module 5 -_gprs_architecture

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GPRS Architecture

© Nokia Siemens Networks 2008

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GPRS Architecture

Contents1 Objectives...................................................................................................22 GPRS Subscriber Profile..........................................................................33 GPRS QoS Profile..................................................................................... 53.1 Rel'99 QoS parameter set .........................................................................123.2 Traffic classes............................................................................................163.3 Ranges of Rel'99 attributes.......................................................................183.4 Mapping between QoS parameters in Rel'97 and Rel'99......................... 19

4 GPRS Logical Functions........................................................................ 214.1 Logical Functions in the GPRS Network .................................................. 214.2 Network Access Control Functions........................................................... 224.3 Packet Routing and Transfer Functions................................................... 244.4 Mobility Management Functions............................................................... 264.5 Logical Link Management Functions........................................................ 264.6 Radio Resource Management Functions..................................................264.7 Network Management Functions.............................................................. 27

5 Network elements................................................................................... 285.1 Packet Control Unit (PCU-PIU of BSC) .................................................... 295.2 Channel Codec Unit (CCU).......................................................................305.3 Serving GPRS Support Node (SGSN)......................................................305.4 Gateway GPRS Support Node (GGSN)................................................... 315.5 GPRS MS..................................................................................................325.6 Domain Name Servers..............................................................................355.7 Firewalls.................................................................................................... 355.8 Border Gateway........................................................................................ 365.9 Charging Gateway.....................................................................................36

6 GPRS Interfaces...................................................................................... 377 Transfer of Packets between GSNs...................................................... 40

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1 Objectives After completing this learning element, the student should be able to:

Explain the GPRS subscriber profile, GPRS QoS profile, and GPRS logicalfunctionsName the GPRS specific network elements and their most importantfunctionsName and explain five important open interfaces in the GPRS networkExplain the principle of transfer of packets between GSNs

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2 GPRS Subscriber Profile

The GPRS Subscriber Profile is the description of the services a subscriber isallowed to use. Essentially, it contains the description of the packet data protocolused. A subscriber may also use different packet data protocols (PDPs), or onePDP with different addresses. The following parameters are available for eachPDP:

The packet network address is necessary to identify the subscriber in the publicdata net. Either dynamically assigned (temporary) addresses or (in the future)static addresses are used in case of IP. The problem of the dynamic addresseswill be overcome with the change from Ipv4 to IPv6. In GPRS is two layer 2protocols are allowed, X.25 or IP.

The quality of service QoS : QoS describes various parameters. The subscriberprofile defines the highest values of the QoS parameters that can be used by thesubscriber.

The screening profile: This profile depends on the PDP used and on thecapacity of the GPRS nodes. It serves to restrict acceptance duringtransmission/reception of packet data. For example, a subscriber can berestricted with respect to his possible location, or with respect to certain specificapplications.

The GGSN address: The GGSN address indicates which GGSN is used by thesubscriber. In this way the point of access to external packet data networks PDNis defined. The internal routing of the data is done by IP protocol; the GSNs willhave IP addresses. A DNS function is needed to find the destination of the datapackets (address translating: e.g. www.gsn-xxx.com → 129.64.39.123)

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Fig. 1 Part of the GPRS subscriber profile are the PDPs and their parameters

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3 GPRS QoS Profile

The different applications that will make use of packet-oriented data transmissionvia GPRS require different qualities of transmission. GPRS can meet thesedifferent requirements because it can vary the quality of service (QoS) over awide range of attributes. The quality of service profile (Rec. 02.60, 03.60) permitsselection of the following attributes:

Precedence classDelay classReliability classPeak throughput classMean throughput class.

By combining the variation possibilities of the individual attributes a large numberof QoS profiles can be achieved. Only a limited proportion of the possible QoSprofiles need PLMN-specific support.

Fig. 2 Quality of service parameters

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Precedence Class

Three different classes have been defined to allow assessment of the importanceof the data packets, in case of limited resources or overload:

High precedenceNormal precedenceLow precedence

Delay Class

GSM Rec.02.60 defines 4 delay classes (1 to 4). However, a PLMN only needsto realize part of these. The minimum requirement is the support of the so-called„best effort delay class“ (Class 4). Delay requirements (maximum delay) concernthe delay of transported data through the entire GPRS network (the first twocolumns refer to data packets 128 bytes in length, while the last two columnsapply to packets 1024 bytes in length).

Delay Class Mean transferdelay (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

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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, datadelivery 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, 1guaranteeing the highest and 5 the lowest degree of reliability. Highest reliability(Class 1) is required for error-sensitive, non-real-time applications, which have nopossibility of compensating for data loss; lowest reliability (Class 5) is needed forreal-time applications which can get over data loss.

The reliability classes (see Table 2) define the probability of:

Lost dataDuplication of dataData arriving out of sequenceCorruption of data

The reliability class specifies the requirements of the various network protocollayers. The combinations of the GTP, LLC, and RLC transmission modes supportthe reliability class performance requirements.

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ReliabilityClass

GTP Mode LLC FrameMode

LLC DataProtection

RLC BlockMode

Traffic Type

1 Acknowledged

Acknowledged

Protected Acknowledged

Non-real-time traffic,error-sensitive applicationthat cannot cope with dataloss.

2 Unacknowledged

Acknowledged


Non-real-time traffic,error-sensitive applicationthat can cope withinfrequent data loss.

3 Unacknowledged

Unacknowledged


Non-real-time traffic,error-sensitive applicationthat can cope with dataloss, GMM/SM, and SMS.

4 Unacknowledged

Unacknowledged

Protected Unacknowledged

Real-time traffic,error-sensitive applicationthat can cope with dataloss.

5 Unacknowledged

Unacknowledged

Unprotected Unacknowledged

Real-time traffic, errornon-sensitive applicationthat can cope with dataloss.

Table 2 Reliability classes

Note : Signalling and SMS are transferred with reliability class 3.

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Throughput classes

The throughput class indicates the data throughput requested by the user.Throughput is defined by two negotiable parameters:

Maximum bit rateMean bit rate. This includes, for example for "bursty" transmissions, theperiods in which no data is transmitted.

The maximum and mean bit rates can be represented by a parameter known asthe Information Transfer Rate.

It is possible for the network to re-negotiate the throughput parameters at anytime during a session. User data throughput is specified in terms of a set ofthroughput classes that characterise the expected bandwidth required for a PDPcontext.

Maximum bit rate

The maximum bit rate is measured in octets per second at the Gi and Rreference points. It specifies the maximum rate at which data is expected to betransferred across the network for an individual PDP context. There is noguarantee that this maximum rate will be achieved or sustained for any timeperiod as this depends upon the MS capability and available radio resources. Thenetwork may limit the subscriber to the negotiated maximum data rate, even ifadditional transmission capacity is available. The maximum throughput isindependent of the particular delay class being used. The maximum (peak)throughput classes are defined in Table 3.

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Max. Throughput Class Max. Throughput in octets persecond

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

Mean bit rate

The mean bit rate (throughput) is measured at the Gi and R reference points inunits of octets per hour . It specifies the average rate at which data is expectedto be transferred across the GPRS network during the remaining lifetime of anactivated PDP context. The network may limit the subscriber to the negotiatedmean bit rate (for example, for flat rate charging), even if additional transmissioncapacity is available.

A 'best effort' means bit rate class may be negotiated. This means that bandwidthwill be made available to the MS on a need and availability basis. The meanthroughput classes are defined in Table 4.

Note : ETSI GPRS specifications define several QoS classes which areassociated with each PDP context, covering priority, reliability, delay, andthroughput. The NSN GPRS system release 1 does not support this QoSfunctionality. The GPRS QoS can be considered as ‘best effort’.

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Mean Throughput Class 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).

Table 4 Mean bit rate classes

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Fig. 4 QoS is an assumption of several parameters, which are defined in the recommendations

3.1 Rel'99 QoS parameter set

Rel'99 parameters are specified for UMTS. NSN GPRS Release 2 also supportsthese parameters. The list of attributes in Rel'99 are given below:

Maximum bit rate specifies the maximum rate at which the data is expectedto be transferred in the network for a PDP context. The subscribed transferrate is not guaranteed; it just specifies the limit that cannot be exceeded. Itspurpose is to limit the delivered bit rate to applications or external networkswith such limitations and to allow maximum wanted user bit rate to bedefined for applications able to operate with different rates, for example,non-transparent circuit switched data. Compare Rel'97/98, similar as 'Peakthroughput class'.

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Guaranteed bit rate specifies guaranteed bit rate delivered in the networkfor the PDP context. Guaranteed bit rate may be used to facilitate admissioncontrol based on available resources, and for resource allocation. Qualityrequirements expressed by, for example, delay and reliability attributes onlyapply to incoming traffic up to the guaranteed bit rate.

Delivery order (y/n) indicates whether the bearer shall provide in-sequenceSDU delivery or not. The attribute is derived from the user protocol (PDPtype) and specifies if out-of-sequence SDUs are acceptable or not. Thisinformation cannot be extracted from the traffic class. Whetherout-of-sequence SDUs are dropped or re-ordered depends on the specifiedreliability required for the application. Compare Rel'97/98, similar as'Reordering required'.

Maximum SDU size (maximum allowed SDU size, octets) is used foradmission control and policing. Policing makes sure that bandwidth limits ofthe PDP context are not exceeded to protect radio interface. Admissioncontrol calculates what network resources are required to provide therequested QoS, determine if resources are available, and reserve them. Theadmission controller in SGSN has the responsibility to accept or reject PDPcontext activation and the requested QoS parameter values.

SDU format information (list of possible exact sizes of SDUs, bits) isneeded because network needs SDU size information to be able to operatein transparent RLC protocol mode, which is beneficial to spectral efficiencyand delay when RLC re-transmission is not used. Thus, if the application canspecify SDU sizes, the bearer is less expensive. SDU format info is notsupported 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 theloading conditions, whereas without reserved resources, such as inInteractive 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 thedelivered SDUs. If no error detection is requested, residual bit error ratioindicates the bit error ratio in the delivered SDUs. Residual bit error ratio ismapped with Rel'97/98 'Reliability class'.

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Delivery of erroneous SDUs (y/n/-) indicates whether SDUs detected aserroneous shall be delivered or discarded. Delivery of erroneous SDUs isused to decide whether error detection is needed and whether frames withdetected errors shall be forwarded or not. A 'yes' value implies that errordetection is employed and that erroneous SDUs are delivered together withan error indication, and 'no' implies that error detection is employed and thaterroneous SDUs are discarded, and '-' implies that SDUs are deliveredwithout considering error detection. Residual bit error ratio is mapped withRel'97/98 'Reliability class'.

SDU error ratio Residual bit errorratio

Delivery oferroneous SDUs

Traffic type

10 -6 10 -5 No Non-real-time traffic,error sensitiveapplication thatcannot cope withdata loss

10 -6 10 -5 No Non-real-time traffic,error sensitiveapplication that cancope with infrequentdata loss

10 -4 10 -5 No Non-real-time traffic,error sensitiveapplication that cancope with data loss

10 -3 10 -5 No Real-time traffic,error sensitiveapplication that cancope with data loss.

10 -3 4*10 -3 Yes Real-time traffic,error non-sensitiveapplication that cancope with data loss.

Table 5 Traffic examples mapped to Rel'99 attributes

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Note : For real-time traffic, the QoS profile also requires appropriate settings fordelay and throughput. Signalling and SMS are transferred with reliability class 3.

Transfer delay (ms) indicates maximum delay for 95% of the distribution of delayfor all delivered SDUs during the lifetime of a bearer service. Transfer delay isused to specify the delay tolerated by the application.

Traffic handling priority specifies the relative importance for handling of allSDUs belonging to the radio access bearer compared to the SDUs of otherbearers. Traffic handling priority is mapped with Rel'97/98 'Delay class'.

Allocation/Retention priority is used for differentiating between bearers. Insituations where resources are scarce, the relevant network elements canprioritise bearers when performing admission control. Attribute has threecategories:

High . Users whose packets will never be discardedNormal. Users whose packets will be discarded sometimesLow . The low priority class users whose packets will be discarded

The Allocation/Retention priority attribute is a subscription attribute which is notnegotiated from the mobile terminal. The addition of a user-controlledAllocation/Retention priority attribute is for further study in future releases.Allocation/Retention priority is mapped with Rel'97/98 'Precedence class'.

Source statistics descriptor is used for conversational and streaming classesfor ('speech'/'unknown'). Since conversational class is not supported by GPRS,NSN 2G-SGSN does not support Source statistics descriptor.

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3.2 Traffic classesEnd-user applications can be categorised in major groups according to their mainQoS requirements. There are four different Rel'99 QoS traffic classes:

Conversational classStreaming classInteractive classBackground class

Fig. 5 Rel'99 QoS traffic classes

The main distinguishing factor between these QoS traffic classes is how delaysensitive the traffic is: Conversational class is meant for traffic which is very delaysensitive while Background class is the most delay insensitive traffic class.

Conversational and Streaming classes are intended to be used to carry real-timetraffic flows. The main difference between them is how delay sensitive the trafficis. Conversational real-time services, like video telephony, are the most delaysensitive applications and those data streams should be carried in Conversationalclass.

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Interactive class and Background are mainly meant to be used by traditionalInternet applications like WWW, e-mail, Telnet, FTP, and News. The maindifference between Interactive and Background class is that Interactive class ismainly used by interactive applications, for example, interactive e-mail orinteractive 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 separatinginteractive and background applications. Traffic in the Interactive class has higherpriority in scheduling than Background class traffic, so background applicationsuse transmission resources only when interactive applications do not need them.This is very important in wireless environment where the bandwidth is lowcompared to fixed networks.

Although the bit rate of a conversational source codec may vary, conversationaltraffic is assumed to be relatively non-bursty. Maximum bit rate specifies theupper limit of the bit rate with which the bearer delivers SDUs. The bearer is notrequired 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 remoteequipment, such as web browsing. The source characteristics are unknown butmay be bursty.

The background class is optimised for machine-to-machine communication that isnot delay sensitive, such as messaging services. Background applicationstolerate a higher delay than applications using the interactive class, which is themain difference between the background and interactive classes.

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3.3 Ranges of Rel'99 attributes

Traffic class Conversationalclass

Streamingclass

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 SDUsize (octets)

<=1 500 or 1 502(4)

<=1 500 or 1502 (4)

<=1 500 or 1 502(4)

<=1 500 or 1 502(4)

SDU formatinformation

(9) (9)

Delivery oferroneous SDUs

Yes/No/- Yes/No/- Yes/No/- Yes/No/-

Residual BER 5*10 -2 , 10 -2 , 5*10 -3

, 10 -3 , 10 -4 , 10 -65*10 -2 , 10 -2 ,5*10 -3 , 10 -3 ,

10 -4 , 10 -5 , 10 -6

4*10 -3 , 10 -5 , 6*10 -8

(6)4*10 -3 , 10 -5 , 6*10 -8

(6)

SDU error ratio 10 -2 , 7*10 -3 , 10 -3 ,10 -4 , 10 -5

10 -1 , 10 -2 ,7*10 -3 , 10 -3 ,

10 -4 , 10 -5

10 -3 , 10 -4 , 10 -6 10 -3 , 10 -4 , 10 -6

Transfer delay (ms 80-100 up to FFS(8) (5)

250 up toFFS (8)

(10) (10)

Guaranteed bit rate(kbps)

< 2 048 (1) (2) < 2 048 (1) (2) (11) (12)

Traffic handlingpriority

1,2,3 (7) (12)

Allocation/Retention priority

1,2,3 (7) 1,2,3 (7) 1,2,3 (7) 1,2,3 (7)

Source statisticdescriptor

Speech/unknown(1) Speech/unknown (1)

Table 6 Value ranges of Rel'99 attributes

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3.4 Mapping between QoS parameters inRel'97 and Rel'99

Since there are two different parameter sets (Rel'97 and Rel'99) and they mightbe used simultaneously in a same network, these parameter must be mappedwith each other.

Fig. 6 Rules for determining Rel'99 attributes from Rel-97/98 attributes

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Fig. 7 Rules for determining Rel'97 attributes from Rel'99 attributes

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4 GPRS Logical Functions4.1 Logical Functions in the GPRS Network

The tasks required for the handling of processes in the GSM-/GPRS network arestructured into logical functions. These functions may contain a large number ofindividual functions. Logical functions are:

Network access control functionsPacket routing and transfer functionsMobility management functionsLogical link management functionsNetwork management functions

Fig. 8 Logical functions of the GPRS network

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4.2 Network Access Control Functions

Network access means the way or manner in which a subscriber gains access toa telecommunication network to make use of the services this network provides.An access protocol consists of a defined set of procedures, which makes accessto the network possible. Network access can be obtained both from the MS andfrom the fixed network part of the GPRS network. Depending on the provider, theinterface to external data networks can support various access protocols, e.g. IPor X.25. The following functions have been defined for access to the GPRSnetwork:

Registration function: Registration stands for linking the identity of themobile radio subscriber to his packet data protocol (or protocols), thePLMN-internal addresses and the point of access of the user to external dataProtocol (PDP) networks. This link can be static (HLR entry), or it can beeffected on demand.Authentication and authorization function: This function stands for theidentification of the subscriber and for access legitimacy when a service isdemanded. In addition, the legitimacy of the use of this particular service iscontrolled. The authentication function is carried out in conjunction with themobility management functions.Admission control function: Admission control is intended for determiningthe network resources required for performing the desired service (QoS). Italso decides whether these resources are available, and lastly it is used forreserving resources. Admission control is effected in conjunction with theradio resource management functions to enable assessment of radioresources requirements in each individual cell.Message screening function: A "screening" function is combined with thefiltering of unauthorized or undesirable information/messages. In theintroduction stage of GPRS a network-controlled screening function issupported. Subscription-controlled and user-controlled screening may beadditionally provided at a later stage.Packet terminal adaptation function: This function adapts data packetsreceived/transmitted from/to the terminal equipment TE to a form suited fortransport through the GPRS network.Charging data collection function: This function is used for collecting datarequired for billing.

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Fig. 9 Network access control functions

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4.3 Packet Routing and Transfer Functions

A route consists of an orderly list of nodes used for the transfer of messageswithin and between the PLMNs. Each route consists of the node of origin, nonode, one or several relay nodes, and the node of destination. Routing is theprocess of determining and using the route for the transmission of a messagewithin or between PLMNs.

Relay function: Transferring data received by a node from another node to thenext node of the route.

*Routing function: Determining the transmission path for the next hop on theroute towards the GPRS support node (GSN) the message is intended for. Datatransmission between GSNs can be effected via external data networkspossessing their own routing functions; e. g. X.25, Frame Relay or ATMnetworks.

Address translation and mapping function: Address translation meanstransforming one address into another, different address. It can be used totransform addresses of external network protocols into internal network addresses(for routing purposes). Address mapping is used to copy a network address intoanother network address of the same type (e.g. for the routing and transmitting ofmessages from one network node to the next).

Encapsulation function: Encapsulation means supplementing address- andcontrol information into one data unit for the routing of packets within or betweenPLMNs. The opposite process is called decapsulation. Encapsulation anddecapsulation is effected between the GSN of the GPRS-PLMN as well asbetween the SGSN and the MS.

Tunneling Function : Tunneling means the transfer of encapsulated data units inthe PLMN. A tunnel is a two-way point-to-point path, only the endpoints of whichare identified.

Compression function: for the optimal use of radio link capacity.

Ciphering function: preventing eavesdropping

Domain name server function: Decoding logical GSN names in GSNaddresses. This function is a standard function of the internet.

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Fig. 10 Packet routing and transfer functions in the GPRS network

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4.4 Mobility Management Functions

Mobility management functions are used to enable tracing the actual location of amobile station in either the home-PLMN or a Visited-PLMN.

4.5 Logical Link Management Functions

Logical link management functions concern maintenance of a communicationchannel between an MS and the PLMN via the radio interface Um. Thesefunctions include the coordination of link state information between the MS andthe PLMN and the monitoring of data transfer activities via the logical link.

Logical link establishment function: Building up a logical link by during GPRSattach.

Logical link maintenance function: Monitoring of the state of the logical linkand state modification control.

Logical link release function: De-allocation of resources associated with thelogical link.

4.6 Radio Resource Management Functions

Radio resource management functions include allocation and maintenance ofcommunication channels via the radio interface. The GSM radio resources mustbe divided /distributed between circuit switched services and GPRS.

Um management function: Managing available physical channels of cells anddetermining the share of radio resources allocated for use in the GPRS. Thisshare may vary from cell to cell.

Cell selection function: Allows the MS to select the optimal cell for acommunication path. This includes measurement and evaluation of the signalquality of neighboring cells and detection and avoidance of overload in theeligible cells.

Um-tranx function: Offers capacity for packet data transfer via Um. The functionincludes a. o. procedures for multiplexing packets via shared physical channels,for retaining packets in the MS, for error detection and correction, and for flowcontrol.

Path management function: Management of packet data communicationbetween BSS and serving GSN node. Establishing and canceling these paths canbe effected either dynamically (amount of traffic data) or statically (maximum loadto be expected for each cell).

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4.7 Network Management Functions

Network management functions provide mechanisms for the support ofGPRS-related operation & maintenance functions.

Fig. 11 Mobility management, logical link, radio resource and network management functions

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5 Network elements

Figure 12 shows the architecture of a GPRS network. The GPRS system bringssome 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 networkGateway GPRS Support Node (GGSN): gateway to external networksBorder Gateway (BG): a gateway to other PLMNIntra-PLMN backbone: an IP based network inter-connecting all the GPRSelementsCharging Gateway (CG)Legal Interception Gateway (LIG)Domain Name System (DNS)Firewalls: used wherever a connection to an external network is required.

Not all of the network elements are compulsory for every GPRS network.

Fig. 12 GPRS architecture

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5.1 Packet Control Unit (PCU-PIU of BSC)

The PCU separates the circuit switched and packet switched traffic from the userand sends them to the GSM and GPRS networks respectively. It also performsmost of the radio resource management functions of the GPRS network. ThePCU can be either located in the BTS, BSC, or some other point between the MSand the MSC. There will be at least one PCU that serves a cell in which GPRSservices will be available. Frame Relay technology is being used at present tointerconnect the PCU to the GPRS core.

Fig. 13 PCU - its position within the BSS

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5.2 Channel Codec Unit (CCU)

The CCU is realised in the BTS to perform the Channel Coding (including thecoding scheme algorithms), power control and timing advance procedures.

5.3 Serving GPRS Support Node (SGSN)

The SGSN is the most important element of the GPRS network. The SGSN ofthe GPRS network is equivalent to the MSC of the GSM network. There must atleast one SGSN in a GPRS network. There is a coverage area associated with aSGSN. As the network expands and the number of subscribers increases, theremay be more than one SGSN in a network. The SGSN has the followingfunctions:

Protocol conversion (for example IP to FR)Ciphering of GPRS data between the MS and SGSNData compression is used to minimise the size of transmitted data unitsAuthentication of GPRS usersMobility management as the subscriber moves from one area to another, andpossibly one SGSN to anotherRouting of data to the relevant GGSN when a connection to an externalnetwork is requiredInteraction with the NSS (that is, MSC/VLR, HLR, EIR) via the SS7 networkin order to retrieve subscription informationCollection of charging data pertaining to the use of GPRS usersTraffic statistics collections for network management purposes.

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5.4 Gateway GPRS Support Node (GGSN)

The GGSN is the gateway to external networks. Every connection to a fixedexternal data network has to go through a GGSN. The GGSN acts as the anchorpoint in a GPRS data connection even when the subscriber moves to anotherSGSN during roaming. The GGSN may accept connection request from SGSNthat is in another PLMN. Hence, the concept of coverage area does not apply toGGSN. There are usually two or more GGSNs in a network for redundancypurposes, and they back up each other up in case of failure. The functions of aGGSN are given below:

Routing mobile-destined packets coming from external networks to therelevant SGSNRouting packets originating from a mobile to the correct external networkInterfaces to external IP networks and deals with security issuesCollects charging data and traffic statisticsAllocates dynamic or static IP addresses to mobiles either by itself or with thehelp of a DHCP or a RADIUS serverInvolved in the establishment of tunnels with the SGSN and with otherexternal networks and VPN.

From the external network's point of view, the GGSN is simply a router to an IPsub-network. This is shown below. When the GGSN receives data addressed to aspecific 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 isinactive, the data is discarded. The GGSN also routes mobile originated packetsto the correct external network.

Fig. 14 GPRS network as seen by another data network

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5.5 GPRS MS

Different GPRS MS classes were introduced to cope with the different needs offuture subscribers. The mobiles differ in their capabilities.

Fig. 15 GPRS network as seen by another data network

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Three GPRS MS classes were defined:

Class A:With a class A mobile GSM circuit switched services and GSM GPRS servicescan be simultaneously activated. A subscriber can get data from an active GPRSlink while simultaneously making a phone call. A class A mobile allows also asimultaneous attach, activation and monitor of the classical GSM and GPRSservices.

Class B:

A class B mobile allows a simultaneous attach, activation and monitor of thecircuit switched GSM and GPRS services. It does not allow a simultaneoustransmission of user data on GSM and GPRS. For instance, a subscriber hasestablished a GPRS data connection and receives data packets. A mobileterminating GSM circuit switched call is indicated. The subscriber accepts thecall. While he is making the voice call, the GPRS virtual connection is “held orbusy”, but no packet data transfer is possible. Having terminated the voice call,packet data can again be transmitted via the still existing GPRS virtualconnection.

Class C:

A class C mobile is either a pure GPRS MS or it supports both GSM circuitswitched services and GPRS. If it supports both then it can be used only in oneof the two modes. If a subscriber switches his mobile into GPRS mode, he canoriginate or terminate GPRS calls, but he can no longer originate or terminateGSM circuit switched calls. In GPRS and HSCSD, increased data rates can beachieved by channel bundling. Channel bundling is the allocation of severaltimeslots 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.

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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.

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5.6 Domain Name Servers

These devices convert IP names into IP addresses, for example, server.nsn.comto 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 informationis also found in the IP CORE Course. In the specifications, the DNS functionalityis included in the SGSN. However, the main vendors have chosen to separatethe DNS functions from the SGSN.

5.7 Firewalls

A firewall protects an IP network against external attack (for example, hackersfrom the mobile users or from the Internet). In the case of GPRS, the firewallmight be configured to reject all packets that are not part of a GPRSsubscriber-initiated connection. The firewall can also include NAT (NetworkAddress Translation), see the Introduction to TCP/IP module. In the specificationsfor GPRS, the firewalls are not included. It is however included here due to thefact that operators usually need to implement firewalls in their GPRS network (forsecurity reasons).

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5.8 Border Gateway

The Border Gateway (BG) is a router that can provide a direct GPRS tunnelbetween different operators' GPRS networks. This is referred to as an inter-PLMNdata network. It is more secure to transfer data between two operators' PLMNnetworks through a direct connection rather than via the public Internet. TheBorder Gateway will commence operation once the GPRS roaming agreementsbetween various operators have been signed. It will essentially allow a roamingsubscriber to connect to company intranet through the Home GGSN via thevisiting PLMN network.

5.9 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, GPRSoffers connectionless service to users, so it not possible to charge subscribers onthe connection duration. Charging has to be based on the volume, destination,QoS, and other parameters of a connectionless data transfer. These GPRScharging data are generated by all the SGSNs and GGSNs in the network. Thisdata is referred to as Charging Data Records or CDRs. One data session maygenerate a number of CDRs, so these need to collected and processed. TheCharging 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 thedata transaction. All CDRs contain unique subscriber and connection identifiers todistinguish it. A protocol called GTP' (pronounced GTP prime) is used for thetransfer of data records between GSNs and the Charging Gateway.

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6 GPRS Interfaces

The GPRS system introduces new interfaces to the GSM network. Figureillustrates the logical architecture with the interfaces and reference points of thecombined GSM/GPRS network.

Fig. 16 GPRS interfaces

Connections from the GPRS system to the NSS part of the GSM network areimplemented through the SS7 network. The GPRS element interfacing with theNSS is SGSN. The important interfaces to the NSS are the SGSN-HLR (Gr),SGSN-EIR (Gf), and SGSN-MSC/VLR (Gs). The other interfaces areimplemented through the intra-PLMN backbone network (Gn), the inter-PLMNbackbone network (Gp), or the external networks (Gi).

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The interfaces used by the GPRS system are described below:

Um between an MS and the GPRS fixed network part. The Um is the accessinterface the MS uses to access the GPRS network. The radio interface tothe BTS is the same interface used by the existing GSM network with someGPRS specific changes.Gb between a SGSN and a BSS. The Gb interface carries the GPRS trafficand signalling between the GSM radio network (BSS) and the GPRSnetwork. Frame Relay based network services is used for this interface.Gn between two GSNs within the same PLMN. The Gn provides a data andsignalling interface in the Intra-PLMN backbone. The GPRS TunnellingProtocol (GTP) is used in the Gn (and in the Gp) interface over the IP basedbackbone network.Gp between two GSNs in various PLMNs. The Gp interface provides thesame functionality as the Gn interface, but it also provides, together with theBG and the Firewall, all the functions needed for inter-PLMN networking, thatis, security, routing, etc.Gr between an SGSN and the HLR. The Gr gives the SGSN access tosubscriber information in the HLR. The HLR can be located in a differentPLMN than the SGSN (MAP).Ga between the GSNs and the CG inside the same PLMN. The Ga providesa data and signalling interface. This interface is used for sending thecharging data records generated by GSNs to the CG. The protocol used isGTP', an enhanced version of GTP.Gs between a SGSN and a MSC. The SGSN can send location data to theMSC or receive paging requests from the MSC via this optional interface.The Gs interface will greatly improve the effectiveness of the radio andnetwork resources in the combined GSM/GPRS network. This interface usesBSSAP+ protocol.Gd between the SMS-GMSC and an SGSN, and between SMS-IWMSC andan SGSN. The Gd interface is available for more efficient use of the SMSservices (MAP).Gf between an SGSN and the EIR. The Gf gives the SGSN access to GPRSuser equipment information. The EIR maintains three different lists of mobileequipment: black list for stolen mobiles, grey list for mobiles underobservation and white list for other mobiles (MAP).Gc between the GGSN and the HLR. The GGSN may request the location ofan MS via this optional interface. The interface can be used if the GGSNneeds to forward packets to an MS that is not active.

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There are two different reference points in the GPRS network. The Gi is GPRSspecific, but the R is common with the circuit switched GSM network:

Gi between a GGSN and an external network. The GPRS network isconnected to an external data networks via this interface. The GPRS systemwill support a variety of data networks. Because of that, the Gi is not astandard interface, but merely a reference point.R between terminal equipment and mobile termination. This reference pointconnects terminal equipment to mobile termination, thus allowing, forexample, a laptop-PC to transmit data over the GSM-phone. The physical Rinterface follows, for example, the ITU-T V.24/V.28 or the PCMCIA PC-Cardstandards.

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7 Transfer of Packetsbetween GSNs

User data packets are sent over the GPRS backbone in 'containers'. When apacket coming from an external packet network arrives at the GGSN, it isinserted in a container and sent to the SGSN. The stream of containers inside theGPRS backbone network is totally transparent to the user: To the user, it seemslike he/she is connected directly via a router (the GGSN) to external networks. Indata 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’

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The protocol that performs the tunnelling in GPRS is called GPRS TunnellingProtocol (GTP). We can say that we transport GTP packets between the SGSNand the GGSN.

Over the GPRS backbone, IP packets are used to carry the GTP packets. TheGTP 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 GPRSbackbone using IP and TCP or UDP (in the example, UDP).

The GTP packet headers, including the tunnel ID (TID), will tell the receiving GSNwho the user is. The tunnel ID includes the user IMSI (and another user specificnumber). The TID is a label that tells the SGSN and the GGSN, whose packetsare inside the container.

Fig. 18 GTP container

From the point of view of the user and the external network, the GTP packets thatcontain the user packets could be transferred between the GSNs using anytechnology, for example, ATM, X.25, or Frame Relay. The chosen technology forthe GPRS backbone is IP.

All the network elements (the GSNs, the charging gateway, etc.) connected to theGPRS backbone must have an IP address. IP addresses used in the backboneare invisible to the MS and to the external networks. They are what we callprivate IP addresses . That is, the user packets are carried in the GPRS corebetween the SGSN and the GGSN using the private IP addresses of the GPRSbackbone.

This concept of tunnelling and hiding backbone addresses ('private') to the userlevel is illustrated in the following figures. Figure 20 shows a close-up of the userand backbone IP address levels. Figure 20 shows the GTP tunnel related to theuser payload, and the relationship between the protocol stacks in the Gi and Gninterfaces.

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Fig. 19 Transfer of packets between the GGSN and the MS

Fig. 20 GTP tunnelling and user payload

module 5 -_gprs_architecture

Technology

gprs network

gprs logical functions

gprs nodes

gprs ms

gprs specific network

network management functions

transfer functions

packet network address