By GRL Team on January 17, 2023
USB

Welcome to the 80Gpbs Ultra-High Speed Era of USB4

Granite River Labs (GRL)

With the release of the latest USB4® Version 2.0 specification on October 18, 2022, USB-IF has once again doubled the USB bandwidth. This allows USB Type-C® cables and connectors to provide 80Gbps performance and DisplayPort 2.1 support, which will benefit higher performance monitors, storage devices, and USB-based hubs and docks. On the same day, USB Type-C® cable and connector specification 2.2 and USB PD 3.1 specification 1.6 were also released to support higher data transmission performance. VESA Association released the latest DisplayPort 2.1 specification on October 17, 2022. In the same week, Intel conducted a prototype demonstration of its next-generation Thunderbolt™ that is consistent with USB4® Version 2.0 specification and can support 80Gbps.

Since the launch of USB4 specification in September 2019, data and video can be combined for simultaneous transfer, thus increasing the transfer speed to USB 20Gbps (10Gbps x2) and USB 40Gbps (20Gbps x2). It also has fast charging capabilities of up to 240W PD, which will fully satisfy gamers’ needs for high resolution display and fast charging. In response to the continuous acceleration of global high-performance computing (HPC) applications, the latest USB4® Version 2.0 specification provides ultra-high transfer speed of USB 80Gbps (40Gbps x2). This allows the USB4 to continue dominating in external interface applications.

This article will focus on these topics:

  1. Introduction to USB4 Gen4
    • PAM3 vs. PAM4 vs. NRZ
    • Data Rate and Baud Rate
    • Bit Error Rate BER vs. TER
  2. Why USB4 Gen4 uses PAM3
    • Total Loss considerations
    • Bit Error Rate considerations
  3. USB4 Gen4 bandwidth optimization
    • Asymmetric Link support to increase bandwidth up to 120 Gbps
    • USB3 Gen T support to optimize bandwidth utilization

迎接 USB4 - 80 Gbps 超高速時代來臨-1

Figure 1. USB4 80Gbps PAM3 Eye Diagram

 

Introduction to USB4 Gen4

The newly released USB4 Version 2.0 specification doubles the transfer rate to 80Gbps (40G bps x2) and is denoted as USB4 Gen4. The electrical layer uses Pulse Amplitude Modulation 3-level (PAM3) signal coding, which utilizes three voltage states for transmission. This creates a two-eyed diagram as shown in Figure 1.

Each channel transmits at a Baud rate of 25.6GB, and the transmitter encodes binary bit signals in an 11-bits to 7-trits (ternary digit) mapping configuration to achieve a dual-channel 80Gbps transmission speed with PAM3 signals. Gen4 Link receivers require Trit Error Ratio (TER) of 1E-8 or lower without Forward Error Correction (FEC).

USB4 supports USB3, DP, PCIe (option), and host-to-host tunneling. Version 2.0 also enhances the USB3 tunneling bandwidth utilization by adding the USB3 Gen T protocol adapter, which supports the latest DP 2.1. The new Asymmetric Link support further increases the bandwidth up to 120Gbps.

Read on to understand some common terms related to the new PAM3 signal coding.

 

PAM3 vs. PAM4 vs. NRZ

迎接 USB4 - 80 Gbps 超高速時代來臨-4

Figure 2. Comparison between PAM2, PAM3, and PAM4 

 

PAM3, Pulse Amplitude Modulation 3-level, ternary code, usually -1, 0, +1 or 0, 1, 2 to represent ternary values. Each PAM3 Symbol can transmit 1.58 bits (log2 3 = 1.58).

PAM4, Pulse Amplitude Modulation 4-level, usually represented by 00, 01, 10, 11. Each PAM4 symbol can transmit 2 bits per Symbol (log2 4 = 2).

NRZ, Non-Return-to-Zero, also known as Pulse Amplitude Modulation 2-level (PAM2), binary code, uses high and low level to represent logic 1 and 0. NRZ can transmit 1 bit per Symbol (log2 2 = 1).

 

Data Rate and Baud Rate

High-speed interface data rate is usually expressed in bps (bit per second). However, with the change in signal coding, such as Ethernet link transmitting with 50GBaud PAM4, it can reach a data rate of up to 100Gbps.

Symbol Rate is expressed in Baud. 1 Baud is equal to one Symbol per second. The Data Rate is then calculated according to the number of bits carried by each Symbol under different coding mechanisms. The conversion relationship between the two is explained in this formula:

Data Rate = Baud Rate * Symbol carries the number of bits

  • When the signal coding method is NRZ, the Baud Rate will be equal to Data Rate, such as NRZ (PAM2) at 25.6 GBaud, the Data Rate is 25.6 Gbps (25.6 GBaud * log2 2).
  • When the signal coding method is PAM3, the Data Rate of PAM3 at 25.6 GBaud is 40.575 Gbps (25.6 GBaud * log2 3).
  • USB4 Gen4 transmits at 25.6 GBaud speed with PAM3 signal, through 11-bits to 7-trits mapping, the data rate is 40.2 Gbps (25.6 GBaud* 11/7). It can achieve 80Gbps with dual-lane transmission.

Note: PAM3 Symbol can carry a maximum of 1.58 bits (log2 3 = 1.58), and USB4 Gen4 has a Symbol carrying bit of 1.57 (11/7 bits), which utilizes 99% of the PAM3 transmission bandwidth.

 

Bit Error Rate BER vs. TER

In an NRZ signal, which is transmitted in bits, the error rate will be represented by BER (Bit Error Ratio). For ternary signal transmission of PAM3, the error rate is represented by TER (Trit Error Ratio), where trit is an abbreviation for ternary digit.

 

Why USB4 Gen4 uses PAM3

For Gen4 to reach 80Gbps while using the same PCB and cable as Gen3, it must use the new signal coding of either PAM3 or PAM4 to meet the SI criteria with the following considerations:

 

Total Loss considerations

The signal is transmitted from the TX side of the Host through the connector, cable, and then to the RX side of the Device. The maximum allowable loss of the cable and PCB is the same as Gen3. If NRZ is used to transmit at 40Gbps, the transmission loss will exceed 40dB at the Nyquist frequency (20GHz). As the chipset cannot compensate for this excessive loss, it will incorrectly receive a signal that NRZ has failed to meet Gen4 requirements. For PAM4 and PAM3, at Gen4 Nyquist frequencies of 10GHz and 12.8GHz, the total insertion loss is about 23dB and 28dB respectively. This can be taken into consideration for analysis since the chipset is able to compensate for these amounts of loss.

 

Bit Error Rate considerations

For receiver BER, the eye height of PAM3 is half of NRZ while the eye height of PAM4 is 1/3 of NRZ. This means that PAM3’s eye height and signal-to-noise distortion ratio (SNDR) are superior to those of PAM4.  By conducting simulations and actual circuit experiments, it has been found that the uncoded BERs for PAM3 and PAM4 are 10E-8 and 10E-6 respectively. As such, PAM3 has been determined to be more compatible with USB4 Gen 4.

 

USB4 Gen4 bandwidth optimization

For Gen4 to reach 80Gbps while using the same PCB and cable as Gen3, it must use the new signal coding of either PAM3 or PAM4 to meet the SI criteria with the following considerations:

  • New Asymmetric Link operation to increase bandwidth up to 120 Gbps
  • New USB3 Gen T for optimized bandwidth utilization

 

Asymmetric Link support to increase bandwidth up to 120 Gbps

USB4 Link can be either Single-Lane Link or Aggregated Link, as shown in Table 1. In Single-Lane Link, only Lane 0 is enabled while Lane 1 is disabled. In Aggregated Link, both  Lane 0 and Lane 1 are enabled and have a connection that is either symmetric or asymmetric. Symmetric Link means that the number of TX lanes is same as the number of RX lanes, e.g. 1*TX/1*RX or 2*TX/2*RX.

USB4 80 Table-1

Table 1.  USB4 Symmetric Link and Asymmetric Link

 

While USB4 Gen2 and Gen3 can only support Symmetric Link, Gen4 can also support Asymmetric Link. This allows Gen4 to have both high image resolution transmission DP 2.1 and high speed data transmission without reducing display quality. In Asymmetric Link, one pair of TX/RX is used for DP transmission as TX/TX or RX/RX. As shown in Figure 3, one side transmits “3*TX/1*RX”, while the other side transmits “1*TX/3*RX”. This enables a transmission of up to 120Gbps (40Gbps x3) in one direction while maintaining a rate of 40Gbps in the other direction. The conversion from Symmetric Link to Asymmetric Link is controlled by the Connection Manager.

迎接 USB4 - 80 Gbps 超高速時代來臨-2

Figure 3.  USB4 Symmetric Link and Asymmetric Link  

 

USB3 Gen T support to optimize bandwidth utilization

USB4 adds support for USB3 Gen T protocol tunneling, mainly to allow USB3 tunneling to make better use of USB4 transmission bandwidth. Here are the definitions of USB3 Gen X and USB3 Gen T, which are newly added in USB4 Version 2.0:

  • USB3 Gen X: USB3 tunnel architecture that uses the existing USB 3.2 protocol.
  • USB3 Gen T: USB3 tunnel architecture that uses a modified USB 3.2 protocol to allow the use of the maximum bandwidth available for USB4.

USB4 needs to be backward compatible with USB3, so the USB4 Gen2/Gen3 router must be equipped with a USB3 Protocol Adapter, which encapsulates the native USB3 protocol data traffic and LFPS in the USB4 data package. The USB3 tunneling protocol that is currently used is represented as USB3 Gen X in USB4 Version 2.0 to differentiate it from the new USB3 Gen T.

Under the current USB3 Gen X tunneling protocol, the hub has only one Embedded SS Hub for uploading. As shown on the left-hand side of Figure 4, when the hub is connected to 2 devices (20Gbps) at its downstream ports for a total bandwidth of 40Gbps, the upstream bandwidth would still be limited to the embedded SS hub upstream maximum bandwidth of 20Gbps. This creates a bottleneck as the higher bandwidth of USB4 cannot be fully utilized.

迎接 USB4 - 80 Gbps 超高速時代來臨-3

Figure 4. Comparison between USB3 Gen X and USB3 Gen T Tunneling

 

The USB3 Gen T tunneling protocol uses a modified version of USB 3.2 protocol, which is mainly achieved by adding a USB3 Gen T protocol adapter. As shown on the right-hand side of Figure 4, the hub is connected to two devices, with both running at 20Gbps (USB3 10G x2). When both the host and devices support the USB3 Gen T, the hub upstream and downstream transmission will pass through directly (without going through the USB3 embedded SS hub) such that the upstream bandwidth is also 40Gbps as opposed to the 20Gbps bandwidth of the original USB3 Gen X. USB3 Gen T Tunneling is an optional support feature for Host, Hub, and Device, which can more effectively utilize the bandwidth of USB4.

 

Conclusion

With PAM3 signal coding, the USB4 Gen4 data rate has been increased to 80Gbps. The bandwidth of USB4 Gen4 has been further optimized with the addition of Asymmetric Link support, which increases the bandwidth up to 120Gbps, and USB Gen T protocol adapter for optimized bandwidth utilization. While only 2 pairs of TX/RX need to be tested for signal quality, by supporting Asymmetric Link, all 4 pairs of TX/RX will need to be tested. This increases the test complexity and duration.

For designers and engineers, PAM3 signal coding presents new challenges. With half of the original NRZ eye height, the signal-to-noise distortion ratio (SNDR) has become a much more important consideration. Moreover, the more complex eye diagram and jitter analysis will require more time to understand.

GRL has extensive experience and deep expertise in USB compliance testing and certification, and has set up a USB4 80Gbps test environment. Feel free to subscribe to our newsletter here to receive the latest GRL News for the next technical update on USB4 Gen4 physical characteristics and PAM3 measurement and design considerations.  

 

References:

  • Universal Serial Bus 4 (USB4®) Specification, Version 2.0, October 2022

 

Author: Sandy Chang, Technical Director GRL Taiwan

  • Experts in testing high-speed interfaces such as Thunderbolt™ 4, USB4™, USB3, DisplayPort, HDMI, PCI Express, etc.

 

Specifications and descriptions in this document can be subjected to changes by GRL without prior notice.

2022/10/20 AN-221020-EN

Published by GRL Team January 17, 2023