![ There are four different types of handover in the GSM system , which
involves transferring call in between
Channels in the same cell
Cells under the control of same BSC
Cells under the control of different BSC, but belonging to same MSC
Cells under the control of different MSC
The first two are called internal handovers [handled by BSC]and next
two are called external handovers [handled by MSC]
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Module1 part2
- 2. Introduction It covers around 71% of the market today People not only use for business but also personal life Used for wireless telephony and messaging through SMS Supports facsimile (Fax) and data communication GSM is based on set of standards. The proposed GSM had to meet certain business objectives, these are: Support for international roaming Good speech quality Ability to support handheld terminals Low terminal and service cost ISDN compatibility 2
- 3. GSM history timeline 3
- 4. Frequency reuse concept led to the development of Cellular technology by AT&T and Bell Labs in 1947 Characteristics were: The area covered is subdivided into radio zones or cells. Cells can be of any shape, for convenient it is considered as hexagon. Base stations are placed at the centre of these cells Each cells i receives a subset of frequencies from the total set assigned to respective mobile network. To avoid any type of interference, no to neighbouring cells can use same frequency Only at distance of D (known as frequency reuse distance) , the same frequency from the set can be reused. When moving from one cell to another during an outgoing conversation , an automatic channel change occurs. This phenomenon is called handover 4
- 5. Regular repetition of frequencies in cells result in a clustering of cells The size of cluster is defined by k, the number of cells in the cluster This also defines the frequency reuse distance D The following fig is an example of cluster size of 4 5
- 6. Cell clusters in GSM 6
- 7. GSM Architecture Are structured in hierarchic fashion Administrative region assigned to one MSC(mobile switching centre) Administrative region is called PLMN i.e. Public Land Mobile Network It is subdivided into one or many Location Area (LA).One LA consists of many cell groups Each cell group is assigned to one BSC (base station controller) Each LA there will be at least one BSC Cells in one BSC can belong to different LA’s Cells are formed by the radio areas covered by a BTS (Base Transceiver Station) Several BTs are controlled by one BSC Traffic from the MS is routed through MSC 7
- 8. GSM System hierarchy 8
- 10. All subscribers with cellular network operator , permanent data such as the service profile is stored in the Home Location Register (HLR) Authentication information like International Mobile Subscriber Identity (IMSI) Identification information like name, address etc of the subscriber Operator selected denial of service to the subscriber Billing information like prepaid or postpaid customer 10
- 11. Handling supplementary services like CFU (Call forwarding Unconditional), CFB (Call Forwarding Busy), CFNR (Call Forwarding Not Reachable ), CFNR( Call Forwarding Not Reachable)or CFNA (Call forwarding Not Answered) Storage of SMS number in case the mobile not connectable so that whenever the mobile is connectable , a paging signal is sent to the SC 11
- 12. GSM entities 12
- 13. GSM entities The GSM technical specifications define different entities that form the GSM network by defining their functions and interface requirements Can be divided into five different groups: The Mobile Station (MS): Includes the Mobile Equipment (ME) and the Subscriber Identity Module(SIM) The Base Station Subsystem (BSS) : includes the Base Transceiver Station (BTS) and Base Station Controller (BSC) 13
- 14. The Network and Switching Subsystem (NSS) : it includes MSTN, HLR,VLR, Equipment Identity Register (EIR) , Authentication Centre (AUC) The Operation and Support Subsystem (OSS): includes the operation and Maintenance Centre (OMC) The data infrastructure: include PSTN, ISDN , PDN(Public Data network) 14
- 15. Mobile Station Technical name for mobile or the cellular phone Earlier days , mobile phone were large in size and now they come in smaller in size It has two important components in it The mobile equipment or device or phone without SIM Subscriber Identity Module (SIM) The weight and volume of the mobile phones are decreasing Life of Battery charging is also increasing. 15
- 16. SIM is installed in every GSM phone, they are smart processor card They have a processor and small amount of memory i.e 32K bytes By inserting SIM card, user have access to all subscribed services. It has International Mobile Subscriber identity used to identify the subscriber to the system, a secret key for authentication and other security information Applications are developed and stored in SIM cards using SAT (Sim Application Toolkit) 16
- 17. The base station subsystem Connects the Mobile station and the NSS (Network and Switching Subsystem) Can be divided into two parts The base transceiver station (BTS) or base station The base station controller (BSC) The base station corresponds to the transceivers and antennas used in each cell of the network Urban areas large number of BTS will be deployed Usually placed in the centre of the cell 17
- 18. Its transmitting power defines the size of cell Each BTS has one to 16 transceivers depending on the density of users in a cell BSC is the connection between the BTS and Mobile service Switching Centre (MSC) It manages the radio resources for one or more BTS It handles handovers, radio-channels setup, control of radio frequency power levels of BTSs, exchange function and frequency hopping. 18
- 19. The network and switching subsystem The central component of the network subsystem is the Mobile Switching Center (MSC) Functions are: It acts like a normal switching node for mobile subscribers of the same network Acts like a normal switching node for PSTN fixed telephone Acts like a normal switching node for ISDN Provides all the functionality needed to handle a mobile subscriber such as registration, authentication, location updating handovers and call routing. It includes databases needed in order to store information to manage the mobility of a roaming subscriber. 19
- 20. The MSC with Home Location Register (HLR) and Visitor Location Register (VLR) databases, provide the call routing and roaming capabilities of GSM HLR stores the important information of subscribers belonging to the covering area of a MSC It contains all administrative information like current location of the mobile, authentication data HLR is always fixed an stored in the home network and VLR logically moves with subscriber. 20
- 21. VLR considered as temporary copy of some important information stored in HLR VLR is similar to cache, HLR is persistent storage It contains the selected administrative information borrowed from HLR, necessary for control and provisioning of the subscribed services. When a subscriber enters a covering area of new MSC, the VLR associated with this MSC will request information about the new subscriber from its corresponding HLR in home network 21
- 22. Another component called (GMSC) Gateway MSC that is associated with MSC It is a gateway is node interconnecting two networks The GMSC is the interface between the mobile cellular network and the PSTN It is in charge of routing calls from fixed network towards a GSM user and vice versa 22
- 23. The operation and support subsytem It controls and monitors the GSM system Is connected to different components of the NSS and to the BSC Controls the traffic of load of the BSS Provisioning information for different services is managed in this subsystem Equipment Identity Register (EIR) is a database that contains a list all valid mobile equipment's within the network, where each mobile station is identified by its IMEI (international Mobile Equipment identity) EIR consists a list of IMEI all valid terminals 23
- 24. An IMEI is marked invalid if it reported as stolen or not type approved. EIR allows the MSC to forbid the calls from the stolen or unauthorized terminals. Authentication Center(AUC) is responsible for authentication of subscribers It is a protected database and stores a copy of secret key stored in each subscribers sim card These data help in verifying the user’s identity 24
- 25. Message centre Popular service provided by GSM is SMS i.e. Short Message Service. Is a data service and allows a user to enter a text message upto 160 characters in length when 7-bit English characters are used Is always ON network Message center is also called as Service Center (SC) or SMS controller There are two types of SMS SMMT SMMO 25
- 26. 26
- 27. SMMT- Short Message Mobile Terminal Is an incoming short message from the network Terminated in the MS SMMO-Short Message Mobile Originated Is an outgoing message Originated at MS Forwarded to network for delivery 27
- 28. Call routing in GSM Human interface is analog There are many complex technologies used between the human analog interface in mobile and digital network The following figure is sequence of operation from speech to radio wave 28
- 29. 29
- 30. Digitizer and source coding User speech is digitized at 8 KHz sampling rate using Regular Pulse Excited-Linear Predictive Coder (RFE-LPC) with a Long Term Predictor loop Here information from previous sample is used to predict the current sample Each sample is then represented in signed 13-bit linear PCM value It is then passed to coder with frames of 160 samples 30
- 31. Channel coding Introduces redundancy information into the data for error detection Gross bit rate after channel coding is 22.8 kbps 456 bits are divided into eight 57-bit blocks, and result is interleaved among eight successive time slots bursts for protection against burst transmission. Interleaving Rearranges a group of bits in a particular way Improves performance of the error correction mechanisms Decreases possibility of whole bursts during the transmission 31
- 32. Ciphering Encrypts block of user data using a symmetric key shared by the mobile station and BTS Burst formatting Adds some binary information to the ciphered block Is used for synchronization and equalization of the received data Modulation Technique chosen for the GSM system is Gaussian Minimum Shift Keying(GMSK) The binary data is converted back into analog signal to fit the frequency and time requirements for multiple access rules 32
- 33. Multipath and equalization GSM frequency bands gets reflected from buildings, hills etc, so not only the right signal received by an antenna but many reflected signals which corrupt the information are also received An equalizer is responsible for extracting the right signal from the received signal It estimates the channel impulse response of the GSM system and then constructs an invers filter 33
- 34. Synchronization For successful operation of a mobile, time and frequency synchronization is needed. Frequency synchronization is needed so that transmitter and receiver frequency match Time synchronization is necessary to identify the frame boundary and bits within the frame 34
- 35. An example Assume that a party called a GSM directory number +919845052534 The directory number dialled to reach a mobile subscriber is called the Mobile Subscriber ISDN (MSISDN) It includes country code and national destination code which indicated subscriber’s operator MSISDN number of subscriber in Bangalore associated with Airtel network is +919845XYYYYY + means the prefix for international dialling like 00 in UK / India or 011 in USA 35
- 36. 91 is the country code for India 98 is the network operator code, here Airtel X is the level number managed by the network operator ranging from 0 to 9 YYYYY is the subscriber code managed by operator 36
- 37. Call routing for mobile terminating call 37
- 38. Call first goes to local PSTN exchange PSTN exchange looks at the routing table and determines that is a call to mobile network It forwards call to Gateway MSC of the mobile network It decides whether to route this call or not If the user has not paid the bills, the call may not be routed If the phone is powered off then , a message may be played or call is forwarded to voice mail 38
- 39. If MSC finds that the call can be processed, it will find out address of the VLR where mobile is expected to be present If the VLR is of different PLMN , it will forward the call to foreign PLMN through gateway MSC If VLR is in the home network, it will determine the location Area (LA) Within the LA it will page and locate the phone and connect the call 39
- 40. PLMN interfaces Basic configuration of PLMN consists of a central HLR and central VLR Contains all security, provisioning and subscriber-related information MSC needs subscriber parameter for successful call set-up 40
- 42. GSM addresses and identifiers To manage all complex functions, GSM deals with many address and identifiers, they are IMEI IMSI MSISDN LAI MSRN TMSI LMSI Cell identifier Identification of MSCs and location registers 42
- 43. International Mobile Station Equipment Identity (IMEI) Every mobile equipment in this world has a unique identifier, called as IMEI Allocated by the manufacturer and registered by the network operator in Equipment Identity Register (EIR) To see your IMEI number, type *#06# on the phone 43
- 44. International Mobile Subscriber Identity (IMSI) When registered with a GSM operator, each subscriber is assigned with a unique identifier IMSI is stored in SIM card and secured by the operator A mobile station can operate only when it has a valid IMSI There are several parts in IMSI, which are as follows 44
- 45. Three decimal digits of Mobile Country Code(MCC), for India it is 404 Two decimal digits of Mobile Network Code (MNC) Uniquely identifies a mobile operator within a country For airtel in Delhi it is 10 Maximum 10 digits of Mobile Subscriber Identification Number (MSIN) This is unique number of the subscriber within a home network 45
- 46. Mobile Subscriber ISDN numbers (MSISDN) Real telephone number that is known in this external world It is public information where as IMEI is private information to a operator The MSISDN categories follow the following international ISDN (Integrated Systems Data Network) numbering plan as the following Country Code (CC): one of three decimal digits of the country code Eg: USA-1, India-91, Finland-358 National Destination Code (NDC) : typically 2 to 3 decimal digits In India 94 for BSNL, 98 for all other operators Subscriber Number (SN) : Maximum 10 decimal digits Eight digit number, consist Two decimal digits of operator code Followed by one decimal digit level number 5 digit subscriber number 46
- 47. Location Area Identity (LAI) Each LA in PLMN has its own identifier It is unique Consists three digits of CC, two digits of Mobile Network Code i.e. MNC, maximum five digit of Location Area Code. 47
- 48. Mobile Station Roaming Number (MSRN) When a subscriber is roaming in another network, he is assigned with a temporary ISDN number This ISDN is used by the Local VLR in charge of the mobile station 48
- 49. Temporary Mobile Subscriber Identity (TMSI) Temporary identifier assigned by the serving VLR Used in place of the IMSI for identification and addressing of the mobile station It is never stored in HLR Difficult to determine the identity of subscriber by listening to the radio channel Stored in the SIM card 49
- 50. Location Mobile Subscriber Identity (LMSI) Assigned by the VLR and stored in the HLR Used as searching key for faster database access in VLR 50
- 51. Cell identifier (CI) Within a LA , every cell has unique cell identifier Together with a Lai a cell can be identified uniquely through Global Cell Identity 51
- 52. Identification of MSC and location registers MSC, HLR,VLR,SCs are addressed by ISDN numbers They may have Signaling Point Code (SPC) within a PLMN 52
- 53. Network aspects in GSM When a call is in progress and the user is on the move, there will be a handover mechanism from one cell to another Resembles to relay race, where one athlete passes the baton to another Roaming and handover are performed by NSS, mainly using the Mobile Application Part (MAP) built on the Signalling System #7 (SS7) protocol The signalling protocol is GSM has three layers, shown in the following figures 53
- 54. 54
- 55. Layer 1 is the physical layer, which uses the channel structures over the air interface Layer 2 is data link layer, called LAPDm i.e. modified version of LAPD protocol Layer 3 is divided into three layers Radio Resources Management Controls set-up, maintenance, termination of radio and fixed channels including handovers Mobility management Manages location updating and registration procedures such as security and authentication Connection Management Handles general call control, manages supplementary services, SMS 55
- 56. Mobility management Person travelling on train without GSM phone or wireless communication device If he wants to talk, how will he talk If the train stops at a station for a long time, he can try to locate a public call office in a station and make the call He must finish the call before the train departs There is no way any person can make a call when they want 56
- 57. Using MM , one can make outgoing calls and receive incoming calls while in motion In wired network where device is stationary, the point of attachment to the network is also fixed Address of the device is sufficient to locate the device in routing table and establish the connection Incase of wireless devices, device moves from one location to another making the old routing table invalid Therefore establishing and maintaining a connection is complex Paging, location updates, handover and roaming are performed to provide mobility 57
- 58. Paging For getting a call connected, a MS has to be traced, located and then call connected The MS is traced through a paging process within a location Using the BSS signalling channel the paging message for an MS is sent that includes the IMSI as the identifier of the MS A single paging message across the MSC to BSS interface contains information of the cells in which the page shall be broadcast In paging the making a decision is difficult i.e which cell to start paging from, this is because there is huge number of cells 58
- 59. If we cannot locate the mobile quickly, the call cannot be connected resulting the lost revenue Eg: it can start from the centre of the network and keep searching each and every cell for long time Such global paging system is going to cost more and consume lot of time To optimize the cost and response time, paging starts at location where the MS was present last. The location of MS is recorded in the HLR and updated through Location Update The MS is searched in these cells where it has the highest probability of being present 59
- 60. Location update Concerned with procedures that enable the network to know the current location of powdered on MS so that mobile terminated call routing can be completed. If the location of MS is not known, tracking the MS through the paging costs in terms of radio and backbone SS7 signalling bandwidth To optimize this, location information is regularly updated within core network The presence and location information is kept up to date within VLR and HLR 60
- 61. Assume the MS is willing to communicate, the MS must be powered off-on and attached to the network If the MS is attached to the network , it must be located through the paging before successful communication can take place for mobile terminated calls and mobile terminated SMS When MS is powered off, the HLR is updated with an explicit IMSI detach which indicates that MS is not available and connection cannot be established. 61
- 62. Handover When the call is going on in a moving vehicle, the relationship between the radio signals and user is dynamic The user movements can make the user to move away from a wireless tower, causing the radio signals to reduce and ultimately break The user has to be moved to another cell where the signal strength is higher Will result in changing the association of the resources to another channel within the same cell or different cell altogether, this is called handover 62
- 63. Handover has to be made very fast without any disruption to the service to the higher layer In north America handover is called as “handoff” A handover can be initiated by a mobile or network A mobile initiated handover is based on radio subsystem criteria of Radio Frequency (RF), quality of signal or distance from tower Network initiated handover is associated by a mobile device is based on traffic loading per cell, maintenance request etc 63
- 64. There are four different types of handover in the GSM system , which involves transferring call in between Channels in the same cell Cells under the control of same BSC Cells under the control of different BSC, but belonging to same MSC Cells under the control of different MSC The first two are called internal handovers [handled by BSC]and next two are called external handovers [handled by MSC] 64
- 66. Authentication and Security issues during handover GSM uses A3,A8 and A5 algorithms for security A3 algorithm is used to authenticate the subscriber A8 algorithm is used to generate Cipher key Kc A5 algorithm is used to cipher everything that is transmitted over the air that include both signal and traffic When handover occurs necessary information is transmitted within the system infrastructure to enable communication to proceed from the old BSS to new BSS, synchronization process is resumed The key remains unchanged at handover 66
- 67. Roaming When handover happens between one network to another network is called roaming When a mobile is powered off , it performs an IMSI detach to say the network that is no longer connected. When a mobile station is turned on in a new network or the subscriber moves to different operators PLMN , then the subscriber must register with the new network to indicate its current location. The first location update is called IMSI attach procedure where the MS indicates its IMSI to the new network. 68
- 68. A location update is performed periodically A location update message is sent to new MSC/VLR which records the location area information and then sends this information to the subscriber’s HLR If the mobile station is authenticated and authorized in the new MSC/VLR the subscriber’s HLR cancels the registration of the mobile station with the old MSC/VLR 69
- 69. Two types of roaming Horizontal Roaming Roaming between two networks of the same family GSM to GSM roaming Vertical Roaming Roaming between two networks of different families When vertical roaming happens without any disruption it is called seamless roaming. GSM to CDMA, GPS to WIFI 70
- 70. GSM Frequency Allocation GSM in general uses 900 MHz band 890-915 MHz are allocated for uplink i.e. MS to BS 935-960 MHz are allocated for downlink i.e. BS to MS Mobile to Base Ft(n)=890.2+0.2(n-1) MHz Base to Mobile Ft(n)=Fr(n)+45MHz 71
- 71. 72
- 72. 73
- 73. 3 (tail) These tail bits at the start of the GSM burst give time for the transmitter to ramp up its power 57 (data): This block of data is used to carry information, and most often contains the digitised voice data 1 (steal bit )is bit within the GSM burst indicates the type of data in the previous field. 26 (Midamble): This training sequence is used as a timing reference and for equalisation. 1 (steal bit ) Again this flag indicates the type of data in the data field. 57 (data) Again, this block of data within the GSM burst is used for carrying data. 3 (tail) These final bits within the GSM burst are used to enable the transmitter power to ramp down. They are often called final tail bits, or just tail bits. 8.25 (guard bits) At the end of the GSM burst there is a guard period. This is introduced to prevent transmitted bursts from different mobiles overlapping. 74