With the rapid development of high-definition video and high-speed data services, people have higher and higher requirements for bandwidth, and the existing 10GE has shown its limitations. Therefore, it is very necessary to conduct research on the next-generation high-speed Ethernet technology. The IEEE 802.3ba standard, the 40G/100G Ethernet standard, was officially approved on June 17, 2010, and the first specification will use two new Ethernet speeds simultaneously. These two transmission rates are mainly for different requirements of servers and networks, 40GbE is suitable for server and storage applications, and 100GbE is suitable for aggregation and core network applications.

The architecture of 40G/100G Ethernet Technology

40/100GE technology is developed on the basis of 10GE technology, and their architectures have a certain degree of similarity. As shown in Figure 1, the common parts of 10GE, 40GE, and 100GE architectures are: PHY represents the physical layer device of Ethernet, corresponding to the first layer of the OSI model; PHY is connected to the MAC layer through the connection medium (fiber or copper wire), and the MAC layer corresponds to the second layer in the OSI model. PHY is further divided into Physical Medium Dependent Sublayer (PMD), Physical Medium Attachment Sublayer (PMA), and Physical Coding Sublayer (PCS).

In order to achieve high-speed transmission, the 40GE and 100GE architectures have been improved on the basis of 10GE, and the MLD protocol architecture is the key. As shown in Figure 1, for 40G and 100G Ethernet, the L virtual channel data in the PCS layer is bit-multiplexed to form M channels of XLAUI interface channel data (40G) or CAUI interface channel data (100G).

The interface is connected to the PMA layer corresponding to the optical module device, and then the M-channel XLAUI (or CAUI) interface data is converted into N-channel channels and connected to the PMD layer. Four virtual channels are designed in the PCS layer of 40GE transmission, and 20 virtual channels are designed in the PCS layer of 100GE transmission to adapt to different optical modules to realize network interconnection. The MLD mechanism is flexible and can support any physical type based on electro-optical transmission.

For 10GE, 40GE, and 100GE, the logical interfaces connecting the MAC and the physical layer are XGMII, XLGMII, and CGMII interfaces respectively. The transmission and reception of the interface are independent of each other. In the IEEE802.3ba standard, the bit width of the interface is defined as 64 bits, while the XGMII corresponding to 10G is an interface with a signal width of 74 bits. In which the data paths for transmission and reception occupy 32 bits each.

Compared with 10GE technology, 40GE/100GE puts forward higher requirements on the performance of the transmission medium. Mainly using optical fiber for transmission, among which 40G SR4 and 100G SR4 can reach 100m on multimode fiber (MMF), and 40G LR4 can reach 100m. The transmission distance of 100G LR4 on single-mode fiber (SMF) can reach 10km.

 MLD Mechanism Analysis

The MLD mechanism is a key technology introduced to achieve 40G/100G high-speed transmission. Using the MLD mechanism to process high-speed 40G/100G data streams can reduce the requirements for hardware clock frequency and facilitate the adaptation of multi-channel connections from the PMD layer to the optical fiber medium. The MLD mechanism performs 64B/66B encoding on the data stream from the MAC through the PCS layer to form a code block, and then distributes the code blocks to multiple virtual channels (Virtual Lanes) in turn, and then converts the corresponding channel data through the optical modules PMA and PMD. and transmission.

The data stream is first encoded into a 64B/66B continuous code block stream and scrambled. The PCS layer distributes the scrambled data to L virtual channels (40G and 100G virtual channels are 4 and 20 respectively) through the round-robin distribution mechanism and adds special flag bits to each channel to record correctly and arrange data. The PMA layer multiplexes the L channels of virtual channel data to form M channels of XLAUI or CAUI interface channel data. For 40GE, the channel number M is 4, and the 4:4 channel conversion is performed at the PMA layer; for 100GE, the channel number M is 10, and 20:10 channel conversion is performed at the PMA layer.

Then connect to the PMA layer corresponding to the optical module device through the XLAUI (or CAUI) interface, and convert the XLAUI (or CAUI) interface data of M channels into N channels and connect them to the PMD layer (40G for 4:1 conversion, 100G for 10:4 conversion). The least common multiple of the number M of interface channels between the electrical layers in the PMA layer and the number of channels N connected to the optical fiber medium in the PMD layer is the number of channels L in the virtual channel, that is, L=LMC(M, N). Commonly used optical modules for 100G include 10×10G, 4×25G, and 5×20G. M is usually 10, so the PCS layer of 100G should be designed with 20 virtual channels.


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