DSL (digital subscriber line) applications, such as ADSL (asymmetrical digital subscriber line) are an important part of the present telecommunications infrastructure, and there is ever reason to believe that DSL's significance will only continue to increase. Many protocols have been suggested to help more efficiently and accurately transmit and process data in the DSL systems. The world standards for ADSL are defined in ITU 992.1 and 992.2, each of which is incorporated herein by reference.
One such protocol deals with a concept known as “bit swapping.” Bit swapping is a process or protocol for multiple carriers in a transmission line, as opposed to a single carrier. Bit swapping occurs when a transceiver decides to switch (or alter) the transmission of data from one carrier to a second carrier, for such reasons as noise, etc.
Bit swap or other modifications to the transceive parameters can be applied during normal operation to compensate for changes to the communication channel, caused by many things such as changes in loop temperature, network traffic. Referring to FIG. 6, the receiver in transceiver B monitors some measure of the relative error performance of each of the received carriers in a multi-carrier modulation such as is used in ADSL. If the receiver in transceiver B determines that a change is desirable, e.g. to decrease the overall error level, then transceiver B sends a request message that contains the proposed change to transceiver parameters to transceiver A, which sends an acknowledge/will comply (hereafter called ack/comply message) back to the transceiver B if it wishes to make the requested change.
A protocol for this using an operations and management (OAM) overhead channel for the request and ack/comply messages is described in U.S. Pat. No. 5,479,447, incorporated herein by reference. With this protocol, the reconfiguration of transceive parameters (the bit swaps) are synchronized between transceiver A transmitter and transceiver B receiver by having the new parameters become effective a fixed number of data symbols following the ack/comply message.
A modified protocol is described in U.S. Pat. No. 5,400,322, incorporated herein by reference. This protocol requires that both transceivers A and B count transmitted and received data symbols, i.e., transceiver A counts the data symbols it transmits and counts the data symbols it receives; transceiver B does similarly. The bit swap ack/comply message sent by transceiver A contains a data symbol count (technically, a superframe count identification) that tells transceiver B, which receive data symbol, is the first use the new transceive parameters (bit swap). The protocol allows for the overhead messages to be sent repeatedly as a way of reducing the chance that the message is either not received or that the message is not received correctly. The current generation of ADSL equipment uses a scheme based on this protocol.
Another method, proposed for ADSL equipment does not use the OAM channel for the ack/comply message. Instead, sending a particular bit pattern, sometimes called the Sync Flag, in place of the normal Sync Symbol indicates the ack/comply message. The Sync Symbol is a predetermined bit pattern, applied to 4QAM constellations on the carriers, that normally is sent every 69 data symbols for purpose of transceiver synchronization. Sixty-nine data symbols, with the last symbol being the Sync Symbol, form a superframe. Occurrence of the Sync Flag in place of the Sync Symbol instructs transceiver B to begin using new transceive parameters with the first data symbol following the Sync Flag symbol.
A simple framework for communications systems in general is useful for describing functionality as well as for comparing methods and protocols. Communication systems are often described and/or implemented in a layered way. The Open Systems Interconnection (OSI) reference model is one such way of layering and is used in this document. The lowest layer, sometimes called the physical media dependent (PMD) layer, is the layer where bits of information are transformed by the transmitter into modulations of physical properties such as voltage and current on the physical media (e.g., the wire pair loop) and transformed back into bits at the other end. The PMD layer typically includes functions such as symbol timing generation and recovery, encoding and decoding, modulation and demodulation, echo cancellation (if implemented) and line equalization, link startup, and physical layer overhead (superframing). A higher layer, sometimes called the PMS-TC (Physical Media Specific-Transmission Convergence) layer, typically includes functions such as the data framing, frame synchronization, error correction, error detection, data scrambling, and data descrambling. Generally, operation and maintenance information, if it is transmitted across the channel, is done via an interface to the PMS-TC or higher layer, rather than directly to the PMD layer, because the PMD layer is not a convenient interface for variable length, multi-bit messages. FIG. 7 illustrates a communications system and the PMD and PMS-TC layers.
The layered model described above is now applied to ADSL. The current generation of ADSL equipment typically uses the two-wire telephone cable (the loop) as the physical media. In ADSL, the Sync Symbol and Sync Flag replacement for Sync Symbol are PMD layer signals, meaning that each corresponds to a specific physical signal put onto the loop. The PMS-TC layer provides the OAM overhead channel. User data derives from a hierarchy of higher layers that feed data to the PMS-TC layer. Additionally, a transceiver management layer provides interface protocol for the OAM provided by the PMS-TC.
Typical multi-carrier or ADSL transceive parameters to reconfigure, such as the number of bits per carrier, gain of carrier, and the order of the data applied to carrier, are closely associated with the PMD layer. Changing these parameters must be data frame synchronous at both transceivers; otherwise the connection is at risk to fail.
In ADSL, the OAM overhead channel has some embedded error checking and error mitigation functionality. Additionally, the error correcting and detection capabilities of the PMS-TC layer are applied to OAM data. Normally, this would be a reasonably reliable protocol. However, a fairly typical situation for reconfiguration is when receiver in transceiver B is experiencing higher than normal errors such that a clear OAM overhead message cannot reliably be received. This increased error event is what can trigger a receiver to want to reconfigure in order to bring the errors back to a normal level. Therefore, for this situation, it is desirable to increase the reliability of the reconfiguration ack/comply as this is a message that transceiver A will assume that transceiver B will correctly receive.
In current generation ADSL equipment, the ack/comply message is sent by transceiver A over the OAM overhead channel, and the message is repeated several times in succession to increase the reliability that the message will be correctly received by transceiver B receiver. However, it can be demonstrated in practice that in typical situations, the receiver can miss the ack/comply message. In such an event, transceiver A switches to new transceiver transmit parameters but transceiver B does not switch to corresponding receive parameters. This causes errors in the link that degrade performance, often requiring the equipment to leave ShowTime and retrain.
CRC (cyclic redundancy code) error detection employed by the transceivers does not prevent this problem. The CRC is used to detect if there are errors but not to correct errors. It is possible to improve the protocol over OAM but it would require adding additional messaging between transceivers, which would take more time, and would still not provide sufficient reliability.