Rockwell SoniCrafter BT8960 - Manual (Page 15)

5
1.0 System Overview
1.2 Applications
Bt8960
Single-Chip 2B1Q Transceiver
N8960DSB
1.2 Applications
1.2.1 Voice/Data Pairgain
A well-established market exists for voice pairgain systems. These systems trans-
port several simultaneous phone conversations over a single twisted pair. They 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
possible with ordinary 4 kHz analog transport.
The external interfaces of voice pairgain systems, at both the central office and
remote ends, are analog POTS lines. Various carrier techniques exist to facilitate
the single-pair transmission such as: the Frequency Domain Multiplexed (FDM)
systems and Time Domain Multiplexed (TDM) systems. In FDM systems, each
voice channel is modulated by a successively higher carrier, therefore the com-
posite 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
due to the inherent advantages associated with digital transmission.
Traditional 1 + 3, also called PCM4 voice pairgain systems, use a combina-
tion of 2:1 ADPCM compression and basic rate 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 transmissions
from being transported reliably. Regarding the Bt8960, an alternate way exists to
implement this type of voice pairgain equipment. A Bt8960-based system can
transport four or six clear 64 kbps channels on a single pair. Clear 64 kbps trans-
port assures the transmission of any baud-rate facsimile or can be used to provi-
sion special data services such as switched 56, clear 64, and frame relay.
Figure 1-3 shows the architecture of a PCM6 voice pairgain system. As illus-
trated, six analog Subscriber Line Interface Cards (SLIC) are connected to a con-
centrating framer. The function of this framer is to time-multiplex the PCM data
from the SLICs, create a transport frame, and handle signaling information. The
output of the framer is then passed on to the Bt8960 for conversion into the 2B1Q
code suitable for long-reach transport over the loop plant.