PoC: An IMS-based Wireless Value-added Service

In the 3GPP Release 6 (R6), the Push-to-talk over Cellular (PoC) service has been listed as one of the most important applications of the IP Multimedia Subsystem (IMS). In fact, PoC is not a brand-new concept. It stems from the trunking communication technology for Push-to-talk (PTT). Motorola released its PTT solution based on its iDEN trunking system as early as 1995. One year later, the operator Nextel bought the iDEN trunking system, and launched a PTT service called Direct Connect. This fact has proved the wisdom of Nextel. The success of Direct Connect helped Nextel obtain the highest ARPU and the lowest ratio of subscribers´ switching to other networks in the telecom industry. Moreover, Direct Connect has hardly any influence on the traffic of traditional voice services.

    Therefore, the PoC service is expected to be a new killer application, following the short message service.

    The PoC service discussed here is based on 2.5G and 3G networks. It is different from the traditional PTT service implemented in trunking communication networks. However, they still have many functional similarities. The PoC technology integrates the functions of both mobile phones and walkie-talkies. It is a half-duplex communication technology. With PoC-supported user equipment, a user can make one-to-one (or one-to-multiple) instant conversations by just pressing one key. The call connection is almost instant, and it is unnecessary to specially receive the call. Therefore, the communication is easier. However, if a popular public mobile phone is used to make a call, you have to dial an 11-digit number first, and wait for several seconds or longer before conversing. Once the receiving party is on the line, you have to keep waiting patiently. When the PoC function is added to the existing mobile communication system, the mentioned embarrassing situation may be well changed. In addition, based on IP networks, the PoC service costs much less than ordinary mobile phone voice services. Its potential for bringing income and new profit-making opportunities are attracting more and more attention from operators and vendors.

1 Implementation Solution of PoC Service
Several vendors offered their private solutions for PoC service before the industrial and international PoC standards were released. These solutions are called Packet Switched (PS) based solutions. With them, the PoC service can be implemented in the GSM/GPRS and CDMA2000 1x networks. However, due to the bad interoperability between different solutions, the service interoperability is hardly implemented except that different operators adopt the PoC equipment from the same vendor. Therefore, the standardization of PoC technologies has become an emergent task.
Ericsson, Motorola, Nokia and Siemens jointly released the PoC Release 1.0 (V1.0) specification in August 2003[1,2]. Based on the IMS as defined by 3GPP and 3GPP2, the specification emphasizes openness and standardization of the PoC system. It is a bundle of six specifications including user requirements, architecture, signaling flows, group/list management, do-not-disturb, and two user-plane ones (transport protocols and GPRS/UMTS specification). The four companies are cooperating with such international organizations as Open Mobile Alliance (OMA) and 3GPP, and trying to make the specification an international standard. The PoC V1.0 specification can be called “the IMS-based PoC scheme”[3,4].

    Figure1 illustrates the architecture of the IMS-based PoC system. The mobile User Equipment (UE) with PoC functions accesses the PS/IMS of the mobile communication network via the wireless access network, and connects to the PoC server by Session Initiation Protocol (SIP) and Real-time Transport Protocol (RTP). As shown in Figure 1, the IMS core has multiple SIP agents and registration servers. It implements SIP signaling routing, as well as SIP signaling decompression, authorization and maintenance registration of the UE. It also supports SIP conversation and a  billing system interface. The presence server provides the SIP core server with access to information about reachable UEs, which is similar to the presence function of instant messaging software. In addition, the Group/List Management Server (GLMS), PoC UE and PoC server make up a core functional entity for PoC implementation (see the boldfaced frames in Figure 1).

    The PoC UE is integrated into the cellular phone. It is required to support and operate client software, to set up conversation and to control and release call.

    The PoC server implements network functions of the PoC application layer. These include: PoC conversation control; distribution of RTP voice packets, especially for one-to-multiple group calls or for the situation that the object PoC UE fails to reach (it is also possible to cache the audio flows); conversation authority control, especially management and coordination of conversation authorities in a call group; permission strategy for incoming PoC calls;
call-in control according to the black and white lists and reachable status; control strategy for joining a call group; offering information about conversation participants; obtaining and offering information about QoS of media; and providing billing reports.

    GLMS supports users´ management of group information, as well as of the contact, black and white lists. PoC users are also allowed to create, modify, obtain and delete group and list information through GLMS. Moreover, GLMS works as a database to store group and list information of all the users.

2 Characteristics of PoC Service

2.1 Basic PoC Service
According to the PoC V1.0 specification, the PoC service may provide four communication models, out of which one-to-one personal communication is the most important model.

    When User A sets up a one-to-one talk, he can choose User B to talk with from the contact list, and then press the PoC key of his UE to start speaking. The talk connection is almost instantly completed. When User A finishes the
one-to-one conversation, he releases the PoC key to let User B speak. The conversation proceeds in this cycle.
Moreover, there are three one-to-multiple communication models: Chat Group Talk, Instant Group Talk and Ad-hoc Group Talk. Chat Group Talk is divided into the open talk and the restricted talk. Without any restrictions, the open chat group talk allows anyone to join. However, only stated members can join the restricted chat group. Instant Group Talk is the internal communication of a group. The group members can join or quit the talk at any time. Ad-hoc Group Talk is communication between members of a temporary group. The group would be cancelled after the talk. And only with invitation from the group members could people out of the group join the talk.

    There are two call-answering models: automatic answering and manual answering. The former means that the UE directly plays the incoming talk when the talk is set up, while the latter requires the user to play the incoming talk manually when he is first called. Both models support direct play of the following conversation.

2.2 Typical Service Flow
Figure 2 illustrates the typical PoC service flow. It takes one-to-one communication with early media as an example, and describes the SIP and voice floor control singnaling, during the communication. It assumes all UEs adopt automatic answering. All SIP 100 Trying messages are omitted to simplify the flow.

    In the signaling flow of the one-to-one PoC talk, User A first selects User B to talk with from the contact list, and then presses the PoC key of the UE. This action triggers Terminal A to send a SIP Invite request to the PoC Server A of its home network (see Step 1 in Figure 2). Server A then initiates a new SIP dialog to User B (see Step 2 in Figure 2). The request in Step 2 is caught by the PoC Server of User B, which used to check whether User A is in the talk refusal list or in automatic answering list of User B. Here we assume that User A is in the automatic answering list. The PoC Server therefore sends a SIP Invite (see Step 3 in Figure 2) to User B, and simultaneously responds to the message (see Step 2 in Figure 2) with a SIP 183 Session Progress (see Step 4 in Figure 2). When Server A receives the response that indicates User B´s will to make call connection, it sends a SIP 202 Accepted response (see Step 5 in Figure 2), and assigns the floor to User A (see Step 6 in Figure 2).

    When the terminal receives the messages at Step 5 and Step 6, it will send the announcement tone to User A to notify that he may start the conversation. It may be observed that User A starts his conversation before the end-to-end talk channel is completely set up. This is called Early Media. In this way, the PoC server must make a cache of the voice flow until the session set-up is completed (see Step 10, Step 15 and Step 16 in Figure 2). Terminal A´s receiving SIP Notify (see Step 11 in Figure 2) indicates the successful end-to-end session set up. Simultaneously, Terminal B receives the SIP Invite (see Step 3 in Figure 2) and is notified that User A is going to talk. Since the automatic answering model is adopted, Terminal B, responding to the Invite at Step 3, will send back the OK message at Step 9 without asking for confirmation.

    The floor is released once User A ceases his speaking and releases the PoC key (see Step 19-22 in Figure 2). The Floor Idle messages at Step 20-22 indicate that the floor is free. In Figure 2, User B subsequently presses the PoC key of his UE and begins to speak. Later on, the floor goes back to idle after User B releases his PoC key (see Step 31-35 in Figure 2). It is unnecessary to close the talk manually when the conversation ends, because the PoC server will automatically send SIP BYE to close the talk once the floor idle interval is beyond a pre-configured value.

3 Strengths and Weaknesses of  PoC
It is undeniable that the PoC technology still has some unsolved problems. First, the PoC service takes more time in setting up a call than the PTT service based on trunking communication systems (such as the iDEN system developed by Motorola).Notably, the time of setting up the first call for the PoC service is seven to ten seconds, while that for the PTT service under the same conditions is less than one second. The reason is that the terminal of the iDEN system communicates with the base station once per second, while the UE of cellular mobile communication systems communicates with the base station once per six to ten seconds. Moreover, voice delay constitutes another weakness. PoC adopts the packet switching technology, and needs to do a series of tasks including voice encoding, message packaging, data transporting and regrouping and converting during the course of voice message transport. Therefore, its data transport efficiency is much lower than that by circuit switching. Furthermore, its performance will be much worse when operators make network interworking. Undoubtedly, this will more or less reduce the enjoyment users can get from the service and encumber the popularity of the service.

    However, PoC, as an attractive new service, has its special strengths. First, PoC has wide coverage. Since the PoC service implements conversation through mobile communication networks, users in all the network-covered areas can enjoy it. The traditional PTT service cannot match it in coverage at all. Second, PoC adopts packet switching technology. Therefore, the conversation does not occupy independent channels. This greatly improves the usage of radio channels. Third, PoC adopts VoIP transmission. The PoC service belongs to wireless value-added data services, so operators can flexibly design the service charging models, such as charging with flat rates and charging according to talk duration. Last, the most important strength is its low price. Experts have estimated that the PoC service in commercial use would charge 30% less than the existing voice services. This will play a key role in people´s acceptance of the service.

4 Market Positioning of PoC Service
With regard to market positioning of the PoC service, this author thinks the PoC service should be differentiated from the PTT service based on trunking communication systems. The traditional PTT service can set up a talk in an extremely short time (generally less than one second). It supports rapid communication between groups once emergent events take place. Therefore, it is applicable in some special industries such as military, police, power supply, transportation, emergency and disaster relief. These PTT service users pay attention to the real-time capability, voice quality, security and secrecy of communication, rather than to some assistant functions such as online status and location information. Moreover, they would like to protect the invested capital in PTT. Therefore, these users are not expected to transfer to use the PoC service, abandoning the traditional PTT service.

    Accordingly, the PoC service application should aim at ordinary enterprises, associations and individual user groups composed of white-collar employees and teenagers. These users are insensitive to real-time voice, but are concerned with the service price. Besides, they are interested in assistant functions such as online status. Therefore, the PoC service is expected to be popular with these users and have great market potential.

5 Conclusions
PoC is expected not only to offer voice transferring but also to become a value-added service platform in the future. Telecom-equipment giant Nokia declared that all its GPRS/CDMA mobile phones had supported PoC since 2005.

    In addition, Nokia is doing the research in the PoC based multimedia platform. It is estimated that its PoC based multimedia platform could implement application in one or two years. The transport of PoC text and voice has been implemented. In the future, the PoC service platform will support abundant services, including such multimedia services as Push to Watch and Push to Media. PoC is poised to become a tool that integrates multiple new services including multimedia messaging, instant messaging, presence, picture receiving and sending. Just like the short message service, the PoC service will attract terminal vendors, equipment vendors, service providers and operators to make a win-for-all PoC industrial value chain [5,6]. A brand-new PoC epoch will be created.

References
[1] Erisson, Motorola, Siemens, Nokia. Push-to-talk over Cellular (PoC) Release 1.0 [EB/OL]. http://www.ericsson.com/multiservicnetworks/distr/poc.2003.
[2] OMA POC WG. Push to Talk over Cellular(PoC) Architecture, Draft Version 1.0 [S].2004.
[3] 3GPP TS 23.228. IP Multimedia Subsystem (IMS): Stage 2 [S].2002.
[4] Kim P, Balazs A, van den Broek E, et al. IMS-based Push-to-talk over GPRS/UMTS [A]. Proceedings of 2005 IEEE Wireless Communications and Networking Conference,Vol 4 [C]. New Orleans(LA),2005.Piscataway(NJ,USA):IEEE Press,2005:2472—2477.
[5] 徐童, 廖建新. PTT/PoC技术综述 [J]. 电信工程技术与标准化, 2004,(5):56—60.
[6] 何斌, 黄本雄, 胡广, 等. 基于蜂窝移动通信的PTT研究与实现 [J]. 计算机应用, 2004,24(9):82—84.

Manuscript received：2005-06-20