Network Procedures
6Network Procedures6.1 IntroductionSatellite personal communication networks (S-PCN) have two main objectives [ANA-94]:†To extend existing and possibly future services provided by public networks to mobileusers.†To complement terrestrial mobile networks by providing analogous services in areaswhere satellite technology is more effective and economic.With the rapid development in Internet protocol (IP) technologies in recent years, the firstobjective requires satellite systems to interwork with both public circuit-switched networks,such as the PSTN and ISDN, and packet-switched networks such as the Internet, and toincorporate intelligent network capabilities to support mobility.The second goal sets the objective of providing a truly universal personal communicationsystem to mobile users. This can be achieved by the provision of dual-mode user equipmentwhich communicates with both the satellite and terrestrial mobile networks so that whenusers roam outside of the terrestrial coverage, their requested services can still be supportedvia the satellite segment.The concept of mobility support is central to the development of personal communicationsystems. Mobility encompasses personal mobility and terminal mobility. Personal mobility issupported by associating a logical address with a user, instead of with a physical terminal[ANA-94]. In this way, a user can access services through different networks irrespective oftheir geographical location and terminal type (fixed, portable or mobile), i.e. users can usetheir services independent of the access network technology. Terminal mobility refers to themobile terminal’s capability of accessing the network from different geographical locationsor physical access points within the service coverage, i.e. a logical address is associated witha mobile terminal. To achieve a truly universal personal communication system, both perso-nal mobility and terminal mobility, with wireless access, have to be supported.For S-PCN, the main network functions that need to be implemented are envisaged tobe the same as those in terrestrial mobile networks, although the procedures may beslightly different in order to take into account the satellite system characteristics. Suchfunctions include call handling, mobility management, resource management, security andnetwork management. Second-generation terrestrial mobile networks have now reachedmaturity, the development of current and future generation mobile systems, including bothMobile Satellite Communication Networks. Ray E. Sheriff and Y. Fun HuCopyright q 2001 John Wiley & Sons LtdISBNs: 0-471-72047-X (Hardback); 0-470-845562 (Electronic) satellite and terrestrial need to be compatible with such networks. In Europe, it is envi-saged that the underlying network of future generation mobile systems will be based onthat of the GSM/GPRS, and therefore the network of satellite systems need to be designedon a par with the GSM/GPRS network protocols so that interworking between the twosegments (terrestrial and satellite) can be more efficient and less complicated. Bearing thisin mind, network protocols and functions for call handling, mobility management andresource management in this chapter are studied in parallel with the GSM network. Theextension and modifications of such network functions required for a GSM compatible S-PCN are discussed.6.2 Signalling Protocols6.2.1 Overview of GSM Signalling Protocol ArchitectureGSM communication protocols are structured on the ISO/OSI reference model (ITU-TRecommendations X.200–X.219) with other protocol functions specific to cellular radionetworks being developed. The layer 1 to layer 3 GSM signalling protocol architectureand its distribution among the network nodes is shown in Figure 6.1. Interfaces Um andAbis are GSM specific while interfaces A, B, C, and E are based on common channelsignalling systems No. 7 (SSN7) reported in ITU-T Series Q.700–795. The Um-interfaceis defined between an MS and a BTS, whereas the Abis-interface is located between a BTSand a BSC. Signalling exchange between a BSC and an MSC is through the A-interface.Communication between an MSC and the VLR, HLR and other MSCs are via the B-, C- andE-interfaces, respectively.Mobile Satellite Communication Networks198Figure 6.1 GSM signalling protocols and distribution among network elements. In layer 2 of the MS and BTS protocol stacks, a modified link access protocol on theD channel (LAPDm) is used. LAPDmis a modified version of the ISDN LAPD protocolspecifically for use in mobile applications. It protects the data transfer between the MSand BTS over the radio interface.A mobile application part (MAP) is specifically developed in order to accommodate radiosignalling in GSM networks. It is implemented in all the switching centres directly linked tothe mobile network. MAP groups a number of protocols which are able to support mobilitycontrol functions and is specified in GSM Recommendation 09.02 [ETS-94]. It consists ofseveral application service elements (ASE) necessary for registration transaction and data-base inquiry and for the determination of a mobile station’s current location.Of particular interest to this chapter is the functions defined in the radio resource (RR)management, mobility management (MM) and the connection management (CM) layers.These three layers are sub-layers to the network layers or layer 3 of the ISO/OSI referencemodel.The RR layer handles the administration of frequencies and channels. It is responsible forthe set-up, maintenance and termination of dedicated RR connections, which are used forpoint-to-point communication between the MS and the network. It also includes cell selectionwhen the mobile station is in idle mode (the term idle mode refers to the state when the mobileis switched-on but is not in the process of a call) and in handover procedures. It also performsmonitoring on the broadcast control channel (BCCH) and common control channel (CCCH)on the downlink when there is no active RR connection.The MM layer is responsible for all functions that support mobility of the mobile terminal.It includes registration, location update, authentication and allocation of new temporarymobile subscriber identity (TMSI).The CM layer is responsible for the set-up, maintenance and termination of circuit-switched calls. It provides the transport layer with a point-to-point connection betweentwo physical subsystems.6.2.2 S-PCN Interfaces and Signalling Protocol ArchitectureIn analogy, the GMR specifications (ETSI GEO-Mobile Radio Interface Specifications),which describe the requirements and network architecture for a geostationary satellite systemto interwork with GSM, use the GSM protocols as the baseline protocols for carrying out thesatellite network control functions. ETSI has produced a series of recommendations for twoGMR configurations, known as GMR-1 and GMR-2, respectively. Figure 6.2 shows thenetwork interfaces defined in the GMR network architecture [ETS-99a, ETS-99b]. A briefdescription on the interfaces follows:†S-Um-interface – similar to the Um-interface as defined in GSM protocols, this is used forsignalling between a Gateway Transceiver System (GTS) and an MS.†A-interface – this is the interface between the Gateway Subsystem (GWS) and the Gate-way Mobile Switching Centre (GMSC), although strictly speaking, it lies between theGateway Station Control (GSC) and the GMSC. This interface is used to carry informationon GWS management, call handling and mobility management.†Abis-interface – this is an internal GWS interface linking the GTS part to the GSC part.Network Procedures 199 This interface is used to support the services offered to the GMR users and subscribers. Italso allows the control of radio equipment and radio frequency allocation in the GTS.†B-interface – this interface uses the MAP/B protocol allowing the GMSC to retrieve orupdate local data stored in the VLR. When an MS initiates a location updating procedurewith an GMSC, the GMSC informs its VLR which stores the relevant information. Thisprocedure occurs whenever a location update is required.†C-interface – this interface uses the MAP/C protocol allowing the GMSC to interrogatethe appropriate HLR in order to obtain MS location information. Additionally, the GMSCmay optionally forward billing information to the HLR after call clearing.†D-interface – this interface uses the MAP/D protocol to support the exchange of databetween an HLR and VLR of the same GMSC. It also supports the MAP/I protocol for themanagement of supplementary services.†E-interface – this interface uses the MAP/E protocol to support the exchange of messagesbetween the relay GMSC and the anchor GMSC during an inter-GMSC handover.†F-interface – this is the interface between the GMSC and the AuC/EIR. It uses the MAP/Fprotocol for user authentication.†G-interface – this interface uses the MAP/G protocol between VLRs of different GMSCsin order to transfer subscriber data.†H-interface – this is the interface between the HLR and the AuC. When an HLR receives arequest for authentication and ciphering data for a mobile subscriber and if the datarequested is not held at the HLR, it will send a request to the AuC to obtain the data.The GMR signalling protocol architecture is also similar to that of the GSM. Althoughmost of the functionalities in each layer of the signalling protocol stack remain the same,some modifications have been made in order to accommodate satellite specific functional-Mobile Satellite Communication Networks200Figure 6.2 Functional interfaces of a GMR system. ities. In particular, additional functions need to be included in the physical layer (layer 1), thedata link layer (layer 2) and the network layer (layer 3) of the MS and the GTS to take intoaccount the satellite channel characteristics and the satellite network architecture. Specifi-cally, the location information of both satellites and MSs need to be measured and reported.Additional logical channels have been specified to take into account such characteristics, asdescribed in Table 6.1.6.3 Mobility Management6.3.1 Satellite Cells and Satellite Location AreasMobility management consists of two components: location management and handovermanagement. Mobility management strategies have been extensively researched in the pastin both land mobile and mobile-satellite networks, some of which are studied in conjunctionwith resource management and call control strategies [BAD-92, DEL-97, EFT-98a, EFT-98b,HON-86, HU-95a, HU-95b, JAI-95, MAR-97, SHI-95, WAN-93, WER-95, WER-97]. Afundamental part in deriving mobility management strategies is the definition of a satellitecell and a satellite location area. In cellular land mobile networks, such as the GSM, a cell isdefined as the locus where a broadcast channel transmitted by a BTS is received with signalquality at or above a pre-defined threshold level [DEL-97]. With this definition, a cell issuitable for a given MT if the relevant broadcast signal received by the MT is at or above thispre-defined threshold quality. In essence, in land mobile networks, a cell is characterised bythe presence of a broadcast channel, the basic functions of which are as follows [DEL-97].†Broadcast of data concerning the cell organisation, such as the cell identifier, the locationarea identifier for a given cell, the service type in a given cell, the MT identifier andresource organisation.Network Procedures 201Table 6.1 Additional logical channels in the physical layerGroup Channel DescriptionsDCCH Terminal-to-terminal associatedcontrol channel (TACCH)Dedicated for terminal-to-terminal call set-up. Itmay be shared among several such callsBCCH GPS broadcast control channel(GBCCH)Broadcast of GPS time and satellite ephemersisinformation in the forward linkHigh-margin synchronisationchannel (S-HMSCH)Provides time and frequency synchronisationand spot-beam identification in the forward linkHigh-margin broadcast controlchannel (S-HMBCCH)Contains information for an MS to register inthe system. This channel provides an alternativeto the MS when the S-BCCH cannot be detectedCCCH High-power alerting channel(S-HPACH)A special paging channel to provide highpenetration alerting when an MS cannot bereached through normal pagingBACH Reserved for alerting messages †Broadcast of paging messages, for example in mobile terminated call set-ups.†Provision of a reference signal for the MT to carry out measurements in cell re-selectionand handover procedures.More specifically, a one-to-one correspondence among cells, broadcast channels and BTSsexists. In other words, in each cell there exists a single BTS which transmits on a singlebroadcast channel.However, in a multi-FES, multi-spot-beam satellite system, the definition of a satellite cellis more complicated. There is no one-to-one correspondence among satellite cells, satellitespot-beams and FESs [DEL-96]. More than one FES can access a given satellite spot-beam.In [DEL-97], a satellite cell is identified in association with both FES i and spot-beam j at agiven time t, whereby FES i transmits on a broadcast channel toward spot-beam j at time t.Ina full FES-to-spot connection, each FES can be connected to any spot-beam. In this case, asatellite spot-beam can be considered as the overlap of N satellite cells, N being the totalnumber of FES. Each of these N satellite cells has the same coverage, the spot-beam cover-age, served by a different FES [DEL-94a].With the above satellite cell definition and denoting AFESpot(i, t), AFESact(i, t), AFEStar(i, t)asthe potential coverage area, the actual coverage area and the target coverage area of FES i attime t, respectively, it follows that [DEL-97]:AFESpotði; tÞ¼ <j¼Nj¼1Asatðj; tÞCði; j; tÞð6:1ÞAFESactði; tÞ¼ <j¼Nj¼1Asatðj; tÞBði; j; tÞð6:2ÞAFEStarði; tÞ # Asatðj; tÞBði; j; tÞð6:3ÞAFESpotði; tÞ $<j¼Nj¼1AFEStarðj; tÞð6:4Þwhere Asat(j, t) is the area covered by spot-beam j at time t; C(i, j, t) is a binary function equalto 0 when FES i cannot be connected to spot-beam j at time t and equal to 1 otherwise; andB(i, j, t) a binary function equal to 1 when FES i is actually connected to spot-beam j at time tand equal to 0 otherwise.6.3.2 Location Management6.3.2.1 OperationsLocation management is concerned with network functions that allow mobile stations toroam freely within the network coverage area. It is a two-stage process that allows thenetwork to locate the current point of attachment (in public land mobile network (PLMN)terms, the point of attachment refers to the base station) of the mobile for call delivery [AKY-98]. The first stage is location registration or location update, while the second stage is the calldelivery as shown in Figure 6.3 [AKY-98].In the location registration stage, the mobile station periodically notifies the network of itsnew point of attachment, allowing the network to authenticate and to update the user’sMobile Satellite Communication Networks202 location profile. In the call delivery stage, the network queries the user’s location profile andlocates the current position of the mobile terminal by sending polling signals to all candidateaccess ports through which an MS can be reached.An important issue for designing a location management strategy is the amount of signal-ling load and delay involved in both location registration/update and call delivery processes.The definition of satellite location areas, in conjunction with the definition of a satellite cell,has a direct impact on the signalling load.6.3.2.2 Location Update and Terminal Paging StrategiesLocation registration is a location monitoring procedure which is invoked automatically bythe mobile terminal. This procedure ensures that the network can identify the location of amobile terminal as accurately as possible. Traditionally, location areas are used for identify-ing the MT’s position. A location area (LA) is the smallest area unit that can be used to locatethe MT when it is in idle mode.In cellular land mobile networks, an LA is usually defined as the area wherein an MT canroam without having to perform a location update. Such an area is normally bounded by acluster of cells. A possible parameter used to trigger a location update could be the signalstrength quality received by the MT from the base station. The current location of the MT isconstantly compared with the location information broadcast by the surrounding basestations. The mobile terminal continuously monitors the signal quality of the broadcastcontrol channel in order to decide whether it is out of reach of the current LA. When theMT decides to access a new location area, a location update is required.Similar location update procedures need to be implemented in mobile-satellite networks.For non-geostationary satellite systems, the satellite network has to cope with both thesatellites’ motion and that of the terminal. This has an impact on the location registration/update and handover (which will be discussed in a later section) procedures. If inter-satellitelinks (ISL) are deployed, it adds further dimensions to the definition of location areas andmobility management in satellite networks. In the sections that follow, it is assumed that thereNetwork Procedures 203Figure 6.3 Location management operations. is no ISL. There are two basic approaches [CEC-96] for defining the location of the MT inrelation to a location area:†The guaranteed coverage area (GCA) approach [CEC-96].†The terminal position (TP) approach [CEC-96].GCA ApproachIn the GCA based approach, an LA is the static area surrounding an FES where for aguaranteed 100% of the time, the FES can reach an MT with an elevation angle at bothsides of the link (FES-satellite and MT-satellite) above a pre-defined threshold value. Thisapproach uses the satellite cell definition discussed in the previous section. With thisapproach, AFEStar(i, t) becomes independent of t such that:AFEStarðiÞ¼AFEStarði; tÞ ;t ð6:5Þand that AFEStar(i) contains all the points on the Earth’s surface such thatAFEStarðiÞ¼{Pj;tP[ AFESpotði; tÞ} ð6:6ÞWith this approach, the location information to be stored in the network database is that ofthe FES, with which the MT is registered. Since this LA definition guarantees that the FESwill be able to contact the MT at all times, the paging of the MT through the registered FESwill be guaranteed to be successful. In this way, an MT terminated call can be directly routedto the registered FES. This approach is quite similar to the approach adopted in terrestrialmobile networks. The GCA based approach is shown in Figure 6.4.The GCA approach requires a continuous service area coverage, resulting in a largenumber of FESs. In order to reduce the number of FESs, and consequently the number ofGCAs, a possible scenario is to provide GCA in regions where the traffic density is high andto provide intermediate coverage area (ICA) in between GCAs [CEC-96]. This scenario istermed the partial GCA, as shown in Figure 6.5. With partial-GCA, the system has toimplement extra functionalities in order to page the MT. The traffic signalling associatedwith tracking MTs outside a GCA is greater than that inside a GCA. However, the trafficdensities on the ICAs are much lower than those in the GCAs, thus the extra signalling shouldMobile Satellite Communication Networks204Figure 6.4 Location area under guaranteed coverage area based approach. not have a great impact on the system performance. The GCA approach can be terminalposition based or BCCH based.GCA-Terminal Position (GCA-TP) Based Approach In this approach, a location updateis triggered by the position of the MT, i.e. when the MT roams outside of a specific GCA, alocation update is performed. The location information may be computed by the MT or by theFES, depending on the intelligence of the MT and the distribution of functionality among thenetwork elements. If the FES is in charge of the location computation of the MT, any locationupdate decision will be made by the FES.The MT requests its position from the network at regular intervals. A suitable FES will beselected among a group of FESs on the basis of the signal strength of their broadcast signalreceived by the MT. This group of FESs includes the serving FES and all the other adjacentFESs. When the serving FES detects that an MT has reached the border of its controlling area,it will inform the MT of its new FES identification, which will take over responsibility forcommunicating with the MT. The MT then contacts the new FES, which updates its databasesaccordingly. Once this location update procedure is completed, the MT will communicatewith the new FES to enable subsequent position identification and paging. The connectionbetween the MT and the old FES will then be released. However, if the MT is unable tocontact the new FES due to network congestion, the databases of the new FES cannot beupdated. In this case, the connection between the MT and old FES will be maintained until theMT can contact the new FES.Since the FES measures the MT position on a regular basis, this information is prefer-ably stored in the FES and can be re-used to optimise the paging procedure. The FESidentifier must be stored in the network database. Since the FES has the knowledge of thesatellite ephemerides, it will be able to map the position of the MT onto the ,satellite,spot-beam. co-ordinates pair. This mapping can be performed by the FES during thepaging process.Network Procedures 205Figure 6.5 Partial GCA. The main disadvantage of this approach is that it requires periodic position calculationsindependent of the MT’s movement. This will have an impact on the signalling load. The timeinterval between two successive position requests from the MT has to be traded-off betweenthe frequency of location updates and paging efficiency. The more frequent the positionrequest, the more accurate the position information, which in turn increases the pagingefficiency. However, it also implies a heavier signalling load.GCA-BCCH Based Approach This approach is based on the monitoring of the BCCHchannel broadcast by each spot-beam. The FES identifier is broadcast only over its GCAthrough the BCCH channels of the spot-beams that cover the GCA. It is possible that morethan one satellite will cover a GCA. The BCCH channels containing the same FES identifierare bounded within the FES GCA. As result, a terminal within the GCA will always receivethe same FES identifier.When an MT is switched on, it selects the best BCCH channel and decodes the FESidentifier associated with that channel. Registration with the FES then occurs. Locationupdate occurs when the MT receives a BCCH with a new identifier with better quality. Inthis case, the MT triggers the location update procedure. This should only happen when theMT is approaching the border of the GCA. Normally, there are overlapping areas betweentwo adjacent GCAs. When the MT is within these overlapping areas, the MT detects morethan one FES identifier. When the BCCH with a new FES identifier is received with betterquality than the one with the old FES identifier, the MT triggers a location update request withthe new FES. The new FES then updates the network databases accordingly. This involvesthe updating of the HLR associated with the MT and the deletion of the registration informa-tion in the old FES. Once this is completed, the new FES acknowledges the MT and the MTwill start to listen to the BCCH of the new FES.Spot-beams covering more than one GCA are required to broadcast all the correspondingFES identifiers. In this situation, more than one FES controls the spot-beams.In order to optimise the paging procedure, two techniques can be used to restrict the areawhere the MT is located.1. Terminal position: in this technique, the MT’s position will be measured on a regular basiseither by the FES or by the MT which then reports to the FES of its most up-to-dateposition. The area surrounding the latest measured MT position will be paged first. If theMT does not respond, a wider area is paged.2. Spot-beam area: the FES stores the spot-beam footprint area through which the MT madeits last contact. Spot-beams that overlap that particular area are paged first.The main disadvantage of the GCA-BCCH approach is that the borders of a GCA aredifficult to be precisely defined by an FES. This may lead to frequent location updatesbetween two adjacent GCAs.Partial GCA-TP Based Approach This should be used in conjunction with the GCAterminal position based approach. When the MT roams inside a GCA, the GCA-TPapproach is used. When the MT roams outside of the GCA, the last FES associated withthe MT will continue to measure the MT’s position. After each measurement, the FES decidesand informs the MT of the best FES for it to be associated with. Location updates then occurwith the new FES. The FES decision should also take into account the satellite ephemerides.Partial GCA-BCCH Based Approach In this approach, the FES identifier is broadcastbeyond its GCA, but only to those areas that are not covered by any GCAs. When an MTMobile Satellite Communication Networks206 [...]... concentrated on the most heavily used spot-beam. 6.4.5 Network Operations and Procedures 6.4.5.1 Overview In order to play a complementary role to terrestrial networks, the design of the network operations and procedures in satellite networks will, in as far as possible, re-use those of the terrestrial networks, to facilitate integration. Several network procedures need to be consid- ered in order to establish... traffic channels in one cell to be used in another at any given Network Procedures 229 6 Network Procedures 6.1 Introduction Satellite personal communication networks (S-PCN) have two main objectives [ANA-94]: † To extend existing and possibly future services provided by public networks to mobile users. † To complement terrestrial mobile networks by providing analogous services in areas where satellite... terminal can be updated by the network according to the mobility behaviour of the MT. Network Procedures 207 Figure 6.6 Location area for terminal position approach. satellite and terrestrial need to be compatible with such networks. In Europe, it is envi- saged that the underlying network of future generation mobile systems will be based on that of the GSM/GPRS, and therefore the network of satellite systems... rights to the selected network. Thus, the access control function will first select a network according to the access mode selected by the user. Given the selected access mode, it will then select the user’s preferred network, check- ing whether the selected network is reachable by the terminal and whether the user has access rights to the selected network. Once the access to the network is verified, the... the main network functions that need to be implemented are envisaged to be the same as those in terrestrial mobile networks, although the procedures may be slightly different in order to take into account the satellite system characteristics. Such functions include call handling, mobility management, resource management, security and network management. Second-generation terrestrial mobile networks... agreement between a terminal and a network; (2) the retrieval of the TMTI when the access agreement exists. When these two procedures are completed, communication between the network and the terminal can start. Depending on the complexity of the terminal, it can have simultaneous access to multiple networks. For example, in an integrated satellite and terrestrial network environment, a dual-mode terminal... terrestrial network at the same time. This creates the virtual terminal concept in which a single physical terminal can provide users with multiple virtual terminals, each virtual terminal accesses one particular network environment. Before the creation of an access agreement between a terminal and a network, several information and verification procedures are required. The user has to select a network. .. past. The following summarises a few techniques, which have been reported in Ref. [AKY-98], for minimising the signalling delay and traffic in PLMN networks. More detailed explanation can be obtained from the referenced papers. Network Procedures 209 Network Procedures 213 Figure 6.8 Database partitioning. Figure 6.9 Database hierarchy. † Combined diversity implies that communication is carried out through... Akyildiz, ‘‘ Local Anchor Scheme for Reducing Signalling Cost in Personal Commu- nication Networks’’ , IEEE/ACM Transactions on Networking, 4(5), October 1996; 709–726. [HO-97] J.S.M. Ho, I.F. Akyildiz, ‘‘ Dynamic Hierarchical Database Architecture for Location Management in PCS Networks’’ , IEEE/ACM Transactions on Networking, 5(5), October 1997; 646–661. [HON-86] D. Hong, S.S. Rappaport, ‘‘Traffic Model... Employing Forwarding Pointers to Reduce Network Impact of PCS’’ , IEEE Journal on Selected Areas in Communications, 12(8), October 1994; 1434–1444. [JAI-95] R. Jain, Y.B. Lin, ‘‘An Auxiliary User Location Strategy Employing Forwarding Pointers to Reduce Network Impact of PCS’’ , ACM-Baltzer Journal Wireless Networks, 1(2), July 1995; 197–210. Mobile Satellite Communication Networks244 system capacity is subsequently . 6Network Procedures6 .1 IntroductionSatellite personal communication networks (S-PCN) have two main objectives. circuit-switched networks,such as the PSTN and ISDN, and packet-switched networks such as the Internet, and toincorporate intelligent network capabilities