Method for synchronizing a wide area network without global synchronizing

• US 5,130,987 A
• Filed: 09/11/1991
• Issued: 07/14/1992
• Est. Priority Date: 03/23/1990
• Status: Expired due to Term

• Current Assignee: Proxim Wireless Corporation
• Original Assignee: Metricom, Inc.
• Inventors: Flammer, George H.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): KIZOU, HASSAN
• Application Number:
  US07/758,193
• Time in Patent Office: 307 Days
• Field of Search: 370/103, 370/100.1, 370/94.1, 370/94.2, 370/11, 375/1, 375/107, 455/51, 455/71, 455/166, 455/179, 455/62, 455/63
• US Class Current: 370/436
• CPC Class Codes: H04B 1/713 using frequency hopping
• CPC Class Codes: H04B 1/7156 Arrangements for sequence synchronisation
• CPC Class Codes: H04J 13/00 Code division multiplex systems for frequency hopping H04B1/713
• CPC Class Codes: H04J 3/0664 unidirectional timestamps
• CPC Class Codes: H04J 3/0676 Mutual

Abstract

A frequency-hopping packet communication system without a master clock or master control unit is based on use of a receiver'"'"'s frequency hopping timing and identification to control communication. A frequency-hopping band plan, involving the number of channels and the pseudo-random pattern of frequency change and nominal timing of changes, is universally known to each node in the network. A transmitter acquires synchronization with a target node by use of information previously received from or about a target indicating timing of present idle frequency hop of the target receiver. Each receiving node establishes in each station or node a table of receiver frequency hopping sequence offsets (hop timing offsets) of each other node within its communication range, and each node announces its presence on each frequency in a packet with a hop timing offset indicator. The hop timing offset indicator is a key used to read a table to allow nodes to set themselves in synchronization with one another. A location indicator built into the address of each packet is used to randomize an ordered frequency-hopping table at each node. Frequency-hopping is implemented by the division of communicaton slots and the accumulation of slots into epochs, wherein each epoch equals the total number of available slots (number of channels times the number of time frames per channel). The transmitting node tracks the pre-established frequency-hopping pattern for its target receiver based on previously-acquired information.

Claims

1. In a packet communication network having a plurality of communication nodes, each node having a terminal node controller incorporating a central processing unit (CPU) and memory means and a clock, a method for maintaining frequency and time synchronization within said network comprising the steps of:

generating at each node an age value by means of the clock, said age value being representative of an age of said node, and the rate of the clock being communicated within said network;

at each node, supplying a representation of said age value to any other node as part of transmission of a packet;

at each node, collecting said representation of said age value for other nodes within said network;

at each node, changing, according to a known pattern and independently of each other node of said network, slots of transmission and reception of signals, a slot being a frequency channel during a preselected time period, said known pattern being known to each other node in said network; and

transmitting packets of information from a source node to an addressed node at a frequency and for a duration synchronous with the slot of said addressed node, such that the source node tracks changing frequency of the receiving node.

2. In a packet communication network having a plurality of communication nodes, each node having a terminal node controller incorporated a central processing unit (CPU) and memory means and a clock, a method for maintaining frequency and time synchronization within said network comprising the steps of:

generating at each node an age value by means of the clock, said age value being representative of an age of said node, and the rate of the clock being communicated within said network;

at each node, supplying a representation of said age value to any other node as part of transmission of a packet;

at each node, collecting said representation of said age value for other nodes within said network;

at each node, changing, according to a known pattern and independently of each other node of said network, slots of transmission and reception of signals, a slot being a frequency channel during a preselected time period, said known pattern being known to each other node in said network;

transmitting packets of information from a source node to an addressed node at a frequency and for a duration synchronous with the slot of said addressed node, such that the source node tracks changing frequency of the receiving node;

at each node, determining from said age value of each said other node and said age value of said node a difference in age value; and

storing said difference in age value for each other node.

3. The method according to claim 2 wherein said difference in age value is within a fractional value of duration of a slot such that the transmission of said packets of information is synchronized with step of changing said slots of transmission and reception of signals.

4. The method according to claim 2 wherein said age value for each node is a modulo value based on a repetition of said known pattern.

5. The method according to claim 2 wherein said age value is based on geographic location of said node, in order to assure uniqueness.

6. The method according to claim 2 further including the step of:

at each node, selecting a random slot in said known pattern to serve as a home slot in order to minimize the number of nodes simultaneously occupying identical slots.

7. A node apparatus for a wide area packet communication network, said node apparatus comprising:

a frequency agile transceiver means for transmitting and receiving packet signals on a plurality of frequencies; and

a terminal node controller means for controlling frequency of transmission and reception of said frequency agile transceiver and for tracking changing frequencies of other nodes within said packet communication network, wherein said terminal node controller means in each local node includes means for processing and storing age information specifying age of remote nodes within said network, for assembling packets containing age information specifying age of said local node, and for selecting frequency and time of transmission of packets to an addressed remote node based on said age information of said addressed remote node.