LTE - EPS Bearer Flow

The simple definition of EPS Bearer is process UE to get IP address (this is definition by me). What actually happen inside EPS bearer?

# The EPS Bearer #

UE ---- eNodeB
->1. Attach request
->Wireless interface

eNodeB ---- MME
->2. Attach request
->S1-MME interface (SCTP)

MME ---- SGW
->3. Create session request
->S11 interface (GTPv2-C)

SGW ---- PGW
->4. Create session request
->S5 interface (GTPv2-C)

PGW ---- SGW
->5. Create session response
->S5 interface (GTPv2-C)

SGW ---- MME
->6. Create session response
->S11 interface (GTPv2-C)

MME ---- eNodeB
->7. Attach accept
->S1-MME interface (SCTP)

eNodeB ---- UE
->8. Attach accept
->Wireless interface

LTE Traffic Flow inside IP Backbone

I have explained about EPS in several posted include each LTE interface for simple architecture. Example implement Cisco & Juniper L3 Switch for EPC and E-UTRAN, we can called that is IP Backbone (IPBB). The LTE traffic flow inside IP Backbone is simple. It can ping between node in virtual routing, example S11_VR virtual routing: MME can ping to SGW and vice versa. The previous example is very simple, only one node each, that is not reflect the live network, may be implemented for very simple lab. For expand network you can enable OSPF or MPLS inside IPBB as you need.

Before UE can access internet or VPN use LTE, there are some step will be happen. The simple step for traffic flow, I will divide it become 2, the EPS bearer and the access internet. The EPS bearer is reflect to control plane process and the access internet is user plane process (payload).

The simple definition of EPS Bearer is process UE to get signal and IP address. The access internet is generic process IP flow use TCP or UDP. Each interface between two node use different protocol. If you dump traffic between them you will see encapsulation/tunneling based on below description.

For example:
S11 interface use GTPv2-C protocol to transfer between MME and SGW. The GTPv2-C isn't native encapsulation based on OSI layer but tunneling in layer 4. Layer 3 is general IP flow and layer 2 is ethernet. Basicly GTP packet based on UDP for transfer. But GTP-C and GTP-U has different port for transfer, GTP-C use UDP 2123 and GTP-U use UDP 2152.

# The EPS Bearer #

UE ---- eNodeB
->Wireless interface

eNodeB ---- MME
->S1-MME interface (SCTP)

MME ---- HSS
->S6a interface (Diameter)

MME ---- SGW
->S11 interface (GTPv2-C)

SGW ---- PGW
->S5 interface (GTPv2-C)

# The Access Internet #

UE ==== eNodeB
=>Wireless interface

eNodeB ==== SGW
=>S1-U interface (GTPv1-U)

SGW ==== PGW
=>S5 interface (GTPv2-U)

PGW ==== Gateway to Internet/VPN/etc.
=>SGi interface (IP)

Implement Juniper L3 Switch for EPS E-UTRAN (S1-MME,S1-U Interface)

Another example config on Juniper layer 3 switch (L3_Switch) for implement E-UTRAN network (S1-MME, S1-U interface).

Please refer to LTE Interface and Virtual Routing for picture. Config for S11 and SGi, S5, S6a Interface.

#### Parameter (S11, SGi, S5, S6a, S1-MME, S1-U)

L3_Switch (JunOS)
ge-0/0/1: S11_MME

ge-0/0/2: S11_SGW

ge-0/0/3: S11_DNS

ge-0/0/4: S6a_HSS

ge-0/0/5: S6a_MME

ge-0/0/6: S5_SGW

ge-0/0/7: S5_PGW

ge-0/0/8: SGi_PGW

ge-0/0/9: S1_MME

ge-0/0/10: S1_ENODEB

ge-0/0/11: S1U_SGW

ge-0/0/12: S1U_ENODEB

IP Address & Routing

MME_S11 192.168.16.2/28, gateway (default route) 192.168.16.1

SGW_S11 192.168.20.2/28, gateway (default route) 192.168.20.1

DNS_S11 192.168.21.2/28, gateway (default route) 192.168.21.1

HSS_S6a 192.168.8.2/28, gateway (default route) 192.168.8.1

MME_S6a 192.168.9.2/28, gateway (default route) 192.168.9.1

SGW_S5 192.168.24.2/28, gateway (default route) 192.168.24.1

PGW_S5 192.168.28.2/28, gateway (default route) 192.168.28.1

PGW_SGi 192.168.32.2/28, gateway (default route) 192.168.32.1

MME_S1 192.168.0.2/28, gateway (default route) 192.168.0.1

ENODEB_S1 192.168.2.2/28, gateway (default route) 192.168.2.1

SGW_S1U 192.168.4.2/28, gateway (default route) 192.168.4.1

ENODEB_S1U 192.168.6.2/28, gateway (default route) 192.168.6.1

#### Config

set interfaces ge-0/0/9 description "S1_MME"

set interfaces ge-0/0/9 ether-options no-auto-negotiation

set interfaces ge-0/0/9 ether-options link-mode full-duplex
set interfaces ge-0/0/9 ether-options speed 100m

set interfaces ge-0/0/9 unit 0 family inet address 192.168.0.1/28

set interfaces ge-0/0/10 description "S1_ENODEB"

set interfaces ge-0/0/10 ether-options no-auto-negotiation

set interfaces ge-0/0/10 ether-options link-mode full-duplex

set interfaces ge-0/0/10 ether-options speed 100m

set interfaces ge-0/0/10 unit 0 family inet address 192.168.2.1/28

set interfaces ge-0/0/11 description "S1U_SGW"

set interfaces ge-0/0/11 ether-options no-auto-negotiation

set interfaces ge-0/0/11 ether-options link-mode full-duplex

set interfaces ge-0/0/11 ether-options speed 100m

set interfaces ge-0/0/11 unit 0 family inet address 192.168.4.1/28

set interfaces ge-0/0/12 description "S1U_ENODEB"

set interfaces ge-0/0/12 ether-options no-auto-negotiation

set interfaces ge-0/0/12 ether-options link-mode full-duplex

set interfaces ge-0/0/12 ether-options speed 100m

set interfaces ge-0/0/12 unit 0 family inet address 192.168.6.1/28

set routing-instances S1_VR instance-type virtual-router

set routing-instances S1_VR interface ge-0/0/9.0

set routing-instances S1_VR interface ge-0/0/10.0

set routing-instances S1_VR interface ge-0/0/11.0

set routing-instances S1_VR interface ge-0/0/12.0