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several group number, then with b ± a, =c,c is is methyl b two vertical box between of accurate size. Per-2~3 measurement, such as proceeds of c values are equal and equal to the design value, then the vertical installation accurate. For example a, b, and c valueswhile on horizontal vertical errors for measurement, General in iron angle code bit at measurement level points grid errors, specific method is from baseline to methyl vertical box center line distance for a,, to b vertical box distance for b, list can measured
“交流”。协议（Protocol）是网络设备用来通信
的一套规则，这套规则可以理解为一种彼此都能
听得懂的公用语言。关于网络中的协议可以概括
为两类：“内部协议”和“外部协议”。
Second, the classification of the communication
protocol to form a network, you must select a
network communication protocol, allows the
user between "communication" to each other.
Communication Protocol (Protocol) is a network
device used for a set of rules, the rules can be
understood as a kind of common language can
understand each other. About the network
protocol can be summarized as two types:
"internal agreement" and "external
agreement".
1、内部协议 1978 年，国际标准化组织
（ISO）为网络通信制定了一个标准模式，称为
OSI/RM（Open System Interconnect/Reference
Model，开放系统互联参考模型）体系结构。该
结构共分七层，从低到高分别是物理层、数据链
路层、网络层、传输层、会话层、表示层和应用
层。其中，任何一个网络设备的上下层之间都有
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