In a cellular system, it is common for mobiles to move from cell to cell while an active data connection is in progress. In GPRS, this happens with a cell reselection during which there is a short break in communication. This is called service outage and is defined as [TR 44.901],
the time between the last received uplink RLC block from the mobile station in the old cell and the mobile station’s first uplink RLC block received in the new cell.
Service outage dampens user experience. It can be of the order of a few seconds, certainly a bad thing for real-time or streaming services. GPRS has been designed in such a way that many parameters that control communication with a particular mobile are optional. If these optional parameters are absent during the setup phase of a TBF (Temporary Block Flow), equivalent parameters from SI/PSI are used. This means that an MS that does cell reselection must first acquire a defined minimum set of SI/PSI in the target cell before resuming the TBF in the new cell. This contributes to the long service outage.
An equivalent problem in UMTS does not exist or has been mitigated by sensible design. In UMTS, the UE does handover (hard or soft) when RRC is in CELL_DCH. This is the common case. Occasionally, when RRC is in CELL_FACH with an active connection on PS domain, the UE does cell reselection. However, CELL_FACH allows the mobile to acquire SIBs of neighbour cells before cell reselection is triggered. The problem in UTRAN occurs only if the chosen cell for reselection is of a different frequency. Service outage within UTRAN is less but is still a problem when UTRAN-GERAN cell reselection is triggered.
Network Assisted Cell Change (NACC) was introduced in Release 4 to mitigate this problem. Network assists the MS so that it has necessary SI/PSI of the target cell in advance of the cell reselection. Service outage is reduced from the order of a few seconds to 300 – 700 ms [TR 44.901] . NACC has two main procedures:
- One part that can assist an MS in packet transfer mode with neighbour cell system information required for initial packet access after a cell change.
- One part in which the mobile station notifies the network when the cell change criteria is fulfilled and delays the cell re-selection a short time to let the network respond with neighbour cell system information (CCN mode procedures).
The need for two variants can be understood by the NC modes which controls GPRS cell reselection. There are three NC modes: NC0, NC1 and NC2. In the case of NC0 and NC1 MS does reselection autonomously. In these two modes, the MS enters the CCN (Cell Change Notification) mode which is new for NACC. The MS indicates to the network that it has chosen a cell for reselection but delays the procedure by entering CCN mode. This gives the network time to send SI/PSI of the selected neighbour cell. In NC1 and NC2 MS sends measurement reports to the network, which can then initiate a cell reselection process. Before such an initiation, the network sends SI/PSI to the MS.
NACC adds the possibility to let the MS operate in NC0 and reduces the loading on the signalling in terms of measurement reporting. Even in NC0, the use of NACC enables the network to direct the MS to select a cell that could be different from the one chosen by the MS.
Another important detail is that the MS delays the cell reselection until complete RLC SDU has been transferred. This is only best-effort. If the connection to the current cell is really bad to make further transfer impossible, the MS has to quickly move to the target cell.
The following RLC/MAC messages have been added to enable NACC:
- Packet Cell Change Notification (PCCN): MS indicates to network that it wants to move to another cell and enters CCN mode
- Packet Cell Change Continue (PCCC): network asks MS to continue with cell reselection after sending necessary SI/PSI of neighbour cell
- Packet Neighbour Cell Data (PNCD): contains SI/PSI or neighbour cell
- Packet Serving Cell Data (PSCD): contains SI/PSI of current/serving cell: this is useful if only a minimum set has been sent before cell reselection and the rest is sent in this message after cell reselection
- Packet SI Status: contains SI of current cell: purpose is similar to PSCD
- Packet PSI Status: contains PSI of current cell: purpose is similar to PSCD
A typical scenario is described in Figure 1. The MS in packet transfer mode determines that a new cell (N1) is to be selected. It delays the reselection and sends PCCN to the serving cell indicating its selection. The serving cell contains the system information of the selected cell N1. Therefore it sends PNCD messages to the MS. Once the required PNCD messages have been sent and once complete RLC SDUs have been transferred, the BSC sends PCCC message. MS switches to the new cell and immediately performs uplink access to restart the TBF. Subsequently, the MS requests the new cell to send the missing system information messages on the PACCH. These are sent to the MS. Some messages may be encapsulated into PSCD messages.
Figure 1: Typical Message Sequence for NACC
Figure 1 points to a key fact: “BSC Serving Cell” need not be the same as “BSC Cell N1″. However, in Release 4 the source and target cells must be part of the same BSC. The reason is simple. Unlike UTRAN, in which Iur interfaces one RNC to another, BSCs in BSS are not connected to each other. Thus, SI/PSI of one cell cannot be sent on another cell that’s under a different BSC. Future releases of the specifications extends NACC in many ways:
- NACC for Geran Iu mode
- NACC across different BSC
These enhancements are not really mature in the specifications at this point. In other words, it is not clear if CN should be involved in transferring SI/PSI or would it be sufficient to use the Iur-g interface. Should SI/PSI be stored in the SRNC or in the DRNC? Should O&M be involved in storing SI/PSI statically in each CRNC? However, UTRAN RRC [TS 25.331, Rel5] already has updates for NACC for Cell Change Order From UTRAN procedure. Once these are in place and the parameters tuned by study, we can expect smoother data transfer when cell change occurs between UTRAN and GERAN.
What was the need to introduce a new message PSCN, When network could have sent the PSI/SI on PACCH to MS in the new cell
Good observation. There was no need to introduce PSCN but I guess the designers wanted to clearly separate signalling that’s specific to NACC. PSI is sent on PBCCH and PACCH. If an MS requires updated PSI it would be sent to it on PACCH while GPRS Transfer Mode. For the purpose of debugging, it is difficult to ascertain if the PSI transmission was triggered because of an update or because of the requirements of NACC. I expected sensible implementations would always use PSCD if the transmission has been triggered for NACC.