Wednesday, 16 May 2012

What is MAC layer ACK in wireless ? ACK in wireless is generated by MAC or PHY layer

This is one basic question of wireless. Which layer generates  ACK for unicast packets?

Is it physical layer or the MAC layer. When we say MAC layer, is it controlled by software or hardware?

When i asked this question to few team members, they were like confident that it is "physical layer". As the software does not seem to control this functionality, and it is done at hardware level.

I dig through the standard and no where in physical layer specification, i found the ACK generation. So it has to be "MAC layer". But how?

The answer after researching and searching through Google was : it is generated at MAC level but done at hardware level.

Generally if it is hardware people tend to think its only physical layer. But its not the case now a days many chip sets has advanced capability of doing things at hardware level. The main advantage of this is, it is very fast. The same thing, you do, at hardware level and software level. The hardware level implementation will always be fast, so to make things easy and fast, few things are implemented at hardware level. This functionality is one of them.

IF you go through TI  chipset specification you get below points

http://focus.ti.com/pdfs/bcg/ti_acx100.pdf


"The MAC hardware can directly generate ACK, RTS, CTS, probe response and beacon packets. It also provides wired equivalent privacy (WEP). The MAC can perform encryption and decryption on frames using keys of up to 256 bits in length"


This clearly states that MAC hardware is generating the ACK packet. So if someone ask you this question you should say :

"The generation of ACK is done at MAC layer, but this MAC layer is part of hardware which is physical layer."


 Don't get confused !!!


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