Communications switch permitting transparent maintenance of switch control system

A switch architecture that permits maintenance of switch signaling interfaces in switches for communication networks. The architecture provides a spare switch signaling interface that is enabled when a regular switch signaling interface is disabled. A switch matrix routes signaling channels from their dedicated position in a trunk to the spare switch signaling interface when the spare is enabled. A control processor updates a memory log associating the signaling channel and the trunks from which they originate with the spare switch signaling interface.

The present invention relates to switching equipment for communications 
systems and, more particularly, to a switch architecture that permits 
equipment maintenance and installation to be performed on a switch without 
interruption in communication services to customers. 
BACKGROUND OF THE INVENTION 
In integrated services digital network ("ISDN") applications, a customer is 
interconnected with a communications network through a trunk line, such as 
a T1 trunk. Shown in FIG. 1, a T1 trunk 10 ("T1") contains 24 channels 
multiplexed according to time division multiplexing. The T1 uses 
out-of-band signaling: Of the 24 channels, a single "D" channel contains 
control signaling that is necessary for the customer to interact with the 
communications network. For example, call set up and tear down is 
accomplished via the D channel. The D channel occupies a specific time 
slot on the T1. The remaining 23 "B" channels contain voice, data or any 
kind of user communications data. 
FIG. 1 illustrates the architecture of an end office switch 20 in known 
communication networks. An end office switch is a communication network 
switch connected directly to a customer. The customer's communication 
equipment, such as a telephone, computer, facsimile machine or a private 
branch exchange, interfaces with the end office switch 20. The end office 
switch 20 is the first of what may be several switches that route calls 
through the network. 
T1's also interconnect switches within a communication network. In long 
distance telecommunication services, a call may be routed by several 
switches to connect the two parties to the call. A call may be assigned to 
a first time slot on a first T1, between the customer and the original 
switch then routed to a second time slot on a second T1 by a first switch. 
A second switch connected to the first switch by the second T1 may route 
the call to a third switch on a third time slot on a third T1. A call 
experiences as many switching stages as are necessary to route a call to 
its destination. The T1s interconnecting the switches may also use 
out-of-band signaling, providing a separate path as a signaling channel to 
communicate control information between the switches. 
The end office switch 20 includes a plurality of digital interface units 30 
("DIUs"), also known as "T1 interface units," in communication with a 
switch matrix 25, such as a time division switched network (TDSN). One DIU 
30 interconnects a plurality of T1s to the TDSN. For example, in the 4ESS 
switch available from Lucent Technology, Inc., a DIU 30 interfaces with as 
many as five T1's. Some of the T1's may connect to customers as described 
above, others may connect to other switches within the network. In the 
4ESS, the output of the DIU is another trunk carrying 120 multiplexed time 
slots. The 120 multiplexed time slots are input to the TDSN. 
The switch 20 contains a switch signaling interface 40 ("SSI"). The SSI 40 
is a hardware element that permits the switch 20 to monitor and respond to 
control signals contained within the D channel of the trunks 10 connected 
to the switch 20. The switch 20 routes the D channels of the T1's input to 
the DIU 30 to a single SSI 40. Each SSI 40 includes a T1 facility access 
41 (T1FA), three node processors 42, and six line interfaces 43. The SSI 
is organized as three packets, each including a pair of line interfaces 43 
and one node processor 42, that serve up to eight customers. A failure of 
the T1FA causes the switch 20 to be isolated from the 24 D channels of 
various trunks; failure of a packet causes the switch to lose track of 
eight D channels. Alternatively, the switch 20 may interface with a high 
density switch signaling interface("HD") 50 as a substitute for the SSI 
40. A particular switch 20 may contain more than one SSI 40 or HD 50 (not 
shown). 
The switch 20 can switch any time slot from any T1 to any time slot of any 
other T1. However, the switch 20 establishes a dedicated pathway that 
routes the D channels of the T1's to the SSI 40. As traffic on a single T1 
10 enters the switch 20, the DIU 30 multiplexes the channels of the T1 
with channels from the other T1's connected to the DIU 30. The DIU 30 
generates an output carrying the multiplexed output of the several T1's. 
The switch 20 switches the D channels contained in the DIU output to the 
SSI 40. Also, D channels from other DIUs 30 may be switched to the same 
SSI 40. A control processor (not shown) interfaces with the SSI 40 to 
monitor the status of the D channels and execute commands contained 
therein. 
The architecture of these known switches suffer from a disadvantage because 
SSI maintenance disables the trunks that the SSI 40 services. If the 
switch 20 is an end office switch, SSI maintenance causes an interruption 
in service to the customer. During maintenance, all or part of the SSI 40 
is disabled, severing the connection between the SSI and the D channels 
and isolating the D channels from the control processor. Depending upon 
the type of maintenance, either an individual packet or the T1FA 41 is 
disabled, disconnecting from eight to twenty-four customers. Because 
customers expect and demand continuous, uninterrupted communication 
service, a communication service provider may notify and negotiate 
maintenance times with its customers. Using the current architecture, the 
provider must negotiate a single time when up to twenty-four customers 
consent to termination of service. Customers rarely entertain such 
negotiations with good humor. 
Accordingly, there is a need in the art for a switching architecture that 
permits SSI maintenance without interruption of customer service. 
SUMMARY OF THE INVENTION 
The disadvantages of the prior art are alleviated to a great extent by a 
switch architecture that introduces a spare SSI to the switch. During 
maintenance, the switch changes the routing assignment of the D channels 
assigned to the SSI. The D channels are routed to the spare SSI. 
Alternatively, if SSI service disables only a portion of the SSI, the D 
channels of that portion could be rerouted to the spare SSI. The present 
invention achieves a benefit of permitting maintenance to be made to an 
SSI without requiring the customer service provider to negotiate a 
maintenance schedule with its customers. Indeed, maintenance of SSI under 
the present invention goes unnoticed by the customer.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The architecture of a switch 100 constructed in accordance with the present 
invention is shown in FIG. 2. A switch matrix 110 interconnects with a 
plurality of DIUs 30 which, in turn, are interconnected with the T1's 10. 
A spare SSI 120 shelf is provided in communication with one of the DIUs 
30. The spare SSI 120 is identical to other SSIs in all respects. 
The structure of the switch matrix 110 is shown in FIG. 2. A switch matrix 
110 is a three stage switch including a first time slot interchange 111 
("TSI"), a time multiplex switch 112 ("TMS") and a second time slot 
interchange 113. The TSIs 111, 113 reorganize time slots of a trunk 
carrying the multiplexed output from a DIU 30. The TMS 112 switches time 
slots from one trunk 114 to another. Using the three stage switch matrix 
110, data in any time slot from any T1 10 may be switched to any time slot 
of the same or any other T1 10. 
The control architecture of the switch is shown in FIG. 2 as well. The 
switch 100 includes a hierarchy of a control processor and an attached 
processor that collectively operate as a control processor 130 to 
configure the switch. The control processor 130 causes the switch matrix 
110 to route calls among the T1's. The control processor 130 maintains a 
memory log (not shown) that associates the D channels in the SSIs 40 
(multiple SSIs not shown) with their associated T1's. The D channels are 
read from the SSI 40 to the control processor 130. When the control 
processor 130 decodes an instruction contained in a D channel, it refers 
to the memory log to identify the T1 on which the instruction should be 
executed, and executes the instruction on that T1. 
The SSI shelves are in communication with a control network interchange 140 
("CNI"). The CNI 140 is linked to signal transfer points (not shown) over 
A links that provide connections to other switches in the communication 
network. The CNI also connects to the control processor 130 of the switch 
itself. 
To service an original SSI 40, the switch 100 reassigns the D channels 
assigned to the original SSI 40. The D channels are assigned to the spare 
SSI 120. The control processor 130 causes the switch matrix 110 to route 
the D channels from DIU 30 to the DIU 30 associated with the spare SSI 
120. The control processor 130 updates the memory log relating the spare 
SSI 120 to T1's 10 to reflect the new assignments for the D channels. From 
the spare SSI 120, the switch processors 130 read signaling contained in 
the D channel and perform network operations as appropriate. Switching 
from the original SSI 40 to a spare SSI 120 occurs without loss of data 
and is transparent to the customers serviced by the reassigned D channels. 
After the D channels are routed to the spare SSI 120, a system operator is 
free to conduct whatever operations are necessary on the original SSI 40. 
Upon conclusion of the SSI maintenance, the system operator may return the 
D channels from the spare SSI 120 back to the original SSI 40. However, 
the D channels need not be switched back; instead, the original SSI 40 may 
sit "in reserve" as the "spare SSI" and remain idle until other SSIs are 
scheduled for maintenance. 
The switch architecture also permits instantaneous recovery from an SSI 
equipment failure. If the control processor 130 detect an equipment 
failure in an SSI 40, the processor 130 may switch the D channels from the 
failing SSI 40 to the spare SSI 120. Thus, the switch 100 may recover from 
loss of an SSI 40 with minimal interruption of customer service. 
The description of the present invention has been made in the context of 
4ESS switch, a product of Lucent Technologies. However, the principles of 
the present invention find application with a host of other switches found 
in communication networks. Also, the present invention may find 
application with any kind of trunk using out-of-band signaling, not merely 
T1 trunks.