Pulse or digital communications – Transceivers – Modems
Reexamination Certificate
1999-12-20
2004-04-13
Tse, Young T. (Department: 2634)
Pulse or digital communications
Transceivers
Modems
C375S260000, C375S377000
Reexamination Certificate
active
06721355
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to adaptive power management in a modem. More particularly, the present invention relates to a method and apparatus for detecting and adapting to changing data rates in a telecommunications network using a modem. Such detection and adaptation can take advantage of reduced power modes in digital subscriber line communication networks, or other telecommunications schemes having reduced power modes.
BACKGROUND OF THE INVENTION
Much interest has been expressed recently in DMT modems to increase bandwidth with various communication schemes, especially those digital subscriber line schemes commonly referred to as xDSL systems, such as ADSL. For example, asymmetric digital subscriber line (ADSL) was conceived originally for video-on-demand type applications, but the focus is now on providing higher speed Internet services, such as the World Wide Web. The asymmetry in ADSL refers to the allocation of available bandwidth and means that it is faster (i.e.—has more allocated bandwidth) in the downstream (towards the subscriber) direction and slower in the upstream (towards a central office) direction. Some applications, such as browsing on the Internet, do not generally demand symmetric data rates and can take advantage of an asymmetric system.
ADSL converts existing twisted-pair copper telephone lines into access paths for multimedia and high-speed data communications. ADSL can transmit more than 6 megabits per sec (Mbps) (optionally up to 8 Mbps) to a downstream subscriber from a central office, and as much as 640 kilobits per second (kbps) (optionally up to 1 Mbps) upstream from a subscriber to the central office. Such rates expand existing access modem capacities by a factor of 50, or more, without new cabling.
ADSL was designed for residential or small-office, home-office type services and thus, it was designed from the outset to operate with the analog voice signals of Plain Old Telephone Service (POTS) simultaneously on the same line, such that an additional copper line is not needed. Generally, the POTS channel is split off from the digital modem by filters to provide uninterrupted POTS, even if the ADSL circuit fails.
Unlike previous copper line technologies, an ADSL system does not need manual pre-adjustment to accommodate line conditions. Instead, the ADSL modem automatically analyzes the line, as part of the process of establishing a connection, and adapts itself to start up the connection. This adaptation process can continue, once the connection is started, as the modem compensates for ongoing changes, such as those due to temperature or other environmental factors. Factors that can affect ADSL transmission include the gauge thickness of the copper cable, the distance between the central office and the subscriber and the amount of interference present on the line.
To support bi-directional channels, ADSL modems can allocate the available bandwidth by FDM, where non-overlapping bands are assigned for the downstream and upstream data. DMT, which has now been accepted by ANSI as the standard line code for ADSL transmission, divides an input data stream among several sub-channels, each sub-channel having the same amount of bandwidth but at different center frequencies. Sub-channels can have different bit rates, as discussed below. Using many sub-channels with very narrow bandwidths means the theoretical channel capacity, as calculated according to Shannon's law, can be approached. Generally, DMT was chosen because it is particularly well suited for transmission over copper line at the operating frequency bands. DMT also copes well with the typical noise and impulses that exist in the residential (subscriber) twisted-wire pair environment.
The sub-channels into which a channel is divided, commonly referred to as tones, are quadrature amplitude modulation (QAM) modulated on a separate carrier, commonly called a subcarrier, and the subcarrier frequencies are multiples of one basic frequency. The ANSI standard ADSL system has a theoretical maximum of 256 frequency sub-channels for the downstream data and 32 sub-channels for the upstream, though, in reality, line conditions, interference and other considerations reduce the actual available number of sub-channels. The frequency difference between two successive sub-channels is 4.3125 Khz. In a DSL-Lite or G.Lite system, the number of downstream data streams is halved, eliminating those at the higher frequencies.
As mentioned above, data to be transmitted is QAM modulated so that each sub-channel can transmit multiple bits and bit rates can vary between sub-channels. As the subscriber loops between the central office and a subscriber generally exhibit variations in gain and phase with frequency, each sub-channel can be arranged to carry a different number of bits appropriate for its frequency on the particular subscriber line. By assigning different numbers of bits to different sub-channels, each sub-channel can operate at an optimal, or near optimal, bit rate for the bandwidth available in the subscriber loop. Sub-channels at frequencies where the signal-to-noise ratio is low can have lower numbers of bits assigned to them, while sub-channels at frequencies with higher signal-to-noise ratios can have higher numbers of bits assigned to them, to keep the probability of a bit error constant across the subcarriers.
Generally, the actual user data traffic over a communication link established between two DMT modems is non-constant. The necessary bandwidth, data rate and event frequency can all vary. A data event is a single Protocol Data Unit (PDU) or a cluster of PDUs. In ATM, a PDU is a fixed length cell; in Internet Protocol, a PDU is a variable length IP packet A particular data event can be characterized as isochronous or asynchronous, and both data events can occur simultaneously over different channels, or groups of channels. A regular, or isochronous, data event, such as voice or compressed interactive video information, typically requires a relatively low bandwidth, but is not tolerant of delay greater than approximately 300 msec. “Bursty”, or asynchronous, data events, which are characteristic of interactive human-machine sessions such as world wide web sessions, can occur at random intervals, and can range from a low bandwidth and data rate requirement, such as a keystroke, to a high bandwidth and data rate requirement, such as a JPEG image transfer. In addition, very high bandwidth asynchronous data events, such as large file transfers and network backups, occur infrequently but require significant network resources, both in terms of data rate and bandwidth.
In the interest of conserving power and reducing system cooling requirements at the central office end, it is desirable to operate a DMT modem at a lower power when the data bandwidth is being underutilized. A number of power management states are defined in the current splitterless DMT ADSL (a.k.a. G.Lite or G.992.2) draft recommendation. In a “full on” state (L
0
), the link is fully functional and the linked subscriber and central office modems are capable of delivering the maximum downstream and upstream rates possible under the given loop conditions, given the presence of any simultaneous active POTS devices and service provider restrictions. In the “idle” state (L
3
), the communication link is not active, and requires no power. Both the receiving and central office modems are transmitting idle (zero) signals. An optional “low power” state (L
1
), is also defined, in which the communication link would be operational, but only require enough power to maintain the embedded operations control (EOC) channel and a low-rate data stream. State changes between full on, idle and low power management states are initiated under control of a higher layer function, typically at the application layer, and take on the order of hundreds of milliseconds to complete. As a result, these power management state changes are relatively infrequent when compared to the rate of change of actual user data traffic demands. In addition,
McClennon Robert Scott
Wingrove Michael
Lugo David B.
Nortel Networks Limited
Tse Young T.
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