MT92220BG Zarlink Semiconductor, MT92220BG Datasheet - Page 110

• Manufacturer Part Number: MT92220BG
• Description: Description = 1023 Channel Voice Over IP/AAL2 Processor ;; Package Type = Epbga ;; No. Of Pins = 608
• Manufacturer: Zarlink Semiconductor

MT92220
Data Sheet
9.3
xxPCM Treatment
In xxPCM, packet loss and adjustment can be performed by using the timestamps or UUI sequence numbers of the
received packets. If the LE (Loss Enable) bit is set, the module will perform packet loss/misinsertion compensation
by adjusting its pointer by the number of TDM frames that have elapsed. This is very useful when performing
silence suppression, where the loss of packets is not only accepted but expected. In practice, Loss Enable should
always be set except in UDP, where there is no timestamp or UUI sequence number to measure time with. The Last
Packet Remote Timestamp, as well as the current timestamp of the packet, are used for this purpose in RTP; in
AAL2, the Last UUI and current UUI of the packet are used.
The remaining fields in the xxPCM RX Channel structure are mostly used for error reporting and monitoring. AR
(Always Report) and SR (Slip Report) each indicate whether or not an error of the given type will cause an error
structure to be generated. Note that if an error structure is generated, all errors that occurred on that packet will be
reported, regardless of their Enable bit.
The extension structure contains a 16-bit Received Packet count and a 32-bit Received Octet Count. These have
the same definition as those in the disassembly structure (i.e. count UDP payload), but since all packets that are
treated by this structure are valid UDP/RTP packets, they will all be included in the counts. It is important that the
size of the payload in each packet be a multiple of the number of bearers present on the connection: if not, the BL2
bit (Bad Packet Length 2) in the RX error report structure will be flagged.
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 256 byte
boundaries, which is the smallest allowed size for any RX circular buffer. The buffers can be between 256 bytes and
8K bytes in size, as indicated by the Buffer Size field.
xxPCM channels can receive CN or SID packets in addition to normal packets filled with voice payload. Two types
of CN/SID packets can be received: single-byte, “white” packets that indicate only an energy level, and multi-byte,
“spectral” packets containing both an energy level and spectral information.
RX xxPCM channel structures can generate clock recovery pulses to the clock recovery module to indicate the
arrival of packets. The MT92220 allows 2 redundant clock recovery circuits to run simultaneously, so the extension
structures contain 2 bits, clock recovery A and clock recovery B, which the structure used to generate pulses. When
it generates packet arrival pulses to the clock recovery module, the disassembly process also sends it the
timestamp and the sequence number of the current packet (if RTP is being used). This will more accurately allow
the clock recovery process to reconstruct the relationship between mclk and the packet arrival rate.
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