The Roots of GPRS:
`
`The first System for Mobile Packet based
`Global internet Assessi
`
`Waike B H.
`
`Electrical Engineering and information Technology, RWTH Aachen University, ComNets Research
`Group, Kopernikusstrasse 5, Aachen, 52062, Germany.
`walke@comnetsrwth-aachende
`
`Keywords: GPRS, General Packet Radio Service, GPRS Fundaments, Enabler of Mobile
`
`Internet Access, Multiple-Access Protocol, EDGE, Packet~SWitched Cellular Radio Network.
`
`Abstract
`
`GPRS, the General Packet Radio Service in GSM was the enabler of the mobile lnternet.
`
`The origins of key radio access functions employed for packet-switching in GPRS are
`identified by reviewing state-of—the-art on random access protocols applied in cellular radio
`data networks existent or proposed before GPRS specification started. A table is provided
`
`showing the degree of conformance to GPRS of the respective systems. Besides the type of
`demand assigned multiple access protocol used in a system, dynamic placement of control
`channels to the packet data channel and statistical multiplexing of fractions of [P packets of
`simultaneously transmitting mobile stations to the same packet data channel appear to be
`key differentiators, besides others. CELLPAC by comparing its functions to that of GPRS is
`shown to comprise what
`is called here the Fundaments of the GPRS Radio interface
`
`Protocol. The history of ETSI GPRS standard development is described. Although GPRS is a
`result of cooperation of many actors which contributions are valued,
`it appears possible to
`identify the roots of its radio access protocol and thereby main contributors.
`
`1. Introduction
`
`The General Packet Radio Service (GPRS) was launched worldwide in 2001 as a service
`
`provided by the Global System for Mobile (GSM) to provide mobile internet access. Later,
`adaptive modulation and coding for higher data rate was introduced to GPRS under the
`
`leaving the access protocol
`name Enhanced Data Rate for GSM Evolution (EDGE),
`unchanged. Concepts enabling packet data communication in cellular radio networks were
`
`kept and further developed from GPRS/EDGE when specifying 3G Universal Mobile
`
`Telecommunications System (UMTS) and 4G system Long Term Evolution (LTE).
`
`1.1
`
`Early Concepts for Wide Area Mobile Data Networks
`
`The architecture of a Public Land Mobile Network (PLMN) is shown in Figure 1', where the
`Access Network (AN) is made-up from Mobile Stations (MSs) connected to the Base Station
`Subsystem (BSS) across the Radio Interface (RI). The BSS is part of both AN and Core
`Network (CN), and comprises multiple Base Stations (885) each serving a radio cell
`
`1IEEE Wireless Communications, October 2013
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`1536—1284/13/S25DO © 2013 lEEE
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`connected star-shaped to a Base Station Controller not shown in the figure.
`
`In the core
`
`network, mobility supporting functions are found like Subscriber Register (SR) responsible for
`roaming, authentication and billing of M85, and switching nodes dedicated to circuit— and
`packet-switched services, respectively. Gateway Circuit— I Packet-Switched Exchange nodes
`hosting Interworking Functions (lWFs) shown in Figure 1
`interface to external networks to
`connect a MS to MSs of other PLMNs and to fixed subscriberterminals.
`
`PLMNs support roaming where the MS’s current location is stored in SR so that an incoming
`call can be routed to a MS. Roaming requires the MS to update SR when entering another
`cell not belonging to the location area of the previous cell. Advanced PLMNs besides
`roaming also support handover for keeping service quality of a MS when communicating on
`the move. Handover provides continuation of communication within and across cells with
`small service interruption, only.
`
`Roaming of movable wireless terminals (WTs) connected directly by protocol lEEE 802.11
`WLAN to the Internet is provided by Mobile Internet Protocol versions 4 (Mva4) and MIPVS.
`Since lnternet access routers typicatty do not provide cellular radio coverage, roaming of
`WT5 is supported only when associated to an access router and handover of WT3 is not
`provided at all. Therefore, wireless networks are not considered to be mobile networks.
`
`Access Network
`
`Care Network
`
`Other Networks
`
` Radio
`
`
`
`
`,
`Interface .
`
`888 = Base Station Subsystem (Base Station (BS) plus BS Controller);
`
`CSXIPSX = Circuit-fPacket—Switched Exchange; MS = Mobile Station;
`
`GCSX/GPSX = Gateway CSX/PSX;
`PSTN = Public Switched Telephone Network;
`PDN = Public Data Network;
`
`SR = Subscriber Register;
`PLMN = Public Land Mobile Network;
`
`Figure 1: Generic architecture of a cellular mobile radio network (PLMN)
`
`The network elements shown in Figure 1 have its own protocol stack for both control and
`user data exchange. PLMNs differ much in the protocol stacks used at the RI but extensively
`rely on fixed network protocol stacks known from PDNs. What is PLMN specific are network
`elements for mobility management in the core network and the protocol stack at the RI. The
`focus in this study is mainiy on the protocols applied at the RI in the access network.
`
`Mobile stations having data to send will request transmission at random times. Since MSs
`
`have no knowledge of each other’s existence or status, management of the mobile random-
`access to the uplink (UL) channel by multiple concurrent M85 is a major challenge in radio
`access protocol design. Aloha and slotted (S) Aloha are the simplest multiple~access
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`protocols to a mobile radio channel, but these are considered inefficient when used for data
`
`transfer, where MSs contend directly with their data messages. in [1] it is shown that radio
`access protocols that combine S—Aloha request channels with separate traffic channels can
`achieve very high utilisation in a stable way. Typically. a request channel has only to transmit
`small amounts of control data and so requires a small bandwidth compared to the user data
`channels. If sufficient bandwidth is allocated to the request channel for it to operate stable (at
`low utilisation) then the data channels may be operated at high utilisation. This is the reason
`why modern mobile radio networks provide random access control channels besides traffic
`channels (TCHs) to carry speech and user data transfer.
`
`1.1.1 Mobile circuit-switched data networks
`
`Mobile networks originally were designed for circuit-switched speech communication and
`later offered data as an add-on. A simple form of mobile data communication is data
`
`transmission using modems over analog cellular
`telephone links.
`In
`this
`form of
`communication, the mobile user accesses a cellular channel just ashe would in making a
`standard voice call over the cellular network. Mobile terminals typically operate at 9.6 — 14.4
`kbitls data rate using error correction protocols like MNP—tO, v.34 and v.42 for reliable data
`transmission. Modem based circuit—switched transparent data service was provided by
`analog and digital cellular networks, e.g., ElA—553 AMPS and ETSI GSM shortly after start of
`the respective network. The user then operates the modem just as would be done from office
`
`to office over the PSTN. In this form of communication the network is not actually providing a
`data service but simply a voice link over which the mobile data modem can interoperate with
`a corresponding data modem in an office or computer center.
`
`A data modem uses a traffic channel on the RI in the same way as the voice service.‘A traffic
`channel for exclusive use for the data transfer of one mobile user is reserved when the data
`
`arrives.
`
`It will be released when the data message is transferred. This traffic channel
`
`is
`
`estabiished between the MS and the interworking Function (IWF) located in the GCSX in
`Figure 1.
`
`Data transmission on top of an underlying cellular telephone service has limitations imposed
`by the characteristics of the voice—circuit connection. The service might be cost effective it
`long data files are transmitted on a connection. However, the service is costly if only short
`messages are exchanged over the network during a (long) session supporting an interactive
`service, where the circuit-mode connection is mostly unused but charged by the operator.
`This is the reason for development of mobile data networks that apply end-to-end packet
`switching based on, e.g., X25, IP or proprietary protocols.
`
`1.1.2 Mobile packet-switched data networks
`
`Mobile packet-switched networks enable MSs to exchange packet data over radio. Besides
`
`stand-alone networks there exist packet—switched networks integrated to circuit—switched
`networks occupying some of its radio channels.
`
`Before work started to specify GPRS in 1993, a number of concepts were known for packet
`or message switching in a mobile radio network as discussed in the following. But first,
`multiple-access (MA) protocols to a request channel are introduced.
`
`1.2 ALOHA, SwALGi-EA, DAMA
`
`The birth of mobile radio and MA to a radio channel dates back to 1897 when Marconi was
`
`credited with the patent for wireless telegraph. Marconi MSs mounted on ships, sharing the
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`same radio channel were the first to contend to a shared channel for transmitting a sequence
`of Morse coded telegraphy characters. Like with the Aloha protocol Marconi MSs repeat
`transmission if no response is received to a message sent.
`
`ln 1970 the ALOHANET was opened to connect multiple low data rate stations through a
`single radio channel to a central host. For that purpose the MA-protocol Aloha [2] was
`designed, where stations transmit their data packets at random times. Under Aloha the
`station having a data packet ready transmits it on the channel to the central host without
`
`considering any synchronisation or access rule. The packet also contains identification,
`
`control and parity check information. Packets sent by different stations may partly overlap
`and collide at the receiver. A station waits for a timeout to happen or for receiving an
`acknowledgement from the central host. After time-out the packet is retransmitted after a
`random pause interval. This process is repeated until successful transmission or until the
`
`process is terminated by the station. The randomly transmitted Aloha packet is a user data
`message.
`it
`is not a signalling message to prepare for packet data exchange.
`In [2]
`it
`is
`shown that the effective channel capacity is 1i(2e).
`
`The S-Aloha protocol proposed 1972 is applied to a time-slotted channel and thereby
`doubles channel capacity [3]. Stations apply the Aloha protocol but in addition are required to
`synchronise their packet transmissions into fixed length channel time slots. Thereby, partial
`overlap of packet transmission of different stations is avoided.
`
`Most cellular radio data networks assign radio channels to MSs based on a demand-
`
`assigned multiple-access (DAMA) protocol {1] where an UL request channel is shared by
`many MSs through contention based on S—Aloha. A data channel is assigned by the BS in
`response to a successful request and the requesting MS will start to use the channel
`assigned for the duration of its data communication.
`
`With the DAMA protocol, user data on UL may be transmitted outband (U0) on a TDMA
`channel different from the shared request channel, or inband (Ui) on the shared channel.
`
`Cellular systems based on DAMA protocol require, besides time-slotting, the channel to be
`' organised in TDMA frames so that slots can be identified by their position in a frame.
`If the
`
`frame length is longer than the maximum channel propagation delay, each MS can be
`informed of the status of each time slot of the preceding frame. A slot in the frame provides a
`TDMA channel which may be used as a control or packet data channel (PDCH).
`
`DAMA based systems with explicit reservation in response to a request sent on a contention
`channel assign an UL TDMA channel for packet data transmission by explicit communication
`to the MS via a BL control channel. The PDCH typically is then different from the contention
`channel.
`
`is
`request by an MS on a contention channel
`With implicit reservation a successful
`acknowledged by the BS on the corresponding DL channel. This results in an automatic
`
`reservation of the same channel used for the request to be used also for user packet data
`transmission on UL. Accordingly, two DAMA types on DL are to differ: De and Di for explicit
`(e) and implicit (r) realization, respectively, of the DL control channel granting a MS a data
`channel. Further, the Di. control channel used to grant a MS a channel for UL packet data
`transmission may be realized outband or
`inband to the DL packet data channel
`corresponding to the potential UL data channel.
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`
`1.2.1
`
`RwALOi-EA and PRMA
`
`R—Aloha [4] and PRMA [5} are DAMA protocols type (iii, air). The RwAloha protocol was
`designed to connect MSs generating long multi—packet messages via transponder based
`satellite systems to a central host. The channel is operated without central control since MSs
`can hear each other.
`in cellular radio networks where MSs cannot hear the UL channel
`
`central control by the BS is required to inform MSs via a broadcast control channel on the
`status of each slot of the forthcoming UL frame.
`
`The PRMA protocol is widely known, although not implemented in a real system. There the
`DL control channel is assumed able to immediately broadcast to all MSs the status of an UL
`slot in a preceding frame, UL slots broadcast by the BS to be “available” for random access
`
`in a frame may be accessed by an MS. Collisions of M85 are resolved by back-off and
`repeated transmission. A successful MS is confirmed by the BS to use the slot that it had
`
`used for MA for data transmission as a TDMA channel in the next and subsequent frames
`until the MS‘s data expire.
`
`1.3 Early Mobile Packet Data Networks
`
`The most important early packet data networks discussed in the following were closed after
`
`GSM/GPRS started its operation in the respective regionicountry.
`
`full—duplex wide area packet-
`The Advanced Radio Data Information Service (ARDIS)
`switched cellular radio service of Motorola and [BM that is based on Motorola DataTAC was
`
`launched in 1983 in large US cities [6]. The service connects MSs by radio under control of
`the proprietary Radio Data (RD) Link Access Procedure (LAP) offering 8kbit/s user data rate.
`RD-LAP covers lSO/OSi network (layer—3) and link layer (layer-2). Connectionless and
`connection-oriented communication based on virtual circuits is supported. Mobiiity and radio
`resource management is provided covering roaming but not handover. RD-LAP layer—2
`provides ARQ and access control at the R! by the Digital Sense Multiple Access (DSMA)
`protocol. With DSMA the BS provides in each DL slot, besides user data for a MS addressed
`in a slot, the channei status symbol (088) indicating whether the slotted UL channet is idle or
`
`busy. Free UL channels are used in contention mode to transmit a request packet. if a MS
`has data to transmit,
`it randomly waits up to 50 ms before it reads out the CSS.
`if 088
`signals an idle UL channel, the MS transmits immediately its data as RD-LAP blocks, 12 byte
`each, resulting in a message of up to 512 byte transmitted. If the channel was detected busy,
`the MS waits for a random time—duration and then again looks for the value of the 088. A
`
`coliision during contention to the UL is resolved by a random back—oft time until the MS
`
`retries again. During transmission of RD-LAP blocks by a M8 the receiving BS transmits
`CSS=busy information on DL. DSMA is a DAMA {tit De!) protocol. Packet data is transmitted
`by concurrent MSs one-by-one (see Figure 2) where one common traffic channel of a cellular
`radio system is alternatingiy used as a PDCH by two MSs to transmit data packets with some
`idle gaps in between. The other common traffic channels may also be used as PDCHs or
`may be used for circuit-switched services.
`
`The MOBITEX packet data service for digital speech and data communication developed by
`Swedish operator Teiia and Ericsson was first
`launched in 1986 in Sweden providing
`country-wide cellular data services supporting roaming but not handover. Since in US the
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`system was introduced by RAM Mobile Data in 1990 it is also known as RAM Packet Data
`
`Network. The RI data rate is 8 kbitfs half-duplex supporting fiies of up to 20 kByte. The
`network layer supports datagram transfer by the proprietary protocol MPAK and the link layer
`provides ARQ. Access to the shared radio channel is by a DAMA protocol type (tit, riled)
`cailed Reservation TDMA. The BS on DL of the RI provides the number of slots of the FDMA
`channei available for random access [7}. A iVlS randomly picks a slot to transmit an access
`
`request on UL while the BS may send DL traffic. At the end of the period reserved for
`random access,
`the BS grants permissions to MSs one-by—one resulting in sequential
`transmission of data of concurrent MSs, see Figure 2.
`
`f
`
`Common control
`
`_
`
`Figure 2: Packet channel (PCl-i) reserved for duration of packet transfer of single user
`
`The COGNETO cellular mobile packet switching network was operated until 2003 in UK for
`
`datagram transfer before it was replaced by GPRS {8]. MSs may transmit in slots or minislots
`(four to a slot). 64 byte user data are carried in a siot. Minisiots are used for contention on UL
`and acknowledgement on DL. Periodic slots in the TDMA frame on UL and DL are dedicated
`
`by means of the Slot Map to be control or data channels. Random access is by S-Aloha to a
`control channel and collisions are detected by MSs from absence of an acknowledgement.
`The BS will acknowledge a request on UL and direct the MS to a free UL slot (TDMA channel
`
`to transmit its user data. This DAMA protocol is type (Uo, Dec). MS are served one-by—one,
`see Figure 2.
`
`1.4 Cellular Radio integrating circuit- and packet-switching
`
`1.4.1 Concepts not implemented
`
`to integrate circuit-switched digital
`Local Cellular Radio Network (LCRN) [9} is the first
`speech/data and packet-switched services in a mobile radio system based on FDMNTDMA
`
`channels. Virtual connection and datagram service are supported. Some TDMA channeis are
`provided for control and others for packet data transfer. S-Aloha is used for MA in a DAMA
`
`(U0, Dec) protocol. The trunk of TDMA channels is dynamically assigned according to needs
`to circuit- and packet-switched services.
`
`radio system integrating circuit— and packet-switched data transmission is
`A cellular
`introduced by Ken Felix [10] where channels can be used for voice, dedicated data or
`
`packet—switched data. Extensions to the signaiing standard of US digital cellular phone
`standard (1993) TIA lS-54 are proposed to enable a mobile packet service. MSs transmit on
`a packet-switched radio channel one by one, see Figure 2. Access to the UL channel is
`
`eitherthrough polling IVISs by the BS, or by a MA protocol not specified in detail in [10], which
`appears to be DSMA.
`If polling is used, random access is switched off and the protocol is
`then not MA at all.
`
`Improvements to PRMA are proposed in [11] by Mitrou (MLP) for an integrated system
`supporting both circuit— and message-switching voice and data. Siots of a TDMA-frame are
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`dedicated to be control or data channels as known from PRMA and COGNITO. UL control
`
`slots used for MA are subdivided into minislots to each carry a miniburst request message.
`Once a request miniburst was successful, the MS is assigned by the BS a periodic packet
`data slot for the duration of its data transmission. The protocol is DAMA (Uo, Lied). MSs are
`served one by one as shown in Figure 2.
`
`1.4.2 Systems Implemented
`
`The cellular digital packet data (CDPD) service was specified in 1993 as an overlay to the
`advanced mobile phone service (AMPS) [12] to provide 19.2 kbit/s data rate. Some FDM
`
`channels of AMPS carry the connection~less CDPD service. MA at the RI by DSMA protocol
`prepares transmission of up to 64 blocks each 54 Byte without multiplexing data blocks of
`concurrent stations.
`
`Standards TIA IS-54 and TIA IS-95 specify a three-slot per TDMA frame and a CDMA (code
`division multiple access cellular radio system, respectively. Like ETSI GSM, around 1992
`
`these networks were prepared to carry circuit-switched data services besides speech. Data
`services were offered from about 1993194 on, where a channel is dedicated to a point-to-
`point connection. Since many mobile data applications generate bursty traffic, market
`acceptance of the service was low. In all these systems a channel is shared by MSs on a call
`by call basis. A DAMA (Uo, Dec) protocol is used to provide circuit—switched data service.
`
`2. CELLPAC a first Version of GPRS
`
`To ease understanding of GPRS, the Fundaments of the GPRS Radio Interface Protocol
`
`(“GPRS Fundaments’) are introduced in the foliowing with reference to the roots where the
`respective functions were proposed first. The first full GPRS specification Release ‘99
`provided in 200 kHz bandwidth a symbol rate of271kbitls resuiting in 22.8kbiUs data rate of a
`full-rate TDMA traffic channel (TCH). Multi-slot operation is an option.
`In a later GPRS
`Release (EDGE) the data rate of a TCH increased to 69kbit/s.
`
`It appears that most GPRS Fundamenrs have been first proposed for CELLPAC [13] - [15]
`introducing packet—switching in GSM. In what follows the CELLPAC functions are explained
`and compared to GPRS and to other systems known earlier. Table 1 (discussed later)
`summarizes the results.
`
`GPRS is based on a new protocoi for radio access and on provisions introduced to the GSM
`core network to enable packet data transmission [16]. Since packet-switched data networks
`
`and IP tunneling were known when GPRS was designed, the hardest part
`GPRS was to introduce
`
`in designing
`
`(1) Packet radio access of GPRS enabled M85 without changing GSM layer-1
`functions implemented in hardware.
`
`(2) A protocol suite for the network elements of the access and core networks to
`support packet-switching.
`
`2.1 Protocol Suite
`
`Protocoi stacks for network elements required for packet—switching did not exist in GSM [18}.
`Figure 3a shows the protocol suite with a protocol stack per network element as introduced
`in [15], which is close to GPRS, see Figure 3b.
`It
`is worth noting that layer-2 at the radio
`
`in both protocol stacks
`layer (iayer—i),
`interface (RI) Um running on top of GSM physical
`shows two sub-layers, namely Medium Access Control (MAC) in Figure 3b, called ‘Packet
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`Access: in Figure 3a, and Radio Link control (RLC) in Figure 3b called Radio Link Protocol
`(RLP) in Figure Be. There the MS is split into data terminal equipment (DTE) and mobile
`terminal (MT).
`
`in Figure 3a called
`‘Packet Access’ protocol data units are transmitted across the RI
`RLCIMAC data block in GPRS, see Figure 8. The GPRS stack compared to that of
`CELLPAC is further optimized to contain the Logicai Link Control (LLC) protocol, and the
`
`Sub-network—Dependent Convergence Protocol (SNDCP), both operating between MS and
`SGSN, not affecting the RI.
`In network layer (layer-3) ITU-T protocol X25 is used in both
`CELLPAC and GPRS, besides lP. During specification of GPRS Rel.’99 it turned out that iP
`
`would be the major network layer protocol. An X25 like virtual connection established during
`association of a MS to GPRS was kept to allow for fast link establishment of a MS having
`data ready to send. The virtual connection later was specified as semi-permanent, thereby
`providing the ‘Always On’ property of GPRS.
`
`The protocol suite proposed in [l5] was foilowed by the GPRS standard at the Rl and in part
`in the core network.
`
`MSC/IWF
`
`
`
`Um
`
`ISBN/PSDN
`
`a) PSDN = Packet Switched Data Network, MSC/IWF 2 Mobile Switching Center/Interworking
`Function
`
`
`
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`_;Z;‘ Scope of GPRS
`
`b) 388 = BS Subsystem, SGSN = Serving GPRS Support Node, GGSN = Gateway GPRS SN
`Figure 3: Protocol Suites for CELLPAC (a) and GPRS (b)
`
`(32 kbit/s) but Internet application
`Since the packet data rate in early GPRS is small
`protocols like SMPT, HTTP, etc. should be supported,
`the Wireless Application Protocol
`(WAP) was specified by the WAP Forum to enable ‘Thin Clients' with small screens to run
`
`Internet applications on [VISs with low processing power across low rate data links [20].
`
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`GSIWEDGE with its data rate of up to 384kbiti’s on smart communications devices enables
`Internet application protocols more comfortable.
`
`2.2 GPRS as a Service in the Circuit~Swttched GSM Network
`
`GSM with its voice and data services was deployed when discussion started on how to
`
`reatize a packet radio service. Eight physical TDlVEA radio channels per FDlVl channel are
`provided in GSlVI per transmit direction, see Figure 4. A time-slot may carry a Normal Burst
`(NB) as used by GSM control and TCHs, or may carry a Random Access Burst (RAB) or
`other burst type. In [13] - [15] one (or more) time slot of the GSM TDlle-frame representing
`in GSM a physical TDMA channel
`is dedicated as a combined packet data and control
`channel, called the CELLPAC dedicated TCl—E. Thereby the GPRS Packet Data Channel
`(FDCH) is anticipated.
`
`Dedication of some TDMA channels of a circuit—switched cetlular network for packet—
`switching is known from [9], [10], [12], and [18} of which [9] was the first to propose this.
`
`
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`Figure 4: FDIVI and TDMA channels in GSM
`
`2.3
`
`PDCH Shared by Control and User Data
`
`Multi-frames as known from GSM but with different structure are introduced in [15] for both
`UL and DL of the slot used as a CELLPAC TCH. A CELLPAC multi—frame specifies the roles
`of a slot of a TOMA-frame over time to be either control channel or packet data traffic
`channel
`(PDTCl—i), see Figurefi. The CELLPAC multi—frame comprises 26 (35M TDMA
`frames. In vertical direction slots 0 to 7 represent the respective TDMA frame. TDMA frames
`
`13 and 26 of DL and UL, respectively, carry the fixed packet control channel (PCCH) on DL
`and the packet random access channel on UL. Slots on the DL may carry the PDTCH or the
`
`dynamic packet control channel (PCCH). UL slots may be reserved by the BS as PDTCH for
`a MS to transmit packet data or may be dynamicalty be dedicated for random access. GPRS
`specifies a 52 muiti-frame.
`
`Page 9 of 19
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`9
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`Figure 5: CELLPAC multi-frame. ‘Reserved’ means a slot is dedicated as PDTCH
`
`Both the GSM control channels for virtual connection setup and the CELLPAC dedicated
`TCH are shown in agate '6 [15}. The latter comprises logical control channels ('Access‘ and
`‘Control’) and voiceldata channels for packet data transmission. The logical channels
`mentioned correspond to Packet Random Access Channel
`(PRACl-l), Packet Common
`Control Channel (PCCCH) and Packet Data Traffic Channel (PDTCH) specified by the GPRS
`muiti-frame to be carried on the Packet Data Channel (PDCH) that is a frame-periodic slot.
`
`
`
`MobileStation
`
`occa/nnca
`
`
`
`dedicatedTCHlsl
`
`DccH/RACH
`
`
`
`BaseStation
`
`Voice/Data
`
`VoicelData
`
`dedicatedTCi-lls)
`
`Figure 6: GSM Dedicated Control Channel
`
`(DCCH) and Random Access Channel
`
`(RACH) used for virtual call set-up, and
`dedicated Traffic Channel (TCH) for data
`transmission in CELLPAC [15].
`
`No cellular pa met—switched system besides
`
`GPRS applies a mold—frame for mapping
`loglcal control and data channels to one
`
`TDMA channel as introduced in [t5].
`
`2.4
`
`Packet Data Context Establishment
`
`Association of a MS to the mobile network and packet data context (PDC) establishment
`before transmitting GPRS packets avoids that radio resources have to be kept reserved in
`circuit~switched mode during transmission gaps. In GPRS radio resources are only assigned
`by the BS when needed to receive/transmit packet data by an MS, whereby the PDC is
`referenced in packets transmitted. GSM control channels and GSM protocols are used in
`GPRS for association of an MS to the network. Before an MS may transmit/receive packet
`data, establishment of the PDC between SNDCP entities of MS and SGSN, see Figure 3b, to
`be extended by the GPRS Tunnel Protocol (GTP) to the GGSN is also required. The PDC
`specifies the functions of a tayer~2 logical
`link and relates it to the route of the respective
`layer-3 virtual connection.
`
`Page 10 of 19
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`Virtual connections based on X25 are introduced by CELLPAC [14], [15] to connect MS and
`
`IWF, see Figure 3a using GSM signaling, before data packets are transmitted on the packet
`dedicated TCH assigned. Context establishment before data transmission was known before
`GPRS.
`
`2.5
`
`Control of two M33 by one Di. Control Burst
`
`in Figures 3b
`The logical link between MS and 888 operated by the RLC/MAC protocol
`carries RLCIMAC blocks, see Figure 8 and is known as GPRS Temporary Block Flow (TBF)
`identified by the TBF Flow Identity (TFI) carried in the block. TFI is unique among concurrent
`TBFs in a cell and replaces the complete GPRS MS identity known as Temporary Logical
`
`in layer-2 segments of
`Link Identity (TLLI). Packet Transmission is performed in layer-3.
`packets are transmitted as RLC/MAC blocks that we cali packet data here.
`
`The control burst
`
`(CB)
`
`transmitted in CELLPAC on PCCH has two sub-bursts, each
`
`comprising (1) 1 bit for uplink state indication (USI), (2) 8 bit identification of the packet data
`context by a 'MS Random Number” (MRN),
`(3) 6 bit for time advance info for slot
`synchronization, (4) 5 bit for power level and (5) 1 Paging Bit (PB), see Figure: 7 [15]. The
`next and following UL slots for packet data transmission may be assigned to an M8 by a
`control sub—burst with {USI=1, Pi3=1}. An MS may be informed to receive in the next and
`
`following DL slots packet data by a control sub-burst with {USt=1, PB=O}K Further, a MS may
`be paged to show-up to the BS by setting {USl=0, PB=1} to transmit a random access burst.
`
`reservation for packet data in CELLPAC by setting {MRN, USI=1, PB=1}
`UL channel
`functionally corresponds to the Upiink State Flag (USF) in GPRS. DL channel reservation for
`
`in GPRS‘ RLC/MAC data
`packet data by setting {MRN, USiet, PB=O} corresponds to TFl
`blocks in GPRS carry both USF and TFI, see Figure 8. Data packets destined for MSs are
`queued by the-BS.
`
`HRChanneEl
`
`HR Channel 2
`
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`
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`
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`
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`
`PCCHaInfO: {1] Uplink State indication Bit
`{8] MS—Random Number
`{6] MSeTiming Advance
`{5] answer Level
`t1] Paging Bit
`
`'
`
`{21} for one HR Channel
`
`,y
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`we
`
`Packet Control Channel Burst
`(Normal Burst)
`
`\
`
`bit[5 set to 1 if bu rstIS not transmitted on the provided slot
`
`Figure 7: Packet Control Channel (PCCH) on DL of CELLPAC [15]
`
`A PCCI—i CB in CELLPAC may provide control to two different M83. The RLC/MAC header in
`
`GPRS, Figure 8, carries USF and TFI that also may address two different MSs.
`
`CELL Pitt g l5] anticipate scontlot of two {US$84store En: ccnfioimessage as used :1, (3533‘s
`
`Since the PCCH may be dynamically placed by the BS on any DL time stot of the CELLPAC
`
`multi-frame not reserved as a fixed DL