An introduction to Synchronized Ethernet
Traditional versus Synchronized Ethernet
Traditional Ethernet was originally intended for transmission of asynchronous data traffic, meaning there was no requirement to pass a synchronization signal from the source to destination. In fact, the old 10Mbps (10Base-T) Ethernet is not even capable of synchronization signal transmission over the physical layer interface because a 10Base-T transmitter stops sending pulses during idle periods.
A 10Base-T transmitter simply sends a single pulse ("I am alive" pulse) every 16 ms to notify its presence to the receiving end. Of course, such infrequent pulses are not sufficient for clock recovery at the receiver.
Idle periods in faster Ethernet flavors (100Mbps, 1Gbps and 10Gbps) are continuously filed with pulse transitions, allowing continuous high-quality clock recovery at the receiver--good candidates for synchronized Ethernet.
Figure 1 highlights Gigabit Ethernet over copper (1000Base-T). To reduce clutter, each node has only two ports, although typically each node has multiple ports. Gigabit Ethernet over copper provides an additional challenge for SyncE implementation, which does not exist in Ethernet over fiber.
Gigabit Ethernet over copper uses line coding as well as transmission over all four pairs of CAT-5 cable to compensate for limited bandwidth of twisted pairs used in CAT-5 cables. The transmission is done in both directions simultaneously, similar to ISDN and xDSL where digital signal processing algorithms have to be used for echo cancellation.
The echo cancellation is greatly simplified if the symbol rate (frequency at which data is transmitted) is identical in both directions. This is accomplished with a gigabit Ethernet master/slave concept.
The master generates a transmit clock locally from free-running crystal oscillator and the slave recovers the master clock from the received data and uses this recovered clock to transmit its own data. Master and slave are determined during the auto-negotiation process. The master is generally assigned randomly using a seed value but it can also be set manually.
Figure 2: Physical layer timing in traditional Ethernet
Figure 2 illustrates that synchronization does exist in Ethernet on each hop between two adjacent nodes, but it is not passed from hop to hop. Passing synchronization is relatively simple – take the recovered clock from the node receiving synchronization, and with this clock, feed all nodes that are transmitting synchronization (Figure 3).
Figure 3: Physical layer timing in synchronized Ethernet
Of course, the recovered signal needs to be cleaned with a PLL to remove jitter generated from the clock recovery circuit before being fed to the transmitting device. As well, ports need to be manually set in the clock path to alternate the master and slave function (only for 1000Base-T).
This is not an issue for gigabit Ethernet over fiber (1000Base-X) or for 10 gigabit Ethernet (10GBASE) because one fiber is always used for transmission and the other for reception--there is no bi-directional transmission on a single fiber. Therefore, there is no need for master and slave functions.
Any Gigabit or 10 Gigabit Ethernet PHY device should be able to support synchronized Ethernet, so long as it provides a recovered clock on one of its output pins. The recovered clock is cleaned by the PLL and fed to the 25MHz crystal oscillator input pin on the PHY device. Some new Ethernet PHY devices provide a dedicated pin for the synchronization input. The advantage of this approach is that frequency input can be higher than 25MHz--higher clock frequencies usually have lower jitter. In addition, this approach avoids any potential timing loop problems within the PHY device.
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