Table of Contents
Chapter 2 BSS Functions ... 2-1 2.1 Basic Functions... 2-1 2.1.1 Overview ... 2-1 2.1.2 Channel ... 2-2 2.1.3 System Information ... 2-9 2.1.4 Idle Mode Behavior ... 2-15 2.1.5 PLMN Selection... 2-18 2.1.6 Cell Selection and Reselection ... 2-19 2.1.7 Location updating... 2-24 2.1.8 Access... 2-32 2.1.9 Paging ... 2-33 2.1.10 Immediate assignment ... 2-35 2.1.11 Assignment... 2-44 2.1.12 Authentication... 2-45 2.1.13 Ciphering ... 2-48 2.1.14 DTX ... 2-52 2.1.15 Frequency hopping ... 2-55 2.2 Extended Functions ... 2-60 2.2.1 Handover... 2-60 2.2.2 Power Control... 2-74 2.2.3 Extended Cell... 2-86 2.2.4 IUO ... 2-89 2.2.5 "HW-IUO Property"Satellite Transfer ... 2-95 2.2.6 Diversity Receiving... 2-97 2.2.7 Aggressive Frequency Reuse Pattern ... 2-99 2.2.8 Multiband Network ... 2-104 2.2.9 Carrier Mutual-assistance ... 2-116 2.2.10 Cell Broadcast... 2-119 2.2.11 Radio Channel Allocation... 2-121 2.2.12 Half Rate ... 2-125 2.2.13 E1 Ring Topology... 2-127 2.2.14 GSM900/GSM1800 Co-cell... 2-129 2.2.15 Multi-MNC ... 2-131 2.2.16 E-GSM/R-GSM... 2-135 2.3 GPRS Function... 2-137 2.3.1 Supported Packet System Information ... 2-137 2.3.2 Supported GPRS MS Modes ... 2-1412.3.3 Supported RLC Modes... 2-143 2.3.4 Supported Channel Coding Scheme ... 2-144 2.3.5 Supported Network Control Modes... 2-148 2.3.6 Supported Network Operation Mode ... 2-148 2.3.7 Supported QoS... 2-150 2.3.8 Supported Assignment... 2-150 2.3.9 Supported Paging ... 2-151 2.3.10 Timing Advance ... 2-152 2.3.11 Measurement Report ... 2-153 2.3.12 Supported Flow Control ... 2-153 2.3.13 Supported Dynamic Handover between TCH and PDCH ... 2-155 2.3.14 Supported Packet Access Function ... 2-155
Chapter 2 BSS Functions
BSS is a bridge between MS and NSS, which performs mainly the management of radio links and conversion of radio links and wire links. It is responsible for the communication of MS. BSS system functions can be divided into basic functions, extended functions and GPRS functions.
2.1 Basic Functions
2.1.1 Overview
Figure 2-1 illustrates the GSM Protocol.
CM MM RR LAPDm Sign. Layer1 L3 L2 L1 BTSM MS Um SCCP MTP BTSM RR BSSMAP Abis BTS B MSC A BSC Sign.
Layer1 Sign. Layer1 Sign. Layer1 RR LAPD LAPDm LAPD CM MM BSSMAP SCCP MTP
MS: Mobile Station BTS: Base Transceiver Station BSC: Base Station Controller RR: Radio Resource Management MSC: Mobile services Switching Centre, Mobile Switching Centre
MTP: Message Transfer Part (MTP) SCCP: Signaling Connection Control Part LAPD: Link Access Procedure on the D channel MM: Mobility Management
LAPDm: Link Access Procedure on the Dm channel CM: Connection Management BSSMAP: Base Station Subsystem Management Application Part
BTSM: Base Transceiver Station Site Management Figure 2-1 GSM protocol stack
According to GSM 04.07, the functions of BSS on layer 3 and related sub-layers on the radio interface (Um) are classified into:
1) RR: Radio Resource Management 2) MM: Mobility Management
3) CM: Communication Management
maintenance and release of radio resources are mainly carried out by BSS. There are corresponding communication management protocol for A interface and Abis interface to realize the air interface between GSM network and MS. The other functions of BSS are also essential for establishing communication between the GSM network and MS.
The functions (RR) that BSS involves are mainly as follows: z Radio channel management
z Channel coding/decoding z Transcoding & Rate Adaptation
z Full-rate & half-rate coding of speech and enhanced full-rate coding z Encryption/Decryption
z Frequency hopping z Antenna Diversity
z RF Power control and handover management
2.1.2 Channel
I. Types of Radio Channels
According to GSM/GPRS specifications, the radio channels fall into two major categories, which are Traffic Channel and Control Channel. A traffic channel s further divided into Speech Traffic Channel, Circuit Data Traffic Channel and Packet Data Traffic Channel, while the Control Channel is subdivided into Broadcast Channel, Common Control Channel and Dedicated Control Channel.
Logical channel CCH CCCH DCCH TCH SDCCH ACCH BCCH Downlink Uplink FCCH (BCCH1) SCH (BCCH2) BCCH
(BCCH3) PCH AGCH RACH SACCH FACCH
Figure 2-2 illustrates the logical channels. Below is the introduction.
II. Traffic Channel
1) Speech traffic channels
In the latest GSM 05.02, the speech traffic channels are divided into: z TCH/FS: full rate traffic channel for speech.
z TCH/HS: half rate traffic channel for speech.
z TCH/EFS: enhanced full rate traffic channel for speech. z TCH/AFS: adaptive full rate traffic channel for speech. z TCH/AHS: adaptive half rate traffic channel for speech.
Huawei BSS currently supports three types of traffic channels for speech: TCH/FS, TCH/HS and TCH/EFS.
2) Circuit data traffic channel
In the most updated GSM 05. 02, the circuit data traffic channels are divided into:
z TCH/F9.6: full rate traffic channel for 9.6 kbit/s user data. z TCH/F4.8: full rate traffic channel for 4.8 kbit/s user data. z TCH/H4.8: half rate traffic channel for 4.8 kbit/s user data. z TCH/H2.4: half rate traffic channel for 2.4 kbit/s user data. z TCH/F2.4: full rate traffic channel for 2.4 kbit/s user data. z TCH/F14.4: full rate traffic channel for 14. 4 kbit/s user data.
z E-TCH/F28.8: enhanced circuit switched full rate traffic channel for 28.8 kbit/s user data.
z E-TCH/F32.0: enhanced circuit switched full rate traffic channel for 32.0 kbit/s user data.
z E-TCH/F43.2: enhanced circuit switched full rate traffic channel for 43.2 kbit/s user data.
Huawei BSS currently supportsTCH/F9.6, TCH/F4.8 and TCH/F2.4. 3) Packet Data Traffic Channel
There are two rates for the PDTCH:
PDTCH: full-rate PDTCH. With GMSK modulation it can carry packet data whose momentary rates are 0~22.8 kbit/s, while PDTCH with an 8PSK modulation system can carry packet data whose momentary rates are 0~69.6 kbit/s.
PDTCH is a one-way channel and categorized by the direction as:
z PDTCH/D: downlink PDTCH, for MS terminated packet transmission. z PDTCH/U: uplink PDTCH, for MS originated packet transmission.
III. Broadcast Channel (BCH)
BCH is used to transmit broadcast messages to the MS in down link direction. It includes the following logical channels:
1) FCCH (Frequency Correction Channel): This channel is responsible for transferring the frequency correction signals to the MS so that the MS can be adjusted to the corresponding frequency.
2) SCH (Synchronization Channel): This channel is responsible for transmission of the frame synchronization number (TDMA frame number) and the Base Station Identity Code (BSIC) to the MS.
3) BCCH (Broadcast Control Channel): This channel transmits the information common to all cells, such as Location Area Identity (LAI), cell maximum allowable output power, BCCH carrier frequency of the adjacent cells, and packet service system parameters.
4) PBCCH (Packet Broadcast Control Channel): This channel transfers the messages related to packet services.
5) Cell Broadcast Channel (CBCH): This channel is used for the cell broadcast short message services. It uses the same physical channels as SDCCH.
The channels introduced above are downlink channels.
IV. Common Control Channel (CCCH)
CCCH are classified into the following four channels:
1) Paging Channel (PCH): Downlink channel. MS tunes to and receives the information from this channel to check for any call from MSC at regular intervals. 2) Random Access Channel (RACH): Uplink channel, through which an MS
accesses the network and requests for allocating SDCCH.
3) Access Grant Channel (AGCH): Through which the network notifies the MS about the allocation of the dedicated channel.
4) NCH (Notification Channel): Downlink channel used for Voice Group Call Service (VGCS) and Voice Broadcast Service (VBS).
V. Packet Common Control Channel (PCCCH)
PCCH includes the following four channels:
1) PPCH (Packet Paging Channel): Downlink packet paging channel. MS tunes to the PPCH channel at a regular interval to check if there is any call from SGSN. 2) PRACH (Packet Random Access Channel): Uplink packet random access
channel. MS requests to access the network via the PRACH channel.
4) PNCH (Packet Notification Channel): Downlink channel, designed for point-to-multipoint multicast call.
Huawei BSS supports PPCH, PRACH and PAGCH.
VI. Dedicated Control Channel (DCCH)
DCCH consists of the following channels:
1) SACCH (Slow Associated Control Channel): Associated with the SDCCH or TCH. This channel is designed for MS to send received signal quality and signal intensity of adjacent BTSs to the network, and meanwhile receives the system information including transmission power, power adjustment and timing advance. SACCH can be further divided into:
z SACCH/TF: SACCH associated with TCH/F. z SACCH/TH: SACCH associated with TCH/H. z SACCH/C8: SACCH associated with SDCCH/8. z SACCH/C4: SACCH associated with SDCCH/4.
z SACCH/M: SACCH associated with TCH/F for multi-TS configuration.
2) FACCH (Fast Associated Control Channel): FACCH implements transmission by occupying a part on TCH, mostly for transmitting handover command.
FACCH can be further divided into:
z FACCH/F: FACCH associated with TCH/F; z FACCH/H: FACCH associated with TCH/H.
3) SDCCH (Standalone Dedicated Control Channel): it serves to transmit the signaling such as short message information, location updating information, etc. between the MS and the network, prior to the call setup.
z SDCCH/8SDCCH/8
z SDCCH/4SDCCH/4
VII. Packet Dedicated Control Channel
1) PACCH (Packet Associated Control Channel): Downlink channel serving to transmit the signaling, including response messages and power control messages, to the MS. PACCH can also transmit the resources allocation and re-allocation messages. PACCH shares the resource with the PDTCH currently allocated to MS. When MS is in transmission mode, SGSN can page the MS via PACCH to initiate CS service.
2) PTCCH/U (Packet Timing Advance Control Channel Uplink): PTCCH/U sends the timing advance by way of random access burst when the MS operates in a transmission mode.
3) PTCCH/D (Packet Timing Advance Control Channel Downlink): PTCCH/D is designed to send transmission timing advance to several MSs. One PTCCH/D corresponds to several PTCCH/Us.
VIII. Radio channel management
Radio channel management involves the management of diverse radio channels in the GSM/GPRS. This process occurs in the phase of connection setup, maintenance, modification and release.
IX. Radio channel combination
As per the logical channel types as listed above, a user can configure the following channel combinations in the M900/M1800 BSS.
z TCH/F+FACCH/F+SACCH/TF z SDCCH/8+SACCH/C8 z FCCH+SCCH+BCCH+CCCH z FCCH+SCCH+BCCH+CCCH+SDCCH/4+SACCH/C4 z BCCH+CCCH z BCCH+CBCH z SDCCH+CBCH z PBCCH+PCCCH+PDTCH+PACCH+PTCCH z PCCCH+PDTCH+PACCH+PTCCH z PDTCH+PACCH+PTCCH
X. Traffic channel management
BSS is in charge of all the configured traffic channels. When a call is established, MSC sends the channel type, channel code and other parameters regarding the call to BSS, which chooses a traffic channel based on the messages. BSS also assumes the task for the measurement and release of these traffic channels.
XI. Dedicated control channel management
BSS manages all the available dedicated control channels. After MS has sends a random access request via RACH or PRACH, BSS will allocate a DCCH for the MS. Besides, BSS is also responsible for monitoring and releasing the link of DCCH.
XII. Broadcast channel and common control channel management
The management of the available broadcast channels and common control channels by the BSS involves DRX management, paging message dispatching, AGCH and PAGCH control, RACH and PRACH control, and BCCH message broadcast.
XIII. Terrestrial channel management
available when MSC makes a call (“assign circuit”) and when MS performs handover (“assign terrestrial circuit”). This is to ensure the success for the call and the handover.
Procedures included in the A-interface circuit resource management are Circuit Block/Unblock, Circuit Group Block/Unblock, Unequipped Circuit, and Reset Circuit.
General principles of the circuit control includes:
z Circuit management message is normally initiated by BSC. While resetting circuit can be initiated either by MSC or BSC,
z MSC can only block or unblock its circuits without affecting the circuits at the BSS side.
z The BSS can not change the circuit state that has been changed at the local end of the MSC. For circuits blocked on the maintenance console at MSC side, the BSS has no authority to unblock or reset the circuit.
XIV. Channel Coding & Decoding
The messages are encoded/decoded before being transmitted on the radio channel to avoid radio channel interference. There are various coding and interleaving methods for different logical channels (speech, data and signaling). For a detailed description of the coding methods for various channels, please refer to the specifications GSM 05. 03.
XV. Transcoding & Rate Adaptation
Transcoding (TC) and Rate Adaptation provides an interface between the standard 64 kbit/s transmission at NSS side and the lower rate transmission at BSS side.
The conventional voice-coding mode is PCM with a rate of 64 kbit/s. It is widely applied to PSTN. Pulse Code Modulation (PCM) is used for normal speech in PLMN, at a rate of 64 kbit/s whereas in GSM, RPE-LTP or CELP coding with much lower rate (16 kbit/s) is used due to the limitation of radio channel resources. To further improve the voice quality, EFR (Enhanced Full Rate) is introduced. To implement EFR, newly designed algorithms are used but it does not affect the coding rate on the Um interface. When adopting EFR, the compression algorithm for the MS and Transcoder & Rate Adaptor Unit (TRAU) must be modified.
Generally, 3.6 kbit/s and 6 kbit/s data rates on the Um interface are arranged for the 8 kbit/s or 16 kbit/s channel (for transmission either on the full-rate channel or the half-rate channel), while the 12 kbit/s rate is for the 16 kbit/s channel.
If a PSTN subscriber wants to call an MS, rate adaptation must be performed for the voice. The TRAU is introduced to complete this function. When the BTS and the TRAU are physically detached, these conversions will be especially important. A detailed description of the conversions on the interfaces is given in the related GSM
Since the rate of each channel of existing terrestrial lines is 64 kbit/s, it is a waste if one channel is used to carry one 16 kbit/s GSM channel. To save terrestrial line resources, sub-multiplexer (SMUX) is used between MSC and BSC to multiplex 4 % 16 kbit/s channels to transmit four speech channels over one terrestrial channel.
In general, TRAU and SMUX are integrated in one unit called TCSM, i. e., it handles both rate conversion and multiplexing.
Table 2-1 introduces the full-rate coding/decoding process and enhanced full-rate coding/decoding process.
Table 2-1 Voice coding comparison
FR (Full Rate) EFR (Enhanced Full Rate)
Algorithm RPE-LTP algorithm (regular impulse excitation-long term prediction) ACELP algorithm (arithmetic code book excitation linear prediction)
Coding Process
TRAU converts the voice signal received from MSC into frames in the format of 20 ms/fr. A frame of voice data contains 160 PCM sampling points, making up 1280 bit. The output parameters after encoding are 260 bit, making up the 320 bit TRAU frame together with the synchronous header and control parameter.
TRAU converts the voice signal received from MSC into frames in the format of 20 ms/fr. A frame of voice data contains 160 PCM sampling points, making up 1280 bit. The output parameters after encoding are 244 bit, making up the 320 bit TRAU frame together with the synchronous header and control parameter.
Decoding Process
Decoding is a reverse process of coding. After TRAU receives the TRAU frames sent from the BSC, it restores them into speech data by applying decoding algorithm before sending them to MSC.
Decoding is a reverse process of coding. After TRAU receives the TRAU frames sent from the BSC, it restores them into speech data by applying decoding algorithm before sending them to MSC.
In the occasion of MS-MS session, the TRAU coding / encoding can be omitted. As the coding / encoding process will degrade the voice quality, it is possible to improve the voice quality by removing TRAU coding/decoding with Tandem Free Operation (TFO).
TFO is implemented by FTC via in-band signaling to reduce the primary coding/decoding during MS-MS session and improve the voice quality.
To set up TFO status, the following should be realized: Both parties of the session should subscribe to the same service (i.e. both to FR or EFR service). The FTCs seized by the two MSs should support TFO function. There should be no other equipment that is capable of changing the PCM signal on the PCM link between the FTCs of the MSs, i.e., it should be a direct link, because TFO message and frame are transmitted with the low bit of the PCM sampling value. If these conditions are not satisfied, FTC will perform the normal coding/decoding.
TFO can improve the voice quality of both FR and EFR, especially the former with the MOS can be improved by 0.5 points (totally 5 points).
2.1.3 System Information
I. OverviewSystem information contains the major wireless network parameter on the air interface, including network identifier parameter, cell selection parameter, system control parameter and network function parameter. By receiving system information, MS can be properly accessed and perform network selection so that it can make full use of the services and cooperate with network.
There are two modes for the transmission of system information: broadcast message and channel associated message.
In idle mode, MS communicates with the network via the broadcasting of system information. The network sends system information to MS so that MS knows its current position and the service type available. Some parameters can also control the cell reselection of MS.
When MS is establishing calls, the communication between network equipment is realized with the channel associated system information. Network equipment sends some contents in the channel-associated message to MS so as to control the behaviors such as transmission, power control and handover of MS.
The broadcast system information is closely related to the channel-associated message. The content in the broadcast system information can overlap with that in the channel associated message. While the content in the channel associated message can be inconsistent with that in the broadcast system information, because the channel associated message has the effect on only one MS, while the broadcast system information affects all MSs in idle mode.
II. Types and content of system information
There are totally 13 types: 1, 2, 2bis, 2ter, 3, 4, 5, 5bis, 5ter, 6, 7, 8 and 9. Among them, 1, 2, 2bis, 2ter, 3, 4, 7, 8 and 9 are broadcast information transmitted via BCCH under idle mode; 5, 5bis, 5ter and 6 are channel associated information transmitted via SACCH in active mode.
Type 1: Cell channel description + RACH control information (optional)
Cell channel description: all frequencies used by this cell, including BCCH frequencies and FH frequency to provide the frequency reference for MS Frequency Hopping (FH).
RACH control information: parameters such as maximum times of retransmission (MAX RETRANS), number of transmission timeslots (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), bit allowed for emergency call (EC) and access restricted user level (AC). These parameters are used to control the behavior of MS in the initial access.
Type 2: Adjacent cell BCCH frequency description + Network color code allowed +
RACH control information (mandatory)
Adjacent cell BCCH frequency description: the BCCH frequency used by the adjacent cell.
Network color code allowed: NCC allowed for the MS test on the BCCH carrier in the cell.
Type 2bis: Adjacent extended cell BCCH frequencies description + RACH control
information (optional)
Extended adjacent cell BCCH frequency description: the number of frequencies described in the frequency allocation table in system information type 2 is limited, therefore system information type 2bis contains the information of other frequencies in BA1 which are in the same frequency segment as system information type 2.
RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access.
Type 2ter: Attached multi-frequency information + extended cell BCCH frequency
description 2 (optional)
Attached multi-frequency information: Number of the multi-frame measurement needed.
Extended adjacent cell BCCH frequency description 2: describes the extended frequency allocation table of the adjacent cell (part of BA1 table). The frequency contained in this information is located at the different frequency segment as the current cell. Therefore, only the multiband MS can read this information. The single-band GSM 900 of GSM 1800 MS will skip this information.
Type 3:Cell ID + LAI + control channel description + cell option + cell selection
parameter + RACH control information (mandatory)
Cell ID: identifier of the current cell.
common control channel configuration (CCCH CONF), number of 51 TDMA multi-frames reserved for the same paging group in the paging information (BA PA MFRMS) and the interval of periodic location update.
Cell option: includes the power control indication (PWRC), discontinuous transmission (DTX) and radio link timeout value (Radio Link Timeout).
Cell selection parameter: includes the cell reselection hysteresis value, maximum Tx power level allowed for MS access to the cell (MS TXPWR MAX CCH) and minimum access level allowed for MS to access system (RXLEV Access MIN).
RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access.
System information type 3 rest bytes: cell reselection parameter information and type 3 MS control information.
Type 4: LAI + cell selection parameter + RACH control information + CBCH description
+ CBCH dynamic allocation information (mandatory)
LAI: the location area identifier of the current cell.
Cell selection parameter: includes the cell reselection hysteresis value, maximum Tx power level allowed for MS access to the cell (MS TXPWR MAX CCH) and minimum access level allowed for MS to access system (RxLEV Access MIN).
RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access.
CBCH description: includes the channel type and TDMA offset (which type of dedicated channel combination), timeslot No. (TN), training sequence code (TSC), FH channel indication (H), mobile allocation index offset (MAIO), FH serial No. (HSN) and absolute RF channel No. (ARFCN).
CBCH mobile allocation information: the relation between the sequence of frequencies used for FH and cell channel description.
System information types 4 rest bytes: cell reselection parameter.
Type 5: Adjacent cell BCCH frequency description (mandatory)
Adjacent cell BCCH frequency description: the BCCH frequency used by the adjacent cell. Comparing with system information type 2, the difference is that MS can get the frequencies described in system information type 5 in active mode, and report the
handover. Similarly, the GSM900 MS in Phase 1 recognizes only the adjacent cell frequencies described in system information type 5 and ignore those contained in 5bis and 5ter.
Type 5bis: Extended adjacent cell BCCH frequency description (optional)
Extended adjacent cell BCCH frequency description: the number of frequencies described in the frequency allocation table in system information type 5 is limited, therefore system information 5bis contains the information of other frequencies in BA2 which are in the same frequency segment as system information 5.
Type 5ter: Attached multi-frequency information + extended cell BCCH frequency
description 2 (optional)
Attached multi-frequency information: Number of the multi-frame measurement needed.
Extended adjacent cell BCCH frequency description 2: describes the extended frequency allocation table of the adjacent cell (part of BA2 table). The frequency contained in this information is located at the different frequency segment as the current cell. Therefore, only the multiband MS can read this information. The single-band GSM 900 of GSM 1800 MS will skip this information.
Type 6: Cell ID + LAI + cell option (mandatory)
Cell ID: identifier of the current cell.
LAI: the location area identifier of the current cell.
Cell option: includes the power control indication (PWRC), discontinuous transmission (DTX) and radio link timeout value (Radio Link Timeout).
Type 7: Cell reselection parameter
Cell reselection parameter: includes cell reselection indication (PI), Cell Bar Qualify (CBQ), Cell Reselect Offset (CRO), Temporary Offset (TO) and Penalty Time (PT).
Type 8: Cell reselection parameter
Cell reselection parameter: includes cell reselection indication (PI), Cell Bar Qualify (CBQ), Cell Reselect Offset (CRO), Temporary Offset (TO) and Penalty Time (PT).
Type 9: RACH control information + broadcast channel parameter
RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (Tx Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency
III. Meaning and function of wireless network parameter
1) Network identification parameters
Network identification parameters include CGI and BSIC.
CGI consists of LAI and CI. LAI is composed of MCC, MNC and LAC. System
information type 3,4 and 6 include all or part of CGI information. MS decodes the system information to get the CGI. MS decides whether to connect to the network in this cell according to the MCC and MNC indicated by CGI. It is also used to check whether the current location area has changed so as to initialize the location updating process.
MCC, consisting of three decimal digits, is allocated worldwide in unified way. MNC, consisting of two decimal digits, is allocated by the country in unified way. LAC and CI, both consisting of 2 bytes, are arranged by GSM carrier in unified way. Note that the value range of CI is 0X0001~0XFFFE, while 0X0000 and 0XFFFF are reserved.
BSIC identifies the local color code of each BTS in the GSM system. In GSM system,
frequencies are multiplexed to different extents according to the different requirements in network plan. MS differentiates two cells' same frequency with their BSICs. Therefore, it is necessary to guarantee the uniqueness of BSICs of the cells using the same BCCH carrier frequency.
BSIC is transmitted on the SCH of each cell. It consists of NCC (3 bits) and BCC (3 bits). Note that the TSC described in system information type 4 is the BCC of the current cell. 2) System control parameter
System control parameter is transmitted to MS with system information via air interface by BTS. It serves to keep contact between MS and BTS. Besides, these parameters have the direct effect on the service bearing and signaling flow of various part of system. Therefore, reasonable setting of these parameters is important in maintaining of the normal operation of GSM system.
IMSI attach and detach allowed (ATT) is used to notify MS whether the local cell
allows IMSI attach/detach process. It is transmitted in control channel description in the system information type 3. ATT has 1 bit. "0" stands for IMSI attach/detach process not allowed, and "1" stands for the process allowed.
CCCH CONF decides the integration mode of the CCCH in the cell. It is transmitted in
the control channel description in the system information type 3. CCCH CONF is a 3 bit code. For details, see Table 2-2.
Table 2-2 CCCH code meaning CCCH CONF Meaning Number of CCCH information blocks in BCCH multiframe
000 CCCH uses a basic physical channel which is not shared with SDCCH. 9
001 CCCH uses a basic physical channel which is shared with SDCCH. 3
010 CCCH uses two basic physical channels which are not shared with SDCCH. 18
100 CCCH uses three basic physical channels which are not shared with SDCCH. 27
110 CCCH uses four basic physical channels which are not shared with SDCCH. 36
Others Reserved
Note:
The CCCH CONF setting of a cell should be in line with the actual setting of the cell's CCCH. It is decided by the traffic module of the cell.
BS AG BLKS RES is transmitted in the control channel of system information type 3. It
is used together with CCCH CONF to decide the number of information blocks in each BCCH of the current cell. After setting CCCH CONF, BS AG BLKS RES will be used to arrange the occupancy ratio between AGCH and PCH on CCCH. It is possible to adjust this parameter to achieve the bearing balance between AGCH and PCH.
BS PA MFRAMS is transmitted in the control channel description in system information
type 3. It decides how many multiframes making up a cycle of a page sub-channel. This parameter actually decides how many sub-channels the PCH of a cell will be deviled into. BS PA MFRAMS is a 3 bit code. The value range is 0~7, respectively meaning that the number of multi-frame of a paging group cycled on the PCH is 2~9.
Periodic location updating timer (T3212) decides the frequency of periodic location
updating. It is transmitted in the control channel description in system information type 3. It is an 8-bit code. The value range is 0~255, each unit of which is the duration of six minutes, and 0 means no location updating.
Cell Channel Description, transmitted in system information type 1, describes the RF
Neighbor Cells Discretion, transmitted in system information type 2, 2bis, 2ter, 5, 5bis
and 5ter, describes the absolute channel No. of the BCCH TRX of the cell adjacent to the current cell. Huawei BSS supports at most 32 adjacent cells.
Extension Indication, transmitted in system information type 2 and 5, indicates
whether there are still extended adjacent cells to be transmitted in system information type 2bis and 5bis. It is a 1-bit code. "0" means that system information type 2 and 5 contains the complete BA table, and "1" means that type 2 and 5 contains part of BA table.
BA Indication transmitted in system information type 2 and 5. It is a 1-bit code, used
for MS to select the data in BA 2 before or after modification. In another word, if the adjacent cell relation of the current cell and the BA2 table is changed during a session, the BA Indication in system information type 5 will be 1 instead of stead of 0. This indicate that MS perform decoding in the adjacent cell indicated in the system information type 5 again.
Multiband Reporting (MBR), transmitted in system information type 2ter and 5ter. It is
a 2-bit code, indicating MS to report adjacent cell information on multiple frequency bands. It is applicable to multiband MS only.
2.1.4 Idle Mode Behavior
I. OverviewA powered on mobile station (MS) that does not have a dedicated channel allocated is defined as being in idle mode. The purpose of the tasks performed in the idle mode is to be able to access the system and be reached by the system from any location in the network.
When a mobile is powered on, it immediately attempts to make contact with a GSM Public Land Mobile Network (PLMN). The particular PLMN contacted may be selected either automatically or manually. The MS will look for and select a suitable cell of the chosen PLMN. It will then tune to the control channel of the cell to receive information about the available services provided by the PLMN. This selecting is known as “camping” on a cell. When an MS is in idle mode it will always try to camp on the best cell according to a signal level based criterion.
The idle mode behavior is managed by the MS. It can be controlled by parameters which the MS receives from the base station on the Broadcast Control Channel (BCCH). All the main controlling parameters for idle mode behavior are transmitted on the BCCH carrier in each cell. When the MS is powered on but neither making nor receiving any calls (idle mode) there has to be a mechanism that always selects the best cell on which to camp. Moreover, to be able to access the system from anywhere in the
specific GSM base station, tune to its frequency and listen to the system message informations transmitted in that cell. It must also be able to register its current location to the network so that the network knows where to route incoming calls. The PLMN selection mechanism, the cell selection and reselection algorithms in addition to the location updating procedure are the core of the idle mode behavior. The purpose is to always ensure that the mobile is camped on the cell where it has the highest probability of successful communication.
II. Usage
1) High signal level when accessing the system
The MS will at all times try to obtain the highest possible signal level when accessing the system. This is achieved by means of the idle mode cell selection and reselection algorithms. These algorithms will enable the MS to choose the most suitable cell to camp on, based on signal level. A cell is suitable if certain criteria are satisfied. Camping on the most suitable cell provides the MS with a high probability of good communication with the system.
The cell selection and reselection algorithms are governed by parameter settings. Using these parameters an operator can, on a per cell basis, make a specific cell more or less attractive to camp on for the MS. This makes it possible for the operator to achieve similar behavior for MSs in idle mode as in active mode. Well-designed parameter settings for cell selection and reselection in idle mode, will make the MS to camp on the cell that would have been chosen if the MS had been in active mode. 2) Control of the paging load
In idle mode the MS will notify the network whenever it changes location area by the location updating procedure. Thus, the network will be kept updated concerning which location area the MS is presently in. When the system receives an incoming call it knows in which location area it should page the MS, and does not need to page it throughout the whole MSC service area. This reduces the load on the system. If the MS does not respond to the first paging information, then the network can send a second paging information.
The MS can also, periodically and when powered on or off, notify the network of its present status by the location updating procedure. This prevents the network from doing unnecessary paging of MSs that have been powered off or left the coverage area. This would otherwise cause unnecessary load on the system.
3) Low idle mode power consumption
In idle mode, the MS only occasionally monitors the system information being transmitted in the current cell or does measurements on neighboring cells to see if a
However, most of the time it will be in “sleep mode”. Hence, the power consumption during idle mode will be low. This is also referred to as discontinuous reception (DRX).
III. Technical description
While the MS is in idle mode it will continuously make measurements on the BCCH-carriers of serving and neighboring cells to decide on which cell to camp on. It will also, if necessary, register its presence in the location area of the chosen cell by performing a location updating.
The purpose of camping on a cell is threefold:
1) It enables the MS to receive system information from the PLMN
2) The MS can initiate a call by accessing the network on the Random Access Channel (RACH) of the cell on which it is camped,
3) The PLMN will know the location area of the cell in which the MS is camped (unless the MS has entered a limited service state) and can therefore page the MS when an incoming call is received.
The idle mode task can be subdivided into four processes: z PLMN selection
z Cell selection z Cell reselection z Location updating.
Location Updating Cell Reselectin Automatic/Manual Mode Selection Indication to User User Selection of PLMN Service indication to User PLMN Selection PLMN Available PLMN Selection Cell Selection New Location
Area Initial Cell Selected
Periodic Registration
Location Updating Responses Cell & Location
Area Changes
Figure 2-3 Overall idle mode processes
2.1.5 PLMN Selection
I. OverviewThe MS will select a PLMN when it is powered on or upon recovery from a lack of coverage. It will first try to select and register on the registered PLMN if one exists. If registration on a PLMN is successful, the MS indicates this PLMN (the “registered PLMN”) and is capable of making and receiving calls on it. If there is no registered PLMN, or if the registered PLMN is unavailable, the MS will try to select another PLMN either automatically or manually depending on its operating mode, The MS normally operates on its home PLMN. However, another PLMN may be selected if, for example, the MS loses coverage. The MS will register on a PLMN if the MS finds a suitable cell to camp on and if a location-updating request is accepted. Registration has to be successful in order for the MS to be able to access that network.
However, it does not need to perform location updating if it is in the same location area belonging to the same PLMN as it was before it entered the inactive state.
The interval between attempts is stored in the Subscriber Identity Module (SIM). Only the service provider is able to set the timer value for return to home PLMN.
There are two modes for PLMN selection; automatic and manual. The automatic mode utilizes a list of PLMNs in an order of priority whereas the manual mode leaves the decision to the user and only indicates which PLMNs that are available.
II. Automatic mode
In automatic mode, the MS will select PLMN if available and allowable, in the following order if no registered PLMN exists or is available:
Home PLMN
1) Each PLMN that has been stored in the Subscriber Identity Module (SIM) in priority order
2) Other PLMNs with received signal level above -85 dBm in random order 3) All other PLMNs in order of decreasing signal level.
III. Manual mode
In manual mode, the MS will first try to select the registered PLMN or home PLMN (if no registered PLMN exist). If this registration fails or if the user has initiated a PLMN reselection the MS will indicate to the user all available PLMNs. The user can then select a desired PLMN which causes the MS to initiate a registration on this PLMN. If the selected PLMN is not allowable, an indication to the user to select another PLMN will be made.
The user can at any time request the MS to initiate reselection and registration onto an alternative available PLMN. This is done either using automatic or manual mode, depending on the mode selected by the user.
2.1.6 Cell Selection and Reselection
I. OverviewThe purpose of cell selection and reselection is to enable MS to find a most suitable cell on which MS can reliably decipher the downlink data and maintain a high communication rate on uplink (so as to realize various telecom services). Once MS has selected a cell as its serving cell, its communication with the network becomes possible on this cell.
MS will tune to the BCCH to receive the paging message and the system information broadcast on BCCH and use the RACH to send access request after it has selected this cell.
MS implements cell reselection according to the message in BA table in the system broadcast information from the serving cell. There are two BA tables in GSM network. One is transmitted in the system information via BCCH. It includes the BCCH carrier used in a certain physical area for the MS in idle mode to implement cell selection and reselection. The other one is transmitted in the system information via SACCH. It is used to indicate the MS in active mode about the BCCH carrier for handover monitoring.
In active mode, MS obtains the information of adjacent cell BCCH frequency through BA (BCCH). The process will not stop until MS receives the first BA (SACCH) information.
II. Cell selection procedure
When MS is powered on and move from blind spot of coverage to the serving area, it will search for all available frequencies in the PLMN and select the suitable cell to camp on. This is the procedure of "cell selection".
Cell selection procedure in the case of no BCCH information in MS
MS first searches the 124 RF channels of GSM 900(if the MS is a multiband one, MS searches 374 GSM 1800 RF channels), and compares the signal level on the channels to calculate the average level. The entire measurement procedure lasts 3~5 s, during which, at least five sampling points will be extracted from different RF channels.
After MS has tuned to the maximum carriers of the receiving level, it will first judge which one is the BCCH carrier (by searching FCCH burst). If so, MS will attempt to decode SCH to obtain the BCCH system broadcast information synchronous with this carrier. If the MS can properly decode BCCH data, and make sure that this cell belongs to the selected PLMN, parameter C1>0, and this cell has no access barring, MS can camp on this cell. Otherwise, MS will keep tuning to the next highest carriers until it reaches the available cell.
If no suitable cells are found after searching 30 RF channels with the highest level, MS will monitor the level of all channels and search for the BCCH of C>0 and no access barring. After finding this carrier, MS will camp on this cell without considering its PLMN ID. In this occasion, only emergency call can be implemented.
Case 1: If the access level of the MS is barred at the cell, the cell selection algorithm will not be affected, i. e., when the cell satisfies the criterion, MS will still camp on it.
Case 2: If the cell selected by MS belongs to PLMN, but access is barred (parameter CBA is set as "bar") or algorithm C1<0, MS will use the BA table obtained from this cell to search for these BCCH carriers.
If the BCCH carrier information has been stored in MS during the last powering off, MS will first search the stored BCCH carrier. If MS can decode the BCCH data of the cell, but cannot camp on it. MS will check the BA table of this cell. If no suitable cells found after all BCCH carriers have been searched, the previous procedure will be implemented.
C1 is the path loss criterion as the reference of cell selection and reselection. C1 of the serving cell should be larger than 0. The formula is as follows:
(
)
(
_ _ _ ,0)
_ _
1 RxLEV RxLEV Access MIN MAX MS TxPWR MAX CCH P
C = − − −
See Table 2-3 for formula explanation.
Table 2-3 Name of powers
Name Meaning (Unit d Bm)
RxLEV Average level MS received
RxLEV Access MIN Maximum receiving level allowed for MS to access
MS TxPWR MAX CCH Maximum transmitting power level allowed for MS to access the system
P Maximum output power of MS
C1 algorithm is used during cell selection procedure, as shown in Figure 2-4.
C1=15 C1=8
Cell1 Cell2
Figure 2-4 Cell selection
MS select the suitable cell to camp on according to the priority and C1. The selected cell is the main serving cell Figure 2-4, MS will select Cell 1 as the main serving cell to if the priorities are the same.
III. Cell reselection procedure
After MS has selected a serving cell, it will camp on this selected cell and continue the monitoring on all BCCH carriers configured in the adjacent cell frequency configuration table indicated in the BCCH system information of the serving cell if the conditions are
When monitoring these BCCH carriers, the measurement of their receiving level should base on at least the average of 5 sampling points, and the number of measured sampling points extracted from all BCCHs should be the same. The sampling points allocated to each carrier should be as even as possible in each measurement period. The six strongest BCCH carriers should be refreshed at least once per minute.
To lower the power consumption of MS, MS should measure the receiving level of each carrier in BA table when performing decoding page group. It is possible to obtain some BCCH frequencies and sample values of receiving level on the BCCH frequency of the serving cell during the appearance of MS page group.
The MS routine measurement program also includes the measurement of the BCCH carrier of the current serving cell. MS should attempt to decode all system informations broadcast on BCCH of the serving cell at least every 30 s. MS should implement decoding of BCCH data block to the BCCH carriers of the six strongest non-serving cells at least every 5 min. This data block contains the parameter concerning cell reselection. After MS has found a new BCCH carrier as one of the strongest carriers, it will decode the BCCH data of the new carrier within at least 30 sums. MS should check the BSIC of one of the six strongest carriers within at least 30s to verify that the monitored objective is the same cell. If BSIC is changed, MS will regard the carrier as a new one, and decode the BCCH data again. During the process above, MS tries not to interrupt the monitoring to PCH.
Under the following occasions, the procedure of cell reselection will be initiated. (If C2 algorithm has not been activated, C2 = C1).
MS finds that the C2 value of a cell (in the same location area as the serving cell) has been larger than that of the serving cell for 5 seconds.
MS finds that the C1 value of a cell (not in the current location area) has been larger than the sum of the C2 value of the serving cell and the cell selection hysteresis for five seconds.
The current cell barred.
MS finds the downlink failure: the criterion of downlink signaling failure is based on the downlink signaling failure counter DSC. If MS has selected a cell, DSC is set as [90/BS PA MFRMS] round number. BS PA MFRMS is the number of multiframes of the 51 TDMA frame for the BTS transmission paging information for the MSs of the same paging level. Therefore, when MS is decoding on the PCH, if succeeded, add 1 to DSC; if failed, subtract 4 from DSC. When DSC = 0, there is downlink signaling failure.
Note that after MS reselection and camping on the cell, MS should decode all of the BCCH data of the new cell to check whether the parameter concerning cell reselection has changed. If it is changed, MS will decide whether this change satisfies the criterion of cell reselection. If the criterion is satisfied, MS will camp on this cell. If MS finds that LAI has changed, it will initialize location updating.
C2 algorithm is used in cell reselection, as shown in Figure 2-5
C2=4 C2=18
Cell1 Cell2
Figure 2-5 Cell Reselection
MS selects the cell to camp on according to the priority and C1 value. The camped-on cell becomes the main serving cell. See Figure 2-5. With the same priority, MS will select Cell 2 as the main serving cell if reselection hysteresis and the reselection time are both satisfied.
IV. The impact of the network to the MS in idle mode
Network side is responsible for completing system informations broadcast and paging task for idle MSs in downlink.
System information type 2~4 and the optional type 1, 2bis, 7 and 8 are broadcast periodically from the network via BCCH. The MSs in idle mode decides whether and how to access the network according to these information.
MS of GSM 900 supports the band of GSM 900 only. It regards the EXT IND bit described in adjacent cell in system information type 2 as the standby bit. If the information sent from the multiband network is received, MS will regard that the information unit in system information type 2 contains the complete BA table and will ignore the system information type 2bis.
V. Definition of discontinuous receiving mode (DRX) and PCH
If MS in idle mode has selected its serving cell, it is ready to monitor the paging information from this cell. To lower the power consumption of MS, the GSM specification adopts the discontinuous receiving mechanism, i. e., each subscriber (IMSI) corresponds to a dedicated paging group. Each group corresponds to a paging
sub-channel of the cell. MS recognizes its paging group and the corresponding paging sub-channel according to the last three digits of the IMSI. MS in idle mode uses its own paging sub-channel to receive the paging information (or to monitor the receiving level of the BCCH carrier of the non-serving cell). MS ignores the information from other paging sub-channel or even shuts down the power of some hardware to lower its power consumption during the broadcasting of other paging sub-channels. But MS must measure the network information task periodically.
The number of the paging sub-channels can be calculated according to the configuration type and BS AG BLKS RES (how many AGCH blocks for 51 multiframes), BS PA MFRMS (how many 51 multiframes to make up a cycle of the paging sub-channel).
Common Control Channel (CCCH) includes AGCH and PCH. It is used to transmit the immediate assign information and paging information. CCCH can be bearded by a physical channel or shared by multiple physical channels. CCCH can share the same physical channel with SDCCH. The combination mode of CCCH is decided by the parameter CCCH CONF. The configuration of CCCH CONF should be consistent with that of CCCH. For the cell with one TRX, the recommended CCCH configuration is sharing one physical channel with SDCCH (3 CCCH information blocks in this case).
For some location area with very heavy paging traffic, only one physical timeslot is insufficient to transmit the paging information. Therefore, the GSM specification allows configuring extra CCCHs on the TS0, TS2, TS4 and TS6 of the carrier.
2.1.7 Location updating
Location updating is an important task of Mobile Management (MM).
I. Location Area
To locate MS, each GSM PLMN domain is divided into locations areas covering one or more cells. The location area of each MS is recorded by the network as the location reference for paging this MS. With the introduction of the concept of location area, the paging MS can be implemented with a location area instead of all cells controlled by MSC, thus lowering the paging load. Each location area is assigned with a Location Area Code (LAC), which is broadcast with the system information via BCCH.
The size of a location area has a great effect on the system. The design of location area is very important in network planning. If the coverage of a location area is too small, the location updating of MS will trigger frequently, which will increase the signaling flow of system. On the other hand, if the coverage of a location area is too large, the load of
Therefore, optimization of location area is a very important task in network planning. When designing the location areas, it is necessary to lessen the frequency of location updating on the basis of no overweigh paging load, so as to avoid waste of network resource.
II. Location updating
When MS roams from one location area to another, it is to be registered in the new location area. In other words, once driven by certain needs or finding that the LAI stored is different from that of the current cell, MS will notify the network to change the stored location area. The procedure is called location updating.
If the MS in idle mode triggers cell reselection when moving within the same location area, MS will not notify the network about this change although the serving cell has changed. If the two cells before and after reselection are not in the same location area, MS will notify the network about this change. This is "forced register".
According to the labels of location updating in the network, there are three types of location updating: generic location updating (i.e. inter-location area location updating), periodic location updating (T3212 timeout) and IMSI attach (MS powered on). Their specific differences are whether only one VLR is involved in the location updating process and whether IMSI is used in the process.
III. Generic location updating
Generic location updating is for the purpose of updating the actual MS's location registered in the network. The information unit of type of location updating in "location updating" should be indicated as generic location updating.
If the network indicates that the status of MS in VLR is unknown, the generic location updating will also be initiated as a response to the request of MM connection setup. 1) Intra-VLR location updating
This is the simplest location updating process, in which, MS does not need to provide its IMSI. It is implemented within the current VLR, and HLR will not be notified about the process.
During the initialization process, the access cause indicated in the initialization information contained in SABM frame sent from MS to the network is Location updating Request. This information also contains MSTMSI and LAI noted as for generic location updating. After receiving this information, MSC will send MAP Update Location Area to VLR. VLR, after receiving this information, will implement the location updating. It will update the location information of the MS and store the new LAI and then allocate a new TMSI for MS if necessary (TMSI can also be absent in the TMSI reallocation command. In this case, MS uses the former TMSI). After receiving TMSI Reallocation
Complete from MS, VLR sends Location updating Accept to MS, and then release the channel to end the process.
IV. Inter-VLR location updating
If MS roams to a cell whose LAI is different from the current one, it will send the old LAI and stored TMSI via MSC to VLR in the process of location updating If TMSI cannot be identified, MS can also be identified with its IMSI. See Figure 2-6.
MS MSC VLR PVLR HLR
Location Update Request
MAP Update Location Area
MAP Update Location
MAP Insert Subscriber Data MAP Insert Subscriber Data ACK
MAP Update Location ACK MAP Update Location Area ACK
Location Update Accept
MAPCancel Location MAPCancel Location ACK
A B D
D
Figure 2-6 Interfaces and process of inter-VLR location updating
1) Update with TMSI
If VLR finds that the TMSI is unknown after receiving MAP Update Location Area from MSC, it will label the "VLR Location Information Acknowledge" as "Unacknowledged" for the subsequent updating in HLR. If the subscriber has not registered in that VLR, "HLR Location Information Acknowledge" will be labeled as "Unacknowledged". And then, according to the address of the previous VLR (PVLR) indicated in TMSI and LAI, VLR will send MAP Send Identification to PVLR to request for IMSI and authentication parameter, and as a response PVLR will return the IMSI and authentication parameter to the new VLR. If the new VLR fails to get the IMSI, it will then sends Identity Request
MSC/VLR has the normal service authority, HLR will store the new VLR No., and sends MAP Cancel Location to HLR. After receiving this information, PVLR will delete all information related to this MS, and sends MAP Cancel Location ACK to HLR. The new VLR continues to handle the processes of authentication, ciphering and TMSI reallocation. When these processes are done, HLR sends MAP Insert Subscriber Data to VLR to provide the subscriber information needed, including authentication information. After receiving the response from VLR, HLR will send Location updating Ark to that VLR.
2) Update with IMSI
If the identification of the subscriber is IMSI, VLR will check whether this subscriber is unknown. If so, it will be labeled the "HLR Acknowledge" as "Unacknowledged", and then initializes HLR updating. If the IMSI is a known one, VLR will check whether the previous LAI provided in the information from MSC belongs to this VLR. If not, it will label "HLR Acknowledge" as "Unacknowledged", and then initialize HLR updating. Authentication is needed in these two cases.
V. IMSI attach and detach process
IMSI attach and detach means to attach a binary mark to the subscriber record in MSC/VLR. The former one is marked as access granted, and the latter one is marked as access denied.
IMSI attach and detach is an option of system. If the cell where MS is powered on supports this function, it will notify its power-on status to the network, i. e. sending the information of "IMSI Attach" to notify the network about the change of its current status. When the network receives this indication, it will note down the subscriber status in the system data so as to initialize the paging process when there is an paging information of this MS.
If MS finds that the stored LAI is the same as the current LAI when powered on, it will initialized the process of IMSI attach. The process is almost the same as INTRA VLR Location Updating. The only difference is that the type of location updating marked in Location Updating Request is IMSI attach.
VI. Periodic location updating
Periodic location updating is used to periodically notify the network about the accessibility of MS. MS sends Location Updating Request to the network, in which the information unit of the type of location updating is periodic location updating.
In the following cases, network will lose contact with MS:
A powered on MS roams to the area beyond network coverage (blind spot). Since the network is not notified about the current status of MS, it still considers the MS in the
1) When MS is transmitting "IMSI Detach", if there is interference to the radio uplink path, the network may not be able to decode this information. This means that system still regards this MS in the IMSI attach status.
2) In the case of MS power failure, MS cannot notify the network about its current status, resulting in the loss of contact with the network. If the above cases happen and the MS is paged, system will still sends the paging information to the location area where the subscriber registered. This paging will sure end up with paging timeout, and system resources are wasted.
To tackle this problem, the corresponding measure is taken in GSM system to make the MS automatically reports its current location information to the network periodically. N this way, the network can have the timely information of the current location status of MS. This process is called periodic location updating. BSS sends the period of periodic location updating (T3212) to all subscribers in the cell with system broadcast system via the cell's BCCH, so that MS will automatically initialized location updating request to the network when the timer times out. After cell selection or cell reselection, MS will read T3212 from the system information of the serving cell, and then activate this timer and store it in SIM. After that, whenever T3212 times out, MS will automatically initialize location updating. At NSS side, the network will periodically query the subscribers marked as IMSI attach in its VLR to mark those without any contact with it witting this period as implicit power-off in order to avoid paging these MSs and wasting system resources.
Periodic location updating is an important measure to keep the contact between the network and MSs, therefore, the more frequent periodic location updating is, the better overall performance of network can be achieved. However, frequent periodic location updating has two drawbacks:
Increase of signaling flow which may lower the processing power of MSC/BSC/BTS and the utilization of radio resources if the situation is serious;
Increase of MS power consumption which will shorten the standby time of the MSs served by this system. Therefore, the setting of T3212 should be based on the actual situation.
In the following cases, T3212 will be reset to 0:
When receiving "Location Updating Request" or "Location Updating Refuse",
Ciphering mode complete when receiving the first MM message, or MM connection being established.
MS responds to its paging, and after which, it receives the first correct L3 message (excluding RR message).
When T3212 times out, MS will initialize periodic location updating.
If T3212 times out when MS is in the status of "no available cell", or "service restricted" or "searching for PLMN, MS will delay location updating will be delayed until these status changed. If BCCH information indicates periodic location updating not applied, this process will not be activated. T3212 timeout value is broadcast in the CCH description in "system information type 3".
In the status of "no available cell", "service restricted" and "searching PLMN", T3212 cannot be changed.
MS, after cell reselection, may find that the T3212 of the new cell is different from the previous one (sharing the same LAC) or the broadcast T3212 of the current cell is manually changed). In this case, assumed that "t1" is the new T3212 timeout value and "t" is the current value of T3212, the timer of MS will be restarted with the value of t mod t1.
If MS is in the activated status, or it is necessary to change T3212 value, and the timer is not running, then the new timer will be started with a random number whose value range is 0~t1 ("t1"is new T3212 timeout value.
The signaling flow of periodic location updating is the same as that of generic location updating.
VII. Generic location updating (specification)
MS initialize location updating
If there is no RR connection available when initializing location updating, MM sub-layer of MS will request RR sub-layer to establish RR connection.
MS sends "Location Updating Request" to the network, and start T3210. The information unit of location updating type in this message will indicate the type of this location updating. On this occasion, the network can initialize the type querying procedure (e. g. to get the ciphering capability of MS). If the network cannot obtain the IMSI according to TMSI and LAI, the network can initialize the identification process. After receiving "Location Updating Request" from MS, the network will initialize the authentication process. If it is necessary to reallocate TMSI, the network will initiate the process of ciphering mode setting.
1) Attempt counter
To restrict the frequency of location updating attempt, the attempt counter is recommended in the specification. It is used to count the number of consecutive unsuccessful location updating. When a location updating failure occurs, the counter will add one. The attempt counter will be reset in the following cases:
z SIM inserted.
z Location update successfully completed, and the service statuses switch from Attempting to Update.
z MS roaming into a new location updating area. z T3212 timeout.
z Location update initiated by CM sub-layer.
Attempt counter is used to decide whether to implement another attempt after T3212 timeout.
2) Location update accepted by the network
If the network accept location updating, it will send "Location Updating Accept" to MS. When authenticating the validity of security, TMSI reallocation is a part of location updating process. "Location Updating Accept" contains the TMSI allocated for MS and the current LAI. In this case the network will initialize T3250.
If the network needs to prolong the RR connection so that MS can initialize MM connection (e.g. MS sends a request subsequent to "Location Updating Request"), the network will attach "Continue" to "Location Updating Accept" and initiate T3255. After receiving "Location Updating Accept", MS stores LAI, terminates T3210, restarts the attempt counter, and sets the status in SIM as Updated. If what contained in the message is IMSI, MS will delete the corresponding TMSI stored in SIM. If the message contains TMSI, MS will store it in the SIM and send "TMSI Reallocation Complete" to the network. If neither of them can be received, MS will delete the original TMSI stored in SIM. If the LAI or PLMN identifier in "Location Updating Accept" is one of "Barred series", all of original input will be deleted. After that, MS will use "Continue" to direct it action. If this unit exists, and MS has the underway CM service request, it will send "CM Service Request" to the network.
3) Location update denied by the network
If location updating is denied, the network will send "Location Updating Denied" to MS. After receiving this message, MS will terminate T3210, store the reject cause, activate T3240, enter location denied status and wait for the network to trigger RR connection release.
a) If the reject cause is IMSI unknown to HLR, invalid MS, invalid ME.
MS will set the location updating status as Roaming not Allowed, and store it in SIM. Delete TMSI, stored LAI and ciphering SN and regard the SIM as an invalid one until MS powered off or SIM removed.
b) If the reject cause is : PLMN not allow, location area not allow, international roaming not allowed in this location area.
allowed in this location area", it will return to MM Idle and then implement PLMN selection instead of cell selection.
Other situations will be treated as abnormal ones. 4) RR connection release after location updating
After location updating, MS will set T3240, enter "wait for network command phase" and wait for the release of RR connection.
If MS cannot receive RR connection release command from the network within a period of time (controlled by T3240), it will terminate RR connection. No matter RR connection is released by MS or the network, MS will enter "idle status".
5) Abnormality at MS side
a) Access denial controlled by access level, unable to initiate location updating. MS camps on the current serving cell, and implements normal cell reselection. Try to initiate before denial status ends or cell changed.
b) Random access delayed (after receiving Immediate Allocation Denied): unable to initiate location updating. MS stays in the selected cell and initializes normal cell selection. When changing, initialize location updating before T3122 timeout.
c) Random access failed: activate T3213. Activate location updating after it times out.
d) RR connection failure: terminate location-updating process.
e) T3210 timeout: terminate location updating process and RR connection.
f) RR released before normal termination: terminate location-updating process.
g) Location update denied caused by other reasons: MS waits for RR connection release.
For (d) ~ (g) and random access occurring for many times, MS will terminate T3210. When T3210 times out, RR connection will be canceled, and attempt counter adds 1. The action afterwards is decided by LAI and the record of the attempt counter:
a) The update status is "Updated", the stored LAI equals to the one received from the previous cell, and the record of attempt counter is four. MS will maintain the "Updated" status. The MM idle status after RR connection release is "Normal Service". MS stores the type of location updating. After RR connection release, T3211 will be activated. After T3211 timeout, MS will reinitiate location-updating process (adopting the stored type).
b) If the update status is not "Updated", or the stored LAI is different from the one received from BCCH, or the record in the attempt counter is larger than 4.
After RR connection release, MS will delete LAI, TMSI, ciphering SN in SIM, set the update status as "Not updated", and enter MM idle sub-status "Attempt update". If the
record in attempt counter is smaller than four, T3211 stored in MS will be initiated during RR connection release, otherwise the stored T3212 will be initiated.
6) Abnormality at NSS side a) RR connection failure
If the RR connection failure occurs successively when there is a common program, the network should implement according to the common program description. If RR connection failure occurs successively and there is no common program, the location updating process should be terminated.
b) Protocol error
If protocol error exits in "Location Updating Request", the network should return "Location Updating Denied". The reject cause is
z Mandatory information unit incorrect
z Information unit not exist or unable to be realized z Invalid information unit content
z Protocol error, not regulated
When these errors occurs, the network will initialize the process of channel release.
2.1.8 Access
I. Circuit service access
An MS can be either in "active" state or in "idle" state. In idle mode, MS is not allowed to implement any transmission. In the "dedicated/active" mode, the MS can make effective transmission to the network through an allocated channel.
In idle mode, MS gives the access cause and analysis of the cause in the 8-bit information during access request, and gets the channel for access after channel allocation. If the network cannot select the suitable channel type with limited cause analysis, it will allocate an SDCCH by default. If there is no available channel during channel allocation, the network will notify the MS to implement access attempt after a period of time with the command "Immediate Assign Denied". After the channel activation via Abis interface, the network sends "Immediate Assign" to MS. After receiving "Immediate Assign", MS sets up a dedicated channel to the network with "Setup Indication" and enters active mode. After receiving the setup indication reported by the MS, BSC analyzes the contents of the setup indication, including the processing of MS class mark, power control record, and encryption information. Then BSC transmits the setup indication reported by the MS to MSC.
