4. Traffic Flow on the LTE Network

Figure 4 shows the flow of user plane traffic accessing the Internet in the LTE network reference architecture. Figure 4 (a) shows the traffic flow from a UE to the Internet and Figure 4 (b) shows one from the Internet to a UE. IP packets are forwarded through the GTP tunnel over S1-U and S5 interfaces. These GTP tunnels are established per EPS bearer when a user is attached to the LTE network.

More than one EPS bearer is established on each of the S1-U and S5 interfaces. So, in order to identify these bearers, a Tunnel Endpoint Identifier (TEID) is assigned to the end points (UL and DL) of each GTP tunnel (When identifying a GTP tunnel, a TEID, IP address and UDP port number are used in general.

Here, however, for convenience of description, only a TEID is used for this purpose). The receiving end side of the GTP tunnel locally assigns the TEID value the transmitting side has to use. The TEID values are exchanged between tunnel endpoints using control plane protocols

Figure 4. Traffic flow on the LTE network

When a GTP tunnel is established on the S1-U interface, the S-GW assigns a TEID (UL S1-TEID in Figure 4(a)) for uplink traffic and the eNB assigns a TEID (DL S1-TEID in Figure 4(b)) for downlink traffic. The TEID values of the S1 GTP tunnel are exchanged between the eNB and the S-GW using S1AP and GTP-C messages.

Likewise when a GTP tunnel is established on the S5 interface, the P-GW assigns a TEID (UL S5-TEID in Figure 4(a)) for uplink traffic and the S-GW assigns a TEID (DL S5-TEID in Figure 4(b)) for downlink traffic. The TEID values of the S5 GTP tunnel are exchanged between the S-GW and the P-GW using GTP-C protocol.

When a user IP packet is delivered through a GTP tunnel on the S1-U and S5 interfaces, the eNB, S-GW and P-GW forward the user IP packet by encapsulating with the TEID assigned by the receiving peer GTP entity. In uplink direction, the S-GW builds a one-to-one mapping between an S1 GTP tunnel (UL S1-TEID) and an S5 GTP tunnel (UL S5-TEID) to terminate the S1 GTP tunnel and forward the user IP packet into the S5 GTP tunnel.

Likewise in downlink direction, the S-GW builds a one-to-one mapping between a S5 GTP tunnel (DL S5-TEID) and a S1 GTP tunnel (DL S1-TEID) to terminate the S5 GTP tunnel and forward the user IP packet into the S1 GTP tunnel. In figure 4, the procedure through which each EPS entity forwards Internet traffic flow is as follows:

a) Traffic flow in uplink direction: from UE to the Internet

1. A UE transfers user IP packets to an eNB over LTE-Uu interface.
2. The eNB encapsulates the user IP packets with the S1 GTP tunnel header and forwards the resulting outer IP packets to the S-GW. Here, the eNB selected a “TEID” value (i.e. UL S1-TEID), “Destination IP Address” (i.e. S-GW IP address), and “Source IP Address” (i.e. eNB IP address) to make the S1 GTP tunnel header.
3. After receiving the outer IP packets, the S-GW strips off the S1 GTP tunnel header, encapsulates the user IP packets (the inner IP packets) with the S5 GTP tunnel header and forwards the resulting outer IP packets to the P-GW. Here the S-GW selected a “TEID” value (i.e. UL S5-TEID), “Destination IP Address” (i.e. P-GW IP address), and “Source IP Address” (i.e. S-GW IP address) to make the S5 GTP tunnel header.
4. After receiving the outer IP packets, the P-GW gets the user IP packets by stripping off the S5 GTP tunnel header and transfers them to the Internet through IP routing.

b) Traffic flow in downlink direction: from the Internet to UE

1. A P-GW receives IP packets destined for a UE over the Internet.
2. The P-GW encapsulates the user IP packets with the S5 GTP tunnel header and forwards the resulting outer IP packets to the S-GW. Here, the P-GW selected a “TEID” value (i.e. DL S5-TEID), “Destination IP Address” (i.e. S-GW IP address), and “Source IP Address” (i.e. P-GW IP address) to make the S5 GTP tunnel header.
3. After receiving the outer IP packets, the S-GW strips off the S5 GTP tunnel header, encapsulates the user IP packets (the inner IP packets) with the S1 GTP tunnel header and forwards the resulting outer IP packets to the eNB. Here, the S-GW selected a “TEID” value (i.e. DL S1-TEID), “Destination IP Address” (i.e. eNB IP address), and “Source IP Address” (i.e. S-GW IP address) to make the S1 GTP tunnel header.
4. After receiving the outer IP packets, the eNB gets the user IP packets by stripping off the S1 GTP tunnel header and transfers them to the UE through the Data Radio Bearer (DRB) over the radio link³.