Medium Medium Access Access Control Control (MAC) (MAC)

Handover ControlHandover Control

5.2.5 Medium Medium Access Access Control Control (MAC) (MAC)

5.2.5 Medium Medium Access Access Control Control (MAC)(MAC)

The main functions of the MAC protocol are to perform multiplexing of The main functions of the MAC protocol are to perform multiplexing of one or several upper layer PDUs onto transport blocks, to perform UL and one or several upper layer PDUs onto transport blocks, to perform UL and DL resource allocation through dynamic scheduling and to handle error DL resource allocation through dynamic scheduling and to handle error correction through HARQ operation.

correction through HARQ operation.

The scheduling function manages DL-SCH and UL-SCH radio resources The scheduling function manages DL-SCH and UL-SCH radio resources between HARQ entities (i.e. between UEs). The scheduler determines between HARQ entities (i.e. between UEs). The scheduler determines which UE (or group of UEs) to be serviced each 1ms TTI. The exact which UE (or group of UEs) to be serviced each 1ms TTI. The exact scheduling algorithm used is implementation dependent but should scheduling algorithm used is implementation dependent but should preferably take into account:

preferably take into account:

•

• Availability of radio resourcesAvailability of radio resources •

• Data stream priority levels (for each UE)Data stream priority levels (for each UE) •

• The current channel conditions (for each The current channel conditions (for each UE)UE) •

• The amount of data awaiting transmission (for each UE)The amount of data awaiting transmission (for each UE) •

• How long time since UE X was last How long time since UE X was last servedserved •

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Copyright © © Apis Apis Technical Technical Training Training AB AB 2007. 2007. All All rights rights reserved. reserved. 5-75-7 The scheduler selects a proper Modulation and Coding Scheme (MCS) and The scheduler selects a proper Modulation and Coding Scheme (MCS) and Redundancy Version (RV) for each scheduled MAC PDU based on each Redundancy Version (RV) for each scheduled MAC PDU based on each scheduled UEs current channel condition, the retransmission status and, scheduled UEs current channel condition, the retransmission status and, possibly, on the UE capabilities. The RV is used as input to the HARQ possibly, on the UE capabilities. The RV is used as input to the HARQ layer 1 rate matching function discussed in chapter 4.

layer 1 rate matching function discussed in chapter 4.

One HARQ Entity within MAC handles the HARQ functionality for one One HARQ Entity within MAC handles the HARQ functionality for one user. The HARQ protocol selected for E-UTRA is of the ‘Stop-and-Wait’ user. The HARQ protocol selected for E-UTRA is of the ‘Stop-and-Wait’ type (SAW). This means that it is not allowed to transmit a PDU with type (SAW). This means that it is not allowed to transmit a PDU with sequence number ‘N’ until the PDU with sequence number ‘N-1’ is sequence number ‘N’ until the PDU with sequence number ‘N-1’ is positively

positively acknowledgedacknowledged..

Remember that the TTI used in E-UTRA is only 1ms. Each time the UE Remember that the TTI used in E-UTRA is only 1ms. Each time the UE receives data in a 1ms TTI it must, according to the SAW protocol, send receives data in a 1ms TTI it must, according to the SAW protocol, send back either an ACK (‘everything OK, please send next PDU’) or a NACK back either an ACK (‘everything OK, please send next PDU’) or a NACK (‘please retransmit the PDU’). The creation and sending of an (‘please retransmit the PDU’). The creation and sending of an ACK/NACK takes a certain amount of time. So does the processing of the ACK/NACK takes a certain amount of time. So does the processing of the ACK/NACK in the NodeB. And so does the scheduling of a new ACK/NACK in the NodeB. And so does the scheduling of a new re/transmission to this UE.

re/transmission to this UE.

All this is simply impossible to execute before the start of the next 1ms All this is simply impossible to execute before the start of the next 1ms TTI. The consequence is then that it becomes impossible to schedule TTI. The consequence is then that it becomes impossible to schedule transmissions in consecutive 1ms TTIs to the same UE, resulting in waste transmissions in consecutive 1ms TTIs to the same UE, resulting in waste of resources- or at least waste of time. (The same logic holds, of course, of resources- or at least waste of time. (The same logic holds, of course, for uplink transmissions).

for uplink transmissions).

The solution is to allow each HARQ Entity to work with several processes The solution is to allow each HARQ Entity to work with several processes simultaneously. When one

simultaneously. When one HARQ process HARQ process is awaiting ACK/NACK for ais awaiting ACK/NACK for a transmitted MAC PDU, the scheduler can order transmission of the next transmitted MAC PDU, the scheduler can order transmission of the next MAC PDU from the next HARQ process, that then stops and awaits MAC PDU from the next HARQ process, that then stops and awaits ACK/NACK, and so on. It is expected that 8 HARQ processes will be ACK/NACK, and so on. It is expected that 8 HARQ processes will be sufficient to allow continuous transmission to/from a given UE. Thus, the sufficient to allow continuous transmission to/from a given UE. Thus, the shortest

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5.3 UE UE States States (RRC (RRC and and NAS)NAS)

LTE DETACHED LTE DETACHED LTE ACTIVE

LTE ACTIVE RRC CONNECTEDRRC CONNECTED L

LTTEEIIDDLLEE RRRRCCIIDDLLEE

LTE DETACHED LTE DETACHED LTE ACTIVE

LTE ACTIVE RRC CONNECTEDRRC CONNECTED L

LTTEEIIDDLLEE RRRRCCIIDDLLEE

LTE DETACHED LTE DETACHED LTE ACTIVE

LTE ACTIVE RRC CONNECTEDRRC CONNECTED L

LTTEEIIDDLLEE RRRRCCIIDDLLEE

Figure 5-3: UE states and state transitions Figure 5-3: UE states and state transitions

From a radio resource point of view

From a radio resource point of view there are two operational states for thethere are two operational states for the UE:

UE: RRC Idle State RRC Idle State andand RRC Connected State RRC Connected State. In the RRC Idle state the. In the RRC Idle state the UE is unknown in E-UTRAN and will remain so until it requests the UE is unknown in E-UTRAN and will remain so until it requests the establishment of an RRC Connection. Such a request can be triggered by establishment of an RRC Connection. Such a request can be triggered by higher protocol layers in the UE (

higher protocol layers in the UE (i.e. mobile originating service request) ori.e. mobile originating service request) or by the paging procedure (initiated from the EPC).

by the paging procedure (initiated from the EPC).

In RRC Idle state the UE moves around in the network and change from In RRC Idle state the UE moves around in the network and change from one cell to another through the process of cell reselection. It continuously one cell to another through the process of cell reselection. It continuously monitors the broadcasted system information and the paging channel. No monitors the broadcasted system information and the paging channel. No data/signalling transmission or reception, except paging and system data/signalling transmission or reception, except paging and system information, is possible in the RRC Idle state.

information, is possible in the RRC Idle state.

The RRC Connected state allows data or signalling to be sent or received. The RRC Connected state allows data or signalling to be sent or received. The UE enters the Connected state through the establishment of an RRC The UE enters the Connected state through the establishment of an RRC connection. The UE is always allocated a cell specific identifier, the Cell connection. The UE is always allocated a cell specific identifier, the Cell Radio Network Temporary Identity (C-RNTI) when in Connected state. Radio Network Temporary Identity (C-RNTI) when in Connected state. The C-RNTI is, among other things, used for addressing the UE on the The C-RNTI is, among other things, used for addressing the UE on the downlink resource assignment channel, the PDCCH. UE mobility is downlink resource assignment channel, the PDCCH. UE mobility is network controlled through handovers. The UE may have a DRX cycle network controlled through handovers. The UE may have a DRX cycle configured in order to allow ‘sleep periods’ in-between monitoring the configured in order to allow ‘sleep periods’ in-between monitoring the PDCCH. RRC Connection Release brings the UE back to RRC Idle state. PDCCH. RRC Connection Release brings the UE back to RRC Idle state. The NAS states (EPC related states) are aligned with the RRC states. A The NAS states (EPC related states) are aligned with the RRC states. A UE in RRC Idle state is, from the MMEs point of view, in the NAS state UE in RRC Idle state is, from the MMEs point of view, in the NAS state LTE Idle

LTE Idle. In this state the UE is registered in the MME and has an IP-. In this state the UE is registered in the MME and has an IP- address allocated. Whenever the UE detects a change of Tracking Area it address allocated. Whenever the UE detects a change of Tracking Area it performs a Tracking Area update towards the MME.

performs a Tracking Area update towards the MME.

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Paging or a request from higher layers to transmit uplink data or signalling Paging or a request from higher layers to transmit uplink data or signalling will cause a transition from LTE Idle to the

will cause a transition from LTE Idle to the LTE Active state LTE Active state. In LTE. In LTE Active state the UE has at least one SAE bearer allocated, allowing uplink Active state the UE has at least one SAE bearer allocated, allowing uplink or downlink data/signalling transfer to take place. The S-TMSI is used to or downlink data/signalling transfer to take place. The S-TMSI is used to identify/ address the UE in NAS signalling messages. The UE can never identify/ address the UE in NAS signalling messages. The UE can never be in LTE Active state without also being in RRC Connected state. be in LTE Active state without also being in RRC Connected state. Transition from LTE Active to LTE Idle can, for example, be triggered by Transition from LTE Active to LTE Idle can, for example, be triggered by user inactivity.

user inactivity. In the

In the LTE Detached LTE Detached state there is no information known about the UE instate there is no information known about the UE in the eNodeB or the MME. No data or signalling transfer is possible. This the eNodeB or the MME. No data or signalling transfer is possible. This state is left/entered t

state is left/entered through the Attach/Detach procedures.hrough the Attach/Detach procedures.

5.4 PDU PDU FormatsFormats

Seeqq..NNoo PDCP SDUPDCP SDU MMAACC--II

Seeqq. . NNoo EE LLeennggtth h IInndd.. EE RLC SDU 1RLC SDU 1 ...

LCID

LCID11 LL11 EE11 MAC SDU 1MAC SDU 1 ... PadPad

RRC Message or IP Packet RRC Message or IP Packet PDCP PDU PDCP PDU RLC PDU RLC PDU MAC PDU MAC PDU S

Seeqq..NNoo PDCP SDUPDCP SDU MMAACC--II

Seeqq. . NNoo EE LLeennggtth h IInndd.. EE RLC SDU 1RLC SDU 1 ...

LCID

LCID11 LL11 EE11 MAC SDU 1MAC SDU 1 ... PadPad

RRC Message or IP Packet RRC Message or IP Packet

Seeqq..NNoo PDCP SDUPDCP SDU MMAACC--II

Seeqq. . NNoo EE LLeennggtth h IInndd.. EE RLC SDU 1RLC SDU 1 ...

LCID

LCID11 LL11 EE11 MAC SDU 1MAC SDU 1 ... PadPad

RRC Message or IP Packet RRC Message or IP Packet

Seeqq..NNoo PDCP SDUPDCP SDU MMAACC--II

Seeqq. . NNoo EE LLeennggtth h IInndd.. EE RLC SDU 1RLC SDU 1 ...

LCID

LCID11 LL11 EE11 MAC SDU 1MAC SDU 1 ... PadPad

RRC Message or IP Packet RRC Message or IP Packet PDCP PDU PDCP PDU RLC PDU RLC PDU MAC PDU MAC PDU

Figure 5-4: layer 2 and layer 3 PDU formats Figure 5-4: layer 2 and layer 3 PDU formats

Figure 5-4 shows the PDU formats for (from top to bottom) the PDCP, Figure 5-4 shows the PDU formats for (from top to bottom) the PDCP, RLC and MAC protocols. The payload of a given protocol is referred to as RLC and MAC protocols. The payload of a given protocol is referred to as a Service Data Unit (SDU). PDCP PDUs only carry one SDU while RLC a Service Data Unit (SDU). PDCP PDUs only carry one SDU while RLC and MAC PDUs may carry multiple SDUs.

and MAC PDUs may carry multiple SDUs.

The PDCP protocol takes as input either an RRC message (CP) or an IP The PDCP protocol takes as input either an RRC message (CP) or an IP packet (UP). RRC messages are encrypted and integrity protected. The packet (UP). RRC messages are encrypted and integrity protected. The integrity protection results in a Message Authentication Code for Integrity integrity protection results in a Message Authentication Code for Integrity (MAC-I) field being added at the end of the PDCP PDU. User plane (MAC-I) field being added at the end of the PDCP PDU. User plane packets are encrypted and compressed but never integrity protected. The packets are encrypted and compressed but never integrity protected. The PDCP protocol also adds a one or two byte long sequence number, unless PDCP protocol also adds a one or two byte long sequence number, unless configured for transparent operation where no sequence number is

configured for transparent operation where no sequence number is present.present. Apis Technical Training AB

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Copyright © © Apis Apis Technical Technical Training Training AB AB 2007. 2007. All All rights rights reserved. reserved. 5-105-10 The RLC protocol takes as input PDCP PDUs. Several PDCP PDUs may The RLC protocol takes as input PDCP PDUs. Several PDCP PDUs may be concatenated into one and the same RLC PDU. The RLC protocol may be concatenated into one and the same RLC PDU. The RLC protocol may also perform segmentation, meaning that only part of a given PDCP PDU also perform segmentation, meaning that only part of a given PDCP PDU is fitted within one RLC PDU. An RLC sequence number is added for is fitted within one RLC PDU. An RLC sequence number is added for ARQ operation, sequence control and SDU reassembly purposes. One or ARQ operation, sequence control and SDU reassembly purposes. One or more length indicator is added to indicate the presence of multiple SDUs, more length indicator is added to indicate the presence of multiple SDUs, or SDU segments. The presence of the length indicators themselves is or SDU segments. The presence of the length indicators themselves is indicated with an extension bit (E) following the sequence number and indicated with an extension bit (E) following the sequence number and each present length indicator. Thus, the E-bit following the

each present length indicator. Thus, the E-bit following the last last lengthlength indicator will indicate ‘no more length indicator fields present’.

indicator will indicate ‘no more length indicator fields present’.

The MAC protocol takes as input RLC PDUs. Several RLC PDUs may be The MAC protocol takes as input RLC PDUs. Several RLC PDUs may be concatenated into one and the same MAC PDU. One MAC PDU is the concatenated into one and the same MAC PDU. One MAC PDU is the same as one Transport Block. Thus, one and the same Transport Block same as one Transport Block. Thus, one and the same Transport Block may carry information from more than one logical channel (figure 5-1). may carry information from more than one logical channel (figure 5-1). The identity number of the logical channel where a given MAC SDU The identity number of the logical channel where a given MAC SDU originated is indicated with the Logical Channel Identity field (LCID). The originated is indicated with the Logical Channel Identity field (LCID). The length (in bits) of each MAC SDU is indicated with the Length field (L). length (in bits) of each MAC SDU is indicated with the Length field (L). There is one LCID/L pair for

There is one LCID/L pair for eacheach MAC SDU in the payload field. TheMAC SDU in the payload field. The presence of yet another LCID/L pair is indicated with the extension bit (E) presence of yet another LCID/L pair is indicated with the extension bit (E) following the previous pair. Thus, the E-bit following the

following the previous pair. Thus, the E-bit following the last last LCID/L pairLCID/L pair will indicate ‘no more LCID/L fields’. Padding may be added if the total will indicate ‘no more LCID/L fields’. Padding may be added if the total length of the LCID/L/E fields and the associated MAC SDUs do not length of the LCID/L/E fields and the associated MAC SDUs do not exactly match the number of bits to be

exactly match the number of bits to be transmitted on the assigned physicaltransmitted on the assigned physical resource.

resource.

One or two Transport Blocks per 1ms TTI are delivered to the physical One or two Transport Blocks per 1ms TTI are delivered to the physical layer for further processing, as described in chapter 4.

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5.5 References

24.801

24.801 3GPP 3GPP System System Architecture Architecture Evolution: Evolution: CT CT WG1 WG1 aspects aspects (NAS)(NAS) 33.821

33.821 Rationale Rationale and and track track of of security security decisions decisions in in LTE/SAELTE/SAE 36.321

36.321 E-UTRA; E-UTRA; Medium Medium Access Access Control Control (MAC) (MAC) protocolprotocol 36.322

36.322 E-UTRA; E-UTRA; Radio Radio Link Link Control Control (RLC) (RLC) protocolprotocol 36.323

36.323 E-UTRA; E-UTRA; Packet Packet Data Data Convergence Convergence Protocol Protocol (PDCP)(PDCP) 36.331

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6 X2 X2 and and S1-interfaceS1-interface

6.1

6.1 IINTRODUCTIONNTRODUCTION... 6-26-2

6.2

6.2 TTHEHEX2-X2-INTERFACEINTERFACE ... 6-36-3

6.2.1

6.2.1 X2-interface Protocols...6-3X2-interface Protocols...6-3

6.3

6.3 TTHEHES1-S1-INTERFACEINTERFACE ... 6-56-5

6.3.1

6.3.1 S1-interface Protocols...6-5S1-interface Protocols...6-5

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