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QOS in Passive Optical Networks

Ali Razmkhah

Akbar Ghaffarpour RahbarComputer Networks Research Lab, Sahand University of Technology

Dec. 2010

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Agenda

I. Introduction

II. QOS methods

III. Other issues

IV. Simulation

V. Conclusion

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Introduction

• Ethernet PONs provide high bandwidth Ethernet.

• As a full service access network, the PON should support communications with diverse QOS requirements, i.e., VBR and CBR traffic, gaming, video conferencing, etc.

• QOS is provided by the way of bandwidth allocation – The length of cycle– The size of transmission window– The order of scheduling

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QOS methods – Priority Queues

• Priority Queues– A useful and relatively simple method for supporting differentiated

service classes– Network traffic is classified into a set of classes with diverse QOS

requirements– Each ONU can maintain up to 8 queues for different classes of QOS

• Typical priority classes:– Best effort(EF), lowest priority– Assured forwarding(AF), medium priority, bursty– Guaranteed forwarding(GF), highest priority, CBR traffic

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QOS methods – Priority Queues

• Strict priority scheduling– lower-priority queue is scheduled only if all queues with higher

priority are empty

• Non-strict priority scheduling– packets that were reported are transmitted first as long as they can be

transmitted within the allocated time slot

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QOS methods – Assi

• Assi method– In Assi method, each ONU assigned a weight w based on SLA

agreement. ∑ w = 1– Minimum bandwidth for each ONU at peak times is computed by Eq.1

(( (1)

– Then excess bandwidth calculated by Eq.2

(2)

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QOS methods – Assi

– Excessive bandwidth can be distributed among the heavyloaded ONUs

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QOS methods – IPACT-GE

– Report message used by an ONU to demand bandwidth– OLT grants each ONU by Gate message

– IPACT-GE estimates the amount of new packets arriving between two consecutive pollings, and grants to the ONU the additional estimated size besides the ONU’s requested size in the previous cycle

– Arrival rate can be obtained for the n-th cycle by

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QOS methods – IPACT-GE

– The ONU reports estimated amount E(n+1) to the OLT, together with the instant buffer length at the n-th cycle R(n)

– The OLT decides the grant size for the (n+1)-th cycle

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Other Issues – ONU assignment

• Multiple granting schemes were studied and designed– Fixed granting, fixed size window without considering what size

requested by the ONU– Gate assignment, the ONU granted whatever it has requested– limited granting, each ONU is granted minimum of the requested size

and Wmax

– Simulation results shows that limited scheme has the best performance!

• Example– ONU buffer contains 5 packets: (1000, 1500, 800, 700, 1500) totally

5500 bytes – Wmax = 5000 bytes

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Other Issues – ONU assignment

– The ONU demands 5500 bytes and the OLT grants 5000• The ONU can not transmit the last packet and transmits only 4000 bytes• 1000 bytes of the allocated window waste!

• Solution– First, the OLT announces Wmax to all ONUs – Each ONU computes what size it can upload by tacking Wmax into

account, then reports computed size– In the previous example, the ONU reports 4000 instead of 5500 and

now there is no bandwidth wasting

1000 1500 800 700 1500

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Other Issues – Multiple Wmax

– Also limited granting scheme has the best performance, but Wmax is equal for all ONUs

– Assi method is suitable for offline scheduling at peak times

• Solution– Assign a weight to each ONU based on some parameters, i.e., number

of LANs connected to that ONU, time of day and other statistics gathered in advance by the OLT

– Instead of assigning a minimum bandwidth for each ONU at peak time, assign a different Wmax for each ONU

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???

Questions!?