M23 Multiplexing
M23 multiplexing involves the combination, using bit interleaving, of seven DS2 signals into a single DS3 signal stream. Hopefully, you are beginning to see a pattern. DS1 to DS2 multiplexing is M12, and DS2 to DS3 multiplexing is M23. The first number in these acronyms stands for the DS level that is multiplexed, and the second number stands for the new DS hierarchy level that is created. Thus, M23 is the function of multiplexing DS2s into a DS3. Figure 7-7 shows the M23 process.
Figure 7-7 M23 Multiplexing
M12 Multiplex
4 DS1s 4 DS1s 4 DS1s 4 DS1s 4 DS1s 4 DS1s
4 DS1s
Bit stuffing adds 3671 bps per DS2 for a total of 6.315671 Mbps.
Again in M23, you use bit stuffing to remove clocking variations between the separate DS2 signals. Each DS2 runs at a nominal rate of 6.312 Mbps, and 3761 bps is added to each circuit to bring them to a total bandwidth rate of 6.315671 Mbps. If you combine the total rate of seven DS2s, you get a total bandwidth rate of 44.209697 Mbps. The final addition to the DS3 signal is the remaining overhead of 526,306 bps, which gives you the bandwidth rate that you are accustomed to seeing—44.736003 Mbps. You most commonly see this displayed as either 44.736 Mbps or simply 45 Mbps.
The frame structure of a DS3 is similar to that of a DS2, but it is significantly larger. Within each DS3 frame, there are a total of 4760 bits. Each DS3 frame is also referred to as an M-frame and is further segmented into seven subframes, as shown in Figure 7-8.
NOTE Multiplexing is achieved by using bit-by-bit interleaving, from DS1 to DS2, and from DS2 to DS3, which makes it difficult to extract a single T1 from a DS3. It requires demultiplexing the entire T3.
M12 Multiplex
Bit stuffing adds 3671 bps per DS2 for a total of 6.315671 Mbps.
Figure 7-8 Breakdown of a DS3 M-Frame
Subframe
Identification
\ M-Frame Total = 4760 Bits
Link Alarm Status
As with DS2, the DS3 M-frame sequence is made up of a series of header bits followed by blocks that are set aside for the actual payload of the circuits. Each of the overhead bits serves a specific function, as noted in Table 7-2.
Table 7-2 Bit Types Found in DS3 Frames Bit Description
X The X-bit identifies any significant error detected on the circuit such as an alarm indication signal (AIS). Also called the remote alarm indicator (RAI), X1 and X2 are always set to the same value, either both to 0 during normal operation or both to 1 during the detection of an error.
P The P-bits function as a way for monitoring the performance of the circuit. The continuous transmission of DS3 frames allots the P-bits for a parity check on the previous frame. In other words, the value that is stated in the current frame is the parity information from the last frame. The values are the same on both bits, either 0 or 1.
M The M-bit is used much the same way as it is in the DS2 frame structure to align each
DS3 M-frame. The difference is that not all subframes contain an M-bit in the DS3 frame structure. The alignment pattern is M1=0, M2=1, M3=0.
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Bit |
Description |
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F |
The F-bits are used for the subframe alignment sequence. The alignment pattern is F1=1, |
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F2=0,F3=1,F4=0. |
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C |
C-bits are used in M23 framing to identify whether or not bit stuffing has occurred in the |
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M23 multiplex. In the absence of bit stuffing, C-bits can be used for other functions |
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within the signal stream such as network monitoring and management. |
DS3 signals also need to use bit-stuffing techniques for synchronization of the DS2 signals. The bit stuffing is a bit more involved because there are seven subframes instead of four and a total of 84 bits per payload block, as shown in Figure 7-9. However, the process is the same. Again, you should pick up on the pattern here; there are seven subframes and each subframe stuffs bits in its own position. In other words, subframe #3 stuffs the third bit in its own subframe.
With modern equipment, the DS2s are handled within the same device and then share the same clock source. In that case, bit stuffing at M23 level is not required (it is still required for M12, because the T1s can have different clock sources), and the C-bits can be used for different purposes.
Figure 7-9 Bit Stuffing in a DS3 M-Frame
Figure 7-9 Bit Stuffing in a DS3 M-Frame
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X1 |
s |
o |
1 |
o |
o |
1 |
o |
0 |
1 |
M1 |
o |
o |
1 |
o |
s |
1 |
0 |
0 |
0 |
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... |
... |
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Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
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X2 |
o |
s |
1 |
0 |
0 |
1 |
o |
1 |
0 |
M2 |
1 |
o |
1 |
o |
o |
S |
0 |
1 |
0 |
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... |
... |
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Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
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P1 |
0 |
S |
0 |
0 |
1 |
0 |
1 |
1 |
M3 |
o |
o |
1 |
o |
o |
1 |
g |
0 |
0 |
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... |
... |
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Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
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P2 |
1 |
o |
1 |
s |
o |
1 |
o |
1 |
0 |
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... |
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Bit |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
83 |
84 |
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