reducing multicast traffic load for cellular networks using ad hoc networks

Reducing Multicast Traffic Load

for Cellular Networks using Ad Hoc Networks

Li Lao (UCLA)Jun-Hong Cui (UCONN)

August 23, 2005 QShine 2005 2

Background Hybrid cellular/ad hoc networks for unicast applicati

ons Increase the coverage of base stations Avoid dead spots Re-direct traffic from congested cells to non-congested Improve system throughput

Hybrid cellular/ad hoc networks for multicast applications Enhance network performance, especially for heterogeneou

s receivers (Park & Kasera, WCNC’05) Focus on individual groups only and do not consider QoS wh

en multiple groups co-exist in the network


Our Focus A base station handles multiple multicast

groups simultaneously Cellular mode: base station may be overloaded

NOTE: we use point to point link for multicast Ad hoc mode: ad hoc net may be congested

NOTE: broadcast is not assumed Our approach: to balance between two modes

Goal: Minimize the workload on the base station while

maximizing the utilization of the ad hoc network (without exceeding its capacity)


The ProblemGroup Selection Problem:

Base station determines:

How many groups? Which groups?

To be switched to ad hoc mode


An Example

DE

C

A

FG

H

BT1

T2

T3


Roadmap Problem Formulation

Network Model Problem

Proposed Algorithms Performance Evaluation Conclusions


Network Model Network N(V, E) (for a cell)

|V| = m, |E| = n For a node i, its capacity is Ci

Multicast groups G |G| = ng

For a group gj G, its required data rate is rj

Bandwidth for gj in cellular mode is: |gj|x rj

A multicast routing algorithm


Wireless Interference IEEE 802.11 uses CSMA/CA

RTS/CTS/DATA/ACK

X X

X

AB

Observation I: Neither the sender’s neighbors nor the receivers’ neighbors can transmit or receive data

C

D

E

FG

Observation II: If we want to reserve a unit of bandwidth at two nodes, we must also reserve a unit of bandwidth at their neighbors


Bandwidth Requirement of Multicast Groups For a multicast group gj,

Compute its multicast tree tj

For each link on tj, compute the required bandwidth at corresponding nodes

Obtain a bandwidth vector bwj = (bw1j, bw2j, …, bwm

j), where bwij represents the required bandwidth at node i for gj

For a set of multicast groups G’ The required bandwidth at node i for these groups:

'Gg

ij

j

bw


Example

AB

C

D

EF

G

LinkAffected nodes

C-A A,B,C,D,E

A-B A,B,C,E,F

B-E A,B,E,F

B-F A,B,E,F,G

Bandwidth Vector for (A,B,C,D,E,F,G): (4,4,2,1,4,3,1)


Problem Formulation Multicast Group Selection Problem

Input: Ad hoc network with Ci (iV), a set of multicast groups G with bwj and rj (gjG)

Output: a subset G’ G to maximize the bandwidth savings Constraint: (iV)

Essentially a Multi-dimensional Knapsack Problem Input: A knapsack with m-dimensional size (b1, …, bm), and a s

et of items S = 1, …, n, each having a size rj = (r1j, …, rmj) and a value vj

Output: A subset S’ S that maximizes the values Constraint: (i[1,m])

'

||Gg

jj

j

rg

'Gg

iij

j

Cbw

Group Item Ad hoc network Knapsack

'Sj

iij br 'Sj

jv


Roadmap Problem Formulation Proposed Algorithms

Integer Linear Program Dynamic Algorithm Naive Dynamic Algorithm

Performance Evaluation Conclusions


Integer Linear Program Define:

Objective: maximize bandwidth savings

Constraint: node capacity

otherwise ,0

selected is group if ,1 jxj ],1[ gnj

gn

j

ijij Cxbw1

],1[ mi

gn

j

jjj xrg1

||max


Dynamic Algorithm Utility function for each group gjG

Group g join: O(mn+mng) Admit g in ad hoc mode and reserve bandwidth if enough resource Otherwise, try to swap g with an existing group g’ in ad hoc mode s

uch that u(g’) < u(g) If g’ releases its bandwidth, g can be admitted If there are more than one such groups, the one with the smallest utility s

hould be selected as g’ Group g leave: O(mnng)

Release bandwidth Try to select a group g’ to be swapped in

Vi

ij

jjj

bw

rggu

||)( Bandwidth savings if the group is selected

Total amount of required bandwidth


Naive Dynamic Algorithm Group join

If enough resource in the ad hoc network, admit this group in ad hoc mode and reserve bandwidth

Otherwise use cellular mode O(mn)

Group leave If ad hoc mode, release bandwidth O(m)


Roadmap Problem Formulation Proposed Algorithms Performance Evaluation Conclusions


Simulation Settings Wireless network

Up to 120 nodes A cell of 500*500 m2

Communication range: 115m Channel capacity: 100~500 units

Multicast groups Group size uniformly distributed (mean: 20~100) Group members randomly distributed in the network and

one member randomly selected as source Group arrivals follow a Poisson distribution and lifetime

follows an exponential distribution Each group requires 1 unit of bandwidth 80 active groups


Metrics Number of admitted members Number of admitted groups


Impact of Network Density

0

100

200

300

400

500

600

700

800

900

1000

20 40 60 80 100 120 140Total number of nodes

Nu

mb

er o

f ad

mit

ted

mem

ber

s

ILP

Dynamic

Naive

0

5

10

15

20

25

30

35

40

45

50

20 40 60 80 100 120 140Total number of nodes

Nu

mb

er o

f ad

mit

ted

gro

up

s ILPDynamicNaive

Average group size: 20, Channel capacity: 500


Impact of Channel Capacity

0

100

200

300

400

500

600

0 100 200 300 400 500 600Channel capacity

Nu

mb

er o

f ad

mit

ted

mem

ber

s

ILPDynamicNaive

0

5

10

15

20

25

30

0 100 200 300 400 500 600Channel capacity

Num

ber

of a

dmitt

ed g

roup

s

ILPDynamicNaive

Network nodes: 120, Average group size: 20


Impact of Group Size

0

50

100

150

200

250

0 20 40 60 80 100 120Average group size

Nu

mb

er o

f ad

mit

ted

mem

ber

s

ILPDynamicNaive

0

1

2

3

4

5

6

0 20 40 60 80 100 120Average group size

Nu

mb

er o

f ad

mit

ted

gro

up

s

ILP

DynamicNaive

Network nodes: 120, Channel capacity: 100


Roadmap Problem Formulation Proposed Algorithms Performance Evaluations Conclusions


Conclusions Developed a simple and effective model for

computing bandwidth requirement of multicast groups in wireless networks

Formulated the multicast group selection problem as a multi-dimensional knapsack problem

Proposed an ILP formulation and a utility-based dynamic algorithm

Simulation study has shown that the dynamic algorithm can achieve near-optimal solutions

Future Work: Member dynamics Distributed implementation


Questions?

reducing multicast traffic load for cellular networks using ad hoc networks

Documents

required bandwidth

reserve bandwidth

unit of bandwidth

set of multicast groups

bandwidth savingsconstraint

multicast group gj

subset g g

individual groups