application layer multimedia multipoint conferences using...

2005-05-23Network Database Lab.

Dep. of Electrical Engineering, National Taiwan University 1

Application layer multimedia multipoint conferences using

Megaco/H.248

Speakers: 戴碧如, 黃仁暐, 戴志華, 林與絜F89921035, F91921025, F92921029, B90901033



References‧張任君，應用層多點多媒體會議方法研究與其在Megaco/H.248之應用 Application layer multimedia multipoint conferences using Megaco/H.248，國立臺灣大學電機工程學研究所--碩士論文，民國90年6月。

• Jen-Chun Chang and Wan-Jiun Liao. Application-layer conference trees for multimedia multipoint conferences using Megaco/H.248. In Proc. of IEEE ICME, 2001.



Outline • Motivation• Previous Works• Overview of Megaco/H.248• Proposed Algorithm• Practical Issues• Experiments• Conclusions



Motivation • Multimedia conference using Megaco/H.248

– VoIP should offer the same function/service as PSTN– VoIP should provide more attractive services



Motivation • Multiple control unit (MCU), which centralizes

the control of conference calls– Waste a lot of bandwidth to send replicate data– Inefficient for capability exchanging



Motivation • Tree topology, rather than a star topology.

– Data destined to the conference goes to the tree in one copy directly from the branch of which the sender is located.



Previous Works• KMB, which builds a near optimal minimum

spanning trees– Re-build whole tree while any join or leave happens



Previous Works• Dynamic greedy algorithm

– Join: the joiner connects to the nearest node on the tree using the shortest path

– Leave: mark the node as deleted if it can’t be removed directly; else, remove it

– May waste lots of bandwidth after a series of departure operations



Previous Works• Join time local optimal algorithm mitigate the

skewed tree problem. – Leave: same with dynamic greedy algorithm– Join: locally reshape the tree while the join step– Increases the join latency– Can not solve the problem of wasting bandwidth

entirely



System Architecture• Gateway decomposed

– Separate call control from media transcoding• MGC (Media Gateway Controller)• MG (Media Gateway)

– Residential Gateway– Trunking Gateway



Connection Model (1/3)• Termination and Context

– Termination• Source or destination of media streams

– Context• Association with a collection of Terminations



Connection Model (2/3)



Connection Model (3/3)• Descriptors

– Describe properties of Termination of Context– Encapsulated in commands

• Eight important control commands– Add, Modify, Subtract, Move, AuditValue,

AuditCapabilities– Notify– ServiceChange



Call Processing (1/2)



Call Processing (2/2)



Proposed Algorithm• A conference tree is rooted at the root-MG

where the conference initiator is located, and spans all the participating MGs.



Proposed Algorithm• A multipoint conference tree grows or is

trimmed when members join or leave the conference, respectively.– Conference tree is reshaped using local

reshaping algorithm when a node leaves in order to achieve better performance.



Conference Operations• Create a multipoint conference• Terminate a multipoint conference• Join a multipoint conference• Leave a multipoint conference



Create a Multipoint Conference• A conference will be initiated when a

member (i.e., initiator) starts with a Termination in a null Context on the root-MG, with which the initiator connects.



Example - Create

MG-11 MG-21

MG-12

MGC-1 MGC-2

User1 Null ContextT1

The initial state of the conference



Create a Multipoint Conference• Each conference is associated with a conference

number.• The initiator dials the conference number to

connect to the root-MG.• On receiving a notified number, an MGC performs

the authentication checking, and informs the root-MG of a success in conference creation.

• The initiator waits until other members join and the conference begins.



Terminate a Multipoint Conference

• A conference will be terminated when its initiator leaves the conference because the initiator is considered as the host of the conference.

• The initiator’s MG notifies its MGC of a Notify message if the MG is a Residential Gateway, or an REL message if the MG is an SS7 gateway.



Terminate a Multipoint Conference

• The MGC then forwards the conference message to all the other MGCs involved in the conference.

• Each MGC then in turn sends Subtract commands to the corresponding on-tree MGs to tear down connections and release the resources previously allocated.

• Host change scenario: the conference operates continuously with another host when the initiator leaves.



Join a Multipoint Conference• A Termination is added to the Context

associated with the conference.• Upon receipt of a joining request from a

user, an MG notifies its MGC of a connection being created.



Example - Join

MG-11 MG-21

MG-12

MGC-1 MGC-2

Context 1T1User1

Both MGs are in the same MGCUser2

Context 2

T4

T2*

T3*



Example - Join

MGs are in different MGCs

MG-11 MG-21

MG-12

MGC-1 MGC-2

Context 1T1User1

Context 2

T4User2

T2

T3

*

*

User3

Context 3

T7T5 T6 *

Information Exchange



Example - Join

Context 3

MG is on-tree

MG-11 MG-21

MG-12

MGC-1 MGC-2

Context 1T1User1

Context 2

T4User2

T2

T3

*

*

T7 User3T5 T6

Information Exchange

User4

*T8



Leave a Multipoint Conference• Upon detecting a Termination going on-

hook, the Termination’s MG notifies its MGC of the departure of the Termination.

• An MG at which a departing Termination is located is called a departing MG.



Leave a Multipoint Conference• Case1: Still other Terminations in the Context

when a user departs from its MG– Simply remove the departing Termination.

• Case2: The departure of a leaf MG and no other Terminations in a departing MG – The departing MG just leaves the conference via a

Subtract command to tear down the connection. • Case3: The departure of an intermediate MG

– The MGC applies a local reshaping algorithm to avoid the growth of a skewed tree.



Local Reshaping Algorithm• To retain the on-going conference activities,

the connection between the departing nodeand its parent node should be torn down only after those connections between the departing node and all its immediate child nodes have been glued back to the tree.



Local Reshaping Algorithm

2

1

2

3 4

5 6 7 8

Departing MG

(Departing node)

(a)

1

2

3 4

5 6 7 8Rejoin nodes(b)

1

2

3 4

5 6 7 8

Glued back

(c)

1

2

3 4

5 6 7 8

Glued back

(d)

1

3 4

5 6 7 8

(e)



Context 5

Example - Leave

Conference topology

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

Context 3

T6

User3

T7*

User4

MG-23

T14 User6T13 *

Context 1

MG-12

T2

User1

T1T5

*

T8

T12

T4

User2

T3 *T10

T9T11

*User5



Example - Leave

Case1: Still other Terminations in the Context when departing

Context 5

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

Context 3

T6

User3

T7*

User4

MG-23

T14 User6T13 *

Context 1

MG-12

T2

User1

T1T5

*

T8

T12

T4

User2

T3 *T10

T9T11

*User5



Context 5

Example - Leave

Case1: Still other Terminations in the Context when departing

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

Context 3

T6

User3

T7*

MG-23

T14 User6T13 *

Context 1

MG-12

T2

User1

T1T5

*

T8

T12

T4

User2

T3 * T9 T11* User5



Context 5

Example - Leave

Case2: The departure of a leaf MG and no other Terminations in the departing MG

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

Context 3

T6

User3

T7*

MG-23

T14 User6T13 *

Context 1

MG-12

T2

User1

T1T5

*

T8

T12

T4

User2

T3 * T9 T11* User5



Example - Leave

Case2: The departure of a leaf MG and no other Terminations in the departing MG

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

Context 3

T6

User3

T7*

MG-23

Context 1

MG-12

T2

User1

T1T5

*

T8

T4

User2

T3 * T9 T11* User5



Example - Leave

Case3: The departure of an intermediate MG

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

Context 3

T6

User3

T7*

MG-23

Context 1

MG-12

T2

User1

T1T5

*

T8

T4

User2

T3 * T9 T11* User5

Child of MG-22

T15 T16



Example - Leave

The resultant conference tree after departure

Context 4

MG-11 MG-21

MG-22

MGC-1 MGC-2

Context 2

MG-23

Context 1

MG-12

T2

User1

T1 *

T4

User2

T3 * T11*

User5

T15 T16



Host Change• Approach

– The same root-MG• Choose another Termination as the new initiator

– A new root-MG• Select a child MG of the root-MG as the new root-

MG

1

2 3

4 5 6 7



Determination of the Parent MG (1/3)

• Approach– On the same MGC

• Nearest MG under the same MGC

– On another MGC• Nearest MGC from its MGC

2 3

4 5 6 7

1

N

?



• Network Layer Tree– Shortest path of total distance of all branches

• Application Layer Tree– Simple to implement– Application compatible




• Properties– Less Memory– No calculation– Hierarchical topology– May increase the total distance of all branches




Experiments• Evaluate the following perspectives

– Join latency– Bandwidth efficiency– Cost effectiveness



Join Latency• Formal definition

– Time elapsed between when a user joins a conference and when a user has received data

– Including user request time, processing time, and response time

• In this paper– Distance from the member to the MG on the

tree



Simulation Topology*

*Interested reader can refer to the paper



Join Latency

Flat topology



Join Latency

Drop at the point where on-tree MGs is small



Bandwidth Efficiency• In terms of total number of links required

per approach• Compare the tree approach to the MCU

approach



Bandwidth Consumption• For tree

– N-1 shortest path to the conference tree, Ci– Require

• For MCU– N-1 shortest path connections between the MCU and all

the other MGs, Bi– Require Bk to send the data to the MCU and then use

to duplicate to all the others– Total

∑−

=

1

1

N

iiCs

∑−

≠=

1

,1

N

kiiiBs

s : time duration to send data

∑−

=

1

1

N

iiBs



Parameters• The process an MG joining the conference

is a Poisson distribution (1.5s)• The time an MG staying in the conference

is an Exponential distribution (106s)• Data sent by each member is a Poisson

process (20s)• The data length is an exponential

distribution (320Kb)



N=100, some of them are randomly selected as MG



Cost Effectiveness• Compare the tree size of dynamic greedy

algorithm and that of KMB to the conference tree

• Run a sequence of 1000 events. – Each may be a Join or Departing– The probability is proportional to the number of

MGs on the tree



Ratio to dynamic greedy

Conference TreeDynamic Greedy



Ratio to KMB

XKMB



Conclusions• Propose a new conference tree structure for

multipoint conferences– Capability exchange for communications is

carried out through the Session Description Protocol in the Megaco/H.248

• Develop an analytical model to evaluate the performance of the proposed approach



Conclusions• The proposed approach enjoys the

advantages of lower join latency, better bandwidth efficiency and it’s also cost effective.



References (1/2)‧張任君，應用層多點多媒體會議方法研究與其在Megaco/H.248之應用 Application layer multimedia multipoint conferences using Megaco/H.248，國立臺灣大學電機工程學研究所--碩士論文，民國90年6月。

• Jen-Chun Chang and Wan-Jiun Liao. Application-layer conference trees for multimedia multipoint conferences using Megaco/H.248. In Proc. of IEEE ICME, 2001.



References (2/2)• L.Kou, G. Markowsky, and L.Berman, “A Fast Algorithm

For Steiner Trees,”Acta Informatica, Vol. 15, pp.141-145, 1981.

• B. M. Waxman, “Routing of Multipoint Connections,”IEEE JSAC, Vol. 6, No. 9, pp. 1617-1622, 1988.

• Yang Ming and Xie Xiren, “A Optimal Dynamic Multicast Routing Algorithm,” Comminications, 1999. APCC/OECC’99. Fifth Asia-Pacific Conference on … and Fourth Optoelectronics and Communications Conference, Volume:2, 1999, Page(s): 1130-1133vol.2.



Thank You



Appendix• Message exchange flow chart of each

operation in the proposed conference tree algorithm



Create a new conference and the second user join the conference with MG (root) that is already on tree

Initiator MG(root) MGC Second user1. Notify

4. Reply

2. Reply3. Modify

Off hook

Dial group #Stop dial tone and collect digits

Analyze digits and do authentication

5. Notify6. Reply

8. Reply7. Modify

WaitOff hook

9. Similar with step 1~6

11. Reply10. Add



Join an existing conference with MG-new that is not on tree

New user MG-new MGC MG(to be connected)1. Notify

4. Reply

2. Reply3. Modify

Off hook

Dial group #Stop dial tone and collect digits

Analyze digits and do authentication

5. Notify6. Reply

8. Reply7. Add

Ring 12. Reply11. Modify

9. Add10. Reply

Stop Ring 14. Reply13. Modify

Connected



Leave operation (there are still other users in the MG)

User MG MGC1. Notify

4. Reply

2. Reply3. Subtract

On hook



Leave operation (there is no other user in the MG and the MG is a leaf MG)


4. Reply

2. Reply3. Subtract

On hookMG-parent

5. Subtract6. Reply



Leave operation (there is no other user in the MG and the MG is an intermediate MG)


12. Reply

2. Reply

11. Subtract

On hook

MG-child’s-new parent

3. Add4. Reply

MG-parent MG-child

5. Add6. Reply

7. Modify8. Reply

Connection established

9. Subtract10. Reply

14. Reply13. Subtract



Termination

Root MG (root) MGC1. Notify2. Reply

On hookAll MGs

3. Subtract4. Reply

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