GB2382273A

GB2382273A - Abnormal release recovery in a UTRAN of a UMTS network

Info

Publication number: GB2382273A
Application number: GB0127830A
Authority: GB
Inventors: Bjoern Ehrstedt
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2001-11-20
Filing date: 2001-11-20
Publication date: 2003-05-21
Anticipated expiration: 2021-11-20
Also published as: GB0127830D0; GB2382273B; WO2003045088A1; AU2002351799A1

Abstract

A method of recovering from the abnormal release of a transport bearer established between a SRNC 4 and a DRNC 5 of a UTRAN 2, where a plurality of UEs 7 are allocated to the transport bearer so that common channel signalling and user data may be transported between the SRNC 4 and the DRNC 5 via the transport bearer. The method comprises, at the SRNC 4, detecting loss of the transport bearer, in response to detection of the loss, initiating a RNSAP re-establishment procedure comprising sending a re-establishment request from the SRNC 4 to the DRNC 5, the request containing the identities of the UEs 7 allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs or the UEs shall be allocated to an existing transport bearer or existing transport bearers. A re-establishment response is sent from the DRNC 5 to the SRNC 4, and the SRNC 5 then sends a transport bearer establishment request to the DRNC 5.

Description

Abnormal Release Recovery in a UTRAN of a UMTS network Field of the Invention The present invention relates to the recovery from an abnormal release of transport bearers in a UTRAN of a UMTS network.

Background to the Invention The organisation known as 3GPP is currently in the process of standardising a new set of protocols for mobile telecommunications systems. The set of protocols is known collectively as the Universal Mobile Telecommunications System (UMTS) or, more commonly, 3GPP. The architecture of a UMTS network is based upon a UMTS core network and a UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN comprises a number of Radio Network Controllers (RNCs), each of which is coupled to a set of neighbouring Base Transceiver Stations (BTSs) or Node Bs. Each Node B is responsible for a given geographical area or cell (or a number of areas or cells), and the controlling RNC is responsible for routing user and signalling data between that Node B and the core network. The interface between an RNC and a Node B is referred to as the lub interface. A general outline of the UTRAN is given in Technical Specification TS 25.401 (UTRAN overall description) of the 3rd Generation Partnership Project, 3GPP.

User and signalling data is carried between an RNC and a mobile terminal (referred to in UTRAN as User Equipment (UE)) using Radio Bearers (RBs). Typically, a UE is allocated one or more RBs, each of which is capable of carrying a flow of user or signalling data. At a Radio Link Control (RLC) entity, RBs are mapped onto respective logical channels. At a Media Access Control (MAC) entity, a set of logical channels is mapped in turn onto a transport channel, of which there are two types: a"common" transport channel which is shared by different UEs and a"dedicated"transport channel

which is allocated to a single UE. One type of common channel is a Forward Access Channel (FACH). Another is the Random Access Channel (RACH). Transport channel data streams are carried between the RNC and the Node B via transport bearers, e. g. AAL2 connections. At the Node B, several transport channels (e. g. FACHs) are

mapped at the physical layer onto a Secondary Common Control Physical CHannel (S- CCPCH) for transmission over the air interface between a Node B and a UE.

When a UE registers with an RNC, via a Node B, that RNC acts at least initially as both the serving and controlling RNC for the UE. The RNC both controls the air interface radio resources and terminates the layer 3 intelligence (Radio Resource Control (RRC) protocol). L3 signalling between the UE and the core network is routed transparently via the serving RNC. Figure I illustrates the protocol model for the FACH transport channel when the serving and controlling RNCs are coincident and where Uu indicates the interface between the UTRAN and the UE, and Iub indicates the interface between the RNC and a Node B. It will be appreciated that the MAC (MAC-c) entity in the RNC transfers MAC-c Packet Data Units (PDUs) to the peer MAC-c entity at the UE, using the services of the FACH Frame Protocol (FACH FP) entity between the RNC and the Node B. The FACH FP entity adds header information to the MAC-c PDUs to form FACH FP PDUs which are transported to the NodeB over an lub transport bearer. An interworking function at the Node B interworks the FACH frame received by the FACH FP entity into the PHY entity. An analogous procedure is employed for RACH transport channels.

Consider now the situation which arises when a UE leaves the area covered by a RNC with which the terminal is registered, and enters the area covered by a second RNC. Under the UTRAN protocols, the RRC remains terminated at the first RNC whilst the terminal takes advantage of a cell and common transport channel of the second RNC. Thus, the first RNC remains as the serving RNC with a connection to the core network whilst the second RNC becomes the controlling RNC. The controlling RNC is in control of the NodeB where the UE is located and in particular of the logical resources (transport channels) at that Node B. In this scenario the controlling RNC is referred to as a"drift"RNC (the controlling RNC will also be acting as a serving RNC for UEs registered with that RNC).

The protocol model for the FACH transport channel when the serving and controlling RNCs are separate is illustrated in Figure 2. It will be noted that a new interface lur is exposed between the serving and the controlling RNCs. The RNSAP protocol is used

for control plane signalling on the lur interface. Data for incorporation in FACH transport channels is carried to the Node B via separate transport bearers extending between the SRNC and the DRNC and between the DRNC and the Node B. Again, an analogous procedure is employed for RACH transport channels.

When a UE first moves into the coverage area of the (new) DRNC, the UE must send an RRC Cell Update message to the SRNC to inform the SRNC of its new location. The RRC Cell Update message is forwarded by the DRNC to the SRNC using the RNSAP UL Signalling Transfer message. Upon receipt of a Cell Update, the SRNC must determine the configuration (scheduling priority, MAC-c/sh SDU length (s), and initial window size) of the FACH transport channel used by the DRNC for the cell to which the update relates. According to the current standard, the SRNC responds to receipt of a Cell Update by sending a RNSAP Common Transport Channel Resource Request message to the DRNC over the lur interface. The DRNC responds with a RNSAP Common Transport Channel Resource Response message containing the configuration information.

Summary of the Invention A fundamental concept of UMTS is that known as"always connected"-whilst the UE is switched on it will always have a signalling connection to the network. When a UE is connected to the network via a DRNC, a context is established between the DRNC and the SRNC. The context allocates the UE to a given transport bearer, e. g. AAL2 connection. The number of UEs which can be allocated to a transport bearer is not limited by the standard. It is possible for abnormal release of a transport bearer to occur due to physical layer problems (e. g. due to damage to a cable or to a hardware failure).

One possible recovery mechanism made available by the current standard involves the sending of an RNSAP Downlink Signalling Transfer (RRC Connection Release) message from the SRNC to the DRNC following detection of the loss of the transport bearer, for all affected UEs. The DRNC then sends RRC Connection Release message to the UEs. This is illustrated in Figure 3. The result is that the RRC signalling connection and any Radio Bearers allocated to the UEs (assigned to the lost transport

bearer) are released, along with the Iu connections to the core network and the UE contexts in the DRNC and SRNC. The UEs return to the IDLE state. This obviously incurs considerable signalling overheads both on the air interface and in the UTRAN which is proportional to the number of UEs multiplexed onto the lost transport bearer.

Moreover, it will then be necessary to re-establish the RRC signalling connection to the network as well as any lost radio bearers for the UEs, incurring yet more signalling overheads. In an alternative approach, the RRC Connection Release message may be initiated by the DRNC as this to may detect loss of the transport bearer. In this case, the SRNC must take local action to release the UE contexts and Iu connections. However, the same disadvantages apply.

An alternative recovery mechanism is illustrated in Figure 4. Following the loss of the transport bearer, the SRNC waits until it has received an RNSAP Uplink Signalling Transfer message in response to a cell update received from a UE allocated to the lost bearer (the cell update may arise from a cell reselection or may be a periodic cell update). The procedure to allocate the UE to an existing transport bearer or to establish a new bearer is then carried out. This process must be repeated for each UE affected by the bearer loss. As with mechanism described in the preceding paragraph, this second recovery mechanism will incur significant signalling overheads.

Under the current standard, there exists no mechanism for the efficient re-establishment of the user plane. In addition, those mechanisms which do exist may result in a delay in reestablishment, and the consequent disruption of the service provided to users.

It is an object of the present invention to overcome or at least mitigate the problems identified above. In particular, it is an object of the present invention to provide a mechanism for recovering from the abnormal release of a transport bearer which minimises associated signalling and the interruption of services to users. These and other objects are achieved by introducing a new RNSAP procedure facilitating reestablishment of the Iur user plane for all UEs affected by an abnormal transport bearer release.

According to a first aspect of the present invention there is provided a method of recovering from the abnormal release of a transport bearer established between a serving RNC, SRNC, and a drift RNC, DRNC, of a UTRAN, where a plurality of UEs are allocated to the transport bearer so that common channel signalling and user data may be transported between the SRNC and the DRNC via the transport bearer, the method comprising: at the SRNC, detecting loss of the transport bearer; in response to detection of the loss, initiating a RNSAP re-establishment procedure comprising sending a re-establishment request from the SRNC to the DRNC,

the request containing the identities of the UEs allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs or the UEs shall be allocated to an existing transport bearer or existing transport bearers; and returning a re-establishment response from the DRNC to the SRNC, and if a new transport bearer or bearers are to be established sending a transport bearer establishment request or requests to the DRNC.

The UEs affected by the loss of the transport bearer are identified to the DRNC in a single RNSAP messages. This message is sent immediately after the loss of the transport bearer is detected by the SRNC. Thus, the signalling associated with recovery from the loss is minimised, and recovery itself is not delayed (e. g. pending receipt of cell updates at the DRNC from UEs).

In certain embodiments of the invention, the transport bearer (s) is (are) an AAL2 connection. Other transport bearers may however be employed, e. g. ATM. The signalling and user data transported by the transport bearer (s) is associated with FACH/RACH common transport channels.

The re-establishment request may identify a plurality of new or existing transport bearers together with an indication of which UEs are to be allocated to which of these transport bearers. In the case that a plurality of transport bearers are lost, a single reestablishment request may relate to all of the lost transport bearers, the request allocated affected UEs to new or established transport bearers.

According to a second aspect of the present invention there is provided a radio network controller, RNC, for use in a UTRAN and having means for recovering from the abnormal release of a transport bearer established between the RNC acting as a serving RNC, SRNC, and a drift RNC, DRNC, or between the RNC acting as a DRNC and a SRNC, where a plurality of UEs are allocated to the transport bearer so that common channel signalling and user data may be transported between the SRNC and the DRNC via the transport bearer, the means comprising: means for detecting loss of the transport bearer when the RNC is acting as an SRNC, and in response to detection of the loss, for initiating a RNSAP re-establishment procedure comprising sending a re-establishment request to the DRNC, the request containing the identities of the UEs allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs or the UEs shall be allocated to an existing transport bearer or existing transport bearers, for receiving a re-establishment response from the DRNC, and for sending a transport bearer establishment request or requests to the DRNC if a new transport bearer or bearers are to be established; and means for receiving a re-establishment request from the SRNC when the RNC is acting as an DRNC and following detection of the loss of a transport bearer by the SRNC, the request containing the identities of the UEs allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs or the UEs shall be allocated to an existing transport bearer or existing transport bearers, and for receiving a transport bearer establishment request or requests from the SRNC if a new transport bearer or bearers are to be established.

Brief Description of the Drawings Figure I illustrates a protocol model for a FACH transport channel when serving and controlling RNCs of the UTRAN are coincident ; Figure 2 illustrates a protocol model for a FACH transport channel when serving and controlling RNCs of the UTRAN are separate; Figure 3 illustrates the signalling associated with a first abnormal release recovery mechanism using the current standard;

Figure 4 illustrates the signalling associated with a second abnormal release recovery mechanism using the current standard; Figure 5 illustrates schematically a part of a UMTS network and shows signalling associated with an efficient abnormal release recovery mechanism; and Figure 6 is a flow diagram illustrating the abnormal release recovery mechanism associated with Figure 5.

Detailed Description of a Preferred Embodiment Protocol models for the FACH/RACH transport channel of a UMTS network have been

described with reference to Figures 1 and 2 for the cases where the serving RNC and z : l controlling RNC are both coincident and separate. Figures 3 and 4 have been used to illustrate abnormal release recovery mechanisms available with the current standards.

Figure 5 illustrates a part of a UMTS network comprising a core network 1 and a UMTS terrestrial radio access network (UTRAN) 2. Shown in the UTRAN 2 are a pair of Radio Network Controllers (RNCs) 4,5, each of which has control of a set of Node Bs 6. Each Node B 6 provides radio coverage of a particular geographic area, with the cells of neighbouring Node Bs overlapping.

In the scenario illustrated in Figure 5, a UE 7 is shown communicating with one of the Node Bs of a second of the RNCs 5. The UE is assumed to have moved from the coverage area of the Node B of the first RNC 4 into the coverage area of a Node B of the second RNC 5. The effect of the UE 7 roaming in this way is to cause the UE to take advantage of common transport channels (RACH/FACH) of the second RNC 5, the drift RNC (DRNC). As described above, the RRC protocol remains terminated at the first RNC, the serving RNC (SRNC). The UE 7 will send an RRC Cell Update message to the SRNC 4 to inform the SRNC of its new location. This message is sent via the DRNC 5 across the Iub and lur interfaces (and is the first connection between the UE and the DRNC).

The Cell Update message contains the U-RNTI (SRNC Id + RNTI (Radio Network Temporary Identity) ) which identifies the UE within UTRAN. At reception of the Cell

Update message, the DRNC checks the U-RNTI. As the UE is not registered in the DRNC, the DRNC forwards the Cell Update message, using the RNSAP Uplink Signalling Transfer message, to the SRNC based on the SRNC Id in the U-RNTI.

When the SRNC 4 receives the first Cell Update message from the DRNC, the SRNC 4 updates its cell location register, and initiates the RNSAP Common Transport Channel Resources Initialisation procedure. This involves the sending of a RNSAP Common Transport Channel Resource Request to the DRNC. This message is a request to establish the user plane towards the DRNC 5 and for the DRNC to return to the SRNC 4 the FACH/RACH transport channel configuration for the cell in which the UE 7 is now located. As already stated above this configuration information includes the scheduling priority, MAC-c/sh SDU length (s), and initial window size. The DRNC 5 responds to receipt of the RNSAP request message by generating and returning to the SRNC 4 a RNSAP Common Transport Channel Resource Response.

It is the SRNC that assigns the UE to a transport bearer. In the RNSAP request the SRNC sends the transport bearer ID to the DRNC. In the response the DRNC sends a binding identity to the SRNC. This binding identity is returned by the SRNC to the DRNC in the q. aal2 establishment request so that the DRNC can associate the binding identity to the previously received transport bearer ID. If the SRNC requests usage of an already existing transport bearer then the association between the transport bearer ID and the transport bearer will already exist in the DRNC, so that the DRNC already knows on which transport bearer to receive data for the UE.

This process effectively allocates the UE 7 to a transport bearer extending between the DRNC and the SRNC. In this case, the transport bearer is an AAL2 connection. It will be appreciated from the preceding discussion that a number of other UEs may be allocated to the same AAL2 connection.

It is possible for an abnormal release of an AAL2 connection to occur. For example, this might result from a hardware failure or from a fault in a cable connecting the SRNC 4 and the DRNC 5. Such a release will of course initially interrupt the transmission of data, associated with F ACHIRACH channels, between the SRNC and the DRNC. The

recovery mechanism proposed here detects the release at the radio network layer of the SRNC 4 (which is notified by the transport network layer). Upon detection of the release, a new RNSAP procedure is initiated. This is referred to here as the Common Transport Channel Resource Reconfiguration procedure. The procedure results in a Common Transport Channel Resource Reconfiguration Request being sent from the SRNC 4 to the DRNC 5. This message has the following structure:

IE/Group Name Presence Range IE type Semantics Criticality Assigned and description Criticality referen ce Message Type M Transaction ID M D-RNTI M 1.. < maxno of UEs > Transport Bearer M Request a new Request Indicator transport bearer or to use an existing bearer for the user plane.

Transport Bearer M Indicates the lur ID transport bearer to be used for the user plane.

One of the information elements (IEs) contained in the message is a request for a new transport bearer (AAL2 connection) to be created for the UEs affected by the release of the bearer, or a request for an existing bearer to be assigned to the affected UEs. The Transport Bearer ID IE identifies the new or existing bearer. In addition, the D-RNTI IE lists the affected UEs. Upon receipt of this RNSAP Request by the DRNC 5, the DRNC returns a Common Transport Channel Resource Reconfiguration Response having the following structure:

IE/Group Name Presence Range IE type Semantics Criticality Assigned and description Criticality reference Message Type M Transaction ID M Transport Layer 0 Address Binding Identity 0

The Transport Layer Address IE and the Binding Identity IE are only included if the request indicated that a new transport bearer should be established.

There then follows an AAL2 establishment procedure. The q, aal2 (Q. 2630.1) protocol is used for establishing AAL2 connections. It involves a connection establishment request message sent from the SRNC and a response sent from the DRNC. The request includes the Binding Identity received in the RNSAP reconfiguration response message.

It will be understood that the DRNC 5 will also detect the abnormal release of the transport bearer at the radio network layer thereof. The DRNC starts a timer upon such detection and awaits reception of the Common Transport Channel Resource Reconfiguration Request from the SRNC 4. In the event that the Request is received before the timer expires, the above procedure is carried out. However, if the timer expires before the Request is received, the DRNC takes local actions to release the common transport channel resources for the UEs affected by the transport bearer release.

If for some reason the DRNC cannot accept the RNSAP reconfiguration request it responds with an RNSAP Reconfiguration Failure message. This message has the following structure:

IE/Group Name Presence Range IE type Semantics Criticahty Assigned and description Criticality reference Message Type M TransactionID M Cause M

Figure 6 is a flow diagram illustrating the mechanism described above.

In a modification to the above described procedure, the re-establishment request (i. e. Common Transport Channel Resource Reconfiguration Request) may have the following structure:

IE/Group Name Presence Range IE type Semantics Criticality Assigned and description Criticality referen ce Message Type M Transaction ID M TransportBearer I.. < maxno Info o/TrBearer Info > > D-RNTI M 1.. < maxno of UEs > > Transport M Request a new Bearer transport bearer or Request to use an existing Indicator bearer for the user plane.

> Transport M Indicates the lur Bearer ID transport bearer to be used for the user plane.

The response has the following form : IE/Group Name Presence Range IE type Semantics Criticality Assigned and description Criticality reference Message Type M Transaction ID M Transport Layer I.. < maxno Info o/TrBearer Info > > Transport M Indicates the Bearer ID lur transport bearer to be used for the user plane.

> Transport O Layer Address > Binding O Identity It is noted that the Transport Bearer ID in the response is required in order for the SRNC to be able to make the association between the correct transport bearer and the binding identity (which it sends back to the DRNC in the transport bearer establishment request).

This structure allows a plurality of new or existing transport bearers to be identified in the message, with a mapping between UEs and transport bearers. Such a message structure will prove useful when it is necessary or desirable to allocate different UEs to different transport bearers. Such a message structure is particularly appropriate when a plurality of transport bearers are lost at the same time (indeed this is the most likely scenario resulting from a failure at the physical layer).

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.

Claims

Claims I. A method of recovering from the abnormal release of a transport bearer established between a serving RNC, SRNC, and a drift RNC, DRNC, of a UTRAN, where a plurality of UEs are allocated to the transport bearer so that common channel signalling and user data may be transported between the SRNC and the DRNC via the transport bearer, the method comprising: at the SRNC, detecting loss of the transport bearer; in response to detection of the loss, initiating a RNSAP re-establishment

procedure comprising sending a re-establishment request from the SRNC to the DRNC, the request containing the identities of the UEs allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs and/or whether the UEs shall be allocated to an existing transport bearer or existing transport bearers; and returning a re-establishment response from the DRNC to the SRNC, and if a new transport bearer or bearers are to be established sending a transport bearer establishment request or requests to the DRNC.
2. A method according to claim 1, wherein the transport bearer is an AAL2 connection.
3. A method according to claim 1 or 2, wherein the common channel signalling and user data transported by the transport bearer is data associated with FACH/RACH common transport channels.
4. A method according to any one of the preceding claims, wherein the reestablishment request identifies a plurality of new or existing transport bearers and a mapping between these transport bearers and the UEs.
5. A radio network controller, RNC, for use in a UTRAN and having means for recovering from the abnormal release of a transport bearer established between the RNC acting as a serving RNC, SRNC, and a drift RNC, DRNC, or between the RNC acting as a DRNC and a SRNC, where a plurality of UEs are allocated to the transport bearer

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so that common channel signalling and user data may be transported between the SRNC and the DRNC via the transport bearer, the means comprising: means for detecting loss of the transport bearer when the RNC is acting as an SRNC, and in response to detection of the loss, for initiating a RNSAP re-establishment procedure comprising sending a re-establishment request to the DRNC, the request containing the identities of the UEs allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs or whether the UEs shall be allocated to an existing transport bearer or existing transport bearers, for receiving a re-establishment response from the DRNC, and for sending a transport bearer establishment request or requests to the DRNC if a new transport bearer or bearers are to be established; and means for receiving a re-establishment request from the SRNC when the RNC is acting as an DRNC and following detection of the loss of a transport bearer by the SRNC, the request containing the identities of the UEs allocated to the lost transport bearer and an indication of whether a new transport bearer or new transport bearers shall be established for the UEs or whether the UEs shall be allocated to an existing transport bearer or existing transport bearers, and for receiving a transport bearer establishment request or requests from the SRNC if a new transport bearer or bearers are to be established.