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Step I — Subscriber p is in GSM location area (LA) A, and the HLR indicates that the person is in LA A. The subscriber then moves to the IP net­work(LA B) without turning off the GSM MS. Step II — The subscriber registers to the IP net­work with the procedure described earlier. After registration, the HLR record is modified and p’s record in VLR A is removed. Step III — The subscriber moves back to the GSM MS at LA A. Since the GSM MS is still on, the subscriber p does not notice that an explicit registration is required. Thus, the HLR indicates that subscriber p is still in LA B.	第一步：用户p位于GSM位置区（LA）A，且HLR指示这个人处于LA A。接着用户在不关闭GSM MS的情况下移到IP网络（LA B）。 第二步：用户按照上文描述的步骤注册到IP网络。注册完之后，HLR记录被修改且p在VLR A中的记录被删除。 第三步：用户移回到LA A处的GSM MS。由于GSM MS仍然打开，用户p没有注意需要显式注册。于是，HLR指示用户p仍然位于LA B。
When someone attempts to call p, the call is misrouted to LA B. The misrouting problem is avoided if the sub­scriber explicitly or implicitly registers with the GSM MS at step III. Implicit registration occurs in two cases. Case 1 — The subscriber originates a call. In this case VLR A finds that the VLR record for the subscriber does not exist. VLR A will ask the MS to perform a registration operation as described in the VLR failure restoration procedure [3]. Case 2 — The subscriber moves to another LA in the GSM network. Registration is automati­cally initiated by the GSM MS.	如果某人试图呼叫p，呼叫将被错误发送到LA B。 如果用户在第三步使用GSM MS进行显式或隐式注册，那么就可以避免错打问题。隐式注册在以下两种情况下发生。 第一种情况：用户发起呼叫。如果这样的话，VLR A会发现用户的VLR记录不存在。然后VLR A会叫MS注册，如VLR失败恢复规程所述。 第二种情况：用户移到GSM网络中的另一个LA处。GSM MS自动进行注册。
In both cases, after the HLR has modified p’s record, it also cancels p’s VLR record in VLR B, as described earlier. We have conducted analytic modeling which indicates that the misrouting probabilities increase as the variance of user LA residence time decreases. Our study also indi­cates that if subscribers originate calls much more frequently than they receive calls, the mis­routing effect is insignificant. If the user LA resi­dence times, intercall origination times, and intercall delivery times are of the same order, the misrouting effect cannot be ignored. This problem exists for all approaches based on the concept of universal personal telecommunications to support user mobility.	在这两种情况下，当HLR修改完p的记录之后，它还会删除p在VLR B中的VLR记录。我们已经进行了分析建模，建模显示：错打的可能性会随用户LA驻留时间减少而增多。研究还发现，如果用户发起的呼叫次数大大超过它们接收呼叫的次数，错打的几率很低。如果用户LA驻留时间、拨打内部电话的次数以及内部电话传递次数属于同一级，那么错打的几率将不可忽略。这种问题存在于所有基于“通用个人电信”概念的方法中。
To eliminate the misrouting problem, iGSM subscribers are responsible to perform the explicit registration operation. That is, the subscribers should turn off their MSs when moving to the IP network, and turn on the MSs when they move back to the GSM network. Another alternative is to have the network automatically turn off the MS imme­diately after the subscriber registers to the IP network. This operation can be achieved as fol­lows. At the end of the registration procedure described earlier, the IP network queries the HLR to see if the MS has been detached (when the MS is turned off by the user, a detach mes­sage is sent to the HLR). If not, the IP network sends a GSM short message [10] to the MS. The short message instructs the SIM card of the MS to run an applet that shuts down the MS. As a final remark, misrouted calls are not necessarily lost. With features such as call for­warding on no reply, these calls can be forward­ed to appropriate destinations or mailboxes.	为了消除错打问题，iGSM用户负责执行显示注册操作。即用户在移到IP网络时应该关闭他们的MS，在移回GSM网络时再打开MS。另外一种方法是在用户注册到IP网络之后让网络立即自动关闭MS。这个操作可以按照以下方法完成。在之前描述的注册规程的末尾，IP网络查询HLR，检查MS是否已经被释放（当MS被用户关闭时，向HLR发送一条detach消息）。如果没有释放，那么IP网络会发送一条GSM短消息给MS。这条短消息命令MS的SIM卡运行applet，将MS关闭。 最后需要说明的是，错打的呼叫未必会丢失。因为具有“Call forwarding on no reply（无应答呼叫前转）”功能，这些呼叫会被转发给合适的接收站或邮箱。
CONCLUSIONS This article describes iGSM, a VoIP service for GSM. This value-added service supports user mobility. That is, iGSM allows a GSM user to access VoIP service when he/she moves to the IP network. The iGSM tracks the locations of iGSM subscribers as long as the subscribers turn off/on the terminal (GSM MS or IP terminal) when he/she leaves/arrives at the network. To inter-work GSM and IP networks, we proposed the iGSM protocol translation mechanism between GSM MAP and H.323. Based on this mecha­nism, we showed how iGSM registration, dereg-istration, and call delivery procedures work. The implementation of iGSM does not require modi­fications to the GSM network. The iGSM proto­col translation mechanism can be implemented using the standard H.323 gateway and gatekeep­er. We studied how the tromboning effect in a standard GSM system can be avoided when accessing the IP network. Then we investigated the misrouting problem caused by user mobility.	小结 本文介绍了iGSM，即GSM的VoIP服务。这一增值服务支持用户移动性。即iGSM使得GSM用户在移到IP网络时可以接入VoIP服务。当用户离开/到达网络时，只要他关闭/打开终端（GSM MS或IP终端），iGSM就会跟踪iGSM用户的位置。为了让GSM和IP网络互相作用，我们提出了在GSM MAP和H.323之间使用iGSM协议翻译机制。在这个机制的基础上，我们介绍了iGSM的注册、取消注册以及呼叫传递规程如何工作。iGSM的实现并不需要修改GSM网络。iGSM协议翻译机制可以使用标准的H.323网关和网守来实现。此外还探讨了在接入IP网络时如何避免在标准GSM系统中产生拉长号效应以及由用户移动性导致的错打问题。

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