Design and performance analysis of sched

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
1
Design and Performance Analysis of Scheduling
Algorithms for WDM-PON under
SUCCESS-HPON Architecture
Kyeong Soo Kim, Member, IEEE, David Gutierrez, Student Member, IEEE, Fu-Tai An, Member, IEEE,
and Leonid G. Kazovsky, Fellow, IEEE, Fellow, OSA
Abstract— We report the results of our design and performance
analysis of two new algorithms for efficient and fair scheduling of variable-length frames in a wavelength division multiplexing (WDM)-passive optical network (PON) under Stanford
University aCCESS-Hybrid PON (SUCCESS-HPON) architecture. The WDM-PON under the SUCCESS-HPON architecture
has unique features that have direct impacts on the design of
scheduling algorithms: First, an optical line terminal (OLT) uses
tunable transmitters and receivers that are shared by all the
optical network units (ONUs) served by the OLT to reduce the
number of expensive dense WDM (DWDM) transceivers. Second,
also for cost reduction, ONUs have no local DWDM light sources
but use optical modulators to modulate optical continuous wave
(CW) bursts provided by the OLT for upstream transmissions.
Therefore, the tunable transmitters at the OLT are used for
both upstream and downstream transmissions. To provide efficient bidirectional communications between the OLT and the
ONUs and guarantee fairness between upstream and downstream
traffic, we have designed two scheduling algorithms – batching
earliest departure first (BEDF) and sequential scheduling with
schedule-time framing (S3 F). The BEDF is based on the batch
scheduling mode where frames arriving at the OLT during a
batch period are stored in virtual output queues (VOQs) and
scheduled at the end of the batch period. It improves transmission
efficiency by selecting the frame with the earliest departure
time from a batch of multiple frames, which optimizes the
usage of tunable transmitters in scheduling. Considering the high
complexity of the optimization process in BEDF, we have also designed the S3 F based on the sequential scheduling mode as in the
original sequential scheduling algorithm proposed earlier. In S3 F
we use VOQs to provide memory space protection among traffic
flows and a granting scheme together with schedule-time framing
for both upstream and downstream traffic to reduce framing
and guard band overhead. Through extensive simulations under
various configurations of the tunable transmitters and receivers,
we have demonstrated that both the BEDF and S3 F substantially
improve the throughput and delay performances over the original sequential scheduling algorithm, while guaranteeing better
fairness between upstream and downstream traffic.
Index Terms— Access, media access control (MAC) protocols,
passive optical network (PON), scheduling, wavelength division
multiplexing (WDM)
This work was supported in part by the Stanford Networking Research
Center and STMicroelectronics.
K. S. Kim is with the Advanced System Technology, STMicroelectronics,
Stanford, CA 94305, USA (e-mail: [email protected]).
D. Gutierrez and L. G. Kazovsky are with the Photonics and Networking
Research Laboratory, Stanford University, Stanford, CA 94305, USA (e-mail:
{degm,kazovsky}@stanford.edu).
F-T. An is with the Marvell Technology Group Ltd. (email:[email protected]).
This paper was presented in part at GLOBECOM 2004, Dallas, TX,
November, 2004.
I. I NTRODUCTION
Efficient and fair scheduling of variable-length messages
under the constraints of shared resources is critical for the success of advanced, next-generation wavelength-routed optical
networks where tunable transmitters and receivers are shared
by many users in order to reduce the high cost of wavelength
division multiplexing (WDM) optical components.
The scheduling problem we study in this paper is for
a WDM-passive optical network (PON) under Stanford
University aCCESS-Hybrid PON (SUCCESS-HPON) architecture, which was proposed for next-generation hybrid
WDM/time division multiplexing (TDM) optical access networks [1].1 The SUCCESS-HPON architecture is based on a
topology consisting of a collector ring and several distribution
stars connecting a central office (CO) and optical networking
units (ONUs). By clever use of coarse WDM (CWDM) and
dense WDM (DWDM) technologies, it guarantees the coexistence of current-generation TDM-PON and next-generation
WDM-PON systems on the same network. The semi-passive
configuration of remote nodes (RNs) together with the hybrid
topology also enables supporting both business and residential
users on the same access infrastructure by providing protection
and restoration capability, a frequently missing feature in
traditional PON systems.
In designing the SUCCESS-HPON architecture, we mainly
focused on providing economical migration paths from the
current-generation TDM-PONs to future WDM-based optical
access networks. This has been achieved by sharing some
high-performance but costly components and resources in
SUCCESS WDM-PON2 : First, an optical line terminal (OLT)
uses tunable transmitters and receivers that are shared by
all the optical network units (ONUs) served by the OLT to
reduce the number of expensive DWDM transceivers. Second,
also for cost reduction, ONUs have no local DWDM light
sources but use optical modulators to modulate optical continuous wave (CW) bursts provided by the OLT for upstream
transmissions. Therefore, the tunable transmitters at the OLT
are used for both upstream and downstream transmissions.
The sharing of tunable transmitters and receivers at the OLT
and the use of tunable transmitters for both upstream and
1 We have changed the name of the architecture from SUCCESS to
SUCCESS-HPON to distinguish it from other architectures under the same
research initiative of SUCCESS at PNRL, Stanford.
2 In this paper we use the term SUCCESS WDM-PON to denote the WDMPON under the SUCCESS-HPON architecture.
2
downstream transmissions, however, pose a great challenge in
designing scheduling algorithms: A scheduling algorithm for
the SUCCESS WDM-PON has to keep track of the status of all
shared resources (i.e., tunable transmitters, tunable receivers
and wavelengths assigned to ONUs) and arrange them properly
in both time and wavelength domains to avoid any conflicts
among them for both upstream and downstream transmissions.
While many researchers have studied the issue of scheduling
messages in both time and wavelength domains in network
architectures based on tunable transmitters and/or receivers
(e.g., [2]–[5]), only a few schemes have been proposed to support variable-length message transmissions without segmentation and reassembly processes. In [4], we studied scheduling
algorithms for unslotted carrier sense multiple access with
collision avoidance (CSMA/CA) with backoff media access
control (MAC) protocol to address the issues of fairness and
bandwidth efficiency in multiple-access WDM ring networks.
In [5], the authors studied distributed algorithms for scheduling
variable-length messages in a single-hop multichannel local
lightwave network with a focus on reducing tuning overhead.
To the best of our knowledge, however, scheduling algorithms
for a network where tunable transmitters are used for both
upstream and downstream transmissions as in the SUCCESS
WDM-PON, have not been investigated by other researchers.
In [1] we proposed a sequential scheduling algorithm for
the SUCCESS WDM-PON, which emulates a virtual global
first-in-first-out (FIFO) queueing for all incoming frames. In
this algorithm incoming frames are scheduled sequentially
in the order of arrival at the OLT. This original sequential
scheduling algorithm is simple to implement, but suffers from
poor transmission efficiency and fairness guarantee between
upstream and downstream traffic.
To address the limitations of the original sequential scheduling algorithm, we propose in this paper two new scheduling
algorithms – batching earliest departure first (BEDF) and
sequential scheduling with schedule-time framing (S3 F). The
key idea in the design of BEDF is to provide room for
optimization and priority queueing by scheduling over more
than one frame: In BEDF, frames arriving at the OLT during
a batch period are stored in virtual output queues (VOQs)
and scheduled at the end of the batch period, which allows
in scheduling to select the best frame according to a given
optimal scheduling policy from the batch of multiple frames
in the VOQs. We choose the EDF as an optimal scheduling
policy to minimize the unused time of the tunable transmitters.
The throughput versus scheduling delay tradeoff is a major
design issue in BEDF.
In S3 F, considering the high complexity of the BEDF
optimization process, we adopt the sequential scheduling mode
as in the original sequential scheduling algorithm, but use
VOQs to provide memory space protection among traffic flows
as in BEDF and a granting scheme together with schedule-time
framing for both upstream and downstream traffic to reduce
overhead due to framing and guard bands.
The rest of the paper is organized as follows. In Section
II we provide a high-level overview of the SUCCESS-HPON
architecture and review the MAC protocol, frame formats and
original sequential scheduling algorithm for the WDM-PON
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
under the SUCCESS-HPON architecture. In Section III we
describe the BEDF and S3 F scheduling algorithms based on
the system model and procedures used in the description of
the original sequential scheduling algorithm in Section II. In
Section IV, we provide the results of the performance analysis
of the designed scheduling algorithms through simulations.
Section V summarizes our work in this paper and discusses
future directions for further studies.
II. WDM-PON U NDER SUCCESS-HPON
A RCHITECTURE
A. Overall Architecture
A high-level overview of the SUCCESS-HPON, including
TDM-PONs and WDM-PONs as its subsystems with wavelength allocations, is shown in Fig. 1. A single-fiber collector
ring with stars attached to it formulates the basic topology.
The collector ring strings up RNs, which are the centers of
the stars. The ONUs attached to the RN on the west side of
the ring talk and listen to the transceivers on the west side of
the OLT, and likewise for the ONUs attached to the RN on
the east side of the ring. Logically there is a point-to-point
connection between each RN and the OLT. No wavelength is
reused on the collector ring. When there is a fiber cut, the
affected RNs will switch to the transceivers on the other side
of the OLT for continuous operations as soon as they sense a
signal loss.
The RN for TDM-PON has a pair of CWDM band splitters
to add and drop wavelengths for upstream and downstream
transmissions, respectively. On the other hand, the RN for
WDM-PON has one CWDM band splitter, adding and dropping a group of DWDM wavelengths within a CWDM grid,
and a DWDM MUX/DEMUX device, i.e., arrayed waveguide
grating (AWG), per PON. Each ONU has its own dedicated
wavelength for both upstream and downstream transmissions
on a DWDM grid to communicate with the OLT. Since the
insertion loss of a typical AWG is roughly 6 dB regardless of
the number of ports, AWGs with more than eight ports will
likely be employed to enjoy better power budget compared to
passive splitters.
Fig. 2 shows block diagrams of the portion of the OLT and
the ONU for the SUCCESS WDM-PON. Tunable components,
such as fast tunable lasers and tunable filters are employed for
DWDM channels. Because the average load of the network is
usually lower than the peak load [6], we can expect statistical
multiplexing gain by sharing tunable components at the OLT,
which also reduces the total system cost by minimizing the
transceiver count for a given number of ONUs and user
demand on bandwidth. Downstream optical signals from the
tunable transmitters in DWDM channels enter both ends of
the ring through passive splitters and circulators. Upstream
optical signals from the ring pass the same devices but in
reverse order and are separated from the downstream signals
by the circulators. The scheduler controls the operation of
both tunable transmitters and tunable receivers based on the
scheduling algorithms that will be described in Section III.
Note that the tunable transmitters at the OLT are used
for both downstream frames and CW optical bursts to be
KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON
3
λ 3, λ 4, …
Central
Office
λ1, λ2
λ*3
λ3
λ*3, λ4, …
λ*1, λ2
RN
λ41
λ4
λ1
λ*1 RN
λ42
RN
λ43
λ2
RN
λ21
Fig. 1.
TDM-PON ONU
RN TDM-PON RN
λ22
λ23
WDM-PON ONU
RN WDM-PON RN
Overview of SUCCESS-HPON.
Overhead
M:2 Passive Splitter
For
Downstream
...
Tunable
Transmitter M
Upstream
Grant Queues
8-Bit
Preamble
...
...
Downstream
Traffic Queues
Tunable
Transmitter 1
8-Bit
Flags
Ethernet Frame
16-Bit
Delimiter
…
Ethernet Frame
or
Overhead
8-Bit
Preamble
Circulator
16-Bit
Delimiter
8-Bit
Flags
16-Bit
Grant
Grant (CW)
Scheduler
...
Tunable
Receiver 1
Upstream Traffic
To the Network
Tunable
Receiver N
(a)
N:2 Passive Splitter
Overhead
Upstream Traffic
Queue
For
Upstream
8-Bit
Preamble
16-Bit
Delimiter
16-Bit
Report
Ethernet Frame
…
Ethernet Frame
Order of Transmission
Modulator
Fig. 3.
Frames formats for SUCCESS WDM-PON MAC protocol.
MAC
Circulator
of the SUCCESS WDM-PON, readers are referred to [1].
Burst-Mode
Receiver
Downstream
Traffic
1:2 Passive Splitter
(b)
Fig. 2. Block diagrams of (a) the portion of OLT and (b) the ONU for
SUCCESS WDM-PON.
modulated by the ONUs for their upstream frames. With this
configuration only half-duplex communications are possible at
the physical layer between the OLT and each ONU using a
variation of the time compression multiplexing (TCM) scheme
[7]. Compared to a similar architecture with a two-fiber ring,
two sets of light sources and two sets of MUX/DEMUX
for full-duplex communications [8], our design significantly
lowers deployment cost. As a tradeoff, however, we need a
careful design of a scheduling algorithm to provide efficient
bidirectional communications at the MAC layer.
As discussed before, the ONU has no local light source
and uses an optical modulator to modulate optical CW bursts
received from the OLT for its upstream transmission. A semiconductor optical amplifier (SOA) can be used as a modulator
for this purpose [9]. The ONU MAC protocol block not
only controls the switching between upstream and downstream
transmissions but also coordinates with the scheduler at the
OLT through a polling mechanism.
For implementation details, especially at the physical layer
B. MAC Protocol and Frame Formats for SUCCESS WDMPON
Like APON and EPON systems [10], the SUCCESS WDMPON OLT polls to check the amount of upstream traffic stored
at the ONUs and sends grants – but in the form of optical CW
bursts in this case – to allow the ONUs to transmit upstream
traffic. Since there is neither a separate control channel nor
a control message embedding scheme using escape sequences
as in [11], the MAC protocol has to rely on in-band signaling
and uses the frame formats shown in Fig. 3, where the report
and grant fields are defined for the polling process.3 Note that
the 1-bit ‘ID’ field in [1] for downstream frames has been
extended to 8-bit flags for future extensions: Now the ‘Frame
Type’ field of the flag is used to indicate whether this frame is
for normal data traffic or not. Usage of the fields in the 8-bit
flags is summarized in Table I.
Each ONU reports the amount of traffic waiting in its
upstream traffic queue in octets through the report field in an
upstream frame when the ‘Force Report’ field of a received
downstream frame is set4 , and the OLT uses the grant field
3 In this paper, we assume that Ethernet frames are carried in the payload
part of SUCCESS WDM-PON frames. Note that, however, any other protocol
frame or packet. e.g., IP packets, can be encapsulated and carried in a
SUCCESS WDM-PON frame because it does not depend on any specific
layer 2 or 3 protocols unlike APON or EPON.
4 In this paper, we assume this field is always set.
4
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
TABLE I
8-B IT F LAGS IN D OWNSTREAM SUCCESS WDM-PON F RAME
Bit
Field
0-3
Frame Type
4
Force Report
5
6
7
Unused
Unused
Unused
0
1
2-15
0
1
-
Values
Normal Data
Grant
Unused
No action required
ONU should report in the corresponding
upstream frame
-
to indicate the actual size of each grant (also in octets). Note
that, as shown in Fig. 3, the length of the whole CW burst
corresponds to that of all upstream Ethernet frames (i.e., the
size of grant) plus the report field and the overhead.
We use two control parameters to govern the polling process
consisting of reporting and granting operations as follows:
• ON U T IM EOU T : The OLT maintains one timer per
ONU and resets it whenever a grant frame is sent downstream to an ONU. It clears the timer when the corresponding upstream frame with a nonzero report field is received. If the timer expires after the ON U T IM EOU T
period, which means either there was no upstream traffic
when the ONU received a grant frame or the report
message was lost during the transmission to the OLT,
the OLT sends a new grant to poll that ONU again and
resets the timer. This parameter keeps the polling process
going on even in the case of the loss of polling messages
and bounds the maximum polling cycle. It also affects the
average packet delay of upstream traffic when the system
is under light load.
• M AX GRAN T : This parameter limits the maximum
size of a grant (i.e., the payload part of the CW burst)
for ONU upstream traffic.
C. Original Sequential Scheduling Algorithm
Here we describe how the scheduling of transmission and/or
reception of a SUCCESS WDM-PON frame is done under
the original sequential scheduling algorithm proposed in [1].
This will be the basic building block of the new scheduling
algorithms in Section III. For this purpose, we consider
a SUCCESS WDM-PON system with W ONUs (therefore
W wavelengths), M tunable transmitters, and N tunable
receivers. Because the tunable transmitters are used for both
upstream and downstream traffic but tunable receivers are only
for upstream traffic, we usually need more transmitters than
receivers, i.e., W ≥ M ≥ N . We include in the algorithm
description the guard band of G ns between consecutive
SUCCESS WDM-PON frames that accounts for the effects
of unstable local ONU clock frequencies and tuning time of
tunable transmitters and receivers at the OLT.
We define the following arrays of global status variables
used in the algorithm description:
• CAT: Array of Channel Available Times. CAT[i]=t,
where i = 1, 2, ..., W , means that the wavelength λi will
be available for transmission after time t.
TAT: Array of Transmitter Available Times. TAT[i]=t,
where i = 1, 2, ..., M , means that the ith tunable transmitter will be available for transmission after time t.
• RAT: Array of Receiver Available Times. RAT[i]=t,
where i = 1, 2, ..., N , means that the ith tunable receiver
will be available for reception after time t.
• RTT: Array of round trip times (RTTs) between the OLT
and the ONUs. RTT[i] denotes the RTT between the OLT
and the ith ONU.
When scheduling each SUCCESS WDM-PON frame, we
first select the earliest available transmitter and receiver. Assuming that the ith transmitter and the jth receiver are the
earliest available transmitter and receiver respectively, we can
obtain the transmission time t of a SUCCESS WDM-PON
frame destined for the kth ONU as follows:

max(RAT [j] + G − RT T [k] − GOH ,




T AT [i] + G, CAT [k] + G)

if the frame is a grant for upstream traffic,
t=


max(T AT [i] + G, CAT [k] + G)



if the frame is for downstream traffic,
(1)
where GOH is a transmission delay for the grant overhead
consisting of the overhead, 8-bit flags and grant fields of the
SUCCESS WDM-PON grant frame at a line rate R bit/s. If
the frame is a grant frame for upstream traffic, the reception
of the corresponding upstream frame from the ONU should
be scheduled at t + GOH + RT T [k].
After scheduling the frame transmission and/or reception,
the related status variables should be updated as follows:
•
T AT [i] = t + l/R
,
CAT [k] = t + l/R
(2)
and if the frame is a grant frame for upstream traffic,
RAT [j] = t + l/R + RT T [k],
(3)
where l is the length of the whole frame in bits.
Fig. 4 illustrates the timing relations among tunable transmitters and receivers, and frames over channels through an
example: At t1 , a report for upstream traffic from ONU4
arrives at the OLT. First, the scheduler at the OLT checks
the transmitter availability and finds that TX3 is available
now. Then, it checks the receiver availability and finds that
RX1 will be available at t0 + GOH + RT T1 + lcw1 . Then,
it also checks the channel availability and finds that λ4 is
available now. Finally, based on all these information, the
scheduler schedules the transmission of a grant frame at
t0 + RT T1 + lcw1 + G − RT T4 through TX3 on λ4 and the
reception of a corresponding upstream frame from ONU4 at
t0 + GOH + RT T1 + lcw1 + G. Pseudocode for the whole
procedure is given in Fig. 5.
III. D ESIGN OF N EW BATCH AND S EQUENTIAL
S CHEDULING A LGORITHMS FOR SUCCESS WDM-PON
In this section we describe two new scheduling algorithms
– the BEDF and the S3 F – designed in order to improve the
following performance measures over the original sequential
scheduling algorithm: 1) Fairness guarantee between upstream
KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON
λ1
RTT1
λ4
lcw2
RX2
λ2
λ1
TX1
λ1
A. Batching Earliest Departure First (BEDF) Scheduling
CW1
t0
TX2
TX3
transmission capacity. To address the issue of memory space
protection among traffic flows, we base both the scheduling
algorithms on VOQing with one VOQ per traffic flow either
upstream or downstream for an ONU.
lcw3
lcw1
RX1
λ2
CW2
λ4
CW3
G
t1
New transmission
scheduled!
RTT4
5
Time
Grant Overhead
(= OH + Flags + Grant)
Fig. 4. An example of the original sequential scheduling at t1 for a system
with W = 4, M = 3 and N = 2.
begin
k ←− destination(frame);
l ←− length(frame);
tnow ←− current time;
for i = 1 to W do CAT [i] ←− max(tnow , CAT [i]);
for i = 1 to M do T AT [i] ←− max(tnow , T AT [i]);
for i = 1 to N do RAT [i] ←− max(tnow , RAT [i]);
select i s.t. T AT [i] ≤ T AT [m] ∀m = 1, . . . , M ;
if the frame is a grant for upstream traffic then
select j s.t. RAT [j] ≤ RAT [n] ∀n = 1, . . . , N ;
t ←− max(RAT [j] + G − RT T [k] −
GOH , T AT [i] + G, CAT [k] + G);
schedule reception at time t + GOH + RT T [k] with
the jth receiver via the wavelength λk ;
RAT [j] ←− t + l/R + RT T [k];
else
t ←− max(T AT [i] + G, CAT [k] + G);
end
T AT [i] ←− t + l/R;
CAT [k] ←− t + l/R;
schedule transmission at time t with the ith transmitter via
the wavelength λk ;
end
Fig. 5: Pseudocode for the original sequential scheduling
algorithm.
and downstream traffic flows for a given ONU and 2) overall
throughput. Here we use a simple but intuitive definition of
’fairness’: On the assumption that all received traffic flows
are legitimate, the scheduler assigns bandwidth so that the
resulting throughput of a traffic flow should be in proportion to its incoming rate. By ‘traffic flow’ we mean the
aggregated traffic between the OLT and each ONU in each
direction (upstream or downstream); thus, the scheduler at
the OLT deals with a total of 2W separate traffic flows. In
the original sequential scheduling algorithm, a downstream
Ethernet frame is encapsulated in a SUCCESS WDM-PON
frame immediately after its arrival and put into a global
FIFO queue that is shared by all upstream and downstream
traffic. As the simulation results in [1] show, the lack of
protection for memory space among traffic flows leads into
poor fairness between upstream and downstream traffic. Also,
because there is no room for optimization in scheduling, the
maximum achievable throughput is much lower than the total
The idea of batch scheduling, where a batch of arrived
messages during a certain period forms a task set to which
a scheduling algorithm is applied, has been already studied
in [12], but in a slightly different context where the main
concern is the reduction of the frequency and complexity of
the scheduling algorithm at the cost of deferring consideration
of new tasks. On the other hand, the major concern in our
design of the BEDF is to provide room for optimization in
scheduling by forming a task set consisting of multiple frames
by batching process. Rather than sequentially scheduling each
frame in the order of arrival, by forming a batch of arrived
frames and searching for a frame with an optimal value
according to a given scheduling policy, we can optimize
the scheduling performance. Because transmission efficiency
under the constraint of sharing limited resources is one of
the major design goals, we select the EDF as an optimal
scheduling policy to minimize the time when transmitters and
channels are wasted.
Building upon the basic sequential scheduling algorithm description in Section II, we can describe the BEDF scheduling
algorithm as follows: At the end of each batch period,
Step 1 Choose the earliest available transmitter and receiver
(i.e., whose TAT and RAT are minimum).
Step 2 Given the earliest available transmitter and receiver,
calculate a possible transmission time using Eq. 1 for
the first unscheduled frame in each VOQ that is not
marked as ‘Unschedulable’.
Step 3 Select the frame with the minimum transmission time
(i.e., the earliest departure time) and if the transmission
time is within the boundary of the next batch period,
schedule its transmission; otherwise, cancel its scheduling and mark the corresponding VOQ as ‘Unschedulable’. If the scheduled frame is a grant for upstream
traffic, schedule the reception of the corresponding upstream frame from the ONU after GOH + RT T from
its transmission time.
Step 4 Update the status variables using Eq. 2 for the transmitter, the channel and if needed, the receiver.
Step 5 Repeat the whole procedures from the steps 1 through
4 until there is no unscheduled frame or all VOQs are
marked as ‘Unschedulable’.
Note that in contrast to the batch scheduling scheme
proposed in [12], once the scheduled transmission time exceeds the boundary of the next batch period, we cancel
the scheduling of that frame, mark the corresponding VOQ
as ‘Unschedulable’, and exclude the frames in that VOQ
in further scheduling during the current batch period. This
prevents the frames arriving in the current batch period from
consuming the resources available in the next batch period
and therefore provides some protection for network resources
between batches of frames. The interleaving of scheduling and
6
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
Snapshots of VOQs
at the beginning of batch periods
Scheduling results of
the 1st batch
1st batch period
2nd batch period
Scheduling results of the
2nd batch + remnants
from the 1st batch
3rd batch beriod
Time
Fig. 6. A scheduling example showing the interleaving of scheduling and
transmission phases in BEDF.
transmission phases in BEDF is illustrated through an example
in Fig. 6.
B. Sequential Scheduling with Schedule-time Framing (S3 F)
The major downside of BEDF compared to the original
sequential scheduling algorithm, is the higher computational
complexity due to the optimization process in scheduling to
search for the frame with the earliest departure time. For
example, in the worst case where all VOQs have frames
to schedule, the BEDF needs roughly 2W times as many
calculations as the original sequential scheduling algorithm to
schedule one frame.
Here we propose S3 F, an improved sequential scheduling
algorithm. S3 F is based on the sequential scheduling mode,
but unlike the original sequential scheduling algorithm, the
scheduling is done at the end of each frame transmission
(except in the case when a frame arrives at an empty VOQ,
where the scheduling is done immediately after its arrival). It
also uses grants for downstream traffic as well as upstream
traffic to provide better fairness guarantee and schedule-time
framing of downstream Ethernet frames in the VOQs to overcome the low transmission efficiency of the original scheduling
algorithm. Due to the memory space protection among traffic
flows through VOQing, the S3 F can provide better fairness
guarantee than the original sequential scheduling algorithm.
For the purpose of granting downstream traffic, we maintain
a downstream transmission counter per downstream VOQ.
When granting upstream traffic based on a received request
from an ONU, we also grant downstream traffic as well based
on the VOQ status at the time of the arrival of the report
message. Granting downstream traffic is done by setting the
said grant counter to the minimum of the queue length of
the VOQ and M AX GRAN T . When scheduling downstream
transmission, the grant counter value controls the number
of Ethernet frames to be scheduled and transmitted in one
SUCCESS frame through the procedure shown in Fig. 7.
Note that the procedure in Fig. 7 allows at least one Ethernet
frame transmission to be scheduled, irrespective of the value
of the downstream transmission counter (dsT xCtr[i]). This
allows the OLT to transmit downstream traffic for a particular
ONU even when there is no granting for the ONU: In the
case where there is no request for upstream traffic from that
begin
if VOQ[i] is not empty then
numBits ←− 0;
pos ←− 0;
ptr ←− &ethF rame(V OQ[i], pos);
repeat
dsT xCtr[i] ←− dsT xCtr[i] − length(∗ptr);
numBits ←− numBits + length(∗ptr);
pos ←− pos + 1;
ptr ←− &ethF rame(V OQ[i], pos);
if ptr is NULL then
// no more frames to schedule
exit the loop;
end
until dsT xCtr[i] < length(∗ptr);
schedule the transmission of a SUCCESS frame whose
payload length is numBits;
// using the sequential scheduling
algorithm in Fig. 5
store pos for the scheduled transmission later;
end
end
Fig. 7: Pseudocode for the scheduling of downstream data
frame transmission for a given channel i in S3 F. Note that
pos denotes the relative position of an Ethernet frame from
the head of the VOQ (e.g., pos = 0 means it is the headof-line (HOL) frame.).
ONU and therefore no granting, it is still possible to transmit
downstream frames, but one at a time.
The benefit of granting and schedule-time framing of downstream traffic is three-fold. First, by encapsulating multiple
Ethernet frames in one SUCCESS WDM-PON frame as in
upstream transmission, we can reduce the overhead due to
the SUCCESS WDM-PON framing and the guard bands.
Second, we can also reduce the waste of tunable transmitters
and channels and therefore minimize scheduling delays by
preventing spread of smaller frames over multiple transmitters
and channels. This is well illustrated in the examples in Fig.
8. Here we assume that for both the framing schemes, there
are three Ethernet frames at t1 in the VOQ for channel 1,
four Ethernet frames at t2 for channel 2 and one Ethernet
0
frame at t3 for channel 3. ti denotes the resulting scheduled
transmission time of the first frame for channel i, so the
0
corresponding scheduling delay is given by ti − ti . The effects
of the inefficient use of transmitters in the arrival-time framing,
where each incoming Ethernet frame is encapsulated in a
SUCCESS WDM-PON frame at the moment of its arrival,
become clear when we compare its scheduling delay of the
frame for channel 3 in (b) to that of the schedule-time framing
in (a). Third, by integrated and intelligent granting of both
upstream and downstream traffic, we could better control the
whole traffic flows for guarantee of fairness and support of
quality of service (QoS) in the future.
IV. P ERFORMANCE A NALYSIS
We have developed a simulation model for the performance
evaluation of the designed scheduling algorithms using Objective Modular Network Testbed in C++ (OMNeT++) [13].
The simulation model is for a WDM-PON system under the
KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON
Time
7
Time
EF
H2
G
EF
H3
t 3’
EF
EF
H2
G
EF
EF
EF
H2
H1
G
EF
EF
EF
EF
t3’
t3
t 2’
t2
t 1’
t1
EF
H3
EF
EF
EF
t 2’
Ethernet
Frame
EF
G
t2
H1
Hi
SUCCESS WDM-PON
Frame Header for Channel i
G
1
H1
G
t3
H2
EF
H2
2
3
(a)
TX
t 1’
t1
H1
G
1
2
(b)
3
TX
0
Fig. 8. Effects of the framing on the scheduling delays where ti − ti is the scheduling delay of the first frame for channel i: (a) For the schedule-time
framing and (b) for the arrival-time framing.
TABLE II
D EFAULT PARAMETER VALUES FOR S IMULATIONS
TABLE III
T UNABLE T RANSMITTER AND R ECEIVER C ONFIGURATIONS FOR
S IMULATIONS
Parameter
Value
R
10 Gbps
W
16
G
50 ns
Q
10 Mbytes
ON U T IM EOU T
1 ms
Description
Line rate for upstream and
downstream transmissions
Number of ONUs and Wavelengths
Guard band between adjacent
SUCCESS WDM-PON frames
(including tuning overhead of
tunable transmitters and receivers)
Size of OLT VOQs and ONU
upstream traffic queue
Expiration time of ONU timer
SUCCESS-HPON architecture with 16 ONUs. The ONUs are
divided into four groups with 4 ONUs per group and placed
from the OLT 5 km, 10 km, 15 km, and 20 km, respectively.
The line rate R for both upstream and downstream transmissions is set to 10 Gbps. The ON U T IM EOU T and the guard
band G are set to 1 ms and 50 ns respectively.
As for traffic modeling, we choose a simple Poisson process
for IP packet generation because the major purpose of the simulations in this paper is to compare the relative performances
of the designed scheduling algorithms rather than to investigate
the actual performances under realistic conditions. The packet
size distribution is configured to match that of a measurement
trace from one of MCI’s backbone OC-3 links [14], and the
destination distribution for downstream packets at the OLT
follows a uniform distribution. The generated IP packets are
encapsulated in Ethernet frames before their arrival at the OLT
and ONUs. The size of VOQs at the OLT and the upstream
traffic queue at the ONU is set to 10 megabytes. The default
parameter values for the simulations are summarized in Table
II.
For each scheduling algorithm, we ran simulations for
several different configurations of tunable transmitters and
Number of
Transmitters (M )
4
4
8
8
Number of
Receivers (N )
2
4
4
8
Total Offered Load
[Gbps]
1, 2, ..., 40
1, 2, ..., 40
1, 2, ..., 80
1, 2, ..., 80
receivers. Due to space limitation, however, we show the
simulation results for the chosen subsets of configurations
summarized in Table III in this paper. The total offered load
is the sum of the arrival rates for downstream and upstream
traffic, where we fix the ratio of the former to the latter
to 2 considering that there is more downstream traffic than
upstream traffic in access networks. The maximum load for
each configuration is set to the total transmitter capacity
(= M × R), which slightly overloads the system.
We first investigate the effects of important control parameters on the performances of the scheduling algorithms
– the batch period for the BEDF and the maximum grant
size (M AX GRAN T ) for the S3 F – to determine optimal
parameter values. Then, based on the optimal values of those
control parameters, we compare the performances of the two
scheduling algorithms.
A. Effects of Batch Period on BEDF Performance
To investigate the effects of the batch period on the BEDF
performance, we ran simulations for three different batch periods and show the throughput and delay results in Figs. 9 and
10, respectively. In the simulations we set M AX GRAN T to
120 percent of the average amount of ONU incoming traffic
during a given batch period when the system load is maximum.
For example, with M = 8, the maximum system load is
80 Gbps and the average ONU incoming rate is 1.67 Gbps.
For a batch period of 10 ms, because the average amount of
8
the ONU incoming traffic during this period is 16.7 Mbits,
M AX GRAN T is set to 20 Mbits. In this way we can
minimize the effects of M AX GRAN T in our investigation
of the effects of the batch period on the BEDF performances.
Note that M AX GRAN T should be large enough to
handle the longest possible Ethernet frame. Otherwise that
long Ethernet frame in an ONU upstream queue would block
the whole upstream traffic from the ONU. Likewise, the batch
period should be long enough to handle the maximum size
requests. This implies that there is a limit in the minimum
length of the batch period.
From the simulation results we found that the batch period
of 1 ms provides the best overall performances of the three
periods considered. Unlike our intuitive expectation, the effects
of longer batch period on the actual transmission performances
are not always positive: As the batch period increases, the
negative effect of increasing delay becomes dominating over
the effect of better optimization in scheduling with a bigger
task set consisting of more frames. In general, however,
the performance differences are not significant especially in
throughput. We also observed that the number of receivers,
given the number of transmitters, has minor impacts on the
BEDF performances.
B. Effects of Maximum Grant Size on S3 F Performance
In Figs. 11 and 12 we show the throughput and delay performances of S3 F with four different values of M AX GRAN T .
In Fig. 11 we can see that the total throughput approaches
the total transmitter capacity in most of the cases except
when the maximum grant size is less than 5 Mbits and the
number of receivers is half the number of transmitters. This
higher transmission efficiency has been achieved because as
we expected, the schedule-time framing combined with the
granting scheme efficiently reduces the framing and the guard
band overhead in downstream transmission by encapsulating
multiple Ethernet frames in one SUCCESS WDM-PON frame.
Note that the upstream traffic is no longer penalized as in
[1] by the downstream traffic even when the system is highly
overloaded, which results from the memory space protection
by VOQing as well as the change in downstream frame
scheduling. From the results, we also observed that when
the maximum grant size is less than 5 Mbits and there are
fewer receivers than transmitters, the downstream throughput
actually decreases after reaching its maximum as the system
load further increases.
Although the overall throughput and the fairness between
upstream and downstream traffic have been greatly improved
by the granting with schedule-time framing, one may wonder
whether there is any downside in the delay performance due
to the effects of large transmission delay of possibly very long
SUCCESS WDM-PON frames. From the delay performances
shown in Fig. 12, we can verify that this is not the case: In
fact, the total average packet delay is maintained well below
5 ms until the total offered load exceeds around 95 percent
of the total transmitter capacity, again except for those cases
where the maximum grant size is less than 5 Mbits and the
number of receivers is half the number of transmitters. Unlike
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
traditional TDM-PONs, because there can exist multiple channels simultaneously between the OLT and the ONUs in the
SUCCESS WDM-PON, giant frames using one channel hardly
block other frames. The effects of the number of simultaneous
channels between the OLT and the ONUs, which is directly
related with the number of transmitters and receivers, become
clear when we compare the results in Fig. 12 where the average
packet delay for the larger number of transmitters and receivers
in (a) is less than that for the smaller number of transmitters
and receivers in (b).
Note that the effects of the maximum grant size on the
downstream packet delay are opposite to those on the upstream
packet delay: As the maximum grant size increases, the
downstream packet delay decreases while the upstream packet
delay increases. In our simulations where the downstream
traffic dominates over the upstream traffic, the opposite effects
on the upstream traffic are negligible in the total average
packet delay. But with different traffic conditions, one may
have to take into account these opposite effects.
Also note that the initial dip in uptream packet delay under
light system load: Under the current granting scheme described
in this paper, there is no further grant frame generated for an
ONU when the ONU reports to the OLT no frame waiting in
the upstream traffic queue. Therefore the regular polling cycle
of granting and reporting pauses until the ONU timer at the
OLT expires, when the polling cycle is restarted by sending
a new polling message to the ONU. This whole procedure
related with the ONU timer expiration results in the increase
in upstream packet delay. This can be controlled by adjusting
the ON U T IM EOU T value or a new granting scheme that
generates a certain minimum size grant even when the ONU
reports no frame in the upstream traffic queue.
In general the effects of the maximum grant size are
salient with less number of receivers for the given number
of transmitters, which is different from the results for the
original sequential scheduling algorithm where the number
of receivers, given the number of transmitters, has marginal
impacts on the overall performances: This is because, when
there is less number of receivers, the polling period for the
upstream traffic becomes longer and the maximum grant size
is a more limiting factor in this case than in the case with a
longer polling period.
Considering throughput and delay performances altogether,
we can conclude that the maximum grant size of 5 Mbits
provides the best overall performance for all the configurations
considered in the simulations.
C. Performance Comparison between BEDF and S3 F
Scheduling Algorithms
We compare the performances of the BEDF and the S3 F
with the optimal parameter values – 1-ms batch period for
the BEDF and 5-Mbit M AX GRAN T for the S3 F – through
simulations and show the results in Figs. 13 and 14. The results
for the original sequential scheduling algorithm in [1] are also
shown in the figures for the purpose of comparison.
From the results we can see that both the BEDF and S3 F
greatly improves the scheduling performances over the original
KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON
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N=4, BP=5 ms
N=4, BP=1 ms
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12
Upstream Throughput [Gbps]
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0
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10
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40
50
60
70
80
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(a)
Fig. 9.
N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
N=2, BP=10 ms
N=2, BP=5 ms
N=2, BP=1 ms
35
Total Throughput [Gbps]
Total Throughput [Gbps]
40
N=8, BP=10 ms
N=8, BP=5 ms
N=8, BP=1 ms
N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
70
9
0
5
10
15
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(b)
Throughput of BEDF scheduling algorithm for total, downstream and upstream traffic: (a) For M = 8; (b) for M = 4.
sequential scheduling algorithm. The maximum achievable
total throughput now approaches the total transmitter capacity
and the upstream traffic is no longer penalized by the downstream traffic as the system load increases. The performance
improvement of the new scheduling algorithms becomes also
clear in average packet delay. Both the BEDF and S3 F maintain the total average packet delay below 2 ms until the system
load exceeds 87.5 percent of the total transmitter capacity for
all the configurations in consideration.
The comparison study shows that, of the two scheduling
algorithms, the S3 F provides better overall performances than
the BEDF in terms of both throughput and average packet
delay. Considering the lower complexity of the sequential
scheduling mode and the potential for better control of QoS
and fairness through integrated granting of both upstream
and downstream traffic flows, we can conclude that the S3 F
is a better choice for a scheduling algorithm in practical
implementations.
V. C ONCLUSIONS
We have presented the results of the design and performance
analysis of the two new scheduling algorithms – BEDF and
S3 F – providing efficient and fair bidirectional communications between the OLT and the ONUs in WDM-PON under
the SUCCESS-HPON architecture. The major design goal is
to overcome the low transmission efficiency and the poor
10
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N=8, BP=10 ms
N=8, BP=5 ms
N=8, BP=1 ms
N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
20
N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
N=2, BP=10 ms
N=2, BP=5 ms
N=2, BP=1 ms
35
Total Average Packet Delay [ms]
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N=4, BP=5 ms
N=4, BP=1 ms
N=2, BP=10 ms
N=2, BP=5 ms
N=2, BP=1 ms
40
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N=8, BP=10 ms
N=8, BP=5 ms
N=8, BP=1 ms
N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
20
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Upstream Average Packet Delay [ms]
Upstream Average Packet Delay [ms]
10
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N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
N=2, BP=10 ms
N=2, BP=5 ms
N=2, BP=1 ms
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30
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50
60
70
80
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(a)
Fig. 10.
5
50
N=8, BP=10 ms
N=8, BP=5 ms
N=8, BP=1 ms
N=4, BP=10 ms
N=4, BP=5 ms
N=4, BP=1 ms
25
0
Downstream Average Packet Delay [ms]
Downstream Average Packet Delay [ms]
30
20
0
5
10
15
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(b)
Average packet delay of BEDF scheduling algorithm for total, downstream and upstream traffic: (a) For M = 8; (b) for M = 4.
fairness guarantee between upstream and downstream traffic
flows of the original sequential scheduling algorithm proposed
in [1]. To achieve this goal, we adopt batch scheduling mode
in BEDF to do optimization in scheduling with a batch of
frames. In S3 F we maintain sequential scheduling mode as in
the original sequential scheduling algorithm but use grants for
downstream traffic, in addition to upstream traffic, together
with schedule-time framing to reduce the overhead due to
framing and guard bands. We base both scheduling algorithms
on VOQing to separate and protect memory spaces among
traffic flows.
Through simulations we found that in BEDF, the effects
of the batch period on the throughput are not significant; on
the other hand, the average packet delay is strongly dependent
upon the size of the batch period. The simulation results also
showed that the number of receivers, given the number of
transmitters, has negligible effects on the performances. Of the
three batch periods considered, 1-ms batch period provides the
best overall performances.
In S3 F we investigated the effects of the maximum grant
size, (i.e., M AX GRAN T ). The simulation results showed
that as M AX GRAN T increases, the throughput also increases; as for delay, the downstream delay decreases but the
upstream delay increases. In most of the cases we can see that
the total throughput approaches the total transmitter capacity
except when the maximum grant size is less than 5 Mbits and
KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON
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N=8, MAX_GRANT=5 Mb
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N=8, MAX_GRANT=1 Mb
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N=4, MAX_GRANT=5 Mb
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N=4, MAX_GRANT=1 Mb
N=2, MAX_GRANT=∞
N=2, MAX_GRANT=5 Mb
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N=2, MAX_GRANT=1 Mb
12
Upstream Throughput [Gbps]
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14
N=8, MAX_GRANT=∞
N=8, MAX_GRANT=5 Mb
N=8, MAX_GRANT=2 Mb
N=8, MAX_GRANT=1 Mb
N=4, MAX_GRANT=∞
N=4, MAX_GRANT=5 Mb
N=4, MAX_GRANT=2 Mb
N=4, MAX_GRANT=1 Mb
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Upstream Throughput [Gbps]
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Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(a)
Fig. 11.
N=4, MAX_GRANT=∞
N=4, MAX_GRANT=5 Mb
N=4, MAX_GRANT=2 Mb
N=4, MAX_GRANT=1 Mb
N=2, MAX_GRANT=∞
N=2, MAX_GRANT=5 Mb
N=2, MAX_GRANT=2 Mb
N=2, MAX_GRANT=1 Mb
35
Total Throughput [Gbps]
Total Throughput [Gbps]
40
N=8, MAX_GRANT=∞
N=8, MAX_GRANT=5 Mb
N=8, MAX_GRANT=2 Mb
N=8, MAX_GRANT=1 Mb
N=4, MAX_GRANT=∞
N=4, MAX_GRANT=5 Mb
N=4, MAX_GRANT=2 Mb
N=4, MAX_GRANT=1 Mb
70
11
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20
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(b)
Throughput of S3 F scheduling algorithm for total, downstream and upstream traffic: (a) For M = 8; (b) for M = 4.
the number of receivers is half the number of transmitters. In
the case of unlimited granting, we observed minor decrease
in downstream throughput only when the system is highly
overloaded but the overall performance is as good as in the
best case. Considering the throughput and delay performances
altogether, we found that the maximum grant size of 5 Mbits
is the best of the four values considered.
The comparison study with the optimal parameter values –
1-ms batch period for the BEDF and 5-Mbit M AX GRAN T
for the S3 F – showed that S3 F provides better overall performances than BEDF in terms of both throughput and average
packet delay, although the differences between the two are not
significant. Considering the lower complexity of the sequential
scheduling mode and the potential for better control of QoS
and fairness through integrated granting of both upstream and
downstream traffic flows, we can conclude that the S3 F is a
better choice for practical implementations.
As for the fairness issue, we have mainly focused on
the guarantee of fairness between upstream and downstream
traffic flows for a given ONU in this paper. We demonstrated
through simulations that both BEDF and S3 F can guarantee
good fairness between upstream and downstream traffic with
proper choice of control parameter values except when the
system is severely overloaded. Later in practice, however,
fairness guarantee with QoS support among individual user
connections with a guaranteed minimum bandwidth and a
12
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N=4, MAX_GRANT=1 Mb
15
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Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(a)
Fig. 12.
5
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N=8, MAX_GRANT=5 Mb
N=8, MAX_GRANT=2 Mb
N=8, MAX_GRANT=1 Mb
N=4, MAX_GRANT=∞
N=4, MAX_GRANT=5 Mb
N=4, MAX_GRANT=2 Mb
N=4, MAX_GRANT=1 Mb
30
0
Downstream Average Packet Delay [ms]
Downstream Average Packet Delay [ms]
35
20
0
5
10
15
20
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(b)
Average packet delay of S3 F scheduling algorithm for total, downstream and upstream traffic: (a) For M = 8; (b) for M = 4.
weight per connection will be important, which is beyond the
scope of the current paper. In this regard the extension of
the results in [15] for the cousin-fair hierarchical scheduling
in access networks – mainly in the context of the currentgeneration TDM-PONs – to the case of the next-generation
hybrid WDM/TDM networks with shared tunable transmitters
and receivers, including WDM-PONs under the SUCCESSHPON architecture, could be a solution and an interesting
topic for further research.
ACKNOWLEDGMENT
The authors would like to thank the Associate Editor and
anonymous reviewers for their constructive comments. The
authors would also like to thank Mr. Salvatore Rotolo of
STMicroelectronics for his encouragement and support for this
work.
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0
0
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S33F with N=4
S F with N=2
BEDF with N=4
BEDF with N=2
Sequential with N=4
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12
Upstream Throughput [Gbps]
25
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8
6
4
2
0
0
0
10
20
30
40
50
60
70
80
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
0
5
10
15
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(a)
Fig. 13.
15
S33F with N=4
S F with N=2
BEDF with N=4
BEDF with N=2
Sequential with N=4
Sequential with N=2
25
Downstream Throughput [Gbps]
50
Downstream Throughput [Gbps]
30
20
0
Upstream Throughput [Gbps]
S33F with N=4
S F with N=2
BEDF with N=4
BEDF with N=2
Sequential with N=4
Sequential with N=2
35
Total Throughput [Gbps]
Total Throughput [Gbps]
40
S33F with N=8
S F with N=4
BEDF with N=8
BEDF with N=4
Sequential with N=8
Sequential with N=4
70
13
(b)
Comparison of throughput for total, downstream and upstream traffic: (a) For M = 8; (b) for M = 4.
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S3F with N=8
S3F with N=4
BEDF with N=8
BEDF with N=4
Sequential with N=8
Sequential with N=4
50
S33F with N=4
S F with N=2
BEDF with N=4
BEDF with N=2
Sequential with N=4
Sequential with N=2
50
Total Average Packet Delay [ms]
Total Average Packet Delay [ms]
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Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
0
5
45
S33F with N=8
S F with N=4
BEDF with N=8
BEDF with N=4
Sequential with N=8
Sequential with N=4
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Downstream Average Packet Delay [ms]
Downstream Average Packet Delay [ms]
25
80
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35
40
20
25
30
35
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S33F with N=4
S F with N=2
BEDF with N=4
BEDF with N=2
Sequential with N=4
Sequential with N=2
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S3F with N=4
S3F with N=2
BEDF with N=4
BEDF with N=2
Sequential with N=4
Sequential with N=2
15
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5
0
0
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30
40
50
60
70
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Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(a)
Fig. 14.
5
20
S3F with N=8
S3F with N=4
BEDF with N=8
BEDF with N=4
Sequential with N=8
Sequential with N=4
25
0
Upstream Average Packet Delay [ms]
Upstream Average Packet Delay [ms]
30
50
0
5
10
15
20
25
30
Total Offered Load [Gbps] (Downstream Rate:Upstream Rate=2:1)
(b)
Comparison of average packet delay for total, downstream and upstream traffic: (a) For M = 8; (b) for M = 4.
Y. Ye, “Fair queueing with service envelopes (FQSE): A cousin-fair
hierarchical scheduler for subscriber access networks,” IEEE J. Select.
Areas Commun., vol. 22, no. 8, pp. 1497–1513, Oct. 2004.
Kyeong Soo Kim (S’89-M’97) received the B.S.,
M.E., and Ph.D. degrees, all in electronics engineering, from Seoul National University, Seoul, Korea, in 1989, 1991, and 1995, respectively. From
1996 to 1997, he was engaged in development of
multi-channel ATM switching systems as a Post-Doc
researcher at Washington University in St. Louis,
Missouri, where he also taught undergraduate and
graduate courses as an Instructor of Washington
University and Adjunct professor of University Missouri, St. Louis. From 1997 to 2000, he was with
the PON Systems R&D organization of Lucent Technologies as a Member
of Technical Staff and co-developed the first commercial APON-based FiberTo-The-Home/Business (FTTH/B) system, which won the 1999 Bell Labs
President’s Silver Award. Since 2001 he has been with STMicroelectronics,
working on next-generation access and metro area networks as Researcherin-Residence at Stanford Networking Research Center. Dr. Kim has served as
KIM et al.: SCHEDULING ALGORITHMS FOR WDM-PON UNDER SUCCESS-HPON
a Member of the Technical Program Committee for ICC 2005, STFOC 2005,
GLOBECOM 2004, and JCIS 2005, 2003 and 2002. Dr. Kim is a member of
IEEE.
David Gutierrez (S’93) received the B.S. degree
in electrical engineering from the Universidad de
los Andes, Columbia, in 1998, and the M.S. degree
in electrical engineering from Stanford University,
Stanford, CA, in 2002. He is currently working
toward the Ph.D. degree with the Electrical Engineering Department, Stanford University. He has
previously worked with such companies as Nortel,
Reuters, BASF, and AT&T. At Stanford, he has
worked with the Stanford Learning Laboratory and
the Stanford University Medical Media and Information Technologies (SUMMIT) Laboratory. He is a Member of the Photonics
and Networking Research Laboratory (PNRL), where he is working on access
networks. Mr. Gutierrez is also a Fellow of STMicroelectronics, Stanford, CA.
Fu-Tai An (S’98-M’04) received the B.S. degree
in electrical engineering from National Taiwan University, Taiwan, in 1996, and the M.S. and Ph.D.
degrees in electrical engineering from Stanford University, Stanford, CA, in 1998 and 2004. During
the summer of 1999, he helped to start a company,
Excess Bandwidth Company. He was a Member
of Research Staff of the Analog-Front-End Group
for DSL applications. During the summers of 2000
and 2001, he was with Sprint ATL to investigate
high-performance optical transmission gears. He is
now with Marvell Technology Group Ltd. His research interests include
photonic networking, optical communication system design, wireless and
wired communication system design, and mixed-signal circuit design. Dr.
An received the IEEE Lasers & Electro-Optics Society (LEOS) Japanese
Chapter Student Award at the IEEE OptoElectronics and Communication
Conf. (OECC).
Leonid G. Kazovsky (M’80-SM’83-F’91) is a Professor of electrical engineering at Stanford University, Stanford, CA, since 1990. After joining Stanford, he founded the Photonics and Networking Research Laboratory (PNRL). Prior to joining Stanford,
he was with Bellcore (now Telcordia) conducting
research on wavelength-division-multiplexing, highspeed and coherent optical fiber communication systems. He has authored or coauthored two books,
some 150 journal technical papers, and a similar
amount of conference papers. Dr. Kazovsky is a
Fellow of the Optical Society of America (OSA).
15