The 10 Gigabit Ethernet standard encompasses a number of different physical layer (PHY) standards. As of 2008 10 Gigabit Ethernet is still an emerging technology with only 1 million ports shipped in 2007, and it remains to be seen which of the PHYs will gain widespread commercial acceptance. A networking device may support different PHY types by means of pluggable PHY modules.
At the time the 10 Gigabit Ethernet standard was developed there was much interest in 10GbE as a WAN transport and this led to the introduction of the concept of the WAN PHY for 10GbE. This operates at a slightly slower data-rate than the LAN PHY and adds some extra encapsulation. The WAN PHY and LAN PHY are specified to share the same PMDs (Physical Medium Dependent) so 10GBASE-LR and 10GBASE-LW can use the same optics. In terms of number of ports shipped the LAN PHY greatly outsells the WAN PHY.
From its inception, 10G Ethernet was intended to retain backward compatibility and full interoperability with 10/100/1000M bit/sec Ethernet while adding a tenfold increase in performance.
In 10/100 and Gigabit Ethernet, the MAC layer works in a linear manner - data moves serially in and out of the MAC layer with all the starting and ending control messages (including clocking and synchronization) embedded inside the datastream. With 10G Ethernet, it is much more complex.
To attain a 10G bit/sec bandwidth rate, the IEEE altered the way that MAC layer interprets signaling. Rather than producing a serial stream, the 10G Ethernet layer operates in parallel to interpret data. The transmit and receive paths each comprise four data lanes, and the datastream broken down into bytes is handled in round-robin fashion across the four lanes, numbered 0 to 3. On the transmit path, for example, the first byte aligns to Lane 0, the second byte to Lane 1, the third byte to Lane 2, the fourth byte to Lane 3, the fifth byte back to Lane 0, and so on.
10 Gigabit Ethernet
Ethernet frames have clearly defined beginning and ending boundaries, or delimiters. These are marked by special characters and a 12-byte interpacket gap (IPG) that dictates the minimum amount of space or idle time between packets.
Because of the parallel nature of the 10G Ethernet MAC layer, it is impossible to predict the lane in which the ending byte of the previous datastream will fall. This makes finding the starting bit - a requirement for maintaining timing and synchronization - more difficult. The 802.3ae standard mandated an elegant solution: the "start control character," or very first byte of a new data frame, must always align on Lane 0. However, this solution complicates the way the MAC handles the IPG, directly affecting performance. Nevertheless, the IEEE provided three options for the vendors to address this issue: 1) pad (increase), 2) shrink, or 3) average the "minimum" IPG.
Because of the parallel nature of the 10G Ethernet MAC layer, it is impossible to predict the lane in which the ending byte of the previous datastream will fall. This makes finding the starting bit - a requirement for maintaining timing and synchronization - more difficult. The 802.3ae standard mandated an elegant solution: the "start control character," or very first byte of a new data frame, must always align on Lane 0. However, this solution complicates the way the MAC handles the IPG, directly affecting performance. Nevertheless, the IEEE provided three options for the vendors to address this issue: 1) pad (increase), 2) shrink, or 3) average the "minimum" IPG.
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