Multiplex communications – Fault recovery – Bypass an inoperative channel
Reexamination Certificate
1997-12-05
2001-03-13
Rao, Seema S. (Department: 2732)
Multiplex communications
Fault recovery
Bypass an inoperative channel
C370S537000, C370S907000, C359S199200, C714S004110
Reexamination Certificate
active
06201788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a transmission device and a communication system, and more particularly to an optical communication system employing a synchronous digital hierarchy and a transmission device suitable for such an optical communication system.
An optical communication network has been practically used as means for providing broadband services in which a variety of data on telephone, facsimile, images and so on is integrated. The user
etwork interface in the optical communication network has been internationally standardized, and is known as a Synchronous Digital Hierarchy (SDH), as defined in the CCITT recommendations G707, G708 and G709, the disclosure of which is hereby incorporated by reference. A network which conforms to the SDH has been practically used as SONET (Synchronous Optical NETwork) in the North America.
2. Description of the Prior Art
First, a description will be briefly given of the SONET. The SONET is described in, for example, William Stallings, “ISDN and Broadband ISDN, Macmillan Publishing Company, 1992, pp. 546-558.
In the SONET, a multiplexed optical carrier (OC) is transmitted. The transmission device converts the optical signal (carrier) into an electric signal and vice versa. The electric signal is called a synchronous transport signal (STS). The basic bit rate of the SONET is 51.84 Mbps. The optical carrier having the above basic bit rate is expressed as OC-1. Generally, an optical carrier or signal is expressed as OC-N where N (optical carrier level N) is an integer, and a corresponding electric signal is expressed as STS-N (synchronous transport carrier level N). For example, the optical carrier OC-12 is an optical carrier or signal having a bit rate of 622.080 Mbps (=12×51.84 Mbps). In the SONET, signals having bit rates which are integer multiples of the basic bit rate. The optical carrier OC-12 is obtained by multiplexing 12 STS-1 signals at the byte level to thereby generate an STS-12 signal and by converting the STS-12 signal into an optical signal. Generally, the multiplexing of STS-N signals employs a byte-level interleave process.
It will be noted that the STS-3 in the SONET corresponds to a synchronous transport module STM-
1
in the SDH. Similarly, the STS-12 corresponds to the STM-
4
.
The signal STS can be obtained by, for example, sequentially multiplexing digital signals having lower bit rates, such as DS-0 (64 Kbps), DS-1 (1.5 Mbps), DS-2 (6.3 Mbps) and DS-3 (45 Mbps).
FIG. 1
is a block diagram showing the outline of a network of the SONET. Electric signals from terminals
1
and
2
are respectively multiplexed by transmission devices
3
and
7
, and resultant multiplexed signals are converted into light signals, which are then sent to transmission paths
8
formed of optical fiber cables. Repeaters
4
,
5
and
6
are provided in the transmission paths
8
. Particularly, the repeater
5
has a function of terminating the optical signals (the above function is called an add/drop function). As shown in
FIG. 1
, terms “section”, “line” and “path” are defined in the SONET. The section corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater. The line corresponds to an optical transmission part between transmission devices, between repeaters or between a transmission device and a repeater, each having the terminating function. The path indicates the end-to-end optical transmission part.
FIG. 2A
is a diagram showing the frame format of the signal STS-1. As shown in
FIG. 2A
, the signal STS-1 consists of 810 octets, and is transferred every 125 &mgr;s. The 810 octets consists of nine rows arranged in a matrix formation, each of the rows consisting of 90 octets. In other words, the signal STS-1 has a 9×9 matrix formation. The first three columns (three octets×nine rows) forms an overhead in which a variety of control information concerning transmissions. The first three rows of the overhead forms a section overhead, and the remaining six rows forms a line overhead. The control information forming the overheads is also referred to as overhead information.
FIG. 2B
is a diagram showing the frame format of the signal STS-3. In the SDH, a new format is not created during the hierarchically multiplexing operation. That is, the signal STS-3 can be formed by simply byte-multiplexing the signals STS-1 including the headers thereof without forming a new header specifically directed to the signal STS-3.
FIG. 3A
shows the section overhead and the line overhead, and
FIG. 3B
shows the path overhead. The bytes forming these overheads are well known, and a description thereof will be omitted here.
FIG. 4
is a block diagram of a practical SONET system. Transmission devices
10
A,
10
B,
10
C and
10
D, each capable of operating at a highest bit rate, are connected in a dual loop (ring) formation by means of optical fiber cables
11
1
and
11
2
. The dual loop formation facilitates to the flexibility and expansibility of constructing the system. As will be described later, reference numbers
20
A-
20
D indicate transmission devices according to the present invention.
A transmission device having a bit rate equal to or lower than that of the highest bit rate can be connected to each of the transmission devices
10
A-
10
D. In the case of
FIG. 4
, transmission devices
12
a
,
12
b
,
12
c
,
12
d
, . . . , each having a bit rate lower than that of the transmission device
10
A are connected to the transmission device
10
A. The transmission device
10
A multiplexes signals sent by the transmission devices
12
a
-
12
d
and receives via optical fiber cables
13
a
,
13
b
,
13
c
and
13
d
, and sends a multiplexed optical signal to either the transmission device
10
B and
10
D or both thereof. In
FIG. 4
, for the convenience sake, one of two input/output sides of each of the transmission devices
10
A-
10
D is called an east side, and the other side is called a west side. For example, the transmission device
10
D is located at the east side of the transmission device
10
A, and the transmission device
10
B is located at the west side thereof.
Although not shown in
FIG. 4
, transmission devices having a bit rate lower than those of the transmission devices
12
a
-
12
d
can be connected thereto by optical fiber cables or electrically conductive cables. Signals from terminals such as telephone sets, facsimile machines and personal computers are multiplexed in accordance with a given hierarchy, and multiplexed optical signals are transferred via the transmission devices
10
A-
10
D. In practice, the transmission devices
10
B and
10
D, for example, may be regenerators (repeater devices).
As shown in
FIG. 5
, a network can be constructed by combining a plurality of loops. In
FIG. 5
, transmission devices
10
E and
10
F form a loop together with the transmission devices
10
A and
10
D.
The hierarchy employed when the transmission devices
10
A-
10
D transmit OC-48 light signals is as shown in FIG.
6
. Each of the transmission devices
10
A-
10
D transmits an OC-48 light signal, which corresponds to an STS-48 electric signal having 48 multiplexed channels. The OC-48 light signal can be produced by, for example, multiplexing four OC-12 light signals from the transmission device
12
a
or the like. Each OC-12 light signal can be produced by multiplexing four OC-3 light signals from a transmission device (not shown in
FIG. 1
) having a lower bit rate.
FIG. 7
shows a hierarchy employed when the transmission devices
10
A-
10
D transmit OC-192 light signals. The OC-192 light signal can be produced by multiplexing four OC-48 signals, which can be produced by multiplexing four OC-12 signals, which can be produced by multiplexing four OC-3 signals. The hierarchy shown in
FIG. 7
enables a frame structure called a concatenated STS-N signal (expressed as STS-Mc). In
FIG. 7
, a STS signal having three channels and corresponding to the OC-3 light signal, that is
Fujitsu Limited
Helfgott & Karas P.C.
Rao Seema S.
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