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| 型號： | DS3120N |
| 廠商： | Maxim Integrated Products |
| 文件頁數(shù)： | 30/133頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC FRAMER T1 28-CHANNEL IND |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 控制器類型： | T1 調(diào)幀器 |
| 接口： | 并行/串行 |
| 電源電壓： | 2.97 V ~ 3.63 V |
| 電流 - 電源： | 300mA |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 316-BGA |
| 包裝： | 管件 |
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DS3112
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14.5
T3 Framing Structure
As with M12, to understand the M23 function requires an understanding of T3 framing. The T3 frame
structure is very similar to the T2 frame structure; however, it is made up of seven M subframes (see
). The seven M subframes are transmitted one after another (...M1/M2/M3/.../
M6/M7M1/M2...) to make up the complete T3 M frame structure. Each M subframe is made up of eight
blocks and each block is made up of 85 bits. The first bit of each block is dedicated to overhead and the
next 84 bits are the information bits where the T2 data will be placed for transport. Table 14-3 shows the
definitions of the overhead bits, and Figure 14-5 shows the placements of the overhead bits.
Table 14-3. T3 Overhead Bit Assignments
OVERHEAD
BIT
DESCRIPTION
M Bits
(M1/M2/M3)
The M bits provide the frame alignment pattern for the seven M subframes. Like all framing
patterns, the M bits are fixed to a certain state (M1 = 0/M2 = 1/M3 = 0).
F Bits
(F1/F2/F3/F4)
The F bits provide the frame alignment pattern for the M frame. Like all framing patterns,
the F bits are fixed to a certain state (F1 = 1/F2 = 0/F3 = 0/F4 = 1).
C Bits
(C1/C2/C3)
In the M23 application, the C bits are used to indicate when stuffing occurs. If all three C
bits within a subframe are set to 1, then stuffing has occurred in the stuff block of that
subframe. If all three C bits are set to zero, then no stuffing has occurred. When the three C
bits are not equal, a majority vote is used to determine the true state.
In the C-Bit Parity application, the C bits are defined as shown in Table 14-4.
P Bits
(P1/P2)
The P bits provide parity information for the preceding M frame (not including the M, F, X,
and C overhead bits). P1 and P2 are always the same value (if they are not the same value,
this implies a parity error).
X Bits
(X1/X2)
The X bit is used as a Remote Alarm Indication (RAI). It will be set to a zero (X1 = X2 = 0)
when the T3 framer cannot synchronize or detects AIS. It will be set to a one (X1 = X2 = 1)
otherwise. The value of the X bits should not change more than once per second. X1 and X2
are always the same value.
14.6 M23 Multiplexing
The M23 function multiplexes seven T2 data streams into a single T3 data stream. The seven T2 data
streams are bit interleaved into the T3 framing structure. A bit from T2 line #1 is placed immediately
after the overhead bit in the information bit field, followed by a bit from T2 line #2, and so on. Since
there are 84 information bits in each block, there are 12 bits from each T2 line in a block.
The seven T2 lines are mapped asynchronously into the T3 data stream. This implies that there is no T2
framing information passed to the T3 level. The seven T2 lines can have independent timing sources and
they do not need to be timing locked to the T3 clock. To account for differences in timing, bit stuffing is
used. The last block of each M subframe is the stuff block (
). In each stuff block there is an
associated stuff bit (Figure 14-6) that will be either an information bit (if the three C bits are decoded to
be zero) or a stuff bit (if the three C bits are decoded to be a one). As shown in Figure 14-6, the position
of the stuff bit varies depending on the M subframe. This is done to allow a stuffing opportunity to occur
on each T2 line in every T3 frame. For example, if the C bits in M Subframe 5 were all set to one, then
the fifth bit after the F4 overhead bit in the last block would be a stuff bit instead of an information bit.
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