Industrial > Modems, CSU/DSUs, Muxes

Local multiplexors extend the distance between computers and terminals or printers that are connected via customer-installed or telco-supplied cable.

Like line drivers, local multiplexors extend RS-232 communications and must be used in pairs. The difference between the two is that multiplexors merge several transmissions into one transmission over a single channel; line drivers generally transmit data over a single channel.

Local multiplexors operate over ordinary twisted-pair copper cable or fiber optic cable. Copper cable is typically used within buildings while fiber optic cable is the most common choice for connecting buildings in a campus environment. For in-building connections, copper cable is widely used because it’s comparatively inexpensive and easy to install. Your building might even have unshielded twisted-pair cable already in place.

The twisted-pair copper cable used for local multiplexors is run throughout buildings from the wallplates of each office or work area to a central wiring closet within the building. Wiring closets have centrally located punchdown blocks where all cables from the building are terminated. That way, when a connection needs to be changed or a new one needs to be made within the building, wiring can be easily rerouted on the punchdown blocks.

Selecting a local multiplexor.
When selecting a local multiplexor, keep in mind that copper-based multiplexors come in a vast array of types. You’ll find multiplexors available with RJ-11, RJ-45, or terminal block connections for your in-house wiring and with RS-232 connections for your computer equipment. All these multiplexors can be used to link a local device to a remote device within a building.

ISDN stands for Integrated Services Digital Network. It’s a high-speed digital data service provided by most phone companies. With ISDN you can transmit large amounts of data, voice, and video signals up to 128 kbps over a single phone line.

The most common (and least expensive) ISDN service is Basic Rate Interface, or BRI. It’s usually used to combine voice and data circuits over one line between small-scale ISDN sites. BRI consists of two 64-kbps B channels plus a 16-kbps D channel to support system “overhead” functions, such as signaling the telecomm switching system to initiate a call.

What makes ISDN unique is that each B channel is a separate communication circuit. That means just one ISDN line can support simultaneous two-way communication for two devices, such as a computer and a telephone or a computer and a video camera for teleconferencing.

If you need to send more data than one 64-kbps B channel can handle, ISDN also supports BONDing for inverse multiplexing. This links the two B channels into a single logical circuit that can support data rates up to 128 kbps.

ISDN lines are terminated at your location with a special RJ-45 jack. There are two main interfaces. The U interface consists of two wires (one twisted pair) and is common in North America. The S/T interface consists of four unshielded wires (two twisted pairs) and is more common outside North America. Unless you already own U-compatible ISDN phones or PCs, you’ll need a terminal adapter to make the connection.

ISDN is the perfect choice when faster data rates, lower prices, and guaranteed data integrity are required. Consider it for high-volume datacomm applications such as Internet access and on-line service, telecommuting, remote-office routing, and disaster recovery. Also consider ISDN for a high-speed backup line—because you never know when you’ll need one.