Multiplex communications – Diagnostic testing – Fault detection
Reexamination Certificate
1998-09-28
2002-09-24
Marcelo, Melvin (Department: 2663)
Multiplex communications
Diagnostic testing
Fault detection
C370S522000
Reexamination Certificate
active
06456595
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to digital telecommunications. More particularly, the invention relates to methods and apparatus for detecting alarm indication signals when such signals are overwritten by other special codes.
2. State of the Art
The first commercial digital voice communications system was installed in 1962 in Chicago, Ill. The system was called “T1” and was based on the time division multiplexing (TDM) of twenty-four telephone calls on two twisted wire pairs. The digital bit rate of the T1 system was 1.544 Mbit/sec (±200 bps), which was, in the nineteen sixties, about the highest data rate that could be supported by a twisted wire pair for a distance of approximately one mile. The cables carrying the T1 signals were buried underground and were accessible via manholes, which were, at that time in Chicago, spaced approximately one mile apart. Thus, analog amplifiers with digital repeaters were conveniently located at intervals of approximately one mile.
The T1 system is still widely used today and forms a basic building block for higher capacity communication systems including T3 which transports twenty-eight T1 signals. The designation T1 was originally coined to describe a particular type of carrier equipment. Today T1 is often used to refer to a carrier system, a data rate, and various multiplexing and framing conventions. While it is more accurate to use the designation “DS1” when referring to the multiplexed digital signal carried by the T1 carrier, the designations DS1 and T1 are often used interchangeably. Today, T1/DS1 systems still have a data rate of 1.544 Mbit/sec and support typically twenty-four voice and/or data DS0 channels. Similarly, the designations DS2 and T2 both refer to a system transporting up to four DS1 signals (96 DS0 channels) and the designations DS3 and T3 both refer to a system transporting up to seven DS2 signals (672 DS0 channels). The timing tolerance for modern T1 equipment has been raised to ±50 bps.
The most recent standardized specifications for T1/DS1 systems are contained in several published standards including ANSI T1.102, ANSI T1.231, ANSI T1.403 and ITU-T Recommendation Q.921, the complete disclosures of which are hereby incorporated herein by reference. It is worth noting that the T1 system is substantially the same in North America and Japan but is different in Europe where it is known as “E1”, has a data rate of 2.048 Mbit/sec and multiplexes up to thirty voice and/or data channels.
The current standard for T1/DS1 systems incorporates many improvements and enhancements over the original T1 system. The basic T1 system is based on a frame of 193 bits, i.e. twenty-four 8-bit channels (the payload) and one framing bit (F). According to today's standards, the 192 bit payload need not be “channelized” into 24 DS0 channels. In addition, superframe and extended superframe formats have been defined. The superframe (SF) format is composed of twelve consecutive T1 frames, i.e. approximately 1.5 milliseconds of a T1 signal. In the SF format, the twelve framing bits F are divided into two groups, six terminal framing bits F
t
and six signalling framing bits F
s
. The F
t
bits are used to identify frame boundaries and the F
s
bits are used to identify superframe boundaries. When the frames are DS0 channelized, the F
s
bits are also used to identify signalling frames. The extended superframe (ESF) format is composed of twenty-four consecutive T1 frames, i.e., approximately 3 milliseconds of a T1 signal. In the ESF format, the twenty-four F bits are divided into three groups. Six F bits are used to provide a 2 kbps framing pattern sequence (FPS) which is used to identify the frame and ESF boundaries. When the frames are DS0 channelized, the FPS is to identify signalling frames. Another six of the F bits are used to provide a 2 kbps CRC (cyclic redundancy check-error checking) channel utilizing a CRC-6 code. The remaining twelve F bits are used to provide a 4 kbps data link (DL) channel.
In addition to modern framing conventions, the present T1 specification also includes provisions for different “line codes”, sometimes referred to as “transmission codes”. It will be appreciated that the T1 signal is a plesiochronous (tightly controlled asynchronous) signal and, unlike a synchronous signal, is still subject to wander, jitter, and slips. Line codes are signalling conventions which are designed to facilitate frame synchronization and error detection. One popular line code is known generally as alternate mark inversion (AMI or bipolar line code). AMI utilizes a ternary signal (positive, negative, and null) to convey binary digits (zero and one). Successive binary ones are represented by signal elements of alternate polarity and of equal magnitude. Binary zeros are represented by signal elements having zero amplitude. Under the AMI line code, a non-zero signal element which follows a non-zero signal element of the same polarity is called a “bipolar violation”.
The T1 signal is also conditioned by pulse density requirements, i.e. the minimum number of “ones” (marks or pulses) which must be present in given number of binary digits or “time slots”. Pulse density requirements prevent a lost signal from being mistaken for a long string of zero bits. An enhancement to the basic AMI line code which helps meet pulse density requirements is known as “bipolar with 8-zero substitution” (B8ZS). The B8ZS line code provides that blocks of eight consecutive zeros are replaced with a unique eight bit code, i.e. 000VB0VB, where B represents a non-zero signal element conforming to the bipolar rule and V represents a bipolar violation. Another system for meeting pulse density requirements is known as “zero-byte time slot interchange” (ZBTSI). According to ZBTSI, eight consecutive zeros are replaced by an address chain that is decoded by the receiving terminal. As mentioned above, these transmission codes are based on the nature of the T1 carrier and not on the DS1 multiplexing scheme. Today, a DS1 transmission path may be provided on media other than a T1 carrier. A DS1 transmission path which is synchronous (e.g. via SONET) and does not utilize line codes or pulse density requirements is said to have “clear channel capability”.
The present standards for SF and ESF formats provide means for sending maintenance signals. Exemplary maintenance signals include Remote Alarm Indication (RAI, or “yellow alarm”), Alarm Indication Signal (AIS), and, more recently, trouble sectionalization signals (RAI-CI and AIS-CI) which identify whether trouble exists at the customer installation (CI) or in the network. Other maintenance signals include loopbacks and loopback control signals. In the SF format maintenance signals are transmitted in-band (in one or more DS1 channels or in a T1 frame). In the ESF format; maintenance signals are transmitted in the DL channel.
The RAI signal is transmitted in the outgoing direction when DS1 terminal equipment located in either the network or the customer installation has effectively lost the incoming signal. The detailed requirements for sending an RAI signal are contained in previously incorporated ANSI T1.231. An RAI is transmitted to the NI in several forms. In the SF format, for the duration of the alarm condition, but for at least one second, bit two in every channel time-slot shall be a zero. In the ESF format, for the duration of the alarm condition, but for at least one second, a repeating 16-bit pattern of eight “ones” followed by eight “zeros” is transmitted continuously on the ESF DL channel, but may be interrupted for a period of 100 milliseconds per interruption for “bit patterned messages”. Bit patterned messages are preemptive messages which will overwrite other signals in the DL channel.
The AIS maintenance signal (also known as a blue alarm) is transmitted in place of the normal T1 signal under certain specified conditions such as when an equipment experiences a loss of signal (LOS) at its input or is being placed in a maintenance state such as a loop
Bartholomay William G.
Bhattacharya Santanu
Varthanan Balaraj Vishnu
Yadav Pushkal
Abelson Ron
Gallagher Thomas A.
Gordon David P.
Jacobson David S.
Marcelo Melvin
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