The SFP+ module form factor is 30% smaller than its predecessor, the SFP optical transceiver. It uses less power, requires fewer components, supports module stacking, and is less expensive than XFP. SFP+ improves the port side densities and reduces the overall 10 GE system cost. However, it puts new burden on the PHY’s design in the interfacing IC to the module. Compared to XFI, SFI electrical has higher requirements on the transmitter side, the output eye is smaller, the incoming jitter from the SFP+ module is higher, and issues such as pulse width shrinkage due to pattern dependency would need to be addressed by the interfacing device that is referred to as the host device in the electrical specification that defines SFI and SFP+, SFF-8431.
SFI is defined for both limiting and linear mode modules. In the limiting mode, SFI supports PHY connections to the limiting SFP+ optical modules such as the short reach 10GBASE-SR (MMF 300m), the long reach 10GBASE-LR (SMF 10km), and the extended long reach 10GBASE-ER (SMF 40km), that have limiting receivers. In the linear mode version of the SFI, further signal conditioning capabilities are required by the interface PHY to compensate for electrical dispersion. The linear version of SFI supports connection to both the optical 10GBASE-LRM (MMF 220m) that has a linear receiver, and the passive Direct Attached Copper (DAC) TwinAx cabling (1m to 7m) that is a linear media.
To improve the port side densities in a chassis, the new XLPPI (40 Gbps Parallel Physical Interface) electrical specification was defined by IEEE 802.3 for a direct connection to a Quad Small Form Factor Pluggable (QSFP or QSFP+) module, as defined in SFF-8436. By using these smaller form factor modules in the port side, more 10 Gbps modules can be placed in the same space. This will allow more 10G connections to the face plate of a chassis. QSFP offers three times the density of the traditional SFP port and is lower in both power and cost than a 40 Gbps CFP solution. QSFP also supports module stacking to enable higher port side densities. Unlike CFP, QSFP supports copper cable connections in addition to optical interconnect that are attractive lower cost options for short-reach interconnects within a data center.
XLPPI is derived from the SFI electrical interface and therefore places higher signal integrity requirements on the interfacing PHY. However, few of the SFI parameters were further relaxed in this 10 Gigabit aggregated interface so that the same XLPPI electrical definition would cover both the optical and copper based modules. Therefore, XLPPI is the electrical specification to both passive copper based 40GBASE-CR4 QSFP+ module (TwinAx copper cable, 1 to 7m) and the optical modules such as the short reach 40GBASE-SR4 (MMF 150m) and the long reach 40GBASE-LR4 (SMF 10km).