Quality of Service Guarantee over 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 Mobile Terminal Wired Ethernet Router Access Point Access Point Media Server Conference Server Router Mobile Terminal

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 order VFT(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 BW i 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 Virtual Time 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 over Credit = Prev_Credit + BW * Elaspe_Time If (PacketExists and Packet_Size < Credit) Transmit Packet; Credit = Credit – Packet_Size Credit = 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 W i, then M =GCD(W i ) 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 2 K-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 p 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 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 Access Point Wireless Rether Server Wireless Rether Client Wireless Rether Client Wireless Rether Client Wired Network

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 downstream Packet size: 1444 bytes Cycle time: 33 ms

Wireless LAN QoS Three senders 1.1Mbps sending rate

Wireless LAN QoS Cycle time: 33 ms 1444 bytes 812 bytes 172 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 b and 2G/GPRS/3G networks Porting to a is straightforward Leveraging e 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