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Quality of Service in LTE full explained

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LTE Quality of Service (QoS)

QoS has become an important part of network planning & design. Every user has their own requirements. There are subscribers who use LTE services for critical operations (e.g. voice calls, bank transactions, hospital operations) and there are subscribers who just want to enjoy premium Internet. LTE was designed to meet these increased data and application demands with reliable connections and low cost of deployment. A best case scenario would feature a highly-flexible QoS framework that is built to withstand future challenges. Advanced LTE QOS defines priorities for certain customers or services during congestion.
In LTE Broadband Network QoS is implemented between CPE and PDN Gateway and is applied to a set of bearers. 'Bearer' is basically a virtual concept and is a set of network configuration to provide special treatment to set of traffic e.g. VoIP packets are prioritized by network compared to web browser trac. In LTE, QoS is applied on Radio bearer, S1 bearer and S5/S8 bearer, collectively called as EPS bearer as shown in figure below:

QoS in LTE

Understanding Default & Dedicated Bearer:

To best implement the concept of Qos in a LTE network, we must understand the bearer types and properties associated with each bearer. Two types of Bearer exist - Dedicated bearer and Default bearer. Default bearer is established when a CPE is initially attached to LTE network while dedicated bearer is always established when there is need to provide QoS to specific service (like VoIP, video etc).

QoS in LTE

Bearer in fixed broadband LTE network is similar to virtual traffic management concept. Bearers pre-determine how the user end data is treated when it travels across the LTE network. Network might treat certain type of data in a prioritized or different way and treat other data normally. Some flow of data type might be provided guaranteed bit rate while other may assign lower transfer rate. In a simply explanation, LTE bearers are sets of network parameter that defines data specific treatment.

One example of using bearers to provide tiered service packages – A premium subscriber will always get at least 5Mbps download speed on his LTE broadband service while for a basic package subscriber there is no guaranteed bit rate and his speed may be subjected to network traffic conditions.

Default Bearer in LTE:

When a LTE broadband CRP initiates connection to the network for the first time, it will be assigned default bearer depend on the service subscribed and remain connected until service is changed or terminated. For many carriers default bearer is best effort service which means speed and quantity may vary depend on network usage and time of the day. Each default bearer comes with an IP address. Each LTE broadband subscriber can have additional default bearers as well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non- GBR) can be assigned to default bearer.

Default Bearer in LTE

Dedicated Bearer in LTE:

In a simple term, dedicated bearers provide dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc.). Dedicated bearers are the secondary bearers that are created on top of the existing default bearer. Dedicated bearer shares the IP address previously established by the default bearer therefore dedicated bearer does not require to occupy additional IP address.

Dedicated bearer are mostly used as GBR (Guaranteed Bit Rate) services although it can also can be a non-GBR service. On the other hand, Default bearer can only be non-GBR. Carries or service provides that offer voice over LTE would find dedicated bearer useful to maintain high voice quality and improve user experiences. Carrier or SP can use a TFT within dedicated bearer to assign special rule of treatment to specific data or services. SIP or VoIP services can be .

For services like VoLTE we need to provide better user experience and this is where Dedicated bearer would come handy. Dedicated bearer uses TFT to give special treatment to specific services.

Dedicated bearer in LTE

Dedicated bearer can be subdivided into Non-GBR and GBR types.
GBR - The minimum guaranteed bit rate per EPS bearer. GBRs are specified individually for uplink and downlink.

A value of a GBR QoS parameter is pre-determined and associated to the bearer. If the traffic carried by that GBR bearer conforms to the value of associated with the bearer, congestion-related packet losses will not occur on the services utilizing that GBR bearer. This can be initiated on the network end (e.g. the access base stations) during activation or admission control processes, and are executed when the bearer is established.

A GBR bearer typically is established “on demand” because it blocks transmission resources by reserving them in an admission control function.

An LTE fixed broadband carrier may leverage GBR bearers to implement “service blocking” rather than “downgrade service”. For most carriers this is a preferred user experience, in which network carriers block a service request rather than enabling all services with degrades quality and performance.

This is relevant in scenarios in situations voice services will remained up while data and other traffics will be discontinued during an emergency or extreme heavy network loading.

Non-GBR – No minimum guaranteed bit rate per EPS bearer.

On the other hand Internet network service utilizing a non-GBR bearer is prone to congestion related packet losses. Non-GBR does not block any network specific traffic or transmission resources. A non-GBR is established in the default or dedicated bearer and however, can remain established for a long period of time.

Whether a service is realized based on GBR or non-GBR bearers, the policy of a carrier or service provider is depend on anticipated trac load versus dimensioned capacity. Assuming sufficiently dimension capacity is available, any service, both real time and non-real time, can be realized based on non-GBR bearer.

Other parameter associated with all bearers - QoS class of identifier (QCI) which defines IP level packets characteristics:
Dedicated Bearers in LTE

The expansion and migration burden on carriers’ cellular infrastructure continues to grow rapidly. While advanced modulation techniques and antenna technologies have made great improvement in increasing RF & network efficiency, limitation on usable radio spectrum and back-haul capacity is a everyday challenge and expensive.

If voice & video traffic were treated as normal packet data, an LTE network would deliver it (alongside every other type of traffic) via physical shared channels using the default evolved packet system (EPS) bearer originally designed to deliver regular data trac to an application server such as a Web server. These “best effort” IP delivery methods provide no control over service quality.

BEC’s 4G/LTE CPE products introduce the concept of an EPS dedicated bearer to establish specific tolerances for packet delay, error loss, and guaranteed versus non-guaranteed bit rates. Dedicated bearer setup along with BEC’s Dual APN service enable quality controlled services with highest user experiences and satisfaction.

posted May 14, 2014 by Tapesh Kulkarni

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VoLTE - VoLGA

  • Both CS Fallback (CSFB) and VoLGA rely on the existing circuit voice network, but VoLGA is not approved by 3GPP yet.

Overview:

  • VoLGA is architecture independent and uses the UMA/GAN (Unlicensed Mobile Access/ Generic Access Network) protocol.
  • This was originally adopted for Wi-Fi/3G fixed-mobile convergence and as such did find its way into the 3GPP.
  • VoLGA does not require modifications in the LTE RAN or Core, or the MSC, but uses a separate gateway controller.
  • A technological approach for delivering voice and SMS services over LTE access networks
  • Leverages a mobile operator’s existing core voice network
  • Derived from the existing 3GPP GAN standard
  • A technological approach for delivering voice and SMS services over LTE access networks
  • Leverages a mobile operator’s existing core voice network
  • VoLGA is architecture independent and uses the UMA/GAN (Unlicensed Mobile Access/Generic Access Network) protocol - Derived from the existing 3GPP GAN standard (originally adopted for Wi-Fi/3G fixed-mobile convergence)
  • VoLGA re-uses this principle by replacing the Wi-Fi (GAN-based) access with LTE access on an LTE/UE new dual mode mobile device (both GSM/UMTS and LTE).
  • VoLGA does not require modifications in the LTE RAN or Core, or the MSC, but uses a new separate gateway controller (VANC).
  • The VoLGA Access Network Controller (VANC) , as a GAN gateway between LTE and CS domain, securely connects a subscriber to the infrastructure of a network operator and voice calls and other circuit switched services such as SMS are then securely transported between the mobile device and the Gateway.
  • VoLGA is a stronger contender than CSFB. From technical view, VoLGA seems to be a much better starting point. VoLGA would further delay IMS deployment.

VoLGA

VoLGA Pros and Cons:

Pros:

  • Voice and Data over LTE
  • Call setup times as good as 3G
  • Preserves CS core investments
  • External controller (VANC) minimizes impact to core network - No MSC upgrades
  • Supports simultaneous voice/ data over LTE
  • VoLGA will support all existing circuit services as well as IMS RCS - Supports combinational IMS/ RCS + Voice over LTE
  • Delivering voice over LTE validates LTE QoS capabilities
  • Voice services delivered natively through LTE femtocell
  • The VoLGA forum decided to use the SRVCC as the means to handover VoLGA calls from LTE to GSM or UMTS.
  • No VoLGA specific features required in the MSC or SGSN for VoLGA is a great plus for deployment in a running network.

Cons:

  • Not 3GPP standardized yet - VoLGA is currently not a work item in 3GPP
  • Not fully standardized yet as the stage 3 specification has not yet been finalized
  • Limited operator support
  • GAN-based dual-mode mobile phones is required
  • SRVCC-capable mobile is required
  • Only T-Mobile strongly enthusiastic right now
  • Scaling and Roaming
  • Limited LTE Coverage if only hotspots at the initial phase
  • It also requires changes to handsets, as well as a mechanism for allowing the network to trigger LTE-to-3G/2G handovers for VoLGA calls, originally defined as part of SR-VCC (single radio voice call continuity).

VoLGA Interface:

  • All other network elements and the interfaces between them already exist and are reused without any modifications.
  • A-interface is used to connect the VANC to a GSM MSC (Mobile Switching Center).

  • The Iu-interface is used to connect the VANC to the UMTS MSC.

  • No changes are required on these network nodes to support voice, SMS and other services over the LTE network.

  • SGi: SGi is defined in TS 23.401. It is the reference point between the P-GW and the packet data network. The "packet data network" is the CS core network connected by VANC in this specification.

  • Sv: Sv is defined in TS 23.216, where it is defined as the reference point between the MME/SGSN and MSC Server. In this specification, Sv applies to two interfaces:

  1. between the MME and HOSF, and
  2. between the HOSF and MSC Server.
    HOSF = Handover Selection Function
  • Z1 (UE – VANC): Z1 is the reference point between the UE and VANC, which is based on the Up interface defined in TS 43.318.

  • Z3 (VANC – HOSF): Z3 is the reference point between the VANC and HOSF. It is based on GTPv2-C as specified in TS 29.274. The Z3 reference point is used for the creation and deletion of VANC-UE bindings in the HOSF, and to route the SRVCC PS to CS Handover Request message to the VANC.

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