MT92210 Zarlink Semiconductor, MT92210 Datasheet - Page 77

• Manufacturer Part Number: MT92210
• Description: 1023 Channel Voice Over ip (VoIP) Processor
• Manufacturer: Zarlink Semiconductor

Data Sheet
offset that says: “the first packet should be assembled this many frames after the first event is read”. It allows a
start-up delay on the connection.
7.4.2
The Valid bit is used to ensure that no corrupt structures will be read while the scheduler is being programmed.
While this bit is '0', any event that is read will simply be ignored (i.e. it will not even set the Initialized bit). The order
of events should be the following: program the assembly structure (with Valid = '0'), then program the scheduler
events, and finally set the Valid bit to '1'.
7.4.3
The Buffer Size field indicates how large the TX Circular buffer containing the data is, from a minimum of 512 bytes
up to a possible 4K bytes. The compression rate is determined implicitly from the data in the circular buffer: when it
writes to the circular buffer, the TX TDM also uses auxiliary information to indicate the compression rate of the data
in the buffer. The packet will then be assembled according to this compression rate. Note that a scheduling
mechanism must be used to ensure that all data contained within a single packet is coded in the same compression
format.
This is discussed in further detail in the TX TDM section. Note that the Total number of frames keeps its definition
regardless of the compression rate, so with ADPCM 16 kbps, 4 frames of data only come out to 1 byte. The
assembly process can generate one of 6 payload types for xxPCM packets: one for PCM, one for each ADPCM
compression rate, and one for CN packets. In the TX RTP header structure, the payload types are written in the
RTP header.
7.4.4
The TX Silence Suppression Structure Base points to a structure that will be used to perform silence suppression
on the xxPCM channel carried over this connection. When its value is 0000h, it means that no silence suppression
will be done on this channel. Silence suppression cannot be used on channels carrying more than 1 bearer.
7.4.5
The Extra Delay Frames are also related to the silence suppression calculations: they indicate that the packet
assembly should not use the most recent xxPCM data to assemble its packet, but instead should back up by a
certain number of frames. This allows the silence suppression calculations on a block to be performed fully before
the data is sent in an IP packet, ensuring, for example, that there is no start-up delay between the moment that
voice begins again and the moment that packets start being sent again.
7.4.6
The RTP timestamp sent in PCM packets is calculated in the following way: global timestamp corresponding to the
first byte in the packet + Timestamp Offset written in the structure. The global timestamp is fed to the assembly
module by the TX TDM and can either be a free-running counter within the chip or a value extracted from 4
time-slots on the TDM bus. See the TX TDM section for more information on sourcing the timestamp.
7.4.7
Finally, at the very end of the structure, are a certain number of Circular Buffer Base Addresses that indicate, for
each channel in the connection, where is the circular buffer associated to it. These addresses point to 512 byte
boundaries, which is the minimum size for any circular buffer, since the xxPCM data is only contained in the left
byte of the circular buffer. The buffers may be larger, in which case the lower bits of the base address will not be
used: the Buffer Size field indicates the size for all the circular buffers used by this connection.
7.5
HDLC and CPU-sourced packets are not payload-formatted by the TX connection structure. They are simply
packaged in the link, IP and UDP (or null) headers and transmitted as such. The assembly process, in addition to
encapsulating the payload in these protocols, may perform some RTP functions. The Sequence Number Insert bit
indicates whether the RTP sequence number should be generated by the structure or kept as it is in the payload.
Valid Bit
Buffer Size
TX Silence Suppression Structure Base
Extra Delay Frames
RTP Timestamp
Circular Buffer Base Addresses
HDLC Packets
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MT92210
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