LTE - EPC
Published: 2023-12-11 11:22:35


EPC and it's Interfaces

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/czc5mn05pg672l5bf8rp.png)


Dotted line denotes signalling path and solid lines represent user data path.



![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/rvycll6nkuludce9mr5s.png)


**_<u>MME</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ailwlqqlsu6ga4brdqu1.png)


EXamples of NAS signalling -> Happens between ue and MME through enodeB.

NAS Msgs are -
-> Attach process 
-> Bearer establishment.
-> PDN connection establishment.
-> Service request
-> Tracking area update.

Security procedures -> methods of performing mutual authentication between ue and the network. Establishing a se3cure connection network. MME is responsible for this.

TRacking area list management -> area comes under mme.
1 mme srever serveral TA 


Selection of Serving and PDN gateway is necessary to establish a default bearer btwn UE and network during the attach process.

SGSN -> for legacy networks like 2g/3g and inter-rat handover


Roaming  -> MME handles this when the URE moves from the home PLMN to a visitor PLMN.

For auth -> MME Interacts with HSS to authenticate the UE.

Signalling process for the bearer establishment for UE is done by MME.

Dedicated bearer is established specially during a VoLTE call
.

MME Manages EPS bearers.


access to 3gpp2 networks -> CDMA and EBDU are 2g and 3g technologies respectively.


CS Fallback -> support for legacy networks -> 2g and 3g.



**_<u>S-GW</u>_**
LTE 

With P-GW handles user data traffic btwn UE and external ntwk.

A particular S-GW can handle many UE at the same time.

downlink Pkt routed from -> PDN gateway to enodeB then transmitted to UE via radio network(E-UTRAN).


Routes the uplink pkts from UE to s-gw via enodeB
S-GW routes this packet to PDN gateway

S-GW buffers the downlink pkts when UE Is in ECM-idle 
state until the UE is in active state.


It also stores the uplink and downlink data shared per UE  per bearer.


**_<u>P-GW(PDN Gateway)</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/mhflo2eotsrmkxy9rmjm.png)



A UE can have more than 1 PDN gateway.
Eg. for -> internet access and VoLTE calling.

Functions ->
Managing and locating IP address. Sends reponse to the PDN connectivity request and send in reposnse the IP address to the UE.
PDN supports pkt filtering and screening and routes them to the data path meant for that user.

PDN suports the service level charging based on u and user's subscription and uplink and downlink traffic transferred.

During handover if the s-gw is changed, it sends end marker to the source s-gw  to  notify that np further donwlink pkt shd be send to that ue. The remaning pkts will be send by the target s-gw to that ue.

It is also involved in accounting per ue per bearer.
Collects the data transferred for both uplink and downlinkdata volume per eps bearer, send and receive from S-GW.


**_<u>HSS(Home Subscriber Server</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/kl88w4pbfc3mk4b01ijb.png)

MMe when receives attach request from UE interacts with HSS for user's authenication.
Both the UE and the core network authenticate each other through HSS.


**_<u>S10 Interface</u>_**

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**_<u>S11 Interface</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ywa5ljx08ti1m7al11da.png)



MME connects with S-GW via this interface
It connects with this interface during bearer establishment, modification and release. 


**_<u>S6a Interface</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/9edghuma00rjn8ltorh7.png)


**_<u>S5/S8 Interface</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/60jej66jog0iqtzkrssp.png)



**_<u>SGi Interface</u>_**


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/11hxuuwr9j7daqbs5vc3.png)