GPRS is sometimes considered as the first step in converging between the GSMand the IP networks. It will permit the user equipped with a specific GSM/GPRSmobile terminal to use classical data services such as the SMS, mail, fax and websurfing and in addition new data services such as the file transfer services, the websurfing, mailing, e-application and value added services at high transmissionservices. The problem of limited GSM transmission rate is then overcome and newservices requiring high quality of services become achievable.The high transmission rates it can offer, added to the sophistication resources andmobility management procedures it include, makes of the GPRS an interestingsolution that can help operators to meet subscriber’s satisfaction and generatemore revenues.GPRS constitutes also a supplementary step in the way of transmitting multimediatraffics. It can also serve to an operator to test certain applications as a test forfuture UMTS networks.
HSCSD (High Speed Circuit Switched Data)
The HSCSD is an enhancement of the data services in circuit mode of the GSM
standard phase 2. It intends to reach transmission rates higher than the basicservice at 9,6 kbit/s which can reach theoretically 115,2 kbits/s.
In GSM networks each carrier is divided in eight time intervals (time slots),representing the elementary resources. Each of this elementary resources can offera maximum transmission rate equal to 14,4 kbit/s, with the use of a specialchannel coding scheme.
During a classic telephonic communication or a low rate data communication, eachterminal uses only one elementary resource. The HSCSD allow the use per data
communication up to eight of these elementary resources permitting to reach atheoretical transmission rate up to 8 * 14,4 kbit/s (115,2 kbit/s). Note that with thecombination of only 4 timeslots, the ISDN transmission rate can be matched.
Note also that in the case of data services in transparent mode, it is possible to varythe number of timeslots used during a communication. This point is particularlycritical during the movement from one cell to another (handover procedure). Infact, it is sometimes difficult to guarantee that the new cell will have the possibilityto offer the same number of timeslots reserved in the old one.
Unfortunately, this process of increase and decrease of the number of timeslots is
slow and not very reactive because it is based on the traditional mechanisms ofallocation and liberation of resources in the GSM networks. This characteristic ismaking the HSCSD not adapted for many applications used today on Internetcharacterized by a low mean rate and high peak rates. It is mainly suited forapplications with high but constant transmission rates such as video telephony.
1.2 StandardizationTwo phases characterized the introduction of the GPRS into the GSMRecommendations. The first phase or Phase 1 of GPRS introduction was completedby the ETSI (European Telecommunication Standards Institute) in the AnnualRelease 1997 (03/98) and includes all central GPRS characteristics such as:
- Point-to-point transfer of user data- TCP/IP and X.25 bearer services- GPRS identities- GPRS safety (the introduction of a new ciphering algorithm specially
designed for packet data)- Volume-oriented billing
More extensions and new functions were integrated in the GPRS Phase 2 such as:- Support of point-to multipoint (PTM) services- Support of special point-to-point and point-to-multipoint services for
applications such as traffic telematics and GSM-R (PTM-Group Call: PTM-Multicast)
- Support of further additional services- Support of additional interworking functions (e.g. ISDN)
The interested reader could find a complete overview of the GPRS principles in thefollowing recommendations:- Rec. 01.60 GPRS Requirements- Rec. 02.60 General GPRS Overview- Rec. 03.60 GPRS System and architecture description- Rec. 03.64 Radio architecture description
As mentioned before, the GPRS network is implemented on the actual GSMstructure. Both switching modes (Circuit of classic GSM and packet of GPRS) shallshare the available radio resources available in a cell.
The GPRS defines new radio channels that are distinct from the GSM channels,due to the fact of the packet data switching which characterize the GPRS.The GPRS radio channels present the following characteristics:
- Each logical GPRS channel is shared between many mobile terminals andone base station. Note that a direct communication is only possible betweenthe mobile terminal and the base station.
- The GPRS channel consists in a non-symmetric channel and the radiochannels of transmission (Uplink) and reception (Downlink) are allocatedindependently.
- The Downlink channel carries information from the base station to themobile terminals and doesn’t need any contention technique.
- The Uplink channel is shared between many terminals, thus, it needsprocedures for the contention control.
- 1 to 8 timeslots can be allocated per GSM frame (TDMA)- The timeslots are shared between many users.- The access resources to the radio interface can be distributed dynamically
between the voice and data services, depending on the traffic load and thestrategy used by the operator for the offer of new services.
- Different types of radio channels coding schemes are defined.
The structure of the GPRS timeslot is the same as in GSM. In the contrary, theGPRS defines a new burst operation mode in which a burst corresponds to 456coded information bits, which corresponds to 4 normal bursts. The GPRS uses the
same modulation scheme as in the GSM standard and defines 4 levels ofinformation coding.
The possible transmission rate varies between 9,05 kbit/s and 171,2 kbit/sdepending on the number of used timeslots (1 to 8) and the type of the usedcoding scheme (CS1, CS2, CS3, CS4). These rates does not correspond to the ratesoffered to the user because they depends on the signal to Interference ratio, theload of the network,…Added to this, the network does not carry only the user
information, it carries also the signaling data. The utile transmission rate (which is
really used by the user) is then less than the available rate.
As it was said, the GPRS defines 4 coding schemes: CS1, CS2, CS3, CS4 for the datatransmission on the radio channels. Each one provides different level of errorcorrection and effective bandwidth on the same radio channel:
• CS1 and CS2 offer transmission rates equal respectively to, 9,05 and 13,4kbit/s per each used timeslot. Within these two modes, the mobileterminals located at the borders of the cell can be reached. Any corruptionor loss of data due to a low Signal to Interference ratio is managed by theerror correction.
•
CS3 and CS4 offer transmission rates equal respectively to, 15,6 and 21,4kbit/s, but the obtained value is lower due to the weak protection ensuredby these coding schemes.
Depending on the chosen CS and per timeslot reserved to the data transmission,the transmission rate can reach a maximum value comprised between 9,05 and21,4 kbit/s. With a sophisticated GPRS terminal capable theoretically to combine 4timeslots and use CS4, a transmission rate of 85kbit/s can be reached on thedownlink.In the practice the situation is different, and for economical and technical reasonsmost of the mobile operators have chosen to use mainly the first two codingschemes. In fact, in the first GPRS networks implementations, CS3 and CS4 areused only in the zones with high signal levels (near the centers of the cells) becausethe implementation of these coding schemes would require important investmentsand perfect transmission conditions without interferences and perturbations.
CS-1 makes use of the same coding scheme as has been specified for SDCCH inGSM Rec. 05.03. It consists of a half rate convolutional code for forward errorcorrection FEC. The same thing for CS-2 and CS-3 which represent puncturedversions of the same half rate convolutional code. CS-4 has no redundancy intransmission (no FEC)
2.2 GPRS Subscriber ProfileEvery GPRS subscriber has a profile describing the set of services that he can use.Mainly this profile includes the description of the one or several PDPs used by theGPRS subscriber. Note that he can also use one PDP with different addresses. Thefollowing parameters are available for each PDP:The packet network address permits the identification of the user in the publicdata network. Note that in case of GPRS, either IP or X.25 data networks are used.In case of an external IP data network, this address can be assigned dynamically orstatically.
Note also that the problem related to the availability of IP addresses in Ipv4 will beovercome with the implementation of Ipv6.The quality of service QoS: specifies various parameters specifying the highestQoS that can be asked by the user.The screening profile: depending on the used PDP and the GPRS nodes capacity,the screening profile serves to restrict acceptance during transmission/reception ofpacket data. For example, a subscriber can be restricted with respect to his possiblelocation, or with respect to certain specific applications.The GGSN address: this address specifies which GGSN is used by the subscriber.In this way the point of access to external packet data networks PDN is defined.
Each GSN will have an IP address and the internal routing of the data isperformed by IP protocol. A DNS function is then needed to make the addresstranslation and find the destination of the data packets (e.g. www.gsn-xxx.com →
2.3 Quality of Service (QoS) ProfilesThe data transmissions via the GPRS network can be achieved according todifferent profiles of the quality of services. A profile of quality of service isassociated to each PDP (Packet Data Protocol) context. Note that the GPRS allowthe reservation of resources before starting the transmission of packets. Theamount of reserved resources depends on the type of the data application and itsrequirements.In the GPRS standard, a Quality of Service profile is defined by the followingattributes:
- Precedence Class;- Delay Class: 4 values are possible including the one corresponding to the
best effort;- Reliability Class: 5 values are possible including the Data Loss Ratio, the
Corrupted Data Ratio, etc.;- Peak Throughput Class: 9 values are possible comprised between 8 kbit/s
and 2048 kbit/s;- Mean Throughput Class: 19 possible values.The combination of the values that can be taken by these attributes defineslarge variety of QoS profiles.
More information concerning the QoS definition in the GPRS networks can befound in the recommendations Rec. 02.60, 03.60.
The standard precise three different classes to allow assessment of the importanceof the data packets, in case of limited resources or overload situation:1. High precedence2. Normal precedence3. Low precedence
Delay Class
Four delay classes were specified in the GSM Recommendation 02.60. Each classhas its own requirements concerning the delay of the transport of the data throughthe GPRS network. The best effort corresponds to the class 4 and needs theminimum of requirements.
The GPRS standard specifies five reliability classes. Each class has its specificreliability characteristics such as the probability of data loss, data deliverybeyond/outside the sequence, twofold data delivery, and data falsification(probabilities 10-2 to 10-9).The first class guarantees the highest degree of reliability and is used for error-sensitive, non real-time applications without capabilities of compensating dataloss; and the fifth the lowest degree and is used for real-time applications that canaccept data loss.
Peak Throughput ClassDepending on the mobile terminal capacities and the radio resources availability,the GPRS standard specifies 9 throughput classes. Each class specifies in bytes/sthe maximum expected data rate without any guarantee that this rate can bereached over a certain period of time. Note that the maximum data rate doublesfrom one class to the next.The first class allows a peak transmission rate ranging between 1000bytes/s (8kbit/s) and 256,000 bytes (2048 kbit/s).
Mean Throughput Class
The GPRS standard specifies 19 mean throughput classes. Each class represents theexpected mean data rate /throughput for data transport via the GPRS network
during an activated link. The first class represents the best effort service and meansthat the data rate for the MS is made available on the basis of demand andavailability of resources. Class 2 stands for 100 bytes/h (0.22 bit/s), class 3 for 200bytes/h, class 4 for 500 bytes/h and class 5 for 1000 bytes/h, etc. till Class 19which stands for 50000000 bytes/h (111 kbit/s).
3.1 GPRS ArchitectureTo support the GPRS service, two GPRS Support Nodes (GSN) extends the GSMarchitecture: the SGSN and GGSN.
A GPRS Support Node GSN is an equipment of the type of an IP router equippedwith specific functions to meet the needs of GSM and GPRS networks (e.g MobilityManagement and Access Control)
The Serving GPRS Support Node SGSN or GSN Server is equivalent to an MSC
and ensures functions comparable to those of a Visited MSC. From the point ofview of mobile terminal moving in a given cell, an SGSN is like an access server tothe GPRS service. The SGSN is also responsible for the security and access controlissues.
The Gateway GPRS Support Node GGSN: it represents the interconnection point(like a Gateway) between the GPRS network and an external packet switchingnetwork (IP or X.25) and offers comparable functions of those of the GatewayMSC. The GPRS is connected to the others GPRS nodes through an IP backbone. Itcontains the routing information which will be used to directly route the data
packets transmitted from external network to the SGSN to which is related thedestination mobile terminal. The GGSN can ask the HLR localization informationconcerning a mobile terminal via the optional interface Gc.
In addition to the introduction of the GSNs, the Base Station Controllers will needan upgrade in their capacities for the control of the channels used for the handlingof the data packets and the introduction of new equipments like the Packet ControlUnit (PCU). The PCU is an equipment dedicated to the GPRS and is equivalent tothe GSM Transcoder and Rate Adapter Unit (TRAU) that permits the transmissionrate adaptation.The PCU can be located at the level of the SGSN or at the level of the RadioSubsystem causing some hardware changements.The HLR contains the information concerning the GPRS subscriber profile and willbe termed in the following pages by GPRS Register (GR).
3.2 GSM Phase 2+, InterfacesIn addition to the A-G interfaces defined in the GSM standard, different interfacespermit to the SGSN and GGSN to function with the existing GSM physical entities:
Gb – this interface is defined between the GSM radio part, adapted to support theGPRS, and the GPRS Network Sub-System. Located between the SGSN and theBSS part, the Gb interface allows the exchange of both signaling and user data. Aresource on the Gb interface is assigned only when data are being transmittedor/and received in contrary to the GSM A-interface which reserves physical
resource for the user for the complete duration of a connection.
Gc – this interface defined between the GGSN and the HLR is optional and servesthe GGSN to ask the HLR localization information concerning a certain mobileterminal.Gd – this interface is defined between an SMS-GMSC / SMS-IWMSC and an SGSNGf - this interface is defined between an SGSN and an EIRGi - this interface is defined between GPRS and an external packet data networkPDNGn - this interface is defined between two GPRS support nodes GSN within the
same PLMNGp - this interface is defined between two GSN located in different PLMNs. Itallows the supporting of GPRS services over an area of cooperating GPRS PLMNs.Gr - this interface is defined between an SGSN and a HLRGs - this interface is defined between an SGSN and an MSC/VLR. This interface isoptional and serves to support an MS using both GPRS and circuit switchedservices (e.g. update of location information).
3.3.1 Serving GPRS Support Node (SGSN) FunctionsAs mentioned before, the SGSN is equivalent to an MSC and is responsible forhandling many functions comparable to those of a Visited MSC in order toperform GPRS service.It is responsible for serving the GPRS mobiles located within the area affected to it,tracing their location (Mobility Management Functions) and performing thepaging, security, access control, and charging data collection procedures
(authentication/cipher setting procedures,...).During the mobile terminal declaration phase (GPRS Attach Procedure), the SGSNestablishes mobility context grouping information on the authentication andlocalization of the terminal.During the PDP context activation, the SGSN establishes a PDP context groupingrouting information to the GGSN that will be used by the mobile terminal.The SGSN can send localization information of the mobile terminal to theMSC/VLR via the optional Gs Interface and receive paging requests coming fromthe MSC/VLR.
3.3.2 Gateway GPRS Support Node (GGSN) FunctionsAs it was mentioned before, the GGSN is the node allowing contact/interworkingbetween a GSM network and a packet data network via the Gi interface. Thefunctions that it realizes are similar to those of a gateway MSC. These functionsinclude:The GGSN contains the routing information for GPRS subscribers available in thePLMN which will serves to contact the respective SGSN in the providing areawhere was located the mobile terminal.It also, ensures a screening function, can ask location information from the HLRvia the optional Gc interface and transfers data and signaling information to the
3.3.3 Physical Realization SGSN/GGSNThe SGSN and GGSN functions can be located physically in the same node or inseparated ones that can eventually belong to different mobile networks. The GPRSnodes are interconnected by an IP network and form then a private IP-based GPRSbackbone network).HLR (GPRS Register GR)The HLR contains the GPRS subscriber information. This information includesgeneral data such as the the subscriber identification and his terminal number; andGPRS specific data such as the GPRS address and number, the PDP address andtype, the quality of service profile and the Access Point Name.
3.3.4 Packet Control Unit PCUThe PCU is located in the BSS and serves mainly for:- The GPRS radio channels management functions, e.g. power control,
congestion control, broadcast control information for the temporalorganization of the packet data transfer for uplink and downlink.
- The channel access control functions, e.g. access request and grants.- The conversion of protocols from the Gb interface to the Um radio interface.
Depending on the different solutions of the vendors, the PCU can be positioned inthe BTS, the BSC or in spatial connection with the SGSN. Note that the choice ofone of the three mentioned options for positioning the PCU depends also of thetraffic that has to be handled by the PCU/BSS.
3.3.5 Channel Codec Unit CCUThe CCU is responsible to perform the channel coding, including the FEC (forwarderror correction) function and interleaving Radio channel measurements (RxQual,Rxlev, TA, etc)
3.3.6 GPRS Mobile Stations MSA Mobile Terminal GPRS can work in one of the three following operationalmodes:
• The Class A Operational Mode: the terminal can offer simultaneously GPRSand GSM services.
• The Class B Operational Mode: the terminal can be declared in the twonetworks (GSM and GPRS) and listen simultaneously to their signalizations,but it can offer only one service (GSM or GPRS) at the same time.
• The Class C Operational Mode: the terminal is only used to offer GPRSservices.
The Class A terminals are characterized by a high capacity. They can emit andreceive voice, data or fax in a circuit mode simultaneously with packet datatransfer. This type of terminal is expensive and its use can be highly charged.
The Class C terminals manage only packet data and do not have the capability tosupport voice communications. Such type of terminal is ideal for the laptops andPDAs.
The Class B terminals constitute a compromise between the two precedent types.
They can manage paging channels for, both, data and voice requests, but they caneffectively ensure only one of them. For example, the data transfer would beinterrupted during a voice communication.
4.1 Logical Functions in the GPRS NetworkThe introduction of the GPRS has two types of impacts: at the level of the GSMarchitecture and at the functional level. The first type of impact was described inthe third chapter; the second type concerns some GSM functions and procedureswhich have to evolve to support the new specific GPRS functions and procedures.Thus, added to the functions similar to those of the GSM such as theauthentication, the ciphering, attach/detach specific functions and procedureshave been added for the GPRS.
In the GPRS networks we distinguish 6 types of functions:- The Network Access Control Functions- The Packet Routing and Transfer Functions- The Mobility Management Functions- The Logical Link Management Functions- The Radio Resources Management Functions- The Network Management Functions
The following table indicates the list of the GPRS functions included in thefunctions types listed above.
Function Type Included Functions
Network Access Control User Registration, access authentication andauthorisation, admission control, messagefiltering, terminal packet adaptation,charging data recuperation
Packet Routing and Transfer Relay, routing, addresses corresponding andtranslation, encapsulation, tunnelling,compression, ciphering
Mobility Management Same functions as the GSM mobilitymanagement (Location Update, Paging…)
Logical Link Management Logical Link establishment, Logical LinkMaintenance, Logical Link Liberation
Radio Resources Management Um interface Management, Cell Selection,Um-Tranx Function, Route Management
Network Management Functions of Operation & Maintenancerelated to the GPRS networks.
4.1.1 Network Access Control FunctionsThe GPRS network Access Control functions pilot the network access which meansthe items via which a user will be connected to the telecommunication network inorder to use the services offered by the operator.
The GPRS subscriber is identified by his IMSI as in the GSM network. Thedeclaration of the GPRS subscriber is realised in the same HLR of the existing GSMnetwork.
Added to the PTP (Peer to Peer) standard, the GPRS specifies an anonymousaccess to the network, allowing a mobile terminal to exchange data with apredefined machine. A limited number of PDP destination addresses can be usedby this service. In this case, the IMSI and IMEI are not used during the procedureof the access to the network, in order to grant a high level of confidentiality. Theauthentication and ciphering do not concern such particular type of access.
Registration function: like in the GSM networks, this function references to aprocedure which is not described by messages and results from a LUP (LocationUpdate) procedure. Thus, a LUP permit the registration of the mobile terminal in aspecific Routing Area. It permits also to associate the identity of a mobile terminal
to the user packet data protocols and the PDP addresses used within the PLMNand at the level of the access points to the external packet data networks.This association can be static (Configured and registered within the HLR) ordynamic (established on demand). In the last case, the addresses can be chosenwithin a group of addresses reserved to this issue.
Authentication and authorization function: this function permits to verify thatthe user is well authorized to use the service he is asking. Both, the user identityand his affiliation to the requested service are verified. The authentication functionis carried out in conjunction with the mobility management functions.
Admission control function: this function permits to evaluate of the networkresources needed to provide the desired QoS, determine if these resources areavailable or noand finally reserve them. This function is carried out in conjunctionwith the radio resources management functions to enable assessment of radioresources requirements per each individual cell.
Message screening function: it permits to avoid the transmission of unauthorized
or undesirable messages. A packet blocking mechanism can support this function.This function can be used also in order to restrict the type or volume of messages
that can be sent via the GPRS network transmitted or received to/from asubscriber. We distinguish three types of message screening functions:
- The screening controlled by the network (defined by the Operator)- The screening controlled by the subscriber (defined in the contract)- The screening controlled by the user (defined in the contract and approved
by the subscriber)The first type of control is included in the phase 1 of GPRS and the two otherstypes are included in the ulterior phases.
Packet terminal adaptation function: this function permits the adaptation of thedata packets received or sent by the terminal in an adequate form suited for thetransport via the GPRS network.
Charging data collection function: This function is used for the collection of dataserving for the charging of the service offered within the contract.
4.1.2 Packet Routing and Transfer FunctionsThese functions pilot the data routing from the expeditor to the destination withall the needed relay, addresses translation and tunneling functions.The Packet Routing and Transfer Functions are:
- The Relay Functions- The Routing- The Address Translation and Mapping- The Encapsulation- The Tunneling- The Compression- The Ciphering
The Relay function is the mean via which a node sends the received data ofa precedent node to the next node on the route. Note that a route is anorderly list of nodes used for the transfer of information within andbetween networks. Each route is formed of:- An Origin node;- One or several Relay nodes;- A destination node.
The Routing Function is the process of determining and using, based on somerules, a route for the transmission of information within or between networkstowards the destination node. In the GPRS networks, the routing consists in theselection of the transmission path to carry the incoming flow of data. Thisprocedure determines the GSN node to which the data has to be transmitted anduses the service offered by the lower layer protocols to reach the identified GSNbased on the destination address indicated in the message. The data betweenGSNs can transit via external networks that are processing their own internalrouting functions: e.g, X.25, Frame Relay or ATM.
Address translation function: it consists in converting an address into anotheraddress. Note that the two addresses are issued from different protocols. Thisfunction is used for example to convert an address used in an external networkinto an internal address for the routing of the packets within and between PLMNs.
Address mapping function: this function permits the association of a networkaddress to another network address of the same type. An example of the utility ofthis function can be the routing and transmitting of messages from one networknode to the next.
Encapsulation function: corresponds to the addition of information (related to the
addressing and the control) to a data unit (a packet for example) for the routing ofpackets within and between PLMNs. The opposite function is calleddecapsultation and both functions are effected the GSNs as well as between theSGSN and the MS.
Tunneling Function: it designates the transfer of encapsulated data between thepoint where was made the encapsulation and the point where will be realized thedecapsulation. It is achieved thanks to the mechanism of tunnel in a bi-directionalpoint-to-point connection. Please note that in a tunnel only the endpoints areidentified.
Compression function: this function serves for the optimization of the radio linkcapacity by transmitting the minimum number of SDU (Service Data Unit) whilepreserving the transmitted information.The compression function is achieved by the SNDCP and is applied between themobile terminal and the SGSN to the brut data issued from the application and thecontrol data when the used protocol is IP.
Ciphering function: preserves the user and signaling data confidentiality throughthe radio channels, and thus, it protects the network against the intrusions andprevent eavesdropping. In the GPRS, the ciphering is realized by the LLC layer
and is applied between the mobile terminal and the SGSN differently from theGSM where this function concerns only the radio subsystem: MS, BTS and BSC.
Domain name server function: this function serves to decode logical GSN names inGSN addresses.
4.1.3 Mobility Management FunctionsThe Mobility Management Functions are used to enable the network to keep acurrent trace of each mobile terminal within its databases independently of theirmovements in either the home or visited PLMN.The GPRS M
4.1.4 Logical Link Management FunctionsThe GPRS Mobility Management can be combined with the GSM MobilityManagement by using the Gs interface, defined between the MSC/VLR and theSGSN.
A logical link represents the connection of a data application through the GPRSnetwork between a mobile terminal and an external PDN (Packet Data Network)The Logical Link Management functions consist in the maintenance of acommunication channel between a mobile terminal and the network via the radiointerface. These functions include the coordination of the information concerningthe state of the link between the mobile terminal and the network, and thesupervision of the data transfer activity over the logical link.The Logical Link Management functions are:
- Logical Link Establishment- Logical Link Management
-
Logical Link ReleaseThe Logical Link Establishment is performed when the terminal is attached to theGPRS service.The Logical Link Maintenance supervises the Logical Link State and its variations.The Logical Link Release is used to liberate the resources allocated to the logicalLink Connection.
4.1.5 Radio Resource Management FunctionsThese functions include the allocation and maintenance of communicationchannels via the radio interface. The GSM radio resources are divided between the
circuit mode (voice) and the packet mode (data).The Radio Resource Management functions are:- Um Management: manages the physical channels used within each cell and
determines the quantity of resources to allocate to be able to use the GPRSservices. This quantity of resources can vary from a cell to anotherdepending on the demand of the users or other strategies established by thenetwork operator.
- Cell Selection: allows a mobile terminal to select the optimal cell to use to
establish a communication channel with the network. This function includesthe measurement and evaluation of the signal quality within the
neighboring cells. It permits the detection and the avoidance of the overloadwithin the eligible cells. More details can be found in the recommendationsGSM 03.22 and GSM 03.64.- Um-Tranx: offers capacity for the transfer of data packet via the radio
interface Um. This function includes procedures for the medium accesscontrol via radio channels, for the packets multiplexing via shared physicalradio channels, for error detection and correction and for flow control.
- Path Management: manages the communication paths of packet databetween the radio subsystem and the GPRS nodes. The establishment andrelease of the paths can be dynamic depending of the importance of the datatraffic or static based on the maximum load expected in each cell.
Added to these functions, the GPRS specifications defined a flow control functionin different equipments and at the level of several protocols. This function can acton the following protocols and layers:
- LLC between the mobile terminal and the SGSN which means at the level ofthe interfaces Um and Gb.
- BSSGP between the radio subsystem and the SGSN on the Gb interface.
- Network Service between the radio subsystem and the SGSN over the Gbinterface.
4.1.6 Network Management FunctionsThese functions provide the essential mechanisms for the support of the GPRSOperation & maintenance functions.
4.2 Allocation of Logical FunctionsIn the GSM/GPRS networks, the tasks described in the logical functions can beallocated to various functional units such as the mobile station MS, the base stationsubsystem BSS (with the packet control unit PCU and channel codec unit CCU),the serving GPRS support node SGSN and the gateway GPRS support nodeGGSN. The table below resumes the functions handled by each of these units.
Function MS BSS SGSN GGSN HLR
Network Access Control:Registration X
Authentication & Authorization X X X
Admission Control X X X
Message Screening X
Packet Terminal Adaptation X
Charging Data Collection X XPacket Routing & Transfer:
Relay X X X X
Routing X X X X
Address Translation & Mapping X X XEncapsulation X X X
Tunneling X X
Compression X X X
Ciphering X
Domain Name Server X X X XMobility Management
Logical Link Management: X X
Logical Link Establishment X X
Logical Link Maintenance X X
Logical Link ReleaseRadio Resource Management: X X
