參數(shù)資料
型號： MT90500
廠商： Mitel Networks Corporation
英文描述： Multi-Channel ATM AAL1 SAR(多通道 ATM AAL1分段及重組設備（基于通訊總線的系統(tǒng)與ATM網(wǎng)絡的接口）)
中文描述： 多通道自動柜員機AAL1特區(qū)（多通道自動柜員機AAL1分段及重組設備（基于通訊總線的系統(tǒng)與空中交通管理網(wǎng)絡的接口））
文件頁數(shù)： 60/159頁
文件大小： 514K
代理商： MT90500
MT90500
60
4.4.2.3
Receive Overruns and Underruns
The “First Entry” and “Last Entry” fields in the RX_SAR Control Structure point to the first and last RX Circular
Buffer Base Address pointers in the RX_SAR Control Structure. The “Minimum Lead”, “Maximum Lead”, and
“Average Lead” entries define the window within the circular buffer within which cell data can be received
without generating an underrun or overrun condition. This window is defined relative to the TDM Circular Buffer
Read Pointer, as described in Figure 23. Whenever data from a newly received cell is to be written to the
Receive Circular Buffers, the location of the RX_SAR Write Pointer is checked against the Minimum and
Maximum Lead Pointers.
When a new cell is received, the hardware checks for the location of the RX_SAR Write Pointer, which
indicates where the new cell should be written within the associated Receive Circular Buffer(s). The VALID
bytes shown in the figure above indicate bytes which have been written by the RX_SAR and have yet to be
read by the External Memory to TDM Data Output Process. Consequently, INVALID bytes represent those that
have already been read or, in the case of start-up, have never been written. If the pointer falls within the window
defined by the Min. Lead and Max. Lead parameters, the new data is written immediately following the old data.
If the RX_SAR Write Pointer falls after the Maximum Lead Pointer, an overrun condition is detected, and the
new data is written starting at the location of the Average Lead Pointer. (Some addresses containing previously
received, unread data bytes are overwritten.) If the RX_SAR Write Pointer falls before of the Minimum Lead
Pointer, an underrun condition is detected, and the new data is written starting at the location of the Average
Lead Pointer. (Some addresses containing already-read data bytes are “skipped” and left unwritten.) Figure 23
depicts the Write Pointer to Read Pointer comparison that occurs at cell receive time.
The External Memory to TDM Data Output Process (Section 4.1.4) has its own TDM Read Underrun Error
indication (see register 6000h) which works in parallel to the mechanism described above. The ninth bit of the
external memory byte is not used for parity, but is used to indicate whether each TDM byte has been previously
transferred to the TDM bus. When the External Memory to TDM Data Output Process reads a byte which has
already been transferred (has the ninth bit set), an underrun condition is flagged, if enabled by that TDM
channel’s entry in the External Memory to Internal Memory Control Structure. Since this TDM Read Underrun
Error functions as each byte is read (and not just when a cell arrives, as the RX_SAR errors do) it is useful to
indicate dropped VCs (no cells), and excessive CDV (late cells).
Data bytes which have been read out to the TDM bus by the External Memory to TDM Data Output Process are
handled according to the programming of the External Memory to Internal Memory Control Structure.
Depending on the value of the write-back disable bit for each individual TDM channel, bytes read out to the
TDM bus will either be replaced by silence (FFh) or left unchanged. This has the effect that in the event of an
underrun, either silence (FFh) will be read out of the “skipped” area, or the old data in the “skipped” area will be
repeated on the TDM bus.
RECEIVE BUFFER
TDM CIRC. BUFFER
READ POINTER
Min. Lead
Max. Lead
Figure 23 - Overrun and Underrun Situations
WRITE POINTER
RECEIVE BUFFER
Max. Lead
WRITE POINTER
(OLD)
Normal
Overrun
Underrun
RECEIVE BUFFER
Avg. Lead
INVALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
INVALID BYTE
Avg. Lead
WRITE POINTER
(NEW)
INVALID BYTE
VALID BYTE
VALID BYTE
VALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
INVALID BYTE
Min. Lead
WRITE POINTER
(NEW)
WRITE POINTER
(OLD)
Min. Lead
Max. Lead
RX_SAR
RX_SAR
RX_SAR
RX_SAR
RX_SAR
TDM CIRC. BUFFER
READ POINTER
TDM CIRC. BUFFER
READ POINTER
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MT90500AL 制造商:MITEL 制造商全稱:Mitel Networks Corporation 功能描述:Multi-Channel ATM AAL1 SAR
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