1. Understanding 3GPP Bearers LTE / HSPA / EPC ‘knowledge nuggets’ Neil Wiffen - More free downloads at Public Seminar details –

2. Bearers for Quality of Service
QoS Characteristics
QoS Characteristics
QoS Characteristics
The principles that underpin the provision of services within UMTS and LTE networks are based on providing mechanisms to support the information transfer characteristics required for required services. This differs in several respects from second generation systems , and enables a relatively unrestricted approach to the development of new and improved service types, creating a competitive environment for network operators, service providers and content providers.
During the establishment of a connection, in order to ensure that adequate network resources are allocated to the user to support the required UMTS or LTE Bearer Service, a set of (QoS) parameters are determined and assigned
To realise a certain network QoS, a Bearer Service with clearly defined characteristics and functionality is set up from the source to the destination of a service.
Traffic class, bit rate, delivery order, reliability, delay characteristics, priority etc. 2

3. UMTS Bearer Hierarchy End to End Service Local Bearer Service UMTS
Terminal
Equipment
Mobile
Equipment
UTRAN
Core Network
Edge Node
Core Network
Gateway
Terminal
Equipment
End to End Service
Local
Bearer
Service
UMTS
External
Radio Access Bearer Service
Core Network Bearer Service
Backbone
Bearer
Service
Radio Iu
The Bearer Hierarchy in UMTS consists of a layered provision of information transfer capabilities between various peer entities. These peers communicate using appropriate protocols that have been designed to support the environment in which they exist (e.g. Radio Resource Control (RRC) in the UTRAN, GPRS Tunneling Protocol (GTP) in the Core Network etc.)
Each of the protocols has a clearly defined scope and can be configured to provide QoS required to deliver many service types via appropriately optimized bearers
UTRA FDD/TDD
Service
Physical
Bearer

4. UMTS Bearer Components
RNC
CN node
Iu Signalling bearer
Signalling Radio Bearers
CS Radio Access Bearer (RAB)
MSC
CS Data bearer
CS Radio Bearers
PS Radio Access Bearer (RAB)
Appropriate bearers are required to support both signalling and user data transfer between the UE and the CN. The form these bearers take depends upon several factors, including:
The purpose of the bearer
The domain to which it is providing service
The interface(s) over which the bearer is carried
The slide above shows that both signalling and data radio bearers are created between the UE and the RNC to support transfer across the Air interface for both PS and CS connections. These radio bearers are mapped to appropriate Iu / Iur bearers within the UTRAN, and collectively both the radio and UTRAN bearers support flows between the UE and the CN which are termed Radio Access Bearers (RABs)
SGSN
PS Radio Bearer
PS Data bearer
PS Radio Access Bearer (RAB)
SGSN
PS Radio Bearer
PS Data bearer 4

5. EPS Bearer Hierarchy e-NB UE Application Peer S-GW P-GW
End-to-end Service
EPS Bearer
E-RAB
S5/S8 Bearer
The EPS Bearer Service Architecture and QoS control mechanisms operate according to a simpler set of principles with fewer attributes when compared to UMTS and GPRS. The EPS supports the principle of a UE being ‘always-on’ and each registered UE has at least a default bearer, and potentially other bearers for different services with the bearer setup process typically consisting of a single transaction to the UE from the EPC. (i.e. An EPS bearer is setup in tandem with the Radio Bearer, S1 Bearer and S5/S8 Bearer according to the end-to-end QoS requirement).
Radio Bearer
S1 Bearer
LTE-Uu S1
S5/S8
SGi
E-UTRAN
EPC 5

6. IP-Connectivity Access Network
Internet
GERAN,
UTRAN,
EUTRAN
GPRS CN,
UMTS CN,
EPC
SGSN,
S-GW
GGSN,
P-GW
IMS
Platform
BTS,
Nb,
eNb
The complete solution for the support of IP multimedia applications consists of terminals, IP-Connectivity Access Networks (IP‑CAN), and the specific functional elements of the IM CN subsystem. An example of IP-Connectivity Access Network is the GPRS core network with GERAN and/or UTRAN radio access networks
Intranet
IP-CAN
IP-PDN

7. IP-CAN Bearers 7 SGSN GGSN PS-RAB GTP-Tunnel
IP-CAN Bearer (PDP Context)
S-GW
P-GW
LTE-Bearer
GTP–Tunnel
IP-CAN Bearer
(EPS Bearer)
Interconnect / Roaming support required
An IP-CAN bearer within the cellular domain is comprised of the various PS Radio-bearers, Radio Access Bearers, and Core Network bearers / connections that my be assigned to a UE across both the relevant RAN and CN elements.
SGSN
S-GW
P-GW
PS-RAB
GTP-Tunnel
GTP–Tunnel
(EPS Bearer)
IP-CAN Bearer 7

8. LTE Connection components
SDFs
E-RABs
IP-CAN Bearer
IP Flow(s)
GTP
Tunnel(s)
RBs
ISP / Transit
Networks(s)
SCTP
Association
Application
Server
SGW
PGW
MNO
EDGE
Router
Application / SP
EDGE Router
GTP-C
Tunnel
Connectivity across the Evolved UMTS Terrestrial Radio Access Network (EUTRAN) is achieved by combining individual connection components across the various links between the UE and the Packet Data Network Gateway (PGW). The various components are discussed in more detail in the following sections.
E-RAB : EPS-Radio Access Bearer SCTP: Stream Control Transmission Protocol
EPS: Evolved Packet System PCRF: Policy and Charging Rules Function
IP-CAN: IP Connectivity Access Network MME: Mobility Management Entity
IP: Internet Protocol SPR: Subscription Profile Repository
RB: Radio Bearer SGW: Serving Gateway
SDF: Service Data Flow HSS: Home Subscriber Service
GTP: GPRS Tunnelling Protocol PGW: PDN Gateway
GPRS: General Packet Radio Service PDN: Packet Data Network
MNO: Mobile Network Operator SP: Service Provider
ISP: Internet Service Provider
MME
MNO
Domain
HSS
PCRF
SPR

9. Radio Bearer (RB) Radio Bearer Example
L2 service provided for transfer of info on the Air Interface
Describes L2 processes per bit-stream (flow)
Different QoS requirements supported by different RBs
Example
Flow A requires low latency but can tolerate packet loss of 10-2
Flow B has less stringent latency requirements, but can only tolerate a packet loss of 10-6
Flow A and B must be supported by 2 separate Radio Bearers
The term Radio Bearer (RB) can be considered to be the definition of a set of processes that occur at layer 2 and 1 in the Air Interface (AI) protocol stack on a bit-stream. These processes include such things as prioritization, sequencing, error correction etc. and impact the Quality of Service (QoS) that is provided for that bit-stream. Using the Radio Bearer approach allows the differentiation of flows with dissimilar QoS requirements to be handled by the assignment of separate RBs to flows with different QoS characteristics.
For example, if we take the case where two concurrent information flows are to be supported over the Air Interface with the following characteristics:
Flow A is supporting packetized voice where low latency important, however a packet loss ratio of 10-2 is acceptable.
Flow B is supporting a file transfer application, where low latency is less important, but greater reliability is desired and in this case a packet loss ratio of now worse than 10-6 is to be achieved.
Supporting the different QoS requirements of these two flows can be achieved by assigning each flow to a separate RB, each of which would be configured to perform the relevant processes at L2 (and L1) to manage, in this case, the dissimilar latency and accuracy needs of the two applications.

10. LTE Radio Bearer categories
Signalling Radio Bearers (SRB)
SRB0 is for RRC messages
Using the CCCH logical channel
SRB1 is for RRC / NAS messages
Used prior to the establishment of SRB2
Using DCCH logical channel
NAS messages may be piggybacked inside RRC messages
All messages are Integrity Protected and Ciphered after security activation
SRB2 is for NAS messages
May be contained in RRC messages, but with no RRC control content
Lower priority then SRB1
Data Radio Bearers (DRB)
Carry User Plane content on the Air Interface
Signalling Radio Bearers (SRBs) are defined as Radio Bearers (RB) that are used to support control message flows for specific signalling peers. The are only assigned for the transmission of Radio Resource Control (RRC) and Non Access Stratum (NAS) messages.
Three SRBs are defined:
SRB0 is for RRC messages using the CCCH logical channel
SRB1 is for RRC messages (which may include piggybacked NAS messages) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel
SRB2 is for NAS messages, using DCCH logical channel.
SRB2 has a lower-priority than SRB1 and is configured by E-UTRAN after security activation.
In the downlink, piggybacking of NAS messages is used only for one procedure dependant (i.e. with joint success/ failure) : bearer establishment, modification or release
In the uplink NAS message piggybacking is used only for transferring the initial NAS message during connection setup.
The NAS messages transferred via SRB2 are also contained in RRC messages, which however do not include any RRC protocol control information.
Once security is activated, all RRC messages on SRB1 and SRB2, including those containing NAS or non-3GPP messages, are integrity protected and ciphered by the Packet Data Convergence Protocol (PDCP). NAS independently applies integrity protection and ciphering to the NAS messages.
Data Radio Bearers (DRBs) are used to carry User Plane (UP) content on the Air Interface

11. Radio Bearers SRB1 (RRC / NAS messages on DCCH) SRB0
(RRC messages on CCCH)
Used for RRC connection setup
DRB1
(User content with QoS ‘A’ on DTCH1)
In order to support the establishment of a Radio Resource Control (RRC) connection and the on-going transfer of both signalling and data for a User Equipment (UE), several Radio Bearers (RBs) are created and managed by the UE and Evolved Node B (ENB). Each of the Signalling Radio Bearers (SRBs) have defined uses and characteristics specified in TS , with the RRC and Non Access Stratum (NAS) control messages sent in using SRB0, 1 or 2 as appropriate.
The number and type of Data Radio Bearers (DRBs) that are required will be dependant on the number of user plane flows with differing QoS requirements, and each will be established from the network side using context / bearer setup or context / bearer modification message sequences. Each DRB is then associated with a Traffic Flow Template (TFT) at the UE, and is mapped to a GPRS Tunnel Endpoint ID (TEID) at the ENB.
For a given DRB, the Quality of Service (QoS) is managed by the ENB scheduler on the radio link side, and by the GPRS Tunnelling Protocol (GTP) mechanisms between the ENB and Serving Gateway (SGW)
Multiple SRBs/DRBs to support on-going user connection
SRB2
(NAS messages on DCCH)
DRB2
(User content with QoS ‘B’ on DTCH2)

12. Bearer Mapping DRBs Mapped to / from S1 Bearers SRB RRC messages
Interpreted by UE / ENB
S1 GTP Tunnels
(S1 Bearers)
S5/S8 GTP Tunnels
(S5/S8 Bearers)
SGW
PGW
GTP-C
Tunnel
SCTP
Association
S1 Bearers
Mapped to/from
S5/S8 Bearers
Radio Bearers are assigned by the Evolved Node B (ENB) according to the service requirements of the UE and Network. The General Packet Radio Service (GPRS) tunnels between ENB, Serving Gateway (SGW) and Packet Data Network Gateway (PGW) are setup based on rules and parameters established by the PGW. The tunnelling protocol itself is used to provide flow control, identification and reliability for the IP packets that are to be transported between these nodes, with these mechanisms used in conjunction with prioritisation and buffer control at the nodes themselves to deliver these packets through the Evolved Packet System (EPS) with the appropriate Quality of Service (QoS).
Each of these nodes is responsible for using the bearer identities to map the various flows on each link as required (DRBS1 bearerS5/S8 bearer in the uplink, S5/S8 bearerS1 bearerDRB in the downlink)
Radio Bearer (RB) IDs and Tunnel Endpoint IDs (TEIDs) are used for this mapping.
A signalling connection between the MME and ENB is supported using Stream Control Transmission Protocol (SCTP)
MME
SRB NAS messages
Mapped to SCTP association
Interpreted by UE / MME

13. QoS and TFT EPS Bearer with defined QoS UL Traffic Flow Aggregates
DL Traffic Flow
Aggregates
UL / DL QoS managed by ENB scheduler
UL QoS managed by ENB ‘router’
SGW
PGW
UL TFT
DL TFT
UL TFT  RB ID
RB ID  S1 TEID
S1 TEID  S5/S8 TEID
Uplink and Downlink Traffic Flow Templates are the sets of packet filters that allow mapping of traffic to EPS bearers in the Uplink and Downlink directions. Every dedicated EPS bearer is associated with a TFT. The UE uses the UL TFT for mapping traffic to an EPS bearer in the uplink direction. The PCEF (for GTP-based S5/S8) or the BBERF (for PMIP-based S5/S8) uses the DL TFT for mapping traffic to an EPS bearer in the downlink direction.
The UE may use the UL TFT and DL TFT to associate EPS Bearer Activation or Modification procedures to an application and to traffic flow aggregates of the application. Therefore the PDN GW shall, in the Create Dedicated Bearer Request and the Update Bearer Request messages, provide all available traffic flow description information (e.g. source and destination IP address and port numbers and the protocol information)
The EPS bearer traffic flow template (TFT) is the set of all packet filters associated with that EPS bearer, and for the UE, the evaluation precedence order of the packet filters making up the UL TFTs is signalled from the P‑GW to the UE as part of any appropriate TFT operations
RB ID  S1 TEID
S1 TEID  S5/S8 TEID
S5/S8 TEID  DL TFT
Packet Filters - applied to each uplink IP-datagram
(QoS control)
Packet Filters - applied to each downlink IP-datagram
(QoS control)
DL QoS managed by SGW ‘router’