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Use: To follow up the performance of SDCCH from technical point of view. Experiences on use: 1) High SDCCH drop rates usually result from ghost accesses. A BTS decodes
them from environmental or background noise and filters out most of them. However, all of them cannot be filtered out and the RACH request is passed on to the BSC for processing and for the allocation of a SDCCH channel.
The counter GHOST CCCH RES (3030) is updated each time a CHANNEL REQUIRED is rejected because of an invalid establishment cause. In GSM ph.1 there exist altogether eight establishment causes, three of which are undefined as invalid, for example,resulting in that this counter shows only 3/8 of all the ghost accesses the BTS has decoded. For the rest, a SDCCH is allocated and this will result in SDCCH_ABIS_FAIL_CALL failure. Because of ghost attempts the SDCCH drop ratio is high with low traffic. As the amount of call attempts increases, the influence of ghosts becomes smaller and the drop ratio approaches its real value.
2) The rate of ghosts coming to SDCCH dropped when BTS B9 with improved ghost filtering was
Higest value of ave_rach_busy during the meas. period
BTS sw B9.0 and DF2.0 has enhanced filtering of ghosts- energy level check with previous burst info- All 41 bits of training sequence used
GHOSTS= SDCCH seizures that do not lead to a useful benefit - thermodynamical type of noise - in band noise from network or environment- MS at the limit of sensitivity
GHOSTSorMS
Before B9 around 10 ghosts per hour to BSC
CCCHLOADIND (every 30s)
After B9 around 1 ghosts per hour to BSC
Max Number Retransmission (RET) (1,2,4,7) defines nbr of repetition by MS
Rach_3 Rach_4*) T3126
*) T3126: In GSM Ph.1 waiting period is not defined (e.g. a Nokia MS has 800ms) but in GSM ph.2 it is defined by a complex algoritmand can be as long as 5 sec.GSM 04.08 says: This timer is started either after sending the maximum allowed number of CHANNEL REQUEST messages during an immediate assignment procedure or on receipt of an IMMEDIATE ASSIGNMENT REJECT message,whichever occurs first.It is stopped at receipt of an IMMEDIATE ASSIGNMENT message, or an IMMEDIATE ASSIGNMENT EXTENDED message.At its expiry, the immediate assignment procedure is aborted.
Immediate Assignment
Number of Slots Spread Trans (SLO) (3..12, 14, 16, 20, 25, 32, 50 TDMA frames between retransmissions) MML default 10.
In BTS sw B12 and DF5 the filtering of accesses with illegal establishment causes will be moved to BTS
Known phenomenom: Handover Access bursts causing SDCCH failures, blocking and Ghosts
Channel Required
NOTE: triggered related to level based RACH detection(not what is accepted as RACH andpassed as Channel Req)
If RACH passed all tests
NOTE: triggered related to what is accepted as RACH
One message can include 1 - 4 Immediate AssignmentReject -Messages (from S6) !
Immediate Assignment
Immediate Assignment Rejected
OR
MS BTS
Overflow of AG buffer
Parameters:AG = = Number of Blocks Reserved for Access Grant (0..7 if noncombined, 0..2 if combined, 1..7 if SBCH used in SDCCH/8)NOTE: If AG=0, Access Grant has priority over Paging If AG<>0, Paging has priority on PCHIf AG<>0, only Access Grant is sent on AG blocks
Buffer capacity =8 Abis messages
Immediate Assignment
Immediate Assignment Rejected
Delete indication
OR
Sent on AGCH or alternatively on PCHif there is free capacity
Returned for each deleted AGCH message
MS is not allowed to attempt new RR-establishment i.e channel requestuntil T3122 expires
Channel Request (RACH) Channel Required
Channel Activation
Channel Activation Ack
RACH on CCCH/c3004ch_req_msg_rec
/c1007sdcch_assign
Ghosts having valid establishmentcause get trhough even if cell wasbarred.