Multiple Access and High Density 802.11 Wireless Access Networks

Multiple Access and High Density 802.11 Wireless Access

Networks

Dina Papagiannaki

Intel Research Cambridge

November 15th 2006

Konstantina Papagiannaki2

Multiple Access

In broadcast environments we need a mechanism to coordinate access among devices (Ethernet, Wireless LANs, Cellular networks)

Every transmission is overheard by all other devices in range

Simultaneous transmissions lead to collisions that waste network resources

Two primary ways of mediating access:• Centralized

• Distributed

Design goal:• Maximize the number of messages

• Minimize a station’s waiting time

November 15th 2006


Centralized vs. Distributed

Centralized scheme

• One node is assuming the role of the master node and determines the order by which slave nodes access the medium.

• May lead to low medium utilization.

Distributed scheme

• All nodes are equivalent and can talk to each other.

• Need to coordinate access in order to avoid collisions.

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Circuit-mode vs. packet-mode

Such a choice depends on the intended workload

Circuit-mode allocates part of the medium to a source for its exclusive use – cellular network

Packet-mode operates on a per-packet basis, more appropriate for bursty, non-persistent traffic types.

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Further constraints

• Spectrum scarcity

• Radio channel impairments– Fading – degradation of the signal due to the environment– Multipath interference – reception of signal along multiple paths

that may interfere and potentially cancel each other out– Hidden terminal – a transmission may not be overheard by all

potentially interfering stations– Capture – the strongest signal at the receiver may be properly

decoded (strongest sender has captured the receiver)

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Applying those concepts to 802.11 wireless networking

IEEE 802.11 is used for Wireless LANs

-Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

- Three variations – 802.11b at 2.4GHz and 11 Mbps, 802.11g at 2.4 GHz and 54 Mbps, 802.11a at 5 GHz and 54 Mbps

-Channel impairments dealt using rate adaptation- Different modulation and coding schemes employed that result in different

effective transmission rates

- Hidden terminal mitigation using Request to Send/Clear to Send (RTS/CTS) control frames

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The 802.11 MAC protocol

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

• Before a transmission sender senses the medium

• If the energy level lower than Clear Channel Assessment (CCA) threshold – medium idle

• If not, medium busy

• When the sender wishes to transmit it randomly draws a waiting time [0, CWmin]

• Each idle slot allows the sender to reduce its CW by 1 slot

• Upon each unacknowledged transmission the sender doubles its CW up to CWmax (back off)

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Spectrum scarcity

802.11 success primarily due to low cost and no licensing fees to use the 2.4 Ghz and 5 GHz bands

Small number of operating frequencies in 802.11b/g – slightly more in 802.11a

Sharing with non 802.11 devices (microwaves, cordless phones, BT devices, etc.)

November 15th 2006


The effect of contention

In a single contention domain each sender has an equal probability of accessing the medium

The greater the number of senders the smaller the throughput

Mechanisms for robustness to errors may lead to smaller effective transmission rates• Nominal transmission rate of 802.11a/g: 54 Mbps, effective

~30 Mbps, lowest encoding rate 1 Mbps, under contention even lower…

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Research Challenges

Density of 802.11 APs increases in urban areas Low cost/ease of deployment

No coordination in deployment

May feature manufacturer default settings

Campus and enterprise networks go wireless Higher density could lead to better performance

Network management is an ART, especially due to medium dynamics

There is no equivalent to over-provisioning Adding APs may be counter-productive

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Self-organization in 802.11 networks

Tuneable knobs

• AP frequency (frequency selection)

• Association of clients to APs (user association)

• Transmission power and CCA threshold (Power control/MAC layer tuning)

Performance evaluation

Requirements from existing platforms

Further Challenges

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The problem statement

High density 802.11 wireless networks suffer from sub-optimal performance due to their static configuration (i.e. maximum transmission power, default operating channel,

default aggressiveness to access the medium)!

AP client

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Self-organization objectives

Develop fully decentralized algorithms for the self-organization of infrastructure 802.11

wireless networks

Seeking mechanisms that aim to optimize global performance using local information alone

Robust to changes in the medium and the topology

Can be implemented using today’s technology

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3 facets to the problem

A. Frequency Selection by APs

Identify the appropriate frequency to use so as to minimize overall interference across the network

B. User association

When user association is flexible, balance the users across APs so as to maximize the long-term overall network capacity

C. Transmission Power and Aggressiveness to access the medium (CCA threshold)

Identify the appropriate level of transmission power and CCA threshold for the APs and clients so as to maximize overall network capacity

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Frequency selection formulation

P1

P2

Measure noiseMeasure power

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A. Frequency Selection

Minimize interference by operating on orthogonal frequencies. Minimize overlap when required to reuse a

frequency.

Mbps/11g Client1 Client2 Client3

Before 11.81 6.86 14.37

After 30.51 (*3) 30 (*5) 29.45 (*2)

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User throughput

Internet

1. Channel access time2. Aggregated transmission delay3. Wireless channel quality

State of the art can lead to unnecessarily

low throughput!

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Analytical model

When wireless the bottleneck…

… traffic is downlink – APs are the only senders in the medium

… fully saturated traffic conditions – interference caused by APs does not depend on the #clients

All clients receive the same long-term throughput if rate adaptation employed

In a reference period of time T

ET

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User association formulation

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B. User association

Balance the user associations for minimal potential delay fairness. Users take into account the personal and social

cost of different association rules.

Mbps/11g Client1

Before ~ 5

After ~ 8

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Overall network fairness improved

Mean:1428, variance:4378031

Mean:1559, variance: 627638

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Implementation on Intel 2915ABG

AP Capacity (APC) - MAC

• Modify firmware to compute fraction of access time, i.e. number of busy slots in a reference period of time (M(a))

• Nominal capacity given by 11a/b/g (C(a))

Aggregated transmission delay (ATD) – MAC/PHY

• Modify firmware/ucode to measure amount of time between queueing the packet towards a client and the reception of the ACK (rate scaling, and retransmissions)

• Keep a list of client MACs and delay, compute sum of delays

Transmission rate for new client approximated using RSSI - PHY

APC/ATD advertized through Beacon frames

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Experimental Results

AP1 AP2 AP3

Ch10 Ch10 Ch3

C1 C3C2

4 Mbps 4 Mbps4 Mbps

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Power Control in 802.11

Heterogeneous transmit powers across nodes can lead to node starvation!

1st order starvation

We need to ensure that there is symmetry in the nodes’ contention domains.

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What is the benefit of power control?

Reducing transmission power can reduce interference in the network

Increasing transmission power can improve client SINR thus allowing for higher transmission rates

There is a tradeoff between the amount of interference we introduce in the network and the additional throughput benefit at the client

November 15th 2006


Condition for starvation free power control

We need to ensure network symmetry

We have proven that for starvation-free power control we need to keep the product of CCA threshold and transmission power constant

CCA * P = C

The louder you are going to shout the more carefully you should listen for the nodes that whisper

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How do we maximize network capacity?

We need to identify these values of C that result in the greatest transmission concurrency

We can optimize C using Gibbs sampling in order to maximize network capacity

Input: channel gains between APs, channel gains from AP to clients, number of clients per AP, transmission power

Output: APs select transmission power and CCA. Clients follow the setting of their AP.

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Experimental Testbed

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C. Power Control / CCA adaptation

Tune power to offer the best transmission rate to the farthest client while not introducing excessive interference

to neighboring co-channel devices. Adjust CCA to increase transmission concurrency across the network.

Mbps/11g Client1 Client2 Client3

Before 11.81 6.86 14.37

After 29.45 (*3) 22.59 (*4) 30.51 (*2)

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Experimental Results

Gain Default CCAClient SS03: 149% 15%Client SS15: 228% 34%Client SS24: 112% 3%

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Simulation Results (topology – 8 APs, 26 STAs, 802.11a, AP-STA: 3.5m)

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Simulation Results (power, CCA)

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Simulation Results (throughput)

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Summary Results

Gibbs also leads to the use of a smaller transmission power that can extend client’s lifetime

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Implementation Requirements

AP Capacity (MAC)

Aggregated Transmission Delay (PHY/MAC)

Number of users

Worst Client Channel Gain (PHY)

Introduction of new Beacon fields (CCA, TxPower, auxiliary variables)

Channel Switch Announcements (802.11h/DFS/TPC)

Measurements (802.11k/802.11e)

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Larger Scale Experimentation

SWAN testbed at William Gates Building

• 80 Soekris dual mini-PCI boards

• Intel 2915 ABG cards with modified ucode/firmware

• PoE switches for ease of manageability

Investigation of benefits of the three different algorithms compared to the state of the art and their incremental benefits

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Floorplans

Ground floor1st floor 2nd floor

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More exciting problems….

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Community mesh networking

Internet

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Questions?

November 15th 2006


BACKUP

Multiple Access and High Density 802.11 Wireless Access Networks

Documents

transmission sender

unacknowledged transmission

medium busywhen

decoded strongest sender

low medium utilization

wireless networkingieee

bt devices

devices ethernet