Multiplex communications – Channel assignment techniques – Messages addressed to multiple destinations
Reexamination Certificate
1999-05-24
2004-06-15
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Channel assignment techniques
Messages addressed to multiple destinations
C370S475000
Reexamination Certificate
active
06751230
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates to the field of data communication and data-over-cable systems. More particularly, the invention relates to a method of broadcasting a message from a head-end or cable modem termination system of a data-over-cable system to a plurality of remotely located cable modems which happen to share a common channel in an upstream direction.
B. Description of Related Art
Cable modem systems for wide area network access, e.g., Internet access, are now being rolled out in selected metropolitan areas of the United States. Basically, these systems provide high-speed data communications over a cable television infrastructure. Background information related to cable modem systems in general is described in the Data-Over-Cable Service Interface Specifications (DOCSIS)—Radio Frequency Interface Specifications, Interim Draft, dated Jul. 24, 1998, issued by Cable Television Laboratories, Inc. This document, known to persons working in the art, is incorporated by reference herein in its entirety.
The basic overall architecture of a data over cable system is shown in FIG.
1
. The system of
FIG. 1
provides a mechanism by which a computer
10
connected to a backbone network
12
(either directly or indirectly by intermediate networks) may communicate with another computer
14
via a cable television infrastructure indicated generally by reference numeral
16
. The cable television infrastructure
16
includes a distribution hub or “head-end”
18
that is connected to the backbone network
12
via a wide area network and a switch or router
20
. A cable system head-end is a central location in the cable television network that is responsible for sending cable signals in the downstream direction. The head-end
18
modulates digital data into analog form and supplies analog signals to a fiber network
22
, which is connected to a plurality of O/E nodes
24
. The O/E nodes
24
convert optical signals in the fiber network
22
to electrical signals for transmission over a coax cable network
26
to a cable modem
28
at the customer's location. The cable modem
28
demodulates the analog signals and extracts the digital data and supplies the data to the customer promises equipment
14
, which, in a typical situation, is a general purpose computer in a home environment.
The head-end
18
includes a cable modem termination system or CMTS
30
. This device provides a network side interface to a wide area network, indicated at
32
, and an RF interface between the cable modem termination system and the cable network in both the downstream and upstream directions, indicated at
34
and
36
. The term “downstream”, as used in the present document, refers to transmission in the direction from the head-end
18
or cable modem termination system
30
to the cable modem
28
at the customer premises. The term “upstream” refers to transmission in the direction from the cable modem
28
at the customer premises to the cable modem termination system
30
.
For transmission in the downstream direction, the CMTS
30
supplies data from the computer
10
to a modulation circuit MOD and to a combiner
38
, where the data is combined with video signals for the cable television system. The combined signals are sent to a transmission module
40
where they are imparted onto the fiber network. In the receiving direction, data from the CPE
14
is received from the fiber network at a receive module
42
, sent to a splitter and filter bank
44
and sent to a demodulation circuit DEMOD in the CMTS
30
. The data is processed by a network termination unit
46
, sent to the switch or router
20
and routed onto the WAN for transmission to the remote computer
10
.
Many cable television networks provide only unidirectional cable systems, supporting only a “downstream” cable data path. A return data path via a telephone network (i.e., a “telephony return”), such as a public switched telephone network provided by AT&T, GTE, Sprint, MCI and others, is typically used for an “upstream” data path. A cable television system with an upstream connection to a telephony network is called a “data-over-cable system with telephony return.” Such a return system is indicated at
48
where the cable modem
28
is also shown connected to the public switched telephone network (PSTN).
An exemplary data-over-cable system with telephony return includes customer premises equipment (e.g., a customer computer), a cable modem, a cable modem termination system, a to cable television network, a public switched telephone network, a telephony remote access concentrator (TRAC
49
in
FIG. 1
) and a data network
12
(e.g., the Internet). The cable modem termination system
30
and the telephony remote access concentrator
49
together are called a “telephony return termination system.”
In a two-way cable system without telephony return, the customer premises equipment
14
sends data packets to the cable modem
28
, which sends the data packets upstream via the cable television network
22
and
26
to the cable modem termination system
30
. Such a system is shown in FIG.
1
. The cable modem termination system
30
sends the data packets to appropriate hosts on the data network
12
. The cable modem termination system
30
sends the response data packets back to the appropriate cable modem
28
.
Like all network devices, cable modems filter at the hardware level based on the media access control (MAC) destination address contained in messages received at the cable modem. The MAC layer in the ISO reference model consists of the lower half of the data link layer as defined in the IEEE 802.3 standard. Generally speaking, the MAC layer contains specifications for frame formats as well as the rules for accessing the physical portions of the network. The MAC destination address is matched against addresses stored in the cable modem to determine if the message is intended for the cable modem. These addresses that are stored include the well-known broadcast, multicast and all stations addresses. For cable modems compliant with the DOCSIS specifications, the stored addresses also include the multicast MAC address of the hexadecimal form 01-E0-2F-00-00-01, which enables the addressing of all cable modems on a MAC sublayer domain.
Within this prior art addressing scheme, a message can be sent from a cable modem termination system
30
to a single cable modem (a “unicast message”), and to all the cable modems on a given MAC sublayer domain (i.e., a multicast message directed to all the cable modems in a given MAC domain), but there is no way to address a given subset of cable modems within the MAC domain. For example, there is no way to associate all cable modems on a given upstream channel with a singe multicast address. This would be a useful capability, as it would (1) conserve downstream bandwidth, (2) conserve CPU processing time in the cable modem termination system, and (3) reduce the time for the message to be acted upon, since a single multicast message could be sent quickly to a group of cable modems on an upstream channel, instead of sending a unicast message to each cable modem as would be required by prior art methods. This latter capability of sending a single multicast message to a group of cable modems on an upstream channel rather than sending a series of unicast messages to each cable modem, is critical to systems implementing redundancy or noise avoidance algorithms to facilitate increased reliability, since data links can be lost if timing or nose/power limits are exceeded. However, to the inventors knowledge, the prior art has not provided such a capability.
Thus, in a principal aspect of the invention, improvements in a data-over-cable system are described below which allow for a cable modem termination system or other point in the cable network to send a single multicast message to a group of cable modems on a given upstream channel.
SUMMARY OF THE INVENTION
In a first aspect of the invention, an improvement to a cable modem for a data-over-cable system is provid
Akgun Ali
Vogel Mark O.
3Com Corporation
Hsu Alpus H.
McDonnell Boehnen & Hulbert & Berghoff LLP
Nguyen Toan
LandOfFree
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