SFP ONU as CPE for GE-PON
Nobuhiko Hattori *, Kunio Odaka *, Naoto Nakamura *,Kazutaka Shimoosako *2, Katsuya Aboshi *2

In order to play a part in the expansion of data communication networks, wehave developed ONU equipment for GE-PON in the form of SFP transceivers
that are standardized by the MSA. By simply mounting this ONU on any network equipment
provided with an SFP interface port, it becomes possible to configure the integrated equipment
under a GE-PON system. Since this equipment ensures high system reliability despite its simple
implementation, it is expected that the equipment will be widely used in its best suited
applications such as, for example, the line service for enterprises, and furthermore that it plays arole in developing new GE-PON markets. In this paper, its elementary technologies will be
described and transmission characteristics etc. will be presented.

ABSTRACT1.

INTRODUCTION

Recently, the advancement of broadband access systems
is encouraging individual users to enjoy broad band services, and this is promoting further expansion of data
communication networks. Among many access systems,
the optical access system centered on FTTP (Fiber To
The Premises) is playing a major role taking advantage of
its superior communications throughput and immunity
against external noise, and in particular, GE-PON (Gigabit†
Ethernet Passive Optical Network) specified in IEEE
802.3 constitutes the mainstream. PON adopts a networkarchitecture such that an optical transmission line fromtelecommunications providers is shared by multiple users,
and is physically mid-branched to each user by using
passive optical components. Therefore, the PON configuration is, as shown in Figure 1, categorized into point-to1)
multipoint (P2MP) network topology . This configuration
involves a variety of asymmetry such as, for example, thedistinction of upstream and downstream in a communication system, and the difference in the number of line terminals installed on the telecommunications provider side
Downstream

Upstream
OLT

ONU

Figure 1

System of PON architecture.

* FITEL-Photonics Lab., R&D Div.
*2 Broadband Products Dept., Telecommunications Co.
†

Ethernet is a registered trademark of Xerox Corporation.

(i.e., optical line terminal, OLT) and on the subscriber side(i.e., optical network unit, ONU). When we focus ourattention to the latter aspect, it may be possible for us toadvantageously develop new GE-PON markets by imparting novelty to the ONU that occupies the majority side.
Furukawa Electric has long been involved with theresearch and development of optical sub-assemblies(OSAs) and optical transceiver modules on PMD (Physical2)
Medium Dependent) sublayer , and is also marketing
GE-PON-related products. In the development program
here, after seeking the novelty mentioned above, we haveidentified beforehand a direction for development bymerging our long-cultivated equipment technology andsublayer module technology. In this paper, individual elemental technologies will be described together with thecharacteristics of the developed product.

2.

INTERNAL CONSTRUCTION AND ITSAIMSIt holds true for industrial products in general that, while
those in the lower assembly level (i.e., parts) are mostly
specified in their forms by the standards, equipment on
which these parts and modules installed are less conscious of specifications with respect to their forms. The
same can be said of optical networks.
For optical transceivers in the sublayer of PHY (PhysicalLayer) and optical subassemblies like TOSA and ROSA,
there are standards formulated by standardizing organizations, along with MSA (Multi Source Agreement) inwhich suppliers formulate standards as their common
3)
specifications . But, there are few discussions on theform of ONU as a CPE (Customer Premises Equipment).
This may be justified from a viewpoint that places little
significance on form as long as standardized input- andoutput-interfaces are equipped with. However, the present

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SFP ONU as CPE for GE-PON

development starts from placing significance on form.
This idea will be explained in turn in the following.
SFP (Small Form Factor Pluggable) enclosure has been
adopted for the housing of the equipment. The SFP is
standardized as a “SPF Transceiver” in the MSA, targetingat transceivers on PMD sublayer. As can be seen from theterm “PMD,” the transceiver is an accurate conversion
function block with bit-level transparency, handling medium-dependent portion in the transmission of information
signals. What should be noted here is that “SPF” is originally a term to be combined with “transceiver,” not being
intended to be combined with “equipment” as is in thetitle of this paper.
It is not only the enclosure that has been utilized. For
example, when the equipment entirety is viewed as a
black box, the relationship between input and output isidentical with respect to the transceiver. The optical transceiver on PMD sublayer generally consists of functional
blocks shown in Figure 2. On its one side, optical semiconductor devices transmit and receive optical signals via
an external optical fiber, while on the other side, a driverand an amplifier transfer serial high-speed differential signals with external circuits, interfacing optical semiconductor devices using analog values. Thus, if we focus ourattention to the input and output, we can represent theentirety as a functional block that converts signals while
making it adapted to the signal transmitting media.

grated equipment under a GE-PON system. This
leads to facilitating the introduction of a GE-PON
system.
• Power supply to this ONU can be done via network
equipment. In contrast to normal ONUs power ofwhich is supplied via an AC adapter on a commercial
power line, this contributes to enhancing system reliability.
Taking the security performance of GE-PON systems
into consideration, the reliability of an application systemthat employs this ONU is high. Therefore, this ONU is best
suited for such applications as, e.g., enterprise-targeted
services and MDU (Multi Dwelling Unit) for complex housings, and is expected to develop wide-ranging new
GE-PON markets.

P2MP MACLD

LD Driver

Optical Signal

PD

PD ROSA

Figure 2

Tx̲In
Controller

Ampliﬁer

8B/10B

Parallel/
SerialParallel/
SerialLD Driver

Tx̲In

Ampliﬁer

Rx̲Out

OSA

Optical Signal

LD TOSA

8B/10B

Figure 3

CTRL-IO

Controller

Functional block diagram of SFP ONU.

CTRL
-IO
Rx̲OutFunctional block diagram of optical transceiver.

Replacing all the functional blocks in Figure 2, excluding those for the both ends, using the functional blocks
describing the communication layer and sublayer in the
GE-PON ONU, Figure 3 is obtained. This is the block diagram of ONU developed here. Although Figures 2 and 3
show different block diagrams, they are identical in thesense that they can perform signal conversion in accordance with the signal medium. In other words, as long as
the interface conditions at the SFP port on which an SFP
module is to be installed are the same with respect to theoptical transceiver, they are equivalent to each other, sothat it is not required to be conscious of whether an SFP
transceiver or a developed ONU is installed.
We have described up to here that, in terms of its outerform and input and output, this ONU can function as an
SPF transceiver. Their developmental aims are as follows.
• This ONU can be installed on any network equipment having an SFP port adapted to Gigabit
Ethernet, making it possible to configure the inte-

3.

SIGNAL TRANSMISSION AS ONU

Thus far, we have described ONU’s common elementswith the SPF transceiver. However, although it is used likean SPF transceiver, the equipment is absolutely an ONU.
Thus in terms of its function to perform signal conversion
in accordance with the specified medium, there lies, during conversion, intervention from another layer which ishigher ranked than PMD sublayer. From such a view
point, its difference in function from transceiver may besummarized as follows.
• With regard to signal transmission, it has no bit-leveltransparency which a transceiver should have.
• Concerning its output timing of optical transmission
signals, it is under control of OLT equipment to which
it is optically connected.
The former is because the transmitted information signals are controlled as a frame by the upper DATA LINK
layer, so that, as described later, electrical characteristicsof the function blocks that handle signal conversion is notmutually affected. The latter is because the underlying
system is P2MP, which uses TDMA (Time Division Multiple
Access) for communications from the ONU to the OLT. As
has been mentioned before, an intervening layer higher
ranked than the PMD sublayer is involved in its function of

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SFP ONU as CPE for GE-PON

Input Power General Optical TransceiverSFP ONU‒2 dBm‒15 dBm‒28 dBm

It should be pointed out here that in making suchobservations on this equipment, for the reasons mentioned above, it is necessary to input electrical signals in
the Ethernet frame format, as well as to make optical connection of the equipment with an OLT for GE-PON, placing the equipment under control thereof. Unless these
conditions are met, optical signals will not be transmitted.
The same holds true in bit error rate measurements aimed
at reliability testing of transmission signals, where it is
impossible to make measurements by simply connecting
a bit error tester as in the case of normal optical transceivers. In such measurements, it is required to use an
Ethernet tester or the like via a GE-PON OLT, as shown in
Figure 6.
Downstream
Upstream

Figure 4 Waveform comparison of electrical output.

1000Base-LX

OpticalAttenuator

EUT(SFP ONU)

TestBoardsignal conversion. This means that the signals before andafter conversion are isolated with each other. For example, when an optical signal of identical power is input to anormal optical transceiver and to this SPF ONU, and theirsignal waveforms are observed while changing theirpower, the difference can be seen as shown in Figure 4.
It can be seen that, in the case of normal optical transceiver that is a PMD, when the optical input power is nearits minimum operating value, a variation of timing in signal
waveform occurs in the time axis under the influence ofnoise, while in the case of ONU equipment such variation
can not be observed. The same holds for the reversed
direction. When electrical signals with different amount oftiming variation are input, as can be seen from Figure 5,
no difference can be observed in the optical output signals converted from each input. Thus, it can be understood that there is isolation.

Input( Electrical Waveform )EPON-OLT[ 1000BASE-PX ]Ethernet Tester[ Traffic Generator/Analyzer ]Figure 6

Test setup for measurement of signal transmission.

The reliability of signal transmission was evaluated inorder to confirm the equipment function of signal conversion via the above-mentioned higher ranked layer. The
test set up shown in Figure 6 was used, and the optical
input level to this equipment from OLT was set to approximately –20 dBm, so as not to be affected by the SNR(Signal to Noise Ratio) in the optical receiving circuit. The
results are shown in Table 1, indicating that a sufficient–14
level of reliability is achieved, i.e., of the order of 10 atleast when converted in bit error rate.

Output( Optical Waveform )Table 1Results of transmission function test.
Downstream

Upstream

Frames Sent

Count Item of Tester

71,114,562,290

71,114,562,290

Valid Frames Received

71,114,562,290

71,114,562,290

00Frames Loss Count•Frame Construction: 8Bytes_Preamble + 64Bytes

4.

Figure 5 Waveform comparison of optical output.

OPTICAL RECEIVING CHARACTER­
ISTICSIn the field of optical network systems where optical signals of extremely low levels are handled, it is essential,
just like wireless systems, to suppress the crosstalk components caused by various signals below allowable limits
so that they do not interfere as noise with the receiving

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SFP ONU as CPE for GE-PON

circuit. Therefore, considerable work has been done until
now addressing this issue. Moreover, the GE-PON systemprescribes the use of single optical fiber to transmit bidirectional optical signals. In most cases this has led to,
together with the requirement for size reduction, the useof an OSA module that integrates both the transmitting
and receiving circuits for GE-PON optical transceivermodules. For such an integrated transmitting-receiving
OSA module, theoretical research work is also done by
developing a model to simulate the crosstalk between the4)transmitting and receiving terminals .
As is described in such a model, there exists a kind ofcrosstalk in an integrated transmitting-receiving OSA
module, in which its own optical transmitting signals
directly couple into the photodiode of the optical receiving circuit. This type of crosstalk that depends on theinternal structure of OSA can not be ignored in somecases, so that it is one of the important issues for designing an OSA. Accordingly, we investigated in advance thedegradation of optical receiving characteristics due to this
optical crosstalk within the OSA.
As shown in Figure 7, the theoretical model for the bit
error in an ideal receiving circuit can generally bedescribed by a probability density distribution with double
peaks, in which random noise having Gaussian distribution (σ) is superimposed on the signal (2a in peak topeak). Here, it is depicted that the signals “High” and“Low” included in the optical received signals each occurs
with the same probability. However this model does notinclude optical crosstalk.

own optical transmitting signals, from which the interfering components (2b in peak to peak) are derived, individually occur with the same probability.
Meanwhile, optical beat noise between the transmission
and receiving signals is not taken into account, since thislies outside the receiving bandwidth. It is also probablefor optical crosstalk components within the OSA to reachthe receiving device via multipath. In such a case, the following two items need consideration, but neither of themis serious enough to abandon the model mentioned
above.
• Bit time scale
When one compares the size of an OSA with the
one-bit time of GE-PON electric signal (i.e., 800ps), one can reason that they can be consolidated
into a single path.
• Optical carrier time scale
Depending on the phase summation of opticalcrosstalk components, the total crosstalk can beobserved to have different intensities. In total, however, so long as the model described here is concerned, the crosstalk for a single path can be used
as an appropriate parameter.

2b

2a

σ
2b

a/σ=2.5a/b=2.52aσ

2a: Received signal amplitude
2b: Optical crosstalk amplitude

Figure 8a/σ=2.5

2a: Received signal amplitude

Figure 7

Probability density distribution without optical crosstalk.

On the other hand, since optical crosstalk is an optical
coupling to the optical receiving device, the photocurrentof the interference includes direct current components. In
this case, each of the double-peaked distribution shownin Figure 7 shifts into a double-peaked distribution again.
An example of probability density distribution for this caseis shown in Figure 8. Here too, to set limits to the discussions, it is depicted that the signals "High" and "Low" in its

Probability density distribution with optical crosstalk

For proper comparison of the two Figures, the ratio of
signal amplitude to random noise intensity has not beenchanged. When the distribution crosses over the decisionthreshold value that is an average of “High“ and “Low“
used for amplitude decision, a wrong decision withrespect to threshold value is made, resulting in occurrence of bit errors. While this situation corresponds to thepeak tail portion of the probability density distribution
crossing over the threshold, it would be clearly understood, comparing the two Figures, that the existence ofcrosstalk in the OSA has increased the probability ofcrossing over the threshold. In order to verify the conceptof recognizing the adverse influence of internal opticalcrosstalk, the degradation degree was obtained by simulation, and was compared with the measurements. The

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SFP ONU as CPE for GE-PON

Penalty (dB)6
5

Simulation
Test Result

4
3
2
1
0
-15

-12

-9
-6
-3
0
Relative internal optical crosstalk (dB)3

Figure 9 Effect of internal optical crosstalk.

results are shown in Figure 9, where the relative opticalpower is on the horizontal axis, and the reference level of
0 dB is defined as the optical input power to obtain a bit–12
error rate of 10 under the absence of internal opticalcrosstalk. Close agreement can be seen between thesimulated and measurement results, thus confirming thevalidity of the degradation model.
In view of this simulation results, the target value for theallowable internal optical crosstalk in the OSA was set toaround –10 dB, according to the index defined here.
When the downstream optical input signals containother signals of different wavelengths that turn into opticalcrosstalk components, the way the distribution changes
corresponding to the input signal power changes is different from that mentioned above, since the signal to interference ratio remains constant no matter how the input
signal power into the optical receiver changes. Also, thehorizontal axis of Figure 9 no longer shows the signal tointerference ratio. For this reason, Figure 9 can not beapplied as it stands.
Until now, the effects of crosstalk on optical receiving
characteristics have been presented. In the actual process of signal recovery, on the other hand, the clock
recovery function block in this equipment functions after
the amplitude is fixed using the decision threshold value.
The function performance is generally argued on thebasis of jitter that represents the amount of timing variation of signal waveforms in the direction of time axis.
However, in an ONU for GE-PON, the existence period for
upstream signals becomes bursty, since the upstream
relies on TDMA as mentioned before. So we have to berather careful in discussing the influence of internal optical crosstalk on jitter.
Let us move from the theoretical aspects discussed so
far to the evaluation results of receiving characteristics.
Here the optical receiver sensitivity is defined as the minimum optical input power level in Figure 6 corresponding–12
to a bit error rate of 10 converted from the number oflost frames. Table 2 and Figure 10 show the receiver sensitivity measured with upstream signals transmitted withthe maximum load, and the typical temperature dependence of such receiver sensitivity, respectively.
Also, with an intention of evaluating the upper bound of

the optical input power range mentioned above, bit error
rate was measured by converting from measurements
while inputting a fixed optical power of 0 dBm in Figure 6.
The result is shown in Table 3. Meanwhile, in evaluating
the optical receiving characteristics in this equipment one
has to be somewhat careful in that, while, as mentioned
before, this equipment has to be under control of an OLT
for GE-PON in such evaluation measurement, the system
limit in the upstream direction can be reached first
depending on the setting conditions for optical insertion
loss—this may have wavelength dependence—thus
breaking the logical link with the OLT. For this reason, theoptical transmission line in the GE-PON section should be
separated based on wavelength, between the upstream
and the downstream, and the insertion loss of the
upstream transmission line is fixed at an approximately
middle value.

Table 2

Receiver sensitivity at room temperature.
Item

Value

Sample size

47

Range (dBm)–27.5 to –26.2

Average (dBm)–26.90Std. Deviation (dB)

0.27

-23
Receiver sensitivity ( dBm )7-24

Speciﬁed value of IEEE802.3

-25
-26
-27
-28
-29

0

35

70

Case temperature ( ℃ )Figure 10 Temperature dependence of receiver sensitivity.

Table 3

Evaluation results for the upper bound of receiving
range at room temperature.
Item

Value

Input Optical Power
Case Temperature
Equivalent Bit Error Rate

0 dBm
0 °C

35 °C

70 °C

< 10–12< 10–12< 10–12•Specified value of IEEE802.3 : –3dBm   ,  Sample size : n=5

From the results shown above, it can be seen that thisequipment has a sufficient margin to meet the specifications for 1000BASE-PX10U/20U specified in IEEE 802.3.
This ONU equipment as an optical receiver has an additional function beside that of outputting received Ethernet

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SFP ONU as CPE for GE-PON
5
Average launch power (dBm)frames in the form of electrical signals, such that it
receives signals to maintain a GE-PON link with an OLT,
thereby continuing to stay under the control of the OLT. In
this context, in order to evaluate the limiting point of
GE-PON link, we have measured the minimum opticalreceiving power in Figure 6 needed to maintain a GE-PON
link with an OLT. Table 4 shows the results. The GE-PON
link is seen to be maintained down to a considerably
lower power with respect to the receiver sensitivity above
mentioned. In this measurement also, the downstream
and the upstream of the GE-PON optical transmission line
are separated so as to make the results reflect the opticalreceiving characteristics only.

4

Speciﬁed value of IEEE802.3

3
2
1
0
-1
-2

0

35
Case temperature ( C)70

20

Table 4

Evaluation results for the lower bound of GE-PON
link at room temperature.
Item

Value

Extinction ratio (dB)

18
16
14
12
10
8

Sample size

10

Range (dBm)–31.5 to –30.3

6

Average (dBm)–30.994

Std. Deviation (dB)

0.40

OPTICAL TRANSMITTING CHARACTERISTICS

Optical transmission signals are generated by the lightemitting element, which is driven by the optical transmitting function block under the control of GE-PON system.
Figure 11 shows the temperature dependence of typical
optical transmitting characteristics, indicating that thisequipment has a sufficient margin over the specificationsfor 1000BASE-PX10U/20U defined by IEEE 802.3.
In an ONU for GE-PON, the existence period for
upstream optical signals becomes bursty, since theupstream relies on TDMA. We have observed the opticalburst signals while randomly changing the information
amount in the upstream direction, and the results areshown in Figure 12, in which the vertical direction represents optical power. Comparisons of each optical powerof burst signals indicate that they are stabilized exhibiting
no change. In order to make quantitative evaluation, theaveraged optical power in the burst span during a time
period of 0.6 μs ~ 2.6 μs after rising burst was measured
as an averaged electrical voltage. The measurements for1000 sweeps are plotted on the vertical axis against the
number of sweeps on the horizontal axis, resulting in a

35
Case temperature （ C）

70

80
60
40
20
0

5.

0

100
Eye mask margin (%)

As has been noted above, satisfactory receiving characteristics have been obtained. Considering that this has
been achieved with extremely compact equipment as this
ONU, it can be thought that this is the fruit of our successively accumulated technologies starting from theresearch and development of GE-PON-related OSAs andoptical transceiver modules.

Speciﬁed value of IEEE802.3

Speciﬁed value of IEEE802.3035Case temperature （ C）

70

Figure 11 Temperature dependence of typical optical transmitting characteristics.

trend graph shown in Figure 13. The vertical axis has ascale of 2 mV/DIV, with its center line corresponding to
166 mV. The fluctuations shown in the Figure lie within±1 DIV over the centerline, which is equivalent to lessthan ±0.05 dB.
Finally, let us briefly discuss jitter that represents theamount of timing variation of transmission optical signals
in the time axis direction. Although, as noted before, theoptical transmitting signals are bursty, their timing is synchronized with the clock for transmission in the ONU.
Since this clock operates in continuous oscillation rather
than in a bursty mode, the timing of optical burst signals
is based on a stabilized clock, so that the jitter of the optical transmitting signals in this ONU is basically equivalent
to that of normal optical transmitter equipment, in terms ofdesign concepts. Functional parts of optical burst signals
during information transmission to the OLT were measured for estimation of their jitter, and the results are

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SFP ONU as CPE for GE-PON

Figure 12 Launching burst signal from optical transmitter.

shown below. What is sought is TJ (Total Jitter) 1) specified-12
at a BER of 10 . Here, the TJ is derivered, as shown in
Figure 14, based on the bathtub curve given by the measurement equipment. In the Figure, the upper plots standfor the bathtub curve (in brown) together with the trend (inblue) of 1000 TJs for each burst section, while the lower
plots show the optical burst signal at a certain instant. The
vertical axis of the trend graph has a scale of 10 ps/DIV,
with its center line corresponding to 100 ps. The averaged
TJ is calculated from the trend graph to be 104 ps.
Comparison on a burst-to-burst basis shows that they arestabilized confirming sufficient characteristics. Considering
an incremental jitter that is estimated to be +40 ps, whichis inherent to jitter evaluation systems for optical burst signals compared with those for continuously emitted opticalsignals, the TJ for this ONU is thought to be fully 65 ps.

6.

CONCLUSIONS

We have developed a SFP ONU for GE-PON by applying
the PMD sublayer technology. As can be seen from thefact that the enclosure for SFP transceivers is used, thisequipment densely integrates the functions of ONU equipment into the size of an optical transceiver. This is about
one eighties the volume of our normal ONUs, although asimple comparison may be difficult to make. Also thisequipment is provided with a number of features integrated in its simple form. It is expected that this equipment
will take advantage of its features to play a role in developing new GE-PON markets.

REFERENCES
Figure 13 Trend chart of optical average launch power at the
beginning of each burst span.

Bathtub curve

Trend of 1000TJs

1) IEEE Standard 802.3 - 2005
2) Iwase et al.: Optical Transceiver Modules for Gigabit Ethernet PON
FTTH System, Furukawa Review, No. 28, (2005), p. 83) Yuji Akatsu: Recent Progress in Optical Module and TransceiverTechnology, NTT Technical Review, Vol. 4, No. 10, (2006), p. 124) Mori et al.: An Optical Module for Optical Line Terminal on
155Mb/s and 622Mb/s Downstream ATM-PON Systems, Technical
Report, IEICE, Vol. 100, No. 611, (2001), OCS2000-97, p. 37 (inJapanese)

Figure 14 TJ measured in optical burst signals.

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