Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
2001-05-25
2002-10-08
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C370S505000
Reexamination Certificate
active
06463111
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to telecommunications. More particularly, the invention relates to methods and apparatus for desynchronizing a DS-3 signal and/or an E3 signal from the data portion of an STS/STM payload.
2. State of the Art
Since the early nineteen sixties, three different digital multiplexing and signalling hierarchies have evolved throughout the world. The hierarchies were developed in Europe, Japan, and North America. Fortunately, all are based on the same pulse code modulation (PCM) signalling rate of 8,000 samples per second, yielding 125 microsecond sampling slots (1 second/8,000 samples=0.000125). Japan and North America base their multiplexing hierarchies on the DS-1 rate of 1.544 Mbit/sec±20 ppm, although the higher data rates in Japan do not correspond to the higher rates used in North America. Europe bases multiplexing on a rate of 2.048 Mbits/sec called E
1
which carries thirty voice circuits compared to the twenty-four carried in the DS-1 rate. The next most common higher rates in the U.S. and Europe are DS-3 and E3, respectively, which have rates of 44.736 Mbit/sec±20 ppm and 34.368 Mbit/sec±20 ppm, respectively.
The Synchronous Optical Network (SONET) or the Synchronous Digital Hierarchy (SDH), as it is known in Europe, is a common transport scheme which is designed to accommodate both DS-1 and E-1 traffic as well as multiples (DS-3 and E3) thereof. Developed in the early 1980s, SONET has a base (STS-
1
) rate of 51.84 Mbit/sec in North America. In Europe, the base (STM-
1
) rate is 155.520 Mbit/sec, equivalent to the North American STS-
3
rate (3*51.84=155.520). The abbreviation STS stands for Synchronous Transport Signal and the abbreviation STM stands for Synchronous Transport Module. STS-n signals are also referred to as Optical Carrier (OC-n) signals when transported optically rather than electrically.
Prior art
FIGS. 1 and 2
illustrate the basic STS-
1
signal which has a frame length of 125 microseconds (8,000 frames per second) and is organized as a frame of 810 octets (9 rows by 90 byte-wide columns). It will be appreciated that 8,000 frames*810 octets per frame*8 bits per octet=51.84 Mbit/sec. The first three columns of each row consist of transport overhead (TOH). Of these twenty-seven octets, nine are allocated for section overhead and eighteen are allocated for line overhead. The remainder of the frame (9 rows of 87 columns=783 octets) is referred to as the envelope or Synchronous Payload Envelope (SPE) or, in Europe, the Virtual Container. The first column of the envelope is reserved for STS path overhead (POH) and is referred to as the transport part of the envelope. The remaining 86 columns is referred to as the user part of the envelope. The difference between path overhead, line overhead, and section overhead is illustrated by FIG.
3
. Path represents the complete transit through the SONET network. Line represents transit from one multiplexer to another. Section represents transit from one network element to another.
In order for data to be accommodated efficiently in the SPE, the 87 bytes of the SPE are divided into three blocks each including 29 columns. The POH occupies column
1
and “fixed stuff” (bytes which convey no information) is inserted into the 30th and 59th columns. Data is accommodated in the remaining 3*28=84 columns=756 bytes. An STS-n signal is n STS-
1
signals which are frame aligned and byte-interleaved. Currently, the highest level STS signal is STS-
192
which has a line rate of 9,953.28 Mbit/sec.
These various synchronous optical network signals contain payload pointers (prior art
FIG. 2
) which provide a method of allowing flexible and dynamic alignment of the SPE (Virtual Container) within the envelope or container capacity, independent of the actual contents of the envelope or container. Dynamic alignment means that the STS or STM respective SPE or Virtual Container is allowed to float within the STS/Virtual Container envelope capacity/container. For example, as shown in prior art
FIG. 4
, STS-
1
SPE may begin anywhere in the STS-
1
envelope capacity. Typically, it will begin in one STS-
1
frame and end in the next frame as shown in prior art FIG.
4
. The STS payload pointer is contained in the H1 and H2 bytes (the first two bytes) of the line overhead (prior art FIG.
2
). These two bytes designate the location of the payload byte (the J1 byte) where the STS SPE begins.
When first generated, an SPE is aligned with the line overhead at the originating node (i.e., the pointer value is 0). As the frame is carried through a network, however, it arrives at intermediate nodes (e.g., multiplexers or cross-connects) having an arbitrary phase with respect to the outgoing transport framing of the intermediate nodes. If the SPE had to be frame-aligned with the outgoing signal, the frame would need to be buffered and delayed. Thus, the avoidance of frame alignment allows SPEs on incoming links to be immediately relayed to outgoing links without artificial delay. The location of the SPE in the outgoing payload envelope is specified by setting the H1, H2 pointer to the proper value (0-782). The pointer values are reset at each intermediate node in the network.
In addition, if there is a frequency offset between the frame rate of the transport overhead and that of the STS SPE, then the pointer value will be incremented or decremented, as needed, accompanied by a corresponding positive or negative stuff byte. If the frame rate of the STS SPE is too slow with respect to the transport overhead, then the alignment of the envelope must periodically slip back in time, and the pointer must be incremented by one. This operation is indicated by inverting selected odd bits (I-bits) of the pointer word to allow five-bit majority voting at the receiver. A positive stuff byte appears immediately after the H3 byte in the frame containing inverted I-bits. Subsequent pointers will contain the new offset value. Consecutive pointer operations must be separated by at least three frames in which the pointer value remains constant. This implies a very wide tolerance of clock accuracy required for maintaining SPE data, i.e., ±320 ppm. In comparison, a SONET node is specified to maintain a minimum timing accuracy of ±20 ppm if it loses its reference.
If the frame rate of the STS SPE is too fast with respect to that of the transport overhead, then the alignment of the envelope must be periodically advanced in time, and the pointer must be decremented by 1. This operation is indicated by inverting selected even bits (D-bits) of the pointer word to allow five-bit majority voting at the receiver. A negative stuff byte appears in the H3 byte in the frame containing the inverted D-bits. Subsequent pointers will contain the new offset value.
Both the SONET and SDH standards define a mechanism for mapping the DS-3 or E3 signal into the SONET/SDH payload. For DS-3, positive bit stuffing is defined in which each of the nine rows contain 622 bit positions, including one bit which may be data or stuff. A frequency of 44.736 MHz is achieved if one-third of the rows contain 621 information bits and one stuff bit, and two-thirds of the rows contain 622 information bits and no stuff bits. For E3, a positive-zero-negative mapping mechanism is used, with two stuff opportunities per every three rows. Two rows of 477 bits and one row of 478 bits transport a 34.386 MHz signal. Adding one extra data or one extra stuff allows transported frequency flexibility.
Thus, it will be appreciated that a SONET/SDH signal which carries a DS-3 or an E3 signal will contain overhead bytes, stuff bytes, and other control information. When the DS-3 or E3 signal is extracted from the SONET/SDH signal, these additional bytes must be stripped out, thereby producing gaps in the 51.84 MHz clock of the extracted signal. This “gapped” DS-3 or E3 must be reconstituted into a slower (44.736 MHz or 34.368 MHz, respectively) signal having no gaps. This pro
Corrielus Jean
Gallagher Thomas A.
Gordon David P.
Jacobson David S.
Transwitch Corporaton
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