<<

. 18
( 132 .)



>>

Message Transfer
MAP: Send Routing Info
for SMS
Forward Short Message

Send Info for MT SMS
Page

Message Transfer
Delivery Report

Delivery Report SM Delivery Report


Figure 3.29 Mobile terminated SMS transfer

SMS information such as whether the SMS should be sent via the SGSN or via the MSC.
The HLR will return the address of the serving MSC or SGSN and this can then be
contacted by the SMS-GMSC. The MSC will query its VLR with the IMSI of the mobile
subscriber. The VLR will return the location of the subscriber and the MSC can initiate
a page procedure to contact the mobile device. If the mobile device is in idle mode it
will perform an IMSI attach and the SMS can be received from the MSC. Once the
message has been transferred successfully a delivery report can be sent from the MSC to
the SMS-GMSC. The SMS-GMSC can then send an indication to the HLR and also back
to the SC. This process is illustrated in Figure 3.29.



3.8 SUMMARY
This chapter describes the principles of operation of the GSM network. For evolution to
UMTS, much of the functionality of GSM is retained in the core network, and several key
concepts such as the use of the SIM, and security and mobility procedures are also retained.
The TDMA/FDMA air interface for GSM is explained in some detail, including the
physical, control and transport channels implemented. Each of the key network elements
is described, as well as the interfaces between them. The various protocols that comprise
GSM are described, including the use and extension of SS7 in the mobile context. Mobility
management is explained, along with a description of a mobile terminated call and a short
message transfer.



FURTHER READING
D. J. Goodman (1997) Wireless Personal Communications Systems. Addison-Wesley,
Reading, MA.
FURTHER READING 77


3GPP TS03.04: Signalling Requirements Relating to Routing of Calls to Mobile
Subscribers.
3GPP TS03.09: Handover Procedures.
3GPP TS04.01: Mobile Station “ Base Station System (MS “ BSS) Interface General
Aspects and Principles.
3GPP TS04.04: Layer 1 “ General Requirements.
3GPP TS04.05: Data Link (DL) Layer General Aspects.
3GPP TS08.01: General Aspects on the BSS “ MSC Interface.
3GPP TS08.02: Base Station System “ Mobile Services Switching Centre (BSS“MSC)
Interface “ Interface Principles.
3GPP TS08.04: Base Station System “ Mobile Services Switching Centre (BSS“MSC)
Interface Layer 1 Speci¬cation.
3GPP TS23.003: Numbering, Addressing and Identi¬cation.
3GPP TS24.008: Mobile radio interface Layer 3 speci¬cation; Core network protocols;
Stage 3.
A list of the current versions of the speci¬cations can be found at http://www.3gpp.org/
specs/web-table specs-with-titles-and-latest-versions.htm, and the 3GPP ftp site for the
individual speci¬cation documents is http://www.3gpp.org/ftp/Specs/latest/
4
General Packet Radio Service


4.1 INTRODUCTION TO GPRS

The rapid growth in cellular voice services has led to high user penetration, particularly for
the global system for mobile communications (GSM), as has previously been seen. From
a user perspective, there is now a growing demand for value-added non-voice services,
which the existing GSM infrastructure is not well suited to deliver effectively due to its
circuit switched nature. From the cellular operators, perspective, with such high mobile
penetration in most developed countries, they are seeing a plateau in revenue streams,
and must now turn to non-voice services to create additional revenue.
The general packet radio service (GPRS) is a data service allowing traf¬c in the form of
packets (usually IPv4 or IPv6 packets) to be sent and received across a mobile network.
The point-to-point protocol (PPP) has recently been introduced, allowing the transparent
transportation of protocols such as AppleTalk and IPX. It is designed to supplement the
circuit switched mobile telephone system and the short message service (SMS) system,
as well as enable new services. In many cases it is seen as an evolutionary step towards
3G, and hence is often referred to as 2.5G. It has been termed always connected, since
after the initial connection delay subsequent connections are almost instantaneous. This
is in contrast to the GSM call or a traditional ¬xed-line call where every time a new
connection is made, there is a considerable delay. Although originally speci¬ed by the
European Telecommunications Standards Institute (ETSI), in the summer of 2000 the
standardization of GPRS was moved to the Third Generation Partnership Project (3GPP).
GPRS works by introducing the services of a packet switched network to the user
over the existing GSM network (IS-136 TDMA, as used in North America, also supports
the GPRS technology). This enables the user to continue to use the GSM network for
voice but if a data transfer is required this can be passed via the GPRS system. In this
way the existing base station system (BSS) infrastructure can be reused, as illustrated in
Figure 4.1.

Convergence Technologies for 3G Networks: IP, UMTS, EGPRS and ATM J. Bannister, P. Mather and S. Coope
™ 2004 John Wiley & Sons, Ltd ISBN: 0-470-86091-X
80 GENERAL PACKET RADIO SERVICE




Operator
Mobile ISDN/PSTN
Network Circuit Switched
(GSM)


BSS

Packet Switched
(GPRS)
Internet




Figure 4.1 Network infrastructure


Voice traf¬c will continue to be passed between the BSS and the circuit switched GSM
core network. This is generally referred to as the CS core network (CS-CN). GPRS traf¬c
will be redirected, usually within the BSC via a new unit called a packet control unit
(PCU) and be passed on to the packet switched GPRS network. This is generally referred
to as the PS core network (PS-CN). It should be noted that the external networks are
connected together via gateways. This allows the option of sending Internet protocol (IP)
data over the GSM network through the ISDN/PSTN and on to the Internet. This system
is in use today and supported by most mobile devices and mobile networks, it is known
as circuit switched data (CSD).
GPRS introduces a packet-based core network but still uses much of the GSM func-
tionality, including the home location register (HLR), equipment identity register (EIR)
and authentication centre (AuC) (3G systems also use this GSM functionality). What
GPRS introduces is the capability to transport different traf¬c types with more ef¬ciency
in network resource usage, and allow the introduction of a wide range of services. How-
ever, the general higher-layer functionality does not need to change and can thus be
reused. By way of example, a user who is sending an email does not need their loca-
tion information handled any differently from a user making a phone call. The network
is designed to support a number of different quality of service (Qos) classes and these
will be gradually implemented throughout the various releases of the standard in order
to enable the ef¬cient simultaneous transfer of both real-time and non-real-time traf¬c. A
common GPRS network can be used for both GSM and UMTS; however, some vendors
may not support both the GSM Gb interface and the UMTS Iu interface on a single piece
of equipment, resulting typically in some hardware changes or additions in migrating
to UMTS.
For GPRS/GSM the air interface is allocated in a ¬‚exible manner with 1 to 8 of
the time division multiplexing (TDM) channels being allocated to GPRS traf¬c. The
active users share the time slots and these are allocated independently in the uplink and
downlink. These radio interface resources are shared dynamically between speech and
data users, the exact method used being dependent on operator preference and service
4.2 GENERAL ARCHITECTURE 81


load of the network. Enhanced GPRS (EGPRS) is an enhancement to the system, which
allows higher bit rates through the use of different modulation techniques and coding
schemes (see Section 4.11.12).
The concept of a GPRS handover is referred to as a cell reselection procedure and
is normally performed by the mobile device. The handover timing for GPRS is not so
critical when compared to GSM since the traf¬c is not real time, and can thus be buffered.
In this cell reselection procedure, the mobile device makes measurement reports, as with
GSM; however the mobile station (MS) is more involved in the decision process, and can
even initiate the procedure for handover. It is, nevertheless, still the responsibility of the
network the serving GPRS support node (SGSN) to allow the handover to occur.
Security functions are essentially quite similar to those for GSM services. The SGSN
is responsible for authenticating the subscriber as well as encrypting/decrypting of data
towards the mobile device (regular GSM encryption is only between the mobile device
and the base station). The SGSN and mobile device can also compress data to make more
ef¬cient use of the Gb and air interface. A mobile device containing a standard GSM
SIM can connect to the GPRS network and use the services, depending on the speci¬cs
of the operator network settings.



4.2 GENERAL ARCHITECTURE
Figure 4.2 shows the general architecture of a GPRS network and its interface to other
IP-based networks such as the Internet. The GPRS network makes use of much of the
existing GSM infrastructure. The HLR, AuC and EIR may require minor modi¬cations to
support GPRS, generally in the form of a software upgrade. In the diagram, the different
equipment within the GPRS backbone network is connected together using an Ethernet
switch. Within the GPRS standard, there is no stipulation as to what Layer 2 technology

NSS
BSS
PSTN

MSC/VLR GMSC
Abis

HLR AuC EIR
Mobile
BSC
Station BTS



External
Network e.g.
switch
Internet
SGSN GGSN


GPRS IP
CG LIG DNS
Backbone

Figure 4.2 GPRS General Architecture
82 GENERAL PACKET RADIO SERVICE


infrastructure should be used to interconnect the IP backbone, since to support roaming,
the SGSN and GGSN will be connected through an internetwork. Most current networks
are using an Ethernet network to implement this local backbone, as it is a cost-effective
architecture choice. This Ethernet switch can be a weak link as it is a single point of
failure in the network, so usually it will contain a great deal of redundancy to ensure
reliability.



4.3 GPRS NETWORK ELEMENTS
To enable GPRS over an existing second generation network such as GSM, a number
of additional network elements are required in the GPRS backbone network. These are
described in the following subsections.



4.3.1 Serving GPRS support node (SGSN)
The SGSN serves the mobile devices within its BSS/RAN, and provides authentication,
and mobility management, which are derived as much as possible from the GSM mobile
application protocol (MAP). It is the connection point between the BSS/RAN and the
CN, and at a high level the SGSN provides a similar role for the packet switched network
as the MSC/VLR provides to the circuit switched network. When a mobile device is
packet switch attached, the SGSN is said to provide a mobility management context and
it then keeps track of the mobile device to a routing area (RA) or speci¬c cell. The SGSN
connects to GGSNs and also to other SGSNs via an IP network. When a mobile device
is furnished with a session management context a connection is established between the
SGSN and corresponding GGSN so that the mobile device may transfer data to and
from an external network. An SGSN is not restricted to communication with one single
GGSN and will in practice communicate with many GGSNs, which may not even be
within the same public land mobile network (PLMN) as the SGSN. The SGSN has a
dynamic database which stores information about the current mobile devices it is serving.
This database will contain the location of the device to an RA or speci¬c cell, security
information, such as the ciphering key, charging information, current connections and the
QoS being used, etc.



4.3.2 Gateway GPRS support node (GGSN)
The GGSN provides the interface between the mobile and the external packet switched
network. Packets are routed across the GPRS IP-based packet network between the SGSN
and GGSN using the GPRS tunnelling protocol (GTP). Like the SGSN, the GGSN
also stores information about mobile devices that have established a session with the
SGSN. The database will store the international mobile subscriber identity (IMSI) of
the mobile device, QoS negotiated, charging information, as well as the address of the
4.3 GPRS NETWORK ELEMENTS 83


SGSN serving a particular mobile device. When packets arrive for the mobile device
from an external network it is the GGSN which will receive them and route them to
the correct SGSN for ¬nal delivery to the mobile device. The GGSN does not need to
know the location of the mobile device, only the address of the SGSN which is serving
the mobile device. The SGSN and GGSN are collectively referred to as GPRS support
nodes (GSNs).


4.3.3 Charging gateway (CG)
A charging gateway (CG) is not required in the speci¬cations but is generally imple-
mented since it takes processing load off the SGSN and GGSN. It also introduces a
single logical link to the operator™s billing system and reduces the number of physical

<<

. 18
( 132 .)



>>