PCM frame slip detection in a channel

In the particular embodiment of the invention described in the specification, a frame slip detector for a detecting frame slip in a 24 channel PCM transmission system using an 8 KHz frame rate supplies a 1 KHz test tone to one channel of the system. A phase shift detector detects phase shifts in the same channel and supplies them to a frame slip indicator which responds to phase shift signals within the range from 40 degrees to 50 degrees which recur at periodic intervals of at least one second and supplies corresponding indications of frame slip for each phase shift detected.

BACKGROUND OF THE INVENTION 
This invention relates to pulse code modulation (PCM) frame slip detection 
and, more particularly, to an arrangement for detecting the occurrence of 
a PCM frame slip solely within a single voice channel in contrast to the 
customary detection of frame slip by comparision of PCM bit stream rates. 
In the most common PCM transmission system, each voiceband customer signal 
is sampled at a frame rate of 8,000 times per second and each sample is 
encoded into eight bits. These eight-bit samples of 24 customer channels 
plus a single frame bit are combined into a 193 bit frame format lasting 
1/8,000 second or 125 microseconds. The resulting bit rate thus is 
193.times.8,000=1,544,000 bits per second or, equivalently, 1.544 megabits 
per second (Mb/s). Although the details of the frame structure may be 
different for various PCM systems throughout the world, the channel frame 
rate of 8,000 per second is observed almost universally, i.e., the 
incidence of other than 125 microsecond frame duration is almost 
inconsequential. 
Whereas a connection with digital carriers and multiple digital switches 
all of which have perfectly synchronized bit streams will show no evidence 
of frame slip, (i.e., repetition or deletion of a full 125 microsecond 
frame of data), it has been found that a slight lack of synchrony (and the 
slips which result) is not detrimental to voice traffic in a PCM channel. 
Moreover, elimination of synchronization equipment represents an element 
of cost saving at no apparent penalty to the typical voice customer. 
However, the effect of even a single frame slip on the transmission of 
digital data over the voice channel by data modems may be devastating 
insofar as data errors are produced. To see how this arises, it is 
necessary to note how asynchrony between the received PCM stream and the 
local PCM receiver are handled. 
If we assume that both the incoming PCM rate and the PCM receiver clock 
rate are within the established standards, they will differ by only a 
slight amount with one running relatively faster than the other. Should 
the incoming PCM signal rate exceed the receiver clock rate, at periodic 
intervals related to the difference in the rates a whole 125 microsecond 
frame will have to be deleted in the channel in order to coordinate to 
incoming signals with the clock at the receiver. On the other hand, should 
the receiver clock rate exceed the incoming PCM signal rate, at periodic 
intervals there will be no new and valid PCM sample to be offered to the 
receiver. At those times, it is again impossible to materialize a proper 
input because of a frame slip. In those circumstances, it is customary for 
the PCM receiver to retain and repeat the previous frame's data for use as 
the missing data. This results in an apparent time stretch of 125 
microseconds. As said before, the effect of such "controlled" slips, as 
they are known, is of no consequence on voice traffic but may be very 
serious to data traffic. Such a slip-induced 125 microsecond shift in the 
sampling times in the modem receiver will cause numerous errors until the 
timing circuit recovers. 
Various techniques have been used to identify a slip at the 1.544 Mb/s 
pulse rate, using external test equipment or equipment which receives the 
1.544 Mb/s pulse stream and compares it to a reference clock rate, such as 
a digital switch clock rate. None of these conventional techniques, 
however, allows a customer whose signal is confined to one of the voice 
channels, and who typically does not have access to the 1.544 Mb/s bit 
stream, to identify that a controlled slip has occurred. Moreover, this 
controlled slip may occur in an interior portion of the total transmission 
system so that the end user has no means available to detect slip in the 
PCM portion. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a new and 
improved PCM frame slip detection system which is capable of detecting a 
frame slip within a single voice channel without reference to any other 
information in a PCM bit stream. 
Another object of the invention is to provide a PCM frame slip detection 
system which is capable of detecting a frame slip in a signal after it has 
left the PCM portion of a transmission system. 
These and other objects of the invention are attained by providing a phase 
shift detector to detect phase shifts in the received signal and select 
those phase shifts within a narrow range of phase angles corresponding to 
the ratio of the frequency of a test tone being used to test the system 
and the frame transmission rate of the system. In a particular embodiment 
using a clock rate of 1.544 megahertz (MHz) and a frame rate of 8 
kilohertz (KHz), a 1 KHz test tone produces a phase shift at an angle of 
+45 degrees whenever a positive frame slip (repetition of a frame) occurs 
and a phase shift at an angle of -45 degrees whenever a negative frame 
slip (deletion of a frame) occurs. In actual practice, a tone a few Hertz 
higher than 1000 Hz such as 1004 Hz is used to avoid synchrony of sampling 
with the 8000 Hz sampling rate. 
Accordingly, a frame slip indicator is set to respond to phase shift 
signals received from the phase shift detector having phase shift angles 
in a narrow window centered on 45 degrees, such as between 40 and 50 
degrees, to produce an indication of a frame slip each time the phase 
shift detector detects a phase shift having an angle within that window. 
Since a frame slip in a 1.544 Mb/s bit stream will cause a slip in all 24 
channels, detection of a slip in one channel is sufficient to identify 
that a frame slip has occurred in the 1.544 Mb/s bit stream.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the representative system for detecting frame slip in a PCM voice 
channel illustrated in FIG. 1, a 24 channel digital bank 30 having an 
internal crystal controlled clock creates a 1.544 Mb/s digital bit stream 
32 from 24 separate customer lines designated 1 . . . n . . . 24. The bit 
stream 32 is transmitted to a digital switch 34 which has its own internal 
clock governed by a crystal (or an external source) nominally operating at 
the same 1.544 Mb/s rate. Any difference between the sampling rate of the 
clock in the digital switch 34 and that in the digital bank 30 causes the 
switch 34 to insert controlled frame slips into the digital bit stream. 
The frame slips will occur at a rate equal to the difference in the 
frequencies of the two crystals as divided down to produce the frame or 
sampling rate of nominally 8,000 per second. A 1.544 Mb/s bit stream 36 
from the digital swtich 34 passes on to a 24 channel digital bank 38 which 
supplies 24 separate voice channel signals to 24 corresponding customer 
lines 1' . . . n'. . . 24' for distribution of the signals to separate 
customers. 
If the digital bank 30 were forced into synchronism with the digital switch 
34 by external means, such as loop timing, there would be no frame slip in 
the bit stream 36 leaving the digital switch. On the other hand, as is 
frequently the case, if the digital bank 30 is not forced into synchronism 
with the digital switch 34 by external means, then the clock in the switch 
34 can have a different frequency from the clock in the bank 30 which 
determines the sample rate for the 24 channels in the system. This 
difference in frequency forces the digital switch 34 to insert controlled 
frame slips which appear in the bit stream 36 transmitted from the switch 
34 to the digital bank 38. For example, such frame slips will occur at a 
rate of one every 10 seconds if the two bit stream clock rates differ by 
193/10=19.3 Hertz. 
In order to detect such frame slips in one of the channels of the output 
from the digital bank 38 in accordance with the invention, a conventional 
phase shift detector 40 responsive to voice frequency signals is connected 
to one of the voice lines, designated n' in the illustrated example, in 
the output of the digital bank 38 and a test tone 42 of approximately 1 
KHz is supplied to the corresponding channel n at the input to the digital 
bank 30. The phase shift detector 40 can be of any conventional type and 
may, for example, satisfy the requirements set forth in Section 4.4.4 of 
the ANSI/IEEE Standard 743-1984, "IEEE Standard Methods and Equipment for 
Measuring the Transmission Characteristics of Analog Voice Frequency 
Circuits", November, 1984. This standard puts bounds on the frequency of 
the "1 KHz" holding tone of 990 Hz to 1020 Hz. 
The output of the phase shift detector 40 is a signal having a magnitude 
corresponding to the angle of the phase shift. In order to select only 
those phase shifts corresponding to a frame slip, the system is arranged 
to respond to phase shift signals only within a narrow phase angle range 
embracing the phase shift angle corresponding to a frame slip. For this 
purpose, a frame slip indicator 44 is arranged to respond whenever it 
receives a signal from the phase shift detector having a magnitude 
corresponding to a phase shift of at least 40 degrees but no more than 50 
degrees. 
Conventional equipment can detect whether a phase shift exceeding a preset 
threshold occurs on a 1 KHz test tone. In accordance with the invention, 
the frame slip indicator 44 may utilize such equipment to register a count 
if a phase shift (positive or negative) occurs in a narrow window around 
45 degrees, such as 40 to 50 degrees. 
With this arrangement, each time a frame is repeated, a phase change occurs 
in the signal in the channel n from the bank 38 and the magnitude of the 
phase change is equal to the ratio of the test tone frequency to the frame 
rate multiplied by 360 degrees. For example, if a 1004 Hz test tone is 
used, the ratio 1004/8000 is 0.1255 which when multiplied by 360 gives 
45.18 degrees. This is illustrated in FIG. 2, which is a graphical 
representation of the phase 0 of the received signal as a function of 
time, showing an ascending staircase of approximately 45 degree phase 
jumps 46 at times t.sub.1, t.sub.2 and t.sub.3, i.e., each time a frame is 
repeated. If the received frame rate is faster than 8 KHz, the receiver 
will delete a frame periodically and the change in phase will be in the 
opposite direction from that shown in FIG. 2 and will be represented by a 
descending staircase of phase jumps. 
Since phase shifts exceeding the lower limit of the window, i.e., 40 
degrees, could occur on radio facilities which might be connected in 
tandem with a PCM transmission facility, it is not sufficient in such 
systems to know that a phase shift of 40 degrees magnitude or more had 
occurred. In PCM systems, however, there are no other phenomena, such as 
quantizing noise, impulse noise, out-of-frames or digital bit stream 
errors which can cause repeated phase jumps of nominally 45 degrees. In 
practice, using a window about 45 degrees which is to narrow will prove 
unusable and impractical because of the inescapable effects of quantizing 
noise present at the time of encoding or decoding of the test tone, the 
effects of occasional random bit errors in the PCM system, imperfections 
in the test tone itself, and the inevitable PCM quantization errors at 
encoding. As a result, a window having a range of about 40 to 50 degrees 
is deemed practical. 
Since all phase jitter and phase shift measurements are made relative to a 
local tracking replica (or phase locked loop), the long-term or 
steady-state error will decay towards zero. This means that the effect of 
a true phase step will be evidenced as jump followed by a slow return to 
zero. Accordingly, it is also necessary for the frame slip indicator 44 to 
determine that the time trajectory of the phase parameter has entered this 
window from below the lower threshold (40 degrees) and did not exceed the 
upper threshold (50 degrees) before again dropping below the lower 
threshold. 
It should be noted, that when frame slips occur, it should be as a result 
of asychronism between two relatively stable clocks. In PCM systems the 
absolute value of the maximum frequency error at 1.544 Mb/s is 100 Hz. 
Consequently, if system standards are met, the worst-case clock frequency 
difference is 100 Hz. If frame slip occurs at the maximum rate of 100 Hz, 
the minimum time interval between such slips is 
##EQU1## 
Therefore, 45 degree phase shifts resulting from such slips will occur 
with a degree of regularity and at intervals of 1.93 seconds or longer. 
PCM systems having defects in them may deviate more, of course, so that a 
somewhat more frequent occurrence may be noted. Consequently, the frame 
slip detector 44 would produce an output indicating a frame slip whenever 
phase shifts detected within the window are repeated at periodic intervals 
of, for example, one second or longer. 
An output from the slip detector 44 is obtained by causing the output from 
the phase shift detector 40 to arm the system as the signal from the 
detector increases in amplitude past the level corresponding to a 40 
degree phase shift. If the amplitude continues to increase to a level 
corresponding to at least a 50 degree phase shift, the indicator is 
disarmed. In addition, the indicator 44 will also be disarmed if a loss of 
tone, or dropout, occurs, or if an excessive phase shift at an amplitude 
between 90 degrees and 180 degrees is detected. If the signal falls below 
the level corresponding to a 40 degree phase shift while the indicator is 
armed, a frame slip is indicated. 
Although the invention has been described herein with reference to a 
specific embodiment, many modifications and variations therein will 
readily occur to those skilled in the art. Accordingly, all such 
variations and modifications are included within the intended scope of the 
invention as defined by the following claims.