# Wireless Access

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Frequent Contributor II

## Relation between MIMO, Radios & Rate

why is first AP 300 Mbps per radio while second is 450 Mbps while both has same parameters ( dual radio & 3*3 MIMO ) ?

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MVP Guru

## Re: Relation between MIMO, Radios & Rate

It is dependant on the spatial streams supported on the AP. The 125 supports 2x SS and the 135 is 3 SS.
300Mbs = 2 Spatial Streams

ACMP, ACSA, ACDX #985
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Frequent Contributor II

## Re: Relation between MIMO, Radios & Rate

Thx , and spatial stream is what ?

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MVP Guru

## Re: Relation between MIMO, Radios & Rate

Short answer is that it's kind of like a stream of data

&colon;)

ACMP, ACSA, ACDX #985
MVP

## Re: Relation between MIMO, Radios & Rate

spatial stream is a data that has been broken up and sent at the same time over wireless. WiFi is a half duplex connection, which basically means a device can either send or receive but not both at the same time. To speed up throughput 802.11n uses multiple radios to send chunks of data at the same time.

As an example, lets say you have 25 seconds of a video. In older wireless standards you would send a chunk of the data (say 5 seconds), wait for it to get there, the receiving device would respond that it got it, and then the next chunk would be sent. Lets say it takes 5 seconds to send the data and 3 seconds your device to respond back it got the last chunk and it was ok. In which case it would take 40 seconds (5x5+3x5) to get the video.

Using multiple spatial streams I can send multiple chunks at the same time. Lets say the wireless router has 2 radios capable of sending and receiving and your device has the same. In this case the router will send two chunks of data (two spatial streams) at the same time. Now you will be receiving the data twice as fast. There would be to transmissions with two streams, a single stream, and your device would only need to respond 3 times, for a total time of 36 seconds. You saved 4 seconds.

This was a very basic analogy and I hoped it helped. In reality the times are in milli and microseconds, and we are more concerned with the amount of data moved. Also both the wireless router and the client have to support it to really see an increase in throughput. If I have a router that transmits 2 spatial streams but the client can only receive one, then only one is received. Quality is increased because you have twice the chance to receive the signal.

Hope this helps.

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Contributor I

## Re: Relation between MIMO, Radios & Rate

you stated: "spatial stream is a data that has been broken up and sent at the same time over wireless. WiFi is a half duplex connection, which basically means a device can either send or receive but not both at the same time. To speed up throughput 802.11n uses multiple radios to send chunks of data at the same time.

As an example, lets say you have 25 seconds of a video. In older wireless standards you would send a chunk of the data (say 5 seconds), wait for it to get there, the receiving device would respond that it got it, and then the next chunk would be sent. Lets say it takes 5 seconds to send the data and 3 seconds your device to respond back it got the last chunk and it was ok. In which case it would take 40 seconds (5x5+3x5) to get the video.

Using multiple spatial streams I can send multiple chunks at the same time. Lets say the wireless router has 2 radios capable of sending and receiving and your device has the same. In this case the router will send two chunks of data (two spatial streams) at the same time. Now you will be receiving the data twice as fast. There would be to transmissions with two streams, a single stream, and your device would only need to respond 3 times, for a total time of 36 seconds. You saved 4 seconds."

QUESTION.

Wouldn't the math equate to this: In this case the router will send two chunks of data (two spatial streams) at the same time. Now you will be receiving the data twice as fast. There would be 2 transmissions with two streams (10 seconds), a single stream (5 seconds), and your device would only need to respond 3 times 3x3 =9 seconds, for a total time of 24 seconds. You saved 16 seconds. Please correct me if I am wrong. Thanks