ISR- Idle state Signalling Reduction

What is ISR ?

Idle state Signalling Reduction

Why ISR ?

1. To reduce the frequency of signalling between UE and Network and also Context transfer between networks

2. Prevent Ping-Pong effect

Requirements -

SGW should support ISR

ISR Mandatory for 4G UEs that support GERAN/UTRAN and whereas for network it's optional

Release 8 SGSN and above

Procedure -

Lets assume UE is in 4G coverage area,

UE attaches to E-UTRAN and sets TIN to GUTI

Now if UE has moved to an area where it receives a stronger 3G signal and reselects GERAN/UTRAN,it will initiate RAU. This is when ISR is activated.As TIN is set to GUTI,UE maps P-TMSI from GUTI and sends the same in RAU Request

After the SGSN receives the context from MME,both CN nodes keep the context depending on if SGW would have indicated ISR supported to MME

RAU accept indicates ISR activation as a result UE stores both GUTI,received during the attach and P-TMSI,received during 3G RAU as registered and sets TIN as 'RAT related TMSI'

MME and SGSN are both registered to HSS and both have connection to SGW

Once this is done,there is no need for UE to update the network [E-UTRAN or UTRAN ] as long as it is within the TA/RA s which it has registered to.

Any context transfer that involves RAU/TAU can be the point where this ISR activation may take place.

If UE is camped on to UTRAN,TAU initiated ISR [ First attach to 3G and then moving to 4G ] works in similar way as RAU initiated ISR

#################### Downlink Data ####################

Downlink data is sent to SGW.If ISR is activated,the SGW has connection to both MME and SGSN and sends notification to both nodes.MME and SGSN page in their respective TA/RA

Depending on from where UE responds,the connection is established and data transfer begins

#################### ISR deactivation ###################

Depending on indication in RAU or TAU Accept,the ISR will be deactivated. If there is no explicit indication of ISR activated,then it means it is deactivated

UE sets TIN to GUTI or P-TMSI in this case depending on whether it is connected to U-TRAN or E-UTRAN

If UE performs a RAU with a Gn/Gp SGSN of any standards release as these SGSNs never indicates "ISR Activated" to the UE.

Abbreviations -

ISR : Idle mode Signalling Reduction

TIN : Temporary Identification used in Next update

GUTI : Globally Unique Temporary Identifier

CN : Core Network

EPS Mobility Management and Connection Management

Difference between EMM and ECM

EMM [EPS Mobility Management ]

There are two states :

1.Registered

2.DeRegistered

1.Registered :

Ue can enter into this state by,

a.Attach

b.I-RAT[Inter RAT ] TAU

In this state UE atleast has 1 active PDN connection and EPS security context

The UE location is known in the MME to at least an accuracy of the tracking area list allocated to that UE

2.DeRegistered :

Ue can enter into this state by,

a.Attach Reject

b.TAU Reject

c.On Detach

d.On successful I-Rat Handover

In this case Routing info is not available to MME and UE is not reachable

In the EMM-DEREGISTERED state, some UE context can still be stored in the UE and MME, e.g. to avoid running an AKA procedure during every Attach procedure

ECM [EPS Connection Management]

There are two states :

1.IDLE

2.Connected

1.IDLE :

A UE is in ECM-IDLE state when no NAS signalling connection between UE and network exists.

There is no S1_MME and no S1_U connection for the UE in the ECM-IDLE state.

The signalling connection is made up of two parts: an RRC connection and an S1_MME connection.

The UE shall enter the ECM-IDLE state when its signalling connection to the MME has been released or broken.

The S1 release procedure changes the state at both UE and MME from ECM-CONNECTED to ECM-IDLE.

In the EMM-REGISTERED and ECM-IDLE state, the UE shall

a.Do a periodic TAU to notify EPC that UE is available

b.Do a TAU when the current TA is not in the list of TAs received when UE was registered

c.When RRC connection was release with 'LOAD Balancing TAU required' cause

d.Send Service request when it wants to send uplink user data or upon receiving paging

e.Do TAU when change of the UE's Core Network Capability information

Initial NAS messages that initiate a transition from ECM-IDLE to ECM-CONNECTED state are Attach Request, Tracking Area Update Request, Service Request or Detach Request

2.Connected

Here UE's location is known to the accuracy of serving ENB

There exists s1-MME connection and RRC connection
 
ECM-UE

                                                                      





ECM-MME

EMM-MME

EMM-UE

IMSI-MSISDN-IMEI

IMSI [ International Mobile Subscriber Identity ]

IMSI = MCC+MNC+MSIN

MCC : Mobile Country Code [3 digits]

MNC : Mobile Network Code [2 or 3 digits]

MSIN: Mobile Subscriber Identification Number[ 9 0r 10 digits]

If MNC is 3 digits,then MSIN will be 9 digits.The total number of digits should be 15

This is burnt into your SIM[ Subscriber Identity Module ] card.When a mobile is switched on,it needs to be authenticated in order to access the network.

The IMSI is used for authentication.Once MS is authenticated and any further transaction will use TMSI[Temporary Mobile Subscriber Identity ] alloted by the network in order to prevent eavesdropping

NMSI stands for National Mobile Subscriber Identity formed by the combination of MNC+MSIN

MSISDN [ Mobile Station International Subscriber Directory Number ] or [ Mobile Subscriber Integrated Services Digital Network Number ]

MSISDN = CC + NDC + SN

CC = Country Code

NDC = National Destination Code

SN = Subscriber Number

Country Code (CC) : Up to 3 decimal places.

National Destination Code (NDC): Typically 2-3 decimal places.

Subscriber Number (SN): Maximum 10 decimal places

An MSISDN is limited to 15 digits.This is the number which you enter to make a call on your phone.

IMEI [ International Mobile Equipment Identity ]

IMEI =TAC+FAC+SNR+CD+SVN

The IMEI of your cellphone can be checked by keying in *#06#

It generally is of 16 bits,the first 14 indicates Equipment Identity and the last 2 indicates software version

When a Network queries for this IMEI,MS [Mobile Station ] sends this IMEI/IMEI-SV {SV :Software Version} and network can query EIR [Equipment Identity Register ] to get the status of

the equipment.EIR can reply back as,

0: Whitelisted In this case mobile is allowed service

1: Blacklisted In this case mobile is denied service.Blacklisted phones includes the stolen phones

2: Greylisted In this case mobile is allowed service but will be tracked. Grey listed phones might include those mistakenly left in restaurants, pubs

IMEI code 35-209900-176148-1 or IMEISV code 35-209900-176148-23 tells us the following:

TAC [Type Allocation Code]: 35-2099 - issued by the BABT (code 35) with the allocation number 2099

FAC [Final Assembly Code]: 00 - indicating the phone was made during the transition period when FACs were being removed.

SNR [Serial Number]: 176148 - uniquely identifying a unit of this model

CD: 1 so it is a GSM Phase 2 or higher

SVN: 23 - The "software version number" identifying the revision of the software installed on the phone. 99 is reserved.

Difference between IMSI,MSISDN and IMEI

There are three identities IMEI, IMSI and MSISDN. The IMEI is the mobile phone's identity which is burned into the phone.

The IMSI is the identity of the SIM card. And MSISDN is the phone number

MNP [Mobile Number Portability]

In case of MNP,you will have to change the SIM. The MSISDN will be mapped to a new SIM.Since IMSI is specific to SIM card,it changes in case of MNP