Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
2000-11-01
2004-05-18
Rao, Seema S. (Department: 2666)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C370S244000, C370S253000, C709S224000
Reexamination Certificate
active
06738355
ABSTRACT:
TECHNICAL FIELD
The invention relates to monitoring communications networks. In particular, the invention relates to synchronizing the operation of multiple measurement probes in a distributed communications network monitoring system.
BACKGROUND ART
Modern communications networks, especially those that carry broadband data over large distances, are often large, complex multi-nodal networks. The complexity of these modern networks often benefits from or even requires the utilization of a network monitoring system (NMS) to monitor the activity within and operation of the network. In particular, the NMS is used in many modern communications networks to assist in the coordination of the activities of the network, to locate and identify faulty equipment and/or problem channels, and to optimize the performance of the network.
As used herein, the term communications network refers to a collection of nodes or elements interconnected by a plurality of paths or communication channels. The communication channels carry data traffic or simply data from one node to another. Nodes may be either the source or destination of data traffic. When a node is acting as either a source or a destination (i.e. sink) for data, the node is often referred to as an endpoint or an end station. The data moving between a source and a destination node is referred to as a data stream. More than one data stream may exist between a pair of nodes at any given time.
In addition to nodes that are endpoints, networks may contain a plurality of nodes that act as switching elements. Switching elements or simply switches are nodes used for routing or relaying data streams from one communication channel to another within the network. As such, switches are responsible for dynamically routing data streams through the network from their source to their destination. Switching elements can also act as a source or destination of a data stream combining the switch and endpoint in a single node.
Since the purpose of a communications network is to communicate data from one point to another, the nodes of the network are usually remotely located from one another. In some networks, such as those employing earth orbiting satellites as nodes and/or those which carry data across continents, the distances between nodes can be very large.
Similarly, the NMS is generally a distributed system having multiple measurement points that are often separated by large distances. The network measurements performed by such a distributed NMS include single point measurements and multi-point measurements. A single point measurement is one in which all of the information produced by the measurement can be derived from the single measurement. The measurement of data rate at a given point in the network is an example of a single point measurement.
On the other hand, a multi-point measurement is one that consists of multiple sub-measurements taken at multiple, physically separated points in a network that are subsequently combined to yield a single measurement result. The network measurements produced by such multi-point measurements include but are not limited to such network parameters as ‘network delay’, ‘delay variance’, and ‘dropped packet rate’. These network measurements are useful to the network operator in identifying service-affecting problems in the network as well as for optimization of the network operation.
Typically, an individual measurement probe performs each of the sub-measurements that make up a multi-point measurement. These measurement probes are each connected to or associated with one or more physical links in the network. Preferably, the connection is passive and does not interfere with normal network operation. As used herein, the term ‘measurement probe’ refers to a means for sampling or collecting data packets passing a given point in a network. Therefore, probes can take the form of an apparatus that is inserted into a network such as a conventional logic probe or can be a built-in apparatus that enables copies of packets to be routed to an auxiliary output or sample port of a node.
Given the distributed nature of the network being measured, the measurement probes are generally located at physically separated points within the network. Therefore, probes at separate physical locations in the network produce each of the sub-measurements. Moreover, the sub-measurements are indicative of the data traffic passing the probe location during a measurement time or interval.
Furthermore, since it is intended in a multi-point measurement that the sub-measurements ultimately be combined to yield a single measurement result, each sub-measurement must be performed in a manner that is, in some way, synchronized or coordinated with other sub-measurements. Generally this entails synchronizing multiple probes such that they observe and collect the ‘same’ set of packets when making a measurement. Thus, since the probes are located at different points in the network, they are observing the ‘same’ set of packets at different physical points in the network and potentially at different points in time as well. Often, in fact, it is desirable that all of the sub-measurements be made simultaneously in time across the network.
The term ‘same’, as used herein with reference to sets of data packets, means that if a pair of sets of data packets are collected from a given data stream, at least a predefined sub-set of the packets in each of the sets will match each other. In other words, the ‘same’ data is being observed at different points in the path of a data stream through a network. Conversely, if a pair of sets is collected with the intent of collecting the ‘same’ data and the sets are collected from two different data streams, in general, the pre-defined sub-sets within the sets will not match each other.
As noted above, normally for a multi-point measurement to be carried out successfully, the set of measurement probes and the sub-measurements that they produce must be synchronized to collect the ‘same’ packets. This form of synchronization is sometimes called packet synchronization to distinguish it from time-synchronization. Conventionally, the level of accuracy of the synchronization is a function of the network data rate among other things. However, in general, a high level of synchronization accuracy is typically required for meaningful multi-point measurements.
Achieving a sufficiently high level of synchronization accuracy can be challenging in many practical network implementations. In particular, networks with widely spaced nodes present an acutely difficult problem for synchronization of sub-measurements given the inherent propagation delay that may be observed between measurement probes. In the conventional NMS, sufficiently accurate synchronization is achieved only through the use of complex, usually expensive, equipment including highly accurate clocks associated with the measurement probes.
One conventional method of synchronization that achieves packet synchronization through precise time-synchronization is known as the time plus offset method. In the time plus offset method, each measurement probe maintains a local clock or other method of measuring a global time. The local clocks of the measurement probes are synchronized by using information regarding known propagation delay within the network. Typically, a first probe nearest the source of a data stream starts recording data at a time chosen in advance. A second measurement probe located at a remote point in the network subsequently begins its data recording activities at the chosen time plus an offset time. The offset time is determined such that it is equal to a known propagation time or delay from the point in the network where the first probe is located to the point in the network where the second probe is located. Since the offset is chosen to be equal to the propagation delay, the data recorded by the remote probe is time-synchronized to that of the first probe.
The drawback to this method is the need to know the precise propagation time or delay between probes. In many netw
Love Simon
Pollock Graham S.
Agilent Technologie,s Inc.
Rao Seema S.
Scheibel Robert C
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