Data synchronization unit for a headend

Details Data synchronization unit for a headend Data synchronization unit for a headend

1997-08-25

2001-03-27

Nguyen, Chau (Department: 2663)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via frequency channels

C370S516000

Reexamination Certificate

active

06208665

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a data synchronization unit for a headend. More particularly, the present invention is directed to a data synchronization unit for a headend in a TDMA-based communication system that transmits timing information that is multiplexed with other data streams.
BACKGROUND OF THE INVENTION
Connecting a computer to the Internet and other public networks is an increasingly popular way for home computer users to receive multimedia information and communicate with other computer users. As a result, higher and higher data bandwidth is needed by home computer users. Many efforts are being made to use the already existing communication infrastucture to provide high-speed access to network services. The plurality of existing digital TV Hybrid Fiber/Coaxial (“HFC”) cable networks is one of the most suitable communication mediums available to home users for connecting to the Internet.
An HFC headend is the originating point of a signal in an HFC cable network, and it is also responsible for maintaining synchronization among the users to ensure that they do not create contentions in the distributed system when attempting to send data to the headend via a shared transmission medium. The synchronization is difficult because it must account for variable and differing data propagation delays.
FIG. 1
is a block diagram that illustrates a typical headend for providing Internet access to a plurality of user computers, referred to as “client computers”. In
FIG. 1
, a headend
10
is coupled to an Internet access point
12
. Headend
10
transmits a packet data stream
32
over a transmission medium
30
to client computers
21
-
24
. In the embodiment shown in
FIG. 1
, headend
10
is adapted for a HFC cable system, so transmission medium
30
is an optical and coaxial cable, and client computers
21
-
24
each include cable modems for transmitting and receiving a data stream.
Headend
10
transmits (broadcasts) payload and auxiliary data streams which can be received by the plurality of client computers
21
-
24
. To provide an upstream channel (i.e, a channel that permits data to be transmitted from client computers
21
-
24
to headend
10
) headend
10
also remotely controls how client computers
21
-
24
access and transmit on transmission medium
30
. Typically, Time Division Multiple Access (“TDMA”) is used to provide the upstream channel.
With TDMA, each client computer transmits to headend
10
only during a specific time interval called a time slot. Client computers
21
-
24
keep track of time slot synchronization by maintaining and adjusting their internal clock reference, based on Media Access Control (“MAC”) information broadcast by headend
10
. The time slots are assigned to the client computers by a MAC function within headend
10
.
As a result, client computer slot markers are synchronized to headend
10
's master slot markers, which are based on a common system clock provided by the MAC function. Headend
10
transmits information about the nominal position and current sequential number of the slot markers in a special packet called a Timing Information Element (“TIE”) packet.
When received by client computers
21
-
24
the TIE packet provides a time stamp of when it was generated based on the system clock. Each client computer
21
-
24
can synchronize its own slot marker from the information in the TIE packet.
Headend
10
, which is a “data-only” headend, receives network data from a network data source
14
, modulates a radio frequency (RF) carrier with a modulator
34
, and transmits the resultant signal to the transmission medium
30
. The network data, which is generated by Internet
12
, is then received by client computers
21
-
24
.
Network data source
14
generates packet data stream
32
and includes the system clock
20
, a TIE packet generator
18
that generates TIE packets, and a data generator
16
that formats data packets received from Internet
12
. Modulator
34
converts packet stream
32
generated by network data source
14
into the appropriate form (e.g., radio frequency) so that it can be transmitted on transmission medium
30
.
For the upstream channel, additional functions performed by network data source
14
include upstream signal demodulation and reformatting data into a stream suitable for forwarding to Internet
12
. The receive side of headend
10
, which receives data streams transmitted from client computers
21
-
24
, is not shown in FIG.
1
.
When a TIE packet is transmitted to client computers
21
-
24
from headend
10
, the TIE packet arrives at different times to each client computer
21
-
24
due to different propagation delays among the client computers. For example, client computer
24
will receive a TIE packet at a later time than client computer
21
because it is located at a further distance from headend
10
. However, the delay to each client computer
21
-
24
is fixed because the delay due to modulator
34
, transmission medium
30
, and each client computer
21
-
24
does not change. Therefore, the delay can be measured and factored.
The delay can be determined by each client computer by comparing the TIE packet with a synchronization symbol sent from headend
10
to all client computers. The client computers then compensate for the delay when performing slot marker synchronization. A known procedure, referred to as “ranging”, is used to adjust the synchronization in client computers
21
-
24
to compensate for the constant delay of any origin.
FIG. 2
is a block diagram that illustrates a typical multi-purpose headend. In a multi-purpose headend, the data service and regular audio and video services may coexist on the same frequency channel if audio and video services are digital (e.g., as defined by the Digital Video Broadcaster's consortium (“DVB” under European Telecommunication Standards Institute ETS 300 429)) This allows bandwidth to be used economically and provides rich content services to subscribers (client computers). The network data stream may be either used to enhance the video stream (i.e., to provide additional on-screen information, to provide a database that may be accessed while watching sports, etc.), or used independently.
Multi-purpose headend
50
shown in
FIG. 2
includes a video data source
40
that generates a video data stream and an audio data source
42
that generates an audio data stream. The data streams generated by video data source
40
and audio data source
42
typically are transmitted on the same transmission medium
30
as the data stream generated by network data source
14
. One example of headend
50
is a headend that provides cable television and Internet access over the same coaxial cable.
In order to combine the three data streams generated by headend
50
, the data streams are multiplexed together by a multiplexer
46
before being sent to modulator
34
. However, multiplexer
46
multiplexes input data streams on a packet-by-packet basis and has to account for potentially variable data rates for each data source. Therefore, multiplexer
46
introduces a variable (and in most cases unknown) propagation delay due to the internal data buffering required to synchronize incoming data streams. The time delay for the TIE packets generated by network data source
14
to reach the client computers is no longer a fixed delay as it is for headend
10
of FIG.
1
. Therefore, the ranging procedure cannot properly compensate for the delay when performing slot marker synchronization, and headend
50
cannot maintain the proper TDMA synchronization with the client computers.
Based on the foregoing, there is a need for a method and apparatus for allowing a headend which transmits a TDMA-based data stream multiplexed with other data streams to maintain TDMA synchronization with client computers.
SUMMARY OF THE INVENTION
One embodiment of the present invention is a data synchronization unit that provides Time Division Multiple Access (TDMA) synchronization for a headend. The headend includes a network data source and tra

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