Sriram Lakshmanan Zhenyun Zhuang A Unified Architecture for the Design and Evaluation of Wireless Fair Queueing Algorithms Sriram Lakshmanan Zhenyun Zhuang

Introduction In wireline networks resource reservation and allocation are typically used for QOS Difficulty of performing reservation and fair scheduling in wireless networks due to Location-dependent and bursty errors Channel contention Joint scheduling of uplink/downlink flows

Motivation for a unified Framework Need for unified Architecture Over the recent past, several algorithms have been proposed for wireless scheduling Notion of fairness for the wireless domain unclear Identifies Generic blocks Also enables comparison of algorithms

Wireless Channel Model Packet cellular network Single-hop network Neighboring cells transmit on different logical channels Every mobile host in a cell can communicate with the base station but two hosts are not necessarily within their ranges Each stations performs the scheduling of both uplink and downlink transmissions in its cell

Key Characteristics of Wireless Channel Wireless channel capacity is dynamically changing Channel errors Contention for channel among hosts Hosts don’t have global knowledge Scheduling has to take care of uplink and downlink processing power and battery constraints

Service Model- Fluid Fair Queueing Properties Fairness among backlogged flows over infinitesimal time Bounded delay channel access Guaranteed minimum throughput for backlogged flows In Summary: Full separation between flows Minimum guarantees provided for a flow is unaffected by others Assumes: Error free and no location dependent All flows can transmit at a given time Or none can transmit

Fairness criterion Each flow i, j satisfies the following property:  i, jB(t1,t2) Wi(t1, t2) Wj(t1,t2) ri rj Eq.(1) B(t1, t2): set oh backlogged flows Wi(t1, t2): channel capacity granted to each flow i [t1, t2]: time interval

Wireline fair queuing fails over wireless channels (Illustration 1) Flows f1, f2, f3 in [0,2] with r1=r2=r3 Error –free channel W3=1/3 Error –free channel W2=1/3 Error –free channel W1=1/3 [1, 2] Error –free channel W2=5/6 Error –free channel W1=5/6 [0, 2] Channel error W3=0 Error–free channel W2=1/2 Error–free channel W1=1/2 [0, 1) f3 f2 f1 flow Time Does not satisfy the fairness property of Eq.(1)

Wireline fair queuing fails over wireless channels (illustration 2) Flows f1, f2, f3 in [0,2] with r1=r2=r3 Error –free channel W3=2/3 Error –free channel W2=0 Error –free channel W1=1/3 [1, 2] Error –free channel W2=2/3 Error –free channel W1=2/3 [0, 2] Channel error W3=0 Error–free channel W2=2/3 Error–free channel W1=1/3 [0, 1) f3 f2 f1 flow Time - Is it acceptable for f1 to be impacted? - Over what time period should f3 be compensated for its loss? - In this scheme separation of flows is lost. - Tradeoff between separation and compensation

Wireless Fair Service Model Short-term fairness for backlogged flows when perceive a clean channel Long-term fairness for backlogged flows with bounded channel error Channel-conditioned delay bounds for packets Short-term throughput bounds for flows with clean channels

Wireless Fair Service Model - Continued Long-term throughput bounds for all flows with bounded channel error Support for both delay sensitive and error sensitive Very useful for handling both delay sensitive and error sensitive flows in error-prone channels Optionally, optimization of the schedulable region by decoupling the delay and bandwidth requirements of flows

Definitions in wireless fair queueing algorithms Error-free service: service it would have received if all channels had been error-free Leading flow: received channel allocation excess of it’s error-free service Lagging flow: received channel allocation less than it’s error-free service In sync flow:its channel allocations exactly the same as its error-free service

Unified Wireless Fair Queueing Framework Emulate fluid fair queueing when all flows perceive error-free channels Its goal is to minimize: Wi(t1, t2) Wj(t1,t2) ri rj

Seven Algorithms CSDPS IWFQ WPS Channel State Dependent Packet Scheduling IWFQ Idealized Wireless Fair Queueing WPS Wireless Packet Scheduling CBQ-CSDPS: Enhanced Class Based Queueing SBFA Server Based Fairness Approach WFS Wireless Fair Service Algorithm CIFQ Channel-condition Independent Fair Queueing

Wireless fair queuing algorithms The different proposals differ in terms of : how the channel access swapping takes place Between which flows the swapping tacks place How the compensation model is structured

Five Components Error-free service Lead and lag model Compensation model Slot queues and packet queues Channel monitoring and prediction

1. Error-free service How much service a flow should receive In an ideal error-free channel environment Packetized approximation of fluid fair queueing WFQ by IWFQ, SBFA WRR by WPS …..

WFQ

2. Lead and Lag Model Lag of a lagging flow Lead of a leading flow The amount of additional service to which it is entitled in the future in order to compensate for lost service in the past Lead of a leading flow The amount of additional service that the flow has to relinguish in the future In order to compensate for additional service in the past Two approaches to compute Difference between the error-free service and received Number of slots allocated to the flow during its channel error Upper bounded

3. Compensation model Determines: Three main issues has to address: how lagging flows make up their lag how leading flows give up their lead Three main issues has to address: When does a leading flow relinquish the slots that are allocated to it ? All slots till it becomes in sync.(IWFQ) Fraction of slots.(constant CIF-Q proportion WFS) Never, assume there is some reserved BW.(SBFA)

Compensation model (cont) When are slots allocated for compensating lagging flows ? Preferentially allocated till there is no lagging flow. (IWFQ) Allocated only when leading flows relinquish slots. (CIF-Q & WFS) Allocated from reserved BW. (SBFA) How are compensation slots allocated among lagging flows ? Flow with the largest lag. (CIF-Q) According to the order they became backlogged. (IWFQ & SBFA) In proportion to its lag. (WFS)

4. Slot queues and packet queues Allow for the support of both delay sensitive and error sensitive flows in a single framework Decouple connection-level packet management policies from link-level packet scheduling policies Each flow has 1 slot queue and 1 packet queue

Cont’ Slot queues: Packet queues: Slot is the unit of channel allocation Delay-sensitive flows Delete packets once it violates its delay bound Packet queues: Packet is the unit of data transmission Error-sensitive flows Do not delete head-of-line packet upon channel error during transmission

5. Channel monitoring and Prediction Perfect channel-dependent scheduling depend on accurate channel state Errors typically occur over bursts and highly correlated in successive slots Channel prediction can be achieved by using: N-state Markov model One step prediction algorithm

One Example:IWFQ Idealized Wireless Fair Queueing Error-free WFQ Lag and Lead The difference between service tags Compensation Model Favors lagging flows But may starve leading flows in the short run Comments Short-term Fairness and Throughput Bounds are violated Provides Long-term fairness and bounded delay channel access

Simulations WFS and CIF-Q achieve all the properties Short-term and long-term fairness Short-term and long-term throughput bounds Tight delay bounds for channel access