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Le tecnologie dei ricetrasmettitori ottici di prossima generazione – PAM4 e 64QAM

Tecnologie PAM4 e 64QAM

Il passaggio ai servizi cloud e alle reti virtualizzate ha messo il data center nel mezzo del nostro mondo e ha significato che la connettività all’interno dei data center e tra i data center ha un enorme impatto sulla fornitura di servizi aziendali e personali. I data center Hyperscale vengono installati in tutto il mondo e questi hanno tutti bisogno di essere connessi. Per soddisfare questa domanda, i fornitori di ricetrasmettitori ottici stanno offrendo nuove soluzioni basate su PAM4 e 64QAM, fornendo una modulazione coerente che ridurrà il costo della connettività e aumenterà la larghezza di banda di ciascuna connessione.

Le connessioni a molti server sono già 25G e i collegamenti tra switch in data center di grandi dimensioni sono già 100G. L’introduzione dei transceiver SFP28 e QSFP28 che integrano le nuove tecnologie e sono state costruite utilizzando tecniche di produzione efficienti hanno ridotto il costo di queste connessioni e consentito una crescita massiccia nel mercato. La fase successiva è l’introduzione di soluzioni a singola lambda 100G e ricetrasmettitori a basso costo 400G per i collegamenti tra switch. I dispositivi PHY necessari per questo passaggio successivo sono già disponibili, i dispositivi switch da 12,8T sono in produzione e i primi ricetrasmettitori ottici QSFP-DD e OSFP 400G sono di campionamento.

QSFP-DD

L’ascesa dell’operatore di data center hyperscale ha cambiato radicalmente il mercato. Il passaggio a 25G e 100G da 10G e 40G è avvenuto molto rapidamente. L’ampiezza e il numero di data center installati o aggiornati significa che le nuove tecnologie possono essere spedite in volume non appena il prezzo è giusto, i componenti sono stati qualificati e le linee di produzione sono operative. Ora stiamo vedendo i primi dispositivi 400G PHY e ricetrasmettitori ottici per la disponibilità di data center e le aziende stanno gareggiando per la posizione di mercato in attesa che i principali operatori hyperscale si impegnino a implementazioni di grandi dimensioni.

Molte di quelle aziende che hanno beneficiato di 25G e 100G stanno investendo i propri investimenti in soluzioni Lambda PAM4 100G e 400G per il data center . Ciò ha richiesto nuovi dispositivi PAM4 PHY progettati per soddisfare i limiti di potenza dei transceiver OSGP e QSFP-DD 400G. Alcune aziende hanno anche investito in PHG 50G e 200G PAM4, consentendo un aggiornamento economico da 25G e 100G. Si prevede che i ricetrasmettitori 50G SFP56 e 200G QSFP56 siano soluzioni provvisorie, ma non è chiaro quanto sia diffuso il loro uso o per quanto tempo. 40G è stata una soluzione provvisoria che è durata per molti anni.

La tecnologia coerente, originariamente sviluppata per reti a lungo raggio da 100G, è ora ampiamente utilizzata per le connessioni a lungo raggio, comprese le reti sottomarine, metropolitane e Data Center Interconnect (DCI) tra i data center. Il mercato per DCI è cresciuto rapidamente, con molti fornitori di sistemi che offrono soluzioni con una copertura da 80 a 500 km. Per le applicazioni a lungo raggio e metropolitane, numerosi produttori leader di apparecchiature continuano a utilizzare progetti di DSP (Digital Signal Processor) coerenti all’interno dell’azienda. La soluzione coerente DSP è ora disponibile per i fornitori di ricetrasmettitori ottici come Gigalight che invierà transceiver 400Gbasato su questo disegno. Gli ultimi ASIC DSP stanno abilitando le soluzioni 600G (64Gbaud 64QAM) e i transceiver CFP2-DCO. Il passo successivo è l’introduzione dei DSP 7nm che consentiranno l’utilizzo di transceiver ZR 400G a costi contenuti per collegamenti 400G fino a 100 km a partire dal 2020.

Questo continua ad essere un mercato in divenire. Lumentum ha completato l’acquisizione di Oclaro, Cisco ha completato l’acquisizione di Luxtera e diversi fornitori di ricetrasmettitori ottici cinesi hanno aderito alla carica di 400G nel data center. I dispositivi PAM4 PHY richiesti per 100G single lambda e 400G nel data center si stanno dimostrando molto impegnativi da consegnare. Le soluzioni PAM4 PHY in tecnologia a 28 nm e 14/16 nm sono state sottoposte a campionamento per oltre sei mesi e ora vengono affiancate da soluzioni 7nm.

PAM4 — The High-Speed Signal Interconnection Technology of Next-Generation Data Center

What Is PAM4?

PAM4 (4-Level Pulse Amplitude Modulation) is one of PAM modulation technologies that uses 4 different signal levels for signal transmission. Each symbol period can represent 2 bits of logic information (0, 1, 2, 3), that is, four levels per unit time.

In the data center and short-distance optical fiber transmission, the modulation scheme of NRZ is still adopted, that is, the high and low signal levels are used to represent the (1, 0) information of the digital logic signal to be transmitted, and one bit of logical information can be transmitted per signal symbol period.

However, as the transmission rate evolves from 28Gb/s to a higher rate, the electrical signal transmission on the backplane will cause more severe loss to the high-frequency signal, and higher-order modulation can transmit more data in the same signal bandwidth. Therefore, the industry is increasingly calling for higher-order PAM4 modulation. The PAM4 signal uses four different signal levels for signal transmission, and each symbol period can represent 2 bits of logical information (0, 1, 2, 3). Since the PAM4 signal can transmit 2 bits of information per symbol period, to achieve the same signal transmission capability, the symbol rate of the PAM4 signal only needs to reach half of the NRZ signal, so the loss caused by the transmission channel is greatly reduced. With the development of future technologies, the possibility of using more levels of PAM8 or even PAM16 signals for information transmission is not ruled out.

NRZ vs. PAM4: The comparison of waveforms and eye diagrams between NRZ and PAM4 signals

And then, if the optical signal can also be transmitted by using the PAM4, the clock recovery and pre-emphasized PAM4 signal can be directly realized when the electro-optical transmitting is performed inside the optical module, therefore, the unnecessary step of converting the PAM4 signal into the NRZ signal of 2 times the baud rate and then performing related processing is eliminated, thereby saving the chip design cost.

Why PAM4?

The end-to-end transmission system includes fiber optic and fiber-optic transmission systems. Since the fiber transmission can easily reach the rate of 25Gbd so that the research progress of transmitting PAM4 on the fiber has been progressing slowly. For fiber-optic transmission systems, from NRZ moving to PAM4 is considered in terms of cost. If you do not need to consider the cost, there are other related modulation technologies can be used in the long-distance range, such as DP-QPSK, which can transmit the baud rate signal above 50Gbd for several thousand kilometers. However, in the data center field, the transmission distance is generally only 10km or less. If the optical transceiver using PAM4 technology is adopted, the cost can be greatly reduced.

For 400GE, the largest cost is expected to be optical components and related RF packages. PAM4 technology uses four different signal levels for signal transmission. It can transmit 2 bits of logic information per clock cycle and double the transmission bandwidth, thus effectively reducing transmission costs. For example, 50GE is based on a single 25G optical device, and the bandwidth is doubled through the electrical layer PAM4 technology, which effectively solves the problem of high cost while satisfying the bandwidth improvement. The 200GE/400GE adopts 4/8 channel 25G devices, and the bandwidth can be doubled by PAM4 technology.

For data center applications, reducing the application of the device can significantly reduce costs. The initial goal of adopting higher order modulation formats is to place more complex parts on the circuit side to reduce the optical performance requirements. The use of high-order modulation formats is an effective way to reduce the number of optics used, reduce the performance requirements of optics, and achieve a balance between performance, cost, power, and density in different applications.

In some application scenarios, high-order modulation formats have been used for several years on the line side. However, since the client side needs are different from the line side, so other considerations are needed.

For example, on the client side, the main consideration is the test cost, power consumption and density. On the line side, spectrum efficiency and performance are mainly considered, and cost reduction is not the most important consideration. By using linear components on the client side and the PAM4 modulation format that is directly detected, companies can greatly reduce test complexity and thus reduce costs. Among all high-order modulation formats, the lowest cost implementation is PAM4 modulation with a spectral efficiency of 2 bits/s/Hz.

PAM4

Conclusion

As a popular signal transmission technology for high-speed signal interconnection in next-generation data centers, PAM4 signals are widely used for electrical and optical signal transmission on 200G/400G interfaces. Gigalight has a first-class R&D team in the industry and has overcome the signal integrity design challenges of PAM4 modulation. Gigalight’s 200G/400G PAM4 products include 200G QSFP56 SR4, 200G QSFP56 AOC, 200G QSFP56 FR4, 400G QSFP56-DD SR8, 400G QSFP56-DD AOC, etc.

All of the PAM4 products from Gigalight can be divided into digital PAM4 products and analog PAM4 products. The digital PAM4 products adopt DSP solutions which can support a variety of complex and efficient modulation schemes. The electric port has strong adaptability and good photoelectric performance. And the analog PAM4 products simulate CDR with low power consumption and low cost. Gigalight always adheres to the concept of innovation, innovative technology, and overcomes difficulties. It invests a lot of human resources and material resources in the research and development of next-generation data center products.

Originally published at dci.ti-da.net

Which Is Better? PAM4 or Coherent Technology for 80km Links

A significant portion of Data Center Interconnections (DCIs) and telecom router-to-router interconnections rely on simple ZR or 80km transceivers. The former is mostly based on 100Gbps per 100GHz ITU-T window C-band DWDM transceivers, while the latter is mostly 10G or 100G grey wavelength transceivers. In DWDM links, the laser wavelength is fixed to a specified grid, so that with DWDM Mux and Demux 80 or more wavelength channels can be transported through a single fiber. Grey wavelengths are not fixed to a grid and can be anywhere in the C-Band, limiting capacity to one channel per fiber. DCI links tend to use DWDM because they have to utilize the optical fiber bandwidth as much as possible due to the extremely high-volume traffic between data centers.

Another emerging 80km market is the Multi-System Operator (MSO) or the CATV optical access networks. This need emerges because MSOs are running out of their access optical fibers and they need a transmission technology which would allow them to grow to a very large capacity by using the remaining fibers. For this reason they need to use DWDM wavelengths to pack more channels in a single fiber.

The majority of the 10G transceivers on 80km links will be replaced by 100G or 400G transceivers in the coming years. For that to happen, there are two modulation techniques to enable 80km 100G transceivers.

50G PAM4 with two wavelengths in a 100G transceiver
Coherent 100G dual-polarization Quadrature Phase Shifted Keying (DP-QPSK)

Generally speaking, PAM4 is a low-cost solution but require active optical dispersion compensation (which could be a big headache as well as extra expense to data center operators) and extra optical amplification to compensate for the dispersion compensators. By contrast, Coherent approaches do not need any dispersion compensation and the price is coming down rapidly, especially when the same hardware can be configured to upgrade the transmission data rate per wavelength from 100G to 200G (by using DP-16QAM modulation).

When 400G per wavelength is needed in a DCI network within a 100GHz ITU-T window, coherent technology is the only cost-effective solution, because it will not be feasible for PAM4 to achieve the same high spectral efficiency of 4 bit/sec/Hz.

On the standards front, many standards organizations are adopting coherent technology for 80km transmission. The Optical Inter-networking Forum (OIF) will adopt coherent DP-16QAM modulation at up to 60Gbaud (400G per wavelength) in an implementation agreement on 400G ZR. This is initially for DCI applications with a transmission distance of more than 80km, and vendors may come up with various derivatives for longer transmission distances. Separately, CableLabs has published a specification document for 100G DP-QPSK coherent transmission over a distance of 80km aimed at MSO applications. In addition, IEEE802.3ct is in the process of adopting coherent technologies for 100G and 400G per wavelength transmissions over 80km.

As data rates increase from 100G to 400G and capacity requirements per fiber are driven by DCI needs, and assisted by volume driven cost reductions in coherent optics and in coherent DSPs, we expect coherent transmission to be the technology of choice for 80km links.