1 Supporting different Supporting different iated classes of ser iated classes of ser vice in Etherent pas vice in Etherent pas sive optical network sive optical network s s Adviser: Ho-Ting Wu Adviser: Ho-Ting Wu Presenter: Zen-De Liu Presenter: Zen-De Liu Institute of Computer Science and Institute of Computer Science and Information Engineering Information Engineering National Taipei University of Technology National Taipei University of Technology
Transcript
11
Supporting differentiated claSupporting differentiated classes of service in Etherent sses of service in Etherent passive optical networkspassive optical networks
Adviser: Ho-Ting WuAdviser: Ho-Ting WuPresenter: Zen-De LiuPresenter: Zen-De Liu
Institute of Computer Science and Institute of Computer Science and Information EngineeringInformation Engineering
National Taipei University of TechnologyNational Taipei University of Technology
22
ReferencesReferences [1] [1] G.Kramer, B. Mukherjee, “ Supporting differentiated classe
s of service in Ethernet passive optical networks” August & September 2002 / Vol. 1, Nos. 8 & 9 / JOURNAL OF OPTICAL NETWORKING.
[2] G. Kramer, B. Mukherjee, and G. Pesavento, “IPACT: a dynamic protocol for an Ethernet PON (EPON),” IEEE Commun. 40(2), 74–80 (2002).
[3]G. Kramer, B. Mukherjee, and A. Maislos, “Ethernet passive optical networks,” in Multiprotocol over DWDM: Building the Next Generation Optical Internets.
[4]ANSI/IEEE Standard 802.1D, 1998 ed., “IEEE standard for information technology— Telecommunications and information exchange between systems—Local and metropolitan area networks—Common specifications. Part 3: media access con-trol (MAC) bridges.
33
OutlineOutline
IntroductionIntroduction Overview of EPON’S principle of Overview of EPON’S principle of
operationoperation MPCP MPCP System Architecture:Intergrating Priority System Architecture:Intergrating Priority
Queuing in EPON NetworksQueuing in EPON Networks Packet Delay AnalysisPacket Delay Analysis Optimization SchemesOptimization Schemes Bandwidth UtilizationBandwidth Utilization ConclusionConclusion
44
IntroductionIntroduction
PON: PON: A PON is a point-to-multipoint (PtMP) optical network with no active elements in the signals’ path from source to destination. All transmissions in a PON are performed between an optical line terminal (OLT) and optical network units (ONUs)
55
IntroductionIntroduction
EPON:EPON:
66
IntroductionIntroduction
EPON is expected to be a truly converged network, supporting voice communications, standard and high-definition video (STV and HDTV), video conferencing (interactive video), real-time and near-real-time transactions, and data traffic.
To support this multitude of applications, EPON must exhibit an appropriate performance for each such application.
IntroductionIntroduction To support diverse application requirements, such net
works separate all the traffic into a limited number of classes and provide differentiated service for each class. Such networks are said to maintain classes of service (CoS).
eduling. inter-ONU scheduling: IEEE 802.3ahMPCP arbitrates (gates) trans
mission from multiple nodes to allow dynamic sharing of bandwidth while avoiding packet collisions.
99
IntroductionIntroduction Combination of the limited service scheme
(inter-ONU scheduling:IPACT,Ref2) and a priority queuing (intra-ONU scheduling) results in quite an unexpected network behavior.
find that the queuing delay for some traffic classes increases when the network load decreases.
light-load penalty How to improve the performance and
eliminate the light-load penalty? optimization schemes
1010
MPCPMPCP(Multipoint Control Protocol)(Multipoint Control Protocol)
MPCP:MPCP:To support a timeslot allocation. This protocol relies on two Ethernet messages:
GATE and REPORT. A GATE message is sent from the OLT to an ONU,
and it used to assign a transmission timeslot. A REPORT message is used by an ONU to conve
y its local conditions (such as buffer occupancy, and the like) to the OLT to help the OLT make intelligent allocation decisions.
1111
MPCPMPCP
1212
MPCPMPCP
1313
MPCPMPCP
Some small deviation of the new RTT from the previously measured RTT may be caused by changes in fiber refractive index resulting from temperature drift.
A large deviation should alarm the OLT about the ONU’s potential mis-synchronization and should prevent the OLT from further granting any transmissions to that ONU until it is reinitialized (resynchronized).
1414
MPCPMPCP
1515
System Architecture:Intergrating System Architecture:Intergrating Priority Queuing in EPON NetworksPriority Queuing in EPON Networks
1616
System Architecture:Intergrating System Architecture:Intergrating Priority Queuing in EPON NetworksPriority Queuing in EPON Networks ModelModel:: limited service discipline described in Ref. 2. Under this discipline, the OLT assigns to an ONU a slot
of size equal to what the ONU had requested (through a previous REPORT message), but not greater than some predefined maximum WMAX.
The limit WMAX is needed to guarantee maximum interval between slots (cycle time) TMAX and to avoid bandwidth hogging by a “hungry” ONU. This scheme was shown to share the bandwidth efficiently while maintaining fairness among ONUs.
1717
System Architecture:Intergrating System Architecture:Intergrating Priority Queuing in EPON NetworksPriority Queuing in EPON Networks the default system parameters used
in the simulation experiments.
1818
Packet Delay AnalysisPacket Delay Analysis
Traffic Modeling: divide our data into three priority classes: P0,
P1, and P2. The three classes may be used for delivering voice, video, and data traffic. Using three classes also allows easy mapping of DiffServ’s Expedited Forwarding (EF), Assured Forwarding (AF), and Best Effort (BE) classes into 802.1D classes.
1919
Packet Delay AnalysisPacket Delay Analysis
Class P0 is used to emulate a circuit-over-packet connection. P0 traffic has CBRconsume 4.48 Mbit/s of bandwidth.
Class P1 in our experiment consists of VBR video streamsPacket sizes in P1 streams range from 64 to 1518 Bytes.
Class P2 is used for non-real-time data transfer. There are no delivery or delay guarantees for this servicevariable-size packets ranging from 64 to 1518 Bytes.
2020
Packet Delay AnalysisPacket Delay Analysis performance analysis by investigating
the limited service discipline. As our performance measures, we consider average and maximum packet delay
2121
Packet Delay AnalysisPacket Delay Analysis
As the load decreases from moderate (∼0.25) to very light (∼0.05), the average delay for the lowest priority class (P2) increases significantly. Similar behavior is observed for the maximum packet delay for P1 and P2 classes. We refer to this phenomenon as light-load penalty.
2222
Packet Delay AnalysisPacket Delay Analysis Why light-load penalty? (1) Whatever slot size an ONU requested in the REPOR
T message, the OLT will grant the requested slot size through the next GATE message to that ONU.
(2) However, during the time lag between ONU’s sending a REPORT and the arrival of its assigned timeslot (i.e., between sending a REPORT and transmission of the reported data), more packets arrive to the queue. Newly arrived packets may have higher priority than some packets already stored in the queue, and they will be transmitted in the next transmission slot before the lower-priority packets.
2323
Packet Delay AnalysisPacket Delay Analysis
(3) Since these new packets were not reported to the OLT, the given slot cannot accommodate all the stored packets.
(4) This causes some lower-priority packets to be left in the queue. This situation may repeat many times, causing some lower-priority packets to be delayed for multiple cycle times.
2424
Optimization Schemes
Two-Stage Buffers:
2525
Optimization Schemes
2626
Optimization Schemes
The light-load penalty has been eliminated. The drawback of this scheme, however, is the increased delay for the highest-priority class (P0).
2727
Optimization Schemes
Constant-Bit-Rate Credit: predict the amount of high-priority pack
ets that are expected to arrive at the ONU and to adjust the granted timeslot size accordingly
without increasing the delay of the highest-priority class beyond one cycle time as in a two-stage scheme.
2828
Optimization Schemes To predict the traffic with any reasonable
accuracy, we need to have some knowledge about the traffic behavior. In our case, we have this knowledge about the P0 traffic.
Therefore, when deciding on the size of the next timeslot for an ONU, the OLT can estimate the time of the next transmission and increase the timeslot size by the amount of CBR data it anticipates.
We call this scheme CBR credit, since the additional timeslot size increment (credit) is based on the known CBR arrival rate.
2929
Optimization Schemes
How to compute the size of the credit? The OLT first determines the credit inter
val τ The credit interval can be calculated as f
ollows. Given that tR is timestamp in a received
REPORT message, tS is start time of a granted slot, s is slot size, and RN is EPON line rate
3030
Optimization Schemes
3131
Optimization Schemes
Taking TCBR to be the period of CBR packet arrivals (in s/packet), we can expect nCBR CBR packets to arrive during the interval τ
But the slot size s itself depends on the number of additional CBR packets it should accommodate.
3232
Optimization Schemes
where SCBR is the size of CBR packets and v is a number of bytes (requested slot size) reported by the ONU.
3333
Optimization Schemes Thus the OLT assigns the slot size on the basis
of the following formula:
IFG is the minimum interframe gap WMAX is the maximum limit on slot size.
3434
Optimization Schemes
The tS,i value is updated after each slot assignment as:
the OLT expects the data (first bit) from ONU i+1 to arrive exactly after the guard time B after the data (final bit) from ONU i.
3535
Optimization Schemes
3636
Optimization Schemes
To illustrate the advantages of the CBR scheme, we built a distribution (histogram) for the unused slot remainder and compared it with this distribution for the limited service.
3737
Bandwidth UtilizationBandwidth Utilization Bandwidth utilization in EPON is determined by the cy
cle time, the guard time, and the size of unused slot remainder.
at high loads, the ONUs request windows larger than WMAX.the OLT will grant them a WMAX-Byte slot, ignoring the credit value. Therefore the CBR credit scheme’s performance at high loads is the same as in the limited service scheme.
3838
Bandwidth UtilizationBandwidth Utilization
in the CBR credit scheme, even though a larger slot size is granted to the ONUs, the cycle time is reduced.
At a load of 0.35, the average cycle time for the CBR credit scheme is 301 μs, a 30% reduction from 422 μs in the limited service scheme.
3939
Bandwidth UtilizationBandwidth Utilization
This advantage is gained through the reduction in unused slot remainder, which results in increased network utilization. The two-stage buffer system is found to have even larger cycle-time reduction.
4040
Bandwidth UtilizationBandwidth Utilization
Both the cycle time and the slot remainder affect the bandwidth utilization in EPON. The following formula allows us to compute bandwidth utilization U:
where N is number of ONUs, B is guard time, R is remainder, and tC is cycle time.
4141
Bandwidth UtilizationBandwidth Utilization Both two-stage buffer scheme and the CBR credit
scheme result in considerable improvement in bandwidth utilization at light loads. The two-stage buffer scheme also shows slightly better utilization at high loads, because of its complete elimination of the unused remainder.
4242
ConclusionConclusion
a combination of a default scheduling algorithm (priority scheduling) and MPCP (limited service) results in an interesting phenomenon in which some classes of traffic are treated unfairly when the network load is light.
In fact, under light loads, ONUs with FCFS queue perform better than ONUs with priority scheduling. We call this phenomenon light-load penalty.
4343
ConclusionConclusion
To alleviate this penalty, we proposed and examined the characteristics of two optimization schemes with different trade-offs.
A two-stage queuing scheme eliminates the light-load penalty completely at the expense of increased packet delay for all the classes of traffic.
4444
ConclusionConclusion
Another scheme (CBR credit) attempts to predict high-priority packet arrivals. This scheme eliminates light-load penalty for most packets.
The limitation of this scheme is that some external knowledge of the traffic-arrival process is needed.
4545
ConclusionConclusion
A large amount of research has been done in developing scheduling algorithms with improved fairness in resource sharing.
Integration of such schedulers in EPON is not a trivial task, because of EPON’s distributed nature and unique properties such as limited control-plane bandwidth, large propagation delay, and significant switching overhead.
In a forthcoming study, investigate the feasibility of implementing a fair-queuing scheduling in EPON.
