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LTE Network Architecture: Basic
July 10, 2013 | By Netmanias (tech@netmanias.com)
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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.

 

5. Closing

 

The LTE network architecture has been presented as the first document of the “LTE” technical document series. The LTE network architecture explained in this document applies to a LTE only network provided by a single operator and thus has covered the most basic components of the EPS system.

 

To be able to move on to other LTE technical documents that follow, fundamental understanding of the entities and interfaces of the EPS system is required.

 

The next technical document, consisting of three companion documents, is another basic LTE document and will discuss the LTE identification applied to the LTE network reference model. These basic documents would be helpful in better understanding of subsequent documents, which will discuss more advanced functions of the LTE architecture including LTE interworking and roaming.

 

References

 

[1] 3GPP TS 36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description”.

[2] 3GPP TS 23.401, “GPRS Enhancement for E-UTRAN Access”.

[3] Magnus Olsson, et. al., SAE and the Evolved Packet Core – Driving the Mobile broadband Revolution, AP, 2009.

[4] NMC Consulting Group Confidential Internal Report, “E2E LTE Network Design”, August 2010.

 

 

Footnotes

 

1. A simple example for packet forwarding over GTP tunnel is described in Section IV.

2. It, although not one of the protocol layers that form the LTE-Uu interface, is described here for the sake of convenience. The NAS layer of a UE communicates with its counterpart of an MME through the radio link and over the LTE-Uu interface.

3. For DRB, refer to the technical document, “LTE Identification III: EPS Session/Bearer Identifiers

 

 

Page 2 of 2
SB00335 2014-05-23 18:10:00
Hi Netmanias,
You guys made LTE learning Easy for freshers also. Please Upload some Technical Documents related to PCRF call flows in Detail. because i am working on PCRF project.


Thank you.
Netmanias 2014-05-23 23:49:55
We are preparing a technical document on Policy and Charging Control (PCC), and it will be posted by mid-June.
Thank you..
Rakesh 2014-07-21 08:26:12

On S11 interface, does it provide GTP tunnel per bearer?? I think it should be GTP tunnel per user on S11 Interface.

Netmanias 2014-07-28 18:36:42

Hi Rakesh,


Thank you for noticing the error.

We have fixed it and posted the updated file. 
Thank you again for your kine comment and sorry for the late reply.  

Pinal Patel 2014-09-03 21:20:28

Good One.

JP Singh 2014-09-16 17:10:51

Hi,

 

Could you please provide LTE roaming call establishing e2e doccuments.

 

Thanks for sharing valuable doccuments on LTE.

email id: singh.jp111@gmail.com

 

Mohammad Arabi 2014-11-03 21:12:05

Hi

In LTE network how many MME is needed? or How many user one MME can support?

Vincent 2014-12-17 17:54:27

Netmanias LTE series would be best of the best to understand LTE network. Thank you alot!

NGUYEN THI TAM 2015-02-06 01:53:21

hi, I think that: there are two way for offline charging is OFCS and CG. They use interface Gz (incluce Ga and Rf). If it is OFCS, the interface is Rf and uses Diameter protocol. If it is CG, the interface is Ga and uses GTP' protocol. Please check again!

Rajeswari.pothuraju@gmail.com 2015-05-04 22:57:33

In Above Diagram it shows Interface b/w OFCS and P-GW is Gz which will use GTP Protocol .. but Gz is actually uses diameter protocol isn't it ?? got confused can u pls explain it clearly

Chris Yoo 2015-05-08 04:07:52

Hi, Nguyen & Rajeswari,


As far as I know OFCS can have both Gz (GTP') and Rf (Diameter) interfaces as shown figure below.

If my understading is correct, the Netmanias document (figure 3 above) is not wrong.

Kim Son Nguyen 2015-05-05 17:42:46

Hello everyone!

I don't understand this paragraph

"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 eNB encapsulates the user IP packets with the S1 GTP tunnel header and forwards the resulting outer IP packets to the S-GW.

So why S-GW assigns TEID for uplink traffic? Why not eNB assigns?? 


John Lee 2015-05-09 03:54:50

The S-GW, based on the value "UL S1-TEID" in the received packet from the eNB, figures out to which P-GW and with what "UL S5-TEID" to send the packet. If eNB assings "UL S1-TEID", there's no way to know mapping between "UL S1-TEID" and "UL S5-TEID" in S-GW.


That is, packet receving node should assigns TEID as shown below:

- UL S1-TEID is assinged by S-GW during attach procedure, then eNB uses UL S1-TEID when it sends packet to S-GW

- UL S5-TEID is assinged by P-GW during attach procedure, then S-GW uses UL S5-TEID when it sends packet to P-GW

- DL S5-TEID is assinged by S-GW during attach procedure, then P-GW uses DL S5-TEID when it sends packet to S-GW

- DL S1-TEID is assinged by eNB during attach procedure, then S-GW uses DL S1-TEID when it sends packet to eNB


For more information you can refer to the link below:

https://www.netmanias.com/en/?m=view&id=blog&no=5928

Do Van Quang 2017-03-16 18:24:18

good, thanks

Kim Son Nguyen 2015-05-12 11:24:01

Hi John Lee

Thank you very much! 

Srilakshmi 2015-07-09 19:04:10

Hi,

This is srilakshmi.
could you please share a document on LTE roaming and how does charing works for roaming in lte.

Fabrice 2015-12-20 03:46:43

This introduction was well done. Please help us with more insight with topic like DRX, Power, scheduling and others more. 

Hari 2017-04-04 18:46:50

Hi,

Thanks for sharing the Info on LTE,,its worth usefull.Thanks a Lot.

Hari
 

Sumanthkumar 2017-10-29 04:20:47

HI, 

thanks for the info..

can we download the pdf files for the same.

vineet_agr@yahoo.com 2018-01-16 15:00:07

The follwoing queries are on UE side.

1) What is the mechanism used at UE to select the PDCP Entity for UL IP pakcets? 

2) Is DHCP used to get the assined at UE with DHCP server running on PGW and client on UE?

John 2018-01-16 15:09:06

According to the 3GPP standards, UE can get IP address using DHCP protocol, but it is not common in real network. Usually UE will get IP address from P-GW during LTE attach procedure (with no DHCP procedure). You can find IP address allocation document at https://www.netmanias.com/en/?m=view&id=techdocs&no=7246

vineet_agr@yahoo.com 2018-01-19 20:44:15

Thanks for providing information on query #2.  It was really helpful 

Any information on #1?

Thank you for visiting Netmanias! Please leave your comment if you have a question or suggestion.
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