LTE-Default and dedicated bearer

By : Manish Arora

EPS• EPS stands for Evolved packet system. • whole LTE system is developed to provide all types services through

packet switching. • To achieve this GTP (GPRS tunneling protocol is being used).• GTP is application layer protocol which differentiate session based

on TEID (tunnil end identifier). • UDP is used as a transport layer protocol.• A bearer is a pipe that carry data from one node to another node. • There are two types of GTP messages used one for control plane

and one for user plane.• EPS bearer has following components like TEID , Transport layer

address, etc

GPRS Tunneling Protocol (GTP) in LTE

• GPRS Tunneling protocol is an important IP/UDP based protocol used in GSM, UMTS and LTE core networks.

• It is used to encapsulate user data when passing through core network and also carries bearer specific signalling traffic between various core network entities.

• This protocol has several advantages which will be discussed later.

Bearer in LTE

• UE <------------------------------>enb<---------------------------------->SGW <---------------------->PGW<------->Internet

• The pipes, which are carrying data from PGW to UE and vice versa called EPS bearer.

• when data reaches at PGW from external world it process and create a GTP-U packet and sent to SGW.

• SGW does the same thing it remove the GTP-U header received from PGW and create a new GTP-U header then forward packet to eNB.

• Once eNB receives GTP-U packet it removes header GTP-U header and then PDCP layer at enodeb performs IP header compression and ciphering and map to logical channel

• then message sent to UE through radio channel. radio bearers identified by bearer Identity (1-32) and logical channel identity(3-11).

GPRS Tunneling Protocol Types

Why is GTP used in LTE?

• It provides mobility. When UE is mobile, the IP address remains same and packets are still forwarded since tunneling is provided between PGW and eNB via SGW

• Multiple tunnels (bearers) can be used by same UE to obtain different network QoS

• Main IP remains hidden so it provides security as well• Creation, deletion and modification of tunnels in case

of GTP-C

GTP Interfaces in LTE• In LTE, version 2 is used for GTP-C and version 1 is used for GTP-U• In simple LTE network implementation, GTP-v2 is used on S5 and S11 interfaces and GTPv1 is used on S1-U,

S5, X2-U interfaces (as shown below). In inter-RAT and inter PLMN connectivity, S3, S4, S8, S10, S12 and S16 interfaces also utilize GTP protocols

version 1 GTP-U

version 2 GTP-C

How GTP-U Works ?• GTP-U encapsulation of UE user plane traffic can be easily understood by taking any simple example. Lets

see what happens when IP packet generated by UE reaches to eNodeB and is then forwarded to SGW.

• Consider any application on UE creates an IP/TCP packet. This packet consist of actual data by application, TCP or UDP header and then IP field information which has source address of UE and destination address of application server (e.g. Facebook)

• When the eNodeB receives this packet over air interface, it will put the IP packet inside GTP header which has information related to tunnel IDs. Then further, it is encapsulated inside UDP and IP header and forwarded as ethernet frame towards SGW. Here the IP header contains eNodeB IP as a source address and SGW IP as a destination address

GTP-C signalling messages

As GTP-Cv2 in LTE is used for tunnel management, some of the signalling messages are listed below which use GTP-Cv2 protocol

What is Bearer..??• Bearer is just a virtual concept.• It defines how the UE data is treated when it travels across the network. • Network might treat some data in a special way and treat others normally.• Some flow of data might be provided guaranteed bit rate while other

may face low transfer. • In short, bearer is a set of network parameter that defines data specific

treatment • e.g. Person A will always get at least 256 Kbps download speed on his LTE

phone while for person B there is no guaranteed bit rate and might face extremely bad download speed at times

Default Bearer

• When LTE UE attaches to the network for the first time, it will be assigned default bearer which remains as long as UE is attached.

• Default bearer is best effort service. • Each default bearer comes with an IP address. • UE 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 is one of the main bearer which is created at the time of initial

attach procedure or at the time of new PDN connection. Default bearer represents a PDN connection and exists until UE gets detached from network or UE initiated PDN dis-connectivity explicitly or network force fully trigger release for the default bearer due to policy control. Default bearer is a non-GBR bearer and provide always on IP connectivity.

Dedicated Bearer

• To put it simple, dedicated bearers provides dedicated tunnel to one or more specific traffic (i.e. VoIP, video etc).

• Dedicated bearer acts as an additional bearer on top of default bearer.• It does not require separate IP address due to the fact that only additional default bearer

needs an IP address and therefore dedicated bearer is always linked to one of the default bearer established previously.

• Dedicated bearer can be GBR or non-GBR (whereas default bearer can only be non-GBR). • For services like VoLTE we need to provide better user experience and this is where

Dedicated bearer would come handy. • Dedicated bearer uses “Traffic flow templates (TFT)” to give special treatment to specific

services• Dedicated bearer is created when the requested service can't be fulfilled through default

bearer. Some services required a high level of QoS like voice call. so network create a dedicated bearer with required QoS . Dedicated bearer may be Non-GBR or GBR depend of qci(QoS class identifier) value. dedicated bearer can be created/release based on requirement but default bearer is created only all on IP connectivity and released only at the time of detach/PDN disconnection.

Two default and One dedicated bearer

• Usually LTE networks with VoLTE implementations has two default and one dedicated bearer

•Default bearer 1: Used for signaling messages (sip signaling) related to IMS network. It uses qci 5

•Dedicated bearer: Used for VoLTE VoIP traffic. It uses qci 1 and is linked to default bearer 1

•Default bearer 2: Used for all other smartphone traffic (video, chat, email, browser etc)

Why dont we use dedicated bearer for ims signaling as well ?

•IMS network is separate network from normal internet.

• And it comes with its own APN. • Therefore we need separate default bearer

for IMS network. • This also helps in separating IMS traffic from

normal internet traffic as well

How it is decided to send voip traffic on dedicated bearer?

•Both UE and eNB has TFT which has rules for certain services. For example, incase of VoLTE voip traffic, the rule are defined on the basis of protocol number , destination network ip network etc

• TFT----Traffic flow Template

How is dedicated bearer linked to default bearer?

• The value of "Linked EPS bearer identity" defined in setup info of dedicated bearer is used to link dedicated bearer to default bearer

FAQ• What are the information that Default EPS has but Dedicated EPS bearer does not ?• The answer is "APN Name" and "PDN type/PDN Address".

• Then What kind of PDN address does Dedicated EPS Bearer will use ? • The answer is "Dedicated EPS Bearer uses the PDN address of Default EPS Bearer it is linked to".

• Then how a Dedicated EPS Bearer knows which Default EPS Bearer it is linked to ? • The answer is that it is via a information element called "Linked EPS Bearer Identity" (See the arrow shown in

the picture above)

• What are the information that both Default EPS Bearer and Dedicated EPS Bearer has ?• The answer is "QCI".

• Can Default EPS Bearer and Dedicated EPS Bearer can have the same QCI value ?• The answer is "No, they must have different QCI value".

• What is the relation between Default EPS Bearer and an APN ? • The answer is "there is one-to-one relationship between them. One Default EPS Bearer is alllocated for one

APN.