**3.1 System heterogeneity**

The end devices typically possess a range of distinct hardware, which can vary in their capabilities regarding computation, communication, energy, and other factors.


locations (indoor vs. outdoor), and encounter different wireless channel conditions (congested vs. clear), their transmission speeds can differ greatly. By analyzing hundreds of end devices in a real-world FL deployment [26], it has been observed that there is a substantial order-of-magnitude difference in the network bandwidth [27].

• *Other Factors*. Apart from computation and communication capabilities, the availability and capability of end devices are influenced by many other factors. For instance, when the battery of an end device is low, its computation and communication capabilities are reduced to conserve power. Furthermore, end devices running heavy applications in the background can substantially limit the available computing resources.
