광모듈에서 CDR (Clock Data Recovery) 사용하는
이유
What do I use a CDR for?
Physical impacts are influencing
the optical signal during the transmission over fiber optic cables. Due to the
length of an optical link the signal suffers a time delay deviation which leads
to problems in synchronization. In addition an optical fiber attenuates the
signal which leads to a degradation of the signal. The physical impacts are
linear and non-linear effects which are leading to wave distortion.
This is why a 3R-regeneration will be used:
1. Re-Amplification
(regeneration of amplitude)
2. Re-Shaping (regeneration of signal
waveform)
3. Re-Timing (regeneration of synchronization)
The transceiver includes a Limiting Amplifier (*1), which compensates step 1
& 2 at receiver side. For step 3 (Re-Timing) there is an additional CDR chip
set integrated in the transceiver.
When do I have to use a transceiver with CDR?
This
question was raised with the introduction of SFP+ modules.
Short summary:
XFP modules are built with CDR on-board as defined in the
related MSA-Standard.
To realize a cost effective and smaller form-factor
this functionality doesn't become part of the SFP+ module.
The idea was implementing CDR directly in the host-system of the SFP+ if
needed.
This would increase the price of the end-devices like switches and
routers.
A 2R-regeneration (Re-Amplification, Re-Shaping) is already part of
the transceiver so it is not mandatory to implement a CDR function.
To realize a standard link this is usually sufficient. Standard links are
typically in-house wiring (Multimode), 10km, 20km and 40km point-to-point
connections.
But if you are planning to set up a wavelength multiplexing
(DWDM), an optical amplifier or transmitting over long distances, you should
consider a complete 3R-regeneration. This can be achieved with a CDR chip set
either in your DWDM-transponder, host-system or transceiver. Most of the DWDM
transponder-systems do have a CDR functionality on board.
Which advantages come with a CDR?
There are a lot of
effects on an amplified DWDM link which have a lot of impact on the transmission
signal. For an error-free detection parameters like:
ONSR (Optical signal-to-noise ratio)
BER (Bit-error-ratio)
Input
power value
Data rate
Degradation of the signal
have impact on each other. A stable reliable DWDM link can be achieved,
if you take the first four points into consideration by planning an amplified
DWDM link and in addition eliminating the last point by using a 3R-regeneration
with CDR.
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