The process of initialization is actually the process of random access. Whenever MS needs to set up connection with the network, it has to send a message to the network via RACH to request for a signaling channel. The network will decide the type of the channel to be allocated according to the channel request. This message sent via RACH is called Channel Request. In this message, there is only 8 bits of meaningful signaling message. In which, 3 bits are used to the minimum indication of the access cause (in Phase 1, the cause occupies only 3 bits; in Phase 2, due to concept of half rate, the bit occupied by the cause is not a fixed, and the maximum one can be 6 bit). Such as emergency call, location updating, response to paging or caller request, etc. In the case of network congestion, system can implement different processing (which type of call will be accepted or denied) to the channel request of different access purposes according to this rough indication, and allocate the most suitable channels for them. In this indication, due to the capacity limit of the channel, it is impossible to transmit all information to be transmitted, such as the specific cause of channel request, subscriber identity and the feature of mobile equipment (all transmitted in SABM) to the network.
The other 5 bits is the identification code selected by MS at random (for Phase 1 standard). It is not used to notify the network about the MS's location but to enable the network to identify the request initialized by different MSs. After that, the network will send "Immediate Assign Command" (includes the information of the allocated channel) to MS. The identification code will be returned to MS in this message. MS judges whether the information is for it by comparing the identification code it sent and the one returned from the network. But it has only 5 bits, which can be used to differentiate 32 MSs simultaneously. Two MSs initializing calls simultaneously do not necessarily have the random identification codes different from each other. To further differentiate MSs initializing calls simultaneously, the response messages on Um interface are used as another reference. The channel request message is processed only within BSS.
All MSs with SIMs belong to a level among Level 0~9. The access level is stored in SIM.
MS can also belong to one of the 5 special access levels (Level 11~15). Such level is also stored in SIM.
In BCCH system information, the information, such as the access levels and special access levels allowed by the network, and whether all MS or only those of special levels is allowed to initiate emergency calls, will be broadcast.
If the setup cause requested by MM is not emergency call, then only when MS belongs to the access level or special access level, can its access be grated. If the setup cause requested by MM is emergency call, then only when all MSs in the cell are allowed to initialize emergency call, or belongs to the allowed special access level, can their access be grated.
Since the network cannot control the access time of MS, the event of two MSs
receive a burst level from this timeslot obviously higher than the other. In this case, the network will process the random access request with higher level. The other one is that the network can receive neither of them due to their mutual interference. With the increase of traffic, the possibility of loss of message due to collision will become higher.
This will become the major problem of network capacity. Therefore, it is necessary to introduce the mechanism of retranslating channel request.
MS figures out that it allowed to transmit "Channel Request" via RACH for at most M + 1 times with the following methods:
z The timeslot No. Between the assign process and a "Channel Request" (not including the timeslot containing the information itself) is selected at random from {0, 1 MAC (T, 8)-} with the same probability.
z The timeslot No. between the two consecutive "Information Request" of MS is selected at random from {S, S + 1, , S + T – 1} with the same probability.
T is the parameter "Tx integer" broadcast on BCCH; M is "Max Retrans"; the value of S depends of the configuration of CCCH. See Table 2-4.
Table 2-4 Value of Parameter S
Tx Non-combined CCCH Combined CCCH/SDCCH
3, 8, 14, 50 55 41
4, 9, 6 76 52
5, 10, 20 109 58
6, 11, 25 163 86
7, 12, 32 217 115
If the immediate assign command is not received even after Max Retrans, MS will return to idle mode.
After transmitting initial channel request, MS will activate T3120 and stay on the entire downlink CCCH (to receive answer) and BCCH.
When T3120 times out while RACH retransmission times has not exceeded "Max Retrans", MS will retransmit the channel request message containing a new random reference, and activate T3120 with a new value.
When T3120 times out, and the Max Retrans is reached, MS will activate T3126, and then wait for a period of time and allow network to give up. If no network response received after T3126 timeout, MS will give up request attempt and perform cell reselection.
2) The allocation of the initial channel
After correctly decoding the Channel Request of MS, BTS will send Channel Required to BSC via Abis interface. This message contains important attachment information and the estimation to TA that is important to activating timer advance control. After receiving this message, BSC will select a corresponding idle channel for MS according to the judgement to the existing radio resources. However, the availability of the allocated channel and the related terrestrial resources is to be acknowledged with the response from BT. This process is realized by sending "Channel Active" from BSC to BTS to query the availability of corresponding terrestrial resources (e. g. transmission circuit). This message indicates all properties needed in activating the channel, including channel type, working mode, physical feature and initial lead. When the corresponding resources are ready, BTS will return "Channel Active ACK" as a response to BSC.
a) Immediate assignment
After BSC receive Channel Active ACK from BTS, will send Immediate Assignment or Extended Immediate Assignment to allocate dedicated signaling channel for MS in the non-acknowledge mode, via the CCCH for MS receiving Channel Request. Immediate Assignment contains the assignment information of only one MS, while Extended Immediate Assignment contains the assignment information of two MSs. BTS can send Immediate Assignment or Extended Immediate Assignment on any message block of downlink CCCH. Therefore it is necessary for MS to monitor all information block on CCCH. The allocated channel type (TCH or SDCCH, channel mode is set as signaling) is decided by the carrier.
Normally, if there is an idle SDCCH available that can satisfy the access request, BSC will allocate SDCCH. The process of requesting for SDCCH connection includes location updating, IMSI detach, supplementary service, short message at non-session status and services only supported by SDCCH. MS initializes access request, and BSC allocates a SDCCH for this call. This channel seizes 1/8 sub-timeslots of a timeslot. The signaling interaction necessary for call establishment is implemented on that channel.
The signaling flow of SDCCH immediate assignment is illustrated in Figure 2-7.
SDCCH: Assignment Command SDCCH: Ciphering Mode Complete
SDCCH: Authentication Response SDCCH: Authentication Request
MS BTS BSC MSC
Channel Request
Ciphering Mode Command Channel Required
Channel Active Channel Active ACK Immediate Assignment Immediate Assignment
SDCCH: SABM
SDCCH: UA Establishment Indication
Complete Layer3 Informaiton
Encryption Command SDCCH: Ciphering Mode Command
SDCCH: Setup SDCCH: Call Proceed
Assignment Request Channel Active
Channel ACK Assignment Command TCH: SABM
TCH: UA
TCH: Assignment Complete Assignment Complete
TCH: Alert TCH: Connect TCH: Connect ACK
Figure 2-7 Immediate assignment
If TCH has been allocated before immediate assignment, there is no need to reallocate TCH during the process of assignment. Mode conversion process can be used to change the function of TCH from signaling transfer to voice transmission. Signaling flow of TCH immediate assignment is illustrated in Figure 2-8.
TCH: Ciphering Mode Complete
Messages of immediate assignment or extended immediate assignment contain:
z Description of assigned channel.
z Information field of channel request and abbreviated frame No. of the received channel request frame (abbreviated frame No. is a frame No. with narrow value range calculated from the TDMA frame No. received by BTS during channel request.)
z Initial lead; Start time indication (optional).
The random reference and abbreviated frame No. are directly related to the MS channel request. They are used to reduce the conflict of request among MSs. TA is the initial lead calculated from equalizing the channel request information received by BTS on RACH. MS figures out the next initial lead for transmitting according to TA.
After receiving immediate assignment or extended immediate assignment, MS switches to the channel assigned by the network, sets the channel modes as signaling only and sends the SABM with information field via the allocated channel to establish the main signaling link.
Immediate assignment or extended immediate assignment message, containing the start time and description of possible alternative channel, can be used to indicate the frequency change in the process.
If the received immediate assignment or extended immediate assignment message contains only the description of the channel used after start time, MS will access to the channel during the time waiting for start. If it misses the time, MS will immediately switch to this channel after receiving the message.
If the message contains the description of the channel used after indication time as well as that before the indication time, MS will access the channel after receiving the message. If MS is ready for access to the channel upon the indicated time, MS will first access to the channel used before the indication time, and switch to the one after the indication time when the time comes (new frequency series, MAIO and HSN). If MS is ready after the specified time it will access the channel after the indication time.
If MS has already sent the channel requests for maximum allowed times RR entity will start T3126. After T3126 timeout, immediate assignment process will be terminated. If the random access program is initiated by MM, system indicates failure of random access to MM.
After sending the first channel request message, MS starts to monitor the system message on BCCH as well as the CCCH timeslot corresponding to its paging group, i. e.
immediate assignment command may appear in any CCCH message block in 51 multiframe. Therefore it is necessary for MS to monitor the entire CCCH block after sending channel request, i. e. decode the messages of the entire paging sub-channel for response from the network.
If the network adopts FH, MS will decode MA with the CA got from BCCH system message. CA refers to all the frequencies used in the cell (including FH frequencies).
MA refers to all FH frequencies used in the cell.
b) Immediate assignment denied
If there is no available channel for BSC to allocate, the network can send the immediate assignment denied message in non-acknowledge mode to MS via CCCH. The reject cause can be MSC traffic closed, radio resources shortage, TA value exceeding limit, channel activation no response and BSC traffic overload. But system does not specify the part on downlink CCCH for immediate assignment denied transmission. The message of immediate assignment denied contains request reference and waiting indication.
After receiving Immediate Assignment Denied, as the response to one of the last three channel requests, MS terminates T3120, activates T3122 with the specified value and returns CCCH idle mode. MS cannot start RR connection attempt until T3122 timeout.
MS is not allowed to initialize another call attempt except for emergency calls until
T3122 timeout. Emergency call attempt can be established in the same cell before T3122 timeout as long as no "Immediate Assignment Denied" of RR emergency attempt received.
It corresponds to the immediate assignment extension. In order to improve AGCH efficiency, the format of extended immediate assignment denied is introduced. The message of extended immediate assignment denied can contain information of rejecting at most four MSs.
The value of the wait indication information unit (T3122) depends on the cell receiving this message.
After T3122 timeout, MS will not respond to paging but sends "Channel Request"
instead till MS receives "Paging Request".
c) Signaling channel assignment overlap
The system may have a slow response to the channel request of MS, which results in request retransmission. In this case, system do not know whether a channel request message is a retransmitted one, so it may send the immediate assignment command to the MS for multiple times. MS will use the channel in the first assignment message it decoded. The others are regarded invalid ones. But according to the specification, MS should receive the last three network response messages to the channel request. This is called allocation overlapping. It is possible to cope with CCCH congestion caused by to many overlapped allocations by reducing the retransmission of MS or shorten T3101.
This measure can avoid the waste of system resources.
3) Initialization message
After receiving immediate assignment command, MS will decode this message. If the random identification code and the abbreviated frame No. satisfy the requirement, MS will tune its transceiver equipment to the specified channel and start to transmit signaling according to TA specified by BSC and maximum transmitting power (defined in the parameter "MS TxPWR MAX CCH" in BCCH system broadcast message). The first task for MS on the allocated SDCCH/TCH is to send a SABM frame to establish asynchronous balance mode (service access point type: SAPI = 0) so as to establish signaling message link connection in acknowledge mode. In GSM specification, SABM has a signaling message, i. e. initialization message. On Um interface, SABM frame is a message requesting for the establishment of a multiframe response operation mode on LAPDm. This message contains the L3 service request message. The reason for different standards about standard HDLC is to guarantee the correctness of MS receiving. If two MSs send the channel requests with the same message content at the same (possible in the case of high load), BSS will repines to one of them only. While these two MSs can both be allocated with the same dedicated channel. To settle this
completely the same as that of SABM frame after the cell has received the SABM frame.
MS compares it and the SABM information. If the content is completely the same, the access will proceed. Otherwise, it will give up this channel and repeat the immediate allocation process. Only when the consistency is guaranteed, will MS stay on the channel.
According to different request causes, the initialization messages in SABM can be divided into four types: CM service request (call establishment, short message, supplementary service management), location updating request (generic location updating, periodic location updating, IMSI attach), IMSI detach and paging response.
All these messages contain the identity of MS, detailed access cause and Classmark of MS (used to indicate some key features of MS, such as transmission power level, ciphering algorithm, short message capability and frequency capability).
Upon receiving SABM frame, BTS will send a message "Establishment Indication" to BSC. On Abis interface, this message is used to notify LAPDm that the connection has been established. It is a response to the immediate assignment message. After receiving the indication message of establishment, BSC will send a L3 service request message (Complete Layer3 INFO) to MSC. To be specific, this message is Location Updating Request, CM Service Request, Paging Response and IMSI Detach. This message contains the SCCP connection request (SCCP CR), cause of CM service request (e.g. MO call, emergency call, location updating and short message service), ciphering key sequence No., LAC, CI, physical information of this MS (e.g. transmitting power level, ciphering algorithm support, pseudo-synchronous capability and short message capability) and the ID of MS.
Although the MTP connection at An interface has been established before the session, there should still be a SCCP connection on L2 for each call. This establishment request message will be transmitted in the SCCP CR message via A interface. If the request is permitted, the first downlink message at An interface will be contained in the CC frame at SCCP layer. For SCCP layer, the exchange between CR and CC is the exchange between original reference address and destination reference addresses. For different calls, the same SPC may refer to different original addresses and destination addresses. If SCCP cannot be established, MSC will send the message SCCP Refused. The access ends at this step. The signaling link between MS and MSC has been established, MSC at this phase is able to control the transmission feature of the RR management, and BSS is in the status of monitoring transmission quality and ready for handover.
4) Phase1 and Phase2 MS
BSC cannot differentiate whether a call is for voice, data or signaling completely according to MS establishment cause. In the case of Phase 2 MS, BSC can obtain the access request cause more detailed than that of Phase1 MS. For Phase 2 MS, BSC is able to recognize the information unit "Channel Needed" in the paging message. This
information unit indicates whether the current channel is for voice/data or signaling. MS selects a suitable establishment cause to response according to its own capability.
Huawei BSC supports the information unit "Channel Needed" in the paging message.
III. Parameter
Huawei BSC controls the function of immediate TCH assignment with the switch of
"Immediate assign TCH. The detailed configuration process, as well as the data table and parameters involved are listed below:
[Cell/Modify Cell's Call Control Parameter/Modify Cell Call Parameter/Call Control]
Parameter: Immediate Assignment of TCH
If “Immediate Assignment of TCH” is set as “No”, this indicates that the function of immediately assigned TCH is disabled. All call access requests use SDCCH.
If “Immediate Assignment of TCH” is set as “Yes”, this indicates that the function of immediately assigned TCH is enabled. For emergency call and call re-establishment, BSC will preferentially assign TCH for them. If no idle TCH is available, the BSC assigns SDCCH for them. For other the channel access request of calls, SDCCH will be preferentially assigned and then TCH.
2.1.11 Assignment
I. Overview
BSS switches MS to TCH by means of assignment. Normally, the assignment is finished at the cell where the call is initialized. Huawei BSC supports the function of direct retry, which can assign MS to other cells.
Huawei BSC supports re-assignment process. When BSC receive ASSIGNMENT FAILURE from UM interface, BSC allocate a new radio channel and initiate the second assignment process.