Note: Descriptions are shown in the official language in which they were submitted.
~~9~~~
1
8-151-3
Title: Method And Apparatus For Providing On-Hook
Transmission For Digital Loop Carrier
Channel Units
SPECIFTC'ATTnN
This invention relates-to a method for improving
the on-hook transmission response of SLC5 Remote
Terminal (RT) Plain Old Telephone Service (POTS) type
channel units to received standard SLC series 5
. 5 supervisory signaling codes.
BACKGROUND OF THE INVENTION
Tri the beginning, the telephone companies relayed
'voice frequency (VF) information over copper cables.
.As the demand for telephone lines increased and
'transmission technology improved, a means for digitally
'transmitting VF information was developed. The digital
'technology enabled twenty-four VF signals, i.e., the VF
signals carried by 24 telephone lines, to be sampled
and multiplexed into.one DS-1 (1.54.4 Mb/s rate) digital
line. The economical advantage of this is readily
apparent. A fairly recent development of this digital
advancement is the Digital Loop Carrier (DLC) system.
The DLC system is used in local switching services
to connect the subscriber (or customer) to the local
central office (CO). VF signals are converted to
digital form for transmission over a digital
transmission medium. In this digitizing process,
supervisory signals are incorporated into the
transmitted signal in the form of codes in standardized
0~9~3~
2
formats which provide to the receiving end of the medium
information about the status of or commands from the
other end.
In the overall communication process it is possible
to provide two modes of transmission, off-hook and
on-hook. Off-hook transmission is defined as transmission'
during the period the subscriber's equipment is
requesting service, has closed the loop, or has seized a
telephone line. On-hook transmission (OHT) is defined as
transmission during the period before the subscriber has
gone off-hook. OHT is essential for providing certain
services, such as calling party identification (CPI) and
other features . However, RT equipment of the prior art is
not capable of providing OHT under all circumstances or
in response to ali supervisory signalling codes.
SUMNLARY OF THB INVENTION
An object of the present invention is to provide a
_/
method of arranging and operating a POTS-type RT channel
unit for use in a SLC series 5 DLC system which maintains
OHT while receiving the ~~Ground Start - Idle~~ code to
make the system fully compatible with the requirements of
Bellcore TR-TSY-000030.
Briefly described, the invention includes providing
a control means in the remote terminal for passing
signals in both directions, and generating with that
control means control signals responsive to ~~Ground Start
- Idle~~ codes to energize apparatus for completing VF
connections through the RT, thereby allowing OHT.
2a
In one broad aspect, the invention provides a method
of providing a voice frequency (VF) path foron-hook VF -~-
transmission through each, of a plurality of channel
units, each including a subscriber line interface circuit
chip (SLIC), to a subscriber telephone instrument in a
digital loop carrier (DLC) system for providing POTS-type
telephone service using DC signalling to the subscriber
telephone instrument. The method comprises the steps of
providing a control unit operatively associated with each
channel unit for passing signals including VF and DC
signals in both directions for transmit and receive
communication channels, and generating with the control
unit control signals for each channel unit for causing a
portion of the channel unit to establish a communication
..
path through each such portion in response to receipt by
the control unit of an "idle" code in ground start mode
during idle condition regardless of whether the
subscriber instrument is on-hook~or off-hook, thereby
facilitating on-hook transmission.
In another broad aspect, the invention provides an
apparatus for providing a voice frequency (VF) path for
on-hook VF transmission through each of a plurality of
channel units,' each including a subscriber line interface
circuit chip (SLIC) , to a subscriber telephone instrument
in a digital loop carrier system for providing POTS-type
telephone service using DC signalling to the subscriber
telephone instrument. The apparatus comprises the
combination of control unit means operatively associated
with each channel unit for passing signals including VF
2b
and DC signals in both directions for transmit and
receive communication channels. The apparatus further
comprises means in each control unit for generating
control signals for the associated channel unit for
establishing through a portion of the channel unit a
communication path in response to receipt by the control
unit means of an "idle" code in ground start mode during
idle condition regardless of whether the subscriber
instrument is on-hook or off-hook, thereby permitting
on-hook transmission.
The invention further provides a digital loop
carrier system comprising a source of voice frequency
(VF) signals, a subscriber instrument having on-hook and
off-hook states, and a remote terminal coupled to the
subscriber instrument. A plurality of channel units are
provided in the remote terminal for transmitting VF
signals including VF and DC signals from the source to
the instrument through the remote.terminal, each of the
channel units including a subscriber line interface
circuit (SLIC) chip. Means coupled to each of the
channel units are provided for enabling the channel units
to pass the VF signals through the remote terminal in
response to a ground start idle code received by the
means from the source in ground start mode during idle
condition whereby the VF signals are transmitted to the
subscriber whether the subscriber instrument is on-hook
or off-hook.
In order to impart full understanding of the manner
in which these and other objects are attained in
accordance with the invention, a particularly
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advantageous embodiment thereof will be described with
reference to the accompanying drawings, which form a
part of this specification, and wherein:
Fig. 1 is a schematic block diagram of a digital loop
carrier (DLC) system in universal configuration;
Fig. 2 is a schematic block diagram of a digital loop
carrier (DLC) system in integrated configuration;
Fig. 3 is a simplified schematic block diagram of a
typical Series 5 channel bank;
Figs. 4 and 5 are diagrams illustrating, respectively,
the Super Frame DS-1 format and the Extended Super
Frame DS-1 format;
i
Fig. 6 is a table of the signaling codes normally used
with POTS-type channel units;
' 15 Fig. 7 is a time diagram of a typical ringing signal
illustrating the relationship of ring and silent
intervals;
Fig. 8 is a schematic block diagram of an RT unit
incorporating a command arrangement in accordance with
the present invention;
Fig. 9 is a schematic block diagram of relevant
portions of a control unit usable in the RT unit of
Fig. 8 to produce the control signals in accordance
with the invention; and
4
Fig. 10 is diagram illustrating the RPCM and TPCM 16-
bit words which appear in the apparatus of Figs. 8 and
9; as shown with Fig. 7.
DESCRIPTION OF THE PRE ERRED EMBODIMENT
There are two basic DLC configurations: the
universal configuration and the integrated
configuration.
In the universal configuration, a block diagram of
which is shown in Fig. 1, a Central Office Terminal
s
(COT) 20 located at the central office (CO) 22 is
connected to the analog port of a central office switch
. , 21 and is also connected by a digital transmission '
medium 24 (DS-1 interface to T1, optical fiber or the
like) to a Remote Terminal (RT) 26 located in the
vicinity of the subscriber. VF copper lines indicated
g enerally at 28 from the CO switching system analog
port are routed to the COT. The COT performs an analog .
to di ital
g (A-D) conversion on the VF signals and
multiplexes them for DS-1 transmission on the digital
<>' 20 medium to the RT. The RT demultiplexes the digital
signal and carries out a digital to analog (D-A)
"'~ conversion. The resultant VF signals are routed via
v;
' copper lines indicated generally at 30 to the
subscribers. VF signals from the subscribers arrive at
the CO switch analog port in a similar manner.
In an integrated configuration, a block diagram of
which is shown in Fig. 2, an RT 26 is directly linked
to the digital port of a CO digital switching system 32
. via the digital transmission medium 24. VF information
which is already in digital format at the CO switching
system digital port can be sent on medium 24 directly
to the RT where a D-A conversion takes place. The
'resultant VF signals are sent on copper lines 30 to the
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subscribers. VF signals generated by the subscriber
are converted to digital format at the RT and forwarded
to the CO switching system digital port via the digital
transmission medium. In this form of the system, no
channel bank equipment is needed at the CO end of the
link because the information is all digital, in and
out. The CO facility may, of course, also have
separate equipment for handling analog inputs.
The COT (in the universal configuration) and the
RT include channel banks. Physically, the channel
banks consist of multiple slotted shelves designed to
accommodate plug-in channel units and common equipment
units. Functionally, the channel units interface the
subscriber or CO VF signals with the common equipment,
and the common equipment interfaces the digital
facility with the channel units. Twenty-four channel
samples are multiplexed onto one digital line and are
known as a digroup.
Figure 3 shows a basic, simplified functional
block schematic diagram of a typical channel bank.
Various types of channel units and common equipment are
used to suit the types of service offered the
subscriber, and to suit the method of digital
_ transmission. In the channel bank illustrated therein,
a total of 192 VF telephone lines are connected to two
system groups identified as the white and blue systems,
96 lines going to each system. In each system are four
digroups identified as A, 8, C and D. Within each
digroup (digroup A of the white system being used as an
example) are 12 channel units 34, each of which
,, receives two telephone lines consisting of a pair of
wires. The outputs of the channel units are connected
to the input of a transmit-receive unit (TRU) 36 (each
one of which is shared by a pair of digroups) which
6 ~~~~33
accomplishes the multiplexing necessary to prepare the
signals for transmission and provides the necessary
timing signals. The multiplexed signals are connected
through a digital line interface unit (LIU) 38 onto the
digital line. Bank control units 40 and 41 monitor and
supervise the white and blue systems, respectively, for
correct operation.
Channel units known as POTS-type units provide VF
2-wire local service between a CO and a subscriber and
have two basic modes of transmission: off-hook
transmission and on-hook transmission (OHT). ---
As indicated above, off-hook transmission is
transmission during the period the subscriber is
requesting service, has closed the loop, or has seized
a telephone line, and on-hook transmission (OHT) is
transmission during the period before the subscriber
has gone off-hook.
Two DC supervisory signaling methods are employed
in order to allow the subscriber equipment to request
service on POTS-type telephone lines and the nature of
these signaling methods should be understood in
conjunction with the transmission types and the
signaling codes. The DC supervisory signaling methods
are Loop Start (LS) and Ground Start (GS).
In Loop Start, the subscriber equipment signals
_~ ~. . .
__, . _ _,
the CO of a line request by closing the loop with a DC
termination. LS signaling is usually used with, but is
not limited to, single party services operating into
conventional telephones and key set systems.
In Ground Start, the subscriber signals the CO of
a line request by grounding one leg of the telephone .
line and subsequently closing the loop with a DC
termination. GS signaling is usually used with, but is
'not limited to, private~branch exchanges (PBX), wide
area telephone service (WATS) lines, and WATS trunks
operating into a local CO (end office).
DC supervisory signaling is easily accomplished
when the subscriber and CO are directly linked by a
metallic pair (copper cables). However, in a DLC
system, the charmel unit at the COT or RT has to '
recognize the DC supervisory signaling on the VF line,
encode it and digitally transmit the codes to the far
end. The far end channel unit must decode this
information to produce the correct DC signaling state
on the associated VF line. Some of the digital codes
result in channel unit internal activity which may
result in the VF interface being powered down or the
voice path disabled, or the like. Following is a brief
description of the codes and how they are arranged.
As previously stated, twenty-four VF channels are
sampled and multiplexed onto a digital line. Each -
channel sample contains eight contiguous bits, and the
consecutive bits from the twenty four channels, plus
one framing or synchronizing bit at the end, are known
as a frame. All 193 bits in the frame [(24*8)+1] are
clocked on to the digital line every 125 microseconds.
This process continues frame after frame.
Every sixth frame, the eighth bit, also known as
the least significant bit (LSB), in each of the twenty-
four channels is "robbed" and replaced by a supervisory
signaling bit. This signaling bit is used to build
codes which represent the DC supervisory signaling
information for transmission over the digital line.
Depending on how the 193-bit frames are formatted into
larger groups, this bit can represent one of four
possible positions in the signaling code word.
These larger groups of formats are standardized
-into two basic framing formats: Super Frame and
8
Extended Super Frame (ESF). The Super Frame signaling
format, described in Bell Publication 43801 and
commonly known as the A-B signaling format, groups
twelve consecutive frames into a "super frame". The
robbed bit of each channel sample in the sixth frame is
the "A" signaling bit and the robbed bit of each
channel sample in the twelfth frame of the super frame
is the "B" signaling bit. These two bits form an A-B
code that is used to encode the metallic line DC
l0 signaling states onto the digital facility.
The ESF format, described in Hellcore Technical
Reference TR-TSY000194 and commonly known as the A-B-C-
D signaling format, groups twenty-four consecutive
frames into an extended super frame. The robbed bit
sample in the eighteenth frame of the extended super
frame contains the additional "C" signaling bit, and
the robbed bit sample in the twenty-fourth frame of the
extended super frame the additional "D" signaling bit.
These four bits form an A-B-C-D code which is used to
2o encode the metallic-line DC supervisory signaling
states onto the digital line. Fig. 4 shows the Super
Frame DS-1 frame format, and Fig., 5 shows the ESF DS-1
frame format. The table in Fig. 6 shows the signaling
codes normally used ~iith POTS-type channel units.
In the lower portion of Fig. 4 is shown one
superframe having twelve frames. In the upper portion
of the figure,- frame 5 is "expanded" as an illustration
of the details of each channel word of 8 bits. Each of
the 24 channels is substantially the same as each other
in format although the information content of each may y
be unique. The 24 channels together, along with the
framing bit, constitute the 193 bits of one frame. The
LSB's in each channel of the sixth and twelfth frames
'are used for the A and B signalling bits of the codes
m ~ .~. ey !d e~1 ep
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9
shown in Fig. 6. The other LSB's in the other channels
are used, along with the other bits, to encode the VF
information.
Fig. 5 is arranged in a similar fashion as Fig. 4,
showing the 24 frames of an extended superframe, frame
5 again being expanded to illustrate the arrangement
and content of a single frame. In this case, the LSB's
in each channel of the sixth, twelfth, eighteenth and
twenty-fourth frames are used to create the A-B-C-D
codes.
The upper portion of Fig. 6 contains the codes
which are used in connection with loop start channel
units in single party mode and the lower portion shows
those used in ground start mode with GS/LS channel
units. The codes for both superframe and extended
superframe formats are shown.
Certain of the codes which are shown in Fig. 6 are
associated with specific intervals as will be described
with reference to Fig. 7 which shows a typical ringing
sequence. The beginning of the ringing sequence is
shown at time tl. This initiation of the ringing
sequence can occur anywhere relative to the bursts of
energy which are the actual ringing signals and the
location of tl in the Figure is chosen arbitrarily.
Two ringing signals Rl and R2 are shown in Fig. 7 and
the ringing sequence ends when the subscriber's '
instrument goes off-hook or if the party initiating the
ringing goes on-hook at time t2. Between, before and
after the ringing signals are silent intervals Sl, s2
and S3.
Relating this pattern now to the table in Fig. 6,
the codes identified as "Idle" codes are transmitted to
the RT in a loop start signaling method during
intervals Sl, S2 and S3. In LS circuits, the idle
w~~~w~.~
l0 018233
codes may also be transmitted to the RT before tl and
after t2.
Traditionally, transmission capability to a
subscriber was required only while the subscriber was
in the off-hook state. Or, stated conversely, this
means there was no requirement for on-hook state
transmission. For digital loop carrier systems
employing the robbed-bit signaling method, this means
no VF transmission was required while the RT channel
unit was receiving the "Idle" or "Ringing" A-B or A-B-
C-D codes from the CO end and sending the "On-Hook"
code back.
However, recent advances in switching system
capabilities have made it possible and desirable to
transmit information to subscribers before they enter
the off-hook state or, while they are on-hook, hence
the term "On-hook Transmission" (OHT). The present
invention provides a technique for transmitting data
such as CPI or the like during intervals Sa and Sg
while the telephone connected to the RT is still on-
hook, a function which cannot be performed by the
apparatus of the prior art. Typically, only S2 is used
for CPI. Silent intervals other than S2 and S3 can
also be used.
Following is a summary of AT&T Technologies, Inc.
SLC Series 5 POTS-type RT channel units, in
chronological order of initial availability, showing
their OHT capabilities.
1. AUA51 (GS/LS) No OHT
2. AUA58 (LS) OHT when receiving
"Ground Start - Idle"
codes or "Single Party -
Idle" codes (See Fig. 6
for codes).
~~4~3~
11
3. AUA59 (GS/LS) OHT only when receiving
"Single Party - Idle"
codes.
The present invention provides OHT when receiving
either single party or ground start codes.
More specifically, some of the prior art equipment
had OHT capability only when receiving "Single Party -
.:~.~
Idle" codes, not when receiving "Ground Start - Idle" -"'
codes. Other prior art equipment had no OHT.
.' 10 Typically, during the Ground Start ringing silent
interval, the "Ground Start - Idle" code is sent to a
Loop-Start/Ground-Start POTS type channel unit
manufactured by AT&T and identified as the AUA59. This
means that the AUA59 will be unable to transmit VF
information during the ringing silent interval on a
. ground start line or when connected to any device
transmitting Ground Start codes. Therefore, the AT&T
AUA59 is incompatible with and does not meet the
requirements of Bellcore TR-TSY-000030.
Fig. 8 is a simplified block diagram of one
channel of a channel unit apparatus in accordance with
_ .
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Y
the present invention. The tip and ring wires from a
telephone instrument are connected to the input/output
terminals of a subscriber line interface circuit (SLIC)
unit 46, the tip conductor being connected through the
contact set 47 of a relay 48. The contacts are
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normally in the open state for GS operation and
normally in the closed state for LS operation,
"normally" being used here to mean the state before a
line is seized. The SLIC performs a 2 to 4-wire
conversion and the separate transmit/receive paths are
connected through the transmit and receive attenuators
50 and 51 to a combo unit 54, which is a combined PCM
filter and codec (coder-decoder). One of the channels
interconnecting these units is a transmit channel TX
and the other is a receive channel RX, as marked.
The transmit and receive paths from the combo unit
54 are connected to a control unit gate array device 56
and then to the transmit pulse code modulation (TPCM)
and receive pulse code modulation (RPCM) lines of the
common equipment. The gate array device 56 comprises a
control unit which performs a number of control
functions relating to the operation of the channel unit
which are conventional, do not relate directly to the
present invention and therefore will not be described.
However, in connection with the present invention,
gate array device 56 produces a set of signals, in
response to receipt of an idle code in a GS circuit to
open up paths (i.e., make the paths complete) to insure
information continuity of VF paths in the SLIC and the
combo unit and the appropriate level of attenuation in
t.
13
attenuation circuits 50 and 51 so that OHT can take
place.' These signals are illustrated by the
connections 60, 61 and 62 in Fig. 8. An off/on hook
signal is supplied to gate array unit 56 on a line 63. '
In a preferred current embodiment, SLIC unit 46 is
active except when receiving GS-GS codes (in Fig. 6,
under "Channel Unit Condition", "Ground Start" in the
Ground Start Codes section) when it is in a standby
mode, or except when there is a serious fault in the
lp system when it is powered down. The term "powered
down" in this context means that it is put into a state
in which it draws very little current and is waiting to s
be activated by an appropriate code or codes. It is
not actually deenergized in a powered-down state.
Referring now to Fig. 9 which shows relevant
' portions of gate array device 56, RPCM and TPCM are
connected from the TRU to the channel unit logic
t
circuits. The digital information is in 16 bit words
and is fed serially to all the channel units in the
digroup. Figure 10 shows the format of each RPCM or
TPCM 16 bit word. The first eight bits of the word
contain the voice information, the 9th through 12th
bits contain the A-B-C-D signalling information, the
13th through 15th bits contain digital line failure
14
supervision information, and the last (16th) bit is a
parity bit for error checking purposes.
Timing signals NPA, NPB and NQ are generated by
the TRU to address each channel in each time slot. NPA
specifically addresses the odd (A) channel of each
- . channel unit and NPB specifically addresses the even
(B) channel of each channel unit. As both odd and even
channels are essentially identical in operation, only
the odd (A) channel operation will be described. Fig.
9 is a block diagram of the gate array logic circuits
which control the A channel voice and signaling.
RPCM from the TRU is serially fed to two shift
registers, the Receive Signaling Shift Register 7o and
the Receive Voice Shift Register 71.
During the correct time slot, a counter A allows
the first eight bits of voice of the appropriate RPCM
word to be clocked into the Receive Voice Shift
Register 71 at a 4MHz rate. Counter A also generates a
2MHz clock fox use by combo control circuits 73. The
eight voice bits are sent in parallel to the combo
control circuit and are converted back into a serial
format to be clocked into the combo (Receive Data) at a
2MHz rate. The Receive Voice Shift Register circuit 71
calculates RPCM parity and the result is fed to the
Combo Control circuit. If a parity error is detected,
15
receive data to and transmit data from the combo are
interrupted.
During the correct time slot, counter B allows the
last eight bits (9-16) of the appropriate RPCM word to
be clocked into the Receive Signaling Shift Register 70
at a 4MHz rate. The Receive Signaling Shift Register
stores and then converts the bits into a parallel
format and forwards bits 9-12 (A-B-C-D signaling) to a
Receive Signaling Decoder 75 and bits 13-15 (E,F,and G)
to the SLIC & Relay Control Logic circuit 77. The
Receive Signaling Decoder 75 decodes bits 9-12 and _
sends instructions to the SLIC & Relay Control Logic 77
which controls the SLIC chip 46 and Tip Relay 48 (Fig.
8) accordingly. Receive Signaling Shift Register 70
detects parity and if the parity is bad, a Signaling
Clock 79 is instructed to freeze the Receive Signaling
Decoder 75 until good parity is received. The Receive
Signaling Decoder 75.also instructs the SLIC & Relay
Control circuit 77 to cut the voice path when the
ground start code is received. The only other time the
signaling bits cause the voice path to be interrupted
is during a carrier failure at which time bits 13 and . ~,.._.y""._
15 (E and G bits) command the SLIC & Relay Control
Logic circuit 77 to issue a voice cut command to the
combo control circuit 73 and a TPCM logic circuit 81.
~fl~~~~
16
TPCM is serially sent from the TPCM Logic circuit
to the TRU in 16 bit words during the appropriate time
slot. The first 8 bits (voice information) of the 16
bit word are sent from the combo 54 (transmit data) via
the Combo Control circuit 73 to the TPCM Logic circuit
81. Bits 9-15 are generated by a Transmit Signaling
Generator 83 and sent in serial form to TPCM Logic
circuit 81. The TPCM Logic circuit creates a parity --''~~
' J J.
bit (bit 16) and combines bits 1-16 for serial
transmission at 4 MHz on the TPCM line.
It will be recognized by those skilled in this art
that a gate array is not the only device which can be
used to implement this invention although a gate array
is certainly a preferred implementation. On the
contrary, a dedicated microprocessor, a programmable
processor or a hard-wired unit can be substituted for
the gate array, so long as the appropriate signals are
provided to complete.the data path under the described
conditions.
Following is a glossary identifying abbreviations
commonly employed with reference to the equipment
described herein:
GLOSSARY
CLASS CUSTOM LOCAL AREA SIGNALING SERVICE
(CLASS is a Service Mark of Bell
Communication Research, Inc.)
CNI CALLING NUMBER IDENTIFICATION
.r
17
CO CENTRAL OFFICE
COMBO COMBINATION CODEC AND PCM FILTER
COT CENTRAL OFFICE TERMINAL
CPI CALLING PARTY IDENTIFICATION
S CSA CUSTOMER SERVING AREA
DLC DIGITAL LOOP CARRIER
DS-1 DIGITAL SIGNAL STANDARD, LEVEL
1
GS GROUND START
LIU LINE INTERFACE UNIT
LS LOOP START
OHT ON-HOOK TRANSMISSION
PBX PRIVATE BRANCH EXCHANGE
POTS PLAIN OLD TELEPHONE SERVICE
RT REMOTE TERMINAL
SLC SUBSCRIBER LOOP CARRIER (SLC
is a
registered trademark of AT&T
Technologies Inc.)
SLIC SUBSCRIBER LOOP INTERFACE CIRCUIT
TDM TIME DIVISION MULTIPLEX
TRU TRANSIT RECEIVE UNIT
VF VOICE FREQUENCY
While certain advantageous embodiments have been
chosen to illustrate the invention, it will be
understood by those skilled in the art that various
changes and modifications can be made therein without
departing from the scope of the invention as defined in
the appended claims.
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