Method and apparatus for providing reliable voice and voice-band data transmission over asynchronous transfer mode (ATM) network

A clock recovery apparatus for providing a clock recovery function in the receiving entity of a communication system to implement adaptation of constant bit-rate (CBR) services over an asynchronous transfer mode (ATM) or ATM-like network comprising:

• a cell buffer into which ATM cells received over said network are controllably stored and read out for delivery to a destination communication circuit;
• an ATM cell receiver, which is operative to generate an input clock signal in accordance with times of receipt of ATM cells from said network, transitions in said input clock signal depending upon cell delay variations through said network;
• a reference clock signal generator which is operative to generate a reference clock signal based upon a relatively high frequency service clock signal from which an output clock signal, having a frequency corresponding to a source clock frequency for transmitting said ATM cells over said network, may be derived;
• and
• a phase locked loop (PLL) having a phase detector having respective inputs coupled to receive said input clock signal and said reference clock signal, and having an output coupled to control said clock signal generator, so that said PLL generates said output clock signal in accordance with the number of cycles of said service clock signal that occur between respective transitions in said input clock signal and said reference clock signal.

A method of controlling the operation of a phase locked loop (PLL) used to provide a clock recovery function in the receiving entity of constant bit-rate (CBR) communication signals transported over an asynchronous transfer mode (ATM) or ATM-like network, in order to provide reliable voice and voice-band data communications in a manner that is effectively transparent to an associated modem serving customer premises equipment, said method comprising the steps of:

• (a) in response to the start of a new voice/voice-band data call, determining whether said PLL is already locked;
• (b) in response to step (a) indicating that said PLL is already locked, setting parameters of said PLL to values associated with a tracking mode of operation of said PLL, but otherwise setting parameters of said PLL to values associated with an acquisition mode of operation, until said PLL is locked, and thereafter changing parameters of said PLL to values associated with said tracking mode of operation;
• and
• (c) in response to termination of said call, storing a current operational parameter of said PLL, so as to reduce the time required for the PLL to acquire lock for the next incoming call.