Communication Systems for the Mobile Information Society phần 6 ppt

Universal Mobile Telecommunications System (UMTS) 167 Figure 3.26 Discontinuous transmission (DTX) on a dedicated channel reduces the interference for other subscribers While in the Cell-DCH state, the mobile continuously measures the reception quality of neighboring cells and reports the results to the network. Based on these values the RNC can then decide to start a handover procedure when required. While the GSM radio network uses a static reporting interval, a much more flexible approach was selected for UMTS. In the first step the RNC instructs the terminal, similar to the GSM approach, to send periodic measurement reports. The measurement interval itself is now flexible and can be set by the network between 0.25 and 64 seconds. Furthermore, the network can also instruct the terminal to send measurement reports only if certain conditions are met. This way, it is possible to send measurement reports for neighboring cells to the network only if the measurement values reach a certain threshold. This removes some signaling overhead, which can be used to send more user data over the bearer instead. Another advantage of this method for the RNC is the fact that it has to process fewer messages for each connection compared to periodical measurement reports. Depending on the requirements of the data to be sent, different properties can be assigned to a dedicated channel. One property for example is the length of the spreading code, which affects the maximum bandwidth available for user data. Therefore, depending on the length of the spreading code, data rates in the range of a few kilobits per second up to several hundred kilobits per second are possible for a single connection (see Section 3.3.2). While the Idle and Cell-DCH RRC states are mandatory for the network all other states like the Cell-FACH, Cell-PCH, and URA-PCH states, which are further described below, are optional. The Cell-FACH state is mainly used when only a small amount of data needs to be transferred to or from a subscriber. In this mode, the subscriber does not get a dedicated channel but uses the FACH to receive data. As described in Section 3.4.5, the FACH’s primary task is to carry RRC connection setup messages for subscribers that have requested access to the network via the RACH. If the Cell-FACH state is implemented in the network the channel can also be used to send user data or signaling messages from the MSC and SGSN to the terminals. The FACH is a ‘common channel’ as it is not exclusively assigned to a single user. Therefore, the MAC header of each FACH data frame has to contain a destination ID, which consists of the S-RNTI (serving-radio network temporary ID) which was assigned to a terminal during connection establishment, and the ID of the S-RNC. The terminals therefore have to inspect the header of each FACH data frame and only forward those frames to higher layers of the protocol stack that contain the terminal’s ID. The approach of Cell-FACH RRC state is thus similar to Ethernet (802.11) and GSM/GPRS for 168 Communication Systems for the Mobile Information Society packet-switched data transmission. If data is received in the downlink direction, no resources have to be assigned and the data can be sent to the subscriber more or less quickly depending on the current traffic load of the FACH. As several subscribers share the same channel, the network cannot ensure a certain data rate and constant delay times for any terminal in Cell-FACH state. Furthermore, it should be noted that the FACH usually uses a high spreading factor which limits the total available bandwidth for subscribers on this channel. See Figure 3.27. Compared to the Cell-DCH state in which the mobility of the subscriber is controlled by the network no such control has been foreseen for the Cell-FACH state. In the Cell-FACH state the terminal is responsible for changing cells which is therefore called cell update instead of handover. As the network does not control the cell update it is also not possible to ensure an uninterrupted data transfer during the procedure. Due to these reasons the Cell-FACH RRC state is not suited for real-time or streaming applications. For bursty and low-speed data transmission such as WAP browsing, the Cell-FACH state is an alternative to the establishment of a dedicated bearer. As the displays of mobile devices are usually quite small the amount of data that has to be transferred for a WAP page is usually also quite small. Therefore, a dedicated transmission channel is not strictly necessary. In operational networks, it can be observed that a dedicated channel is established even for WAP browsing but is released again very quickly after the data transfer. More about the use of the different RRC states in operational networks can be found in Section 3.9.2. The Cell-FACH state is also suitable for the transmission of mobility management and packet mobility management signaling messages between the terminal and the MSC or SGSN. As the terminal already indicates the reason for initiating the connection to the network in the RRC connection setup message, the network can flexibly decide if a dedicated channel is to be used for the requested connection or not. If the Cell-FACH channel is implemented in the network and used for signaling exchanges, no dedicated channel has to be assigned for example for a location update procedure. If the terminal is in Cell-FACH state, uplink data frames are sent via the RACH whose primary task is to forward RRC connection setup request messages. As has been shown in Section 3.4.5, access to the RACH is a time intensive procedure which causes some delay before the actual data frame can be sent. This is another reason why the Cell-FACH state is not suited for real-time applications. There are two possibilities for a terminal to change to the Cell-FACH state. As already discussed, the network can decide during the RRC connection setup phase to use the FACH Figure 3.27 Data of different subscribers is time multiplexed on the FACH Universal Mobile Telecommunications System (UMTS) 169 for MM/PMM signaling or user data traffic. Furthermore, it is possible to enter the Cell- FACH state from the Cell-DCH state. The RNC can decide to modify the radio bearer this way if, for example, no data has been sent or received by the terminal for some time. The spreading code which is thus released can then immediately be used for another subscriber. Furthermore, a fallback to the Cell-FACH state reduces the power consumption of the terminal. As long as only small amounts of data are exchanged, the Cell-FACH state is usually maintained. If the data volume increases again, the network can immediately establish a new dedicated bearer and instruct the terminal to enter Cell-DCH state to be able to transfer data more quickly. The optional Cell-PCH (cell-paging channel) RRC state and the URA-PCH (UTRAN registration area – paging channel) RRC state can be used to reduce the power consumption of the terminal even further during extended times of inactivity. Similar to the Idle state, no resources are assigned to the terminal. If data arrives for a subscriber from the network, the terminal needs to be paged first. The terminal then answers the paging request with an RRC connection request message which allows the RNC to establish a new connection. Depending on the decision of the RNC, the terminal then either changes to the Cell-FACH or Cell-DCH state. As the name Cell-PCH already indicates, the subscriber is only paged in a single cell if new data from the core network arrives. This means that the mobile station has to send a cell update message to the RNC whenever it selects a new cell. In the URA-PCH state, the mobile only informs the RNC whenever it enters a new UTRAN registration area (URA). Consequently the paging message needs to be sent to all cells of the URA in case of incoming data (see Section 3.7.3). The difference between the Cell-PCH and URA-PCH state compared to the Idle state is that the network and terminal still maintain a logical connection. As the RRC states are managed by the RNC, the SGSN as a core network component has no information on the RRC state of the terminal. Therefore, the SGSN simply forwards all incoming data packets from the GGSN to the RNC regardless of the current state of the mobile. If the mobile is currently in either the Cell-PCH or the URA-PCH state the RNC needs to buffer the packets, page the terminal, wait for an answer, and then establish a physical connection to the terminal again. If the terminal is in the Cell-DCH or Cell-FACH state the RNC can directly forward any incoming packets. The distinction between a logical and physical connection has been made in order to separate the connection between the terminal and core network (SGSN and MSC) on the one hand and the connection between the terminal and the radio network (RNC) on the other hand. The advantage of this concept is the decoupling of the MSC and SGSN from the properties and functionality of the radio network. Thus, it is possible to evolve the radio network and core network independently from each other. In an operational network the difference between the Idle, Cell-PCH, and URA-PCH is very small from a user point of view. Both the power consumption of the terminal as well as the resumption of a data transfer are only slightly different. Therefore, it is questionable if the Cell-PCH and URA-PCH states will ever be implemented. At the time this book was published, only the Idle state, the Cell-DCH state, and the Cell-FACH state were used in operational networks. As described in Chapter 2, the GSM/GPRS SGSN is aware of the state of a terminal as the Idle, Ready, and Standby states as well as the Ready timer is administered by the SGSN. Thus, a core network component performs tasks of the radio network such as cell 170 Communication Systems for the Mobile Information Society Table 3.4 RNC and SGSN states RNC state SGSN state Idle Not connected Not connected Cell–DCH Connected, data is sent via the DCH or HS-DSCH Connected Cell-FACH Connected, incoming data is sent immediately via the FACH (common channel) Connected Cell-PCH Connected, but subscriber has to be paged and needs to reply before data can be forwarded. Once the answer to the paging has been received the subscriber is put in either the Cell-FACH or Cell-DCH state Connected URA-PCH Same as Cell-PCH. Furthermore, the network only needs to be informed of a cell change if the terminal is moved into a cell which is part of a different UTRAN registration area Connected updates. On the one hand this has the advantage that the SGSN is aware of the cell in which a subscriber is currently located, which can be used for supplementary location-dependent functionalities. The advantage of implementing the UMTS state management in the RNC is the distribution of this task on several RNCs and thus a reduction of the signaling load of the SGSN as well as a clear separation between core network and radio access network responsibilities. See Table 3.4. 3.6 Core Network Mobility Management From the point of view of the MSC and the SGSN, the terminal can be in one of the following mobility management (MM) or packet mobility management (PMM) states. The MSC knows the following MM states: • MM detached: the terminal is switched off and the current location of the subscriber is unknown. Incoming calls for the subscriber cannot be forwarded to the subscriber and are either rejected or forwarded to another destination if the call forward unreachable (CFU) supplementary service is activated. • MM idle: the terminal is powered on and has successfully attached to the MSC (see Attach procedure). The subscriber can at any time start an outgoing call. For incoming calls, the terminal is paged in its current location area. • MM connected: the terminal and MSC have an active signaling and communication connection. Furthermore, the connection is used for a voice or a video call. From the point of view of the RNC, the subscriber is in the Cell-DCH RRC state as this is the only bearer that supports circuit-switched connections. Universal Mobile Telecommunications System (UMTS) 171 The SGSN implements the following PMM states: • PMM detached: the terminal is switched off and the location of the subscriber is unknown to the SGSN. Furthermore, the terminal cannot have an active PDP context, i.e. no IP address is currently assigned to the subscriber. • PMM connected: the terminal and the SGSN have an active signaling and communication connection. The PMM connected state is only maintained while the subscriber has an active PDP context, which effectively means that the GGSN has assigned an IP address for the connection. In this state, the SGSN simply forwards all incoming data packets to the serving-RNC (S-RNC). In contrast to GSM/GPRS the UMTS SGSN is only aware of the S-RNC for the subscriber and not of the current cell. This is due to the desired separation of radio network and core network functionality and also to the soft handover mechanism (see Section 3.7). The SGSN is also not aware of the current RRC state of the terminal. Depending on the QoS profile, the network load, the current data transfer activity, and the required bandwidth, the terminal can be either in Cell-DCH, Cell-FACH, Cell-PCH or URA-PCH state. • PMM idle: in this state, the terminal is attached to the network but no logical signaling connection is established with the SGSN. This can be the case for example if no PDP context is active for the subscriber. Furthermore, the RNC has the possibility to modify the RRC state of a connection at any time. This means that the RNC, for example, can decide after a period of inactivity of the connection to set the terminal into the RRC Idle state. As the RNC no longer controls the mobility of the subscriber it requests the SGSN to set the connection into PMM Idle state as well. Therefore, even though the subscriber no longer has a logical connection to either the RNC or the SGSN, the PDP context remains active and the subscriber can keep the assigned IP address. For the SGSN, this means that if new data arrives for the subscriber from the GGSN, a new signaling and user data connection has to be established before the data can be forwarded to the terminal. 3.7 Radio Network Mobility Management Depending on the MM state of the core network, the radio network can be in a number of different RRC states. How the mobility management is handled in the radio network depends on the respective state. Table 3.5 gives an overview of the MM and PMM states in the core network and the corresponding RRC states in the radio network. 3.7.1 Mobility Management in the Cell-DCH State For services like voice or video communication it is very important that no or only a very short interruption of the data stream occurs during a cell change. For these services, only the Cell-DCH state can be used. In this state the network constantly controls the quality of the connection and is able to redirect the connection to other cells if the subscriber is moving. This procedure is called handover or handoff. In UMTS a number of different handover variants have been defined. Hard handover as shown in Figure 3.28: this kind of handover is very similar to the GSM handover. By receiving measurement results from the terminal of the active connection and measurement results of the signal strength of the broadcast channel of the neighboring cells, 172 Communication Systems for the Mobile Information Society Table 3.5 Core network and radio network states MM states and possible RRC states MM idle MM connected PMM idle PMM connected Idle X X Cell-DCH X X Cell-FACH X Cell-PCH X URA-PCH X RNC Iu(cs), Iu(ps) User moves to the coverage area of a new cell. The network performs a hard handover Iub Figure 3.28 UMTS hard handover the RNC is able to recognize if a neighboring cell is more suitable for the connection. In order to redirect the call into the new cell a number of preparatory measures have to be performed in the network before the handover is executed. This includes for example the reservation of resources on the Iub interface and if necessary also on the Iur interface. The procedure is similar to the resource reservation of a new connection. Once the new connection is in place the terminal receives a command over the still established connection to change into the new cell. The handover command contains, among other parameters, the frequency of the new cell and the new channelization and scrambling code to be used. The terminal then suspends the current connection and attempts to establish a connection in the new cell. The interruption of the data stream during this operation is usually quite short and takes about 100 milliseconds on average, as the network is already prepared for the new connection. Once the terminal is connected to the new cell the user data traffic can resume immediately. This kind of handover is called UMTS hard handover as the connection is shortly interrupted during the process. Soft Handover: with this kind of handover, user data traffic is not interrupted at any time during the procedure. Based on signal quality measurements of the current and neighboring cells, the RNC can decide to set the terminal into soft handover state. All data from and to the terminal will then be sent and received not only over a single cell but also over two or even more cells simultaneously. All cells that are part of the communication are put into the so-called active set of the connection. If a radio connection of a cell in the active set deteriorates, it is removed from the connection. Thus it is ensured that despite the cell Universal Mobile Telecommunications System (UMTS) 173 change, the terminal never losses contact to the network. The active set can contain up to six cells at the same time although in operational networks no more than two or three cells are used at a time. Figure 3.29 shows a soft handover situation with three cells. The soft handover procedure has a number of advantages over the hard handover described before. As no interruption of the user data traffic occurs during the handover procedure the overall connection quality increases. As the soft handover procedure can be initiated while the signal quality of the current cell is still acceptable the possibility of a sudden loss of the connection is reduced. Furthermore, the transmission power and thus the energy consumption of the terminal can be reduced in some situations as shown in Figure 3.30. In this scenario, the subscriber first roams into an area in which it has a good coverage by cell 1. As the subscriber moves, there are times when buildings or other obstacles are in the way of the optimal transmission path to cell 1. As a consequence, the terminal needs to increase its transmission power. RNC Iu(cs), Iu(ps) Iub Data is received by the RNC but is discarded as the same frame is received from another Node-B with a better signal quality rating User moves through the coverage areas of several Node-Bs. Network activates the software handover mode Figure 3.29 Connections to a terminal during a soft handover procedure with three cells Figure 3.30 Soft handover reduces energy consumption of the mobile due to lower transmission power 174 Communication Systems for the Mobile Information Society If the terminal is in soft handover state, however, cell 2 still receives a good signal from the terminal and can thus compensate for the deterioration of the transmission path to cell 1. As a consequence, the terminal is not instructed to increase the transmission power. This does not mean, however, that the connection to cell 1 is released immediately, as the network speculates on an improvement of the signal conditions. As the radio path to cell 1 is not released, the RNC receives the subscriber’s data frames from both cell 1 and cell 2 and can decide, based on the signal quality information included in both frames, that the frame received from cell 2 is to be forwarded into the core network. This decision is made for each frame, i.e. the RNC has to make a decision for every connection in handover state every 10, 20, 40, or 80 milliseconds depending on the size of the radio frame. In the downlink direction, the terminal receives identical frames from cell 1 and cell 2. As the cells use different channelization and scrambling codes the terminal is able to separate the two data streams on the physical layer. This means that the terminal has to decode the data stream twice, which of course slightly increases the power consumption as more processing power is required. See Figure 3.31. From the network point of view, the soft handover procedure has the following advantages: as the terminal uses less transmission power compared to a single cell scenario in order to be able to reach at least one of the cells in the active set, the interference is reduced in the uplink direction. This increases the capacity of the overall system, which in turn increases the number of subscribers that can be handled by a cell. On the other hand, there are some disadvantages for the network as well: in the downlink direction, data has to be duplicated so it can be sent over two or even more cells. In the reverse direction, the RNC receives a copy of each frame from all cells of the active set. Thus, the capacity that has to be reserved for the subscriber on the different interfaces of the radio network is much higher than for a subscriber that only communicates with a single cell. Therefore, good network planning tries to ensure that there are no areas of the network in which more than three cells need to be used for the soft handover state. A soft handover gets even more complicated if cells need to be involved that are not controlled by the S-RNC. In this case, a soft handover is only possible if the S-RNC is connected to the RNC that controls the cell in question. RNCs in that role are called the drift RNCs (D-RNC). Figure 3.32 shows a scenario that includes an S-RNC and a D-RNC. If a foreign cell needs to be included in the active set, the S-RNC has to establish a link to Figure 3.31 Use of scrambling codes while a terminal is in soft handover state Universal Mobile Telecommunications System (UMTS) 175 Figure 3.32 Soft handover with S-RNC and D-RNC the D-RNC via the Iur interface. The D-RNC then reserves the necessary resources to its cell on the Iub interface and acknowledges the request. The S-RNC then in turn informs the terminal to include the new cell in its active set via an ‘update active set’ message. From this point onwards, all data arriving at the S-RNC from the core network will be forwarded via the Iub interface to the cells that are directly connected to the S-RNC and also via the Iur interface to all D-RNCs which control a cell of the active set. These in turn forward the data packets to the cells under their control. In the reverse direction, the S-RNC is the point of concentration for all uplink packets as the D-RNCs forward all incoming data packets for the connection to the S-RNC. It is then the task of the S-RNC to decide which of the packets to use based on the signal quality indications embedded in each frame. A variation of the soft handover is the so-called softer handover, which is used when two or more cells of the same Node-B are part of the active set. For the network, the softer handover has the advantage that no additional resources are necessary on the Iub interface as the Node-B already decides which of the frames received from the terminal via the different cells to forward to the RNC. In the downlink direction, the point of distribution for the data frames is also the Node-B, i.e. it duplicates the frames it receives from the RNC for all cells which are part of the active set of a connection. One of the most important parameters of the GSM air interface is the timing advance. Terminals that are further away from the base station have to start sending their frames earlier compared to terminals that are closer to the base station due to the time it takes the signal to reach the base station. This is called timing advance control. In UMTS controlling the timing advance is not possible. This is due to the fact that while a terminal is in soft handover state, all Node-Bs of the active set receive the same data stream from the terminal. The distance of the terminal to each Node-B is different and thus each Node-B receives the data stream at a slightly different time. For the terminal, it is not possible to control this by starting to send data earlier, as it only sends one data stream in the uplink direction for all Node-Bs. Fortunately, it is not necessary to control the timing advance in UMTS because all active subscribers are transmitting simultaneously. As no time slots are used, no collisions 176 Communication Systems for the Mobile Information Society can occur between the different subscribers. In order to ensure the orthogonal nature of the channelization codes of the different subscribers it would be necessary, however, to receive the data streams of all terminals synchronously. As this is not possible, an additional scrambling code is used for each subscriber that is multiplied by the data that has already been treated with the channelization code. This decouples the different subscribers and thus a time difference in the arrival of the different signals can be tolerated. The time difference of the multiple copies of a user’s signal is very small compared to the length of a frame. While the transmission time of a frame is 10, 20, 40, or 80 milliseconds, the delay experienced on the air interface of several Node-Bs is less then 0.1 milliseconds even if the distances vary by 30 kilometers. Thus, the timing difference of the frames on the Iub interface is negligible. If a subscriber continues to move away from the cell in which the radio bearer was initially established, there will be a point at which not a single Node-B of the S-RNC is part of the transmission chain. Figure 3.33 shows such a scenario. As this state is a waste of radio network resources, the S-RNC can request a routing change from the MSC and the SGSN on the Iu(cs)/Iu(ps) interface. This procedure is called a serving radio network subsystem (SRNS) relocation request. If the core network components agree to perform the change, the D-RNC becomes the new serving RNC and the resources on the Iur Interface can be released. An SRNS relocation is also necessary if a handover needs to be performed due to degrading radio conditions and no Iur connection is available between two RNCs. In this case it is not the optimization of radio network resources that triggers the procedure but the need to maintain the radio bearer. Therefore not only is an SRNS relocation necessary but also a hard handover into the new cell, as a soft handover is not possible due to the missing Iur interface. When the first GSM networks were built at the beginning of the 1990s, many earlier generation networks already covered most parts of the country. The number of users was Figure 3.33 SRNS relocation procedure [...]... shown in Figure 3.39 for clarity After successful authentication and activation of the encrypted radio channel, the terminal then proceeds to inform the MSC of the exact reason of the connection request The call control (CC) setup message contains among other things the telephone number (MSISDN) 184 Communication Systems for the Mobile Information Society Figure 3.39 Messaging for a mobile originated... of the GSM cell are available for the handover decision Figure 3.34 3G to 2G handover 178 Communication Systems for the Mobile Information Society The advantage of this procedure, of course, is simple implementation in the network and in the terminals However, there are a number of problems linked to a blind intersystem handover: • The network has no information if the GSM cell can be received by the. .. direction, which the receiver cannot decode correctly The receiver therefore sends an error indication to the Node-B, which then in turn retransmits the frame In detail, the process works as follows Before the transmission of a frame, the Node-B informs the terminal of the pending transmission on the HS-SCCH Each HS-SCCH frame thus contains the following information: • ID of the terminal for which a frame... update will fail As the new RNC cannot inform the 182 Communication Systems for the Mobile Information Society Figure 3.37 Cell change in PMM connected state to a cell that cannot communicate with the S-RNC S-RNC of the new location of the subscriber it will reset the connection and the terminal automatically defaults to Idle state In order to resume data transmission, the terminal then performs a location... block bits per TTI 7298 (16QAM) 363 0 (QPSK only) 363 0 (QPSK only) 200 Communication Systems for the Mobile Information Society A Category 11 terminal on the other hand, which is limited to QPSK modulation, can only receive data in every second frame Thus for this category of terminals the maximum speed is: 363 0 bits every 4 milliseconds = 1/0 004 × 363 0 = 900 kbit/s In the future there may also be devices... 202 Communication Systems for the Mobile Information Society than the current cell The RNC can then decide to redirect the data stream to a different cell As the concept is different from the UMTS soft handover, the standards refer to this operation as cell change procedure Compared to the cell update procedure of (E)GPRS, the cell change procedure of HSDPA is controlled by the network and not by the. .. milliseconds 198 Communication Systems for the Mobile Information Society is controlled by the network However, the mobile can indicate to the network during bearer establishment which of the two methods it supports 3.10.3 Node-B Scheduling The HS-DSCH channels have been designed in a way so that different channels can be assigned to different users at the same time The network then decides for each frame... connection to the RNC that transparently forwards the message to the MSC DTAP messages are exchanged between the RNC and the MSC via the connection-oriented SCCP protocol (see Section 1.4.1) Therefore, the RNC has to establish a new SCCP connection before the message can be forwarded Once the MSC has received the CM service request message, it verifies the identity of the subscriber via the attached... slots The information in the control frame is arranged in a way that the terminal has all information necessary to receive the frame once it has received the first two of the three slots Thus, the network does not wait till the complete control frame is sent but already starts sending the user data on the HS-PDSCH once the terminal has received the first two slots of the control frame This means that the. .. dedicated bearer for the duration of a web page download, for example, and to put 188 Communication Systems for the Mobile Information Society the terminal into Cell-FACH state as quickly as possible This is done in order to support as many users as possible in a single cell Table 3 .6 shows how two network operators have set their air interface parameters and timers and how they perform their radio resource . terminal. Therefore, the SGSN simply forwards all incoming data packets from the GGSN to the RNC regardless of the current state of the mobile. If the mobile is currently in either the Cell-PCH or the. other things the telephone number (MSISDN) 184 Communication Systems for the Mobile Information Society Figure 3.39 Messaging for a mobile originated voice call (MOC) of the destination. If the. However, as the bearer for a packet- switched connection uses other QoS attributes, the parameters inside the messages will be different. 1 86 Communication Systems for the Mobile Information Society Figure