HYBRID WIRELESS NETWORK: CHALLENGES AND SOLUTIONS

Hybrid wireless networks provide combined medium access methods selected from cellular, MANET, IP and 802. 1X. Effective resource management, route scheduling and relay strategies are key aspects in facilitating this heterogeneous environment. The hybrid relays have received significant research interest as a consequence of system capacity advantage and reasonable infrastructure cost. The novel HWN* infrastructure proposed here is also expected to provide stable, high-speed and user satisfied communication services in most situations including urban city with both cellular and MANET coverage, indoor environment and remote areas without cellular coverage thus is a challenging design goal. The section starts with a state of the art review for hybrid wireless networks with different design objectives. It then provides the rationale which motivates us to develop HWN* with system concept and architecture. The HWN* infrastructure is further supported by the novel algorithms and protocols proposed at Medium Access layer (MAC), NETWORK transport layer, and cooperation between cellular network, MANET and relay structure. Therefore, algorithms and protocols related to hybrid wireless networks are also briefly discussed.

Hybrid Wireless Networks

Hybrid wireless networks are defined as an integrated infrastructure that provides seamless services over several networks. However, most of research focuses on the infrastructure research itself and few algorithms have been proposed to explore hybrid wireless network practical usage at medium access layer and routing layer. Current algorithm suites are normally proposed for system capacity, relay station placement plan, cooperative resource sharing and path discovery to improve only the cellular network performance. These solutions leverage the presence of persistent resources to support relay networks, but rely on the fact that ad hoc multi-hop services being underestimated and multiple interface technology is not comprehensively considered. The service-oriented algorithm for hybrid wireless networks was still left for further investigation as most previous proposals only concentrate on the infrastructure design, which assume such services would be supported or extended based on existing cellular differentiated service methodologies.

For the design of effective resource and routing management algorithms in a complex system, centralised control approaches should be avoided as network scalability issues largely jeopardises the hybrid wireless network performance. The introduction of system flexibility by avoiding central control motivates this research to propose algorithms that consider cross layer communication issues and differentiated services in a decentralised manner that influence the global system performance through local conditional changes. The challenges for communication service provision have increased in many respects in recent years. Competition among providers demands a continuous reduction of production costs and improvement of service quality, which motives us to propose the cost-effective novel HWN* infrastructure and develop associated algorithms for the infrastructure realise with guaranteed QoS. The algorithm development does not require a brand new ubiquitous radio access technology or focusing on multi-hop based cellular networking technologies, but rather, the work integrally utilises existing cellular, ad hoc and relay technologies in an integrated fashion, and combines their advantages and overcomes their inefficiencies.

The research addresses several issues from handover management and routing perspectives, evaluates proposed algorithms in the HWN*, and finalises the HWN* management framework towards 4G mobile system. It is also concerned with the provision of fixed relay nodes to subscribers in both urban and remote areas and also implementing coordinated cellular and MANET radio access. It devises a framework for studying the trade-offs of interworking between the two active systems, identifies and answers the specific design space questions such as: How to share the resource between different service classes? How many relay nodes are sufficient in a reasonable cost? Where should we place RNs? What protocols are necessary to facilitate the load balancing between the two systems? How to handle network routing in a heterogeneous environment? Why the infrastructure uses relays and MANET to reduce the number of wired BSs? The study answers these questions in order to demonstrate that the proposed integrated infrastructure can be used for providing the enhanced data communication services.

Recent research attempts for MANET-cellular infrastructure integration include Multi-hop Cellular Network (MCN), Multi-Power Architecture for Cellular Networks (MuPAC), integrated Cellular and Ad hoc Relaying system (iCAR), Self-organising Packet Radio Networks with Overlay (SOPRANO) and the IST-WINNER project. The basic rational of the MCN is a cellular network evolution that concentrates on cellular radio access technology. Traditionally, a MT and a BS have a direct link in 2G or 3G cellular networks, but in the MCN a MT may reach the serving base station by using multi-hop relaying. The relay is called soft MT based relay which refers to other MTs act as relay clients using ad hoc frequencies. The proposal also states that the relay node could be an infrastructure node if the condition allows. However, it only generally concludes that the relay has capabilities like those of the BS such like an ad hoc node access point. It does not exactly solve problems such as how to choose packet delivery route or when relay mode should be used other than original cellular communications. The analysis and simulation results present the throughput comparison between conventional Single hop Cellular Network (SCN) and MCN. The throughput of MCN is better than that of SCN and increases as the transmission range decreases. The node uses a transmission range rthat is a fraction 1/k of the cell radius R where r = R/k. The parameter k is referred to as the reuse factor. The research explains these two observations by illustrating the different increasing orders, of mean number of channels such as simultaneous transmissions in a cell, and mean hop count, as the transmission range between source node and destination node decreases by k times. But unfortunately the actual gain will be lower. First of all, large control overheads are produced since every node may perform routing updates even when there is no topology change. If a MANET routing protocol e.g. ADOV is used for multi-hop routing, there is a high possibility of relay client MTs absence. The main disadvantage of the relay is the latency in route discovery and link failure. On the other hand, as the ad hoc frequency is assumed for packet relaying, a small topology change results in medium access failure on the fragile link.

MuPAC is an extended MCN with focus on node flexible power management. The system architecture is exactly the same as MCN and each MT uses a separate 802.11X interface. The multiple transmission powers helps MuPAC achieve maximised spatial reuse without substantially increasing the number of hops. The work certainly improves the system capacity performance since the possibility of a path break in MuPAC is lower than MCN once the algorithm is used. Throughput Enhanced Wireless in Local loop (TWiLL) is also an extended work based on MCN. It has been proposed by the same researchers and focuses on the power management of wireless multi-hop local loop to increase the hybrid system capacity. However, here it is highly possible that nodes increase the transmission power to reduce the path break probability and hop distance. It should also implement a distributed power management algorithm at each MT which is computationally expensive. Otherwise the power management has to be coordinated by BS central control because no dedicated relay node is used.

iCAR is not difficult to be evolved from the cellular network and it can be also categorised as one type of MCNs. Other than increase the system capacity through node power management or a longer hop distance. The infrastructure works on adaptive traffic load balancing in this multi-hop cellular infrastructure. It is the first hybrid wireless network that looks at the horizontal handovers between cellular accesses through MANET access. Extra cellular bandwidth available in surrounding cells can be borrowed to the congested cell through dedicated ad hoc relay modes named primary, secondary and cascaded relaying. The channel borrowing evaluation results indicate the improvement on packet congestion delay over conventional cellular networks and MCN, and it verifies that with a limited number of relay nodes, the call blocking and dropping probability in a congested cell as well as the overall system can be reduced. However, the simulation evaluation of iCAR system suffers unfair packet contention problem as all packets are treated exactly the same. An extra bandwidth can be allowed to any packets without considering packet priority, packet transmission requirement and QoS. For example, an urgent communication request or instant request can be blocked or terminated due to the contention and unreasonable channel borrowing. And this is also why differentiated traffic input should be introduced for hybrid wireless network evaluations. The iCAR has introduced a novel concept called managed mobility for relay nodes based on its signalling protocol and node mobility model. The relay node movement provides assistance on channel borrowing in congested communication areas but at the expense of complex and two layered route management (MT layer and Relay Layer). Furthermore, each relay station in iCAR must be equipped with location tracking system with extra cost. Another drawback of iCAR is that the system does not explore or underestimate the dedicated relay node assisted MANET communication mode capability. It only intends to reuse the cellular resource via multi-hop ad hoc relaying.

SOPRANO is a scalable architecture that assumes the use of multi-hop cellular and asynchronous CDMA with spreading codes to support high data rate Internet and multimedia traffic. The general idea of SOPRANO is not much different from iCAR other than IP network support and cross active network connections. It focuses on connection establishment and node power control based self-organisation, and investigates the formulation for an optimum transmission strategy. Again, a power management algorithm is proposed to adaptively selection transmission power to improve the system capacity, which is similar to MuPAC. The system presents high capacity bounds that illustrate how the technique helps in trading off conserved power for a multi-fold capacity advantage. However, both SOPRANO and iCAR rely heavily on soft MT based multi-hop traffic relay underestimate the usage of an alternative MANET with dedicated and location fixed RN infrastructure since fixed relay nodes are not considered.

European Commission Information Society Technologies (IST) WINNER project proposed the WINNER system. What WINNER focuses on is the Research & Development of a brand new beyond 3G radio interface technologies needed for a ubiquitous radio system. The RNs are deployed to incorporate with BSs realising an efficient and flexible spectrum usage and spectrum sharing environment, where the RNs only implement their new ubiquitous radio interface. The RNs are planned and share the same Radio Access Technology (RAT or refereed as interface) with BSs and MTs. Ad hoc communication is prohibited in this architecture and one can see the WINNER as an evolving node-oriented multi-hop cellular network with relay support. One disadvantage of WINNER can be the implementation cost. Significant hardware and software updates are required at base station radio network controller, relay node and end-user equipment to apply the novel radio interface. The telecom providers may not ready to replace anew radio access deployment without significant system performance improvement compared to current 3G cellular system.

The proposed HWN* has major differences from other hybrid wireless networks motioned previously. It has been summarised in a nutshell in the important issues comparison between HWN*, WINNER and SOPRANO architectures. Detailed comparison between iCar, MuPAC and MCN. Although advanced technologies such as location tracking make soft relay based infrastructure feasible, the route recalculating and reconfiguring in systems such as MCN, TWiLL and MuPAC, without dedicated relay nodes, are unstable. Meanwhile, there are other fundamental problems such as relay node absence and third party terminal relay security. Using dedicated and location fixed relay node provides straightforward method to enable reliable communication. The research will later compare system performance between HWN* with dedicated RNs, MCN with soft relay, SOPRANO and WINNER. SOPRANO is considered other than iCAR since CDMA based SOPRANO gives better results in terms of system capacity and network delay.

With dedicated and location fixed RNs support, the adaptive and scalable HWN* has four basic communication modes, which is cellular communication (also named BSON), RN supported cellular communication (BSON RN), MANET communication and RN supported MANET. With the assumption of coded and modulated digital communications, Node Xcan transmit information to Node Y via one or more RNs. The dedicated RN can be part of a cellular network and a MANET. The nodes X and Y can be a BS, a MT or a dedicated RN. And, the term communications include uplink communications (link from MT to BS), downlink communications (link from BS to MT), MT to MT communications or BS to RN communications. Two MTs may communicate directly or through an intermediate node (The node can be a RN or a group of RNs). The MT can be also accommodated into cellular network with dedicated RNs assistance. Therefore, the relay structure is viewed as a means of extending the communication coverage of either a cellular network or MANET. MANET and cellular network are mutually supported through the use of RN structure. The performance of individual MANET or cellular network, respectively, is also enhanced by RN support. Figure 1 presents the topology of the HWN* used for handover mobility management and cross-layer routing. The RNs create a mesh structure to support node communication through RN infrastructure. The procedure is similar to 802.1 IX node-to-infrastructure communication but a virtual backbone is constructed between RNs. The BSs may connect to an IP network via fixed lines or switching nodes. The HWN* assumes that there exists wireless connections between RN and BS, and between RN and RN.

 
Figure 1: Hybrid wireless network with dedicated and fixed relay nodes (HWN*)(© 2008, Chong Shen. Used with permission.)

The HWN* is a generic system and works with any off-the-shelf air interfaces, as an example, for the cellular part of HWN*, Time Division Multiple Access (TDMA) based system is used. This means that every individual transmitting channel required as part of the chain between any two terminals is created by allocation of time slots such as multiple time slots in Enhanced Data rate for GSM Evolution (EDGE). Variability of data rate is achieved by allocating differing number of time slots. The TDMA cellular interface allows the HWN* to take advantage of multi hop connections formed through RNs with more flexible implementation compared to Frequency Division Multiple Access (FDMA) (3GPP). The HWN* network deployment scenarios against physical layer link duplex model, medium access method, spectrum usage, node movement speed, transmission rate and overall scenario capacity. Traditional MANET scenario can not support high node mobility speed and data transmission rate without the presence of infrastructure node such as 802.11g access point or dedicated RN, thus high mobility and data rate can be realised in RN supported MANET. System capacity of cellular network or RN supported cellular are optimised as the introduction of RN optimises the resource sharing performance. We implement standard CSMA/CA for pure MANET and synchronised CSMA/CA for RN supported MANET mode considering IEEE 802.11e QoS standard for delay-sensitive traffic and the RN is given priority in terms of medium access. The RN priority access will be detailed in cross-layer routing algorithm proposal. Large scale deployment of dual-interfaced RNs is cost-effective as the equipment can be based on an integration of a modified 802.11 access point and cellular packet relaying function node. Each RN associates with one BS so that the radio resource usage in each RN can be coordinated. A decrease in BS density can be compensated by an increase in RN density, in order to maintain constant performance. The RN has properties and functionaries of:

• Two radio interfaces: The cellular interface and the MANET interface.
• The RN extends the cellular service range and optimises cell capacity.
• The RN minimises node transmitted power.
• The RN covers remote areas, supports inter network load balancing.
• The meshed RNs provide an alternative communication mode which is MANET with RN infrastructure for MANET based resource management and routing.

Theoretically, both the HWN* system capacity and the average packet delivery ratio per MT, compared to traditional 2G and 3G cellular networks, should be improved because the RNs provide relay capability as the substitution of a poor quality single-hop wireless link with a better-quality link encouraged whenever possible. The disadvantage of the RN integration is that whether in reality the infrastructure can be realised or not due to feasibility issues. Telecom providers should first agree, design and prototype such equipment. After identify air interfaces for MANET access and cellular access, both software and hardware are required to be upgraded on actual relay nodes.

HWN* Framework towards 4G Mobile Communication Networks

The evolving cellular communications industry was originally focused on providing voice services. However, ever increasing demand for multimedia mobile services required new architectures to extend indoor and outdoor coverage, increase system capacity, improve service quality and reduce transmission delay. For example, a large amount of bandwidth is needed to support mobile broadband applications. Current solutions require the installation of expensive BSs to increase the system capacity with reduction of cellular cell sizes, which result in large costs in equipment, wiring and complex system management. The alternative approach is to install wirelessly linked multi-functional Relay Nodes (RN), which not only creates smaller cell sizes but also provides packet relay functions, introduces networking flexibility, saves end terminal power and largely mitigates the pass loss. In this work, we propose to realise high quality mobile communication services in a novel system infrastructure that includes a cellular network, a Mobile Ad hoc Network (MANET) and an integrated dual radio access interface on each RN and MT

The proposal brings out many research challenges as the infrastructure lends itself to complex change in topology, medium access, resources sharing and routing path selection. The ad hoc interface has limited transmission range thus multi-hop communication and RN infrastructure support are required for data exchange. The cellular interface offers robust communication but the bottleneck is at the resource sharing between base stations and relay stations, which require the development of a traffic sharing approach between entities. In order to exploit the advantages of the infrastructure, algorithms and protocols have been proposed to tackle resource management problems e.g. inter-network traffic management and heterogeneous network route selection in a distributed manner.

Hybrid wireless networks with multiple active interfaces towards 4G wireless networks are still being developed and as yet no real world prototype has been deployed except the 3GPP Long Term Evolution (LTE) test operating by Motorola, Nortel and NTTDoCoMo, the IEEE 802.16j multimedia traffic relaying project and Alcatel-Lucent 3G Femto high speed mobile home access where a large number of Femto relay cells are required on per house basis. The HWN* represents another possible realisation of the hybrid wireless network concept. The HWN* and associated algorithms are proposed and evaluated by means of computer simulation (Rea, 2006) to analyse node mobility, scheduling algorithm, handovers, routing, resource sharing and topology design schemes. The complexity of the HWN* system is such that performance evaluation does not lend itself to pure mathematical treatment but more accurate evaluation is only possible by means of simulation, which also provides more practical options for parameter changes.

Briefly review the state of the art of the cellular network, MANET, relay concept, recent hybrid network system approaches, and hybrid wireless networks related algorithms to provide an understanding of the effectiveness of our HWN* infrastructure. The major achievement of our research is then discussed. Apart from the cost-effective HWN* architecture proposal with minimal change on existing cellular and MANET structures, other core contributions can be summarised as:

• Network Design: In order to maximise the spectrum usage and facilitate load balancing, RN positioning planning has been developed and investigated. Our approach focuses on heuristic relay placement to explore the node mobility pattern's impact, cellular system, MANET and relay characteristics.
• Solving complexity: For large scale hybrid networks, more components are added to the system. Therefore simple route management algorithms may produce larger end-to-end transmission delays, which results in a slower application response time. The design of cascaded routing algorithm has contributed a novel approach to deal with complex systems over several layers. The scheme includes several route change triggers and considers cross layer design in overlayed networks.
• Quality of Service: The HWN* is expected to provide services with different QoS requirements, which results in complex user management. The research provides a user classification strategy with differentiated service class profiles for the evaluation of traffic admission, mobility management, handover management and cascaded routing. For the inter-network handovers, congestion control between service classes has been introduced, which discourages applications from using heavily loaded cellular resources and encourages the MANET usage.

The algorithms and strategies developed as part of this research aim to benefit resource management and routing for the HWN*. These methodologies, meanwhile, have potential to be applied on cooperative vehicle networks including vehicle-to-vehicle communication and vehicle-to-infrastructure communication, and wireless sensor network packet relay infrastructure and related medium access & route optimisation.

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.

Interconnect Session Border Controller (ISBC)

This SBC addresses the boundary requirements where service provider networks interconnect and exchange inbound and outbound SIP sessions. It integrates the following three IMS functional elements related to the 3GPP Release 8 ("Exploring IMS security mechanisms"; Acme Packet):

• Interconnect Border Control Function (IBCF): The IBCF offers boundary control between various service provider networks, which provides IMS network security in terms of signaling information. This is done by implementing a Topology-Hiding Inter-network Gateway (THIG) sub-function, which performs signaling-based topology hiding, session screening and IPv4-IPv6 translations based on source and destination signaling addresses. When connecting non-SIP or non-IPv6 networks, the IBCF also invokes the Inter-Working Function and performs bandwidth allocation and admission control using local policies or through the interface to PCRF elements. The IBCF may also interact with TrGW for control of the boundary at the transport layer including NAPT, pinhole firewall, and other features.
• Inter-Working Function (IWF): The IWF provides signaling protocol mechanism between the SIP-based IMS network and other service provider networks, which uses other SIP profiles or H.323.
• Transition Gateway (TrGW): The transport boundary at layer 3 and layer 4 is controlled by the TrGW among various service provider networks having similar media functions as in the AGW.

Multiservice Security Gateway

In the SAE architecture, the Evolved Packet Data Gateway (ePDG) is the functional element, which delivers voice and data services over the un-trusted Wi-Fi networks and Internet to femtocells and dual-mode handsets. The ePDG is responsible to authenticate subscribers and use IPsec to tunnel voice and data securely to devices over the Wi-Fi networks and Internet. For conforming to the 3GPP standards, the ePDG system also supports the I-WLAN Tunnel Terminating Gateway (TTG) functional elements based on the 3GPP Release 7, and UMA/GAN Security Gateway (SeGW) based on the Release 6 and the Femtocell Security Gateway (SeGW) based on the Release 8 specifications.

Session Routing Proxy (SRP)

The SRP deals with interactive communication sessions between SIP network border points, including: SBCs, IMS subscriber call control elements, mobile switching centers (MSC), and controlling media gateways.

Access Session Border Controller (ASBC)

Session border controllers (SBCs) offer required functionalities at the border to the applications and subscribers who access the IMS services. The SBC is responsible for connecting mobile devices to the SIP-based IMS services and related applications, including IPTV, interactive gaming, voice, video, and messaging. In SAE, the access SBCs are responsible to connect all access networks to the IMS network, including the 3G RAN, the LTE RAN, the trusted non-3GPP IP access networks (such as DSL, FTT_x and WiMAX), and un-trusted non-3GPP IP access network (such as the Internet and Wi-Fi networks).

The SBCs are located at the border point of IMS and SAE networks and integrate two functional elements from the IMS Release 8 architecture. A few architectural entities are introduced below:

• Topology Hiding Internetwork Gateway (THIG): THIG can be used with I-CSCF, which hides the capacity, configuration, and topology of the network from the outside. The P-CSCF forwards the SIP messages received at the I-CSCF and/or the S-CSCF from the User Equipment, depending on the procedure and the type of message. The I-CSCF offers a contact point within an operator's network, which allows registration to the subscribers of that network operator and roaming subscribers. The S-CSCF will maintain session state for all IMS services, once registered. Other elements include: The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), which is also called a Master User Database (MUD) that supports the IMS network entities, actually handling calls. The HSS contains the subscription-related information, which performs user-based authentication and authorization and provides user's physical location information.
• BGCF (Border Gateway Control Function): The BGCF is used to select the network where PSTN connection is going to be made. It is responsible to forwards to another BGCF or to a MGCF controlling the access in regards to the PSTN.
• MGCF (Media Gateway Control Function): The MGCF controls the media gateway call management, which is responsible of sending or receiving calls from or to PSTN or other circuit switched networks. MGCF uses SIP messages to or from the BGCF/CSCF and uses Media Gateway Control (MGWC) messages to or from the Media Gateway.
• MGW (Media Gateway): The MGW is responsible for call media processing to or from PSTN/Circuit Switched Network (CSN).
• MRF (Media Resource Function): The MRF provides media related functions such as media manipulation.
• MRFC (Media Resource Function Controller): The MRFC is responsible for signaling plane node, which acts as a SIP User Agent to the S-CSCF. The MRFC also controls the MRFP.
• MRFP (Media Resource Function Processor): The MRFP is a media plane node, which is responsible for the implementation of media-related functions.
• MSG (Multiservice Security Gateways): The MSG delivers service provider's voice and data services securely over un-trusted Wi-Fi access networks and Internet links to dual mode handsets and femtocells.
• SRP (Session Routing Proxy): The SRP selects the destination for incoming and outgoing SIP sessions and provides core session routing. It also deals with the traffic to or from media gateways and interconnects session border controllers.
• AGWF (Access Gateway Function): The transport boundary at layers 3 and 4 between the service provider's network and the subscribers is being controlled by the AGWF. It acts as a NAT device and a pin-hole firewall, which protects the service provider's IMS network. The AGWF also controls the access through IP address/port packet filtering and opening/closing gates into the network. It uses NAPT to hide the IP addresses/ports relate to the service elements in the IMS network. Other features of the AGWF include: bandwidth and signaling rate policing, QoS packet marking, usage metering and media flows QoS measurements.

Types of CSCF | IMS-IPV6 MULTIMEDIA SUBSYSTEM

The first point of contact for the IMS terminal is a Proxy-CSCF (P-CSCF), which is a SIP proxy assigned to an IMS terminal during the registration, which does not change during the registration duration. It also involves authenticating the user and establishing IPsec security integration with the IMS terminal. The other nodes trust the P-CSCF, and do not have to re-authenticate the user.

The Policy Decision Function (PDF) is also involved, which is used for policy control, bandwidth management, and authorizes media plane resources, such as QoS over the media plane.

Proxy-Call Session Control Function (P-CSCF)

P-CSCF serves as a Back-to-Back User Agent (SIP B2BUA) and is the initial SIP signaling contact point for subscribers. The P-CSCF is responsible to forward SIP registration messages from the the User Element (UE) and subscriber's endpoint, to the Interrogating-CSCF (I-CSCF) and as a consequence; call set-up requests and responses to the Serving-CSCF (S-CSCF). The P-CSCF maintains the mapping between physical UE IP address and logical subscriber SIP URI address and a security association, for both confidentiality and authentication. It supports the E911 emergency call and local routing within the visited network, session timers, accounting, and admission control. The DIAMETER protocol (Rx interface) is used by the session admission control to query the external Policy and Charging (PC) Rule Function (PCRF) element for resource reservation and bandwidth-based admission control. The P-CSCF interacts with AGW (defined in subsection 3.4.5) for the boundary control at the media layers and signaling, including Port Translations (NAPT) lawful intercept and pinhole firewall, Network Address and etc.

Serving-CSCF (S-CSCF)

A S-CSCF serves as a central node in the signaling plane. It also serves as a SIP server, which performs session control. S-CSCF uses Diameter D_x and C_x interfaces in connection to the HSS to upload and download user profiles, without having a local storage for the user. All necessary data is loaded from the HSS.

The SIP registrations are handled at this signaling plane. This permits to bind the SIP address, the user location, and to decide the destination application servers for which the SIP message will be forwarded to. This is done in order to provide the routing services using Electronic Numbering lookups and enforcing the network operator's policy.

To offer load distribution and high availability, multiple S-CSCFs can coexist in the same network, however the S-CSCF is assigned to the user through the HSS when I-CSCF is queried.

Interrogating-CSCF (I-CSCF)

An I-CSCF is another SIP proxy that provides service locator functionality. The followings are within its major functions:

1. Registration: Registration is assigning a S-CSCF to a user via SIP registration.
2. Session Flows (SFs): A routing message may include a session flow, which is part of a SIP request received at the S-CSCF from another network. It can also be a part of routing intra-domain SIP requests between users on different S-CSCFs.

IMS-IPV6 MULTIMEDIA SUBSYSTEM

IMS, also known as IP Multimedia Subsystem is an IP-based multimedia and telephony core network technology, which was introduced by 3GPP and 3GPP2 standards and is based on IETF Internet protocols. IMS is based on a set of specifications describing the Next Generation Networking (NGN) architecture involving the implementation of IP based telephony and multimedia services. It contains the specification of a framework and a complete architecture enabling the integration of video, voice, data and mobile network technology on top of an IP-based infrastructure (Amirth, n.d.).

IMS is access technology independent since it supports IP-to-IP session over wired IP, wireless (i.e., 802.11,802.15, CDMA, etc), and packet data along with GSM/EDGE/UMTS and other packet data applications.

 IMS Architecture

In IMS, the networking infrastructure is subdivided into individual functions with standardized interfaces (reference point) between each of them. Every reference point defines both the operating functions and the protocol over the. Figures 1 and 2 show the IMS architecture overview.

The IMS architecture is split into three main layers containing a number of individual entities.

 

Figure 1: IMS architecture

Figure 2 presents a graphical overview of the IMS core entities. The combination of legacy mobile signaling networks, other IP multimedia networks, and the PSTN entities form the external interface functions. The combination of BGCF, CFCF, MGCF, and MRCF form the Sequencing and Control Functions. The combination of HSS and SLF form the Storage and Reference Functions. UE covers the User Interface Functions and the combination of MGW and MRFP form the Media Processing Functions. The interfaces between BGCF and MGCF and CDCF are of Ml type. The interface between MGCF and CSCF is Mg and the interface between WGW and MGCF is of M1 type.

 

Figure 2: IMS core network 

The traffic types are of two major categories; Data/Bearer and signaling/control types. The PSTN link between PSTN and MGW entities contain CSD (TDM) traffic and the Data/bearer, such as Mb interface, contains IPv6 traffic. The rest of the interfaces contain signaling and control traffic, including: MEGACO (H.248), ISUP, SIP, QoS (COPS), MAP/TCAP, and other types of traffic.

Transport and Endpoint Layer

This layer is involved in setting up sessions and providing bearer services and initiating and terminating SIP signaling procedures. This layer is also responsible for providing the media gateways to convert the VoIP data to the PSTN TDM format.

Session Control Layer

This layer includes Call Session Control Function (CSCF), which provides the routing for the SIP signaling messages and the endpoints for the registration. The routing functionality enables the SIP signaling to be routed to the correct application servers. Through communicating with the transport and endpoint layer, the CSCF is able to guarantee QoS.

Mobile-IPv6 Considerations | LTE AND IPV6

Mobile IPv6 (MIPv6) allow mobile nodes to roam between Internet domains while maintaining ongoing sessions and reachability. This is done by using an IPv6 home address or prefix. Since IPv6 is not widely deployed, however it is unlikely that mobile nodes will use IPv6 addresses just for their connections alone. It is assumed that mobile nodes be requiring an IPv4 home address for a long time, which can be used by upper layers. It is fair to assume that mobile nodes will be moving to networks that might not be IPv6-ready and would therefore require an IPv4 Care-of need capability support.

IPv6 compared to IPv4, offers a number of functional improvements, mostly due to its large address space and additional functional fields. The same goes for Mobile IPv6 compared to Mobile IPv4, which offers a number of improvements inheriting from IPv6 additional capabilities, including: Dynamic Home Agent Discovery (DHAD) and route optimization, which can only be achieved in Mobile IPv6 systems.

An advantage of having a large address space (as in IPv6) is to allows mobile nodes to obtain a globally unique care-of address disregarding their current location Hence, there is no need to use Network Address Translator (NAT) traversal techniques, which are required for Mobile IPv4. This further simplifies Mobile IPv6 system architectures and increases the efficiency of mobility management protocol and bandwidth allocations. However for existing private IPv4 networks, NAT traversal needs to be considered during the transition towards IPv6.

In order to minimize the need to changing the mobility stack because of the IPv6introduction within a deployed network and to allow for a long lasting mobility solution Mobile IPv6 should be used with dual stack mobile nodes capability.

Security Considerations | LTE AND IPV6

Privacy and security are among the most important and major concerns in LTE/SAE/IPv6 adoption strategy. The security mechanism involving IP networks slows down the further network technology adoption. Due to the fear of privacy breaches and putting the businesses at risk and potentially cause significant financial loss, operators and enterprises may be reluctant to adopt new network requirements though they are clearly aware of the cost saving benefits and productivity improvements of using convergent communication technologies on a single infrastructure, which enables universal connectivity for users. Therefore it is fundamentally essential to adopt an end-to-end system approach to security for the next-generation wireless networks. A number of mandatory security measures/requirements include:

• Secure protocols, infrastructure, communication, and data storage
• User authorization, authentication, and auditing
• End-to-end compliance
• Secure signaling, network management and control
• Unsolicited traffic protection
• Software integrity

Voice Core Network (VCN)

In a 3GPP network, voice telephony services are traditionally provided by through a Mobile-Services Switching Centre (MSSC) and supported infrastructure. This can also be provided through an IP Multimedia System (IMS).

IMS relies heavily on IP protocol. According to 3GPPP standards, IMS should be based on IPv6, which avoids future IPv4-to-IPV6 migration requirements.

Though new IMS-compliant equipments will typically be deployed using IPv6, existing voice core equipments will still be utilizing IPv4.

The voice core IPv4 to IPv6 migration is a complex task, which requires the cooperation of various protocols and layers. Therefore alleviating the IP address consumption is one of the primary motivations for the IPv4 to IPv6 migration.

Due to the facts that the voice core migration from IPv4 to IPv6 is a complicated task and the end-user devices have almost consumed all IP addresses, it may not be feasible to migrate the entire 3GPP voice core of operators from IPv4 to IPv6. However instead, it would be feasible to perform such migration within the voice core at specific interfaces.

An operator, for instance, may wish to leave intra-call server MGW and all Iu-CS(IP) interfaces to the MGW traffic as IPv4 transfers all inter-call server MGW to MGW traffic using IPv6. This type of architecture requires dual stack capable MGWs only to be on the network bound interfaces and would doesn't require IPv6 capable RNCs.

For the LTE requirements, the carrier may dictate that the device to be assigned an IPv6 address whenever it attaches to the LTE network.

Through the LTE migration, Ipv6's adoption will become increasingly important due to the fact that they will require more IP addresses for all the network connected devices. Therefore the number of unused IPv4 addresses will be reducing more and more, which requires a new standard to address new waves of data-capable devices.