EPS introduces an E2E QoS model based on the EPS Bearer, which replaces the PDP Context of GPRS. The EPS Bearer is a logical link between the UE and the PDN GW providing a specific QoS along the whole path. The EPS Bearer Architecture consists of multiple layers as in UMTS and the service of each layer is provided through the service of the layer immediately below it. Thus the required QoS to the end user is provided by each Bearer across all layers. The E2E Service is completed by an external Bearer from the edge of PLMN to the final external destination and not controlled by the PLMN.
Bearer Level QoS Paremeters
The QoS of an EPS Bearer is defined by the following QoS parameters:
- QCI (Quality Class Identifier)
QCI is an index referring to a number of different sets of minimum QoS characteristics, such as priority, delay, etc. required by a service. The different QCI’s can be achieved by different packet forwarding treatment (e.g. scheduling, queue management, RLC config, etc.) at the network nodes. The network has to be pre-configured to provide the supported QCI’s. There are 9 standardised QCI’s and associated with example services. The characteristics of the QCI are:
It is a priority indicator in order to allow the network to reject the establishment or modification of new Bearers or discard existing ones in cases of limited resources. After the Bearer establishment, it does not affect routing.
Applies to Bearers with Resource Type = GBR and indicates the minimum bitrate to be provided for this service.
Applies to Bearers with Resource Type = GBR and indicates the maximum bitrate to be provided for this service. GBR=MBR for now.
Applies to Bearers with Resource Type = non-GBR. It is the maximum bitrate allowed across all Bearers of the same UE for each UE-APN connection and it is stored in HSS. One non-GBR Bearer can have the whole capacity if all other non-GBR are zero.
Applies to Bearers with Resource Type = non-GBR. It is the maximum bitrate allowed of the same UE and it is stored in HSS. One non-GBR Bearer can have the whole capacity if all other non-GBR are zero. UE-AMBR = sum(APN-AMBR) for all APN connections of the UE.
QoS Mechanism
When an UE is attaching to the network, an EPS Bearer is always set up and lasts until the UE detaches in order to provide an ‘Always On’ IP connectivity, reducing set up latency and excessive signaling. This Bearer is the Default Bearer and its QoS parameters are set by the network based on the UE subscription profile stored in the HSS. The Default Bearer is always non-GBR. More EPS Bearers can be established for one UE, known as Dedicated Bearers. The Dedicated Bearers are modified or created always after network trigger. The GBR Bearers have fixed allocated dedicated network resources related to the GBR, while non-GBR not.
In order to map the incoming data packets to the correct EPS Bearer, the packets need to have a QoS requirement which will then be satisfied by the Bearer. All the data packets from one application protocol (e.g. FTP) within the same IP-CAN session constitute an SDF (Service Data Flow) and the SDF has a QoS requirement as defined by its QoS parameters, described before: QCI, ARP, MBR, and GBR. A set of SDF’s within the same IP-CAN session with the same QCI and ARP constitutes an SDF Aggregate which can be treated in the same way as a single SDF. One EPS Bearer can then carry all the SDF’s which have exactly the same QoS parameters as the Bearer. Thus an EPS Bearer carries the data from one application or from multiple applications which share exactly the same QoS requirements.
The exact mechanism for the mapping of SDF’s to EPS Bearers is using the TFT concept and is described in detail here.
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