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 optimised 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+. The SFP+ module does not have internal CDR retiming which means the high speed electrical characteristics are very different to the XFI and as such a new interface was created, the SFI. The SFI only species the electrical requirements at point B’/B’ and C/C’, the PMA function in the Ethernet host to both operate with these signals and to compensate for any additional distortion created by the PCB traces between the ASIC and the SFP+ module.
In the transmit direction the SFI requirement for the signal eye at B’ are much greater than those for the XFI, thus the host PMA must implement pre-emphasis to compensate for the signal distortion due to the PCB traces. On the receive side the SFI output eye at point C is not only smaller than with the XFI but the incoming jitter is higher, in addition a new element needs to be taken care of which is the pulse width shrinkage due to pattern dependency (DDPWS). In order to reliably recover the data from the SFI the host PMA needs to implement signal equalization and for some applications (like 10GBase-LRM) EDC is required.
The above eye diagrams give a visual indication of the difference between the XFI and SFI; the actual parameters can be found the table at the end of the blog.
The XLPPI and how it compares with the XLAUI and CFP modules
The XLPPI stands for XL(40Gbps) Parallel Physical Interface and is defined in 802.3ba Annex 86a as the interface between the PMA and PMD functions (where as the XLAUI dissects the PMA). The XLPPI is derived from the SFI interface and places higher signal integrity requirements on the host PMA than the XFI based XLAUI.
The diagram above shows the differences between how a QSFP+ module uses the XLPPI versus a CFP module, which uses the XLAUI. The diagram also shows the 802.3ba test points which use a different naming convention than the SFF XFP and SFP+ specifications. As with the XFI and SFI the main difference is the increased signal integrity requirements for the host PMA and PCB signal traces.
Using the XLPPI with QSFP+ modules
The XLPPI interface is defined as being AC coupled on the host system, since the signals are running at 10Gbps (max 5GHz) it normal to route the traces as strip line on the top layer avoiding vias. When laying out the traces it is important to get both the impedance and differential trace length as close as possible to their target values. The impedance of the TXD and RXD pairs is 100Ω, with a termination mismatch of +/-5% at 1MHz; the closer your trace impedance is to 100Ω the more margin you’ll have for the modules and ASIC’s to vary their termination. The same applies for the length of the +ve and –ve traces, these need to be as close in length as possible since as any difference will eat away at the signal eye (note the effect of DDPWS in reducing the UI). However don’t spend time trying to match the length of the four TXD+/- or RXD+/- pairs, this is not necessary as the MLD function takes care of this.
Currently QSFP+ modules support 40GBase-CR4 (both passive and active) and 40GBase-SR4 (either AOC or using the MPO/MTP®connector). However in June 2010 Colorchip announced their QSFP+ based 40GBase-LR4 module which is stated to be available in 2011, thus a system using the XLPPI/QSFP+ interface will soon be able to support the complete range of 40gBase-CR4, SR4 & LR4 – Excellent!
In addition to straight 40G interfaces there are also 4:1 hydra cables which have a QSFP+ at one end and four SFP+ cables at the other, these cables are typically direct attach copper and are used for aggregating servers to a top of rack switch. When being used for this application the high speed interface of the QSFP+ needs to conform to the SFI specification rather than the 802.3ba XLPPI.
Whilst the future trend is to adopt the XLPPI the XLAUI will probably still survive for special telecom orientated modules like SEI’s 40Km 40GBase-LR4 CFP (more like a 40GBase-ER4), or any future 40GBase-FR module.
Short note on the CPPI and CXP
The CPPI is the equivalent interface to the XLPPI for 100G, currently it is only usable with the CXP module and it’s not clear if the CXP module (defined initially for infiniband) will be used much for 100GigE due to concerns of reach and reliability of the MPO connector. The new 10x10MSA is currently defined as using CAUI though in the future this may move to CPPI for cost reduction. In the future 100G modules with a 4x28Gbps (CEI-28) interface will appear using a CFP2 format (similar in size to the X2 module).
Summary of electrical parameters
The above table captures only a small portion of the high speed interface requirements, when designing a board it is important to get the complete specification. For SFI and XFI the relevant specifications can be downloaded free at http://www.sffcommittee.org whilst the XLAUI and XLPPI specification is included in the (also free) 802.3ba specifications at http://standards.ieee.org/about/get/802/802.3.html .
Overview of TLA’s
CAUI : 100Gbps (C) Attachment Unit Interface
CPPI : 100Gbps (C) Parallel Physical Interface
EDC : Electronic Dispersion Compensation
DDPWS : Data Dependant Pulse Width Shrinkage (in UI)
SFI : Small form Factor Interface
SFP : Small form Factor Pluggable
UI : Unit interval of a bit period, for all the interface above this is 96.97ps (10.3125Gbps)
XFI : 10Gbps(X) form Factor Interface
XFP : 10Gbps(X) form Factor Pluggable
XLAUI : 40Gbps (XL) Attachment Unit Interface
XLPPI : 40Gbps (XL) Parallel Physical Interface