Transport of NAS Message

• This procedure is used to carry SMS messages in an encapsulated form between UE and the MME.
• Can be initiated by the UE or the network and can only be used when UE is attached for “EPS services and Non EPS services” or “EPS services with SMS only” and UE in connected mode.

UE initiated:

• To send an SMS UE iniates UPLINK NAS TRANSPORT message including the SMS message in NAS message container IE.

Network Initiated:

• Network sends DOWNLINK NAS TRANSPORT message which will be sent to SMS entity by EMM entity at the UE.

Generic NAS transport message :

• Used to send other type of data, example an LCS application to send LPP message or a location service message.
• Should not send uplink NAS transport message when timer T3346 is running unless

i)                    UE configured to use access class AC11-AC15

ii)                  UE has PDN connection established for emergency services.

EMM Status Message

• EMM status message is used to report errors detected on the received EMM protocol messages.
• This message can be initiated by either UE or MME.
• No state transitions are seen from radio interface on receiving this message.
• Shall trigger in connected mode. 

  

•  If UE receives a EMM or ESM message with with message type not defined for protocol discriminator or not implemented by the receiver, UE will send EMM status message with EMM cause as #97 Message type not existent or not implemented.
•   Other EMM causes which will be sent in EMM status are

          #96 Invalid mandatory information
          #100 conditional IE errors
          #95 semantically incorrect message

GUTI Reallocation Procedure

• GUTI Reallocation procedure is used to allocate GUTI and optionally new TAI list to the UE.
• This procedure is always initiated by MME in EMM-REGISTERED state
• This procedure is performed in chipered mode, normally takes place in conjunction with other mobility management procedure.
• If UE lower layers indicate transmission failure of GUTI REALLOCATION COMPLETE message with TAI change and is not part of new TAI list UE should abort current procedure and starts TAU.
• IF lower layer failure detected at NW before T3450 timer expiry,NW should consider both old and new GUTI and old and new TAI list as valid.
• For paging first UE should try with S-TMSI from old GUTI and try in old and new TAI list, if response received from UE network should restart the GUTI reallocation procedure again
• In no response received network should use S-TMSI from new GUTI and page in new TAI list.
• If UE sends attach, detach or tracking area update request while on going GUTI procedure network should abort current GUTI procedure and proceed with specific EMM procedure. 

  

                               

Figure : GUTI Reallocation Procedure:

GUTI:

                          

• GUTI size is 80 bits and is composed of Globally Unique MME Identity (GUMMEI) which is of 48 bits long and M-TMSI (Temporary Mobile Subscriber Identity)which of 32 bits long.
• GUMMEI : used to identify the MME which allocated this GUTI. Composed of PLMN (MMC MNC) code, MME Group Id and MME code

T3346 - NAS Level Mobility Management Congestion Control

T3346 timer value may be included in Attch Reject , Tracking Area Update (TAU) Reject or Service Request Reject sent by network to UE. 

During EMM signaling congestion detected network will perform NAS level mobility management congestion control.

By sending reject message with cause value #22 Congestion

If the reject message from network is not integrity protected UE starts T3346 with random value.

if it is integrity protected UE uses the value present in reject message.

NAS Mobility Management Congestion Control consists of

a) general NAS level mobility management congestion control

b) Subscribed APN based congestion control

During general NAS level congestion control

Network shall not reject 

i)attach requests for emergency bearer services.

ii) NAS signaling connections with RRC establishment cause as High priority(Access class 11-15).

iii) Requests for CS fall back emergency services

When APN based congestion control is active the network may reject attach requests from UE's with a subscription to this APN.

While assigning T3346 value network send different values for different UE's so that time outs are not synchronized.

Cases when NAS signalling allowed during T3346 running

•  Attach for emergency services can be initiated even if T3346 timer is running.
•  Attach can be trigger in a new PLMN which is not part of EQPLMN list and also not part of forbidden PLMN list.
•  TAU can be initiated in response to paging if UE in attempting to update state.
• Deatch request can be initiated even T3346 is running

Reasons to stop T3346:

Upon reception of paging, UE shall stop T3346  and send service request or TAU request.

If UE switched off while T3346 is running, on switch on UE should restart the timer with value T3346-switchoff(if the value non zero) time,if UE not able to detect switch off time UE should restart with same T3346 value before switch off.

Though all NAS timers shall be stopped,UE should not stop T3346 in Power Saving Mode(PSM)

PLMN selection

At switch on or recovery from loss of coverage UE should select the last registered-PLMN (RPLMN) if exists and start with the PLMN selection operating mode that was used before switch off/loss of coverage
Two different modes of PLMN selection
i)Automatic PLMN Selection Mode
This mode utilizes a list of PLMN's in priority order. The highest priority PLMN which is available and allowable is selected.
Ms selects and attempts registration on other PLMN/access technology combinations, if available and allowable in the following order
1. Either HPLMN  (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available (if EHPLMN list present)
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order 
5. Other PLMN/access technology combinations in order of decreasing signal quality

ii) Manual PLMN Selection Mode
Here the MS indicates to the user which PLMN's are available, Only when the user makes a manual selection does the MS try to obtain service on the PLMN.
MS indicates the list of PLMN's with all supported access technologies, this list includes PLMN's present in the "Forbidden PLMNs" list, "Forbidden PLMNs for GPRS Service" list
and PLMNs which offers services not supported by the MS.
Plmns meeting above criteria are presented in following order.
1. Either HPLMN  (if EHPLMN list not present/empty on SIM) or the highest EHPLMN that is available, if more EHPLMNs are available display either the highest priority one or all based on the flag on the SIM.
2. PLMN/access technology combination in "User Controlled PLMN selector with access technology" data file in SIM.
3. PLMN/access technology combination in "Operator Controlled PLMN selector with access technology" data file in SIM.
4. Other PLMN/access technology combinations with received high quality signal in random order 
5. Other PLMN/access technology combinations in order of decreasing signal quality

Once the UE has registered on a PLMN selected by the user, the UE shall not automatically register on a different PLMN unless
i) The new PLMN is indicated as an equivqlent PLMN by the registered PLMN or
ii) The user selects automatic mode

If no such PLMN selected or that PLMN is no longer available, the MS shall attempt to camp on any acceptable cell and enter the limited service state.

Roaming:
If the MS is roaming and in registered on a VPLMN, the MS shall periodically attempt to select HPLMN (if EHPLMN list is empty) or one of its EHPLMNs (if list is available on SIM) or a higher priority PLMN/access technology combinations listed
in "User and Operator controlled PLMN's list.
A value T minutes stored in the SIM which can range from 6 minutes to 8 hours in steps of 6 minutes to perform periodic attempts.
If no value is present in SIM a default value of 60 minutes is used for T.
The attempts to access the HPLMN or an EHPLMN or higher priority PLMN shall be as specified below:
a) The periodic attempts shall only be performed in automatic mode when the MS is roaming, and not while the MS is attached for emergency bearer services or has a PDN connection for emergency bearer services;
b) The MS shall make the first attempt after a period of at least 2 minutes and at most T minutes:
c) The MS shall make the following attempts if the MS is on the VPLMN at time T after the last attempt;
d) Periodic attempts shall only be performed by the MS while in idle mode

PLMN - Public Land Mobile Network
HPLMN - Home PLMN
EHPLMN - Equivalent HPLMN
RPLMN - Registered PLMN


Reference : 3GPP TS 23.122

Push To Talk Over Cellular (POC)

Push To Talk Over Cellular (POC) is the communication method similar to walkie-talkie style of communication using cellular networks and mobile phones instead of stand alone hardware. In normal Push To Talk (PTT) communication there were limitation like the range and stand alone device for communication and huge technological gap when compared to commercial tele communication devices.

PTT communication is half duplex communication. where only one way traffic is allowed at the same time. POC works based on Voice Over IP (VoIP) technology.In PTT user presses a button and start communication immediately to a specific contact or predefined group. No call set up delay as in case of normal voice call.

Fast Dormancy (FD)

Fast Dormancy (FD) is a wireless technology feature introduced to improve battery life on the mobile devices.

Fast dormancy is a way by which UE  informs the network to release the RRC connection. (Termination of RRC connection is always triggered by network)

UE consumes more battery when in connected state, so when there is no data to transfer UE informs to network to release its RRC connection to save the battery power.

However with bursty data traffic nature UE's may end up sending moreRRC connection request and  SCRI(signalling connection release indications) to network.

From Rel-8 3GPP inroduced a timer T323 whose value is brodcasted in SIB1.

If network supports this rel-8 fast dormancy feature UE should not trigger fast dormancy request until the T323 timer expiry.

Device to Device Communication

• Device to Device Communication has been the latest reasearch topic for LTE advanced. Which enables UE's those are in the proximity to discover and communicate with each other.
• Various use cases involved are
• Social gaming
• Commercial advertisements
• Pubic safety purposes
• Network offloading  

• Enormous operator benefits involved using device to device communications such as
• Peak data rates: as the UE's are near by due to which good radio conditions can enables higher modulation schemes.
• Low latency: No need to go through all the network entities involved in data communication.
• Capacity: Due to spectrum re use UE can accommodate more users
• Extended Coverage : UE's in network coverage can provide services to UE's which are present out side of network coverage (Relay operation)
• With the advancement in the technology and social life opened new ways to communication.
• Prose is a feature that enables operator network controlled discovery and communications between UEs that are in proximity.
• It allows UE's in proximity will discover other UE's in their vicinity and start communication directly with less involvement of network.
• Various use cases involved such as location based advertising (Ads by shops to customers passing near by), social networking applications (facebook, twitter),network off loading and Public safety etc.
• The communication path could for example be established directly between the UEs or routed via local eNB(s).

Discovery Types : 

• Open ProSe Discovery: is ProSe Discovery without explicit permission from the UE being discovered.
• Restricted ProSe Discovery: is ProSe Discovery that only takes place with explicit permission from the UE being discovered. UE's can have a feature enabled whether it can be discovered by other UE's are not (privacy).
• Operator can change dynamically the range of the proximity per user/application. 

Disovery Models :

Model A:

• Model A is push model like "i am here". used for both open and restricted discovery.

Model B:

• Model B is pull model like "who are there" or "Are you there". it is used for restricted discovery. 

3GPP TR 22.803,3GPP TS 23.303

T3412 - Periodic Tracking Area Update timer

Periodic Tracking Area Update procedure to used to periodically notify the availability of the UE to network.

UE needs to notify its presence to the Network periodically and is controlled using T3412 timer in LTE.

One of the reasons for this is that at any time network will be aware of the UE's presence at a granularity of Tracking Area. Which will be needed for cases such as paging.

Default value of this timer is 54 min.

The value of this timer is provided by the network operator during the attach and tracking area updating procedures

If this value is not present or value is zero (0) UE should not trigger periodic tracking area procedure.

T3412 contains two parameters

i)timer_unit

   If timer unit is 2 it represents that timer value is represented in deci hours.

ii)timer value

    If timer value is 1 it means 6 mins (if timer unit is 2)

T3412 will be stopped when UE enters EMM-CONNECTED or EMM-DEREGISTERED mode.

T3412 will be reset and started with initial value when UE moves from EMM-CONNECTED to EMM-IDLE mode.

When the network includes T3412 extended value IE in the ATTACH ACCEPT message or TRACKING AREA UPDATE ACCEPT message, the network uses timer T3412 extended value IE as the value of timer T3412

On expiry of the timer UE initiates Periodic TAU Request if the UE is not registered for the emergency bearer services or T3423 running.Implicit detach from network if the UE is attached for emergency bearer services.

Network will maintain a mobile reachable timer which is 4minutes greater than T3412. If UE not updated TAU with in mobile reachable timer network shall implicitly detach the UE

According to 3GPP 24.301