EIARS-232C定义了DTE和DCE之间的接口，其机械特性规定RS-232C的D型连接器有(46)个插脚，其电气特性与CCITT的(47)兼容。DTE和DCE之间的接口信号线按功能一般可分为(48)4类，使用EIARS-232C接口进行数据通信时，至少需用的信号线有(49)。V.11建议中规定的电气特性逻辑“1”的电平电压是(50)。

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Frame. Relay is simplified form. of(71), similar in principle to(72), in which synchronous, frames of data are routed to different destinations depending on header information . Packets are routed throught one or more Virtual Circuits known as(73). Most Virtual Circuits are(74),which means that the network provider sets up all DLCI connections at subscription time.(75)are also part of the Frame. Relay specification . They privide a link that only lasts only as long as the session.

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Networks can be interconnected by different devices．In the physical layer，networks can be connected by(66)or Hubs, which just move the bits from one network to an identical net-work. One layer up we find bridgas and swiehes, which operate at data link layer. They can accept(67), examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two netwoks. If two networks have(68)network layer, the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which can interface between the two transport connections. Finally, in the application layer, application gateways translate message(69). As an example, gateways between Internet e-mail and X.400 e-mail must(70) the e-mail message and change various header fields.

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