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Send 10GbE over optical networks

Posted: 17 Oct 2005 ?? ?Print Version ?Bookmark and Share

Keywords:applied micro circuits? gbe? optical? networks? otn?

The escalating deployment of 10GbE lan technologies in enterprises presents new challenges and opportunities for optical transport network (OTN) providers. As such, the ability to transport 10GbE signals transparently is becoming a critical competitive factor for carriers that serve corporations, governments and other large organizations.

From the end customer's perspective, the OTN should appear as a seamless extension of its existing 10GbE LAN infrastructure without imposing either restrictions on the native protocols or special handling requirements before transport. For example, if the customer organization or equipment vendor uses the 10GbE preamble to track fault info or manage interbox communication links, it is important that the integrity of the information in the preamble be preserved intact throughout transport over the OTN. Similarly, if the customer's LAN uses jumbo packets to maximize data-to-payload efficiency within the 10GbE LAN, the ability to transport jumbo packets must be seamlessly maintained across the OTN as well.

The inherent line-rate differences between 10GbE LAN and OTN networks make it more difficult to achieve these goals. The 10GbE LAN signal operates at a nominal line rate of 10.3125Gbps (10Gbps x 66/64) while the OPU-2 payload operates at a nominal data rate of 9,995,276,963bps (OC-192 x 238/237). Thus, it is impossible to maintain "bit transparency" while transporting the full 10GbE LAN signal with generic framing procedure (GFP) wrapping in an OPU-2 payload as a constant bit-rate signal, while maintaining the industry-standard 10.7GHz line rate.

It is possible, however, to achieve the desired objectives by using the "information transparency" described in this article. The key is to use the inherent digital flexibility within the OTN standard in conjunction with intelligent signal-mapping techniques to "right size" the 10GbE signal for transport in a standard 10.7GHz OTU-2 signal.

Leveraging existing standards

Many standards fall under the OTN umbrella. Since the "information-transparent" technology focuses on adapting L1 standards for 10GbE transport, no changes are needed to the physical or optical layers, or to any layers implemented in software. From an implementation standpoint, this also offers the advantage that all changes are in the digital layer, so the mapping functions can be efficiently done in ASIC hardware.

The optical transport hierarchy (OTH) defined in ITU G.872 ("Architecture for the OTN") establishes the transport technology for OTN. The G.872 standard defines an architecture that comprises the optical channel (OCh), optical multiplex section (OMS) and optical transmission section (OTS), and describes the functionality needed to make OTN work.

It is important to note that the G.872 development focused on building in the inherent flexibility of a digital approach. This work allows for FEC while also introducing two digital-layer networks: the optical data unit (ODU) and the optical transport unit (OTU). The intention is that all client signals would be mapped into the optical channel via the ODU and OTU layer networks.

Thus, OTN can offer key advantages relative to SONET/SDH. These include stronger FEC, transparent transport of native client signals, more levels of tandem connection monitoring and switching scalability. The flexibility in the OTN standard can be leveraged to transport 10GbE signals while maintaining information transparency.

Balancing trade-offs

The basic challenge is how to pass the preamble and handle jumbo frames without requiring changes to the standard 10.7GHz OTU line rate. If one just wanted to pass the preamble, it is possible to use GFP encapsulation and the 10GbE signal would fit into an ODU-2 at 10.7GHz. However, if you also want to include jumbo frames and maintain the existing 10.7GHz transmission rate, the 10GbE signals will no longer fit.

One approach to overcoming this problem is to send both the preamble and jumbo frames, but map it to the ODU-2 at a higher rate. The entire 10.3GHz signal (10GbE data and encoding) is encapsulated in a synchronous OTN wrapper, which results in an output rate of 11.1GHz for the OTU signal instead of the industry-standard 10.7GHz. While there are a number of proposals to the ITU for using this approach, it is not optimal because the change to a higher line rate poses significant issues for many carriers that have built their infrastructures around the 10.7GHz standard.

Also, while changing to 11.1GHz might seem like a workable short-term solution to some carriers, it is a shortsighted approach because it will not migrate smoothly to future 40Gb transport environments. It will not be possible to take multiple OTUs from different customers and multiplex them together if some are at 10.7GHz and others are at 11.1GHz.

The alternative information-transparency approach enables the complete 10GbE information stream (including data, control, preamble and jumbo packets) to be seamlessly transported over a standard OTN 10.7GHz signal with full G.709 compliance. The only difference is in the number of "idles" found within the signal.

Standard 10GbE 66B/64B encoding specifies data, idle and ordered sets. This approach takes the 10GbE signal at 10.3125GHz (10Gbps data x 66/64 encoding) and intelligently segments it to map all of the information into the OTN signal while minimizing idles. By stripping off the 66B/64B encoding and doing a GFP wrap of the data packet with one payload type and a GFP wrap of the ordered sets with a different payload type, all of the information can be efficiently mapped to a standard 10.7GHz OTN signal.

This approach maintains complete compatibility with the 802.3 specification because it fully conforms to the minimum instantaneous inter-packet gap (IPG) of 5bytes and the specified average minimum IPG of approximately 10bytes.

From the customers' perspective, with information transparency, there is no difference between the traffic streams that move entirely within their private 10GbE LANs and traffic streams that have been transported over their carrier's OTN network. This allows customers to take optimal advantage of jumbo packets while also leveraging the advantages of platforms that use embedded preamble information to provide carrier-grade communications integrity.

The information-transparent approach also lends itself to highly efficient implementation as an integral element within hardware ASICs. The use of high-performance digital processing enables transparent mapping and enhanced FEC capabilities to be tightly integrated within a single device to provide maximum flexibility for mapping 10GbE signals into various OTN line rates.

- Timothy Walker

Principal Engineer

Applied Micro Circuits Corp.

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