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Quantizing variations are sometimes employed, although they are neither generally accepted nor widely deployed. Those variations include the following [8-5]:
T-carrier employs a very specific set of conventions to transmit information, with framing being one example. Using T1 as an illustration, each channel of input is time division multiplexed into a T1 frame, or set of data. In other words, conversation 1 might be allocated time slot 1 (channel 1), conversation 2 might be allocated time slot 2 (channel 2), and so on through conversation 24 and channel 24. That set of sampled data is inserted into frame 1, which is appended by a framing bit in order to distinguish it from subsequent frames of data. The process is then repeated for frame 2, and so on.
The combined processes of voice encoding and framing yield a total T1 bandwidth of 1.544 Mbps. Of that total, 1.536 Mbps is available for information transfer, as noted in the following calculation; the remaining 8 Kbps is required for framing.
4,000 Hz x 2 8,000 Samples/Second x 8 Bits per Sample 64 Kbps = DS-0 x24 Channels 1.536 Mbps + 8 Kbps Framing 1.544 Mbps = DS-1 (T1)
There exist several generations of framing conventions, designated as D1, D2, D3, D4, and Extended SuperFrame (ESF). Additionally, the ITU-T has developed an international set of recommendations for framing digital carrier signals.
D1 Framing
Developed in 1962, D1 framing robbed the Least Significant Bit (LSB), the eighth bit, in order to insert a signaling bit in each channel of each frame; signaling bits were alternating 1s and 0s. The quality of digitized voice conversation was not affected, as 7 bits are satisfactory for reconstructing a high-quality approximation of the analog voice input. Data, however, is seriously impacted by truncating an 8-bit value. As a result, data transmission was limited to 56 Kbps. Data remains limited to 56 Kbps in many carrier networks due to similar limitations imposed in D4 framing. While D1 framing no longer is used, the least significant bit is still robbed, even in the contemporary D4 framing technique.
D2 Framing
D2 Framing was used to create a superframe, a 12-bit pattern for framing locators. Information was transmitted in a 12-frame sequence or superframe.
D3 Framing
D3 Framing, which is still in use, assumes that all inputs are analog, whether they are voice or data. It uses a superframe format and sequence bits.
D4 Framing (M24 Superframe)
D4 Framing (M24 Superframe) uses a repeating 12-bit sequence (1000 1101 1100), repeated every 12 frames, to allow robbing of the least significant bits of the sixth and twelfth frames only. Voice and data are accommodated, with data being treated as a digital input. While this improves available signal capacity and yields better voice transmission, data is still limited to 56 Kbps. Additionally, ones density must be maintained through the insertion of stuff bits. Considered together, the twelve frames are designated a superframe.
Extended Superframe (ESF)
Extended Superframe (ESF), originally tariffed by AT&T in 1985, is now widely available. ESF superframes are 24 frames in length; signaling is accomplished in frames 6, 12, 18, and 24. The bit positions liberated by ESF are used for error checking and network management. ESF offers the advantages of nondisruptive error detection (6-bit CRC) and network management, using only 8 Kbps of overhead [8-2], [8-3], and [8-4].
ITU-T Framing Conventions
ITU-T Framing Conventions are quite different from those described above, which are used in North America and Japan (modified). ITU conventions call for Level 1 (E1) to employ 32 DS-0 channels, 30 for information and two specifically designated for signaling and control. The functional equivalent of framing bits are carried in the first such DS-0 channel, while the second carries signaling bits [8-5].
Transmission
Transmission facilities can include unshielded twisted-pair (22 or 24 gauge), shielded copper, coaxial cable, microwave, satellite, infrared or fiber optic cable. Therefore, Digital Carrier is medium-independent. A T-carrier circuit is also know as a T-span or T1 pipe. Regenerative repeaters are used to reshape and boost the signal at regular intervals, every 6,000 ft. in a twisted-pair system. The repeaters are powered by the CO exchange at levels up to 100 volts. The repeaters maintain their synchronization through the transmission bit stream. Therefore bipolar transmission is critical, as is the 1s density rule.
DS-1 equipment is required both for end-user organizations and for carriers. That equipment must be of the same generation in order to effect compatibility. Ideally, within the user organization, it should be of the same origin and software generic as is the carrier side in order to deliver the same functionality and feature set.
Channel Banks
Designed for voice-only service in analog applications, channel banks are used for interfacing analog switches (PBXs and COs) to DS-1 circuits. Channel banks perform two functions in sequence. First, they multiplex up to 24 (T1) analog signals on a common Pulse Amplitude Modulation (PAM) electrical bus. Second, they encode the individual PAM channels into a digital format, using PCM, for transmission over a DS-1 circuit [8-3] and [8-4].
Channel banks also will accommodate digital data. As relatively unintelligent devices, channel banks place each conversation on a separate channel; for instance, a 9.6 Kbps data conversation occupies a 64 Kbps channel, just as does a 56 Kbps data transmission or a digitized voice conversation. Therefore, channel banks do not make efficient use of available bandwidth. Combined channel banks and CSUs often are in the form of printed circuit boards which fit into PBX card slots for seamless interface to a network T1 circuit.
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