opnetwork - qos bandwidth allocation for pon - final draft3

7/31/2019 OPNETWORK - QoS Bandwidth Allocation for PON - Final Draft3

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QoS Based Bandwidth AllocationSchemes for Passive Optical Networks

Nadav Aharony, Eyal Nave, Nir Naaman and Shay AusterDepartment of Electrical Engineering

Technion Israel Institute of Technology

Haifa, 32000, IsraelE-mail:{ nadav@, eyal@, auster@, cn17s02@}comnet.technion.ac.il

Web: www.comnet.technion.ac.il/~cn17s02

Abstract

"Ethernet in the First Mile" (EFM) is an upcomingstandard currently being drafted by the 802.3ah task force in

the IEEE organization. One of the tracks in the standard dealswith Ethernet over Passive Optical Network (EPON), a

centralized point-to-multi-point network with a shared

upstream channel. This paper deals with the problem ofupstream bandwidth allocation in EPON networks. Wesuggest, analyze, and compare several bandwidth allocation

algorithms. As a novel type of network, EPON presents manynew challenges, and one of the main goals of this project was

to get a feel for the network in order to decide on the nextsteps in its exploration. To that end, we have constructed an

OPNET model of an EPON network based on the IEEEstandard. The model contains one module for the network's

central-office, the Optical Line Terminal (OLT), and anothermodule for the customer premises device, the Optical

Network Unit (ONU). The network model implements

EPONs Time Division Multiple Access (TDMA)

multiplexing scheme and supports the relevant messages asdefined by the standard. Although designed for EPON, our

OPNET model can be easily converted to simulate othershared, TDMA based, networks.

The new OPNET modules were designed as a test-bed for

bandwidth allocation algorithms; they enable smooth

insertion of a wide range of algorithms and easy extension ofthe currently supported features. We used our model to

simulate several bandwidth allocation algorithms startingfrom a simple static algorithm and advancing to more

sophisticated algorithms that support Quality of Service(QoS) and guarantee fair bandwidth allocation. Our results

demonstrate the impact of the bandwidth allocation algorithmon the overall network performance.

1. Introduction

Passive Optical Network (PON) is an emerging access

technology, offering a high bandwidthpoint-to-multipoint

optical fibernetwork. It is called "passive", since there are noactive devices (e.g. repeaters) along the route apart from the

end units. The only interior elements used in such networks

are passive combiners, couplers, and splitters. This greatlyreduces the costs and complexity of the deployment and

maintenance of the network. PONs are intended to solve theaccess networks bandwidth bottleneck by offering a cost-

effective, flexible, and high bandwidth solution. Today newhousing developments in many places around the world are

built with fiber-based connections to the home, and networkproviders are conducting field-tests and experiments with fiber

access. Eventually, fiber access is predicted to replace the old

copper infrastructure the world over.

PON consists of two main types of end devices: An OpticalLine Terminal (OLT) and an Optical Network Unit (ONU).

The OLT resides in the Central office (CO) and is connectedto an uplink fiber and a downlink fiber, linking it to the

network end-units. The OLT is responsible for control andmanagement of the PON, and also acts as the gateway to the

outside world and adjacent networks.

The ONU is the client-side of the network that resides near

or inside the Customer Premises (CP). An ONU may serve asingle residence or business, or it may serve several subscriber

residences or an entire apartment building. PONs use point-to-

multipoint topologies they may be connected as a tree, a bus,or a ring. Figure 1 illustrates the components of an EPON

deployment.

Figure 1 - EPON Network Illustration

The term uplink refers to information flowing from the

end units the ONUs in our case, to the central officeequipment - the OLT. The term downlink refers to the

information flowing from the OLT to the ONUs.

All communication within a PON is mediated by the OLT.

On the downlink the OLT broadcasts all information to thefiber. Each ONU filters out only the transmissions that are

directed to it (see Figure 2). Encryption and authenticationfeatures may be applied to the traffic in order to make sure

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that only the intended party/parties have access to the

information. On the uplink the ONUs send their traffic to theOLT which then forwards it to its destination. ONUs are not

able to directly "see" the upstream traffic from their networkpeers (see Figure 3).

The OLT is responsible for all bandwidth allocations in thenetwork. An ONU is allowed to transmit only in time slots

that have been assigned to it by the OLT. The OLT has greatamount of flexibility in implementing a bandwidth allocation

algorithm; allocations may change dynamically over time toadapt to the different bandwidth requirements of the ONUs.

The bandwidth allocation algorithm may implement anythingfrom a strict and even division of the bandwidth among all

ONUs, to giving all bandwidth to a single ONU (both uplinkand downlink). It is also up to the algorithm to maintain the

level of QoS that has been guaranteed to each ONU. Thiswork inspects different aspects of bandwidth allocation in

PONs.

Figure 2 - EPON TDMA Downlink

Figure 3 - EPON TDMA Uplink

1.1 Ethernet over PON (EPON)

"Ethernet in the First Mile" (EFM) is an upcomingstandard currently being drafted by the 802.3ah task force in

the IEEE organization. It will define an access standard basedon the Ethernet protocol for several media architectures and

physical link types. One of the tracks in the standard dealswith Ethernet over Passive Optical Network (EPON).

EPON deals with a symmetric point-to-multi-pointconnection at very high speeds (Currently 1Gb/s, in the future

10Gb/s and possibly even more). The protocol does not limitthe number of users, though a typical scenario refers to the

connection of up to 64 end points per PON fiber. A mainadvantage of using Ethernet datagram over the PON is the

fact that most networks on both sides of the PON (the

customer and the service providers) are based on Ethernet.Using Ethernet in the access link will save unnecessary format

conversions. Another great benefit is that Ethernet equipmentis widely available "off the shelf" and is manufactured by

many vendors.

A unique network management protocol has been devised

by the EFM task force Multi Point Control Protocol(MPCP) [1]. MPCP defines a TDMA multiplexing scheme, in

which the upstream channel is divided into time-slots. EachONU is allocated time-slots in which to send its uplink traffic.

The time slots are in granularity of a Time Quanta (TQ),which is defined as the time that it takes to broadcast 2 octets

of data. The MPCP constitutes an absolute timing model. Aglobal clock exists in the OLT, and the ONUs set their local

clock to OLT clock. All control messages in the network aretime-stamped with the local clock, and through them the

devices are able to synchronize their network clocks.

According to the standard, ONU devices must support the

802.1Q queuing, meaning a queuing system that supports 8priorities (or traffic classes). There is no definition about how

the priorities must be used. The priority queue may also beused for QoS purposes, where each traffic type is given a

distinct priority.

1.2 Bandwidth Allocation in EPON

This project concentrates on upstream bandwidth allocation

in EPON. It is important to note that the 802.3ah standard

does not dictate the bandwidth allocation algorithm and leavesit open to the implementation of each vendor/manufacturer.

There are many possibilities for the management of the

bandwidth.

The simplest regime is "static allocation", in which each

ONU is allocated a constant bandwidth allocation, orgrant,which is re-allocated at constant intervals. This type of

allocation is very inefficient, since the end-stations may not

require the entire grant bandwidth all the time, and the wastedbandwidth might have been allocated to someone else. For this

reason more advanced allocation algorithms have been

devised, that are dynamic in nature. These algorithms usuallyhave access to input about the current, near real-time needs of

the end stations, and also have access to the service levelagreement (SLA) definitions of each end-user. These schemes

usually attempt to achieve fairness in the allocations to end-users of the same class.

Dynamic algorithms may work either in a continuoustimeline or as cycle based. A continuous timeline algorithm

receives bandwidth requests from the end stations andallocates bandwidth according to them in a continuous

fashion. A cycle based algorithm divides the timeline intoconsecutive "cycles", and calculates the bandwidth allocation

for an entire cycle at a time.

Ultimately, the bandwidth allocation processes in the OLT

are supposed to reflect the QoS and SLA requirements andimplement them in this segment of the network. The OLT is

also responsible for taking into consideration all aspects of

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equipment and physical delays in order to avoid collisions.

Collisions occur when more than one ONUs signal reach theOLTs receiver at the same time.

The grants information is sent in MPCP GATE messages,

which tell each ONU its start time to transmit and the length

of the allocated transmission. In order to make correctallocation decisions, the ONUs send MPCP REPORT

messages, informing the OLT of their queue status. Eachallocation algorithm might use this information in a different

way. Even if an ONU did not make request for any bandwidthfor the next cycle, it would typically be allocated a minimal

grant that would allow it to at least send a REPORT messagefor the cycle afterwards.

EPONs bandwidth allocation scheme is different from thatof other shared access networks such as data over cable

networks. The standard for data over cable, DOCSIS [3],allows contention between end stations on the available

bandwidth. In EPON there is no contention and thereforethere is no danger of data collision. The downside to this

mode is obligation to allocate the minimal grant to each end-station. The difference between the two networks is that the

total available bandwidth of EPON is very large compared tocoaxial cables (1 Gbps compared to only 30 Mbps), and the

number of users is much smaller (a few dozens compared to afew hundreds or thousands). Because of this, the minimal

grants for the ONU reports are negligible.

1.3 Goals

In this project we attempt to explore some of the aspects ofQoS bandwidth allocation in the 802.3ah EPON architecture.

Since this is a novel network model and a very extensive

field, this project can offer a starting point for more advancedresearch on the subject.

The bandwidth allocation process can be divided into several

sub-processes (that may or may not be independent to eachother):

Gathering of the input for the decision making process

(such as bandwidth requests or the bandwidth definitionsfor each end unit).

Dividing the available bandwidth between the end units

(determining the quota for each) within a defined time-frame.

Scheduling the allocated quotas of all end units for the

defined time frame. Informing the end units when they are allowed to

broadcast (parallel to the frequency of the scheduling

changes).

For each of these sub-processes there are many work modes

and algorithms that may be thought up and compared. Sincethere are numerous approaches that can be used, the goal of

this project is not to provide a complete and optimal solution,

but rather to present a preliminary comparison of severalalgorithms, in order to get a feel for where to go next. The

current project concentrates on a narrow section of the

network mainly the bandwidth allocation in the uplink

direction.

OPNET has been chosen as the simulation environment for

the project. Since EPON is still a novel technology and thestandard is still being developed, there were no ready-made

simulation modules. A new environment and set of moduleshad to be designed and implemented.

2. Problem Definition

The problem we set out to investigate is the problem ofbandwidth allocation of the EPONs uplink, with the

following guidelines:

Two types of network traffic are defined:

o Committed Rate (CR) A specified bit rate

that the ONU must be allocated by the network if it

requests it. The requirement is for the average bit rate

over one second, and there are no requirements forminimum delays between the bandwidth allocations

within this timeframe.o Best Effort (BE) - Traffic that is not

promised to the ONU in the SLA, and will be

allocated only if all CR requirements in the networkhave been fulfilled and there is still available bandwidthto allocate. The only guideline is the attempt to allocate

the available traffic as fairly as possible between theONUs.

Fairness is defined as follows:

o Dont ask dont get An ONU that did

not request any BE bandwidth will not be allocated any.

o History window A certain history will be

kept regarding the amount of BE bandwidth allocated

to each ONU. ONUs that were allocated less bandwidthin this history window will be entitled to receive

bandwidth until they are aligned with the others.The window size is not defined; it is a parameter to be

tested, but note that too large a window might enable asingle ONU that was quiet for a long time to take over

all of the available BE bandwidth and deny service tothe others.

o Among ONUs that have the same amount of

allocated history, fairness will be defined as an equal

division of the bandwidth.

3. OPNET Implementation

In this section we describe the EPON environment that was

implemented and its components. Since there was no availableOPNET module for the EPON standard, we constructed anEPON modeling environment from scratch. This required

simulating both the physical PON characteristics and theupcoming 802.3ah standard. Even though the standard is not

complete at this time, the aspects of the 802.3ah that wereimplemented were ones that are at final stages of ratification.

They were also designed with the option to easily modifythem, if necessary.

3.1 Network Model Assumptions

Several assumptions have been made in order to simplify the

simulation environment and to allow better focus on the

bandwidth allocation problem:

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Static network configuration The number of ONUsremains constant, i.e., no ONUs are joining or leaving the

network. Accordingly, the network registration processesdefined in 802.3ah were not implemented.

Single subscriber ONUs Each ONU is assumed to be

serving only a single subscriber (and not, for example, anentire apartment building). When more than one distinct

subscriber is served by an ONU, each subscriber willhave his own SLA definitions and will need to receive

specific allocations according to it. This will require

increased complexity from the network devices and theallocation algorithms.

Bandwidth allocated per ONU All bandwidth

allocations are done per ONU. The OLT does not instructthe ONU how to utilize the granted bandwidth. This is

the simplest work mode and probably the most common -at least for the initial deployments of EPON. More

advanced allocation models are also available. Forexample, the "granted entity" might not be the ONU but a

specific IP address at the customer's premise or specifictraffic types. Another example is the allocation of a

separate grant for each queue priority.

Global Clock All simulation modules use the same

global clock provided by the network. Consequently, theexchange of synchronization information, which is done

in reality, is not implemented.

No Packet Loss The initial model assumes that no

packets are lost. Future versions would take into account

the realistic bit error rate of about 10 -12.

No Propagation Delays To simplify the initial model,

propagation delays have been ignored. Propagation

delays will be added to future versions.

Part of the plans for future work include expansions of thesimulation to include some of the leniencies that were stated,

in order to make it more realistic.

3.2 TDMA Model and the Bandwidth AllocationConcept

The TDMA timing regime was implemented usingOPNET's global simulation time and the scheduling of self

interrupts and remote interrupts in advance. These interruptsinvoke processes that remove packets from the queue and

transmit them, make REPORT packets, schedule futuregrants, etc. The ONUs set interrupts to start transmitting in

offsets based on their GATEs start time and length. With theassumption that the OLT allocates the grants without

overlaps, there should be no collisions of uplink data betweendifferent ONUs.

The timeline is divided into cycles, which are defined as anattribute of the OLT. The division to cycles is not mandatoryand is a matter of our implementation of the standard. TheOLT utilizes several "markers" to schedule actions for itself

in the future. The most significant marker on the timeline is apointer to the time of the next cycle's start time. All the other

action markers are set accordingly. As in reality, the GATE

allocations must be received by the ONUs before the cycle

starts, which provides them with ample time to transmit attheir assigned time. For transmissions, there is another marker,

"send_grants" which schedules an interrupt at a certain offsetbefore the next cycle's start. This interrupt tells the OLT to

send the GATE messages that have already been prepared bythe algorithm. Before this can happen, another marker,

"do_schedule" must instruct the OLT to actually "execute" thealgorithm and make the necessary calculations.

There are also other time markers that are used by the ONUsto schedule their GATE start and stop times, and to simulate

the REPORT message generation and the actual sending of theoutstanding packet queue in a realistic manner.

As mentioned above, the basic unit of time is a TimeQuanta, or TQ. All allocations are in multiplications of TQ

units. All control packets created are time stamped with thecurrent simulation time. Figure 4 depicts an example of the

timeline from the OLT and ONU point of view (in a 3 ONUscenario):

Figure 4 - Bandwidth Allocation timing model; a 3 ONUs

example.The top timeline depicts the OLTs point of view, the

bottom timeline is seen by the ONUs. As mentioned, thetimelines are divided into cycles. The cycles are marked in the

OPNET environment by next_cycle_start interrupt. TheOLTs interrupts are scheduled in advance, one cycle at a

time. As described above, before the cycle starts, two actionsmust be completed the actual allocation for that cycle, and

the broadcast of all GATE messages for the upcoming grant.

Each ONU is allocated its grant in which it is allowed to sendits data. At the beginning of each grant (or at its end) the ONU

sends its REPORT message. The reports reach the OLT and

the most recent report from each ONU is saved. When the

do_schedule interrupt is invoked, the last report received isused by the allocation algorithm. In the depicted example, theREPORTs from ONUs 1 and 2 are received in time to be

considered for the cycle N-1, whereas ONU 3s REPORT hasto wait until cycle N.

Note: The described timing model may easily be changed bychanging the interrupt allocation scheme. For example, the

allocation algorithm may be executed several times before thegrants are sent, optimizing the original execution.

3.3 Modeling MPCP in OPNET

As mentioned earlier, the MPCP defines different types of

messages for managing the EPON. Currently the only MPCP

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data passed through the fiber, and passes it on to the

packet_classifierprocess, which determines if the packet is acontrol packet destined to the OLT, or a data packet. The data

packets are routed to the port_1_transmitterand passed on,out of the EPON network. Control packets are passed on to

the schedulerprocess, which is the main process of the OLTdevice. The PON downstream is connected to the downlink

fiber of the network through the pon_transmitter. It receivespackets from olt_q, the OLTs queue process, which is based

on OPNETs abc_fifo process model. The queue receivesGATE messages from the scheduler, and optional downlink

traffic from the outside. Since we concentrate on the upstreamprocess, the downstream is used mostly to deliver the OLTs

control messages. The external ports Tx and Rx can be usedto test a TCP/IP connection between a station on the PON to

the outside.

Figure 8-OLT node model implemented in OPNET

The resource_manageris responsible for holding node-wideinformation (such as its MAC address) and execute nodeinitialization actions, if needed.

The scheduler (Figure 9), is responsible for managing theEPON upstream, and for implementing the timing model

described earlier. It is composed of the following states:

init Initializes the process model and specifically theselected allocation algorithm. Also jump-starts the

TDMA scheme by allocating the initial cycle interrupts.

idle The default state in-between process interrupts.

msg_arrival Receives control messages that wererouted to the scheduler process, and distributes them

according to the message type. Currently it handles only

REPORT messages, but it may be easily adapted tohandle more.

store_reports This process receives the REPORT andprocesses it as required by the selected allocation

algorithm.

schedule The key state of the scheduler process. Theactual execution of the bandwidth allocation algorithm is

done within this state. It relies on the formatted REPORTdata from the store_reports process, and its output is in

the format of a list of grant data for the upcoming cycle.

send_grants Uses the output of the Schedule state to

prepare standard GATE messages, and broadcast them tothe PON.

Figure 9-OLT scheduler state machine

3.5 ONU Node Module

Similar to the OLT, the ONU device is also composed of

PON upstream and PON downstream data paths (see Figure10). The packets received from the pon_receiverconnected to

the PONs downlink are passed to the packet_filter, which

passes on only the packets destined to the specific ONU.Other packets are destroyed in the packet_filter_sink. Next,

the packet_classifier determines if the packet is a control

packet or a data packet. The data packets are routed to theappropriate external port. Control packets are passed on to the

scheduler.

The different external ports are used to easily insertsimulation traffic into the different priority queues, each ofwhich corresponds to a different port number (currently only

four priorities are used).

Figure 10-ONU node model implemented in OPNET

The resource_manager is responsible for holding node-wide

information (such as its MAC address) and executing nodeinitialization actions if needed.

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The scheduler (Figure 11) is responsible for the

implementation of the timing model from the ONUs point ofview. It is composed of the following states:

init Initializes the process model.

idle The default state in-between process interrupts.

msg_arrival Similar to the OLT state of the same name.

Currently, it handles only GATE messages.

gate_arrival This process receives the GATE messageand is responsible for extracting the allocated grants data

and for scheduling interrupts accordingly.

gate_start Conducts necessary actions at the start-time

of each grant.

Figure 11-ONU scheduler state machine

onu_q, the ONUs queue process (as depicted in Figure 12),

is implemented as an active queue, meaning that it is

autonomous in the insertion and extraction of packets,according to its limitations and the allocated grants. As

mentioned earlier, it comprises 8 sub-queues, in accordance

with to 802.1Q. Control messages receive the highest priority.The queue may perform in one of three modes:

Unlimited sub-queue size.

Limited queue size with specific size for each sub-

queue.

Limited queue size with shared memory for the sub-queues, meaning that their size may vary and a high

priority packet in one sub-queue may cause a tail-drop of a lower priority packet in another sub-queue.

The queue performs three main tasks:

Insertion of packets managed by the ins_tail state,according to the selected work-mode for the queue size.

Packet transmission - the beginning of the GATE is set up

by thestart_gate state, and the actual transmission is

managed bysend_headWithin the GATE, packets aretransmitted one-by-one. A single packet is sent and then the

process sets an interrupt for the next packet to be sent at theactual time that the previous packet finishes, according to

the ONU line-rate. For this implementation the queue has 2idle states one for when the ONU is not transmitting

(branch state) and one for when the ONU is within a

grants timeframe (gate_idle).

Generation of a REPORT message At the appropriateinterrupt, make_report state creates a REPORT packet

and assigns its fields with appropriate values.

Figure 12-ONU queue state machine

4. Bandwidth Allocation Algorithms

Three bandwidth allocation algorithms have been

implemented: static, semi-static, and dynamic. The algorithmsare described in the following subsections.

4.1 Static Allocation

This is perhaps the simplest algorithm possible, and the one

we used in our model development and validation stage.

A cycle size parameter is specified, setting the size of each

allocation cycle in units of TQ.

The allocated grant size for each ONU is the same, and is set

by the simple calculation:_

_ _

cycle sizegrant

number of ONUs=

This division implements fairness in the grant allocation.

However, other divisions are also possible, as long as the sumof the static allocations does not exceed the cycles size. There

is no consideration of the actual needs of each ONU, and eachreceives the same allocation in every cycle. Figure 13 depicts

an example of the algorithm's allocation for 3 ONUs.

Figure 13-Static Allocation Illustration

4.2 Semi-Static Allocation Algorithm

This simple algorithm is somewhat of a hybrid between thestatic allocation and the dynamic one. The semi-static

algorithm, as opposed to the static algorithm, uses the

REPORTs collected by the OLT from the ONUs to determine

which ONUs requested bandwidth. Each REPORT acts as aBoolean variable that is False if the ONU did not request any

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bandwidth, and True if the ONU requested bandwidth. The

algorithm ignores the actual size of the bandwidth requests.

Each ONU (both idle and requesting) is granted a minimalallocation in every cycle, which is sufficient for it to send a

REPORT packet with requests for the next cycle. The

remainder of the cycles length is divided equally among theONUS that had bandwidth requests for this cycle. Unlike the

static algorithm, the semi-static does not allocate bandwidthto ONUs that are idle .Figure 14 presents a sample output of

the semi-static algorithm for 3 ONUs in the network.

Figure 14-Semi-Static Allocation Example

4.3 Dynamic Allocation Algorithm

The dynamic algorithm allows for a dynamic allocation ofbandwidth, which varies from cycle to cycle and is adapted to

the end-units SLA, the networks current requirements, andfairness in the division of excess bandwidth. The algorithm

currently supports two types of traffic - best effort traffic (BE)and committed rate traffic (CR), which is bandwidth that

ONU is entitled to but does not have delay/jitter limitations.Delay and jitter constraints force the allocation algorithm to

schedule the grants with added definitions of mandatorylengths and intervals, which add a great deal to the

complexity of the allocation problem.

Three important elements are defined:

Window A window is defined as the time interval to

be used for the measurement of whether the networkcomplies with the ONUs SLA constraints. In addition, it

defines the history interval during which the algorithmenforces the fairness in the allocation of BE traffic

among the networks ONUs.

Cycle Each window is divided into several equallength cycles (the total number of cycles per window is a

parameter of the algorithm). The cycle bounds the totalamount of bandwidth that can be allocated to the entire

network for each execution of the algorithm. Thus, the

delay between the ONU requests and the correspondingresponses is controlled.

Sub-cycle A sub-cycle is enabled when the total sumof requests is lower than the remaining cycle size. In this

case, each ONU receives a minimal grant and all requestsare granted.

The algorithm has two work modes, based on the networkload: a simple mode for low traffic loads and a complex mode

for high traffic loads. Decision points are defined along thetime-line, where the algorithm chooses the work-mode for the

next cycle or window. Low-load is selected if the sum ofrequests is less than cycle length; in this case the algorithm

allocates a sub-cycle. If the sum of requests is more than

cycle length the OLT enters high-load mode and remains in

it for at least the duration of one window (see Figure 15).

Figure 15- Dynamic algorithm mode arbitration

As mentioned, each window is divided into consecutivecycles. Within them, the algorithm allocates the CR requests

first; if there is room left in the cycle the algorithm allocatesthe BE requests, according to the algorithms fairness

guidelines. Due to length constraints, the details of the BEallocation mechanism are omitted from this paper. The main

idea is that an ONU is allocated BE bandwidth according tothe history of the BE allocations that it received since the

beginning of the current window. ONUs that have beenallocated less BE bandwidth get higher priority.

Figure 16-Dynamic algorithm bandwidth allocation to threeONUs in low-load (sub-cycles) and high-load (windows and

sub-cycles)

5. Simulation Results

5.1 Initial testing

Before starting the algorithm tests and comparisons, asimple test scenario was constructed in order to give a general

perspective on how the prototype functions and to convey the

general characteristics of the EPON as reflected by thesimulation. The following section shows examples of several

of the properties tested.

ONU queue population vs. grant size

These tests were conducted in order to verify the behavior of

the ONU according to the allocated grant size. The allocationwas done using the static allocation algorithm. A constant

priority 1 source is used for all the simulations. The sourcewas active between seconds 1 to 2 and then halted. The tests

differ in the size of the allocated grant. The size of the grantwas set by setting the number of ONUs in the PON for the

allocation algorithms. The more ONUs are connected, thesmaller the grant that each one receives. The queue size is

unlimited for these tests, in order to see how high the queuefills up at each case.

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Three scenarios were tested: one where the grant allocation

rate is about the rate of the source bandwidth (titled large-GATEs; one where the allocated rate is much smaller than

the source bandwidth, so queue explosion was expected(titled small-GATEs); and one where the grant size is in

between (titled medium-GATEs). The results are depictedin Figure 17, showing the network acts as expected.

Figure 17 - Queue size vs. sim time, TOP - "large-GATEs",

MIDDLE - "medium-GATEs", BOTTOM - "small-

GATEs"

ONU queue overflow mode test

In this experiment, we set out to test the shared-memory

mode of the ONUs sub-queues. The total queue size was setto 400,000 bits, and two priorities were fed with source data.

The source for the higher priority was active between seconds1-2, and the lower priority source was active between seconds

1-2.5. Figure 18 shows how the higher priority dominates thetotal queue, dropping the lower prioritys packets. When the

source is done and the packets are transmitted, the lowerpriority is enabled to add packets to the queue and transmit

them.

Figure 18 - ONU's sub-queues' size in bits vs. simulation

time

End-to-end delay vs. the number of active ONUs

This simple test shows how the end-to-end delay increases

with the number of ONUs. All sources are constant bit-rate of

about 30 Mbps. The static allocation algorithm is used. Notethat the delay starts to increase only after crossing thethreshold of about 30 ONUs (which is parallel to a static

allocation of about 30Mbps, same as the traffic sources rate),and continues with a linear increase. The specific delay value

is a factor of the cycle size, but the main idea here was to seethe networks trend. Data was collected through the execution

of a series of simulations with a varying number of ONUs.

Figure 19 - End-to-end delay in seconds vs. number of

ONUs in PON

5.2 Algorithm ComparisonFor the comparison of the three algorithms behavior, a 16

ONU scenario was constructed. We wanted to create a diverseenvironment that would allow us to examine as many aspects

of comparison as possible within the same simulation:

The networks packets sources generated Ethernet packets.

The packet length is drawn by an exponential distributionwith a mean of 3000 bits, but packets of over 1500 bytes are

discarded (Ethernet MTU size). The packet inter-arrival

time is exponentially distributed. The average source bit-rate was determined through the setting of different inter-

arrival mean values. Three source modes were defined:

o High Load 100 Mbpso Medium Load 50 Mbps

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o Low Load 5 Mbps

The 16 ONU sources were configured as follows:

o 8 ONUs High load

o 4 ONUs Medium load.

o 3 ONUs Low Load

o

1 ONUs source was idle throughout thesimulation.

Roughly, half of the ONU sources of each load type were

defined to be stable they had the same packetgeneration parameters throughout the simulation. The rest

of the ONUs were defined to have a burst that begins

some time after the simulation starts, and ends some timebefore it finishes.

The simulations duration was 0.69 seconds. The timelinewas divided as follows:

o Segment 0 - 0 0.0001 seconds Init margin, all

sources idle.

o Segment 1 - 0.0001 0.05 sec Only stable ONU

sources are active. The mean of the networks total bit-

rate was 510 Mbps.

o Segment 2 - 0.05 0.3 The bursty ONU sources

join the stable ones, raising the mean total bit-rate to

a peak of 1015 Mbps.

o Segment 3 - 0.3 0.5 The bursty ONU sources all

go down to a mean bit-rate of 5Mbps. The mean total

bit-rate is now at 545 Mbps.

o Segment 4 - 0.5 0.69 The bursty ONU sources stop

transmitting, only the stable remain. Mean total bit rateof 510 Mbps.

Allocation algorithm configuration:

o TQ 16*10-9 [sec]

o Cycle size 500 [sec]

o Cycles per Window 10 (for dynamic alg.)

Sample Results:

Figure 20 and Figure 21 show the total number of bits that ahigh-load ONU was allocated and its queuing delay,

respectively, produced by each algorithm in accordance with

the timeline described above. Other results, not shown here,indicate that the queue size presents similar behavior to that

of the queuing delays.

Note that the queuing delay is proportional to the queuesize. Figure 22 and Figure 23 show the same for low load

ONUs, and Figure 24 and Figure 25 show the same for

medium load.

Figure 20-Total Bits Granted [bits] vs. Time [sec] for a

High-Load ONU

Figure 21-Queueing Delay [sec] vs. Time [sec] for a High-

Load ONU

In a 16 ONU scenario, the static algorithm would allocateabout 30 Mbps per ONU. Clearly, a 100 Mbps high load will

not be able to send all of its data and its queue wouldexplode.

During time segment 1, the dynamic and semi-staticalgorithms utilize the fact that they do not allocate bandwidth

to non-requesting ONUs and are thus able to allocate morebandwidth to requesting ONUs. In this segment, the total

requests are smaller than the network capacity, so the dynamic

algorithm is able to allocate each ONU the amount itrequested. The semi static division of available bandwidth isalso sufficient for even the high-load ONUs request. Thus,

the queuing delay of packets for these two algorithms duringthis segment tends towards zero.

During segment 2, the bursty ONUs start requestingbandwidth, so the dynamic and semi-static algorithms have to

take them into consideration. The semi static algorithmgenerates an output very similar to the static algorithm (all

ONUs except the idle one are requesting some amount, so the

cycles bandwidth is divided equally among 15 ONUs -instead of 16 in the static). The dynamic algorithm is able to

allocate the low and medium loads according to their specificrequests. When these ONUs request less bandwidth than their

10


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allowed limit, the dynamic algorithm is able to allocate the

additional bandwidth to the high load ONUs.

During segment 3, the bursty ONUs requests falls down to 5Mbps, but they are still requesting bandwidth. So while the

dynamic algorithm adapts its allocations to the decrease in

requests, the semi-static algorithm still allocates the sameamount as before.

During segment 4, the bursty ONUs stop requesting

bandwidth. Here, the semi-static algorithm can stop taking

them into consideration and can allocate more bandwidth tothe other ONUs.

Some additional observations:

According to Figure 22, the low load ONU seems to be

allocated smaller grants by the semi-static than by the staticalgorithm. This seems to contradict the fact that the

smallest allocation possible for a requesting ONU is_

_ _

Cycle SizeGrant

Number of ONUs=

, the same as the static, and there shouldbe no possibility that the static will allocate more than the

semi-static. A closer look reveals the explanation: the low-loads request rate may sometimes be smaller than the

REPORT packet frequency in the semi-static algorithmThis means that there will be some cycles where the low-

load request will be zero even when the source is active. Inthese cycles the ONU will not be allocated any bandwidth

for data. Consequently, the total allocation slope is moremoderate


Low-Load ONU

Figure 23-Queueing Delay [bits] vs. Time [sec] for a Low-

Load ONU

The phenomenon described in the previous observation alsoaffects the queuing delay. A packet arriving during a cycle

in which the ONU was not allocated any bandwidth will be

delayed more than when there is an allocation (such as inthe static case). This is seen in Figure 23: there is anincrease in the delays for the semi-static and the dynamic

algorithms in time segment 2, where the ONU operates inhigh-load mode, in comparison to the static algorithm.

The delays in the dynamic algorithm during segment 2 arehigher than the semi-static delays. This is because the semi-

static algorithm allocates more bandwidth than actually

requested, so packets arriving after REPORT was sent mayalso be transmitted before they are actually reported. This

lowers the overall delay of a packet in the queue.

When the dynamic algorithm operates in low-load mode,

the sub-cycles allocate all requests, and the next sub-cyclestarts right after the current one ends. As seen in Figure 21,

Figure 23, and Figure 25, the delays tend towards zero afterthe queues stabilize, a feature that is not possible in a work-

mode that defines a fixed cycle size, such as the static andsemi-static algorithms and the high-load mode in the

dynamic algorithm.


Medium-Load ONU

11


12/13

Figure 25-Queueing Delay[sec] vs. Time [sec] for a

Medium-Load ONU

Figure 26 shows the queue delay for a high load burstONU. At 0.3 sec the source drops from 100 Mbps to 5

Mbps, and shuts down completely after 0.5 seconds. In thefigure we see that all three algorithms manage to empty the

queue and handle the burst event. What is seen clearly isthat the dynamic algorithm manages to empty the queue

very fast, followed by the semi-static and the staticalgorithm.

Figure 26-Queueing Delay [sec] vs. Time [sec] for a High-

Load Burst ONU

Figure 27, Figure 28, and Figure 29 show the totalallocation of each algorithm to each of the ONU types.

Figure 27 shows the results for the dynamic algorithm. Init we see how the allocation is dependant on the ONUs

specific needs, so each of the six types is treated separately.The reason that burst ONUs continue to receive noticeable

allocations even after they shut down is that the totalallocation also includes allocations for REPORT messages,

and the dynamic algorithm working in low-load moderequests a large amount of REPORTS from all network

stations (but remember that the bandwidth dedicated tothose REPORTs is not utilized in low network loads).

Another observation is that the low-load of 5 Mbps isnegligible

Figure 27-Bits Granted [bits] vs. Time [sec] for Each Load

Type Using Dynamic Algorithm

In Figure 28 we see that the semi-static algorithm groups

together the medium and high load ONUs, and that the

allocations increase at similar rates for all requesting ONUs.The exception is the low load mode, which displays a lowerslope, as was explained previously.

Figure 29 shows the expected result that the staticallocation treats all ONUs in the same manner and allocates

the same bandwidth to all ONUs.


Type Using Semi-Static Algorithm

12


13/13


Type Using Static Algorithm

6. Conclusions

Although the research is only in its beginning, severalconclusions are already eminent:

For heterogeneous-source networks, the dynamic algorithmachieves the best division of the network bandwidth as it

adapts the allocations to each end-stations needs. It alsohandles high-load bursts most effectively.

For ONUs with low load sources in a highly loaded

network, the dynamic algorithm shows the worst delayperformance. This is because the other algorithms allocate

more than the low load ONUs need, and they may use theextra allocations to send new packets faster without the

need to report them. On the other hand, the rest of theONUs in the network suffer more because there is wasted

bandwidth while they still have outstanding requests.

For the same reason, the static allocation provides betterdelay results for the low-load ONUs than the semi-static

algorithm since it keeps allocating bandwidth even if itdoes not receive a request for it.

The downsides of the static algorithms are clear: it wastes alot of empty grants that may be needed by other ONUs, and

it prevents over-subscription to the network. It limits the

amount of ONUs/allocated bandwidth per ONU for the

network.

For ONUs that request less bandwidth than the semi-static

algorithm eventually allocates them, the semi-staticalgorithm takes on the downsides of the static algorithm as

described above. On the other hand, it handles situationswith idle ONUs well. Time segments 3 and 4 show these

two behavior characteristics.

7. Current Activities and Future Work

We are currently in the process of exploring further thedynamic algorithm, mostly learning its behavior

characteristics with different attribute parameters and workmodes (such as testing the algorithms behavior without the

low-load mode, or with a limit on the minimal size of a sub-

cycle). Concurrently, we are working on enhancements to the

simulation environment, adding more features and bringingthe simulation closer to realistic results (such as the addition

of propagation delays to the network).

Future work includes adding traffic types to the algorithm,

such as delay-critical traffic. Another future direction is thetesting of additional allocation algorithms of different natures.

References

[1] Point to Multipoint Ethernet Passive Optical Network

(EPON) Tutorial, by Gerry Pesavento, EFMTaskforce

http://www.ieee802.org/3/efm/public/jul01/tutorial/p

esavento_1_0701.pdf

[2] MPCP State of the Art, by Ariel Maislos et. al., EFMTaskforce

http://grouper.ieee.org/groups/802/3/efm/public/jan02/maislos_1_0102.pdf

[3] MPCP: Message Format, by Onn Haran et al, EFM

Taskforcehttp://grouper.ieee.org/groups/802/3/efm/public/jan0

2/haran_1_0102.pdf

[4] IEEE 802.3ah Ethernet in the First Mile Task Force

web site, including task force meetings' presentation

material and meeting summaries.http://grouper.ieee.org/groups/802/3/efm/public/

[5] CableLabs, Data-Over-Cable Service InterfaceSpecifications - Radio Frequency Interface

Specification (status: interim), March 1999.

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