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Ethernet's most significant limitation is the lack of support for quality of service (QoS), which means it can't supplant other network fabrics, most notably Fibre Channel (FC). QoS refers to the ability to create and manage networks to deliver different levels of service for different types of traffic. Currently, Ethernet QoS doesn't do enough to distinguish between types or classes of traffic. While it's true that Ethernet can achieve some level of QoS, native Ethernet gives all classes and/or types of traffic equal access to bandwidth.
IN DEPTH: High-speed Ethernet planning guide
Another key Ethernet limitation is the fact that, when over-saturated with traffic, buffers overflow and packets are dropped. Ethernet tries to compensate by resending dropped packets, but that often only exacerbates the problem. Although these resends happen quickly (<25 milliseconds), they contribute to the lack of consistent response times.
There are, however, ways to address these issues and build out converged data centers on an Ethernet base.
Ethernet utilizes upper-level layer protocols (TCP) to manage end‐to‐end data delivery and integrity. FC, by comparison, provides a buffer‐to‐buffer credit that ensures packets aren't dropped due to network congestion, making it lossless. As Ethernet is an 802-based network, we can only make Ethernet lossless by adopting higher-level protocols such as TCP/IP which have adapted to the intent of IEEE 802-based networks by incorporating end‐to‐end congestion avoidance and flow control algorithms.
Data Center Bridging (DCB) is an architectural extension to Ethernet designed to improve and expand its role in the data center. With DCB, organizations can logically manage networks end-to-end with QoS through:
• Quantized Congestion Notification (QCN ) -- provides end-to-end congestion management mechanisms to enable throttling of traffic in the event of traffic congestion.
• Priority‐based Flow Control (PFC) -- enables control over individual data flows on shared lossless links.
• Enhanced Transmission Selection (ETS) -- Control/Negotiation protocol to negotiate QoS parameters and queuing and provides discovery exchange protocol to ensure consistent configurations across the network. [Also see: "Ethernet Adapts for Data Center Applications"]
With these enhancements, we now only need one type of network adapter eliminating the redundant infrastructure cost by using one cable infrastructure to support both storage and IP traffic.
It should be noted that Ethernet does support storage, but mainly through iSCSI, the server‐to‐storage protocol designed to transport SCSI block storage commands over Ethernet using TCP/IP. iSCSI adoption is growing and has found considerable acceptance in small and midsize enterprises. (At Advanced Systems Group [ASG], we've documented an increase in10Gbs iSCSI deployments over the past 18 months.)