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ITU-T G.709光传输网络(OTN)概述.pdf
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SONET/SDH 网络最初为采用每光纤单波长的光接口而设计,但现在已经成为大部分现代电信网的基石。随着光纤元件技术的不断发展,采
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Application Note: 1379
An overview of ITU-T G.709
As increasing demands are made
on the world’s communications
networks, new standards emerge to
cater for the challenges that these
demands make. ITU-T G.709
“Interface for the optical transport
network (OTN)” is one of the latest
recommendations. It has been
developed to meet two requirements:
to cater for the transmission needs of
today’s wide range of digital services,
and to assist network evolution to
higher bandwidths and improved
network performance. Furthermore,
it takes another step towards the
all-optical network.
This paper provides a brief overview
of ITU-T G.709 signal structures, and
examines the testing requirements
for ITU-T G.709 compliant network
equipment.
2
Introduction
SONET/SDH is now a mature digital
transport technology, established
in virtually every country in the
world. When SONET/SDH was
first conceived in the early 1980s,
telecommunications traffic was
predominantly voice. During the
last five years there has been
an explosion in the demand for
bandwidth driven mainly by internet
access, e-commerce and mobile
telephony. This increase in demand
has, so far, been satisfied through a
combination of increased line rates
(TDM – time division multiplexing)
and transmitting multiple wave-
lengths through a single fiber
(DWDM – dense wave division
multiplexing).
But as the network evolves to higher
line rates, the physical limits of the
transport medium (optical fiber)
becomes critical. And, there remains
an over-riding requirement to control
the cost of providing an improving
service to customers.
The latest recommendation from the
ITU is G.709 “Interface for the optical
transport network (OTN)” builds on
the experience and benefits gained
from SDH and SONET to provide a
route to the next-generation optical
network. Indeed, the OTN is widely
regarded as the lifeline to increased
bandwidth capacity. Many of the
concepts in ITU-T G.709 have their
roots in SDH/SONET, for example a
layered structure, in-service perfor-
mance monitoring, protection and
other management functions.
However, some key elements have
been added to continue the cycle of
improved performance and reduced
cost. These include:
● Management of optical channels
in the optical domain
● Forward error correction (FEC) to
improve error performance and
enable longer optical spans
ITU-T G.709 also provides a
standardized method for managing
optical wavelengths (channels) end
to end without the need to convert
an optical signal into the electrical
domain. (Today’s DWDM networks
are typically managed as a series
of point-to-point links with a path
through the network requiring many
expensive optical/electrical/optical
{O/E/O} conversions.) Thus, ITU-T
G.709 along with the advent of all-
optical switches (using MEMs and
bubble technology) opens the door to
potentially extensive cost savings in
the network.
3
The FEC scheme used in the ITU-T
G.709 standard is a Reed-Solomon
RS(255,239) code. This means that
for every 239 bytes of data, an
additional 16 bytes (255-239=16) of
data is added for error correction.
The RS(255,239) code can correct up
to eight symbol errors in the code
word when used for error correction,
and can detect sixteen symbol errors
in the FEC code word when used for
error detection only.
In the optical transport unit (OTU)
frame, each row contains 16 FEC
blocks of 16 bytes for the row, thus
making 64 FEC blocks (4 x 16) for
every OTU frame.
The FEC for the OTU frame uses
16-byte interleaved codecs. This
results in the serial bit stream
(10.71 Gb/s for example) being
converted into 16 parallel signals for
processing. This architecture helps
improve the error correction on error
bursts and countering interleaving
that may split up closely spaced
errors.
The size of the frame is four rows of
4080 bytes (figure 2). Data is trans-
mitted serially beginning at the top
left, first row, followed by the second
row and so on.
ITU-T G.709 framing structure and byte definitions
The ITU-T G.709 frame (figure 1) has
three distinct parts, two that are
broadly similar to a SDH/SONET
frame:
● Overhead area for operation,
administration and maintenance
functions
● Payload area for customer data
In addition, the G.709 frame also
includes a foward error control
(FEC) block.
FEC has been used in telecommuni-
cations for many years, mainly in the
areas of satelite communications and
undersea transport. FEC has been
important in enabling communica-
tions to maintain acceptable perfor-
mance quality in ‘noisy’ environ-
ments at the same time as keeping
infrastructure costs in check.
As transmission bit rates increase
to 10 Gb/s and beyond, physical
parameters of the optical fiber play
a more significant part in degrading
transmitted pulses of light. FEC
provides additional coded data to
enable error checking and correction
by a receiving device. ITU-T G.709
includes a standardized method
of FEC that enables long haul
transmission at higher line rates
without degraded performance.
Figure 1
There are three line rates currently
defined in ITU-T G.709:
1. 2,666,057.143 kbit/s – optical
channel transport unit 1 (OTU1)
2. 10,709,225.316 kbit/s – optical
channel transport unit 2 (OTU2)
3. 43,018,413.559 kbit/s – optical
channel transport unit 3 (OTU3)
Unlike SONET/SDH, as the line
rate increases, the G.709 frame size
(4 x 4080) remains the same and the
frame rate increases. This is a
departure from the fundamental
8 kHz frame rate that has been a
foundation of digital telecommunica-
tion networks designed to carry
predominantly voice traffic.
The three frame rates (and period)
are:
1. 20.420 kHz (48.971 ms) for OTU1
2. 82.027 kHz (12.191 ms) for OTU2
3. 329.489 kHz (3.035 ms) for OTU3
Note: The period is an approximated
value, rounded to three digits.
This means that to carry one SDH/
SONET 10 Gb/s frame, for example,
requires approximately eleven OTU2
optical channel frames.
The optical transport module
overhead consists of four functional
areas (figure 3):
OTU – Optical transport unit
ODU – Optical data unit
OPU – Optical payload unit
FEC – Forward error correction
Figure 2. ITU-T G.709 Figure 11-1
Figure 3. ITU-T G.709 Figure 5-1
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资源评论
- 郭逗2023-07-28这份文件提供了关于ITU-T G.709光传输网络的全面介绍,让读者能够快速了解相关知识。
- 高中化学孙环宇2023-07-28该文件结构清晰,条理分明,能够循序渐进地引导读者深入了解光传输网络。
- 老光私享2023-07-28文件内容准确无误,语言简洁明了,非常适合初学者学习和理解。
- 叫我叔叔就行2023-07-28对于想要了解OTN的人来说,这是一份非常有用的参考资料,内容详尽但不啰嗦。
- 俞林鑫2023-07-28作者在这份文件中为读者提供了许多实际应用案例,使得理论知识更具实际意义,易于理解。
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