Network modems enable computers to transmit data over analog telephone wires or digital telephone lines. They are used to connect to the Internet and to local or wide area networks (LANs and WANs), where users can access and share computing resources. Modem speed, the maximum rate at which devices can deliver data, is typically measured in kilobits per second (kbps).

How Network Modems Work

Some network modems are designed to send digital data from a computer over traditional analog phone lines in the public switched telephone network (PSTN). These analog modems convert the digital data to sound waves (modulation) and then, on the receiving end, another network modem decodes or demodulates the transmitted information.

Dial-up Internet Access. Image Credit: Bell Canada

Other types of network modems use various communications standards to send voice, video and data over analog, digital and/or fiber optic telephone lines. These communication products offer much faster transmission speeds than dial-up access, where a computer dials a phone number to an Internet Service Provider (ISP) and then sends and receives data over analog phone lines.

High-Speed Internet Access. Image Credit: Bell Canada

Form Factors and Interface Types

As the diagrams above indicate, there are two form factors for network modems: internal and external.

• Internal modems are computer cards that attach to a computer through a slot. The modem in Figure 1 is an example of this type.

• External modems are modules or devices that connect to a larger system through a physical port. The modem in figure 2 is an example of this type.

Both types of products also have a bus or interface type that specifies the type of communication and/or connection. For example, some external modems have serial ports for RS232, RS422, RS485, or USB connections. Other use RJ-45 connectors or work with ISA or PCI buses.

Network Modem Types

The Engineering360 SpecSearch database lets industrial buyers select modems according to type, a key specification which indicates whether the device is designed for use with analog, digital, and/or fiber optic phone lines.

Telephone modems transmit analog data across copper telephone wires in the public switched telephone network (PSTN). Analog modems, as they are sometimes called, typically have connection speeds ranging from 2400 bps to 56 Kbps.

ISDN modems use the integrated services digital network (ISDN) standard and can transmit data over either analog phone lines or digital phone lines. Typical ISDN speeds range from 64 Kbps to 128 Kbps. An ISDN variant, broadband ISDN, transfers data only fiber optic telephone lines.

DSL modems use digital subscriber lines (DSL) and support standard xDSL technologies to provide "always-on" Internet connections. Variants include network modems for asynchronous DSL (ASDL) and ASDL+2, symmetric DSL (SDSL) or high data rate DSL (HDSL), and very high DSL (VDSL). xDSL connection speeds range from 128 Kbps to 8 Mbps.

Fiber optic modems are used in optical networks and are not suitable for use with either analog or digital lines.

Modem Technologies

Modem technologies are standards for uploads, downloads, data signaling rates, and encapsulation. When selecting products, buyers may specify V.120, V.92, V.90, V.90, V.34 and V.34+.

• V.120 enables two devices (typically PCs) to communicate over an ISDN connection by using their standard asynchronous-only COM ports (an external V.120 adapter is then used) or an internal board that plugs into the PC's bus (the board then includes the COM port and V.120 adapter functionality).
• V.92 provides for an increase in upload speeds, quicker connect times, and a modem-on-hold feature that allows the device to work in conjunction with call-waiting provided by the phone company. V.92 doesn't increase the download speed over V.90, but does increase the maximum upload speed to 48 kbps.
• V.90 is the standard serial line protocol for modems that allow download speeds of up to 56 kbps and upload speeds of 33.6 kbps.
• V.34 is a serial line protocol for modems operating at data signaling rate of up to 28,800 bit/sec.
• V.34+ is similar to V.34, but supports a data signaling rate of up to 33,600 bit/sec.

Fax modes include V.17, V.29, V.27ter, and V.21.

Resources

Virginia Tech - Network Infrastructure & Services

Image credits

Patton Electronics | Moxa, Inc.