Mitigating Signalling Overhead from Multi-Mode Mobile ... · SGSN MME UTRAN EUTRAN P-GW SGi ... •...

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Mitigating Signalling Overhead from Multi-Mode Mobile TerminalsIndra Widjaja and Carl Nuzman

Bell Labs, Alcatel-Lucent

Background

MT

2



• Multiple technologies (2G/3G/4G) are increasingly deployed simultaneously by cellular-network operators.

• Mobile terminals (MTs) have multi-mode capability and can switch from one technology to another seamlessly.

Problem and Motivation

Overlay

Coverage hole

Update

MT

2

3

4

5

6

7

Upd

ate

rate

(pe

r M

T p

er h

our)

case 1case 2

4G

3



Frequent updates can stress the control plane of the network

Overlay

Underlay 0

1

0 50 100 150 200 250 300

Upd

ate

rate

(pe

r M

T p

er h

our)

Time (hour) 3G

4G

• Can we characterize the update rate due to registration ping-ponging?

• Can we minimize the impact on signaling load?

Solution from Standards - Idle-state Signaling Reduction (ISR)

S-GWS1-U

Iu-PS

S5 PDN

SGSN

MME

UTRAN

P-GW SGiEUTRAN

S1-MMES11

S4

HSSS6a

S6d

S3

MT

4



When ISR is not activated, MT is registered with either technology:

- An MT moving from one cell of a given technology to another cell of a different technology has to perform a location update (TAU/RAU).

When ISR is activated, MT is registered with both technologies:

- No update is triggered when an MT moves from one cell to another cell of a different technology.

ISR is currently perceived to be a good thing

ISR Activation Example

MT SGSNeNB

TAU Request

TAU Accept (GUTI, ISR)

HSSMME

TAU RequestContext Request

Context Response (ISR supported)

Context Acknowledge (ISR activated)

Update Location

TAU Complete

TAU triggered

5



- During an update, the network decides whether to activate ISR individually for each MT

- Once ISR is activated, it remains active until the network decides not to re-activate during the next update

- MT and the network run periodic update timers. The network performs implicit detach which deactivates ISR if it does not receive a periodic update after the timer expires. MT deactivates ISR if it cannot perform a periodic update.

MT won't initiate location update within the registered area

except for periodic update

Analysis of Update Rate

• Mobility model

- MT location is uniform with density ρ- Direction of travel is uniformly distributed over [0, 2π]- In a closed region with perimeter length L, the average rate MTs with velocity V cross the perimeter is

VLR

ρπ

=

6



- The average update rate per MT without ISR can be expressed by

π

(1) due to MT entering an overlay region

(2) due to timer-triggered updates (period T) while in overlay

(3) due to MT exiting overlay coverage holes

Simulation of Update Rate

• Random waypoint on a torus

- MT’s journey consists of a sequence of flights with a pause between two consecutive flights.

- In each flight, the duration and velocity are picked independently according to given density

functions, and flight direction is uniformly distributed over [0, 2π]. The path of each flight follows a straight line.

- The pause time is independently chosen according to a given density function.

- When MT hits the boundary of a rectangular region, it is wrapped at the opposite side.

7



- When MT hits the boundary of a rectangular region, it is wrapped at the opposite side.

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8

y-p

osi

tion

x-position

(1) Entering overlay

(2) Timer-triggered update

(3) Exiting coverage hole

Average Update Rate per MT

Update rate grows as a square root of number of coverage holes without ISR for a fixed total hole area

5

6

7

8

9

10

Up

da

te r

ate

p

er

MT

(p

er

ho

ur)

With ISRWithout ISR (α=0.2)Without ISR (α=0.4)

8



ISR is beneficial

Assumptions:

- Overlay size = 100 eNBs, V=10 km/hr, Cell perimeter length = 3.5 km, Periodic timer = 3 hrs

- Note: α is the ratio of total area of coverage holes to overlay area

Simulation

1

2

3

4

0 20 40 60 80 100

Up

da

te r

ate

p

er

MT

(p

er

ho

ur)

Number of coverage holes

What Happens with Paging?

MT SGSNeNB S-GWMME

Downlink data

RNC P-GW

Downlink notification

Downlink notificationPaging

PagingPaging

Paging

Service request

Ack

Ack

9



- When the network receives a packet but does not have a connection for

a given MT with ISR activated, it buffers the packets and pages both

overlay and underlay.

- with ISR deactivated, the network pages overlay or underlay.

Ghosting Effect for Overlay Paging with ISR

UnderlayPeriodic timer expires

“Ghost MT” paged onoverlay andunderlay

UpdatePeriodic timer restarts

Upper and lower bounds on the relative increase in number of MTs registered with overlay due to ghosting is given by

6

7

8

Effe

ctiv

e A

rea/

Act

ual A

rea

Upper bound

10



Update

Overlay

With ISR activated, ghosting increases paging load on overlay

ISR is harmful

0 5 10 15 200

1

2

3

4

5

6

Speed (km/h)

Effe

ctiv

e A

rea/

Act

ual A

rea

Simulation

Lower bound

MT paged onunderlay

MT pagedon underlayand overlay

Tradeoff between Paging and Updating

40

60

80

100

Nh

V=20V=40V=80

V=120

No ISR is better

ISR is better

in this region

Incre

ase

d p

atc

hin

ess

Nh = number of

coverage holes

Increased mobility

11



0

20

0 0.5 1 1.5 2 2.5

λ

- Break-even is when Load of Updating-and-Paging with ISR = Load of Updating-and-Paging

without ISR.

- Let λ*(V, Nh) be λ at break-even point.

− λ*(V, Nh) is sensitive to patchiness but insensitive to velocity.

No ISR is better

in this regionIncre

ase

d p

atc

hin

ess

λ = average rate of

incoming call arrivals

Increased chattiness(calls/hr)

Signaling Load per MT for a Given Overlay

Signaling load vs λ

for a given overlay

deployment (Nh=60)Break-even point

600

800

1000

1200

1400

1600

1800

2000O

vera

ll m

essa

ge r

ate

ISR, V=20No ISR, V=20



12



0

200

400

600

0 0.5 1 1.5 2 2.5 3 3.5 4

Ove

rall

mes

sage

rat

e

λ

- With ISR activated, the signaling load depends strongly on λ, while with ISR deactivated, the load is insensitive to λ.

- Using ISR is beneficial when λ is below the break-even point but harmful above it.

Threshold-Based ISRλ∗ vs velocity

(paging dominates)

ISR reduces batterypower consumption

Battery power consumption is not anissue compared toheavy usage by the user

1

1.5

2

2.5

3

3.5

4

λ∗

No ISR is better for chatty users

13



− λ*(V) = λ(V) at break-even point.- Choose a global λ = λ*(V) at high-velocity value as high-velocity MT has more impact on load.

- This results in threshold-based ISR that is independent of MT velocity:

- Activate ISR when λ ≤ λ- Deactivate ISR when λ > λ

~

~

~

(updating dominates)power consumption

0

0.5

10 20 30 40 50 60 70 80 90 100

V

ISR is better quiet users

Experiment Setup

Distribution of call arrival rates of MTs follows a generalized Pareto:

where σ =Λ (1-ξ), and Λ is aggregate call arrival rate per MT per hour.

0.1

1

14



Comparison between

generalized Pareto and

data from a real trace.

1e-07

1e-06

1e-05

1e-04

0.001

0.01

0.1

0.01 0.1 1 10

CC

DF

rate

DataGeneralized Pareto ξ=0.21

Result

0.8

1

1.2

1.4

1.6

1.8

Rel

ativ

e lo

ad

No-ISRISR

T-ISR, ξ=0T-ISR, ξ=0.5T-ISR, ξ=0.7T-ISR, ξ=0.9

Comparison of

No-ISR, ISR and

Threshold-based ISR.

15



0.2

0.4

0.6

0 1 2 3 4 5 6 7

Λ

Assumptions:

- Load normalized to no-ISR case.

- λ = 1.7, Number of MTs = 500,000

-

~

Practical Setting of ISR

• Previous open-loop approach requires knowledge of overlay parameters (e.g., overlay size, number of holes, sizes and shapes of holes) to find the threshold λ that minimizes signaling load, Μ(λ).

• In reality, the parameters of the overlay deployment are generally not known.

~

~

16



generally not known.

Measurement-based Approach

• An alternative closed-loop approach is to adopt a stochastic approximation algorithm (motivated by Kiefer-Wolfowitz algorithm) to iteratively optimize the threshold based on noisy observation M(λ, t

n, τ) – an empirical estimate of measured

signaling load taking into account random call arrivals in time interval (tn, tn+τ) with a control variable λ.~

~^

17



interval (tn, tn+τ) with a control variable λ.

• Let y=F(λ) be the fraction of MTs with call arrival rate lower than λ and let q(y) = F-1(y). Starting at y0= 0, the algorithm iteratively evaluates:

~

perturbation step sizeincrement step size

~

Result

40

50

60

70

80

90

100

Sig

nalli

ng L

oad

(mes

sage

s/hr

/MT

)

• Progress of the stochastic approximation algorithm.

• After convergence, a single iteration can be run each day during the busy-hour period.

18



0 10 20 30 40 500

10

20

30

40

Iteration

Sig

nalli

ng L

oad

(mes

sage

s/hr

/MT

)

Λ = 1Λ = 3Λ = 7

Assumptions:

- System with 10,000 MTs with individual {λ} drawn from a generalized Pareto distribution with

shape parameter ξ=0.7.- δ=0.1, β=0.05, τ=30min

during the busy-hour period.

Conclusions

• The proliferation of multi-mode mobile terminals (MTs) can significantly stress signaling load in wireless networks.

• 3GPP has devised a mechanism to reduce signaling load called ISR, but no approach on how to set ISR is given or known.

• We analyze the tradeoff between updating and paging and quantify a single threshold value to decide on ISR activation.

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single threshold value to decide on ISR activation.

• We develop a practical algorithm to activate or deactivate ISR for each MT without requiring knowledge of network deployment or terminal mobility.

Thanks!

20



Thanks!

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