EVOLVED PACKET SYSTEM (EPS)

Architecture & Attach Procedure

ByAnirudh Yadav

WHAT IS EPS ?

• Release 8 of 3GPP• E-UTRAN (Radio N/W) + EPC (Core N/W) = EPS• Flow of EPS architecture.

User Radio N/W Core N/W IP N/W(UE, e-nodeB) (E-UTRAN)

(EPC) (IMS)

EPS ARCHITECTURE

Continued…..

• UE: refers to the device controlled by user Eg.- mobile, tablet, USB dongle etc.

• E-nodeB: – formal name for cell tower.– X2 interface with other e-nodeBs.– WCDMA vs. EPS architecture.

• MME: – Keeps track of user state whether ON or OFF– Communicates with HSS for security keys & encrypted data– User can be registered with 1 MME at a time.– Primary responsibilities include EMM (user’s location & state?) &

ESM (create actual data session)

Continued….

• HSS: – Similar to HLR– Contains all encrypted data related to user.– Acts as a database information storage node.

• S-GW:– 1 UE can be served by 1 S-GW.– MME tells it what P-GW to connect to.– Handles user IP packets b/w e-nodeB & P-GW.

• PDN-GW:– Known as PCEF, connected to PCRF, OCS (billing server)– Controls amount of QoS dictating its services as a DICTATOR.– Functionally an IP router.

CONTROL & USER PLANE

ATTACH PROCEDURE

Continued….

1. ATTACH REQUEST:– UE wants to connect, sends an attach request to e-nodeB– E-nodeB determines which MME to connect to.

2. AUTHENTICATION INFO REQUEST/RESPONSE– MME queries the HSS for key information– Pre-shared key concept– MME asks e-nodeB what specific it wants to ask?– Authentication successful.

3. UPDATE LOCATION REQUEST– HSS declares specific MME for the UE– In case of old MME it cancels it & update the location register.

Continued….

4. UPDATE LOCATION ANSWER– HSS forwards the UE’s subscription services, QoS networks &

information to the MME.

5. CREATE SESSION REQUEST– MME figures out what P-GW can serve that external network &

what S-GW can serve what e-nodeB to which subscriber is connecting on.

– Tunnels points/legs get created b/w e-nodeB & S-GW & b/w S-GW & P-GW.

6. CREATE SESSION RESPONSE– P-GW- Dictator, billing with OCS sever, controls QoS settings– MME gets this response from P-GW through S-GW.

Continued….

7. ATTACH ACCEPT:– MME sends IP in ATTACH ACCEPT to UE. – Also include IP of S-GW for e-nodeB– E-nodeB forwards ATTACH ACCEPT as part of RRC message.

8. ATTACH COMPLETE– UE sends ATTACH COMPLETE to MME – Connection b/w UE & e-nodeB & b/w S-GW & P-GW gets created.

9. MODIFY BEARER REQUEST– MME sends this request to S-GW – Includes TEID of e-nodeB – Connection b/w e-nodeB & S-GW gets completed – UE is able to exchange data

HISTORICAL CONTEXT

GPRS

• MS (Mobile sation)

• nodeB• RNC• SGSN• GGSN• HLR

EPS• UE• E-nodeB• E-nodeB• S-GW & MME• P-GW• HSS