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 schedul-
ing of variable-length frames in a wavelength division multi-
plexing (WDM)-passive optical network (PON) under Stanford
University aCCESS-Hybrid PON (SUCCESS-HPON) architec-
ture. 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 ef-
ficient 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 (S3F). 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 de-
signed the S3F based on the sequential scheduling mode as in the
original sequential scheduling algorithm proposed earlier. In S3F
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 S3F substantially
improve the throughput and delay performances over the orig-
inal 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. (e-
mail:[e-mail]).
This paper was presented in part at GLOBECOM 2004, Dallas, TX,
November, 2004.
I. INTRODUCTION
Efficient and fair scheduling of variable-length messages
under the constraints of shared resources is critical for the suc-
cess 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) archi-
tecture, which was proposed for next-generation hybrid
WDM/time division multiplexing (TDM) optical access net-
works [1].1The 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 coex-
istence 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 con-
tinuous 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
1We 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.
2In this paper we use the term SUCCESS WDM-PON to denote the WDM-
PON under the SUCCESS-HPON architecture.

2 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
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 sup-
port variable-length message transmissions without segmenta-
tion 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 schedul-
ing algorithm, we propose in this paper two new scheduling
algorithms – batching earliest departure first (BEDF) and
sequential scheduling with schedule-time framing (S3F). 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 S3F, 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
under the SUCCESS-HPON architecture. In Section III we
describe the BEDF and S3F 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 UNDER SUCCESS-HPON
ARCHITECTURE
A. Overall Architecture
A high-level overview of the SUCCESS-HPON, including
TDM-PONs and WDM-PONs as its subsystems with wave-
length 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 drop-
ping 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
RN
RN
RN
RN
λ
*1,
λ
2
λ
1
λ
2
λ
21
λ
22
λ
23
λ
*1
λ
*3,
λ
4, …
λ
1,
λ
2
λ
3,
λ
4, …
λ
4
λ
*3
λ
3
λ
41
λ
42
λ
43
TDM-PON ONU
RN TDM-PON RN
WDM-PON ONU
RN WDM-PON RN
Central
Office
Fig. 1. Overview of SUCCESS-HPON.
Scheduler
Downstream
Traffic Queues
Tunable
Transmitter 1
. . .
. . .
Tunable
Transmitter M
Tunable
Receiver 1
. . .
Tunable
Receiver N
Upstream
Grant Queues Circulator
M:2 Passive Splitter
. . .
Upstream Traffic
To the Network N:2 Passive Splitter
(a)
Burst-Mode
Receiver
Modulator
MAC
Downstream
Traffic
Upstream Traffic
Queue
Circulator
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 semi-
conductor 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
8-Bit
Flags
Ethernet Frame
Grant (CW)
or
8-Bit
Preamble 16-Bit Delimiter
Overhead
16-Bit
Report Ethernet Frame …
For
Downstream
For
Upstream
8-Bit
Preamble 16-Bit Delimiter
Overhead
16-Bit
Grant
8-Bit
Flags
8-Bit
Preamble 16-Bit Delimiter
Overhead
Ethernet FrameEthernet Frame …
Order of Transmission
Fig. 3. Frames formats for SUCCESS WDM-PON MAC protocol.
of the SUCCESS WDM-PON, readers are referred to [1].
B. MAC Protocol and Frame Formats for SUCCESS WDM-
PON
Like APON and EPON systems [10], the SUCCESS WDM-
PON 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.3Note 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
3In 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.
4In this paper, we assume this field is always set.

4 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. X, MONTH 2005
TABLE I
8-BIT FLAGS IN DOWNSTREAM SUCCESS WDM-PON FRAME
Bit Field Values
0 - Normal Data
0-3 Frame Type 1 - Grant
2-15 - Unused
0 - No action required
4 Force Report 1 - ONU should report in the corresponding
upstream frame
5 Unused -
6 Unused -
7 Unused -
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 EOUT : The OLT maintains one timer per
ONU and resets it whenever a grant frame is sent down-
stream to an ONU. It clears the timer when the corre-
sponding upstream frame with a nonzero report field is re-
ceived. If the timer expires after the ON U T IM EOUT
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.
•MAX GRANT : 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 WONUs (therefore
Wwavelengths), Mtunable transmitters, and Ntunable
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 Gns 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 λiwill
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 trans-
mitter 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. As-
suming that the ith transmitter and the jth receiver are the
earliest available transmitter and receiver respectively, we can
obtain the transmission time tof a SUCCESS WDM-PON
frame destined for the kth ONU as follows:
t=
max(RAT [j] + G−RT T [k]−GOH ,
T AT [i] + G, CAT [k] + G)
if the frame is a grant for upstream traffic,
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 Rbit/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 lis the length of the whole frame in bits.
Fig. 4 illustrates the timing relations among tunable trans-
mitters 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 TX3is available
now. Then, it checks the receiver availability and finds that
RX1will be available at t0+GOH +RT T1+lcw1. Then,
it also checks the channel availability and finds that λ4is
available now. Finally, based on all these information, the
scheduler schedules the transmission of a grant frame at
t0+RT T1+lcw1+G−RT T4through TX3on λ4and the
reception of a corresponding upstream frame from ONU4at
t0+GOH +RT T1+lcw1+G. Pseudocode for the whole
procedure is given in Fig. 5.
III. DESIGN OF NEW BATCH AND SEQUENTIAL
SCHEDULING ALGORITHMS FOR SUCCESS WDM-PON
In this section we describe two new scheduling algorithms
– the BEDF and the S3F – 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 5
RX1
RX2
TX1
TX2
TX3t1
RTT4
λ
1
λ
2
λ
4
G
λ
4
λ
1
New transmission
scheduled!
lcw1
RTT1
λ
2
λ
1
Time
Grant Overhead
(= OH + Flags + Grant)
CW1
CW2
CW3
lcw3
lcw2
t0
Fig. 4. An example of the original sequential scheduling at t1for a system
with W= 4,M= 3 and N= 2.
begin
k←− destination(frame);
l←− length(frame);
tnow ←− current time;
for i= 1 to Wdo CAT [i]←− max(tnow , CAT [i]);
for i= 1 to Mdo T AT [i]←− max(tnow , T AT [i]);
for i= 1 to Ndo RAT [i]←− max(tnow , RAT [i]);
select is.t. T AT [i]≤T AT [m]∀m= 1,...,M;
if the frame is a grant for upstream traffic then
select js.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];
elset←− max(T AT [i] + G, CAT [k] + G);
end
T AT [i]←− t+l/R;
CAT [k]←− t+l/R;
schedule transmission at time twith 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 propor-
tion 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 2Wseparate 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
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.
A. Batching Earliest Departure First (BEDF) Scheduling
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 de-
scription 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 schedul-
ing and mark the corresponding VOQ as ‘Unschedu-
lable’. If the scheduled frame is a grant for upstream
traffic, schedule the reception of the corresponding up-
stream frame from the ONU after GOH +RT T from
its transmission time.
Step 4 Update the status variables using Eq. 2 for the transmit-
ter, 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 ex-
ceeds 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