Adaptive Registration in TWDM-PON
With ONU Migrations
Jun Li, Weiqiang Sun, Hongyang Yang, and Weisheng Hu
Abstract—In time and wavelength division multiplexed
passive optical networks (TWDM-PONs), optical network
unit (ONU) migration (equivalently retuning working
wavelength pairs) is an effective approach to reducing
power consumption at the optical line terminal (OLT) side.
Migration delay, the time from when a migration is initi-
ated until the migrating ONU(s) have successfully joined
the target wavelength pair, may affect the user service level
agreement (SLA) and should be minimized. After an ONU
migration process has been triggered, both the ONUs to
be migrated (the migrating ONUs) and the newly incoming
ones will have to register (referred to as activation in the
ITU-T G.984.3 standard) in the upcoming quiet windows.
This makes the number of ONUs competing in a single quiet
window much more dynamic and leads to unpredictable
registration performance and longer registration delay,
which in turn will have a negative impact on the migration
delay. In this paper, we propose an adaptive registration
scheme in a TWDM-PON with ONU migrations. In the pro-
posed scheme, the size of the quiet window is adaptively
adjusted according to the up-to-date number of involved
ONUs (including both ONUs to be migrated and the newly
incoming ones). We show through simulations that the
proposed scheme reduces average migration delay and in-
creases the bandwidth utilization, especially when a large
number of ONUs are involved in a registration process. In
addition, we further investigate the length of the discovery
cycle and find a shorter discovery cycle is needed to reduce
migration delay and satisfy the requirement of the ONU
SLA for a large number of involved ONUs. To realize a
99.999% ONU SLA, the length of the discovery cycle has
to be shorter than 0.1 s for 128 involved ONUs and 0.15 s
for 32 involved ONUs.
Index Terms—Adaptive registration scheme; High band-
width utilization; Low migration delay; Time and wave-
length division multiplexed passive optical network
(TWDM-PON).
I. INTRODUCTION
T
he time and wavelength division multiplexed passive
optical network (TWDM-PON) is considered to be one
promising candidate of next-generation passive optical net-
works, because it can combine the flexibility of the time
division multiplexed access passive optical network
(TDMA-PON) with the increased overall capacity of wave-
length division multiplexed (WDM) technology [
1]. Due to
the uneven distribution of online optical network units
(ONUs) at different times in a day, wavelength utilization
is often low in a TWDM-PON. An ONU migration mecha-
nism within TWDM-PON has been proposed in [
2]. As
shown in Fig. 1, ONU(s) can be migrated to a relatively
smaller number of active wavelength pairs, and the re-
maining ones are turned off to save power. It has been
proved that as much as 60% of wavelengths can be shut
down under typical ONU online profiles [
2]. Moreover,
an ONU migration based on a wavelength relocation
mechanism is proposed in [
3], in which the percentage of
wavelength and transceivers that need to be deployed
can be further reduced to as low as 30%.
According to the ONU migration mechanism, the mi-
grating ONUs first deregister from the original wavelength
pairs and register at the target wavelength pairs with
newly incoming ONUs. In the registration mechanism de-
fined in the ITU-T G.984.3 standard, the optical line termi-
nal (OLT) periodically allocates a fixed time interval, called
a quiet window, to register the newly incoming ONUs and
migrating ONUs. During the quiet window, the OLT tem-
porarily suspends the authorization for upstream trans-
mission, and all involved ONUs send responses back to
the OLT after receiving a serial-number (SN) request.
Collisions may occur if two or more ONUs send responses
at the same point of time and they will fail to register [
4].
The ONUs that failed to register in the first attempt will
continue to register in a series of successive discovery
cycles until completing the migration. Collision can be seri-
ous when a large number of migrating ONUs need to be
migrated. For ONUs undergoing a migration, failure to
register means unsuccessful migration and may lead to
service disruption at the user side.
As discussed in [
5–9], increasing the size of the quiet
window can increase the probability of successful registra-
tion and thus is helpful to reduce migration delay. However,
an oversized quiet window will also reduce the available
bandwidth. In the conventional registration processes,
when the average number of incoming ONUs is stable over
time and can be estimated a priori, an optimal quiet win-
dow size statistically meeting the requirement of ONU
registration delay can be easily determined. The problem
becomes difficult when ONU migrations are taken into ac-
count, as the total number of ONUs competing in a quiet
window cannot be determined in advance. Apparently,
http://dx.doi.org/10.1364/JOCN.6.000943
Manuscript received June 18, 2014; revised September 6, 2014; accepted
September 6, 2014; published October 1, 2014 (Doc. ID 213919).
The authors are with the State Key Laboratory of Advanced Optical
Communication Systems and Networks, Shanghai Jiao Tong University,
Shanghai 200240, China (e-mail: sunwq@sjtu.edu.cn).
W. Hu is also with the Key Laboratory of Optical Communication Science
and Technology, Liaocheng University, Liaocheng, Shandong 252000,
China.
Li et al. VOL. 6, NO. 11/NOVEMBER 2014/J. OPT. COMMUN. NETW. 943
1943-0620/14/110943-09$15.00/0 © 2014 Optical Society of America