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Transmission Technologies

An Introduction to LTE QoS

by Abel M. – Instructor of Telefocal

Unlike previous technologies, such as UMTS or GSM/GPRS, which offer dedicated Circuit-Switch domains to carry delay sensitive traffic (Voice, Video, Streaming), LTE carries traffic through a 100% Packet-Switch domain. This presents a new challenge for LTE from the customer service assurance perspective, and Quality of Service (QoS) in LTE networks becomes an important issue that needs to be handled with due care and attention.

In an LTE network, parameters are used to distinguish between different types of services. There is end-to-end QoS from the mobile device (UE) to the Packet Switching Gateway.

IP packets from different services are mapped into EPS (Evolved Packet System) bearers, which are assigned a different QoS. Traffic Flow Templates (TFTs) are in charge of filtering packets in EPS Bearers. TFTs are based on IP header information (IP Address, TCP ports)

QoS Class Identifier (QCI) identifies the QoS applied for a specific service. There exists a limited number of QCI (currently nine). QCI specifies several indicators to be met during a service, such as priority, service type (GBR or Non-GBR), maximum packet delay and maximum packet loss. This is propagated through the network to guarantee a service with this set of characteristics. For example, eNodeB will map QCI into the RLC layer (AM/UM) and MAC layer (scheduler to select priorities between services).

QCI are applied to the EPS Bearer (from the UE to the P-GW), which can be separated in GBR (Granted Bit Rate) and Non-GBR. GBR reserves a set of resources which impacts on admission control (e.g. VoLTE), whereas Non-GBR services do not.

ARP (Allocation and Retention Priority)
is a tag applied to an EPS Bearer for admission control purposes. It applies priorities when services are established. For example, in case of congestion a new service with higher ARP could pre-empt an established service with a lower ARP.

The optimization of QoS parameters (e.g. QCI, ARP, etc) will have a direct impact on LTE user experience. Therefore QoS in LTE networks, while adding value, is an important feature that warrants great attention from telco operators to ensure superior customer experience.

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