Cortina unveils multi-channel dispersion compensation chip

Cortina Systems has announced its latest electronic dispersion compensation (EDC) chip.  The CS4342 is a compact device that supports eight duplex 10 Gigabit-per-second (Gbps) links. 

"Some customers are doing 2,000 signals at 10 Gig across the backplane"

Scott Feller, Cortina 

The chip is suited for use with optical modules and on line cards to counter the effect of transmission distortion where a bit's energy leaks into one or more adjacent bits, known as inter-symbol interference (ISI).

The Cortina device can be used for 10, 40 and 100Gbps line card and backplane designs and supports copper cable and optical fibre standards such as the multimode 10GBASE‐LRM and the 80km 10GBASE‐ZR interface. 

Significance

Routeing high-speed signals from an ASIC to the various high-speed interfaces - 10Gbps and greater - is becoming harder as more interfaces are crammed onto a card. 

"Boards are getting denser: from 48 ports to 96," says Scott Feller, director of the EDC product line at Cortina Systems. The issue with an ASIC on the board is that the distance it can span to the modules is only about 6-8 inches (~15-20cm). Placing the PHY chip on the board relaxes this constraint.

The use of the octal EDC chip between a line card IC and SFP+ optical transceivers. Source: Cortina Systems

Vendors also gain greater flexibility in terms of the interfaces they can support. "These types of PHYs allow them [designers] to avoid having to make hard decisions," says Feller. "They put the PHY in front of the optical connector and they almost get every single optical format on the market."  

The platforms using such EDC PHYs include data centre switches and telecom platforms such as packet optical transport systems (P-OTS). Data centre switches typically support Direct Attach Copper cable - a market area that has been growing significantly, says Cortina - and short-reach optical interfaces. For P-OTS the interfaces include the 10GBASE-ZR where EDC is a necessity.

The device is also being used for system backplanes where bandwidth requirements are increasing significantly. "Some customers are doing 2,000 signals at 10 Gig across the backplane," says Feller. "Now that there are so many signals - so much crosstalk - and the ASICs are further away from the backplane, so PHYs are starting to be put into systems."

EDC employed in a backplane design. Source: Cortina Systems

Chip details

Cortina claims the 17x17 ball grid array CS4342 is a third smaller than competing devices. The chip compensates the received signal in the analogue domain.  An on-chip DSP calculates the filter's weights to counter ISI while the filtering is performed using analogue circuitry. As a result, the EDC has a latency of 1ns only.

Cortina has dual, quad and now octal EDC ICs. It says that the delay between the different devices is the same such that both an octal and dual chip can be used to implement a 10-channel 100 Gig interface, for example the 10x10 MSA. In turn, future line cards supporting four 100Gbps interfaces would use five octal PHYs ICs.

The CS4342 is available in sample form and will enter production from October. 

What next

"This type of product is at the very end of the food chain so there is always macro developments that could change the market," says Feller. Silicon photonics is one but Feller expects that it will be years before the technology is adopted widely in systems. 

The external EDC PHYs must also compete with PHYs integrated within custom ASIC designs and FPGAs.  "We always have to be ahead of the cost and performance curves on the PHY," says Feller. "If not, they [companies] are just going to integrate PHYs into their ASICs and FPGAs."

Meanwhile, Cortina says it has two more EDC devices in development.