PROCEDURE FOR SETTING UP AN EMERGENCY CALL IN A WIRELESS LOCAL LOOP

Abstract: Procedure for setting up an emergency call in a wireless local loop, in which subscriber terminal units (TU) are connected over a wireless link via a base station (BS) to an access node (AN) and further from the access node to a local exchange (LE) via a standard V5.2 interface, in which, when the interface is congested and a subscriber (TU) connected to the access node (AN) is attempting to make an emergency call to a public emergency number, the call control system of the local exchange (LE) cannot at once find a free time slot for the setup of the emergency call. In the procedure, call setup requests for other than emergency calls over the V5.2 interface are disabled during a predetermined period of time t2, and attempts to allocate a time slot in the V5.2 interface for the emergency call are repeated during said predetermined period of time t2.

Description: PROCEDURE FOR SETTING UP AN EMERGENCY CALL IN A WIRELESS LOCAL LOOP The 'present invention relates to a procedure as defined in the preamble of claim 1. The invention concerns a wireless local loop system, in which a subscriber is connected over a wireless link via an access node (AN) to a wired network local exchange. The subscriber terminal devices are connected to the access node via a GSM/DCS 1800 interface (GSM, Global System for Mobile Communications; DCS, Digital Cellular System) . The access node is connected to the local exchange via a V5.2 interface consistent with the ETS 300 347 standard series. GSM is a European digital mobile communication system standardized by ETSI. DCS-1800 is a mobile communication system standardized by ETSI, which is based on the GSM specification and aims at a more effective use of microcells and which works in the frequency range of 1800 MHz. Between the terminal device and the ac- cess node there is a base station, through which call signals sent by the terminal device over a radio channel are transmitted via the access node to a public telephone network and vice versa. The access node can be connected to the telephone exchange using e.g. the V5.1 or V5.2 protocol. Open interf ces (V5.1 and V5.2) between an access node and a telephone exchange are defined in the ETSI (European Telecommunications and Standards Institute) standards of the ETS 300 324 and ETS 300 347 series. V5 interfaces enable subscribers belonging to a physically separate access network, which may be either a wired or a wireless network, to be connected to a telephone exchange using a standard interface. A dynamic V5.2 concentrator interface consistent with the standards ETS 300 347-1 and 347-2 consists of one or more (1 - 16) PCM (Pulse Code Modulation) cables. One PCM cable comprises 32 channels, each of which with a transfer rate of 64 kbit/s, i.e. 2048 kbit/s in all. The V5.2 interface supports analogue telephones as used in the public telephone network, digital, such as ISDN (Integrated Services Digital Network) basic and system subscriptions as well as other analogue or digital terminal equipment based on semi-fixed connections . A call initiated with an EMERGENCY_SETUP mes- sage as described in the GSM system specification may, if the resources prove to be insufficient, start a process of forced release of a call having a lower priority. When a subscriber connected to the access node of a wireless local loop tries to make an emergency call to a public emergency number and if the call control system of the local exchange is unable to find a free time slot in the V5.2 interface within a predetermined period of time, then the call setup will f il. The subscriber will keep trying again until the call is successfully set up. The standard V5.2 interface contains no definition of forced call setdown or an emergency call as such. When the setup of an emergency call fails due to congestion, the WLL terminal will generate a busy tone for the subscriber. Consequently, the subscriber will try to call again. In practice, the entire signalling required for call setup is generated anew, starting from the allocation of a communication channel, resulting in unnecessary burdening of the wireless network resources. The object of the present invention is to eliminate the drawbacks described above. The object of the invention is to disclose a procedure that makes it possible to increase the prob- ability of successful setup of an emergency call and allows an emergency call to be set up over a congested V5.2 interface without repeated allocation of radio resources and without repeated call control signalling within the access node. The procedure of the invention is characterised by what is presented in claim 1. In the procedure of the invention, an emergency call is set up in a wireless local loop in a congested situation. In the wireless local loop, WLL terminals are connected over a wireless link via a base station to the access node and further from the access node to a local exchange via a standard V5.2 interface. In a situation where the V5.2 interface is congested, a subscriber connected to the access node tries to make an emergency call to a public emergency number and the call control system of the local ex- change is unable to find a free time slot, i.e. a channel for the setup of the emergency call in the V5.2 interface within a predetermined period of time tl. According to the invention, new call setup requests for other than emergency calls over the V5.2 interface are disabled during a predetermined period of time t2 and attempts to allocate a time slot in the V5.2 interface for the emergency call are repeated during said predetermined period of time or until a channel is successfully allocated. However, it is also possible to implement the procedure of the invention by disabling only new outgoing non-emergency calls from the access node. In this case, new non-emergency calls addressed to subscribers connected to the access node could be enabled. A corresponding arrangement is feasible in all the embodiments described in the present application. In an embodiment of the procedure, when a free time slot has been allocated in the V5.2 inter- face for the emergency call within a predetermined period of time t2 as mentioned above, call setup over the V5.2 interface is enabled. In an embodiment of the procedure, when the subscriber who initiated the emergency call hangs up (subscriber on-hook) before a free time slot is found in the V5.2 interface, the disabled state of setup of new calls is cancelled and call setup is enabled. In an embodiment of the procedure, when the attempts to allocate a time slot in the V5.2 interface fail during a predetermined period of time t2 as mentioned above, the emergency call is released, the dis- abled state of setup of new calls is cancelled and call setup is enabled. In an embodiment of the procedure, a message indicating failure of an attempt to set up an emergency call is given by the V5.2 interface signalling program to the process responsible for the setup of the emergency call, and, via the process responsible for the setup of the emergency call, the higher-level process responsible for signalling is notified about the failed attempt to set up an emergency call . In a preferred embodiment, no such notification is given, but the V5 interface signalling program keeps sending standard establish messages until the attempt is cancelled e.g. when the subscriber hangs up (AN subscriber on-hook) . In this case, the hanging up or the tripping of a timer tl corresponds to a failure of call setup. A corresponding arrangement is also possible in other embodiments described in this application. In an embodiment of the procedure, new call setup requests are disabled by a higher-level process and attempts to allocate a time slot in the V5.2 interface are continued by the lower-level process responsible for the emergency call. In an embodiment of the procedure, the prede- termined period of time tl is defined using a first timer controlling normal call setup time, which is started at the beginning of call setup, and the prede- termined period of time t2 is defined using a second timer controlling the disabled state of call setup, which is -started and/or stopped by the higher-level process responsible for signalling. In an embodiment of the procedure, when a free time slot for the emergency call has been found in the V5.2 interface within a predetermined period of time t2 as mentioned above, a message indicating allocation of a time slot is given by the process respon- sible for the emergency call to the higher-level process responsible for signalling, the second timer is stopped by the higher-level process, and setup of new calls is enabled by the higher-level process. In an embodiment of the procedure, when the subscriber who initiated an emergency call hangs up before a time slot in the V5.2 interface is released, the higher-level process responsible for signalling is given an on-hook message, the second timer is stopped by the higher-level process, and setup of new calls is enabled by the higher-level process. In an embodiment of the procedure, in a situation where the lower-level process responsible for the emergency call does not receive a message indicating allocation of a time slot in the V5.2 inter- face from the local exchange within the predetermined period of time t2, the lower-level process is instructed by the higher-level process responsible for signalling to disconnect the emergency call. In an embodiment of the procedure, one of the ongoing non-emergency calls is selected, the selected non-emergency call is disconnected via a forced process to release a time slot in the V5.2 interface, the emergency call is set up using the released time slot, and the disabled state of setup of new calls is can- celled and call setup is enabled. In an embodiment of the procedure, if the attempts to reserve a time slot in the V5.2 interface fail during the first and second periods of time tl+t2 combined, then the emergency call is released and the disabled state of setup of new calls is cancelled and call setup is enabled. In an embodiment of the procedure, after the lapse of the predetermined period of time t2, forced call setdown is started again only once for the same call, and the failed emergency call to the wireless network is disconnected after forced disconnection of a call has failed again. In an embodiment of the procedure, after the lapse of the second predetermined period of time t2, the emergency call is released and the disabled state of setup of new calls is cancelled and call setup is enabled. In an embodiment of the procedure, the above- mentioned non-emergency call is selected by the higher-level process responsible for signalling, an instruction to disconnect the call is given by the higher-level process responsible for signalling to the lower-level process responsible for the selected non- emergency call, forced disconnection of the selected non-emergency call is effected by the lower- level process, and the second timer controlling the disabled state of call setup is started by the higher-level process responsible for signalling. In an embodiment of the procedure, the higher- level process responsible for signalling is informed of the disconnection of the call by the lower- level process responsible for the selected disconnected call, the timer controlling the disabled state of call setup is stopped and a second timer for a time-out for the disabled state of call setup is started. In an embodiment of the procedure, in a situation where the lower- level process responsible for the emergency call does not receive a message in- dicating allocation of a time slot in the V5.2 interface within the combined predetermined period of time tl+t2, the lower-level process responsible for the emergency call is instructed by the higher-level proc- ess responsible for signalling to disconnect the emergency call upon receipt of the next response to a failed request for a time slot in the V5.2 interface if the allocation of a time slot has failed again. In an embodiment of the procedure, after the predetermined period of time t2 when timer T2 has tripped, forced call disconnection is started again only once for the same call by the higher-level process responsible for signalling, and the failed emergency call to the wireless network is disconnected af- ter forced call disconnection has failed again. In an embodiment of the procedure, after the predetermined second period of time t2 when the second timer trips, setup of new calls is enabled by the higher-level process responsible for signalling, and the lower-level process responsible for the emergency call is instructed by the higher-level process responsible for signalling to disconnect the emergency call. In the following, the invention will be described in detail by the aid of a few examples of its embodiments by referring to the attached drawing, in which Fig. 1 presents a diagram representing a wireless local loop in which an emergency call is being set up using the procedure of the invention, Fig. 2a and 2b present a diagram representing a first embodiment example of the procedure of the invention, and Fig. 3a and 3b present a flow chart representing a second embodiment example of the procedure of the invention. Fig. 1 is a diagrammatic representation of a wireless local loop in which a number of subscriber terminal units TU are connected over a radio link via a base station BA to an access node AN and from the access node to a local exchange LE over a V5.2 interface consistent with the ETS 300 347-2 standard, which comprises 1 - 16 PCM lines (2 Mbit/s) . EXAMPLE 1 In the examples presented in Fig. 2a and 2b, the solution used for the setup of an emergency call in a congested situation is to queue up the call in the access node AN until a time slot is released in the V5.2 interface. Phase 21: A subscriber starts an emergency call, in which an emergency call setup message EMERGENCY_SETUP is used. Phase 22 : A timer Tl is started to control the duration of call setup and to make sure that no extra time is spent for the call setup. Phase 23: If the call is successful, i.e. if a free time slot is found in the V5.2 interface before the timer Tl trips, the normal call setup procedure will be initiated, phase 24. If no free time slot is found in the congested V5.2 interface, the process will go on to phase 25. Phase 25: The V5 interface signalling program gives the process responsible for the setup of the emergency call a message indicating failure of the first setup attempt concerning the emergency call and notifies the higher-level process responsible for sig- nailing about the event. As stated above, this notification may be omitted, in which case the expiration of time tl corresponds to such notification. Phase 26: The higher-level process responsible for signalling disables new call setup requests. The lower-level process responsible for the emergency call goes on attempting to allocate a time slot in the V5 interface. Phase 27: The higher-level process starts the timer T2 controlling the disabled state of call setup. Phase 28: If the process responsible for the emergency call is able to set up the emergency call over the V5.2 interface, phase 29, it will give a corresponding message to the higher-level process responsible for signalling. Phase 30: Via the higher- level process, the disabled state of setup of new calls is cancelled and call setup is enabled, and timer T2 is stopped. On the other hand, if at phase 28 the process responsible for the emergency call is unable to set up the call over the V5.2 interface, a test is carried out to establish whether the subscriber has hung up or is still off hook, phase 31. If the subscriber has hung up, a message as at phase 29 is given and setup of new calls is enabled as at phase 30. EXAMPLE 2 In this example, the solution used for the setup of an emergency call in a congested situation is forced disconnection of a call, effected by the access node AN. Phase 41: A subscriber starts an emergency call, in which an emergency call setup message EMERGENCY_SETUP is used. Phase 42 : A timer Tl is started to control the duration of call setup and to make sure that no extra time is spent for the call setup. Phase 43: If the call is successful, i.e. if a free time slot is found in the V5.2 interface before the lapse of time tl, the normal call setup procedure will be initiated, phase 44. If no free time slot is found in the congested V5.2 interface, the process will go on to phase 45. Phase 45: The V5 interface signalling program gives the process responsible for the setup of the emergency call a message indicating failure of the first setup attempt concerning the emergency call and notifies the higher-level process responsible for signalling about the event. Phase 46: The higher-level process responsible for signalling disables new call setup requests. The lower-level process responsible for the emergency call goes on attempting to allocate a time slot in the V5 interface. Phase 47: The higher-level process starts a timer T2 controlling the disabled state of call setup. Phase 48: A counter n=0 is reset. The counter value n is used to ensure that forced disconnection is performed only once on the call selected to release a time slot. Phase 49: The counter value n is checked to see if it is higher than one. As the counter value n was initially set to zero, the process will first proceed through this phase directly to phase 51. If the counter value n is 2 or higher, i.e. n>l, setup of the emergency call is terminated, phase 50, and setup of new calls is enabled, phase 54. Phase 51: The higher-level process responsible for signalling selects one of the non-emergency calls (other than emergency calls) and instructs the lower-level process responsible for this call to disconnect it . Phase 52: If the process responsible for the emergency call is now able to set up the call over the V5.2 interface, i.e. makes use of the free time slot within time t2, execution will proceed to phase 53, at which the process responsible for the emergency call informs the higher-level process responsible for signalling about the event. At phase 54, the higher-level process enables setup of new calls and stops the timer T2. Phase 55: If no free time slot is found, i.e. the emergency call is unsuccessful, a test is carried out to establish whether the subscriber has hung up or is still off hook. If the subscriber who started the call has hung up before a time slot is released in the V5.2 interface, the process responsible for the emergency call will notify the higher-level process about the event, phase 53. The higher-level process will then enable setup of new calls and stop the timer T2 , phase 54. If the test at phase 55 indicates that the subscriber is off hook and the lower-level process responsible for the call selected for disconnection has disconnected the call, it will give a corresponding message to the higher-level process responsible for signalling. Phase 56: A test is carried out to establish whether the lower-level process responsible for the emergency call has received from the local exchange a message indicating allocation of a V5.2 interface time slot within the time t2 measured by timer T2 , in other words, whether tp is smaller than t2. If the lower-level process responsible for the emergency call does not receive a message indicating allocation of a time slot in the V5.2 interface from the local exchange within the period of time t2, in other words, if tp > t2, the higher-level process responsible for signalling will instruct the lower- level process responsible for the emergency call to disconnect the emergency call, phase 57, upon receiving the next response to a request for allocation of a time slot in the V5.2 interface if allocation has failed again. After that, setup of new calls will be enabled, phase 54. If there is any time left, in other words, if the period of time t2 has not yet elapsed (tp < t2) , then the counter value n will be increased by 1, phase 58, and action will be resumed at phase 49, from where the process will go on again as described above. The invention is not restricted to the exam- pies of its embodiments described above, but many variations are possible within the scope of the inventive idea defined by the claims.

Claims: CLAIMS 1. Procedure for setting up an emergency call in a wireless local loop, in which subscriber terminal units (TU) are connected over a wireless link via a base station (BS) to an access node (AN) and further from the access node to a local exchange (LE) via a standard V5.2 interface, in which, when the interface is congested and a subscriber (TU) connected to the access node (AN) is attempting to make an emergency call to a public emergency number, the call control system of the local exchange (LE) is unable to find a free time slot for the setup of the emergency call within a predetermined period of time tl, charac t e ri s ed in that call setup requests for other than emergency calls over the V5.2 interface are disabled during a predetermined period of time t2, and attempts to reserve a time slot in the V5.2 interface for the emergency call are repeated during said predetermined period of time t2. 2. Procedure as defined in claim 2, char ac t eri sed in that, when a free time slot in the V5.2 interface has been allocated for the emergency call within said predetermined period of time t2, call setup over the V5.2 interface is enabled. 3. Procedure as defined in claim 1, char ac teri s ed in that if the setup of the emergency call is interrupted when the subscriber hangs up before a time slot is released in the V5.2 interface, then call setup over the V5.2 interface is enabled. 4. Procedure as defined in claim 1, char ac teri sed in that, if the attempts to allocate a time slot in the V5.2 interface fail during said predetermined period of time t2 , then the emergency call is released and call setup over the V5.2 interface is enabled. 5. Procedure as defined in any one of claims 1 - 4, charact eri sed in that a message indicating failure of an attempt to set up the emergency call is given by the V5.2 inter- face signalling program to the process responsible for the setup of the emergency call, and, via the process responsible for the setup of the emergency call, a message indicating failure of an attempt to set up the emergency call is given to a higher-level process responsible for signalling. 6. Procedure as defined in any one of claims 1 - 5, charact eri sed in that call setup requests are disabled by the higher-level process and attempts to reserve a time slot in the V5.2 interface are continued by the lower-level process responsible for the emergency call . 7. Procedure as defined in any one of claims 1 - 6, characteri sed in that the predeter- mined period of time tl is defined using a timer (Tl) controlling the time used for normal call setup, said timer being started at the beginning of call setup and/or stopped by the higher-level process responsible for signalling. 8. Procedure as defined in any one of claims 1 - 7, c ha r ac t e r i s e d in that the predetermined period of time t2 is defined using a timer controlling the disabled state of calls, said timber being started after time tl if normal call setup fails. 9. Procedure as defined in any one of claims 2 - 8, charac teri sed in that when a free time slot has been allocated for the emergency call in the V5.2 interface within said predetermined period of time t2, a message indicating allocation of a time slot is given by the process responsible for the emer- gency call to the higher- level process responsible for signalling, timer (T2) is stopped by the higher- level process, and call setup is enabled by the higher- level process . 10. Procedure as defined in any one of claims 3 - 8, charact eri s ed in that, when the subscriber who initiated the emergency call hangs up be- fore a time slot in the V5.2 interface is released, the higher-level process responsible for signalling is given an on-hook message, timer (T2) is stopped by the higher-level process, and call setup is enabled by the higher-level process . 11. Procedure as defined in any one of claims 4 - 8, charac t e ri s ed in that, if the lower- level process responsible for the emergency call does not receive a message indicating allocation of a time slot in the V5.2 interface from the local exchange (LE) within the predetermined period of time t2 , then the lower-level process is instructed by the higher- level process responsible for signalling to release the emergency call. 12. Procedure as defined in any one of claims 1 - 11, charact eri sed in that, after time tl, of the ongoing calls, a call other than an emergency call is selected, the selected call is disconnected via a forced process to release a time slot in the V5.2 interface, the emergency call is set up using the released time slot, and call setup is enabled. 13. Procedure as defined in any one of claims 1 - 12, charac t e ri s ed in that if the attempts to allocate a time slot in the V5.2 interface for the emergency call fail during the combined period of time tl+t2, then the emergency call is released and call setup is enabled. 14. Procedure as defined in claim 12 or 13, charac t eri sed in that after the lapse of the predetermined period of time t2, forced disconnection is started again only once for the same call, and the failed emergency call to the wireless network is released if forced disconnection of a call fails again. 15. Procedure as defined in any one of claims 12 - 14, charact eri sed in that after the lapse of the second predetermined period of time t2, the emergency call is released and call setup is enabled. 16. Procedure as defined in any one of claims 12 - 15, characteri sed in that said call other than an emergency call is selected by the higher-level process responsible for signalling, an instruction to disconnect the call is given by the higher-level process responsible for sig- nailing to the lower-level process responsible for the selected call, forced disconnection of the selected call is effected by the lower-level process, and the timer (T2) controlling the disabled state of call setup is started by the higher-level process responsible for signalling. 17. Procedure as defined in claim 16, charac t e ri sed in that the higher-level process responsible for signalling is informed of the dis- connection of the call by the lower-level process responsible for the selected disconnected call. 18. Procedure as defined in any one of claims 13 - 17, charac t eri s ed in that, if the lower- level process responsible for the emergency call does not receive a message indicating allocation of a time slot in the V5.2 interface from the local exchange (LE) within the predetermined period of time t2 , then the lower-level process responsible for the emergency call is instructed by the higher-level process responsible for signalling to release the emergency call upon receipt of the next response to a failed allocation request for a time slot in the V5.2 interface if the allocation of a time slot has failed again. 19. Procedure as defined in any one of claims 14 - 18, charact eri s ed in that after the predetermined period of time t2 , forced call disconnection is started again only once for the same call by the higher-level process responsible for signalling, and the failed emergency call to the wireless network is disconnected after forced call disconnection has failed again. 20. Procedure as defined in any one of claims 15 - 19, characterised in that after the lapse of the second predetermined period of time t2, setup of new calls is enabled by the higher-level process responsible for signalling, and the lower-level process responsible for the emergency call is instructed by the higher-level process responsible for signalling to release the emergency call.

Inventors: PENTTINEN SIMO, PENTTINEN, SIMO
Applicants: NOKIA TELECOMMUNICATIONS OY, PENTTINEN SIMO, PENTTINEN, SIMO
Application year: 1998
Main classification: H04Q7
IPC codes: H04W4, H04W28, H04W74, H04W84