UTRAN RNC (Tutorial)

7/30/2019 UTRAN RNC (Tutorial)

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UTRAN RNC TypesNovember 16, 2007 by arvindpadmanabhan

The Radio Network Controller (RNC) is a key element within UTRAN for it controls all theradio resources of UTRAN. However, each RNC controls only resources under its control. There

are multiple RNCs in UTRAN and as such, control of radio resources is done with a distributedarchitecture. In this article, we will focus on the different types of RNCs that are defined in thestandards. As a preliminary let us start with the UTRAN architecture and the interfaces relatingto an RNC (Figure 1).

Iub Interface: connects to a Node-B. An RNC controls one or more Node-Bs.

Iur Interface: connects to other RNCs. This interface is not necessarily a point-to-pointlink.

Iu Interface: interfaces to the Core Network (CN). This is usually broken down into Iu-CS and Iu-PS for the respective domains. There is also the Iu-BC component thatconnects to the Broadcast Domain.

Iur-g Interface: connects to GERAN BSS which is possible from Release 5 onwardswhen GERAN can operate in Iu mode.

Iupc Interface: connects to Stand-Alone SMLC (Serving Mobile Location Center) toenable location-based services.

Figure 1: UTRAN Architecture

The specifications frequently mention the following logical separation:

CRNC: Controlling RNC

SRNC: Serving RNC

DRNC: Drift RNC

The CRNC is responsible for the controlling the resources of a Node-B. It is responsible for loadand congestion control. If new radio links (RLs) are to be established CRNC does the job.
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On the other hand, SRNC and DRNC are concepts that are tied to a UE-UTRAN connection onthe Uu interface. In other words, an SRNC serves a particular UE and manages the connectionstowards that UE. Likewise, DRNC fulfils a similar role to the SRNC except that it is involvedonly in the case of a soft handover. Thus the UE initially starts a connection with an RNC thatbecomes its SRNC. If the UE moves towards a cell edge, SRNC may decide to put the UE in softhandover state. If the new RLs added to UEs Active Set are under the control of a differentRNC, that RNC becomes the DRNC.

The important thing here is that CRNC is logically tied to Node-Bs, not connections. On thecontrary, SRNC and DRNC are tied to connections to the UE. This implies that CRNC managescommon and shared resources while SRNC and DRNC manage dedicated resources. This doesnot imply that SRNC will be involved only when UE RRC is in CELL_DCH state. Even inCELL_FACH state, the UE being allocated Signalling Radio Bearers (SRBs), dedicated logicalchannels (DCCH and even DTCH) would have been established between UE and SRNC. DRNCcannot be involved in this state because the principle of soft handover applies only to dedicatedphysical channels.

Once this is understood it becomes easy to make sense of many UTRAN procedures and

protocol termination points within UTRAN. As an example, let us look at the case of FACH(Figure 2).


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Figure 2: FACH Protocol Architecture

Figure 2(a) is a case in which the CRNC and SRNC are physically on the same RNC. So all thatFACH FP (Framing Protocol) does is to transfer Mac Pdus from the RNC to Node-B. In theNode-B, an interworking function translates the FACH frames to PHY frames on the Uu. Figure2(b) is a case in which CRNC and SRNC are separate and connected by Iur. A FACH FP on Iurcarries the Mac-d Pdus. In the CRNC, this is interworked with Mac-c/sh/m. Subsequently,FACH FP on the Iub carries it to the Node-B. In general, Mac-c/sh/m will reside on the CRNCsince they relate to the common resources of the cell. Mac-d (and Mac-es in the case of E-DCH)resides on the SRNC.
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To understand the difference between SRNC and DRNC, we take the example of a SRNSRelocation procedure (Figure 3).

Figure 3: SRNC Relocation
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In Figure 3(a), the UE is in SHO in which 2 RLs are under the SRNC and one under the DRNC.As the UE continues to move away from its original cells, all RLs in its Active Set are now underthe DRNC (Figure 3(b)). Since the SRNC no longer manages any of its own radio resources forthe UE, at some point it decides to transfer the complete UE context to the DRNC. At this point,the original DRNC becomes the SRNC. The new SRNC establishes connection on the Iuinterface to the CN. It is always the SRNC that manages the RANAP signalling towards the CN.The RRC layer in UTRAN resides in the SRNC which takes care of mapping Radio AccessBearer parameters to air interface transport channel parameters. The SRNS Relocation procedureis invoked within the SRNC RRC. Ultimately, it is the SRNC that has an RRC connection withthe UE.

While the UE is in Inter-RNC SHO, Iur is invoked to transfer data between the SRNC andDRNC. The DRNC routes data transparently between the Iub and Iur interfaces. The DRNC mayoptionally (if configured by the SRNC) combine or split data when relaying data between Iuband Iur interfaces. This has the advantage of reducing load on the Iur. It also distributes theprocessing load between the SRNC and the DRNC. In fact, these may be considerations onwhich the SRNC decides if combining/splitting is to be performed at the DRNC. Naturally,combining is done on the UL and splitting is done on the DL. Two types of data combining ispossible: maximum ratio combining or selection combining based on quality informationassociated with each TBS (Transport Block Set).

One important exception to this rule is E-DCH data handling at the DRNC. DRNC does not docombining. Combining is done in the Node-Bs under the DRNC. This makes sense for E-DCHfor which PHY layer terminates at Node-B. The E-DCH FP carries only Mac-es PDUs across theIub. This means that the SRNC can receive duplicates of Mac-es PDUs. Thus one of thefunctions of the Mac-es layer in the SRNC is to perform reordering for in-sequence delivery.This is not done at the DRNC which transparently relays the E-DCH FP from Iub to Iur (Figure4). This is not the case with DCH for which PHY is distributed between Node-B and RNC.

Figure 4: E-DCH Protocol Architecture

In conclusion, the terms CRNC, SRNC and DRNC are only logical definitions based onfunctionality. Physically, an RNC may be a CRNC+SRNC or CRNC+DRNC. CRNC managescommon transport resources, code allocation, load and congestion control. SRNC manages the


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connection to the UE and maintains the UE context within UTRAN. (For this purpose, S-RNTIallocated to the UE identifies the UE uniquely within the SRNC. This identity is managed by theSRNC and will be reallocated during SRNC Relocation.) Thus, outer loop power control,handover and RANAP signalling are managed from the SRNC. DRNC manages resources underits control but relies on SRNC for most decisions and commands.

Figure 2(b) is an example of what you are looking for. In this case, CRNC is responsible forcontrolling all common resources on the cell such as FACH. SRNC contains the dedicatedcontext. In this example, SRNC carries the dedicated logical channels DTCH and DCCH. CRNCis unaware of these logical channels since they are transparently passed via FACH-FP. Finally, itis Node-B that maps these logical channels to the FACH.

In one network implementation, we could have one CRNC for a one or more Node-Bs and feweror less powerful SRNCs. Network designed this way would be more robust (no single point offailure) than a network with just SRNC entities. It also makes sense for load sharing.

Having said that, HSPA R7 and LTE networks are evolving in such a way that RNCfunctionality is going to become part of Node-B.

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UTRAN RNC (Tutorial)