MT92220 Zarlink Semiconductor, MT92220 Datasheet - Page 89

• Manufacturer Part Number: MT92220
• Description: 1023 Channel Voice Over IP/AAL2 Processor
• Manufacturer: Zarlink Semiconductor

Data Sheet
8.5
In the case of an connection carrying PCM or ADPCM, the packet assembly module uses the TX connection
structure to determine how the bytes should be assembled, as well as all the different headers needed by the
multiple protocols (Link, IP, UDP, RTP). The two fields that will determine the shape and size of the packet payload
are the Total number of frames, that determines how many payload samples per channel will be carried by the
packet, and the Number of Bearers, which tells how many xxPCM bearers will be included in the packet.
The total amount of payload samples in the packet will be Total number of frames multiplied by Number of Bearers.
This is converted into bytes according to the compression rate.
8.5.1
The first word of the structure contains the Next TDM write pointer on which the next packet can be assembled.
This is used when receiving a packet assembly event from the queue to determine if the packet assembly
requested is valid or not. If the current TDM pointer is greater or equal to this value, then the event is valid;
otherwise, it is ignored. The Initialized bit is used to detect the start-up of the connection: when a packet assembly
event is read and the Initialized bit is '0', the first packet will be discarded, and the Next TDM write pointer will be
initialized to the current TDM pointer + Next TDM write pointer. This means that this field should be initialized as an
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.
8.5.2
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'.
8.5.3
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.
CPS Timer
FL
x kbps/SID UUI
Bearer X - Circular
Buffer Base Address
Bearers
PCM Packets
Next TDM Write Pointer
Valid Bit
Buffer Size
Field
CPS Timer applied to this packet when waiting for cell completion to be transmitted.
Units are in frames.
Flush Pending Packet. When ‘1’, pending packets will be sent in a zero padded cell
before this packet is sent. When ‘0’, normal TX CPS processing will be done to try to
maximize bandwidth utilization.
These fields contain the fixed part of the UUI field (i.e. the bits that are not part of the
UUI sequence number field) for each CODEC rate and for SID packets. These fields
are usually set to all zeros.
Pointer to each TX xxPCM circular buffer that must be read in order to assemble a
packet. There must be one of these addresses per Bearer in the xxPCM channel. This
points to circular buffers in increments of 512 byte of addressing in SSRAM A.
For each bearer in this xxPCM channel, a single circular buffer base address must be
specified in one of these extension words. All xxPCM channels may have between 1
and 255 bearers, thus between 1 and 255 extension words.
Table 37 - Fields and Description (continued)
Zarlink Semiconductor Inc.
Description
MT92220
89