Access Control Class (ACC), which is often reduced to Access Class, for users of GSM SIM has been mentioned from time to time at this Forum and although its importance has been highlighted, in a broader context an understanding of how Access Class can be used has not been fully discussed. Below is a further expansion of knowledge connected to ACC.
Wiki has posts for ACCOLC and MTPAS you may like to read. Below are some extracts including links to Wiki to read more on the subject.
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ACCOLC
ACCOLC (Access Overload Control) is a British procedure for restricting mobile telephone usage in the event of emergencies. It is similar to the GTPS (Government Telephone Preference Scheme) for landlines.
This scheme allows the mobile telephone networks to restrict access in a specific area to registered numbers only and is normally invoked by the Police Incident Commander (although it can be invoked by the Cabinet Office). The emergency services are responsible for registering their key numbers in advance.
ACCOLC is being replaced by MTPAS (Mobile Telecommunication Privileged Access Scheme) in 2009.
Purpose
The purpose of the ACCOLC in the UK is to restrict civilian access to cell phone networks during emergencies. This actively prevents civilian usage from congesting the cell networks, thus allowing emergency services personnel priority for communications.
How it works
Mobile telephones work on a cell basis. The cells adjacent to the incident are identified and ACCOLC is implemented on those cells alone. ACCOLC-enabled telephones are allowed access to the network and all other users will receive a fast beep (called a Fast Busy Signal). Once the call is connected to the network it is routed like any other. If the user receives a recording that all lines are busy or engaged tone then ACCOLC is not being utilised.
ACCOLC is indicated on the SIM card by a number in the range 0 - 15 giving a total of 16 flag bits in the Global action message. It is not hierarchical so it can allow level 1 access while disallowing level 6 access. If the 16 bit control word is, 1010-0000-0011-1111 only phones with the ACCOLC of 1, 3, 11, 12, 13, 14, 15 & 16 will accept request for placing a call.
Ordinary cellphone users have numbers in the range 0 - 9. Higher priority users are allocated numbers 10 to 15 [1]. During an emergency, some or all access classes in the range 0 - 9 are disabled. If the overload condition continues, mobiles with access classes level 10, 11, then 12 and so on may also be disabled by the operator.
As ACCOLC can be a frustration to normal network users, it is normally only initiated after careful consideration. In the UK, the authority of a Police "Silver" is required (major incident control is named in three tiers in the UK, gold, silver and bronze, in accordance with the London Emergency Services Liaison Panel, a group responsible for creating best-agreed procedures for dealing with various emergency situations) after consideration with the co-ordinating group.
Not all calling by regular mobiles is prevented. Calls to an emergency services number (911, 112, 999) will ignore all ACCOLC or global action messages.
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MTPAS
MTPAS (Mobile Telecommunication Privileged Access Scheme) is a British procedure for prioritising access to the mobile telephone networks for privileged persons (members of emergency services as designated at a local level). It is replacing ACCOLC in 2009, and during the crossover period sim cards registered in the ACCOLC scheme will continue to gain priority.
The changeover is being made in order to "devolve responsibility and management of the Scheme to the local level", "coordinate a common approach to the Scheme in England and Wales", improve the effectiveness by further limiting the number of users, and to "ensure clarity regarding activation of the scheme."
Activation of MTPAS is the responsibility of the mobile networks, who after receiving a signed proforma from a Gold Commander have the responsibility in implementing it by whatever means they deem suitable. Unlike ACCOLC, MTPAS appears to not go straight to full blocking of all civilian (unprivileged) users but rather prioritizing privileged users by increasing the bandwidth available to them.
Good read!