Quality of Service Guarantee over 802.11 Wireless LAN

Tzi-cker Chiueh

Wireless LAN QoS

Introduction Multimedia applications requires QoS

support, specifically bandwidth guarantee But radio link is a shared resource and its

access is through CSMA/CA protocol packet collision on the channel

Collision causes two problems: Non-deterministic access delay Lower effective link throughput

Wireless LAN QoS

WLAN Configuration

MobileTerminal

Wired EthernetRouter

AccessPoint

AccessPoint

Media Server

Conference Server

802.11

Router

MobileTerminal

802.11

Wireless LAN QoS

Quality of Service Metrics Bandwidth Delay Delay jitter Packet loss rate

Wireless LAN QoS

Theory Fluid fair queuing: standard weighted round

robin with two exceptions: Infinitesimally small granularity Simultaneous service

No queuing delay Impossible to impelement in practice

Wireless LAN QoS

Approximation I Packetized weighted fair queuing (WFQ) Simulate FFQ by computing the virtual finish

time of an incoming packet, and servicing packets based on finish time orderVFT(i) = Max{VFT(i-1), VAT(i)} +

Packet_Size/BW

Delay Bound: Burst/BW + SUM(Packet_Size/BW + Packet_Size/Capacity)

Wireless LAN QoS

Virtual Time In each virtual time unit, each backlogged

connection i gets a BWi share Allows the finish time of a packet to be

independent of existence of other connections The number of real time units required to service a

virtual time unit of work depends on the number of backlogged connections

VT-RT mapping requires O(N) overhead because of iterative deletion Various approximations, e.g., SCFQ

Wireless LAN QoS

Virtual-Real Time Mapping

Real Time

VirtualTime

VT-RT mapping overhead could be spread out when queues evolve from backlogged to non-backlogged

Wireless LAN QoS

Real Bottleneck VT-RT mapping overhead is probably not

that important in practice VFT sorting takes O(logN) and is the real

scalability limit Can “locality” help? How big can N be realistically?

Wireless LAN QoS

Approximations II Weighted round robin (WRR)

Simple to implement Cycle time: tradeoff between

efficiency and delay bound Variations:

Deficit RR Discrete Fair Queuing: non-packet-

based Smooth RR: still packet-based

Wireless LAN QoS

Deficit Round Robin Allow unused credit from previous cycles to

carry overCredit = Prev_Credit + BW *

Elaspe_TimeIf (PacketExists and Packet_Size <

Credit) Transmit Packet; Credit = Credit – Packet_SizeCredit = Cap(Credit)

Wireless LAN QoS

Discrete Fair Queuing Discard packet-based assumption FFQ with small scheduling quantum Rely on link-layer multiplexing/demultiplexing

support O(1) implementation complexity Delay bound is proportional to quantum size Easy to implement in hardware

Wireless LAN QoS

Smooth Round Robin Schedule across multiple (M) cycles of WRR Assume weights are Wi, then M =GCD(Wi) NxK scheduling matrix, where K = log(M) Each of M slots is marked with one of the K

labels and the distance between consecutive slots marked with the d-th label is 2K-d slots

O(1) complexity and pretty good delay bound compared to WFQ

Wireless LAN QoS

QoS on WLAN A wireless channel vs. a wired link Queues are fundamentally distributed Raw bandwidth from the AP to different wireless

stations may be different Raw bandwidth from the AP to the same wireless

station may be different at different points in time Interactions with media access control protocol Hidden node problem

Wireless LAN QoS

Wireless Rether Rether is a software-only token passing protocol

originally developed for shared-segment Ethernet adapted to WLAN

Provides bandwidth guarantee to individual applications, both upstream & downstream

Requires changes to AP and every wireless node No changes to applications are required Interoperable with wired network’s DifferServ or

802.11p mechanisms

Wireless LAN QoS

Wireless Rether A WLAN node can send traffic only when it receives

the token Token circulates among real-time (RT) nodes in a

periodic fashion Token holding time depends on the total bandwidth

reservation on each node Whatever residual cycle time left by RT nodes are

used by the NRT nodes Requires explicit registration from WRC with WRS

Wireless LAN QoS

Link Scheduling DRR but based on channel usage rather than

number of bits transmitted Per-connection packet queuing on each node Need to dynamically estimate and measure per-

packet channel usage time Overflowed packets are redirected to NRT queue How many NRT packets should be allowed to be

dispatched at a time? Based on global knowledge of NRT queue lengths

Wireless LAN QoS

Architectural Decisions Hardware vs. Software implementation Peer-to-peer vs. Centralized token passing

Essentially the polling mode in 802.11 standard

Is it necessary in infrastructure mode? Work-conserving vs. Non-work-conserving

network link scheduling To ACK or Not to ACK

May not be necessary always Implicit vs. Explicit bandwidth reservation

Wireless LAN QoS

Rether System Architecture

Router

802.11

AccessPointWireless

RetherServer

WirelessRether Client

WirelessRether Client

WirelessRether Client

WiredNetwork

Wireless LAN QoS

Bandwidth Reservation Reservation policy table

SrcAddress/Mask, DestAddress/Mask, SrcPortRange, DestPortRange, Bandwidth Requirement

Statistical admission control: based on actual usage rather than reservation sum

Leave slack to avoid starvation of NRT traffic Automatic two-way reservation for TCP Intra-LAN connection requires twice the amount of required

bandwidth reservation Special packet queues for Rether packets and other network

control packets (ARP and ICMP)

Wireless LAN QoS

Transparent Packet Scheduling

Wireless LAN QoS

Wireless Rether Client

Wireless LAN QoS

Wireless Rether Server

Wireless LAN QoS

Prototype and Test-bed Implemented under Red Hat 7.0 WRS is a 400-MHz Pentium-II machine

with 128 Mbytes of memory WRC is 650-MHz Pentium-III portable

machine with 64 Mbytes of memory Orinoco wireless LAN cards and access

point (AP-1000) Wired network is Fast Ethernet

Wireless LAN QoS

2 upstream and 1 downstreamPacket size: 1444 bytesCycle time: 33 ms

Wireless LAN QoS

Three senders 1.1Mbps sending rate

Wireless LAN QoS

1 2 3 4 5 6 7 8 9 10 11 12 13No. of Clients

0

1

2

3

4

5

6

Thr

ough

put

(Mbi

ts/s

ec)

Throughput Vs. No. of ClientsWith different packet size

64 bytes172 bytes812 bytes1444 bytes

Cycle time: 33 ms1444 bytes

812 bytes172 bytes

64 bytes

Throughput vs. Number of Clients

Wireless LAN QoS

16Kbps

84Kbps

300Kbps

1Mbps

Wireless LAN QoS

Improvements WRS can be readily used as a traffic manager for

downstream traffic on a wireless LAN; no WRC is needed on the mobile terminal

TCP-aware good-put management Automatic content-based bandwidth reservation Low-latency hand-off for infrastructure-mode wireless

LAN, from 2-3 sec to under 100 ms Vertical hand-off between 802.11b and 2G/GPRS/3G

networks Porting to 802.11a is straightforward Leveraging 802.11e standard

Wireless LAN QoS

In Retrospect,…. Major performance problem lies in token passing overhead

due to buffering delay at access points; scheduling and buffering cause no performance problems

Redundancy between link-layer, WRether-layer and network layer mechanisms: registration and ACK

How to leverage MAC-layer header information: Eliminate token ACK overhead Turn on the token passing mechanism only

when necessary: determine the extent of collision

Trade off between degree of QoS guarantee and QoS mechanism overhead