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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to real time testing for continuity in transmission
paths through a digital concentrator in subscriber loop digital carrier
transmission systems.
2. Description of the Prior Art
Testing for continuity in the path through a telephone switching office has
been known for a long time. However, these tests usually involve the use
of additional equipment, even for testing in time division multiplex
switching offices. One such method is disclosed in U.S. Pat. No. 4,064,369
granted to Mr. Frank E. Battocletti on Dec. 20, 1977.
Similarly, in the subscriber loop plant using digital multiplexed
transmission systems, it is likewise desirable to test the path through a
digital trunk prior to establishing a talking connection.
SUMMARY OF THE INVENTION
In accordance with the illustrative embodiment of the present invention, a
looping test is performed to test the assigned transmission path for
continuity and to test the associated equipment just prior to establishing
the connection for every call.
More particularly, such continuity testing for concentrated pulse code
modulated (PCM) transmission paths is accomplished using the same terminal
equipment necessary for establishing the initial connection assignment.
Even more particularly, the assignment sequence includes a preconnection
test subsequence in which test codes are exchanged between the two
terminals of the system during a time-out interval. Failure to
successfully complete the subsequence within the time-out interval
interrupts the assignment sequence and forces the test to be repeated
periodically until the fault is cleared.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a general block diagram of a digital concentrator for a
subscriber loop digital transmission system using a plurality of pulse
code modulated (PCM) bit streams,
FIG. 2 is a graphic diagram of the data messages transmitted from the
central office terminal of the concentrator system of FIG. 1 to the remote
terminal;
FIG. 3 is a graphic diagram of the data messages transmitted from the
remote terminal of the concentrator system of FIG. 1 to the central office
terminal;
FIG. 4 is a timing diagram showing how the data message link is derived for
the concentrator system of FIG. 1;
FIG. 5 is a timing diagram for the transmit time slot interchanger of FIG.
1;
FIG. 6 is a timing diagram for the receive time slot interchanger of FIG.
1;
FIG. 7 is a general block diagram showing address and data buses
interconnecting the major elements at each terminal of FIG. 1;
FIG. 8 is a block diagram of the time slot interchangers of FIG. 7;
FIG. 9 is a storage map for the section of the transmit time slot
interchanger;
FIG. 10 is a storage map for a section of the random access memory in the
receive time slot interchanger of FIG. 7;
FIG. 11 is a data storage map for the random access memory of the
microcomputer of FIG. 7;
FIG. 12 is a detailed memory map of the line group data words shown in the
memory map of FIG. 11;
FIG. 13 is a block diagram of the time slot interchanger random access
memory;
FIG. 14 is a block diagram of the common control of the time slot
interchanger of FIG. 8;
FIG. 15 is a more detailed block diagram of the sequencer shown in the
common control of FIG. 14;
FIG. 16 is a detailed block diagram of the supervision collection circuitry
shown in the common control of FIG. 14;
FIG. 17 is a detailed block diagram of the microcomputer--time slot
interchanger interface circuitry shown in the common control of FIG. 14;
FIG. 18 is a timing diagram for the microcomputer--time slot interchanger
interface circuitry shown in FIG. 17;
FIG. 19 is a detailed logic diagram of the synchronizing circuit for the
transmit time slot interchanger of FIG. 8;
FIG. 20 is a timing diagram for the transmit time slot interchanger
synchronizing circuit shown in FIG. 19;
FIG. 21 is a detailed block diagram showing the derivation of a data link
clock in the time slot interchanger of FIG. 8;
FIG. 22 is a timing diagram showing the timing signals for the data link
messages, useful in understanding the operation of the data link clock
circuits of FIG. 21;
FIG. 23 is a detailed block diagram showing the reframing circuitry in in
FIG. 14;
FIG. 24 is a timing diagram associated with the reframing circuitry in FIG.
23;
FIG. 25 shows a detailed block diagram of the initialization circuitry for
the time slot interchanger of FIG. 8; and
FIG. 26 is a timing diagram associated with the initialization circuitry of
FIG. 25.
DETAILED DESCRIPTION
Concentrator System
FIG. 1 is a general block diagram representation of a storage and retrieval
concentrator system comprising a central office terminal (COT) 100, a PCM
digital transmission facility 108, and a remote terminal (RT) 102.
Referring more particularly to COT 100, there is shown a first group 126 of
twenty-four so-called "D-type" channel units CU1, CU2,-CU24 for
periodically producing and receiving pulse amplitude modulation (PAM)
samples from voice messages on lines 1, 2,-24, respectively, for producing
per line signaling information, and for producing per line class of
service information (herein called "transmit not enable" (TNEN) signals).
A second group 128 of twenty-four D-type channel units CU25, CU26,-CU48
performs a similar function for lines 25,26,-48. A more detailed
description of the D-type channel units can be found in U.S. Pat. No.
4,059,731 granted to J. H. Green and J. E. Landry on Nov. 22, 1977.
The first group 126 of channel units is connected with a master
transmit-receive unit (master TRU) 114 through leads 101, 103,-105 for
bidirectional PAM transmission. Similarly, the second group 128 of channel
units is connected with a slave transmit-receive unit (slave TRU) 116
through leads 107, 109,-111 for bidirectional PAM transmission.
In the transmit direction, the master TRU 114 performs the functions of
generating timing pulses to direct sampling and supervision by the first
group of channel units 126, encoding the PAM samples from the first group
of channel units 126 into PCM code groups, multiplexing the PCM code
groups to form a PCM bit stream, and inserting framing bits into the PCM
bit stream to derive a multiplexed digital pulse stream, sometimes
referred to as a DS1 signal. In the receive direction, the master TRU 114
performs the functions of demultiplexing the received DS1 PCM bit stream
to derive the separate PCM code groups, decoding the PCM code groups into
PAM samples, and extracting the timing, framing, and signaling
information. The slave TRU 116 performs similar functions for the second
group of channel units 128. A more detailed description of the
transmit-receive units 114 and 116 can be found in the above-mentioned
Green et al patent.
Both TRU 114 and TRU 116 are connected to a transmit time slot interchanger
(transmit TSI) 120 by leads 113 and 115, respectively, for transmission of
the outgoing multiplexed PCM streams. A central office terminal
concentrator (COT concentrator) 110 comprises the transmit TSI 120, a
receive time slot interchanger (receive TSI) 122 and a microcomputer 124,
interconnected by a bus 121 for address and data communication. The
receive TSI 122 and the TRU's 114 and 116 are connected by leads 117 and
119, respectively, for transmission of incoming multiplexed PCM bit
streams.
The multiplexed outgoing PCM bit streams on leads 113 and 115 are
demultiplexed at the transmit TSI 120 and the PCM code groups are
individually stored. Up to twenty-four of the stored PCM samples are
selectively retrieved from the transmit TSI 120, multiplexed, and the
framing bit from the PCM bit stream on lead 113 inserted to derive a
concentrated PCM bit stream for transmission on lead 123 to the remote
terminal concentrator (RT concentrator) 112. Each of the twenty-four PCM
samples selectively retrieved from the transmit TSI 120 is assigned to a
unique one of the twenty-four time slots available in each frame on
transmission facility 108. Each time slot in facility 108 is referred to
as a concentrator trunk.
The assignment of a subscriber line to a concentrator trunk is controlled
by the microcomputer 124 at the COT concentrator 110. The assignment
information is transmitted to the RT concentrator 112 for duplicating the
same assignments at the remote terminal. This assignment information is
transmitted from the COT concentrator 110 to the RT concentrator 112 via
data messages. These data messages are assembled at the microcomputer 124
and transmitted through the transmit TSI 120 embedded, as will be
described, in the concentrated PCM bit stream. Similarly, other
information is transmitted between the concentrator terminals 110 and 112
via data messages.
DATA LINK MESSAGES
Central Office Terminal to Remote Terminal
Referring temporarily to FIG. 2, there are shown a plurality of
concentrator data messages that are transmitted from COT concentrator 110
to RT concentrator 112. Message I is a trunk assignment data message
comprising three words. Each word comprises 11 bits divided into two
fields. The first field comprises five bits for a trunk identification
number (2.sup.5 =32). The second field comprises six bits for the line
identification number that is assigned to the trunk identified in the
first field (2.sup.6 =64). The second and third words repeat the same
information contained in the first word, protecting against errors in
transmission by triple redundancy. If the RT concentrator 112 receives two
out of the three words which match, it assumes the message has been
correctly received.
Message II comprises a data message to convey trunk deassignment
information. There are again three words, each comprising two fields. The
first field is a trunk identification field comprising five bits. The
second field is a line identification deassignment code comprising six
bits. To protect against errors in transmission, the information contained
in the first word is repeated in the second and third words.
Message III is a data message for an assignment update. Assignment update
messages are sent periodically to assure that the recorded assignment at
the RT concentrator 112 is accurate. The assignment update message
comprises three words. The first word is a distinguishable header A. The
second word comprises two fields. The first field is a trunk
identification field comprising five bits and the second field comprises
six bits, representing the line identification assigned to the trunk in
the first field. Word three comprises the complement of the second word
for error protection.
Message IV is a data message for a data link looping test. The data link
looping test (not to be confused with the PCM looping test which will be
described in detail later and which is performed after trunk assignment
but before establishing connection at the beginning of every call) is an
autonomous test for verifying the integrity of the data link connection
itself. This message is sent from the COT concentrator 110 to the RT
concentrator 112 and, as will be described in connection with FIG. 3,
Message IV, is transmitted back from the RT concentrator 112 to the COT
concentrator 110 to establish the integrity of the data link. If either
terminal does not receive the looping test message at least once every
second, an alarm is sounded. The looping data link test message IV
comprises three words, each having eleven bits. The first word is the
header A. The second word comprises a distinguishable request code B; and
the third word is a compliment of the request code B in the second word,
again, for error protection.
Message V is a data message for an activity update request. Activity
information refers to the on-/off-hook status of the subscriber stations
and is part of the line-associated signaling information mentioned earlier
in connection with FIG. 1. Periodically, activity information stored in
the transmit TSI 120 and in the microcomputer 124 is updated. Activity
information generated at the remote terminal must be transmitted from the
remote terminal to the central office terminal because all assignment
decisions are made at the COT concentrator 110. In response to a command
from the microcomputer 124, an activity update request is transmitted to
RT concentrator 112. The activity update request Message V is a data
message comprising three words, each eleven bits long. The first word is a
distinguishable request code C. Words two and three of Message V are
repeats of the request code C in word one.
Message VI is a data message representing an idle state. The idle message
comprises a word eleven bits long for transmission of an idle code D. An
idle code is transmitted repetitively whenever none of the other data
messages are being transmitted.
REMOTE TERMINAL TO CENTRAL OFFICE TERMINAL
Referring temporarily to FIG. 3, there are shown the data messages that are
transmitted from the RT concentrator 112 to the COT concentrator 110.
Message I represents an activity data message for activity information to
be transmitted from RT concentrator 112 to COT concentrator 110. Activity,
as stated earlier, represents the on-/off-hook status of the subscriber
stations. Activity information is transmitted from the RT concentrator 112
in response to a change in status of a subscriber line, as will be
described hereinafter.
The activity Message I of FIG. 3 comprises three words. The first word
comprises two fields. The first field identifies a line group and
comprises three bits. Lines 1, 2-48 are divided, for convenience, into six
groups of eight lines each. Hence, three bits are required to identify any
of the six line groups (2.sup.3 =8). The second field comprises eight bits
to convey activity information (1 or 0) for all eight lines in the
identified line group. Each bit represents the on-/off-hook status for one
line in the identified line group. A "1" indicates off-hook and a "0"
indicates on-hook. Words two and three are repeats of word one to guard
against errors in transmission. If the COT concentrator 110 receives two
out of the three words which match, it assumes that the message has been
correctly received.
Message II represents an activity update message. Activity update messages
are transmitted in response to activity update request messages from the
COT concentrator 110, shown in FIG. 2, Message V, to insure that COT 110
has registered the correct activity information. Activity update messages
comprise three words, each eleven bits long. The first word is the header
A. The third word comprises two fields. Field one contains three bits and
represents the line group identification as was described in connection
with Message I. Field two contains eight bits and represents the activity
of the eight lines in the line group identified in field one. The second
word is the complement of the information contained in the third word.
Message III is a data message for transmission of the "no alarm" condition,
sent periodically by RT 112 to COT 110. The first word represents a no
alarm code E which is repeated in the second and third words. The periodic
transmission of the no-alarm code provides fail-safe alarm information
from the RT 102 to COT 100.
Message IV represents the data message for the data link looping test
described earlier in connection with FIG. 2, Message IV. The looping test
message comprises three words. The first word is the header A. The second
word is a compliment of the request code B and word three is the request
code B (See Message IV in FIG. 2).
Message V represents an assignment update request data message. The
assignment update request is transmitted from the RT concentrator 112
whenever the memory is initialized (as when there is a momentary loss of
power) or whenever the assignment information is determined to be
outdated. This message is required only for applications where the central
office switch (not shown) is digital and can itself perform COT
concentrator 110 functions. As mentioned in connection with Message III of
FIG. 2, COT concentrator 110 periodically transmits assignment update
information to the RT concentrator 112 without the need for assignment
update requests. Consequently, when the central office switch is an analog
machine, the assignment update requests from RT concentrator 112 are
ignored. Assignment update request messages comprise three words, each
eleven bits long. A request code C is transmitted as the first word. The
second and the third words repeat the information contained in the first
word.
Message VI represents an idle code data message comprising eleven bits. The
idle code D is transmitted whenever none of the previous messages is
needed.
DATA LINK DERVIATION
Referring temporarily to FIG. 4, there are shown timing diagrams that
illustrate how the data link message channel is derived. Timing diagram I
shows a PCM word comprising eight bits. As mentioned earlier in connection
with FIG. 1, information signals on lines 1, 2-48 are sampled by the first
group 126 of channel units and the second group 128 of channel units to
produce PAM samples. These PAM samples are then encoded by the TRU's 114
and 116 to produce eight-bit PCM words. In every sixth frame, the eighth
bit is used for transmitting per line signaling information. The signaling
information transmitted relates, for example, to ringing signals to be
applied to subscriber lines.
Timing diagram II represents one frame of information transmitted in 125
microseconds. One frame comprises twenty-four eight-bit PCM words
(24.times.8=192 bits) and one framing bit in the 193rd bit position. Each
PCM word has a format shown in diagram I. Every sixth frame, the eighth
bit in all twenty-four PCM words is used for transmitting per channel
signaling information, one bit for each of the twenty-four channels.
Diagram III shows seventy-two consecutively numbered frame, (72.times.1/8=9
ms) each frame having a format as shown in diagram II. In diagram IV there
is shown the terminal framing bit pattern F.sub.T appearing in the 193rd
bit positions in odd numbered frames. The terminal framing bit pattern
comprises an alternating sequence of "1's" and "0's" and permits overall
framing at the receiving terminal.
Diagram V represents the supervisory framing bit pattern F.sub.S appearing
in the 193rd bit positions of even numbered frames. The pattern comprises
three consecutive zeros followed by three consecutive ones. The change
from zeros to ones and the change from ones to zeros in the supervisory
framing bit pattern mark every sixth frame for recovering the per line
signaling information contained in the eighth bit position of the PCM
words therein. Out of seventy-two consecutive frames, only twelve of the
thirty-six supervisory framing bit positions are needed for framing. The
remaining twenty-four bit positions may therefore be used for other than
framing information. Use is made of some of these bit positions to derive
a data link for transmission of the data messages of FIGS. 2 and 3 between
COT 100 and RT 102. Eleven of the twenty-four unused supervisory framing
bit positions constitute a 1,222 bps data link ((11 bits.div.9
ms).times.1000 ms/sec=1,222 bits/sec) for transmitting data messages
relating to the concentrator functions. These data messages were described
in detail in connection with FIGS. 2 and 3. In the U.S. patent application
of Mr. J. E. Landry, Ser. No. 966,637, filed Dec. 5, 1978, and assigned to
applicant's assignee, the derived data link is described in more detail.
CONCENTRATOR SYSTEM (CONTINUED)
Referring back to FIG. 1, there is shown a lead 127 for transmitting data
messages originating at microcomputer 124, passed via bus 121 to TSI 120
and, one eleven bit data word at a time, from transmit TSI 120 to data
link unit (DLU) 118. The eleven bits in DLU 118, are transmitted, one bit
at a time, over lead 129 to the master TRU 114 for insertion in the
appropriate 193rd bit positions constituting the data link on the pulse
stream on lead 113. The slave TRU 116 inserts normal framing bit patterns
in the 193rd bit positions of the pulse stream on lead 115. Synchronizing
signals are transmitted from the master TRU 114 to the receive TSI 122, as
will be described later. It is for these reasons that the master TRU 114
is called the master unit.
The PCM streams from TRU's 114 and 116 are transmitted over leads 113 and
115, respectively, to transmit TSI 120 where up to twenty-four PCM samples
may be selectively multiplexed for transmission over lead 123, thereby
achieving the concentrator function at the COT concentrator 110. At
transmit TSI 120, the framing bits from the master PCM stream from master
TRU 114 are inserted in the 193rd bit positions of the concentrated PCM
bit stream, while the framing bits from the slave TRU 116 are discarded.
To minimize errors in transmission, the outgoing unipolar PCM bit stream on
lead 123 is converted to a bipolar pulse stream by the line interface unit
(LIU) 126. The outgoing bipolar concentrated PCM pulse stream transmitted
to RT 102 from LIU 126 is passed through a pulse repeater 104 which is one
of a plurality of such repeaters. The incoming bipolar concentrated PCM
stream, received from the RT 102 is similarly passed through a pulse
repeater 106 which is one of a plurality of such repeaters. The received
concentrated bipolar bit stream is then converted from bipolar to unipolar
signals at LIU 126. The received concentrated PCM stream is transmitted
from LIU 126 to the receive TSI 122 over lead 125. The LIU 126 also
generates a 6.176 MHz transmit clock signal for the transmit TSI 120, and
the transmit portion of master TRU 114, and the transmit portion of the
slave TRU 116. From the concentrated PCM stream received from the remote
terminal 102, a receive clock signal is extracted at LIU 126 for the
receive TSI 122 and the receive portions of the master TRU 114 and slave
TRU 116.
The concentrated PCM stream received on lead 125 is selectively stored in
the receive TSI 122 and sequentially retrieved to derive two separate
multiplexed PCM pulse streams of twenty-four words per frame each, thereby
performing the expansion function. The expansion function performed at the
receive TSI 122 is the exact opposite of the function performed during the
concentration stage at transmit TSI 120. Receive TSI 122 and transmit TSI
120, however, operate independently and asynchronously of each other. The
expanded PCM streams are transmitted over leads 117 and 119 to master TRU
114 and slave TRU 116, respectively.
Lead 131, bridged to lead 117, is connected to DLU 118. In order to extract
the data link messages from the received PCM stream, timing pulses
occurring in the 193rd bit position of the received pulse train from
master TRU 114 are transmitted over lead 135 to DLU 118. Data link
messages, extracted by using these timing pulses, are transmitted back
from DLU 118 to receive TSI 122 over lead 137 in order to take advantage
of the interface with microcomputer 124 in receive TSI 122. At the receive
TSI 122, the data messages are read by microcomputer 124, decoded and
appropriate action taken. A similar function is performed at the RT
concentrator 112.
Signaling information, e.g., ringing signals, from the first group 126 of
channel units is detected and sequentially and periodically transmitted
from each channel unit over a common bus 139 to transmit TSI 120.
Similarly, signaling information from each channel unit in the second
group 128 is detected and sequentially and periodically transmitted over a
common bus 141 to transmit TSI 120. Signaling information thus collected
from all forty-eight lines is stored in six activity words of eight bits
each in TSI 120. In a signaling activity word, each bit represents the
activity of one line. Off-hook information, collected in TSI 164 at RT
102, is also transmitted to COT 100 via the data link and stored as
activity words in microcomputer 124.
On command, an activity word is transmitted from the transmit TSI 120 to
microcomputer 124 in order to determine if a change in status of the
activity for a line has occurred since the last time the activity word was
examined. Similarly, activity words from RT 102 stored in the
microcomputer memory, RAM 704, is examined. If the status of a line has
changed, i.e., if a line is determined to have received a ringing signal
or has gone off-hook, thereby requesting service, an idle trunk must be
assigned to that line. If a line has gone on-hook, the trunk assigned to
that line must be deassigned. Consequently, trunk assignment and
deassignment data words (FIG. 2) are assembled in microcomputer 124 for
transmission through transmit TSI 120 and the data link to the RT
concentrator 112. Activity status for all forty-eight lines are thus
maintained at the COT concentrator 110 for both originations at the COT
100 and originations at the RT 102.
In the transmit direction, synchronization is obtained by pulses
transmitted from transmit TSI 120 over lead 143 to master TRU 114 and to
slave TRU 116 (lead not shown). In the receive direction, TRU's 114 and
116 and receive TSI 122 are synchronized to the concentrated PCM stream
received from the RT 102. If framing is lost at the master TRU 114, an
out-of-frame (OOF) signal is transmitted over lead 145 to the receive TSI
122; normal processing is inhibited and a special out-of-frame mode of
operation is entered until framing is recovered at the master TRU 114.
When framing is recovered at the master TRU 114, the out-of-frame signal
is removed from lead 145. On recognition of this state at the receive TSI
122, the out-of-frame mode of operation is discontinued and the normal
mode of operation is resumed.
Referring to the RT 102 in FIG. 1, subscriber stations 49, 50-72 are
connected to a third group 150 of channel units CU1, CU2,-CU24,
respectively. Similarly, the subscriber stations 73, 74,-96 are connected
to a fourth group 152 of channel units CU25, CU26-CU48. The third group
150 of channel units is connected to a master TRU 154 through leads 151
for bidirectional PAM transmission. Similarly, the fourth group 152 of
channel units is connected to a slave TRU 156 through leads 153 for
bidirectional PAM transmission.
RT 102 is identical in most respects to the COT 100 and performs the same
functions as described earlier in connection with COT 100. However, RT 102
may be distinguished from the COT 100 in the following functions. All
information necessary for the assignment of idle trunks to busy lines is
stored only at COT concentrator 110. Necessarily then, all assignments are
also performed only at COT concentrator 110. After assignment of an idle
trunk to a busy line, but before establishing a connection, a PCM looping
test is performed to ensure continuity of the path and to verify that all
essential components are indeed functional for establishing a connection.
This looping test is performed under control of the microcomputer 124 at
the COT concentrator 110.
TRANSMIT TSI TIMING
Referring to FIG. 5, there are shown timing diagrams associated with the
functions performed at the transmit TSI 120. Diagram I is a 1.544 MHz
master transmit clock derived from the transmit clock obtained from LIU
126 in FIG. 1. A control counter (discussed later) in the transmit TSI 120
generates frames of 193 clock pulses to correspond with the 193 bit
positions in a frame. The states corresponding to each of the clock pulses
are numbered consecutively 0, 1, 2,-192 (see diagram II). Each control
counter state corresponds to a control period or memory cycle
(1.div.1,544,000=648 nanoseconds) during which a function must be
performed at the TSI's in terminals 100 and 102.
Some functions are performed more often than others. For purposes of
understanding, then, the control counter states are divided into groups,
words and substates. Control counter states 0, 1, 2-7 are called substates
X0, X1, X2,-X7, respectively, and are repeated regularly as shown in
diagram III. The eight substates are grouped to form a control counter
word (corresponding to a PCM word). Four control counter words W0, W1, W2,
and W3 form a control counter group.
Since there are 193 control counter states, there are seven groups G0, G1,
G2,-G6. In the first six control counter groups, all four words and all
eight bits in each word are legitimate and correspond to the first 192
control counter states (6.times.4.times.8=192). Therefore, in the seventh
group, G6, only the first substate, X0, in word W0, is legitimate. All
other words and substates in G6 are illegitimate.
For example, control counter state 190 will correspond to group G5, word
W3, and substate X6, or G5W3X6. Control counter state 192 will correspond
to group G6, word W0 and substate X0 or G6W0X0.
Referring to diagram IV, there are shown the individual bit positions in
control counter word format received at transmit TSI 120 in FIG. 1 from
master TRU 114 and from slave TRU 116. The multiplexed PCM stream
PCM.sub.M, from master TRU 114, and the multiplexed PCM stream PCM.sub.SL,
from slave TRU 116, arrive simultaneously at transmit TSI 120 shown in
FIG. 1. At transmit TSI 120, the PCM.sub.M stream is clocked with the
master clock shown in diagram I to derive the word format shown in diagram
V. This word format appears with approximately a one-half memory cycle
delay. Similarly, the PCM.sub.SL stream is shown in waveform VI. However,
as only one operation can be performed during any one memory cycle at the
transmit TSI 120, the PCM.sub.SL bit stream is delayed by one and one-half
memory cycles.
Referring to diagram VII, there are shown the bit positions in the
concentrated PCM bit stream, PCM.sub.LIU, for transmission on lead 123
shown in FIG. 1.
Referring to diagram VIII, there is shown a segment, in block format, of a
series of control counter words corresponding to the PAM sampling periods
during which supervisory information from channel units in the first group
126 and the second group 128 shown in FIG. 1 is transmitted over common
buses 139 and 141 to the transmit TSI 120. Because about four substates in
each control counter word are required to permit the information to settle
down (shown as Xs in diagram VIII), only during the last four substates of
a PCM word is the information made available for use at the transmit TSI
120. Diagram IX shows some of the functions that are performed in the
substates and will be described in detail later.
RECEIVE TSI TIMING
Referring to FIG. 6, there are shown timing diagrams for receiving TSI 122
shown in FIG. 1. Diagram I is a 1.544 MHz master receive clock received
from LIU 126. Diagram II represents control counter states similar to the
control counter states in timing diagram II shown in FIG. 5. During each
control counter state, a function is performed at the receive TSI 122. The
control counter states are grouped into substates, words, and groups in
the same manner as was described in connection with FIG. 5.
Diagram III of FIG. 6 shows a segment of the sequence of the control
counter substates in a frame. Diagram IV is the sequence of the bit
positions in multiplexed PCM words, PCM.sub.LIU, received on lead 125 from
LIU 126. The PCM.sub.LIU pulse stream is clocked with the master receive
clock shown in Diagram I. This results in a delay and is shown shifted by
half a memory cycle in timing diagram V.
Receive TSI 122 expands the received concentrated bit stream PCM.sub.LIU
into two PCM streams. One PCM stream, PCM.sub.M, is transmitted over lead
117 to master TRU 114. The second PCM stream, PCM.sub.SL, is transmitted
over lead 119 to slave TRU 116. Both of the expanded PCM streams,
PCM.sub.M and PCM.sub.SL, are transmitted simultaneously and in
synchronism and are shown in diagram VI.
Referring to diagram VII of FIG. 6, there is shown a different segment of
the same sequence of the control counter states shown in diagram II.
Diagram VIII is the PCM.sub.M and PCM.sub.SL bit positions corresponding
to the control counter states shown in diagram VII. When master TRU 114 in
FIG. 1 loses framing, an out-of-frame signal is transmitted over lead 145
to receive TSI 122. After framing is recovered, the out-of-frame signal
present on lead 145 of FIG. 1 is removed. Simultaneously, a clock signal,
RIFT.sub.M, shown in diagram IX, is transmitted from master TRU 114 to
receive TSI 122 (lead not shown). The RIFT.sub.M clock signal is gated
with the master clock shown in diagram I to produce a clock pulse one
memory cycle wide shown in diagram X as RIFT.sub.M, MC clocked. The
RIFT.sub.M clock pulse presets the control counter to a preselected state,
e.g., to state fifteen.
MICROCOMPUTER
Referring to FIG. 7, there is shown in block diagram representation a
concentrator terminal for use at either COT 100 or RT 102 in FIG. 1. At
the concentrator terminal, a microcomputer (i.e., microcomputer 124 at COT
110 or microcomputer 160 at RT 112 in FIG. 1) comprises a microprocessor
700 connected to a read only memory (ROM) 702, a random access memory
(RAM) 704, and input/output ports 710 through a common address bus 701 and
a common data bus 703. A master PCM bit stream, PCM.sub.M, from master TRU
114 and a slave PCM bit stream, PCM.sub.SL, from slave TRU 116 in FIG. 1
are transmitted over leads 705 and 707, respectively, to transmit TSI 706
(TSI 120 at COT 100 or TSI 164 at RT 102). Under direction of the
microprocessor 700, up to twenty-four PCM words from the PCM.sub.M and the
PCM.sub.SL bit streams are selectively retrieved from transmit TSI 706 and
multiplexed to derive a concentrated PCM bit stream PCM.sub.LIU for
transmission over lead 709 (i.e., lead 123 in FIG. 1) to RT 102 shown in
FIG. 1.
The concentrated PCM stream, PCM.sub.LIU, received from RT concentrator
112, shown in FIG. 1, is transmitted over lead 711 to a receive TSI 708
for expansion of the received concentrated PCM stream. The two expanded
PCM bit streams, PCM.sub.M and PCM.sub.SL, are transmitted over leads 713
and 715, respectively, to master TRU 114 and slave TRU 116 of FIG. 1.
Communication between equipment other than the TSIs 706 and 708 and
microprocesso | | |
