物理层的电气特性有多种标准，其中非平衡型标准规定(65)，电缆最大长度为(66)m。新的非平衡标准规定(67)，距离为10m时的最高数据率为(68)。在多种标准中，数据率最高的是新的平衡型标准，近距离传输其最高数据率可达(69)。

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VRML is a(71)for 3D multimedia and shared virtual worlds on the WWW. In comparison to HTML, VRML adds the next level of interaction, structured graphics, and extra(72)(z and time)to the presentation of documents. The applications of VRML are(73), ranging from simple business graphics to entertaining WWW page graphics, manufacturing, scientific, entertainment, and educational applications, and 3D shared virtual worlds and communities.

 X3D is the name under which the development of VRML is continued. X3D is based on XML and is backwards(74)with VRML. Furthermore, it is componentized, profiled, and extensible, which makes it possible to use X3D in very different(75), from high-end visualizations to lightweight applications.

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Most IP layer-based proxy mechanisms, such as network address translation(NAT), only support uni-directional proxy, from the internal network to external network(the Internet). The proxy establishes the communication channel by(66)IP addresses. The IP addresses must be(67). These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software(68)for each application that uses(69)channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with(70)IP addresses.