The EPS bearer service layered architecture is depicted in Figure 1. Besides the different names of bearers and reference points, this architecture does not look very different from the bearer service architecture defined in Release 99. However, there is a major difference that is not obvious at first sight.
In 3G UMTS the request of a subscriber for a defined QoS of an end-to-end service starts the QoS negotiation procedure. This depends on the subscriber's subscribed QoS stored in the HLR and the available network resources which QoS is granted to a particular connection at the end. The QoS negotiation and control process starts on the NAS layer with the first SM message sent by the UE.
In LTE – different to 2.5 and 3G PS connections – a default bearer with a default QoS is already established when the UE attaches to the network. The QoS attributes of this default bearer are determined by the subscribed QoS parameters stored in the HSS. This is still as seen in 2.5/3G networks.
However, if now the first user plane packet is sent by the UE it is routed toward the PDN where the PCRF analyzes the requested end-to-end service. Depending on this service, the PCRF may now trigger a modification of QoS parameters in all the involved bearers. There is no option for the subscriber to request a particular QoS; only the network is in charge of QoS control. There is also no way for the UE to request something known as a secondary context in 3G (see Section 3.26 in Kreher and Ruedebusch, 2007). In LTE all QoS management is tied to the application, not to SM signaling.
It is important to understand that one UE in LTE can have multiple end-to-end services active and each of these services will have its own individual bearer. It is not intended by LTE standards that, for example, non-real-time services like web-browsing and e-mail will be mapped onto the same bearer (e.g., the same S1-U GTP tunnel) as we have seen in 3G UMTS. For this reason also 256 individual E-RABs for a single UE can be addressed by E-UTRAN protocols while in UMTS only 15 different RAB-IDs had been defined by the standard organizations.
In the 3GPP specs there is also a Traffic Flow Template (TFT) mentioned for the UL as well as for the DL part of the connection. These TFTs are bound to the EPS bearers. In general, a TFT can be described as a set of filters for a particular end-to-end service. Each TFT consists of a destination IP address and a set of source/destination port numbers. On the DL, the IP address is the address assigned to the UE; on the UL, it is the address of a server on the PDN. If we assume, for example, an HTTP 1.1 end-to-end service, the DL TFT of this service consists of the UE's IP address, the TCP source port number is 80, and the TCP destination port number is 80. On the UL, the port numbers are the same, but the IP address is the address of the server that hosts the web site.
To standardize the QoS handling, a set of nine QCIs have been defined by 3GPP. There are four classes with a Guaranteed Bit Rate (GBR) and five classes with a Non-Guaranteed Bit Rate (Non-GBR).
Besides the bit rate, the parameter priority, packet delay budget, and packet error loss rate are critical factors as given in Table 1.
QCI
|
Resource type
|
Priority
|
Packet delay budget (ms)
|
Packet error loss rate
|
Example services
|
|---|---|---|---|---|---|
1
|
GBR
|
2
|
100
|
10−2
|
Conversational voice
|
2
|
4
|
150
|
10−3
|
Conversational video (live streaming)
| |
3
|
3
|
50
|
10−3
|
Real-time gaming
| |
4
|
5
|
300
|
10−6
|
Non-conversational video (buffered streaming)
| |
5
|
Non-GBR
|
1
|
100
|
10−6
|
IMS signaling
|
6
|
6
|
300
|
10−6
|
Video (buffered streaming) TCP based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.)
| |
7
|
7
|
100
|
10−3
|
Voice, video (live streaming) interactive gaming
| |
8
|
8
|
300
|
10−6
|
Video (buffered streaming) TCP based
| |
9
|
9
|
(e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.)
|
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