IEEE 1588 Layer 2 - Description

IEEE Layer 2 offers the smallest and thus cheapest clock synchronisation.

More Details
Since its publication in 2002, the IEEE1588 Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems has sparked tremendous interest. It comes at no surprise though that with the latest revision of the standard in 2008 quite a number of new features and options have come into the official document. Among them is the normative specification of Transport of PTP over IEEE802.3 Ethernet in Annex F, also know as IEEE1588 Layer 2, in reference to the Open Systems Interconnection (OSI) reference model. It includes the Ethertype, which shall be set to 0x88F7, Multicast MAC addresses (01-80-C2-00-00-0E peer delay messages, 01-1B-19-00-00-00 for all other messages), and the settings of a view transport specific field values. Thus the payload of the Ethernet frame does not differ much from the Precision Time Protocol (PTP) frames normally used. The message length of IEEE1588 Layer 2, however, is significantly smaller since the entire Internet Protocol (IP) header is obsolete. A layer 2 PTP sync message hence has an overall length of 64 bytes, already including two bytes for padding, compared to the 90 bytes for the normal sync message.

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