| 单词 | T1 (Terrestrial 1) |
| 释义 | T1 (Terrestrial 1) Corresponds to DS-1 (Digital Signal level One) in the North American digital sig- nal hierarchy.The fundamental level of the T-carrier digital carrier system.A T1 system comprises circuit-terminating equipment in the form of a combination of a channel service unit (CSU) and data service unit (DSU) that jointly serve to interface a device to a full-duplex (FDX) four-wire digital circuit and to perform various signal-formatting, signal-timing, monitoring, and diagnostic functions.T1 operates at a signaling rate of 1.544 Mbps, which supports a frame rate of 8,000 frames per second (fps), with each frame comprising a framing bit followed by 192 bits of user payload, at least potentially.The framing bits are used for synchronization and, in some cases, for monitoring, diagnostic, and other network management purposes.The 192 bits of user payload are organized into 24 time-division multiplexed (TDM) time slots, each of which is eight bits wide. See Figure T-1. At a rate of 8,000 fps, each time slot is repeated 8,000 times per second, which translates into a DS-O channel at 64 kbps (8 bits × 8,000 per second = 64,000 bps). Taken together, the 24 8-bit TDM channels at 8,000 fps yield an aggregate payload transmission rate of 1.536 Mbps. Adding the 8,000 framing bits (one per frame) per second, yields the aggregate signaling rate of 1.544 Mbps. Actually, different generations of CSUs, DSUs, and channel banks operate on different framing conventions (D1, D2, D3, D4, and ESF). In some cases, a process of bit robbing reduces the amount of user payload to seven bits per eight-bit time slot of each frame, thereby restricting user payload to 56 kbps. Another convention bit robs only certain frames, and yet another bit robs not at all. T1 was designed to operate over an unshielded twisted-pair (UTP) circuit comprising two two-wire pairs, each of which operates in simplex mode. One pair supports transmission in one direction, and the other pair in the opposite direction. In the aggregate, the physical four-wire circuit supports full-duplex (FDX) transmission. The line coding technique employed in traditional T1 is alternate mark inversion (AMI), which yields 1.544 Mbps at a nominal carrier frequency of 784 kHz, which is exactly half the T1 bit rate, plus some overhead for error control. At such a high frequency, issues of attenuation are significant, and mutual interference between cable pairs must be considered, so repeaters must be placed every 6,000 feet. Contemporary T1 circuits typically are provisioned using high-bit-rate digital subscriber line (HDSL) technology, which mitigates these issues.T1 was initially designed to operate over a physical four-wire twisted-pair copper circuit.The interface, more correctly known as DSX-1, is medium-independent and will run over coaxial cable, optical fiber, microwave, satellite, and free space optics (FSO) just as well. T1 generally is used in local loops and other short haul applications. In long-haul applications, T3 and other, higher speed, standards generally are employed. The T for Terrestrial was to distinguish the system from satellite transmission as Bell Laboratories both activated the first T1 system and launched Telstar I, the first communications satellite, in 1962. See also AMI, attenuation, bit robbing, carrier, channel bank, CSU, D1, D2, D3, D4, DS-0, DS-1, DSU, error control, ESF, FDX, four-wire circuit, fractional T1, frame, framing bit, frequency, HDSL, line coding, overhead, payload, signaling rate, simplex, synchronization, T3, T-carrier, TDM, time slot, transmission rate, two-wire circuit, and UTP. 74570c20.qxd 9/12/07 12:44 AM Page 472 |
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