MIGRATING ON PON TECHNOLOGIES,
BASED ON ITU STANDARDIZATION
PON technologies have been standardized by the
International Telecommunications Union (ITU) since
2003, and there are now a series of G.984 standards
covering the first version of GPON technology ratified
in 2003.
The most commonly deployed PON today is
GPON technology that delivers 2.5 Gb/s downstream
and 1.25 Gb/s upstream in point-to-multipoint
networks. It is a shared medium and supports optical
budgets of 28 and 32 dB and a split ratio of 1:128;
in other words up to 128 ONTs can be connected
to a single PON. However, the optical budget for the
technology is the maximum attenuation of the optical
signal that can be supported using splitters. Every time
the signal is split, the signal is reduced on every strand
of fiber. When the signal passes through a splitter with
a split ratio of 2, then the optical budget is reduced
by 3dB; a split ratio of 4 reduces the optical budget
by 6dB and so on. The reach of an optical signal
is a tradeoff between the split ratio and optical budget.
The larger the optical budget, the longer the reach.
Typically, with a 1:32 split ratio, 20 km can be covered.
MIGRATING TO 10 GB/S
Considering the evolution of PON technology, ITU has
defined the next step as 10 Gb/s technology: XG-PON
(ITU-T G.987.x) and XGS-PON (ITU-T G.9807.1).
XG-PON was the first standardized next generation PON
technology. It is an asymmetrical solution that delivers
10 Gb/s downstream towards the endpoints and 2.5 Gb/s
in the opposite direction. The more popular variation
is XGS-PON, which supports 10 Gb/s in both directions
and delivers four times more upstream bandwidth.
50 I ICT TODAY
MIGRATING TO 40 GB/S
To achieve 40 Gb/s, PON technology uses time
wavelength division multiplexing (TWDM-PON).
Defined in the ITU-T 989 series, it is the most advanced
of the next generation PON technologies. 40 Gb/s is
achieved by a combination of four dedicated wavelengths
per fiber (on top of GPON wavelengths), each of which
is capable of delivering symmetrical or asymmetrical bit
rates of 2.5 Gb/s or 10 Gb/s. TWDM-PON supports
flexible bitrate configurations (e.g., 2.5/2.5G, 10/2.5G,
10/10G) and tunable lasers that allow to dynamically
assign and change the wavelength on which a business
is connected.
The next generation PON technologies support split
ratios of up to 1:256 per channel group and optical
budgets of 29 and 31 dB. They will support 33 and 35 dB
in the future.
One of the built-in advantages to upgrading to the
newer PON technologies is that their split ratios
and optical budgets can be higher than the first
generation of GPON. This means that the optical fiber
cabling design done today is forward compatible with
the new technologies, because these new technologies
support at least the same split ratios and the same optical
budgets as the existing technologies.
Another aspect of PON that facilitates the evolution
to higher data rates is that these technologies use
dedicated wavelengths in the optical bands as shown in
Figure 2. Notice where the different bands are located for
GPON upstream and downstream, 10G PON, and where
XGS-PON, and XG-PON use the same balance. Notice
also the TWDM wavelengths. Different PON technology
generations use separated wavelength bands and can
coexist on the same fiber plant as the OLT and ONT
which implement filters that remove unwanted
wavelengths. It is interesting to note that XGS-PON
and XG-PON use the same wavelengths, but dual-rate
optics allow both on the same network.
XGS PON/XGPON1
up
TWDM PON
up CATV
XSG PON
XG-PON1
down
TWDM PON
down
GPON
up
GPON
down
1260
1280
1290
1300
1320
1330
1480
1500
1524
1540
1550
1560
1575
1580
1596
1603
(in nm)
FIGURE 2: PON technologies use dedicated wavelengths in the optical bands.