10GBase-T

 

Xmultiple's Engineering Department


The IEEE 802.3an, 10-Gigabit Ethernet over twisted pair standard, also known as 10GBase-T is now emerging on a compliant of switches and servers in data centers products. Advances in semiconductor designs, and sophisticated algorithms intended to increase electromagnetic interference (EMI) immunity and lower operating power have provided networking equipment manufacturers the technology to bring to market these high speed new switches and servers. It is apparent 10GBase-T is becoming the connectivity technology of choice. Data centers can reduce overall costs and improve flexibility by implementing 10GBase-T. The operation of a 10GBase-T transceiver and the inherent advantages of 10GBase-T technology has many advantages over the alternatives, such as optical fiber and coaxial copper. Twisted pair is in most homes and businesses today and it replacing and installing fiber or coaxial cable would be very costly. Power-reducing algorithms such as Energy Efficient Ethernet (EEE) and Wake-on-LAN (WoL) are two elements of these new products which will impact the installations of 10GBase-T.

The main benefit of 10GBase-T is for data-center, businesses and home who want high speed connectivity. 10GBase-T provides the most flexible, economical, backward-compatible, and user-friendly 10G Ethernet connectivity option available. It delivers the ability to interoperate with legacy slower technologies, the use of ubiquitous and inexpensive cabling and connectors, the flexibility of full, structured wiring reach, and the ease of Cat6A cabling deployment. 10GBase-T¡¦s cost- and power-saving benefits are certain to enable designers to develop state-of-the-art 10G system.

The growing importance of cloud computing and increasing utilization of unified data/storage connectivity are reasons for the installation of 10Gbps Ethernet. While several connectivity options are available for 10Gbps Ethernet, both over optical fiber and copper cables, 10GBase-T, is arguably the most flexible, economical, backward-compatible, and user-friendly connectivity option available. 10GBase-T is designed to operate with the unshielded twisted pair cabling technology, which is already used for 10/100Mbps and 1G Ethernet. 10GBase-T has the advantage also of interoperating directly with 1G Ethernet. It operates over a existing cabling type which is used worldwide today. It operates up to 100 meters and thereby reaches 99 percent of the distances required in data centers and enterprise environments.

10GBase-T is the fourth generation of IEEE standardized Base-T technologies which all use RJ45 connectors and unshielded twisted pair cabling to provide 10Mbps, 100Mbps, 1Gbps, and 10Gbps data transmission. All these generations are backward-compatible with prior generations. Because Base-T devices have used an auto-negotiation protocol defined by IEEE to determine the capabilities supported by the other end of the link, this backward-compatibility means upgrading to 10GBase-T is easy.

The 10GBase-T transceiver uses full duplex transmission with echo cancellation on each of the four twisted pairs available in standard Ethernet cables; thereby transmitting an effective 2.5Gbps on each pair. These bits are transformed into a bandwidth-reducing line code called 128-DSQ (for double square), which limits the analog bandwidth utilization of the 10GBase-T modem to 400MHz. High-performance line equalization countermands the low-pass filter effects of the transmission channel, and additional digital signal processing (DSP) functions cancel the crosstalk and echo impairments present in the cabling. Additionally, powerful Low-Density Parity Check, or LDPC, forward error correction coding rounds out some of the DSP functions and allows nearly error-free detection at close to fundamental limits in signal-to-noise ratio.

The major DSP blocks responsible for line equalization, LDPC forward error correction, and analog line code data transformation.


EMI Issues and Concerns

The 128-DSQ line code, used by 10GBase-T systems increases the number of bits per symbol when compared to prior Base-T standards. Therefore an external signal with EMI which couples to the cable¡¦s common mode and gets converted to a differential signal may cause errors on a 100-meter 10GBase-T link.

Common EMI tests, such as those mandated by the Telcordia GR1089 standard, call for testing with field strengths of 8.5V/m. Measurements using Cat5 and Cat6 unshielded twisted pair cabling in 8.5V/m EM fields indicate that differential pickup can easily reach 60mV, thereby exceeding the voltage margin at the receiver of a 100-meter 10GBase-T link. To contend with such EMI events, particularly in an unshielded cabling system, 10GBase-T transceivers support adaptive interference cancellation. Such an algorithm requires three key steps: detecting the interferer, identifying it, and then removing it.

Detection can be based on analysis of the differential signal available at each receiver. This detection must operate in the presence of the desired signal being sent from the transmitter on the other side, thus requiring sophisticated signal processing. A Fourier transform or other techniques applied to the received differential signal can reveal an interferer that would otherwise go undetected because its voltage levels are lower than that of the transmitter signal.

Once the EMI signal is detected, adaptive filtering techniques can learn the frequency and relative amplitude of the interference and filter it out. The latest generation of 10GBase-T equipment can recover from large EMI events (10V/m fields) in less than 10£gsec, and simultaneously cancel multiple EM interferers and operate error-free after adaptation to the EMI. Test chambers are used with 10GBase-T equipment to make sure it meets the Telcordia GR1089 standard for electromagnetic Interference. This assures users of such equipment in a data center that EMI will not be a problem.

10GBase-T Technology Advantages

When compared to other 10Gbps connectivity solutions, one of the most important advantages of 10GBase-T is the ability to communicate and interoperate with legacy, often-slower Base-T systems. Most commercially available 10GBase-T transceivers are perfectly capable of reversion to both 1000Base-T (1Gbps) and 100BAase-TX (100Mbps) protocols. Data centers, therefore, can ¡§future proof¡¨ their switching architectures. A 10GBase-T switch purchased today can communicate effectively with all legacy 1G and 100M servers, while providing the infrastructure to upgrade to 10G switching when commensurate speed servers are introduced. This also means that data center expenditures can grow incrementally. Rather than a wholesale conversion of all servers and switches to 10G speeds, which would be required with a non-compatible technology such as SFP+ Direct Attach, 10GBase-T switching systems can convert only those links that truly need upgrades to 10G speeds, while maintaining 1G speed on legacy servers that don¡¦t require such data rates.

Unlike direct-attach twin-ax cabling systems, which constrain full-performance-supported distances to seven meters depending on cable thickness, 10GBase-T allows cable spans to reach to the full 100-meter length permitted by structured cabling rules. This extra reach affords data center managers the flexibility of locating switches away from server racks and opens up the data center to architectures that may be more amenable to accommodating legacy configurations, which rely on more centralized switching. Heretofore, the lack of economical cabling options for 10G Ethernet beyond a single or adjacent rack has led to the popularity of Top-of-Rack (ToR) architectures, in which a stack of rack mounted servers are connected with short cables to a fixed configuration switch in close proximity -- typically on top of the server rack. However, such architecture has the drawback of increased management domains, with each rack switch being a unique control plane instance that must be managed and updated. A more centralized switching architecture known as End-of-Row (EoR), in which server ports are routed to a larger switch servicing several racks of servers, can have the benefit of a singular entity for management with commensurate reduction in maintenance costs. Also, because larger switches amortize the cost of common elements such as power supplies and cooling fans, the per-port cost of a larger EoR switch may be lower than the equivalent number of ports in a collection of ToR switches.

10GBase-T enables uniform transmission media. The alternative in use today relies on a hodgepodge of cabling types, lengths and connectors: Cat6 for 1000BAase-T, twin-ax with SFP+ connectors for short rungs of 10G, optical modules and multimode fiber for longer runs of 10G. By standardizing on 10GBase-T the data center manager can focus on only one cabling system for all speeds and all distances. And, as luck would have it, that cabling system can be inexpensive Cat6A with RJ45 connectors that are familiar, cheap, and easy to use and install.

This, of course, leads to another great advantage of 10GBase-T technology -- its ability to use ubiquitous and inexpensive cabling and, in many cases, the existing installed base of cabling already in the data center supporting 1000Base-T systems. Even if a data center does not currently have Cat6 or Cat6A cabling as part of its existing cabling plant, the purchase price of unshielded twisted pair UTP cable is at least a factor of three less expensive than connecting twin-ax cable of the same length and up to factor of 10 less expensive than fiber solutions, when the necessary optical modules are factored in. 10GBase-T has advantages over SFP+ DA cables.

Since twisted-pair cabling and RJ45 connectors have been a part of the data center infrastructure for many years. Techniques and tools for terminating (attaching connectors to) Cat6 and Cat6A cables on the data center floor exist and are used widely. Such tools give mangers the flexibility of cutting spooled cable to needed lengths rather than ordering and keeping an inventory of pre-defined lengths of terminated cable, as would be the case for either optical or twin-ax counterparts. Another advantage of UTP cable is that, unlike optical fiber requires tight control of bend radius, deployment rules for bending and twisting UTP are significantly relaxed, allowing easy installation in even the tightest places.

Power-Issues for 10GBase-T

One of the arguments against 10GBase-T has been power dissipation, but this perspective is rooted mostly in early implementations of the technology. Recent advances in semiconductor lithography have allowed 10GBase-T transceivers to enjoy a dramatic reduction in the power they dissipate during normal operation. From a per-port power of over 6W just a few years ago (interestingly enough, the same power per port that 1000Base-T initially shipped at), the new 40-nanometer (nm) devices are capable of sub-4W performance today. And due to continuing shrinkage of chip feature sizes and the famous ¡§Moore¡¦s Law,¡¨ the 28nm devices that will become available in the 2012 time frame promise to bring power dissipation down further to about 2.5W per port when operating over a 100-meter line. For shorter length lines, most modern transceivers allow a tradeoff between power dissipation and reach. In a 30-meter mode, for example, a 28nm device¡¦s power dissipation is expected to be in the 1.5W range. Figure 4 depicts the power dissipation of 10GBase-T transceivers as semiconductor lithography has improved.

10GBase-T Transceiver power per port. The reductions in per-port power demonstrated over three prior generations are expected to continue in future lithography generations.

In addition to the reductions afforded by advances in semiconductor technology, Base-T systems in general, and 10GBase-T systems in particular, are able to take advantage of some unique and standards-based algorithms, which exploit the nature of computer traffic to further reduce power dissipation.

WoL is a standard in which a network element, such as a server, is put to sleep until awakened by a special signal called a ¡§magic packet.¡¨ The server¡¦s network interface card reverts to a very low power dissipation mode during the sleep period but remains alert and waiting for the magic packet. Once it arrives, the server is awakened and normal operation is resumed. Since the wakeup time associated with WoL is typically tens of seconds, it is designed for long periods of time when servers are idle, such as at night or during other lengthy periods of inactivity.

WoL is designed for lengthy idle periods, whereas another technology, EEE, is specifically designed to take advantage of the burst nature of computer traffic. Typical Ethernet traffic contains many gaps, which can range in duration from microseconds to milliseconds. Until recently, these gaps have been filled with so-called ¡§idle patterns,¡¨ in which no real computer information exchange takes place but whose waveform transitions can be used for maintaining clock synchronization between transceivers. The EEE algorithm exchanges those idle patterns for a Low Power Idle (LPI) mode, in which very little power is dissipated.

EEE was developed by the IEEE 802.3az task force and issued as a completed standard in November 2010. The LPI mode used during idle periods requires a new signaling scheme composed of alerts over the line, and to and from station management. During the LPI mode, a Refresh signal is used to keep current such receiver parameters as timing lock, equalizer coefficients, and canceller coefficients. These are also critical to enable fast transitions from LPI to Active modes. Typical transition times from Active to LPI mode and back are in the three-microsecond range. The bottom line is that transceiver power savings utilizing the EEE algorithm can range between 50 to 90 percent, depending on actual data patterns.

To put all this in quantitative terms, a 28nm 10GBase-T transceiver with a typical Active power dissipation of 1.5W for 30-meter reach will dissipate only 750mW when utilizing the EEE algorithm with typical computer data patterns. System-level optimizations in switches and Ethernet controller silicon are expected to take advantage of EEE¡¦s low-power idle signaling, and save far more power than the transceiver, since they can leverage the consumption of the entire switch or server, which is more than double the power per port of even the previous generation of transceivers.

 

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