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devices. These edge devices continuously collect a variety of data, including images, videos, audios, texts, user logs, and many others with the ultimate goal to provide a wide range of services to improve the quality of people's everyday lives.

Although the Internet is the backbone of edge computing, the true value of edge computing lies at the intersection of gathering data from sensors and extracting meaningful information from the collected sensor data. Over the past few years, deep learning (i.e. deep neural networks [DNNs]) [4] has become the dominant data analytics approach due to its capability to achieve impressively high accuracies on a variety of important computing tasks, such as speech recognition [5], machine translation [6], object recognition [7], face detection [8], sign language translation [9], and scene understanding [10]. Driven by deep learning's splendid capability, companies such as Google, Facebook, Microsoft, and Amazon are embracing this technological breakthrough and using deep learning as the core technique to power many of their services.

      Deep learning models are known to be expensive in terms of computation, memory, and power consumption [11, 12]. As such, given the resource constraints of edge devices, the status quo approach is based on the cloud computing paradigm in which the collected sensor data are directly uploaded to the cloud; and the data processing tasks are performed on the cloud servers, where abundant computing and storage resources are available to execute the deep learning models. Unfortunately, cloud computing suffers from three key drawbacks that make it less favorable to applications and services enabled by edge devices. First, data transmission to the cloud becomes impossible if the Internet connection is unstable or even lost. Second, data collected at edge devices may contain very sensitive and private information about individuals. Directly uploading those raw data onto the cloud constitutes a great danger to individuals' privacy. Most important, as the number of edge devices continues to grow exponentially, the bandwidth of the Internet becomes the bottleneck of cloud computing, making it no longer feasible or cost-effective to transmit the gigantic amount of data collected by those devices to the cloud.

      In this book chapter, we aim to provide our insights for answering the following question: can edge computing leverage the amazing capability of deep learning? As computing resources in edge devices become increasingly powerful, especially with the emergence of artificial intelligence (AI) chipsets, we envision that in the near future, the majority of the edge devices will be equipped with machine intelligence powered by deep learning. The realization of this vision requires considerable innovation at the intersection of computer systems, networking, and machine learning. In the following, we describe eight research challenges followed by opportunities that have high promise to address those challenges. We hope this book chapter act as an enabler of inspiring new research that will eventually lead to the realization of the envisioned intelligent edge.

3.2 Challenges and Opportunities

      3.2.1 Memory and Computational Expensiveness of DNN Models

Memory and computational abilities are expensive for DNN models that achieve state-of-the-art performance. To illustrate this, Table 3.1 lists the details of some of the most commonly used DNN models. As shown, these models normally contain millions of model parameters and consume billions of floating-point operations (FLOPs). This is because these DNN models are designed for achieving high accuracy without taking resources consumption into consideration. Although computing resources in edge devices are expected to become increasingly powerful, their resources are way more constrained than cloud servers. Therefore, filling the gap between high computational demand of DNN models and the limited computing resources of edge devices represents a significant challenge.

Table 3.1 Memory and computational expensiveness of some of the most commonly used DNN models.

DNN	Top-5 error (%)	Latency (ms)	Layers	FLOPs (billion)	Parameters (million)
AlexNet	19.8	14.56	8	0.7	61
GoogleNet	10.07	39.14	22	1.6	6.9
VGG-16	8.8	128.62	16	15.3	138
ResNet-50	7.02	103.58	50	3.8	25.6
ResNet-152	6.16	217.91	152	11.3	60.2

To address this challenge, the opportunities lie at exploiting the redundancy of DNN models in terms of parameter representation and network architecture. In terms of parameter representation redundancy, to achieve the highest accuracy, state-of-the-art DNN models routinely use 32 or 64 bits to represent model parameters. However, for many tasks like object classification and speech recognition, such high-precision representations are not necessary and thus exhibit considerable redundancy. Such redundancy can be effectively reduced by applying parameter quantization techniques that use 16, 8, or even fewer bits to represent model parameters. In terms of network architecture redundancy, state-of-the-art DNN models use overparameterized network architectures, and thus many of their parameters are redundant. To reduce such redundancy, the most effective technique is model compression. In general, DNN model compression techniques can be grouped into two categories. The first category focuses on compressing large DNN models that are pretrained into smaller ones. For example, [13] proposed a model compression technique that prunes out unimportant model parameters whose values are lower than a threshold. However, although this parameter pruning approach is effective at reducing model sizes, it does not necessarily reduce the number of operations involved in the DNN model. To overcome this issue, [14] proposed a model compression technique that prunes out unimportant filters which effectively reduces the computational cost of DNN models. The second category focuses on designing efficient small DNN models directly. For example, [15] proposed the use of depth-wise separable convolutions that are small and computationally efficient to replace conventional convolutions that are large and computationally expensive, which reduces not only model size but also computational cost. Being an orthogonal approach, [16] proposed a technique referred to as knowledge distillation to directly extract useful knowledge from large DNN models and pass it to a smaller model that achieves similar prediction performance as the large models, but with fewer model parameters and lower computational cost.

      3.2.2 Data Discrepancy in Real-world Settings

The performance of a DNN model is heavily dependent on its training data, which is supposed to share the same or a similar distribution with the potential test data. Unfortunately, in real-world settings, there can be a considerable discrepancy between the training data and the test data. Such discrepancy can be caused by variation in sensor hardware of edge devices as well as various noisy factors in the real world that degrade the quality of the test data. For example, the quality of images taken in real-world settings can be degraded by factors such as illumination, shading, blurriness, and undistinguishable background [17] (see Figure 3.1 as an example). Speech data sampled in

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