What is the difference between sending dedicated preamble & common preamble in RRCconectionreconfiguration?

Question

I just found an IE AS Context which contains ReestablishmentInfo in Handover Preparation message ?

Can any one signify the importance of this IE ?

HandoverPreparationInformation-r8-IEs ::= SEQUENCE {
ue-RadioAccessCapabilityInfo UE-CapabilityRAT-ContainerList,
as-Config AS-Config OPTIONAL, -- Cond HO
rrm-Config RRM-Config OPTIONAL,
as-Context AS-Context OPTIONAL, -- Cond HO
nonCriticalExtension HandoverPreparationInformation-v920-IEs OPTIONAL
}

AS-Context ::= SEQUENCE {
reestablishmentInfo ReestablishmentInfo OPTIONAL -- Cond HO
}

ReestablishmentInfo ::= SEQUENCE {
sourcePhysCellId PhysCellId,
targetCellShortMAC-I ShortMAC-I,
additionalReestabInfoList AdditionalReestabInfoList OPTIONAL,
...
}

AdditionalReestabInfoList ::= SEQUENCE ( SIZE (1..maxReestabInfo) ) OF AdditionalReestabInfo

AdditionalReestabInfo ::= SEQUENCE{
cellIdentity CellIdentity,
key-eNodeB-Star Key-eNodeB-Star,
shortMAC-I ShortMAC-I
}

Answer 1

When allocating a dedicated preamble for the random access, E-UTRAN ensures it is available from the first RACH occasion the UE may use. So in short network allocates a reserved preamble to the UE thus drastically reducing the possibility of collisions with other UEs and sharply reducing the access delay.

Now with above definition we can deduce what the common preamble is i.e. the collision can occur hence the access delay.