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參數(shù)資料
| 型號： | MT92220 |
| 廠商： | ZARLINK SEMICONDUCTOR INC |
| 元件分類： | 數(shù)字傳輸電路 |
| 英文描述： | 1023 Channel Voice Over IP/AAL2 Processor |
| 中文描述： | ATM NETWORK INTERFACE, PBGA608 |
| 封裝： | 31 X 31 MM, 2.50 MM HEIGHT, MS-034, EPBGA-608 |
| 文件頁數(shù)： | 64/210頁 |
| 文件大?。?/td> | 1536K |
| 代理商： | MT92220 |
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Data Sheet
MT92220
64
Zarlink Semiconductor Inc.
9.0
The MT92220 packet assembly module is responsible for collecting the bytes written in the circular buffers by the
TX TDM and assembling them into RTP or AAL2 packets. For RTP packets, bytes will usually be encapsulated into
RTP, UDP and IP, and then be shipped off on Ethernet, ATM or Packet over SONET; the packet assembly can also
support a multitude of other header combinations. In addition to generating voice packets, the packet assembly
must also perform silence suppression calculations on the incoming bytes to determine if the RTP or AAL2 packets
it assembles should be transmitted or not. All of this must be scheduled correctly so that packets are generated at
correct times and silence suppression information is available when necessary.
Packet Assembly
9.1
Service Timer
To ensure that all events in the packet assembly occur on time, the module contains a connection service timer that
times both PCM packet assembly events and silence suppression treatment. The principle of the service timer is
the following: a service timer is contained in external memory and has a length of N indicators. M indicators are
read every frame. Therefore, the service timer is cyclic over N/M frames. Note that up to 16 service timers can be
contained in external memory, each one of which can have any length N and any M.
Each time the TDM write pointer increments, it generates a pulse indicating to the service timer to read an entire
frame of the service timer. Thus M event indicators are read and each of them can request the packet assembly
module to try to assemble an xxPCM packet, and optionally, perform silence suppression calculations on a block of
PCM data.
The service timer must schedule events in such a way as to minimize the end-to-end delay that CBR data
encounters through the system. For example, if each RTP packet on a given connection contains 80 bytes of
payload from a single channel, then an event signaling the assembly of one of these packets will be contained in a
service timer of 80 frames in length. Thus, this event will be scheduled to occur once every 80 frames, matching the
data rate of the incoming TDM traffic.
Because the number of frames in an RTP or AAL2 packet can vary and that finding a combination of 16 service
timer lengths that could accommodate all possible frame fills between 1 and 1500 would be impossible, the events
indicating packet assembly do not force the transmission of a packet. Instead, they indicate to the assembly module
to attempt the assembly of a packet. The module will verify what is the current value of the TDM pointer and where
the previous packet assembly ended, and based on this, will determine if enough TDM bytes are available to
generate the desired packet. If a limited number of packet sizes is used, then the 16 service timers may be able to
accommodate all connections in an exact way, adding no extra delay.
Because the assembly of RTP or AAL2 packets containing HDLC data is asynchronous, it does not need to be
scheduled in the service indicator.
When the service timer requests a packet assembly, the event may be preceded by a silence suppression
calculation event. For channels on which silence suppression is performed, a separate process takes care of
reading the local and remote PCM bytes from their circular buffers, performing energy calculations on them and
returning a silent/non-silent result so that the assembly module knows whether or not to send packets containing
that channel. The silence suppression calculations are performed on blocks of the same size as the number of
frames of data contained in the RTP packet in which these bytes will be transmitted. In this manner, the silence
suppression event makes sure that the suppression decision (send / suppress) is up to date when the packet is
transmitted or not transmitted. Silence suppression can only be used with connections carrying a single channel.
Apart from IP/RTP and AAL2 packet assembly events, the service timer can also contain Start of Frame events.
These are used to cause time-outs on AAL2 cell assembly. Since scanning the AAL2 VC for time-outs requires a
certain amount of bandwidth, the scanning process can be configured to operate less often than every frame by
only scheduling a Start of Frame event every N frames.
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