Gsm Immediate Assignment

 

 

 

This page is based on a call processing sequence between an MS and GSM Network Simulator. The details of the sequence and contents of the messages would differ from those from live network, but I think this would contain all the essential components of the call processing. The value in the "Time(sec.ms)/Memo" column is just for a reference to give you rough timing between each step and the next step. It is not the clean-cut/absolute value.

(This is at very early stages of the whole page that I am planning, so I will have on-going update for a long period).

 

 

 

RR  : System Information Type

 

NAS:RR,System Information Type 1

System Information Type 1 ::= DIVISION

  +-L2 Pseudo Length ::= V

  | +-L2 Pseudo Length value ::= INT (0..63) [21]

  | +-N(SD)or0_1 ::= FIX [0]

  | +-N(SD)or0_2 ::= FIX [1]

  +-Skip Indicator ::= V

  | +-Skip Indicator ::= FIX [0]

  +-RR management Protocol Discriminator ::= V

  | +-Protocol Discriminator ::= PD [6]

  +-System Information Type 1 Message Type ::= V

  | +-Message Type ::= MSG [19]

  +-Cell Channel Description ::= V

  | +-Octet1-16 ::= DIVISION

  |   +-Format-ID bit128-127 ::= SWITCH [bit map 0]

  |   +-(Format-ID bit128-127)=0 ::= BRANCH

  |     +-spare ::= FIX [0]

  |     +-Cell Allocation Absolute RF Channel Number bit124-121 ::= INT (0..15) [0]

  |     +-Cell Allocation Absolute RF Channel Number bit120-001 ::= OCTETARRAY SIZE(15..15)

                                                                 [000000000000000000000000000000]

  +-RACH Control Parameter ::= V

  | +-Octet1 ::= DIVISION

  | | +-Max retrans ::= CHOICE [Maximum 1 retransmission]

  | | +-Tx-integer ::= CHOICE [3 slots used to spread transmission]

  | | +-CELL_BAR_ACCESS ::= CHOICE [The cell is not barred]

  | | +-Call reestablishment allowed ::= CHOICE [Call Reestablishment not allowed in the cell]

  | +-Octet2-3 ::= DIVISION

  |   +-Access Control Class C15-C11 ::= INT (0..31) [0]

  |   +-Emergency Call allowed ::= CHOICE [Emergency call allowed in the cell to all MSs]

  |   +-Access Control Class C9-C0 ::= INT (0..1023) [0]

  +-SI 1 Rest Octets ::= V

    +-flg0 ::= SWITCH [L]

    +-Band indicator ::= CHOICE [ARFCN indicates 1800 band]

    +-spare padding ::= PAD SIZE(23..23) [2B]

 

 

NAS:RR,System Information Type 2

System Information Type 2 ::= DIVISION

  +-L2 Pseudo Length ::= V

  | +-L2 Pseudo Length value ::= INT (0..63) [22]

  | +-N(SD)or0_1 ::= FIX [0]

  | +-N(SD)or0_2 ::= FIX [1]

  +-Skip Indicator ::= V

  | +-Skip Indicator ::= FIX [0]

  +-RR management Protocol Discriminator ::= V

  | +-Protocol Discriminator ::= PD [6]

  +-System Information Type 2 Message Type ::= V

  | +-N(SD)or0 ::= FIX [0]

  | +-Message Type ::= MSG [1A]

  +-BCCH Frequency List ::= V

  | +-Octet1 ::= DIVISION

  | | +-bit128-127 ::= INT (0..3) [0]

  | | +-EXT-IND ::= CHOICE [The information element carries the complete BA]

  | | +-BA-IND ::= INT (0..1) [0]

  | | +-bit124-121 ::= INT (0..15) [0]

  | +-Octet2-16 ::= DIVISION

  |   +-bit120-001 ::= OCTETARRAY SIZE(15..15) [000000000000000000000000000200]

  +-NCC Permitted ::= V

  | +-NCC permitted ::= INT (0..255) [255]

  +-RACH Control Parameter ::= V

    +-Octet1 ::= DIVISION

    | +-Max retrans ::= CHOICE [Maximum 1 retransmission]

    | +-Tx-integer ::= CHOICE [3 slots used to spread transmission]

    | +-CELL_BAR_ACCESS ::= CHOICE [The cell is not barred]

    | +-Call reestablishment allowed ::= CHOICE [Call Reestablishment not allowed in the cell]

    +-Octet2-3 ::= DIVISION

      +-Access Control Class C15-C11 ::= INT (0..31) [0]

      +-Emergency Call allowed ::= CHOICE [Emergency call allowed in the cell to all MSs]

      +-Access Control Class C9-C0 ::= INT (0..1023) [0]

 

 

NAS:RR,System Information Type 3

System Information Type 3 ::= DIVISION

  +-L2 Pseudo Length ::= V

  | +-L2 Pseudo Length value ::= INT (0..63) [18]

  | +-N(SD)or0_1 ::= FIX [0]

  | +-N(SD)or0_2 ::= FIX [1]

  +-Skip Indicator ::= V

  | +-Skip Indicator ::= FIX [0]

  +-RR management Protocol Discriminator ::= V

  | +-Protocol Discriminator ::= PD [6]

  +-System Information Type 3 Message Type ::= V

  | +-Message Type ::= MSG [1B]

  +-Cell Identity ::= V

  | +-CI value ::= INT (0..65535) [1]

  +-Location Area Identification ::= V

  | +-Octet1 ::= DIVISION

  | | +-MCC digit 2 ::= INT (0..15) [0]

  | | +-MCC digit 1 ::= INT (0..15) [0]

  | +-Octet2 ::= DIVISION

  | | +-MNC digit 3 ::= INT (0..15) [15]

  | | +-MCC digit 3 ::= INT (0..15) [1]

  | +-Octet3 ::= DIVISION

  | | +-MNC digit 2 ::= INT (0..15) [1]

  | | +-MNC digit 1 ::= INT (0..15) [0]

  | +-Octet4-5 ::= DIVISION

  |   +-LAC ::= INT (0..65535) [1]

  +-Control Channel Description ::= V

  | +-Octet1 ::= DIVISION

  | | +-MSC Release ::= CHOICE [MSC is Release '98 or older]

  | | +-Attach-detach allowed ::= CHOICE [MSs in the cell shall apply]

  | | +-BS-AG-BLKS-RES ::= INT (0..7) [0]

  | | +-CCCH-CONF ::= CHOICE [1 basic physical channel used for CCCH: not combined with SDCCHs]

  | +-Octet2 ::= DIVISION

  | | +-spare ::= FIX [0]

  | | +-CBQ3 ::= CHOICE [Iu mode not supported]

  | | +-spare ::= FIX [0]

  | | +-BS-PA-MFRMS ::= CHOICE [2 multiframes period]

  | +-Octet3 ::= DIVISION

  |   +-T3212 time-out value ::= INT (0..255) [0]

  +-Cell Options ::= V

  | +-DN-IND ::= CHOICE [Dynamic ARFCN mapping is not used by the PLMN]

  | +-PWRC ::= CHOICE [PWRC is set]

  | +-DTX ::= CHOICE [2-The MS shall not use uplink discontinuous transmission]

  | +-RADIO-LINK-TIMEOUT ::= CHOICE [16]

  +-Cell Selection Parameters ::= V

  | +-Octet1 ::= DIVISION

  | | +-CELL-RESELECT-HYSTERESIS ::= CHOICE [0 dB RXLEV hysteresis for LA re-selection]

  | | +-MS-TXPWR-MAX-CCH ::= INT (0..31) [8]

  | +-Octet2 ::= DIVISION

  |   +-ACS ::= CHOICE [System information type 16 and 17 are not broadcast on the BCCH]

  |   +-NECI ::= CHOICE [New establishment causes are not supported]

  |   +-RXLEV-ACCESS-MIN ::= INT (0..63) [0]

  +-RACH Control Parameters ::= V

  | +-Octet1 ::= DIVISION

  | | +-Max retrans ::= CHOICE [Maximum 1 retransmission]

  | | +-Tx-integer ::= CHOICE [3 slots used to spread transmission]

  | | +-CELL_BAR_ACCESS ::= CHOICE [The cell is not barred]

  | | +-Call reestablishment allowed ::= CHOICE [Call Reestablishment not allowed in the cell]

  | +-Octet2-3 ::= DIVISION

  |   +-Access Control Class C15-C11 ::= INT (0..31) [0]

  |   +-Emergency Call allowed ::= CHOICE [Emergency call allowed in the cell to all MSs]

  |   +-Access Control Class C9-C0 ::= INT (0..1023) [0]

  +-SI 3 Rest Octets ::= V

    +-Optional selection parameters ::= DIVISION

 

 

NAS:RR,System Information Type 4

System Information Type 4 ::= DIVISION

  +-L2 Pseudo Length ::= V

  | +-L2 Pseudo Length value ::= INT (0..63) [12]

  | +-N(SD)or0_1 ::= FIX [0]

  | +-N(SD)or0_2 ::= FIX [1]

  +-Skip Indicator ::= V

  | +-Skip Indicator ::= FIX [0]

  +-RR management Protocol Discriminator ::= V

  | +-Protocol Discriminator ::= PD [6]

  +-System Information Type 4 Message Type ::= V

  | +-Message Type ::= MSG [1C]

  +-Location Area Identification ::= V

  | +-Octet1 ::= DIVISION

  | | +-MCC digit 2 ::= INT (0..15) [0]

  | | +-MCC digit 1 ::= INT (0..15) [0]

  | +-Octet2 ::= DIVISION

  | | +-MNC digit 3 ::= INT (0..15) [15]

  | | +-MCC digit 3 ::= INT (0..15) [1]

  | +-Octet3 ::= DIVISION

  | | +-MNC digit 2 ::= INT (0..15) [1]

  | | +-MNC digit 1 ::= INT (0..15) [0]

  | +-Octet4-5 ::= DIVISION

  |   +-LAC ::= INT (0..65535) [1]

  +-Cell Selection Parameters ::= V

  | +-Octet1 ::= DIVISION

  | | +-CELL-RESELECT-HYSTERESIS ::= CHOICE [0 dB RXLEV hysteresis for LA re-selection]

  | | +-MS-TXPWR-MAX-CCH ::= INT (0..31) [8]

  | +-Octet2 ::= DIVISION

  |   +-ACS ::= CHOICE [System information type 16 and 17 are not broadcast on the BCCH]

  |   +-NECI ::= CHOICE [New establishment causes are not supported]

  |   +-RXLEV-ACCESS-MIN ::= INT (0..63) [0]

  +-RACH Control Parameters ::= V

  | +-Octet1 ::= DIVISION

  | | +-Max retrans ::= CHOICE [Maximum 1 retransmission]

  | | +-Tx-integer ::= CHOICE [3 slots used to spread transmission]

  | | +-CELL_BAR_ACCESS ::= CHOICE [The cell is not barred]

  | | +-Call reestablishment allowed ::= CHOICE [Call Reestablishment not allowed in the cell]

  | +-Octet2-3 ::= DIVISION

  |   +-Access Control Class C15-C11 ::= INT (0..31) [0]

  |   +-Emergency Call allowed ::= CHOICE [Emergency call allowed in the cell to all MSs]

  |   +-Access Control Class C9-C0 ::= INT (0..1023) [0]

  +-CBCH Channel Description ::= TV OPTIONAL:Omit

  | +-Octet1 ::= DIVISION

  | | +-Channel Description IEI ::= IEI [64]

  | +-Octet2 ::= DIVISION

  | | +-Channel type and TDMA offset ::= INT (0..31) [1]

  | | +-Timeslot number ::= INT (0..7) [0]

  | +-Octet3-4 ::= DIVISION

  |   +-Training Sequence Code ::= INT (0..7) [0]

  |   +-Hopping channel ::= SWITCH [Single RF channel]

  |   +-(Hopping channel)=0 ::= BRANCH

  |     +-spare ::= FIX [0]

  |     +-Absolute RF Channel Number ::= INT (0..1023) [0]

  +-CBCH Mobile Allocation ::= TLV OPTIONAL:Omit

  | +-Octet1 ::= DIVISION

  | | +-Mobile Allocation IEI ::= IEI [72]

  | +-Octet2 ::= DIVISION

  | | +-Length of mobile allocation contents ::= LEN (0..255) [0]

  | +-Octet3- ::= DIVISION

  |   +-Mobile Allocation RF Channel ::= OCTETARRAY SIZE(0..8)

  +-SI 4 Rest Octets ::= V

    +-SI4 Rest Octets_O ::= DIVISION

    | +-Optional selection parameters ::= DIVISION

    | | +-flg21 ::= SWITCH [L]

    | +-Optional Power offset ::= DIVISION

    | | +-flg31 ::= SWITCH [L]

    | +-flg11 ::= SWITCH [L]

    +-flg1 ::= SWITCH [L]

    +-(flg1)=L ::= BRANCH

    | +-Break indicator ::= CHOICE [Additional parameters are not sent in SYSTEM INFORMATION TYPE 7 and 8]

    +-spare padding ::= PAD SIZE(23..23) [2B]

 

 

NAS:RR,System Information Type 5

System Information Type 5 ::= DIVISION

  +-L2 Pseudo Length ::= V

  | +-L2 Pseudo Length value ::= INT (0..63) [18]

  | +-N(SD)or0_1 ::= FIX [0]

  | +-N(SD)or0_2 ::= FIX [1]

  +-Skip Indicator ::= V

  | +-Skip Indicator ::= FIX [0]

  +-RR management Protocol Discriminator ::= V

  | +-Protocol Discriminator ::= PD [6]

  +-System Information Type 5 Message Type ::= V

  | +-N(SD)or0 ::= FIX [0]

  | +-Message Type ::= MSG [1D]

  +-BCCH Frequency List ::= V

    +-Octet1 ::= DIVISION

    | +-bit128-127 ::= INT (0..3) [0]

    | +-EXT-IND ::= CHOICE [The information element carries the complete BA]

    | +-BA-IND ::= INT (0..1) [0]

    | +-bit124-121 ::= INT (0..15) [0]

    +-Octet2-16 ::= DIVISION

      +-bit120-001 ::= OCTETARRAY SIZE(15..15) [000000000000000000000000000200]

 

 

NAS:RR,System Information Type 6

System Information Type 6 ::= DIVISION

  +-L2 Pseudo Length ::= V

  | +-L2 Pseudo Length value ::= INT (0..63) [11]

  | +-N(SD)or0_1 ::= FIX [0]

  | +-N(SD)or0_2 ::= FIX [1]

  +-Skip Indicator ::= V

  | +-Skip Indicator ::= FIX [0]

  +-RR management Protocol Discriminator ::= V

  | +-Protocol Discriminator ::= PD [6]

  +-System Information Type 6 Message Type ::= V

  | +-Message Type ::= MSG [1E]

  +-Cell Identity ::= V

  | +-CI value ::= INT (0..65535) [1]

  +-Location Area Identification ::= V

  | +-Octet1 ::= DIVISION

  | | +-MCC digit 2 ::= INT (0..15) [0]

  | | +-MCC digit 1 ::= INT (0..15) [0]

  | +-Octet2 ::= DIVISION

  | | +-MNC digit 3 ::= INT (0..15) [15]

  | | +-MCC digit 3 ::= INT (0..15) [1]

  | +-Octet3 ::= DIVISION

  | | +-MNC digit 2 ::= INT (0..15) [1]

  | | +-MNC digit 1 ::= INT (0..15) [0]

  | +-Octet14-5 ::= DIVISION

  |   +-LAC ::= INT (0..65535) [1]

  +-Cell Options ::= V

  | +-DTX_1 ::= BITARRAY SIZE(1..1) [0]

  | +-PWRC ::= CHOICE [PWRC is set]

  | +-DTX_2 ::= BITARRAY SIZE(2..2) [10]

  | +-RADIO-LINK-TIMEOUT ::= CHOICE [16]

  +-NCC Permitted ::= V

  | +-NCC permitted ::= INT (0..255) [255]

  +-SI 6 Rest Octets ::= V

    +-flg1 ::= SWITCH [L]

    +-flg2 ::= SWITCH [L]

    +-DTM_support ::= SWITCH [DTM is not supported in the serving cell]

    +-Band indicator ::= CHOICE [ARFCN indicates 1800 band]

    +-flg3 ::= SWITCH [L]

    +-flg4 ::= SWITCH [L]

    +-implicit spare ::= PAD SIZE(19..19) [2B]

 

 

 

RR  : Channel Request

 

Channel Request has various functionalities and the one right after power-on is is equivalent to RACH process of UMTS and LTE. Simply put, this message is to tell the network "I want to camp on you, please allocate me a radio resource that I initiate the connection process".

 

UE did not receive immediate assignment for channel request(RACH)

All the wireless devices intended to use the channel or common resources first send request to obtain the one. The network assigns the resources to the requested device (MS or UE).

In the ideal scenario channel request is transmitted by mobile subscriber (MS) also referred as UE in certain systems. It is received by Base station (BTS) or NodeB or eNodeB designated in different wireless systems.

In response to the received channel request, the network transmits immediate assignment or any other suitable message as per wireless system.

Let us understand this scenario with respect to GSM system. CHANNEL REQUEST is a request of an MS for a channel when in the idle state. Although only 1 byte long this message already contains the reason for the connection request (answer to PAGING, Emergency Call, etc.) and an identifier for the channel type that the MS prefers. The CHANNEL REQUEST has no hexadecimal message type, because the message does not conform to the regular format and is sent via an access burst.

First of all, depending on the channel configuration, it is possible to send a RACH, practically anywhere within a 51-multiframe.

One of the RACH control parameters sent in the BCCH/SYSTEM INFORMATION MESSAGES 1-4. MAX_RETRAN indicates how many consecutive times an MS may send the CHANNEL REQUEST message to the BTS. Valid values are 1, 2, 4, and 7.

If UE does not receive IMMEDIATE ASIGNMENT for the multiple channel requests (RACH), timer T3126 gets expired. This results into UE connection failure with the GSM network (I.e. base station or cell).

Following are the checkpoints to be analyzed for this issue. Check for both the transmit and receive path.

Let us examine transmission of channel request from the UE:
•  Check for whether channel request has been scheduled on the proper slots in the 51 frame multi frame as per combined or non combined channel type.
•  Check whether the power level for the access burst is fair enough.
•  Check for proper gain settings in the RFIC.
•  Check for upper layer values which need to be embedded into the access burst is as expected or not.
•  Check the channel conditions at the time of channel request initiation.

Let us examine reception of the same at the network side:
•  Check whether the Channel request has been received at the network side. Network log captured can help for the same. Equipments such as Agilent 8960 and Anritsu 8480 help capture the network log. If the channel request is not received then the problem might be with UE transmission or in the channel itself.
•  If the channel request has been received then whether response for the same has been transmitted back to UE/MS or not.
•  If the response such as IMMEDIATE ASSIGNMENT in GSM is already sent by network but is not received by the UE then there might be problem with the UE receive part of channel can be the cause of the issue.

Timer T3122 is used during random access or during dedicated channel assignment while in packet transfer mode (PS mode), after the receipt of an IMMEDIATE ASSIGN REJECT message at UE.

Often it has been observed that poor C/I can results into channel requests not reaching the network.

In LTE RACH is transmitted using PRACH physical channel. The similar issue if observed in LTE system can also be analyzed with the checklists mentioned above.

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