Synchronization method as well as synchronizing units, terminal and exchange therefor

• US 5,654,815 A
• Filed: 05/15/1995
• Issued: 08/05/1997
• Est. Priority Date: 05/21/1994
• Status: Expired due to Fees

• Current Assignee: Alcatel NV (Nokia Corporation)
• Original Assignee: Alcatel NV (Nokia Corporation)
• Inventors: Bunse, Stephan
• Primary Examiner(s): NEGASH, KINFE MICHAEL
• Application Number:
  US08/440,822
• Time in Patent Office: 813 Days
• Field of Search: 359/110, 359/135, 359/158, 359/152, 359/162, 370/103, 370/503, 375/356, 375/357, 375/358, 327/146, 455/51.1
• US Class Current: 398/154
• 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 7/0075 with photonic or optical means
• CPC Class Codes: H04L 7/042 Detectors therefor, e.g. correlators, state machines digital correlators in general G06F17/15

Abstract

Rather than synchronizing clocks between two stations in an optical communication system by correlating a received synchronization signal with an own-station synchronization signal and controlling the own-station clock accordingly, the correlation takes place in one station (EX), but the correlation product is evaluated in the other station (E1-E3), and the clock signal of the latter is controlled accordingly. This may be done by sending a first synchronization bit sequence from a terminal (E1-E3) and looping it back in an exchange (EX) at the bit rate of a second synchronization bit sequence, wherein the clock generator of the terminal (E1-E3) is controlled according to the received correlation bit sequence. This has the advantage at the exchange that synchronization requires only optical switches and no optical-to-electrical transducers. It permits all optical exchanges and eliminates one optical-to-electrical transducer per connected subscriber. The only disadvantage is that for a transmitter without receiver, an additional reverse channel is necessary.

Claims

1. A method of synchronizing at least a first clock signal of a first station (E1 to E3) with at least a second clock signal of a second station (EX), comprising the steps of sending a first synchronization signal containing the first clock signal from the first station (E1 to E3) to the second station (EX) and correlating the first synchronization signal in the second station (EX) with a second synchronization signal containing the second clock signal to obtain a correlation signal,

wherein the correlation signal is sent from the second station (EX) back to the first station (E1 to E3) in response to the transmission of the first synchronization signal,

wherein the correlation signal is evaluated in the first station (E1 to E3), and wherein the result of said evaluation is used to adjust the first clock signal of the first station (E1 to E3),

wherein the first and second synchronization signals are formed by a first synchronization bit sequence and second synchronization bit sequence, respectively, and

wherein the two synchronization bit sequences are correlated by passing the first synchronization bit sequence through a switching device (SUB1 to SUB3) controlled by the second synchronization bit sequence, said switching device generating a correlation bit sequence and sending it back to the first station (E1 to E3).

2. A method as claimed in claim 1, wherein a first bit- and frame-rate clock of the first station (E1 to E3) is synchronized with a second bit- and frame-rate clock of the second station (EX).

3. A method as claimed in claim 2, wherein the first station (E1 to E3) sends first data packets to the second station (EX) at the first bit clock rate and at the first frame clock rate, and wherein a first synchronization bit sequence is sent from the first station (E1 to E3) to the second station (EX) between a first sequence of data packets.

4. A method as claimed in claim 2, wherein the second station (EX) sends second data packets to the first station (E1 to E3), and wherein a correlation bit sequence is delayed by a constant time interval which is chosen so that when the clock signals are in synchronism, the correlation bit sequence is sent from the second station (EX) to the first station (E1 to E3) between the second data packets.

5. A method as claimed in claim 1 wherein the first station (E1 to E3) sends a first sequence of data packets to the second station (EX) at the first bit clock rate and at the first frame clock rate, and wherein the first synchronization bit sequence is sent from the first station (E1 to E3) to the second station (EX) between the first sequence of data packets.

6. A method as claimed in claim 1, wherein the second station (EX) sends second data packets to the first station (E1 to E3), and wherein the correlation bit sequence is delayed by a constant time interval which is chosen so that when the clock signals are in synchronism, the correlation bit sequence is sent from the second station (EX) to the first station (E1 to E3) between the second data packets.

7. A synchronizing unit (SYNC2) for a second station (EX), for synchronizing at least a first clock signal of a first station (E1 to E3) with at least a second clock signal of the second station (EX), comprising a generator (GEN2) for generating a second synchronization signal containing the second clock signal and a correlator (SW2) for correlating the second synchronization signal with a first synchronization signal containing the first clock signal, wherein the synchronizing unit (SYNC2) is provided with a transmitting device (SW1) which is adapted to receive from the correlator (SW2) a correlation signal corresponding to the result of the correlation of the first and second synchronization signals, and transmitting the correlating signal to the first station (E1),

wherein the first synchronization signal and the correlation signal are optical signals, and wherein the correlator (SW2) and the transmitting device (SW1) each contain an optical switch controlled by the second synchronization signal.