COL 9(6), 060602(2011) CHINESE OPTICS LETTERS June 10, 2011
Analysis of IPACT network performance considering
channel switch latency for WDM-EPON
Zhao Zhou (±±± mmm)
∗
, Shilin Xiao (), Min Zhu (ÁÁÁ ¯¯¯)
∗∗
, and Meihua Bi (...{{{uuu)
State Key Laboratory of Advanced Optical Communication Systems and Networks,
Department of Electronic Engineering, Shanghai Jiaotong University, Shanghai 200240, China
∗
Corresponding author: zhouzhao@sjtu.edu.cn;
∗∗
corresponding author: zhuminxuan@sjtu.edu.cn
Received October 26, 2010; accepted January 14, 2011; posted online May 6, 2011
In wavelength-division multiplexing (WDM) ethernet passive optical networks (EPONs), to realize the
statistical multiplexing of upstream wavelength resources, some optical tunable components are introduced
in the optical network units. However, the switch latency (SL) of these tunable components constrains the
performance of WDM-EPON. In this letter, we extend the mathematical model of the WDM interleaved
polling with adaptive cycle time (IPACT) scheme by additionally considering the SL conditions. We
also investigate the effect of channel SL on network performance. The simulation results show that the
performance of WDM-IPACT-SL deteriorates as the SL increases.
OCIS codes: 060.4250, 060.4264, 060.4510.
doi: 10.3788/COL201109.060602.
Ethernet passive optical networks (EPONs) have
emerged as promising solutions re cently due to their in-
herent advantages, such as longevity, low attenuation,
high bandwidth, and cos t-sharing among subscribers.
However, the access to network traffic has grown ex-
plosively with the ever-increasing number of end users
and proliferation of emerging applications , such as triple
play, video on demand, and so on. Thus, current EPONs
that dictate a single channel for both downstream and
upstream transmissions are not adequate. Hence, the
wavelength-division multiplexing (WDM) EPON, which
combines WDM with EPON technology, is a pr omising
upgrade solution to achieve the next-generation s calable
and flex ible passive optical network (PON)
[1,2]
.
Previous studies on the pr oblem of channel sharing in
WDM-EPON usually fall into two categories: static and
dynamic wavelength assignments. In static wavelength
assignment, each optical network unit (ONU) is assigned
a fixed pair of upstr eam and downstream wavelengths
[3]
.
However, this architecture may not utilize bandwidth
efficiently, esp e c ially when some wavelengths are over-
loaded while others have light loads
[4]
. In dynamic wave-
length assignment, all ONUs can have full access to all
wavelengths, which necessitates that each O NU trans-
mitter c an switch to any designed wavelength channel
for upstream transmission. One obvious feature of this
category is that each ONU is equipped with one tunable
laser, whose tuning speed generally ranges from dozens to
hundreds of microseconds
[5,6]
. Jayasinghe et al. proposed
another approach to dynamically select the assigned up-
stream channel from multiple seeding wavelengths by us-
ing a tunable filter and a re flec tive semiconductor opti-
cal amplifier
[7]
. The cost-effective filters mentioned in
technological reports also have tuning times in the scale
of microseconds
[8]
. Therefore, the limited tuning speed
of these optical components constrains the performance
of WDM-EPONs. While the packet length in EPONs
ranges from 64 to 1518 bytes, the corresponding trans-
mission time approximately ranges from 0.512 to 12.144
µs on a 1-Gb/s link. Thus, the switch latency (SL) eq uiv-
alent for the transmission time of a data packet cannot
be abso lutely neglected while considering the problem
of bandwidth management in practical applications of
WDM-EPONs, beca us e channel SL has a vital effect on
the dynamic bandwidth allocation. However, previously
proposed dynamic bandwidth-allocation schemes only fo-
cus on the scheduling strategy, without considering the
impact of SL on the schedule perfo rmances
[9]
.
In this letter, we investigate the effect of channel SL
on network-performance parameters, s uch as average de-
lay and packet-loss ratio, based on the extended WDM
interleaved polling with adaptive cycle time (IPACT)
[10]
scheme. First, we extend the WDM-IPACT scheme in-
corporating the SL consideration (named WDM-IPACT-
SL) and elaborates the operation process of a n optical
line terminal (OLT). Then, we simulate the performance
of the WDM-IPACT-SL model for varying SL values un-
der different simulation conditions.
In the WDM-IPACT-SL scheme, the OLT decides not
only to alloca te the upstream transmission window in
terms of the start time and le ngth, but also to schedule
the upstream channel with the first-available wavelength
channel (FAWC) scheme for ea ch ONU. In the following
text, we describe the detailed explanatio n regarding how
SL is additionally considered into the FAWC scheme. Af-
ter receiving the GATE message from the OLT , the ONU
sends Ethernet fr ames according to its granted transmis-
sion window size on the assigned wavelength channel.
Because the OLT knows the round-trip time (RTT) of
all ONUs and keeps track of all upstream channels, it
can calcula te the next idle time o f every upstream chan-
nel exactly by using the following equatio n:
t
Idle
(C
i
) = t
j
G
(C
i
) + t
j
T
(C
i
) + t
j
P(G)
(C
i
) + RTT
j
+ B, (1)
where t
Idle
(C
i
) is the next idle time ep och of the channel
C
i
. We assume that ONU-j is the last ONU assigned
on channel C
i
to transmit upstr e am data. Here, t
j
G
(C
i
)
is the time epoch when the la st GATE is sent to inform
ONU-j to transmit on channel C
i
; t
j
T
(C
i
) is the trans-
mission time spent by ONU-j, which is decided by the
transmission-window size assigned by the OLT and the
1671-7694/2011/060602(4) 060602-1
c
2011 Chinese Optics Letters