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Table of Contents 1. Introduction |
2. EPS Session and EPS Bearer: Overview
Before we discuss IDs relating EPS sessions and EPS bearers, an overview of what the EPS sessions and EPS bearers are and what they are like and a description of the relationship among the IDs will be given.
Figure 1 shows the EPS sessions and EPS bearers of a user, with their IDs shown underneath.
Figure 1. Overview of Session/Bearer IDs
2.1 EPS Session
IP connection between a UE and a PDN is called PDN connection or EPS session. Each PDN connection (or EPS session) is represented by an IP address of the UE and a PDN ID (in other words, Access Point Name (APN)). It has more than one EPS bearer to deliver user traffic (IP packets), and applies the service quality (QoS) policy obtained from a PCRF to the EPS bearers. The minimum fundamental bearer that an EPS session has for a PDN is called a default EPS bearer.
Having an EPS session established means i) a PDN through which a user is to use services has been selected (by the user’s input or based on the subscription information provisioned by an HSS), ii) an IP address to be used in the PDN has been assigned to the user, iii) policy rules to be applied to the user IP packets (QoS and charging rules) have been selected, and iv) a default EPS bearer for delivering IP packets over the LTE network has been established. Through this EPS session established, IP packets can be exchanged between the user and the PDN according to the rules set by the operator.
Management and operation of sessions, including PCRF, will be explained in other document, and a PDN ID (APN) will be discussed as an ID relating to the EPS session in this document.
2.2 EPS Bearer
An EPS session is in charge of delivering and handling flows of the IP packets that are labeled with UE IP addresses and travel between a UE and a PDN (UE – P-GW – PDN). On the other hand, an EPS bearer is a pipe through which IP packets are delivered over the LTE network, i.e., between a UE and a P-GW (UE – eNB – S-GW - P-GW). A UE can have multiple EPS bearers concurrently. So, different EPS bearers are identified by their EPS bearer ID, which is allocated by an MME.
As seen in Figure 1, an EPS bearer actually is a concatenation of the following three bearers (DRB, S1 bearer and S5 bearer):
EPS bearer established over LTE-Uu interface. User traffic (IP packet) is delivered through a DRB. Different DRBs are identified by their DRB ID, which is allocated by an eNB.
EPS bearer established over S1-U interface. User traffic is delivered through a GTP tunnel. Different S1 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the endpoints (eNB and S-GW) of the GTP tunnel.
EPS bearer established over S5 interface. User traffic is delivered through a GTP tunnel. Different S5 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the endpoints (S-GW and P-GW) of the GTP tunnel.
E-RAB is a bearer that has two endpoints of a UE and an S-GW, and consists of a DRB and an S1 bearer. Technically, E-RAB is a concatenation of a DRB and an S1 bearer, and connects from a UE to an S-GW (UE – eNB – S-GW). Different E-RABs are identified by their E-RAB ID, which is allocated by an MME. DRB IDs and E-RAB IDs are mapped with EPS bearer IDs on 1:1 basis.
2.3 Types of EPS Bearers
Before we go ahead and describe EPS bearer-related IDs, we will look at different types of EPS bearers and how they work. Figure 2 shows two different types of EPS bearers: default and dedicated. Each PDN must have one default EPS bearer, but may have none to many dedicated EPS bearers.
Figure 2. EPS Bearer Types
The LTE network is an all-IP network, and provides its users with always-on IP connectivity. This means, once a UE connects to a PDN using the IP address assigned at its initial attach to the network, the IP connection remains connected after a default EPS bearer is established over the LTE network and until the UE detaches from the LTE network (i.e., the PDN connection is terminated). Even when there is no user traffic to send, the default EPS bearer always stays activated and ready for possible incoming user traffic.
Additional EPS bearer can be established if the default EPS bearer itself is not sufficient enough to obtain QoS (see LTE QoS document). The additional EPS bearer established is called a dedicated EPS bearer and multiple dedicated bearers can be created if required by the user or the network. When there is no user traffic, these dedicated EPS bearers can be removed, whereas the default one is never removed and keeps the user staying connected to the network unless the user detaches from the network. Dedicated EPS bearers are linked to a default EPS bearer. The linked bearers are represented by a Linked EPS Bearer Identity (LBI), indicating they are all associated with the same default EPS bearer.
IP traffic from or to a UE is delivered through an EPS bearer appropriately depending on QoS class over the LTE network. Uplink IP traffic is mapped from a UE up to the EPS bearer while downlink IP traffic is mapped from a P-GW down to the EPS bearer.
As discussed in Sections 2.2 and 2.3, IDs relating to EPS bearers, such as EPS bearer ID, E-RAB ID, DRB, TEID, and LBI are described in this document. The following Chapter III will further explain about these EPS session/bearer IDs.
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How are an EPS bearer and an EPS session different?
A session refers to user traffic (an IP flow) while a bearer refers to a path that user traffic (IP flows) uses when passing an LTE transport network (between UE and P-GW). In other words, an EPS bearer is a pipe that carries session(s), and hence more than one session can use the same EPS bearer.
Both the session and bearer are related to user traffic delivery, but in different ways: Sessions are about WHO uses WHAT service while bearers are about HOW user traffic is delivered in the LTE network. Bearers only care about “how to deliver user traffic to its destination?”, and not about “who uses what service?”. This is why, when user traffic is delivered through bearers, user IP addresses are not shown, and only the information about the pipe that carries the user traffic, such as DRB ID or GTP tunnel (i.e. S1 or S5 bearer) IP address, is shown.
But, once arrived at P-GW (or UE), different sessions carried by the bearer are identified and processed individually.
hi Netmainas
thanks you nice statement what‘s bearer and session
but I have some doubts about your answer.
such as“hence more than one session can use the same EPS bearer”
“But, once arrived at P-GW (or UE), different sessions carried by the bearer are identified and processed individually.”
for example：A subsrciber Use the mobile phone to surf the web and play games and watch video.
At this point, multiple IP flow are created that is not session
you say session refer to EPS session？
so A EPS session= PDN connection= single or multiple EPS bearers=A lot of multiple IP flows（An EPS bearer can have single or multiple IP flows）
I think a subsrciber only one session attach to PDN（cmnet）that could be multiple bearers，pelease correct me
Hi, Netmanias Team.
Thanks for your good response.
One question:
About "more than one session can use the same EPS bearer. ":
One session means one PDN connection. Is it right?
If so, different session may not use same EPS bearers as each EPS bearers can only have one LBI. In other words, each beaer is linked to only one PDN. Maybe I am wrong if one session does not mean one PDN connection.
1. Thanks for the good work from Netmanias team.
2. I do have the same doubt about this. Different PDN connections will setup different default EPS bearers, how can an EPS session use the same EPS bearer if one EPS session means one PDN connection?
First of all Thankyou for such wonderful presentations. Its a delight to study these documents.
In the presentation you have mentioned "The E-RAB ID is assigned by an eNB, generally with the same value as the EPS bearer ID". Is the E-RAB ID not assigned by the MME. Spec 36.413 says that ERABList ( in InitialContextSetupRequest or E-RAB Setup Request ) is sent by MME to eNB and one of the mandatory parameters of this list is E-RAB Id. Moreover, eNB would never know what EPS Bearer ID is because it is generated by MME and sent to UE in NAS message. It is not part of S1ap Message sent to eNB.
Please correct me if I am wrong and Thankyou again.
Thanks for your comment. i just want to clarify some of your comment. basically, E-RAB interface is between UE and SGW which is NAS. Furthermore, E-RAB establishment should be done by EPC Side. I agreed with your this point. but, according to E-RAB Procedure, EPC (MME is part of EPC) send E-RAB Setuprequest to eNB with e-RAB ID after PDN connectivity request from UE side through NAS signaling. after that, if eNB send back to EPC with "E-RAB Setup response" after eNB performed RRC-Reconfiguration complete. and then finally, E-RAB will be established.
Here is some of clarificaiton from myside for your concern and hopefully NETMANIAS can response for our query.
1. E-RAB ID Allocator is not eNB.
2. eNB would know E-RAB ID. (If you are familiar with S1 - signaling, then you can verify by yourself)
Thanks, Bred
Thank you for your comments and clarifications. As you pointed out, E-RAB ID is allocated by MME. The error has been corrected on the post and the attached file.
The below is our understanding of “E-RAB ID allocator” and “whether eNB knows EPS bearer ID”.
- E-RAB ID allocator: MME
E-RAB ID is allocated by MME (TS 23.401, 5.2.1) and the value of E-RAB ID is the same as the value of EPS bearer ID (TS 36.300, 8.2).
- Whether eNB knows EPS bearer ID:
When an eNB receives S1 signaling (e.g. Initial Context Setup Request message or E-RAB Setup Request message) from MME, the eNB can know E-RAB ID i.e., EPS bearer ID. When the eNB establishes a DRB, the EPS bearer ID and the corresponding DRB ID are sent to UE through RRC Signaling (e.g. RRC Connection Reconfiguration).
Thanks again for your kind notice.
Thanks for your prompt response and for making things so clear.
Netmanias is one of the best things that has happened to people trying to learn LTE. Thanks again and please keep up the good work.
Vimal
I think there is a tiny mistake in this excellent paper:
Namely:
An APN is a combination of a network ID and an operator ID. The network ID is used when identifying PDNs such as Internet or cooperate VPNs or identifying services like IMS that the PDN provides.
Should read:
An APN is a combination of a network ID and an operator ID. The network ID is used when identifying PDNs such as Internet or Corporate VPNs or identifying services like IMS that the PDN provides.
Hi,
Thank you for your kind comment. The typo has been corrected (as Corporate VPNs) on the post and the attached file.
Hello NetManias,
Excellent documentation.
Do you have any plan for VoLTE Technical Documents? Thank you.
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rintho
Hello, experts,
Thanks for the good paper.
I have one stupid question: what's the difference between the EPS sessions and IP flows?
Thanks.
Wei
Hello, Wei,
I'd expect you can find answer in link below:
https://www.netmanias.com/en/?m=view&id=techdocs&no=5908 (LTE QoS: SDF and EPS Bearer QoS)
In my understanding,
- EPS session = PDN connection
- An EPS session can have single or multiple EPS bearers
- EPS bearer is transmission path between UE and P-GW to deliver user traffic (IP flows) with specficied QoS
- An EPS bearer can have single or multiple IP flows
Hi, Chris,
Thanks for your explanation. I'll check the paper you recommended.
Thanks.
BR
Wei
Dear guys,
can we think of gtp load balancing between eNB and S-GW.
I really need it.
Thank you
Hi Netmanias,
I love your answers, especially this one:"whether eNB knows EPS bearer ID". Thank you!
Hi Netmanias,
Why do have concept of ERABID and EPS Bearerid when there is 1 to 1 mapping b/w two and also there value is always same. We could have used any one of the IDs.
Regards,
Arjun
Refer to the TS 23.401 as shown below:
5.2.1 EPS bearer identity
An EPS bearer identity uniquely identifies an EPS bearer for one UE accessing via E-UTRAN. The EPS Bearer Identity is allocated by the MME. There is a one to one mapping between EPS RB and EPS Bearer, and the mapping between EPS RB Identity and EPS Bearer Identity is made by E-UTRAN. The E-RAB ID value used at S1 and X2 interfaces to identify an E-RAB is the same as the EPS Bearer ID value used to identify the associated EPS Bearer.
The description is perfect and well structured, as always.
Keep up the good work!
If say GTPU and PDCP on eNB are not on same system (dis-integrated stack), for DL traffic after GTPU decapsulation when UE IP clear packet is submitted for PDCP prorcessing, what is the best way to identify DRB in PDCP leayer? Can we assume UE IP address and ToS value in IP header is unique for every DRB?
Hi,
In Figure 4. Bearer IDs allocators, I think P-GW should be responsible for S5 TEID DL allocation, not UL. S5 TEID UL allocation is done by S-GW which is already indicated there.
Regards,
Melih
Can a single APN have two IP-CAN sessions for a UE?
Thanks for the explanation. It inc. my basic understanding
Hi,
when you do sizing for EPC, how many bearer per user you used? Tks
What happens to the dedicated bearers when the linked default bearer is deactivated ? If the dedicated bearer will also gets deactivated, then how will it be done ? how will dedicated bearer comes to know that default has been deactivated ? What action does it take ? Whats the procedure ?
Hi， there is one thing confuse me that what is the relationship between the Eps and SRB(singling ridio bearer)。 thank you
EPS bearer is the pipeline from UE To PDN gateway, UE send IP packets to PDN GW through that bearer.
SRB is signal radio bearer, it is bearer between UE to EnodeB. That bearer is used to send signaling messages between UE to EnodeB during attach procedure. This bearer is only used for attach purpose and not related with EPS bearer.
Hi, I would like to say this is the complete package to understand LTE. I found this document very useful.Thanks a lot for clearly explaining each and everything and putting a great effort to prepare a document like this.
Hi Netmanias,
Thanks for providing excellent docuementation.
In Figure 4. Bearer IDs allocators, S-GW is responsible for allocating Ids S1 TEID Allocation(UL) and S5 TEID Allocation (DL), but not as showin in the picture S1 TEID Allocation(DL) and S5 TEID Allocation (UL). Please comment on this.
Thanks,
RAM
Hi Team Netmanias,
Can you clarfify how MME allocates E-RAB ID or EPS bearer Identity? Is it any way related to QoS? What is the bandwidth of the default E-RAB bearer for the very first time? How is it decided?
Hi Netmanias & All,
Can you please clarify on 3 Default PDN connetions ?
There will be 3 Default Bearers per 3 PDN /APN can be established, 1 is for Internnet 2. for IMS 3 for what kind of APN can be established ? Which bearer can be established during the flightmode ?
Hi NetManias,
In figure 4 S-GW will allocate S5 TEID for DL not for UL , similarly it will allocate for S1 UL TEID
Thanks a lot for your contribution. Could you pls cleaify my some doubts
1) when UE is deattach from EPC network, EPS default bearer will be removed. As EPS default bearer is removed, EPS session will also removed, Right sir?
2) When UE is idle for about an hour, is there EPS default Bearer and Session still active, sir?
Hi Netmanias,
Thanks for such a wonderful explanations, really appriciate your effort.