A rapid prototyping system preferably includes a basic component in powder form selected from the group consisting of a metal oxide, and one or more aluminosilicate glasses; an acidic component (polymeric, oligomeric or polymerizable low molecular weight acid or hydrolyzable acidic metal salt); and an aqueous binder capable of stimulating a crosslinking reaction between the basic component and the acidic component to form a three-dimensional printed object.

The present invention relates to the field of rapid prototyping. More specifically, the present invention relates to a material system for use in rapid prototyping.
Rapid prototyping is fast becoming a popular process for manufacturing three-dimensional objects including prototype parts and working tools such as structural ceramics and ceramic shell molds. One form of rapid prototyping involves a process of sequentially forming layers.
In this process, a powdery material is used to form each individual layer of the desired product.
Such a printing process offers the advantages of speedy fabrication and low materials cost. It is considered one of the fastest rapid prototyping methods, and can be performed using a variety of colors as well.
However, there are several disadvantages in conventional powder based rapid prototyping processes including the fragility of the resulting product. Poor mechanical properties in the final product are characterized by a low modulus of elasticity and low fracture strength. Weakness in compression and tensile failures at low stress may be due to low density, poor adhesion between powder particles, low density of particles, and the presence of voids. In both the intralayer and interlayer levels, the powder particles are only loosely glued together. More particularly, powders that are presently being used in the market are based on gypsum and/or water swellable polymers such as starches, PVA, etc. Interaction of these powders with an aqueous binder results in poor mechanical strength as well as high porosity of the green object. Also, parts made by powder based rapid prototyping as well as jetted, direct build-up type rapid prototyping suffer from poor strength. The latter is due to the fact that only lower molecular weight polymers (namely their solutions) can be jetted since high molecular weight polymers have viscosities that are too high.
Further, the poor mechanical properties in the resulting product lead to the fact that the base or xe2x80x9cgreenxe2x80x9d object, which is fabricated by printing layers in a powder bed, must be subjected to labor intensive post-processing. This post-processing often involves reinforcing the printed object by soaking it in binding or strengthening agents such as cyanoacrylate glue, etc. which penetrate the surface and fill the interconnected pores within the bulk. Gypsum based powders and water swellable polymers currently available have long swelling times, which can be thirty minutes or more. Another disadvantage of this and similar processes is that the resulting products can have a poor resolution, represented by a grainy texture of the product.
While post-processing drying of the resulting article improves the mechanical properties slightly, the improvements are minimal and the drying process is very slow. Other post-processing measures include reinforcing with polymerizable glues such as cyanoacrylate, or surface finishing, but these measures are costly and labor intensive as well. Ultimately the mechanical properties and surface finish depend on the properties of the system of materials in concert with their ability to intermix uniformly and react sufficiently.
In one of many possible embodiments, the present invention provides a rapid prototyping system including a basic component in powder form selected from the group consisting of a metal oxide, and one or more aluminosilicate glasses; an acidic component; and an aqueous binder capable of stimulating a crosslinking reaction between the basic component and the acidic component to form a three-dimensional printed object.
Additional advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The advantages of the invention may be achieved through the means recited in the attached claims.
In a wide variety of embodiments, the present invention provides a system for rapid prototyping, the compositions included in the system, and a rapid prototyping method incorporating the system. The system for rapid prototyping is preferably leveraged from so-called acid-base cements. The components included in the system preferably include a base such as a metal oxide or an aluminosilicate glass, a polymeric acid or other acid, and an aqueous binder. The basic powder interacts with the acid in the presence of water, causing the formation of an ionically crosslinked hydrogel salt. Formation of the crosslinked hydrogel causes setting of the mixture.
There are three general possibilities for implementation of the system and the materials included in rapid prototyping systems according to the present invention. First, both reactive components, i.e., the acid component and the basic component, can be present in a dry powder mixture. The powder is then inkjet printed with an aqueous or polar solvent binder solution that does not contain any of the reactive components that form the cement. Second, a dry powder mixture can contain only the basic component. An aqueous or polar solvent binder solution that includes the acid component is then jetted onto the powder bed during the printing process. Third, a dry powder can contain the basic component as well as some of the acid component as a mixture. An aqueous or polar solvent binder solution is then used that includes some of the acid component dissolved therein, so that both the powder mixture and the aqueous binder include some of the reactive acid component. The aqueous or polar solvent binder solution and acid component mixture is then jetted onto the powder bed containing the mixture of the acid and basic components.
Apart from the chemical aspects of the present invention, the printing process is similar to the conventional mechanics associated with rapid prototyping that uses a printing process. Multiple planar layers are printed and adjoined together to form a three-dimensional object. Printing is performed layer-by-layer, with each layer representing a cross section of a portion of the final desired product. The powder material forms each individual layer, and is evenly distributed and compressed by compression means such as a roller. When the printer used in the rapid prototyping method is an ink jet printer, a printer head deposits the binder onto the powder in a two-dimensional pattern, and the powder is bonded in the areas,where the adhesive is deposited, thereby forming a printed layer of the final object to be produced. Predetermined portions of the adjacent printed layers are adhered one to another by the use of an aqueous binder, resulting in the joining of the individual cross sections of the final product. The binder is applied simultaneously with the printing of each individual layer. The xe2x80x9cun-printedxe2x80x9d regions where no adhesive has been applied are then separated from the printed regions where adhesive binder has been applied, leaving a three-dimensional printed base or xe2x80x9cgreenxe2x80x9d product.
Next, the chemical aspects of the present invention will be described. The acid component of the rapid prototyping system is water/solvent soluble, and is acidic relative to water/solvent. Consequently, contact of the acidic component with the aqueous or polar solvent binder solution causes protons to dissociate from the acidic component. The free protons are immediately attracted to the basic component of the rapid prototyping system, and the basic component releases multivalent cations (Me+Z) as they are replaced by the protons.
The released cations from the basic component of the rapid prototyping system mediate the crosslinking of the compounds that make up the acid component. Ionic crosslinking of the acidic compounds reduces the mobility of the acid component. Eventually the crosslinking process results in solidification of the acid in the solution because of formation salt hydro (or solvent) gel, followed by setting and further hardening of the cement product.
A representative (and the most simplified) example of: this chemical process involves glass-ionomer chemistry, where the acid component in the rapid prototyping system is a polyacid such as polyacrylic acid. The basic component in this example is aluminosilicate glass, for example. The protons from the polyacrylic acid release upon the dissolving of the polyacrylic acid in the aqueous binder, and the protons attack the glass, which releases multivalent cations. The cations then crosslink the polyacid through formation of ionic bonds, and the bonding causes the polyacid components to compress and solidify until the cement is completely solidified and extremely hard.
There are three basic chemical components of the rapid prototyping system, namely, an active acidic component, an active basic component, and an aqueous binder. The basic component can be a metal oxide, and can also be an aluminosilicate glass. The aqueous binder must be capable of stimulating a crosslinking reaction between the basic component and the acidic component to form a three-dimensional printed object. The acidic component can be one or more acidic components such as an organic polyacid, a monomer acid, an oligomer acida monomer having anions capable of forming hydrogel (or solvent-gel) salts that are cross-linkable with metal ions from said basic metal oxide, and a hydrolyzable metal salt capable of forming an oxysalt polymer matrix with said basic metal oxide.
Examples of acid-base combinations that form a cement system for rapid prototyping include the following.
a. Zinc oxidexe2x80x94polycarboxylic acid cements.
b. Metal oxide (i.e., oxides of Be, Zn, Cu, Mg, Ca, Sr, Ba, or other metal oxides.)xe2x80x94orthophosphoric or poly (phosphoric acid) cements. In this case, metal cations crosslink phosphate anions resulting in the formation of a hydrogel matrix.
c. A mixture of reactive aluminosilicate glasses, (i.e., xCaO*yAl2O3*ZSiO2*nCaF2, and i.e. sometimes containing fluorine) with orthophosphoric or poly (phosphoric acid). In this case, setting of the cement involves the formation of a hydrogel matrix of silica gel and ionically crosslinked phosphate ions. The average particle size for the glass is preferably approximately 30-50 xcexcm or less, as glass particles with a smaller diameter can be difficult to spread.
d. Oxysalt-bonded cements. These are formed by acid-base reactions of metal oxide powder such as ZnO or MgO, although the metal oxide powder is not limited to these oxides, and a concentrated solution of metal chloride or sulfate where the metal is, for example, Zn or Mg.
e. Glass-ionomer cements. In this case, the basic component of the system is reactive aluminosilicate glass (i.e., xCaO*yAl2O3*zSiO2, and frequently containing fluorine, i.e., in the form of CaF2, and the acidic component of the system is organic polyacid containing functional groups such as xe2x80x94COOH, xe2x80x94SO3H, and xe2x80x94PO3H2. The glass ionomer mixture may also contain small amounts of low molecular weight complexing agent such as L- or D-tartaric acid for adjusting the kinetics of the cement setting process. In some cases, the glass-ionomer cements should be pretreated to make the surface of the polyacid powder less hydrophilic and therefore les susceptible to clumping due to moisture absorption. A preferable pretreatment includes the addition of some anti-caking hydrophobic agent to the dry cement mix. The agent could include some stearate salts (Mg, Ca, Zn) or lecithin at a concentration of between 0.01 and 13.0 wt %.
The above types of cements provide superior compressive strength and significantly better mechanical properties relative to common systems typically used in rapid prototyping systems. Using these cements, there is no need for any reinforcing post-treatment. The cements have a very short setting/curing time. No drying is necessary because water in the aqueous binder is consumed and becomes part of the solid phase during the acid-base setting reaction, which generally proceeds to completion much faster than drying of the green object composed of the materials currently present on the market. Further, the material produced by the cement has a continuous texture.
The cements of the present invention cure by means of ionic reactions like neutralization, salt formation, chelation, crystallization, or ionic and covalent cross-linking, specifically in the presence of water, or other polar solvent. As discussed above, the components included in the system preferably include a base such as a metal oxide or an aluminosilicate glass, an acidic component, and an aqueous binder. The acidic component is usually a polymeric acid (polycarboxylic, polysulfonic, polyphosphonic acids) or other acid (phosphoric acid, derivatives of salicylic acid), or a hydrolysable metal salt. The binder is not limited to an aqueous one. Any polar solvent capable of interacting acid and base components may be effective, so long as it can dissolve or solubilize the components and promote the cross-linking reaction.
In one of the embodiments of current invention, acid or polymeric acid component of the acid-base cement could be partially or fully substituted with unsaturated polymerizable acidic moieties of a monomeric or oligomeric nature, as well as their salts or other acid derivative groups. In such a case, a cross-linked hydrogel formed after the acid-base interaction of the cement components could be further fortified by polymerization and, hence, covalent cross-linking of the unsaturated moieties. Examples of polymerizable unsaturated monomers, oligomers or prepolymers with acid groups or reactive acid-derivative groups may include:
unsaturated organic esters of phosphoric and phosphonic acids (German AS No. 2 711 234 and German OS No. 3 150 285),
unsaturated organic esters of monofluorophosphoric acid (U.S. Pat. No. 3,997,504),
unsaturated organic esters of phosphoric acids that contain either chlorine or bromine bonded directly to the phosphorus (Eur. Pat. No. 0 058 483),
unsaturated organic esters of phosphoric acid in the form of pyrophosphates (anhydrides) (German OS No. 3 048 410),
unsaturated carboxylic acids,
unsaturated sulfur-containing organic acid moieties with groups of xe2x80x94SO2H, xe2x80x94SO3H, xe2x80x94Oxe2x80x94SO3H type,
unsaturated organic derivatives of boric acid i.e. the ones containing groups: xe2x80x94B(OH)2, B(OH)(OR), xe2x80x94Oxe2x80x94B(OH)2, xe2x80x94Oxe2x80x94B(OH)(OR) wherein R is H or alkyl,
unsaturated organic moieties containing cationic acid radicals like NR2H+, xe2x80x94RR2H+ (wherein R is H or alkyl), and/or
unsaturated organic moieties containing different combinations of the acidic species listed in the a) xe2x88x92h).
The reactive acid derivatives can be substituted with acid halides, with acid anhydrides, and with acid amides, nitrites, and esters, that readily hydrolyze into acid in the presence of water or other polar solvent, as such can enter into ion-exchange, neutralization, salt formation, or chelation reactions with the base component of the acid-base cement, i.e. metal oxides, ceramics, zeolites or leachable reactive glasses. Especially preferred are acid groups or reactive acid derivatives in the form of carboxylate, phosphate, phosphonate, sulfonate, or borate acid radicals or of their reactive derivatives.
The polymerizable unsaturated monomers, oligomers, or prepolymers in the polymerizable cement mixtures in accordance with the invention can carry alkenyl, alkenoxy, cycloalkenyl, aralkenyl, or alkenaryl radicals, with acryl, methacryl, vinyl, or styryl radicals being preferable and, of these, the acryl and methacryl radicals which constitute the polymerizable groups in many monomers are especially preferable. Especially appropriate are compounds that contain at least two polymerizable groups or at least two acid groups or acid-derivative groups. Examples are phosphoric-acid esters of glycerine dimethacrylate or 1-methacryloxyethane-1,1-diphosphonic acid.
The presence of polymerizable unsaturated acidic moieties in the acid-base cement systems is highly desirable, as well as the presence of a polymerization initiator in the mixture. The role of the initiator is to enable triggering of polymerization of the unsaturated species after the initial setting caused by the interaction of the acid and base components of the cement. The covalent polymerization of the unsaturated component of the cement could be initiated either by photoirradiation (light) or heat. An example of the initiator used for the light-triggered polymerization is mixture of a-diketones and tertiary amines. Typical initiators used for the heat-triggered polymerization include but are not limited to organic or inorganic peroxides such as benzoyl peroxide or ammonium persulfate.
The major purpose for the aqueous or polar solvent-based binder is to deliver and/or enable interaction of the acidic component of the cement with the basic component. Apart from water and/or solvent, the liquid binder may also contain:
a. surfactants/wetting agent to facilitate quick wetting of the powder surface by the binder,
b. colorants such as dyes or pigments to provide color for the printed object,
c. co-solvents to improve dye solubility in the binder,
d. soluble polymers to modify rheological behavior and improve jettability of the binder,
e. complexing agent, i.e. tartaric acid or EDTA, to control the setting behavior and rate of the acid-base reactive system.
In the case where the acidic part of the system contains covalently polymerizable acidic moieties, the initial acid-base interaction of the components is still used to print and produce the so-called xe2x80x9cgreen object.xe2x80x9d After the initial printing, the mechanical properties, the ease of handling, and the resistance to environmental factors (moisture and/or humidity) of the xe2x80x9cgreen objectxe2x80x9d is significantly enhanced by the post-treatment involving curing material of the object through exposure to light or heat. Polymerization of the unsaturated moieties in the xe2x80x9cgreen objectxe2x80x9d results in covalent cross-linking and further fortification of the hydrogel salt matrix formed by the initial acid-base interaction.
In another embodiment of the current invention, covalent cross-linking of the unsaturated polymerizable moieties could be initiated immediately after the delivery of the aqueous or polar solvent-based binder into the powder. In this case covalent cross-linking happens in parallel with ionic cross-linking cased by acid-base interaction. The mechanical embodiment of this approach implies the presence of a source of light in the visible or UV range, or heat from, for example, IR radiation, above the printed powder surface. The xe2x80x9cgreen objectxe2x80x9d in this case is cured at the same time as it is printed on the layer by layer basis.
An example reaction mixture involving reactive glass-ionomer chemistry in one embodiment of the invention includes between about 60% and about 90% by weight of a reactive aluminosilicate glass. An acidic powder having an average molecular mass of between about 2,000 about 1,000,000 is present at about 5 wt % to about 40 wt %. It is preferred that in this case the acid component is a polyacrylic acid having an average molecular weight that is between about 10,000 and about 150,000. L- or D-tartaric acid is also included. Finally, an ink-jettable aqueous binder is present at between about 5 wt % and about 50 wt % of the dry mixture.
There are other cement systems that can be used in accordance with the principles of the present invention. For example, acid-base cements that have previously been used for dental and surgical applications may be used with the present rapid prototyping system, and include polycarboxylate cements such as zinc oxide and polyacrylic acid-based surgical cements such as those disclosed in U.S. Pat. No. 3,751,391 which is hereby incorporated by reference, and glass-ionomer cements such as those disclosed in U.S. Pat. No. 3,814,717 which is hereby incorporated by reference, and in British Patent No. 1,316,129 which is also hereby incorporated by reference.
Several applications can be created and modified using the above-described materials in the present rapid prototyping system. According to a first application, the acidic and the basic component are mixed together in a dry powder form prior to the addition of the aqueous binder. Preferably in this application, the basic component is a metal oxide or a reactive glass as described above, and the acidic component is an organic polyacid or a metal salt. The surface of the powder is printed with ink-jettable aqueous binder, which dissolves the acidic component and causes initiation of the setting reaction. This approach is especially useful when the acidic component is a high molecular weight polyacid.
According to this first application and other applications, the aqueous binder may be delivered by an inkjet and may contain complexing agent(s) and coloring agent(s) as well. In the case where the chemistry involves a glass-ionomer system, the polyacid dissolves upon contact with the aqueous binder. A viscous liquid phase is formed, binding together partially reacted glass particles. When the polyacid is a high molecular weight compound the mechanical properties of the final product are significantly improved. The organic polyacid is preferably of an average molecular weight ranging from about 10,000 to about 150,000, although the range can be expanded to range from about 2,000 to about 1,000,000. Most preferably, the organic polyacid is of a molecular weight that is less than 100,000.
According to a second application of the present rapid prototyping system, the acidic component is stored separately from the powder, in a liquid form. Preferably, the acidic component is mixed with the aqueous binder. While not so limited, this approach could be typical for cases where the acidic component is of a relatively low average molecular weight. In any respect, the acid component is dissolved in the liquid binder and consequently is delivered to the basic component-containing powder by an inkjet in the case where inkjet printing is applied. One advantage of this approach is a more efficient reaction as there is no need for the acidic component to dissolve in the aqueous binder during printing.
According to a third application of the printing system of the present invention, the first two applications are combined, so that while some of the acidic component and all of the basic component are combined together in a dry powder form prior to the addition of the aqueous binder. Further, the aqueous binder is separately mixed with additional amounts of the acidic component prior to printing. Under this approach, it is preferred that the acidic component in the dry powder has a higher average molecular weight than that of the acidic component that is mixed with the aqueous binder. This approach combines the advantages of the first and second approaches. Further, the integrity of the finally produced object is improved because of the initial presence of the acidic polymer in the binder, and the ability for a relatively high average molecular weight acid polymer to mix with the powder. An additional benefit of this approach is improved solubility of the acidic component present in the powder. The acidic component present in the liquid binder helps to solubilize the acidic binder in the powder, and results in better structural uniformity of the printed object.

Hewlett-Packard Development Company, L.P.


A method is provided for allocating a radio resource in a packet switched data transmission system, such as GPRS (General Packet Radio Service), which data transmission system includes terminals and a network. In the method, terminals communicate with the network over the radio interface using the packet transfer mode, and for communication, a radio resource is allocated to the terminal. For allocating the radio resource, for the packet switched implementation of a realtime service, a first message is transmitted from the terminal to the network and the first message is received in the network. The network identifies the first message as a radio resource request for the packet switched implementation of a realtime service and allocates to the terminal the requested radio resource for the packet switched implementation of the realtime service. A wireless terminal and network element relating to the method are also provided.

In packet switched data transmission, the data to be transmitted in a network is divided into small data units, called packets. These packets that comprise the address information of a recipient, are transmitted from a sender to the recipient by routing their path in the network on the basis of the recipient's address. In packet switched data transmission, the same radio resources can be divided among multiple users, as necessary.
GPRS is the GSM (Global System for Mobile communications) network's packet switched data transmission service that complements the existing services, such as conventional circuit switched data transmission and the short message service (SMS). In conventional circuit switched data transmission, the allocation of radio resources between a wireless terminal, such as a mobile station or a computer terminal, and a base station subsystem (BSS) is typically carried out by reserving a so-called physical (radio) channel for the duration of the call, where a physical channel means a specific time slot of a transfer frame on a given frequency band. GPRS, which is defined in general in the GSM recommendation 03.60, enables the dynamic allocation of physical channels for data transmission. In other words, a physical channel is being reserved for a particular MS-BSS link only when there is data to be transmitted. Thus, the unnecessary reservation of radio resources when there is no data to be transmitted is avoided.
GPRS is intended to operate in conjunction with conventional GSM circuit switched transmission to efficiently use the air interface for both data and voice communications. GPRS therefore uses the basic channel structure defined for GSM. In GSM, a given frequency band is divided in the time domain into a succession of frames, known as TDMA (Time Division Multiple Access) frames. The length of a TDMA frame is 4.615 ms. Each TDMA frame is in turn divided into eight consecutive time slots of equal duration. In the conventional circuit switched transmission mode, when a call is initiated, a physical channel is defined for that call by reserving a given time slot (1-8) in each of a succession of TDMA frames. Physical channels are similarly defined for conveying various signalling data in a network.
With taking GPRS into use in the GSM system, radio resources for data transmission are reserved by dynamically assigning physical channels either for circuit switched transmission mode or the packet switched transmission mode. When the network requirements for circuit switched transmission mode are high, a large number of time slots may be allocated to that mode. The GSM network service that offers multiple time slots for the use of the same circuit switched connection in the same TDMA frame, is called a HSCSD (high speed circuit switched data) service. On the other hand, when demand for GPRS transmission mode is high, a large number of time slots may be allocated to that transmission mode. In addition, a high-speed packet switched transmission channel may be provided by assigning two or more slots in each of a succession of TDMA frames to a single wireless terminal. A series of four consecutive time slots on a physical channel is known as one data block, and it represents the shortest packet switched data transmission unit on a physical channel.
FIG. 1 shows telecommunication network connections in a packet switched GPRS service. The main element of the network's infrastructure for GPRS services is a GPRS support node, which in packet switched data transmission corresponds to the GSM network's mobile switching center MSC, known in connection with circuit switched data transmission. GPRS support nodes are divided into serving GPRS support nodes SGSN and gateway GPRS support nodes GGSN. SGSN is a support node that transmits data packets to a wireless terminal MS (mobile station) and receives the data packets sent by the MS, through a base station sybsystem BSS that is formed of base transceiver stations BTS and base station controllers BSC. In this description, by a wireless terminal MS is meant all data terminal equipment that communicate over a specific radio interface. Thus, also a computer terminal that communicates through a mobile station coupled thereto is called a wireless terminal. SGSN also maintains together with GPRS registers (not shown in the figure) the location data of wireless terminals that move in its service area. Physically, SGSN is typically implemented as a separate network element. GGSN that communicates with SGSN implements the connection and co-operation with other networks. These other networks may be, among others, the GPRS network of some other operator or some private network, Internet network/public switched packet data network PSPDN or X.25 network.
The GPRS radio interface between a BTS and an MS is called the Um interface. Said Um interface for GSM Phase 2+ (GSM 03.64) can be modelled as a hierarchy of logical layers with specific functions. As is shown in FIG. 2, a wireless terminal (mobile station, MS) and a network have identical layers, which communicate via the Um interface between the mobile terminal and the network. It should be understood that the model shown in FIG. 2 does not necessarily represent the hardware contained by the mobile station and the network, but it rather illustrates the flow and processing of data through the system. Each layer modifies data received from the neighbouring layer. Typically, the received data passes through the logical layers from the bottom to the top layer and the data to be transmitted passes from the top to the bottom layer.
Under the application layer, in the top logical layer shown in FIG. 2, the MS has multiple packet data protocol (PDP) units. Some of these PDP units use point-to-point protocols (PTP) adapted for sending packet data from one wireless terminal to another or from a wireless terminal to a fixed terminal. Examples of PTP protocols are IP (Internet Protocol) and X.25, which are capable of establishing an interface with applications of the application layer. In the network, similar protocols with which protocols of the top layer of the mobile station communicate are typically located in a gateway GPRS support node (GGSN).
The top layer units use a subnetwork dependent convergence protocol (SNDCP, GSM 04.65), one task of which is to compress and divide, as well as compile data into SNDCP Packet Data Units. In the network, a similar SNDCP layer is typically located in a serving GPRS support node (SGSN).
The logical link control (LLC, GSM 04.64) layer provides a reliable encrypted logical connection between the MS and the SGSN. LLC frames, formed by the LLC layer, are used for conveying SNDCP packet data units (or other GPRS endpoint protocol units) over the radio interface.
The RCL/MAC (radio link control/medium access control, GSM 04.60) layer provides services for transmitting information over the physical layers of the GPRS radio interface between the MS and a base station subsystem. The RLC/MAC layer comprises two different functions: RLC function comprises, among other things, procedures for segmenting LLC layer data blocks and reassembling them into RLC data blocks. The RLC function also comprises procedures for re-transmitting unsuccessfully delivered RLC blocks. The MAC function acts above the Phys. link layer and defines the procedures that enable radio resources to be allocated and divided among multiple users. The MAC function also arbitrates between wireless terminals that are trying to transmit data simultaneously, by providing collision avoidance, detection and recovery procedures. Physically, the network's RLC/MAC layer is typically located in a base station subsystem BSS, in a base station controller BSC, wherein it is typically implemented by a so-called packet control unit (PCU). It is also possible to place the PCU in a SGSN or a BTS.
The physical link layer provides a physical channel between the MS and the network. The physical RF (radio frequency) layer defines, among other things, the carrier frequencies and GSM radio channel structures, the modulation of the GSM channels and the transmitter and receiver characteristics.
When the MS has data to transmit, the RLC/MAC layer's MAC function allocates, from the network, the radio resources necessary for transmitting the data over the radio interface. Typically, in this case, a TBF (Temporary Block Flow, GSM 03.64) connection is being established, which is a temporary physical one-way connection between the MS and the network for transmitting data blocks on a physical channel over the radio path. Here, its temporary nature means that TBF is only maintained for the duration of data transmission.
There are two types of TBF connections: close ended TBF and open ended TBF. In a close ended TBF, the network allocates to the MS for the transmission of data blocks a pre-determined number of time slots in a succession of TDMA frames, depending on the number of data blocks to be transmitted. In an open ended TBF, the number of data blocks to be transmitted during the connection is typically not known to the network in advance. Therefore, in an open ended TBF, the network allocates time slots to the MS until the open ended TBF connection is released either by the network or the MS. The open ended TBF is released, for example, if the network detects that the MS has not transmitted data during a given number of frames. Unlike in the close ended TBF, in the open ended TBF, the duration of the connection is not known to the network in advance. Therefore, the network prefers to allocate closed TBF connections to the MS, because then it has better opportunities to efficiently divide radio resources among different users.
For allocating radio resources (for establishing a TBF connection), there are substantially two different alternatives (FIG. 3a-3b): 1-phase access and 2-phase access.
In 1-phase access (GSM 04.60), the MS sends the network a packet channel request. The packet channel request is in its encoded form eight or eleven bits long, depending on the network. Among other things, a so-called Multislot Class parameter is encoded by five bits in the packet channel request, which parameter indicates how many time slots the MS is able to use at a maximum, but due to the shortness of the message, not much other data can be supplied to the network therein. In response to the packet channel request, the network typically sends the MS a specific assignment message wherein the network assigns radio resources to the MS by typically establishing a close ended TBF connection for the MS. Typically, said assignment message is a packet uplink assignment message, wherein the network assigns the radio resources to the MS for an uplink radio transmission. Among other things, the time slots during which the MS can transmit appear from the message.
In 2-phase access, the MS sends the network two messages. First, the MS sends a packet channel request, wherein it only asks the network for radio resources for the transmission of a packet resource request. After again receiving a specific assignment message from the network, the MS sends the packet resource request, which is one radio block in length. In the packet resource request, the MS may send the network a lot of information (values of different parameters) on the basis of which the network decides on the allocation of the radio resources.
In the one-bit long RLC_MODE frame (GSM 04.60), the MS may propose a specific transfer mode for the TBF connection it requested. If the bit is 1, the MS is proposing the unacknowledged RLC mode. If the bit is 0, the MS is proposing the acknowledged RLC mode. In the acknowledged transfer mode, acknowledgements are used so as to find out the error-free delivery of the RLC data blocks. The acknowledged transfer mode also provides the RLC function an opportunity to retransmit unsuccessfully transferred data blocks.
In the sixteen-bit long RLC_OCTET_COUNT field, the MS may propose the TBF connection being established to be either a close ended TBF connection of specific duration or an open ended TBF connection. However, the network may ignore the MS's proposal and decide alone what type of TBF connection will be established.
In response to the packet resource request, the network again sends the MS a specific assignment message, wherein the network assigns radio resources to the MS establishing a close or open ended TBF connection for the MS.
GPRS was initially designed for non-realtime data services, such as e-mail services. However, the pressure for using GPRS in realtime services that require a short delay, such as in the transmission of speech (voice) and a video image, is continuously increasing. In the following, by the term realtime data transmission is meant explicitly data transmission for realtime services. For realtime services, the following three requirements are set on GPRS:                use of open ended TBF        short access delay        short transmission delay
The use of an open ended TBF connection in realtime services is important in order to avoid, for example, constant breaks due to the establishment and release of close ended TBF connections of specific lengths, e.g. in speech transmission. However, the problem is that according to current GPRS specifications, the network can decide alone what type of TBF will be established. But, as has already been mentioned, said network prefers to allocate to the MS close ended TBF connections, because then it has better possibilities to efficiently divide radio resources between different users.
Achieving a short access delay is important because, for example, when transmitting speech the TBF connection will be cut off during a quiet period so that a new TBF connection must again be established when the silence ends. A short access delay is attained by using 1-phase access. But when using 1-phase access, obtaining an open ended TBF connection is not at all certain, because the network decides what type of TBF connection will be established at any given time.
Achieving a short transmission delay is a distinct requirement in connection with realtime services. A short transmission delay is attained by using the unacknowledged RLC mode. However, according to the current GPRS specification (GSM 04.60 version 6.4.0), the acknowledged RLC mode should be used when requesting a TBF connection with 1-phase access.

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Allocation of radio resources from a network in a packet switched data transmission