内容发布更新时间 : 2024/5/19 16:04:00星期一 下面是文章的全部内容请认真阅读。

The difference b/w XFP and SFP+

Take a look at the XFP block digram below which shows the XFI electrical interface from an XFP module connecting to the the host device (ASIC or PHY chip) on left. The test-points (arrows marked A,B,B’,C,C’& D) aren’t part of the physical interface but are markers that we can use to discuss the diagram. The four boxes on the right represent transmit and receive sub-assemblies, drivers and amps. You’ll find similar functional blocks in all optical transceivers. Focus on the CDR (re-timer) blocks to the left of the XFP module. The first thing to note is that there are two re-timers, one for the transmit path and one for receive. The Receive Optical Sub Assembly (ROSA) translates the optical signal received from the fiber into an electrical signal. The signal is then boosted by the Post Amp, before being re-timed by the CDR block. The CDR block is acting solely as a re-timer here, reconditioning the signal received from the ROSA. The CDR re-transmits this signal using the XFI serial interface to the ASIC. Similarly the CDR on the XFP’s transmit path, receives a degraded signal from the ASIC and applies re-timing to prepare the electrical signal for optical conversion by the the Transmit Optical Sub Assembly (TOSA).

XFP Block Digram – source: http://40gethernet.wordpress.com/

At the bottom of the diagram you’ll see the PMD, PMA and PCS markers showing you which PHY-layer sub-functions are mapped to each block. You should note that XFI is a ‘just’ a set of electrical requirements and tolerances expressed as compliance masks in the XFP MSA Specification. The serial bit stream sent on the XFI interface is exactly the same 64b/66b encoded bit stream received on the wire. XFI isn’t ‘intelligent’ and the IEEE may no reference to XFI nor the SFI interface shown below.

SFP+

SFP+ transceivers are cheaper, smaller and less power-hungry than XFP modules, and performs similar duties. How is this possible? The XFP MSA specification was agreed many years before SFP+, so Moores Law would definitely have helped to shrink the internal circuitry.

However the main reason for the size, power and cost improvements over XFP is that the SFP+ simply removes the CDR blocks from the module. Tada!! In a classic case of lazy engineering the SFP+ module makes the signal conditioning the responsibility of the upstream ASIC or signal conditioning chip.

SFP+ Block Diagram – source: ://40gethernet.wordpress.com

The SFI interface almost identical to the XFI interface you saw in the first diagram. If you zoom into the PMA block now you’ll see that it has two new sub-functions. The signal conditioning responsibility has shifted from the SFP+ module to the device performing the PMA function. On the TX side, the signal needs pre-emphasis to prepare it for the transmission to the SFP+ module. On the RX side the signal needs serious clean-up as it now contains noise and jitter from the circuit and the optical channel.

Sherpa Summary

The SFP+ module performs less functions than the XFP module. The CDR blocks which were included in XFP module are discarded by the SFP+ which shifts the responsibility of signal conditioning to other components on the circuit board. The PMA sub-layer must take up the signal conditioning role, but needs to adopt different techniques as it working at the other end of the noisy circuit-board.

Pre-emphasis and Equalization are the tools employed at the PMA and they will be the subject of the next hardware post.

Overview of the XLPPI and using it with QSFP+ modules

In the initial development of the 100/40G Ethernet 802.3ba specification the intended interface to the optical module were the XLAUI and CAUI, which were derived from the 10Gbps XFI used on XFP transceivers. As with the XFI, both the XLAUI and CAUI require that the module perform retiming on both transmit and receive data. This retiming function is found in CFP modules but not in the popular QSFP+ module, which means that the XLAUI is not compatible with the quad 10Gbps interface of the QSFP+ module. To address the incompatibility between the XLAUI and the QSFP+ module the final version of the 802.3ba includes the description of the optional XLPPI (and CPPI) which allows direct connection to optics without the necessity of a re-timer function. This blog will look at the XLAUI and XLPPI with respect to their 10G predecessors (the XFI and SFI) and how the XLPPI is used with QSFP+ modules.

Background on the XFP/XFI and SFP+/SFI

The XFP was the first 10Gbps pluggable module with a serial interface called the XFI and in order to enable the connecting ASIC’s to support the 10Gbps serial XFI the XFP module supported a PMA function for retiming of both the transmit and receive data. By introducing the retiming function into the XFP, the signal eye at the test points B’ /B and C/C’ were optimized for the benefit of the host PMA SERDES at the expense of the cost and size of the XFP. The diagram below shows the functional blocks of the XFP module, the location of the test points and how it connects to a Ethernet host.

The next step in 10G module evolution was to enhance the SFP module (used for 1Gbps Ethernet and 1-4Gbps FC) to support 10Gbps thus creating the SFP+.