Dynamic Bandwidth Allocation and Quality of
Service Guarantees in Passive Optical Networks
Tomaz Berisa*
*
Department of Telecommunications, Faculty of Electrical Eng. and Computing, University of Zagreb, Zagreb, Croatia
Phone: +385 (1) 6129 843, Email: tomaz.berisa@fer.hr
Abstract—This paper gives an overview of current work and
research conducted on passive optical networks, namely on
dynamic bandwidth allocation and providing quality of
service guarantees. The main areas of interest for
investigating dynamic bandwidth allocation are grant sizing,
grant scheduling and optical network unit queue scheduling.
Regarding quality of service parameters - bandwidth, delay
and jitter are of most interest. Future avenues of work are
also pointed out.
Taking into account the fact that most data originates
and terminates in Ethernet networks, EPONs and GPONs
have turned out to be the more promising technologies.
APONs introduce typical issues regarding segmentation
and reassembly of data packets, while EPONs support
Ethernet natively and GPONs support it through GEM.
PON architecture is presented in section II, sections III
and IV cover dynamic bandwidth allocation and quality of
service, while a conclusion is given in section V.
I.
INTRODUCTION
Throughout the past decade, wide area networks
(WANs) and metropolitan area networks (MANs) have
experienced significant bandwidth improvement. Local
area networks (LANs) also haven’t been bypassed by
large bandwidth upgrades (from 10Mb/s up to 1 Gb/s).
The access network that connects LANs to MANs and
WANs has not experienced such a level of progress and is
still copper based, relying on digital subscriber line (DSL)
technology. This technology has made incremental
improvements in throughput, but hasn’t been able to keep
up with LAN, MAN and WAN, thus becoming a
bottleneck.
Many experts agree that a fiber infrastructure is needed
to mitigate this issue in the access network. In contrast to
MANs and WANs which carry bit streams of numerous
subscribers, the access network carries a single or several
users bit streams, making it very sensitive to cost. This has
a large influence on operator policy from a revenue
perspective and has impaired the penetration of fiber in
the “first mile”.
Passive optical network (PON) technology is getting
more and more attention by the telecommunication
industry as the “first mile” solution. PONs deploy a shared
fiber medium between service providers and customer
premises utilizing passive optical splitters/combiners.
Sharing the fiber medium has a consequence of reduced
cost in the physical fiber deployment and amount of active
optical equipment at the service provider premises, while
using passive optical components reduces costs that are
tied to maintaining remote facilities with power.
PONs are usually classified by the link-layer protocol
they use. Asynchronous transfer mode (ATM) PON
(APON) uses ATM, Ethernet PON (EPON) uses Ethernet
and gigabit PON (GPON) uses the GPON encapsulation
method (GEM) and ATM. The International
Telecommunications Union (ITU) has standardized
APON and GPON in the ITU-T G.983 and G.984
standards, respectively. IEEE has standardized EPON in
IEEE 802.3ah.
II. PON ARCHITECTURE
A PON is a point-to-multipoint (P2MP) optical network
with a physical tree topology and no active elements in the
signal’s path from source to destination. The only interior
elements used in PON are passive optical components,
such as optical fibers, splices and splitters. All
transmissions in a PON are performed between the optical
line terminal (OLT), which is located in the local
exchange (central office), and optical network units
(ONUs) that reside at each end-user premises (Figure 1).
Figure 1. Passive Optical Network scheme
The transmission direction from OLT to ONU is
referred to as downstream and operates as a broadcast
medium. The transmission direction from the ONUs to
OLT is referred to as upstream. The upstream signals
propagate from ONU to OLT but are not reflected back to
each ONU, thus the PON is not a broadcast medium in the
upstream direction and standard collision detection based
medium access control (MAC) protocols are not
applicable. In order to avoid collisions in the upstream
direction Time Division Multiplexing (TDM) or
Wavelength Division Multiplexing (WDM) can be used
[1]. Current PON technology uses WDM to separate
upstream and downstream channels, while TDM is used to
avoid upstream collisions between ONUs. Due to these
facts, a centralized polling-based MAC is utilized.
A PON can formally be defined as a directed
hypergraph H=(V,E), where V={olt, onu1, onu2, …, onuN},
and E={d, u} with the incidence matrix I:
d u
I=
olt
onu1
onu2
...
onu N
⎡
⎢
⎢
⎢
⎢
⎢
⎣
−1
1
1
...
1
1
−1
−1
...
−1
⎤
⎥
⎥
⎥
⎥
⎥
⎦
(1)
Edge d represents the downstream transmission link
from olt to {onu1, onu2, …, onuN} (point-to-multipoint –
broadcast medium), while edge u represents the upstream
transmission link (multipoint-to-point – shared, but not
broadcast medium).
The olt may use d without any time constraints to
transmit data to onui, i∈{1,2,…,N} (further on in this
paper onui, i∈{1,2,…,N} will be referred to as simply
onui), but at any given moment t ∈ 0, ∞ ) only one onui
may use edge u to transmit data to the olt.
[
III. DYNAMIC BANDWIDTH ALLOCATION
A great deal of attention has been directed to traffic
management and upstream link utilization in PONs.
Because of bursty traffic sources, bandwidth requirements
vary largely with time. Therefore the static allocation of
bandwidth to the individual subscribers in a PON is
typically inefficient [2]. Statistical multiplexing that
adapts to instantaneous bandwidth requirements is
therefore more efficient. The OLT may use a Dynamic
Bandwidth Allocation (DBA) algorithm for providing
statistical multiplexing. The OLT accomplishes this by
receiving bandwidth information from each ONU and
allocating resources based on that information.
Figure 2. Multi-Point Control Protocol
Ethernet PONs (EPONs) implement Multi-Point
Control Protocol (MPCP) in their Medium Access Control
(MAC) to facilitate DBA. In MPCP, an ONU reports its
queue occupancies using REPORT messages transmitted
within its allocated time slot, whereas the OLT arbitrates
ONU transmissions through GRANT messages (Figure 2).
Apart from MAC, MPCP also facilitates ONU discovery
and registration.
The Gigabit PON (GPON) standard also provides for
the implementation of DBA. In GPON, the OLT allocates
bandwidth among ONUs through the bandwidth map field
of the physical control block (PCBd). ONUs may
optionally publicize their bandwidth requirements upon
OLT request. That is carried out through the dynamic
bandwidth report unit (DBRu) field of the physical layer
overhead (PLOu), hence enabling DBA algorithms to be
run at the OLT [3].
Generally, a PON is a remote scheduling system [4]
that has the following problems:
1. Significant queue switchover overhead [4] due to
guard times between ONU transmissions. Guard
times are needed for the previously transmitting
ONU to power off its laser, the next ONU to
power on its laser and for the OLT to adjust its
receiver.
2. Large control plane propagation delay because of
the distances between the ONUs and OLT.
3. Limited control plane bandwidth.
A DBA algorithm named Interleaved Polling with
Adaptive Cycle Time (IPACT) [5] mitigates all of the
mentioned issues inherent to PONs.
When utilizing interleaved polling (to mitigate large
RTTs) the grants provided by the OLT are interleaved,
thus the decisions for providing those grants are based on
individual REPORT messages. This prevents the OLT
from making bandwidth allocation decisions based on the
bandwidth requirements of all ONUs. An alternative
approach is for the OLT to wait for the receival of all
REPORT messages before starting a new polling cycle.
The former approach is referred to as the online DBA
framework, while the latter is the offline (interleaved
polling with stop) DBA framework. The offline DBA
framework provides the OLT with global knowledge of
ONU bandwidth requirements, which allows it to
distribute bandwidth in a fair manner, but this introduces a
tradeoff in upstream link utilization because of the
additional walk time at the end of every cycle.
Figure 3. DBA Taxonomy
Topics regarding DBA research are organized as shown
in Figure 3.
A. Grant sizing
Grant sizing refers to the act of determining the size of
the grant to be given to an ONU in a cycle. It can be
divided into 5 categories: 1) Fixed, 2) Gated, 3) Limited,
4) Limited with excess distribution and 5) Exhaustive
using queue size predicition.
1.
2.
3.
4.
5.
Fixed – in the fixed grant sizing scheme, the grant
size is fixed for an ONU in every cycle. If all
ONUs are given grants according to the fixed
scheme, this scheme is equivalent to basic TDM.
Analysis in [6] has shown that this scheme
underperforms compared to the schemes presented
further on in this paper.
Gated – in the gated grant sizing scheme, the grant
size for an ONU is the queue size reported by the
ONU. Analysis of the gated scheme can be found
in [7] and [8].
Limited – the limited grant sizing scheme [5]
defines a maximum grant size Wmax which cannot
be breached. The ONU is granted a transmission
window equal to the queue size reported by the
ONU if that value is less than Wmax, if the value
reported by the ONU is greater than Wmax, Wmax
bytes are granted.
Limited with excess distribution – this scheme [9]
is an extension of the limited scheme. Overloaded
ONUs (REPORT > Wmax) share the unused
(excess) bandwidth left over from underloaded
ONUs (REPORT < Wmax).
Exhaustive using queue size prediction – this
scheme is based on predicting the traffic that is
generated between the ONU queue report and the
actual transmission grant. [10] and [11] are
examples of this approach.
B. Grant scheduling
When utilizing the online DBA framework, the round
robin scheduling scheme is used since the OLT does not
have a complete overview of every ONUs queue status.
In order to change the scheduling scheme from round
robin, the offline framework should be utilized in order to
be able to determine the order of grants. The Largest
Queue First (LQF) and Earliest Packet First (EPF)
scheduling disciplines have been examined in [12] and
[13]. Results have shown that both LQF and EPF provide
lower average delay at medium loads, while not showing
any improvement at low and high loads.
Grant scheduling is also referred to as inter-ONU
scheduling.
C. Queue scheduling
Queue scheduling determines how frames from
different ONU queues are scheduled inside a given
transmission window. It is also referred to as intra-ONU
scheduling.
There are generally two classes of scheduling that
tackle this issue: 1) Strict Priority (SP) scheduling, and 2)
Weighted Fair Queuing (WFQ) scheduling. The ideal
scheduler should provide statistical multiplexing and
guarantee a minimal portion of bandwidth to each queue.
Due to cost concerns, the complexity of the ONU
should be kept as low as possible, which has an impact on
the choice of the intra-ONU scheduling scheme. In order
to keep ONU complexity low, intra-ONU scheduling can
be left over to the OLT, but this can result in scalability
issues.
IV. QUALITY OF SERVICE
IPACT mitigates many important issues inherent to
PONs, but does not address quality of service (QoS)
guarantees in terms of bandwidth, delay, etc. which are
required in typical service level agreements (SLAs).
QoS topics are organized as shown in Figure 4.
Figure 4. QoS areas of interest
A. Differentiated Service
This is the simplest manner of providing QoS in PONs.
It assumes the differentiation of traffic and providing
different service to each traffic class.
All of the conducted research in this area [9], [14], [15],
[16], [17] suggests separating traffic at the ONU into
classes, separately reporting queue occupancies of each
class and allowing the OLT to provide separate grants for
each class.
B. Bandwidth Guarantees
Significant algorithms that have been proposed for
supporting bandwidth guarantees are Bandwidth
Guaranteed Polling (BGP) [18] and its admission control
augmented counterpart [19].
BGP provides bandwidth guarantees by dividing a fixed
length polling cycle into bandwidth units. The bandwidth
required by an ONU (as defined in a SLA) determines the
number of bandwidth units from the polling table that are
allocated to that ONU. This presents a compromise
between fixed TDM and statistical multiplexing.
C. Delay Guarantees
An algorithm named Dual DEB-GPS (Deterministic
Effective Bandwidth – Generalized Processor Sharing)
Scheduler, is presented in [20]. It uses DEB to determine
the scheduling weights used in a GPS scheduler.
This algorithm performs scheduling in two layers:
1. Class level multiplexing at the OLT
2. Source level multiplexing at the ONU.
Dual DEB-GPS addresses the issue of providing delay
guarantees, but does not provide absolute delay bound
guarantees.
D. Delay Jitter Guarantees
The Hybrid Slot Size/Rate (HSSR) algorithm [21] can
stabilize packet delay variation in EPONs for jitter
sensitive traffic, while the Hybrid Granting Protocol
(HGP) [22] can ensure QoS by minimizing jitter and
guaranteeing bandwidth.
E. Admission Control
A novel admission control framework along with an
appropriate DBA algorithm was introduced and studied in
[23]. The algorithm determines whether or not to admit a
real-time traffic stream based on the requirements of the
stream and the upstream channel utilization. The authors
conclude that the presented admission control scheme is
able to satisfy QoS requirements in terms of delay and
bandwidth.
V. CONCLUSION
From the presented, it can be concluded that grant
sizing, grant scheduling and queue scheduling have been
extensively covered in existing work, but there are still
areas that need further work to be done. An interesting
topic for future work is exploring scheduling schemes for
single- and multi-channel PONs.
It can also be seen that significant work has been
conducted in the area of providing differentiated service
QoS, while providing bandwidth, delay and jitter
guarantees requires further work to be conducted. As M.
McGarry, M. Reisslen and M. Maier point out in [24]:
“providing guaranteed service across a PON is going to be
critical as the access network is required to be an
integrated services network carrying packetized voice and
video along with data traffic”.
Additionally, DBA for multi-wavelength PONs
represents a broad area for future research [24].
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