Mobile Systems


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Mobile Systems

  1. 1. 7.5 Location Management The remainder of this Chapter is concerned with system aspects: location and roaming management, and handover. The terminals of a mobile network are allowed to receive services at every point in the network where radio signals are received. In principle, this requirement is independent of which operator is offering the service1. The capability to receive services over a wide geographic area is usually called roaming. Roaming consists of two elements: a localisation service allowing the network to know where each mobile terminal is located, and a calling services allowing the mobile user to make and receive calls at different locations independent of which operator owns the access point. Cell Cell Cell Cell Cell Cell Location Location area Cell area Cell Operator Operator Cell Cell Cell Cell Cell Cell Location Location area Cell area Cell System 63 Figure 7.14 Hierarchical composition of mobile network Figure 7.14 shows a hierarchical decomposition of a mobile system. This model was first developed by the ITU in 1980, that is, before GSM was conceived, with the purpose of deriving a general definition the term location within a system. The model is independent of the geographic location but defines the coordinates relative to the infrastructure of the system. The particular coordinate system used in GSM is shown in Figure 7.14. The location of a mobile terminal is determined from four coordinates: system, operator, location area and cell. The uppermost construct is the system. This coordinate is usually implicitly understood. The coordinate represents the whole area in which the mobile terminals can roam. All the GSM networks worldwide constitute a system in this definition. The GSM system is homogeneous because the technology is the same everywhere. Future mobile network with multipurpose terminals capable of accessing GSM, UMTS, WLAN and low-earth orbit satellites (LEOs) constitutes also a system. This system is technologically heterogeneous since the same terminal can access different subsystems designed with different technologies. The next coordinate is the operator. This is related to the ownership of a particular network within the system. Several operators can offer services in the same geographic area. 1 The constraint in GSM that a mobile terminal cannot receive services from a competing operator in the same geographic area is not a technical constraint. The reason is competition and the constraint was introduced in order to maintain customer relationships by forcing them to use the particular network with which they have subscription. The users can roam freely between networks that are not competing in the same geographical area. The restriction may not be applied in new systems because it is counterintuitive to the user.
  2. 2. In the model, these operators have distinctly different system coordinates though their geographic coordinates are equal. Each network (or subnetwork) owned by an operator is divided into location areas. The location areas consist of cells where each cell corresponds to an antenna site (in some cases just a part of an antenna site). The cell is the finest granularity by which the location of a mobile terminal can be defined relative to the system. The purpose of the location area is to allow a coarser granulation of location required for roaming. The reason is trade-off between different types of traffic load on the radio system. This trade-off will be explained below. The location coordinate system is illustrated in Figure 7.15. The coordinates are written as the triplet operator.location_area.cell (ignoring system). Cells with coordinates A.1.1 and A.1.2 belong to location area 1 of operator A; the cell with coordinate A2.2 belongs to location area 2 of operator A; cells with coordinates B.2.1, B.2.2 and B.2.3 belong to location area 2 of operator B. The coordinates of a cell are broadcast to the mobile terminals able to receive radio signals in that cell. The coordinates enable the mobile terminal to determine when it leaves one cell and enters another. For example, the mobile terminal knows that it entered a new cell if the coordinate changes from B.2.1 to A.3.1 or from A.3.1 to A.3.2. Operator A A.3.2 A.1.2 A.1.1 A.2.2 A.3.1 B.1.4 B2.2 B.2.1 B.2.3 Operator B 64 Figure 7.15 Location co-ordinates In the first case, the mobile terminal entered the network of a new operator. In the second case, the mobile terminal moved from one cell to another within the same location area. The convention used in GSM is that if the mobile terminal moves from one location area to another, the change of location must be reported to the network. If the mobile station moves between cells in the same location area, the location change is not reported. This means that the change from cell B.2.1 to cell A.3.1 is reported while the change from cell A.3.1 to cell A.3.2 is not reported. Before explaining how updating takes place, we have to define the architecture of the mobile system. The general architecture of mobile systems is shown in Figure 7.16. The different entities are called by generic names in order to avoid confusion with existing architectures. The GSM architecture contains these elements but with different names. The
  3. 3. architecture proposed for UMTS is much more complex but all the functions explained here are included. The elements of this architecture are: • Subscription database. There is one2 such entity for each operator delivering services to mobile subscribers. The database contains all information required for handling the subscription and the related services. The database also contains the location of the mobile subscriber. There may be operators only offering mobile services without owning any other infrastructure of the mobile network than the subscription database (virtual mobile operators). In GSM, this database is called home location register or HLR. • Local control computer controls service execution and switching in the mobile network. The database contains the identity of all mobile stations in the location areas it controls. It may also contain, as in the GSM network, subscription data replicated from the subscription database (HLR) in order not to request such information for every call event. This is only a design concern where the trade-off is between the cost of local storing and replication of data and the cost of making remote database queries for every call. In the GSM system, the first solution is normally used because it is cheapest; but the latter solution has been implemented in order to support new services that are not generally available. The local control computer is called visitor location register or VLR in the GSM network. • Switch and base station controller acts both as switch between the fixed network and the base stations and as resource manager of the base station infrastructure. In GSM, this function is divided between base station controller (BSC) and mobile-services switching centre (MSC). • Base station offering the radio interface to the mobile terminals. Base station Subscription database Switch/ Cell base station controller Fixed network Local control Local control 65 Figure 7.16 Infrastructure of mobile network 2 The database can be distributed consisting of several interconnected elementary databases. Therefore, we can safely say that there is only one such database per operator.
  4. 4. Note that the entities described above are just computers and databases in a distributed system. They perform a set of functions: information storing, location management, switching and service control, and access and configuration management. There are additional functions not included here such as security management, fault management, recording of data for charging and statistics, and tracing of equipment and users. These databases and computers in mobile systems must offer important characteristics that are not common for databases in administrative systems because of extreme real-time requirements. The subscription database must be able to handle several thousand transactions per second with response time of 10 ms or less for each transaction. The data must also be accessible by several database keys, some of which being random. Figure 7.17 shows how this infrastructure can be used for location updating. The local control database (VLR in GSM) contains the identity of the user, the current location coordinate, and possibly some user related service execution data. The identity of the subscription database (HLR in GSM) is derived from the identity of the user. The subscription database contains the current location coordinate of the mobile station (coordinate B.2) with granularity corresponding to location area. Subscription database Identity, B.2, all service data New Identity, A.3, all service data location <identity, A.3.1> Update <identity,A.3> Cancel Data location <identity> Local Terminal control registration A.3 Local A.3.1 <identity,A.3.1> control Identity, A.3, some service data B.2 New entry B.2.1 Identity, B.2, some service data 66 Figure 7.17 Location updating
  5. 5. Base station Subscription database Establish call <identity, Provide info called number> <identity, other para> Cell Fixed Call request Switch network <identity, call called no> Local control 67 Figure 7.18 Call from a mobile terminal In the figure, the mobile terminal detects that it accesses a new base station because the coordinate information contained in the coordination message received from the base station changes from B.2.1 to A.3.1. The mobile terminal then sends a message containing the coordinate of the new cell and its own identity3 to the local control computer. The local control database stores the new registration and sends the new coordinate (A.3) of the mobile terminal to the subscription database where it replaces the previously stored coordinate (B.2). If the new registration is accepted, the subscription database provides the new local control computer with service execution data for this mobile user. The previous local control computer is requested to remove the stored location entry. The location coordinate of the mobile user is then stored in two places: in the control database of the current location area (VLR in GSM), and in the subscription database of the user (HLR). Figure 7.18 shows how calls originating in the mobile terminal are handled. The call processing is simple and does not deviate significantly from that of calls in the fixed network. The local control computer receives the call request from the mobile terminal and processes of the call before it is routed into the fixed network. The call processing includes execution of call restriction such as not allowing calls to certain numbers, nondisclosure of the identity of the mobile user, and restricted access to the network, for example to a competing network in the same geographic area. Remember that the subscription database sent the call processing information to the local control computer during location updating. 3 Note that the public identity may not be sent but an encrypted version of it or a temporary identity as in GSM. This will prevent that anyone can trace the caller by intercepting to the radio communication.
  6. 6. Base station Subscription database Provide info Provide Cell routing information <number> Fixed network Call control Local control 68 Figure 7.19 Call to a mobile terminal Figure 7.19 shows a call originating in the fixed network. The network exchange (in the fixed network or in a mobile network) recognising that this is a call to a mobile user requests the subscription database to provide routing information (the message provide routing information <number>). The routing information must be such that the network can route the call to the local control computer in charge of the current location of the user. This capability is constrained by the numbering used in different networks. In the telephone network, the routing address must be a telephone number; in the Internet the routing address must be an IP number. The coordinate indicating the location is meaningless as a routing address in these or other networks. The problem was solved in the GSM system in the following way. When the request arrives at the subscription database (HLR), the database uses the stored location coordinate to identify which local control computer (VLR) is in charge of the location area. This computer is then requested to allocate a routing number to the call (the message provide info). The routing number is called roaming number in GSM. Having received the roaming number, the subscription database forwards the roaming number the exchange requesting the routing information. The number is then used by this exchange to forward the call to the destination. In GSM, this roaming number exists until the call has been established (duration of the order of a second). If the call is not established for some reason, the allocation of the roaming number is removed after about one minute. Roaming numbers can then be reused frequently so that only a small number of them need to be allocated to each VLR. A similar method is used for allocation of temporary IP addresses (care-of address) in the mobile IP system. The resemblance with the GSM is striking, except that the care-of address is allocated for the time the terminal is in its new location. The reason for quasi- permanent allocation of care-of addresses is that the IP service is connectionless requiring that the full address of the termination is included in each datagram. If a new address had to be allocated for each dagram by the remote server, this would have resulted in a very inefficient communication system requiring much time and much network resources for delivering one datagram. In stead, the remote server allocates the care-of address for the time the terminal is registered in the remote server and announces this address to the home server of the terminal.
  7. 7. Datagrams are then sent to the home server. The home server inserts the complete datagram in the information field a new IP datagram containing the care-of address as routing address. The remote server then retrieves the original datagram and delivers it to the terminal. This procedure is called tunnelling. Quasi-permanent allocation of the routing address was not done in the GSM system for particularly two reasons. First, two telephone numbers had then to be allocated at all time to each mobile user, that is, one invariant number identifying the user and the subscription, and one roaming number identifying the location of the terminal. Second, the restart of a VLR computer could easily cause numbering confusion since a roaming numbers has to be allocated whenever the terminal enters a new area, and removed when the terminal leaves this area. Previously used roaming numbers must be reallocated to new terminals entering the area in order not to waste too many numbers. This represents a particular problem in GSM because the time a terminal spends within a location area can be very short, requiring frequent location updating. The relationship between roaming number and the real identity of a terminal is therefore a very dynamic one. If a failure of one of the databases occurs, the database is restarted automatically with location data read from a data dump to a backup storage device at previous instant. The data dump takes place at regular intervals, e.g., every 15 or 30 minutes. Because of the dynamics of the mobile units, the location information may have changed considerably from one data dump to the next and the probability that calls are routed to the wrong user becomes significant. The solution developed for call handling in the GSM system is similar to that of the intelligent network (IN) developed by Bellcore during the same period. The development of GSM and IN were independent but the motives were the same, that is, providing efficient routing for calls with nongeographic addresses, and remote processing of switching and services. 7.6 Handover Handover4 is also an aspect of roaming. In small-cell systems, the likelihood that a mobile terminal is leaving one cell and entering another during conversation is high. If it is not possible to hand over the call from the first cell to the next, the call will be interrupted. Handover is, as the terminology suggests, the mechanism that ensures that calls are handed over from one cell to another in order to ensure continuity of the conversation. Handover was developed already for the first-generation systems in order to avoid this problem. It is an essential procedure in all land mobile systems. It is also an essential procedure for satellite systems using low-earth orbit (LEO) satellites since the time each such satellite is visible above the horizon is less than ten minutes. The components of the system (mobile terminal, base station and so on) must perform a number of tasks in order to perform the handover: • Establishment of the handover criterion by monitoring the quality of the signals received from different base stations, that is, determination of when and to which new cell handover should take place. • Rearrange the access and network resources in order to establish the new connectivity. • Perform fast switchover in order to avoid that the connection is interrupted or prematurely terminated. 4 Usually called handoff in the USA.
  8. 8. A.3.1 Measures quality Communication link B.2.2 B.2.1 69 Figure 7.20 Determination of handover criterion The mechanisms required for implementing handover are different in different systems. In NMT, the base station determines when handover shall take place; in GSM, the mobile terminal detects when handover is required. The criterion used in land mobile systems can be based on measuring the received signal level, the signal-to-noise ratio, the bit error rate and the distance from the base station5. The handover criterion used in LEO systems can be the combination of high bit error rate and low elevation angle of the satellite. Handover then takes place when the elevation angle is below a certain lower limit or if the received signal power drops below a certain level. Figure 7.20 shows the handover method used in GSM. The mobile terminal determines both when handover shall take place and to which base station the call shall be handed over. In the figure, the terminal is located in area where it receives signals from cells B.2.1, B2.2 and A.3.1, where B.2.1 etc are the cell coordinates defined above. The mobile terminal undertakes comparative measurements (power level and bit error rate) of the signals received from the different base stations and determines the preferred handover candidate from these measurements. Preferably, the handover should be between cells in the same location area, that is, from B2.1 to B.2.2. If this is not possible, the mobile terminal chooses a base station in another location area (A.3.1). 5 A TDMA system such as GSM must be synchronised. The way this is done in GSM establishes limits to how far from the base station a mobile terminal can synchronise to the frame pattern. This limit is about 35 km in GSM. For this reason, handover must take place even if the received signal level is well above the receiver threshold.
  9. 9. Subscription database Local control Cell Allocate A.3.1 resources Local B.2.1 control Signalling link 70 Figure 7.21 Just before handover Since GSM is a TDMA system, the mobile terminal can use the time between frames to perform measurements on adjacent cells. Each base station also broadcast the cell coordinates and the channel arrangement of adjacent base stations in order to facilitate the measurements required for taking the proper handover decision. The next step is then to perform the handover. Figures 7.21 and 7.22 show the case where handover takes place between base stations in different location areas. Handover may also take place between base stations in the same area. In both cases, the procedure is the same on the radio path but the procedures in the network are different. The bold full lines show the speech connection6; the bold dotted lines indicate control channels (or signalling channels). Suppose that the mobile terminal has found that handover from cell B.2.1 to cell A.3.1 is required. The mobile terminal then requests the local control computer of cell B.2.1 to initiate the handover. The local control computer of cell B.2.1 requests cell A.3.1 to allocate a new channel for the call and prepare it for the following handover activity. When this is done, the local control computer establishes a connection between the exchanges of cells B.2.1 and A.3.1 and reserves it for the call to be handed over. The connection is extended all the way to the radio interface in cell A.3.1. The connection is established in the usual way using resources in the fixed network. When all this has been completed, the base station in cell B.2.1 signals to the mobile terminal that the call can be handed over to the radio channel assigned for it in cell A.3.1. As soon as the call is established in cell A.3.1, the radio connection in cell B.2.1 is released. The resulting connection is shown in Figure 7.22. The new speech path is complex. However, the major points are: . 6 A data connection will have the same configuration.
  10. 10. Subscription database Local control Cell A.3.1 Signalling link Local B.2.1 control 70 Figure 7.22 After handover • The call control is retained in the control computer of the original cell (B.2.1). This means that service information and call states are not transported from one computer to another during the conversation. That process would have been tremendously complex and difficult to develop further for new versions of the system. Subsequent signalling between the mobile terminal and the network is between the mobile terminal and the control computer of cell B.2.1. The exchange of cell A.3.1 is only a passive relay remotely connecting base station A.3.1 to the control computer of cell B.2.1. • Not more than two mobile-services exchanges will be involved in the call. If the mobile terminal moves on to a cell connected to yet another mobile-services exchange (say, A.4.5), a connection will be established between the original exchange and the new exchange in the same way as just described. The connection to the exchange of A.3.1 is released because this exchange is no longer needed in the connection. If the mobile terminal moves back to the original location area, the handover will again result in the configuration as shown in Figure 7.21. If handover takes place within one location area, the exchange simply switches the call from the outlet to one base station to the outlet to another base station. The handover procedure just described was developed by the ITU in 1980 in the first effort to specify an international land mobile system. The principle is implemented in the GSM system and the UMTS system. 7.7 General Mobility We have still not answered the essential question: what is mobility? Radio-mobile systems as described above offer terminal mobility. GSM and UMTS also offers continuous mobility since the location updating procedure and the handover procedure together offer access to and continuity of services everywhere. Mobility may also be discrete. One obvious example is the use of payphones or telephones owned by third parties, that is, to move from one apparatus to another. The
  11. 11. problem with this type of mobility is that the user does not get access to his or her services. This capability is, however, not what we understand by mobility. Another example is to move a PC from one ISDN plug to another. If the user is a member of a local area network, he or she may gain access to services by calling the destination number of the local area network. This type of mobility is called discrete mobility because it is only available at certain locations. The user may not move the PC at all but employ any suitable PC in the location where he or she is, dial up the local area network and log in remotely as user. This is called personal mobility. Management of home offices is an example of discrete and personal mobility. Discrete mobility means in this case that the same PC can be connected both at work and at home; personal mobility means that the person may move between different PCs, one at home and one at work. It is often a problem to offer discrete and personal mobility because of protection systems such as firewalls. This is particularly difficult if mobility takes place across firewalls since one screening mechanism is to restrict access depending on the IP number of origin. The user may also move active applications or sessions from one computer to another. Sessions may be suspended at one place and resumed at another. This is called application mobility or session mobility. User User profile Owns Application Application parameters Runs on Terminal Terminal capabilities Connected to Network access point Network characteristics Interfaces that may be mobile 72 Figure 7.23 Relationships across mobile interfaces Note that terminal mobility, personal mobility, application mobility and session mobility may be continuous or discrete. The user referred to in these concepts may be a physical device, a software program or a human being. Figure 7.23 shows the relationships that may exist between the different entities. The philosophy is as follows. The user (in this case a person) “owns” the application, which can be e-mail, voice mail, file management, database management and executable programs. The user has a user profile stored in a database somewhere. The user profile can be simple or complex depending on the system and services required. Typical elements of a user profile can be private address directories, service options, access control information related to these services and to management information, charging and usage conditions, and security management. In GSM, the profile is stored in the home location register (HLR). In local area networks the information may partly be stored in the user’s PCs and in servers such as e-mail server, firewall, directory server and file server.
  12. 12. The application (e.g., Microsoft Office, Internet, e-mail) is run in accordance with parameters stored in a server in a local area network or in the local PC. Every application is run on a terminal, for example a PC, offering a set of capabilities. Some of the terminal capabilities are stored in the PC; other capabilities are stored in the server to which the PC is connected. Finally, the PC is connected to a network access point (the socket in the wall). The access point has some characteristics that must be stored somewhere. Such characteristics include physical address, data rate and protocol. In principle, the interfaces shown in the figure can be mobile interfaces of some kind. The user may move between PCs. The PC may be connected at different access points at different times. The network access point can be an access point in a land mobile system. Land mobile systems with this degree of flexibility are UMTS and WLANs. The full exploitation of mobility requires dedicated software that takes care of the different types of mobility independently of the applications running in the system. One way of achieving this is to develop a middleware offering a mobility independent interface toward the applications and a machine dependent interface toward the operating system of the computer. Still such middleware does not exist but it is required in order to support peer-to-peer communication and communication between sensors, control systems and actuators.