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Wireless networking – When do devices transmit data?

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“Stop interrupting!”

Previously I wrote about why high data rates are beneficial for increasing network capacity and reducing power consumption.

Here I’ll look at two common ways of determining when a device accesses the radio channel and starts transmitting its data.

The first method is ALOHA. Named after one of the world’s first packet based networks in Hawaii. ALOHA says:

  1. If a devices has a packet to send then send it immediately
  2. If a collision is detected then try sensing the packet again

ALOHA is a super simple method which is cheap and easy to implement. As such it is found in early packet based networks (like the first packet based network in Hawaii) and cheap unsophisticated technologies.

The disadvantage of ALOHA based networks is that collisions occur. Collisions occur more frequently as the network get more congested. Collisions degrade the network efficiency as packets that collide have to be resent.

ALOHA networks operate efficiently, with reasonable low number of collisions and repeats up to around 10% utilization of the radio channel. At utilizations greater than 10% packets start colliding more frequently triggering more and more repeated transmission. This is a vicious circle of deteriorating performance as the required performance increases! In practice the maximum radio spectrum utilization of ALOHA networks is around 18%

Fortunately a better alternative to ALOHA is Listen Before Talk (LBT).

As the name implies, devices using this protocol first listen to the radio spectrum. If they detect a packet on the spectrum they wait until it is complete before turning on their transmitter to send their packet.

Wi-Fi uses LBT to decide when to access the radio spectrum. It also uses some other smart techniques to schedule time slots for certain packets and to alleviate various other problems (such as hidden nodes).

Wi-Fi devices require a computer processor and accurate time synchronization in order to adhere to the strict protocol requirements. However, these complications are taken care of by the IC chip designer and due to advanced semiconductor manufacturing processes the unit cost impact is minimal.

The advantage of the efficient Wi-Fi protocol is far greater utilization of the radio spectrum, and far lower requirements to retransmit data. Wi-Fi networks can achieve radio spectrum utilization of around 75%

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