GPRS Architecture Overview

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© Nokia Siemens Networks 2008
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GPRS Architecture
Contents
1 Objectives...................................................................................................2
2 GPRS Subscriber Profile..........................................................................3
3 GPRS QoS Profile..................................................................................... 5
3.1 Rel'99 QoS parameter set.........................................................................12
3.2 Traffic classes............................................................................................16
3.3 Ranges of Rel'99 attributes.......................................................................18
3.4 Mapping between QoS parameters in Rel'97 and Rel'99.........................19
4 GPRS Logical Functions........................................................................ 21
4.1 Logical Functions in the GPRS Network.................................................. 21
4.2 Network Access Control Functions...........................................................22
4.3 Packet Routing and Transfer Functions................................................... 24
4.4 Mobility Management Functions............................................................... 26
4.5 Logical Link Management Functions........................................................ 26
4.6 Radio Resource Management Functions..................................................26
4.7 Network Management Functions.............................................................. 27
5 Network elements................................................................................... 28
5.1 Packet Control Unit (PCU-PIU of BSC).................................................... 29
5.2 Channel Codec Unit (CCU).......................................................................30
5.3 Serving GPRS Support Node (SGSN)......................................................30
5.4 Gateway GPRS Support Node (GGSN)................................................... 31
5.5 GPRS MS..................................................................................................32
5.6 Domain Name Servers..............................................................................35
5.7 Firewalls.................................................................................................... 35
5.8 Border Gateway........................................................................................ 36
5.9 Charging Gateway.....................................................................................36
6 GPRS Interfaces......................................................................................37
7 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 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

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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.
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)

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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 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.
Fig. 2 Quality of service parameters

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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).
Delay Class
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

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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, 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.

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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
ed
that can cope with
infrequent data loss.
3 Unacknowle
dged
Unacknowle
dged
ed
that can cope with data
loss, GMM/SM, and SMS.
4 Unacknowle
dged
Unacknowle
dged
Protected Unacknowle
dged
Real-time traffic,
loss.
5 Unacknowle
dged
Unacknowle
dged
Unprotected Unacknowle
dged
Real-time traffic, error
non-sensitive application
loss.
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 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
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.

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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
Mean bit rate
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’.

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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 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'.

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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.
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'.

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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
10
-6
10
-5
No Non-real-time traffic,
error sensitive
application that
cannot cope with
data loss
10
-6
10
-5
error sensitive
application that can
cope with infrequent
data loss
10
-4
10
-5
error sensitive
cope with data loss
10
-3
10
-5
No Real-time traffic,
error sensitive
cope with data loss.
10
-3
4*10
-3
Yes Real-time traffic,
error non-sensitive
cope 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 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'.
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.

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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:
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.

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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.
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.

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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)
SDU format
information
(9) (9)
Delivery of
erroneous SDUs
Yes/No/- Yes/No/- Yes/No/- Yes/No/-
Residual BER 5*10
-2
, 10
-2
, 5*10
-3
, 10
-3
, 10
-4
, 10
-6
5*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 to
FFS (8)
(10) (10)
Guaranteed bit rate
(kbps)
< 2 048 (1) (2) < 2 048 (1) (2) (11) (12)
Traffic handling
priority
1,2,3 (7) (12)
Allocation/Retentio
n priority
1,2,3 (7) 1,2,3 (7) 1,2,3 (7) 1,2,3 (7)
Source statistic
descriptor
Speech/unknown(1) Speech/unkno
wn (1)
Table 6 Value ranges of Rel'99 attributes

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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.
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 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
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 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:
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.

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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 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.
*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.
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.

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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 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.
Logical link release function: De-allocation of resources associated with the
logical link.
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).

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4.7 Network Management Functions
Network management functions provide mechanisms for the support of
GPRS-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 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.
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 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.
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 the
coding 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 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:
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.

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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.
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

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5.5 GPRS MS
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

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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:
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.

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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.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).

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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.
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, 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.

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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.
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).

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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 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.

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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.

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7 Transfer of Packets
between GSNs
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’

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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.
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.

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Fig. 19 Transfer of packets between the GGSN and the MS
Fig. 20 GTP tunnelling and user payload

GPRS Architecture Overview

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