This is the first application filed for the present invention.
Not applicable.
The present invention relates to the multiplexing of SONET/SDH data streams and, in particular, to a method of interleaving tributary data streams and recovering the tributary data streams, using unique, independent encoding schemes.
Optical fiber, because of its data transfer capacity and reliability, has become a commonly used medium for data transport. Synchronous Optical NETwork (SONET) and Synchronous Digital Hierarchy (SDH) standards are used to enable network devices, made by different manufacturers, to receive and transmit data streams through optical networks. Certain optical fibers have the capacity to convey independent data streams on multiple wavelength channels. The transmission of data streams onto different wavelength channels of a single fiber is familiar to those skilled in the art, and referred to as Wave Division Multiplexing (WDM).
The data stream with a lowest density defined by current SONET standards is a stream of frames transmitted every 125 microseconds. Each frame includes three 9-byte columns of transport overhead and 87 9-byte columns of payload. This least dense SONET stream is called a Synchronous Transport Signal (STS-)1. A stream of STS-n frames convey n byte interleaved columns of STS-1 frames respectively, at the same rate of 8000 frames/s. For example, the first nine 9-byte columns of an STS-3 frame are transport overhead, and the remaining 261 columns are referred to as the Synchronous Payload Container. The transport overhead consists of section overhead, as defined by the SONET standard.
The first bytes of the an STS-1 frame are A1 and A2 bytes. These bytes are used to detect the beginning of frames, which permits SONET devices to parse SONET frames into constituent bytes.
The SONET standard defines the structure of a the frames, and the key to extracting data from the frames. A set of virtual tributaries (VTs) that make up the payload of a frame are indexed by pointer fields in the section overhead. The SONET standard also provides a protocol for producing higher bit-rate streams from STS-1 frames m Synchronous Transport Signal (STS)-n frames can be byte interleaved to produce an STS-(nxc3x97m) data stream, or can be concatenated to produce an STS-(nxc3x97m)c data stream. The transport, section, line and path overheads of higher bit-rate streams are also defined by the SONET standard. The section and line overhead of higher bit rate SONET frames are not identical to those of the constituent frames, and so when an STS-(nxc3x97m) data stream is formed, the overhead of the component data streams must be replaced. The byte interleaving and replacing of section and line overhead is referred to as xe2x80x9creframingxe2x80x9d.
SONET signals are received by a class of SONET network elements (which includes line and section terminating equipment and routers), and the beginning of a SONET frame is detected by A1 and A2 byte filters.
The SDH protocol is very similar to the SONET protocol, but the two are not compatible to an extent that network elements adapted to propagate or terminate frames can be interchangeably used for either protocol. Consequently, if SONET frames are to be converted to SDH frames, or vice versa, a complex reformatting process is required. An example of a reformatting process is described in Assignee""s co-pending U.S. patent application Ser. No. 09/494,518 which was filed on Jan. 31, 2000 and entitled METHOD AND APPARATUS FOR CROSS-CONNECTING DATA STREAMS WITH EFFICIENT MEMORY UTILIZATION AND TRANSPARENT PROTOCOL CONVERSION.
As is well known in the art, the speed of data transmission over optical fiber has been increasing at a steady and rapid rate. Consequently, as new high-speed equipment is connected into optical networks, it is often desirable to multiplex lower speed data streams for transport across the network in order to take advantage of the transport capacity of high-speed links. This gives rise to certain problems, however. First, in order to multiplex data streams in either SONET or SHD format, the frames must be pointer processed and reformatted as described above. The pointer processing changes certain frame header information, however, and this is not always acceptable to transport service customers. Second, SONET and SHD frames cannot be multiplexed together because of well understood incompatibilities between the two protocols.
It is therefore desirable to provide a method and apparatus that permits a plurality of low-speed data streams to be multiplexed onto a high-speed data channel without pointer processing, so that the multiplexing demultiplexing process is completely transparent to transport service customers. It is also desirable to provide a method and apparatus that permits synchronous data streams that conform to different protocols to be transparently multiplexed together without protocol conversion or pointer processing.
It is therefore an object of the invention to provide a method and apparatus for multiplexing/demultiplexing synchronous data streams in either SONET or SDH protocol, or any combination of the two protocols.
It is a further object of the invention to provide a method and apparatus for transparently multiplexing/demultiplexing synchronous data streams without pointer processing or protocol conversion.
Accordingly, the invention provides a method and system for encoding and multiplexing source data streams to form an aggregate data stream that is demultiplexed into component data streams using a self-inverting encoding scheme.
This is accomplished using a set of self-inverting encoding schemes, one for each of the data streams. The self-inverting encoding schemes are applied to the respective data streams. Each of the self-inverting encoding schemes must adhere to certain criteria. First, the encoding schemes of any given set must be unique. Second, the framing bytes of one data stream encoded by the encoding scheme associated with that stream must not, when decoded by any one of the other data streams, be identical to a framing byte of the one of the other data streams. The restriction of independence of a set of encoding schemes is ensured if the operation of any one of the set of encoding schemes does not influence the operation of the others in the set.
The method begins with parsing each of the m synchronous data streams into data units. This is accomplished by detecting respective framing bytes of each data stream. The data units of respective synchronous data streams are encoded using a respective encoding scheme, and then the encoded data units are interleaved to produce a serialized aggregate data stream.
The recovery of any one of the m synchronous data streams is performed by copying the aggregate data stream and applying a decoding scheme to data units of the copy of the aggregate data stream. The decoded data stream is then passed to a framer, which detects one or more framing bytes. After the framing bytes are detected, the remaining data units are recovered using a simple counting process in which every mth data unit is selected to recover the one of the m synchronous data streams.