The RAB corresponds one-to-one with the service. For example, a voice call needs a rab to carry it, which represents the concept of the Iu port. RB is the radio bearer of RAB and is the concept of Uu. In short, RAB needs RB to carry
The three concepts of RAB, RB, and SRB are both interconnected and different.
First look at the connection between them (common):
They are all services provided by the lower layer protocol to the upper layer protocol. For example, RAB is the service provided by the access layer protocol stack of the user plane to the non-access layer, and RB and SRB are the services provided by the RLC layer to its users.
The key is the difference between them: RAB is a concept applied between the user plane UE and the core network. When a user initiates a service request, the core network requires the access network to provide a corresponding radio access bearer (RAB) according to the characteristics of the requested service, regardless of how the bearer is implemented. The benefits of doing this are The interface between the core network and the access network is simplified, and the independence of the two network elements is better, which facilitates the independent update of each other's technology. So how to realize RAB in WCDMA (TD-SCDMA) system
This leads to the concept of RB. RB is a radio bearer between the UE and the SRNC. It is a service provided by the RLC layer. According to the attributes of the business, the RLC layer uses its three different types of entities (AM / UM / TM), to provide high-level point-to-point data transmission with acknowledgment, data transmission without acknowledgment and transparent transmission, in order to ensure that the user's data reaches the RNC in a reasonable manner. At this point, RAB's "implementation" between the UE and the RNC is very clear, it is "implemented" through the RB. Further, how does RAB "implement" on the IU interface? In fact, it is mapped to an AAL2 bearer through the IUUP protocol. In other words, RAB = RB + AAL2. Now a specific example will make it clearer. Taking the voice service of AMR12.2k, a RAB between the UE and the MSC is mapped to 3 RBs between the UE and the SRNC. Within a TTI, this The three PDUs output by the three RBs are mapped into an IUUP frame, and then sent to the MSC on the AAL2 channel.
As for SRB, it is actually RB, which is also a service provided by RLC, but they are used to carry control plane control messages, and are called SRB to distinguish them from RB.
Simply put, rab is the cargo you want to transfer, srb is the railway built to transport the cargo, and rb is multiple train cars
(1) The RRC connection is to establish the signaling connection between the UE and the UTRAN (SRB1-SRB4), which can be through the CCH or DCH. If it is established on the DCH, the upstream and downstream occupy 1 code channel (SF = 16) [in HS When the service is established, two code channels are occupied; and the PS rate includes the code channel occupied by the signaling connection]. Similar to the Iub connection between RNC and NodeB, the IU connection between RNC and CN, but the Iub connection has always existed, there is no need to establish and release each UE and CN connection, and the Iu connection must be The second time to establish and release (here refers to connection-oriented signaling and data bearer, except for connectionless).
(2) RL is to establish a DCH connection between RNC and NodeB, as long as the data goes to DCH, this link must be configured. RL is a logical concept, in fact, it is a format set, data is transmitted between L2 and L1 (MAC-D-> DCH / HS-DSCH / DSCH / USCH FP-> DPCH / HS-DPSCH transmission channel and MAC-D FLOW (mac-dh-> mac-hs)), DCH FP / HS-DSCH FP should be added / modified during DCH, so it needs to be configured, while in CCH, the link of L2-> L1 has been established, so it is not necessary RL is configured, but the connection between RNC and NodeB exists, so RL is the format set. There is also something called Iub data transmission under RL,
The main difference between it and RL is that the Iub data bearer carries the data of the Iub interface, while RL is the data of the Uu port, and RL is located on the Iub bearer. (It can be understood as follows: the Iub bearer is the AAL2 bearer, and the FP of the common channel is established on the AAL2. Therefore, some bearers will be established when the cell is established. If you want to use the DCH channel, you must also establish the DCH AAL2 bearer.
(3) RB is the connection format set between the UE and UTRAN, which is the format problem of the UU ports L1 and L2, that is, the configuration problem of the physical channel, transmission channel, and logical channel. If there is no service, RB is not needed, so if you want to transmit signaling between CN / URTRN and UE, as long as there is RRC connection (in fact, there is also a radio bearer, or SRB), but as long as there is service, it must be configured RB, too, must configure Iub bearer (as long as there is DCH, you must configure DCH FP bearer, same as RL).
(4) RAB is the agreement of the connection between the UE and the CN, which is reflected in the business, mainly the configuration of Qos. In order to transmit in a wireless environment, you must use the radio access network, so RAB is divided into RB between UE and UTRAN and IU bearer between CN and UTRAN.
(5) Related issues can be understood by:
If there is no service to be established, such as location area registration and update, only RRC connection and Iu connection need to be established, and RL, Iub bearer, Iu bearer, RAB, RB need not be established. If you want to build services on CCH, such as PS8k services, you must establish RRC connection, Iu connection, and then establish RAB, RB, Iub bearer, Iu bearer, but do not need to establish RL. At this time, once the RB fails and the service has failed, the RNC will reply to the CN message RAB assignment response indicating failure, and the CN decides whether to release the service. If you want to build a CS service on the DCH, all connections and bearers must be established, and the RRC connection must be established on the DCH, so that: once the RRC connection fails, all connections and bearers are released, including RL; once the RL configuration fails, if If the original configuration exists, it will be restored to the original configuration. The RNC will not release RRC, RL, Iu, etc. by itself, replying to the CN RAB assignment response indication failure, and executing according to the new CN instructions. The business must fail. Generally speaking, If the CN finds that the service has failed, it will initiate the IU release process. Once the RB configuration fails, the CS service will not work. If the RB configuration failure is received before the RL RECFG COMMIT, and the RL is restored to the original RL, the RNC responds to the CN RAB assignment response indicating failure, and the CN determines the new process; After receiving the RB configuration after RL RECFG COMMIT, because the RNC cannot restore the original RL, all services have failed. The RNC responds to the CN with a RAB assignment response indicating failure, and releases all connections and bearers. If you want to build a PS service on the DCH, all connections and bearers must be established. If the RRC connection is established on the DCH, the RRC, RL, and RB configuration failure handling is the same as the CS. If the RRC connection is established on the CCH, in fact The treatment is still the same. Note: When the PS service is released, the CN sends RAB assignments. At this time, it is necessary to re-apply for the resources of the two code channels to transmit signaling. If there are no resources, it will be released abnormally and RL reconfiguration and RL will not be performed. deleted.
(6) In TD-SCDMA, a UE has only one RRC connection at most. When performing hard handover, there can be up to two RLs, but generally there is only one RL (including combined services), multiple RABs, and two Iu connections 1CS + 1PS), multiple RBs.
(7) SRB in a sense, SRB and RB are twin brothers, SRB is the signaling RB, when the RRC connection is established, the SRB is established, in order to transfer the signaling between the UE and UTRAN, and The RB is a user plane RB, and the RB is established only when a service is established, that is, when user plane data needs to be transferred. When the cell is established, SRB0 has been established and transmits CCCH signaling. SRB1-SRB4 is established when RRC is established and transmits DCCH signaling. Above SRB5 are all service RBs, which are actually RBs.
(8) Related news:
RRC connection related messages: including RRC connection request, RRC connection establishment (release), RRC connection establishment (release) completed, in these messages, there are mainly these contents:
RB pre-stored instructions
UE information
Measurement information
SRB information
Transmission channel information
Physical channel information
Wireless resource information
RL related messages: RL establishment, addition, reconfiguration, deletion, etc.
Transmission channel information (including MAC-D FLOW)
RL information
RB related messages: RB establishment, release, reconfiguration
UE information
CN information
UTRAN mobility information
RB information
Transmission channel information
Physical channel information
Wireless resources
RAB related messages: RAB assignment request, RAB assignment response, RAB release request
RAB information
Iu connection related messages: Iu release command, Iu release complete, Iu release request
RAB information__
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