Arrangement for improving availability of services in a communication system

ARRANGEMENT FOR IMPROVING AVAILABILITY OF SERVICES IN A COMMUNICATION SYSTEM

FIELD OF THE INVENTION

0003The present invention relates to an arrangement for improving availability of services in a communications system, especially a telecommunications system, said system comprising distributed hardware and software components which interact in order to provide services to one or more users .

0004More specifically the invention concerns the user mobility support in distributed systems.

0005Still more specifically the invention relates to an improved utilisation of agent technology.

STATE OF THE ART

0007Open distributed system is a category of distributed systems which are made up of components that may be obtained from a number of different sources, which together work as a single distributed system.

0008The open distributed system provides an application interface to the distributed system and the application may run on any local operating system which is appropriate. The TINA architecture (Telecommunications Information Network Architecture) is a specialisation of open distributed system for telecommunications applications, and the present invention has primarily been developed in connection with inter alia such TINA architecture. The TINA-C architecture does address the personal (user) mobility issue, but does not use multiple agents for each user. Further, this architecture does not address the allowance of additional constellations, for example by let- ting one user using several terminals of same or different capabilities and the allowance of several users using the same terminal .

0009Besides TINA, there is no architecture addressing user mobility on distributed systems.

0010Consequently, there exists an increasing demand for user mobility, especially in connection with distributed systems, and hitherto no proper solution for distributed system has been suggested.

OBJECTS OF THE INVENTION

0012An object of the present invention is to suggest imple- mentations of the improved availability of services in a communication system, especially the user mobility support in distributed systems.

0013Another object of the present invention is to provide an arrangement allowing a user to move and still allow access to applications/services.

0014Still another object of the present invention is to provide an improved availability by which one user can use several terminals of same or different capabilities and whereby several users can use the same terminal .

0015Yet another object of the present invention is to provide— an arrangement allowing the separation and protection of objects belonging to different users but running on the same terminal .

0016A further object of the present invention is to suggest an improved use of multiple agents for each mobile user.

SUMMARY OF THE INVENTION

0018The above objects are achieved in an arrangement of the type as stated in the preamble, which primarily is characterised by introducing in said system a user mobility support, for thereby enabling application availability including personal mobility.

0019More specifically the objects are achieved by introducing said user mobility support for distributed system.

0020Further features and advantages of the present invention will appear from the following description taken in con- junction with the enclosed drawings, as well as from the enclosed patent claims.

BRIEF DISCLOSURE OF THE DRAWINGS

0022Fig. 1 is a schematical diagram illustrating the user mobility defined in terms of interactions.

0023Fig. 2 is a schematical diagram illustrating an embodiment of the present invention especially the introduction of user agents necessary for user mobility support.

0024Fig. 3 is a schematical diagram illustrating operations initiated by the mobile user. ~ Fig. 4 is a schematical diagram illustrating operations initiated by an object in the telecom system domain.

0025Fig. 5 is a schematical diagram illustrating a specific embodiment of the present invention comprising a dedicated object user registration means for storing registration data.

0026Fig. 6 is a schematical diagram illustrating interactions with the default user domain.

0027Fig. 7 is a schematical diagram illustrating the functionality of the user interface object.

0028Fig. 8 is a schematical diagram illustrating an embodiment of local registration.

0029Fig. 9 is a schematical diagram illustrating a procedure for the local registration.

0030Fig. 10 is a schematical diagram illustrating an embodiment for remote registration.

0031Fig. 11 is a schematical diagram illustrating a procedure for remote registration.

0032Fig. 12 is a schematical diagram illustrating an embodiment of local deregistration.

0033Fig. 13 is a schematical diagram illustrating an embodiment for a procedure for the local registration.

0034Fig. 14 is a schematical diagram illustrating an embodi- - ment for remote deregistration. Fig. 15 is a schematical diagram illustrating a procedure for remote deregistration.

0035Fig. 16 is a schematical diagram illustrating an example of multiple terminal registration.

0036Fig. 17 is a schematical diagram illustrating an embodiment of a user registration object having object regis- tration capability.

0037Fig. 18 is a schematical diagram illustrating an embodiment of a registration of a user object.

DETAILED DESCRIPTION OF EMBODIMENTS

0039Definition of user mobility

0040Let us now consider an example consisting of a telecom system_N domain, a Terminal_m domain and a User, domain. The

0041User_a domain consists of the User_a object and an arbitrary computational object called COl. Note that the object COl may or may not reside physically in the terminal_m. In the case where the user is also a terminal or hardware de- vice, COl may reside on the user itself and not on the Terminal_m domain.

0042To offer user mobility means to allow the whole user domain including COl, to move and change terminal and still be able to interact with an object C02 residing in the telecom system_N domain. By interactions it is meant both operations and stream flows. This is depicted in Figure 1. ~ Use of agents

0043Let us now introduce the agents necessary to enable interactions between domains . To represent the User, an agent called PD_UA_a (ProviderDomain_User_Agent_a) is introduced in the telecom system_N domain. This agent assumes security functions such as identification, authentication, encryption, etc. and is also responsible for the functions supporting the mobility of the user. Another agent called TDm_UA_a (TerminalDomain_User_Agent_a) is introduced in the Terminal,, domain to represent the User. and to assume the security functions of the Terminal,.. If the User_a is also using another terminal, namely Terminal_n then a TDn_UA_a is also defined for him on the Terminal,, do- main.

0044We assume further that every terminal must have an owner and it is reasonable to define this owner as the default user of the terminal. A legal and operative terminal will always have a default user. This default user is permanently registered at the terminal. Definition of a default user for a terminal allows us to implement access control procedures which may restrict the usage of the terminal. Of course, the default user can also be regis- tered at another terminal but the former registration is always valid and we have a multiple terminal registration case. There is therefore introduced in the Terminal_m domain an agent TD_U- - representing the default user. Correspondingly, an agent PD_UA-. is introduced in the telecom system_N domain.

0045In addition, we have objects such as SPAN representing the telecom system_N domain in the Terminal., domain, TAm representing the Terminal,, in the telecom system., domain, TAP representing a Terminal Access Point (port) , NAP representing a Network Access Point. This is depicted in Figure 2.

0046Enabling operations between the mobile user and the telecom system

0047Since operations can be initiated either by the User, or by objects in the telecom system_N domain, we will consider the two cases separately.

0048Operations initiated by the mobile user

0049Suppose now that COl wants to invoke an operation OpY() on the object C02. The invocation is not done directly.

0050First, an operation Call (C02.OpY () ) is issued to the object TD_UA_a. This shows that there must be an association between C02 and TD_UA_a . The definition of this association will be discussed later. The TD_UA_a will then invoke the operation Call (02.OpY () ) on its peer-object PD_UA_a . What TD_UA_a needs in order to issue the operation request is the identifier of the object PD_UA_a . Since both TD_UA_a and PD_UA_a represent the same user and there is only one unique PD_UA_a, it is not a problem for TD_UA_a to find the identifier of PD_UA_a . Indeed, both of these two objects hold the identity of the user whom they represent.

0051The operation Call (02. OpY ( ) ) on PD_UA_a is converted to an operation Call (PD_UA_a . Call (C02. OpY ()) ) of the SPAN. The SPAN issues a operation request

0052Call (PD_UA_a.Call (C02. OpY () ) ) on its peer-object TA_m. This request is converted to the operation -—

0053Call (TA_m.Call (PD_UA_a. Call (C02. OpY () ) ) on the TAP. The TAP will then forward this operation to the NAP to which it is currently connected with. The NAP will then issue an operation request Call (PD_UA_a . Call (C02.OpY ()) ) to the TA_πι. The TA„, will then invoke Call (C02.OpY () ) of the PD_UA_a. The PD_UA_a will now call the operation OpY ( ) of the object C02. This concludes the conveyance of the operation OpY ( ) .

0054The invocation of operations by objects in the user do- main are seamless supported. The only information required by the user domain is that C02 is residing on the fixed kTN (or located in the telecom system domain) . No additional information is required. This is depicted in Figure 3.

0055Operations initiated by an object in the telecom system domain

0056Reference is made to Fig. 4.

0057Suppose now that an object C02 in the telecom system domain wants to invoke the operation OpX ( ) of COl. This operation will be converted to the operation Call (COl .OpX () ) of the PD_UA_a . In order to make the con- version possible the information that COl belongs to the User_a domain and not to any other user domain, must be available in the telecom system domain.

0058The PD_UA_a will then invoke the operation Call (COl . OpX () ) on the object TD_UA_a. The first requirement is to find the identifier of TD_UA_a. For the time being we just suppose that such information is available. We shall soon come back to how it can be found. The operation request is thereafter converted to the operation Call (TDJAa. Call (COl. OpX () ) on the TA_m. To be able to do that, the PD_UA_a must have the identifier of TA„, or the identity of the terminal, namely Terminal., where the User_a is located.

0059The procedure to acquire the necessary information to reach the mobile user is commonly known as location tracking and is often supplemented by a location registration and deregistration procedure.

0060Let us suppose for the time being that the PD_UA_a somehow gets sufficient information and calls the correct Termi- nal_agent , namely TA_ra. From this step to step 6, all the operations are related to the handling of terminal mobil- ity. The TA., will transfer the call to its peer-to-peer object SPAN by issuing the operation

0061Call (SPAN. Call (TD_UA_a.Call (COl. OpX () ) to the NAP. The NAP transfers the call to the corresponding TAP. The TAP invokes then the operation Call (TD_UA_a . Call (COl . OpX ()) ) of the SPAN. The SPAN calls the operation Call (COl .OpX () ) of the TD_UA_a. Finally, the TD_UA_a invokes the operation OpX() of the object COl. This concludes the convey of an operation initiated by an object in the telecom system domain.

0062We have seen that the condition for the success of these procedures is the availability of information about the current terminal which the user is using or having access to. Formally speaking, the success of operations initiated by objects residing on the telecom system domain de- pends on the definition of the association between the PD_UA_a and a TA.

0063When the User_a is moving, the association between PD_UA_a - and TA is changing correlatively and may sometimes be un- defined. The operations necessary to determine this association are parts of the procedures commonly referred to as user registration and deregistration.

0064User location registration and deregistration

0065The user location registration and deregistration is the procedure to determine the association between the PD_UA and the TA. There are many methods that will be described in the following sections.

0066The "on-the-Fly" method

0067The most straightforward method is the "on-the-fly" method. This method can also be called "lazy" since nothing is to be done unless it is necessary, e.g. a request is issued. When an object C02 residing in the telecom system domain wants to call an operation of an object COl belonging to the User_a domain, a request is issued to the PD_UA_a obj ect . The PD_UA_a object will issue a broadcast to all the TAs asking them to search for the mobile user. The TAs can ask their counterpart, namely the SPA, to search for the corresponding TD_UA_a . If none of the SPAs succeed, then the User_a cannot be reached. Nothing else can be done. If one SPA succeeds in finding the mobile

0068User_a, it will answer to the corresponding TA which again informs the PD_UA_a . Then the PD_UA_a can send the operation invocation to the TA in question which forwards it to the corresponding SPA. The SPA forwards the operation to the TD_UA_a object which delivers it to the object COl. Interactions are then enabled.

0069This method is acceptable for small and "less geographi- — cally distributed" system, i.e. system with small number of terminals and small number of users. It can be time consuming for larger or more geographically distributed systems. It may take a while to find the user or to know that it is not possible to find him. This method requires also much activity in the telecom system domain. If there are n terminals and m users in the system, then, in the worst case, m x n search procedures can be initiated simultaneously to find all the m users. This method is used in Internet when a search engine is used to identify the location of another web page or place of information.

0070Method based on the predetermination of the association between the PD_UA and the TA

0071There is another method based on the predetermination of the association between the PD_UA and the TA. The PD_UA knows in advance whether the mobile user has contact with any TA or not, and which specific TA instance it is associated with. In other words, the PD_UA object has the ability to store the association between the PD_UA and the TA.

0072A simple implementation is to use a TA identifier which is a kind of "distribution transparent" pointer to a TA. By this method, much time will be saved at run-time when an operation request is issued.

0073The TA identifier does not necessarily have to be incorporated inside the PD_UA but can be realized as a sepa- rate computational object, say User_Registration offering registration information service to the PD_UA object. The User_Registration object may contain several TA identifiers since a user may be allowed to use several terminals-— at the same time. The second piece of information required is the identifier of the corresponding TD_UA. Although this information can be deduced from the identity of the user and the identity of the terminal, it is more convenient to store it. The less the processing required at run-time, the shorter response time will be obtained. A User_Registration object containing a information about the TD_UAs and the TAs is shown in Figure 5.

0074The questions now are: how can the PD_UA get the information about the TD_UAs and the TAs and when should the updating be done to ensure consistency with the real movements of the user?

0075Default registration

0076As mentioned earlier, every terminal is associated with a default user who is the owner of the terminal. In this case, the association between the PD_UA and the TA is always defined. We can refer to this as default registration.

0077Let us consider again the example where an object C02 wants to invoke the operation OpX of an object COl belonging the User, domain. Let us suppose now that the User_a is the default user of the Terminal.,. When receiving the operation request Call (COl . OpX ()) , the PD_UA_a call the operation Get_Registration ( ) of the User_Registration_a . The User_Registration_a checks its TD_UA and TA table and return the identifiers of the default TD_UA and the TA, namely TD_UA_a and TA„ to PD_UA_a . The PD_UA_a proceeds to call the operation Call (TD_UA_a. Call (COl . OpX () ) on the TA„, — object. The procedure will continue as described earlier until the whole operation OpX is accomplished. This is depicted in Figure 6.

0078Local registration

0079The local registration is the case where a User_a wants to use a Terminal,, belonging to a User_m. Actually the local registration belongs to the Access Session which purpose is to enable the user to access services.

0080Before continuing, some requirements have to be imposed on the terminal. In order to allow several users to use the same terminal to access the telecom system domain, the terminal must have some more local "intelligence", i.e. more processing and storage capability than the current terminal used in the fixed network. We introduce an object called UI (User_Interface) in the Terminal,, domain. This object is responsible for the supervision and control of interactions between the terminal and the users. UI has the functionality to switch between different users in the case where several users share the same terminal. Upon request, UI can also create new TD_UA object to serve new arriving user.

0081In Figure 7 we show an example with three users. User_b and User, are already registered at Terminal,.. Upon request, UI can switch between these two users, i.e. connecting the input/output channel respectively to TD_UA_b or TD_UA_C . The switch request can be realised in different ways such as entering command New_User() , depressing a key or by using a mouse to click on an icon, etc.

0082When User, arrives and wants to use the terminal he/she can issue a request New User. The UI can create a new and "temporary" TD__UA object. By temporary it is meant that this TD_UA is not yet associated with any user or more specific to any PD_UA which is known and recognisable for the telecom system domain. The association will only be done after the local registration has been accomplished successfully. The UI will also connect the input/output channel to this TD_UA. The User_a can now interact with the temporary TD_UA.

0083Let us now return to our example. The User, wants to register himself at the Terminal,.. After calling the operation New_User() and getting a temporary TD_UA assigned to him, the User_a enter his User Identifier (or name) to the temporary TD_UA (see Figure 8) .

0084From the User Identifier the temporary TD_UA deduces the Computational Interface Identifier (CII) of the corresponding PD__UA_a knowing that the operation LocalRegister () has to be invoked on that PD_UA_a . The temporary TD_UA in- vokes therefore the operation

0085Call (PD_UAa. LocalRegister () ) on the SPAN, to start the convey of the operation LocalRegister ( ) . The invocation is conveyed through the TAP, NAP and arrives at TA.,. The TA_m will then invoke the LocalRegister ( ) operation on the PD_UA_a. The PD_UA_a will start the security procedures. As shown in Figure 9, we assume now that the security procedures are successful . The PD_UA_a will call the operation Set_Registration (TD_UA, TAJ on the User_Reg_a which saves the identifiers of the TD_UA and TA__. The PD_UA_a will answer back to the TD_UA with a status Ok. The TD_UA will save the identifier of PD_UA_a and becomes a permanent agent associated to User_a. The TD_UA will remain alive until an eventual deregistration of the User, for Terminal.,.- Remote registration

0086The remote registration is the case where a User_a wants to run or receive an application on a remote Terminal., where he has not logged in. User, is actually logged in at a

0087Terminal,.. In order to perform a remote registration User. must have a registration application running on Terminal_n. A registration application is a special outgoing application which allows the registration for the delivery of application. The design of the registration application falls, however, beyond the scope of this document.

0088Let us for the time being assume that the Registration application allows User_a to enter the identity of the wanted remote terminal, namely Terminal.,.

0089The Registration application will then invoke Register (Term.,) on the PD_UA_a.

0090The PD_UA_a will invoke the operation Register (User on TA-_, the terminal agent of Terminal.,. Here again, before granting the registration, the TA_m initiates the access control of the user for the use of the terminal to check whether User, is allowed to register himself at Terminal., for example to protect the privacy of the terminal owner. If the User, is not allowed, TA_„ returns a NotAllowed status in the response to PD_UA_a which informs the registration application. The registration application will then deliver a NotAllowed message to the User_a.

0091Let us assume that the access control is successful. The TA_m will invoke the operation Register (PD_UA_a) on its counterpart SPAN. As usual, the operation invocation musfc- of course be conveyed through the NAP and TAP . In Figure 10, we present briefly some operations belonging to the terminal mobility support in italics in order not to burden our explanation.

0092Upon receipt of the call, the SPAN will create a TD_UA_a2 and invoke the operation Associate_with (PD__UA_a) on the TD_UA_a2 causing the saving of the identifier of PD_UAa . The TD_UA_a2 will then persist until a deregistration for Terminal-.. The SPAN will return an Ok status and the identifier of TD_UAa2 to the TA,. which also returns the same information to the PD_UA_a.

0093The PD_UA_a will then call the operation Set_Registration (TD_UA_a2, TAJ on the User_Reg_a which saves the identifiers of TD_UA_a2 and TA_m. The PD_UA_a returns then an Ok status to the Registration application. The Registration application delivers an Ok Status to the User, at Terminal,.. This completes the remote registration. The procedure for remote registration is shown in Figure 11.

0094Local deregistration

0095When the user wants to terminate all his activities at a terminal, he can just log out from this terminal. The deregistration is initiated locally from the same terminal.

0096Let us consider the example shown in Figure 12. A User_a registered at the Terminal,, wants to log out. He termi- nates his access session (see Paragraph 9.4) which results in a termination request to the TD_UA. Before terminating, the TD_UA invokes the operation LocalDeregis- ter() on the PD_UA_a. The operation request is converted — into the operation Call (PD_UA_a. LocalRegister ( ) on the SPAN. The operation is conveyed through the TAP, NAP and arrives at TA_,. The TA„, will then invoke the operation Lo- calDeregister () on the PD_UA_a. The PD_UA_a will invoke the operation Reset_Registration (TD_UA, TAJ of the User_Rega which removes the identifiers of the TD_UA and TA.. from its internal table. The PD_UA_a will return to TD_UA a status Ok. The TD_UA can now terminate itself. The procedure for local deregistration is shown in Figure 13.

0097Remote deregistration

0098To terminate all his activity at a terminal, the user can also initiate the deregistration remotely from another terminal . Let us consider the example where a User, wants to deregister himself at a Terminal_m belonging to a User_m (the default user), from another terminal, namely Terminal-..

0099In order to do such deregistration User, must have access to a deregistration application running on Terminal,, so that he can enter the identity of the remote terminal he wants to deregister from. The deregistration application will then invoke the operation Deregister (TermJ on the PD_UA_a obj ect .

0100The PD_UA_a will call the operation Deregister (User.) on the TD_UA_a2, the terminal domain user agent on Terminal,, assigned to User.. This operation is shown by a dotted arrow in Figure 14. Physically it is converted to an opera- tion on the TA„, and then conveyed via the NAP, TAP, SPAN before arriving at TD_UA_a2. The TD_UA_a2 object will remove the identifier of the PD_UA_a, return an Ok status to the PD_UA_a object and terminate itself. — The PD_UA_a object will call the operation Re- set_Registration (TD_UA_a2, TAJ on the User_Rega object causing the removal of the identifiers of the TD_UA_a2 and TA_. from its internal table. Upon receipt of an Ok status, the PD_UA_a object can return the Ok status to the deregistration application which may forward it to User_a. This completes the remote deregistration. The procedure for deregistration is shown in Figure 15.

0101Supporting multiple terminal registration

0102As stated earlier, a user must be allowed to register at and use several terminals at the same time.

0103To clarify the situation, let us consider the example shown in Figure 16. A User_a is registered at two terminals Terminal, and Terminal., and is associated with two terminal agents TD_UA_al and TD_UA_a2 respectively. He has two computational objects COl and C03 running on Terminal,, and Terminal., respectively. COl is interacting with C02 , C03 with C04 , where C02 and C04 belong to the telecom system domain.

0104Suppose first that COl requests an operation OpX ( ) on

0105C02. The operation request is translated to the operation Call (C02. OpX () ) on TD_UA_al . The TD_UA_al object will then invoke the equivalent operation Call (C02. OpX () ) on its peer-object PD_UA_a in the telecom system domain which in turn delivers the request to C02. This is done without difficulty since TDJUAal knows the identifier of PD_UA_a. Similarly, C03 can invoke any operation on C04 without any problem. The existing mechanism is sufficient to sup-- port invocation initiated form the user domain. The oppo- site that is, object C02 invoking an operation on COl or object C04 on object C03 requires additional information and functionality.

0106Suppose next that C02 requests an operation OpY ( ) on COl. Again, the operation request is translated to the operation Call (COl .OpY () ) on PD_UA_a. The problem now is that PD_UA_a does not know on which terminal COl is located. It is unable to decide whether it should transfer the opera- tion request to TD_UA_al or to TD_UA_a2. The PD_UA_a holds only the information that User, is registered at both terminals Terminal,, and Terminal,, but it has no knowledge concerning the location of each object of User_a. It is therefore obvious that such information must be supplied to it.

0107Every object in the user domain wishing to receive request from object on the telecom system domain must therefore be registered by some object in the telecom system domain so that the PD_UA_a object can interrogate in order to obtain the location of the user object. A convenient place to store this information will be the User_Registration_a object.

0108The user agents TD_UA_al and TD_UA_a2 must be equipped with an additional operation permitting the registration of the user objects in the terminal domain. Such operation, say ObjReg may have the following IDL/ODL specification in which IDL is defined as Interface Definition Language defined in CORBA, and ODL is defined as Object Definition Language defined by TINA-C which is an extension of IDL allowing the definition of object [TIN95h] . ObjReg(in Objectld objectname, in Objectld useragentname) objectname is the Computational Interface Identifier of the registered object.

0109useragentname is the Computational Interface Identifier of the user agent to where the object will be registered. This argument is not necessary when the registration is done locally and may therefore be empty.

0110This registration of the user objects (COl or C03) may be done by the user objects themselves or by some proxy objects .

0111The registration procedure for the object COl is shown in Figure 18. The user agents (TD__UA and PD_UA) act as relays for passing the registration information to the User_registration object.

0112When receiving the operation request Ob- jReg (objectname, TD_UAname) , the TD_UA will call the operation UserObj Reg (objectname, TD_UAname) on its peer- object PD_UA. The PD_UA will in its turn call a similar operation UserObjReg (objectname, TD_UAname) on its User_Registration object. Upon receipt of the request, the User_Registration object shall save the object registration information. Its storage capability must therefore be expanded to include such information. The internal data structure can be implemented in different ways. An example is shown in Figure 17. For each row of the main table, there is assigned an ObjectList containing the identifiers of all object registered at a terminal. To enhance further the functionality, a multiple object registration operation may also be defined for each of the objects TD_UA, PD_UA and User_Registration.

0113An object can also be deregistered. The procedure for object deregistration and the required operations are quite similar to these for registration. Upon receipt of an operation UserObjDereg (objectname, TD_UAname) , the User_Registration fetches the corresponding ObjectList and removes the identifier of the specified object. When a row of the main table is remove, i.e. a user deregistration for a terminal is executed, the corresponding ObjectList will also be deleted.

0114In addition to the object registration and deregistration operations, the User_registration object must also have an operation other objects may use in order to request object registration information:

0115GetObjReg (in Objectld objectname, out Objectld useragentname)

0116When receiving an operation request Call (COl . OpY () ) from C02 addressed to COl, and COl is not in the telecom sys- tem domain, the PD_UA_a invokes GetObjReg (COl , AgentToCall) on its User_Registration object. The identifier of TD_UA_al is returned in the parameter AgentToCall. The PD_UA_a can proceed to invoke Call (COl . OpY () ) on TD_UA_al . The operation is finally conveyed all the way to TD_UA_al which call OpY on COl. The result will be returned using the same path back to C02. MERITS OF THE INVENTION

0117• The present invention provide user mobility in distributed system.

0118• It is quite flexible since it is using agent technology which allows modification of the internal implementation of the agents without affecting the rest of the system.

0119• It can be used in any type of networks, fixed or mobile, public or private, local-area or wide-area, wireline or wireless.

0120• It also allows several users to use same terminal

0121• It also allows one user to use several terminals simultaneously.

0122• It ensures the user privacy by recording and separating objects belonging different users.