This chapter is from the book
This chapter is from the book
This chapter is from the book
Packet Flow
Whenever a network device receives data, it is in the form of either electrical signals or optical signals. A signal goes through multiple stages before it reaches the packet-forwarding engine. Similarly, a packet goes through multiple stages before it is transmitted out. Figure 4-3 illustrates the major components related to the optics in a switch.
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In the following sections, we’ll discuss packet flow for the newer 400 Gbps and onward optics.
Demultiplexers and Multiplexers
As shown in Figure 4-4, when a signal—either in electrical form when using copper cables or optical form when using optical fiber—reaches the pluggable optics, it goes through a demultiplexer (demux) function that splits the signals.
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The PFE ASIC has multiple SerDes (serializer/deserializers), which form a high-speed interface used to convert data streams received between serial and parallel forms. As the PFE ASIC processes packets serially, the SerDes performs the job of serializing the data received from multiple SerDes links in parallel. If the PFE ASIC supports ~50 Gbps SerDes, a demux could perform the demultiplexing to 8 × 50 Gbps from 400 Gbps optics. If the PFE ASIC supports ~100 Gbps SerDes, a demux could perform the demultiplexing to 4 × 100 Gbps from 400 Gbps optics or 8 × 100 Gbps from 800 Gbps optics.
After the PFE ASIC processes the data, the SerDes again parallelizes the data into separate streams. Either single or multiple SerDes can be mapped to a single optics unit to achieve the required rate. This process is used for creating N:1 conversion of signals, and it is achieved with the help of multiplexing, as illustrated in Figure 4-5.
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With AI/ML requirements of 400 Gbps, 800 Gbps, and beyond, the SerDes links are now at 200 Gbps and moving toward higher speeds. Further improvements in the mux and demux are required to be able to achieve higher speeds.
Digital Signal Processors (DSPs)
The signals from the demultiplexer are passed to the digital signal processor (DSP), which carries out several functions. Let’s investigate each of them in detail.
Modulation and Demodulation
DSPs are responsible for dealing with sophisticated modulation methods in high-speed optical communications, as illustrated in Figure 4-6. They encode digital data onto the optical carrier wave by converting it into diverse amplitude and phase states. On the receiving side, DSPs decode the optical signal by interpreting the detected changes in amplitude and phase to retrieve the original data.
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Modulation is the process of converting data or information to electrical or optical signals. Modulation is required from the ASIC to the electrical signals and then later from electrical signals to optical signals. The modulation at each layer may be different. NRZ, also known as PAM-2, is a traditional modulation technique that does not support higher-bandwidth requirements. PAM-4 and above are being used for newer optics to support the higher-bandwidth requirements of AI/ML data centers.
The following modulations are used in different types of network connectivity:
NRZ: Non-Return-to-Zero (NRZ), which used to be a widely used scheme, has two voltage levels to represent 0 and 1. It is commonly used in the 28 GHz range and for some 56 GHz channels.
PAM-4: Pulse Amplitude Modulation with four levels (PAM-4) is a signal-encoding technique that uses four distinct signal levels (that is, voltage levels) to represent four combinations of two-bit logic (00, 01, 10, and 11). PAM-4 modules double the bandwidth of a connection, with each signal level representing 2 bits of logic information. PAM-4 is used for some 56 GHz channels and all 112 GHz channels.
Higher-order modulation: For greater speeds, modulation schemes such as PAM-8 or QAM (quadrature amplitude modulation) are considered. These methods boost data rates by using additional signal levels or combining various amplitude and phase states.
DWDM (dense wavelength division multiplexing): Coherent DWDM technology uses amplitude, phase, and polarization of light. It supports much higher bit rates on a single wavelength when DSPs are used. DWDM is used for transmitting multiple data channels over a single fiber, expanding data capacity in high-speed connections between data centers. 400G ZR modules have ushered in a new era of DWDM technology marked by open, standards-based, and pluggable DWDM optics, enabling true IP-over-DWDM. 400G ZR modules are used for connectivity between data centers (up to 80 km).
To achieve higher data transmission rates, the industry is trying out different modulation methods that make it possible to transmit more data at a time. We can compare it to adding more seats to carry a larger number of passengers on a flight, where the downside is that privacy is reduced. Similarly, transmitting more data at a time results in more noise to the signals and creates a need for more sophisticated mechanisms to deal with it.
Error Detection and Correction
DSPs are responsible for error detection and handling retransmission or correction of received packets to maintain data accuracy. They use forward error correction (FEC) algorithms to rectify errors that occur during transmission. Methods like low-density parity-check (LDPC) codes and Bose-Chaudhuri-Hocquenghem (BCH) codes are often used to enhance data integrity and reliability. At 400 Gbps and above, FEC is needed for reliability, although it introduces latency.
Clock Data Recovery
DSPs play a critical role in synchronizing the transmitter and receiver to maintain data integrity and minimize errors. A DSP extracts clock signals from the incoming data, which is vital for precise data sampling and decoding. This function becomes especially important at high data rates, where accurate timing is essential.
Equalization
DSPs use equalization to improve the signal-to-noise ratio (SNR). Equalization is a signal processing technique that restores the shape of a signal waveform in optics. Equalization algorithms include feed-forward equalization (FFE) and decision-feedback equalization (DFE).
