- Cisco Aironet Access Point Manual

Cisco Aironet Series 1600/2600/3600 Access Point Deployment Guide, Release 7.5

802.11n Primer - Understanding Spatial Streams

Figure 49 Example of a high gain antenna AIR-ANT2480V-N with cover removed

A high gain antenna may have a null or dead spot directly underneath it as the antenna element is often

much longer with less metal surface area available to conduct the radio wave if you are located directly

underneath it, however; the further away you are from the antenna the more surface is available and so

the better it performs.

802.11n Primer - Understanding Spatial Streams

For a video on understanding the fundamentals of Spatial Streams see the following URL:

http://www.cisco.com/en/US/netsol/ns767/index.html

MIMO, which refers to a radio system that has multiple separate receive and transmit paths, is at the

heart of 802.11n. MIMO systems are described using the number of transmitters and receivers in the

system. For example, "two by one" or 2x1 refers to a system with two transmitters and one receiver

(Figure 50).

Spatial streams, the act of transmitting information out of more than one antenna port concurrently,

requires that the AP have at least two or more transmitters and support elements of 802.11n, e.g., support

of multiple spatial streams.

In the 802.11a/b/g days data rates were actual Mbps rates like 2, 11, 54 Mbps etc., and was done with

one transmitter. In the case of the AP 3500 series it has two transmitters per band so it supports 802.11n

data rates up to 300 Mbps using two spatial streams.

With 802.11n the different rates are called Modulation and Coding Scheme (MCS) index value, and the

value also defines how many streams are used. The AP 3500 supported up to 300 Mbps (MCS rate 15

configured with a bonded channel and short guard interval (GI). Refer to Figure 51. The MCS values

correspond to actual data rates.