Automated line signal processing

Details Automated line signal processing Automated line signal processing

1998-09-02

2004-08-17

Ton, Dang (Department: 2666)

Multiplex communications

Diagnostic testing

Of a repeater

C370S352000

Reexamination Certificate

active

06778503

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to processing of line state signaling in communications networks, and particularly although not exclusively for processing of channel associated signaling in circuit switched networks such as Asynchronous Transfer Mode (ATM) networks.
BACKGROUND OF THE INVENTION
Conventional telecommunication systems handling telephony and other circuit switched data calls comprise a plurality of central office exchange switches each communicating with each other to form a national and/or international network, each central office switch communicating with a large number of subscriber apparatus, for example, telephone handsets, fax machines, modems. In general each subscriber apparatus is connected to a local exchange over one or a plurality of line circuits. Each line circuit is bi-directional, having two channels.
FIG. 1
herein illustrates schematically part of a conventional telecommunications network comprising a plurality of subscriber terminals
100
-
103
, each subscriber terminal connected to a corresponding local exchange apparatus
104
,
105
, the local exchanges being interconnected as part of a larger network, for example an international network comprising a plurality of international exchanges
106
,
107
. Set up and termination of calls between individual user terminals is initiated by a sequence of signals passed between the subscriber terminals, the local exchanges and the international exchanges. Typically, such signaling comprises a number of different tone sequences, for example a call waiting indication tone; a recall dial tone; a message waiting tone; a confirmation tone, a ringing tone; a line busy tone; a reorder tone; a receiver off hook tone; and a continuity check tone. Analog tone levels at transmitting and receiving user terminals are defined in International Telecommunications Union (ITU) recommendations ITU Q.312 and ITU Q.213. For example, to set up a call, a user may pick up a telephone handset which results in an off hook line seizure signal which is converted into a digital bit stream, either at the telephone handset, or at the local exchange, and which is monitored by a call control processor at the local exchange. The off hook signal is followed by a plurality of dial tone signals representing telephone numbers, which are converted into a digital bit stream either at the subscriber terminal, or at the local exchange. The off hook signal is converted at the first local exchange into a digital bit stream and is then routed by the call control processor over the entire telecommunications network to a destination specified by the dial tones. For example in the case illustrated schematically in
FIG. 1
, the off hook signal generated by first user terminal
100
is received at first local exchange
104
. After a predetermined number of digits have been dialed at first user terminal
100
, local exchange
104
recognizes that a call has to be routed to first international exchange
106
and generates a seizing signal, comprising a pattern of bits which is communicated to first international exchange
106
by first local exchange
104
. Similarly, first international exchange
106
on receiving the seizing condition signal from first local exchange
104
recognizes the seizing condition signal and generates a further seizing condition signal to second international exchange
107
, and so on as the call propagates across the network and is routed to second local exchange
105
to which the destination of the call, ie, second user terminal
102
, is connected. At each exchange, there is a delay in processing the signaling. The signals can only change every 1.5 to 2 ms. Further, the signals themselves have a duration, depending upon the type of signal. Each exchange monitors a plurality of line circuit channels, and on receiving a signal on a circuit will only recognize that signal after the signal has persisted for a predetermined duration. For example, where first user terminal
100
generates an off hook signal, first local exchange
104
monitoring a line to which the first user terminal is connected must determine that the off hook signal has persisted for a predetermined duration, before the first local exchange will recognize the signal as a valid off hook signal. Typically, such predetermined duration is of the order of 10 ms. Further, each intervening exchange, ie, first international exchange
106
, second international exchange
107
and second local exchange
107
requires the off hook signal to persist for a minimum duration of 10 ms before the relevant exchange will recognize the off hook signal as being valid. At each exchange, changes in a line state must persist for a predetermined duration before the corresponding call control process of a local exchange recognizes that line change state as being valid. Thus, for a 14 digit dialed national telephone code, typically a maximum of the order of 8 to 10 exchanges would be involved. The delay at each exchange would comprise a 10 ms delay to confirm persistence of a line change state, plus a delay incurred by the local call control processor to process the line change state and signal to a next exchange. Over an extended telecommunications network, call set up may take of the order of 0.5 seconds in all. At each exchange, each line connected to that exchange is periodically polled by the call control processor, checking for changes in line state on each line repetitively every 1 or 2 ms. This places a data processing burden on the call control processors. The data processing requirement for checking each line at an exchange every 1 or 2 ms includes the processing overhead of polling the line, retrieving line state data from the line, comparing the received line state data with a previously received line state data from the same line and determining whether a change has occurred, and if a change is determined to have occurred on the line, performing additional signal processing to check whether that change in line state persists on that line, ie, counting the number of times which a line has been in a particular state. The call control processor carries out the polling procedures on an interrupt basis, interrupting its normal call control data processing operations to check each line individually. Thus, the call control processor is not doing useful call control processing operations during this time, but is performing data processing associated with persistence monitoring operations of line state.
For example, during an average 3 minute call, less than 30 seconds will be spent performing dialing and initial call set up. This requires generation of 5 message events thus, in a typically longest national dialed call there will be 14 digits dialed, giving 14 dialed digit events, plus an initial seizure event. Thus, when a subscriber goes off hook and dials a 14 digit number, assuming each individual line state is aggregated, then in one direction from the subscriber to the exchange, in 3 minutes there are 15 events transmitted on one line. If the signaling is being handled by the conventional call control processor which polls every 1.5 ms, there are of the order 660 polls per second, or about 39,000 polls per minute, so over the duration of the 3 minute call the processor may poll the line state over 100,000 times. Considering that each exchange may handle a large number of calls at any one time, the data processing burden on the call control processor due to the line state persistence measurement consumes a significant proportion of the data processing capacity of the processor.
Thus, as the number of lines which a single call control processor is required to service increases, the data processing burden due to the line state persistence measurement process increases for that processor, at the expense of using data processing capacity which could otherwise be used for call control data processing operations.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a method and apparatus for devolving the processing of persistence mea

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