The hardware required to implement a WAN link can be as simple and readily available as a modem and a telephone line.  On the other hand it can be complicated and expensive, such as satellites and transoceanic cabling.  This webpage will survey some of the most frequently used hardware.

Modems.  The word modem originally derived from modulation/demodulation.  This needs to occur in order to take digital signals from/to computers and transmit/receive them over analog telephone lines.  Currently, they take on an even larger variety of basic approaches to functioning.

They tend to come in two main varieties in terms of their relative position inside or outside the transmitting/receiving device.  Modems can be categorized as internal or external. 

One of the main advantages of an internal modem is compactness.  They are circuit boards that fit into one of two places inside a computer.

• ISA - Industry Standard Architecture slot
• PCI - Peripheral Component Interconnect slot

Since they are inside you do not need to find extra space on the desktop, nor do you need an extra serial cable.

Unfortunately, internal modems are traditionally more difficult to configure than external ones.  Several things need to be set in order to ensure there aren't conflicts with other internal components.

• IRQ - Interrupt Request
  • this is an assigned location where the system expects the device, in this case a modem, to interrupt it when the device sends a signal
  • signals that go to a processor on the same line would interrupt each other
• Input/Output Address
  • this is the memory location where data from the device is stored in order to be processed by the CPU
  • again, if multiple devices try to use the same I/O address it will inevitably result in conflicts
• Virtual Com Port
  • this is a logical port number by which the operating system identifies a serial port
  • you must set each serial device to use a different com port

All modern operating systems provide a means by which you can view how resources are being used.

Modifying these settings depends on the modem.  The following list gives three typical ways these settings can be changed.

• Dip switches - these are small switches directly on the circuit board that can be moved
• Jumpers - pairs of metal pins built into the circuit board that represent an electrical contact point
  • configured so that a small plug is placed on the pins to complete a circuit
• Software - a software program is used to change the configuration

Many modems support PNP - Plug and Play technology which enables the operating system to

• detect the device
• install the necessary software drivers
• detect what resources are available on the computer
• assign particular resources to the device automatically

Little or no intervention is required from the user.  Unfortunately, these sorts of devices require your computer's operating system to have features that are compatible.

External modems have a couple advantages over internal ones.

• most provide status lights
  • power is on
  • connected
  • transferring data
• usually easier to configure and install than internal ones

A couple disadvantages of external modems are

• they require a power cord to plug into an electrical outlet, rather than running off the computer's power like an internal modem
• they require a serial cable to run from the modem to one of the serial ports on the back of the computer

To use an external modem you need to have an available serial port.  Most computers have two built in serial ports, COM 1 and COM 2, with connectors on the back of the computer.  It is important to remember that many other devices such as scanners, digital cameras and serial printing devices also use serial ports.

If your computer doesn't have a serial port then you have a few options.

• use an internal modem
• install an expansion card in your computer
• if you have a USB you can chain several serial devices off a single port

Serial ports make use of a UART - Universal Asynchronous Receiver/Transmitter chip to handle serial communications.  This chip comes in different types and the type used determines how fast data can be transferred over the serial port.

Drivers are software programs that act as liaisons between hardware devices and the operating system.  Some modems require a modem driver that either comes with the modem or can be downloaded from the manufacturer's website.

A computer may also be configured as a dial-up server or remote access server to allow other computers to dial into it and connect over the phone lines.  Computers running particular server software can support many remote access connections simultaneously.  In order to do this the server needs to have a modem bank.  The modem bank allows the server to make use of a group of modems simultaneously.

ISDN and DSL Adapters.  The device used to connect a computer to an ISDN - Integrated Services Digital Network or DSL - Digital Subscriber Line is often referred to as a modem.  It is more accurate to call it a terminal adapter.  It doesn't really modulate and/or demodulate signals because the lines are digital.

ISDN adapters also come in both internal and external varieties.  They are configured similarly to modems, but typical 128 Kbps ISDN service consists of two data channels each running at 64 Kbps.  The two data channels usually have separate phone numbers. 

ISDN adapters are configured with information about the SPID - Service Profile Identifier for each channel.  Each of these SPIDs consists of a telephone number, a two digit sharing terminal identifier and a two digit terminal identifier.  Some more recent models support automated SPID selection and don't require the admin to enter the information.

DSL adapters or endpoints are required at both ends of a DSL connection.  Sometimes these endpoints are external.  Other times they are placed together with the NIC on the same card.

Customer Premises Equipment.  CPE - Customer Premises Equipment is a general term that encompasses several different devices.  These are used for processing transmissions on WAN links such as

• T-1 lines
• T-3 lines
• X.25 connections
• Frame Relay links