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Inter faces and thei r pr otocol stacks | LTE AN D BEYOND | Te Tech- bl og on 4G/LTE and beyond..

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Interfaces and their protocol stacks After familiarising with main network elements, elements, time has c ome to better know interfaces between those elements. Interfaces In terfaces are allowing MME, S GW and PGW PGW cooperating with wi th other network network elements( elements( e.g . HS S or PCRF). Each one of them i s built in standardised way described by 3GPP.org. Each interface described here is taken from fr om 23.401 3GPP.org documentation. 3GPP.org documentation. Please keep in mind, that the documentation documentation is (sometime (sometimes) s) bigger than we need it to be, so not every aspect of interfaces is described here. here.

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Lets start with big picture of situation in which user is not roaming.

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I'm a M.Sc, B.Eng of Telco and Electronics from Poland who graduated gradua ted both of his degree on Military University of Technology in Warsaw, Poland. I've I've c reated t his blog to help myself as others gaining knowledge on LTE/4G and beyond. Hope that it will be, or maybe even was, useful to You. When there is nothing to do, there is always something to do. Except doing the dishes, and so on. ;-)

Fig.1. Non-roaming architecture by 3GPP

Figure above is showing only 4G interfaces, additional interfaces for 2G and 3G are described in TS 23.060. As I probably wrote wrote earlier earlier Serving Gatew Gateway ay (SGW ) and PDN Gateway (PGW) could be put in one chassis. As it comes to s cenario with roaming architectu architecture re standards are describing two ways of dealing with it. Two roaming scenarios:

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where wher e traffic is routed from home network network to UE by S8 interface, PAGES

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attach (4) attach (4) CDF (2) CDF (2) CDR (1) CDR (1) CSFB (2) CSFB (2) CTF (3) CTF (3) DNS in LTE (6) LTE (6) ECM (2) ECM (2) EMM (4) GUTI (3) GUTI (3) Gx (3) Gx (3) Gy (3) Gy (3) Handover (4) Handover (4) Fig. 2. Roaming architecture scenario with home routed traffic

where there is local breakout with home operator's application functions only, and separate from that with vis ited operator's application functions only.

IMEI (1) IMEI (1) Interfaces (4) M M E (9) MME Selection (2) Selection (2)

http://www.l teandbeyond.com/2012/01/i nter faces- and- thei r - pr otocol - stacks.html

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Interfaces and their protocol stacks | LTE AND BEYOND | Tech-blog on 4G/LTE and beyond.. OCF (1) OCS (3) OFCS (2) PCC (2) PCEF (2) PCRF (2) PGW (3) Resource (6) S1 (5) S5/S8 (1) S6a (2) SGs (3) SGW (4) Sy (1) TAU (3) TMSI (2)

BLOG ARCHIVE Fig. 3. Roaming architecture for local breakout, with home operator's application functions only

► 2014 (3) ► 2013 (21) ▼ 2012 (19) ► December (3) ► October (1) ► M ay (1) ► April (1) ► March (4) ► February (1) ▼ January (8) LTE attach proc edure Gy interface - sitting between OCS and PCEF Gx interface - sitting between PCRF and PCEF Offline Charging System (OFCS) - introduction to G... Online Charging System (OCS) - introduction to Gy ... Interfaces and their protoc ol stacks Functions of main LTE packet core elements - MME, ... This is where it all start

Fig. 4. Roaming architecture for local breakout, with home visitor's application functions only

Keeping that in mind, now we can go straight through interfaces their functions and protocol stack. Information flow could be divided into two groups, one is Control Plane, and the other is User Plane. The Control Plane consists of protocols for control and support of the user plane functions: controlling the E-UTRA network access connections, s uch as attaching to and detaching from EUTRAN; controlling the attributes of an established network access connection, s uch as activation of an IP address; controlling the routing path of an established network connection in order to support user mobility; and controlling the ass ignment of network resources to meet changing user demands.

Control Plane interfaces: S1-MME interface between eNodeB and MME.

Reference point for the control plane protocol between E-UTRAN and MME.

http://www.lteandbeyond.com/2012/01/interfaces-and-their-protocol-stacks.html

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Fig. 5. Protocol stack of S1-MME interface

Where: S1-AP (S1 Application Protocol): Application Layer Protocol between the eNodeB and MME. SCTP ( Stream Control Transmiss ion Protocol): This protocol guarantees delivery of s ignalling mess ages between MME and eNodeB (S1). SCTP is defined in RFC 4960 S3 interface between SGSN and M ME.

It enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state.

Fig. 6. Protocol stack of S3 interface

LIVE TRA FFIC

Where: GTP-C ( GPRS Tunnelling Protocol for the Control Plane): This protocol tunnels signalling mess ages between SGSN and MME UDP (User Datagram Protocol): This protocol signaling messages. UDP is defined in RFC 768.

S4 interface between SGSN and S GW.

It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the us er plane tunnelling.


Where: GTP-C (mentioned above): This protocol tunnels signalling messages between SGSN and SGW. UDP: This protocol transfers signalling messages. UDP is defined in RFC 768. S5 or S8 interface between SGW and PGW.

S5 : It provi des user plane tunnelling and tunnel management between Serving GW and PDN GW . It i s us ed for Serving GW relocation due to UE mobility and if the S erving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity. S8 : Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN (Visited PLMN) and the PDN GW in the HPLMN (Home PLMN). S8 is the inter PLMN variant of S5.

Difference between those two interfaces i s S 5 is used in one network entity(no roaming scenario), and S8 is being used to connect Visiting PLMN where user is with his Home PLMN.


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Fig. 8. Protocol stack of interface S5 or S8

Where: GTP-C: This protocol tunnels signalling messages between SGW and PGW. UDP: This protocol transfers signalling messages between SGW and PGW. UDP is defined in RFC 768. S10 interface between MME and other MME.

Reference point between MMEs for MME relocation (e.g handover) and MME to MME information transfer.

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Where: GTP-C: This protocol tunnels signalling messages between MMEs. UDP: This protocol transfers signalling messages between MMEs. UDP is defined in RFC 768. S11 interface between MME and SGW.

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Reference point between MM E and Serving GW.


Where: GTP-C: This protocol tunnels signalling messages between MME and SGW. UDP: This protocol transfers signalling messages between MME and SGW. UDP is defined in RFC 768. S6a interface between MME and HSS.

It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.


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Fig. 11. Protocol stack of S6a interface

Where: Diameter: This protocol supports transferring of subscription and authentication data for authenticating/authorizing user access to the evolved system between MME and HSS (S6a). Diameter is defined in RFC 3588. SCTP: This protocol transfers signalling messages. SCTP is defined in RFC 4960. S13 interface between MME and EIR.

It enables UEEIR.


Where: Diameter: This protocol supports UE identity check procedure between MM E and EIR (S13). Diameter is defined in RFC 3588. SCTP: This protocol transfers signalling messages. SCTP is defined in RFC 4960. SBc interface between CBC and eNodeB.

Reference point between CBC and MM E for warning mes sage delivery and control functions. Cell Broadcast Center (CBC) was a solution for the special requirement of an Earthquake and Tsunami warning system (ET WS ) created for Japan, introduced in Rel. 8. It utilizes the exi sting i nterfaces between UE and MME in control plane. Additionally the MM E is connected to the CBC vi a the S Bc interface. In LTE/4G S Bc interface is fully standardized and based on SCTP.

Fig. 13. Protocol sta ck of SBc interfa ce

Where: SBc-AP (SBc Application Protocol): Application Layer Protocol between CBC and MME. This protocol supports transfer of warning messages. S1-AP (S1 Application Protocol): Application Layer Protocol between the eNodeB and the MME. SCTP: This protocol guarantees delivery of signalling messages between MME and eNodeB (S1). SCTP is defined in RFC 4960.

User Plane interfaces: http://www.lteandbeyond.com/2012/01/interfaces-and-their-protocol-stacks.html

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User Plane in a big picture.

Fig. 14. User Plane

S1-U interface between eNodeB and SGW.

Reference point between E-UTRAN and Serving GW for the per bearer user plane tunnelling and inter eNodeB path switching during handover.

Fig. 15. Protocol stack of S1-U interface

Where: GTP-U (GPRS Tunnelling Protocol for the user plane): This protocol tunnels user data between eNodeB and SGW. UDP: This protocol transfers user data. UDP is defined in RFC 768.

S4 interface between UE with 2G access and PGW.

S4 i nterface is also being us ed to connect UE with 3G access and PGW . It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the us er plane tunnelling.

Fig. 16. Protocol stacks of S4 interfaces used to connect UE from 2G network to PDN

Where: GTP U: This protocol tunnels us er data between SGSN and the S GW as well as between the S GW and the P GW in the backbone network. GTP shall encapsulate all end user IP packets. UDP/IP: These are the backbone network protocols used for routing user data and control signalling. Protocols on the Um and the Gb interfaces are described in TS 23.060.


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Fig. 17. Protocol stacks of S4 interfaces used to connect UE from 3G network to PDN

S12 interface between UE from 3G network and PGW.

Reference point between UTRAN and Serving GW for user plane tunnelling when Direct Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGS N. Usage of S12 i s an operator configuration option.

Fig. 18. Protocol stack of S12 interface used to connect UE from 3G network to PDN

Where: GTP U: This protocol tunnels us er data between UTRAN and the S GW as well as between the S GW and the P GW in the backbone network. GTP shall encapsulate all end user IP packets UDP/IP: These are the backbone network protocols used for routing user data and control signalling. Protocols on the Uu interface are described in TS 23.060. SGSN controls the user plane tunnel establishment and establish a Direct Tunnel between UTRAN and S GW as shown in Fig. 18.

Sources: 23.401 3GPP.org Evolution of 3G Network - book on google books

Posted by Bart Barton at 2:48 PM

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Labels: Interfaces, MME, PGW, S1, S5/S8, S6a, SGW Location: Budapest, Hungary

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Eric Shen 6 days ago - Shared publicly

Is GTP-U (over S1-U between eNodeB and SGW) ciphered? · Reply


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Inter faces and thei r pr otocol stacks | LTE AN D BEYOND | Tech- bl og on 4G/LTE and beyond.. sanket ghate 3 weeks ago - Shared publicly

Very useful indeed

Akhilesh soni 4 months ago - Shared publicly

This page is really helpful. Knowledge has to be shared like this. · Reply

Vijay Reddy 1 month ago - Shared publicly

very useful and organized · Reply

Andrew Caruanav ia Google+ 2 months ago - Shared publicly

Very intersting reading :) · Reply

swagata mukherjee 1 month ago - Shared publicly

Useful article . Thanks · Reply

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Joseph wa kamula 2 months ago - Shared publicly

Excellent effort Bart

muhammad mujtaba 2 months ago - Shared publicly

good one · Reply

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Ranjit kumar 4 months ago - Shared publicly · Reply

Anonymous 5 months ago

Ueful article on Knoweldge sharing.


Thanks for your reply I guess SCTP HBs are only sent in s6a multi-homing connections. Would you know the 3GPP standard that defines the "mandatory" behaviour of SCTP? For some reason I'm not seeing any HB for s6a when in single-homing mode.


Is it mandatory that heartbeats are sent in SCTP s6a single homing connections?

Bart Barton 10 months ago Oh yes they are. As through the heartbeats the SCTP's path management function monitors the reachability of the end nodes. From troubleshooting point of view those messages are helping to define on which layer there is a connectivity problem. I would even said that because of that and probably few others features SCTP was chosen to be used. Please correct me if I'm wrong on the last one (or any of the above).

Anonymous 1 year ago

Very nice document, I like it a lot. But X2 interface is missing


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I love this document, i just read it today, but tomorrow i will try to look for some more details on User-plane. very nice..


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