rom the local unit or a local loopback for testing the T1 or E1 electrical interface. The TC8300p is similar to optical modems produced by the vendor in that the fiber-optic multiplexer can be ordered with an optic redun- dancy option. When an optic redundancy feature is added to the TC8300p, its transmitter sends the same signal via two optical connec- tors, with the built-in optical modem at the receiving fiber-optic multi- plexer determining whether receiver A or B should be used as the valid incoming signal. TRANSMISSION DISTANCE The transmission distance obtainable between a pair of fiber-optic multiplexers varies in essentially the same way as does the distance between a pair of fiber-optic modems since the former includes the latter. Thus, you must consider the type of fiber-optic cable used as well as the wavelength of the optical signal. For the TC8300p you can obtain a transmission distance of 3 km when a wavelength of 850 nm is used with multimode fiber, while a distance of 4 km becomes achievable when a wavelength of 1310 nm is used. When single-mode fiber is used, a transmission distance of 30 km becomes possible. When a wavelength of 1310 nm is used, a transmis- sion distance of 40 km becomes possible when a lightwave of 1550 nm is used with single-mode fiber. THE TC8300s A second member of the TC8300 family is the TC8300s. This fiber-optic multiplexer can be obtained with either four or eight RS-232 asynchronous ports and supports a maximum data transmission rate of 56 kbits/s. The TC8300s brings back fond memo- ries for this author, as an equivalent product from another vendor was used in the late 1990s to solve an interconnection problem between two data centers residing in the same building. At that time communi- cations circuits terminated within one data center, with some circuits connected to a switch that could be considered to represent a data PBX (private branch exchange). By connecting asynchronous data circuits to the data PBX, it became possible to allow remote users located around the 