How to Select Fiber Optic Modems
Fiber optic modems (FOM) are used in fiber optic networks for sending and receiving data. They range from simple devices with just a few ports to multiplexers capable of handling large-scale communication networks. FOMs have a higher bandwidth and greater electromagnetic immunity than wire-based modems, but are sometimes incompatible with existing electronic hardware systems. Fiber optic devices need fewer repeaters along the same length of line, however.
How Do Fiber Optic Modems Work?
Fiber optic modems receive incoming optical signals over fiber optic cables and convert them back to their original electronic form for full duplex transmission. They are available in both single-channel and multi-channel configurations. This diagram shows how fiber optic modems are used with a controller and an AS/400 server in a wide area network (WAN).
Figure 1 - Image Credit: Black Box Network Services
Modem Form Factors
There are two form factors for fiber optic modems: internal and external.
- Internal modems are printed circuit boards (PCBs) that attach to a computer through a slot.
Figure 2 - Image Credit: Bomara Associates
- External modems are self-contained devices that connect to other hardware devices through physical ports. Generally, they have a box-like shape.
Figure 3 - Image Credit: Danforth Corporation
Fiber Types and Modem Speed
Fiber optic modems use either single-mode or multimode fiber.
- Single-mode fiber modems are used in conjunction with laser sources for high speed, long distance links.
- Multimode fiber modems are used mainly with LED sources for lower speed, shorter distance links.
Modem speeds are typically measured in kilobits per second (kbps) or megabits per second (Mbps), depending on the rate of data transfer. Products operate at higher data rates when in synchronous (sync) mode and at lower data rates when in asynchronous mode (async). The reason for this difference in speeds is that in sync mode, an acknowledgement is not required for each byte of transmitted data.
Bus / Interface Type and Applications
The GlobalSpec SpecSearch database allows buyers to source fiber optic modems for specific bus types or communications interfaces. Common choices include Type II cards, CardBus, ISA, ISDN, PCI, RJ-45, ST, and USB. When selecting products, buyers need to specify the network requirements for the equipment to which the FOMs will connect.
Application is also an important selection consideration. Typically, fiber optic modems are used with radio terminals and repeaters, tactical and circuit switches, tactical multiplexers and communications systems, satellite support radios, and WANs.