System for generating a time reference value in the MAC layer of an ISO/OSI communications model among a plurality of nodes

• US 5,386,542 A
• Filed: 10/31/1991
• Issued: 01/31/1995
• Est. Priority Date: 10/31/1991
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Brann, J. Joseph, Ralya, Thomas C.
• Primary Examiner(s): Lee, Thomas C.
• Assistant Examiner(s): Kim, Sang Hui
• Application Number:
  US07/785,713
• Time in Patent Office: 1,188 Days
• Field of Search: 395/550, 395/200, 375/107, 370/100.1, 370/60
• US Class Current: 709/248
• CPC Class Codes: H04J 3/0664 unidirectional timestamps
• CPC Class Codes: H04J 3/0673 using intermediate nodes, e.g. modification of a received timestamp before further transmission to the next packet node, e.g. including internal delay time or residence time into the packet
• CPC Class Codes: H04J 3/0682 by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
• CPC Class Codes: H04L 12/28 characterised by path configuration, e.g. LAN [Local Area Networks] or WAN [Wide Area Networks] wireless communication networks H04W ; arrangements for dividing the transmission path H04W40/00
• CPC Class Codes: H04L 12/433 with asynchronous transmission, e.g. token ring, register insertion

Abstract

A Time Reference Manager for providing a time reference value to the nodes of a ring topology local area network. A time reference is implemented on a distributed system data bus at a low level interface to provide highly accurate time reference values at each node of the LAN with no settling time required. One node on the network is designated as the Time Reference Manager and broadcasts the Time Reference Protocol data around the ring of the LAN network. Time delay correction is provided by an algorithm performed in a time reference software process. Clock accuracy is selectable by the number of bits used in the clock-counter.

Claims

1. A method for generating a Time Reference Value on a plurality of nodes of a ring topology local area network (LAN) implemented in an ISO/OSI communication model comprising the steps of:

at a local node performing the following steps implemented in a media access control (MAC) layer of the ISO/OSI communication model;

acquiring the value of a local clock-counter for use as a local clock time value;

appending the local clock time value to a time, count and correction value received on the LAN in a frame of a time reference protocol data unit (TR-PDU);

incrementing the count by one in the received time, count and correction value;

calculating a correction factor based on a time value correction algorithm;

retransmitting in response to a time reference request the incremented and corrected time, count and correction value in the TR-PDU; and

at a time reference node performing the following steps implemented in a media access control (MAC) layer of the ISO/OSI communication model;

replacing a current Time Reference Value with a received time, count and correction value from a local node;

constructing and transmitting a new Time Reference Value in a modified TR-PDU; and

resetting the clock-counter to a specified value in response to a set time reference period service call.

2. In an ISO/OSI communication model, a system for providing a Time Reference Value to a plurality of nodes on a ring topology local area network (LAN) comprising:

a Time Reference Manager implemented in a media access control (MAC) layer of the ISO/OSI communication model for controlling a transmit state machine and initiating communication bus requests on a communication bus, the Time Reference Manager broadcasting a Time Reference Value onto the LAN in a frame of a Time Reference Protocol Data Unit (TR-PDU);

a state machine implemented in the MAC layer for receiving the Time Reference Value on the LAN in the TR-PDU and periodically transmitting a processed Time Reference Value; and

a local processor at a node of the LAN, the processor implemented in the MAC layer and receiving the broadcast Time Reference Value, incrementing it by one, calculating a correction factor and returning the processed Time Reference Value to the state machine for transmission on the ring topology local area network.

3. A system for distributing a uniform time reference for a plurality of data processors in a data communications network implemented in an ISO/OSI communication model having a network clock source, comprising:

a first node, the first node having a first network adapter coupled to the data communications network, which includes a first adapter clock counter coupled to said network clock source, said first node also having a first data processor coupled to said first network adapter, which includes a first processor clock counter coupled to said network clock source, for representing a reference time at said first node;

a second node, the second node having a second network adapter coupled to said data communications network, which includes a second adapter clock counter coupled to said network clock source, said second node also having a second data processor coupled to said second network adapter, which includes a second processor clock counter coupled to said network clock source, for representing the reference time at said second node;

said first node computing in a media access control (MAC) layer of the ISO/OSI communications model a first difference value as a difference between first processor clock counter value in said first processor clock counter and a first adapter clock counter value in said first adapter clock counter;

said first node forming in said MAC layer a first time reference data unit in a frame of a protocol data unit identified as a time reference-protocol data unit (TR-PDU) which includes said first adapter clock counter value and said first difference value and transmitting in said MAC layer said first time reference data unit over said data communications network to said second node;

said second node in said MAC layer computing in said MAC layer a second difference value from a sum of said first difference value and said first adapter clock counter value minus a second adapter clock counter value in said second adapter clock counter; and

said second node computing in said MAC layer a second processor clock counter value for said second processor clock counter from a sum of said second difference value and said second adapter clock counter value.

4. The system for distributing a uniform time reference for a plurality of data processors of claim 3, which further comprises:

a third node having a third network adapter coupled to said data communications network, which includes a third adapter clock counter implemented in the MAC layer, the third node coupled to said network clock source, said third node also having a third data processor coupled to said third network adapter, which includes a third processor clock counter coupled to said network clock source, for representing the reference time at said third node;

said second node forming in said MAC layer a second time reference data unit which includes said second adapter clock counter value and said second difference value and transmitting said second time reference data unit over said data communications network to said third node in the TR-PDU frame;

said third node computing in said MAC layer a third difference value from a sum of said second difference value and said second adapter clock counter value minus a third adapter clock counter value in said third adapter clock counter;

said third node computing in said MAC layer a third difference value from a sum of said second difference value and said second adapter clock counter value minus a third adapter clock counter value in said third adapter clock counter; and

said third node computing in said MAC layer a third processor clock counter value for said third processor clock counter from a sum of said third difference value and said third adapter clock counter value.

5. A method for distributing a uniform time reference for a plurality of data processors in a data communications network implemented in an ISO/OSI communications model having a network clock source, the network including a first node, the first node having a first network adapter coupled to the data communications network, which includes a first adapter clock counter implemented in a media access control (MAC) layer of the ISO/OSI communications model coupled to the network clock source, said first node also having a first data processor coupled to said first network adapter which includes a first processor clock counter coupled to said network clock source, for representing the reference time at said first node, the network further including a second node, the second node having a second network adapter coupled to said data communications network, which includes a second adapter clock counter implemented in the MAC layer coupled to said network clock source, said second node also having a second data processor coupled to said second network adapter, which includes a second processor clock counter coupled to said network clock source, for representing the reference time at said second node, the method comprising the steps performed in the MAC layer of:

computing in said MAC layer at said first node a first difference value as a difference between a first processor clock counter value in said first processor clock counter and a first adapter clock counter value in said first adapter clock counter;

forming in said MAC layer at said first node a first time reference data unit identified as a TR-PDU in a frame of a protocol data unit which includes a first adapter clock counter value and said first difference value and transmitting said first time reference data unit or TR-PDU over said data communications network to said second node;

computing at said second node a second difference value from a sum of said first difference value and said first adapter clock counter value minus a second adapter clock counter value in said second adapter clock counter; and

computing at said second node a second processor clock counter value for said second processor clock counter from a sum of said second difference value and said second adapter clock counter value.