Spatial streams are separately encoded,independent data signals used for spatial multiplexing.
[Spatial multiplexing is a transmission technique in MIMO(Multi Input Multi Output)]
Spatial streams enable independent transmission and reception of data on a device. The goal is to increase the efficiency of the channel by enabling simultaneous transmission and reception.
[Spatial streams can be visualized as separate radios (and antennas) one handling transmission and the other reception for a device]
Why is it needed?
Traditionally a wifi channel can communicate with one device at a time. Sequentially. When communication is happening all the other devices have to wait in queue[1].
Moreover when a channel is being used by a device it can either transmit or receive but not do both. Spatial streams allow a single device to transmit and receive at the same time. All the other devices however are still waiting in queue.
Thus it can be said that in wireless communication spatial streams are meant for single devices. Client devices receive from and transmit to multiple antennas but still only one at a time.
The parlellism that spatial streams introduce is single device parlellism.This technology is called SU-MIMO where SU stands for single user.
[MU or multi user MIMO extends this parellism to multiple users. Enabling several devices to transmit and receive at the same time. ]
Application
In practical world, available bandwidth is rarely distributed by a single station. A network with its collection of access points is able to serve multiple users simultaneously.
Now while the devices still use channel sequentially, multiple access points mean that there are multiple queues of devices.This technique is the foundation for MU-MIMO which makes a single network capable of serving multiple users at once using streams.
Improvements in Wireless services not only mean a greater throughput to a device but also total increase in network capacity.Where we lack is in distribution of this speed equally among devices. We need to focus more on distribution than throughput.
Aggregation
Link aggregation allows us to introduce network redundancy. Multiple data lines are accessed from a single point. So that a user has a singular view of the network. For them its as if the data was available from a single provider. Although it may be that the data is infact being served by several different service providers.
The real benefit of aggregation is to provide network redundancy so that even if one provider goes down we can use others to keep the system online. Traffic may be distributed across different networks but that is only effective if all the networks are guaranteed to work at all times.
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