UK Resilience
Last updated: 27 October 2008
Terrestrial telecommunications networks are sometimes referred to as ‘fixed line’ or ‘land line’ infrastructures because much of the physical infrastructure consists of copper (regular wire and co-axial cable) and optical fibres. These networks are used to deliver regular services such as voice telephony and ISDN. The physical network used to deliver these services can be considered to consist of core networks and access networks. Core networks, which are sometimes referred to as ‘backbones’, are high bandwidth transmission systems connecting together geographically dispersed locations (also variously referred to as ‘nodes’ or ‘points of presence’) which, for voice services, are commonly called ‘telephone exchanges’. Access networks, often referred to as the ‘local loop’ or ‘last mile’, provide connection between the exchange on the provider’s network and the customer’s premises.
Background information on the role that these structures plain in the resilience of UK telecommunications networks can be found in An introduction to the structure of UK telecommunications sector [PDF, 257KB, 33 pages] and Ensuring Resilient Telecommunications: A Survey of some Technical Solutions [PDF, 37 pages, 480KB].
Core networks generally offer a high level of resilience, however, the ‘last mile’ connection to the customer is invariably the weakest connectivity link. Strategies to enhance the resilience of the ‘last mile’ connection include gaining assurance from your provider that the geographic cable routes and points of presence are physically separate and that arrangements provide diversity of connection in the event of service degradation of failure.
Unless assurance is gained that telecommunications services are delivered to your premises over more than one distinct geographic route you should assume that the connection takes a single path from the point of presence on the providers network to your premises. Outside the customer’s premises, connections are vulnerable to hazards such as road works disturbing cables in ducts or, in rural areas, extreme weather washing out buried cables or bringing down overhead lines. The consequences of these hazards can be mitigated by having more than one physical connection to your service provider’s exchange. This arrangement of diverse routing can be further enhanced by the provision of duplicate connectivity from different points of presence on your provider’s network to different parts of your premises – a scheme that is sometimes referred to as ‘dual parenting’.
Obtaining telecommunications services from more than a single supplier may not enhance resilience. Largely as a consequence of the structure and maturity of the telecommunications industry in the UK, it can be very difficult to gain assurance that two telecommunications links, provided by different suppliers obtained for the purpose of providing resilience, do provide resilience through diverse geographic routing and logical connection. Networks are increasingly becoming virtual with services being delivered over shared infrastructures – for further information see Ensuring Resilient Telecommunications: A Survey of some Technical Solutions [PDF, 37 pages, 480KB].
Does your corporate telephone exchange have stand-by power? While exchanges on the provider’s network generally have robust stand-by electrical supplies – typically being able to work for at least five days from diesel generators – arrangements for a PBX or PABX (Private Automatic Branch eXchange) are the customer’s responsibility.
Wireless handsets connected to fixed lines are dependent on grid-distributed electricity. Predominantly in the domestic environment, very basic telephone equipment wired into the access network is typically powered by the network. In the event of a failure of the local electricity supply the equipment will continue to function as exchanges have robust back-up power arrangements. Handsets that are connected by wireless to a base station (commonly using DECT technology) that is in-turn wired to a ‘wall socket’ are not powered by the network. In both the domestic and corporate environment DECT technology is increasingly being used to provide wireless access for digital portable telephones.
Equipment connected to ADSL Broadband Internet that is delivered over a regular telephone line will require back-up power supply arrangements in order to work if local power fails. While the Broadband and any telephony services that share the line are resilient in the event of a local power failure, additional ‘customer side’ equipment such as a ‘hub’ (containing a modem and router) and computers will require back-up power arrangements to ensure resilience. In the short-term this could be provided by an uninterruptible power supply (UPS) or batteries.
With ISDN services the service provider does not provide supporting power. In order to ensure resilience in the event of local power failure, ‘customer-side’ back-up power arrangements need to be provided for any line termination or interface equipment (usually located where the lines enter the customer’s premises) and for subsequent equipment connected to the service (such as modems).
Review your requirements for resilient telecommunications and interact with your provider. The CPNI [External website] has published the Good Practice Guide for Telecommunications Resilience [External PDF] which provides a helpful starting point from which to address resilience. The Guide contains a risk-based methodology to assist in clarifying business requirements supported by a “Self Assessment Questionnaire”. This process is designed to enable resilience-enhancing dialogues to be established with your telecommunications suppliers. To assist in promoting a meaningful dialogue the Guide offers a list of “Twenty questions to ask your providers”.
Consider arrangements to redirect calls at your provider’s exchange. Equipment can be installed [PDF, 3 pages, 366KB] in your provider’s exchange to redirect in-coming calls to alternate locations in the event that these cannot be forwarded to your premises or, in situations where you are unable to accept them. The type of events that preclude completion of an incoming call include failure of the connection between your provider’s exchange and your premises or failure of the private exchange. Redirecting incoming calls at your provider’s exchange enables business continuity when, for instance, your premises becomes uninhabitable - possibly as a consequence of a major fire or flooding; inaccessible - as a consequence of a Police cordon or, when staff are unavailable to answer the calls – possibly resulting from transport difficulties. The equipment installed in your provider’s exchange is usually programmed in advance to provide call re-direction of DDI (Direct Dial Inward) calls – those starting with 01 and 02 - to alternate locations. These arrangements are then activated with a single call, such pre-planned response arrangements can be customised on ‘the fly’ for unforeseen circumstances.
Ensure that your private exchange has resilient electrical power. Unless your private exchange is provided with a back-up power supply, failure of electricity at your premises will result in failure of telephony equipment connected to the ‘customer-side’. Usually, an uninterruptible power supply (UPS) is installed to provide uninterrupted service in the event of short term loss of power – usually under an hour. For greater resilience, this can be augmented with diesel generation. Facsimile and other equipment may also need to be connected to the back-up supply.
Test back-up power supply arrangements. Untested back-up power supply arrangements invariably fail on demand – batteries and generation equipment either fail to take full load or do not function for the designed time. Confidence in back-up arrangements should be sought through routine stress-testing carried out under realistic load conditions and for a time that tests site specific arrangements. Only under these conditions do problems such as blocked air and fuel filters and contaminated fuel become apparent. For well maintained back-up generation systems practical experience indicates that the likelihood that they fail on demand lies between one in 70 and one in 200. With this evidence it seems worth considering having an additional layer of ‘fast fix’ arrangements in place, such as a call-off rental contract.
Do not rely on GTPS as the cornerstone for providing resilient telecommunications. The current fixed-line privileged access scheme has been used very rarely, largely as a consequence of the draconian consequences of invocation. The core telecommunications networks in the UK are to be renewed. In view of the gradual refresh of the old networks, over which the existing scheme works, we are not encouraging further take-up of GTPS. The new scheme will be called the Fixed Telecommunications Privileged Access Scheme, or FTPAS. FTPAS will be made available as the new networks are rolled-out.
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