Detailed Description of the QoS Mechanism in EPS

WEDNESDAY, 18 NOVEMBER 2009

Detailed Description of the QoS Mechanism in EPS

A Packet Filter has to be set up at the PDN GW (and signaled to the UE) for each SDF or SDF Aggregate in order to allow the correct mapping of data to the EPS Bearer and effectively the correct routing. The EPS Bearer is associated with one TFT (one in UL and one in DL) and thus an EPS Bearer can carry only one SDF or only one SDF Aggregate while all data of the same EPS Bearer will experience the same QoS treatment meaning. The SDF can be mapped to an EPS Bearer only if they have the same QCI and ARP.

The Packet Filters are sequentially applied to the incoming data (on the UE in UL and PDN GW in DL) according to the Evaluation-Precedence-Index values of the Packet Filters and eventually the SDF (if onePacket Filter exists for the TFT) or the SDF Aggregate (If more Packet Filters exist for the TFT) is mapped to a TFT and thus to an EPS Bearer.

In UL the UE creates a binding between the SDF or SDF Aggregate and the Radio Bearer (RB) carrying it, the eNB creates a binding between the RB and S1 Bearer, the SGW creates a binding between the S1 Bearer and the S5/S8 Bearer. In the DL the PDN GW creates a binding between the SDF or SDF Aggregate and the S5/S8 Bearer carrying it, while the rest mappings are as in UL.

Any non-matching data should be sent to the Bearer which has no Packet Filters associated. If no such Bearer exists, data shall be discarded.

Traffic Flow Templates
A TFT (Traffic Flow Template) consists of one or multiple Packet Filters (1-8) and is used to discriminate data packets from different applications, with different QoS requirements, etc. in order to route them appropriately. Each Packet Filter is aimed at isolating the data from one application protocol (e.g. FTP), so effectively a TFT can carry data from more than one application protocol, which have to share the same QoS characteristics though. The TFT’s are pre-configured at the PDN GW.

Packet Filters
The Packet Filter has a unique packet identifier (1-8) within the TFT and consists of one or more of the following attributes depending on its configuration with regards to the application to be carried:

• Source/Destination address with subnet mask
  IP address with subnet mask; source is valid on DL and destination on UL
• Protocol number
  Number of higher protocol (e.g. TCP/UDP)
• Destination port range
  Port range of the application (e.g. HTTP)
• Source port range
  Port range of the application (e.g. HTTP)
• IPsec security parameter index
  Arbitrary number between 256 -16639 to identify the secure connection between two entities
• Type of service
  Identifies the QoS
• Flow level
  IPv6 flow level. Not used in IPv4.

Each Packet Filter has an Evaluation-Precedence-Index which is unique across all TFT’s of the same APN which indicates the priority in which the Packet Filters will be applied to the packets. Highest priority is 0 and lowest is 255. Packet filters are signaled to the UE during NAS procedures.

TFT Example

A PDN GW has 2 different TFT’s stored as follows. The first one is aimed at transferring VoIP, with two different options (over TCP or UDP) and the second one is aimed for FTP services. The PDN GW checks first the two packet filters of the VoIP TFT and then the FTP one.

When data from an FTP service reaches the PDN GW, the PDN GW applies Packet Filter 1 first, then Packet Filter 2 and finally Packet Filter 3 as per their Evaluation-Precedence-Index values. Packet Filter 3 matches its attributes (protocol number and ports) and the PDN GW creates a binding between the FTP SDF and the TFT of Packet Filter 3 (i.e. FTP TFT).Then the PDN GW forwards the data to the EPS Bearer which is associated with that TFT. The data will then follow the appropriate S5/S8, S1 and finally Radio Bearer which constitute that EPS Bearer.

For a description of the EPS Bearer and the QoS Model in EPS click here.