Gigabit Ethernet Physical Layer in the Optical Modules
There are many optical transceivers named according to the different Ethernet physical layers, such as the Cisco compatible SFPs GLC-T 1000BASE-T and SFP-10G-SR 10GBASE-SR SFP module. The 1000BASE-T and 10GBASE-SR are the physical layers which I am going to talk about.
10/100 Mbps Ethernets are mostly deployed on copper medium (coaxial cable or unshielded twisted pair, i.e., UTP). Gigabit Ethernet was first standardized on optical fiber in 1998. Two designs were ratified in IEEE 802.3z to transmit Gigabit Ethernet signals: the 1000BASE-SX uses short-wavelength laser (850 nm) on multimode fiber (MMF), and the 1000BASE-LX uses long-wavelength laser (1310 nm) on the standard single-mode fiber (SMF). At that time, transmitting 1000Mbps signals on the widely deployed Category 5 UTP was a significant challenge for silicon-chip designers. It requires tremendous signal processing to mitigate the channel impairments in copper wires such as ISI (intersymbol interference) introduced by limited channel bandwidth and signal crosstalks between pairs of copper wires. It was not until a year later that the 1000BASE-T standard (IEEE 802.3ab) was finished.
Although Gigabit Ethernet is now mainly deployed with UTP interfaces, early Gigabit Ethernet was mostly deployed with optical interfaces. Fiber has the advantage of little signal impairments and wide bandwidth. It is suitable for backbone transmission which is the major application for early Gigabit Ethernet. To keep the cost of Gigabit Ethernet low, the IEEE 802.3z committee very conservatively defined the transmission distance limit of 1000BASE-SX as 300m, and that of 1000BASE-LX as 5 km.
Both 1000BASE-SX and 1000BASE-LX share the 8B/10B 1000BASE-X PCS line coding. Besides the transmission media, the only difference between 1000BASE-SX and 1000BASE-LX lies in the PMD layer which defines the laser transmitter and photodetector. The interface between the PMA and PMD layer is simply a serial interface. This made it easy to reuse all the designs between 1000BSAE-SX and 1000BASE-LX except the PMD transceiver, which can’t interoperate with each other.
Although the IEEE 802.3z standard committee has made the PMD specification extremely conservative, it still represented a significant portion of the Gigabit Ethernet switches and routers containing high port counts. Luckily, the well-thought layered design of Ethernet allows the optical transceiver modules to be separated from the rest of system.
The IEEE 802.3z standard did not specify an exposed interface between the PMA and PMD. Nevertheless, transceiver manufactures formed MSA consortiums that defined optical transceiver modules (i.e., PMDs) with a common electrical interface and uniform mechanical dimensions. The most commonly seen Gigabit Ethernet MSA PMD modules are GBIC and SFP. SFP modules are much smaller in size and became the most popular Gigabit PMD. To improve system density, SFPs use the compact-form LC connector not specified in the IEEE 802.3 standard. Both GBIC and SFP modules are hot swappable so that a router/switch does not need to be populated with expensive optical modules when they are manufactured. Instead, optical transceivers can be inserted when a port needs to be connected. In addition, one does not need to decide ahead of the time which type of optical PMD to be populated at the time of purchasing a piece of Ethernet equipment.
As shown in the figure below, the GBIC and SFP MSA modules contain no data-rate and protocol-specific processing blocks. Therefore, such modules can also be used for other applications such as Fibre Channel and SONET/SDH. Therefore, the MSA concept not only created a pay-as-you-grow upgrade scenario, but also the economy of scale for optical transceivers which helps to reduce their costs through mass production.
Besides the basic necessary optical-electrical (OE) and electrical-optical (EO) conversion functions, MSA modules also offer a digital diagnostic I2C (Inter-IC bus) interface, which provides information such as PMD type, laser wavelength, input, and output optical power to the host system. This interface can be used for optical link trouble shooting and performance monitoring.
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