BT8954 データシートの表示（PDF） - Conexant Systems

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BT8954

Voice Pair Gain Framer

Conexant Systems 

1

1.0 DSL Systems

N8954DSC

1.1 Voice Pair Gain Applications

A well-established market exists for voice pair gain systems. In such systems,

several simultaneous phone conversations are transported over a single twisted

pair. These systems are used by telecommunications service providers to

maximize the utilization of the existing copper plant and allow it to provision

many more telephone circuits than is possible with ordinary 4 kHz analog

transport. The external interfaces of voice pair gain systems, at both the Central

Office and remote ends, are analog POTS lines. Two carrier techniques facilitate

single pair gain transmission: Frequency Domain Multiplexed Systems (FDM)

and Time Domain Multiplexed Systems (TDM). In FDM systems each voice

channel is modulated by a successively higher carrier such that the composite

transmission consists of several frequency bands. In TDM systems the voice data

is digitized and sampled in a channel-multiplexed fashion. Although FDM

systems are currently fielded, recent trends are clearly toward TDM systems

because of the inherent advantages associated with digital transmission.

Traditional PCM4 (also called “1+3”) voice pair gain systems use a

combination of 2:1 Adaptive Differential Pulse Code Modulation (ADPCM)

compression and basic rate Integrated Service Digital Network (ISDN)

U-interface devices to transport four-voice conversations on one twisted pair. The

disadvantage of this scheme is that clear 64 kbps channel capacity is lost due to

the ADPCM voice compression algorithm. This may prevent high-speed

facsimile and data transmissions from being transported reliably. Since

telecommunication service providers want to provision telephone equipment that

can be used for business purposes, this disadvantage has caused them to seek

alternative solutions that can handle data as well as voice. When used with a

Digital Subscriber Line (DSL) bit pump, such as the Bt8960, PCM4 systems can

be constructed to transmit clear 64 kbps channels, thereby enabling voice, fax,

and data transmission.

The Bt8954 with a higher speed DSL bit pump, such as the Bt8970, allows a

greater number of voice conversations to be simultaneously carried over a single

twisted pair. The Bt8954/Bt8970 combination can facilitate up to 18 64-kbps time

slots. If clear channel capability is needed, this combination results in up to

18 (PCM18) systems. When used with 2:1 ADPCM voice compression,

the Bt8954/Bt8970 combination makes up to 36 voice channels possible.

Conexant

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