GB2061673A - Transmitter-receiver synchronizer - Google Patents

Abstract

A transmitter-receiver synchronizer is described for use in terminating T1 lines from a subscriber line switch at a central time division multiplexed switching system. A shared rate converter provides bidirectional data rate conversion between the 1.544 Mb/s T1 line and the 2.048 Mb/s switching system. This converter uses two buffer memories (1, 2) so controlled as to effect the above conversion. Thus during a first frame time interval a first data frame at the first rate is stored in the first buffer memory and a second data frame is stored at the second rate in the second buffer memory. During a second frame time interval, the frame in the first buffer memory is read at the second data rate, and the frame in the second buffer memory is read at the first data rate. Also during the second interval another frame at the second data rate is read into the first buffer memory and another frame at the first data rate is read into the second buffer memory. Thus the functions of the two buffer memories alternate on successive frame time intervals.

Description

SPECIFICATION Transmitter-receiver synchronizer This invention relates to telephone switching systems, in general, and to apparatus for interfacing digital transmission lines and digital telephone switching systems in particular.

Where a digital transmission line is used to interface a digital switching network of a Central Office with remote equipment such as a remote line switch, a channel bank of another digital office, it is known to provide a carrier terminal unit or span interface to provide terminating functions for the transmission line. The primary digital carrier DS1, used in North America, has 24 pcm channels at a 1.544 Mb/s bit rate with D2/D3 signalling format. In a recently developed family of time-division multiplexed switching systems developed by ITT North Electric Company previously identified as " DSS" and now referred to as "System 1210", it was found to be particularly advantageous to switch voice paths at a 2.048 Mb/s bit rate. Various aspects of the "DSS" or now "System 1210" family of systems have been described by N. J.

Skaperda in "Generic Digital Switching Systems", International Switching Symposium, Vol. 1, October 1976; by C. G. Svala in "DSS-1, A Digital Local Switching System with Remote Line Switches", Proceedings of the National Telecommunications Conference, p 3915-1, 1977; and by F. Fellinger in "Modular Digital Switching Network", International Communications Conference, June 4-7, 1978.

A carrier terminal unit such as mentioned above is particularly suited for interfacing between a remote line switch and a central office. Where a local line switch is used, complex synchronization equipment such as needed to operate the remote line switch is not needed, since due to the proximity of the local line switch to the digital network, the network clock can be distributed directly to the line switch to provide timing, i.e., it is not necessary to derive clock signals from the incoming pcm data stream. Further, framing circuits, and the elastic store arrangement, unipola/bipolar and bipolar/unipolar conversions of the data, as used in a carrier terminal unit, are not needed. However, it is necessary to provide an interface between the local line switch and the network which provides rate conversion, bit alignment and signalling format conversion functions.

An object of this invention is to provide an arrangement for data rate conversion, especially suitable for the above-mentioned application.

According to the present invention there is provided apparatus for providing bidirectional rate conversion between frame of data having a first rate and a first format and frames of data having a second rate and a second format; which apparatus includes: (a) means for storing a first frame of data at said first rate in a first memory means during a first frame interval; (b) means for storing a second frame of data at said second rate in a second memory means during said first interval; (c) means for reading said stored first frame of data stored in said first memory means at said second rate during a second frame interval; (d) means for reading said stored second frame of data stored in said second memory means at said first rate during said second interval; (e) means for storing a third frame of data at said second rate in said first memory means during said second interval; and (f) means for storing a fourth frame of data at said first rate in said second memory means during said second interval.

An embodiment of the invention will now be described in conjunction with the drawings, in which: Figure 1 illustrates in block diagram form a transmitter-receiver synchronizer embodying the invention; Figure 2 is a timing diagram of the read and write operations of the buffer memories of Fig.

1; Figure 3 is a more detailed block and schematic diagram of a transmitter-receiver synchronizer; Figure 4 is a schematic diagram of timing and control circuits for the arrangement of Fig. 3; Figure 5 is a timing diagram of various signals; and Figure 6 is a timing diagram illustrating signalling conversion.

In one system embodying the invention, data is transmitted to and received from a line switch at a 1.544 Mb/s rate as 192 bits of data arranged as 8 bits in each of 24 channels plus an additional bit for signalling or framing, i.e. a total of 1 93 bits every 1 25 microseconds. Data is transmitted and received from the network at a 2.048 Mb/s rate as 240 bits of data arranged as 10 bits (8 bits + parity + signalling) in each of 24 channels and 1 6 "unassigned" bits or a total of 256 bits every 1 25 microseconds.

The transmitter-receiver synchronizer described herein converts data coming from the line switch to the higher rate before transmitting the data to the network. Data from the network is converted to the lower rate before it is transmitted to the line switch.

Fig. 1 is a simplified block diagram of the transmitter-receìver synchronizer. Two buffer memories 1 and 2 are provided for storing data, each being a random access memory which in the arrangement shown is a 256 word by 1 bit commercially available device. Data is stored in a buffer memory one frame at a time and, one frame later, is read out at the different rate.

Associated with the operation of the buffer memories 1 and 2 are two parity registers 3 and 4, each comprising a 24 bit shift register, two data selectors 5 and 6, a two-bit storage register 7 and a Read/write (R/W) flip-flop 8.

The transmitter-receiver synchronizer is connected to the line switch via lines 9 and 10, with data transmitted from the line switch over line 9 and received by the line switch over line 10.

Connections to the network are provided via lines 11 and 12, with data transmitted from the network over line 11 and data received by the network over line 12.

If it is assumed that the R/W flip-flop 8 is set such that its output A' is active, addresses supplies to buffer memory 1 over lead AO-A7 are incremented at a 1.544 MHz rate and addresses supplied to buffer memory 2 over leads AO'-A7' are incremented at a 2.048 Milz rate. The memory addresses are supplied from a set of eight bit counters clocked at the respective frequencies.

With A' active, data selector 5 connects the transmit line 9 from the line switch to the data input to buffer memory 1 and the transmit line 11 from the network to the data input of buffer memory 2. Data selector 6 connects the receive line 10 of the line switch to the data output of buffer memory 1 and connects the receive line 12 of the network to the data output of buffer memory 2.

As shown in the timing diagrams of Fig. 2 during the first half of the 1.544 MHz cycle, data from the network stored during the previous frame is read frorn- buffer memory 1 and transmitted to the line switch; during the second half of the cycle, data received from the line switch is stored in buffer memory 1. During the same frame, data is tra-nsferred from buffer memory 2 to and from the network. During the first half of the 2.048 MHz- cycle, data from the line switch stored during the previous frame is read from buffer memory 2 and transmitted to the network; during the second half of the cycle, data received from the network is stored in buffer memory 2.

At the end af a frame, the R/W flip-flop 8 changes state, As a result, output A becomes active and A'- becomes inactive. Now, the addresses supplied to buffer memory 1 over leads AO-A7 are incremented. at a 2.048 MHz rate while addresses supplied to buffer memory 2- over leads AO'-A7' are incremented at a f .544 I\4Hi rate. Data selector 5 connects the fransmit line 1.1 from the network to the data input of buffer memory I and the transmit line 9 from the line switch to buffer memory 2. Data selector 6 connects the receive line 12 of the network to the.

data output of buffer memory 1 and the receive line 10 of the line switch to the data output of buffer memory 2: Buffer memory- 1 is now- read at a 2.048 MHz rate to supply the previous frame data from the network is written into buffer memory 1. Similarly, buffer memory 2 is now read at a 1 .544 MHz rate to supply the previous frame data from the network to the line switch and data received from the line switch is written into buffer memory 2.

There are- two bits of each frame that are not stored in the buffer memories 1 and 2. The frame of data transmitted from the: line switch is delayed 221 to 3 bit-times from the frame supplied on the receive line of the line switch. This delay represents a 22 bit delay in the line switch itself as well as a variable delay, up to one bit time, due to-cable lengthy variations.

Because ofthis delay, the last two bits in a frame from the line- switch will not have arrived at the. time the R/W flip-flop 8 changes state. In fact, by the time- these last bits- do arrive at the transmitter-receiver synchronizer, the buffer memory that they would have been written into is already being read and receiving? data from the network-at the 2.048 Mb/s rate. Accordingly, these two bits are loaded into the- storage resister 7 from wh-ich they- are subsequently multipled into the data. stream to the network. by the data selector-6.

The delay problem. does not exist- for data- coming from the network because the last data bit of a frame is written six time slots--befdre. the end af the frame Fig. 3 illustrates the transmitter-receiver synchronizer ofFig.- 1 in greater detail.The data selector 5 of Fig. 1 is replaced by its component blocks DS1 and~DS2, and the data selector 6 is replaced by its component blocks. DS3 and DS-4. At~the input from-the line switch, a differential~ receiver 3-1 receives data over the digital lines 9 and feeds the data into the bit alignment flip-flop 33, which reclocks the- data to equalize variations in loop delay. This- is done so that a fixed rather thaJrvariable.delay appears at the input to the- buffer rnemories. The data at the output of the flip-flop 33 is checked by a data line-monitor circuit 34. The D2 framing pattern inserted into the data in the. 193rd bit tim-e every alterna-te:frame- is compared as it comes from the line switch- agairrst a a?pattern.which was inserted from pattern generator 413 (see: Fig 4) of the timing an control circuits; If~a rnismatchoccurs-, an error spinal is-generated The bit aligned data from the flip-flop 33 is-fe-d:to- data selector DS2 a signalling conversion flip-flop 35, and a parity generation flipflqp 36. Data selector DS2 directs data to either buffer memory 1 or buffer memory 2, depending on the state of the R/W flip-flop 8, Fig. 4.The signalling conversion flip-flop 35 extracts the signalling bit from the data and stores it for later insertion into the 2.048 Mb/s data stream to the network.

Odd parity is generated over the input data from the line switch with parity generator flip-flop 36 and is strobed into either parity register 3 or parity register 4 via data selector DS1 as determined by signal WPR' from the delay, decode and gating logic 401 (Fig. 4). The flip-flop 36 toggles for each bit that is a logic "1", such that at the end of eight bits, the output is a logic "1" if the number of bits was even and a logic "0" if the number of bits was odd. At the end of the eighth bit, the flip-flop is reset by the signal PGFCI. This reset signal is generated by the logic 401. This odd parity is generated over each 8-bit channel received from the line.

At the output of the buffer memories 1 and 2, the data selector DS4 selects data at 2.048 Mb/s and gates it to the network, selecting the parity bit (9th bit of every channel) from the parity register and the signalling bit (10th bit of every channel) from the signalling conversion flip-flop 35 thereby converting common channel signalling to inband signalling.

The data selector DS4 is an 8 to 1 data selector, and a three-bit selection address is provided for it over leads BO', B1 and A from the bit address counter 402 and R/W flip-flop 8 of Fig. 4.

At the output of data selector DS4, odd parity is checked by parity checker 50 to insure error free operation of the buffer memories 1, 2 and the parity registers 3, 4. Parity is checked by comparing a regenerated parity bit with the parity bit obtained from the parity registers. The parity check circuit 50 regenerates parity over every channel by means of a flip-flop (not shown).

The parity check logic is strobed by a signal PCA25 during the eight bit of a channel and a parity check alarm PCA is generated if wrong parity is obtained. The parity check circuit 50 is cleared by the PCFCR signal during the tenth bit of every channel.

On the outgoing side of the transmitter-receiver synchronizer, the previously described process is reversed. 2.048 Mb/s data from the network is converted back to 1.544 Mb/s data and transmitted out to the line switch. The data, as it comes from the network, goes to signalling conversion flip-flop 51 and to data selectors DS1 and DS2.

The flip-flop 51 extracts the inband signalling bit, the tenth bit of every channel, and stores it for insertion into every alternate 1 93rd bit position at the outpout to the line switch. Data selector DS1 provides the data from the network to either parity register 3 or 4 and data selector DS2 provides the data to either buffer memory 1 or 2 depending on the state of R/W flip-flop 8. Thus data from the network is present at the inputs of a selected parity register and of a selected buffer memory. At the selected parity register, clock signal B1 is used to clock the ninth bit of every channel, which is the parity bit, into the selected register. Likewise, at the input to the selected buffer memory, write-enable signal WE-2 is used to strobe the eight bits of data per channel into the selected buffer memory.The ninth and tenth bits per channel, i.e., parity and signalling are not stored in the selected buffer memory.

At the output of the buffer memories, the stored data is converted to a 1.544 Mb/s data stream. Data selector DS-3 selects data from one of the buffer memories and sends it to the line switch via data buffer flip-flop 53 and bus driver 54 over line 55. In a manner similar to that described for data selector DS-4, data selector DS-3 selects parity from the appropriate one of parity registers 3 or 4, for comparison against parity regenerated from data read from the associated buffer memory. Parity regeneration and checking is provided by parity check circuit 56. In addition, data selector DS-3 inserts the 1 93rd bit signalling information from signalling conversion flip-flop 51 into every alternate frame and the D2 framing pattern obtained over lead D2FP into the intermediate frames.

The data read from the buffer memories 1 and 2 at 1.544 Mb/s is available only during the read half of a cycle, and the duration of a data bit needed at the line switch is equal to the full cycle. Hence, the data buffer flip-flop 53 is employed at the output of data selector DS-3 to provide the required stable bit duration.

The timing and control circuits for the transmitter-receiver synchronizer are shown in Fig. 4. In addition, Fig. 5 illustrates the timing waveforms for signals provided by the timing and control circuits. A list of the signals and definitions is provided in the table presented below.

TABLE OF SIGNALS Name Definition SESIN Signaling bit extraction signal (incoming FESIN Framing bit extraction signal (incoming) C4 4 KHz clock C8 8 KHz clock C8-1 8 KHz clock delayed 244 nsec C8-2 8 KHz clock delayed 488 nsec C8-3 8 kHz clock delayed 488 + 324 = 812 nsec FASIS Framing and signaling bit insertion signal C2 2.048 MHz clock Cl 1.544 MHz clock PCFCR Parity check flip-flop clear (2.048 Mb/sec data) BI Clock to 2.048 MHz parity register BO LSB on address to DS-4 WE-2 Write enable to the RAMs (2.048 MHz) PCA2S Strove for parity check error (2.048 MHz) C2SC Strobe for 2.048 Mb/sec data into the parity check flip-flop PGFCI Parity generation flip-flop clear signal PGFCO Parity check flip-flop clear (1.544 MHz) WPR Clock to 1.544 MHz parity register WE-1 Write enable to RAMs (1.544 MHz) CIST Strobe for 1.544 Mb/sec data into the parity check flip-flop PCA1 S Strobe for parity check error (1.544 MHz) PIST Strove for 1.544 Mb/sec data into parity generation flip-flop CIDEL Delayed C1 D2FP D2 framing pattern PCA Parity check alarm The main elements in the timing and control circuits are two 1.544 MHz counters 408 and 409, a 2.048 MHz counter 407, a frame reference delay circuit 403, a bit-address counter 402, address selectors 404, 405, 406, the R/W flip-flop 8 and delay, decode and gating circuits 401 and 412. The gates 411 and 422 and error detector 410 are provided for hardware error detection.

The 1.544 MHz counters 408 and 409 respectively provide the read and write addresses to the buffer memories 1 and 2, and they differ by one channel count and are reset at different instants of time. Network data to the transmitter-receiver synchronizer is delayed by one channel and data is therefore written into addresses 8 to 200 of a selected buffer memory. Hence the read counter 408 must be reset at the beginning of each frame to address 8 to properly read the data in the subsequent frame. Because of transmission delays to the line switch and back, the write counter 409 must be initialized to address zero one bit time after the beginning of each frame.

The 2.048 MHz counter 407 generates the 'Read Addresses' for reading data at 2.048 Mb/s from the buffer memories to the network and the 'Write Addresses' for writing data at 2.048 Mb/s from the network to the buffer memories. Counter 407 is clocked with the 2.048 MHz clock, but during every ninth and tenth clock pulses, the counter 407 is disabled and the count is maintained. This is necessary because during reading a buffer memory at 2.048 MHz, parity is added during the ninth bit and signalling during the tenth bit of every channel, and during memory writes at 2.048 MHz the ninth bit parity and the tenth bit signalling per channel are not written into the buffer memory. The counter 407 is reset to zero with a signal from the 'Frame Ref' Delay flip-flop 403.

The bit address counter 402 is a decade counter that provides several timing pulses which repeat every ten clock pulses of the 2.048 MHz clock. The following signals are decoded from this counter: (a) The ninth and tenth clock pulse inhibit for the 2.048 MHz counter 407.

(b) The B1 signal for writing and reading parity bits for the 2.048 MHz data in and out of the parity registers 3 and 4.

(c) The ninth bit parity insertion signal for DS-4.

(d) The tenth bit signalling insertion signal for DS-4.

The two address selectors 405 and 406 alternately switch back and forth between the counter 407 and the counters 408 and 409, i.e., during a frame, address selector 405 selects the counter 407 and address selector 406 selects counters 408, 409. In the next frame, address selector 405 selects counter 408, 409 and address selector 406 selects the counter 407. The two address selectors 405, 406 are switched between the counters by the A and A signals from the R/W flip-flop 8.

Address selector 404 alternately selects the addresses from the counters 408, 409. The 1.544 MHz clock is used as the select control for tHe address selector 404.

The R/W flip-flop 8 provides an important control function. The flip-flop 8 is toggled at the beginning of every frame by C8 (the 8 KHz frame clock), and it is reset every other frame by C4 (the 4 KHz signalling clock). Thus, the outputs of the flip-flop are referenced to C4 in order to distinguish between framing and signalling bits. The A output controls the addresses to buffer memory 1 (Ao-A7) by switching address selector 405 from the counters 408, 409 to the counter 406. The A output controls the addresses to buffer memory 2 (Aol-A7') through address selector 406 in the same manner and also provides control for all the data selectors DS1, DS2, DS3 and DS4.

Two delay lines which are included in the circuits 401 and 402 are needed to give delayed clocks for generating various times pulses, WE-1 and WE-2 signal, etc. These delay lines will require several taps on them to provide various fixed delay points. In addition to the circuits described, there are several flip-flops and gates on this card which provide timing functions.

The preceding description has only briefly mentioned signalling conversion. It is required to convert D2 or Common Channel signalling (CCS) to inband signalling for the incoming data and inband signalling to D2/CCS for the outgoing data. Involved with this conversion is the correct insertion of the signalling bit so that the signalling information can be correctly identified.

Fig. 6 gives a brief outline of this signalling conversion. On the incoming side, 1.544 Mb/s data coming from the digital line is written into a storage buffer and the signalling bit is extracted as shown in waveform A. One frame later, when the data is being read out of the buffer, this signalling bit is inserted into the tenth bit position of every channel for two frames (i.e., until the next signalling bit is extracted) as shown in waveform B.

On the outgoing side, 2.048 Mb/s data coming from the network is written into another storage buffer and the signalling bit is extracted from the tenth bit of the last channel in the frame as shown in waveform E. This signalling bit is inserted into the 193rd bit position of the outgoing data as shown in waveform F. A special 4 KHz signal as shown in waveform C is generated during this bit period and shipped out to the line switch so that this signalling bit can be identified. Waveform D illustrates the waveform of the 4 KHz office clock.

Although the above description indicates that the transmitter-receiver synchronizer is connected to the network, it should be noted that the connection may be via multiplexingdemultiplexing circuits.

It will be obvious to those skilled in the art that various modifications and changes may be made without departing from the spirit and the scope of the invention.

Claims (3)

1. Apparatus for providing bidirectional rate conversion between frames of data having a first rate and a first format and frames of data having a second rate and a second format; which apparatus includes: (a) means for storing a first frame of data at said first rate in a first memory means during a first frame interval; (b) means for storing a second frame of data at said second rate in a second memory means during said first interval; (c) means for reading said stored first frame of data stored in said first memory means at said second rate during a second frame interval; (d) means for reading said stored second frame of data stored in said second memory means at said first rate during said second interval; (e) means for storing a third frame of data at said second rate in said first memory means during said second interval; and (f) means for storing a fourth frame of data at said first rate in said second memory means during said second interval.

2. Apparatus as claimed in claim 1, wherein for data received at the higher of the two data rates there is provided an additional memory means whose capacity depends on the difference in the two data rates, and wherein for each data frame at the higher rate the last bits are stored in said additional memory means and are read therefrom after the read-out of the appropriate one of said first and second memory means.

3. Apparatus for providing bidirection rate conversion between frames of data having a first rate and a first format and frame of data having a second rate and a second format, substantially as described with reference to the accompanying drawings.