中继线和PBX的关系： 可以理解PBX内部分机需要和公共电话网接接，必通过中继端口的线路去实现对接。
狭义的中继线：运营商提供的带中继信号的电话线路称之为中继线。
中继线分二种：数字中继(一般为E1中继);模拟中继(一般环路中继)
环路中继线：可以提供反极信号的模拟中继线。
一个E1中继同时持持30线电话,(就是可以30个外线电话打进来~或者30个内线打出去~当然外线内线加起来36个也是可以的)
同时E1中继理由上可以分配数个--数万个电话号码(类似一个光纤可以分数个或数万个公网IP地址。)
思科的技术文档中说是E＆M接口之间类型不同造成的。
E&M Interface Types and Wiring Arrangement
There are five different E&M interface types or models named Type I, II, III, IV, and V (Type IV is not supported on Cisco platforms) Each type has a different wiring arrangement, hence a different approach to transmit E&M supervision signaling (on-hook / off-hook signaling) The signaling side sends its on-hook/off-hook signal over the E-lead The trunking side sends the on-hook/off-hook over the M-lead
E&M Type I—This is the most common interface in North America
o Type I uses two leads for supervisor signaling: E, and M
o During inactivity, the E-lead is open and the M-lead is connected to the ground
o The PBX (that acts as trunk circuit side) connects the M-lead to the battery in order to indicate the off-hook condition
o The Cisco router/gateway (signaling unit) connects the E-lead to the ground in order to indicate the off-hook condition
E&M Type II—Two signaling nodes can be connected back-to-back
o Type II uses four leads for supervision signaling: E, M, SB, and SG
o During inactivity both the E-lead and M-lead are open
o The PBX (that acts as trunk circuit side) connects the M-lead to the signal battery (SB) lead connected to the battery of the signaling side in order to indicate the off-hook condition
o The Cisco router / gateway (signaling unit) connects the E-lead to the signal ground (SG) lead connected to the ground of the trunk circuit side in order to indicate the off-hook condition
E&M Type III—This is not commonly used in modern systems
o Type III uses four leads for supervision signaling: E, M, SB, and SG
o During inactivity, the E-lead is open and the M-lead is set to the ground connected to the SG lead of the signaling side
o The PBX (that acts as trunk circuit side) disconnects the M-lead from the SG lead and connects it to the SB lead of the signaling side in order to indicate the off-hook condition
o The Cisco router / gateway (signaling unit) connects the E-lead to the ground in order to indicate the off-hook condition
E&M Type IV—This is not supported by Cisco routers / gateways
E&M Type V—Type V is symmetrical and allows two signaling nodes to be connected back-to-back This is the most common interface type used outside of North America
o Type V uses two leads for supervisor signaling: E, and M
o During inactivity the E-lead and M-lead are open
o The PBX ( that acts as trunk circuit side) connects the M-lead to the ground in order to indicate the off-hook condition
o The Cisco router / gateway (signaling unit) connects the E-lead to the ground in order to indicate off-hook condition
从上面来看，华为和北电应该属于类型一，毕竟华为的交换机有3Com的技术成分。
这些都可以实现的, 只要是是有IP交换机, 和电话机做分级使用就可以
外线去电信, 联通申请; 内线就是电话机可以实现内线互拨的
那就要看公司需要的外线数量, 也就是需要几个外线号码
如果一条外线号码, 就可以配AIO100, 一条外线的IPPBX, 可以配到20个内线分机; 如果两条外线, 基本上2个外线, 60个分机可以满足一般小型公司的使用了 分机就配IP话机, 连接网线使用, 或者干脆WiFi话机, 有WiFi信号, 连接电源就可以了, 很方便 如果是4条外线, 可以做到4条外线, 100个分机, 足够一般的中小型规模公司使用了 如果还需要连接普通传真机, 加一个S口网关就可以了
使用IP交换机, 也就是IPPBX的好处就是免布电话线，安装方便，内线互拨免话费, 只要公司有网线或者WiFi信号就可以实现，IP话机的优点就是可以实现声音保真．外企500强现在一般都换成IP话机了，所以是个趋势．
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