Interactive video distribution systems – Video distribution system with upstream communication – Having link to external network
Reexamination Certificate
1998-07-30
2001-05-08
Faile, Andrew (Department: 2611)
Interactive video distribution systems
Video distribution system with upstream communication
Having link to external network
C725S117000, C725S118000
Reexamination Certificate
active
06230326
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of networking and more specifically to provide information over a cable modem network.
2. Description of the Related Art
In a cable modem network, information is carried over a cable from a cable modem termination system (CMTS) to a plurality of cable modems (CMs). The CMs are coupled in communication with the CMTS to receive information on a so-called “downstream” channel and to communicate information to the CMTS on a so-called “upstream” channel. Particular characteristics (e.g., frequency, power levels, etc,) of the upstream channel are determined at the time the CM is initialized.
FIG. 5
illustrates an implementation in which a single downstream channel
501
is utilized in conjunction with four upstream channels
511
-
514
. The four upstream channels
511
-
514
may be, for example, separate fibers serving four geographically separate communities of CMs
521
-
524
. The Radio Frequency Interface Specification, Data-over-cable Service Interface Specifications, SP-RFI-I03-980202 available from the Cable Television Laboratories, Inc. (hereinafter, the “MCNS specification”) describes a basic initialization process for CMs. Initialization of CMs in an implementation as shown in
FIG. 5
is described in greater detail in Appendix G of the MCNS specification.
As is discussed in the MCNS specification, the CMTS
531
periodically broadcasts upstream channel descriptors (UCDs) on the downstream channel
501
in order to describe characteristics of upstream channels to the CMs. As part of the initialization process, a CM waits for a UCD in order to obtain transmission parameters for the upstream channel. When the CM receives a UCD and determines that the transmission parameters are suitable, it waits for a SYNC message from the CMTS in order to achieve time synchronization with the CMTS. It then waits for a bandwidth allocation map for the selected channel. The allocation map allocates periods in the bandwidth to various types of data transmissions. One of these may be an “initial maintenance” request. It is during this initial maintenance request time slot that a new CM may transmit what is termed a “ranging request” which is used to determine network delay. In response to the ranging request, the CMTS transmits a “ranging response” which, may among other parameters, include a parameter redirecting the CM to an appropriate upstream channel. As is discussed in Appendix G to the MCNS specification, this topology imposes a number of constraints including:
All of the upstream channels
511
-
514
must operate at the same frequency because, as a result of the CM essentially choosing a UCD at random (typically, the first UCD received during the initialization cycle), it would be transmitting at the wrong frequency if this constraint were not imposed.
All of the upstream channels
511
-
514
must operate at the same symbol rate because the CMTS would otherwise be not be aware of the symbol rate of the transmissions and would be unable to demodulate the ranging requests;
All of the initial maintenance opportunities must be aligned because the CMTS must be prepared to receive a ranging request at the time designated in the allocation map; and
All initial maintenance opportunities must use the same burst characteristics so that the CMTS can demodulate the ranging request.
These constraints raise a number of issues. For example, when there is excessive ingress (noise and RF interference) on a particular channel, it is desirable to change the frequency of the channel. However, it may be that the ingress is only occurring on one of channels
511
-
514
at a given frequency because of particular characteristics (environmental, etc.) of that channel. However, as a result of the first constraint, if the frequency of one channel is changed, the frequency of all channels must be changed. Thus, not only is the overhead of needing to change the frequency of multiple upstream channels imposed by this constraint, but the likelihood of experiencing ingress at particular frequency is increased because ingress on any channel requires the making a change.
Thus, what is needed is a method and apparatus relieving as many as of the above-recited constraints as possible. It would be desirable if such a method and apparatus allowed operation of a cable modem plant in accordance with the MCNS specification.
SUMMARY OF THE INVENTION
A method and apparatus for initializing cable modems which overcomes certain constraints imposed by prior implementations is described. In the described embodiment, the cable modem termination system transmits UCDs which include management channel information (and the associated maps) only for a management channel and not for normal data channels. The cable modem perform initialization over the management channel and are then transferred to a data channel for operation.
In the described embodiment, an extra (N+1) receiver is used for communication on the management channel.
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Cable Television Laboratories, Inc., Ref: Radio Frequency Interface Specification. SP-RFI-104-980724, Mar. 1997.
O'Neil, Jr. Eugene E.
Sawyer Wilson
Ulm John
Unger J. David
Blakely , Sokoloff, Taylor & Zafman LLP
Faile Andrew
Nortel Networks Limited
Tran Hai V.
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