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System architecture
31
Peripheral Equipment
Peripheral Equipment consists of peripheral controller cards which provide
the timing and control sequences for peripheral circuits, analog and digital
line and trunk cards, which provide the interfaces for terminal devices, and
circuit cards which provide Digital Trunk Interface (DTI),and Primary Rate
Access (PRA) services.
The peripheral controller performs the first stage of multiplexing signals
from the terminals, which are then passed on to the network for digital
switching. The peripheral controller also transmits timing signals, and
carries out other functions associated with the control of the peripheral
circuits.
The line card converts incoming analog signals into digital signals which
are then passed on to the peripheral controller. This conversion is
accomplished by an integrated encoder/decoder (codec) chip. There is a
codec for each port on the line card. Conversion from digital back to analog
is also performed by the codec for outgoing signals. For digital telephones,
the codec is located within the telephone set itself.
PCM
Pulse Code Modulation (PCM) is used to convert analog signals to digital
signals. The PCM method converts the analog signal to digital by samplin.g
the amplitude of the analog signal at a rate of twice the highest signal
frequency and converting the amplitude of each sample into a series of
coded pulses. The PCM sampling frequency standard for
telecommunications is 8 kHz.
Companding (compressing
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expanding) PCM is a standard technique for
using 8 bits words to efficiently represent the wide dynamic range of voice
and data signals. Two standards for companding are internationally
recognized:
Mu-255 law for North American applications
A-law for international service
Meridian 1 codecs conform to both standards and are software selectable (in
Intelligent Peripheral Equipment Modules) for use in North America and
internationally. The codecs are also designed to pass signals up to 3.4 kHz
System overview 553-3001-100
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