Post on 18-Jan-2016
1
QOS in Passive Optical Networks
Ali Razmkhah
Akbar Ghaffarpour RahbarComputer Networks Research Lab, Sahand University of Technology
Dec. 2010
2
Agenda
I. Introduction
II. QOS methods
III. Other issues
IV. Simulation
V. Conclusion
3
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
4
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
5
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
6
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)
7
QOS methods – Assi
– Excessive bandwidth can be distributed among the heavyloaded ONUs
8
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
9
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
10
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
11
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
12
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
13
???
Questions!?