LTE packet services

Eps Bearer & PDP Context
http://beyond-3g-wireless.blogspot.com/

LTE has been designed to support packet services in a more efficient way 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 us first discuss how PDP context works?

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.

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.

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.

An EPS bearer is actually composed of the three following elements:
a) An S5 bearer – implemented by a tunnel which transports packets between the Serving & PDN Gateways.
b) An S1 bearer – implemented by a tunnel which transports packets between the ServingGW & eNodeB.
c) A Radio Bearer – implemented by a RLC connection between the eNodeB & the
UE. There is one RLC protocol machine per Radio Bearer

From the Specs:

1) What is EPS bearer?
As per the doc 23.401-800, Section 4.7.2.1
An EPS bearer is a logical aggregate of one or more Service Data Flows (SDFs), running between a UE and a PDN GW in case of GTP-based S5/S8, and between UE and Serving GW in case of PMIP-based S5/S8. An EPS bearer is the level of granularity for bearer level QoS control in the EPC/E-UTRAN. That is, SDFs mapped to the same EPS bearer receive the same bearer level packet forwarding treatment (e.g. scheduling policy, queue management policy, rate shaping policy, RLC configuration, etc.). Providing different bearer level QoS to two SDFs thus requires that a separate EPS bearer is established for each SDF.

2) What is difference between Default EPS bearer and Dedicated EPS bearer?
As per the doc 23.401-800, Section 4.7.2.1
One EPS bearer is established when the UE connects to a PDN, and that remains established throughout the lifetime of the PDN connection to provide the UE with always-on IP connectivity to that PDN. That bearer is referred to as the default bearer. Any additional EPS bearer that is established to the same PDN is referred to as a dedicated bearer.

The initial bearer level QoS parameter values of the default bearer are assigned by the network, based on subscription data (in case of E-UTRAN the MME sets those initial values based on subscription data retrieved from HSS). The
PCEF may change those values based in interaction with the PCRF or based on local configuration.

3) What is TFT(Traffic Flow Template) ?
TFT is a set of all Packet filters associated with a EPS Bearer. Every dedicated EPS bearer is associated with a TFT. Every EPS bearer is associated with an UL TFT in the UE and a DL TFT in the PCEF.

Maximum number of TFT that can be allocated per UE is 10(assuming one default bearer and 10 dedicated bearers).

Refer Spec 24.007/11.2.3.1.5/Eps Bearer Identity
A L3 protocol may define that bits 5 to 8 of octet 1 of a standard L3 message of the protocol contain the EPS bearer identity. The EPS bearer identity is used to identify a message flow.

From other sources: http://www.lteuniversity.com/blogs/chrisreece/archive/2009/02/06/bearers-questions.aspx

4) Default Bearers are created per UE basis or PDN GW basis?
Default bearers are created on a per PDN basis. So if a UE is connecting to two PDNs it will need to establish two default bearers. The best example that I can come up with for why a subscriber may want to connect to multiple PDNs is if subscriber wants to connect to an IP Multimedia Subsystem (IMS) network and the Internet. The first default bearer will be established during the Attach process when the UE first powers up and the second will be done using Activate default EPS bearer context request procedure. When the second default bearer is established will be dependent on the service and the UE.