Chapter 7: Mobile Communication Systems

1. Chapter 7:Mobile Communication Systems Associate Prof. Yuh-Shyan Chen Dept. of Computer Science and Information Engineering National Chung-Cheng University

2. Introduction • A wireless systems implies support for subscriber mobility and the overall communication infrastructure needed to handle movements not only from one cell to another, • in a cell’s mobile switching center area and • in area controlled by other service providers • Handoff among cells and mobile switching centers (MSCs) that are serviced by the same service providers or different service providers needs to be supported

3. Outline • This chapter • Consider handoff schemes • Allocation of resources • Routing in the backbone network

4. Cellular System Infrastructure • A generic block diagram of a cellular system is given in Fig. 9.1. • Each BS consists of a base transceiver system (BTS) and BS controller (BSC) • Both tower and antenna are part of BTS, and all associated electronics are contained in the BTS • Authentication Center (AUC) unit provides authentication and encryption parameters that verify the user’s identify and ensure the confidentially

5. A detailed block diagram of a cellular system

6. Cellular System Infrastructure • Equipment Identity (EIR) is a database that contains information about the identify of mobile equipment that prevents calls from being stolen • Both AC and EIR could be implemented as individual stand-alone nodes or as a combined AUC/EIR node • The home location register (HLR) and visitor location register (VLR) are two sets of pointers that support mobility and enable the use of the same telephone number over a wide range

7. Cellular System Infrastructure • The HLR is located at the MSC where the MS is initially registered and is the initial home location for billing and access information • Any incoming call, based on the calling number, is directed to the HLR of the home MS, which then points to the VLR of the MSC where the MS is currently located • The VLR contains information about all MSs visiting the particular MSC and hence points to the HLR of the visiting MSs for exchanging related information about the MS

8. Cellular System Infrastructure • Such two-way pointers allow calls to be routed or rerouted to the MS, wherever it is located • There are many control signals that traverse back and forth between the HLR and VLR (including billing and access permissions maintained at the home MSC), bidirectional HLR-VLR pointers help in carrying out various functionalities

9. Redirection of a call to MS at a visiting location

10. Redirection • This works very well if the destination MS has moved from one cell to another • If a call is initiated from a residential telephone, the call is forward through the backbone network to the wireless access point closet to the home MSC, where the MS being called is registered • Thereafter, a similar routing enables connection to the MS • The reverse path connection can be established

11. Redirection • The home MSC also maintains access information about all MSs registered, including of • State of the MS (active/nonactive) • Type of allowed service (local and/or long-distance calls) • Billing information (past credit, current charges, chronological (依年代順序) order of calls made, timings of each call)

12. Redirection • This work even if the home MSC is different from the visiting MSC • As long as two MSCs have information about how to forward message to each other • Mobility can also be supported is an unknown territory (領域)

13. Registration • This is maintained not only for billing, but also for authentication, verification, as well as for access privileges (特權) • The wireless systems needs to know whether the MS is currently located in its own home area or is visiting some other area • This enables incoming calls to be routed to an appropriate location and assures desirable support for outgoing calls

14. Registration • This is done by exchanging signals known as “beacons” between BS and the MS • BSs periodically broadcast beacon signals to determine and test nearby MSs • Each MS listens for new beacon signals, and information is the used by a MS to locate the nearest BS and establish an appropriate rapport to initiate dialogue with outside world through the BS as a gateway • Other information of beacon • Cellular network identifier, timestamp, gateway address, ID (Identification) of the paging area

15. Beacon signals • Beacon signals help synchronize, coordinate, and manage electronic resource using miniscule (微不足道) bandwidth for a very short duration • Researchers continue to improve their functionality by increasing signal coverage while optimizing energy consumption

17. Using a mobile phone outside the subscription area

18. Registration Steps • A mobile phone listens for new beacons, and if it detects one, it adds it to the active beacon kernel table. If the device determines that it needs to communicate via a new BS, kernel modulation initiates the handoff process.

19. Registration Steps 2. The mobile phone locates the nearest BS via user-level processing.

20. Registration Steps 3. The visiting BS performs user-level processing and determines who the user (MS) is, the user’s registered home site for billing purposes, and what kind of access permission the user has.

21. Registration Steps • The home site sends an appropriate authentication response to the BS currently serving the users.

22. Registration Steps 5. The BS at the visited location approves or disapproves user access.

23. Handoff Parameters and Underlying Support • Handoff basically involves change of radio resource from one cell to another adjacent cell • From a handoff perspective, it is important that a free channel is available in a new cell whenever handoff occurs, so that undisrupted service is available

24. Handoff Underlying Support • Hard handoff • Also known as break before make • Is characterized by releasing current radio resources from the prior BS before acquiring resource from the next BS • Both FDMA and TDMA employ hard handoff • Soft handoff • It is possible for an MS to communicate simultaneously with the prior BS as well as the new BS, just for some short duration of time

25. Handoff Underlying Support • In CDMA, as the same channel is used in all the cells, if the code is not orthogonal to other codes being used in the next BS, the code could be changed • In fact, beacon signals and the use of the HLR-VLR pair allow MSs to roam anywhere as long as the same service provider, using particular frequency band in that area (Fig. 9.6)

26. Handoff between MSCs

27. Handoff region

28. Hard handoff

29. Soft handoff

30. Parameter Influencing Handoff • Handoff depends on cell size, boundary length, signal strength, fading, reflection, and refraction of signals, and by man-made noise • The probability of a channel being available in a new cell area depends on the number of channels per unit area

31. Parameter Influencing Handoff • It can be easily observed that the number of channels/area increases if the number of channels allocated per cell is increased or if the area of each cell is decreased • This leads to a smaller cell size may be good • However, this would cause more frequent handoff, especially for MSs with high mobility and speed • Handoff can be initiated either by the BS or the MS and it could be due to • Radio link • Network Management • Service issues

32. Radio link-type factros • Number of MSs that are in the cell • Number of MSs that have left the cell • Number of calls generated in the cell • Number of calls transferred to the cell from neighboring cells by the handoff • Number and duration of calls terminated in the cell • Number of calls handed off to neighboring cells • Cell dwell time

33. Network management • Network management may cause handoff if there is a drastic imbalance of traffic over adjacent cells and optimal balance of channels and other resources are required • Service-related handoff is due to degradation of quality of service (QoS), and handoff could be invoked when such a situation is detected • The factor that define the right time for handoff are • Signal strength, signal phase, bit error rate, distance

34. The need for handoff is determined in two different ways • Signal strength • Carrier-to-interference ratio • A low value of CIR may force the BS to change the channel currently being used between the BS and the MS.

35. Roaming Support Using System Backbone • What happens when a channel and the radio contact is changed from one cell to another when handoff occurs • A number of cells are controlled by an MSC, and depending on the destination, the signals go through the backbone network, interconnecting MSCs with the PSTN • The hardwired network is primarily supported by ultra-high-speed fiber optic cables

36. Roaming Support Using System Backbone • With mobility support, the real problem in routing becomes getting moving packets to appropriate endpoints of the backbone network (Fig. 9.7)

37. Handoff with different degree of mobility

38. Roaming • Assuming MSC1 to be the home of the MS for registration, billing, authentication, and all access information • When the handoff is from a to b, the routing of message can be performed by MSC itself • When the handoff occurs from b to c, the bidirectional pointers are set up to link the HLR of MSC1 to VLR of MSC2 so that information can be routed to the cell where the MS is currently located

39. Information transmission path

40. Roaming • When handoff occurs at locations d and e, routing of information using simply the HLR-VLR pair of pointers may not be adequate • The paging area is the area covered by one or several MSC in order to find the current location of the MS • This concept is taken from the internet network routing area

41. MSC connections to backbone

42. An example: • One involves determining the path along the shortest path • The second involves ascertaining the path according to the current location of the MS • The movement from a to c can be supported effectively by HLR-VLR, wherein MSC1 knows how to route the data to MSC2 • One option is to let all the messages reach MSC1 and forward the messages from there to the MS

43. An example: • Another option is to find a router along the original path, from where a new path needs to be used to reach the destination MSC along the shortest path • If this is done, then part of the message in the pruned tree could be lost if a hard handoff is performed, when breaks the connection before it makes • Therefore, after handoff, it may be desirable to forward message from an old location to a new one, for a short duration of time

44. Home Agents, Foreign Agents, and Mobile IP • In mobile internet protocol (Mobile IP), two important agents are associated with the routers: home agent (HA) and foreign agent (FA) • A router closet to the home MSC can be selected to serve as its HA (Table 9.2) • Once MS moves from the home network (where it is registered) to a foreign network, a software agent, FA, assists the MS by forwarding packets for the MS • The functionality of HA-FA is somewhat analogous to the HLR-VLR pair, as long as there is an agreement and understanding between the home network and the foreign network

45. Home MSC and Home Agent

46. The way it works as follows. • Whenever A MS moves into a new network, it retains its initial HA • The MS can detect the FA of the current domain by the periodic beacon signals that the FA transmits • On the other hand, the MS can itself send agent solicitation messages • When the FA detects that a new MS has moved into its domain, it allocates a care-of-address (CoA) to the MS • The CoA can either be the address of the FA itself, or it maybe a new address called colocated CoA (C-CoA) that the FA allocates to the MS using the dynamic host configuration protocol (DHCP, IETF RFC 2131, Marcg 1997)

47. Registration process

48. Registration process • Once the MS receives the CoA, it registers this CoA with its HA and the time limit for its binding validity • Such a registration is initiated either directly by the MS to its HA, or indirectly through the FA at the current location • The HA the confirms this binding through a reply to the MS

49. Message forwarding • A message sent from arbitrary source to the MS at the home address is received by the HA, binding for the MS is checked, and the HA encapsulates the packet with the CoA of the MS and forwards it to the FA area • If the colocated CoA address is used, the MS receives the packet directly and is decapsulated to interpret the infomration • If CoA for the FA is used, then the packet reaches the FA, which decapsulates the packet and passes on to the MS at the link layer

50. Message forwarding to the MS