Bearer Concept:
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 there is no guaranteed guaranteed bit rate and and might face extremely extremely bad download download speed speed at times
EPS Session IP connection between a UE and a PDN is called PDN connection or E PS session. It has more than one EPS bearer to deliver user traffic (IP packets), and applies the service quality (QoS) policy obtained from a PCRF to the EPS bearers. The minimum fundamental bearer that an EPS session has for a PDN is called a default EPS bearer. EPS session established means i) a PDN through which a user is to use services has been selected ii) an IP address to be used in the PDN has been assigned to the user, iii) policy rules to be a pplied to the user IP packets (QoS and charging rules) have been selected iv) a default EPS bearer for delivering IP packets over the LTE network has been established.
Through this EPS session established, IP packets can be exchanged between the user and the PDN according to the rules set by the operator. EPS Bearer
EPS bearer is a pipe through which IP packets are delivered over the LTE network, i.e., between a UE and a P-GW (UE – eNB – SGW - P-GW). A UE can have multiple EPS bearers concurrently. So, different EPS bearers are identified by their EPS bearer ID, which is allocated by an MME.
There are two types of EPS Bearer. One is 'Default EPS Bearer' and the other one is 'Dedicated EPS Bearer'. Simply put, we can describe as follows. i) Default EPS Bearer :
Be established during Attach Process Allocate IP address to UE Does not have specifc QoS (only Nominal QoS is applied). Similar to Primary PDP Context in UMTS
ii) Dedicated EPS Bearer Normally be established during the call setup after idle mode. (but can be established during the attach as well). Does not allocate any additional IP address to UE Is linked to a specified default EPS bearer Have a specific (usually guaranteed) QoS Similar to Secondary PDP Context in UMTS
An EPS bearer actually is a concatenation of the f ollowing three bearers (DRB, S1 bearer and S5 bearer): [UE] - [eNB]: Data Radio Bearer (DRB) EPS bearer established over LTE-Uu interface. User traffic (IP packet) is delivered through a DRB. Different DRBs are identified by their DRB ID, which is allocated by an eNB. [eNB] - [S-GW]: S1 bearer EPS bearer established over S1-U interface. User traffic is delivered through a GTP tunnel. Different S1 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the Endpoints (eNB and S-GW) of the GTP tunnel. [S-GW] - [P-GW]: S5 bearer EPS bearer established over S5 interface. User traffic is delivered through a GTP tunnel. Different S5 bearers are identified by their tunnel endpoint identifier (TEID), which is allocated by the endpoints (S-GW and P-GW) of the GTP tunnel.
From the LTE specs, the maximum number of bearers that a UE can establish is 11. (The maximum of 11 is based on a parameter called the EPS bearer identity size.It is four bits in length with five values reserved.
ERAB :
E-RAB is a bearer that has two endpoints of a UE and an S-GW, and consists of a DRB and an S1 bearer. Technically, E-RAB is a concatenation of a DRB and an S1 bearer, and connects from a UE to an S-GW (UE – eNB – S-GW). Definition
of
E-RAB:
An E-RAB uniquely identifies the concatenation of an S1 Bearer and the corresponding Data Radio Bearer.When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS bearer of the Non Access Stratum (NAS). E-RAB Setup: E-RAB set up request is initiated by MME towards E-NodeB. Based on the information contains this message.E-NodeB will assign the Data Radio Bearers to UE (Uu Interface). For each E-RAB and based on the E-RAB level QoS parameters IE the eNB shall establish a Data Radio Bearer and allocate the required resources on Uu. The procedure uses UE-associated signalling.
S ig nalling R adio B earer : Signalling radio bearers "Signalling Radio Bearers" (SRBs) are defined as Radio Bearers (RB) that are used only for the transmission of RRC and NAS messages. More specifically, the following three SRBs are defined: - SRB0 is for RRC messages using the CCCH logical channel; - SRB1 is for RRC messages (which may include a piggybacked NAS message) 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 always configured by E-UTRAN after security activation.
When we refer to EPS bearers, it means we always refer to Data Radio bearers (DRBs) when in comes to E-UTRAN Radio/Air interface. EPS never directly refers to SRB. A PDN Connection gets mapped all the way till a DRB and is available only in user plane. Usually EPS bearer identity in EPS = DRB identnty in E-UTRAN, let it be default or dedicated bearers. The DRBs exists in the path UE <--> eNodeB <--> S-GW <--> P-GW. But the story is entirely different in case of SRBs. We can say, SRB exists in the path UE <--> eNodeB <--> MME <--> S-GW <--> P-G, but it is not true, SRBs exists only in the path UE <--> eNodeB. After eNodeB SRBs are not mapped to any other bearers in EPC. SRBs exists only in control plane of UE <--> ENodeB and these are used to exchange the information (through NAS or AS signalling messages) that helps in setup/ modification/ release of DRBs. So when we refer to EPS bearers they are all logical path and in terms of radio bearers they are refered as DRBs. SRB though it is also a logical path, it does not find its context in EPS bearers.
As explained in the forum there are 11 EPS Bearer + 3 Signal bearer. But if we see the RRC structure there is one more field "logicalChannelIdentity (8 integer values)" is present, this means that max 8 logical channels can be created. So how the 11 EPS bearer are mapped with the 8 logical channels ? That’s the good catch. I would like to make one thing sure. 11 EPS bearers are never mapped on 8 logical channel Ids. Each EPS bearer, i.e. DRB is mapped to one DTCH logical channel. So, following t able shows mapping.
DRBs
DTCH Channel (Logical Instance) ---------------------------------------------DRB-1 <--> DTCH-3 DRB-2 <--> DTCH-4 DRB-3 <--> DTCH-5 ... ... DRB-8 <--> DTCH-10 DRB-9 <--> ??? DRB-10 <--> ??? DRB-11 <--> ??? Following description is according to my understanding. Table 4.8.2.1.7-1 of spec 36.508 dictates how the values of DRB id should be used. As per thi s document filling of the DRB-ToAddMod information element (IE) is done as follo ws: Consider we are establishing DRB2 through RRCConnectionReconfiguration Message. Then,
DRB-ToAddMod(drbId) ::= SEQUENCE { eps-BearerIdentity drbID+4 drb-Identity drbID pdcp-Config PDCP-Config-DRB-AM (Considering case of AM) rlc-Config RLC-Config-DRB-AM (Considering case of AM) logicalChannelIdentity drbID+2 logicalChannelConfig LogicalChannelConfig- of DRB for AM } Table 4.5.3.3-1 of same spec which gives detailed procedure of generic radio bearer establishment, indicates the number of drbs to be added (N) is 0 <= N <= 7. Thus, Specification nowhere specifies what is the mapping after all logical Channel IDs are used up? We can expect this field to be updated in upcoming publication of 36.331 spec.
Why SRB0 is used for before SRB1 & SRB2? When UE camps on a cell, it figures out band of frequencies that it should use or subsequent UL and DL. Once this band is acquired, logical channels – CCCH, BCCH and PCCH are up (in RLC TM mode). After this radio connection has be established by UE in-order t o get required service. This is obviously by using some signalling messages. Since BCCH is specifically for broadcast and PCCH is only for paging messages the only way for transmitting signaling messages is on CCCH.Unless SRB1/2 are available. Thus by default all signalling messages go through CCCH--> DL-SCH --> PDSCH until SRB1/2 are made available. This logical path configured b y RRC during startup is termed as SRB0 though it does not maintain any context identified as SRB0.
SRB In The following combination are supported:
SRB1
SRB1+SRB2+Best Effort
SRB1+SRB2+Best Effort + GBR
SRB1+SRB2+Best Effort + VoIP
SRB1+SRB2+Best Effort + Best Effort
SRB1: RRC signaling with high priority SRB2: RRC signaling and NAS signaling (lower priority) Best Effort: also defined as the default EPS Bearer GBR: Radio Bearer with a guaranteed bit rate VoIP: Radio bearer to carry the VoIP GBR: Radio Bearer with a guaranteed bit rate VoIP: Radio bearer to carry the VoIP
LTE Radio Capabilities 4.2.1 Radio Bearers The UE must support the following combination of radio bearers for Voice over IMS profile (see Annex B in 3GPP TS 36.331 [52]): SRB1 + SRB2 + 4 x AM DRB + 1 x UM DRB
The network must support the following combination of radio bearers: SRB1 + SRB2 + 2 x AM DRB + 1 x UM DRB One AM Data Radio Bearer (DRB) is utilized for Evolved Packet System (EPS) bearer with Quality of Service Class Indicator (QCI) = 5 Another AM DRB for EPS bearer with QCI = 8/9. UM DRB is utilized for EPS bearer with QCI = 1. EPS bearer usage is described in Section 4.3.