmission is intended for a station on the opposite side. The main reason for putting a bridge in a network is to connect two segments together, or to divide a busy network into two segments.

Router

A router is a network device that connects multiple, often dissimilar, network segments into an internetwork. The router, once connected, can make intelligent decisions about how best to get network data to its destination based on network performance data that it gathers from the network itself.

Gateways

A gateway is any hardware and software combination that connects dissimilar network environments. Gateways are the most complex of network devices because they perform translations at multiple layers of the OSI model.

For example, a gateway is the device that connects a LAN environment to a mainframe environment. The two environments are completely different. LAN environments use distributed processing, baseband communications, and the ASCII character set. Mainframe environments use centralized processing, broadband and baseband communications, and the EBCDIC character set. Each of the LAN protocols is translated to its mainframe counterpart by the gateway software.

Another popular example is the e-mail gateway. Most LAN-based e-mail software, such as Novell’s GroupWise and Microsoft’s Exchange, can’t communicate directly with Internet mail servers without the use of a gateway. This gateway translates LAN-based mail messages into the SMTP format that Internet mail uses.

Other Devices

In addition to these network connectivity devices, there are several devices that, while maybe not directly connected to a network, participate in moving network data. Some of these devices include:

• Modems

• ISDN Terminal Adapters

• CSU/DSUs

Modems

A modem is a device that changes digital data into an analog form for transmission over an analog medium and then back to digital again at the receiving end. The term “modem” is actually an acronym that stands for MOdulator/DEModulator.

When we hear the term modem, three different types should come to mind:

• Traditional (POTS)

• DSL

• Cable

Traditional (POTS)

Most modems you find in computers today fall into the category of traditional modems. These modems convert the signals from your computer into signals that travel over the plain old telephone service (POTS) lines. The majority of modems that exist today are POTS modems, mainly because PC manufacturers include one with a computer.

DSL

Digital subscriber line (DSL) is quickly replacing traditional modem access because it offers higher data rates for a reasonable cost. In addition, you can make regular phone calls while online. DSL uses higher frequencies (above 3200Hz) than regular voice phone calls use, which provides greater bandwidth (up to several megabits per second) than regular POTS modems. DSL “modems” are the devices that allow the network signals to pass over phone lines at these higher frequencies.

Most often, when you sign up for DSL service, the company you sign up with will send you a DSL modem for free or for a very low cost. This modem is usually an external modem (although internal DSL modems are available), and it usually has both a phone line and an Ethernet connection. You must connect the phone line to the wall and the Ethernet connection to your computer (you must have an Ethernet NIC in your computer in order to connect to the DSL modem).

Tip 

If you have DSL service on the same phone line you use to make voice calls, you must install DSL filters on all the phone jacks where you have a phone. Otherwise, you will hear a very annoying hissing noise (the DSL signals) on your voice calls.

Cable

Another high-speed Internet access technology that is seeing widespread use is cable modem access. Cable modems connect an individual PC or network to the Internet using your cable television cable. The cable TV companies use their existing cable infrastructure to deliver data services on unused frequency bands.

The cable modem itself is a fairly simple device. It has a standard coax connector on the back as well as an Ethernet port. You can connect one PC to a cable modem (the PC will need to have an Ethernet NIC installed), or you can connect the modem to multiple PCs on a network (using a hub or switch).

ISDN Terminal Adapters

Integrated Service Digital Network (ISDN) is another form of high-speed Internet access. It delivers digital services (in 64Kbps channels) over standard telephone copper pairs. The device you must hook up to your computer to access ISDN services is properly known as an ISDN Terminal Adapter. It’s not a modem in the truest sense of the word because a modem changes from digital to analog for transmission, but an ISDN TA isn’t changing from digital to analog. It’s just changing between digital transmission formats.

The box itself is about the size of a modem and looks similar to one. But, like DSL modems, there is a phone jack and an Ethernet jack. You connect a phone cord from the phone jack to the wall jack where your ISDN services are being delivered. Then, you connect an Ethernet cable from your PC to the ISDN TA’s Ethernet jack.

CSU/DSUs

The Channel Service Unit/Data Service Unit (CSU/DSU) is a common device found in equipment rooms when the network is connected via a T-series data connection (i.e., a T1). It is essentially two devices in one that are used to connect a digital carrier (the T-series or DDS line) to your network equipment (usually to a router). The Channel Service Unit (CSU) terminates the line at the customer’s premises. It also provides diagnostics and remote testing, if necessary. The Data Service Unit (DSU) does the actual transmission of the signal through the CSU. It can also provide buffering and data flow control.

Both components are required if you are going to connect to a digital transmission medium, such as a T1 line. Sometimes, however, one or both of these components may be built into a router. If both components are built into a router, you only have to plug the T1 line directly into the router.