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PDP Context vs. EPS Bearer A Battle ofthe Data Session Setups

1Chris Reece 11 Dec 2008 9:40 AM

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. Lets 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 isnt 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 dont do SMS over the packet network and we dont 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 theneeds 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 DedicatedEPS BearerSetup 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.

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