to skip Q, R, S, and to use T, for Time Division. The naming system ended with the letter T, which designated fiber networks. Destined successors of the T1 system of networks, called T1C, T2, T3, and T4 were not a commercial success and disappeared quickly, signals that would have been carried on these systems, called DS1, DS2, DS3, and DS4 are now carried on T1 infrastructure. DS-1 means "Digital Service – Level 1," and had to do with the signal to be sent—as opposed to the network that delivers it (originally 24 digitized voice channels over the T1). Since the practice of naming networks ended with the letter "T," the terms T1 and DS1 have become synonymous and encompass a plethora of different services from voice to data to clear-channel pipes. The line speed is always consistent at 1.544 Mbit/s, but the payload can vary greatly. Alternative technologies Dark fiber: Dark fiber refers to unused fibers, available for use. Dark fiber has been, and still is, available for sale on the wholesale market for both metro and wide area links, but it may not be available in all markets or city pairs. Dark fiber capacity is typically used by network operators to build SONET and dense wavelength division multiplexing (DWDM) networks, usually involving meshes of self-healing rings. Now, it is also used by end-user enterprises to expand Ethernet local area networks, especially since the adoption of IEEE standards for gigabit Ethernet and 10 Gigabit Ethernet over single-mode fiber. Running Ethernet networks between geographically separated buildings is a practice known as "WAN elimination". DS1C is a digital signal equivalent to two Digital Signal 1, with extra bits to conform to a signaling standard of 3.152 Mbit/s. Few (if any) of these circuit capacities are still in use today. In the early days of digital and data transmission, the 3-Mbit/s data rate was used to link mainframe computers together. The physical side of this circuit is called T1C. Semiconductor The T1/E1 protocol is implemented as a "line interface unit" in silicon. The semiconductor chip contains a decoder/encoder, loop backs, jitter attenuators, receivers, and drivers. Additionally, there are usually multiple interfaces and they are labeled as dual, quad, octal, etc., depending upon the number. The transceiver chip's primary purpose is to retrieve information from the "line", i.e., the conductive line that transverses distance, by receiving the pulses and converting the signal which