On the feasability of transmission scheduling

in a code-based transparent passive optical network

architecture

P. M. Santiago del Río, J.A. Hernández, V. López, J. Aracil, B. Huiszoon

14th European Conference on NOCJune 12th, 2009 Valladolid (Spain)

Outline

Introduction Analysis Numerical Experiments Conclusions

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Introduction (I) Architecture

• Each PON contains Ni ONUs

• Multiplexing solutions:• Synchronous: TDMA (well-established technology)

• Asynchronous: OCDMA (simplifies control and management plane)

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Introduction (II) OCDMA:

• Data is signed with unique orthogonal optical code.• Optical carrier can be shared by all the users.

Problem:• Degradation caused by Multiple User Interference

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Introduction (III) Contention resolution:

• Transmission scheduling:– Requires user monitoring functionalities at the ONU– ONU derives accurate information and acts accordingly

• Packet-level analysis

Transmission scheduling:• Before transmitting, it must to check the media

availability (active users < Maximum of users allowed):

» If u<M, it can transmit the data» If u=M, it must wait until one of ONUs finishes.

• The system enters a blocked situation when Mth connection arrives 5

Introduction (IV) Issue:

• The information sent by one ONU, it is sensed 2τ seconds in the future by the remaining ONUs

• Transmission delay• “Old” information available about state at passive

coupler

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Introduction (V)

Goal of this paper:• Analyze blocking duration under several traffic

conditions and varying networking parameters

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Analysis (I)

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Time-slotted scenario:• 1 time-slot = 1packet• Times-slotted scenario to facilitate analysis

Burst length probability distribution:• Geometric distribution:

– Memoryless– Model traffic highly-multiplexed

• Pareto distribution:– Self-similarity– Model traffic from residential end-users (not multiplexed)

Analysis (II) Blocking-time probability distribution:

• When Mth connection arrives to the system sees the residual life of the other M-1 connection

• D is given by the minimum of the M-1 residual lives and the duration of the Mth arrival

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Analysis (III) Evaluation criterium of the Feasibility:

• P(D>2τts)

• We are interested in finding the optimum values for L, B, EXon and M such that P(D>2τts)≥1-ε

• For instance, it is interesting to know, the maximum bit rate, B, such that P(D>2τts)≥0.8 i.e. there is coherence of state 80% of the blocking time

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Numerical Experiments (I) L vs. P(D>2τts) (Geometric):

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• P(D>2τts) does not depend on L for values up to 2 km, because is smaller than one time-slot.

The fiber length is not an important parameter (if L<2km).

• P(D>2τts) decreases as M increases since ED is smaller.

Blocking lasts less time when the number of active users is higher.

• P(D>2τts) increases with an increasing EXon since the value of ED increases.

Blocking lasts more time when the bursts are longer.

Numerical Experiments (II) L vs. P(D>2τts) (Heavy-tailed):

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• Behavior similar to geometric case.

•P(D>2τts) is smaller because the variance of Xon is infinite in the Pareto case.

• P(D>2τts) is too small to do transmission scheduling.

• If the traffic is not highly multiplexed, it is less interesting to deploy the transmission scheduling medium access mechanism.

Numerical Experiments (III) B vs. P(D>2τts) :

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• P(D>2τts) decreases as B increases.

• If we want to assure P(D>2τts)≥0.8:• if M=8 then B must be ≤ 800 Mbps• if M=4 then B must be ≤ 2 Gbps

Conclusions This work provides a set of guidelines for

designing PONs assuming:• Topology constraints: Fiber length (L)

• Observed pattern traffic: EXon

• Maximum number of active users: M

These parameters determine the maximum bit rate such that coherence of state holds (transmission scheduling is feasible)

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Acknowledgements This work was carried out with the support of:

• BONE project (“Building the FutureOptical Network in Europe”), a Network of Excellence funded by the Europea Commission through the 7th ICT-Framework Programme.

The authors would also like to acknowledge the support of the Spanish MEC:: • DIOR project (TEC2006-03246/TCM), • Juan de la Cierva post-doctoral research program.

Thank you for your attention

Questions?

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