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Cisco ONS 15454 DWDM Reference Manual, R8.5
78-18343-02
Chapter 10 Network Reference
10.9.4 ITU-T G.709 Optical Data Rates
The rate is derived from the basic 9.95328 Gbps OC-192 rate. First, it has the 64-bit to 66-bit encoding
added, which brings it to the 10.3125 Gbps rate (10 Gbps x 66/64 = 10.3125 Gbps). Beyond that, the
WIS overhead is added, which is an additional two percent on top of the 10.3125 Gbps. This yields:
10.3125 Gbps x .02 = 0.20625 Gbps
10.3125 Gbps + 0.20625 Gbps = 10.51875 Gbps
10.9.4 ITU-T G.709 Optical Data Rates
To understand optical networking data rates, an understanding of the ITU-T G.709 frame structure,
shown in Figure 10-25, is needed.
Figure 10-25 ITU-T G.709 Frame Structure
Each of the sub-rows in Figure 10-25 contains 255 bytes. Sixteen are interleaved horizontally
(16 x 255 = 4080). This is repeated four times to make up the complete ITU-T G.709 frame.
The Reed Solomon (RS) (255,239) designation indicates the forward error correction (FEC) bytes. There
are 16 FEC, or parity, bytes. The ITU-T G.709 protocol uses one overhead byte and 238 data bytes to
compute 16 parity bytes to form 255 byte blocks—the RS (255,239) algorithm. Interleaving the
information provides two key advantages. First, the encoding rate of each stream is reduced relative to
the line transmission rate and, second, it reduces the sensitivity to bursts of error. The interleaving
combined with the inherent correction strength of the RS (255,239) algorithm enables the correction of
transmission bursts of up to 128 consecutive errored bytes. As a result, the ITU-T G.709 contiguous burst
error correcting capability is enhanced 16 times above the capacity of the RS(255,239) algorithm by
itself.
ITU-T G.709 defines the Optical Transport Unit 2 (OTU2) rate as 10.70923 Gbps. ITU-T G.709 defines
three line rates:
1. 2,666,057.143 kbps—Optical Transport Unit 1 (OTU1)
159457
Sub Row 3
1 239 240 255
Info Bytes RS (255, 239)
Sub Row 2
Info Bytes RS (255, 239)
Sub Row 1
Rows: 1
2
3
4
Columns: 1 17 3825 4080
Info Bytes RS (255, 239)
Info Bytes Payload FEC
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