Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1997-08-18
2001-03-13
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S496000, C370S522000
Reexamination Certificate
active
06201818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates generally to a bi-directional data communication link in a cable television system, and particularly to an increased downstream data transmission capacity in a system having a control channel of the so-called out-of-band type for transmitting information from the central equipment to the terminals.
2. Discussion of Related Art
A cable television system is usually a tree-like distribution network having at the root a main amplifier or the so-called head-end of the operator which controls the network. More generally the main amplifier can be called the central equipment. The distribution lines are branched in a tree-like fashion towards the terminals of the subscribers, of which there can be even hundreds of thousands under the same central equipment. In order to prevent signal weakening and to reduce interference the distribution lines have distribution amplifiers, repeaters and other devices known per se.
Plans to change the cable television systems from unidirectional distribution networks into bi-directional data transmission networks have been presented recently. Then the data transmission direction from the central equipment to the terminals is generally called downstream, and the opposite direction is called upstream. In addition to the high capacity downstream channel the system comprises at least one upstream channel, through which the terminals can transmit information towards the central equipment, and a downstream control channel with a relatively low capacity, through which the central equipment controls the use of the upstream links. The control channel can comprise cyclically repeating fields which the central equipment multiplexes into the digital video picture or other signal transmitted on the main channel by utilizing its frame structure. Such a control channel is of the so-called in-band control channel type. In another embodiment the control channel is located on a frequency band of its own, whereby it is a so-called out-of-band control channel. This invention relates primarily to systems where the control channel is of the out-of-band type.
The invention can be applied e.g. in digital video systems known per se, i.e. DAVIC (Digital Audio Visual Council) and DVB (Digital Video Broadcasting). Definitions relevant to the invention are presented in the publications “DAVIC 1.0 specification part 08; Lower layer protocols and physical interfaces, December 1995”, “DAVIC 1.0 corrigenda part 08; Lower layer protocols and physical interfaces, Edited version after New York meeting, Rev. 2.1, June 1996”, “DAVIC 1.1 specification baseline document #18. Cable modem baseline document, Rev. 2.0, as of New York meeting, June 1996” and “ETSI draft specification prETS 300 800; Digital video Broadcasting (DVB); DVB interaction channel for Cable TV distribution system (CATV). TM
1640
Rev. 4, June 1996”. The cable television system presented in the publications can be based on coaxial cables or at least partly on optical fibers, whereby in the last mentioned case it is also called an HFC network (Hybrid Fiber Coax).
FIG. 1
shows the proposal in the publication prETS 300 800 for dividing the frequencies in the DVB system. The dimensions on the horizontal axis in the figure are illustrative, and the vertical axis only shows which signals are directed from the central equipment to the terminals (DS, upwards in the figure) and which signals are directed in the opposite direction (US, downwards in the figure). The frequency band
101
extends from about 50 MHz to almost 900 MHz, and typically it is divided into channels
102
with a typical width of 6 to 8 MHz, of which only three are shown for the sake of clarity. Each channel carries one QAM modulated (Quadrature Amplitude Modulation) signal, which can contain one or more digital video signals in the MPEG-TS format (Motion Picture Experts Group—Transport Stream) or other information requiring a high transmission capacity. The frequency band
103
extends from 70 to 130 MHz, and it contains channels
104
with a width of one or two MHz, each carrying one QPSK modulated (Quadrature Phase Shift Keying) control channel. Also here only three channels are shown for the sake of clarity. The frequency band
105
extends from 300 to 862 MHz, and its contents corresponds to that of the frequency band
103
. In the figure the frequency band
106
extending from 5 to 65 MHz is reserved for the upstream links, and it contains channels
107
of which the figure shows only three, each having a width of 200 kHz, 1 MHz or 2 MHz. It is planned to use the QPSK modulation also on these upstream links. The object is to use QPSK modulation also on the upstream links. However, the new definitions for the DAVIC system also provides for the use of QAM modulation on the low capacity channels.
The communication on each upstream channel is divided into nine cyclically repeated time slots. The central equipment determines how these time slots are used, so that some of the time slots can be used for ranging, which aims at the measurement and compensation of transfer delays, and some of the time slots can be freely used by the terminals in a kind of reservation contest, and according to a particular reservation list some of them are assigned to be used by terminals which have made reservations, and some of the time slots are used according to a time schedule, which guarantees a certain regular data transmission capacity for a link. In the DAVIC system the central equipment transmits on one downstream control channel information regarding the use of up to eight upstream channels. The transmission of this information on the downstream control channel is described in more detail below.
According to
FIG. 2
the transmission on the control channel comprises SL-ESF frames
108
(Signalling Link Extended Superframe). The length of one SL-ESF is 4632 bits and it is divided into 24 frames of 193 bits. In
FIG. 2
the frames are numbered sequentially from 1 to 24, and further one frame is enlarged and marked by the reference numeral
109
. At the beginning of each frame there is a so-called overhead bit
110
, which is followed by the payload section
111
comprising 192 bits. The significance of the overhead bit depends on which frame in the SL-ESF is considered. In the SL-ESF frames with the sequential number 4, 8, 16, 20 and 24 the overhead bit has a fixed value and functions as a frame alignment bit. Correspondingly, when the overhead bits in the frames 2, 6, 10, 14, 18 and 22 are placed in row they form a CRC checksum, which represents the bit contents of the previous SL-ESF. In every second frame the overhead bit starting from the frame number 1 is a so-called M counter, which indicates the timing of the time slots in the upstream channel controlled by this control channel.
The payload of the SL-ESF is obtained by writing the payload sections of all frames in sequence, beginning with the payload section of frame number 1 and ignoring the overhead bits. The SL-ESF payload is usually written in the form of the table below:
Row No.
2 bytes
53 bytes
2 bytes
1 byte
1 byte
1
R1a
R1b
ATM cell
RS
2
R1c
R2a
ATM cell
RS
R2b
3
R2c
R3a
ATM cell
RS
4
R3b
R3c
ATM cell
RS
R4a
5
R4b
R4c
ATM cell
RS
6
R5a
R5b
ATM cell
RS
R5c
7
R6a
R6b
ATM cell
RS
8
R6c
R7a
ATM cell
RS
R7b
9
R7c
R8a
ATM cell
RS
10
R8b
R8c
ATM cell
RS
T
T
In the table the rows 1, 3, 5, 7 and 9 each have 57 bytes, the rows 2, 4, 6 and 8 each have 58 bytes, and the row 10 has 59 bytes. When the bytes Rxa-Rxc (where x is one of the integers 1 to 8) are placed one after another they form a field with the length of 24 bits, which contain information regarding the upstream channel defined by the integer x. If the downstream channel controls only one upstream channel, then the bytes, except the bytes R1a, R1b, R1c, are insignificant. Of said 24 bits the first bit instructs, when required, the terminals to measure and compensate for the time delays, the next six bits indicate the use of the time slots in the upstream channel,
Kokkinen Heikki
Pihlaja Juha
Hyun Soon-Dong
Nguyen Chau
Nokia Technology GmbH
Ware, Freesola, Van Der Sluys & Adolphson LLP
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