CN8478 Datasheet, PDF (110/221 Pages) Conexant Systems, Inc – Multichannel Synchronous Communications Controller (MUSYCC)

5.0 Memory Organization
5.2 Descriptors
CN8478/CN8474A/CN8472A/CN8471A
Multichannel Synchronous Communications Controller (MUSYCC™)
5.2.2.8 Subchannel Map
To provide the subchanneling feature, MUSYCC shares the time slot mapping
function between a Time Slot Map and a Subchannel Map. The transmit and
receive functions each have a separate pair of maps.
The Time Slot Map indicates if the time slot is enabled and configured for
subchanneling. The TSEN bit field in the Time Slot Descriptor specifies the
mode of operation for the time slot. If TSEN = 6, the time slot is in subchannel
mode with bit 0 disabled. If TSEN = 7, the time slot is in subchannel mode with
bit 0 enabled and mapped to the channel specified in the CH bit field in the same
Time Slot Descriptor. The CHx bit field is also used in the treatment of the
remaining 7 bits in the time slot, specifically as part of an index into Subchannel
Map. The Time Slot Map always manages (disables or enables or maps) bit 0 of
the time slot. The Subchannel Map configures each of the remaining 7 bits of a
time slot.
The CHx bit field (0–31) from the Time Slot Descriptor is linked with the bit
number (1–7) of the time slot being processed to form an 8-bit index (0–255) into
the Subchannel Map for each bit number. The Subchannel Map associates each
bit of a time slot to a channel.
For a time slot configured for subchannel mode, MUSYCC considers each bit
of that time slot independently of any other bit in the same time slot. Any bit can
be enabled or disabled. The time slot is considered to consist of 8 individual
8 kbps bit streams, or subchannels. The subchannels can remain separated or be
concatenated to provide one or more subchannels running at bit rates between
8 kbps and 64 kbps, inclusive, in multiple of 8 kbps.
Note the following special cases for subchannel map assignments:
• Bit 0 is always disabled or enabled/mapped by the Time Slot Map;
therefore, configuring the Subchannel Map for bit 0 is not required.
• Assigning the same channel number for all bits (including bit 0) is
equivalent to disabling the subchanneling feature and treating the time slot
as if in 64 kbps mode.
• Disabling all bits in subchannel mode is equivalent to disabling the time
slot using the Time Slot Map.
The Subchannel Map can map bits 1–7 from each time slot to any of
32 channels in a channel group. The map for bit 0 is allocated, but unused (bit 0 is
mapped by the time slot map). The Subchannel Map consists of 256 descriptors
representing 32 channels and 8 bits per channel. Two subchannel descriptors
(2 dwords) are required to describe the treatment of 8 bits of one channel. The
first dword describes the treatment of the lower 4 bits, and the second dword
describes the treatment of the upper 4 bits. Table 5-16 lists the subchannel
descriptor location and byte offset. Table 5-17 lists the value and description of
each subchannel descriptor.
5-24
Conexant
100660E