2003 Kluwer Academic Publishers. Manufactured in The Netherlands.
vGPRS: A Mechanism for Voice over GPRS
MING-FENG CHANG, YI-BING LIN and AI-CHUN PANG∗
Department of Computer Science and Information Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC
Abstract. This paper proposes vGPRS, a voice over IP (VoIP) mechanism for general packet radio service (GPRS) network. In this
approach, a new network element called VoIP mobile switching center (VMSC) is introduced to replace standard GSM MSC. Both standard GSM and GPRS mobile stations can be used to receive real-time VoIP service, which need not be equipped with the VoIP (i.e., H.323) terminal capabilities. The vGPRS approach is implemented using standard H.323, GPRS, and GSM protocols. Thus, existing GPRS and H.323 network elements are not modified. Furthermore, the message flows for vGPRS registration, call origination, call release and call termination procedures are described to show the feasibility of our vGPRS system.
Keywords: gatekeeper, GGSN, GPRS, SGSN, VoIP
1. Introduction
In early 2000, only a small portion of GSM subscribers used data services. This low data penetration rate is due to the fact that the existing GSM systems do not support easy access, high data rate and attractive prices. GSM operators must offer better services to stimulate the demand. The solution is
Gen-eral Packet Radio Service (GPRS). GPRS reuses the existing
GSM infrastructure to provide end-to-end packet-switched services. The GPRS core network has also been developed for IS-136 TDMA systems, and is anticipated to evolve as the core network for the third-generation mobile systems.
Recently, supporting telephony services over IP network or the so called voice over IP (VoIP) is considered as a promising trend in telecommunication business. Particularly, integrating mobile phone services with VoIP becomes an important issue, which has been intensively studied [3,9,14]. Thus, it is natural to support voice service over GPRS. In 3G TR 21.978 [1], VoIP service for GPRS is defined based on the H.323 pro-tocol [11], which nicely integrates GPRS with H.323-based VoIP network. In this approach, the mobile phones or mo-bile stations (MSs) must be equipped with vocoder and H.323 terminal capabilities. In this paper, we propose vGPRS, a VoIP service for GPRS [13]. Our approach supports stan-dard GSM and GPRS MSs to enjoy real-time VoIP service without modifying the existing GSM/GPRS network nodes as well as H.323 family protocols. Other approaches such as 3G TR21.978 cannot accommodate standard GSM and GPRS MSs to receive VoIP service. Instead, the MSs must be equipped with the H.323 terminal capabilities. Furthermore, this previous approach may not offer real-time VoIP based on the existing GPRS architecture.
This paper is organized as follows. Section 2 introduces the vGPRS network architecture and shows how GPRS can be modified to support vGPRS. Section 3 describes the regis-tration procedure that enables a GSM or a GPRS MS to enjoy VoIP service. Section 4 describes how an MS originates a call
∗Corresponding author.
through vGPRS. Section 5 describes how an incoming call is delivered to an MS. Section 6 compares vGPRS with the 3G TR21.978 approach.
2. vGPRS network architecture
Figure 1 illustrates the GPRS architecture. In this figure, base transceiver station (BTS), base station controller (BSC), mo-bile switching center (MSC), home location register (HLR), and visitor location register (VLR) are GSM network nodes. GPRS introduces two new network nodes; namely, serving
GPRS support node (SGSN) and gateway GPRS support node
(GGSN), to the GSM architecture. An SGSN receives and transmits packets between the MSs and their counterparts in the public switched data network (PSDN). To connect to an SGSN, a packet control unit (PCU) is implemented in the BSC. The BSC forwards circuit-switched calls to the MSC, and packet-switched data (through the PCU) to the SGSN. A BSC can only connect to one SGSN. The GGSN interworks with the PSDN using connectionless network protocols, such as Internet protocol and the OSI connectionless network pro-tocol, or connection oriented protocols such as X.25. Both SGSN and GGSN interact with the GSM location databases, including the HLR and the VLRs, to track the location of the
(b)
Figure 2. (a) the VMSC interfaces and (b) the vGPRS network architecture. MSs. The reader is referred to [12] for details of GSM and GPRS.
The vGPRS approach provides VoIP service to standard GSM and GPRS MSs. In vGPRS a new network node VoIP MSC (VMSC) is introduced to replace MSC. The VMSC is a router-based softswitch and its cost is anticipated to be cheaper than an MSC. Figure 2(a) shows the interfaces be-tween the VMSC and other GSM/GPRS network nodes. The GSM signaling interfaces of the VMSC are exactly the same as that of an MSC. That is, it communicates with the BSC, the VLR, the HLR, and the MSC (either GSM MSC or VMSC) through A, B, C and E interfaces, respectively. The VMSC also interfaces with the public switched telephone network (PSTN) via the SS7 ISUP protocol. Unlike an MSC, the VMSC communicates with SGSN through GPRS Gb inter-face. In other words, while an MSC delivers voice traf-fic through circuit-switched trunks, the VMSC delivers VoIP packets through GPRS network. In figure 2(b), the data path of a GPRS MS is (1) ↔ (2) ↔ (3) ↔ (4). The voice path is (1) ↔ (2) ↔ (5) ↔ (6) ↔ (4). In this voice path,
(1) ↔ (2) ↔ (5) is circuit switched as in GSM. Thus, both standard GSM MSs and GPRS MSs can set up calls as if they are in the standard GSM/GPRS network. At the VMSC, the voice information is translated into GPRS packets through vocoder and packet control unit (PCU). Then the packets are delivered to the GPRS network through the SGSN (see path
(6) ↔ (4)). An IP address is associated with every MS at-tached to the VMSC. In GPRS, the IP address can be created statically or dynamically for a GPRS MS. On the other hand, a standard GSM MS cannot be assigned an IP address through GPRS. Thus, in vGPRS the creation of the IP address is per-formed by the VMSC through the standard GPRS Packet Data
texts such as TMSI (Temporary Mobile Subscriber Identity), IMSI (International Mobile Subscriber Identity), and the QoS profile requested. These contexts are the same as that stored in a GPRS MS (see section 13.4, GSM 03.60 [7]). In vGPRS, H.323 protocol [11,14] is used to support voice applications. The VMSC executes the H.323 protocol just like an H.323 terminal.
A gatekeeper GK in the H.323 network (see figure 2(b)) performs standard H.323 gatekeeper functions (such as ad-dress translation). Figure 3 illustrates the connection between an H.323 terminal and a GSM MS. In this figure, the TCP/IP protocols are exercised in links (1), (2), and (8). The GPRS tunneling protocol [8] is exercised in link (3), and GPRS Gb protocol [6] is exercised in link (4). The standard GSM pro-tocols are exercised in links (5)–(7), (9), and (10). The H.323 protocol is implemented on top of TCP/IP, and is exercised between the VMSC and the nodes in the H.323 network. The H.323 packets are encapsulated and delivered in the GPRS network through the GPRS tunneling protocol.
3. Registration
Several types of messages are described in this and subse-quent sections. A message with the “MAP_”. prefix repre-sents a GSM MAP message. The “Um_”, “A_” and “Abis_” prefixes denote air interface, A interface, and Abis interface, respectively. The Registration, Admission and Status (RAS) and Q.931 messages are defined in H.323 and H.225 proto-cols [10,11]. In GSM, a registration procedure is performed when an MS is turned on or when the MS moves to a new lo-cation area. Without loss of generality, this section describes the registration procedure by assuming that registration is per-formed when the MS is turned on. The registration proce-dure for MS movement is similar and will be briefly elabo-rated at the end of this section. The GSM registration mes-sages (steps 1.1, 1.2, and 1.3) are delivered through the path
(7)↔ (6) ↔ (5) ↔ (9) ↔ (10) in figure 3. The H.323 mes-sages (steps 1.4, 1.5, and 1.6) are delivered through the path
(7)↔ (6) ↔ (5) ↔ (4) ↔ (3) ↔ (2). The message flow is illustrated in figure 4 and are described in the following steps. Step 1.1. An MS is turned on. To join in the network, the MS performs registration following the standard GSM loca-tion update procedure [5]. The MS sends aUm_Location_
Figure 4. Message flow for vGPRS registration.
Update_Requestmessage to the BTS. The BTS forwards this request to the BSC using the GSM Abis message Abis_Location_Update. The BSC then sendsA_Location_ Update message to VMSC through GSM A interface. The VMSC issues GSM MAP message MAP_Update_ Location_Areato the VLR. We assume that the MS pro-vides the IMSI (international mobile subscriber identity) for registration. Our message flow can be easily extended for the case when the TMSI (temporary mobile subscriber identity) is used [12] (which is likely to occur for location update due to MS movement). The standard GSM authen-tication procedure is exercised between the MS and the HLR to authenticate the MS. The details are omitted. Step 1.2. The VLR sends message MAP_Update_Location
to the HLR, and obtains the subscription profile of the MS (the profile indicates, e.g., if the MS is allowed to make international calls) from the HLR through the MAP_Insert_Subs_Datamessages exchanged. The VLR then sets up the standard GSM ciphering with the MS. Then it sends theMAP_Update_Location_Area_ack mes-sage to VMSC to indicate that the registration is success-ful.
Step 1.3. The VMSC performs GPRS attach to the SGSN by exchanging the GPRSAttach RequestandAcceptmessage pair. Following the standard GPRS PDP context activation procedure, the VMSC activates a new PDP context just like
a GPRS MS does. In the activation procedure, the IMSI of the MS is used by the GGSN to retrieve the HLR record to obtain information such as IP address (we assume that the IP address is allocated dynamically). The PDP context record for the MS is created in the GGSN, which includes fields such as IMSI, IP address, QoS profile negotiated, SGSN address, and so on. Since the activated PDP con-text is dedicated for transmitting the H.323 signaling, the QoS profile can be set to low priority and network resource would not be wasted. At this point, an IP session is estab-lished so that the VMSC can communicate with the gate-keeper GK in the external H.323 network.
Step 1.4. By using the H.323 signaling PDP context, the VMSC initiates the end-point registration to inform the GK of its transport address and alias address (i.e., MSISDN) through the RASRegistration Request(RRQ) message. Step 1.5. The GK creates an entry for the MS in the address
translation table, which stores the (IP address, MSISDN) pair. Then it confirms the registration by sending the RAS Registration Confirm (RCF) message to the VMSC. The VMSC then creates the MS MM and PDP contexts for the MS and stores these contexts in its MS table.
Step 1.6. VMSC informs the MS that the location update re-quest is accepted by the HLR. At this point, the registration procedure is completed.
dures [4,5] are performed to set up the radio link between the MS and the BTS. Then the digits dialed by the MS are sent to the BTS in aUm_Setupmessage. The BTS for-wards the message to the VMSC through the GSM Abis and A interfaces.
Step 2.2. The VMSC sends the MAP_SEND_INFO_FOR_ OUTGOING_CALLmessage to the VLR, which asks the VLR to check if the service requested by the calling party
destination through the GGSN. The Q.931Call Proceeding message is sent back to the VMSC to indicate that enough routing information has been received and it does not ex-pect to receive more routing information from the VMSC. The message path is (4)↔ (3) ↔ (8) in figure 3.
Step 2.5. The H.323 terminal exchanges the RAS ARQand ACFmessage pair with the gatekeeper. It is possible that an RAS Admission Reject (ARJ) message is received by
the terminal and the call is released. The message path is (1) in figure 3. If the call admission is accepted, the following steps are executed.
Step 2.6. A ringing tone is generated at the H.323 terminal to alert the called party. The Q.931Alertingmessage is sent to the VMSC.
Step 2.7. The VMSC sendsA_Alertingto the BSC. The BSC sends anAbis_Alertingmessage to the BTS. The message is then forwarded to the MS through messageUm_Alerting. This message triggers the ringback tone at the MS. Step 2.8. When the called party answers the phone, the
H.323 terminal generates the Q.931Connectmessage to the VMSC. The VMSC sends theA_Connectmessage to the BSC. The BSC sends theAbis_Connectmessage to the BTS. The BTS forwards the message to the MS through the air interface Um.
Step 2.9. The VMSC performs the standard GPRS PDP Con-text Activation procedure to create another PDP conCon-text for real-time VoIP packet transmission.
At this moment, the call is established and the conversation begins. When the conversation is over, the call release proce-dure is described as follows (see steps 3.1–3.3 in figure 5). We assume that the calling party (the GSM user) hangs up the phone first.
Step 3.1. The MS sends a Um_Disconnect message to the BTS. The BTS then forwards this message to the VMSC through the BSC.
Step 3.2. The VMSC sends Q.931 Release Complete mes-sage to the H.323 terminal to release the call.
Step 3.3. Both the VMSC and the H.323 terminal inform the GK of call completion by exchanging RASDisengage Re-quest (DRQ) andConfirm(DCF) message pair. The GK records the call statistics for charging.
Step 3.4. At the end of the call, the VMSC performs the stan-dard GPRS PDP context deactivation procedure to deacti-vate the voice PDP context established in step 2.9.
5. The MS call termination
This section describes the message flow for the MS call termi-nation. We assume that the call is originated from an H.323 terminal, and the MS is the called party. The message flow (figure 6) is described in the following steps:
Step 4.1. The calling party sends the RASARQmessage to the gatekeeper with the called party’s address (i.e., the MSISDN of the MS). From the address translation table, the gatekeeper finds the IP address of the MS and returns it to the calling party through the RASACFmessage.
BTS then pages the MS.
Step 4.5. Upon receipt of the paging response from the MS, the network (VMSC and VLR) performs the standard GSM traffic channel assignment, authentication, and ci-phering procedures. The VMSC sends A_Setup to the BSC. The BSC sends anAbis_Setupmessage to the BTS. The BTS forwards this setup instruction to the MS. Step 4.6. The MS rings and sends a Um_Alerting message
to the BTS. The BTS forwards this signal to the VMSC through the BSC. The VMSC sends the Q.931 Alerting message to the H.323 terminal. The H.323 terminal gener-ates the ringback tone.
Step 4.7. The called party answers the call and the MS sends aUm_Connectmessage to BTS. The BTS forwards this message to VMSC, which results in a Q.931Connect mes-sage delivered to the calling party.
Step 4.8. Finally, the VMSC activates another GPRS PDP context for transmitting VoIP packets as in step 2.9. At this moment, the call is established and the conversation begins.
6. Comparing vGPRS and 3G TR21.978
This section compares vGPRS with the 3G TR21.978 ap-proach. The following issues are discussed.
Real-time communication
vGPRS provides real-time communication that is required for VoIP. This is achieved by using circuit-switched mechanism in the GSM air interface. On the other hand, the 3G TR21.978 approach is affected by the non-real-time packet switching nature in the radio interface. Thus, VoIP with required quality can not be satisfied.
PDP context activation
In vGPRS, after an MS is attached to the GPRS network, a signaling PDP context is activated (see step 1.3, section 3). In GPRS, the PDP context must be activated before a rout-ing path can be established between the GGSN and the MS. In vGPRS, when a call (either incoming or outgoing) to the MS arrives, the call path can be quickly established because
Figure 7. Tromboning phenomenon in GSM.
Figure 8. Tromboning elimination in vGPRS.
the PDP context is already activated (see step 2.2, section 4, and step 4.2, section 5). 3G TR21.978 takes a different ap-proach. In this approach, after the MS has registered to the H.323 gatekeeper, due to the network resource consideration, the PDP context is deactivated (see step 6, figure 7 in [1]). The PDP context must activated again when there are call activi-ties to the MS (see step 1 of figure 8, step 6 of figure 9 in [1]). Thus, the SGSN and the GGSN do not need to maintain the PDP contexts of MSs when they are idle. In this scenario, however, the PDP context must be established and deactivated for every phone call. 3G TR21.978 does not provide details about how the PDP context can be activated for phone calls. We briefly discuss the 3G TR21.978 call path setup as fol-lows. For MS call origination, the PDP context is activated as described in step 1.3, section 3. For MS call termination, a network initiated PDP context activation is required. As pointed out in [7], to perform this task, static PDP address is required (which may not be practical for a large-scaled net-work). Furthermore, IMSI is used by the GGSN (through GPRS PDP context activation) and the gatekeeper (through MAP_SENDING_ROUTING_INFORMATIONand other MAP messages) to obtain MS information from the HLR. It implies that the H.323 gatekeeper should memorize IMSI. Since IMSI is considered confidential to the GPRS network operator, this approach may not work if the GPRS network and the H.323 network are owned by different service providers. When an H.323 terminal makes a call to the MS, the gatekeeper must find the IP address of the GGSN and the IMSI based on the dialed MSISDN. The gatekeeper then requests the GGSN to perform PDP context activation. After the PDP context ac-tivation, the routing path between the GGSN and the MS is
(a)
(b)
Figure 9. The routing path (a) before and (b) after intersystem handoff. established, and the network can set up the call to the MS. Clearly, the call setup time is longer in this approach. vGPRS registration and call procedures can be easily modified to de-activate the PDP contexts when the MSs are idle. However, this approach may significantly increase the call setup time and is not considered in the current vGPRS implementation.
Modifications to the existing networks
To receive VoIP service in the 3G TR 21.978 approach, the MS must be an H.323 terminal. Also, the gatekeeper must equipped with GSM MAP because it needs to communicate with the HLR. In vGPRS, on the other hand, standard GSM and GPRS MSs can enjoy VoIP service. Furthermore, the
GK is a standard H.323 gatekeeper, which only communicate
with the GGSN using the standard H.323 protocol. However, vGPRS introduces a new component VMSC that will replace the existing MSC. Since VMSC is a router-based softswitch, this component is anticipated to be much cheaper than the existing MSC.
Tromboning elimination
In GSM, when a GSM user x in one country (say, the UK) roams to another country (say, Hong Kong) and someone y in Hong Kong makes a phone call to x, it will result in two international calls as described in [2]. As illustrated in fig-ure 7, when y in Hong Kong dials x’s MSISDN, the call is first routed to x’s gateway MSC (GMSC), which is located in the UK (see (1), figure 7). After the GMSC has queried the HLR and the VLR (not shown in this figure), the location of x is identified, and a trunk is set up back to Hong Kong (see (2), figure 7). Thus, the call setup results in two inter-national calls. If the local telephone company in Hong Kong connects to VoIP network (many local telephone companies
are evolving into this configuration now), vGPRS can elimi-nate this tromboning situation; in other words, the call from
y to x will be a local phone call, as explained below. When
x roams from the UK to Hong Kong, it registers at the GK of Hong Kong. As shown in figure 8, when y at Hong Kong makes a phone call, the local telephone company first routes the call to the H.323 gateway through VoIP service (see (1), figure 8). The gateway checks with the GK to see if the entry for x can be found in the address translation table (see (2), fig-ure 8). If so, the call setup follows the procedfig-ure described in section 5 (see (3), figure 8), which results in a local phone call, and the tromboning situation is avoided. On the other hand, if x is not found in the GK, the GK will instruct y to con-nect to the international telephone network as a normal PSTN call. Note that 3G TR21.978 cannot eliminate tromboning. To make a local call setup as illustrated in figure 8, the Hong Kong’s gatekeeper needs to communicate with UK’s HLR to set up the call (see step 2, figure 9 in [1]). This implies that the Hong Kong’s gatekeeper needs to keep x’s IMSI that is confi-dential to UK’s HLR, which is not allowed in a real business model.
7. Conclusions
Recently wireless voice over IP (VoIP) has been intensively studied. 3GPP has proposed VoIP service for GPRS in 3G TR 21.978 specification, which nicely integrates GPRS with H.323-based VoIP network. In this approach, the mo-bile phones or momo-bile stations (MSs) must be equipped with vocoder and H.323 terminal capabilities. In this paper, we proposed vGPRS, a VoIP service for GPRS, which allows standard GSM and GPRS MSs to receive VoIP service.
In this approach, a new network element called VoIP mo-bile switching center (VMSC) is introduced to replace stan-dard GSM MSC. The design of VMSC can co-exist with the standard GSM MSCs in our vGPRS system. The issue for co-existence is that the VMSC needs to communicate with the standard GSM MSC for inter-system handoff. This commu-nication is achieved through standard SS7 MAP E protocol. Figure 9 shows the scenario before (see figure 9(a)) and af-ter (see figure 9(b)) inaf-ter-system handoff. In this figure, the VMSC is an anchor MSC, which is always in the call path after inter-system handoff. The circuit-switched trunk con-nection between the VMSC and the GSM MSC is established through the standard GSM inter-system handoff procedure. Note that inter-system handoff between two VMSCs follows the same procedure.
The vGPRS approach is implemented using standard H.323, GPRS, and GSM protocols. Thus, existing GPRS and H.323 network elements are not modified. We described the message flows for vGPRS registration, call origination, call release and call termination procedures. We also showed that for international roaming, vGPRS can effectively elimi-nate the tromboning situation (two international trunks in call setup) for incoming call to a GSM roamer.
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Ming-Feng Chang received the Ph.D. degree in
computer science from the University of Illinois at Urbana-Champaign in 1991. He is currently an Associate Professor in the Department of Com-puter Science and Information Engineering, National Chiao-Tung University, Taiwan Republic of China. His research interests include Internet communica-tion, mobile computing and VLSI system design. On-going research projects include “VoIP for Wire-less Networks,” “Interworking of VoIP Protocols” and “Cache Model for WAP Applications.”
Wireless Series, an editor of IEEE Personal Communications Magazine, an editor of Computer Networks, an area editor of ACM Mobile Computing and Communication Review, a columnist of ACM Simulation Digest, an editor of International Journal of Communications Systems, an editor of ACM/Baltzer Wireless Networks, an Editor of Computer Simulation Modeling and Analy-sis, an editor of Journal of Information Science and Engineering, Program Chair for the 8th Workshop on Distributed and Parallel Simulation, General Chair for the 9th Workshop on Distributed and Parallel Simulation, Program Chair for the 2nd International Mobile Computing Conference, Guest Editor for the ACM/Baltzer MONET Special Issue on Personal Communications, a Guest Editor for IEEE Transactions on Computers Special Issue on Mobile Computing, and a Guest Editor for IEEE Communications Magazine Special Issue on Active, Programmable, and Mobile Code Networking. Lin is the au-thor of the book Wireless and Mobile Network Architecture (co-auau-thor with Imrich Chlamtac; published by John Wiley & Sons). Lin received 1998 and 2000 Outstanding Research Awards from National Science Council, ROC, and 1998 Outstanding Youth Electrical Engineer Award from CIEE, ROC. Lin is an Adjunct Research Fellow of Academia Sinica.
E-mail: liny@csie.nctu.edu.tw
Ai-Chun Pang received her B.S.C.S.I.E. and M.S.
C.S.I.E. degrees from National Chiao Tung Univer-sity in 1996 and 1998, respectively. She is currently a Ph.D. candidate of the Department of Computer Sci-ence and Information Engineering, National Chiao Tung University. Her current research interests in-clude design and analysis of personal communica-tions services network, mobile computing and voice over IP.
