Other Devices

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:

• Modems
• ISDN terminal adapters
• Wireless access points
• CSU/DSUs
• Transceivers (media converters)
• Firewalls

Modems
A modem is a device that modulates digital data onto an analog carrier for transmission over an analog medium and then demodulates from the analog carrier to a digital signal 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 provide while still
allowing the standard voice frequency range to travel at its normal frequency to remain compatible with traditional POTS phones and devices, an advantage over ISDN. 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 a wall jack and the Ethernet connection to your computer (you must have an Ethernet NIC in your computer in order to connect to the DSL modem). Alternatively, a router, hub, or switch may be connected to the Ethernet port of the DSL modem, increasing the options available for the Ethernet network.

Note:

If you have DSL service on the same phone line you use to make voice calls, youmust install DSL filters on all the phone jacks where you have a phone. Or, a DSL filter will be installed after the DSL modem for all the phones in a building. 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). A router may also be used to enhance the Ethernet network’s capabilities.


ISDN Terminal Adapters
Integrated Services Digital Network (ISDN) is another form of high-speed Internet access. It delivers digital services (over 64Kbps channels) over conditioned 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. An ISDN TA doesn’t change from digital to analog. It just changes between digital transmission formats. The box itself is about the size of a modem and looks similar to one. But, as with 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. Older, less-capable TAs used an EIA/
TIA-232 serial port for PC connectivity. 

Wireless Access Points (WAPs)
A wireless access point (WAP) allows mobile users to connect to a wired network wirelessly via radio frequency technologies. WAPs also allow wired networks to connect to each other via wireless technologies. Essentially, they are the wireless equivalent of a hub or a switch in that they can connect multiple wireless (and often wired) devices together to form a network. 
    One of the most popular use for wireless access points is to provide Internet access in public areas, like libraries, coffee shops, hotels, and airports. WAPs are easy to set up; most often, you
just need to plug them in to a wired network and power them up to get them to work. Plus, without the clutter or added expense of cables to hook them up, they make ideal foundations for small business networks.

Note:

You’ll learn the intricate details of wireless access points that a Network+ technician should know in Chapter 6.

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 or other digital serial technology (e.g., a T1 or Digital Data Server [DDS]). 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. Otherwise, some Physical Layer specification, like V.35 or HSSI, will have to be used to cable the interface on the router to the external CSU/DSU.

Transceivers (Media Converters)
Another small device that is commonly seen on a network is the external transceiver (also known as a media converter). These are relatively simple devices that allow a NIC or other networking
device to connect to a different type of media than it was designed for. Many NICs have special connectors that will allow this, as do hubs and switches. For example, if you have a 100Base-TX switch and would like to connect it to another switch using fiber-optic cabling, you would connect a fiber transceiver to each switch’s transceiver port and then connect the two transceivers together with the appropriate fiber-optic cabling. With early Ethernet-style DB-15 female Digital-Intel-Xerox (DIX, or more commonly
Attachment Unit Interface [AUI]) NIC interfaces, which are still available as medium-independent connectors on more advanced NICs and other networking devices, an external transceiver
has to be used to convert the electrical signal from the device to one that is compatible with the cabling medium. Every other popular type of Ethernet technology, such as the xBase-T standards, has a built-in transceiver on the NIC card or device interface. An external transceiver is necessary with these technologies only to act as a media converter.

Firewalls
A firewall is probably the most important device on a network if that network is connected to the Internet. Its job is to protect LAN resources from attackers on the Internet. Similarly, it can prevent computers on the network from accessing various services on the Internet. It can be used to filter packets based on rules that the network administrator sets. These rules state what kinds of information can flow into and out of a network’s connection to the Internet. Firewalls can be either stand-alone “black boxes,” or can be set up in software on a server or router. Either way, the firewall will have at least two network connections: one to the Internet
(known as the “public” side), and one to the network (known as the “private” side). Sometimes, there is a third network port on a firewall. This port is used to connect servers and equipment that can be considered both public and private (like web and e-mail servers). This intermediary network is known as a demilitarized zone, or DMZ.


Firewalls are the first line of defense for an Internet-connected network. If a network was directly connected to the Internet without a firewall, an attacker could theoretically gain direct

access to the computers and servers on that network with little effort.