Silicon and Systems
Silicon and Systems
PTX Deserves an Oscar Award
06.21.16

Still remember when former vice president Al Gore’s movie, “An Inconvenient Truth”, won an Oscar for best documentary? The movie raised the general public’s awareness of the threat of global warming.  If there were an Oscar in the networking industry, Juniper’s PTX-series platform certainly deserves one for its “Go Green” and for preventing global warming with several innovations in cooling and energy consumption:

  • Linear cooling algorithm
  • Multi-zone cooling
  • Section cooling
  • Multi-sensor temperature monitoring

Linear cooling algorithm

Traditionally, Juniper chassis uses a two-speed algorithm to control fan speed. When the boards’ temperatures are below a threshold, the fans run at normal speed; when the temperatures are above the threshold, the fans run at full speed to cool down the overheating boards.

 

BlogPTXCoolingTraditionalFanControl.png

This algorithm is simple and easy to implement, but it makes the chassis consume more power unnecessarily. Moreover, the abrupt fan speed spike between normal and full speed creates large noise variations and decreases fans’ life span. Imagine driving a car that only has two speeds!

 

PTX, in contrast, uses an innovative linear cooling algorithm, which avoids the above drawbacks, as shown below:

 

BlogPTXCoolingLinearFanControl.png

Multi-zone cooling

The traditional Juniper chassis also uses a single zone concept to control fan speed. All the boards are regarded as one zone and all the fans run at the same speed. If any board on the chassis heats up, all the fans on the chassis will run at full speed, even though other boards don’t need the increased cooling as their temperatures are still low. Think about it this way: it’s as if a sheriff were chasing a hit-and-run criminal on the highway at 90 mph and every other car was forced to drive at that speed. It’s a little weird, isn’t it?

 

PTX, on the other hand, introduces multi-zone cooling. The boards and their associated cooling fans are physically divided into multiple cooling zones. If the boards in one cooling zone heat up, only the fans in that zone spin faster, while the fans in the other zones stay at the same low speed. This design saves energy, reduces noise, and expands fans’ life span. Now sheriff can drive fast, and everyone else can drive at 65 mph. Phew!

 

Section cooling

To further reduce energy consumption and noise, each horizontal fan tray is divided into two sections: FPC-left section and FPC-right section. Each section cools five FPCs (with the two middle FPCs being cooled by both sections). This is called section cooling. Section cooling works in this way:

  1. If all five FPCs cooled by a section are not present, the fans in that section will run at a speed 20% lower than the speed of the other section;
  2. If all eight FPCs are not present, the fans in both sections will run at minimum speed;
  3. If FPCs are present in both sections, horizontal fans in both sections will run at the same speed.

Note: section cooling is independent of multi-zone cooling.

 

Isn’t section cooling a brilliant design?

 

BlogPTXCoolingSectionControl.png

 

Multi-sensor temperature monitoring

The innovations of PTX don’t just end there, however. Typically, only one temperature sensor on each board is monitored and used to control fan speed. Because different ASICs on a board may have different temperatures, and these ASICs can tolerate different maximum temperatures, using only one temperature sensor on each board to control fan speed is inaccurate and can’t provide the optimum cooling efficiency. Let’s use a simple example: assume that there are two ASICs on a board, a Q-chip and an F-chip. The Q-chip can tolerate up to 80 C and its current temperature is 72 C; the F-chip can tolerate up to 100 C and its current temperature is 85 C. If we use only one temperature sensor on the board to control fan speed, to prevent the Q-chip from being burnt, temperature threshold for the board must be set to 80 C (the lower of the two ASICs’ maximum tolerant temperatures). But when the F-chip hits 85, which is still within the safe zone, the fan will spin at full speed (unnecessarily) because chassis finds that 85 is above the temperature threshold for the Q-chip.

 

Here comes the innovation: the PTX chassis constantly monitors multiple sensors on the board and compares each sensor’s temperature reading with its temperature threshold, and uses the highest temperature/threshold ratio as the driving force to control fan speed. Again, let’s use the above example: now the Q-chip’s temperature ratio is 72/80 = 0.9, and the F-chip’s temperature ratio is 85/100 = 0.85. Although the Q-chip’s temperature is lower, it is closer to its threshold, so the Q-chip is the driving force for fan speed. Yet the Q-chip is still below maximum temperature threshold, so the fan still spins at a normal speed, whereas it would be spinning at full speed in the traditional design. This saves energy, reduces noise, and expands the fans’ lifespan.

 

PTX’s innovations in cooling truly complement Juniper's Go Green mission. These innovations have also been adopted in the recent MX chassis.

 

By relentlessly looking for opportunities for innovations, Juniper stays ahead of the game, becoming a winner in both markets and on the Oscar stage.

06.16.16
Tobias

You must be talking about PTX5000, because the PTX3000 has a huge cooling problem and Juniper does not do anything about it.

 

It does not deserve a Oscar, it deserve a Golden Raspberry Award

06.21.16
Juniper Employee

Thank you for the feedback. The 2x100G DWDM PIC is compliant to 40° C (not 55° C).

 

We have recently introduced a 5x100G DWDM interface with 2.5X the capacity which operates at 65% of the power consumption of the 2x100G DWDM interface and is fully NEBs compliant for both PTX3K and PTX5K.