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  • 1. GPRS Architecture GPRS Architecture Contents Objectives...................................................................................................2 2 qeletene.und.slo 1 GPRS Subscriber Profile.......................................................................... 3 3 3.1 3.2 3.3 3.4 GPRS QoS Profile..................................................................................... 5 Rel'99 QoS parameter set .........................................................................12 Traffic classes............................................................................................16 Ranges of Rel'99 attributes.......................................................................18 Mapping between QoS parameters in Rel'97 and Rel'99......................... 19 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 GPRS Logical Functions........................................................................ 21 Logical Functions in the GPRS Network .................................................. 21 Network Access Control Functions ........................................................... 22 Packet Routing and Transfer Functions ................................................... 24 Mobility Management Functions ............................................................... 26 Logical Link Management Functions ........................................................ 26 Radio Resource Management Functions ..................................................26 Network Management Functions .............................................................. 27 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 Network elements ................................................................................... 28 Packet Control Unit (PCU-PIU of BSC) .................................................... 29 Channel Codec Unit (CCU) .......................................................................30 Serving GPRS Support Node (SGSN) ......................................................30 Gateway GPRS Support Node (GGSN) ................................................... 31 GPRS MS ..................................................................................................32 Domain Name Servers..............................................................................35 Firewalls.................................................................................................... 35 Border Gateway........................................................................................ 36 Charging Gateway.....................................................................................36 6 GPRS Interfaces...................................................................................... 37 7 Transfer of Packets between GSNs...................................................... 40 © Nokia Siemens Networks 2008 1-1
  • 2. GPRS Architecture 1 Objectives After completing this learning element, the student should be able to: qeletene.und.slo Explain the GPRS subscriber profile, GPRS QoS profile, and GPRS logical functions Name the GPRS specific network elements and their most important functions Name and explain five important open interfaces in the GPRS network Explain the principle of transfer of packets between GSNs 1-2 © Nokia Siemens Networks 2008
  • 3. GPRS Architecture 2 GPRS Subscriber Profile The GPRS Subscriber Profile is the description of the services a subscriber is allowed to use. Essentially, it contains the description of the packet data protocol used. A subscriber may also use different packet data protocols (PDPs), or one PDP with different addresses. The following parameters are available for each PDP: The packet network address is necessary to identify the subscriber in the public data net. Either dynamically assigned (temporary) addresses or (in the future) static addresses are used in case of IP. The problem of the dynamic addresses will be overcome with the change from Ipv4 to IPv6. In GPRS is two layer 2 protocols are allowed, X.25 or IP. The quality of service QoS : QoS describes various parameters. The subscriber profile defines the highest values of the QoS parameters that can be used by the subscriber. qeletene.und.slo The screening profile: This profile depends on the PDP used and on the capacity of the GPRS nodes. It serves to restrict acceptance during transmission/reception of packet data. For example, a subscriber can be restricted with respect to his possible location, or with respect to certain specific applications. The GGSN address: The GGSN address indicates which GGSN is used by the subscriber. In this way the point of access to external packet data networks PDN is defined. The internal routing of the data is done by IP protocol; the GSNs will have IP addresses. A DNS function is needed to find the destination of the data packets (address translating: e.g. www.gsn-xxx.com → 129.64.39.123) © Nokia Siemens Networks 2008 1-3
  • 4. GPRS Architecture qeletene.und.slo Fig. 1 Part of the GPRS subscriber profile are the PDPs and their parameters 1-4 © Nokia Siemens Networks 2008
  • 5. GPRS Architecture 3 GPRS QoS Profile The different applications that will make use of packet-oriented data transmission via GPRS require different qualities of transmission. GPRS can meet these different requirements because it can vary the quality of service (QoS) over a wide range of attributes. The quality of service profile (Rec. 02.60, 03.60) permits selection of the following attributes: Precedence class Delay class Reliability class Peak throughput class Mean throughput class. By combining the variation possibilities of the individual attributes a large number of QoS profiles can be achieved. Only a limited proportion of the possible QoS profiles need PLMN-specific support. qeletene.und.slo Fig. 2 Quality of service parameters © Nokia Siemens Networks 2008 1-5
  • 6. GPRS Architecture Precedence Class Three different classes have been defined to allow assessment of the importance of the data packets, in case of limited resources or overload: High precedence Normal precedence Low precedence Delay Class GSM Rec.02.60 defines 4 delay classes (1 to 4). However, a PLMN only needs to realize part of these. The minimum requirement is the support of the so-called „best effort delay class“ (Class 4). Delay requirements (maximum delay) concern the delay of transported data through the entire GPRS network (the first two columns refer to data packets 128 bytes in length, while the last two columns apply to packets 1024 bytes in length). Mean transfer delay (sec) 95% delay (sec) Mean transfer delay (sec) 95% delay (sec) 1 < 0,5 < 1,5 <2 <7 2 <5 < 25 < 15 < 75 3 < 50 < 250 < 75 < 375 4 (Best Effort) unspecified unspecified unspecified unspecified Table 1 Delay Class 1-6 © Nokia Siemens Networks 2008 qeletene.und.slo Delay Class
  • 7. GPRS Architecture qeletene.und.slo Fig. 3 QoS is an assumption of several parameters, which are defined in the recommendations Reliability class Transmission reliability is defined with respect to the probability of data loss, data delivery beyond/outside the sequence, twofold data delivery, and data falsification (probabilities 10-2 to 10-9):. 5 reliability classes (1 to 5) have been defined, 1 guaranteeing the highest and 5 the lowest degree of reliability. Highest reliability (Class 1) is required for error-sensitive, non-real-time applications, which have no possibility of compensating for data loss; lowest reliability (Class 5) is needed for real-time applications which can get over data loss. The reliability classes (see Table 2) define the probability of: Lost data Duplication of data Data arriving out of sequence Corruption of data The reliability class specifies the requirements of the various network protocol layers. The combinations of the GTP, LLC, and RLC transmission modes support the reliability class performance requirements. © Nokia Siemens Networks 2008 1-7
  • 8. GPRS Architecture Reliability Class GTP Mode LLC Frame Mode LLC Data Protection RLC Block Mode Traffic Type 1 Acknowledg ed Acknowledg ed Protected Acknowledg ed Non-real-time traffic, error-sensitive application that cannot cope with data loss. 2 Unacknowle dged Acknowledg ed Protected Acknowledg ed Non-real-time traffic, error-sensitive application that can cope with infrequent data loss. 3 Unacknowle dged Unacknowle dged Protected Acknowledg ed Non-real-time traffic, error-sensitive application that can cope with data loss, GMM/SM, and SMS. 4 Unacknowle dged Unacknowle dged Protected Unacknowle dged Real-time traffic, error-sensitive application that can cope with data loss. 5 Unacknowle dged Unacknowle dged Unprotected Unacknowle dged Real-time traffic, error non-sensitive application that can cope with data loss. qeletene.und.slo Table 2 Reliability classes Note : Signalling and SMS are transferred with reliability class 3. 1-8 © Nokia Siemens Networks 2008
  • 9. GPRS Architecture Throughput classes The throughput class indicates the data throughput requested by the user. Throughput is defined by two negotiable parameters: Maximum bit rate Mean bit rate. This includes, for example for "bursty" transmissions, the periods in which no data is transmitted. The maximum and mean bit rates can be represented by a parameter known as the Information Transfer Rate. It is possible for the network to re-negotiate the throughput parameters at any time during a session. User data throughput is specified in terms of a set of throughput classes that characterise the expected bandwidth required for a PDP context. Maximum bit rate qeletene.und.slo The maximum bit rate is measured in octets per second at the Gi and R reference points. It specifies the maximum rate at which data is expected to be transferred across the network for an individual PDP context. There is no guarantee that this maximum rate will be achieved or sustained for any time period as this depends upon the MS capability and available radio resources. The network may limit the subscriber to the negotiated maximum data rate, even if additional transmission capacity is available. The maximum throughput is independent of the particular delay class being used. The maximum (peak) throughput classes are defined in Table 3. © Nokia Siemens Networks 2008 1-9
  • 10. GPRS Architecture Max. Throughput Class Max. Throughput in octets per second 1 Up to 1000 (8 kbit/s) 2 Up to 2000 (16 kbit/s) 3 Up to 4000 (32 kbit/s) 4 Up to 8000 (64 kbit/s) 5 Up to 16 000 (128 kbit/s) 6 Up to 32 000 (256 kbit/s) 7 Up to 64 000 (512 kbit/s) 8 Up to 128 000 (1024 kbit/s) 9 Up to 256 000 (2048 kbit/s) Table 3 Maximum bit rate classes The mean bit rate (throughput) is measured at the Gi and R reference points in units of octets per hour . It specifies the average rate at which data is expected to be transferred across the GPRS network during the remaining lifetime of an activated PDP context. The network may limit the subscriber to the negotiated mean bit rate (for example, for flat rate charging), even if additional transmission capacity is available. A 'best effort' means bit rate class may be negotiated. This means that bandwidth will be made available to the MS on a need and availability basis. The mean throughput classes are defined in Table 4. Note : ETSI GPRS specifications define several QoS classes which are associated with each PDP context, covering priority, reliability, delay, and throughput. The NSN GPRS system release 1 does not support this QoS functionality. The GPRS QoS can be considered as ‘best effort’. 1-10 © Nokia Siemens Networks 2008 qeletene.und.slo Mean bit rate
  • 11. GPRS Architecture Mean Throughput in octets per hour 1 Best effort. 2 100 (~0.22 bit/s) 3 200 (~0.44 bit/s) 4 500 (~1.11 bit/s) 5 1000 (~2.2 bit/s) 6 2000 (~4.4 bit/s) 7 5000 (~11.1 bit/s) 8 10 000 (~22 bit/s) 9 20 000 (~44 bit/s) 10 50 000 (~111 bit/s) 11 100 000 (~0.22 kbit/s) 12 200 000 (~0.44 kbit/s) 13 500 000 (~1.11 kbit/s) 14 1 000 000 (~2.2 kbit/s) 15 2 000 000 (~4.4 kbit/s) 16 5 000 000 (~11.1 kbit/s) 17 10 000 000 (~22 kbit/s). 18 20 000 000 (~44 kbit/s). 19 50 000 000 (~111 kbit/s). qeletene.und.slo Mean Throughput Class Table 4 Mean bit rate classes © Nokia Siemens Networks 2008 1-11
  • 12. GPRS Architecture 3.1 qeletene.und.slo Fig. 4 QoS is an assumption of several parameters, which are defined in the recommendations Rel'99 QoS parameter set Rel'99 parameters are specified for UMTS. NSN GPRS Release 2 also supports these parameters. The list of attributes in Rel'99 are given below: Maximum bit rate specifies the maximum rate at which the data is expected to be transferred in the network for a PDP context. The subscribed transfer rate is not guaranteed; it just specifies the limit that cannot be exceeded. Its purpose is to limit the delivered bit rate to applications or external networks with such limitations and to allow maximum wanted user bit rate to be defined for applications able to operate with different rates, for example, non-transparent circuit switched data. Compare Rel'97/98, similar as 'Peak throughput class'. 1-12 © Nokia Siemens Networks 2008
  • 13. GPRS Architecture Guaranteed bit rate specifies guaranteed bit rate delivered in the network for the PDP context. Guaranteed bit rate may be used to facilitate admission control based on available resources, and for resource allocation. Quality requirements expressed by, for example, delay and reliability attributes only apply to incoming traffic up to the guaranteed bit rate. Delivery order (y/n) indicates whether the bearer shall provide in-sequence SDU delivery or not. The attribute is derived from the user protocol (PDP type) and specifies if out-of-sequence SDUs are acceptable or not. This information cannot be extracted from the traffic class. Whether out-of-sequence SDUs are dropped or re-ordered depends on the specified reliability required for the application. Compare Rel'97/98, similar as 'Reordering required'. Maximum SDU size (maximum allowed SDU size, octets) is used for admission control and policing. Policing makes sure that bandwidth limits of the PDP context are not exceeded to protect radio interface. Admission control calculates what network resources are required to provide the requested QoS, determine if resources are available, and reserve them. The admission controller in SGSN has the responsibility to accept or reject PDP context activation and the requested QoS parameter values. qeletene.und.slo SDU format information (list of possible exact sizes of SDUs, bits) is needed because network needs SDU size information to be able to operate in transparent RLC protocol mode, which is beneficial to spectral efficiency and delay when RLC re-transmission is not used. Thus, if the application can specify SDU sizes, the bearer is less expensive. SDU format info is not supported by NSN 2G-SGSN. SDU error ratio indicates the fraction of SDUs lost or detected as erroneous. By reserving resources, SDU error ratio performance is independent of the loading conditions, whereas without reserved resources, such as in Interactive and Background classes, SDU error ratio is used as target value. SDU error ratio is mapped with Rel'97/98 'Reliability class'. Residual bit error ratio indicates the undetected bit error ratio in the delivered SDUs. If no error detection is requested, residual bit error ratio indicates the bit error ratio in the delivered SDUs. Residual bit error ratio is mapped with Rel'97/98 'Reliability class'. © Nokia Siemens Networks 2008 1-13
  • 14. GPRS Architecture Delivery of erroneous SDUs (y/n/-) indicates whether SDUs detected as erroneous shall be delivered or discarded. Delivery of erroneous SDUs is used to decide whether error detection is needed and whether frames with detected errors shall be forwarded or not. A 'yes' value implies that error detection is employed and that erroneous SDUs are delivered together with an error indication, and 'no' implies that error detection is employed and that erroneous SDUs are discarded, and '-' implies that SDUs are delivered without considering error detection. Residual bit error ratio is mapped with Rel'97/98 'Reliability class'. SDU error ratio Residual bit error ratio Delivery of erroneous SDUs Traffic type -6 10 -5 No Non-real-time traffic, error sensitive application that cannot cope with data loss 10 -6 10 -5 No Non-real-time traffic, error sensitive application that can cope with infrequent data loss 10 -4 10 -5 No Non-real-time traffic, error sensitive application that can cope with data loss 10 -3 10 -5 No Real-time traffic, error sensitive application that can cope with data loss. 10 -3 4*10 Yes Real-time traffic, error non-sensitive application that can cope with data loss. -3 Table 5 Traffic examples mapped to Rel'99 attributes 1-14 © Nokia Siemens Networks 2008 qeletene.und.slo 10
  • 15. GPRS Architecture Note : For real-time traffic, the QoS profile also requires appropriate settings for delay and throughput. Signalling and SMS are transferred with reliability class 3. Transfer delay (ms) indicates maximum delay for 95% of the distribution of delay for all delivered SDUs during the lifetime of a bearer service. Transfer delay is used to specify the delay tolerated by the application. Traffic handling priority specifies the relative importance for handling of all SDUs belonging to the radio access bearer compared to the SDUs of other bearers. Traffic handling priority is mapped with Rel'97/98 'Delay class'. Allocation/Retention priority is used for differentiating between bearers. In situations where resources are scarce, the relevant network elements can prioritise bearers when performing admission control. Attribute has three categories: High . Users whose packets will never be discarded Normal. Users whose packets will be discarded sometimes Low . The low priority class users whose packets will be discarded The Allocation/Retention priority attribute is a subscription attribute which is not negotiated from the mobile terminal. The addition of a user-controlled Allocation/Retention priority attribute is for further study in future releases. Allocation/Retention priority is mapped with Rel'97/98 'Precedence class'. qeletene.und.slo Source statistics descriptor is used for conversational and streaming classes for ('speech'/'unknown'). Since conversational class is not supported by GPRS, NSN 2G-SGSN does not support Source statistics descriptor. © Nokia Siemens Networks 2008 1-15
  • 16. GPRS Architecture 3.2 Traffic classes End-user applications can be categorised in major groups according to their main QoS requirements. There are four different Rel'99 QoS traffic classes: qeletene.und.slo Conversational class Streaming class Interactive class Background class Fig. 5 Rel'99 QoS traffic classes The main distinguishing factor between these QoS traffic classes is how delay sensitive the traffic is: Conversational class is meant for traffic which is very delay sensitive while Background class is the most delay insensitive traffic class. Conversational and Streaming classes are intended to be used to carry real-time traffic flows. The main difference between them is how delay sensitive the traffic is. Conversational real-time services, like video telephony, are the most delay sensitive applications and those data streams should be carried in Conversational class. 1-16 © Nokia Siemens Networks 2008
  • 17. GPRS Architecture Interactive class and Background are mainly meant to be used by traditional Internet applications like WWW, e-mail, Telnet, FTP, and News. The main difference between Interactive and Background class is that Interactive class is mainly used by interactive applications, for example, interactive e-mail or interactive web browsing, while Background class is meant for background traffic, for example, background download of e-mails or background file downloading. Responsiveness of the interactive applications is ensured by separating interactive and background applications. Traffic in the Interactive class has higher priority in scheduling than Background class traffic, so background applications use transmission resources only when interactive applications do not need them. This is very important in wireless environment where the bandwidth is low compared to fixed networks. Although the bit rate of a conversational source codec may vary, conversational traffic is assumed to be relatively non-bursty. Maximum bit rate specifies the upper limit of the bit rate with which the bearer delivers SDUs. The bearer is not required to transfer traffic exceeding the guaranteed bit rate. As for conversational class, streaming traffic is assumed to be rather non-bursty. Maximum bit rate specifies the upper limit of the bit rate. This class is optimised for transport of human or machine interaction with remote equipment, such as web browsing. The source characteristics are unknown but may be bursty. qeletene.und.slo The background class is optimised for machine-to-machine communication that is not delay sensitive, such as messaging services. Background applications tolerate a higher delay than applications using the interactive class, which is the main difference between the background and interactive classes. © Nokia Siemens Networks 2008 1-17
  • 18. GPRS Architecture 3.3 Ranges of Rel'99 attributes Traffic class Conversational class Streaming class Interactive class Background class Maximum bit rate (kbps) < 2 048 (2) < 2 048 (2) Delivery order Yes/No Yes/No Yes/No Yes/No Maximum SDU size (octets) <=1 500 or 1 502 (4) <=1 500 or 1 502 (4) <=1 500 or 1 502 (4) <=1 500 or 1 502 (4) (9) (9) Yes/No/- Yes/No/- SDU format information Delivery of erroneous SDUs Yes/No/-2 Yes/No/- -2 -3 -2 -2 -5 -3 -4 -3 -5 -3 -4 10 , 7*10 , 10 , -4 -5 10 , 10 10 , 10 , -3 -3 7*10 , 10 , -4 -5 10 , 10 10 , 10 , 10 Transfer delay (ms 80-100 up to FFS (8) (5) 250 up to FFS (8) (10) Guaranteed bit rate (kbps) < 2 048 (1) (2) < 2 048 (1) (2) (11) (12) 1,2,3 (7) (12) 1,2,3 (7) 1,2,3 (7) -1 -2 Traffic handling priority Allocation/Retentio n priority Source statistic descriptor 1,2,3 (7) 1,2,3 (7) -6 10 , 10 , 10 Speech/unknown(1) Speech/unkno wn (1) Table 6 Value ranges of Rel'99 attributes 1-18 -8 (10) -3 4*10 , 10 , 6*10 (6) © Nokia Siemens Networks 2008 -6 qeletene.und.slo SDU error ratio -3 4*10 , 10 , 6*10 (6) -8 5*10 , 10 , 5*10 -3 -4 -6 , 10 , 10 , 10 -2 5*10 , 10 , -3 -3 5*10 , 10 , -4 -5 -6 10 , 10 , 10 -3 Residual BER
  • 19. GPRS Architecture 3.4 Mapping between QoS parameters in Rel'97 and Rel'99 Since there are two different parameter sets (Rel'97 and Rel'99) and they might be used simultaneously in a same network, these parameter must be mapped with each other. qeletene.und.slo Fig. 6 Rules for determining Rel'99 attributes from Rel-97/98 attributes © Nokia Siemens Networks 2008 1-19
  • 20. qeletene.und.slo GPRS Architecture Fig. 7 Rules for determining Rel'97 attributes from Rel'99 attributes 1-20 © Nokia Siemens Networks 2008
  • 21. GPRS Architecture 4 GPRS Logical Functions 4.1 Logical Functions in the GPRS Network The tasks required for the handling of processes in the GSM-/GPRS network are structured into logical functions. These functions may contain a large number of individual functions. Logical functions are: Network access control functions Packet routing and transfer functions Mobility management functions Logical link management functions Network management functions qeletene.und.slo Fig. 8 Logical functions of the GPRS network © Nokia Siemens Networks 2008 1-21
  • 22. GPRS Architecture 4.2 Network Access Control Functions Registration function: Registration stands for linking the identity of the mobile radio subscriber to his packet data protocol (or protocols), the PLMN-internal addresses and the point of access of the user to external data Protocol (PDP) networks. This link can be static (HLR entry), or it can be effected on demand. Authentication and authorization function: This function stands for the identification of the subscriber and for access legitimacy when a service is demanded. In addition, the legitimacy of the use of this particular service is controlled. The authentication function is carried out in conjunction with the mobility management functions. Admission control function: Admission control is intended for determining the network resources required for performing the desired service (QoS). It also decides whether these resources are available, and lastly it is used for reserving resources. Admission control is effected in conjunction with the radio resource management functions to enable assessment of radio resources requirements in each individual cell. Message screening function: A "screening" function is combined with the filtering of unauthorized or undesirable information/messages. In the introduction stage of GPRS a network-controlled screening function is supported. Subscription-controlled and user-controlled screening may be additionally provided at a later stage. Packet terminal adaptation function: This function adapts data packets received/transmitted from/to the terminal equipment TE to a form suited for transport through the GPRS network. Charging data collection function: This function is used for collecting data required for billing. 1-22 © Nokia Siemens Networks 2008 qeletene.und.slo Network access means the way or manner in which a subscriber gains access to a telecommunication network to make use of the services this network provides. An access protocol consists of a defined set of procedures, which makes access to the network possible. Network access can be obtained both from the MS and from the fixed network part of the GPRS network. Depending on the provider, the interface to external data networks can support various access protocols, e.g. IP or X.25. The following functions have been defined for access to the GPRS network:
  • 23. GPRS Architecture qeletene.und.slo Fig. 9 Network access control functions © Nokia Siemens Networks 2008 1-23
  • 24. GPRS Architecture 4.3 Packet Routing and Transfer Functions A route consists of an orderly list of nodes used for the transfer of messages within and between the PLMNs. Each route consists of the node of origin, no node, one or several relay nodes, and the node of destination. Routing is the process of determining and using the route for the transmission of a message within or between PLMNs. Relay function: Transferring data received by a node from another node to the next node of the route. Address translation and mapping function: Address translation means transforming one address into another, different address. It can be used to transform addresses of external network protocols into internal network addresses (for routing purposes). Address mapping is used to copy a network address into another network address of the same type (e.g. for the routing and transmitting of messages from one network node to the next). Encapsulation function: Encapsulation means supplementing address- and control information into one data unit for the routing of packets within or between PLMNs. The opposite process is called decapsulation. Encapsulation and decapsulation is effected between the GSN of the GPRS-PLMN as well as between the SGSN and the MS. Tunneling Function : Tunneling means the transfer of encapsulated data units in the PLMN. A tunnel is a two-way point-to-point path, only the endpoints of which are identified. Compression function: for the optimal use of radio link capacity. Ciphering function: preventing eavesdropping Domain name server function: Decoding logical GSN names in GSN addresses. This function is a standard function of the internet. 1-24 © Nokia Siemens Networks 2008 qeletene.und.slo *Routing function: Determining the transmission path for the next hop on the route towards the GPRS support node (GSN) the message is intended for. Data transmission between GSNs can be effected via external data networks possessing their own routing functions; e. g. X.25, Frame Relay or ATM networks.
  • 25. GPRS Architecture qeletene.und.slo Fig. 10 Packet routing and transfer functions in the GPRS network © Nokia Siemens Networks 2008 1-25
  • 26. GPRS Architecture 4.4 Mobility Management Functions Mobility management functions are used to enable tracing the actual location of a mobile station in either the home-PLMN or a Visited-PLMN. 4.5 Logical Link Management Functions Logical link management functions concern maintenance of a communication channel between an MS and the PLMN via the radio interface Um. These functions include the coordination of link state information between the MS and the PLMN and the monitoring of data transfer activities via the logical link. Logical link establishment function: Building up a logical link by during GPRS attach. Logical link maintenance function: Monitoring of the state of the logical link and state modification control. 4.6 Radio Resource Management Functions Radio resource management functions include allocation and maintenance of communication channels via the radio interface. The GSM radio resources must be divided /distributed between circuit switched services and GPRS. Um management function: Managing available physical channels of cells and determining the share of radio resources allocated for use in the GPRS. This share may vary from cell to cell. Cell selection function: Allows the MS to select the optimal cell for a communication path. This includes measurement and evaluation of the signal quality of neighboring cells and detection and avoidance of overload in the eligible cells. Um-tranx function: Offers capacity for packet data transfer via Um. The function includes a. o. procedures for multiplexing packets via shared physical channels, for retaining packets in the MS, for error detection and correction, and for flow control. Path management function: Management of packet data communication between BSS and serving GSN node. Establishing and canceling these paths can be effected either dynamically (amount of traffic data) or statically (maximum load to be expected for each cell). 1-26 © Nokia Siemens Networks 2008 qeletene.und.slo Logical link release function: De-allocation of resources associated with the logical link.
  • 27. GPRS Architecture 4.7 Network Management Functions Network management functions provide mechanisms for the support of GPRS-related operation & maintenance functions. qeletene.und.slo Fig. 11 Mobility management, logical link, radio resource and network management functions © Nokia Siemens Networks 2008 1-27
  • 28. GPRS Architecture 5 Network elements Figure 12 shows the architecture of a GPRS network. The GPRS system brings some new network elements to an existing GSM network. These elements are: Packet Control Unit (PCU) Serving GPRS Support Node (SGSN): the MSC of the GPRS network Gateway GPRS Support Node (GGSN): gateway to external networks Border Gateway (BG): a gateway to other PLMN Intra-PLMN backbone: an IP based network inter-connecting all the GPRS elements Charging Gateway (CG) Legal Interception Gateway (LIG) Domain Name System (DNS) Firewalls: used wherever a connection to an external network is required. qeletene.und.slo Not all of the network elements are compulsory for every GPRS network. Fig. 12 GPRS architecture 1-28 © Nokia Siemens Networks 2008
  • 29. GPRS Architecture 5.1 Packet Control Unit (PCU-PIU of BSC) The PCU separates the circuit switched and packet switched traffic from the user and sends them to the GSM and GPRS networks respectively. It also performs most of the radio resource management functions of the GPRS network. The PCU can be either located in the BTS, BSC, or some other point between the MS and the MSC. There will be at least one PCU that serves a cell in which GPRS services will be available. Frame Relay technology is being used at present to interconnect the PCU to the GPRS core. qeletene.und.slo Fig. 13 PCU - its position within the BSS © Nokia Siemens Networks 2008 1-29
  • 30. GPRS Architecture 5.2 Channel Codec Unit (CCU) The CCU is realised in the BTS to perform the Channel Coding (including the coding scheme algorithms), power control and timing advance procedures. 5.3 Serving GPRS Support Node (SGSN) Protocol conversion (for example IP to FR) Ciphering of GPRS data between the MS and SGSN Data compression is used to minimise the size of transmitted data units Authentication of GPRS users Mobility management as the subscriber moves from one area to another, and possibly one SGSN to another Routing of data to the relevant GGSN when a connection to an external network is required Interaction with the NSS (that is, MSC/VLR, HLR, EIR) via the SS7 network in order to retrieve subscription information Collection of charging data pertaining to the use of GPRS users Traffic statistics collections for network management purposes. 1-30 © Nokia Siemens Networks 2008 qeletene.und.slo The SGSN is the most important element of the GPRS network. The SGSN of the GPRS network is equivalent to the MSC of the GSM network. There must at least one SGSN in a GPRS network. There is a coverage area associated with a SGSN. As the network expands and the number of subscribers increases, there may be more than one SGSN in a network. The SGSN has the following functions:
  • 31. GPRS Architecture 5.4 Gateway GPRS Support Node (GGSN) The GGSN is the gateway to external networks. Every connection to a fixed external data network has to go through a GGSN. The GGSN acts as the anchor point in a GPRS data connection even when the subscriber moves to another SGSN during roaming. The GGSN may accept connection request from SGSN that is in another PLMN. Hence, the concept of coverage area does not apply to GGSN. There are usually two or more GGSNs in a network for redundancy purposes, and they back up each other up in case of failure. The functions of a GGSN are given below: Routing mobile-destined packets coming from external networks to the relevant SGSN Routing packets originating from a mobile to the correct external network Interfaces to external IP networks and deals with security issues Collects charging data and traffic statistics Allocates dynamic or static IP addresses to mobiles either by itself or with the help of a DHCP or a RADIUS server Involved in the establishment of tunnels with the SGSN and with other external networks and VPN. qeletene.und.slo From the external network's point of view, the GGSN is simply a router to an IP sub-network. This is shown below. When the GGSN receives data addressed to a specific user in the mobile network, it first checks if the address is active. If it is, the GGSN forwards the data to the SGSN serving the mobile. If the address is inactive, the data is discarded. The GGSN also routes mobile originated packets to the correct external network. Fig. 14 GPRS network as seen by another data network © Nokia Siemens Networks 2008 1-31
  • 32. GPRS Architecture 5.5 GPRS MS qeletene.und.slo Different GPRS MS classes were introduced to cope with the different needs of future subscribers. The mobiles differ in their capabilities. Fig. 15 GPRS network as seen by another data network 1-32 © Nokia Siemens Networks 2008
  • 33. GPRS Architecture Three GPRS MS classes were defined: Class A: With a class A mobile GSM circuit switched services and GSM GPRS services can be simultaneously activated. A subscriber can get data from an active GPRS link while simultaneously making a phone call. A class A mobile allows also a simultaneous attach, activation and monitor of the classical GSM and GPRS services. Class B: A class B mobile allows a simultaneous attach, activation and monitor of the circuit switched GSM and GPRS services. It does not allow a simultaneous transmission of user data on GSM and GPRS. For instance, a subscriber has established a GPRS data connection and receives data packets. A mobile terminating GSM circuit switched call is indicated. The subscriber accepts the call. While he is making the voice call, the GPRS virtual connection is “held or busy”, but no packet data transfer is possible. Having terminated the voice call, packet data can again be transmitted via the still existing GPRS virtual connection. Class C: qeletene.und.slo A class C mobile is either a pure GPRS MS or it supports both GSM circuit switched services and GPRS. If it supports both then it can be used only in one of the two modes. If a subscriber switches his mobile into GPRS mode, he can originate or terminate GPRS calls, but he can no longer originate or terminate GSM circuit switched calls. In GPRS and HSCSD, increased data rates can be achieved by channel bundling. Channel bundling is the allocation of several timeslots to a MS. In other words, the mobile stations have a multislot capability. In the specification 05.02, the individual GSM multislot MS classes are specified. © Nokia Siemens Networks 2008 1-33
  • 34. qeletene.und.slo GPRS Architecture a) = 1 with frequency hopping. = 0 without frequency hopping. b) = 1 with frequency hopping or change from Rx to Tx. = 0 without frequency hopping and no change from Rx to Tx. c) = 1 with frequency hopping or change from Tx to Rx. = 0 without frequency hopping and no change from Tx to Rx. 1-34 © Nokia Siemens Networks 2008
  • 35. GPRS Architecture 5.6 Domain Name Servers These devices convert IP names into IP addresses, for example, server.nsn.com to 133.44.15.5. There is a primary DNS server and a secondary DNS server. Details of DNS were described in Introduction to TCP/IP module and information is also found in the IP CORE Course. In the specifications, the DNS functionality is included in the SGSN. However, the main vendors have chosen to separate the DNS functions from the SGSN. 5.7 Firewalls A firewall protects an IP network against external attack (for example, hackers from the mobile users or from the Internet). In the case of GPRS, the firewall might be configured to reject all packets that are not part of a GPRS subscriber-initiated connection. The firewall can also include NAT (Network Address Translation), see the Introduction to TCP/IP module. In the specifications for GPRS, the firewalls are not included. It is however included here due to the fact that operators usually need to implement firewalls in their GPRS network (for security reasons). qeletene.und.slo © Nokia Siemens Networks 2008 1-35
  • 36. GPRS Architecture 5.8 Border Gateway The Border Gateway (BG) is a router that can provide a direct GPRS tunnel between different operators' GPRS networks. This is referred to as an inter-PLMN data network. It is more secure to transfer data between two operators' PLMN networks through a direct connection rather than via the public Internet. The Border Gateway will commence operation once the GPRS roaming agreements between various operators have been signed. It will essentially allow a roaming subscriber to connect to company intranet through the Home GGSN via the visiting PLMN network. Charging Gateway GPRS users have to be charged for the use of the network. In a GSM network, charging is based on the destination, duration, and time of call. However, GPRS offers connectionless service to users, so it not possible to charge subscribers on the connection duration. Charging has to be based on the volume, destination, QoS, and other parameters of a connectionless data transfer. These GPRS charging data are generated by all the SGSNs and GGSNs in the network. This data is referred to as Charging Data Records or CDRs. One data session may generate a number of CDRs, so these need to collected and processed. The Charging Gateway (CG) collects all of these records, sorts them, processes it, and passes it on to the Billing System. Here the GPRS subscriber is billed for the data transaction. All CDRs contain unique subscriber and connection identifiers to distinguish it. A protocol called GTP' (pronounced GTP prime) is used for the transfer of data records between GSNs and the Charging Gateway. 1-36 © Nokia Siemens Networks 2008 qeletene.und.slo 5.9
  • 37. GPRS Architecture 6 GPRS Interfaces The GPRS system introduces new interfaces to the GSM network. Figure illustrates the logical architecture with the interfaces and reference points of the combined GSM/GPRS network. qeletene.und.slo Fig. 16 GPRS interfaces Connections from the GPRS system to the NSS part of the GSM network are implemented through the SS7 network. The GPRS element interfacing with the NSS is SGSN. The important interfaces to the NSS are the SGSN-HLR (Gr), SGSN-EIR (Gf), and SGSN-MSC/VLR (Gs). The other interfaces are implemented through the intra-PLMN backbone network (Gn), the inter-PLMN backbone network (Gp), or the external networks (Gi). © Nokia Siemens Networks 2008 1-37
  • 38. GPRS Architecture Um between an MS and the GPRS fixed network part. The Um is the access interface the MS uses to access the GPRS network. The radio interface to the BTS is the same interface used by the existing GSM network with some GPRS specific changes. Gb between a SGSN and a BSS. The Gb interface carries the GPRS traffic and signalling between the GSM radio network (BSS) and the GPRS network. Frame Relay based network services is used for this interface. Gn between two GSNs within the same PLMN. The Gn provides a data and signalling interface in the Intra-PLMN backbone. The GPRS Tunnelling Protocol (GTP) is used in the Gn (and in the Gp) interface over the IP based backbone network. Gp between two GSNs in various PLMNs. The Gp interface provides the same functionality as the Gn interface, but it also provides, together with the BG and the Firewall, all the functions needed for inter-PLMN networking, that is, security, routing, etc. Gr between an SGSN and the HLR. The Gr gives the SGSN access to subscriber information in the HLR. The HLR can be located in a different PLMN than the SGSN (MAP). Ga between the GSNs and the CG inside the same PLMN. The Ga provides a data and signalling interface. This interface is used for sending the charging data records generated by GSNs to the CG. The protocol used is GTP', an enhanced version of GTP. Gs between a SGSN and a MSC. The SGSN can send location data to the MSC or receive paging requests from the MSC via this optional interface. The Gs interface will greatly improve the effectiveness of the radio and network resources in the combined GSM/GPRS network. This interface uses BSSAP+ protocol. Gd between the SMS-GMSC and an SGSN, and between SMS-IWMSC and an SGSN. The Gd interface is available for more efficient use of the SMS services (MAP). Gf between an SGSN and the EIR. The Gf gives the SGSN access to GPRS user equipment information. The EIR maintains three different lists of mobile equipment: black list for stolen mobiles, grey list for mobiles under observation and white list for other mobiles (MAP). Gc between the GGSN and the HLR. The GGSN may request the location of an MS via this optional interface. The interface can be used if the GGSN needs to forward packets to an MS that is not active. 1-38 © Nokia Siemens Networks 2008 qeletene.und.slo The interfaces used by the GPRS system are described below:
  • 39. GPRS Architecture There are two different reference points in the GPRS network. The Gi is GPRS specific, but the R is common with the circuit switched GSM network: Gi between a GGSN and an external network. The GPRS network is connected to an external data networks via this interface. The GPRS system will support a variety of data networks. Because of that, the Gi is not a standard interface, but merely a reference point. R between terminal equipment and mobile termination. This reference point connects terminal equipment to mobile termination, thus allowing, for example, a laptop-PC to transmit data over the GSM-phone. The physical R interface follows, for example, the ITU-T V.24/V.28 or the PCMCIA PC-Card standards. qeletene.und.slo © Nokia Siemens Networks 2008 1-39
  • 40. GPRS Architecture 7 Transfer of Packets between GSNs qeletene.und.slo User data packets are sent over the GPRS backbone in 'containers'. When a packet coming from an external packet network arrives at the GGSN, it is inserted in a container and sent to the SGSN. The stream of containers inside the GPRS backbone network is totally transparent to the user: To the user, it seems like he/she is connected directly via a router (the GGSN) to external networks. In data communications, this type of virtual stream of containers is called a tunnel. We say that the GSNs are performing tunnelling of user packets, see Figure 18. Fig. 17 User packets over the GPRS backbone in ‘containers’ 1-40 © Nokia Siemens Networks 2008
  • 41. GPRS Architecture The protocol that performs the tunnelling in GPRS is called GPRS Tunnelling Protocol (GTP). We can say that we transport GTP packets between the SGSN and the GGSN. Over the GPRS backbone, IP packets are used to carry the GTP packets. The GTP packets then contain the actual user packets. This is shown in Figure 19. The user packet, for example, a TCP/IP packet that carries some part of an e-mail, is carried inside a GTP packet. The GTP packet is carried over the GPRS backbone using IP and TCP or UDP (in the example, UDP). The GTP packet headers, including the tunnel ID (TID), will tell the receiving GSN who the user is. The tunnel ID includes the user IMSI (and another user specific number). The TID is a label that tells the SGSN and the GGSN, whose packets are inside the container. qeletene.und.slo Fig. 18 GTP container From the point of view of the user and the external network, the GTP packets that contain the user packets could be transferred between the GSNs using any technology, for example, ATM, X.25, or Frame Relay. The chosen technology for the GPRS backbone is IP. All the network elements (the GSNs, the charging gateway, etc.) connected to the GPRS backbone must have an IP address. IP addresses used in the backbone are invisible to the MS and to the external networks. They are what we call private IP addresses. That is, the user packets are carried in the GPRS core between the SGSN and the GGSN using the private IP addresses of the GPRS backbone. This concept of tunnelling and hiding backbone addresses ('private') to the user level is illustrated in the following figures. Figure 20 shows a close-up of the user and backbone IP address levels. Figure 20 shows the GTP tunnel related to the user payload, and the relationship between the protocol stacks in the Gi and Gn interfaces. © Nokia Siemens Networks 2008 1-41
  • 42. GPRS Architecture qeletene.und.slo Fig. 19 Transfer of packets between the GGSN and the MS Fig. 20 GTP tunnelling and user payload 1-42 © Nokia Siemens Networks 2008