MT92220 Zarlink Semiconductor, MT92220 Datasheet - Page 84

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

84
8.4
The TX AAL2 connection structure uses many of the same basic fields as the TX RTP connection structure to treat
PCM, HDLC and CPU payload, but its header treatment is completely different. The AAL2 CID indicates the CID of
packets generated by this structure; however, CPU-injected packets will contain their own CID in the descriptor
injected into the event queue, since they are often transported using different CIDs than voice packets.
AAL2 voice packets use the low bits of the UUI field as a sequence number, which is used on the receiving end to
monitor traffic, compensate for packet loss and detect silence suppression. When using the low bits as a sequence
number, the high bits are fixed. These fixed bits can be determined on a per-compression type basis, so that PCM
CPS-packets can have different UUI fixed bits as ADPCM-32 packets, for example. This is useful for AAL2 profiles
that use the same length across different compression types. The 64 kbps UUI, 40 kbps UUI, 32 kbps UUI, 24 kbps
UUI, 16 kbps UUI and SID UUI fields indicate the fixed UUI bits for each compression type. The Num Seq Bits field
ranges from 0 to 4 and indicates how many UUI bits are used as sequence number bits. The number of bits is the
same for all compression types: since voice UUI values are only supposed to range from 0 to 15, a value of 4 for
the Num Seq Bits indicates that the UUI counter will be the same for all compression types. If the Num Seq Bits is
3, then bit 3 of the UUI can be used to differentiate between 2 compression types, and so on.
Packet Type
UDP Header Start
Header Length
Minimum Total Packet
Length
Link Header Length
8P
802.3 Length Adjust
802.3 Length Position
ID v6 Position
AAL2 Packets
Field
Table 35 - Fields and Description (continued)
This field indicates the type of the first header present in the packet. This field
is only used when the RTD is set to sent packets to the packet identifier.
"0000" = LLC Header;
"0001" = PPP Header;
"0010" = IP Header;
"0011" = MPLS Unicast Header;
"0100" = MPLS Multicast Header;
"0101" = MPOA Tag;
"0110" = LANEv1-Ethernet/802.3 Header;
"0111" = UDP payload (with or without RTP contained in payload);
others = reserved.
Points to the first dword of the UDP header, relative to header word 0.
Minimum desired packet length including headers+48, in bytes. Hardware will
pad the payload with 0’s up to this length as required, e.g. if a minimum
Ethernet encapsulated packet of 46 bytes is desired, and the Link Header is
14 Bytes, this value should be set to 46+14+48=108. When no minimum
packet length is desired, this field should be set to 0.
if there were any.
802.3 Present. When ‘1’, an 802.3 header requiring a Length is present.
This value is subtracted from hardware calculated packet length to obtain
correct 802.3 length. The hardware calculated packet length = Payload +
Header Words - Padding.
Position in bytes of the first byte of the 802.3 Ethertype/Length with respect to
Header word 0.
Points to the dword of the IPv6 header, relative to Header Word 0, in which the
packet identification field will be written.
This field is the byte counter of all Header words, less the initial padding bytes
Header length in dwords, includes all Header Words.
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