INTELLIGENCE IN SWITCHED AND PACKET NETWORKS

Transcription

1 22-24 Setember, Sozool, ULGRI INTELLIGENCE IN SWITCHED ND PCKET NETWORKS Ivaylo Ivanov tanasov Deartment of teleommuniations, Tehnial University of Sofia, 7 Kliment Ohridski st., 1000, hone: , iia@tu-sofia.bg Key words: Intelligent network, SIP rotool, MEGCO rotool The aer investigates the interation between intelligent network (IN) and the Internet. lthough there are many features of IN that an be reognized in the Internet, there is one imortant differene. In IN servies are entrally ontrolled; in the Internet they are omletely distributed through the network. If the Internet is used for telehony, there is a need of a rotool that sets u a all between omuters or telehones. The interation between IN and the Internet may be illustrated by Conferene all servie modelling. The aer onsiders Conferene all servie imlementation in a ubli telehone swithed network strutured as IN and ossible servie models by the use of Session Initiation Protool (SIP) and MEGCO rotool. 1. Introdution The first ste toward a feature-rih network was made with the introdution of intelligent networks. The main idea of IN is to searate all logi roessing from swithing, and to delegate it to seialized servie ontrol oints (SCPs). IN standards are rodued in the form of aability sets (CSs). The initial CS-1 and CS-2 were oneived for enhaned ontrol of oint-to-oint voie alls. They are based on a all model that desribes the states of setting u a voie all between two arties. IN was not originally intended for onnetionless data servies and does not deal well with onferene tye servies involving multile arties and multile media streams. CS-3 and CS-4 were intended to address some of these features, but do not seem to live u to the exetations. New arhitetures may be neessary in the long term. The Internet ommunity influenes the develoment of IN by seifying new rotools for Internet telehony, suh as SIP, H.323, MEGCO, PINT, and SPIRITS. lthough in a aket network there are no swithing matrixes, there is still a need for nodes that terminate signalling rotools. The soft swithes are nodes that terminate signalling and ontrol network resoures, but they are not neessary diretly ouled to a swithing matrix. The aliation of IN to soft swithes is a hot toi. In this aer some fores that Internet and IN ommunities exert on one another are examined. Models of a onferene all servie are resented to illustrate the different tehnologies. 2. Conferene all model in an intelligent network IN CS-1 deals with oint-to-oint telehone alls and does not offer suffiient features for multiarty and multimedia ommuniations. CS-2 an handle alls bet-

2 22-24 Setember, Sozool, ULGRI ween more than two arties. While in CS-1 the all and the onnetion setu as the result of the all were the same thing, in CS-2 it is ossible to maniulate the individual arties in a all and their onnetions. One of the most imortant requirements is that the servie logi must always have a onsistent view of the onfiguration of the onnetions in a all. When the servie logi loses trak of who is onneted to the all, the servie gets into an inonsistent state and the effet of a join or slit oeration beomes unreditable. For this reason CS-2 defines a formal model for desribing the state of onnetions in a all. This is alled the onnetion view state (CVS) model. shared shared Originating setu On hold On hold Stable 2 arty ending Stable 2-arty C shared Originating setu On hold Stable multi arty ending SSF InitialDP onnet; requestreort eventreort requestreort eventreort requestreort slitleg slitlegresult ontinuewithrgument; onnet; requestreoort eventreort mergecallsegments mergecallsegmentsresult Figure 1 Connetion-state view for a onferene all C C SCF (1) (2) (3) (4) (5) (6) (7) (8)

3 22-24 Setember, Sozool, ULGRI The main elements in CVS are all segment desribing a half all in terms of legs and onnetion oints and all segment assoiation that is a grou of all all segments that are involved in a given servie. Figure 1 illustrates an examle of onferene all between three users,, and C. The initiator of the all is user. The figure shows the dialog between the servie swithing funtion (SSF) that takes are of ontrolling the bearer onnetions in a all, and the servie ontrol funtion (SCF) ontaining servie logi. 1. When the user laes the first all to, the in SSF detets trigger and sends the initialdp message to the SCF. 2. The SCF instruts SSF to onnet the originating and destination arties, and requests the SSF to reort any midall events. 3. When the user uts on hold the urrent onnetion with user, the SSF reorts the midall event to the SCF. 4. When the user dials the number of user C, the SSF reorts the midall event to the SCF. 5. The SCF requests the SSF to searate one arty from its all segment and lae it in a new all segment. 6. The SCF instruts SSF to onnet the originating arty to the third arty C. 7. The user flashes the hook to onnet both arties. 8. The SCF requests the SSF to merge two assoiate all segments into a single all segment. It reestablishes the bearer onnetion between all involved legs. Users, and C are involved in a onferene all. Of ourse, the examle is simlified; the messages between the SSF and SCF are more omlex in reality. The fous is on CVS that reresents how SCF sees the onnetions in SSF. When a ommand from the SCF to the SSF hanges the onnetions in a all, the CVS hanges aordingly. CVSs an be very omlex beause IN CS-2 tries to imlement multiarty ommuniations by using lassi swithing tehnology intended only for oint-to-oint telehone alls. CS-3 is little more than revised version of CS-2. CS-4 was to aommodate the voie, video, multimedia, data ommuniations, ossibly involving multile arties and using aket-based transmission rather than swithed onnetions. CS- 4 introdues the session manager (SM) handling voie and multimedia alls over Internet using either of the two well-known rotools H.323 or SIP. Figure 2 gives a simlified view of SM in CS-4 as it omits some funtional entities already defined in CS-1, CS-2, and CS-3.

4 22-24 Setember, Sozool, ULGRI SIP roxy H.323 gatekeeer SCF Publi swithed telehone network SSF SSF SM IN-IP gateway Internet Figure 2 Partial view of CS-4 distributed funtionality The wide aeted by all standardization bodies distributed arhiteture for the Gateway imlementation is based on three omonents Media Gateway, Media Gateway Controller and Signaling Gateway. Figure 3 deits the Gateway arhiteture distributed into the Call ontrol funtionality and the Media roessing funtionality over different network elements (a onet of soft swith). Media Gateway Controller SIGTRN Signaling Gateway Media Gateway MGCP MEGCO H.248 Paket interfae Figure 3 Gateway referene arhiteture 3. MEGCO onnetion model of a onferene all MEGCO is an aliation rotool for ontrolling Media Gateways from external all ontrol elements alled Media Gateway Controllers or all agents. Gateway is tyially a network element that rovides onversion between the audio signals arried on telehone iruits and data akets arried over the Internet or over other aket networks. The foundation for the seifiation of MEGCO is a onnetion model that desribes terminations and ontext. There is a striking resemblane between the onnetion model for MEGCO and the onnetion view state in IN CS-2.

5 22-24 Setember, Sozool, ULGRI termination is a oint on the media gateway that reeives or sends media streams. termination may be a hysial ort suh as telehony line, but it an also be a media stream of temorary nature, suh as RTP stream. ontext desribes how media flows between terminations in the media gateway. Contexts and terminations an be maniulated with MEGCO ommands. Figure 4 illustrates a MEGCO onnetion model of the onferene all servie. Termination 1 Termination 2 Context 1 Media Gateway Two arty state dd (termination 3, to ontext 1) Termination 1 Termination 2 Context 1 Termination 3 RTP stream Media Gateway Multiarty state Figure 4 Examle of terminations and a ontext for all onferening senario In the to onfiguration termination 1 is onneted to termination 2. In other words, the users onneted to ubli swithed telehone network (PSTN) are ommuniating. In this ase the terminations are hysial orts. dding a aket data network subsriber in a onferene is not different than adding a PSTN subsriber to a onferene. The termination 3 is an ehemeral termination reresenting RTP stream. The dd ommand is used to reate the termination 3 and to add it to the existing ontext. The onet of ehemeral termination brings about uniformity in reresentation in sense that the RTP stream an be oerated uon in a manner similar to any other termination in the Media Gateway. 4. Conferene all model in a SIP network SIP is an aliation rotool for setting u multimedia sessions over the Internet. It is seen as the future of all signalling and telehony. One of the reasons for the raid aetane of SIP is that it is inredibly owerful all ontrol rotool. It allows intelligent end oints to imlement the entire suit of telehony servies without a

6 22-24 Setember, Sozool, ULGRI servie rovider, and without a ontroller or swith for examle. The signalling model for the onferene all servie in SIP telehony network is showed in the Figure 5. SIP user C SIP user PSTN gateway ISUP exhange Invite si @df.arrier.om IM 183 session rogress CM Ringing User PRCK 200 OK RTP akets 200 OK CK Ring tone NM nswer Invite (join) 180 Ringing CK RTP akets PCM seeh nalog seeh Invite 200 OK CK Figure 5 signalling model of a onferene all In the examle shown in Figure 5 the alling SIP hone laes a telehone all to the PSTN through a PSTN gateway. The SIP hone has been reonfigured with the IP address of the PSTN gateway, so it is able to send it Invite request. The gateway initiates the all into PSTN by seleting an SS7 ISUP trunk to the next telehone swith in the PSTN. The dialed digits from the Invite are maed into ISUP IM. The ISUP CM is sent bak by PSTN to indiate that the trunk has been seized. Progress tones are generated in the one-way audio ath established in the PSTN. Ring tone is generated by the far end telehone swith. The gateway mas the CM to 183 session rogress resonse ontaining the RTP ort that the gateway will bridge the audio from the PSTN. Uon reetion of 183, the aller s user agent begins reeiving RTP akets sent from the gateway and resents the audio to the aller so they know

7 22-24 Setember, Sozool, ULGRI that the all is rogressing in the PSTN. The all omletes when the alled arty answers the telehone, whih auses the telehone swith to send NM to the gateway. The gateway then uts the PSTN audio onnetion through in diretions and sends a 200 OK resonse to the aller. Caller s user agent sends CK to omlete the SIP signaling exhange. SIP aller and PSTN subsriber are involved in a dialog. nother SIP user C wants to join to the dialog established. His SIP hone sends Invite request with Join header field. Invite (Join) requests that the dialog be with an existing dialog. s the dialog is already art of a onferene, the Join header field is a request to be added into the onferene. 5. Conlusion IN was born in an era when swithed telehony networks ruled the world and the Internet rotool was still onfined to the interonnetion of data networks. ut the world has hanged. The Internet has beome a general ubli utility. It is now erfetly ossible to enode a voie onversation in akets and send these over an IP network. Today we faed with the hallenge of reating value-added servies in heterogeneous networks with both iruit-swithed and aket-based transmission. rofound analysis shows that servies traditionally offered in telehone networks may be also rovided in IP networks. New rotools for Internet telehony suh as SIP, H.323, and MEGCO ontribute to ombination of Internet aliations and telehony servies. The aer examines some ways of imlementing IN servies in Internet. Referenes [1] Johnston., SIP Understanding the Session Initiation Protool, rteh House, 2004 [2] RFC 3261, SIP: Session Initiation Protool, [3] RFC 2805 Media Gateway Control Protool rhiteture and Requirements, 2000 [4] ETSI EN , INP CS-2 Part 1, Protool Seifiation, 1996 [5] Zuidweg J., Next generation Networks, rteh House, 2002.