TrafficInfraTech Magazine Linking People Places & Progress
Metro Transit
The Delhi Metro has transformed the way people commute in the national capital and has become a symbol of transportation infrastructure development. Many other cities are also in various stages of implementing their own metro systems which have become a strategic part of urban transportation planning. A new dimension to telecom systems is added with metro transit systems. The advantage in green-field metro projects is that the design of the communication system takes place alongside the planning and design of the transit system itself, and can be architected and sized precisely according to the needs from day one. The telecom network in metros is equipped to support triple-play services using current networking technology without facing the challenges of interoperation with legacy networks. In that sense, it is a “neater” network and easier to manage as successive phases are built and commissioned in a growing metro network. At the same time, metro networks see a great degree of integration due to the number of systems that use the communication system. CCTV, ticketing, access-control, and passenger help-phones, advanced passenger information systems, synchronous timing system, and trunked radio systems for staff are just some of the systems that share a common communication backbone with the advanced signalling and train control systems.
Essential Requirements of the Telecom System
The railways is a mission-critical sector, for which the telecom sector must meet these characteristics:
High reliability of the Equipment
India has one of the most vast and complex rail networks in the world – reaching even the remotest corners – and has become synonymous to most popular travel modes among the masses. Such vast transport networks need mission critical applications and systems that are infallible and ubiquitous. Therefore, any technology solutions focused towards this sector must come with high reliability, high life expectancy and stringent environmental specifications. Reliable and rugged networks have shown that they can weather the onslaught of flooding due to a heavy downpour and power loss due to lightning strikes. They can be restored quickly in case of shutdowns.
Redundancy in Network Architecture
After selecting reliable equipment, various protection mechanisms are carefully incorporated while designing a network in order to increase its availability even in the event of failure of its specific parts. This includes both hardware redundancy and link protection. In the case of hardware redundancy, the entire switching matrix, power train and line interface cards are duplicated in main/standby protection mode. So, in case the working component fails, the standby component takes over without any interruption of service. Link protection, on the other hand, provides continuity of service even if the fibre breaks, by providing an alternate path which automatically takes over when there is a fault in the working path. As per the ITU-T standards, the switch-over from the main to the protected path must happen within 50ms, this being the time within which a call-drop does not occur in a telephone call.
With advances in high-speed electronics, this switch-over time in modern equipment is actually in the range of 20ms or less. However, at the network level, the end-to-end switch-over depends on the complexity of the network, the number of nodes that have failed, as well as the presence of Shared Risk Link Groups (SRLGs). Common types of protection mechanisms are Sub-network Connection Protection (SNCP) and Multiplex-Section Protection (MSP). As shown in Figure 2, MSP is implemented by providing two links of identical bandwidth between a pair of nodes, with one serving as the working link and the other as the stand-by. In the event of a failure of the main link, the stand-by link takes over within the specified time to permit continuity of operation.
Furthermore, with many applications moving towards packet-based services, additional protection mechanisms such as Rapid Spanning Tree Protocol (RSTP), Multiple Spanning Tree Protocol (MSTP) and Resilient Packet Ring (RPR) can be implemented at the Layer-2 level. For highly advanced networks, Automatically Switched Optical Network (ASON) based protection can be overlaid on top of layer-1 and layer-2 protection mechanisms.
Intelligent Network Management
An important requirement for railway networks is usability due to the high level of alertness required of the operator to take quick and precise action on alert. The entire network needs to be managed through a user-friendly software interface, with the ability to monitor and act on the five-fold dimensions of fault, configuration, administration, performance and security (FCAPS). This is usually available in most equipment using standardised protocols, so that the management of complex networks with large number network elements from multiple vendors is simplified, and preferably done via a single application. Equipment from different OEMs can be monitored by integrating the individual management systems over Common Object Request Broker Architecture (CORBA) or Simple Network Management Protocol (SNMP) interface.