MT92220BG Zarlink Semiconductor, MT92220BG Datasheet - Page 100

• 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
structures to route various associated signaling or control payload types to other extension structures (which can
then send them to control agents like the CPU, or on the HDLC bus).
The table is pointed to by the Payload Type/Marker Bit Table Base field in the structure. Since the payload type and
marker bit are, together, 8 bits, there are 256 entries in a table.
Conversely, in UDP-only connections, the UDP payload length is used to point in this structure. Since the pointer is
only 8 bits, the UDP length will be capped, and the value 255 will be used if the payload length is greater than 255.
RTP Sequence number checking is also performed on each packet received by the structure. When a new packet
arrives its sequence number is compared to the Last Received Sequence Number, and if they do not follow each
other sequentially, a sequence number error will be reported and an error structure containing both sequence
numbers will be reported. If the LR (Loss Report) bit is set, the module will report any packet loss by generating an
error report structure that will contain both the previous and current sequence number.
Then, the Network Jitter may be calculated. The Network Jitter is a measure of how much the inter-arrival delay
between 2 packets varies on average. When each packet is received, the module calculates the difference between
its current time stamp and the Last Received Timestamp, as well as the difference in actual arrival time in the chip
using the current local timestamp and the Last Local Timestamp.
Once it calculates this inter-arrival delay, it adds it to the previous network jitter using the function J = J*(15/16) +
D*(1/16), where J is the old sum and D is the new value. When the software wishes to generate an RTCP packet, it
can consult this value and insert it in the inter arrival jitter field of the RTCP packet.
Note that packets will only be included in the Network Jitter if, after looking them up in the treatment table, their
Include Jitter bit is '1'. If this bit is '0', then the Network Jitter will not be updated and the Last Received Timestamp
and Last Local Timestamp will not be modified. In other words, from a network jitter standpoint, it will be as if the
packet never arrived.
All valid packets will also be included in the structure's 16-bit Received Packet count and 32-bit Received Octet
Count. These are used to monitor the size and frequency of packets that are received. In the case of the octet
count, it helps to see what is the average size of the packets being received. The RX Channel structures will also
have their own counters.
When packets are looked-up in the table, the result will give an RX Channel Structure Number as well as the type of
the RX Channel Structure. The RX Channel Structure Number is then used to select the correct RX Channel
Structure Address from the RX connection structure and the RX Channel structure is read. This structure may be in
the xxPCM, HDLC or CPU format. If the Type field in the RX Connection Structure associated to the RX Channel
Structure is delete, then no RX Channel structure will be read and the packet will be deleted. Otherwise, the RX
Channel structure is read and interpreted according to its type.
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