• modems
  • internal
  • external
  • PC Card
• ISDN Terminal Adapters
• Cable Modems
• CSU/DSUs

Modems.  A modem is an electronic device that is used to convert digital signals to analog or analog signals to digital.  Since computer's internal signals are digital, and so many phone lines are based on analog signals there needs to be some way to get the two to work together.  The following diagram represents the basic issues.

When attempting to connect a modem sends a tone signal to another.  The tone actually modulates at some frequency relevant to its carrying capacity.  If the modem at the other end can receive the signal then the communication starts.  If it can't then the modem goes  through a process called fallback where it tries slower  modulations until it either matches or fails completely.  Due to this sort of approach the modems are always communicating at their highest common modulation.

Early modems made use of an acoustic coupler which held the telephone handset.  These modems needed to be dialed from the telephone.  These older modems were much slower and more difficult to configure.

One way that most modems can be categorized is by whether they are one of the following.

• internal
  • fit in a slot within the computer
    • ISA
    • PCI
    • PCMCIA (laptop)
  • requires no extra space on the desktop
  • requires someone to open the computer case in order to install it
  • might require you to set IRQ and/or I/O addresses
  • makes use of a COM port not already in use
  • likely to require a software driver
  • most recently there are PnP - Plug and Play devices
• external
  • plug the modem into a port
    • serial
    • USB
  • makes use of IRQ and I/O addresses of the port to which it is attached
  • have status lights
  • likely to require a software driver
• PC Card
  • PCMCIA modems are small cards that fit into special slots on laptops or notebook computers
  • analogous versions for desktops
  • configured in a fashion similar to other modems

Modems are likely to have several features that can be adjusted.  The following list contains some that are often more useful.

• the capability to adjust the volume
• adjust it to dial silently
• ability to specify things like source number to assist in selecting area codes
• ability to specify default disconnect because of idle duration
• ability to specify the number of retries
• maximum port speed

Troubleshooting modems is something else we need to consider.  The following outline surveys some of the more typical problems likely to need resolution.

• modems that won't dial
  • phone line might not be plugged in
  • likely due to lack of communication between software and modem
  • software drivers may not have been installed
  • need to check BIOS to ensure the port is enabled
  • might have a conflict in IRQs
• modem won't establish a connection
  • likely dialing the wrong number
  • might be getting a busy signal
  • might have a remote system configured for callbacks
  • incompatibility problems with other modem
    • data transfer rate
• modem disconnects
  • might not be able to receive account authentication at other end
  • connection properties may be set incorrectly
  • noisy lines
  • if have call waiting you might disconnect due to an incoming call
  • you might have hit a maximum connection duration
• modem connects at low speed
  • noisy lines
  • incorrect configuration
  • incompatibility at other end
  • software drivers might be incorrect or out of date
  • modem might be defective
• COM Port problems
  • settings may be incompatible or limiting to the modem

ISDN Terminal Adapters.  ISDN terminal adapters are sometimes called ISDN modems.  But this phrasing is inaccurate because these adapters do not have to modulate and demodulate the signals since all the signals are digital.

Most ISDN adapters are built to handle two lines simultaneously in order to increase line capacity.  Unfortunately, sometimes only one of the two lines connects.  ISDN adapters are installed in ways that are similar to those of modems.

ISDN lines can also be used to connect analog phones and receive voice calls.  The adapter can be configured so that when a voice call is received the connection drops back to one line at 64 kbps.

ISDN routers can be used to provide Internet access for an entire LAN.  An ISP can assign the LAN a block of IP addresses or some sort of Network Address Translation approach can be used.

Cable Modems.  Cable modems are used to connect to the CATV network.  They are LAN connection like devices.  They are available as internal or external units.  You might be required to lease or buy these devices from the service provider.  They generally provide Internet access only.  The connection can end up seeming very much like a direct LAN connection.

Cable connections do not require dial in and are always on.  In some instances they are purely one way and you also need a phone line to send requests upstream.  Otherwise they are likely to have a built in duplexer to support both upstream and downstream signals.

External cable modems generally connect to a computer through a UTP Ethernet card in the computer.  A splitter divides the incoming cable between the TV and the cable modem.

Internal modems are generally installed as PCI cards.  They are not generally cross compatible between CATV systems.  Unfortunately, there are several standards in use and none has emerged as the clear favorite.

CSUs/DSUs.  CSUs - Channel Service Units are used to connect to leased digital lines, usually T-carriers.  It translates the digital signals used locally to whatever is compatible with the WAN link.  The devices resemble external ISDN adapters and/or modems.

CSU/DSU units are required at both ends.  The CSU protects the T-carrier from electrical interference and is used to echo test signals called loopbacks.  The DSU performs the control duties that manage timing and regeneration of signals.

The CSU/DSU communicates with the computer over DTE - Data Terminal Equipment.  This is a standard RS-232C or serial interface.