What are Access Classes ? How Access Barring(Cell Barring) Takes Place ?

By Azar

A few years back  I and a mentor of mine traveled to New York together for a conference. He made an off-hand comment saying,

Life is so much easier if you accept the fact that there is a hierarchy in this world. There is a privileged class and the others

He said this , when we boarded a plane , we saw first class and economy seating. Later after landing, we waited for our shared transport service to arrive. There was an option for shared transport service and an option to book the whole shuttle .

On an on later that night , we went to see a Broadway show ,there again we found a number of ticket options available. Yes, we did enjoy the show and later discussed handover scenarios in LTE   LOL.    How fun is that ?

The point is, in a world of consumerism , having availability of different options according to our wallets does matter. We all can pick and choose what do we like and want.

In the world of telecommunications. It is not separate from the availability of different options.  We all use different data plans and phones according to our needs ,requirements and /or desires.

User differentiation services exist in the Telecommunication world such as silver, Bronze, platinum subscriber etc.

 

In this article, I am not going to talk about different data plans available etc.
Instead, we are going to dig a little  deeper than that. Deeper to the point that even in Idle mode, not all UEs will not be able to make RRC connection request based on their Access class. That’s right.

There are a total of 15 Access classes defined in LTE . Access class (AC)  information for a UE is stored in the USIM(your UE SIM card).

Even among those 15 categories there are ordinary and Special Categories defined. These access classes help the UE to delay or allow services such as mobile originating signaling, mobile originating data , voice service etc.

 

 

 

 

What is the use of these Access Categories ?

In Idle mode , based on the AC a user will be treated differently when it comes to

  • Emergency Call
  • Mobile Originated (MO) Signaling Setup
  • MO Data Session
  • Voice Call (MMTEL Voice)
  • Video Call  (MMTEL Video)
  • CS Fall back

 

 

 

Parameters for Cell Barring

Cell barring for these Access classes depend on the following parameters.

( If you are not familiar with Cell Barring read here )

Barring Factor : It can have a value from 0 to 0.95 in steps of 0.05.  This factor determines if a UE needs to treat a cell as barred or not.

Barring Time:  It decides how long of a duration the cell will be treated as barred by a UE. It can have a value such as 4, 8, 16, 32, 64, 128, 256 and 512 seconds

Barring for Special AC : This is a Boolean parameter. It has a value either TRUE or FALSE. If cell barring is applied for special Access classes then Barring for Special Access Class will have a value TRUE and vice versa.

How would a UE know if a cell is MO signaling barred, MO data barred , Emergency call barred etc. ?  The answer lies in these parameters. These parameters are broadcasted in SIB 2 to the UE.  After reading SIB 2 . UE finds out if it is barred for MO signaling, MO data, MMTEL Voice , MMTEL Video barred or CS Fallback barred etc.

The table below tells you, these parameters arrive for each of the service such as emergency call, MO Data session, MO signaling session etc.

 

 

access_class_table_lte_in_bullets

Table showing different Information Elements for Different Access Barring Types. Credits LTE in Bullets

 

Access Class Barring for Emergency Call

Access class barring for emergency calls determines whether or not UE with AC 0 to 9 should treat the cell as barred for emergency calls. The same rule is applicable to UE without a USIM.

UEs with Access class 11 to 15 are able to make emergency calls unless

  • the cell is AC class barred for emergency calls and
  • the cell is AC barred for mobile originating data calls

 

 

Access Class Barring for Mobile Originated (MO) signaling (General Procedure)

Access class barring for Mobile Originated signaling determines whether or not UE should treat the cell as barred when initiating an MO signaling connection.

 

How to Determine if a Cell is Barred ?

UEs with Access class 0 to 9 generate a random number between 0 and 1. If it is less than the Barring factor then the cell is treated as not barred.

UE with AC 11 to 15 check the Barring for special Access Classes parameter  to determine whether or not it is necessary to generate a random number with which to compare against the Barring Factor.

 

What if a cell is barred for MO Signaling ?

If a cell is to be treated as barred for MO signaling connection , then the Barring Time is used to define a penalty time during which the UE is not allowed to re-attempt access . The penalty time for MO Signaling is T305. The way timer T305 value is computed is as follows.

 

mo_signaling

Penalty Time for MO Signaling

 

What if a cell is barred for MO Data ?

In case if the cell is to treated as barred for MO data connections. In this case, the penalty time is T303. The way timer T303 is computed is as follows

 

mo_data

Penalty Time for MO Data

 

Service Specific Access Class Barring

In 3GPP release 9, Service Specific Access Class (SSAC) barring for Multimedia Telephony (MMTEL) was introduced for video and voice calls.

The parameters for SSAC barring are also broadcasted in SIB 2.  These parameters in SIB 2 are used to determine whether or not UE should treat the cell as barred when initiating MMTEL voice and data calls.

 

 

Procedure for finding out if a cell is barred or not for SSAC :

The procedure to find out if a cell is barred for SSAC or not is pretty much the same as for regular access class barring.

UE with Access class 0 to 9 generate a random number between 0 and 1. If it is less than the Barring Factor then the cell is treated as not barred.

UE with the Access Class 11 to 15 check the Barring for Special Access class to determine whether or not it is necessary to generate a random number with which to compare against the Barring Factor.

If a cell is to be treated as barred then the barring time is used to define a penalty time during which the UE is not allowed to re-attempt access . The penalty time is defined as

 

 

penalty_time

General Penalty Time computation if a cell is Barred

 

Question for you ?

If you have read down so far. I have a question for you. Among 15 Access classes. Did you see an Access class which is not listed or missing? If so why ?  Reply with your answer in the comments below.

 

 

Interested in getting more information like this. Give me your autograph below.

 

 

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