40G Ethernet over DWDM : Part2

With the technical barriers of how to map a 40G Ethernet signal into an OTU3 pretty much resolved (see part 1), early adopters will face the problem that current 40G DWDM systems support only OC-768/STM-256 or OTU3 client interfaces! Since the DWDM market moves slower in releasing new technology than in the enterprise (due to component complexity and longer qualification times) this problem will be with use for some time.

First it’s best to recap on the two types of interfaces that are currently provided, OC768 and OTU3. The OC768 interface run at 39.8Gbps and is used mainly for connection to Routers for OC768 POS. The second client mode offered by most systems is an 43.02Gbps OTU3 client, more correctly described as an OTM0.3 interface, which uses OTU3 framing with G709 RS-FEC and is used mainly as an internetwork interface between DWDM systems (IaDr or IrDr). The optical interface for both OC768 and OTM0.3 is 40G serial and currently uses 300pin MSA VSR modules which are available from several vendors. Currently OC768 and OTU3 framing are incompatible with the XLAUI system interface, due to the 4x10Gbps data path, however there is talk in ITU about a proposal to map OC768 and OTU3 over XLAUI which we’ll touch on later.

To allow a 40G Ethernet signal to be connected to a current DWDM system with an OTM0.3 client (sorry, but no OC768) a Media Converter would need to provide the PCS transcoding function, bidirectional 40GBase-R MAC monitoring along with an OTU3 framer with G709 RS FEC, this could be realized using one of the latest FPGA’s, especially since the standardization for the 1024B/1027B transcoding is still pending in the ITU, or a combination of FPGA and 40G FEC device. On the optical interfaces the 40G Ethernet would be supported via a 40GBase-xR4 and a  XLAUI interface whilst on the OTU3 side a SFI-5.1 or SFI5.2 interface would connect to 300pin MSA 40G VSR modules to provide the OTM0.3 signal.

Eventually the functions of transcoding and mapping the 40G Ethernet into the OTU3 would get absorbed into new DWDM line cards, however they would also need to resolve the issue of having a common optical interface which would support OC768, OTM0.3 and 40GBase-xR4. As mentioned above one proposal would be to standardize a mapping of OC768 and OTU3 framing over XLAUI using a form of byte interleaving, this however would not be backward compatible with existing DWDM systems! Perhaps a better solution would be the standardization of serial 40G Ethernet (e.g. 40GBase-VSR from an earlier blog) with both a XLAUI and SFI-5.2 system interface, as supported on the new 40G/100G CFP MSA. We think serial 40G Ethernet would provide the best solution for the LAN-WAN interconnect and it is great to hear that the IEEE are going to restart the discussion at their next meeting in April.

Whilst the role for Media Converters to interface to DWDM system may be just stop gap solution a similar function would also be necessary to allow 40GBase-CR4/SR4 interfaces to be converted to future 40GBase-LR4. This would allow large campus networks to take advantage of the 10KM reach and any possible inclusion of 64B/66B inband FEC – let’s see.

40G Ethernet Resource Center

In both part 1 & 2 of this blog we’ve assumed some (if not too much) knowledge of the G709 standards. Rather than try to go into detail on this subject we think it best to refer people to the excellent OTN Tutorial by Tim Walker which is available “free” from the ITU website.
http://www.itu.int/ITU-T/studygroups/com15/otn/OTNtutorial.pdf or just search for “ITU OTN Tutorial”.

Oh yes and some more of those dreaded TLA’s :

DWDM : Dense wave Division Multiplex
FEC : Forward Error Correction
IaDr : Intra Domain interface (a type of interface between DWDM systems)
IrDr : Inter Domain interface (another type of interface between DWDM systems)
POS : Packet over SONET (a method of mapping TCP/IP into SONET)
SFI-5.1 : SERDES to Framer Interface Level 5, interface standard from the OIF for 40G using 17bit datapath
SFI-5.2 : SERDES to Framer Interface Level 5, interface standard from the OIF for 40G using 5bit datapath
VSR : Very Short Reach (VSR2000 means 2KM reach on SMF)

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