Techniques for Labeling of Optical Signals in Burst Switched
Networks
I. Tafur Monroy (1), A. M. J Koonen (1), J. Zhang (2), Nan Chi (2), P. v. Holm-Nielsen (2), C.
Peucheret (2), J. J. Vegas Olmos (1), G-D Khoe (1).
1: COBRA Institute, Eindhoven University of Technology
P. O. Box 513, 5600 MB Eindhoven, The Netherlands, E-mail: i.tafur@tue.nl
2: Research Center COM, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
Keywords: Optical Burst Switching, Optical Networks, IP-over-WDM, GMPLS.
ABSTRACT
We present a review of significant issues related to labeled optical burst switched (LOBS) networks and
technologies enabling future optical internet networks. Labeled optical burst switching provides a quick and
efficient forwarding mechanism of IP packets/bursts over wavelength division multiplexed (WDM) networks
due to its single forwarding algorithm, thus yielding low latency, and it enables scaling to terabit rates.
Moreover, LOBS is compatible with the general multiprotocol label switching (GMPLS) framework for a
unified control plane. We present a review on techniques for labeling of optical signals for LOBS networks,
including experimental results, we discuss as well issues for further research.
1. INTRODUCTION
In a LOBS network, bursts of data are composed by assembling several IP packets in the ingress LOBS nodes
according to their destination or class of service. To each burst a label, of a short, fixed length, is assigned and
used by the core nodes to forward the packet through the network [1]. In this way packet forwarding decisions
are taken by examining only short optical labels, supported by GMPLS, yielding a low latency, low overhead
routing technique that simplifies packet forwarding and enables scaling to terabit rates [2]. This networking
strategy is exemplified in Fig. 1.
Figure 1: Network architecture of an optical labeled burst switched network.
The present contribution focuses on techniques to implement optical labeling of IP bursts and the
corresponding technologies for label swapping in the core nodes. Several techniques have been proposed to
label optical packets or burst of packets. In this paper we report on five basic methods for labeling packets in
a multi-wavelength network: time division multiplexing (TDM) labeling, optical code division multiplexing
(OCDM) labeling, sub-carrier multiplexing (SCM) labeling, orthogonal labeling, and WDM labeling. In the
first four methods, the label is attached to the payload in the same wavelength channel as the one carrying
these payload data, whereas in the fifth method a separate wavelength channel is used to carry the label(s).
We make special emphasis on the orthogonal angle/intensity modulation labeling method that is extensively
being studied in the framework of the IST–STOLAS (Switching Technologies for Optical Labeled Signals)
research project. In this technique, label information is either modulated in FSK (frequency shift keying) or
DPSK (differential phase shift keying) format, while the payload is in intensity modulation format [3]. We
report on experimental results demonstrating the feasibility of combined FSK/IM modulation for the
labeling/payload information. We show also an experimental assessment of the performance of a wavelength
converter based on semiconductor optical amplifiers (SOA) in a Mach-Zehnder interferometer (MZI)
configuration for optical label swapping operating at a payload data rate of 10 Gbit/s and an FSK label at 312
Mbit/s. Moreover, summary conclusions, design guidelines are presented regarding orthogonal modulation
format for labeling of signals. Finally, issues for further research are discussed.
2. OPTICAL SIGNAL LABELING TECHNIQUES
TDM LABELING: In TDM labeling (also called bit-serial labeling) the label information is attached in the
time domain, by putting it in front of the payload and header. The payload/header and the attached label are
encoded on the same wavelength carrier. Guard time bands are used to separate the label bits from the
payload/label and synchronization bits are also used for time-alignment. The guard time may also serve to
reduce the buffer time for the payload data, needed to allow completion of the processing of the label and
preparing of the new label, before the new label can be attached to the payload. The Advantages of these
techniques are related to the coupling of label and payload/header in the same wavelength channel, easing the
bookkeeping in the routing node. The Disadvantages are related to the tight synchronization and delineation
of payload/header and label that is required. Moreover, label/payload delineation for label erasure is needed;
this requirement becomes even more complex as the data rates are increased. It may be noted, that the bit-rate
of the label may be the same (synchronous TDM) or of a lower bit-rate (asynchronous TDM) than the
payload; commonly a low bit-rate for the label is chosen to allow the use of low speed electronics for label
processing and synchronization /delineation at the packet rate. Opto-electronic label swapping is commonly
used. In the KEOPS project the following packet format was used: fixed time slot of 1.64 microseconds and a
payload of 1.35 microseconds, and 14 bytes for header information [4]. A payload rate of 10 Gbit/s and
header rate of 622 Mbit/s was implemented in a lab trial.
All-Optical label swapping: the TDM labels can be swapped by optical means. Two techniques have been
reported:
Wavelength conversion in a fiber loop mirror structure. By using a different pulse format for the header and
payload, the header can be suppressed while wavelength conversion of the payload takes place. After removal
of the header, a new level can be inserted by opto-electronics means or alternatively the new serial header can
be pre-modulated onto the probe signal for the wavelength conversion. Experimental demonstration of label
switching by the latter technique of 40 Gbit/s payload and 2.5 Gbit/s header has been reported[5]. A two-hop
routing experiments of packets at 80 Gbit/s and 10 Gbit/s has also been reported [6]. Although the header
erasure needs no timing information the label insertion does.
Optical XOR operations in a SOA-MZI wavelength converter. An optical scheme has been proposed to
perform label replacement in a semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI)
2
wavelength converter by a logical XOR operation. However, tight synchronization at the bit level is required
and buffering is required. Experiments of routing by time-serial addressing and XOR label swapping have
been demonstrated at 20 Gbit/s payload and 10 Gbit/s label swapping [7].
OCDM LABELING: Optical code division multiplexing (OCDM) has been proposed for labeling in optical
networks [8][9]. The label information is attached by scrambling the payload with a specific code carrying the
label information. Although OCDM is one of the labeling techniques that allow label recognition for routing
instead of look-up table operations, its implementation is still quite complex. For example, if a wavelength
supports N OCDM codes, a bank of N optical autocorrelators per wavelength is needed for every channel and
a replica of every channel should be provided to every autocorrelator of the bank. However, OCDM labeling
offers possibilities to be combined with WDM (WDM sub-bands) and OTDM transmission techniques, and it
has inherently a label recognition property. An experiment of WDM-to-OTDM and back of 4x10 Gbit/s
OCDM coded channels has been published recently [9]. The OCDM coders and decoders are optical
transversal filters in planar lightwave circuit (PLC) technology to perform 8-chips BPSK coding with a chip
interval of 5 ps. Although data rates supported are above 10 Gbit/s with label recognition properties, the
complexity and the amount of coders and decoders are large as well as the need for dispersion compensation.
Large performance penalties are also introduced in each conversion stage, in the order of 8-9 dB for operation
at 10-9 BER. The Advantages of OCDM labeling are the coupling of label and payload/header in the same
wavelength channel, easing the bookkeeping in the routing node. To the Disadvantages belongs the sizeable
increase of the line rate (by scrambling each payload bit with a label code sequence).
SCM LABELING: With subcarrier labeling, the label information is modulated on a subcarrier, which is
positioned in the same wavelength channel, well above the baseband spectrum of the payload data. e.g., in the
HORNET project the payload data rate is 2.5 Gbit/s, and a subcarrier at 3 GHz carries FSK modulated header
data [10]. In a multiwavelength system, the subcarrier frequencies may be chosen to be unique per
wavelength channel, which allows easy recognition by direct detection and bandpass filtering. By intensitymodulating the subcarrier label on the optical carrier, two subcarrier sidebands next to the baseband will
occur, centered around the optical carrier. The wavelength channels need to be spaced by twice the subcarrier
frequency at least. Due to fibre dispersion, fading of the subcarrier signal in a fibre link may occur. More
complicated optical single-sideband modulation techniques have been explored to avoid the fading
problem[12][13]. The Advantages of SCM labeling are related to the coupling of label and payload/header in
the same wavelength channel, easing the bookkeeping in the routing node. Moreover, the label data can be
completely asynchronous to the payload data; no strict synchronisation issues. Futhermore, by optical direct
detection by a single photodiode, the different subcarriers belonging to different wavelength channels can be
detected without wavelength demultiplexing. Some Disadvantages are related to the fading of the subcarrier
that may occur due to fibre dispersion. Moreover, non-linearities may cause intermodulation distortions,
causing interference into other channels by direct simultaneous optical detection of the subcarriers. In
addition the issues above, for high payload data rates, the subcarrier needs to be positioned at a very high
frequency, which requires complicated electronics, and which enlarges the minimum allowed wavelength
channel spacing
Reading the SCM label: Reading the SCM label can be done in two ways: 1) optical direct detection by means
of tapping off part of the multiwavelength signal and converting it to an electrical signal. When the subcarrier
frequencies are chosen such that each wavelength channel uses a unique subcarrier frequency, this electrical
signal contains all the individual labels, and each can be inspected separately by electronic bandpass filtering.
No (complex) wavelength demultiplexing is needed. The demodulation of the subcarrier label information can
be done by means of envelope detection, which requires a carrier to be present. 2) alternatively, a narrow
optical bandpass filter may be centred at the spectral location of a subcarrier band. The filter may pass just
one of the subcarrier bands, and thus no carrier fading effects are noticeable. Then, the filter output signal is
the label baseband signal, so no high frequency receiver is needed.
3
Swapping of a subcarrier label is done in two steps: firstly erasure of the old label, and subsequently insertion
of the new label. The sub-carrier can be suppressed by using a notch filter while the payload is left intact.
Subcarrier erasure: A subcarrier suppression of 25- 32 dB and a payload loss of 2 dB by using a FP filter
have been demonstrated [12]. Label suppression has also been realized by using a fiber Bragg grating (FBG)
[12]. This technique is also pursued in the IST LABELS project. Alternatively, a fibre non-linear mirror can
be used for subcarrier suppression. SCM erasure can also be performed by wavelength conversion by XGM in
a SOA [5][11]. If the corner frequency of the low-pass wavelength conversion response is located at a higher
frequency than the upper frequency of the baseband payload but far enough below the subcarrier channel, the
SOA will efficiently convert the baseband payload to the new wavelength while suppressing the SCM header
signal. SCM Label insertion: Using a MZI LiNbO3 dual drive modulator. The data payload can thus be
differentially modulated, eliminating chirp effects. To achieve optical SSB modulation of the subcarrier, the
subcarrier with the label information is added to the payload and fed to one modulator port, and after shifting
by /2 and adding to the inverted payload fed to the other modulator port [11]. Another technique is to use a
MZI-SOA wavelength converter, by modulation of the current of one of the SOAs in the MZI structure. A
two-stage scheme using XGM + XPM in a MZI-SOA has been demonstrated for a payload at 2.5 Gbit/s and a
SCM label up to 10 GHz [5][11]. Alternatively, using a MZI-SOA wavelength converter and a tunable laser,
the new SCM label can be pre-modulated by means of an external modulator onto the output of the tunable
laser that delivers the probe signal for the wavelength converter.
ORTHOGONAL LABELING: Label information can be conveyed in the phase or frequency of the payload –
carrier by orthogonal modulation to the amplitude modulated of the payload information. In the STOLAS
project, the payload data at 10 Gbit/s is intensity-modulated, and the label data at 155 Mbit/s is modulated
orthogonally in FSK (or DPSK) format on the same wavelength channel. Label erasure is accomplished by
using an MZI-SOA wavelength converter, where only the IM payload information is transposed to a new
wavelength channel, not the label information. Label rewriting in FSK format is done by FSK modulation of
the tunable laser at the wavelength converter; in DPSK format by means of a phase modulator following the
wavelength converter.
The Advantages of this approach are related to the fact that the data payload is coupled to the label in the
same wavelength channel, which eases the bookkeeping in the routing nodes. Moreover, the label and data
payload are decoupled regarding timing, and thus do not need strict synchronisation, only synchronisation at
packet level is needed, not at the bit level. Furthermore, no header/payload delineation is needed for label
erasure and rewriting. Addition of label information does not increase the channel’s bandwidth.
The Disadvantages of orthogonal modulation scheme are related mainly to crostalk: crosstalk of payload to
label by chirp in the wavelength converter used for the label swapping. Also, crosstalk of label to payload by
FM-to-IM conversion, due to e.g. dispersion and interferometric effects in the fiber links during propagation.
WDM LABELING: The labels of the packets in every wavelength channel can be time-multiplexed in a
separate common wavelength channel. Careful synchronisation of the individual label signals with the
respective payload channels needs to be maintained. Therefore, time-slotted operation in all wavelength
channels with careful synchronisation among the channels is required. Chromatic dispersion in the fibre links
may, however, affect the strict synchronisation, by introducing group velocity differences between the labelchannel and the various payload channels.
4
Orthogonal
labeling
SCM
labeling
Synchronous
TDM
labeling
Aynchronous
TDM
labeling
OCDM
labeling
WDM
labeling
Synchronisation
of payload and
label
Not strict, at
packet level
Not strict, at
packet level
Strict, at bit
level
Not strict, at
packet level
Strict, at bit
level
Not strict, at
packet level
Channel
bandwidth *
Payload rate +
FSK tone
spacing
Highest
subcarrier freq.
Payload
rate+label rate
Slightly larger
than payload
rate
Multiple of
payload rate
Payload rate
Net payload
rate *
Line rate
Line rate
Line rate –
label rate
Reduces when
label rate
decreases
Fraction of line
rate
Line rate
Label reading
Demuxing of
all -channels
OE detection
for all channels, no demuxing
Demuxing of
all -channels +
high-speed
electronics
Demuxing of
all -channels +
optical switch +
low-speed
electronics
Demuxing of
all -channels
Demuxing of
labels in
common channel
Label erasing
-demuxing, +
-converter
(XGM or XPM)
-demuxing, +
XGM converter, or
optical notch
filter
-demuxing, +
OE conversion
of
payload+label
required
Separate label
from payload
by slow optical
switch,
triggered by
label sync bits
-demuxing, +
OE conversion
of
payload+label,
+ decoding
OE conversion
+ time
demuxing of
labels in
common channel
Label rewriting
By FSK
modulation of
tunable laser in
-conv., or
DPSK external
modulation
after -conv.
By dual-drive
external
modulator, or
driving SOA in
MZI-SOA conv.
EO conversion
of
payload+label
required, + time
muxing of new
label
Multiplex
payload and
new label with
slow optical
switch
EO conversion
and decoding of
payload+label
required, +
encoding with
new label code
OE conversion
+ time muxing
of new labels
on common channel
Transmission
issues
IM-to-FM
conversion and
v.v.
Fading of
subcarriers
Payload + label
delineation
among channels
Large guard
bands between
payload and
label
High line rate
Multi-channel
delineation of
payload,
chromatic
dispersion
Table 1. Comparison of techniques for labeling of optical signals. * neglecting guard bands, and
assuming label rate is much smaller than payload rate
5
a)
Label, 312 Mb/s
FSK
DFB+EAM
SOA-MZI
Label generation
Label detection
Label erasure
Laser
ECL
Label, 312 Mb/s
b)
Label generation
DFB+EAM
FSK
SOA-MZI
IM
ECL
Label detection
Label insertion
AM
Payload, 10Gb/s
Figure 2: Experimental setups for FSK label (a) erasure and (b) insertion using a SOA-MZI
wavelength converter. ECL: external cavity laser. From [14].
One advantage of this approach is that only the common label-wavelength channel needs to be inspected for
label processing and routing. However, a disadvantage is the fact that strict synchronisation of timemultiplexed labels in the dedicated label-wavelength channel with the payload data in the various datawavelength channels required; also header/payload delineation for label erasure is needed, which becomes
more complex at higher data rates. Moreover, the data payload is not closely coupled to the label, requiring
careful bookkeeping when routing in a node. In Table 1 is presented a comparative study of techniques to
label optical signals.
3. EXPERIMENTAL RESULTS ON FSK/IM LABEL SWAPPING
In this section we present experimental results on the generation of FSK/IM orthogonal labeling of signals.
We present as well results on the performance of a SOA-MZI based wavelength converter label swapping for
a payload rate of 10 Gbit/s and a FSK label rate of 312 Mbit/s.
The experimental setup for the label erasure and label insertion is depicted in Figure 2. The optical FSK
modulation can be achieved simply by directly modulating the electrical current of a distributed feedback
(DFB) laser diode at 1549.3nm. However, the drive current variation always results in a simultaneous
intensity modulation of the emitted light. To remove the intensity variation of the laser’s output, the inverse
electrical data is injected into the integrated electro-absorption modulator (EAM) with appropriate time delay
and modulation voltage. In this way, a constant amplitude optical FSK signal is generated. The payload
6
10-5
back-to-back label
back-to-back payload
inserted label
converted payload
payload after label erasure
10-6
BER
10-7
10-8
10-9
10-10
-36
-32
-28
-24
-20
-16
Average received power (dBm)
Figure 3: BER versus receiver input power for the IM payload and the FSK label a) ■ label in the back-toback configuration b) ▼ payload back-to-back c)  payload after wavelength conversion and FSK label
insertion d) □ inserted FSK label, e)  payload after label erasure. From [14].
information at 10Gbit/s is added by a subsequent Mach-Zehnder modulator, thus producing an optically FSK
labeled signal.
This signal is then injected into the SOA-MZI with a CW light at 1554.1 nm, which is generated by a tunable
external cavity laser (ECL). With the above setup, label insertion and erasure has been experimentally verified
and its performance assessed. More details on the experimental setup and transmission issues will be
presented during the talk and they can also be found in [14][15][16][17].
Because of the wavelength conversion process in the SOA-MZI, the 10 Gbit/s IM payload is copied onto the
CW light while the FSK label will not be converted to the new wavelength. Hence the label erasure is
accomplished. For an input payload with extinction ratio of 4.5 dB, the output signal has an extinction ratio of
12.9 dB, resulting in an increase in receiver sensitivity of 2 dB compared to back–to-back case, clearly
demonstrating the 2R regeneration due to the SOA-MZI. Fig. 3 shows the dependence of the BER on the
received power for a back-to back configuration of the FSK/IM combined modulation format. We can see that
the receiver sensitivity at a BER of 10-9 is –32.6 dBm and –24.2 dBm, for the FSK label and IM payload
respectively (curves marked with ■, and ▼, respectively).
A label insertion scheme was realized using a FSK modulated signal instead of a CW signal at the SOA-MZI
converter. The signal output of the SOA-MZI device is then the original payload data, converted to the FSK
signal wavelength, with the superimposed FSK label modulation. After splitting the signal for FSK and IM
detection, the BER was measured as a function of the average received power. The results are presented in
Fig. 3. As we can see the receiver sensitivity at a BER of 10 -9 is –31 dBm and -23.5 dBm, for the label and
payload, respectively (curves marked □ and , respectively). We observe that no substantial degradation of
the receiver sensitivity takes place in the label insertion process. In fact, the SOA-MZI has a regenerative
character. However, for a proper detection of the FSK signal, a certain power level in the ‘zeros’ bit should be
present. The extinction ratio of the input signal was measured to be 5.1 dB and an extinction ratio of 4 dB was
7
observed at the output of the SOA-MZI. The error free detection of the FSK data experimentally demonstrates
that the chirp introduced by the SOA-MZI wavelength converter has no detrimental influence on the FSK
detection.
FSK label swapping: The complete operation of label swapping (i.e. label erasure and insertion of a new FSK
label) can be performed with the SOA-MZI under study provided a second FSK signal is available at a
different wavelength than the old FSK label to be erased. This second FSK signal is then used instead of the
CW input into the SOA-MZI wavelength converter. Although, a counter-propagation mode of operation could
be used, the SOA-MZI device under study shows insufficient performance for data rates above 5 Gbit/s. To
study the chirp and linewidth properties of the IM converted signal after FSK erasure, and to assess its
suitability for FSK label re-insertion, the experimental setup shown in Fig. 4 was used. The output of the
SOA-MZI wavelength converter was amplified and fed into an EAM where FSK label insertion took place by
cross-absorption modulation (XAM) using an FSK modulated pump signal. Experimental details on the label
swapping process and transmission propertied are presented in [14][15][16][17].
The results for the BER measurements for the FSK and IM signals after a complete label swapping stage are
shown in Fig. 5. We can compare these results with the back-to-back measurements presented in Fig. 3. As
we can see, there is no substantial observed power penalty for both signals (below 0.5 dB). The chirp
introduced by the SOA-MZI has shown no effect on the performance, as confirmed by the error free detection
of the FSK signal. This is in accordance with the previous chirp measurements and simulations results
presented [14].
Packet generation
Label, 320 Mb/s
Label
generation
FSK
DFB+EAM
FSK/IM
AM
Payload, 10Gb/s
SOA-MZI
Optical filter
Label erasure and 2R
ECL
OC
EAM
Payload detection
label detection
Figure 4: Setup for a complete label swapping of FSK/IM labeled signal. OC: optical circulator. From
[14].
8
1.00E-05
label insertion w EAM
1.00E-06
BER
payload
1.00E-07
label
1.00E-08
1.00E-09
Received power, dBm
1.00E-10
-38
-36
-34
-32
-30
-28
-26
-24
-22
-20
Figure 5: BER versus input power for a FSK/IM combined modulation scheme after a complete label
swapping stage. The results are to be compared with the back-to-back curves of Fig. 2. From [14][15]
DISCUSSION OF RESULTS: The extinction ratio of the IM is a crucial design parameter for a combined
FSK/IM system. Namely, there should be enough optical power in the IM ‘”zero”’ binary symbols so that the
FSK data still can be recovered. Experimentally, it has been shown that an ER of 4-5 dB gives proper
performance, this value is in accordance with simulations results [14]. Using the above value for the IM
extinction ratio, transmission of FSK/IM has shown to be feasible over a link of 88 km of standard single
mode fiber with pre-and post dispersion compensation[16][17]. Another aspect relevant to the design of the
combined modulation format may be the synchronization between the FSK bit and the IM bit. With the
reported generation method of the FSK signal, if the FSK and the IM bits are not perfectly synchronized, the
eye pattern may be distorted and errors are detected [14]. This effect is accumulative in the case of label
swapping, and therefore the performance degradation may be severe. However, in recent experiments with
other methods to generate FSK optical signal this effect has not been observed; correspondent details will be
published elsewhere.
4. CONCLUSIONS
Several optical labeling techniques have been reviewed with emphasis on their advantages and weaknesses
for applications in LOBS networks. We have presented an experimental validation of the feasibility of
FSK/IM labeling scheme. A label swapper based on a SOA-MZI has been employed. The presented results
show that the orthogonal labeling scheme is a promising technique for LOBS that offers the possibility for
building label swapping nodes in integrated photonic circuits based on SOA technologies.
9
ACKNOWLEDGMENT
This work was performed in the framework of the IST-STOLAS project, which is partially funded by the IST
Program of the European Community. The authors would like to acknowledge the contributions of their
colleagues within the STOLAS consortium.
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[14] I. Tafur Monroy, E. J. M. Verdurmen, Sulur, A. M. J. Koonen, H. de Waardt, G. D. Khoe, N. Chi, P. V. HolmNielsen, J. Zhang and C. Peucheret, “Performance of a SOA-MZI wavelength converter for label swapping using
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[15] N. Chi, J. Zhang, P. V. Holm-Nielsen, C. Peucheret and P. Jeppesen, “Transmission and transparent wavelength
conversion of an optically labelled signal using ASK/DPSK orthogonal modulation”, IEEE Photonics Technology
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[16] J. Zhang, N. Chi, P. V. Holm-Nielsen, C. Peucheret and P. Jeppesen, “An optical FSK transmitter based on an
integrated DFB laser/EA modulator and its application in optical labeling, to appear in IEEE Photonics Technology
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[17] N. Chi, L. Xu, L. J. Christiansen, K. Yvind, J. Zhang, P. V. Holm-Nielsen, C. Peucheret, C. Zhang and P. Jeppesen,
“Optical label swapping and packet transmission based on ASK/DPSK orthogonal modulation format in IP-overWDM networks, in Technical Digest Optical Fiber Communication Conference, OFC’03, Atlanta, Georgia, U.S.A.,
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