MT92220 Zarlink Semiconductor, MT92220 Datasheet - Page 76

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

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8.3
PCM, HDLC and CPU-generated packets are all treated by the same structure, while there is a different structure
format for IP/RTP and AAL2. In IP/RTP, most of the fields of this structure, the TX RTP connection structure,
manage the encapsulation protocols used (IP, UDP and RTP) as well as containing the link header. The TX RTP
connection structure also contains a pointer to a secondary structure, the TX RTP header structure. This structure
contains all protocol-related information, as well as anything that might need to be changed dynamically.
By having a pointer to another structure, a new TX RTP header structure can be created with new headers and new
characteristics for the packet; once this is done, the pointer in the connection structure can be modified, creating a
glitch-free change between the two.
8.3.1
The TX RTP header structure contains a number of header words (note that headers for any layer at the network
level or above are multiples of dword in length, while link headers can be byte multiples). The structure can indicate
different types of encapsulation: IPv4 with UDP and with/without RTP, IPv6 with UDP and with/without RTP, or null
encapsulation with/without RTP. In addition, the header words in the structure will contain a SNAP/LLC,
LANE/Ethernet, LANE v2 or PPP header, or begin directly with IP, MPOA, MPLS or application data, depending on
the type of the connection on which the packet will be sent.
8.3.2
The Header Length shows how many bytes of total header will be present in the packet, including link header. This
can vary from very short (a few bytes, in the case of ATM carrying null encapsulated data) to very long (when using,
for example, Ethernet 802.1 p/Q with IPv6/UDP/RTP and many extension headers). The first 48 bytes of the header
are reserved for ATM fields: the ATM header is contained in the first dword of link header and the first 2 bytes of the
second dword contain the CPI and UU fields. These will always be ignored in Ethernet or PPP. The other 42 bytes
are not used. After the ATM header, the rest of the link header follows and can be between 0 and 255 bytes in
length, as indicated by Link Header Length. This length includes the 48 bytes used for the ATM header, UU and
CPI, independently of whether the link is Ethernet, PPP or ATM. Note that the IP and RTP headers are always a
multiple of 4 bytes in length, and the UDP header is always 8 bytes, while the link header can end on any byte
boundary. In the structure, padding bytes and will be added to the link header: it must always end on a dword
boundary. The padding bytes will be ignored when the packet is transmitted on the bus. The Header Length is in
dword, including the padding octets.
TDM Write Pointer
Buf Size
CPU Packet Length
CID
TX AAL2 VC Base
RTP Packets
TX RTP Header Structure
Header Length
TDM Write Pointer used to write xxPCM samples in the TX Circular Buffers at the
time of the generation of this event.
Size of the CPU circular buffer in which the CPU packet is contained.
Length of the payload of the CPU packet in bytes. This length includes the UUI
field with is contained the first byte of the packet. Thus the AAL2 LI will be equal
to this field minus 2.
Value of the CID field of this packet.
Address of the TX AAL2 VC Structure that will be used to send this packet.
Table 32 - Fields and Description (continued)
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