What is QoS Class Identifier (QCIs) for LTE?

Multiple applications can run simultaneously on a user device (UE), each requiring different quality of service (QoS). For example, you might be on a VoIP call while browsing the web or downloading a file. VoIP needs low delay and minimal jitter, whereas web browsing and file downloading need low packet loss.

QCI in LTE.

To handle these different QoS needs, different bearers are created within the Evolved Packet System (EPS) or EPC, each linked to a specific QoS.

Bearers can be divided into two main types:

Minimum Guaranteed Bit Rate (GBR) Bearers:

• These are used for applications like VoIP.
• They have a guaranteed bit rate (GBR) for which dedicated transmission resources are permanently allocated by the eNodeB during bearer setup or modification.
• If additional resources are available, the bit rate can exceed the GBR, up to a Maximum Bit Rate (MBR).

Non-GBR Bearers:

• These don’t guarantee any specific bit rate.
• They are suitable for applications like web browsing or file downloads.
• No permanent bandwidth resources are allocated to these bearers.

The eNodeB in the access network is responsible for ensuring the required QoS for each bearer over the radio interface. Each bearer is assigned a QoS Class Identifier (QCI) and an Allocation and Retention Priority (ARP).

Each QCI (QoS Class Identifier) is defined by its priority, packet delay budget, and acceptable packet loss rate. The QCI label assigned to a bearer dictates how it is managed in the eNodeB. Only a limited number of QCIs have been standardized to ensure consistency in service characteristics and treatment across different vendor equipment. This standardization allows LTE operators to expect uniform traffic handling behavior throughout the network regardless of the eNodeB manufacturer.

The QCI table specifies values for priority handling, acceptable delay budget, and packet error loss rate for each QCI label. These values influence the configuration of the Radio Link Control (RLC) mode and how the Medium Access Control (MAC) scheduler handles packets over the bearer. For instance, packets with higher priority are scheduled before those with lower priority. For bearers requiring low packet loss, an Acknowledged Mode (AM) in the RLC layer ensures successful packet delivery across the radio interface.

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

Allocation and Retention Priority (ARP).

The Allocation and Retention Priority (ARP) of a bearer is used for call admission control, determining whether a requested bearer should be established during radio congestion. ARP also controls the prioritization of the bearer for pre-emption in favor of a new bearer establishment request. Once a bearer is established, its ARP does not affect packet forwarding treatment, which is instead determined by other QoS parameters like QCI, GBR (Guaranteed Bit Rate), and MBR (Maximum Bit Rate).

In an LTE network, an EPS (Evolved Packet System) bearer must traverse multiple interfaces:

1. S5/S8 interface: Connects the Packet Data Network Gateway (P-GW) to the Serving Gateway (S-GW).
2. S1 interface: Connects the S-GW to the eNodeB.
3. LTE-Uu interface: The radio interface between the eNodeB and the User Equipment (UE).

Across each of these interfaces, the EPS bearer is mapped onto lower layer bearers, each with a unique bearer identity. Each network node keeps track of the bindings between these bearer IDs across different interfaces.

Bearer Types and Their Roles:

1. S5/S8 Bearer: Transports packets between the P-GW and the S-GW. The S-GW maintains a one-to-one mapping between the S1 bearer and the S5/S8 bearer using the GTP (GPRS Tunneling Protocol) tunnel ID.
2. S1 Bearer: Transports packets between the S-GW and the eNodeB.
3. Radio Bearer: Transports packets between the UE and the eNodeB.

The eNodeB manages the one-to-one mapping between the radio bearer ID and the S1 bearer, ensuring that packets mapped to the same EPS bearer receive consistent packet forwarding treatment, such as scheduling policy, queue management policy, rate shaping policy, and RLC (Radio Link Control) configuration.

Packet Filtering and Traffic Flow Templates (TFTs):

To provide different levels of QoS, separate EPS bearers are established for each QoS flow. User IP packets are filtered into different EPS bearers using Traffic Flow Templates (TFTs). TFTs use IP header information (e.g., source and destination IP addresses, TCP port numbers) to filter packets, such as VoIP traffic from web browsing traffic, ensuring each type of traffic is sent through the appropriate bearer with the suitable QoS.

• UpLink TFT (UL TFT): Filters IP packets to EPS bearers in the uplink direction within the UE.
• DownLink TFT (DL TFT): Performs similar filtering in the downlink direction within the P-GW.

By using these TFTs, the network can effectively manage and prioritize different types of traffic, ensuring that applications with stringent QoS requirements, like VoIP, receive the necessary resources and treatment to maintain performance.

When a User Equipment (UE) attaches to the LTE network, it is assigned an IP address by the Packet Data Network Gateway (P-GW) and establishes at least one bearer, known as the default bearer. This bearer remains active for the duration of the PDN (Packet Data Network) connection, providing the UE with always-on IP connectivity to the network. The initial Quality of Service (QoS) parameters for the default bearer are assigned by the Mobility Management Entity (MME) based on subscription data from the Home Subscriber Server (HSS). The Policy and Charging Enforcement Function (PCEF) can modify these values in interaction with the Policy and Charging Rules Function (PCRF) or through local configuration.

Types of Bearers

1. Default Bearer:

• Always non-Guaranteed Bit Rate (non-GBR) since it must remain established.
• Provides continuous IP connectivity.
• Initial QoS parameters are set by the MME and can be modified by the PCEF.

1. Dedicated Bearer:

• Can be either Guaranteed Bit Rate (GBR) or non-GBR.
• Established during or after the attach procedure.
• Each dedicated bearer has its own QoS parameters and Traffic Flow Templates (TFTs).
• Can be triggered by network requirements (e.g., from the IMS domain) or requested by the UE.
• May be provided by one or more P-GWs.

QoS and Bearer Management

• The QoS parameters for the default bearer come from the MME based on HSS data.
• For dedicated bearers, the P-GW receives QoS parameters from the PCRF and forwards them to the S-GW.
• The MME acts as a transparent conduit, forwarding the QoS parameters received from the S-GW over the S11 reference point to the E-UTRAN (Evolved Universal Terrestrial Radio Access Network).

Key Points

• Default Bearer: Ensures basic connectivity with non-GBR QoS.
• Dedicated Bearers: Provide additional QoS as needed, can be GBR or non-GBR.
• QoS Parameters: Managed by the MME, PCEF, and PCRF to ensure the required service quality.
• Traffic Flow Templates (TFTs): Used to filter and direct traffic appropriately across bearers.

By managing these aspects, the LTE network ensures that different applications running on a UE, each with distinct QoS requirements, are adequately supported, providing a seamless and efficient user experience.

Bearer Establishment Procedure.

The process of establishing a bearer in an LTE network involves several key steps across network nodes, ensuring end-to-end connectivity with appropriate Quality of Service (QoS). Here’s a steps of the bearer establishment procedure:

PCRF to P-GW Interaction:

• PCC Decision Provision: PCRF sends a message specifying required QoS parameters for the bearer to the P-GW (Policy and Charging Rules Function to Packet Data Network Gateway).
• Create Dedicated Bearer Request: P-GW receives the QoS policy and sends a message to the S-GW requesting creation of a dedicated bearer, including QoS parameters and UpLink Traffic Flow Template (UL TFT).

S-GW to MME Interaction:

• S-GW forwards the Create Dedicated Bearer Request message, along with bearer QoS, UL TFT, and S1-bearer ID, to the MME (Serving Gateway to Mobility Management Entity).

MME to eNodeB Interaction:

• MME builds session management configuration information (including UL TFT and EPS bearer identity) and sends a Bearer Setup Request message to the eNodeB.
• Bearer Setup Request includes bearer QoS parameters, used by the eNodeB for call admission control and scheduling of IP packets.

eNodeB to UE Interaction:

• eNodeB configures radio bearer QoS based on EPS bearer QoS received.
• eNodeB sends an RRC (Radio Resource Control) Connection Reconfiguration message to the UE.
• RRC message includes radio bearer QoS, session management configuration, and EPS radio bearer identity.
• This message configures Layer 2 (PDCP, RLC, MAC parameters) and Layer 1 (physical layer) parameters on the UE’s protocol stack.

Confirmation and Acknowledgement:

• Messages 6 to 10 represent responses confirming correct bearer setup across the network.
• These messages ensure that the UE and network nodes acknowledge successful bearer establishment, confirming end-to-end connectivity.

The bearer establishment procedure ensures that each UE receives appropriate QoS for its applications by setting up dedicated bearers as required. This process involves interactions between the PCRF, P-GW, S-GW, MME, eNodeB, and UE, each playing a crucial role in configuring and confirming the bearer parameters across multiple network interfaces. This structured approach guarantees seamless data transmission with prioritized handling based on application requirements.

Note: PCC stands for Policy Control and Charging.