Monday, November 14, 2011

Perfect Companions: IEEE DCB and SAE AS6802

Evolution of Ethernet

With SAE AS6802 (Time-Triggered Ethernet), asynchronous Ethernet networks can utilize TDMA bandwidth partitioning and statistical multiplexing, to support synchronous and asynchronous communication. Synchronous time-triggered traffic will operate with fixed latency and microsecond jitter even if the Ethernet network is overloaded. SAE AS6802 provides robust distributed fault-tolerant time base, by aligning all local (computers & Ethernet device) clocks to microsecond precision - a mechanism which is essential for efficient resource use in highly-available systems. So there is no need for central time or wall-clock dissemination.

Time-triggered communication is driven by time progression only, and not by priorities. It behaves as the highest priority traffic class, but there is really no explicit priority scheme involved, as every message is scheduled only with respect to time. This is the key to control of media access and robust TDMA partitioning among asynchronous and synchronous Ethernet traffic – meaning adding new capabilities to Ethernet as a networking standard.
As shown in the chart below, SAE AS6802 represents a protocol enhancement in domain of congestion-free, low-latency and low-jitter communication. With SAE AS6802, Ethernet can control the timing, latency and jitter of network traffic and efficiently use proven asynchronous lossless communication mechanisms. This can significantly influence the evolution of Ethernet as a deterministic unified network, and enable lossless, deterministic and time-critical (hard real-time) communication. 

(click on image to enlarge!)

Unified Ethernet Networking with SAE AS6802 and IEEE DCB

With SAE AS6802 and IEEE DCB, critical command and control systems, audio/video, SAN and standard LAN applications can safely coexist in one network and Ethernet can efficiently and natively handle both synchronous and asynchronous communication. So, we are not talking only about the convergence-enhanced Ethernet. With SAE AS6802 and IEEE DCB we can design completely unified Ethernet networks capable of handling low-latency, low-jitter and standard LAN applications.

Complementary IEEE DCB and SAE AS6802 Traffic Classes
SAE AS6802 has strict time-critical properties, fixed latency and µs-jitter, and IEEE DCB does not impose narrow time boundaries, but focuses on latency minimization for high-priority traffic and lossless communication (e.g. SAN).
SAE AS6802 supports strictly lossless time-critical operation and TDM-style (synchronous)  network communication. It is a strictly deterministic traffic class with well-defined performance and constant bandwidth use, so it cannot “steal” bandwidth from IEEE DCB and vice versa.

SAE AS6802 can be used in networks with IEEE DCB, to maximize unified networking capabilities and isolation of critical data paths and different traffic classes. It adds properties essential for efficient virtualization and use of all computing and networking resources in the system.
They are perfect companions, fully compatible and have different, but complementary properties.

SAE AS6802 Protocol operation is not dependent on distances between network nodes. In comparison, IEEE DCB switches at 10GbE have distance limitations below 50-100m, as a result of protocol design. Those distances depend on switch buffer size and line speed. Both protocols enable dynamic bandwidth release, but use different mechanisms.

(click on image to enlarge!)
IEEE DCB measures use of bandwidth over a certain period (T) prior to release, while SAE AS6802 releases bandwidth immediately on message basis, meaning if no time-triggered message is sent, its bandwidth is released for asynchronous (also IEEE DCB) traffic. Therefore, the bandwidth reserved for time-triggered messages, which is temporarily unused is free for all other asynchronous traffic.

(click on image to enlarge!)

Comparison: IEEE DCB and SAE AS6802 Traffic Classes

YouTube Videos on Deterministic Unified Networking 

Video1: Deterministic Ethernet

Video2: Use Case: TTEthernet (SAE AS6802) in Converging Networks

Update: Jumbo Frames and SAE AS6802
There are some limitations for use of Jumbo frames in low-speed Ethernet networks (e.g. 100MBit/s), as the Jumbo frame can take upto 750µs to pass through a switch, and it could delay other time-critical (scheduled) traffic or complicate its scheduling. At higher speed (1Gbit/s) this challenge decreases as the maximum delay decreases (<80µs per frame), and disappears almost completely at 10GBit/s (<9µs).

Jumbo frames can be transported if the switch provides the capability to tunnel storage data via time-critical (scheduled, synchronous, time-triggered) links. This enables predictable temporal performance and fixed latency of storage traffic, unaffected by distances between end station and switches. The fragmentation can start at first switch and the storage data is transfered synchronously over a multi-hop network, and assembled at the last hop, and forwarded in initial form to the storage unit or disk.

In case we have an unified network with control applications, IT and manufacturing functions, with significant data storage capability and sensor fusion, this approach can simplify system integration of different traffic classes. This represents a real unified Ethernet network.