Mobile Communication System

1. Mobile Communication System Azizol Bin Abdullah azizol@fsktm.upm.edu.my (A2.04) Rujukan: Text Book Chapter 9

2. Introduction • A wireless system implies support for subscriber mobility and the overall communication infrastructure. • In this chapter we will consider handoff schemes, allocation of resources, routing in the backbone network and security.

3. Cellular System Infrastructure • A Cellular system – complex infrastructure. • Consist of: • BS (BTS and BSC) • contained all associated electronics. • Authentication Center(AUC) • provides authentication and encryption parameters that verify user identity and to ensure the confidentiality of each call. • Protects network operators from different types of frauds and spoofing • Equipment Identity Register (EIR) • database that contains information about the identity of mobile equipment that prevents calls from being stolen, unauthorized or defective MSs

4. Cellular System Infrastructure (cont.) • Home location register (HLR) and visitor location register (VLR) • Support mobility and enable the use of the same telephone number over a wide range. • HLR is located at the Mobile switching Service Center (MSC) where the MS initially registered for billing and access information

5. Diagram of Mobile Communication System: GSM

6. Registration • MSs must be registered at one of MSCs for operation. • It is maintained for: billing, authentication, verification, access privileges and call routing. • Call is route to an appropriate location based on registration information. • Call routing is done by exchanging signals known as “beacons” between the BS and MS. • Information carried by beacon signal: cellular network identifier, timestamp, gateway address, ID of paging area and other BS parameters.

7. Registration (cont.) • BSs periodically broadcast beacons signal to determine and test nearby MSs. • Each MSs that hear from a new BS, it adds BS to the active beacon kernel table for locating the nearest BS as a gateway.

8. Registration (cont.) • Steps used for mobile phones outside their own subscription areas: • MS listen for new beacons, if it detects one, adds it to the active kernel table. If it needs to communicate via new BS, kernel modulation initiates the handoff process. • The MS locates the nearest BS via user-level processing • The visiting BS perform user-level processing and determines the MS, user’s registered home site. • The home site sends an appropriate authentication response to BS currently serving the user. • The BS at the visited location approves of disapproves user access.

9. Handoff Parameters and Underlying Support • Basically, handoff involves change of radio resources from one cell to another cell. • Handoff depend on: cell size, boundary length, signal strength, fading, reflection and refraction of signal, and by man-made noise. • Handoff could be due to: Radio link, Network Management and services issues.

10. Handoff Parameters and Underlying Support (cont.) • Radio link-type handoff due to mobility of MS and depends on the relative value of the radio link parameter: • Number of MSs that are in the cell • Number of MSs that left the cell • Number of call generated in the cell • Number of call transferred to the cell from neighboring cell by the handoff • Number and duration of calls terminated in the cell • Number of calls handed off to neighboring cell • Cell dwell time

11. Handoff Parameters and Underlying Support (cont.) • Network management may cause handoff if there is a drastic imbalance of traffic over adjacent cell • Service-related handoff is due to degradation of quality of Service. • The factor that defined to handoff: • Signal strength • Signal phase • Combination of Signal strength and phase • Bit error rate • Distance

12. Handoff Parameters and Underlying Support (cont.) • The need for handoff is determined in two different ways: • Signal strength • Carrier-to-interference ratio (CIR) • The specific units involved in setting up a call: • Base station controller (BSC) • Mobile station (MS) • Mobile Switching center (MSC)

13. Handoff Parameters and Underlying Support (cont.) • Handoff can be classified into two different types: • Hard handoff • break before make • Employ by FDMA & TDMA • Soft handoff • Make before break

14. Roaming • By using beacons signal and HLR-VLR, MSCs are allow to roam anywhere as long as the same service provider. • It is possible to move from one cell another cell that control by other MSC area.

15. Roaming Support using System Backbone • A number of cell are controlled by an MSC. • Signal go through the network backbone (interconnecting MSC with PSTN) through access points. • Routing process – moving packet to appropriate endpoints of the backbone network. • Paging area: • the area covered by one or several MSC in order to find the current location of MS. • Use the internet network routing area concept.

16. Roaming Support using System Backbone (cont.) • In Mobile IP, two important agents are associated with the routers: home agent (HA) and foreign agent (FA). • MS also registered with a router, a router closest to the home MSC can be selected to serve as its HA. • Once MS moves from the home network to a foreign network, FA will assist the MS by forwarding packets for the MS. • The functionality of HA-FA is analogous to HLR-VLR, except it supports mobility in a much broader sense.

17. Multicasting • A process of transmitting messages from a source to multiple recipients by using a group address for all hosts that wish to be members of the group. • It reduces the number of messages as compared with multiple unicasting for each member – enhancing bandwidth utilization. • Extremely useful in video/audio conferencing, distance learning and multiparty games. • Can be performed by building a source-based tree or by using a core-based tree.

18. Multicasting (cont.) • Source-based tree • Each source of the group maintain shortest path tree, the source being the root of the tree. • Core-based tree • A particular router is chosen as a core and tree is maintain, with the core being the root. • Every source then forwards the packet to the core router, then it forwards to the tree and members. • Tree rebuilding in wireless is more complex than wired – mobility: MS changing from one MSC to another MSC.

19. Multicasting (cont.) • There is a need to route packets to MS and address these problems during multicast tree generation: • Nonoptimal path length • Packet duplication • Distruption of packet delivery • IETF proposed two approach for providing multicast over mobile IP: • The bidirectional tunneling (BT) • The remote subscription approach.

20. Multicasting (cont.) • Bidirectional Tunneling • Whenever an MS moves into a foreign network, HA is responsible for forwarding the multicast packets to MS after due registration process. • HA creates a bidirectional tunnel to that FA that currently serving the MS. • FA encapsulates the packets for the MS, the forward to MS. • BT prevents data distruption due to movement of MS but cause problems: • Packet duplication if several MSs of the same HA, which have subscribed to same multicast group, move to the same HA • Packet may use nonoptimal paths for delivery

21. Multicasting (cont.) • Remote subscription approach • Whenever an MS moves into foreign network, the FA sends a tree join request (if it not already a member of the multicast tree). • MS receives multicast packet through the FA. • Simple and prevents packet duplication & nonoptimal path delivery. • Problems: • However, since the FA needs to join the multicast tree, it can cause data distruption until FA is connected to the tree. • Frequent tree updates when the MSs move frequently

22. Multicasting (cont.) • The Mobile Multicast (MoM) • Attempts to prevent packet duplication by forcing an HA to forward only one multicast packet for a particular group. • The FA select a designated multicast service provider (DMSP) for each group. • DMSP is an HA that is responsible for forwarding a multicast packet to the FA of the group. • Problems: • Selecting a DMSP: if MS of the DMSP that subscribed to FA move out, DMSP may stop forwarding packets to FA – cause data distruption. • Selecting more that one DMSP: cause data duplication.

23. Wireless System Security • Needs to capable of protecting. • The services of security can be classified as: • Confidentiality : only the authorized party can access the information in the system and transmit data. • Nonrepudiation: the sender and receiver cannot deny the transmission. • Authentication: ensure the sender • Integrity : the content of the message or information can only be modified by authorized users. • Availability : the resources should be available only to the authorized users.

24. Wireless System Security (cont.) • Security mechanisms can be divided into three categories: • Security prevention : enforces security during the operation of the system to counter security attacks. • Security detection : Detects both attempts to violate security. • Recovery : use to restore the system to a presecurity violation state.

25. Wireless System Security (cont.) • Threats can be broadly classified in two types: • Accidental threats : due to operational failure. • Intentional threats : any action performed by an entity with an intention to violate security.

26. Wireless System Security (cont.) • Intentional threats can be categorized as: • Interruption : attack by blocking or interrupting system resources. • Interception : System resources accessed by illegal party – attack the confidentiality • Modification : Create anomaly in the network, illegal party transmits spurious message – affect authenticity • Fabrication : unauthorized party transmit counterfeit object into the system – cause an attack on authenticity

27. Wireless System Security (cont.) • Different types of active attack: • Masquerade: Attacker poses as an authorized party to make previledged changes in network. • Replay: Playing back previous sent data to create undeseirable effects. • Modification of data: Original message is tempered to create inconsistency in the network • Denial of service: Hijacking of network resources • Passive attack : listen and monitor the communication between to parties.