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ARQ in NV2 versus Ack in 802.11 legacy

Posted: Wed Jun 01, 2011 1:22 am
by WirelessRudy
Can anybody explain the difference between these two and why the one (ARQ > NV2/nstreme) should be better than the other?

Re: ARQ in NV2 versus Ack in 802.11 legacy

Posted: Wed Jun 01, 2011 11:52 pm
by Mplsguy
From terminology standpoint ACK in 802.11 is also ARQ mechanism. In every protocol ARQ is used to achieve reliable data transmission, so the purpose is the same. The main difference between ARQ mechanism in nstreme/nv2 and 802.11 is that 802.11 basically uses Stop-and-wait ARQ, while nstreme/nv2 uses selective repeat ARQ.

Each of aproaches has its advantages and disadvantages. NV2/nstreme approach is able to provide more throughput, because there is no need to send one ACK per every received frame (multiple frames can get acked). This becomes even more important with increasing distance, because longer propagation delay requires more "wait-for-ack" time after every transmitted frame in 802.11. On the other hand, the approach used in 802.11, ensures lowest latency (where by latency I mean time interval from when frame is first sent by transmitter until it is passed further by receiver, but not the total effect on latency as caused by queues, etc.). Therefore nv2/nstreme will be more sensitive to packet loss from latency standpoint.

It is also worth noting that all of the above holds true for pre-11n 802.11 protocol. 802.11n also adds "selective repeat ARQ" feature, by means of "block acks". More on this can be found in 802.11n standard.

Re: ARQ in NV2 versus Ack in 802.11 legacy

Posted: Thu Jun 02, 2011 12:57 am
by WirelessRudy
Mplsguy: Thanks for the reply. This clears a lot.