the figure, interfaces GSM to a private or public network such as a PSTN, ISDN, circuit-switched public data network, packet- switched public data network, and so on. Because the data communication protocols of a mobile station may be different from those of devices it is in communication with across a network, entity IWF may perform protocol conversion, rate adaptation, and so on. ■ Short messaging service A mobile station in GSM may transmit or receive short alphanumeric messages during both idle and active call states. ■ GPRS This is a new service available with GSM Phase 2ϩ that enables multiple users to transmit packet data over a single slot. In this section, we will present a brief description of GPRS. General Capabilities and Features of GPRS In circuit-switched data services, when a user wants to transmit or receive any data, first a physical channel is set up using the normal GSM call control procedures. Because data usually comes in bursts separated by variable periods of inactivity, the channel may remain idle for a considerable length of time, depending upon the type of data services being used. One could, of course, release the channel during inactive periods of data and reestablish the connection when user data is ready. However, this approach is not very efficient or practical, because delays associated with call control procedures for setting up a physical channel are relatively long. A packet switching system, where multiple users may transmit their data over the same physical channel using the so-called virtual circuits, over- comes this problem by taking advantage of the statistical nature of the traffic arrival process. The virtual circuits may be either perma- nent or switched. But even when they are switched, call control delays for setting up or tearing down a virtual circuit are usually very small. As we mentioned before, GPRS is a new feature of GSM that pro- vides the capability of packet mode transmission of user data and signaling information using the existing GSM network and radio resources. Each physical channel is shared by multiple users. The Chapter 5 174 TEAMFLY Team-Fly ® channel access mechanism has been optimized for intermittent, short bursts as well as large volumes of data, allowing data to be transmitted within about 0.5 to 1.0 seconds of a reservation request. It supports both IP and X.25 protocols and real-time as well as non- real-time data. Both point-to-point and point-to-multipoint commu- nications are possible. There is no restriction on the transfer of SMS messages over GPRS channels. In packet switching, it is necessary to use a set of data communi- cation protocols so that the transmission is efficient and error-free. Protocols that are of interest here are usually the lower-layer proto- cols such as the logical link control (LLC) and medium access control (MAC). Users are allowed to request a desired quality of service (QoS) from the network. However, only a limited number of QoS profiles are supported. Different modes of operation are possible. For exam- ple, in one mode, a mobile station can receive both GSM and GPRS services simultaneously (that is, a voice call and packet mode data transfer at the same time). In another mode, it can only receive the GPRS service. In the third mode, the mobile station monitors control channels of both GSM and GPRS, but can receive services from only one of them at a time (that is, either a voice or packet mode data). Four channel-coding schemes, designated CS-1, CS-2, CS-3, and CS- 4, with coding rates of 1 / 2 , 2 / 3 , 3 / 4 , and 1, respectively, are supported. The throughput depends on the coding scheme used: with CS-1, the maximum throughput is about 9 kb/s, whereas with CS-4, it is 21.4 kb/s. Because a user may be assigned all eight slots of a frame, the per-user throughput may be in excess of 160 kb/s. GPRS Network Architecture Figure 5-13 is the architecture of a general GPRS network. The interface points between different elements of the network have also been indicated. To see the difference between a GSM and a GPRS network, compare this figure with Figure 5-2. Notice that in GPRS, there are only two new entities: ■ Serving GPRS Support Node (SGSN) As the name implies, the SGSN provides GPRS services to a mobile station in the serving 175 The GSM System and General Packet Radio Service (GPRS) area of its associated MSC. A PLMN may have more than one SGSN, in which case, the SGSNs are connected together over an IP-based Gn interface. Two different PLMNs, on the other hand, are connected over a Gp interface. A serving GSN connects to a gateway GSN via a Gn interface and to its BSS over a Gb interface that uses the Frame Relay protocol at the link layer. An SGSN node locates mobile stations subscribing to GPRS services and adds this information to the HLR. Another function of the SGSN is to control user access to the network by performing authentication using the same encryption keys and algorithm as in GSM. Optionally, it can also perform signaling and control for non-GPRS services. For example, it can support short messaging service over a GPRS radio channel and efficiently process paging messages and mobile location information required in GSM circuit-switched calls. Chapter 5 176 HLR MSC A SGSN 1 PSTN/ISDN PDN1 (e.g. The Internet) GGSN Gb Gs Gn Gi Another PLMN Gp Gr Gc PDN2 Gi SGSN 2 Gn To other PDN o o To other SGSN BSS VLR SMS- GMSC Gd D CE TE MT Um R PLMN GGSN Figure 5-13 The architecture of a GPRS network ■ Gateway GPRS Support Node (GGSN) GGSN provides an interface between a GPRS network and any external network such as a packet-switched public data network (PSPDN). Thus, as an example, whenever a PSPDN has a packet to send to a PLMN, it comes first to the GGSN. The gateway GSN contains the routing information of all mobile stations attached to it and forwards an incoming packet appropriately en route to its destination. It may request information from an HLR or provide information to the HLR when necessary. Both SGSN and GGSN have IP routing functionality, and as such may be connected together by an IP router. In the current version of cellular systems (that is, 2Gϩ), GPRS is supported by adding packet-handling capabilities to the base station controller. This is done by means of an interface called packet control unit (PCU) as shown in Figure 5-14. In a fully evolved 3G system, the interface to a GPRS network would be integrated into the UTRA BSS. GPRS Protocol Stacks The GPRS protocol stacks required in a mobile station, BSS, SGSN, and GGSN, are shown in Figure 5-15. Although the networking 177 The GSM System and General Packet Radio Service (GPRS) HLR MSC SGSN PSTN/ISDN The Internet GGSN BSC VLR BTS PCU Figure 5-14 Support of GPRS in 2Gϩ networks protocol is shown in the figure to be either IP or X.25, GPRS is fully capable of supporting applications based on any standard data pro- tocol. GPRS protocols at various layers are thoroughly described in Ref- erences [14] — [22]. Here, we provide only a short description of the protocol at each layer: ■ Subnetwork Dependent Convergence Protocol (SNDCP) SNDCP, which, in the protocol hierarchy, lies between the network layer (that is, IP/X.25) and the LLC layer, takes the network layer PDUs (corresponding to different protocols) and converts them into a format that is suitable for transmission over the underlying radio interface network. For example, if the protocol at the layer above it is IP, the SNDCP will take the IP packet, compress its header, and pass it to the LLC layer. Similarly, when it receives a packet from the LLC layer, it may decompress the header and pass it to the IP layer. User packets may have variable lengths and are segmented, if necessary. Both acknowledged and unacknowledged data transfer is possible. Other functions performed at this layer include ■ Data transfer using negotiated QoS profiles ■ Security and encryption of user data and control to provide protection against eavesdropping ■ Logical Link Control (LLC) The data link layer at the mobile station (the Um reference point) consists of two sublayers: the Chapter 5 178 MS BSS SGSN GGSN Um Gb Gn Gi Application IP/X.25 SNDCP LLC RLC MAC GSM Physical Layer Physical Layer RLC MAC GSM Physical Layer RLC MAC RLC MAC Physical Layer Frame Relay Relay BSSGP Physical Layer Frame Relay BSSGP LLC SNDCP L2 IP TCP/UDP GTP Physical Layer L2 IP TCP/UDP GTP IP/X.25 Relay Figure 5-15 GPRS protocol stacks at a few reference points upper sublayer known as LLC and the lower sublayer consisting of a radio link control (RLC) and a MAC sublayer. The LLC sublayer is based on the link access procedures of the ISDN D channel (LAPD) and supports procedures for the following: ■ Unacknowledged data transfer. The Frame Relay protocol is a subset of LAPD procedures using the unacknowledged information transfer mode. ■ Acknowledged data transfer. ■ Flow control. ■ Error recovery using sequence numbering in the acknowledged transfer mode. ■ Ciphering of logical link PDUs in both acknowledged and unacknowledged transfer modes. ■ RLC The RLC protocol provides a reliable transmission of data blocks over the air interface using a selective automatic repeat request (ARQ)-type procedure, where data blocks received in error are retransmitted by the source. ■ MAC The MAC sublayer controls access of the physical medium by mobile stations using a slotted Aloha scheme by resolving contention among multiple users or among multiple applications of an individual user and then granting the requested access in a manner that ensures efficient utilization of bandwidth. ■ GPRS Tunneling Protocol (GTP) In GPRS, address and control information are added to protocol data units so that they can be routed within a PLMN or between two PLMNs. The protocol that defines this process is known as the GTP. 5 Simultaneous 179 The GSM System and General Packet Radio Service (GPRS) 5 The term tunneling is used to mean encapsulating an original packet with a new header. Its use is quite common in packet-switched networks. Suppose that an IPv6 packet has to be sent over a network that is using the older IPv4 protocol. In this case, we could take the original IPv6 packet, add the IPv4 header to it, and send the result- ing packet over the network. That would be called tunneling. Another example is IP over ATM where an IP packet, when it first enters an ATM device, is encapsulated with an 8-octet header before it is sent out over ATM. operation of two modes is possible — unacknowledged mode for UDP/IP and acknowledged mode for TCP/IP. ■ Relay function It provides a procedure for forwarding a packet received at a node to the next node en route to its destination. In the BSS, LLC PDUs are relayed between Um and Gb interfaces. In the SGSN, packet data protocol (that is, IP and X.25) PDUs are relayed between interfaces Gb and Gn. ■ Base Station System GPRS protocol (BSSGP) The function of this protocol is to provide multiple, connectionless, layer 2 links and to transfer data, QoS-specifying parameters, and routing information between a base station and an SGSN. ■ Frame Relay This is the link layer protocol on the Gb interface. Data is transmitted over one or more permanent virtual circuits (PVCs). Frames received in error are discarded. The data link connection identifier is two octets long. The maximum frame size is 1,600 octets. The physical layer on the Um interface includes the typical, GSM radio link functions such as framing, channel encoding, inter- leaving, modulation, wave-shaping, synchronization, timing recov- ery, and so on. For a description of TCP and IP protocols, see Reference [10]. Figure 5-15 also indicates the need for protocol conversion at dif- ferent points in the network. For example, consider the serving GSN. After receiving a packet from the base station system, it must terminate the five lower layers — physical, frame relay, BSSGP, LLC, and SNDCP — and retrieve the network protocol data units (PDUs). These IP/X.25 PDUs must then be encapsulated in GTP, TCP/UDP, IP, and L2 in that order and sent out over its physical layer to GGSN. Packet Structures The packet structure at each layer of the Um interface is shown in Figure 5-16. PDUs received from the IP or X.25 layer for transmis- sion over the air interface are segmented at the SNDCP layer into smaller packets and passed to the LLC layer where a header and Chapter 5 180 frame check sequence are added to each segment. The maximum size of the LLC data unit is 1,600 octets. Each LLC PDU is further seg- mented, if necessary, into smaller blocks before passing it to the RLC/MAC layer. To each of these blocks are added an RLC header, a MAC header, and a block check sequence (BCS). The resulting frame, after the usual physical-layer processing, is sent out in normal bursts, each consisting of 156.25 bits, of which 114 bits are from an RLC/MAC PDU. Logical Channels Broadly speaking, there are three types of logical channels for trans- mitting packets in GPRS. They are packet broadcast control channel, packet common control channel (PCCH), and traffic channels. Some operate only on uplinks, some on downlinks, and the rest on both uplinks and downlinks (that is, they’re bidirectional). Uplink Channels Packet Random Access Channel (PRACH) This is a common control channel and is used by a mobile station to start a packet transfer process or respond to a paging message. 181 The GSM System and General Packet Radio Service (GPRS) Network Layer PDU Header + User Data + CRC if needed Segment 1 Segment 2 Segment N o o o Header FCS LLC Data Unit LLC PDU SNDCP Layer o o o Segmentation Segmentation 1 n o o o RLC/MAC Data (or Signaling) BCS RLC/MAC PDU Coding, Interleaving, etc. Physical Layer Transmitted over Air Interface in Bursts RLC HeaderMAC Header Figure 5-16 Packet structure at different protocol layers at the Um interface Downlink Channels Packet Broadcast Control Channel (PBCCH) It broadcasts system-specific parameters to all mobile stations in a GPRS serving cell. The following are common control channels: ■ Packet Paging Channel (PPCH) The GPRS network uses this channel to transmit paging messages before sending user packets. ■ Packet Access Grant Channel (PAGCH) When a mobile station wants to initiate a data transfer, it transmits a Packet Channel Request message on a PRACH or on a RACH in the absence of a PRACH. In reply, the base station sends a Packet Immediate Assignment message on a PAGCH, reserving one or more packet data transfer channels for that mobile station. Similarly, the network may send on this channel a resource assignment message to a mobile station. ■ Packet Notification Channel (PNCH) This channel is used to notify a group of mobile stations prior to sending packets to those stations in a point-to-multipoint fashion. Bidirectional Channels A Packet Data Transfer Channel (PDTCH) is allocated to a mobile station for transferring their data packets. A given user may request, and be granted, more than one PDTCH. A Packet Associated Control Channel (PACCH) carries signaling information, such as an acknowledgment (ACK), in response to a data block transfer, a resource assignment message in response to a resource request, or power control information. Only one PACCH is assigned to each mobile station, and is associated with all packet data transfer channels that may be allocated to that station. Logical channels are multiplexed at the MAC layer onto physical channels on a block-by-block basis. Physical channels used for GPRS packet data transmission are known as packet data channels (PDCH). Packet Transmission Protocol Multiple users may transmit packets on a PDCH on a time-shared basis. Each PDCH consists of one time slot of a TDMA frame. How- Chapter 5 182 ever, a mobile station may be assigned up to eight PDCHs for packet data transmission. A cell may permanently set aside a fraction of its available physi- cal channels exclusively for packet data transmission and the rest for the usual voice traffic. Alternatively, it may use a dynamic allo- cation scheme whereby one or more channels out of its available pool of channels are allocated to packet data transmission on a demand basis, and are deallocated and returned to the pool when there is no longer any need for them. The number of packet data channels active at any time depends on the number of simultaneous users and the volume of traffic generated by each user. However, there must be at least one PDCH to enable transfer of control and signaling informa- tion (as well as user data if necessary). It is not necessary that the same PDCH be used to send packets to/from a given mobile station. Multiple users transmit on a PDCH using a slotted Aloha, multi- ple-access reservation scheme. In the event of transmission errors, an ARQ protocol is used that provides error recovery by selective retransmissions of RLC blocks. To this end, GPRS employs the con- cept of a temporary block flow (TBF), which is actually a physical connection between a mobile station and the network, allowing the transfer of RLC/MAC blocks. 6 Each RLC data block or RLC/MAC control block includes in its header a temporary flow identifier (TFI) that indicates the TBF to which the block belongs. 7 Furthermore, all downlink RLC/MAC blocks contain in their header an uplink state flag (USF) that indicates which mobile station (or application) can use the next uplink RLC block on the same time slot. In this way, dif- ferent mobile stations may be multiplexed on the same PDCH when necessary. A mobile station transfers packets to an SGSN following the state diagram of Figure 5-17. The corresponding state machine represen- tation of an SGSN is similar. In the IDLE state, a mobile station may select or reselect a cell, but its location or routing information is not available to the SGSN. 183 The GSM System and General Packet Radio Service (GPRS) 6 It is temporary because it exists only as long as there is an RLC/MAC block to send and is removed when it is no longer needed. 7 On any PDCH, the same TFI may be used in the uplink and downlink directions. Similarly, different PDCHs may use the same TFI. [...]... location and routing information of the mobile In the STANDBY state, the mobile station is still GPRS-attached and sends the SGSN its location and routing information periodically and each time it moves into a new routing area (RA) While in this state, it can transmit a PDU and then transition to the READY state The packet transfer procedure when initiated by a mobile station is shown in Figure 5-1 8 The mobile. .. sensitivity Ϫ121 dBm for base stations and Ϫ117 dBm for UE at a bit error rate of 10Ϫ3 Power control steps 1, 2, or 3 dB for UE and 0.5 or 1 dB for base stations Maximum possible change in the transmit power level on TPC commands 26 dB for UE and 12 dB for base stations Data rates 144 kb/s in rural outdoor, 384 kb/s in urban/suburban outdoor, 2 Mb/s in indoor or low-range outdoor W-CDMA system features... System (UMTS) Wideband Code Division Multiple Access (W-CDMA) , which is based upon a direct-sequence CDMA (DS-CDMA) technology and operates in the frequency division duplex (FDD) mode The other is the UMTS TDD system, which is based on time-division CDMA (TD-CDMA) principles The purpose of this chapter is to present an overview of the W-CDMA UMTS system as specified in the ETSI standards documents [1]—[40]... consult them for greater detail System Features The UMTS operates in two modes—FDD and Time Division Duplex (TDD) In both modes of operation, the information is transmitted usually in 10 ms frames In FDD, two distinct frequency bands, separated by a guard band, are used—one for the uplink and the other for the downlink transmission In TDD, on the other hand, the same frequency band is used for transmissions... Universal Mobile Telecommunications System (UMTS) Copyright 2002 M.R Karim and Lucent Technologies Click Here for Terms of Use Chapter 6 190 As we indicated in Chapter 1, “Introduction,” the European Telecommunications Standards Institute (ETSI)/Special Mobile Group (SMG) developed two standards for International Mobile Telecommunication in the year 2000 (IMT-2000) One of them is the Universal Mobile. .. continue in the READY state for a certain length of time that is marked by starting an associated timer As the timer is running, the network has the capability to preempt the timer and force the mobile station into the STANDBY state When the timer expires, the mobile station changes to the STANDBY state While in the READY state, the mobile station may power down by performing a GPRS-detach procedure It then... allocation FDD mode: 1850—1910 MHz for uplink, 2110—2170 for downlink TDD mode: 1900—1920 MHz and 2010—2025 MHz Each of these bands for the TDD mode is used for both uplink and downlink transmissions Channel spacing 5 MHz Center frequency Integral multiples of 200 kHz Separation between uplink and downlink frequency bands 134.8—245.2 MHz Chip rate 3.84 Mc/s Modes FDD and TDD Transmitter power output... IDLE Timer Expiry or Forced to Standby READY GPRS Detach STANDBY PDU Transmission AM FL Y In other words, it is not attached to the mobility management function, and therefore cannot receive or originate a call When the mobile station establishes a logical link to an SGSN, it enters the READY state The mobile is now attached to the mobility management function and can initiate a mobile- originated call... designed to carry paging messages for mobile- terminated calls The physical layer transmits the information over one or more cells I Forward Access Channel (FACH) This is a downlink transport channel and carries control information such as an acquisition indication from a base station after a UE has randomly selected an access slot on an available random access channel and transmitted a preamble on that... defined on both uplinks and downlinks, are available in both FDD and TDD modes, and carry higher-layer data and control information such as voice, video, handoff orders, and signal measurements destined to any desired user Common Transport Channels I Broadcast Channel (BCH) This is a downlink transport channel that carries UTRAN system and cell-specific parameters Summary of transport channels Transport . reservation request. It supports both IP and X.25 protocols and real-time as well as non- real-time data. Both point-to-point and point-to-multipoint commu- nications are possible. There is no restriction. location and routing information of the mobile. In the STANDBY state, the mobile station is still GPRS-attached and sends the SGSN its location and routing information periodi- cally and each. timing recov- ery, and so on. For a description of TCP and IP protocols, see Reference [10]. Figure 5-1 5 also indicates the need for protocol conversion at dif- ferent points in the network. For example,