PowerPoint Presentation

Slide 06 Notes:

C-RNTI (Cell - Radio Network Temporary Identity). Unique within a cell.U-RNTI (UTRAN - Radio Network Temporary Identity). Unique within a UTRAN.C-RNTI and U-RNTI are temporary identities assigned as a part of RRC connection.

Slide 08 Notes:

In Idle state: Mobile is not transmitting anything in uplink. Mobile is on standby intermittently waking up to listen to paging (on PCH) and going back to sleep to save battery. Mobility aspect in Idle state is handled by NAS layer procedures. If the mobile wakes up to listen to paging and finds out its in a different LA(RA) it will perform relevant LAU(RAU). Addressing uses IMSI, TMSI and P-TMSI for paging in downlink (on PCH) and network access on RACH in uplink. Addressing is needed because we use common channels shared by multiple mobiles. In special cases IMEI can be used for addressing, e.g., when one wants to make an emergency call and there is no SIM in the mobile equipment.

Slide 09 Notes:

In CELL_DCH state:

Mobile is actively transmitting and receiving on uplink and downlink.

This state is used in circuit switched call as well as packet switch call that are transferring a large amount of data.

The only mobility procedure in CELL_DCH is handover procedure as the network knows exactly which cell has dedicated resources allocated to the mobile. There is no need for LAU or RAU.

There is no need for addressing as mobile has a dedicated channel; mobile is not sharing it with any other user.

Purpose of rest of the three connected mode states (CELL_FACH, CELL_PCH and URA_PCH) is only to support packet calls.

Whenever an exchange of data is needed between network and the mobile an RRC connection needs to be setup. Mobile is the one requesting the RRC connection (on RACH); network responds with a setup command (on FACH); the mobile goes into connected state. Network is the one that tears down RRC connection.

Slide 10 Notes:

Packet switched calls have different characteristics than circuit switched calls.

We may think of all these connected mode states as varying degree of data transfers:CELL_DCH: In packet mode transferring a large amount of data.CELL_FACH: In packet mode transferring a small amount of data.CELL_PCH: In packet mode after a small period of inactivity.URA_PCH: In packet mode after a large period of inactivity.

At the time of RRC connection setup the network will tell the mobile which state to be in (either CELL_DCH or CELL_FACH).

It is one of the MAC layer procedures to send "traffic volume measurement reports" to the network. Network resource allocation manager has to take these measurements and make a decision on if the connection needs CELL_DCH (dedicated resources) or it can be moved to one of the other states. These decisions are made via proprietary algorithms of vendors.

Slide 10 Notes (Continued):

FACH is a common channel shared by multiple mobiles so moving to CELL_FACH when there is little data to transfer frees up DCH for more data hungry voice/data calls. When in CELL_FACH mode downlink is on FACH and uplink is on RACH. The addressing is done via U-RNTI or C-RNTI. In downlink the addressing is used by the mobile to know if it is the recipient. In uplink the addressing is used by the network to distinguish between mobiles.

Mobility in CELL_FACH state is handled by cell update. In GSM the cell reselection takes place by the mobile without any network side procedure taking place (as long as the cell remains in same LA and RA). This is also true for WCDMA. However, in WCDMA, if the mobile is in CELL_FACH state it will be having intermittent exchange of data for which network needs to move the logical channel (resources) to the new cell. This is done via cell update after cell reselection.

In CELL_FACH state the mobile is not at standby. Its awake all the time consuming battery; although there are periods of non-activity.

CELL_PCH and URA_PCH allows mobile to go into standby thus conserving battery.Slide 10 Notes (Continued):What is the difference between U-RNTI and C-RNTI?When a mobile sends a signaling message on RACH it uses U-RNTI (instead of IMSI/TMSI). C-RNTI is used at the MAC layer. It is used when there are dedicated logical channels mapped onto a common transport channels.U-RNTI: Signaling layer addressingC-RNTI: MAC layer addressing

From book (Introduction to 3G mobile communications: Juha Korhonen):The C-RNTI identifies a UE within a cell, so it can only be used in paging messages when the UE's location is known (i.e., it must be in CELL_PCH state). This also implies that a new C-RNTI will be allocated to a UE every time it moves to a new cell and conducts a cell-update procedure. The U-RNTI is a UTRAN-wide identity that is used by the UTRAN for paging if it knows that the UE is in the URA-PCH sub-state (i.e., its location is only known at the URA level). Note that U-RNTI is not a URA-specific identity, but it identifies a UE within the whole UTRAN. Therefore, it can be optionally used for CN-originated paging.

IMSI/TMSI are NAS layer concepts; radio access network is not aware of IMSI/TMSI. But in WCDMA we have RRC layer involved in mobility where RRC layer has to address a particular mobile and this message is not coming from NAS layer. U-RNTI and C-RNTI are concerned with UTRAN level mobility.

Slide 11 Notes:

In CELL_PCH state:

Mobile monitors the PCH channel. There is no uplink. There are standby periods. Its much like being in idle state except that in the background it still has the RRC connection (RRC connection is not released). It still has U-RNTI assigned.

It is to support periods of no activity in a packet call.

Cell reselection and cell update will occur as in the case of CELL_FACH state. When a cell reselection occurs the mobile will transition to CELL_FACH state on its own and initiate a cell update procedure. The network will respond with cell update confirm. After the procedure the network will decide whether to send the mobile back to CELL_PCH, CELL_FACH, or CELL_DCH depending on data uplink / downlink requirements.

Addressing is done by U-RNTI unlike in CELL_FACH state where both U-RNTI and C-RNTI are used.

There is no way to go from CELL_PCH to CELL_DCH because in order to get a dedicated channel there needs to be some signaling exchange. In order to do the signaling mobile has to be in CELL_FACH state.

Slide 12 Notes:

URA_PCH state is very similar to CELL_PCH state. The difference is that instead of notifying the network every time mobile changes cell, mobile will only notify when changing URA.

In CELL_PCH state if the user is moving in a car changing cells quickly the mobile will constantly be waking up from standby consuming FACH / RACH resources. This will result in battery drain as well as consumption of common resources (FACH/RACH).

URAs can be nested up to 8 levels deep. This can be useful for example when the network wants to assign different URAs to two different mobiles at the same place depending upon if it is a pedestrian (small URA of say 5 cells) or a moving car (larger URA of say 50 cells).

All these states are to optimize the following when the mobile is in packet switched call:Battery life of the mobileDedicated resources of the networkSlide 12 Notes (Continued):

The network decision to put mobile in a particular state depends upon several things some of which are:

Uplink traffic volume measurements of the mobile by the MAC layerDownlink traffic from the packet core networkSystem capacity - if all OVSF codes are being used up network may decide to put a mobile in CELL_PCH state.

Transition b/w idle and connected modes.Set up radio bearers and as a consequence configure all of logical, transport and physical channels.Handover (idle), Cell update (connected)routing NAS layer messages.

MIB is sent every 80 milli-second. It has information about scheduling of other SIBs.

(Idle Mode)System Information Block type 12- Handover neighbor list (Connected Mode)Tells the UE how the secondary common control channels & RACH are configured.Slide 19 (Notes):

RRC Connection Setup contains information on whether to use common or dedicated channel etc.

RRC Connection Release is different from call clearing which happens in upper layers. When a user disconnects a call RRC Connection Release happens as a consequence of call clearing.

Slide 20 (Notes):

Radio Bearer Management is complex as it entails setting up every layer (radio bearers, logical channels, transport channels and the physical channels). Radio Bearer Setup message contains within its subsections how each of the layers needs to be configured:What mode RLC should operate in?What size blocks should MAC use?What orthogonal codes should be used by physical layer?

Radio Bearer Reconfiguration takes place, e.g., when mobile transitions from CELL_DCH to CELL_FACH.

Radio Bearer Release: Consider a case where we have simultaneous voice and data calls. The mobile is in CELL_DCH state. Now consider that the voice call ends but data call is preserved. In this case the radio bearers related to voice call will be released while the ones related to data call will be preserved.

Slide 22 (Notes):

In idle mode the mobile wakes up and checks the paging indicator channel to see if there is a page. If there is a paging indication mobile checks the paging channel, decodes the paging message and search for an address (IMSI, TMSI, P-TMSI) that is owned by that mobile; then decide if it has received a page.

Paging Type 2 is used in case mobile already has an RRC connection setup.

If mobile is in CELL_DCH no addressing is needed.If mobile is in CELL_PCH U-RNTI is used for addressing.

If a mobile is in dedicated call and there is an incoming page it will be passed onto UTRAN. It is the UTRAN that will realize that it is already connected to that mobile and proceed accordingly.

- NAS layer at the network side (CS and PS core) needs to talk with NAS layer in the mobile. RRC provides a pipe for these NAS layer messages.

Authentication is done at mobility management layer.Ciphering & integrity protection is done at RRC layer.

Security Mode Command tells mobile how to setup ciphering and integrity protection.

There are wide variety of measurements pertaining to:Handover (Neighbor cell measurements)Traffic Volume MeasurementsEtc.

Measurement Control and Measurement Report are type of messages that may contain a variety of information.

e.g. quality targetsThis is outer loop Power Control. SIR targets of both the network and mobile side are adjusted.

Slide 28 (Notes):

RLC supports typical layer 2 functions.

Radio bearers received by RLC, from upper layers, can be RABs (Radio Access Bearers if coming from user plane) or SRBs (Signaling Radio Bearers if coming from RRC).

RLC does not care if it is receiving SRBs or RABs from upper layers; for RLC they are just radio bearers that needs to be mapped to logical channels in three possible ways:Transparent Mode - TMUnacknowledged Mode - UMAcknowledged Mode - AM

For TM and UM the receive and transmit sides are separate.For AM the receive and transmit sides are combined into a single entity.

Slide 29 (Notes):

Transparent Mode: As the name suggests it is as if RLC is not even there. In TM the RLC simply passes the data as is with minimal manipulation like segmentation / de-segmentation.

Two places where TM is used are:Voice ChannelsPaging and broadcast channels

RLC TM mode for voice channel: The VOCODER generates frames every 20 ms. We want to send these frames over to receive side. There is no reason to do any processing for re-transmission as if a frame is lost it need not be re-transmitted. VOCODER will have another frame by then. Therefore VOCODER does not add any extra information or header for voice out of VOCODER.

Slide 30 (Notes):

UM (a bit more complex than TM):Information in a large signaling message can be segmented.Portions in two different signaling messages can be concatenated.

UM is typically used for signaling messages. Small header is added and ciphering done. For TM ciphering is done at MAC layer instead.

Slide 31 (Notes):

AM: Tx and Rx sides are combined in one entity as acknowledgement needs to be transmitted after receiving a message.

Re-transmission Buffer stores the messages sent in case a NACK is received and the message needs re-transmission.

Mux is used to execute the decision on whether a new message is to be transmitted or a re-transmission is needed due to NACK.

Slide 33 (Notes):

The main goal of MAC layer is to take the family of logical channels and map them to transport channels.

Some of these mappings are straight forward like BCCH logical channel is mapped to BCH transport channel.

Things get complicated with dedicated control and dedicated voice channels. This is because they can either be mapped to common transport channels (FACH & RACH) or dedicated transport channel (DCH).

Slide 34 (Notes):

Normally the BCCH logical channel gets mapped to BCH transport channel. However, there is one Broadcast System Information message that needs to be sent on FACH. This is the message sent to mobile when mobile is in CELL_FACH state; it tells the mobile that the System Information has changed and the mobile should listen to the BCH to update it and them come back to CELL_FACH.

UL / DL CCCH logical channels are always mapped to RACH & FACH transport channels. This is when, e.g., during Cell Update procedure signaling is exchanged in CELL_FACH state.

UL / DL DCCH logical channels can be mapped to RACH & FACH transport channels or dedicated transport channels. When do we use either dedicated or common transport channel in CELL_FACH state depends upon whether the network knows where the mobile is. If the network knows location of the mobile it use dedicated transport channel. Let us assume mobile is in CELL_FACH and it just moved to a new cell. In order to perform cell update procedure it does not have a dedicated control channel yet (because resources are not allocated on the new cell yet). The mobile in this case will use a common transport channel.

UL / DL DCCH are SIGNALING Radio Bearers.Slide 34 (Notes) Continued:

The distinction b/w using dedicated or common logical channels in the CELL_FACH state is strictly dependent on whether RRC connection b/w the mobile and a particular cell is setup or not.

In CELL_FACH state:Common logical channels (UL CCCH & DL CCCH) are used for cell update procedure; as it has to take place on common control channel due to lack of dedicated resources allocated on the new cell.When the cell has dedicated resources allocated for the mobile it will use dedicated logical channels.

In CELL_FACH UL DTCH and DL DTCH are mapped to RACH and FACH respectively.

Slide 35 (Notes):

Slide 35 depicts how dedicated logical channels are mapped in case of a voice call. We see four logical DCCH being mapped to single transport DCH; case of a typical voice call. Four SRBs are allocated, two of them are given to RRC and two of them to the NAS layer; depending upon the type of message being sent it will use one of these DCCH. As signaling is intermittent we can get away with single DCH carrying the data from all four DCH logical channels.

In case of a voice call, in user plane, three DTCH logical channels are mapped to three DCH transport channels. The VOCODER divides voice in class A, B & C bits. Each of these set of bits are put on a separate logical channel. MAC layer too maps them on separate transport channels. The reason is that different class bits are handled (coded, CRCd) differently. They are all combined back on a single channel at physical layer.

Slide 36 (Notes):

In slide 34 it is shown that different types of logical channels (DL CCCH, DL DCCH, DL DTCH) gets mapped on FACH. How does the MAC layer on the receive side know which logical channel up the stack should the information be sent? The MAC layer on the transmit side adds a header to the beginning of the data coming down a logical channel. TCTF tells the receive side MAC layer which logical channel (CCCH, DCCH, DTCH) did the data come from.

In slide 35 four DL DCCH logical channels are mapped to one DCH transport channel. How does the MAC at the receive side know which logical channel number of a given type (SRB 1, 2, 3 or 4) to send the data on towards RLC layer. This is taken care of by C/T bits.

Slide 37 (Notes):

We use U-RNTI in signaling messages but there are occasions when we have to put an identifier at the MAC layer. C-RNTI is used in the MAC header for this purpose. This comes up when we have a dedicated control logical channel mapped to a common transport channel (and common physical channel). This only comes up on the CELL_FACH state. In this case the upper layer do not include any addressing as it is sending message in a dedicated logical channel (it thinks it has a dedicated connection to the mobile). When this dedicated logical channel gets mapped to a common transport channel we have multiple mobiles listening to the common channel. Now we suddenly have a need for addressing which is taken care of by the MAC layer.

TODO: Is C-RNTI used in downlink while U-RNTI in uplink. Clarify?

CCTrCH: Coded Composite Transport ChannelThis is a bit complicated concept. It is half way b/w transport and physical channel.Coming from Signaling planeComing from AMR VOCODERHeader added by MAC in signaling messageSignaling is being segmentedThis is in continuation from slide 35.Slide 39 (Notes):From slide 38, MAC layer sends four different channels over to physical layer. At the same it also hints the physical layer as to how these should be multiplexed to go over the air.

TTI: Transmission Time IndicatorPhysical layer radio frame duration (TTI) is 10 ms.A particular transport channel is assigned a TTI of duration: 10, 20, 40 or 80 ms.Voice frame has a TTI of 20 ms; meaning the MAC will get data from RLC every 20 ms.Signaling is typically 40 ms; meaning the MAC will get data from RLC every 40 ms.So there are transport layer frames that only generate data after, say, 40 ms. But the transport layer has to handover something to physical layer every 10 ms.

In slide 38 we have transport blocks pertaining to a voice call. There are four of them that are being generated simultaneously (there may be more in the case of concurrent voice and data calls). So MAC takes multiple transport channels and sends something down to physical layer every 10 ms. The combination of formats being sent can change every 10 ms.

TFCS relates to the multiplexing function that MAC performs.

TFCI is used at the receive side. It tells the format that was used to send the bits. TFCI allows the receive side to look up in the table (example table shown in slide 40) and figure out how to de-multiplex the received bits.

on the same transport channel.MAC may send more than one transport block at a time on a given transport channel; In this case it will be termed as Transport Block Set.How many blocks, bits per block are to be sent on a particular transport channel.

This table is built in such a way that it takes care of all the possible combinations: is there or is there not data to send on the various transport channels.Slide 41 (Notes):

Suppose that 3 transport channels are being multiplexed by MAC layer onto the physical channel. At instance 0 ms, TFCI is as depicted in slide 41. This initial selection restricts the choice of TFCI in subsequent transmissions until the longest TTI frame is cleared. In this case TrCh 1 has a TTI of 80 ms, so at 10, 20, 30, 40, 50, 60, and 70 ms intervals the MAC layer is only allowed to choose one of the TFCIs which contains the transport format (100, 2). So the number of TFCIs to select from reduces.

Slide 42 (Notes):

AICH: Acquisition Indicator Channel

For RACH, mobile sends a preamble at low power and waits for an ACK from the network. If no ACK is received it sends the preamble with a little higher power. This goes on until mobile hears an ACK or mobile has exhausted its max number of preamble cycles.

If the network receives a preamble it sends an ACK on AICH channel. The physical layer after receiving an ACK tells the MAC layer to send the actual message.

The network can possibly send a NACK (e.g., when there are no resources) in which case the mobile will wait a randomized back off interval before re-attempting the preamble.