EPS Bearer  'Bearer' in the dictionary means "Carrier" or "Porter" which carries something from a point to another point. Under the context of communication technology, I would define the 'Bearer' as a 'pipe line' connecting two or more points in the communication system in which data traffic follow through.Taking this definition, we can define 'EPS Bearer' as a pipe line through which data traffic flows within EPS (Evolved Packet switched System).EPS Bearer can be illustrated as the Red path in the following illustration. 

 As shown above, EPS bearer has several components in it. It means EPS beare is a complex of multiple element bearers as in the following diagram. 

 It would seem to be a simple diagram, but as you see EPS bearer includes all the components from Radio Link to the final packet core. It means understanding EPS bearer means understanding the whole LTE network.I would leave it up to you to study the very details of each component. If you have any experience or knowledge on the other technology like WCDMA, you can think of EPS Bearer as an entity similar to WCDMA PS Bearer. If you see the diagram shown above, you would notice that this bearer has two main part. One is 'Radio Bearer' and the other part is Core network bearer.In UMTS case, the 'Radio Bearer' part is configured by 'Radio Bearer Setup' message and the Core Network Bearer is configuredActivate PDP Context procedure .In LTE, the both 'Radio Bearer' part and 'Core Network Bearer' both configured by a single message, 'RRC Connection Reconfiguration'. Actually within 'RRC Connection Reconfiguration' message there is one part for Radio configuration and another part for Core Network configuration. See the following two links for the details.

RRC : RRC Connection Reconfiguration + NAS : Attach Accept + NAS : Activate Default EPS Bearer Context Req RRC : RRC Connection Reconfiguration + NAS : Activate Dedicated EPS Bearer Context Request

There are two types of EPS Bearer. One is 'Default EPS Bearer' and the other one is 'Dedicated EPS Bearer'. Simply put, we can describe as follows. i) Default EPS Bearer :

Be established during Attach Process Allocate IP address to UE Does not have specifc QoS (only Nominal QoS is applied). Similar to Primary PDP Context in UMTS

 ii) Dedicated EPS Bearer

Normally be established during the call setup after idle mode. (but can be established during the attach as well). Does not allocate any additional IP address to UE

Is linked to a specified default EPS bearer Have a specific (usually guaranteed) QoS Similar to Secondary PDP Context in UMTS

 RRC : RRC Connection Reconfiguration + NAS : Attach Accept + NAS : Activate Default EPS Bearer Context Request An important procedure done in this step is "ESM : Activate Default EPS Bearer Context Request". One thing you notice here is that in LTE Packet call is initiated by Network where as in UMST most of the packet call is initiated by UE. Network specifies an IP for the UE here.

 If you have any experience with WCDMA protocol, you may take this message to be similar to 'Radio Bearer Setup' + 'Attach Accept' + Activate PDP Context Accept.At this step, UE gets an IP from the network and this IP does not get returned to Network even after 'RRC connection Release' and UE gets into IDLE mode.

 

 An example of RRC Connection Reconfiguration is as follows. Don't try to look into all the details since this message is one of the most complicated message in LTE. Just try to understand overall structure and compare the tree map shown above and the real messages shown below.Probably it will take several month to understand all the details of these elements, so don't be so hurry.Whenever you study a little bit further details of the topics in the tree diagram shown above, open up this section and see the details under the topics you studied. If you fully understand all the information elements shown below, you can say you mastered the LTE. Again don't try to understand all of these at once. It will just raise your blood pressure. Just look through these items as often as possible and get familiar with the overall structure first.

 RRC : RRC Connection Reconfiguration + NAS : Activate Dedicated EPS Bearer Context Request This is another 'RRC Connection Reconfiguration' message. But you would see some difference between this message and the message at step 15.You don't see any 'Attach Accept' part because you already went through 'attach' process during the registration. And now you created only 'Dedicated EPS Bearer'. Does this mean that you cannot use the 'Default EPS Bearer' you created at step 15) ? No.. the default Bearer is still alive once it is created during the registration. That's why you don't have to recreate the default EPS bearer at this step. 

  One think you would notice would be that 'Activate Dedicated EPS Bearer Context Request' does not have IP address setting. This is because Dedicated EPS is using the same IP configuration specified by Default EPS Bearer. The purpose of Default EPS bearer is to create a data pipe with a different QoS from Default EPS QoS. It means Dedicated EPS Bearer is linked to a specific Default EPS bearer.Then, how do we know which default EPS bearer is linked to which Dedicated EPS Bearer ?This link is specified by 'Linked EPS Bearer Identity'. For example, if 'Linked EPS Bearer Identity' is set to be 5. It means this 'Dedicated EPS Bearer' is linked to the Default EPS Bearer with Bearer ID = 5 and use the same IP configuration as defined in the default EPS bearer. 

PDP Context vs. EPS Bearer – A Battle of the Data Session Setups

Chris 11 Dec 2008 3:40 PM 

2

LTE has been designed to support packet services in a more efficient manor than UMTS. The key service, from a wireless data network perspective, is the establishment of the data session that will be used by the mobile device for data services. In UMTS and GPRS, the key to establishing a data session is the Packet Data Protocol (PDP) Context establishment procedure. In LTE, the procedure has been changed to an Evolved Packet System (EPS) Bearer Setup. Let’s take a look at the differences.

In a UMTS network the data session is established with a PDP Context Activation procedure. But, before the PDP context can be established the UE must do an Attach procedure. The Attach procedure is used to alert the SGSN (Serving GPRS Support Node) that the UE has powered up. The problem is that there isn’t anything the UE can do after an Attach without requesting a PDP Context. To be fair, after an Attach the UE is available to receive a SMS or a Network Initiated PDP Context. The problem is that we don’t do SMS over the packet network and we don’t do Network Initiated PDP Context in practice.

After the Attach procedure is completed the UE will then do a Primary PDP Context that will establish the data session and allocate an IP address to the UE. This PDP Context will have a QoS associated with it based on the needs in the request. If the UE needs to have multiple data sessions, due to various Quality of Service (QoS), the UE will do a Secondary PDP Context activation. For the sake of completeness, it is important to note that there are other reasons to establishing subsequent PDP Context beyond QoS, but that is a good place to start.

In a LTE based system, there are two types of data session setups. The first is called a Default EPS Bearer. The second is the Dedicated EPS Bearer. The first is established as part of the Attach procedure. The Default EPS Bearer will only support a nominal QoS, but that should be sufficient for application signaling. When the UE needs to establish a service a Dedicated EPS Bearer will be established. This will have the QoS requirements needed for the service.

As way of comparison, the LTE Attach/Default EPS Bearer will be equivalent to the UMTS Attach and then doing a Primary PDP Context establishment procedure. The Secondary PDP Context Activation is similar to the Dedicated EPS Bearer Setup procedure. So, there is a variation of a theme here, but not a significant difference.

If we were to look at the key parameters in these messages, we would see that both the UMTS procedures and the LTE procedures still use parameters like an Access Point Name (APN), IP address type, and QoS parameters. Therefore, the only real difference between the two types of procedures is that there has been an optimization in LTE that reduces the number of signaling messages that need to be sent over the air.

Radio Bearer in LTE

There are 2 types of Radio Bearers (RB) in LTE: 

To carry signaling. There are called the SRB (Signaling Radio Bearer)

To carry user data. There are associated with an EPS Bearer

In LA1.X, the maximum number of RB per UE is 4

The following types of Radio Bearer are defined: 

SRB1: RRC signaling with high priority

SRB2: RRC signaling and NAS signaling (lower priority)

Best Effort: also defined as the default EPS Bearer

GBR: Radio Bearer with a guaranteed bit rate

VoIP: Radio bearer to carry the VoIP

In LA1.X the following combination are supported:  SRB1

SRB1+SRB2+Best Effort

SRB1+SRB2+Best Effort + GBR

SRB1+SRB2+Best Effort + VoIP

SRB1+SRB2+Best Effort + Best Effort

At the RRC connection, the eNodeB scheduler creates a context for the UE containing the

UEBearerList.  this list is limited to 4 per user in LA1.X 

Each bearer is identified by the LCID (Logical Channel ID) 

Each bearer is associated with QoS parameters like :  Max bit rate and guaranteed bit rate  VoIP or not  H-ARQ usage

Default Bearer vs Dedicated Bearer

A default bearer is bearer able to carry all kinds of traffic (no filter) without QoS. It is typically created during the attach procedure

A dedicated bearer is a bearer to carry a specific data flow, identify by the TFT (Traffic Flow Template), with a given QoS.

Example: Voice, streaming

It can be established:

During the Attach procedure (depending on the user profile)

After the Attach procedure, on demand

Default Bearer, Dedicated Bearer... What exactly is bearer ?While trying to get a better understanding of VoLTE and IMS , I came across bearers stuff which I thought might be useful to share with you guys. Before we talk about what default and dedicated bearer is, let's understand what bearer itself is :  Bearer is just a virtual concept. It defines how the UE data is treated when it travels across the network. Network might treat some data in a special way and treat others normally. Some flow of data might be provided guaranteed bit rate while other may face low transfer. In short, bearer is a set of network parameter that defines data specific treatment   e.g. Person A will always get at least 256 Kbps download speed on his LTE phone while for person B there is no guaranteed bit rate and might face extremely bad download speed at times

Now let me talk briefly about Dedicated and Default bearer one by one.

Default Bearer in LTEWhen LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached. Default bearer  is best effort service. Each default bearer comes with an IP address. UE can have additional default bearers as well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer. 

Dedicated BearerTo put it simple, dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer on top of default bearer. It does not require separate IP address due to the fact that only additional default bearer needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR). For services like VoLTE we need to provide better user experience and this is where Dedicated bearer would come handy. Dedicated bearer uses Traffic flow templates (TFT) to give special treatment to specific services

ExampleUsually LTE networks with VoLTE implementations has two default and one dedicated bearer

Default bearer 1: Used for signaling messages (sip signaling) related to IMS network. It uses qci 5

Dedicated bearer: Used for VoLTE VoIP traffic. It uses qci 1 and is linked to default bearer 1Default bearer 2: Used for all other smartphone traffic (video, chat, email, browser etc)

Why dont we use dedicated bearer for ims signaling as well ?IMS network is separate network from normal internet. And it comes with its own APN. Therefore we need separate default bearer for IMS network. This also helps in separating IMS traffic from normal internet traffic as well

How it is decided to send voip traffic (for example) on dedicated bearer?Both UE and eNB has TFT which has rules for certain services. For example, incase of VoLTE voip traffic, the rule are defined on the basis of protocol number , destination network ip network etc

How is dedicated bearer linked to default bearer?The value of "Linked EPS bearer identity" defined in setup info of dedicated bearer is used to link dedicated bearer to default bearer

Please also visit Quality of Service (QoS) in LTE to get more better understanding on bearers and QoS

What is the value of GBR in case of dedicate bearer?

The GBR or MBR can be configured on the PCRF per PCC Rule

Hello,

Can you tell me how does the UE know whether it is scheduled on the dedicated bearer or the

defaults one? Is there any indication of it in DCI? From my undertanding it cannot be the TFT

because there is one TFTs for each dedicated bearer.

Lets consider one example.

There is some Uplink data in UE buffer related to VoIP (VoLTE rtp traffic). So now assuming the

dedicated bearer (DRB) is already established previously and there is TFT defined in UE. So UE

checks the ip data and finds that this data should flow on one specific DRB defined. DRB mapping at

MAC layer is done through LCID to provide unique QoS to specific bearer

Hi,

Since there are up to 9 qci standardized values that a bearer can have, and 10-255 are qci values

that an operator can define, how and at which point/network element will the bearer have qci with

such operator defined values?

Hi,

I think the range is from 128-254

Operator specific QCI will BE applied on radio bearer, s1 bearer and s5/s8 bearer

What will be the difference if operator decides to use dedicated bearer with QCI5 for SIP signalling or default bearer with QCI5.Why is it that default bearer is always preffered for SIP signalling?Is traffic separation is the only reason, I mean why cant we use dedicated GBR bearer bearer for this more often?

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Adnan Basir April 25, 2013 at 2:59 PM Well separate default bearer is preferred for SIP signalling because in addition to separating IMS traffic we also want to give SIP signalling priority over other traffic and you know QCI 5 has the highest priority check the QCI table here http://4g-lte-world.blogspot.jp/2013/01/quality-of-service-qos-in-lte.html Hopefully you get more idea

If there are multiple PDN connections, will there be multiple Radio bearers with one-to-one mapping?

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Adnan Basir March 24, 2013 at 10:28 AM Multiple PDN connections means multiple default bearers. Multiple default bearers means multiple radio bearers. So correct, there is separate radio tunnel identified by LCID value for each default beare

How is radio bearer mapped to default/dedicated bearer?will there be a separate radio bearer for each default/dedicated bearer?

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Adnan Basir March 24, 2013 at 10:25 AM Yes there will be separate radio bearer for each default / dedicated bearer. DRBs are mapped via LCID (Logical channel id) at MAC layer. Each LCID defines a radio tunnel to provide QoS to user at radio level.

Quality of Service (QoS) in LTEBackground: Why we need QoS ? There are premium subscribers who always want to have better user experience on their 4G LTE device. These users are willing to pay more for high bandwidth and better network access on their devices. Not only the subscribers but some services itself need better priority handling in the network (e.g. VoIP call). To be able to full fill this, QOS plays the key role. QOS defines priorities for certain customers / services during the time of high congestion in the network

3GPP definition for QoSIn LTE Network QoS is implemented between UE and PDN Gateway and is applied to a set of bearers. 'Bearer' is basically a virtual concept and is a set of network configuration to provide special treatment to set of traffic e.g. VoIP packets are prioritized by network compared to web browser traffic.In LTE, QoS is applied on Radio bearer, S1 bearer and S5/S8 bearer, collectively called as EPS bearer as shown in figure below.

In order to comprehend the concept of QoS , we must understand the bearer types and properties associated with each bearer through hierarchical chart as shown below. First there are two types of Bearer, i.e. Dedicated bearer and Default bearer. There is  at-least one default bearer established when UE is attached to LTE network while dedicated bearer is always established when there is need to provide QoS to specific service (like VoIP, video etc). Please go through the article Default and Dedicated Bearer which hopefully will help to explain the concept in more detail.

Dedicated bearer can be subdivided into Non-GBR and GBR types. 

GBR provides guaranteed bit rate and is associated with parameters like GBR and MBR

- GBR: The minimum guaranteed bit rate per EPS bearer. Specified independently for uplink and downlink

- MBR: The maximum guaranteed bit rate per EPS bearer. Specified independently for uplink and downlink

On the other hand, Non-GBR bearer does not provide guaranteed bit rate and has parameter like A- AMBR and UE- AMBR

- A-AMBR: APN Aggregate maximum bit rate is the maximum allowed total non-GBR throughput to specific APN. It is specified interdependently for uplink an downlink 

- UE -AMBR: UE Aggregate maximum bit rate is the maximum allowed total non-GBR throughput among all APN to a specific UE

As you can see, the default bearer can only be non-GBR type. Some other  important terms associated with each bearer type are discussed below:

- ARP: Allocation and retention priority is basically used for deciding whether new bearer modification or establishment request should be accepted considering the current resource situation.

- TFT: Traffic flow template is always associated with dedicated bearer and while default bearer may or may not have TFT. As mentioned earlier, dedicated bearer provides QoS to special service or application and TFT defines rules so that UE and Network knows which IP packet should be sent on particular dedicated bearer. It usually has rules on the basis of IP packet destination/source or protocol used.

L-EBI: It stands for Linked EPS bearer ID. As I discussed in previous article about dedicated and default bearer, we know that each dedicated bearer is always linked to one of default bearers. L-EBI tells Dedicated bearer which default bearer it is attached to. 

IP Address/ PDN: Each default bearer is attached to some PDN network and has its own IP address while dedicated bearer does not need this since it is linked to default bearer.

You can also see one other parameter associated with all bearers i.e. QoS class of identifier (QCI).This parameter basically defines IP level packets characteristics as shown below

EXAMPLE

Let me try to explain here again with the same example I gave in Default and Dedicated Bearer section

Usually LTE networks with VoLTE implementations have two default and one dedicated bearer

Default bearer 1: Used for signaling messages (sip signaling) related to IMS network. It uses qci 5Dedicated bearer: Used for VoLTE VoIP traffic. It uses qci 1 and is linked to default bearer 1Default bearer 2: Used for all other smartphone traffic (video, chat, email, browser etc), assuming qci 9 is used here

This means that Default bearer 1 is associated with IMS PDN and has specific IP address. It has throughput limitations defined in terms of A-AMBR and UE-AMBR. Since it has qci 5 which means that its IP packets has the highest priority over other IP packets and maximum delay as 100ms between UE and PGW with packet loss percentage up to 10-6

Default bearer 2 is associated with internet PDN and has specific IP. It has throughput limitations defined in terms of A-AMBR and UE-AMBR as well. Since it has qci 9 which means that its IP packets has the lowest priority over other IP packets and maximum delay possible as 300ms between UE and PGW with packet loss percentage up to 10-6

Dedicated bearer will be linked to Default bearer 1 with L-EBI and it also has TFT which basically defines which IP packets should be allowed to travel on this bearer. It has throughput limitations defined in terms of MBR and GBR. Since it is using QCI 1, the IP packets traveling on this bearer have the second highest priority. The maximum delay possible to IP packets on this bearer is 100 ms and the percentage of packet loss will be under 10-2

http://4g-lte-world.blogspot.in/2013/01/quality-of-service-qos-in-lte.html

What are the information that Default EPS has but Dedicated EPS bearer does not ?The answer is "APN Name" and "PDN type/PDN Address".

 Then What kind of PDN address does Dedicated EPS Bearer will use ?The answer is "Dedicated EPS Bearer uses the PDN address of Default EPS Bearer it is linked to".

 Then how a Dedicated EPS Bearer knows which Default EPS Bearer it is linked to ?The answer is that it is via a information element called "Linked EPS Bearer Identity" (See the arrow shown in the picture above)

 What are the information that both Default EPS Bearer and Dedicated EPS Bearer has ?The answer is "QCI".

 Can Default EPS Bearer and Dedicated EPS Bearer can have the same QCI value ?The answer is "No, they must have different QCI value".

 What is the relation between Default EPS Bearer and an APN ?The answer is "there is one-to-one relationship between them. One Default EPS Bearer is alllocated for one APN.