United States Patent c191
`Kanerva et al.
`
`I 11111111111111111111111111111111111111111111111111111111111111111
`US005793744A
`5,793,744
`[11] Patent Number:
`[45] Date of Patent:
`Aug. 11, 1998
`
`[54] MULTICHANNEL HIGH-SPEED DATA
`TRANSFER
`
`7/1991 Goldstein et al ....................... 370/477
`5,029,164
`5,566,208 10/1996 Balakrishnan .......................... 370/468
`
`[75]
`
`Inventors: Mikko Kanerva. Helsinki; Juba
`Risinen. Espoo; Harri Jokinen. Hiisi;
`Harri Honkasalo. Helsinki, all of
`Finland
`
`[73) Assignee: Nokia Telecommunications Oy, Espoo,
`Finland
`
`[22] Filed:
`
`[21] Appl. No.: 690,262
`Jul. 24, 1996
`Foreign Application Priority Data
`
`[30)
`
`Dec. 18, 1995
`Finland .................................... 956087
`[Fl]
`................................ H04J 3/16; H04J 13/00
`Int. CL 6
`[51]
`[52] U.S. Cl . .......................... 370/209; 370/342; 370/433;
`370/468; 371/32
`[58] Field of Search ..................................... 370/345, 347.
`370/348, 412. 417. 433,468. 476. 498.
`517,458.209, 342,441; 371/32. 33
`
`[56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`7/1978 Gindi et al. ·············••u••·········· 395/285
`
`4,103,336
`
`Primary Examiner-Benedict V. Safourek
`Attorney, Agent, or Finn-"JP Group of Pillsbury Madison &
`Sutro LLP
`
`[57]
`
`ABSTRACT
`
`A digital mobile communication system has a high-speed
`non-transparent data connection between a transmitting and
`a receiving party. For the data connection, parallel
`subchannels, corresponding in number to the nominal data
`transfer rate. have been allocated on the radio interface. A
`radio link protocol is responsible for transmitting data over
`the radio interface, and for acknowledging correct data
`frames and for retransmitting defective data frames. A
`transmission buffer buffers the data frames to be transmitted
`and stores the data frames transmitted until it receives an
`acknowledgement of successful reception. In order to reduce
`interference and power consumption. user data is transmit(cid:173)
`ted by using as many of the allocated subchannels as
`required by the actual user data rate at any one time. On the
`other allocated subchannels, transmission is interrupted or
`discontinuous transmission is applied.
`
`25 Claims, 9 Drawing Sheets
`
`DTX-CONTROL
`
`RADIO
`INTERFACE
`
`V.24
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`,TAF
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`63
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`MS
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`
`V.24
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`L2R
`
`Ex.1018
`APPLE INC. / Page 1 of 18
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`

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`U.S. Patent
`
`Aug. 11, 1998
`
`Sheet 1 of 9
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`Ex.1018
`APPLE INC. / Page 3 of 18
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`

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`U.S. Patent
`
`Aug. 11, 1998
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`Ex.1018
`APPLE INC. / Page 4 of 18
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`U.S. Patent
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`Aug. 11, 1998
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`U.S. Patent
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`Aug. 11, 1998
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`Ex.1018
`APPLE INC. / Page 8 of 18
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`

`

`U.S. Patent
`
`Aug. 11, 1998
`
`Sheet 8 of 9
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`5,793,744
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`Ex.1018
`APPLE INC. / Page 10 of 18
`
`

`

`1
`MULTICHANNEL HIGH-SPEED DATA
`TRANSFER
`
`5,793,744
`
`2
`channel for data or speech transmission. Thus. the GSM
`system. for instance. may have up to eight parallel connec(cid:173)
`tions to different mobile stations on a same carrier wave. The
`maximum data transfer rate on one traffic channel is
`restricted to a relatively low level according to the available
`bandwidth and the channel coding and error correction used
`in the transmission, for example in the GSM system to 12
`kbit/s, 6 kbit/s or 3.6 kbit/s.
`A digital mobile communication system typically uses
`10 several connection types which can be divided into two
`categories: transparent and non-transparent connections. On
`a transparent connection. data is transferred through a traffic
`channel of the mobile communication system in a transpar(cid:173)
`ent way, which means that error correction on the radio path
`15 is carried out by employing channel coding only. In the
`GSM system the channel coding is Forward Error Correc(cid:173)
`tion (FEC). A non-transparent connection uses. in addition to
`channel coding. an additional protocol in which the data
`transmission over the traffic channel is repeated in case the
`20 data was not received correctly at the other end. In the GSM
`system, this communication protocol is Radio Link Protocol
`(RLP). used between a terminal adaptor of a mobile station
`MS and an interworking function IWF. which is typically at
`a mobile services switching center MSC. The RLP is a
`25 balanced (HDLC type) data transfer protocol having a frame
`structure. F.rror correction by the RLP is based on retrans(cid:173)
`mission of frames corrupted on the traffic channel. There is
`another protocol, Layer 2 Relay (L2R). above the RLP. In
`the present patent application, the functional part of the T AF
`30 or IWF carrying out these protocols is referred to as an
`L2R/RLP unit.
`In a normal data transfer state, the L2R/RLP unit packs
`user data into 200-bit long protocol data units (PDU), which
`are transmitted in 240-bit RLP frames over the radio inter(cid:173)
`face to a second L2R/RLP unit. If there is no data or other
`information to be transferred between the two L2R/RLP
`units, discontinuous transmission (DTX) may be applied
`DTX refers to a method reducing transmission on the radio
`path to a minimum (i.e. interrupt the transmission) during
`pauses in the data transfer. The aim is to reduce the power
`consumption of the transmitter, a very significant matter for
`the mobile stations, as well as the overall interference level
`on the radio path, which has an effect on the system capacity.
`The DTX operates independently for the uplink and down(cid:173)
`link directions. The mobile communication network may
`either allow or prohibit the use of DTX.
`In normal L2R/RLP operation, the PDUs are possibly
`filled only partially, because the application may limit the
`maximum user rate below the maximum rate on a traffic
`channel. The PDUs may be full if the actual user data rate
`on the terminal interface is high enough or if, due to delays
`caused by re-transmission or some other congestion, the
`L2R/RLP buffer has enough user data to fill one PDU
`completely. It is also possible, depending on the
`implementation, to prefer full PDUs to partially full PDUs.
`This can be accomplished by using a timer or a counter to
`slightly delay the building of a PDU until there is enough
`data available (e.g. in a buffer) for a full PDU to be built, or
`until the building of a PDU, albeit only a partially full PDU,
`cannot be delayed any longer. The timer may have a typical
`value in the order of 20 ms, in other words the repetition
`period of TDMA frames. The timer value should be rela(cid:173)
`tively short in order not to introduce additional data trans(cid:173)
`mission delays.
`However. the data rates of present-day mobile communi(cid:173)
`cation networks are not sufficient for the new. high-speed
`data services. A solution is proposed for introducing higher
`
`65
`
`FIELD OF THE INVENI10N
`The present invention relates to high-speed multichannel 5
`data services (HSCSD) on a radio interface of a mobile
`communication system.
`
`BACKGROUND OF THE INVENTION
`
`There are several multiple access modulation techniques
`for facilitating communications in which a large number of
`mobile users are present. These techniques include time
`division multiple access (TDMA), code division multiple
`access (CDMA) and frequency division multiple access
`(FDMA).
`In mobile telecommunication systems of the time division
`multiple access (TDMA) type, time-division communica(cid:173)
`tion takes place on the radio path in successive TDMA
`frames. each of which consists of several time slots. In each
`time slot, a short information packet is sent as a radio
`frequency burst which has a finite duration and which
`consists of a set of modulated bits. The time slots are mainly
`used for transmitting control channels and traffic channels.
`On the traffic channels, speech and data are transmitted. On
`the control channels. signalling between a base station and
`mobile subscriber stations is carried out. An example of a
`TDMA radio system is the Pan-European mobile commu(cid:173)
`nication system GSM (Global System for Mobile
`Communications).
`CDMA is a modulation and multiple access scheme based
`on spread spectrum communication. Unlike FDMA or
`TDMA, in CDMA a large number of CDMA signals (users)
`simultaneously share the same wide band radio channel,
`typically 1.25 MHz. Pseudorandom noise (PN) binary 35
`codes. so-called spreading codes, are used to distinguish
`between different CDMA signals, i.e. traffic channels on said
`wide band radio channel. A separate spreading code is used
`over each connection between a base station and a sub(cid:173)
`scriber terminal. In other words, the narrow-band data signal 40
`of the user is conventionally multiplied by the dedicated
`spreading code and thereby spread in bandwidth to the
`relatively wide band radio channel. The signals of the users
`can be distinguished from one another in the receivers on the
`basis of the unique spreading code of each connection. by 45
`using a correlator which accepts only a signal energy from
`the selected spreading code and despreads its spectrum into
`a narrow-band signal. The other users' signals, whose
`spreading codes do not match, are not despread in bandwidth
`and as a result, contribute only to the noise and represent a 50
`self-interference generated by the system. The spreading
`codes of the system are preferably selected in such a way
`that the codes used in each system cell are mutually
`orthogonal, i.e. they do not correlate with each other. Thus,
`in the CDMA systems, the spreading code unique to each 55
`user or user's signal provides a traffic channel in a similar
`sense as a time slot in the TDMA systems. CDMA is
`described in more detail in the document: "An overview of
`the application of code division multiple access (CDMA) to
`digital cellular systems and personal cellular networks", 60
`Qualcomm Incorporated, 1992, USA. (Document Number
`EX60-10010).
`In traditional TDMA and CDMA mobile communication
`systems, the maximum data rate on the radio interface is
`relatively low.
`For communication in conventional mobile communica(cid:173)
`tion systems. each mobile station is assigned one traffic
`
`Ex.1018
`APPLE INC. / Page 11 of 18
`
`

`

`5,793,744
`
`5
`
`3
`data rates for mobile communication systems in the appli(cid:173)
`cant's co-pending PCT application W095/31878, unpub(cid:173)
`lished on the priority date of the present application: two or
`more parallel traffic channels (subchannels) are used on the
`radio path for one high-speed data connection. The high-
`speed data signal is distributed to the parallel subchannels at
`the transmitting end for the transmission over the radio path,
`and then combined at the receiving end. In this manner, it is
`possible to provide data transfer services which, depending
`on the number of allocated traffic channels, have a transfer 10
`rate up to 8 times the conventional data rate. In the GSM
`system, for example. the total user data transfer rate 19.2
`kbit/s is obtained with two parallel subchannels. This prin(cid:173)
`ciple is also referred to as a multi-slot channel technique.
`High-speed data service thus obtained are referred to as 15
`HSCSD (High Speed Circuit Switched Data) services.
`If the user data rate in a HSCSD service is lower than the
`maximum capacity of the radio link protocol, partially filled
`PDUs may be built and transmitted over the radio interface
`in the RLP frames. It is also possible that some subchannels
`carry full PDUs in their RLP frames and some subchannels
`occasionally carry partially full or empty PDU s in their RLP
`frames.
`For the mobile station, this inefficient use of transmission
`capacity may lead to an unnecessarily high power
`consumption. heating of RF and other elements, and possi(cid:173)
`bly a more complex scheduling of reception, transmission
`and neighbour cell monitoring than necessary for the actual
`user data rate.
`For the radio interface, this leads to an increased
`interference, either in the uplink or the downlink direction,
`or both. For the base station and the IWF, reducing com(cid:173)
`plexity is not that much of an issue as for the MS.
`The prior art DTX implementation, and the slightly
`delayed building of PDUs can relief the situation somewhat
`but not completely. The DTX is mainly used mainly in cases
`when there is nothing at all to be transmitted. When some
`data has to be transmitted (at a low rate). the transmission
`requiring only a fraction of the allocated bandwidth, the
`conventional DTX does not suffice.
`
`4
`The basic concept of the present invention is to transmit
`frames of the radio link protocol (RLP) selectively only via
`specific subchannels in cases the maximum data transfer
`capacity allocated to the data link is not required This is
`important because interleaving on the radio interface spreads
`an RLP frame over several frames. If the RLP frames were
`transmitted over arbitrarily selected subchannels without
`any consistency, many or maybe all allocated subchannels
`would constantly be "active". According to the present
`invention. transmissions are concentrated on specific sub(cid:173)
`channels only, whereas the other subchannels allocated to
`the connection carry no transmission at all. or they are
`maintained with as little transfer as possible. for example
`they employ subchannel-specific DTX or their data rate
`and/or power is decreased to as low a level as possible. The
`direct benefits of a lower number of active subchannels
`include reduced transmitter power consumption, less tem(cid:173)
`perature problems and a simpler timing of reception.
`transmission, and measuring neighbouring cells. In addition.
`20 as the number of unnecessary transmissions on the radio
`interface is lower, the interference level in the mobile
`communication network will be lower.
`The required minimum number of subchannels can be
`determined by monitoring the data flow into the transmis-
`25 sion buffer, i.e. the actual user data rate. Furthermore, by
`monitoring the amount of buffered data it is possible to
`determine whether more active subchannels will be required
`than said minimum number. and to dynamically increase and
`decrease the number of subchannels in use. Data amount
`30 mentioned above can be represented e.g. data rate, buffer
`status, number of PD Us or RLP frames etc. Determining the
`number of subchannels used, and weightings for different
`subchannels may be based on various mathematical and
`statistical variables of the amount of input and buffered data.
`35 such variables being for example instantaneous value. a
`fixed average, a moving average, or some other statistical
`variable (geometric mean. median. etc). This allows the
`control process to react in a controlled fashion to sudden,
`slow, temporary or long term changes in the transmission
`40 capacity requirements and availability. Such changes may be
`caused by, for example, handover, bad coverage (temporary
`or long term), a data transfer request, requested retransmis(cid:173)
`sion of corrupted data, allocation of new subchannels to the
`connection, removal of subchannels from the connection.
`45 and changes in the radio interface channel coding.
`There are different ways to choose the subchannels on
`which data transfer will be continued. One of the embodi(cid:173)
`ments of the invention utilizes a subchannel preference list
`which organizes the subchannels according to principles
`50 such as: (1) an order based on the position of subchannels in
`a frame, (2) a predetermined order which depends on the
`total number of subchannels on the connection, (3) an order
`to be negotiated during the connection. or (4) an arbitrary
`order. Even an arbitrary order is advantageous if maintained
`55 the same for the duration of several frames.
`
`DISCLOSURE OF TIIE INVENTION
`An object of the present invention is a discontinuous
`transmission suitable for multichannel high-speed data links.
`An aspect of the invention is a method for high-speed data
`transfer in a digital mobile communication system. the
`method comprising the steps of establishing a non(cid:173)
`transparent data connection having a number of parallel
`subchannels allocated on the radio interface, said number
`being determined by a specific maximum transfer capacity;
`transmitting user data over the non-transparent data connec(cid:173)
`tion in data frames by employing a communication protocol
`which acknowledges data frames received correctly and
`retransmits defective data frames; buffering data frames to
`be transmitted in a transmission buffer; storing the data
`frame transmitted in the transmission buffer for a possible
`retransmission until an acknowledgement is received from
`the receiving end; monitoring the actual user data rate on the
`terminal interface; monitoring fill level of the transmission 60
`buffer; transmitting user data in data frames via specific one
`or ones of said allocated subchannels. the number of said
`specific ones of said allocated subchannels depending on the
`actual user data rate and the fill level of the buffer; inter(cid:173)
`rupting transmission on the remaining one or ones of said 65
`allocated subchannels which do not belong to said specific
`ones of said allocated subchannels, if any.
`
`BRIEF DESCRIPI'ION OF THE DRAWINGS
`In the following the invention will be described in greater
`detail by means of the preferred embodiments. with refer(cid:173)
`ence to the accompanying drawings, in which
`FIG. 1 illustrates a part of a mobile communication
`system to which the present invention can be applied on a
`single channel non-transparent connection,
`FIG. 2 is a block diagram illustrating the functional units
`of a single channel non-transparent GSM traffic channel on
`different protocol levels,
`
`Ex.1018
`APPLE INC. / Page 12 of 18
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`

`

`5,793,744
`
`5
`FIG. 3 shows a L2R PDU.
`FIG. 4 shows an RLP frame.
`FIG. 5 illustrates a part of a GSM mobile communication
`system to which the invention can be applied on a multi(cid:173)
`channel non-transparent connection.
`FIG. (i is a block diagram illustration of an arrangement
`for an HSCSD connection in accordance with the invention,
`FIGS. 7. SA and SB illustrate high-speed data transmis(cid:173)
`sion according to the invention over N parallel CDMA traffic
`channels.
`FIG. 9 shows a CDMA transmitter in which four CDMA
`traffic channels can be divided between quadrature (Q) and
`in-phase (I) branches in a QPSK modulator.
`
`PREFERRED EMBODIMENTS OF THE
`INVENTION
`
`5
`
`6
`recommendation 05.02. In operating according to the
`recommendation, at the beginning of a call a mobile station
`MS is assigned a time slot from a carrier wave as a traffic
`channel (Single Slot Access). The MS synchronizes to this
`time slot to transmit and receive radio frequency bursts.
`In the GSM system, a data link is established between a
`mobile station MS network terminal TAP (Terminal Adap(cid:173)
`tation Function) 31 and a network adaptor IWF
`(Interworking Function) 41 in the fixed network (usually at
`10 the MSC). The data link is a circuit-switched connection
`which reserves one (or more) traffic channel(s) from the
`radio interface for the duration of the connection. In the
`GSM network, the data link in data transfer is a V.110 rate
`adapted. V.24 interface compatible, UDI coded digital Full
`15 Duplex connection. The V.110 connection is originally a
`digital transmission channel developed for ISDN (Integrated
`Services Digital Network), specified in the recommendation
`CCITT Blue Book V.110. The terminal adaptor TAP adapts
`a data terminal TE connected to the MS for the V.110
`20 connection which in FIG. 1 is established over a circuit(cid:173)
`switched connection using traffic channel ch0. The network
`adaptor IWF adapts the V.110 connection to another V.110
`network such as an ISDN or another GSM network. or to
`another transit network. e.g. the public switched telephone
`25 network PSTN.
`In addition. the traffic channel employs channel coding
`FEC (Forward Error Correction) with the aim of reducing
`the effect of transmission errors on the radio path. The GSM
`system employs convolution coding according to the GSM
`30 recommendation 05.03, the efficiency of which can be
`illustrated by a convolution coding ratio X/Y. which signifies
`that X data bits are represented in the channel coding by Y
`code bits. On a full-rate GSM traffic channel, on user data
`rates 9.6 kbit/s. 4.8 kbit/s and 2.4. the convolution coding
`ratios of ½ (punctured). 1/, and 1/6, respectively. are
`employed.
`The circuit-switched non-transparent connection between
`the TAP and the IWF on a GSM traffic channel comprises
`4(1 several protocol layers.
`The terminal interface between the MS terminal adaptor
`TAP and the data terminal equipment, as well as the inter(cid:173)
`face between the IWF and e.g. an audio modem MODEM
`are in accordance with CCITT V.24, and in FIG. 2 the
`45 terminal interface is marked with the symbol L2. As far as
`the invention is concerned, the interesting protocols are L2R
`(Layer 2 Relay) and RLP (Radio Link Protocol) which both
`are at the tenninal adaptor TAP and the network adaptor
`IWF at both ends of the connection. In addition. the con-
`50 nection has, as illustrated in FIG. 2. different kinds of Rate
`Adaptation (RA) functions, such as RAl' between the TAP
`and a CCU unit (Channel Codec Unit) located at the BSS.
`RAl between the CCU and the IWF, RAA between the CCU
`and a transcoder unit TRAU placed apart from the base
`55 station. and RA2 between the TRAU and the IWF. The rate
`adaptation RA functions are defined in the GSM recommen(cid:173)
`dations 04.21 and 08.20. Communication between the CCU
`and the TRAU is defined in the GSM recommendation
`08.60.
`The information rate-adapted on the radio interface RAl'
`is furthermore channel-coded the way specified in the GSM
`recommendation 5.03, illustrated by blocks FEC in the MS
`and CCU.
`The present invention. however. only relates to the L2R/
`RLP operation of the TAP and the IWF, and communication
`between them. The other aforementioned lower layer
`protocols. functions and units only provide a transmission
`
`35
`
`In different multiple access methods the physical concept
`of traffic channel varies. being primarily defined by a time
`slot in TOMA systems. a spreading code in CDMA systems.
`a radio channel in FDMA systems, a combination thereof.
`etc. The basic concept of the present invention is. however.
`independent of the type of the traffic channel and the
`multiple access method used.
`The present invention can be used in all digital TDMA
`based data transfer systems on a non-transparent data link
`· comprising several parallel subchannels (e.g. multi-slot
`access).
`The present invention is particularly well suited to data
`transfer applications in digital TDMA mobile communica(cid:173)
`tion systems. such as the Pan-European digital mobile
`communication system GSM. DCS1800 (Digital Commu(cid:173)
`nication System). a mobile communication system accord(cid:173)
`ing to the BIA/TIA Interim Standard IS/41.3. etc. Below. the
`invention will be described by using the GSM system as an
`example but without restricting the invention to it. FIG. 1
`very briefly shows the basic structure of the GSM system.
`not paying closer attention to its characteristics or other
`aspects of the system. For a more detailed description of the
`GSM system. the GSM recommendations and 'The GSM
`System for Mobile Communications", M. Mouly & M.
`Pautet. Palaiseau, France, 1992, ISBN:2-9507190-0-7, are
`referred to.
`A mobile services switching centre MSC handles the
`connecting of incoming and outgoing calls. It performs
`functions similar to those of an exchange of a public
`switched telephone network (PSTN). In addition to these, it
`also performs functions characteristic of mobile communi(cid:173)
`cations only, such as subscriber location management,
`jointly with the subscriber registers (not shown) of the
`network. The mobile stations MS are connected to the center
`MSC by base station systems BSS. The base station system
`BSS consists of a base station controller BSC and base
`stations BTS.
`The GSM system is a time division multiple access
`(TDMA) system in which time-division traffic takes place
`on the radio path in successive TDMA frames each of which
`consists of several time slots. In each time slot, a short
`information packet is sent as a radio frequency burst which 60
`has a finite duration and which consists of a set of modulated
`bits. The time slots are mainly used for transmitting control
`channels and traffic channels. On the traffic channels, speech
`and data are transmitted. On the control channels, signalling
`between a base station and mobile subscriber stations is 65
`carried out. Channel structures used on the radio interface of
`the GSM system are defined in closer detail in the GSM
`
`Ex.1018
`APPLE INC. / Page 13 of 18
`
`

`

`5,793,744
`
`15
`
`30
`
`7
`path according to the GSM recommendations between L2R/
`RLP units, and they are not significant to the present
`invention with the exception of channel coding FEC.
`Consequently, the other functions are not described herein in
`any greater detail.
`L2R (Layer 2 Relaying) functionality for non-transparent
`character-oriented protocols is defined e.g. in the GSM
`recommendation 07 .02. L2R packs the user data and the
`status information originating from the terminal interface
`into 200-bit, 25-octet long PDU s (Protocol Data Units), such
`as the one illustrated in FIG. 3. The octets are numbered
`0-24, octet 0 being transmitted first. The bits in the octets are
`numbered 1-8, bit 1 being transmitted first. In a PDU, the
`octet may be a status octet, a character (higher layer data) or
`fill bits. Octet 0 is always a status octet. A status octet
`comprises 3 bits, SA, SB and X for the status of the V.24
`connection, and 5 bits that indicate the number of data octets
`succeeding the status octet, as well as the special indications
`of the data octets such as empty and PDU. In FIG. 3. status
`octet 0 is succeeded by 3 data octets into which the word
`"GSM" has been packed, after which a new status octet 4
`follows.
`The L2R PDUs are packed in a frame according to the
`RLP protocol. such a frame being shown in FIG. 4. The RLP
`protocol is specified in the GSM recommendation 04.22.
`The RLP is a balanced (HDLC type of) data transfer
`protocol with a frame structure, in which error correction is
`based on retransmitting corrupted frames at the request of
`the receiving party. The RLP extends from the mobile station
`MS terminal adaptor TAP to the network adaptor IWF.
`which is usually located at the MSC. As shown by FIG. 4,
`the RLP frame structure comprises a header field (16 bits),
`an information field (200 bits), and a frame check sequence
`(24 bits). The 200-bit L2R PDU is packed in the information
`field. As a result, the net RLP data rate is clearly above the
`maximum 9.6 kbit/s data rate for one channel, which allows
`a specific number of retransmissions without a decline in the
`nominal user rate. For example, if the user rate on the
`terminal interface is 9600 bit/s and the data rate on the radio
`interface is 12 kbit/s, the "swplus capacity" is, depending on
`the character structure being used, at least 12.5%.
`The transmission buffer buffers the data received from the
`V.24 interface so that data will not be lost even if the MS is
`not able to transmit it instantly over the radio interface. A
`reception buffer buffers data which is transferred to the V.24
`interface so that data received from a traffic channel will not
`be lost even if it cannot immediately be forwarded via the
`V.24 interface to e.g. the terminal equipment TE. The RLP
`protocol also includes a flow control, used to adjust the fill
`level of the transmission and reception buffers. The flow
`control is specified in the GSM recommendation 07 .02. The
`criterion used to activate the flow control may be a half full
`transmission or reception buffer.
`The maximum user data rate on a single GSM traffic 55
`channel is restricted to 9.6 kbit/s.
`In high-speed data services (HSCSD), several traffic
`channels are assigned to a data call; in other words, two or
`more time slots are assigned from the same TOMA frame.
`An example of how