The core concept of the model Driven Architecture — which can solve or alleviate the problems mentioned above — is the platform-independent model. Essentially, the MDA has three tiers.

模型驱动架构的核心概念——可以解决或减少上面提到的问题——是平台无关模型(platform - independent model)。

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This model is composed of three tiers: data tier, application tier and presentation tier.

模型框架结构分为数据层、应用服务层和视图层。

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For more accurate simulation of the head with all levels of organizations, the model from outside to inside is divided into three tiers, namely, the skin layer, bone layer and the brain layer.

为了更准确的模拟头部内各层组织，该模型从外到内分为三层，分别为皮肤层、骨骼层和大脑层。

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