Shared and multi-technology wireless networks for public safety

The gap between the capabilities of 3G/4G cellular networks and PMR networks is considerable. But Neil Wiffen, principal technical consultant at Red Banana Wireless, argues that there are ways to bridge the divide

Shared and multi-technology wireless networks for public safety

Wireless access to high bandwidth, low latency networks via an ever-increasing range of devices is now commonplace for a wide variety of applications. Although services vary greatly, there are often common attributes that can be used to define categories of service, which becomes important in the dimensioning of suitable service provisioning architectures. 

Within the public and private cellular networking arenas there is a growing potential to provide many services in an access-independent fashion using emerging protocols and procedures that increase the flexibility of Quality of Service (QoS) control and delivery mechanisms for many service providers and user groups. These include mobile network operators (MNO), M2M providers, emergency and utility service networks.

Traditionally, cellular networks have had strict architectural and protocol hierarchies which have stimulated the creation of service-specific delivery systems (SMS, MMS, prepay platforms etc.). On the other hand, simple standalone wireless technologies, such as Wi-Fi, often provide a more flexible ‘IP-oriented’ network access capability over which many services can be delivered. However, service provision is usually less co-ordinated. 

PMR networks, such as TETRA, have standardised procedures and protocols and could be viewed as having an enterprise-style service provision model, with the ‘enterprise’ being an organisation such as a regional fire service, for example. 

A key issue for PMR systems is that they usually lag behind the capabilities of publicly available commercial technologies, in particular in terms of broadband service provision, device functionality and application support. This capability gap is readily noticeable when comparing the applications and features that come as standard with today’s low-cost smartphones with those available on professional mobile radio handsets. 

The emerging wireless network architectures, protocols and procedures discussed here provide for a set of service provision principles, which should enable many services to be delivered in a standardised manner irrespective of access network type.

Defining services

An application is considered to be the functionality on a terminal device and /or network server, which interacts with the end user or machine directly, whereas a service is considered to be the delivery of all required bit-streams with the appropriate QoS to support an application. 

It is important to recognise QoS as a set of attributes that collectively describe the essential bit-stream delivery requirements to be assured by networks, including priority, throughput, latency and reliability requirements. A service may have multiple sub-streams, each of which must be carried with appropriate QoS, and it is often more efficient to aggregate bit-streams with similar requirements by defining acceptable QoS tolerances, and dynamically managing resource allocations to support these.

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 platform for more cost-effective group service provision.

As it is common for multiple independent organisations to provide the connectivity across an MNO’s region, service level agreements are required between the service providers and the network connectivity providers. These technical and commercial relationships become increasingly more complex when the MNO is ‘sub-leasing’ their service provision capabilities to entities such as the emergency services. 

The logical and technical responsibilities of the inter-connecting networks become clearer when considering the end-to-end protocol architecture. Deployment of standardised QoS control mechanisms to support multiple service requirements becomes easier to achieve with the advent of the simplified protocol architectures and network structures that have emerged in the LTE/EPC systems.

IP-based QoS

There has been a significant growth in the variety and flexibility of the wired and wireless transmission systems used to provide IP interconnectivity in all parts of cellular networks with the evolution of an IP-based RAN, in conjunction with the migration of MNOs to an Evolved Packet Core (EPC). This has stimulated the drive to manage QoS, including prioritisation, at the IP layer. 

Without QoS, each IP-packet that arrives at a router is treated on a first-in, first-out basis, creating a best-effort service. QoS-based service routers are used in multi-service networks to meet SLAs for different services at the IP layer, while improving network utilisation. Controlling QoS at the IP layer maximises network utilisation by providing priority access to network bandwidth for high-priority traffic, with service routers typically providing high availability and a feature-rich and scalable control-plane.

Networking hierarchy

Cellular networks have traditionally been deployed with strict architectural hierarchies. However, the evolution of both RAN and CN technologies has led to a ‘flatter’ IP-based network architecture, which allows more effective distribution and load-balancing of system functions. 

Consequently, network operators have more flexibility to both reduce the number of QoS control points and optimise the positioning of these. This 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.

It is beneficial if QoS attributes for specialised emergency service applications can be matched to those of standard commercial services, as MNOs have greater capability to assure QoS for services that are similar to those they already offer to the public in a more cost-effective manner. 

An additional approach is to provide some specialised services using the existing PMR technologies such as TETRA, and take advantage of the higher bandwidth commercial 3G/4G networks for more demanding applications such as video streaming. 

For many web-based applications, Over The Top (OTT) provision using a best-efforts service is adequate. However, when features such as high reliability, low latency and prioritised access are required, this is not suitable, and understanding the emerging alternative methods for providing these is vital to selecting the appropriate techniques for service delivery.

Rich Communications Services, (RCS), when coupled with an IP Multimedia Subsystem (IMS), provide baseline features and protocols that enable many services to be offered in a standardised form. 

A major benefit of an all-IP architecture is that if Policy Control and Charging (PCC) gateways are used for externally located services, a network operator’s domain can be almost entirely service-driven. QoS requirements passed to the IMS platform from RCS/OTT application servers, located within, or external to the MNOs network, can be assessed, prioritised and subsequently controlled, per-service, within the operator’s domain. 

Multi-technology access

Many relationships have developed between MNOs and Wi-Fi providers to support such features as handover and Wi-Fi offload, and therefore the utilisation of suitable Wi-Fi ‘hotspots’, should be considered for professional and private mobile radio use. 

Additionally, existing PMR systems and services should continue to be included in the mix of access technologies as they provide robust and reliable connectivity in many areas for specialised and emergency service applications. 

Innovative techniques at the radio interface layer in the device are being developed that enable the radio technology selection to be more responsive to the application selected by the user. Handover techniques already exist between many wireless systems and mechanisms for fast system-reselection causing minimal service interruption are continually being improved. 

To support fast system reselection the integration of security mechanisms is required, and in order to co-ordinate service delivery across multiple network types, integration of the bearer request, media flow and QoS control entities should be carefully considered.

These mechanisms allow optimised devices to make service and application-oriented technology selections which are, to a degree, independent of network-based parameters.  

Small cell/femtocell evolution

The Small Cell Forum reports that globally, femtocells will grow to outnumber macro cells during 2013. Although femtocells are primarily intended for household residents, they can often be configured to allow a number of channels to be used for open access, which could then provide an alternative connection capability for emergency service access in areas where there is poor macro coverage. 

Many small cell vendors have partnered with applications developers to provide network-in-a-box solutions where several, if not all core network components, are integrated into a chipset that resides on the cell itself, or on a low-cost server accessed via IP. These private network solutions can be ideal for small scale deployments, and providing remote connectivity to other networks. 

These solutions present an IP interface to the external networks, hiding the complexity of cellular protocols and reducing the cost of integration. This is of particular interest when considering small cells for self-contained private network installations, and rapid/temporary deployment for non-contiguous areas of coverage.

Multi-provider architecture

The simplified architecture in the figure 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 can include fleet management, asset tracking, M2M, voice applications, IM, video streaming etc.

In the figure, the customer WAN/LAN could be a fire service control room where standardised and non-standardised voice and data applications are used to manage both serious incidents and day-to-day activities.

Prioritisation and service-based QoS can be achieved through event-based PCC rules, with enforcement by service routers. Dynamic delivery and updating of rules can readily be achieved using the DIAMETER protocol, and interconnect SLAs using a combination of IPX and leased line techniques can provide suitable QoS for IP flows between domains.

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