Performance Analysis of the IPACT Dynamic Bandwidth Allocation Algorithm for EPONs

1. Performance Analysis of the IPACT Dynamic Bandwidth Allocation Algorithm for EPONs Joint research between IBBT-UGent and ENST-Paris Dept. of Information Technology (INTEC)

2. Passive Optical Networks • Downstream (P2MP): • Packets filtered in Optical Networks Units (ONUs) • Upstream (P2P): • MAC protocol to avoid collisions • Offers an arbitration mechanism for upstream traffic • Provides signaling infrastructure • Bandwidth allocation algorithm to divide bandwidth Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

3. Performedresearch (IBBT & ENST) Furtherresearch Ethernet PON (EPON) • MAC protocol • EPON uses Multi Point Control Protocol (MPCP) • Ethernet control messages: GATE and REPORT • Dynamic band allocation (DBA) algorithms: 2 examples • IPACT (Interleaved Polling with Adaptive Cycle Time) • OLT keeps a polling table • Contains amount of bytes in each ONU and RTT • Updated by examining each ONU via round-robin • OLT grants a transmission window to each ONU • BGP (Bandwidth Guaranteed Polling) • OLT guarantees certain ONUs with a fixed bandwidth Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

4. Original model (ENST) • Analytical model of IPACT • ONU-architecture supporting three service classes • Expedited ForwardingAssured ForwardingBest Effort • Modeled by Weighted Round Robin (WRR) • Shortcomings ( model differs from reality) • Some oversimplifications in analytical IPACT model • WRR only suitable for a permanently available link • WRR not compliant with IEEE 802.1D standard Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

5. New model (IBBT, ENST) • Problem original model: • Mathematical description differs from reality • Model tries to model too much at once: • IPACT itself already very complex • Differentiated services even more complex • New Approach: • Analyzing simpler cases to provide real insight into the matter • Traffic load: symmetric (i.e. equal for each ONU) • Packet arrivals: Poisson distributed • Packet sizes: constant fixed length • No implementation of differentiated service classes • Validation verified by simulations Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

6. Analysis of IPACT algorithm • Analysis consists of two steps • Thorough analysis of the cycle times • Cf. IPACT, Interleaved Polling with Adaptive Cycle Times • Cycle time = time between the start of two successive transmission windows for a fixed ONU. • Derivation of the packet delay from the cycle times • Several service disciplines exist in IPACT • Service discipline = possibility for the OLT: • to determine the granted window size Wi • dependent on the requested window Vi • Three service disciplines analyzed: • Fixed: Wi = WMAX • Gated: Wi = Vi • Limited: Wi = min(Vi , WMAX) Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

7. Simulations in NS-2 • Simulation parameters • N Number of ONUs 16 • λ Load per ONU 5 → 65 Mbit/s (Poisson – symmetric) • ℓ Distance ONUs - OLT 20 km • TguardGuard time 5 or 1.5 µs • B Packet size 12000 bits • Breq REPORT message size 576 bits • Beth Ethernet overhead 304 bits • RU Upstream bandwidth 1 Gbit/s • PmaxMax. transmission window 10 or 3 packets Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

8. Fixed service • Constant cycle time Tcycle • Average queue size at discrete moments nTcycle • Queue sizes at t = nTcycle form a discrete homogeneous Markov chain  transition probabilities can be defined • Probability formulas: obtained by Poisson properties • Stationary distribution: obtained by linear system of equations • Delay derived from queue sizes (Little’s law) Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

9. Gated service • Varying cycle times: more complex analysis • Successive cycle times influence each other (correlated) • Distinction between low and high traffic load • Low: cycle time mostly determined by the traffic of that ONU (+ in case of clustering, the ONUs that are polled right before) • High: cycle time most often determined by the traffic off all ONUs together (distribution of cycle times via Markov chain) • Delay directly derived from average cycle time Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

10. Limited service • Some properties similar to fixed service and some similar to gated service • Possibility that a packet cannot be sent with its first requested window (cf. fixed) • Cycle time is variable (cf. gated) • System too complex for a complete analytical analysis similar to fixed or gated service • Especially in case of a small transmission window and a high load (high chance that a packet cannot be immediately sent) • In the other cases, a model analogue to gated service gives fairly good results Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

11. Extensions • Asymmetric load • Fixed: analysis still applies • Gated: analysis for high load is valid • Distribution packet size • Fixed: difficult since an unused part of the transmission window • Gated: good results obtained by using an average packet size • Self-similar traffic • too complex for analytical model Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)

12. Conclusion • Analytical model formulated for IPACT • Fixed and gated service: valid model • Starting from the cycle time (very important for gated service) • With the assumptions of symmetric traffic load, Poisson traffic and fixed packet size • Limited service: too complex for numerical analysis • Also a valid model in some more general cases • The importance of simulations • Allows to verify analytical method and limitations • Only possibility in more complex situations • E.g. for the moment: research to the possibilities of the Bandwidth Guaranteed Polling DBA algorithm Performance analysis of the IPACT DBA Algorithm for EPONs Dept. of Information Technology (INTEC)