Until recently, packet networks, such as Ethernet, have been inherently asynchronous. However, as the data communication world moves toward an all Internet Protocol (“IP”) core and Ethernet based edge network, there is a need to provide synchronization to transform information carried on packet based networks for transport on long distance data communication networks. In other words, mapping of Ethernet into a Time-Division-Multiplexed-based (“TDM”) network such as Synchronous Optical Network (“SONET”) or Optical Transport Network (“OTN”) enables transport over very large distances, e.g., cross-continent, but also requires the Ethernet physical layer clock to be transported through the network for use as a TDM or Ethernet reference clock to time a SONET or SDH timing island if required.
Synchronous Ethernet network synchronization is a recently developed technology used to extend the well-known concepts of TDM network synchronization into the domain of packet-based networks, which until now have been specified as asynchronous in nature. The timing standard for synchronous Ethernet implementations has been defined by the Telecommunication Standardization Sector of the International Telecommunication Union (“ITU-T”), in recommendation G.8261, entitled “Timing and Synchronization Aspects in Packet Networks.” ITU-T G.8261 specifies the maximum limits of allowable network jitter and wander through a packet network. Furthermore, G.8262 provides the minimum requirements for an Ethernet slave clock in terms of its wander generation, tolerance and transfer for network equipment at the TDM interfaces at the boundary of these packet networks, and the minimum requirements for the synchronization function of network elements. The goal is enable a Primary Reference Clock (“PRC”) traceable clock for TDM applications to be maintained across an Ethernet switched network.
Timing distribution implies that the required phase stability and frequency accuracy of the reference timing signal is maintained as the data traverses across the Ethernet switched network. Several approaches have been used to transport native Ethernet traffic over a TDM network, including Ethernet over a fiber optic transport signal, e.g., synchronous transport module (“STM-n”) or optical carrier (“OC-n”) using contiguous concatenation (“CCAT”), and virtual concatenation (“VCAT”) approaches.
Ten Gigabit Ethernet (whose physical layer is also referred to as “10GBASE-R”) is the most recent and currently the fastest of the Ethernet standards. It defines a version of Ethernet with a nominal data rate of 10.3125 Gbits/s, i.e., MAC rate is ten times as fast as Gigabit Ethernet (whose physical layer is referred to as “1000BASE-X”). One approach for transporting 10GBASE-R packets over an optical TDM network is simply to over-clock the 10GBASE-R signal, which normally runs at 10.3125 Gb/s, to produce a “pseudo-OTU2” signal, or OTU2e (see ITU G.Sup43 Clause 7.1) running at approximately 11.09557 Gb/s. Since the standard Optical Channel Transport Unit-2 (“OTU2”) signal normally runs at 10.709225 Gb/s, the over-clocked signal results in a further protocol on the OTN side to the standard one, requiring additional circuitry to implement and further complicating network management by creating potential traffic issues at OTU2 network interfaces between equipment providers. As such, the over-clocked OTU2e approach can work in Synchronous Ethernet but has its limitations.
Another common approach for 10GBASE-R clients is Generic Framing Procedure (“GFP”) mapping of the 10GBASE-R client traffic into an OTU2 signal for transport over an OTN network, as outlined in ITU Recommendation G.7041, entitled “Generic Framing Procedure.” Framed GFP mapping is typically preferred over over-clocking because the resulting OTU2 signal then runs at the standard 10.709225 Gb/s line rate. However, stability requirements of the OTN networks require a clock accuracy of at least ±20 ppm, while the 10GBASE-R clients nominally run at ±100 ppm. While G.8261 provides specifications for the timing requirements, it does not address how to actually implement these requirements.
Referring now to FIG. 1, details of a prior art method and 10GBASE-R client/OTN transponder, designated generally as 10, for transforming Ethernet packets into TDM, and vice versa, is displayed. FIG. 1 displays a first data path flowing from an Ethernet network to an optical transport network (right to left). Similarly, a second data path flowing from the optical transport network to the Ethernet network (left to right) is also displayed. To convert from Ethernet to OTN, data enters the 10GBASE-R client/OTN transponder 10 from a 10GBASE-R, or similar, client interface. The data packets, encoded using the 10 Gigabit Ethernet protocol, are first transformed from an optical form to an electrical form by an optical/electrical (“O/E”) converter 12 to produce an electronic data stream having a data rate of 10.3125 Gb/s with an accuracy of ±100 ppm. This data stream then passes through a clocked data recovery (“CDR”) plus 1:16 deserializer 14 to produce sixteen independent data streams running in parallel at a nominal rate of 10.3125/16 GHz (f=644.53125 MHz). This deserialization is necessary because the GFP mapping functions of the client GFP/OTN mapper block 16 are not presently capable of running at the full data rate, thus the framed-GFP mapping functions are performed simultaneously on all sixteen streams at a rate equal to 1/16 of the data rate. Each data stream is then decoded by a 10GE decoder 18 and temporarily stored in a client ingress FIFO 20. The client ingress FIFO 20 is timed by a clock signal recovered by the CDR plus 1:16 deserializer 14 from the incoming data stream. Next, a GFP mapper 22 converts each media access control (“MAC”) frame in the incoming Ethernet packets to a GFP frame. An OTU2 mapper 24 converts the GFP frames to OTU2 frames using a free-running local clock 26 as the timing source. The local clock operates at a frequency of 10.709225×109/16=669.3265625 MHz, with an accuracy of ±20 ppm. The resulting sixteen streams of OTU2 frames are then passed through a dense wave division multiplex (“DWDM”) line transceiver 28, which converts the sixteen parallel data streams back into a single data stream using a Clock Multiply Unit (“CMU”) plus 1:16 serializer 30, which is a PLL that generates a line clock from the incoming 16-bit data clock, e.g., in this case a ×16 multiply function, and transforms the single electrical data signal to an optical signal through a second O/E converter 32. The output data stream is an OTU2 signal with a frequency of 10.709225 Gb/s ±20 ppm for delivery to the OTN network. However, because the timing of the input signal and the output signal are not tied together, the phase and frequency of the reference clocks for the two networks are completely asynchronous. Thus, this approach will not work for synchronous Ethernet.
In the reverse direction, i.e. OTN to Ethernet, OTU2 data frames from the OTN network enter the DWDM line transceiver 28, are converted from optical to electrical signals by the O/E converter 32, and then deserialized to sixteen parallel OTU2 streams by a second CDR plus 1:16 deserializer 34. The OTU2 frames are converted to GFP frames by an OTU2 deMapper 36, and then the GFP frames are converted to MAC frames by a GFP deMapper 38. The GFP deMapper 38 inserts or deletes “idle” frames into the MAC frames in order to maintain the proper timing. The resulting MAC frames are then queued in a client egress FIFO 40 which is timed by a second local free-running clock 42 operating at a frequency of f10GE=10.3125×109/16=644.531 MHz ±100 ppm. The MAC frames are then placed in Ethernet packets by a 10GE encoder 44 which inserts the proper headers and packs the frames as needed for delivery. A second CMU plus 1:16 serializer 46 then serializes the sixteen parallel Ethernet data streams into a single 10GBASE-R data stream operating at a frequency of 10.3125 MHz ±100 ppm. This single data stream is converted from an electrical signal to an optical signal (if necessary) for delivery in the 10GBASE-R packet network by the O/E converter 12.
Because the timing sources in the above prior art method are not synchronized, this method is unsuitable for use with Synchronous Ethernet networks. Therefore, what is needed is a system and method for distributing reference timing between an Ethernet switched network and an optical transport network while maintaining phase stability and frequency accuracy.
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