Mobile networks and mobility management

1. Mobile networks and mobility management Impacts of mobility on networks A few mobility solutions Raimo Kantola raimo.kantola@hut.fi SG210, 4512471

2. Usage of number positions: m13 = 109 13 lg m = 9 m = 4.92 The number of nodes in the analysis tree is: m13 - 1 = 305 million 1 + m + m2 + … m12 = m - 1 Mobility demands logical subscriber numbers that must be mapped to the topology of the network • Routing numbers describe the network topology. • An example: 109 subscribers, the length of subscriber numbers is 13 digits Let’s calculate an approximation of memory requirements : The analysis tree consists of nodes of 64 octets. One digit is analyzed in each node:

3. A B d e f g The number analysis tree of the exchange associates routing with signaling information From signaling: ABC - destination (area) ABCd - the shortest subscriber number ABCdefgh - the longest subscriber number Buckets C We assume a treelike data structure in the analysis. Nodes d,e,f,g are necessary depending both on the number length and the father node. h

4. Update traffic Mbit/s 1000 100 10 1 Partition of the problem required! 0,01 0,1 1 10 updates/subscriber/day Calculation continues ... Memory required for the analysis tree: 64 * 305 * 106 = 19 Gb • Copying this to many places is very expensive. • Analysis of number in this database demands 13 reads, which basically isn’t a problem • The biggest technical problem is updating the database: Let us assume that:- one update demands a message of 50b - it takes 6h to update the entire DB Note: - For reliability, the update frequency per subscriber may need to be substantially higher for security.

5. A solution is to partition by operators and number prefixes • GSM supports this solution: • Every HLR knows locations of some hundreds of thousands of subscribers with the accuracy of a VLR • First digit positions of a subscriber number define which HLR is asked for the location. • The location area hierarchy also decreases the number of necessary updates of HLR. • All changes don’t need to be updated as far as the HLR. • A subscriber has an MS-ISDN “subscriber phone book number” and a separate roaming (i.e. routing) number (MSRN).

6. VLR knows - in practice a set of cells - updates once/6 min….24h and on conditions of switching on/off phones - an update demands also authentication Final locations via paging: - call is sent (paged) in the set of cells known by the VLR - MS responds in its own cell - Result is that the best cell can be chosen Location area hierarchy in GSM HLR knows the exchange/VLR MSC/VLR area Sijaintialue Location area Solu Cell

7. Could be transmitted using one PCM line! It feels like a viable solution. Calculation of location update traffic in a HLR with 200000 subscriber capacity • 200 000 subscribers • 1 update/5min/subscriber • an assumption: 1 update = 100 octets TRAFFIC = 200000 * 100 * 8/(5*60) = 0,53Mbit/s.

8. Number of Hand-overs/call Call duration 3 min 100 10 Speed 150 km/h 1 Speed 100 km/h 0 1 2 3 4 5 6 7 8 Speed 50 km/h 0,1 Speed 15 km/h Speed 5 km/h 0,01 The radius of the cell in km The number < 1 preferably in a viable architecture! It is interesting to study the number of probable handovers during a call

9. GSM architecture HLR/AC/EIR BTS HLR - Home Location Register AC - Authentication Center EIR - Equipment Identity Register MSC - Mobile Switching Center VLR - Visitor location Register BSC - Base Station Controller BTS - Base Transceiver Station BSC MSC BSC MS = ME+SIM VLR cells BTS

10. MSC HLR VLR MS is found in a mobile terminated call using the “Routing information request” MAP/C MAP/D GMSC PSTN ISUP - IAM SendRoutingInformation ProvideRoamingNumber ProvideRoamingNumberACK SendRoutingInformationACK ISUP - IAM (a start-up message of normal signaling) MSRN - Mobile Subscriber Roaming Number is the routing number - supports the E.164 format (ordinary exchanges can handle) - every MSC has restricted number of MSRNs - MSRN has a period of validity - MSRN can be allocated per call or for the duration of roaming

11. More capacity can be built with multi-layer cellular networks GSM900 macro GSM1800 macro GSM1800 micro GSM900 micro Cell selection aims at placing fast moving MSs up into large cells.

12. Usage of hexa positions: m8 = 109 8 lg m = 9 m = 13.34 The number of nodes m8 - 1 = 114 millions 1 + m + m2 + … m7 = No significant improvement! Would binary subscriber numbering change the situation? • Example: 109 subscribers, number length 128 bits (IPv6) Approximate memory need for analysis: The analysis tree consists of nodes of 64 octets. In each node 4 bits are analyzed. m - 1

13. Two solution models: Mobile-IP and GPRS. A “Brute Force” - solution to IP -mobility Memory needed for the analysis tree (=RT) is: 64 * 114 * 106 = 7.3 Gb • In the “Brute Force” - solution this is updated in all routers! In practice this is not feasible! • An analysis of 128 bits in this DB requires 8 reads which basically is not a problem. • The biggest technical problem is updates! • Mobility architectures must decrease mobility update traffic to lower than the user traffic. • Updates to places that have no reads must be eliminated or at least minimized.

14. RT Destination IP-address Outgoing port/ Next. R IP-ad. Mobility in packet networks/background • Routing is based on routing tables that are read per packet. • Routers maintain routing tablesusing routing protocols. • Size of feasible tables is today under 100 000 entries. A search based on destination addresses demands many references (reads) in the memory (<32). - In networks of 100m users a feasible RT-size is reached by using provider addressing and searching from the table with address prefixes. (So, IP-addresses of the full length of 32 bits are hardly ever used)

15. Care-of-Address Mobile Foreign Agent 3 tunnel 2 Tunnel = IP over IP 1 Home Network of the Mobile Correspondent Host Home Agent Home-IP-Addressof the Mobile A mobile has to update its location from time to time. In Mobile-IP user has a home agent and foreign agent. 1 - normal IP -routing 2 - tunnel HA ->FA 3 - normal IP - routing

16. a mobile 3 Foreign Agent 4 5 2a Binding warning 1 Mobile’s Home Network Correspondent Host MAY have a binding cache 2b Home Agent Binding Update In Mobile-IP triangle routing could be avoided

17. Features of the Mobile-IP • Care-of-address changes are authenticated. • Routing Optimization is a draft proposal, not a part of the basic Mobile-IP. • Optimization of routing can also try to retrieve messages going to a moving mobile with help of a negotiation between the old and the new FAs. • Matters of radio technology and other issues of the data link layer are ignored. • Not considered, who owns the networks and who will pay for the operations.

18. Packet traffic extension to GSM is GPRS Every trx has 8 time slots, that are classified: - dedicated to circuit switched service - dedicated to packet service (optional) - packet service as a default (is kept free from calls even using hand-overs - packet service allowed if no circuit switched traffic. Trx 1 Circuit switched time slots Circuit switched time slots max Circuit switched time slots Additional GPRS Default GPRS max Trx n Dedicated GPRS time slots Selecting the size of areas appropriately for circuit switched and packet switched traffic generates an elastic boundary so that the quality of service, traffic revenue and usage of the network are optimized. Definition of areas is an additional step brought by GPRS into cellular network design.

19. A tunnel/context SGSN takes care of the mobility and GGSN takes care of connectivity to other networks in GPRS An interface to a circuit switched network BTS Company Xyz’s Intranet HLR/AC/EIR GGSN BSC BSC MS = ME+SIM SGSN Company ABc’s Intranet GGSN SGSN - Serving GPRS Support Node GGSN - Gateway GPRS Support Node A context is supported from MS to GGSN” GGSN Public Internet cells BTS

20. The network doesn’t monitor the location of the MS A subscriber is active. The network monitors the subscriber with the accuracy of a cell. Mobility management in GPRS IDLE Attach Detach READY Standby timer PDUforwarding Ready timer MS is in the “online” standby state, but isnot forwarding packets right now. The network monitors the subscriber with the accuracy ofa routing area: cell < RA < location area. MS is found by paging in the set of cells. STANDBY GPRS mobility model

21. Features of GPRS • Two different mobility management systems in the network: for circuit switched services and packet switched services. • The GGSN owns the externally visible IP address of the Mobile. Normal IP routing outside the GGSN. • BSC-SGSN-GGSN (+HLR) network takes care of the mobility and uses internal IP addresses aligned with the topology. ( cmp. SGSN/FA, GGSN/HA.) • Two IP networks on top of each other in the tunnel MS - GGSN : an IP transmission network and an IP network seen by the applications. • This solution is a result of the need to manage the issues of networkownership and responsibilities. • Big header overhead (>100 octets)!

22. “GSN to GSN” -networks have a big header overhead E.g. 20ms voice bits Header overhead on different layers RTP RTP 12 A voice packet 6...12kbit/s takes 15 …30 octets. If there is an ATM network beneath (48 octet payload + 5 octets header/cell), How much is the overhead? UDP UDP 8 20 IP IP GTP GTP UDP UDP 8 Gn interface IP IP 20 L2 L2 L1 L1 GSN1 GSN2

23. Summary • Mobility solutions in packet switched networks and circuit switched networks are different from each other. • GMSC asks per call a HLR for routing information: the centralized architecture is feasible. • External nodes can’t be asked about destination of a packet in packet switched networks. A mobility solution is either adaptive or distributed. • GPRS and mobile-IP architectures are quite similar. • GPRS has been carefully adapted to GSM. • GPRS takes care of who owns the equipment and who is allowed to use it and where and with what kind of rights.