The rapid development in data center throughput has led to the increasing usage and demand for higher-performance servers, storage and interconnects. And the old 1G Ethernet cannot handle the heavy-loaded solutions in data center anymore. As a result, datacenter designers are looking to the expansion of higher speed Ethernet solutions, specifically 10 G and 40G Ethernet. As for 10GbE, there are two broad categories—SFP+ optical options and 10GBASE-T available on the market, which pose difficulty in selecting the appropriate 10-gigabit physical media. This article will make a brief introduction to these two 10G solutions to help you choose a suitable one.
What Is 10G SFP+?
10G Small-form-factor pluggable plus (SFP+) is the industry standard for data rates up to 10 Gbps, which is also MSA compliant. SFP+ module is especially standardized for 10 Gbps application and is identical in dimensions to the SFP. To achieve the SFP+ form factor reduction, only the optical-to-electrical and electrical-to-optical conversion functionalities occur inside the optical module. The key advantage of SFP+ over the existing 10 G optical interconnects is the higher port densities enabled by its dimensions, and lower power consumption. The figure shows an SFP+ modules connected with an LC to LC patch cord.
SFP+ transceivers are available in different 10G Ethernet standards—10GBASE-SR, 10GBASE-LR, 10GBASE-LRM, 10GBASE-ER, 10GBASE-ZR, 10GBASE-LX4 and 10GBASE-PR. Each standard has a unique specification that can be suitable for different applications. SFP+ optics are selected more often when designers need faster and more reliable solutions to handle 10 Gigabit Ethernet optical lines. With lower power usage and low latency, SFP+ ports are most commonly used for enterprise switches and also for plug-in cards for servers.
How Does 10G Copper Solutions Compare?
10G Ethernet can also run over twin-axial cabling, twisted pair cabling, and backplanes. SFP+ direct attach cable, 10GbASE-CX4 and 10GBASE-T are the common copper solutions for 10G short-reach interconnect. 10GBase-CX4 achieves the aggregate rate of 10 Gbps by transmitting over four independent cables at 3.125 Gbps. The disadvantage of this solution is the bulkiness of the cables that have eight twin-ax cables within them for a duplex link. And the bulkiness of the cables makes cable management more difficult. This becomes a significant limitation as the port densities and inter-port connection densities increase. Additionally, the number of conductors make these cables expensive compared to SFP+ cables.
10GBase-T enables 10 Gbps transmissions over Cat 6 and higher quality cables using complex signal processing and channel coding. The potential advantage of this technology is its extended reach. The extended reach and ability to enable structured cabling are not required for the short reach interconnects between servers and switches collocated in a rack. The disadvantages are its high power consumption and latency. The high latency, in particular, is a key limitation in latency sensitive data center and storage applications.
SFP+ Direct Attach DAC is another lower cost alternative to fiber with a limited link length of 7 meters. Additionally, it has significant power, cost, and performance advantages over the above media as explained below.
Why Use 10G DAC Cables in the Data Center?
As 10 Gbps interconnects become ubiquitous in servers, providers are looking for a low cost, low power, and space efficient interconnect solution for the short reach (5-7m) links that dominate the data center environment. A length of seven meters covers all connections between server cards and switches, typically mounted on a single data center rack, and a vast majority of inter-rack connections. The SFP+ Direct Attach Copper is the 10G interconnect technology that matches these requirements, playing a vital role in enabling the next generation power and cost efficient data centers.
SFP+ Direct Attach Copper Solution
To further reduce cost and power in interconnect distances of 7 meters, which is sufficient to link server cards and switches, the SFP+ Direct Attach Copper replaces the optical modules and fiber with a passive copper cable with connectors identical to an SFP+ optical module. Figure 2 shows an SFP-H10GB-CU3M plugging in a Cisco switch. The reduction in cost and power are significant since the price of two optical modules required to support a full duplex link is approximately 10 times the cost of a fully connectorized SFP+ Direct Attach cable. In addition, the optical SFP+ modules consume around 1W each, adding 2W per port to the overall system power budget and cooling requirements.
Data is transmitted as 10 G serial NRZ (Non-Return to Zero) symbols with transmitting pre-emphasis and receive equalization compensating for Inter-Symbol Interference caused by the board trace and the copper cable. This choice of serial NRZ transmission over a passive medium makes SFP+ copper both a low power and low latency solution compared to its alternatives.
As more emphasis is placed on energy efficient data centers and higher bandwidth applications, the need for a small form factor, low power, low latency, and low cost interconnect makes SFP+ Direct Attach the optimal solution for short-reach 10G interconnects. While SFP+ fiber options provide a great path for higher performance long haul applications.