Packet Data Transfer in UMTS

Packet Data Transfer in UMTS Cellular Networks
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Published Date:26-10-2017
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Packet Data Transfer in UMTS Cellular Networks This chapter introduces packet data transmission and the protocols used for carrying user plane application data across Universal Mobile Telecommunication System (UMTS) cellular networks. Our target is to give an overview of the roles of the supported protocols and explain the mapping between the service access points (SAPs) of the protocols in questions and the information available in the network elements to classify and collect performance measurements. As explained in Section 9.3.1, such identifiers will allow the network management system (NMS) to monitor the perform- ance of service applications based on a particular subset of quality of service (QoS) profile attributes. The high level functional grouping of Layer 1–3 protocols described in this chapter allows the reader to get a clear view of the protocol architecture and the transfer of a specific type of information over the air (radio) interface on common, shared or dedicated resources. More information on the topics covered in Section 4.1 for enhanced General Packet Radio Service (EGPRS) and Section 4.2 for wideband code division multiple access (WCDMA) can be found in 1–11 and in 12–26, respectively. 4.1 Packet data transfer across EGPRS networks This section introduces the user and control plane protocol stacks implemented in EGPRS networks. Besides this, the radio channels and frame structure are also pre- sented. The radio channels include traffic, control and packet data channels. 4.1.1 Userplaneprotocols The user plane protocol stack for EGPRS networks is depicted in Figure 4.1. The numbers in the figure define the SAPs between the protocol layers where theApplication IP IP 0 1 Relay SNDCP GTP-U SNDCP GTP-U 3 12 2 13 BSC LLC LLC 4 5 UDP UDP Relay RLC BSSGP RLC BSSGP IP IP 7 6 8 9 MAC MAC Network Network L2 L2 service service 10 11 GSM RF GSM RF L1bis L1bis L1 L1 Um Gb Gn Gi BTS MS BSS SGSN GGSN Figure 4.1 EGPRS user plane protocol stacks 1. corresponding performance can be assessed. In the base station system (BSS), we have broken the protocol stack to show how the different entities may be deployed in the radio access network (RAN). The channel codec unit (CCU) – that is, Layer 1 functions – is in the base transceiver station (BTS), and the packet control unit (PCU) – that is, medium access control (MAC) and radio link control (RLC) functions – is deployed in the base station controller (BSC) 1. The Packet Data Protocol (PDP) context between the 2nd-generation (2G) Serving GPRS Support Node (SGSN) and the mobile station (MS) is uniquely addressed with a temporary logical link identity (TLLI) and a network layer SAP identifier (NSAPI) pair. The NSAPI and TLLI are used for network layer routing. An NSAPI/TLLI pair is unambiguous within a routing area (RA). The NSAPI identifies the PDP context associated with a PDP address; it is represented by a transaction identifier (TI) in some session management (SM) signalling messages. The MS produces an unused NSAPI any time it requests the activation of a PDP context. The TI is dynamically allocated by the MS for MS-requested PDP context activation, and by the network for network-requested PDP context activation. The TI is de- allocated when a PDP context has been deactivated. The TLLI unambiguously identifies the logical link between the MS and SGSN. Within an RA, there is a one-to-one correspondence between the international mobile subscriber identity (IMSI) and TLLI, which is only known in the MS and SGSN. The TLLI is derived from the packet temporary mobile subscriber identity (PTMSI), which is allocated by the SGSN, and is valid only in the RA associated with the PTMSI. Sub-network Dependent Convergence Protocol (SNDCP) minimises the transfer of redundant control information (e.g., TCP/IP header) and user data between the SGSN and MS through compression techniques. In addition, the SNDCP multiplexes N protocol data units (N-PDUs) from one or several NSAPIs onto one Logical Link Control (LLC) protocol SAP identifier (SAPI). The NSAPI multiplexed onto the same SAPI must use the same radio priority level, QoS traffic handling priority and traffic class. The output of the compression subfunctions are segmented (reassembled) to LLC frames of maximum length (to SNDCP packets) 2.Packet Data Transfer in UMTS Cellular Networks 93 The relationship between TLLI/NSAPI and LLC/SNDCP is illustrated in Figure 4.2. The figure shows the end-to-end packet data transfer across the EGPRS network and relevant information stored and available in the MS, BSS and packet core network (CN). Besides this, Figure 4.2 depicts how the network layer uniquely identifies the ongoing packets belonging to distinct communications. As described in Section 9.3.1, by means of such identifiers, it is possible to classify measurements to assess the performance of the carried service applications based on a particular combination of QoS attributes. The LLC permits information transfer between the SGSN and one or more MSs using the same physical radio resources with different service criteria. An LLC connection is identified by a data link connection identifier (DLCI), which consists of a SAPI, which identifies the SAP at the SGSN end and the MS end of the LLC interface, and the TLLI, which represents a specific MS. LLC protocol supports acknowledged, unacknowledged and ciphering types of operations. The LLC frames are multiplexed onto BSS GPRS protocol (BSSGP) virtual connections (BVCs) 3. RLC functions are: segmentation of LLC PDUs into RLC data blocks and reassembly of RLC data blocks into LLC PDUs; segmentation of RLC/MAC control messages into RLC/MAC control blocks and reassembly of RLC/MAC control messages from RLC/ MAC control blocks; backward error correction (BEC) enabling the selective retransmis- sion of RLC data blocks. An RLC data block transfer has a unique temporary flow identity (TFI), a set of physical data channels (PDCHs) to be used for downlink transfer; and, optionally, a temporary block flow (TBF) starting time indication. A TBF is comprised of two peer entities, which are the RLC endpoints 4. MAC enables multiple MSs to share a common transmission medium, which may consist of several physical channels. MAC may allow an MS to use several physical channels in parallel – that is, use several time slots within the time division multiple access (TDMA) frame 4. The interface between the SGSN and BSS (denoted by Gb in Figure 4.1) allows many users to be multiplexed over a common physical resource. The communication between BSSGP entities is based on BVCs. Each BVC is used in the transport of BSSGP PDUs between: peer point-to-point (PTP) functional entities; peer point-to-multipoint (PTM) functional entities and peer signalling functional entities. There is a one PTP functional entity per cell, which is identified by a BSSGP virtual connection identifier (BVCI). Each BVC is identified by means of a BVCI, which has end-to-end significance across the Gb interface. Each BVCI is unique between two peer network service entities (NSEs). The identifier of the network service entity (NSEI), together with the BVCI, uniquely identifies a BVC (e.g., a PTP functional entity) within an SGSN. The NSEI is used by the BSS and the SGSN to determine the network service virtual connections that provide the service to a BVCI. In the downlink, the SGSN includes the IMSI (or TLLI) in the PDU and makes the following available to the BSS: MS radio access capability; QoS profile (peak bit rate), type of BSSGP SDU (signalling or data), type of LLC frame (ACK, NACK or not), precedence class (1, 2, 3) and transmission mode to be used when transmitting the LLC-PDU across the radio interface – that is, acknowledged mode (AM) (using RLC/MAC ARQ functionality) or unacknowledged mode (UM) (using RLC/MAC unit data functionality). In the uplink the BSS provides the SGSN with the following: TLLI, received from the MS, and the negotiated QoS profile (peak bit rate, theExternal PDN External PDN Radio block(s) (4 bursts each = 20ms) on PDTCH CS 1 – CS 4 (GPRS) BSS virtual connection MCS 1 – MCS 9 (EGPRS) = Mux of LLC frames (M-CS) (BVCI = Cell ID) LLC connection: BSS packet flow Context (PFC) = Mux of N-PDUs from one or more NSAPIs PCU frames (DLCI(s) = TLLI + SAPI(s)) MS BTS BSC SGSN GGSN CCU PCU CCU Packet-switching Packet-switching Circuit-switching Abis Gn Gb Um One MM context per MS • PDP context(s) One BSS context per MS • PDP context(s) • QoS profile(s) MS • BSS PFCs (PFIs) • QoS profile(s) •TFT(s) • Aggregate BSS QoS profile(s) One MM context • Radio priority (UL) • PDP context(s) • PFI(s) • QoS profile(s) • Aggregate BSS QoS profile(s) •TFT(s) One RR connection (TBF) • Radio priority (UL) RR (RLC/MAC) connection: over one or more PDCH(s) One tunnel per PDP context • PFI(s) Temporary block flow (TBF) ( TS(s) ) (NSAPI, TLLI, TEID) ( TFI) Figure 4.2 EGPRS end-to-end data transmission.Packet Data Transfer in UMTS Cellular Networks 95 precedence used at radio access and the Tx mode used across the radio path). The SGSN obtains the BVCI and NSEI from the underlying network service 5, 6. In packet idle mode, no temporary block flows exist; the upper layers can require the transfer of an LLC PDU that, implicitly, may trigger the establishment of a TBF and transition to packet transfer mode. The MS listens to the broadcast channel and to the paging subchannel for the paging group the MS belongs to in idle mode. In packet transfer mode, the MS is allocated radio resources providing a TBF on one or more physical channels. Concurrent TBFs may be established in opposite directions so that the transfer of LLC PDUs in RLC AM or RLC UM is provided. When selecting a new cell, the MS leaves packet transfer mode, reads the system information and enters packet idle mode where it switches to the new cell 7. One radio block is by definition carried by four normal bursts (20 ms). The MAC header is of constant length, 8 bits, whereas the RLC header is variable in length. The RLC data field contains octets from one or more LLC PDUs. The block check sequence (BCS) is used for error detection 8. More information on frame structure and radio channels is provided in Section 4.1.3. In R98, four different coding schemes (i.e., CS-1, CS-2, CS-3 and CS-4) are defined for the radio blocks carrying RLC data blocks. CS-1 is used for the slow associated control channel (SACCH) – that is, 1/2 convolutional code for forward error correction (FEC) and a 40-bit FIRE code for the BCS. CS-2 and CS-3 are punctured versions of CS-1 for FEC. CS-4 has no FEC. CS-2 to CS-4 use the same 16-bit CRC for the BCS over the whole uncoded RLC data block. Table 4.1 summarises the channel coding for the packet data traffic channel (PDTCH) 8. In R99, the radio block structure for user data transfer is different for GPRS and EGPRS. For EGPRS, a radio block for data transfer consists of one RLC/MAC header and one or two RLC data blocks. Interleaving depends on the modulation and coding scheme (MCS) used. Nine different modulation and coding schemes, MCS-1 to MCS-9, for the EGPRS radio blocks carrying RLC data blocks are defined. For all EGPRS packet control channels the corresponding GPRS control channel coding is used. Details of the EGPRS coding schemes are shown in Table 4.2. Transmission and reception data flows are the same for GPRS and EGPRS, except for EGPRS MCS-9, 8 and 7, where four normal bursts are used for carrying two RLC blocks (one RLC block within two bursts for MCS-9 and 8) 8. Table 4.1 GPRS channel coding for PDTCH 8. Scheme Code rate Radio block Modulation Data rate Data rate excluding size (bytes) (kb/s) RLC/MAC headers (kb/s) CS-1 1/2 23 GMSK 9.05 8 CS-2 2/3 34 GMSK 13.4 12 CS-3 3/4 39 GMSK 15.6 14.4 CS-4 1 54 GMSK 21.4 2096 QoS and QoE Management in UMTS Cellular Systems Table 4.2 EGPRS channel coding for PDTCH 8. Scheme Code rate Header code Modulation RLC blocks Raw data within Data rate rate per radio one radio block (kb/s) block (20ms) MCS-9 1.0 0.36 8-PSK 2 2592 59.2 MCS-8 0.92 0.36 2 2544 54.4 MCS-7 0.76 0.36 2 2448 44.8 MCS-6 0.49 1/3 1 592 29.6 48þ544 27.2 MCS-5 0.37 1/3 1 448 22.4 MCS-4 1.0 0.53 GMSK 1 352 17.6 MCS-3 0.85 0.53 1 296 14.8 48þ248 and 296 13.6 MCS-2 0.66 0.53 1 224 11.2 MCS-1 0.53 0.53 1 176 8.8 Note: The italic captions indicate the 6 octets (48 bits) of padding when retransmitting an MCS-8 block with MCS-3 or MCS-6. For MCS-3, the 6 octets of padding are sent every second block. 4.1.2 Controlplaneprotocols The user plane protocol stack for EGPRS networks is depicted in Figure 4.3. The control plane consists of protocols for control and support of user plane functions 1: . Controlling the packet domain network access connections, such as attaching to and detaching from the packet domain network. . Controlling the attributes of an established network access connection, such as activation of a PDP address. . Controlling the routing path of an established network connection in order to support user mobility. . Controlling the assignment of network resources to meet changing user demands. GMM/SM GMM/SM GTP-C GTP-C BSC LLC LLC UDP UDP 0 1 Relay RLC RLC BSSGP BSSGP IP IP 2 3 6 7 MAC MAC Network Network L2 L2 service service 4 5 GSM RF L1bis L1bis L1 L1 GSM RF Gn Um Gb MS BSS 2G-SGSN GGSN BTS Figure 4.3 EGPRS control plane protocol stacks 1.Packet Data Transfer in UMTS Cellular Networks 97 GPRS Mobility Management and Session Management (GMM/SM) protocol supports mobility management functionality such as GPRS attach, GPRS detach, security, RA update, location update, PDP context activation and PDP context deactivation. GPRS Tunnelling Protocol for the control plane (GTP C) tunnels signalling messages between SGSNs and gateway GPRS support nodes (GGSNs) (Gn), and between SGSNs in the backbone network (Gp). User Datagram Protocol (UDP) transfers signalling messages between GSNs. UDP is defined in RFC 768. The remaining protocols in the stacks are the same as in the user plane (see Section 4.1.1). 4.1.3 Radiochannelsandframestructure Each base station (BS) is assigned one or more carrier frequencies. A carrier frequency is also referred to as a TRX (transmitter–receiver, or better, transceiver), which is a combination of two frequencies (uplink and downlink frequency) carrying certain channel definitions. Each of these carrier frequencies is then divided in time, using a TDMA scheme 9, 10. The fundamental unit of time in this TDMA scheme is called aburstperiod. Each burst period lasts 15/26 ms (or approx. 0.577 ms). A burst is a period of RF carriers modulated by a data stream, and therefore represents the physical content of atimeslot (TS). A time slot is divided into 156.25 symbol periods. For Gaussian minimum shift keying (GMSK) modulation a symbol is equivalent to a bit. A particular bit period within a time slot is referenced by abitnumber (BN), with the first bit period being numbered 0, and the last (1/4) bit period being numbered 156. For octagonal phase shift keying (8-PSK) modula- tion one symbol corresponds to three bits. In this case, the last (3/4) bit is numbered 468. The bits are mapped to symbols in ascending order according to 11. Eight burst periods are grouped into a TDMAframe (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. The time slots within a TDMA frame are numbered from 0 to 7 and are called the time slot number (TN) 10. Radio channels implementing traffic in the radio interface are calledphysicalchannels. A physical channel uses a combination of frequency and time division multiplexing and it is defined as a sequence ofradiofrequencychannels (RFCHs) and time slots. The RFCH sequence is determined by a function that, in a given cell with a given set of general parameters – time slot number, mobile radio frequency channel allocation (MA) and mobile allocation index offset (MAIO) – maps the TDMA frame number onto a radio frequency channel. Therefore, in a cell there is, for a physical channel assigned to a particular mobile, a unique correspondence between radio frequency channel and TDMA frame number. A given physical channel always uses the same time slot number in every TDMA frame. Hence, a time slot sequence is defined by a time slot number and a TDMA frame number (FN) sequence. A physical channel is therefore defined as a sequence of TDMA frames, a time slot number (modulo 8) and afrequency hopping sequence (FHS). Logical channels are defined based on the type of information carried over the air interface. They can be divided into dedicated channels, which are allocated to an MS, and common channels, which are used by MSs in idle mode.98 QoS and QoE Management in UMTS Cellular Systems The detailed mapping of logical channels onto physical channels, the mapping of physical channels onto TDMA frame numbers, the permitted channel combinations and the operation of channels and channel combinations can be found in 10. Hyperframes, superframes and multiframes The organisation of burst, TDMA fames and multiframes for speech and data is illus- trated in Figure 4.4. The multiframe structure for packet data channels is shown in Figure 4.5. The longest recurrent time period of the structure is called ahyperframe and has a duration of 3h 28 min 53 s 760 ms (or 12 533.76 s). The TDMA frames are numbered modulo this hyperframe (TDMA frame number from 0 to 2715 647). One hyperframe is subdivided into 2048 superframes which have a duration of 6.12 s. The superframe is itself subdivided in four types of multiframes: . A 26-multiframe (51 per superframe) with a duration of 120 ms, comprising 26 TDMA frames. This multiframe is used to carry the TCH (and SACCH/T) and FACCH (see the following sections for a description of logical channels). . A 51-multiframe (26 per superframe) with a duration of 235.4 ms (3060/13 ms), comprising 51 TDMA frames. This multiframe is used to carry the BCCH, CCCH (NCH, AGCH, PCH and RACH) and SDCCH (and SACCH/C), or PBCCH and PCCCH. . A 52-multiframe (25.5 per superframe) with duration of 240ms, comprising 52 TDMA frames. This multiframe is used to carry the PBCCH, PCCCH (PNCH, PAGCH, PPCH and PRACH), PACCH, PDTCH, and PTCCH. Time slots and bursts Four different types of bursts exist in the system: . Normalburst: this burst is used to carry information on traffic and control channels, except for the RACH, PRACH and CPRACH. It contains 116 encrypted symbols and includes a guard time of 8.25 symbol duration (30:46ms). . Frequency correction burst: this burst is used for frequency synchronisation of the mobile. It is equivalent to an unmodulated carrier, shifted in frequency, with the same guard time as the normal burst. It is broadcast together with the broadcast control channel (BCCH). The repetition of frequency correction bursts is also named fre- quency correction channel (FCCH). . Synchronisation burst: this burst is used for time synchronisation of the mobile. It contains a long training sequence and carries the information of the TDMA frame number and BS identity code. It is broadcast together with the frequency correction burst. The repetition of synchronisation bursts is also named the synchronisation channel (SCH). . Accessburst: this burst is used for random access and is characterised by a longer guard period (68.25 bit duration or 252ms) to cater for burst transmission from a mobile which does not know the timing advance at the first access (or after handover).B B 1 Hyperframe = 2048 superframes = 2 715 648 TDMA frames (3 h 28 min 53 s 760 ms) 2042 2044 2045 2046 2047 0 1 23 4 5 6 2043 1 Superframe = 1326 TDMA frames (6.12 s) (= 51 (26-frame) multiframes or 26 (51-frame) multiframes) 2 47 48 49 50 01 3 01 24 25 1 (26-frame) multiframe = 26 TDMA frames (120 ms) 1 (51-frame) multiframe = 51 TDMA frames (3060/13 ms) 1 (51-frame) multiframe¼51 TDMA frames (3,060/13ms) 2 2 01 34 22 23 24 25 0 1 3 464748 4950 1 TDMA frame = 8 time slots (120/26 or 4.615 ms) 1 3 4 56 7 0 2 1 Time slot = 156.25 symbol durations (15/26 or 0.577 ms) (1 symbol duration = 48/13 or 3.69 µs) (TB: tail bits - GP: guard period) Normal burst (NB) TBEncrypted bits Training sequenceEncrypted bits GPT 58 26 58 3 8.25 3 TB Fixed bits TBGP Frequency correction burst (FB) 3 3 142 8.25 TBEncrypted bits Synchronization sequence Encrypted bits GTP Synchronization burst (SB) 39 64 39 3 3 8.25 TB TBEncry Sypte nchronization sequenced bits GP Access burst (AB) 41 36 68.25 8 3 Figure 4.4 Organisation of bursts, TDMA frames and multiframes for speech and data 9. The numbers shown in the figure are in symbols. For GMSK modulation, one symbol is one bit. For 8PSK modulation, one symbol is three bits.100 QoS and QoE Management in UMTS Cellular Systems 1 Multiframe = 52 TDMA frames B0 B1B2 TB3 B4 XB6 B5 B7 TB8 B9 B10 B11 X = 1 TDMA frame (8 time slots, 4.615 ms) X = Idle frame T = Frame used for PTCCH (packet timing advance control channel) B0–B11 = Radio blocks Figure 4.5 Multiframe structure for PDCH 10. Traffic channels Traffic channels are used for carrying either encoded speech or user data in circuit- switched (CS) mode. Traffic channels for the uplink and downlink are separated in time by three burst periods, so that the MS does not have to transmit and receive simul- taneously, thus simplifying the electronics. The traffic channels are: . Full-rate traffic channel (TCH/F): this channel carries information at a gross rate of 22.8 kb/s. TCH/Fs are defined using a 26-frame multiframe, or group of 26 TDMA frames. Of the 26 frames, 24 are used for traffic, one is used for the SACCH and one is currently unused (see Figure 4.4). . Half-rate traffic channel (TCH/H): this channel effectively doubles the capacity of a system (i.e., speech coding at 7 kb/s, instead of 13kb/s). . Enhanced full-rate traffic channel (E-TCH/F): this channel carries information at a gross rate of 69.6 kb/s including the stealing symbols. Common control channels Common control channels can be accessed both by idle mode and dedicated mode mobiles. They are used by idle mode mobiles to exchange the signalling information required to change to dedicated mode. Mobiles already in dedicated mode monitor the surrounding BSs for handover and other information. The common control channels are defined within the 51-frame multiframe, so that dedicated mobiles, using the 26-frame multiframe TCH structure, can still monitor control channels. Common control channels include: . Broadcast control channel (BCCH): this channel continually broadcasts, in the downlink, information including BS identity, frequency allocations and frequency- hopping sequences. . Frequency correction channel (FCCH) and synchronisation channel (SCH): these channels are used to synchronise the mobile to the time slot structure of a cell byPacket Data Transfer in UMTS Cellular Networks 101 defining the boundaries of burst periods, and the time slot numbering. Every cell in a GSM network broadcasts exactly one FCCH and one SCH, which are by definition on time slot number 0 (within a TDMA frame). . Randomaccesschannel (RACH): this channel is the Slotted ALOHA channel used by the mobile to request access to the network. . Paging channel (PCH): this channel is used to alert the MS of an incoming call. . Notificationchannel (NCH): this channel exists in the downlink only; it is used to notify MSs of voice group and voice broadcast calls. . Accessgrantchannel (AGCH): this channel is used to allocate an SDCCH to a mobile for signalling (in order to obtain a dedicated channel), following a request on the RACH. Dedicated control channels Dedicated control channels are used for signalling between the network and the MS. They comprise: . Stand-alone dedicated control channel (SDCCH): this channel is used to provide a reliable connection for signalling and short message services (SMS); it may be combined with CCCH (SDCCH/4). The SACCH/C is used to support this channel. . Slow associated control channel (SACCH): this channel provides a relatively slow signalling connection. The SACCH is associated with either a TCH (SACCH/TH or SACCH/TF) or SDCCH (SACCH/C4 or SACCH/C8). The SACCH can also be used to transfer SMS messages if associated with a TCH. . Fastassociatedcontrolchannel (FACCH): the FACH/F or FACH/H appears in place of the TCH/F or TCH/H when lengthy signalling is required between a GSM mobile and the network while the mobile is in call. The channel is indicated by the use of stealing flags in the normal burst. Typical signalling where this may be employed is during call handover. All associated control channels have the same direction (bidirectional or unidirectional) as the channels with which they are associated. Packet data channels are also defined for dedicated and common traffic. They include: . Packet random access channel (PRACH): the uplink PRACH is used by the MS to initiate uplink transfer (signalling) or for sending data; it is mapped onto one or several physical channels. . Packet paging channel (PPCH): the downlink PPCH is used to page an MS prior to downlink packet transfer; it can be used for paging of both CS (Class A and CB GPRS MSs) and packet-switched (PS) data services; it is mapped onto one or several physical channels in the same way as done for the PCH.102 QoS and QoE Management in UMTS Cellular Systems . Packet access grant channel (PAGCH): the downlink PAGCH is used in the packet transfer establishment phase to send the resource assignment to an MS prior to packet transfer; it is mapped onto one or several physical channels. . Packet notification channel (PNCH): the downlink PNCH is used to send a PTM-M (Point To Multipoint–Multicast) notification to a group of MSs prior to a PTM-M packet transfer. The PNCH is mapped onto one or several blocks on the PCCCH. . Packet broadcast control channel (PBCCH): the downlink PBCCH is for system information. If not allocated in the cell, the packet data specific system information is broadcast on the BCCH. It is mapped onto one or several physical channels in the same way as done for the BCCH. The existence of the PCCCH, and consequently the existence of the PBCCH, is indicated on the BCCH. . Packet timing advance control channel, uplink (PTCCH/U): this channel is used to transmit random access bursts to allow estimation of the timing advance for one MS in packet transfer mode. Two defined frames of a multiframe are used to carry the PTCCH. . Packettiming advancecontrolchannel,downlink (PTCCH/D): this channel is used to transmit timing advance information updates to several MSs. One PTCCH/D is paired with several PTCCH/Us. Two defined frames of a multiframe are used to carry the PTCCH. Four normal bursts comprising a radio block are used for carrying the channels. . Packetdatatrafficchannel (PDTCH): this channel is allocated for unidirectional data transfer, either uplink (PDTCH/U) or downlink (PDTCH/D). It is temporarily dedicated to one MS or to a group of MSs in the PTM-M case. One MS may use multiple PDTCHs in parallel for individual packet transfer in multislot operation, and all packet data traffic channels may be used for mobile-terminated packet transfer. Up to eight PDTCHs with different time slots but with the same frequency parameters may be allocated to one MS at the same time. One PDTCH is mapped onto one physical channel. . Packet associatedcontrol channel (PACCH): this channel is of a bidirectional nature and conveys signalling information (e.g., acknowledgements and power control infor- mation) related to a given MS. It carries resource assignment and reassignment messages, comprising the assignment of capacity for PDTCH(s) and for further occurrences of the PACCH. The PACCH shares resources with PDTCHs, which are currently assigned to one MS. An MS that is currently involved in packet transfer can be paged for CS services on the PACCH. This channel is dynamically allocated on the block basis on the same physical channel as carrying PDTCHs. 4.1.4 Mappingofpacketdatachannels As illustrated in Figure 4.5, mapping of logical channels in time is defined by a multi- frame structure of 52 TDMA frames, divided into 12 blocks (of 4 frames), 2 idle frames and 2 frames used for the PTCCH. B0 is used as the PBCCH when allocated, and if required up to 3 more blocks on the same PDCH can be used as additional PBCCHs. On any PDCH with a PCCCH (with or without PBCCH), up to the next 12 blocks in the ordered list of blocks are used for the PPCH, PAGCH, PNCH, PDTCH or PACCH in the downlink. On an uplink PDCH that contains a PCCCH, all blocks in the multiframePacket Data Transfer in UMTS Cellular Networks 103 can be used as the PRACH, PDTCH or PACCH. The mapping of channels onto multiframes is controlled by several parameters broadcast on the PBCCH. On a PDCH that does not contain a PCCCH, all blocks can be used as the PDTCH or PACCH. Two frames are used for the PTCCH and the two idle frames as well as the PTCCH frames can be used by the MS for signal measurements and BSIC identification. When no PCCCH is allocated, the MS camps on the CCCH and receives all system information on the BCCH. The MS monitors the uplink state flags on the allocated PDCHs and transmits radio blocks on those which currently bear the uplink state flag value reserved for the usage of the MS 8. In short, PCCCHs are mapped together with the PBCCH (or BCCH) and PDTCH onto one or several physical channels according to the 52-multiframe. If the PCCCH (PNCH, PAGCH, PPCH and PRACH) is not allocated in the cell, the CCCH (PCH, RACH, AGCH and NCH) is used to initiate the packet data transfer. Possible channel combinations are: . PBCCHþ PCCCHþ PDTCHþ PACCHþ PTCCH . BCCHþ PCCCHþ PDTCHþ PACCHþ PTCCH . BCCHþ CCCHþ PDTCHþ PACCHþ PTCCH: A multislot configuration consists of multiple CS or PS traffic channels together with associated control channels, allocated to the same MS. The multislot configuration occupies up to eight basic physical channels, with different time slot numbers, but with the same frequency parameters – absolute radio frequency channel number (ARFCN) or MA, MAIO and hopping sequence number (HSN) – and the same training sequence code (TSC). 4.2 Packet data transfer across WCDMA networks The section introduces end-to-end packet data transmission and the combined models for protocols used to control, support and carry user plane application data across WCDMA networks. Our target is to explain the mapping between bearer services and the SAPs of protocols, and the information available in the network elements in order to classify performance counters and indicators during measurements. As already pointed out, such identifiers will allow the NMS to measure the implemented service applications based on the corresponding PDP contexts. The concepts of high-speed downlink packet access (HSDPA), introduced in 3rd Generation Partnership Project (3GPP) R5 specifica- tions, and high-speed uplink packet access (HSUPA), defined in 3GPP R6 specifications, are presented in Sections 4.3 and 4.4, respectively, where more details on adopted protocols and radio channels are given. The high-level functional grouping into the access stratum (AS) and non-access stratum (NAS) is defined in 12. The AS is the functional grouping of protocols specific to the access technique (i.e., radio and Iu protocols). The NAS is the functional grouping of protocols aimed at: call control (CC) for CS voice and data; session management (SM), for PS data; mobility manage- ment for circuit-switched MM and PS domains (GMM); Short Message Services (SMS)104 QoS and QoE Management in UMTS Cellular Systems for PS and CS domains; supplementary services (SS) and RAB management for re- establishment of radio access bearer (RABs) which still have active PDP contexts. 4.2.1Userplaneprotocolstack The UMTS user plane protocol stack is depicted in Figure 4.6. The numbers in the figure define the SAPs between protocol layers where the performance of the related bearer and thus the corresponding offered QoS may be assessed. The mapping of bearer services onto protocol SAPs is reported in Table 4.3. The GPRS Tunnelling Protocol (GTP) encapsulates all PDP PDUs – that is, tunnels user data between the RNC and SGSN, and between GPRS support nodes (GSNs) in the backbone network. UDP/IP is the backbone network protocol used for routing user data and control signalling 13. The Packet Data Convergence Protocol (PDCP) exists only in the user plane and only for services from the PS domain. The main PDCP functions are: header compression and decompression of IP data streams (e.g., TCP/IP and RTP/UDP/IP headers) at the transmitting and receiving entity, respectively; transmission of user data means that PDCP receives a PDCP SDU from the NAS and forwards it to the RLC layer andvice versa; support for lossless serving radio network subsystem (SRNS) relocation or lossless downlink RLC PDU size change; and maintenance of PDCP sequence numbers for radio bearers that are configured to support lossless SRNS relocation or lossless downlink RLC PDU size change 14. Broadcast Multicast Control (BMC) protocol provides a broadcast/multicast trans- mission service in the user plane on the radio interface for common user data in UM. The BMC functions are: storage of cell broadcast messages (CBMs); traffic volume monitor- ing and radio resource (CTCH/FACH) request for the cell broadcast service (CBS); scheduling and transmission of BMC messages to terminals; and delivery of CBMs to the upper layer (NAS) 15. Radio Link Control (RLC) protocol provides segmentation/reassembly (payloads units, PUs) and retransmission services for both user (radio bearer) and control data (signalling radio bearer). Each RLC instance is configured by Radio Resource Control (RRC) protocol to operate in one of the three modes: transparent mode (TM), where no protocol overhead is added to higher layer data; unacknowledged mode (UM), where no retransmission protocol is in use and data delivery is not guaranteed; and acknowledged mode (AM), where the Automatic Repeat reQuest (ARQ) mechanism is used for error correction. For all RLC modes, CRC error detection is performed on the physical layer and the results of the CRC are delivered to the RLC together with the actual data. Other relevant functions of the RLC are: in-sequence delivery of upper layer PDUs; duplicate detection of RLC PDUs; flow (rate) control of the peer RLC transmitting entity; sequence number check in AM RLC to guarantee the integrity of reassembled PDUs and provide a mechanism for the detection of corrupted RLC SDUs through checking sequence numbers in RLC PDUs when they are reassembled into an RLC SDU; protocol error detection and recovery; ciphering in the RLC layer for non-transparent RLC mode; and service data unit (SDU) discard. RLC transfer mode indicates the data transfer mode supported by the RLC entity configured for that particular radio bearer. The transfer mode for a radio bearer is the same in both uplink and downlink directions; and it isAppl. prot. Appl. prot. 0 1 TCP/UDP TCP/UDP 2 3 IP IP IP IP IP IP 6 7 4 5 GTP GTP PDCP GTP PDCP GTP UDP UDP UDP RLC RLC UDP Data 10 11 Data Data MAC-d/c/es Data MAC-d/c/es link layer link layer link layer link layer IP IP IP IP MAC-e/hs MAC-e/hs FP FP Data link Data link Data link 8 9 Data link Data link Data link layer layer layer layer layer layer WCDMA WCDMA 15 12 13 14 L1 L1 Physical Physical Physical Physical Physical Physical Physical Physical Physical Physical TE MT 3G SGSN GGSN Peer appl. SRNC Node B R Uu Iub Iu Gn Gi Figure 4.6 PS domain user plane protocol stack 12.106 QoS and QoE Management in UMTS Cellular Systems Table 4.3 Mapping of bearer services onto protocol service access points. Bearer service (BS) Service access point (SAP) Service applications 0 1 Network services 2 3 UMTS bearer service 4 5 Radio access bearer service 4 7 Core network bearer service 7 5 Radio bearer service 4 6 RAN access bearer service 6 7 Backbone network service 10 11 Physical bearer service 12 (14) 13 (15) UTRA FDD 8 9 determined by admission control in the serving RNC (SRNC) from the RAB attributes and CN domain information. RLC transfer mode affects the configuration parameters of outer-loop power control in the RNC and the user bit rate. The quality target is not affected if TM or UM RLC is used, whilst the number of retransmissions should be taken into account if AM RLC is employed. The user bit rate is affected by the transfer mode of the RLC, since the length of Layer 2 headers is: 16 bits for AM; 8 bits for UM and 0 bits for TM. Hence, the user bit rate for network dimensioning is given by the Layer 1 bit rate reduced by the Layer 2 header bit rate. The RLC provides logical link control over the radio interface. There may be several simultaneous RLC links per UE and each link is identified with a bearer ID 16. Medium Access Control (MAC) protocol controls the access (request and grant) procedures for the radio channel. The functionality of the MAC layer includes: mapping of logical channels onto the appropriate transport channels; selection of the appropriate transport format (TF) for each TCH depending on the current source rate; priority handling between data flows of one user equipment (UE), when selecting the transport format combination (TFC) in the given transport format combination set (TFCS); priority handling between UEs by means of dynamic scheduling of common transport channels, shared transport channels and for the dedicated E-DCH transport channel; identification of UEs on common transport channels; multiplexing/demultiplexing of upper layer PDUs onto/from transport blocks delivered to/from the physical layer on common transport channels (service multiplexing for common transport channels, since the physical layer does not support multiplexing of these channels); multiplexing/demul- tiplexing of upper layer PDUs onto/from transport block sets (TBSs) delivered to/from the physical layer on dedicated transport channels (service multiplexing for dedicated transport channels, this function can be utilised when several upper layer services (e.g., RLC instances) can be mapped efficiently onto the same transport channel); traffic volume measures on logical channels and reporting to RRC (based on the reported traffic volume information, RRC performs transport channel switching decisions); transport channel-type switching (execution of switching between common and dedi- cated transport channels based on a switching decision derived by RRC); ciphering (forPacket Data Transfer in UMTS Cellular Networks 107 transparent RLC mode); access service class (ASC) selection for RACH transmission; hybrid ARQ (HARQ) functionality for HS-DSCH and E-DCH transmission; in- sequence delivery and assembly/disassembly of higher layer PDUs on the HS-DSCH; and in-sequence delivery and assembly/disassembly of higher layer PDUs on the E-DCH 17–19. The data stream(s) is/are characterised by one or more frame protocols (FPs) specified for that interface 20. A PDP context is a virtual communication pipe established between the UE and the GGSN (SAPs 4 and 5 in Figure 4.6) for delivering the data traffic stream. The PDP context is defined in the UE, SGSN and GGSN by: . A PDP context identifier (index of the PDP context). . A PDP type (e.g., PPP or IP). . A PDP address (e.g., an IP address). . An access point name (APN) (label describing the access point to the packet data network). . A QoS profile (bearer service attributes). There is a one-to-one correspondence between the PDP context, UMTS bearer and RAB, as well as between the RAB and the radio bearer service, which, however, can be carried by more transport channels of the same type at the radio interface. A QoS profile is associated with each PDP context. The QoS profile is considered to be a single parameter with multiple data transfer attributes, as illustrated in Table 4.4 with RAB attributes. 4.2.2 Controlplaneprotocolstack The UMTS control plane protocol stack is depicted in Figure 4.7. The GMM protocol supports mobility management functionality such as attach, detach, security and RA update. The SM protocol supports PDP context activation, modification, deactivation and preservation procedures. The SMS protocol supports mobile-originated and mobile-terminated short messages. RAN Application Part (RANAP) encapsulates and carries higher layer signalling, handles signalling between the 3G SGSN and Iu mode RAN, and manages the GTP connections on the Iu interface. RANAP is specified in 3GPP TS 25.413. The layers below RANAP are defined in 3GPP TS 25.412 and 3GPP TS 25.414. The RRC protocol handles the signalling of Layer 3 between UEs and the UTRAN. RRC performs the following functions: broadcast of information provided by the NAS (CN); as an example RRC may broadcast CN location service area information related to some specific cells; broadcast of information related to the AS (typically, cell-specific information); establishment, re-establishment, maintenance and release of an RRC connection between the UE and UTRAN; establishment, reconfiguration and release of radio bearers; assignment, reconfiguration and release of radio resources for the RRC connection; evaluation, decision and execution of handover; cell reselection and cell/area update procedures; paging and notification to selected UEs; routing of higher layerTable 4.4 Value ranges for RAB attributes for UTRAN and GERAN in 3GPP R6 20. Attribute/Traffic class Conversational Streaming Interactive Background 3 3 3 3 Maximum bit rate (kb/s) ( 16 000 ( 16 000 ( 16 000 – overhead ( 16 000 – overhead Deliver order Yes/No Yes/No Yes/No Yes/No 2 2 2 2 Maximum SDU size (octets) ( 1500 or 1502 ( 1500 or 1502 ( 1500 or 1502 ( 1500 or 1502 1 SDU format information See 20 See 20 Delivery of erroneous SDUs Yes/No/– Yes/No/– Yes/No/– Yes/No/– 2 6 2 6 3 5 8 3 5 8 Residual BER 5 10  10 5 10 10 4 10 ,10 ,6 10 4 10 ,10 ,6 10 2 5 1 5 3 4 6 3 4 6 SDU error ratio 10  10 10  10 10 ,10 ,10 10 ,10 ,10 Transfer delay (ms) 80 – max value 250 – max value 3 3 Guaranteed bit rate (kb/s) ( 16 000 ( 16 000 Traffic handling priority 1, 2, 3 1 Allocation/Retention priority 1, 2, ...,15 1,2, ...,15 1,2, ...,15 1,2, ... ,15 Source statistic descriptor Speech/Unknown Speech/Unknown Signalling indication Yes/No 1 This parameter is limited to the values 1, 2 and 3 for GERAN when the Gb bearer service is used. 2 Valid for PPP only. 3 In case of GERAN the highest bit rate value is 473.6kb/s.Packet Data Transfer in UMTS Cellular Networks 109 GMM / GMM / GTP-C GTP-C SM / SMS SM / SMS Relay RRC RANAP RRC RANAP UDP UDP RLC-C RLC-C SCCP SCCP MAC MAC IP IP FP FP Signalling bearer Signalling bearer WCDMA WCDMA Data link Data link Data link Data link Data link Data Link L1 L1 layer layer layer layer layer Layer PHY PHY PHY PHY PHY PHY UE Node B RNC 3G SGSN GGSN Uu Iub Iu Gn Figure 4.7 PS domain – control plane protocol stack 12. PDUs; control of requested QoS (this includes the allocation of a sufficient number of radio resources); UE measurement reporting and control of reporting; outer-loop power control (the RRC layer controls setting of the target of the closed-loop power control); control of ciphering; arbitration of radio resources on uplink DCH (this function controls the allocation of radio resources on the uplink DCH on a fast basis, using a broadcast channel to send control information to all involved users); initial cell selection and reselection in idle mode; integrity protection; initial configuration for CBS; config- uration for CBS discontinuous reception; timing advance control; Multimedia Broadcast Multicast Service (MBMS) control (the RRC controls the operation of MBMS point-to- point and point-to-multipoint radio bearers). The RRC is specified in 3GPP TS 25.331. The RLC-C protocol offers logical link control over the radio interface for the transmission of higher layer signalling messages and SMS. RLC-C is defined in 3GPP TS 25.322. GPRS Tunnelling Protocol for the control plane (GTP-C) is used for signalling messages between SGSNs and GGSNs (Gn), and between SGSNs in the backbone network (Gp). User Datagram Protocol (UDP) is the transport protocol for signalling messages between GSNs. UDP is defined in RFC 768. 4.2.3 Radiointerfaceprotocolarchitectureandlogicalchannels The radio interface protocol architecture and the connections between protocols are shown in Figure 4.8. Each block represents an instance of the corresponding protocol. The dashed lines stand for interfaces through which the RRC protocol controls and configures the lower layers. The SAPs between MAC and the physical layer and between the RLC and MAC sublayers provide the transport channels (TrCHs) and the logical channels (LoCHs), respectively. TrCHs are specified for data transport between physical layer and Layer 2 peer entities, whereas logical channels define the transfer of a specific type of information over the radio interface.110 QoS and QoE Management in UMTS Cellular Systems U-plane information C-plane signalling RRC L3 control Radio bearers PDCP L2/PDCP PDCP BMC L2/BMC RLC RLC L2/RLC RLC RLC RLC RLC RLC RLC Logical channels MAC L2/MAC Transport channels PHY L1 Figure 4.8 UTRA FDD radio interface protocol architecture 12. The logical channels are divided into two groups: control channels and traffic channels. The control channels are used for transfer of control plane information and the traffic channels are used for the transfer of user plane information only 12. The control channels are: . Broadcastcontrolchannel (BCCH), for broadcasting system control information in the downlink. . Pagingcontrolchannel (PCCH), for transferring paging information in the downlink (used when the network does not know the cell location of the UE, or the UE is in cell- connected state). . Common control channel (CCCH), for transmitting control information between the network and UEs in both directions (commonly used by UEs having no RRC connection with the network and by UEs using common transport channels when accessing a new cell after cell reselection). . Dedicatedcontrolchannel (DCCH). PTP bidirectional channel for transmitting dedi- cated control information between the network and a UE (established through a RRC connection setup procedure). . MBMS point-to-multipoint control channel (MCCH). Point-to-multipoint downlink channel used for transmitting control information from the network to the UE. This control control control control

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