1. Overview
1.1 Interpretation
10 Giga bit Ethernet, abbreviated as 10GE, 10GbE, commonly known as 10 Gigabit Ethernet, is a transmission standard for Ethernet, which was originally adopted in 2002 and became IEEE Std 802.3ae-2002, which regulates Ethernet transmission at a rate of 10Gbit/s.
1.2 Classification
The physical layer of 10G Ethernet includes 10GBASE-T, 10GBASE-X, 10GBASE-R, and 10GBASE-W.
1.2.1 10GBASE-T
10GBASE-T (IEEE 802.3an) CAN TRANSMIT DISTANCES OF UP TO 55 METERS (180 FEET) FOR CAT-6 AND UP TO 100 METERS (330 FEET) FOR CAT-6A OVER UNSHIELDED TWISTED PAIR OR SHIELDED TWISTED PAIR CABLE. 10GBASE-T uses the RJ45 interface, which is widely used in various Ethernet networks. Depending on the characteristics of the transmission, the frequency of transmission needs to reach 500MHz, and the IEEE 802.3an standard specifies PAM-16 coding for 10GBASE-T. There is no 10GBASE-T in the IEEE Std 802.3ae-2002 standard because it was published in the 2006 standard, and its standard is IEEE 802.3an-2006.
1.2.2 10GBASE-X
10GBASE-X uses an ultra-compact package consisting of a simpler WDM device, four receivers, and four lasers operating at approximately 25nm intervals around 1300 nm wavelengths, with each pair of transmitter/receiver operating at 3.125 Gbit/s (2.5 Gbit/s data stream).
1.2.3 10GBASE-R
10GBASE-R is a serial interface that uses 64B/66B encoding (not the 8B/10B used in Gigabit Ethernet) to support transmission over optical media. The data stream is 10.000 Gbit/s, resulting in a clock rate of 10.3 Gbit/s.
1.2.4 10GBASE-W
10GBASE-W is a WAN interface that is compatible with SONET OC-192 with a clock of 9.953 Gbit/s and a data stream of 9.585 Gbit/s.
1.3 Development
The technological evolution of Ethernet is mainly marked by the continuous improvement of speed, from the initial 1M, 10M to 10G or even hundreds of G, the following figure shows the speed increase and wiring harness change of Ethernet with twisted pair as the transmission medium.
II. Principle
Standard setters relied on four technology building blocks to make 10GBase-T a reality: loss elimination, analog-to-digital conversion, cable enhancement, and coding improvements. 10GBase-T follows the 1000Base-T transmission mode, still using four differential pairs of simultaneous bidirectional transmission, full duplex, but the total transmission rate is up to 10Gbps, and the rate of each pair of lines is up to 2.5Gbps. In terms of encoding, instead of using the original 1000Base-T PAM-5, the PAM-16 encoding method is adopted.
Among the four physical layer technologies introduced earlier, 10GBASE-R and 10GBASE-T are now more widely used, 10GBASE-R is used for the interface of optical modules, and 10GBASE-T is an electrical port, using RJ45 and twisted pair for signal transmission.
2.1 Structure
The diagram below is taken from IEEE 802.3an-2006 to see the differences between the different physical layer standards.
The physical layer, or PHY, is layered into multiple sub-layers, which mainly completes the function of data encoding, checksum, and conversion into analog signals.
XGMII, the 10G media independent interface (where the "X" is 10 in Roman numerals) is used to make the different physical layers below 10G Ethernet transparent to the MAC sublayer above. In fact, it is the same as the RMII interface in the 100 Gigabit network, but the data transmitted is up to 10Gbps, but now it is mostly replaced by XAUI.
PCS, the physical coding sublayer, is used to encode (when data is sent) and decode (when data is received).
PMA, Physical Media Connectivity Sublayer, provides a media-agnostic approach to the PCS sublayer to support the use of serial-bit-oriented physical media.
PMD, the Physical Media Dependent Sublayer, defines the physical layer signaling and Media Dependent Interface (MDI), as well as the supported media types. It should be pointed out that the PMD sublayer is an optical signal sublayer, and its main function is to send and receive optical signals, while the layers above PMD use electrical signals.
WIS, the WAN interface sublayer, is only used in the WAN physical layer, and it sits between the PCS sublayer and the PMA sublayer. The role of the WAN interface sublayer is to perform SONET/SDH framing.
AN, THE AUTO-NEGOTIATION SUBLAYER, ALLOWS THE PHYS AT BOTH ENDS TO ADVERTISE THEIR CAPABILITIES (SPEED, PHY TYPE, HALF-DUPLEX OR FULL-DUPLEX) AND AUTOMATICALLY SELECT THE MODE OF OPERATION TO COMMUNICATE ON THE LINK, ONLY USED IN 10GBASE-T.
MDI, a media-related interface, is used to connect the cables of the PMD sub-layer to the physical layer.
The structure of 10GBASE-T consists of PCS, PMA, and NA, and has two interfaces: MII and MDI. There is an additional LDPC in front of the PCS layer, which is a low density parity check.
2.2 Media independent interface
The Media Independent Interface is the interface between the Media Access Control (MAC) sublayer and the Physical Layer (PHY). XGMII is a simple, inexpensive, and easy-to-implement interconnect. We also have the option to use a 10 Gigabit Connectivity Unit Interface (XAUI) to extend the operating distance of XGMII with a reduced number of pins. Of course, due to the large number of XGMII lines and the short distance, XAUI is basically used now.
The relationship between XAUI and XGMII is both a substitution and an extension, and the graph of XGMII XAUI is given here.
2.2.1 XGMII接口
The single-ended signal of the XGMII interface adopts HSTL/SSTL_2 logic, and the port voltage is 1.5V/2.5V, which is rarely used now due to the high port voltage and high power consumption of the SSTL_2. The communication line of XGMII is divided into data line, control line and clock line, with symmetrical transmission and reception, 32 data lines, 4 control lines and one clock line.
TXD[31:0], send signal line.
RXD[31:0], receive signal line.
TXC [3:0], transmit channel control signal, TXC=0, indicates that data is transmitted on TXD. TXC=1, the control character is transmitted on the TXD.
RXC[3:0], the receiving channel control signal, RXC=0, indicates that the data is transmitted on the RXD. RXC=1, the control character is transmitted on the RXD.
TX_CLK, the reference clock of TXD and TXC, clock frequency 156.25MHz, samples data on both the rising and falling edges of the clock signal.
RX_CLK, the reference clock of RXD and RXC, clock frequency 156.25MHz, samples data on both the rising and falling edges of the clock signal.
156.25MHz×2×32=10Gbps。
The 32 signal lines of XGMII are divided into 4 lanes, each lane has a control signal, and their correspondence is shown in the figure below.
2.2.2 XAUI interface
XAUI borrows the abbreviation of Ethernet "Attachment Unit Interface" (AUI), and the initial letter "X" stands for Roman numeral 10, which means that the transmission rate is 10 gigabits per second. Compared to the XGMII interface, the XAUI interface is greatly simplified. The XAUI interface has only 16 signal lines, of which 4 pairs of balanced differential lines are used for data reception, 4 pairs of balanced differential lines are used for data transmission, and the XAUI interface is a serial bus with its own clock. The differential pair uses CML logic and AC coupling mode, and the coupling capacitance is between 10nF~100nF. The data rate on each pair of differential lines is 3.125Gbps, the total data bandwidth is 12.5Gbps, and the effective bandwidth is 12.5Gbps ×0.8=10Gbps (because the XAUI bus data is converted 8b/10b before transmission).
The following figure shows the specifications of the transmitter of the XAUI interface.
The maximum amplitude of the differential signal is only specified here, and the typical value used in Marvell's 88X3310 is 1000mVpp, the maximum value is 1200mVpp, and the maximum input allowed at the receiver is 1600mVpp, as shown in the figure below.
The impedance requirement for a differential pair is 100 ohms ±5% at 2.5GHz. Compared with the greatly reduced number of XGMII signal lines, the differential signal has stronger anti-interference ability, thus simplifying the circuit routing design. On the FR4 PCB, the XAUI interface can be traced up to 50 cm, while the XGMII interface is only 7 cm. The cable routing of the XAUI interface is relatively long, which greatly facilitates the design of the PCB board, and the backplane routing is possible. For this reason, MAC chips do not provide XGMII interfaces for users. However, inside the chip, XGMII is still used as a standard interface. Inside the MAC chip, the XGXS (XGMII Extender Sublayer) sublayer is integrated to implement the bidirectional mapping between the XGMII interface and the XAUI interface, as shown in the previous figure.
In the process of XGAMII interchange, the source XGMII divides the transmitting and receiving 32-bit width data stream into 4 independent lane lanes, each lane channel corresponds to a byte, and after the XGXS (XGMII Extender Sublayer) completes 8b/10b encoding, the 4 lanes correspond to the 4 independent lanes of XAUI, and the XAUI port rate is 2.5Gbps ×1.25×4=12.5Gbps. In the XGXS module at the transmitter, TXD[31:0]/RXD[31:0], TXC[3:0]/RXC[3:0], TX_CLK/RX_CLK are converted into serial data and sent out from TXLane[3:0]/RXLane[3:0], and in the XGXS module at the receiver, the serial data is converted into parallel, and clock recovery and compensation are carried out to complete clock debounce, and after 5b/4b decoding, it is re-aggregated into XGMII.
There are a series of variants of XAUI interfaces, such as RXAUI, XLAUI (40Gb), and CAUI (100Gb).
The XAUI interface can be directly connected to optical modules, such as XENPAK/X2. It can also be converted into a 10G signal XFI, connected to XFP/SFP+, etc.
2.2.3 MDIO interface
To add a note, MDIO requires a resistor to pull up.
2.3 Coding technology
The PCS layer is a physical coding sub-layer, which is used to encode the data sent by the MAC layer through XGMII or XAUI for transmission through MDI, and of course also undertakes the task of decoding. The task of encoding is to ensure that high-speed data can be transmitted normally over the cable. Encoding plays a critical role in the challenging nature of 10GBASE-T, which requires data at up to 2.5Gbps per twisted pair cable.
The problem of high-speed transmission immunity is also faced in automotive Ethernet, where PMA3 technology is used. PAM5 (5-stage pulse amplitude modulation technology) modulation technology used in 1000Base-T. In PAM5 mode, the signal transmitted in the medium is no longer simply 0 and 1, but is divided into 5 levels (-2, -1, 0, 1, 2). This level signal is divided into 5 levels called symbols, and the number of bits that can be carried by 1 symbol depends on the characteristics of the symbol and the way it is encoded. For example, PAM5, each PAM5 symbol carries a maximum of 2.32 bits (2^2.32=5), considering the efficiency of encoding and the need for error correction code and synchronization code, so the final 1000Base-T each code carries 2 bit information. According to the Nyehl's criterion, the maximum symbol transmission rate under the ideal low channel = 2× bandwidth, we know that the code rate of 1000Base-T is 125M/s, so at least 62.5MHz transmission bandwidth is required.
If the 1000Base-T technology is used, the transmission rate of 10GBase-T is 1250M/s, and the minimum transmission bandwidth of the system is 625MHz. This places high demands on the performance of the transmission system. However, if the performance of the code element is improved, a code element carries more bits, and the minimum bandwidth of the system is reduced, a powerful processor is required for encoding and decoding processing, which means an increase in cost, which is a pair of contradictions. Finally, after balancing performance and cost, 10GBase-T uses PAM16 technology (16 levels of pulse amplitude modulation, using -15, -13, -11, -9, -7, -5, -3, -1, 1, 3, 5, 7, 9, 11, 13, 15) Under PAM16 modulation, the pulse voltage amplitude is divided into 16 levels, so that each voltage amplitude can represent 4 bits of information, of which 3.125 bits are effective data, and the other 0.875 bits are used for auxiliary and verification. Of course, 3.125 and 0.875 are averages, with a code rate of 800Mbit/s and a minimum bandwidth of 400Mhz.
In order for PAM16 to safely transmit 10 Gbps (BER=10^12), certain encoding rules need to be set. In order to improve the BER, a check code should be added for forward error correction, and the LDPC code (low-density parity check code) used by 10GBase-T is a linear block code, which has superior error correction performance and great practical value, and is considered to be the best error correction code so far. The performance of LDPC code can approach the Shannon limit, and at the same time, this approximation is achieved under the low decoding complexity, the hardware implementation is simple, and the performance and cost are also taken into account.
In the 10GBase-T encoding process. Every 64 bits of information, plus control/data flags, form a 65-bit block, and 50 blocks are coded into a group, and each group is added with an 8-bit CRC check code. A total of 65×50+8=3258 bits are generated, and a total of 3259 bits are added to the previous channel. 3259 bits are divided into 2 parts, 3×512bit (including channel additional code) is transmitted in an unprotected mode, and the other 1723bit plus 325 check codes are transmitted through LDPC (1723, 2048) protection mode, so that a total of 512 128DSQ codes (3×512+4×512), that is, 1024 PAM16 symbols. In the end, each PAM16 carries 3.125 bits of information (64×50/1024=3.125), and the transmission rate = 3.125×800M×4=10Gbps. The internal block diagram of the 10G Ethernet PHY chip is shown below.
2.4MDI interface
10GBASE-T uses RJ45 and CAT-6 or higher cables for transmission, and there are four pairs of twisted pair cables for full-duplex communication. The line sequence definition is shown in the following figure.
The following figure shows the spec of the differential pair of MDI interface of a certain chip.
The following figure shows the transmission distance that can be achieved by different specifications of network cables and their corresponding standards.
3. Expansion
3.1 Naming conventions
10G Ethernet uses a variety of fiber optic media. The model number of the fiber media is expressed as 10GBASE-[media type] [coding scheme] [wavelength number], or more specifically 10GBASE-[E/L/S] [R/W/X] [4]. In the media type, S is a short wavelength (850nm) and is used for multimode fiber to transmit data over short distances (about 35m). L is a long wavelength (1310nm) for data transmission between buildings on the campus network or between floors of buildings, and can support a transmission distance of 10 km when using single-mode fiber, and 300 m when using multi-mode fiber. E is the extra-long wavelength (1550nm), which is used for data transmission in WAN or metro network, and when using single-mode fiber with a wavelength of 1550nm, the transmission distance can reach 40km.
In the coding scheme, X is the 8B/1OB code in the LAN physical layer, R is the 64B/66B code in the LAN physical layer, and W is the 64B/66B code in the WAN physical layer (simplified SONET/SDH encapsulation). The final wavelength number can be 4 and uses Wide Wavelength Division Multiplexing (WWDM). WWDM is much cheaper than dense wavelength division multiplexing (DWDM) for short-distance transmissions. If you don't use WDM, the wavelength number is 1 and can be omitted.
3.2IEEE802.3ae port type
Source: Pupil of the Machine
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