concept of default and dedicated bearer in LTE and VoLTE

1. LTE-Default and dedicated bearer
By : Manish Arora

2. 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

3. 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.

4. 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).

5. GPRS Tunneling Protocol Types

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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.

13. 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)

14. 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

15. 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

16. 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

17. 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.