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IMPLEMENTATION ISSUES This section discusses two issues regarding the iGSM implementation: reducing the GSM trom­boning effect and investigating misrouting of user mobility. REDUCING THE GSM TROMBONING EFFECT In the standard GSM call delivery procedure, the tromboning effect occurs if the caller and the called MS are in the same city but the GMSC is in another city (or another country).We assume that both GSM MSCs and iGSM gate­ways can serve as GMSCs of iGSM subscribers. If an iGSM subscriber is assigned a GSM MSC as his/her GMSC, call delivery follows the proce­dure described earlier, and the tromboning effect occurs. On the other hand, if the iGSM gateway is assigned as the GMSC of the iGSM subscriber, call delivery tromboning can be avoided. In this case a PSTN-to-iGSM (IP net­work) call is described in the following steps.	实现过程中的问题 这一节介绍与iGSM实现相关的两个问题：如何降低GSM拉长号效应以及避免由用户移动性导致的错打问题。 降低GSM拉长号效应 在标准GSM呼叫传递规程中，如果呼叫方和被呼叫的MS在同一个城市，GMSC却位于另一个城市（或其他国家），那么会发生“拉长号效应”。假定GSM MSC和iGSM网关都可以当作iGSM用户的GMSC。如果向iGSM用户分配一个GSM MSC用作他/她的GMSC，那么呼叫传递还是按照之前的规程。另一方面，如果向iGSM用户分配一个iGSM网关用作GMSC，那么就可以避免拉长号效应。如果这样的话，使用以下步骤描述PSTN-to-iGSM（IP网络）呼叫。
Step 1 — The originating switch routes the call to the iGSM gateway based on the dialed MSISDN. Step 2 — The iGSM gateway first queries the iGSM gatekeeper to check if the iGSM sub­scriber is in the IP network. If not, the iGSM gateway queries the HLR and performs the stan­dard GSM call delivery procedure. Step 3 — If the iGSM subscriber is in the IP network, the iGSM gateway sets up the call to the H.323 terminal following the standard H.323 call setup procedure.	第一步：发话端交换机根据拨打的MSISDN号码将呼叫发送到iGSM网关 第二步：iGSM网关首先查询iGSM网守，检查iGSM用户是否位于IP网络。如果不在，那么iGSM网关再查询HLR并执行标准GSM呼叫传递规程。 第三步：如果iGSM用户位于IP网络，那么iGSM网关按照H.323呼叫建立规程与H.323终端建立呼叫。
It is clear that at step 3 no resource in the GSM network is consumed, and the call setup cost is cheaper than in Fig. 6. Also, in this case, the cost for an IP-to-iGSM (IP network) call setup is exactly the same as a traditional VoIP call, which is even cheaper than the PSTN-to­iGSM call. Thus, two kinds of iGSM subscribers are anticipated: The GMSCs of the subscribers are standard GSM MSCs. In this case, the subscribers typically subscribe to the standard GSM services at the beginning, and decide to include the iGSM service later. The GMSC of the subscribers is the iGSM gateway. In this case, the subscribers typi­cally subscribe to the iGSM service at the beginning.	显然，在第三步中GSM网络没有消耗任何资源，且呼叫建立的成本低于图6。如果这样的话，IP-to-iGSM（IP网络）呼叫建立的成本与传统VoIP呼叫完全相同，甚至比PSTN-to-iGSM呼叫更加便宜。 因此，预料有以下两类iGSM用户： 1. 用户的GMSC是标准GSM MSC。如果这样的话，用户一般首先订阅标准GSM服务，之后再决定包含iGSM服务。 2. 用户的GMSC是iGSM网关。如果这样的话，用户一般首先就订阅iGSM服务。
For the first kind of subscribers, call delivery follows the standard GSM procedure. When the subscribers visit the IP network, tromboning may occur as in a traditional GSM network. The GSM operator would prefer this scenario if the iGSM gateway and gatekeeper are owned by the ISP. For the second kind of subscribers, call deliv­ery tromboning can be avoided when the sub­scribers visit the IP network. In this scenario, the GSM operator is likely to own the iGSM gate­way and gatekeeper.	对于第一类用户来说，呼叫传递沿用标准GSM规程。当用户访问IP网络时，可能会像在传统GSM网络中一样发生拉长号效应。如果iGSM网关和网守都属于ISP，那么GSM运营商会更加偏爱这种方案。 对于第二类用户来说，当用户访问IP网络时，在呼叫传递中可以避免拉长号效应。在这种情况下，GSM运营商很可能同时拥有iGSM网关和网守。
MISROUTING DUE TO USER MOBILITY To support user mobility, the subscriber needs to explicitly perform registration to inform the sys­tem in which location area he/she resides if the terminal has been changed. If the subscriber for­gets to take this action when he/she changes ter­minals, call deliveries to the subscriber may be misrouted. This problem can be eliminated if the subscriber always turns off the MS when he/she moves to the H.323 terminal. The “turnoff” action results in a GSMdetach message to deregister the MS. The misrouting problem may occur in the fol­lowing scenario for an iGSM subscriber.	由用户移动性导致的错打问题 要支持用户移动性，用户需要进行显式注册，以便在终端发生改变后通知系统他/她位于哪个位置区。如果用户在他/她改变终端后忘记采取这个行动，用户的呼叫传递路线可能会出错。如果用户在他/她移到H.323终端时一直关闭MS，那么可以避免这种问题发生。“turnoff（关闭）”动作会导致产生一条“GSM释放”消息，取消注册MS。 IGSM用户如果处于以下情况，则可能会出现错打问题。