Proceedings ofISCIT2005

Architecture and Performance of SoftSwitchNetworking

XU Peng, SU Sen, CHEN JunliangSchool of Computer Science and Technology

Beijing University of Posts and Telecommunications, Beijing, ChinaTel: +86-010-62251188-2130, Fax: +86-010-62241658

E-mail: xupengpengD)263.net

Abstract-In this paper, new features of SoftSwitch, suchas IP based infrastructure and the separation of signallingand media, are taken into account to propose a variety ofSoftSwitch network architectures. Furthermore, analysis onboth the performance and the applicability of thesearchitectures is presented. In the end, the roadmap for themigration from existing telecommunication networks to NGNis drawn.

Keywords-SoftSwitch, Next Generation Network,networking, signaling routing, subscriber management

I. INTRODUCTION

SoftSwitch controls a variety of gateways, accessdevices and IP terminals with signallings and controlprotocols, thus providing convergence communication.Furthennore, it is also a powerful service platform,integrating lots of service solutions. Therefore, more andmore network equipment providers and carriers havedeemed that next generation network (NGN) would beconstructed based on SoftSwitch.

Considering the forthcoming popularity ofSoftSwitch SoftSwitch networking technologies haveshown its importance. While current research mainlyfocuses on SoftSwitch interconnection protocols andcorresponding perfonnance analysis. At the same time, [Pbased infrastructure and the separation of signalling andmedia provide great flexibility for SoftSwitch networking.In this paper, all these new features have been taken intoaccount and a variety of architectures for SoftSwitchnetworking are further presented as well as theirapplicability and performance analysis.

This paper is organized as follows. In section 2, threekinds of SoftSwitch network architecture are distinctivelyintroduced as well as their applicability and performance.Then in section 3, the migration roadmap from existingtelecommunication networks to SoftSwitch based nextgeneration network is proposed.

II. SOFTSWITCH NETWORKING ARCHITECTURE

Based on different management structures ofsubscriber information, different connectivity of signalingand media, SoftSwitch networking architecture can beclassifled as following.1) Single Layer Architecture (SLA);2) Subscriber Management and Signaling Routing

Layered Architecture (SMSRLA);3) Fully Layered Architecture (FLA);4) Hybrid Layered Architecture (HLA).

A. Single Layer Architecture (SLA)In SLA, every SoftSwitch in the network could

perform signalling routing independently. Thus they haveto manage signalling routing information for allsubscribers in the network. A typical SLA SoftSwitchnetwork is shown in figure 1.

- Media- Signaling

SoftSwitch D

SoftSwitch CSoftSwitch c B

oftSwoitch BFig. 1. Single Layer Architecture

Single layer SoftSwitch networking architecture isthe simplest one in all the architectures presented in thispaper both on signalling and on subscriber management.On the aspect of signalling, call setup takes just one hopin SLA, and signalling routing can be performed locally.

However, to support this architecture, allSoftSwitches have to maintain signalling routinginformation for all subscribers in the network. Once a newsubscriber connects to the network, all databases in allSoftSwitches should be updated. At the same time,signalling connection among each pair of SoftSwitch isalways carried in a dedicate [P tunnel for signallingconnection's tremendous importance. When the number

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of SoftSwitches in the network keeps growing, thenumber of IP tunnels will have to increase exponentially.

To analyze the performance of certain networkingarchitecture, a universal SoftSwitch functional blockdiagram is proposed shown in figure 2.

Subscr i ber Management& Routing

Call Control

Fig. 2. Universal SoftSwitch Functional Block DiagramIn figure 2, Call state machine, with which

consecutive call control operations can be organized, andfollowing which a certain call control message can becorrectly processed, is implemented in Call Control (CC).CC is the central part of a SoftSwitch. SubscriberManagement & Routing (SMR) will fulfill subscriberauthentication, authorization and signalling routing. Thenthe queueing network model for the diagram in figure 2can be composed as shown in figure 3.

Subscr i berCall Management &

Control Routif

Fig. 3. Queueing Network Model for SoftSwitchIn figure 3, CC queue is assumed to be an M/M/1

queueing model. Although the input process of SMR,which is the output process of CC, is no longer Poissonous,since many different traffic streams are usuallysuperimposed in the subsequent queueing models, thePoisson approximation can still be used and leads tosufficiently reliable results [3]. Therefore, both CC queueand SMR queue can be viewed as M/M/1 queueing model.To calculate the call setup delay by SoftSwitch, theaverage service time of CC is assumed to be C; that ofSMR is r. r will increase along with the increase of thesubscribers managed by a SoftSwitch Then it can be giventhat r = f(m), where m is the number of subscribers.

The performance (mainly call setup delay) of SLA isanalyzed in the scenario shown in figure 4.Now, let1) The rate for call setup requests arriving at a

SoftSwitch will linearly increase according to theincreasing of the subscribers directly connected to aSoftSwitch.Let X denote the rate for call setup requests, thenA = ao , where m is the number of subscribers.

Furthermore, based on the traffic theories oftelecommunication networks [2], a can be given by

Traffic SingleSubs criber

TimeLength AvemgeCal I

Where Trafficsi,.iesu,bl,,ber is the traffic of a singlesubscriber; and TimeLengthAvegecajj is the averagelength of time for a call.

2) r is the ratio of the number of outgoing calls to thetotal number of calls.

3) The number of subscribers directly connected to bothSoftSwitch land SoftSwitch 2 is m.

4) The htansfer delay of signalling messages over IPbackbone is d.

5) The total number of subscribers in SoftSwitchnetwork is M.Based on above assumptions, the queueing network

model for the scenario shown in figure 4 is produced(Figure 5).

SoftSwitch 1 SoftSwitch 2

Subscribers of Subscritbers ofSoftSwitch 1 SoftSwitch 2

Fig. 4. Example ofSLA

Cc1 SWlE CC2 SIR2From fh-Subscribers im 11

of 55S1..J4.FToFro iTSubscribers From To

of SS1 Subscribers * Subscribersof SS2 of SS2

Fig. 5. Queueing Network Model for SLAIn figure 5, the dashed line illustrates the path of a

call setup message to pass the SoftSwitch network. Then,the call setup delay T is given by

T = Tcc1 +T1 +TCC2 +d (1)Where Tccj, Ts]n.i, TCC2 are the delay of CC1, SMR1

and CC2.The arriving rate and the service rate for each queue

in figure 5 are summarized in table 1 and table 2.

TABLE IARRiviNG RATE OF EACH QUEUE IN FIGU1RE 5

Acci ASMRI .CC2 )SiJ+ram Ram (1+r)am Ram

TABLE IISERVICE RATE OF EACH QUEUE IN FIGURE 5

FCC] PMRJ P1CC2 MR2I/C 1/n{M) 1/C -1(M)/

It is known from [4] that the delay of CC1 (Tcc1) asan M/Mll queueing model can be given by

TM= 1 c (2)icc, - Acc, I - C(r + I)am

Similarly, TsmR1 and TCC2 are given

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I f(M)I,-WR - ',SMR1 1- ramf(M) (3)

1 C (4)PcC2 -CC2 1- C(r +1)am

Substituting (2), (3) and (4) into (1), the call setupdelay for the example shown in figure 5 can be expressedas

(5)T = 2C + f(m) - + dI-C(r+l)am l-C(r+1)am

SubscriberManagement and SignalingRouting LayeredArchitecture (SMSRL4)A new type of entity will be introduced into

SoftSwitch network in SMSRA. That is routing server(RS), which is used to manage subscriber and signallingrouting information in SoftSwitch network. It should bepointed out that RS does not manage the information indetail as that of a SoftSwitch. It just keeps the relationbetween a certain subscriber and the SoftSwitch directlyaccessing this subscriber.

RS should manage subscriber and signallinginformation in the format of E.164 prefix and IP subnetaddress, thus manage the information in a largergranularity than that of a SoftSwitch. What's more, RSsthemselves can be organized in a layered architectre.SoftSwitch network adopting SMSRLA is shown in figure6.

RoutingServer A

RoutingSprur Ftu. v

-~MediaSignal ing

ISoftSwitch C

SoftSwitch

'-' SoftSwitch BFig. 6. Subscriber Management and Signaling Routing Layered

ArchitectureIn SMSRLA, sialling routing can use proxy

mechanism or redirection mechanism.1) Signalling Routing Proxy

When signalling routing proxy mechanism isemployed, RS will act as a proxy, perform signallingrouting and forwarding signalling messages between twoSoftSwitches.2) Signalling Routing Redirection

When Signalling Routing Redirection mechanism isemployed, RS helps SoftSwitch perform signalling routing.All signalling interactions still exist between the callingSoftSwitch and the called SoftSwitch themselves.

Both Signalling Routig Proxy and SignallingRouting Redirection will complicate the signallinginteraction in SoftSwitch network. However, theintroduction of RS can provide SoftSwitch network greatflexibility on its fial deployment.

The performance (mainly call setup delay) ofSMSRLA is analyzed in the scenario shown in figure 7.

SoftSw itch 1 __SoftSw itch

Subscribers of Subscribers ofSoftSwitch 1 SoftSwitch 2

Fig. 7. Example ofSMSRLABecause the functions of RS are rather simple, it can

be abstracted to an M/MIl queueing model. Then figure 7can be further abstracted to the queueing network modelinfigure 8.

In figure 8, the dashed line illustrates the path of acall setup message to pass the SoftSwitch network. Then,the call setup delay T is given by

TccT +TU1 TT ±TcT 2 +3d (6)Where TcC1, TSM1, TRS, TCC2 are the delay of CCl,

SMR1, RS and CC2.

cci SMR1 1 CC2 SMR2

Subscribers--of 55S LLS iIir

From I ToTo Subscribers Subscribers SubscriberOf SSof SS2 s of SS2

Fig. 8. Queueing Network Model for SMSRLAThe arriving rate and the service rate for each queue

in figure 10 are summarized in table 3 and table 4.

TABLE IIIARRIvING RATE OF EACH QUEUE IN FIGURE 10

)iCCj Aam] APs ACC2c

(f +r)am ram 2ram (1+r)am ram

TABLE IVSERVICE RATE OF EACH QUEUE IN FIGURE 10

Lc.I 1ILM Ulu IUCC2 US_I/C 1/| m I/,lf(M 1 /C I /f(m)

Because RS could manage subscribers in a largergranularity than that of SoftSwitch and the largest feasiblegranularity depends on the number of subscriber directlyaccessed by a SoftSwitch, the average service time of RScan be given asf(M,m), then service rate ofRS is I/f(M,m),where M is the total number of subscribers in SoftSwitchnetwork and m is the number of subscribers accessed byone SoftSwitch.

With the same methodology used to calculate the callsetup delay for SLA, the call setup delay for the exampleshown in figure 7 can be given as

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atffVI

T = 2C + f(m) + f(M,m) +3d (7)I- C(r + l)cam 1- ramf(m) 1- 2ramf(M, m)

Fully LayeredArchitecture (FLA)In FLA, SoftSwitch is organized hierarchically based

on the range of subscribers it managed and capabilities onsignalling routing. Low level SoftSwitch managessubscribers directly connecting to it, while high levelSoftSwitch manages subscribers connecting to all itssubordinate SoftSwitch.

Being alike to routing servers, high level SoftSwitchcould manage subscribers and signalling routinginformation in a larger granularity than low levelSoftSwitch. In FLA, both signalling connection and mediaconnection between two low level SoftSwitch will be viatheir higher level SoftSwitch as shown in figure 9.

FLA is exactly the same as the architecture ofexisting telecommunication networks. Although thisarchitecture might lose some advantages of SoftSwitch,such as a single hop media connection, existing techniquesand tools, especially on billing and network management,can be inherited into SoftSwitch network.

SoftSwitch E SoftSwitch F

- lledia- Signal ingr

SoftSwitch CSoftSwitch A

In figure 11, the dashed line illustrates the path of acall setup message to pass a SoftSwitch network. Then, thecall setup delay T is given byT = TCCI +T.. + T,,. + T.C + T,,2 + 2d (8)Where TCC1, TSMRI, TCCO, TSMRO, TCC2, TSMR are the

delay of CC1, SMR1, CCO, SMRO, CC2 and SMR2.The arriving rate and the service rate for each queue

in figure 11 are summarized in table 5 and table 6.

TABLE VARRivING RATE OF EACH QUEUE IN FIGURE 13

XCC I SMRI CCO SMR0 XCC2 1 2I(I+r)am Ram 2ram 2ram 1(+r)am ram

TABLE VISERVICE RATE OF EACH QUEUE IN FIGURE 13

PCci W1 pCco SMRO l,CC2 MR21/c l/f(m) 1/C 1/f(M,m) 1/C I/f(m)

With the same methodology used to calculate the callsetup delay for FLA, the call setup delay for the exampleshown in figure 10 can be given as

T= 2C + f(m)I-C(r+l)am 1-rcamf(m) (9)

C f(M,m) +2d+ ~+ +21-2Cram 1-2rwonf(M,m)

HybridLayeredArchitecture (HLA)

Fig. 9. Fully Layered ArchitectureThe performance (mainly call setup delay) of FLA is

analyzed in the scenario shown in figure 10.|SoftSw itch 0

Subscribers of Subscribers ofSoftSw itch 1 SoftSw itch 2

Fig. 10. Example of FLAIn figure 10, SoftSwitch 0 is a high level SoftSwitch,

which does not access subscribers directly, but has toperform call processing. Therefore, the queueing networkmodel in figure 3 can be adopted for SoftSwitch 0. Thequeueing network model for figure 10 is shown in figure11.

I CCO SIIROlFrom Subscribe s CC1 SMR1 CC2 SR2

of SS1 o SS2

To Subscribs From Su scrib ribersof SS F of SS2 of SS2

Fig. II. Queueing Network Model for FLA

Above three networking architectures could coexistwith one another to fit various requirements, thusconstructing HLA shown in figure 12.

Routing C SoftSwitchServer A witch DEF

tchSC

Fig. 12. Hybrid Layered ArchitectureThe concepts of "domain" and "region" are

introduced into the HLA. A SoftSwitch manages allsubscribers and signalling routing information in thedomain it resides. In a SoftSwitch domain, SLA is adopted.And a SoftSwitch region is composed of many SoftSwitchdomains. Two SoftSwitch residing in the same SoftSwitchregion but two different domains will rely on routingserver or high level SoftSwitch for signalling routing.

Numerical AnalysisIn this section, a numerical example is presented to

compare the performance of different networkingarchitectures and to further illustrate their applicability. Inthis example, scenarios in figure 4, 7 and 10 are adopted.Then some experimental values are given as following.

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1) Up to 10000 subscribers could directly connect to asingle SoftSwitch, m is 10000.

2) 20 percent of calls are outgoing calls, r is 0.23) The average service time for call control module is

0.03s, C is 0.03s.4) The average time used to inquire a subscriber in a

SoftSwitch will linearly increase according to theincreasing of the size of subscriber database. It meansthat r = f(m) = am, where a = 4.3 x o-8.

5) Trafficsr,8esub.,;,be, is 0.10 Erlang; TimeLegver-,c.jjis lOOs, then a is 0.00 1.

6) d is 0.025s7) Routing Server and high level SoftSwitch manage

subscribers by their E. 164 prefix and adopt the samealgorithm to inquiry subscriber database as that ofSoftSwitch, thenf(M,m) =aAf Im =4.3xIO-8M m

Substituting the above values into (5), (7) and (9), therelation between call setup delay and number ofsubscribers for each SoftSwitch networking architecturecan be drawn as figure 13, figure 14 (a) and figure 14 (b).

It can be seen from figure 13 that call setup delay ofSLA increases remarkably when the number of subscribersranging from 104 to 107, while call setup delay of SMSRAand LFA is primarily stable. When the number ofsubscribers is less than 7 x IO', the performance of SLA isbetter than that of SMSRA and LFA. However, as thenumber of subscribers is more than 9x1iO , theperformance of SLA deteriorates rapidly, and theperformance of SMSRA and LFA become better than thatof SLA. Therefore, a simple conclusion can be drawn thatwhen the number of subscribers is comparatively small(e.g. less than 7 x I0'), SLA should be adopted. While thenumber of subscribers is comparatively large (e.g. morethan 9xl 05), SMSEA or FLA should be adopted. Withfigure 13, the largest number of subscribers in aSnftSwitch iinmnin fnr T-LT.A can albenheniumerateid

Fig. 13. Relations between Call Setup Delay and Number ofSubscribers for SLA, SMSRA and FLA

As to figure 14 (a) and 14 (b), these two figurescompare the performance of SMSRA and FLA. It can beseen that the performance of SMSRA is much the same asthat of FLA. They all keep increasing with the increasingof subscribers. As the number of subscribers increases, thedifference between the two architectures gets less. Bycomparing figure 14 (a) to figure 14 (b), it can be seen thatdifferent IP transfer condition results in the differencebetween the performance of SMSRA and FLA. When IPtransfer condition is good enough (e.g. the transfer delay isless than 0.025s), the performance of SMSRA is betterthan that of FLA; while IP transfer condition is poor (e.g.the transfer delay is more than 0.045s), the performance ofPT A ic hetter th-n th2t af RMAR A

Fig. 14(b). Relations between Call Setup Delay and Number ofSubscribers for SMSRA and FLA (C= 0.03s, d= 0.045s)

With above figures, the performance (mainly call setupdelay) of SLA, SMSRA and FLA is illustrated as well astheir applicability. However, it should be pointed out thatto adopt certain networking architecture call setup delay is

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not the only consideration, some other features, such assignalling complexity, requirements on billing andmanagement are still something which have to be takeninto account.

Ill. MIGRATION ROADMAP FROM ExISTING NETWORKSTO SOFTSWITCH BASEDNGN

Different carriers might adopt different migrationroadmap from existing networks to SoftSwitch based NGNconsidering their difference on both existing networks andexpectation for NGN. However, generally, the deploymentof SoftSwitch always begins with tniking SoftSwitch,which is used to access end offices. Then SoftSwitch'scapabilities on accessing subscribers will be employed. Atthis time, SoftSwitch is the end office. Till now the scaleof SoftSwitch network is still comparatively small, andSLA might be a good choice. As more and moresubscribers connect to SoftSwitch network, routing sewverbegins to be introduced into SoftSwitch thus a SMSRAnetwork can be considered. While at gateway office, FLAmight be the ideal solution for its maturity on billing andnetwork management. Therefore, SoftSwitch based nextgeneration network would be of HLA.

IV. SUMMARY

SoftSwitch could support flexible networkingarchitecture to fit for various network infrastructure andapplications. With a variety of SoftSwitch networkingarchitectures presented in this paper, SoftSwitch could notonly coexist with existing telecommunication networksseamlessly, but also migrate to next generation networkeasily.

REFERENCES

[1] ISC Reference Architecture Description Version 1.1 April30,2002

[2] Zhou Jiongpan, Theory of Telecommunication Networks,ISBN:7-115-04600-X, 12, 1991

[3] Gert Willmann, Paul J. Kuhn, Performance Modeling ofSignaling System No.7, EEE Communication Magazine,July 1990

[4] Gunter Bolch, Stefan Greiner, Hermann de Meer, Kishor S.Trvedi,Queueing Networks and Markov Chains, John Wiley& Sons, Inc, 1998

[5] IETF RFC3261: SIP: Session Initiation Protocol

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