Studying Network Timing With Precision Packet Delay Measurements

摘要

As the transmission of telecommunications data is increasingly reliant on new generation packet network transport, new methods of time and frequency transfer are required. While some of these methods are at the physical layer, many involve the network and data link layers and are affected by packet network behavior. Thus, it has become important to develop instrumentation and analysis techniques applicable to the study of packet latency and packet delay variation. Several of the packet network protocols which have been proposed for precision time and frequency transfer are NTP (Network Time Protocol) and IEEE 1588v2, also known as PTP (Precision Time Protocol). Systems designed for high-performance NTP or PTP contain the necessary elements for precision packet timing measurements, namely a common precision timing reference from a source such as GPS and hardware timestamping capability. Such equipment has been developed over the past several years and has been used to make packet delay measurements, both in the laboratory and in live operating production networks. The interpretation and analysis of packet timing data benefit from a combination of traditional techniques and new techniques. Certain analysis techniques applicable to traditional time and frequency measurements carry over to packet timing data, while others are of little utility or in some cases benefit from modification. In this latter category is a new metric, minTDEV, based on one of the Allan deviation calculations. This paper starts with a discussion of the measurement techniques and systems used in the study of packet network timing. Following this introduction, it presents laboratory and production network measurement results from a number of commercial operators and introduces various techniques for the analysis of such results.