January 2007

Thomson et al

Slide 1

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Idle Mode and Paging Analysis Part 2

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January 2007

Thomson et al

Slide 2

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Abstract

This presentation provides additional points to consider as part of the decision on Idle Mode & Paging proposal

January 2007

Thomson et al

Slide 3

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Summary

Key points to consider• Where do battery savings come

from

• How often are STAs mobile

• How much are the savings

• Do the savings justify the complexity

Reduced AP re-acquisition & re-association for mobile STAs only

Vast majority of 802.11 networks mobility is small %

Minimal even for mobile STAs, no benefit for stationary STAs

No, especially considering complexity tax applied to 802.11 as a whole

January 2007

Thomson et al

Slide 4

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Where do Paging Mode savings come from?

• Paging mode provides no benefit if you are stationary

• Paging mode does not change how often a STA has to wake up

• Paging only changes what the STA does when it wakes up

• With Paging proposal, an idle STA does not have to acquire a new AP and re-associate so long as it hears a Paging beacon in the same paging domain

• Battery can be saved by not having to re-acquire APs or re-associate when mobile– So long as you remain within the same Paging Domain

January 2007

Thomson et al

Slide 5

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Key questions

• How long does AP acquisition take?

• Assumptions:– Today STAs take advantage of idle mode to optimize their algorithms

– Neighbor list provided to STA to help optimize scanning

– Scanning triggers adjusted for lower RSSI threshold

– Further optimizations possible (such as using 11k AP channel reports)

• Case examined 802.11b

• Scanning 11 channels can be optimized to 3 channels based on neighbor list

• Channel Dwell time (10mS + 2mS switch-time)

• Best case scanning time: 36mS

January 2007

Thomson et al

Slide 6

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Key questions

• How long does re-association take?

• Static/Open: 20mS to 30mS

• 11i: 50mS to 60mS

• Expected 11r: 15mS to 30mS

January 2007

Thomson et al

Slide 7

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Static/Open11i

11r

Paging

Static/Open11i

11r

Paging

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5

Reassociation Types

Non-

Pagi

ng R

oam

Tim

e

Savings comparison

• Non-Paging: AP acquisition + Reassociation

– Static/Open• Best (3 channels): 36mS + Static

(20mS) = 56mS• Worst (11 channels): 120mS + Static

(30mS) = 150mS

– 11i • Best (3 channels): 36mS + 11i

(50mS) = 86mS• Worst (11 channels): 120mS + 11i

(60mS) = 180mS

– 11r • Best (3 channels): 36mS + 11r (15mS)

= 51mS• Worst (11 channels): 120mS + 11r

(30mS) = 150mS

• Paging proposal suggests 700mS

Difference of opinion?

January 2007

Thomson et al

Slide 8

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

-2

0

2

4

6

8

10

12

14

16

18

20

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Number of Roams Avoided

% I

mp

rove

men

t

Paging

Worst Case (11i | 11r | Open)

Best Case (11i | 11r | Open)W

700mS Roaming overstates Paging benefit

• Shows IMPROVED AP re-acquisition and reassociation MINIMIZES paging improvements SIGNIFICANTLY

January 2007

Thomson et al

Slide 9

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Keep-Alive

• IP-based Phones require keep-alive• Protocols used SIP and SCCP• SCCP every 30 secs• SIP every 60 secs (typical)• Assume walking speed 1.1m/s• Max distance traveled 66m before SIP keep-alive• Assume -75dbm re-scan trigger on idle mode phone

– -75 equates to approx 35 meters at 2.4GHz– -75 diameter around AP is approx 70m

• An idle mode phone will encounter 2 to 3 AP cells MAX before keep-alive required

• Therefore MAX number of roams saved between keep-alives will be 2 to 3 regardless of other scanning triggers

January 2007

Thomson et al

Slide 10

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

-2

0

2

4

6

8

10

12

14

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18

20

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Number of Roams Avoided

% I

mp

rovem

en

t

Paging

Worst Case (11i | 11r | Open)

Best Case (11i | 11r | Open)W

Keep-Alives limit Paging benefit to 1.5%• Assumptions:

– Keep-Alive every 60 secs is approx 66m total distance

– -75dbm AP Cell Diameter is 70m, so being very generous assume 3 consecutive roams before keep-alive

• Even using 700mS Paging roam = 1.49%

• Worst Case (11i|11r|Open) = 0.25%, Best Case (11i|11r|Open) = 0.01%

January 2007

Thomson et al

Slide 11

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Can keep-alives be proxied? NO

• Purpose of keep-alive is endpoint reachability

• Many phones have a certificate for encryption of signalling traffic– Therefore if keep-alive was proxied by another network entity, that

other network entity would have to have the phone’s certificate to be able to proxy the keep-alive

– SIP will use DTLS as a transport

January 2007

Thomson et al

Slide 12

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

What is a SIP Proxy

• Provides a gateway function between two “user agents”, namely SIP endpoints

• Provides a function of translating SIP URIs to network addresses

• Endpoints register with the SIP Proxy to originate and terminate phone calls

• A keep-alive mechanism is still required to maintain endpoint reachability

SIP Keep-alive

2 SIP Call Managers that include ProxyRelays INVITE to the destination Proxy or UA(end-point)

SIP end-points (UA)

SIP Keep-alive

SIP end-points (UA)

IP

IP

IP

January 2007

Thomson et al

Slide 13

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Paging breaks Call Server roaming

• With paging if a phone roams across a call manager server boundary, NO event to trigger hand-off

• Call Manager never discovers phone has crossed call manager boundary

• -> Phone system breaks

January 2007

Thomson et al

Slide 14

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

IM/Email Applications

• Are phones unique?

• NO

• Smart Handhelds, Laptops…etc all run email/IM programs that periodically get (or pushed) email or instant messaging

• Email and IM applications on idle mode devices will cause them to have similar behavior to keep-alive for phones

January 2007

Thomson et al

Slide 15

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Complexity

• Paging introduces a new function called Paging Server

• Paging Server must be tightly coupled with the data path

• All frames sent to an idle mode device must be stored– 802.11 spec only defines queuing at the AP NOT the DS

– For paging to work, stored frames must be forwarded to new AP paged STA associates to• With non-paging this complexity is not required

• Data path always aware of current associated AP to STA

– Signalling must be introduced between the queuing function and the paging server for paging to schedule proper forwarding of stored frames

January 2007

Thomson et al

Slide 16

doc.: IEEE 802.11-07/0179r1

Idle Mode Analysis

Summary

To reiterate:• Where do battery savings come

from

• How often are STAs mobile

• How much are the savings

• Do the savings justify the complexity

Reduced AP re-acquisition & re-association for mobile STAs only

Vast majority of 802.11 networks mobility is small %

Minimal even for mobile STAs, no benefit for stationary STAs

No, especially considering complexity tax applied to 802.11 as a whole