The ability to conduct high-speed data and voice communications between remotely separated data processing systems and associated subsystems has become a requirement of a variety of industries and applications, such as business, educational, medical, financial and personal computer uses. Moreover, it can be expected that future applications of such communications will engender more systems and services in this technology. Associated with such applications has been the growing use and popularity of the “Internet”, which continues to stimulate research and development of advanced data communications systems between remotely located computers, especially communications capable of achieving relatively high-speed data rates over an existing signal transport infrastructure (e.g., legacy copper cable plant).
One technology that has gained particular interest in the telecommunication community is digital subscriber line (DSL) service, which enables a public service telephone network (PSTN) to deliver relatively high bandwidth signals (including voice and data) using conventional telephone company copper wiring infrastructure. DSL service has been categorized into several different technologies, based upon expected data transmission rate, the type and length of data transport medium, and encoding/decoding schemes.
Regardless of its application, the general architecture of a DSL network essentially corresponds to that diagrammatically shown in FIG. 1 as comprising a pair of remotely separated, mutually compatible digital communication transceiver entities. One entity is located at respective network controller site 10 (such as a telephone company central office (CO)), while a second entity is located at a customer premises site 20. Each transceiver is coupled to a communication link, such as a twisted pair (loop) 30 of an existing copper plant. Using ATM-based, digital subscriber line (DSL) protocol, this telecommunication fabric allows information, such as voice and (Internet-sourced) data (which is readily accessible via a backbone network 15), to be transmitted from the central office site 10 over the DSL loop 30 to an integrated access device (LAD) 21 at the customer site 20.
For this purpose, at the network controller site 10, a DSL transceiver 11 is customarily located in a DSL access multiplexer (DSLAM) 12. Within the communication infrastructure of the telephone company, DSLAM 12 is coupled with the backbone 15, which typically contains one or more of signaling transport devices, such as an asynchronous transfer mode (ATM) switch 31, a voice gateway 33, a Class-5 switch 35, and the like, that are linked to an internet service provider (ISP) 37. Also a data gateway 36 may link the ATM switch 31 to a data network 38.
The other transceiver, serving the customer premises site 20, may comprise an integrated access device (IAD) 21, which is coupled via a plain old telephone system (POTS) interface 23 to a modem 25 (such as a V.90 modem) serving data terminal equipment (DTE) 27.
For transporting data and voice, an ATM network of the type shown in FIG. 1 employs ATM Adaptation Layer 5 (AAL5) for data transport, and AAL2 for voice transport. As ATM is a ‘cell’-based asynchronous transfer protocol, processing at both is the transmit or source site and the receiver entity are necessary to ensure a continuous CBR flow of voice and voice-band data cells across the ATM fabric. Unfortunately, AAL2 protocol-based voice and voice-band data transmission can be disrupted by delays encountered by the ATM cells during their transport over the network. These delays are of two types: 1—fixed delay associated with the configuration of the network (which is predictable and readily accounted for), and 2—variable delay (termed Cell Delay Variations (CDV)) associated with the traffic load on the network switches, causing successive ATM cells to arrive at a receiving or destination entity in an aperiodic manner.
To minimize or eliminate these disruptions, in order to effectively ensure reliable voice-band data transmission, it is necessary to remove the variable delay component of cell arrival time. This is customarily achieved through the use of a cell jitter buffer of sufficient length to accommodate maximum cell delay variation), and synchronizing the receive site's POTS interface (the IAD's CODEC) clock to the far-end or source site's transmitter clock, in a manner that avoids overflow or underflow of the buffer (which will occur if the clocks are not locked together).
One relatively straightforward method to recover the clock is to encode the transported ATM stream with a Synchronous Residual Time Stamp (SRTS) representative of the frequency difference between the source clock and a common reference network clock. At the receiving entity, the SRTS is decoded to regenerate the source clock frequency. Unfortunately, for AAL2-based data transmissions, physical layer timing on the DSL loop may not always be traceable to a primary reference source (transmit site) clock, and must be extracted ‘adaptively’ from the incoming AAL2 cell stream.
In an adaptive clock recovery scheme, no explicit timing information is transmitted from source to destination across the network and no common reference clock is used. Instead, source clock frequency information is derived by monitoring ATM cell arrival activity, and averaging out CDV effects. While there is currently no ‘standardized’ method, adaptive clock recovery has typically involved monitoring the ‘fill’ level of a cell jitter buffer, through which received ATM cells are controllably clocked by an associated clock recovery loop, and adjusting the receive entity clock, so that positions of write/read pointers to the buffer fall within a prescribed error window relative to a selected (e.g. median or statistically averaged) buffer fill level, and avoid overflow or underflow of the buffer.
For an illustration of non-limiting examples of literature describing various clock recovery schemes including both SRTS and buffer fill level-based adaptive mechanisms of the type described above, attention may be directed to the following U.S. Pat. Nos. 5,361,261, 5,844,891, 5,966,387, 6,111,878, 6,188,692 and 6,252,850.