WIDESTAR II Satellite Core Network System

Mobile Satellite Communications Services SIP FAX Gateway

Special Articles on WIDESTAR II High-speed Mobile Satellite Communications Service for Diverse Satellite Communications

WIDESTAR II Satellite Core Network System

†1 The new WIDESTAR II mobile satellite communications ser- Core Network Development Department Takashi Yamamoto vice supporting faster communications was launched in April Takahiro Kawada†0 2010. With the aim of expanding the use of data communica- tions services and providing them at low rates, NTT DOCOMO has upgraded the existing PDC-based satellite system with a Journal new IMS-based mobile communications system, which is also used in the FOMA voice network. This system provides voice, FAX and packet services on GPRS and achieves voice-call control using SIP, which enables S-MSs to access supple- mentary services offered on FOMA and to connect to all FOMA APNs. Technical

nects with the Serving/Gateway GPRS sists of the Access Gateway for Satellite 1. Introduction Support Node (xGSN) in the FOMA (AGS), Satellite-Call Session control As an approach to converting voice service thereby enabling a Satellite- Node (S-CSN), Satellite-Application services provided by circuit switching Mobile Station (S-MS) to connect to Serving Node (S-ASN), Satellite-Media to IP, NTT DOCOMO is conducting a any Access Point Name (APN)*2 avail- Resource Node (S-MRN), which have migration to an IP-based core network able in the FOMA network without been newly developed, and the Media called Circuit Switched over-IP Net- impacting the server side of existing Gateway Node (MGN), Media Process- *3 DOCOMO work (CS-IP NW) for controlling and APNs in any way. A FAX gateway ing Node (MPN) , xGSN and IP Ser- transmitting voice traffic using the IP service, moreover, provides functions vice Control Point (IPSCP), which are Multimedia Subsystem (IMS)*1 [1]. A such as FAX storage not possible with existing equipment to which enhance- similar approach is adopted in the net- the existing satellite system. ments have been made. The S-CSN, S- NTT work handling mobile satellite commu- The network configuration of ASN, and S-MRN have a common nications, and to minimize the develop- NTT DOCOMO’s new satellite system development base with the CS-IP NW, ment load here, a development base is shown in Figure 1. The core network which is a development approach that common to that of CS-IP NW is used. equipment dedicated to this mobile enabled this mobile satellite communi- The packet communication service con- satellite communications system con- cations system to be constructed in an

*1 IMS: A communication system standardized *2 APN: The name of a network connection point by 3GPP for achieving multimedia services by used by corporate users and others to connect integrating communication services of the to the network. fixed-line network, mobile communications network, etc. using IP technology and SIP (see *5) protocol as used in VoIP.

50 NTT DOCOMO Technical Journal Vol. 12 No. 2 Satellite PDC network

S-ASN S-CSN S-MRN MGN FOMA network

Antenna

Customer Business LAN Mopera xGSN S-MS S-AP AGS IP router network （GGSN） MoperaU Internet MPN （FPE）

IPSCP iFAX Other operator Journal networks

Figure 1 Network configuration of satellite communications system

efficient and effective manner. tem, a group of equipment based on a need to establish an IP connection In addition, this system, as its pre- CS-IP NW is installed specifically for between the terminal and IMS, and this decessor, is expected to be available satellite use. The S-CSN, S-ASN and is accomplished through the introduc- during times of natural disasters and S-MRN perform control functions tion of AGS, which provides General

Technical other calamities, and to this end, a equivalent to standard IMS, with S-CSN Packet Radio Service (GPRS). There is redundant configuration has been performing session control*4, S-ASN likewise a need for the new satellite adopted. Specifically, network equip- service control, and S-MRN guidance system to inherit existing satellite ser- ment is installed in two physically transmission control. The MGN, mean- vices like voice mail and call forward- remote sites so that communication ser- while, controls connections with other ing, and this need is met by S-ASN as vices can continue uninterrupted even if networks such as FOMA while also an Application Server (AS) that con- one site should be destroyed. being used in the CS-IP NW. trols voice services. In this article, we describe the The new CS-IP NW introduces Sig- A key feature of the new mobile

DOCOMO core network equipment making up naling Interworking Node for 3G satellite communications system is that NTT DOCOMO’s new high-speed access (SIN) to perform protocol con- an S-MS terminates Satellite-SIP (S- mobile satellite communications system. version so that it does not impact the SIP), a specially-designed SIP for satel- existing 3G access network. In contrast, lite use, but does not terminate IP or NTT 2. Voice Services the new satellite system features a Real-time Transport Protocol (RTP)*6. 2.1 Control System for Satellite newly developed terminal and achieves The protocol stacks indicating which Voice Services end-to-end Session Initiation Protocol protocols each of the above nodes ter- To provide voice services over the (SIP)*5 control between that terminal minate are shown in Figure 2. While mobile satellite communications sys- and the IMS network. There is therefore SIP exchanges information for sending

*3 MPN: A node developed for the purpose of *4 Session control: A function for controlling *6 RTP: A protocol defined by IETF for real-time unifying media service enablers to enhance the end-to-end IP communications over the net- distribution of audio, video or other such value of supplementary services in the future work. media. network. *5 SIP: A call contorol protocol defined by the Internet Engineering Task Froce (IETF) and used for IP telephony with VoIP, etc.

NTT DOCOMO Technical Journal Vol. 12 No. 2 51 WIDESTAR II Satellite Core Network System

（a） U-Plane

G.729 G.729

RTP RTP

IP/UDP IP/UDP IP/UDP IP/UDP

L1 L1 GTP GTP GTP GTP

IP/UDP IP/UDP IP/UDP IP/UDP L1/L2 L1/L2

L1/L2 L1/L2 L1/L2 L1/L2

AGS AGS S-AP S-MRN MGN （SSAR） （GSAR） S-MS

（b） C-Plane Journal S-SIP S-SIP SIP SIP

IP/UDP IP/UDP IP/UDP IP/UDP S-RRC S-RRC GTP-U GTP-U GTP-U GTP-U

L2 L2 IP/UDP IP/UDP IP/UDP IP/UDP L1/L2 L1/L2

L1 L1 L1/L2 L1/L2 L1/L2 L1/L2

AGS AGS S-AP S-CSN S-ASN MGN （SSAR） （GSAR）

Technical S-MS

（c） IP connection setup

SM/GMM SM/GMM GTP GTP S-NWMP S-NWMP S-RRC S-RRC RANAP RANAP

SIGTRAN SIGTRAN L2 L2 IP/UDP IP/UDP IP/UDP IP/UDP IP IP

L1 L1 L1/L2 L1/L2 L1/L2 L1/L2 L1/L2 L1/L2 DOCOMO

AGS AGS S-AP S-CSN （SSAR） （GSAR） S-MS

NTT C-Plane : Control Plane RANAP : Radio Access Network Application Part GSAR : Gateway Satellite Access Router S-NWMP : Satellite-Network Management Protocol GTP : GPRS Tunnelling Protocol SSAR : Serving Satellite Access Router L1 : Layer 1 UDP : L2 : Layer 2 U-Plane : User Plane

Figure 2 Protocol stack for voice communications

52 NTT DOCOMO Technical Journal Vol. 12 No. 2 and receiving media using Session the originating request (INVITE) to the arriving IP packet and performs Description Protocol (SDP)*7, the Satel- S-CSN. paging. Then, on receiving a lite-Access Point (S-AP) terminates ¥ Procedure 2: response from the terminating S-MS, RTP, which means that the SDP infor- The S-CSN sends INVITE to it performs authentication/ciphering mation provided by SIP must be notified S-ASN where registration has been processing and PDP context estab- not only to S-MS but also to S-AP. performed. lishment and sends the queued Thus, on receiving an SIP signal to ¥ Procedure 3: packet to the terminating S-MS. which SDP is attached, the S-CSN noti- The S-ASN identifies the origi- ¥ Procedure 8: fies necessary information to S-AP via nating subscriber and performs The terminating S-ASN which the AGS. service control on the originator’s has received a ringing signal (180) side such as assessing subscription from the terminating S-MS sends 2.2 Originating/Terminating Call status. INVITE to the S-MRN to make a Processing Sequence for ¥ Procedure 4: connection to Ring Back Tone (RBT). Satellite Voice Services The S-CSN accesses the IPSCP The S-MRN, in turn, responds to Journal On power ON, an S-MS sends a accommodating the terminating the terminating S-ASN with a suc- signal in GPRS Mobility Management subscriber, obtains the address of cess response (200) that is set the (GMM) protocol*8 to AGS, which then the S-CSN on which the terminat- SDP for sending out the RBT. performs attach procedure. The S-MS ing subscriber has been registered, ¥ Procedure 9: also sends a signal in Session Manage- and sends INVITE to that S-CSN. Once SDP updating is completed, ment (SM) protocol*9 to AGS to estab- ¥ Procedure 5: the terminating S-ASN sends a ring- lish a Packet Data Protocol (PDP) con- The terminating S-CSN identi- ing signal (180) to the originating

Technical text for a voice-dedicated APN. In other fies the terminating subscriber and S-ASN, which sends the ringing words, an IP connection is provided sends INVITE to the terminating signal to the originating S-MS via through GPRS. After this, the terminal S-ASN, which assesses the sub- S-CSN. Here, the S-CSN (Proxy-Call sends out an SIP signal requesting reg- scription status of the terminating Session Control Function (P-CSCF)) istration*10, which prompts IMS regis- subscriber and sends a provisional sends SDP information to the S-AP tration control to be performed with response (183) to the originating via the AGS. Then, in parallel with respect to upper nodes (S-CSN/S- side. The terminating S-ASN also this, the terminating S-ASN sends ASN/IPSCP) and profile information to performs service control on the ter- back an acknowledgment request

DOCOMO be created at each node. Basic originat- minator's side. (ACK) with respect to procedure 8 ing/terminating call processing after ¥ Procedure 6: to S-MRN, which sends the RBT to registration is shown in Figure 3. The terminating S-ASN sends the originating side. ¥ Procedure 1: INVITE to the terminating S-MS ¥ Procedure 10: NTT In response to an originating via the terminating S-CSN without At the time of subscriber call from an S-MS, the AGS exe- being aware of the existence of the response, the terminating S-CSN cutes security procedures with the terminating AGS. (P-CSCF) which received the suc- terminal such as authentication and ¥ Procedure 7: cess response (200) for the first ciphering processing and transfers The terminating AGS queues INVITE from the terminating S-MS

*7 SDP: Protocol for describing IP addresses and tion and authentication in the packet switched of a contact address by a mobile terminal with other information necessary for beginning a domain. a registrar server, and in IMS, to registration multimedia session. Also used for describing *9 SM protocol: Protocol for connecting with processing by an IMS terminal with respect to session information in SIP. the network including call originating and call its home network. *8 GMM protocol: Protocol for performing terminating in the packet switched domain. mobility management such as location registra- *10 Registration: In SIP, refers to the registration

NTT DOCOMO Technical Journal Vol. 12 No. 2 53 WIDESTAR II Satellite Core Network System

Originating side Terminating side S-CSN S-CSN S-CSN S-CSN S-MS S-AP AGS IPSCP S-MRN AGS S-AP S-MS （P-CSCF） S-ASN S-ASN （P-CSCF）

(1) Service Request

Authentication/ciphering/RAB-settings

INVITE（SDP） (2)INVITE（SDP）

(3)Originating service control (4)Diameter LocationInformationRequest

Diameter LocationInformationAnswer

INVITE（SDP） (5)183 183 183 Terminating service control (6)INVITE（SDP） INVITE（SDP） (7)Paging

Authentication/ciphering/RAB-settings Journal (8)180 180 INVITE[2]（SDP） 200[2]（SDP） (9) 180（SDP） 180（SDP） Policing request GW Setup Request

180（SDP） ACK[2]

RBT (10) 200（SDP） 200（SDP） BYE[2] Policing request GW Setup Request 200（SDP） (11) UPDATE SDP Technical （ ） Policing request GW Setup Request

UPDATE（SDP）

200（SDP） 200（SDP）

ACK ACK ACK ACK ACK

RAB : Radio Access Bearer SIP Satellite-unique signals Other DOCOMO Figure 3 Overview of originating/terminating sequence

sends SDP information to the termi- including the SDP of the terminating minating S-MS are connected. NTT nating S-AP via the terminating S-MS to the originating S-ASN so ¥ Procedure 11: AGS. In addition, the terminating that the state in which the originating The originating S-ASN which S-ASN releases the session release S-MS and S-MRN are exchanging has received the success response with respect to S-MRN and also SDP can be changed to the state in (200) from the terminating S-MS sends the success response (200) which the originating S-MS and ter- sends update information (UPDATE)

54 NTT DOCOMO Technical Journal Vol. 12 No. 2 including the SDP of the terminat- mum 64 kbit/s in the downlink; best- On receiving mail, the MPN noti- ing S-MS to the originating S-MS effort communications). fies the FAX adaptor connected to the and receives a response from the In contrast, the new satellite FAX S-MS of the incoming mail. The FAX originating S-MS. At this time, the gateway service enables the sending adaptor, in turn, establishes a network originating S-CSN sends SDP and receiving of FAXs over the IP net- connection with the MPN to receive the information to the S-AP. After this, work using G3FAX machines. Adopt- mail. Then, after receiving the mail, the the originating S-ASN sends the ing the International Telecommunica- FAX adaptor converts the mail to G3 success response (200) to the origi- tion Union (ITU) T.37*11 standard (mail protocol and passes it to the G3FAX nating S-MS with respect to the system), the FAX gateway service machine, which proceeds to output the original INVITE. The above pro- incorporates a new FAX storage func- FAX. It is also possible to receive cessing enables a call to be made in tion (three-day maximum) in the FAXs from other Internet Service the satellite system. NTT DOCOMO network so that FAX Providers (ISPs) that provide services communications can later be realized if similar to iFAX. 3. Data Communications the other party should be out of range. New Fax Processing Equipment Journal Services This new service also features a short (FPE) has been developed and installed In addition to IP connections for communication time due to a broad in MPNs in the NTT DOCOMO net- voice communications on the satellite bandwidth (maximum 144 kbit/s in the work to provide the FAX gateway system as described above, the AGS uplink and maximum 384 kbit/s in the service. The FPE installation configura- also provides IP connections for data downlink; best-effort communications) tion within an MPN is shown in Figure communications services. These ser- providing users with an even higher 5. This FPE, which has T.37 functions, vices can be broadly divided into the level of convenience. makes effective use of existing software

Technical FAX gateway service, packet commu- An example of a network configu- while making original extensions to nication service, and direct connect ser- ration for providing the FAX gateway general-purpose software and general- vice from the network connection point service is shown in Figure 4. To send purpose interfaces. This approach, of view. and receive FAXs, the user connects a which flexibly utilizes high-value- G3FAX machine to the FAX adaptor of added and inexpensive general-purpose 3.1 FAX Gateway Service the S-MS. The FAX adaptor performs solutions, has been successful in signifi- The launching of NTT DOCOMO’s mutual conversion between G3FAX cantly reducing development time and FAX gateway service in April 2010 signals and mail. There are three pat- expenses.

DOCOMO provided users with an alternative to the terns of service provision: fixed-line existing satellite FAX service. Based on telephone to S-MS, S-MS to fixed- 3.2 Other Services a circuit switched system, the existing line telephone, and S-MS to S-MS. The packet communication service satellite FAX service was unable to The iFAX¨*12 service provided by is the same as the data communication NTT forward FAXs if the other party was in NTT Communications Corporation is service provided by FOMA. It aims to a line-busy or out-of-range state, and it used to send and receive FAXs between provide connections to all APNs provided suffered from a long communication an S-MS and fixed-line telephone, by FOMA with the exception of i-mode time due to a narrow bandwidth (maxi- and the MPN is used to send and receive related communications. To this end, mum 4.8 kbit/s in the uplink and maxi- FAXs between two S-MSs. we adopt a network architecture in

*11 T.37: A transmission-control protocol for achieving G3FAX communications via e-mail on the IP network. *12 iFAX®: A registered trademark of NTT Com- munications Corporation.

NTT DOCOMO Technical Journal Vol. 12 No. 2 55 WIDESTAR II Satellite Core Network System

Satellite

Other ISP servers Satellite network G3FAX G3FAX S-AP

S-MS AGS FAX adaptor MPN Internet iFAX PSTN

NTT Communications, NTT DOCOMO etc. NTT EAST/WEST, etc. Journal T.30 TCP/IP -POP/SMTP- TCP/IP -SMTP- T.30

U-Plane (FAX) PPPoE G3FAX Mail Mail G3FAX

PPP : Point-to-Point Protocol PPPoE : Point-to-Point Protocol over SMTP : Simple Mail Transfer Protocol

Figure 4 Network configuration of FAX gateway service Technical

General/original interface Service control equipment

Maintenance Service control functions functions

General-purpose interface

STM Media control functions FPE (T.37

STM/IP processing) conversion

DOCOMO Media processing Media IP interface IP interface functions (audio/video) processing functions

Database (FAX)

Various media processing equipment NTT

Figure 5 FPE installation configuration in MPN

which the AGS connects to the xGSN is that connections from S-MSs can be ties or change settings on the APN side. (GGSN). The advantage of this scheme made without having to add new facili- Furthermore, by adopting in the AGS a

56 NTT DOCOMO Technical Journal Vol. 12 No. 2 GPRS system that is completely the not covered by mobile-phone networks same as that of the xGSN (SGSN), 4. Conclusion such as off-shore and mountainous communications from the satellite can This article described CS-IP NW areas, as well as enhanced services that be achieved without affecting the equipment supporting NTT DOCOMO’s exploit the capabilities of this new xGSN (GGSN) in any way. new WIDESTAR II high-speed mobile satellite network based on IMS archi- The direct connect service enables satellite communications system. Look- tecture. S-MSs to communicate with each other ing to the future, we can expect the at the IP level as an alternative means to addition of new xGSN (GGSN) func- Reference data communications performed via cir- tions to expand data communications [1] Y. Shimada et. al: “IP-based FOMA Voice Network toward Enhanced Services and cuit switching on the existing satellite and the addition of new MPN functions Improved Efficiencies,” NTT DOCOMO system. In the new service, each terminal to expand FAX communications. We Technical Journal, Vol.12, No.1, pp.4-14, is assigned a fixed IP address enabling can envision a broadcast service for Apr. 2010. an originating S-MS to connect to any providing emergency information by other S-MS. voice, mail and FAX to users in areas Journal Technical DOCOMO NTT

NTT DOCOMO Technical Journal Vol. 12 No. 2 57