MT92210 Zarlink Semiconductor, MT92210 Datasheet - Page 98

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

98
8.3
The MT92210's PDV monitoring writes packets within a PDV window, minimizing delay while trying to avoid slips.
To do this, it writes packets based on their remote timestamp: a packet's position in the circular buffer is decided by
its RTP timestamp. This allows compensation for packet loss and misinsertion.
Because some profiles in RTP use varying-length packets (the term varying-length here refers to changing number
of samples per packet, not length in bytes; compressed packets are converted to a number of samples before PDV
monitoring is applied to them), changing packets lengths will show up as PDV on the network. To compensate for
this, the xxPCM channel structure contains a Max Expected Payload Size in Samples field. Any variation in packet
length up to this size will be compensated for. Setting this field to too large a value will insert delay; setting this field
to too small a value will allow the changing length to show up as PDV. If the value is not known with certainty, a
smaller value is usually better.
The default mode of the PDV monitoring system allows a PDV window of length Maximum Used Samples in
Circular Buffer. (Note: the Max Expected Payload Size in Samples - 1 must be added to Maximum Used Samples
in Circular Buffer to get a window of the desired size. As an example, if the Max Expected Payload Size in Samples
is 160 and the desired PDV window is 10 ms (80 samples) long, then Maximum Used Samples in Circular Buffer
should be set to 239 (80 + (160-1)).
An underrun occurs when not enough data is received by the disassembly module, so the desired write pointer's
lead vs. the TDM read pointer would be smaller than 0. In this case, the write pointer's position is reset to Underrun
Lead + current TDM read pointer.
An overrun occurs when too much data is received by the disassembly module, so the desired write pointer's lead
vs. the TDM read pointer would be greater than Maximum Used Samples in Circular Buffer. In this case, the write
pointer's position is reset to Overrun Lead + current TDM read pointer.
Finally, the Init Lead is used to reset the write pointer when the first packet is received on the channel.
BL2
PLX
BL4
BL3
XD
RX xxPCM Channel
Structure Address
Samples Lost
Remote Time Stamp
Local Bus Time Stamp
Packet Delay Variation (PDV) Monitoring
Field
Table 33 - Fields and Description (continued)
Bad Packet Length 2. When set, the number of “payload bytes” * 8 was not
divisible “Number of channels” * “Number of bits per sample”. Packets affected
by this error will be discarded.
Packet Longer than Expected. The number of samples in the received packet
exceeded the “Max Expected Payload Size in Samples” field of the RX xxPCM
Channel Structure,
Bad Packet Length 4. The number of samples in the received packet exceeded
2047 frames.
Bad Packet Length 3. The number of samples in the received packet was 0
frames.
Extreme Delay. When ‘1’, extreme delay has been detected in the Extended
PDV monitoring structure.
Base address of the RX xxPCM Channel Structure that generated this report
structure.
Number of samples lost due to packet loss as detected by the difference
between remote time stamps in two consecutive packets.
Remote time stamp of the packet received that cause the generation of this
report structure.
Local Bus Time Stamp present at the time of reception of the packet that
caused this report structure to be generated.
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Description