Connections +

Preparing for the 40 Gb/s migration

There is a coordinated effort ongoing to develop the next generation twisted pair copper cabling and Ethernet standards.

January 1, 2013  

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For this issue’s column, I would like to report on the ongoing work and direction of the Next Generation Base-T Study Group in IEEE 802.3.

What is driving the development of the next generation of Ethernet Standard over twisted pair copper cabling? Namely, the need for higher speed Switch-to-Server connections.

Recent trends indicate that a large majority of these network connections will be at 10 Gbps by 2015 and will be migrating to 40 Gbps starting in 2015. The NGBase-T study group is currently focusing on defining the topology, the reach objectives, the cabling requirements and the power consumption of the next generation physical layer interface (PHY).

Let’s start by looking at the evolution of the 10GBase-T Standard. The 10GBase-T standard was published almost six years. Today, 10GBase-T represents about 25% of the Switch-to-Server market. The main factors that slowed the adoption in the market were the relatively high cost and power consumption of the PHYs compared to 1000Base-T. The first generation 10G Base-T PHYs consumed about 12 watts of power.

Current generation 10GBase-T PHYs, using today’s chip technology with 40-nm lithography manufacturing process, consume considerably less power, between 1.5 watts to 4 watts depending on the length of the channel. They also take up less space and are more economical. All these advancements are leading to an accelerated deployment of 10GBase-T in the data centre, particularly with the introduction of LAN-on-motherboard (LoM) chips in 2012.

Having learned the lesson from the evolution of 10GBase-T technology, low power consumption is one of the most important considerations for the next generation 40GBase-T PHY. A contribution by Will Bliss of Broadcom Corp. is illuminating in this respect. Entitled “A Simple Model of Relative Power vs. Reach,” it shows that the relative power consumption for 40 Gb/s with respect to 10GBase-T is about the same for a reach of 22 metres, about two times more for a reach of 34 metres and four times more for a reach of 46 metres. Every 12 metre increase in reach approximately doubles the power consumption.

In another presentation, Scott Kipp, also of Brocade, noted that the power consumption can be related to the product of Length times the Bandwidth. For example, the power consumption of 10G at 100 metres is roughly the same as 40G at 25 metres. The relative power consumption for 40G at 20 metres is approximately 0.8. The relative power consumption for 40G at 10 metres is approximately 0.4. Shorter distances result in considerably less power consumption. The same presentation by Kipp showed that for an “end of row topology” a switch rack at the end of a row can support up to three server racks for a cable distance of 10 metres and 18 server racks for a cable distance of 20 metres. Using a server density of 40 servers/rack, 18 server racks can accommodate 720 servers.

For a “middle of row” topology with the switch rack located in the middle of a row, a switch rack can support up to 36 server racks or 1440 servers. When a “middle of row” switch rack also supports servers in adjacent aisles, 25 additional racks in each adjacent aisle can be accommodated, for a total of 50 server racks, or 1000 additional servers when using a server density of 40 servers/rack. All this means that with a suitable data centre topology, thousands of high density, rack mounted servers can be supported with a 20 metre link to a modular switch.

Another important consideration in the development of the next-generation Base-T Ethernet standard is the ability to interoperate with legacy slower-speed Ethernet technologies through the function of auto-negotiation.

Compatibility, for auto-negotiation, is facilitated by the use of the 8-pin modular (RJ-45) connector that is currently defined for lower speed Ethernet applications. TIA 42.7 subcommittee is currently developing a Category 8 cabling standard that includes the 8-pin modular connector and 4-pair balanced twisted pair cable that are specified to 2 GHz, which is four times the bandwidth for existing Category 6A cabling. An early draft 0.5 of this standard was submitted to the IEEE 802.3 NGBase-T study group for information.

There was also a joint presentation from Belden, CommScope, Intertek and Fluke Networks that demonstrated the technical feasibility of a cabling system using prototype, Category 8, RJ-45 connectors and prototype Category 8 balanced twisted-pair cables from two different vendors for a short reach (5 m) and long reach (40 m) permanent link configuration. The testing was performed at Intertek, an independent third party test laboratory. The test fixtures were provided by Fluke Networks. This demonstration showed that it is feasible to make components that meet the proposed requirements in the draft Category 8 standard.

What is the main takeaway from this meeting? After consideration of all the discussion and associated contributions on topology, power consumption and reach, the NGBase-T Study group agreed to the following motions with greater than 75% support, 1) to focus on presentations regarding reach objective in the range of 20 metres to 30 metres, 2) to adopt a 2-connector channel model as an objective.


Paul Kish is Director, Systems and Standards at Belden.
The information presented is the author’s view and
is not official TIA correspondence.