Basics of real-time measurement, control, and communication using IEEE 1588: Part 2

John Eidson, Agilent Technologies

January 7, 2008

John Eidson, Agilent Technologies

Fundamental Operation of the Protocol
The precision time protocol (PTP) defined in IEEE 1588 is designed to synchronize clocks in devices in a distributed system. PTP is a distributed protocol. Every device in the system that implements IEEE 1588 executes exactly the same protocol.

There is no central authority governing any aspect of the protocol, nor is there any need to configure nodes prior to operation, assuming that the default values of various parameters are instantiated in all IEEE 1588-enabled devices.

The entire operation of the protocol is implemented using only information obtainable from the exchange of PTP-defined messages between these clocks. There are five operational features of the protocol that together allow the synchronization of clocks in a system, and provide sufficient management These features are:

1) Establishing boundaries and communications for the system to be synchronized,
2) Establishing a master-slave synchronization hierarchy,
3) Establishing orderly startup and reconfiguration of the system,
4) Providing the necessary information to allow slave clocks to correctly synchronize to their master, and
5) Providing system and clock management capability when needed by an application.

Figure 3.1. Typical system of network devices, boundary clocks, and ordinary clocks

System Boundaries and Communications
PTP is designed to operate on packet-based networks that support multicast communications. There are five message types defined by PTP:

1) Sync
2) Delay Req
3) Follow Up
4) Delay Resp
5) management

Message types Sync and Delay Req are called "event messages", since they are used as timing events by the PTP protocol. Message types Follow Up, Delay Resp, and management are called "general messages".

Follow Up and Delay Resp messages are used to convey timing information. Both event and general messages are to be communicated using a multicast model of communication to enable the self- configuration objective of the standard to be met.

Management messages are normally communicated using a multicast model, but in addition are permitted to use a point-to-point model. Figure 3.1 above illustrates a typical IEEE 1588 system topology that allows independent synchronization systems to be maintained on the same communication network. Each system maintains its time scale independently of the others. These independent synchronization systems are called "subdomains".

Subdomains are implemented by defining a namespace, so that each subdomain is distinguished by a subdomain name. All PTP messages contain the name of the applicable subdomain. All interactions, communications, and other features of IEEE 1588 occur within a single subdomain, and are logically independent of similar operations in other subdomains.

There may be performance impairments that result from multiple subdomains sharing a common communication fabric. These impairments result from communication or processor loading on system components.

The boundary of a subdomain is determined by the underlying communication fabric and how it responds to multicast messages. The PTP protocol will synchronize all clocks in a subdomain that receive and process PTP event and general messages.

Therefore, to limit the extent of an IEEE 1588 subdomain, it is necessary to correctly limit the propagation of multicast messages in the underlying communication fabric. This may be done physically by isolating the subdomain, or logically by proper configuration of routers, switches, and similar network equipment.

A system such as the one shown in Figure 3.1 above typically contains several end or terminal devices, and several network devices. An end device is a device with only a single network connection. End devices containing a PTP clock are termed "ordinary clocks" in the standard.

Depending on the network technology, a network may also contain nonterminal devices such as routers, switches, and repeaters. These devices are called network devices, and may or may not contain specialized IEEE 1588 functionality.

These devices serve to pass a network message received on one port to one, or more of the remaining ports of the device. Routers and switches make these forwarding decisions based on addressing information contained in protocol headers of the messages.

Repeaters simply forward the message to the other ports on the device. The standard refers to any end or network device that can issue or receive PTP messages as a node. Strictly speaking, any end or network device performs this function, not only devices that support IEEE 1588.

Since devices that do not implement the protocol have no direct effect on the protocol, no confusion arises. However, such non IEEE 1588 devices may adversely influence the performance of the protocol.

Network devices containing specialized IEEE 1588 functionality are called boundary clocks. The PTP protocol uses boundary clocks to logically segment the physical network to create a master-slave synchronization hierarchy of clocks.

In the general case, a node may be connected to a boundary clock or a network device. The network, depicted by the cloud in Figure 3.1, may contain a mixture of boundary clocks and network devices, depending on the complexity of the application and the network technology.

Note that not all potential network technologies have the equivalent of routers and switches. A network in such a technology would appear to PTP as a set of ordinary clocks communicating directly with each other in a multicast manner. An example of such a network would be a set of nodes that communicate using the controller area network (CAN).

To read Part 1, go to "The varieties of system temporal specifications."
Next in Part 3: IEEE 1588's master-slave synchronization hierarchy

Used with permission of its publisher, Springer Science and Business Media, this series of articles is based on material from "Measurement, Control and Communication Using IEEE 1588," by John C. Eidson and can be purchased on line.

John C. Eidson, Ph.D., received a B.S. and an M.S. from Michigan State University and a Ph.D. from Stanford University, all in electrical engineering. He held a postdoctoral position at Stanford for two years, spent six years with the Central Research Laboratory of Varian Associates, and joined the Central Research Laboratories of Hewlett-Packard in 1972. When HP split in 1999, he transferred to the Central Research Laboratory of Agilent Technologies. Dr. Eidson was heavily involved in IEEE 1451.2 and IEEE 1451.1 and is the chairperson of the IEEE 1588 standards committee and a life fellow of the IEEE.