Nowadays, the evaluation of performance measurement in computer networks is an important issue. To ensure the quality of service of the network communication, one of the most important network performance parameters is the one-way delay (OWD). For accurate OWD estimation, it is essential to consider some parameters that can influence the measure, such as the operating system and, in particular, the threads, which are concurrent with the measurement application. Moreover, OWD estimation is not an easy task, because it can be affected by synchronization uncertainties. This paper aims to review the different solutions proposed in the scientific literature for OWD measurement. These solutions adopt different methods to guarantee a reasonable clock synchronization based on the Network Time Protocol, the Global Positioning System, and the IEEE 1588 Standard. These different approaches are critically reviewed, showing their advantages and disadvantages.

One-Way Delay Measurement: State of the Art

Just like Ethernet before, IEEE 802.11 is now transcending the borders of its usage from the office environment toward real-time communication on the factory floor. However, similar to Ethernet, the availability of synchronized clocks to coordinate and control communication and distributed real-time services is not a built-in feature in WLAN. Over the years, this has led to the design and use of a wide variety of customized protocols with varying complexity and precision, both for wired and wireless networks, in accordance with the increasingly demanding requirements from real-time applications. This survey looks into the details of synchronization over IEEE 802.11 with a particular focus on the infrastructure mode which is most relevant for industrial use cases. It highlights the different parameters which affect the performance of clock synchronization over WLAN and compares the performance of existing synchronization methods to analyze their shortcomings. Finally, it identifies new trends and directions for future research as well as features for wireless clock synchronization which will be required by the applications in the near future.

Clock Synchronization Over IEEE 802.11—A Survey of Methodologies and Protocols

In distributed systems, clock synchronization performance is hampered by delays and jitter accumulated not only in the network, but also in the timestamping procedures of the devices being synchronized. This is particularly critical in software timestamp-based synchronization where both software- and hardware-related sources contribute to this behavior. Usually, these synchronization impairments are collapsed into a black-box performance figure without quantifying the impact of each individual source, which obscures the picture and reduces the possibility to find optimized remedies. In this study, for the first time, the individual sources of delay and jitter are investigated for an IEEE 802.11 wireless local area network (WLAN) synchronization system using the IEEE 1588 protocol and software timestamps. Novel measurement techniques are proposed to quantify the hardware- and software-related delay and jitter mechanisms. It is shown that the delays and their associated jitter originate from both the WLAN chipset and the host computer. Moreover, the delay from the chipset cannot be considered symmetric and any such assumption inevitably leads to a residual offset, and thus to synchronization inaccuracy. Therefore, a calibration-based approach is proposed to compensate for these delays and to improve the performance of WLAN synchronization. Experimental results show that with optimal error compensation, a similar synchronization performance as software-based synchronization in Ethernet networks can be achieved.

Delay and Jitter Characterization for Software-Based Clock Synchronization Over WLAN Using PTP

Knowing the time at which sensors acquired data is critical to the proper processing and interpretation of that data, particularly for mobile robots attempting to project sensor data into a consistent coordinate frame. Unfortunately, many popular commercial sensors provide no support for synchronization, rendering conventional synchronization algorithms useless. In this paper, we describe a passive synchronization algorithm that can significantly reduce timing error versus naively time-stamping sensor data when it arrives at the host. It is passive in the sense that the algorithm requires no special cooperation from the sensor. Our method estimates the timing jitter induced by hosts, and thus does not require a real-time operating system. We rigorously derive and characterize the method, proving that it can only improve upon the synchronization accuracy of the standard approach.

http://april.eecs.umich.edu/pdfs/olson2010.pdf?origin=publication_detail

A passive solution to the sensor synchronization problem

Wireless sensor networks are evolving from relatively undemanding applications to applications which have stronger requirements. The coordination of distributed entities and events requires time synchronization. Although a number of methods have been studied for WSNs, some applications require high precision time synchronization. Precision time synchronization enables a variety of extensions of applications. The IEEE 1588 precision time protocol (PTP) provides a standard method to synchronize devices in a network with sub-microsecond precision. This paper deals with precision time synchronization using IEEE 1588 over wireless sensor networks. Precision time synchronization using IEEE 1588 provides compatibility between heterogeneous systems in WSNs. This paper also presents experiments and performance evaluation of precision time synchronization in WSNs. Our result established a method for nodes in a network to maintain their clocks to within a 200 nanosecond offset from the reference clock of a master node.

Precision Time Synchronization Using IEEE 1588 for Wireless Sensor Networks

IEEE 1588 is a standard for precise clock synchronization for networked measurement and control systems in LAN environment. This paper presents the design and implementation of two IEEE 1588 prototypes for wireless LAN (WLAN). The first one is implemented using a Linux PC platform and a standard IEEE 802.11 WLAN with modifications to the network device driver. The second prototype is implemented using an embedded WLAN development board that implements the synchronization functionality using an embedded processor with programmable logic device (PLD) circuits. The measured results show that 1.1 ns average clock offset can be reached on HW based implementation, while Linux PC network driver enables 660 ns with a standard WLAN. Although WLAN is an extremely difficult environment for the synchronization, the results achieved with the prototype are fully comparable to those achieved with wired LAN implementations

J. Kannisto

Papers

Software and hardware prototypes of the IEEE 1588 precision time protocol on wireless LAN