Multiplex communications – Channel assignment techniques – Using time slots
Reexamination Certificate
1998-08-07
2002-05-28
Ton, Dang (Department: 2661)
Multiplex communications
Channel assignment techniques
Using time slots
Reexamination Certificate
active
06396846
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to digital communication systems and networks, and is particularly directed to a mechanism for enabling a channel bank-installed ISDN channel unit to controllably ‘spoof’ a channel bank controller to release time slots assigned to a different card slot, and thereby enable the usurping channel unit to conduct 2B+D ISDN communications without the use of a null card.
BACKGROUND OF THE INVENTION
Integrated services digital network (ISDN) communication networks, such as the ‘extended distance’ ISDN communication network architecture diagrammatically illustrated in
FIG. 1
, enable telephone service providers to supply multiple types of signalling channels from a central office to a network termination interface at a customer premises site. In the reduced complexity network example of
FIG. 1
, a PCM communication link
10
, such as a T
1
data rate (1.544 Mb/s) optical fiber link, provides digital communication connectivity between a central office (CO) channel bank
20
, such as an AT&T SLC series 5 channel bank, at a ‘west’ end of the PCM link, and customer premises equipment (CPE) served by a channel bank
30
located at a remote, ‘east’ end
40
of the link.
The west end central office
20
is coupled by way of a link
21
to a central office switch
22
(such as a 5ESS switch manufactured by AT&T), and includes a line interface unit (LIU)
23
that terminates the west end of the PCM link
10
. The east end channel bank
30
has an LIU
33
coupled to the PCM link, and includes a plurality of U-BRITE circuit cards
35
coupled via associated local loops (twisted tip/ring pair)
37
to customer premises equipment (CPE)
40
. As shown in
FIG. 2
, the line interface unit
33
of the east end channel bank
30
is coupled over an internal PCM bus
34
to a plurality of ISDN channel units or U-BRITE circuit cards
35
installed within card slots of the channel bank's backplane
36
. Each respective U-BRITE circuit card
35
is dedicated to providing extended ISDN service to remote customer premises equipment via a local loop
37
that connects the channel unit with digital communication equipment
40
installed at the customer premises.
Under control of a communications control processor or channel bank control unit (BCU)
38
, a carrier system transceiver within the line interface unit
33
is operative to transmit and receive standard 2B+D ISDN data traffic over the PCM digital data link
10
. In order to interface a digital subscriber loop (DSL) over the local loop (twisted pair)
37
to the customer premises equipment
40
, each U-BRITE card
35
includes a line transceiver and an associated line interface, which are also operative under microprocessor control to interface PCM data with the line interface unit
33
, and to transmit and receive basic rate 2B1Q ISDN signals over the local loop
37
to and from the customer premises equipment
40
.
In some channel banks, the bank controller unit may not be designed to recognize ISDN common cards (ISDN channel units) installed in the channel bank's backplane. Since ISDN channel units require the use of three time slots (i.e., two bearer (B
1
and B
2
) and one data (D)), and the bank controller assigns only two time slots per backplane card slot, it has been customary practice, as shown at
39
in
FIG. 2
, to install what is commonly known in the industry as a ‘null’ card
39
, in a card slot that is immediately adjacent to the card slot containing an ISDN channel unit or U-BRITE card
35
.
While containing no transceiver circuitry of its own, the null card is configured to appear to the BCU as a standard POTS card, so that the bank controller unit will release or assign (a pair of) time slots associated with the null card's backplane card slot. Since the null card does not transmit, these time slots are available for use as third time slots of adjacent U-BRITE cards.
Although installing a null card is one way to induce the bank controller unit to release two otherwise unused time slots for utilization by adjacent ISDN channel cards, it requires that the ISDN service provider purchase and physically install a null card for every two ISDN cards employed.
SUMMARY OF THE INVENTION
In accordance with the invention, the above-described cost and labor penalty of having to purchase and install null cards in an ISDN channel bank is effectively obviated by taking advantage of an a priori search sequence that is conducted by the bank controller unit whenever a service request (SR) signal is generated by a channel unit, irrespective of the backplane card slot in which the channel unit sourcing the service request is installed. In particular, it has been observed that the channel bank controller unit will always respond to a service request by polling the card slots for the presence of channel units, beginning with the lowest numbered card slot, and then proceeding sequentially through the remaining (twelve per digroup) card slots of the backplane.
Unfortunately, even with knowledge of the order of the polling sequence, and the fact that each channel unit can read each of the messages being clocked over a common channel unit, there is the problem that the card slot addresses lines (including a selectively clocked enable or “not message page (NMP)” line and an asynchronous clock or “not message quadrant (NMQ)” line) are not common to all channel units. Instead, for each digroup of card slots, the address lines are selectively wired in sets or groups (two NMQ lines for two sets of six card slots each, and six NMP lines respective assigned to non-adjacent pairs of card slots (
1
/
7
,
2
/
8
,
3
/
9
,
4
/
10
,
5
/
11
and
6
/
12
)). As a consequence, each channel unit cannot determine when the bank controller is addressing another (in particular, the adjacent) card slot.
As will be described, this problem is obviated in accordance with the invention, by employing the NMQ clocking signals generated by the bank controller for a given plurality of channel units, of which an ISDN channel unit of interest is a member, to artificially generate a ‘pseudo NMP’ signal, the duration of which corresponds to a period of time slightly longer than the length of time that the sequence of NMQ signals is being asserted by the bank controller. This combination of NMQ signals on the NMQ lead and the pseudo NMP signal enables the ISDN channel unit of interest to conduct serial message exchanges (read/write operations) with the bank controller during an interrogation interval associated with another (the adjacent) backplane card slot. The bank controller unit is thereby effectively fooled or ‘spoofed’ into thinking that a channel unit is actually installed in that backplane slot, so that it releases the pair of time slots associated with that adjacent card slot, and thereby allows the ISDN channel unit to usurp one of the two released time slots it needs for its set of three (2B+D) ISDN channels.
For this purpose, upon being installed in the channel bank, the modified ISDN channel unit according to the present invention generates a communication service request (SR) signal. As described above, this communication service request signal will stimulate the channel bank controller to poll each channel unit installed in the channel bank, so that the bank controller may ascertain which circuit card slots actually contain channel units. Such polling of the card slots is conducted by means of the set of selectively clocked NMP and NMQ address lines and an associated common message (MSG) line to which the card slots are connected.
The modified ISDN channel unit of the present invention includes a set of interface logic that is coupled in circuit between the NMP and NMQ input leads and the message exchange circuitry of the channel unit to which the NMP and NMQ leads are normally directly connected. The interface logic has an output gate to an input of which the NMP input lead of the backplane communication bus is coupled. The output of this output gate is coupled to control the s
Adtran Inc.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Ton Dang
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