Application Server Layer (ASL) | IMS-IPV6 MULTIMEDIA SUBSYSTEM

The ASL undertakes the control of the end services required by the user. The functions and services supported at this layer include (Amirth, n.d.):

• Telephony Application Server (TAS): The TAS maintains the call state and is a back-to-back SIP user agent. The TAS supports routing, call setup, call forwarding, call waiting, conferencing and contains the service logic that provides the basic call processing services, which includes digit analysis.
• IP Multimedia - Service Switching Function (IM-SSF): The IM-SSF provides SIP message interworking, which corresponds to the Customized Applications for MobileNetworks Enhanced Logic (CAMEL), ANSI-41, Transaction Capabilities Application Part (TCAP), or Intelligent Network Application Protocol (INAP) messages.
• Supplemental Telephony Application Server (STAS): The STAS is a standalone independent server that provides supplemental telephony services at: the beginning, the end, or in the middle of a call, or by triggering.
• Non-Telephony Application Server(NTAS): These application servers, such as NTAS, interwork with endpoint clients, which provide services such as PTT, IM, or presence-enabled services.
• Open Service Access - Gateway (OSA-GW): The interworking between SIP and the Parlay API is provided in the OSA-GW, which is part of the 3GPP IMS architecture application server layer.

1 Benefits and Implementation of IMS

The benefits of IMS in regards to the existing cellular network infrastructure can be shown in the following forms (Amirth, n.d.):

• Reduced time-to-market new multimedia services: The IMS infrastructure provides reusable components and a standardized platform. The common features provided by IMS infrastructure and the related standardized interface help service provider to market new multimedia services in a relatively short period of time.
• Quality of Service (QoS): To improve and guarantee the transmission quality, IMS specifies Quality of Service within the IP network and therefore takes advantage of the QoS mechanism.
• IMS Location Independence: IMS offers service availability irrespective of the users' location. IMS uses specific protocols and Internet technologies, which allows users to roam across different countries and still be able to have access to all the services. Therefore all services are available to the users disregarding their location.

2 Push to Talk over Cellular (PoC)

The following PoC related functionalities are offered:

• Simultaneous Ringing and Multiple / find-me, follow-me: Which involves a predefined list of destinations (sequentially or in parallel) for routed calls.
• Multimedia Push: This service permits users to push some multimedia content.
• Push Ring Tone: Calling party selects a desirable ring tone on the destination number/ address.
• Real Time Video Sharing: This deals with the real time peer-to-peer and multimedia streaming service.

3 Interactive Gaming

Interactive gaming deals with the followings:

• Folder Sharing: Content and folder sharing enables users to share files/folders among terminals.
• Voice Messaging: This involves sharing of audio files instant messaging.
• Instant Messaging Services: This refers to a general communication service that allows end-users to send and receive messages instantly.
• Video-Conferencing: This takes advantage of IP Multimedia Subsystem (IMS) Videoconferencing service, which extends the point-to-point video call to a multi-point service.

4 IMS Enabled Voice and Video Telephony

Multimedia enriched (i.e., voice and video calls) IMS traffic are carried over a packet core network (VoIP). In mobile networks, video telephony is counted as a critical end-user service. The Session Initiation Protocol (SIP) enables Voice and Video Telephony using Person-to-Person and Multiparty sessions over an IP network.

The main issue in VoIP and Video Telephony calls is the Quality of Service (QoS) in packet core networks, as well as on interoperability with the legacy phones and PSTN and inter-working with existing domains for Video Telephony such as H.324M and H.323.

User-based QoS schemes drive bandwidth requirements, thus, with similar QoS schemes, packet-switched video telephony may experience similar bandwidth allocations, similar to the circuit-switched video telephony scenario. However, packet switching provides more freedom in balancing the bandwidth and video quality requirements. The Quality of Service and bandwidth requirements for the connection are requested from the network by the terminal at connection set-up phase (PDP Context activation). The IMS infrastructure is responsible to provide several features to manage QoS.

5 Quality of Service (QoS) - Key to Quality Real-time Service Realization

IMS provides a standardized and effective solution for operators who require implementing real-time IP mobile services with guaranteed customer satisfaction in mind.

Maintaining real-time Mobile IP communication is difficult due to bandwidth fluctuations, which can severely affect the IP packet transmission through the network. In a QoS-disabled IP networks, IP transport is based on the ‘best effort’ setting, meaning that the network will do its best effort in a uniform sense to ensure the required bandwidths are provided without any guarantee. The result is that real-time mobile IP services may function poorly, such as the voice quality may sound poor or with garbled effects and video quality may include ‘jitter’ effects and so on, which depends on the network congestion and bandwidth availability.

The Quality of Service (QoS) mechanisms are used to lessen the degradation effects on the multimedia transmissions and to provide some type of guaranteed sense to the transmission compared to the case of ‘best effort’. QoS ensures that critical elements of IP transmission such as gateway delay, transmission rate, and error rates can be guaranteed, measured, and improved in advance. Users should be able to specify the required quality level, depending on users' circumstances and the type of services used.

IMS is responsible to provide the ‘intelligence’ required to enable QoS within a mobile IP network, which is also known as the Policy Decision Function (PDF). Through the Go interface of the GGSN, the PDF interacts with and controls the underlying packet network.

6 Presence Server

A presence server is an element in the IMS network that offers presence aggregation that can provide network-based presence consolidation. The main components of the presence server include:

• Interfaces: Including Home Subscriber Server (HSS) with Diameter S_h interface to obtain subscription information and SIP interface with presentities and watchers.
• Entities: Including HSS, which is responsible to store service-specific information, Presentities, which can be either real-time services, user devices or in form of applications, which they send availability information to the server, and Watchers, which are services interacting with the presence server and getting presence information from other services or users
• Functional Modules: Including Presence core, which handles all publish, subscription and notification events, Presence database, which is responsible for storing information given by presentities, Presence aggregator that consolidates presence information from multiple identities of the same user, Transport layer, which provides interfaces of SIP, Diameter S_h, XMPP (Extensible Messaging and Presence Protocol), if required, and Presence subscription and watcher module, which are storing a list of subscriptions.

7 IP Centrex

Centrex provides PBX-type services, which provides switching functionality at the central office instead of the customer's premises. In a typical sense, the telephone company owns and manages all the communications hardware and software systems necessary for implementing the Centrex service.

Voice conversations can be digitized and packetized in IP telephony, for across network transmission. In such a context, an IP Centrex refers to a number of IP telephony solutions offering Centrex service for customers transmitting packetized voice streams across a broadband access facility. IP Centrex can be built on top of traditional networks combining them with the benefits of IP telephony. One of these IP telephony benefits is the access capacity increased utilization. Using IP Centrex, a single broadband access technology is used to carry the packetized voice streams for many simultaneous calls. During inactive call sessions (no active calls available), more bandwidth is available for high speed data sessions, such as Internet access. This is a much more efficient use of capacity compared to the traditional Centrex. In analog Centrex, one pair of copper wires is required to serve each analog telephone station, disregarding if the phone has an active call; the bandwidth capacity of those wires is unused, or one of the phones is not engaged in a call.

Other IMS-related functionalities, which may be less IPv6 dependent include: Download parental control and media streaming, programmable control and incoming call screening, outgoing VoIP call control/barring, convergent instant messaging system, including: mixing SIP, SMS USSD, and text-to-speech), and enhanced policy controller, including: controls SIP message sequences and format.