Face off: 10GBase-T and SFP+ Direct Attach

10GBase-T Standard, Interoperability

IT managers value interoperability to leverage their organizations' investments in existing infrastructure and for ease of manageability.  Interoperability can be broken down into two sections:

* Interoperable technology to ensure performance is obtained regardless of which vendor/s equipment is located on either side of the link, and thus assure ease of manageability

* Operability over existing, known medium; leveraging the infrastructure already in place and the knowledge-base for its maintenance

10GBase-T is a fully IEEE compliant Ethernet transport technology standard, as defined by IEEE 802.3an-2006.  As an industry standard, this ensures the technology is interoperable between silicon vendors, NICs, switches, servers and routers.  Table 1 explains the standard's cabling capabilities.

The concept of standard, structured cabling provides a long-term underlying foundation of transmission infrastructure that follows a base set of engineering rules.  10GBase-T retains and abides by this by operating over the installed base of twisted-pair copper cabling already in place for lower-speed applications.  10GBase-T uses the same cabling-link-negotiating concepts as do 1GBase-T, 100Base-T and 10Base-T, providing a clear path of bandwidth upgradeability as needed, by leveraging the existing pair infrastructure.

Within a rack, following link segment specifications, the same 10GBase-T copper twisted-pair cabling will enable use with earlier Ethernet generations - 10MB, 100MB and 1GB operation.  To extend this further, this same copper twisted-pair cabling is utilized outside the equipment distribution area (EDA) zone, connecting with horizontal distribution area in the data center

As a technology, 10GBase-T enables network managers to preserve their knowledge base of Ethernet transport while seamlessly upgrading bandwidth capacity from 1G to 10G rates.  Additionally, network managers preserve their underlying investment in the maintenance of standard, structured cabling systems.

Limitations arise in the use of SFP+ Direct Attach, as it requires non-standard cabling infrastructure (twin-ax cabling directly attached by each vendor to the SFP+ housing cage) and the confinement to 10GE-only operation.  Deviating from standard, structured, cabling contradicts its underlying purpose: to ensure a long-term, infrastructure-orientated, foundation rather than a piece-meal grouping.

Field Termination

As a standard, structured cabling system, twisted pair cables are a known technology to data center cable technicians. Twisted pair cabling offers the ability to field-terminate, as needed, clean lengths in less than a minute.  10GBase-T implementations enable just such a clean, structured cable infrastructure.

SFP+ Direct Attach cables, in contrast, cannot be field-terminated.  SFP+ uses a passive twin-ax cable assembly and connects directly into a SFP+ housing; they are specially terminated at the vendor site and must be purchased at pre-determined lengths.  This adds overhead to cable management inventory, while preventing a clean, efficient cable infrastructure design.

Universal Connectivity RJ45

Twisted pair copper cabling utilizes a standard RJ45 connector for termination.  The RJ-45 connector, already utilized within the data center for 1GBase-T applications, ensures plug-and-play-ability of 1000Base-T and 10GBase-T applications over the twisted pair cable infrastructure regardless of switch, server, or adapter-card vendor on either end of the link.  The IT manager has the flexibility to connect 1000Base-T or 10GBase-T as needed over the same twisted-pair copper cabling (as long as link segment requirements are met) with the universal RJ-45 connector.  This avoids getting bogged down with end-to-end interoperability concerns or upgrade-ability concerns.

SFP+ Direct Attach cables are purchased by IT Managers at pre-determined lengths with SFP+ connector attached to both ends.  Often vendors will include certain "vendor security IDs" to ensure their own connections or performance on both sides of the link; adding overhead and limitations to data center design topologies.  SFP+ Direct Attach inserts an additional form of cabling management, inventory overhead, and interoperability limitations.

For data center IT managers, the importance of technology interoperability, of a standardized structured medium of transport, of plug-and-play-ability through universal connectors, and of the ability to easily upgrade existing bandwidth on an as-needed basis cannot be under-stated.

Reach

The IEEE 802.3an-2006 10GBase-T Ethernet Standard specifies operation over standard, structured twisted-pair copper cable up to 100 meters.  In contrast, 10G SFP+ Direct Attach is limited to 10 meters, with a reach of five to seven meters more commonly available.  SFP+ Direct Attach cables longer than five meters are exponentially higher cost compared to shorter cables (see Table 2).

In the data center environment, rack and equipment placement is a key factor for energy consumption.  Limitations of 10 meters or less with SFP+ Direct Attach directly contradicts the increasingly dire need to meet best practices of grouping and placing of equipment.

IEEE 802.3an and 10GBase-T standards allow for two modes of reach capability:

* 30 meters for short-reach applications

* 55 to 100 meters for full reach applications

By segmenting the reach requirements, 10GBase-T embraces less than 30 meter within a rack, top of rack or rack-to-rack reach applications, as well as end-of-row applications of 30 to 100 meters.

10GBase-T over structured twisted-pair cabling meets the different reach requirements for flexibility in the placement of rack and equipment, while meeting cabling design rules.

Power Dissipation

At a physical layer (PHY) device level, 10GBase-T power consumption, whether at full 100-meter reach or 30-meter short reach mode, will be higher than SFP+ Direct Attach, including the electronic-dispersion-compensation chip.  A 10GBase-T PHY, at the device level, can consume anything from two to four Watts per port compared to two Watts per port for SFP+ Direct Attach.

A costly misconception is viewing the pure wattage without incorporating inherent value-add parameters.   For example, as discussed, 10GBase-T offers significant flexibility in reach to achieve best practices in equipment placement for energy-consumption efficiencies, as well to address multiple configuration usage reaches (within rack-to-rack and end-of-row).

EEE (Energy Efficient Ethernet)

The IEEE Energy Efficient Ethernet (EEE) Task Force recently developed the IEEE P802.3az standard, which calls for Ethernet links to greatly reduce power in underutilized links.  The protocol developed allows a link operating under the scheme to automatically place itself into low-power mode when no traffic is present.  Once traffic is ready to be sent, the link automatically activates and turns itself into higher-power mode.  The standard requires a very low-power protocol communication scheme between two PHY devices on the same link when no data is present.  Power savings vary depending on the link utilization and cable length, though in some cases power savings of more than 50 percent may be obtained for the complete link.  EEE will enable 10GBase-T to be more power competitive against optical and other copper technologies.

1Gigabit Ethernet to 10 Gigabit Ethernet: Same Cable, Same Connector

Today's 10GBase-T solutions offer flexibility for seamless, auto-negotiated operation of 1Gigabit or 10Gigabit Ethernet operation; 10G rates can be utilized when and as needed following link segment guidelines.

This contrasts significantly with SFP+ Direct Attach, for which limitations arise in the use of in a non-standard cabling infrastructure and the confinement to 10 Gigabit Ethernet operation.

Summary

As data center IT managers face options in copper cabling within the EDA, they must take into consideration an array of key factors in deciding between 10GBase-T and SFP+ Direct Attach.  Reach is significant, in that it determines optimal equipment placement and conformance to standard structured cabling practices. Energy consumption is a large concern among the data center community; maintaining the most favorable power-per-meter technology is key.  Field termination, connectors, and standard cabling are all underlying factors in ensuring a clean, solid foundation of cable design.  Finally, the ability to transverse multiple data rates provide the basis for legacy support and future bandwidth growth.

With these considerations as guidance, 10GBase-T is proving itself the forerunner for next-generation data centers.

About the Author

Yogi Patel is field application engineer for 10GBase-T PHYs at PLX Technology, Sunnyvale, Calif.   He can be reached at ypatel@plxtech.com.