Multiplex communications – Fault recovery – Bypass an inoperative switch or inoperative element of a...
Reexamination Certificate
1999-11-03
2004-03-16
Nguyen, Steven H. D (Department: 2733)
Multiplex communications
Fault recovery
Bypass an inoperative switch or inoperative element of a...
C370S222000, C370S230000, C370S237000, C370S352000, C370S360000, C370S389000
Reexamination Certificate
active
06707789
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to communication and, more particularly to cross connect systems.
There are two distinct types of network elements that are used in transport networks: Digital Cross-Connect Systems (DCS) and Add/drop Multiplexers (ADMs). With the advent of the SONET standard, these network elements have SONET capabilities and are called as SONET DCS and SONET ADM. It is to be understood that when the term SONET is used in this document, the corresponding international standard Synchronous Digital Hierarchy (SDH) is also included and it is interchangeable with SONET.
FIG. 1
shows the architecture of a SONET DCS, and
FIG. 2
shows the architecture of an ADM. As can be observed, their architectures are quite similar. The SONET DCS has a switch fabric
100
that is responsive to a controller
101
, and line interface units
104
,
114
, and
124
that are coupled to the switch. Elements
101
,
104
,
114
, and
124
operate under control of controller element
102
. The ADM includes switch fabric
27
, line interface units
21
24
, and controller
28
that communicates with the switch fabric and the interface units.
To elaborate, each interface unit in a SONET DCS contains one or more external system transmission interfaces that can handle different data rates. In SONET, the lowest rate is known as STS-1, and the higher rates that are typically employed are OC-3, OC-12, OC-48, and OC-192 (representing 3, 12, 48, and 192 times the data rate of OC-1). A system can start with one interface unit and grow with the addition of more interface units as needed. Each interface unit has one port at the output of the DCS and one or more ports connected to the cross connect fabric. The signals flowing through the ports that are connected to the cross connect fabric are sometimes referred to as “tributaries.” In a DCS, a tributary from any port can be connected to any other port through the cross connect fabric, under software controlled commands.
In addition to controller
101
, the control structure for a SONET DCS typically comprises a central controller
102
that resides within the DCS, with connections to controllers within the interface unit and controller
101
. The central controller also maintains communications interfaces for communicating to external control systems.
Functionally, the SONET DCS is used to multiplex and groom SONET payloads across the different SONET line rates. The DCS can connect traffic between rings, and manage complicated connections in the office by being a central point that connects SONET ADMs on different rings and that couples other equipment (such as an end office) to the network. The SONET DCS is not required to immediately reroute at the time of the failure. Normally, it reports failure events to a central system and waits to be told how to reconfigure in response to a failure.
It is also possible to reroute traffic through the SONET DCS in case of a failure autonomously, using a distributed communications protocol. However, implementation of distributed restoration using DCS is not so common. One reason is that it is not so simple to implement the distributed rerouting protocol in a DCS network, and the other reason is that it is not easy to make such rerouting as fast as it is done in a SONET ring.
While the SONET DCS and SONET ADM are architecturally quite similar, the DCS and the ADM are targeted for different applications, and correspondingly, their characteristics are quite different. To illustrate, the cross connect fabric of an ADM is extremely small (on the order of a dozen input ports and a dozen output ports) whereas the cross connect fabric of a DCS may be quite large (e.g., thousands of ports). The SONET ADM architecture is, effectively, a lower capacity version of the SONET DCS architecture.
More particularly, the SONET ADM comprises line interface units
21
-
24
, drop interface units
25
-
26
. In a typical four-fiber arrangement there is:
one (service) fiber pair (transmit and receive) that carries service traffic in the “east” direction,
another fiber pair in the east direction whose primary function is to serve as a backup for the east direction traffic, should the service fiber fail (the backup or protection fiber in some applications is allowed to carry low priority traffic that may be preempted),
one (service) fiber pair that carries service traffic in the “west” direction, and
another fiber pair in the west direction whose primary function is to serve as a backup for the west direction service traffic, should the service fiber fail, (the backup or protection fiber in some applications is allowed to carry low priority traffic that may be preempted.
One capability that is usually targeted to the SONET ADM is “ring functionality”. There are several types of SONET ring functionalities that are available in different SONET ADMs—Uni-directional path switched rings (UPSR), 2-fiber bidirectional path switched rings (BLSR), and 4-fiber BLSR. All of these rings comprise a sequence of SONET ADMs arranged in a closed loop. When a facility failure occurs, such an outside plant fiber being cut by construction equipment, the SONET ADMs react to the failure and reroute all of the traffic within 60-100 msec. This is done by the ADM whose one of two ports cannot handle traffic because of a failure condition in the link connected to that port applying that traffic to a spare fiber or to the fiber that carries traffic in the reverse direction in the ring. Although the arguments apply equally well to both 2-fiber and 4-fiber rings, our discussion here particularly adheres to 4-fiber case for simplicity of discussion. Because of this survivable architecture, the SONET ADM and the ring architecture are used in many critical network applications.
An ADM terminal in such an arrangement comprises four high-speed line interface units, one for each of the fiber pairs. Other than the drop-off traffic and the add-on traffic on the protection lines, under normal conditions the ADM cross connect sends all traffic from the east pair of line interface units to the west pair of line interface units. The drop interface units are interfaces that are employed for connecting to DCS elements, or to other elements (such as an end office). For an OC-48 ring typical drop interfaces are OC-
3
and OC-12 and for an OC-192 ring typical drop interface units are OC-3, OC-12 and OC-48.
During the provisioning period of the ADM ring, messages are passed around the ring using the SONET Data Communications Channel (DCC) that is in-band within the SONET overhead. Some of these messages are used to configure the ring. At the time of a failure, bit-oriented messages are passed from node to node using the SONET K-byte protocol (henceforth, “K-byte messages”). The ADM controller reads these messages from one side of the ring, processes the message and passes the message to the other side of the ring or back out in the reverse direction that the message came in. These messages are handled with low latency because they are designed to support the goal of restoration due to fiber failures in as little as 50 msec. As necessary (when a failure occurs), the ADMs cross connect reroutes the traffic to the backup fiber that sends traffic in the same direction as the failed fiber, or sends the traffic over the backup fiber that carries traffic in the opposite direction.
A typical network architecture using SONET DCS is shown in
FIG. 3
, where SONET rings
10
,
11
, and
12
are inter-coupled via SONET DCS
13
. Traffic originating from a node in a ring (e.g., traffic that is placed onto ring
10
by ADM node
101
from drop
1011
) and terminating at another node in the ring (e.g., ADM node
102
) is routed merely over the ring. On the other hand, traffic between two nodes in two different rings is routed through the DCS, which forms an interconnection point between multiple rings.
For example, a signal between ADM nodes
114
and
121
is routed through ring
11
, DCS
13
and ring
12
.
As stated earlier, a conventional DCS primarily provides the functio
Arslan Ahmet Vecdet
Chaudhuri Sid
Cortez Bruce Gilbert
AT&T Corp.
Stevens Roberta
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