What is the difference between a PHY chip in a switch and a switch controller ASIC in the switch?

Question

I am analyzing the block diagram of a switch and I would like to know more about the difference between PHY chip and ASIC (which I think is also called Switch controller).

What is the JOB of each device and how do they communicate?

Answer 1

As its name implies the PHY chip is responsible for getting an actual fully formed Ethernet (or other transport) frame on to / off of the wire. This will tend to include clocking, carrier, calculating checksum, possibly multiplexing (for some types of optics), MAC framing and the like. Its job is generally to present a normalized and (hopefully) correct frame to the fabric or controller, which is tasked with lookup and forwarding - either to another PHY or, possibly, to a higher-order fabric/bus controller.

How do these elements communicate? There are a variety of topologies and types but there are some number of lanes running between the PHY and fabric/controller. Depending on the vendor's particular design and implementation there may be some amount of over- or even under- subscription at this point. Keep in mind that the PHY is likely stripping some amount of extraneous framing information while also potentially adding some amount of locally significant tag info (again - varies by specific design). There are actually a number of different ways in which a given frame could be represented under the covers, either as whole/partial frames or as fixed-sized cells. Each approach has its own set of advantages and disadvantages.

Answer 2

The PHY is the guy at the door of the bar generally checking to see if you're the kind of guy they want to serve inside (e.g. do they like your dress code and the car you came in). The ASIC is the barman. :-)

Note that the term "ASIC" is generic, and could refer to any application-specific integrated circuit on the board. When talking about networking devices, though, "The ASIC" refers to the electronic device that does the bulk of the packet processing work. But there may be other smaller ASICs elsewhere on the board that do some specific (usually smaller) part of the packet processing.