While many critical everyday services are dependent on GPS timing, it is very easy to jam the reception of the Universal Time Code (UTC) signal via the GPS. An alternative delivers UTC in a large public optical network � but it's not economically feasible to build dedicated synchronization networks. Therefore, it is critical that such an approach can also be used to transport data, voice, and video traffic.
By Per Lindgren
Net Insight
Reception of the Universal Time Code (UTC) is mission critical for the synchronization of data in telecommunications networks, power distribution networks, digital TV, radio distribution systems, etc. Without synchronization they stop functioning or they function very poorly. Typically, the distribution of the UTC is performed via the Global Positioning System (GPS). However, it costs only about $50 to $100 to build a device able to jam all GPS reception within a radius of 10 miles or more. Such devices could cause major disturbances to TV distribution, mobile phones, and other daily activities. Additionally, several of these functions are also mission critical for homeland security, which means disruptions could prevent authorities and media from communicating with the public. These concerns have made several governments state that alternatives to GPS should be considered for these vital services.
For a long time the European Union has considered launching its own "GPS" satellite system, a project called Galileo. Launching and maintaining dozens of satellites is a daunting and costly undertaking, which is one of the reasons the project has been continuously delayed. However, such a network would still be sensitive to jamming and spoofing.
An attractive alternative would be to distribute the UTC over an optical network that could also be used to distribute the actual user data. By distributing the UTC over a redundant optical network, jamming is impossible. Additionally, the same infrastructure can be used for data, voice, and video communication at the same time. Such an approach would be cost-effective and could be built robustly.
As of yet, time transfer over communication networks (e.g., digital TV and radio systems or mobile networks) has not provided the reliability and accuracy needed for synchronization. However, a new approach offers significant promise.
Time transfer in optical networks
Over IP networks and through the Internet, the Network Time Protocol (NTP) is used to synchronize different clocks. NTP uses time stamps sent in data messages for time synchronization. In non-dedicated networks, it is difficult to reach accuracies below 10 msec using NTP, which is too low for handling synchronization of TV antennas, mobile phone systems, and in most defense and media applications.
Over synchronous networks it is easier to obtain a higher accuracy using a method called "two-way time and frequency transfer." With the source node transferring its time to the neighboring nodes in the network, the neighboring nodes returning their time, two-way time and frequency transfer compensates so that neighboring nodes work with the same time as the source node. Retransmitting until all nodes in the network operate on the same time ensures accuracy and consistency.
Additionally, the full UTC timescale must be monitored and corrected at link-start, such that correct date and time are coordinated. In case of fiber breakage, the system should automatically re-route the time transfer signals using any alternative path through the network. Correction for intrinsic delays is also done in the nodes. Over time, external influences will cause diurnal wander, but these can be eliminated by continuously monitoring and eliminating this developed wander, enabling a stable time representation across the network. Several sources such as atomic clocks, located far apart from each other, can be used to create a system with full redundancy. The interfaces used in the ingress and egress nodes are the same used by a GPS receiver, i.e., two BNC connectors, one for a 10-MHz signal and one for the 1 pulse/sec input.
Initially, such an approach could be used as a complement to GPS to enhance the strength and resilience of the UTC distribution. However, with redundant network architecture it would be possible to fully rely on the time transfer functionality, since the data signals would not reach the receivers should the network go down.
Per Lindgrenis founder of Net Insight (www.netinsight.se) and head of business development.