Multiplex communications – Fault recovery
Reexamination Certificate
1999-10-19
2004-11-02
Marcelo, Melvin (Department: 2663)
Multiplex communications
Fault recovery
C370S401000, C725S111000
Reexamination Certificate
active
06813238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cable modems and cable-modem systems, and particularly relates to a cable modem and a cable modem system which provide a connection to a LAN via a cable television transmission line.
2. Description of the Related Art
A cable television system (hereinafter referred to as a CATV system) distributes broadcast signals from a headend, which is a center of a CATV provider, to subscribers via trunk lines and branch lines, which are configured in a tree structure or in a star structure. The trunk lines and branch lines are used for transmission of broadband broadcast signals, and, thus, are implemented by using a coaxial cable or an optical fiber cable. This renders a CATV system a superior broadband transmission capacity.
In recent years, the Internet has made a significant market progress. Against this background, a superior broadband transmission capacity of a CATV system has been attracting attention, and efforts are underway to utilize CATV transmission lines as an access network to the Internet. A cable-modem system, in particular, is increasingly used in practice, connecting subscriber households to the Internet via a LAN interface utilizing CATV lines.
FIG. 1
is an illustrative drawing showing a configuration of a cable modem system. A description of the cable modem system will be given with reference to FIG.
1
.
The cable modem system of
FIG. 1
includes a CATV center
10
, a CATV delivery facility
20
, and a subscriber household
30
. The CATV center
10
includes a center device
11
, a headend device
12
, photoelectric converters
13
, a router
14
, a server
15
, personal computers
16
, and bridges
17
. The center device
11
is connected to an intra-CATV-center LAN at one end and to the headend device
12
at the other end. The headend device
12
transmits video signals and the like after demodulation and mixing of signals. The router
14
provides a connection to the Internet. The server
15
stores data therein to provide various services. The bridges
17
are used for connecting different LANs. The bridges
17
monitor packets, and reduces the number of packets in the LANs via a filtering function to dispose of unnecessary packets, thereby insuring transmission efficiency.
The CATV delivery facility
20
includes optical cables
21
, photoelectric converters
22
, coaxial cables
23
, amplifiers
24
, and tap-offs
25
. Here, the tap-offs
25
are nodes or branching devices that connect between the CATV delivery facility
20
and the subscriber household
30
.
The subscriber household
30
includes cable modems
31
a
through
31
c
, terminal devices
32
a
through
32
d
such as personal computers, and a bridge
17
. The terminal device
32
a
, operating standalone, is connected to the CATV delivery facility
20
via the cable modem
31
a
. Further, a subscriber-household LAN
33
including the terminal devices
32
b
through
32
d
and the bridge
17
is connected to the CATV delivery facility
20
via the cable modems
31
b
and
31
c.
When a user accesses the Internet via the terminal device
32
c
, the terminal device
32
c
supplies packets to the cable modem
31
b
or
31
c
, where the packets are converted into RF signals to be supplied to the CATV delivery facility
20
via the tap-offs
25
. The RF signals attenuate as they propagate through the CATV delivery facility
20
, so that the amplifiers
24
are provided to boost signal levels. The RF signals are converted into optical signals by the photoelectric converters
22
, and the optical signals are then sent via the optical cables
21
.
The photoelectric converters
13
convert the optical signals received from the optical cables
21
into electrical signals, and supplies reconstructed RF signals to the center device
11
via the headend device
12
. The center device
11
selects a frequency band that is used as an uplink band for the cable modems
31
a
through
31
c
, and converts the RF signals of this frequency band into packet signals. The reconstructed packets are sent to the Internet via the router
14
. When packets are supplied from the Internet, on the other hand, transfer of the packets is carried out by following steps of the same procedure as described above in a reversed order. In this manner, the user can access the Internet by using the cable modems
31
a
through
31
c.
Unfortunately, there is a case in which a mistake in wire connections ends up causing the subscriber-household LAN
33
to create a closed loop, trapping packets inside. Such a loop may leads to a shutdown of the entire cable-modem system. In consideration of this, the bridge
17
is typically provided with a function to detect and sever a loop.
The bridge
17
in a conventional LAN system exchanges BPDU messages (packets) with other bridges by attaching its own MAC (media access control) address to these packets for the purpose of automatically detecting a loop. When a loop is detected, a bridge constituting part of the loop closes a relevant port thereof in order to sever the loop. This scheme is called a spanning tree scheme.
A cable-modem system is subject to standardization, and is required to serve as a bridge in compliance with standards set forth in the United States. A typical cable-modem system, therefore, adapts the spanning tree scheme in order to provide a function to detect and sever a loop in the same fashion as does a bridge of a conventional LAN system.
FIG. 2
is an illustrative drawing showing a logical configuration of a cable-modem system such as that shown in FIG.
1
. With reference to
FIG. 2
, a description will be given with regard to a function to detect and sever a loop in a cable-modem system.
In
FIG. 2
, a center device
40
and cable modems
43
a
through
43
e
are each configured to serve as a bridge, and, for this purpose, are each provided with a filtering database
44
. The filtering database
44
stores a definition as to whether to transit or dispose of a packet with respect to each MAC address. Such a definition is temporarily stored for a predetermined time period.
In the cable-modem system of
FIG. 2
, the center device
40
serves as a bridge BRIDGE
0
, and the cable modems
43
a
through
43
n
serve as bridges BRIDGE
1
through BRIDGEn, respectively. The center device
40
has a port PORT
0
thereof connected to the Internet via a router
41
and a port PORT
1
thereof connected to a port PORT
0
of each of the cable modems
43
a
through
43
n
. A port PORT
1
of each of the cable modems
43
a
through
43
n
is connected to a terminal device
45
either directly or via a hub
46
and/or a bridge
47
.
As described above, the center device
40
and the cable modems
43
a
through
43
e
are configured to serve as logically separate bridges, and are required to have a function to detect and sever a loop as such a loop may become a problem when implementing a LAN. Namely, each of the center device
40
and the cable modems
43
a
through
43
e
is so designed as to perform the spanning tree scheme.
Another configuration that can take care of the problem of a closed loop in a cable-modem system is to regulate communications between the cable modems
43
a
through
43
e
by requiring all the packets from the cable modems
43
a
through
43
e
to always make a transit at the center device
40
. This can prevent formation of a closed loop.
FIG. 3
is an illustrative drawing showing another example of a logical configuration of a cable-modem system. The configuration of
FIG. 3
has advantages in reduction of the load and conservation of resources.
The cable-modem system of
FIG. 3
includes a center device
51
and cable modems
52
a
through
52
n
, all of which together constitute a logical bridge
50
. The logical bridge
50
is equivalent to a bridge that has a port PORT
0
on the side of the intra-CATV-center LAN and ports PORT
1
through PORTn on the side of the subscriber-household LAN at the end of the cable modems
52
a
through
52
n.
FIG. 4
is a block diagram showing a con
Otani Keiichi
Yamamoto Takaya
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Marcelo Melvin
LandOfFree
Cable modem and cable modem system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Cable modem and cable modem system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cable modem and cable modem system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3327636