Methods and apparatus for retiming and realigning sonet signals

Details Methods and apparatus for retiming and realigning sonet signals Methods and apparatus for retiming and realigning sonet signals

2001-01-24

2003-06-10

Kizou, Hassan (Department: 2662)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S535000

Reexamination Certificate

active

06577651

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 calculating a pointer leak rate for retiming a SONET signal.
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 E1 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.
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). 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. “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 comprised of n STS-1 signals which are frame aligned and byte-interleaved. An STS-nC signal is comprised of n STS-1 signals which are frame aligned and concatenated. 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 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, an 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. The STS payload pointer is contained in the H1 and H2 bytes (the first two bytes) of the line overhead. 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 fixed to some value from 0 to 782). 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 regenerated 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 (or 8 out of 10 as per Bellcore requirements) 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.
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.
U.S. Pat. No. 5,331,641 (the complete disclosure of which is hereby incorporated by reference herein) discloses methods and apparatus for retiming and realignment of STS-1 signals into STS-3 type signal. The SPE of an incoming STS-3 type signal is demultiplexed into three STS-1 payloads and fed to three FIFOs, and a byte which is synchronous with the TOH is tracked through the three FIFOs to provide an indication of the FIFO depth. A frame count is also kept to track the number of frames since a last pointer movement. Stuffs or destuffs are generated based on the FIFO depth as well as based on the frame count, with a stuff or destuff generated as quickly as four frames from a previous pointer movement if the FIFO is close to full or close to empty, and less quickly (e.g., at thirty-two frames from a previous pointer movement) if the FIFO is only starting to empty or to fill. The '641 patent mentions a “pointer recalculation block” but does not disclose details about pointer recalculation.
Pointer recalculation is generally accomplished by summing the positive and negative pointer movements with the previous pointer. There are practical limits as to the frequency and magnitude of pointer movements and these are part of the SONET standards. In order to assure that pointer movements do not exceed the number permitted by the standards and also in order to avoid unnecessary pointer movements, it is known to “leak” pointer movements. Pointer leaking involves buffering several pointer movements, summing them, and leaking the net pointer movement after a period of accumulating pointer movements. The '641 patent does not teach how to leak pointers. Advantageously, the pointer leak rate is adjustable based on the amount of ji

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