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DESCRIPTIONService provision techniques in mulit-technology networks
Service Delivery in Shared and Multi-technology Wireless Networks Neil Wiffen ([email protected])
Outline Many communications-services are currently provided in networks where the predominant access method for the service-users is via one of several common wireless technologies. Wireless access to high-bandwidth low latency networks using devices with an ever-increasing range of form-factors and functionality is commonplace in urban, sub-urban and rural environments for a wide variety of commercial and private sector applications. Although access technologies, delivery mechanisms and user-groups vary greatly, there are often common service attributes that can be used to define distinct categories of service. Defining categories of service can be very useful in the determination of suitable service provisioning architectures and procedures. Increasingly attention is being paid to the control, delivery and charging of services over IP-oriented architectures, with 4G wireless networks having an ‘all-IP’ focus. Within the public and private cellular networking arenas there is the growing potential to provide many services in an access-independent fashion, using protocols and procedures which, despite being common and standardised increase the flexibility of delivery techniques and QoS control mechanisms for many service providers. These providers and user groups include:
- Cellular network operators - Third-party service providers - Emergency service networks - Enterprise / Corporate networks - Utility-service networks - M2M service providers
One of the challenges that is faced when deploying services over multi-technology or shared networks is to assure that appropriate QoS, QoE and priority are provided (and guaranteed where relevant), to the contending organisations and applications. This has to be achieved while also attempting to ensure that resources are only allocated in the areas of the network where and when they are required. The intention of this paper is to:
- present several service provision architectures that are commonly deployed in legacy wireless, cellular and private radio networks;
- present key service provision architectures in the emerging 3GPP LTE, EPC and IMS systems; - discuss common QoS characteristics, including appropriate service provision models, for
applications and services that could be delivered over multi-technology (GSM/GPRS/UMTS/LTE/TETRA) or shared (private/commercial) networks;
- discuss methods of providing and controlling the appropriate QoS, QoE and priorities for contending service types and user-groups;
- present possible solution architectures and examine various technical aspects of these In particular this paper has been written to stimulate more focussed discussion on the techniques required to provide workable solutions which will allow emergency services (Police, Fire, Medical etc.) to utilise a mixture of private and commercial cellular networks that support multiple wireless access technologies. This could include temporary use of commercial networks based on a short term incident, perhaps providing broadband services such as video streaming to enhance incident control, or the use may be continuous to support asset-tracking etc.
Service-delivery background The past 3 decades have seen the introduction and evolution of many communications systems that utilise a variety of wireless access technologies. These provide a vast array of services to private, commercial, government, military and many other end users across the globe. There are a variety of ways in which these systems, and wireless techniques can be categorised and the services themselves characterised. As a consequence of several factors, including the diversity of network architectures and application functionality, many different service delivery mechanisms are now used in thousands of networks around the world. For many years, mobile cellular networks have had strict architectural and protocol hierarchies which, although in the main are specified in great detail by various standards bodies (3GPP, ETSI, ITU etc.), have stimulated the creation of what are often very service-specific delivery techniques and architectures. Examples of these are SMS, MMS, voice-mail servers and pre-pay platforms. On the other hand, simple stand-alone wireless technologies such as WiFi (IEEE802.11) often provide a more flexible ‘IP-oriented’ network access capability over which many services can be delivered, however in general the provision of service is much less coordinated unless it is managed and delivered within a private enterprise. A system environment that could be considered to exist somewhere between those of WiFi and public cellular is a typical professional mobile radio network. These networks have standardised network architectures and protocols (e.g. those used in the TETRA system), yet they are more customised to what could be viewed as enterprise-style service provision models. The ‘enterprise’ in this model however could be a private organisation such as a regional Fire Service. A key challenge for systems of this type however, is that they tend to lag behind the capabilities of publicly available commercial technologies, often by several years, in particular in terms of broadband service provision, device functionality and application support. This capability gap is readily noticeable when comparing the number of applications and features that come as standard with today’s lowest cost smartphones with the standard features and functions available on professional mobile radio hand-sets. These factors often lead to the ‘cobbling-together’ of different technologies in non-standard ways to provide quick-fix solutions for what are often high-priority and mission-critical requirements. This often results in inter-operability, scalability and system evolution issues that are at the very least problematic and commonly un-manageable. The current evolution of wireless network architectures, protocols and procedures discussed in this paper provides for a set service provision principles which should enable many services to be delivered in a standardised manner irrespective of access network type. Defining Services It is useful to distinguish between applications and services before isolating service categories and individual services. For the purposes of this paper an application is considered to be the required functionality that resides either on a terminal device and / or network server, which interacts with the end-user or machine directly. Examples of applications are:
- Instant Messaging client / server - Voice Codec - Video Codec - Video server - Telemetry (M2M) client / server - Positioning / Location tracking client / server
A service however, is a little more difficult to encapsulate, as the attributes that characterise a service are not only dependent on the required end-user functionality, but also on a wide range of networking factors including:
- Network architecture - Service distribution (Point-to-Point / Point-to-Multipoint / Broadcast etc.) - QoS requirements (throughput, reliability, priority, latency) - Service ownership (QoS control / charging / authorisation) - Access capability (2G/3G/4G/WiFi etc.)
For the purposes of this paper however, as the wireless systems under discussion are all digital, a service is considered as the delivery of all required bit-streams through a network with the appropriate Quality of Service to support the required application. Although in some instances, the service end-points may be on wired networks, this paper concentrates on those instances in which at least one end-point uses one of the following wireless systems for network access:
- 3GPP defined GERAN / UTRAN / EUTRAN - IEEE defined 802.11(b/g/n) - ETSI defined TETRA
While other wireless access systems are also possible, it is considered that those listed are the most common, and adequately represent the key architectural components, capabilities and working procedures to provide appropriate service provision models that will be suitable for other common systems. In addition, there is also enough architectural difference between the listed systems to discuss the challenges of inter-working and integration when considering multi-technology solutions for service provision. Further consideration is given to satellite access technologies in a separate paper. Understanding Quality of Service There are a variety of methods used to express the quality of a service, many of which are based on the end user quality of experience and therefore can be very subjective. However in order to examine service delivery mechanisms it becomes relevant to understand QoS as a set of attributes that collectively describe the essential ‘bit-stream delivery requirements’ that should be assured by the network, in order for the application to function appropriately and to therefore ensure that the service is delivered. With this in mind, the service-delivery attributes that need to be managed appropriately typically fall into the following categories:
- Throughput (bit-rate) requirements - Latency characteristics - Bit-level reliability - Bi-directionality delivery characteristics - Query / Response profile - Service Distribution (Unicast / Multicast / Broadcast)
Furthermore, a given service may be delivered using multiple service-delivery components or sub-streams, and not all of these carry application content. For example messages that carry application or network control information, event notifications or performance management details are often associated with service delivery, and these must be carried in sub-streams with appropriate QoS. Each service component will require one or more of the listed attributes to be managed appropriately in order that the entirety of the service is delivered.
It should be noted that priority is not listed above, and the reasoning for this is that priority is relative to other services and the use of this does not assure a required service characteristic. Priority of one service or bit-stream over another can be assigned and managed at the various QoS control routers or radio-resource allocation elements, and this should be coordinated across the network. Many providers already support voice, video and data-oriented services, and a variety of standardised procedures exist to deliver these over both circuit-switched and packet-switched network domains. However, the development of highly adaptive resource-allocation techniques in the HSPA, LTE and EPC systems has resulted in a movement away from conventional dedicated channel usage in order to maximise network capacity. Major emphasis is placed on the optimisation of shared radio network resources to achieve suitable application performance by defining acceptable QoS tolerances, and dynamically managing resource allocations to support these. To better understand QoS characteristics associated with service-delivery, an example voice service is briefly described below, and in order to examine the QoS requirements for this service, it is necessary to consider at least the following:
- Allocation of resources should be managed efficiently (e.g. suitable resources should be available, but not committed until required)
- The vagaries and instabilities of a wireless connectivity should be accounted for (e.g. QoS monitoring and adaptive error correction techniques should be employed)
- Pre-defined tolerances must be established for each QoS attribute (e.g. minimum/maximum latencies, guaranteed bit-rates etc.)
- The ability to support a reasonable set of possible events should be provided An example of a reasonable event would be a handover from one wireless access point or base-station to another if the service-user is expected to be mobile. Therefore appropriate resources and service-delivery procedures must be dimensioned and controlled to support these events should they take place and this has an impact on the overall resource availability and the system should be dimensioned accordingly. However, these ‘handover-resources’ should not be dedicated to this purpose or to a specific user, as the event may not actually occur, and this would not make most efficient use of the system. Legacy cellular wireless systems do not manage events of this nature in a resource efficient manner for multiple subscriber groups or services. Consequently these systems are far from ideal for organisations such as the emergency services or professional mobile radio groups, as the cost of provisioning resources solely for these organisations is often too high. Systems such as HSPA and LTE have evolved with an increasing emphasis placed on dynamic allocation / de-allocation and sharing of resources at a more granular level than in previous systems, and this provides a more suitable platform for more cost effective group-service provision. Example Service Characteristics Bi-directional, one-to-one, conversational voice service This service is one that readers will be very familiar with - the ability for an individual to make a voice call to another individual. In order to examine how this service can be provided in the wireless-access systems of interest, consideration should be given to the individual components of the service, and the QoS requirements of each component characterised. The key components for this service and the typical QoS requirements for these are:
- The signalling messages required to establish and release the voice call. (QoS requirements: Low throughput, very high accuracy, relatively low latency, bi-directional symmetric delivery).
- The media stream that transports the voice itself. (QoS requirements: Low throughput, relatively low accuracy, very low latency, bi-directional symmetric delivery).
- The signalling messages required to support reasonable events. (QoS requirements: Low throughput, very high accuracy, very low latency, bi-directional symmetric delivery)
When allocating resources for a service, including routes, processing power, priorities etc. it should be recognised that so long as individual services and service-components can be identified, it is often more efficient to aggregate bit-streams with similar QoS requirements. This can be beneficial as it allows similar services to be managed collectively, which can often reduce the complexity of transport-network signalling requirements. It can also create a simpler model for statistical determination of resource consumption for a wide range of services or user groups. Service Provision Architecture A simplified view of the service provision architecture traditionally deployed in many public cellular networks is shown in Figure 1. It is often the case that service control and connectivity is provided for individual services from distinct service-specific platforms. Radio access resources for all services are allocated and managed within the Radio Access Network (RAN), and this is usually according to some static QoS parameters established in the Core Network (CN).
Service access via a WLAN using IEEE802.11 technologies has a less defined structure and less coordinated deployment model and a simplified architecture is shown in Figure 2. In general it can be considered that the wireless component is simply supporting access to a LAN, WAN or ISP, and services are delivered over the top of the collection of networks that provide end-to-end connectivity. There is often no single organisation which is responsible for all the network elements that support the service, and therefore no single entity provides comprehensive QoS control. Notable exceptions are VoIP services within an enterprise LAN/WLAN where bandwidth and priority can be managed locally, and also delivery of other LAN-based applications such as email, intranet access, file-transfer, IM etc. In addition, there is no CS ‘domain’ to consider here, simply IP access over a wireless connection to some form of IP network.
Figure 1 - Legacy Cellular Service Provision Architecture
Figure 3 shows a generalised service provision architecture for a professional mobile radio system that utilises gateways to access service platforms residing outside of the network provider’s infrastructure. These services may be accessed via PS or CS backbones, or sometimes both. Example command and control services being provided could be location, status and dispatch services, some of which may be accessed via service-specific application servers and protocol gateways.
Although these examples only highlight a small number of generalised architectures, they capture some important service provision domains of responsibility, which help to then define appropriate network relationships, inter-connectivity requirements, service agreements, and QoS control points. These factors become vital when attempting to provide a range of services across shared-ownership, multi-technology networks. Domains of Responsibility It is very common in today’s heterogeneous networks for different carriers to provide and manage the connectivity across a service providers region, and therefore service level agreements need to
Figure 2 - WLAN Service Provision Architecture
Figure 3 - PMR Service Provision Architecture
exist between end-to-end service-providers and connectivity-providers. For example, a national Mobile Network Operator (MNO) providing end-to-end services may use a variety of leased-lines, microwave links, IP backbones etc. from different regional carriers, and so the appropriate commercial relationships need to be established for the expected services between the MNO and each regional carrier. These relationships become increasingly more complex when the MNO is also acting as a carrier to other organisations, such as the Police, Fire and Ambulance services or large corporate customers. In essence, the MNO can often be considered to be ‘sub-leasing’ their service provision and connectivity capabilities to other organisations. One approach to simplifying these service level agreements is to use a cascaded agreement system as discussed later in the IPX service provision model, but whatever method is used to support the management of these commercial relationships, the technical relationships and protocol parameter set between the various domains have to be configured appropriately. The logical and technical responsibilities of the various inter-connecting networks become clearer when considering the end-to-end protocol architecture and the key points of QoS control can more readily be identified. Deployment of standardised QoS control mechanisms to support multiple service requirements becomes easier to achieve with the advent of simplified protocol architectures and network structures that have emerged to support the new generation of mobile broadband systems. This makes the support of a greater variety of user groups and specialised services in a cost-effective manner more realisable in today’s evolving wireless networks. In the shared network environment, and in particular where the end-user services utilise some form of intermediate server (e.g. SIP proxy, presence-server, PTT-server, asset-management-middleware), it is useful to isolate the various domains of responsibility for service provision, to then determine how QoS can be controlled, and from where.
Figure 4 depicts a high-level view of the protocol domains for a service where application level content is interpreted by a service-specific gateway. An example of this could be a dispatch or asset-tracking service using a variety of thin client devices and a feature-rich server. In this case the gateway converts the application level protocols and content into a suitable form for the appropriate devices and the server. Even in the situation where a gateway is not required, what becomes clear is that the application protocols have no control over resource allocation, and the network specific protocols will differ depending on the network type. If IP is used between the client and the gateway (or server), this is the level at which end-to-end QoS could be managed in a consistent manner.
Figure 4 - Protocol Domains
Migration to IP This paper does not address the rationale or benefits of the evolution or migration of many wireless and wired networks to IP-based architectures, rather it acknowledges the fact that this process has been taking place for many years now, and indeed that the emerging technologies such as LTE are specified as being all-IP. Taking this into account however, has a major impact on resource-dimensioning, resource-allocation, QoS control, service-authorisation, charging and many other aspects of service delivery. This paper assumes that this trend will continue for the foreseeable future, and that the services examined will for the most part, and in many cases in their entirety, be delivered over IP networks. IP-based QoS Without QoS, each IP-packet that arrives at a router is treated on a first-in, first-out (FIFO) basis, and as packets are not assigned priority, based on the type of application they are supporting this creates a best-effort service. QoS-based routers, often referred to as service routers, are used in multi-service networks in order to meet the SLAs for different services while attempting to maximise network utilization. QoS maximizes network resource utilization by providing priority access to network bandwidth for high-priority applications and services, and in the absence of high-priority traffic, allocating the available bandwidth to lower-priority applications, thus optimising revenue generation. An IP network that utilises service-routers facilitates network convergence as they can carry many types of traffic including ATM, Frame-Relay, Ethernet, SDH, SONET etc. traffic. Also, service routers typically provide a more feature-rich and scalable control-plane, optimised routing protocols, and high availability. Networking Hierarchy Cellular networks, both public and private, have traditionally been designed and deployed with a strict hierarchy, which in part has been in order to manage regions of mobility and to coordinate radio coverage. This has resulted in the network architectures themselves having very rigid hierarchical structures, with clear logical distinctions between RAN and CN. A simplified view of a 3GPP release 99 logical network architecture for PS services which highlights this hierarchy is shown in Figure 5.
Figure 5 - UMTS Logical Hierarchy for PS Services
The subsequent evolution of both RAN and CN technologies, including R5 Iu-Flex and pooling options, R6+R7 IP optimisation features and R8 introduction of LTE/EPC topologies, has led to a ‘flatter’ IP-based network architecture as illustrated in Figure 6. This architecture provides the ability to distribute system functions (e.g. mobility management, security, access control etc.) more effectively by employing load-balancing techniques, or to combine multiple functions into one network node to reduce latency. Consequently, network operators have more flexibility to both reduce the number of decision-making nodes and to optimise the positioning of QoS control points, creating a more efficient architecture for service provision. This non-hierarchical approach also creates an environment that more readily supports the introduction of new services and the rapid deployment of connectivity to specialised applications for both temporary and/or permanent access requirements. Standardised and OTT services For services that are required by specialist organisations such as the emergency services, consideration should be given to which of these services are themselves specialised and which are simple variants of standardised services. Network operators have greater capability to assure QoS for services that are similar to those that they already offer to the public (voice, video-streaming, instant messaging etc.). There is also more flexibility to provide these in a cost efficient manner than would be the case for very specialised services. One approach can be to provide the specialised services using the existing PMR technologies such as TETRA, and take advantage of the higher bandwidth afforded by commercial 3G/4G networks for the standardised but perhaps more demanding applications such as video streaming. There are some parallels here with the direction that new services are currently being developed and the service provision mechanisms that are being promoted within organisations such as the GSM Association. For several years the most common method used to support services such as instant messaging is for an operator to simply provide an Internet connection with a best-efforts service, and allow the end-user to run a third party application over the top of this connection. This is commonly referred to as OTT service provision. QoS for OTT For many web-based applications, OTT provision using a best-efforts service is perfectly adequate, however when features such as high reliability, low latency and prioritised access are required, this method is not suitable, and understanding the emerging alternative methods for providing these is vital to selecting the appropriate techniques for service control and delivery. Many issues arise when dimensioning resources and QoS control mechanisms to support OTT services, and one of the key problem areas is that there are many versions of these applications, all of which have their own peculiarities and requirements which are typically not visible to the MNO. It is only necessary to look at the variety of voice codecs, instant messaging applications, video-streaming protocols etc. that are currently used by web-based services to realise that it is an almost impossible task for a network operator to support all variants in a consistent manner or to account
Figure 6 - Evolved Packet System Architecture
for the software updates over which they have no control. As the common OTT applications and services have not typically been specified by recognised standards bodies, managing the deployment of services based on these can require different provisioning and support mechanisms per service, leading to scalability and inter-operability issues and also a lack of consistency of QoE for the end users. One method to address this would be to offer the highest QoS parameter-set to the service and ensure prioritisation for users of that service, however this comes at the cost of in-efficient reservation of resources and potentially poor QoS for other users, both of which are not acceptable for a commercial service provider. RCS and IMS Since 2008, the GSM Association (GSMA) has been driving the development of Rich Communications Services, (RCS) a standardised set of diverse applications that can be delivered, managed and controlled in a consistent manner. These services include voice, video, content sharing, messaging and file-transfer over broadband wireless access networks, and are supported by systems such as LTE. RCS and enhanced RCS (RCS-e), when coupled with the 3GPP specified IP Multimedia Subsystem provides a baseline set of features, protocols and architecture requirements that enable many services that have previously been delivered in an OTT manner, to be offered from within the network operators domain in a standards-based form. RCS-e was initiated in May 2011 for the rapid commercialisation of services into the marketplace, with both RCS-e and RCS being supported by the mobile operator community. Figure 7 shows a simplified view of several possible locations for IMS and RCS entities to provide a range of service delivery options, also including the ability to deliver OTT services over an all-IP wireless access network. Returning to the discussion on QoS control and
domains of responsibility, using the architecture options shown, it now becomes a little easier to identify the key points of QoS control and the domains responsible for these for the various service platform locations. To support RCS working, including voice services via an IMS platform, a basic IMS profile has been defined in GSMA Permanent Reference Document (PRD) IR-92 identifying a minimum mandatory set
Figure 7 - IMS/RCS Service Provision
of features which are defined in 3GPP specifications that a wireless device and network are required to implement in order to guarantee an interoperable, high quality IMS-based telephony service over Long Term Evolution (LTE) radio access. The protocol stack that supports this set of features is depicted in Figure 8, and it should be recognised that as long as the IP-based access bearer supports the appropriate QoS, then this model is suitable for wireless access systems other than LTE.
A major benefit of the architecture shown in Figure 9 is that it allows for QoS control within the network operator’s domain to be totally service-driven, irrespective of where the service end-points exist, or indeed what the purpose of the service is. QoS requirements passed to the IMS platform from internal or external application servers allow the various QoS parameters to be assessed, prioritised and subsequently controlled per-service within the operator’s domain. The IMS platform in this situation contains the Policy Control and Charging (PCC) elements to translate both static and event-based service requirements into rules and packet filters that are then used to control the various service-delivery routers in terms of priority, routing, authorisation and resource reservation and allocation. Multi-technology access The majority of commercial 3GPP-based cellular networks have multiple Radio Access Technologies (multi-RAT) deployed across a region as a consequence of access network evolution and the need to support legacy devices. This paper assumes that many of the networks under consideration will maintain their legacy wireless access systems for a considerable time while expanding and enhancing their IP-based RAN and CN subsystems. The 3GPP evolution path includes inter-working with WiFi access networks which are prolific in many regions. Many relationships have developed between cellular and WiFi providers to support such features as handover and WiFi off-load, and therefore the utilisation of suitable WiFi ‘hot-spots’ should be considered for professional and private mobile radio use. Additionally, existing PMR systems and services should be included in this mix of access technology considerations as they already provide robust and reliable connectivity in many areas, in particular for specialised and
Figure 8 - RCS basic protocol suite
Figure 9 - Simplified QoS Control via IMS
emergency service provision. This gives a wireless access landscape which in many areas includes GERAN, UTRAN, EUTRAN, WiFi and TETRA based systems as illustrated in Figure 10. Access Network Selection When several possible access technologies are available to a device, consideration must be given to which technology is the most appropriate to select, and how this may be influenced either by the user, the device or the network. Historically, in wireless networks, this choice has been driven by signal quality and signal strength related measurements, and although it is becoming increasingly common to
take into account service-related requirements, decisions based on these parameters are made in the network, not by the device. However more innovative techniques such as application-aware radio interface layers, are now being developed to enable the radio technology selection to be more responsive to the application that is selected by the user on the device. These mechanisms allow optimised devices to make service and application-oriented technology selections which are, to a degree, independent of network-based parameters. Inter-system working When presented with multiple possible wireless access technologies, selection of an appropriate system must be coordinated with suitable provision of the service via the selected network, and this should take account of mobility and handover management if these are considered to be reasonable events. Mechanisms have already been specified within 3GPP for handover scenarios between many of the previously mentioned systems and although the author is not aware of specific 3GPP/TETRA or WiFi/TETRA seamless handover procedures, fast system-reselection should be possible in these instances which should cause minimal service interruption. To support fast system reselection, the tight coordination and integration of security mechanisms is required. In order to coordinate service delivery across multiple network types, integration of the bearer request, media flow and QoS control entities should be very carefully considered in order to reduce the number of QoS control points required and the optimal positioning of these.
Figure 10 - Multi-access coverage
One aspect of service provision across multi-access networks that becomes a little clearer from Figure 11 is that within one operator’s domain, irrespective of the access technology type, QoS control mechanisms can be consistently applied in the core network at the points where the access networks converge. This has been one of the driving factors behind the evolution to IP-based, flat network architectures, with the access control and wireless resource management functionality being pushed completely to the radio edge of the network. 3GPP Release 7 has an architecture option that requires no RNC, and Release 8 LTE has no equivalent node to an RNC or BSC, and as such, in both of these cases the base station connects directly to the core network. IP Backbone In order to support services consistently across an operators core network, a high speed, low latency IP backbone is desirable for backhaul, interconnect and distribution purposes. When we take into account the previously mentioned evolution to an IP-based RAN, and the migration of network operators towards an Evolved Packet Core, this then leads to the conclusion that QoS, including prioritisation, can in the main be achieved at the IP layer, between the various edge points. This is providing that suitable lower layer protocols such as MPLS, supporting very fast switching and virtual circuit techniques are deployed and dimensioned to appropriately react to the IP layer requirements.
With the advent and subsequent growth of mobile broadband service provision in the past 5-7 years, many MNOs have continually enhanced their IP backbones in both the core and access portions of their networks to support the ever-increasing demands they face. As the 3GPP specifications from Release 5 onwards do not mandate specific layer 1 or 2 protocols,
there has been a significant growth in the variety and flexibility of both wired and wireless transmission systems used to provide IP interconnectivity in all parts of the network. Another trend that has been growing within the mobile cellular industry is that of pooling core network and RAN resources and providing access to these via highly reliable, low delay IP signalling connections. Figure 12 gives a simplified view of this type of architecture
Figure 11 - Interconnection and QoS control points
Figure 12 - IP Backbone Connectivity
IP Exchange / Interconnect IP interconnect architectures and procedures have been standardised and promoted by the GSMA in order to provide consistent mechanisms for supporting QoS for multiple simultaneous services being delivered between different end-points. The general inter-networking architecture is referred to as an IP Exchange (IPX) and these interconnects can be implemented using proxies as shown in Figure 13. This scalable architecture has been designed so that it can provide either bi-lateral or multi-lateral connectivity between service providers. These service providers may be MNOs, ISPs, Web-based middleware, IMS platforms etc., with the IPX supporting either simple IP transport with a fixed QoS, or service-aware IP transport between these networks. In a cascaded service provision model, technical and commercial agreements are created between connected networks which define the QoS rules and charging mechanisms for bi-directional IP flows across the boundary between them. This minimises the number of Service Level Agreements (SLA) that a service provider needs to establish and maintain, even when supporting access to applications and services hosted at many other locations. As an example, in Figure 14, Service Provider ‘A’ can create a single SLA with IPX Provider ‘1’, and this could incorporate all flows across the gateway. It is then the responsibility of IPX Provider ‘1’ to create and maintain a suitable SLA with IPX Provider ‘2’ etc. These SLA’s cover QoS, prioritisation, event-based service provision and charging rules.
Small Cell / Femto-cell evolution Increasing numbers of small cells are being deployed by network operators globally and many new equipment vendors are entering this market with particular interest in providing femto-cell, (residential small cells) solutions. The Small Cell Forum report that currently Femto-cells constitute over 80% of the 4.6 million small cells currently deployed globally compared to 5.6 million conventional macro-cells. It is estimated that by the end of 2012, there will be 6.4 million small cells
Figure 13 - IPX General Architecture
Figure 14 - Cascaded Service Provision
worldwide, 86% of which will be femto-cells and that globally, femto-cells will outnumber all macro-cells during Q1 2013. Although residential femto-cells are primarily intended for a closed subscriber group (i.e. the residents of a household), they can be configured to allow a number of channels to be used for open access, which is often desirable as it helps to reduce interference between macro and femto layers.
This hybrid configuration could then potentially provide an alternative connection capability for the emergency services to access simple voice services in areas where there is poor or no macro coverage, or the macro-cell is congested. New models for coverage and capacity enhancement are emerging, with auto-configuration and self-optimisation being important techniques in the tool-set available to operators deploying small cells, as these
resolve many of the issues associated with uncoordinated deployment. Typically small cells are designed as small foot-print, low power base-stations, which fit into a modified cellular architecture as shown in Figure 15. However many small cell vendors have partnered with applications developers to provide innovative network-in-a-box solutions, where several if not all core network components are integrated into a chip-set that can reside on the cell itself, or on a low-cost server accessed via an IP connection from the cell. Examples of these architectures are shown in Figure 16
These private network solutions can be ideal for small scale or temporary deployment of both specialised and standardised services, and for providing remote connectivity to WAN services and other networks. They also typically present an IP interface to the external networks, and ‘hide’ the complexity of cellular protocols and procedures from the connecting networks and services, therefore much reducing the cost of integration. Deployments of this nature tend to require either the use of privately owned spectrum, special agreements with the license-owner or suitable roaming agreements, however these ‘hurdles’ can often be easily overcome providing there is a viable commercial incentive, and many systems of this nature are currently deployed.
Figure 15 - Small cell integration
Figure 16 - Private Network 'in-a-box' architectures
This is of particular interest when considering the use of small cells for self-contained private network installations, and rapid / temporary deployment for small or non-contiguous areas of coverage. Site Inter-connectivity With the ever-increasing need to provide high-bandwidth, low-latency connections within the RAN, a variety of techniques are evolving to fulfil this requirement within the wireless access environment which include:
- Line of sight - Near line of sight - Point-to-Point - Point-to-Multipoint - Broadband ADSL/Cable - VSAT (Satellite)
In addition, the 3GPP R8 specifications are such that in addition to providing individual terminals with access to base stations, LTE radio access bearers themselves can be utilised as backhaul from site to site. The introduction of LTE Relay nodes in 3GPP R10 also provides a flexible capability to support dynamic deployment and resource allocation for both device access and site inter-connectivity. When coupled with the auto-configuration and self-optimisation techniques which are currently being developed and enhanced for 3GPP systems, this makes LTE an attractive proposition for service provision in a wide-variety of environments, including those encountered by the emergency services. Security Connection security is always a major consideration when deploying services that are accessed via wireless technologies, and as such various mechanisms can be employed to safeguard against security threats. Security features that can be built in to 3GPP and WiFi systems include:
- Authentication of the device / subscription / user / network - Protection of the transferred content via encryption techniques - Integrity protection of control signalling to prevent message ‘manipulation’ by a 3rd party
Most of these techniques are system-specific, and their scope is often limited to one wireless technology domain. To create end-to-end security is more demanding, and will often require multiple security domains to be managed, and the coordination of this between providers is essential. Although this can be challenging, IP-based techniques such as IPSec VPNs are now common-place, and it is often purely a case of configuring the appropriate mode of operation and suitable security association parameters for the type of service being protected. In some instances, an end-to-end VPN tunnel from a client device to an application server location may be appropriate, however this requires greater bandwidth allocation on the radio link, and as the network has no visibility of the service due to encryption, it is much more difficult to ensure that any QoS provision is working appropriately for the application.
Multi-Provider Architecture The simplified architecture in Figure 17 gives an indication of how different wireless systems can be interconnected to provide access to services that are hosted in various locations over an IP-based architecture. OTT/RCS services could include fleet-management, asset tracking, M2M, database access, standardised and specialised voice applications, Instant Messaging, Video Streaming, File Transfer, Email, Multimedia Messaging etc. In Figure 17, the Customer WAN / LAN could be for example, a Fire Service control room where standardised and non-standardised voice and data applications are used to manage incidents and day-to-day activities.
With the migration towards IP-based interconnection both within and between domains, prioritisation and service-based QoS can be achieved through event-based Policy Control and Charging (PCC) rules, with the enforcement of these rules in the IP-layer at key boundary routers and gateways. Dynamic delivery and updating of these rules can readily be achieved using a standardised AAA protocol such as DIAMETER, which is already deployed in fixed and mobile networks for this purpose. Interconnect SLAs using a combination of IPX and leased line techniques can be used to provide suitable QoS for IP flows between domains, again based on service priorities and requirements, with less critical applications being provided via ISP interconnects where available. It is unrealistic to attempt to guarantee service provision across all areas of a region for various reasons, in particular the cost of creating adequate coverage and capacity in a coordinated form. However, by interconnecting public and private networks, standardising where possible the service delivery and control architecture, and utilising multi-technology devices, there is greater potential for at least one suitable access network to be available which supports the required service.
About the Author Neil Wiffen is the Principle Technical Consultant at Red Banana Wireless Ltd, providing bespoke
training courses and consulting services in a variety of Wireless System architectures, protocols
interfaces and procedures. Neil has recently presented at MBW2011 in Berlin, the 4th International
Femto-cell workshop at Kings College London and he regularly delivers webinars on behalf of the
GSMA, with current topics including VoLTE / RCS(e) / LTE interconnect and Roaming / 3GPP RAN
Figure 17 - Simplified Multi-Provider Architecture
Evolution. Neil’s tutorial on LTE and Femto-cell opportunities in the Military and Intelligence Arenas
has been accepted for delivery at MILCOM2012 in November and he regularly delivers 3G/LTE
protocol lectures at Oxford University. The stimulus for this paper was his attendance as a member
of the Wireless Magazine Round Table ‘Next Generation Technology and the Future of TETRA’ at
BAPCO2012 in Manchester, UK.
3GPP Third Generation Partnership Project
AAA Authentication, Authorisation and Accounting
ADSL Asymmetric Digital Subscriber Line
AS (IMS) Application Server
AS (RAN) Access Stratum
ATM Asynchronous Transfer Mode
BSC Base Station Controller
BTS Base Transceiver Station
CS Circuit Switched
ENB Evolved Node B
EPC Evolved Packet Core
EPS Evolved Packet System
ETSI European Technical Standards Institute
EUTRAN Evolved UMTS Terrestrial Radio Access Network
GERAN GSM / EDGE Radio Access Network
GGSN Gateway GPRS Support Node
GPRS General Packet Radio Service
GSMA GSM Association
HSPA High Speed Packet Access
HTTP Hyper-Text Transfer Protocol
IM Instant Messaging
IMS IP Multimedia Subsystem
IP Internet Protocol
IPX IP Exchange
ISP Internet Service Provider
ISP Internet Service Provider
L2 / L1 Layer 2 / Layer 1
LAN Local Area Network
LTE Long Term Evolution
M2M Machine to Machine
MMS Multimedia Messaging Service
MNO Mobile Network Operator
MPLS Multi Protocol Label Switching
NAS Non Access Stratum
OTT Over The Top
PCC Policy Control and Charging
PCC Policy and Charging Control
PDN Packet Data Network
PGW PDN Gateway
PMR Professional Mobile Radio / Private Mobile Radio
PS Packet Switched
PTT Push To Talk
QoE Quality of Experience
RRM Radio Resource Management
SDH Synchronous Digital Hierarchy
SONET Synchronous Optical Networking
QoS Quality of Service
RAN Radio Access Network
RAT Radio Access Technology
RNC Radio Network Controller
RTCP RTP Control Protocol
RTP Real-time Transport Protocol
SDP Session Description Protocol
SGSN Serving GPRS Support Node
SGW Serving Gateway
SIP Session Initiation Protocol
SIP Session Initiation Protocol
SLA Service Level Agreement
SMS Short Message Service
SwMI Switching and Management Infrastructure
TCP Transmission Control Protocol
TETRA Terrestrial Trunked Radio Access
UDP User Datagram Protocol
UMTS Universal Mobile Telecommunications System
UTRAN UMTS Terrestrial Radio Access Network
VPN Virtual Private Network
VSAT Very Small Aperture Terminal
WAN Wide Area Network
WAP Wireless Access Point
WCDMA Wideband Code Division Multiple Access
WLAN Wireless Local Area Network
XCAP XML Configuration Access Protocol
XML Extensible Mark-up Language