System and method to reduce transmit wander in a digital...

Details System and method to reduce transmit wander in a digital... System and method to reduce transmit wander in a digital...

2000-12-22

2004-11-02

Chin, Stephen (Department: 2634)

Pulse or digital communications

Synchronizers

C375S355000, C375S293000

Reexamination Certificate

active

06813325

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to data communications, and more particularly, to a system and method for achieving reduced timing wander in a digital subscriber line (DSL) communication system.
BACKGROUND OF THE INVENTION
In the field of data communications a transceiver, or modem, is used to convey information from one location to another. Digital subscriber line (DSL) technology now enables DSL transceivers to more rapidly communicate data than previously possible with purely analog modems. DSL transceivers communicate by modulating a baseband signal carrying encoded digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a multi-dimensional multi-level signal space constellation and slicing the received constellation to recover the transmitted information.
The constellation can include both analog and digital information or only digital information.
In the above mentioned communications environment, a central office DSL transceiver is located at a telephone company central office location. Connected to the DSL transceiver via a conventional copper wire pair is a suitably configured remote DSL transceiver. The remote transceiver resides at a location commonly referred to as a customer premise. Before the central office transceiver can exchange information with the remote transceiver, clock timing and synchronization between the central office transceiver and a network master clock should be established.
Timing and synchronization are fundamental to any digital transmission and switching network. In a digital transmission system, timing is encoded with the transmitted signal using a network master clock, such as a T1 or E1 clock as a reference clock. As such, the central office transceiver must recover system timing and synchronization from this system clock. Once frequency synchronization between the central office transceiver and the network clock is achieved, the central office transceiver can identify frame boundaries of downstream data signals designated for further transmission to the remote transceiver. In addition, the central office transceiver can identify frame boundaries of upstream data signals received from the remote transceiver that may be designated for further transmission to other network connected devices.
In the aforementioned communications environment, synchronization is provided in a master-slave relationship such that the network timing (e.g., a T1 clock) is the master allowing it to provide timing information to all the slave data transmission systems connected to the network. Each remote transceiver connected to the network must be synchronized to the network system clock as provided by the central office transceiver.
In order to achieve higher data rates with a fixed distance or with a given non-rate adaptive DSL transceiver technology, two or more DSL lines may be combined. By way of example, high-speed DSL (HDSL) technology uses two pairs of twisted copper wire, HDSL transceivers, multiplexers and demultiplexers at each end of a communication link to provide T1 capacity service over two pairs of twisted copper conductors commonly used in local subscriber loops within the PSTN. The European version of HDSL binds three pairs of twisted copper conductors and their related transceivers, multiplexers, and demultiplexers to provide E1 capacity service.
The prior art HDSL link illustrated in
FIG. 1
is offered by way of example to highlight various interface equipment that may be used to provide a T1 capacity link between a PSTN central office (CO) and a customer premise (CP). In this regard,
FIG. 1
illustrates a basic HDSL network link architecture. As illustrated in
FIG. 1
, a HDSL network link
10
may comprise equipment located within a CO
20
, equipment located within a CP
40
, and HDSL interface equipment
30
as required within each location to transfer data to and from an ATM switch (not shown). More specifically, the central office
20
may comprise a plurality of trunk line interfaces
21
,
23
, and
25
, herein labeled analog trunk card, digital trunk card, and optical trunk card, respectively; a PSTN digital switch
22
; and a plurality of HDSL termination units—central office (HTU-C)
24
a
,
24
b
,
24
c
, . . ., and
24
x
. As illustrated in
FIG. 1
, each HTU-C
24
a
,
24
b
,
24
c
, . . . , and
24
x
may be coupled via two twisted pair telephone transmission lines
31
a
,
31
b
to a dedicated HDSL termination unit—remote (HTU-R)
44
(one shown for simplicity of illustration).
As also illustrated in
FIG. 1
, the combination of the HTU-C
24
c
, the two twisted pair telephone transmission lines
31
a
,
31
b
, and the HTU-R
44
may comprise the HDSL interface equipment
30
. As further illustrated in
FIG. 1
, the CP
40
may comprise a customer interface
46
and customer premise equipment
48
which may contain one or more computing devices (not shown).
It is significant to note that downstream and upstream data transmissions that are transmitted across the HDSL network link
10
of
FIG. 1
must be processed at the HTU-Rs
44
and the HTU-Cs
24
in order to ensure that data transmissions are inverse multiplexed and reconstructed into their original configuration. Each of the HTU-Rs
44
and the HTU-Cs
24
may further comprise a transceiver and a mapper (both not shown). At one end of the HDSL communications network
10
, a first mapper may be used to inverse multiplex or distribute a data transmission across multiple transmit media (i.e., the twisted pair telephone transmission lines
31
a
,
31
b
). At the opposite or receiving end of the HDSL communications network
10
, a second mapper may be used to multiplex or reconstruct the original data transmission. By way of example, a downstream data transmission may be inverse multiplexed such that a portion of the data is transmitted via the HTU-C
24
c
across a first twisted pair telephone transmission line
31
a
with the remaining portion of the data transmission sent via a second twisted pair telephone transmission line
31
b
. After the first and second portions of the data transmission are received and reconstructed by the HTU-R
44
, the first and second portions of the original data stream may be multiplexed before being forwarded to the customer interface
46
and the CPE
48
. Often the customer interface
46
is implemented with a router having a port coupled with one or more HTU-Rs
44
and or other network interface devices.
A common technique for achieving timing synchronization between the network clock and the central office transceiver is based upon the use of an external framer, which performs a bit-stuffing operation. In this arrangement the aggregate bit stream has a higher data rate than the input data rate from the network. This data rate relationship accommodates the additional stuffing and framing bits. Bits are stuffed (inserted) or deleted (removed) from the incoming data stream until a clock rate derived from the incoming data stream is equal to that of the input data rate from the network. This bit stuffing operation permits the transceiver to derive a local clock with a frequency that tracks the frequency of the network clock.
Presently, the add/delete or bit-stuffing mechanism synchronizes a customer interface and a transmit carrier by determining the relative position of a DSL frame reference point to a periodic customer reference point and responding accordingly. When the DSL frame reference point leads the customer reference point, a timing field in the frame is set to 4 bits. Otherwise, the timing field is set to 0 bits. The present bit-stuffing mechanism generates a significant wander in the DSL frame with respect to the customer reference point. The wander is not removable. Accordingly, it is desired to provide a system and method that efficiently and accurately reduces timing reference wand

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