In a typical case, multiple applications may be running in a UE at the same time, each one having different QoS requirements. For example, an UE can be engaged in a VoIP call while at the same time browsing a web page or downloading an FTP file. VoIP has more stringent requirements for QoS in terms of delay and delay jitter than web browsing and FTP, while the latter requires a much lower packet loss rate. In order to support multiple QoS requirements, different bearers are set up within EPS, each being associated with a QoS.
Broadly, bearers can be classified into two categories based on the nature of the QoS they provide:
In the access network, it is the eNodeB’s responsibility to ensure that the necessary QoS for a bearer over the radio interface is met. Each bearer has an associated Class Identifier (QCI), and an Allocation and Retention Priority (ARP).
Each QCI is characterized by priority, packet delay budget and acceptable packet loss rate. The QCI label for a bearer determines the way it is handled in the eNodeB. Only a dozen such QCIs have been standardized so that vendors can all have the same understanding of the underlying service characteristics and thus provide the corresponding treatment, including queue management, conditioning and policing strategy. This ensures that an LTE operator can expect uniform traffic handling behaviour throughout the network regardless of the manufacturers of the eNodeB equipment. The set of standardized QCIs and their characteristics (from which the PCRF in an EPS can select) is provided in Table below.
QCI |
Resource type |
Priority |
Packet delay budget (ms) |
Packet error loss rate |
Example services |
1 |
GBR |
2 |
100 |
10−2 |
Conversational voice |
2 |
GBR |
4 |
150 |
10−3 |
Conversational video (live streaming) |
3 |
GBR |
5 |
300 |
10−6 |
Non-conversational video (buffered streaming) |
4 |
GBR |
3 |
50 |
10−3 |
Real time gaming |
5 |
Non-GBR |
1 |
100 |
10−6 |
IMS signalling |
6 |
Non-GBR |
7 |
100 |
10−3 |
Voice, video (live streaming), interactive gaming |
7 |
Non-GBR |
6 |
300 |
10−6 |
Video (buffered streaming) |
8 |
Non-GBR |
8 |
300 |
10−6 |
TCP-based (e.g. WWW, e-mail) chat, FTP, p2p file sharing, progressive video, etc. |
9 |
Non-GBR |
9 |
300 |
10−6 |
The priority and packet delay budget (and, to some extent, the acceptable packet loss rate) from the QCI label determine the RLC mode configuration, and how the scheduler in the MAC handles packets sent over the bearer (e.g. in terms of scheduling policy, queue management policy and rate shaping policy). For example, a packet with a higher priority can be expected to be scheduled before a packet with lower priority.