WiFi-Nano: Reclaiming WiFi Efficiency through 800 ns Slots Eugenio Magistretti Krishna Kant Chintalapudi * Božidar Radunović # Ramachandran Ramjee * Rice University * Microsoft Research India # Microsoft Research Cambridge
Problem Overview [Mbps] ~55 Mbps 1997 802.11 1999 802.11a/b 2003 802.11g 2009 802.11n 2012 802.11ac/ad 802.11 data-rates have increased from 1 Mbps to 1 Gbps Throughput performance has not seen a commensurate increase
Contribution WiFi-Nano increases 802.11 throughput up to 100%
Why Throughput << Data-rate? Medium Access Preamble Data ACK 54 Mbps DIFS 101.5µs 20µs 224µs SIFS 44µs 7 6 5 4 3 2 1 0 405.5 µs Medium Access Preamble Data ACK 300 Mbps DIFS 101.5µs 32µs 40µs SIFS 36µs 7 6 5 4 3 2 1 0 225.5 µs Medium Access Preamble Data ACK 600 Mbps DIFS 101.5µs 40µs 20µs SIFS 44µs 7 6 5 4 3 2 1 0 221.5 µs
Why Throughput << Data-rate? 54 Mbps DIFS Medium Access 101.5µs Preamble Overhead ~45% Data 20µs 224µs SIFS ACK 60µs 7 6 5 4 3 2 1 0 405.5 µs 300 Mbps Medium Access 101.5µs Preamble Data 32µs 40µs SIFS ACK 52µs Overhead ~82% 7 6 5 4 3 2 1 0 225.5 µs 600 Mbps Medium Access Overhead Preamble Data ~91% ACK 101.5µs 40µs 20µs 60µs SIFS 7 6 5 4 3 2 1 0 221.5 µs 802.11 overhead does not scale with data-rate
Motivation Overhead Components SIFS 16% Preambles 40% Communications Overhead Cannot be removed! ACK 2% Slot Duration 42% Single Link Case Medium Access Preamble Data Preamble+ACK DIFS 101.5µs 40µs 20µs SIFS 44µs 7 6 5 4 3 2 1 0
Motivation Preambles 34% Overhead Components SIFS 14% ACK 2% Slot Duration 23% Collisions 27% Multiple Links Case (30)
Objective SIFS 14% Preambles 34% ACK 2% Slot Duration 23% Collisions 27% Reduce slot duration q and reduce the occurrence of collisions q while preserving fairness Remove SIFS
Challenge: Slot Duration A B 802.11a/n slot duration of 9 µs is close to the minimum feasible 0 1 PropagaDon Clear Channel Assessment (CCA) 4 µs 802.11 Slot û A 3 B 1 PropagaDon Clear Channel Assessment (CCA) ü Rx/Tx Hardware Turnaround 4 µs 4 µs 0 Key assumption: Preamble detection and transmission are serial
Speculative Preambles Preamble detection and transmission occur in parallel Clear Channel Assessment may take multiple slots The slot time can be reduced to a round-trip propagation delay Slot Time (800 ns)
Speculative Preambles Slot Time (800 ns) 1. As soon as the backoff expires, a node transmits its preamble 2. CCA: A node transmitting a preamble continues to attempt to detect incoming preambles 3. A node aborts its transmission if it detects a preceding preamble Node A 1 Node B Node C Preamble 0 3 2 1 0 2 1 0 û Preamble Preamble û Medium Access time decreases from 101.5 µs to 7.6 µs
WiFi-Nano Design Objectives Techniques ü Slot Time Duration Speculative Preambles Collisions Probabilistic Collision Resolution Fairness Counter Roll-back Minimum Slot Size SIFS Speculative ACKs
Implementation Detect preambles and their starting time under interference [PN1] Lattice Correlator PN1 PN2 PN3 [PN2] [PN3] Simultaneously transmit and detect preambles Analog Self-Interference Canceller Rx Δ Δ Δ [PN1,PN2] + Δ [PN2,PN3] + QH Noise Canceller Δ Tx Analog Radio Front-end [PN1, PN2,PN3] + Interference may require longer preambles
Results Experiments q q q Reliability of Preamble Detection Efficiency Gain and Analysis Fairness Lyrtech Qualnet
Preamble Detection Can nodes reliably detect preambles despite self-interference? Slightly longer preambles permit to maintain reliability
Efficiency Efficiency f(data rate, #nodes) WiFi-Nano increases the throughput up to 100%
Efficiency Efficiency f(data rate, slot time) WiFi-Nano.11a/n.11b WiFi-Nano increases the throughput up to 100%
How to Achieve More? Frame Aggregation Works only for single-link bulk downloads Practically difficult to achieve q q q Small packets (TPC ACKs) Short flows (HTTP) Delay sensitive applications (TPC, VoIP)
Frame Aggregation Practically difficult to achieve Related work reports 18 kb as average aggregation At 18 kb, WiFi-Nano gains 25% over 802.11 at 600 Mbps
Summary WiFi-Nano permits to q 802.11 overhead can be > 90% Reduce the slot time to 800 ns q Reduce the occurrence of collisions to nearly 0 q Remove SIFS WiFi-Nano increases 802.11 throughput up to 100%
WiFi-Nano: Reclaiming WiFi Efficiency through 800 ns Slots Q&A Eugenio Magistretti Krishna Kant Chintalapudi * Božidar Radunović # Ramachandran Ramjee * Rice University * Microsoft Research India # Microsoft Research Cambridge