LTE/IMS reference

                                             Basic LTE

LTE /SAE Key Features -

IP network is the data network. There are different type of IP data network like virtual private network, internet , IMS etc.

Evolved packet core- Evolved means advance. Only PS network is used. Evolved UTRAN means modified UTRAN(Evolved UMTS Terrestrial Radio Access Network). There is no RNC. All the RNC functionality put in a Node-B. So LTE system comprises of two main systems. One is EUTRAN and another is EPS.

So, LTE system has got many names .One is EPS, another is SAE and the other is LTE. 

Now mobile equipment would be connected with EUTRAN by using Radio technology like OFDMA/SCFDMA because in 3G System data rate is low, latency is more and physical layer complexity is more. So to minimize these three problems LTE was introduced.

In GSM, 200 KHZ bandwidth is used. In 3G 5MHZ BW is used. In LTE  1.4, 3 , 5 , 10 ,20 ,40 …. 80 MHZ bandwidth can be used. That is why it is called flexible Bandwidth. In LTE we can increase the bandwidth.

MIMO means multiple input multiple output. Mobile equipment can have 1,2,3,4 multiple antenna. If we increase the antenna data rate also increases. E-NB have multiple antenna. 

Mobile equipment can have multiple HARQ processor. If one HARQ processor is busy then another HARQ processor can do the task.

Brief Summary Of LTE -

ØLTE supports flexible bandwidth up to 20 MHZ ( 1.4 MHZ, 3 MHZ,

    5 MHZ, 10 MHZ, 15 MHZ, 20 MHZ)

ØIt use the flat network architecture.

ØIn LTE E-NB is more powerful than UMTS node-B because E-NB contain

   all RNC protocol of UMTS. So there is no RNC concept in LTE.

ØLTE mainly design for packet switch services which can support speed

   up to 300 mbps that is the reason LTE came with completely new core

   network that is called Evolved packet core (EPC). It does not means

   that LTE is not supporting the speech call. LTE can also support circuit

   speech call by using existing CS network through CS Fallback

   procedure.

ØAll the Packet switch (PS) services is initiated by NAS layer under Service request procedure where as circuit switch services will be initiated by NAS layer under extended Service request procedure.

ØWhen LTE mobile switch ON by default it get data connectivity by getting an IP address.

ØLTE cell coverage area is almost 15 times larger than UMTS cell coverage area, so that it can easily control the high mobilized user.

ØGenerally LTE presents air interface technology in 4G and SAE ( System Architecture Evolution ) represent the technology E-UTRAN

     and the EPC. By combining LTE and SAE the entire system is called

     Evolved packet System (EPS).

ØLTE having 2 RRC states-

     - Idle state

     - Connected state

 The NAS layer of UE is communicating to EPC which is divided in to two

 sub- entities EMM and ESM .The Radio protocol of UE will communicate

 with E-NB where as NAS protocol of UE will communicate with EPC.

ØLTE uses  OFDMA in downlink direction  where as SCFDMA in uplink direction.   

LTE Summary-

•Flat architecture optimized PS domain.

•IP based interface.

•IP is widely used as the network layer in the protocol stack of all interfaces.(Both for the control and user plane)

•Simplify the RAN: reduce the different type of RAN nodes and their complexities  / Minimize the no. of RAN  interface type.

•Increase the throughput.

•Reduce latency (User and Control plane)

•Improve spectrum efficiency

•Provide greater flexibility of frequency band ( 1.4 , 3, 5, 10, 20……MHZ)

•Migrate to an optimized PS domain, with no CS domain in the core network.

•CS service will be supported via VOIP etc.

•Interworking with 3GPP access network(UMTS,GSM) and non 3GPP access network ( WiFi, WiMax, WLAN)

•All IP transport network.

LTE Architecture-

LTE Architecture with other 3GPP technology SAE (Service 

Architecture Evolved) -

We can extend the LET SAE as follows to interface with other technology.

Here we would just check the brief summary of functions of each node -

Evolved nodeB (E-NB)-

•Radio Resource Management (RRM)- means E-NB is holding the resource. Everybody will get this resource from E-NB.

•Radio Bearer Control- Setup, modification and release of radio resources means setup of new application, modify the application and close the application.

•Connection Management Control -  E-NB must be playing vital role in connecting b/w MME and UE, because it is the middle component. So whatever NAS messages are send by mobile, should be received and send to MME.

•Radio Admission Control- It is just like a RRM which allows and rejects resources.

•E-NB Measurement Collection and evaluation- Measures the signal strength of UE and based on that it takes the decision of handover.

•Dynamic Resource Allocation (Scheduler)-

Optimizes the resource utilization. Resource will not allocate to the user in a static manner. Dynamic resource means in every 1ms we may or may not get the resource.

•IP header Compression /Decompression-

We use the PDCP layer for header compression and decompression. Previous in 3G PDCP layer was present in UE and RNC but in LTE PDCP layer is present in UE and E-NB.  

•Access layer Security-

 Ciphering and integrity protection is done on the radio interface between UE and E-NB.

•MME selection at the time of attach-

In case of UMTS scenario  the RNC is connected to one MSC and one SGSN or GSM scenario. But in LTE one E-NB is connected to 10 MME, also that is the ultimate goal of flat Architecture if one MME fails nothing will happen, other MME will take over but they are connected by IP network again, it is not connected by any physical cable, every places are connected by router and switches. But mobile equipment is attached  to first time, it is up to E-NB which MME he has to select on what basis. So E-NB select the MME on the basis of load. This is called load balancing. Load balancing means according to load of MME, E-NB has to choose which MME has to attach for the UE.   

•User data routing to the SAE GW.

•Transmission of  Paging message coming from MME.

•Transmission of Broadcasting information (SIBs, MBMS). 

MME (Mobility Management Entity):

This as the most important component of SAE which has the following functionalities. We can take MME as a centre of all signalling messages.

•Idle mode UE (User Equipment) tracking Process

•Paging Process.

•Bearer activation/deactivation process

•Choosing the SGW for a UE at the initial attach

•Core Network (CN) node relocation at time of intra-LTE handover

•Authenticating the user (by interacting with the HSS)

•Destination of NAS message

•Generation and allocation of temporary identities to UEs.

•Authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN)

•Enforces UE roaming restrictions

•Termination Point for Ciphering/Integrity for NAS signalling

•Security key management

•Lawful interception of signalling is also supported by the MME

•Provides the control plane function for mobility between LTE and 2G/3G access networks in connection to SGSN

SGW (Serving Gateway) :

 This is a centre for all user data (packet data).

•Routes and forwards user data packets

•Act as the mobility anchor for the user plane during inter-eNodeB handovers

•Act as the anchor for mobility between LTE and other 3GPP technologies

  

                             

•Terminates the DL data path and triggers paging when DL data arrives for the UE when UE is in Idle mode

•Manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information

•Performs replication of the user traffic in case of lawful interception

PGW (PDN Gateway)- / PCRF

•Provides connectivity from the UE to external packet data networks (IMS,INTERNET, VPN)

•Performs policy enforcement, packet filtering for each user, charging support, lawful Interception and packet screening

     

What is the difference b/w PCRF and  PCEF.

PCRF sets the rule and according to that PGW will follow

PCEF is the software function which is present inside the PDN

Gateway . It helps in billing and quality of service.

(For example, PCRF tells to PDN GW that for every 1MB of data you

charge Rs.5/-. So, PDN GW will generate the bill according to this rule.

That is why PDN SAE gateway is the Implementer and PCRF is a Ruler ). It acts 

as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX

and 3GPP2 (CDMA 1X and EvDO).

HSS (Home Subscriber Server) :

•This is a central database that contains user-related and subscription-related information. It is like "HLR(WCDMA) + Authentication centre AuC(WCDMA) + Additional Information(LTE)"

•Mobility management

•Call and session establishment support

•User authentication and access authorization