ERICSSON CA638, CF688 Introduction CHAP2
LZB 1 15 2508 R1A Introduc tion to GSM/DCS/PCS Systems
Chapter 2 Introduction to GSM/DCS/PCS Systems
Introduction
Cellular telecommunication is one of the fastest growing and most demanding telecommunications applications ever. Digital cellular solutions are successfully competing with traditional wire networks and cordless telephones. Eventually, digital cellular mobile telephones will be the universal way to
telecommunicate.
CME 20/CM S 40 is a GSM-b ased system operating in t he 900, 1800 , or 1900 MHz range (there is a common platform for CME 20 and CMS 40: Telecommunications Standards Institute (ETSI) and American
National Standards Institute (ANSI). It is a "total system" solution including switching centers, location
registers, base station controllers, and base transceiver stations, as well as facilities for centralized operations and maintenance and mobile stations (handsets). The system ensures that operating companies
receive the benefits of a full system standard with open interfaces, complete security, and roaming, among
others. CME 20 and CMS 40 are designed to provide users with advanced telecommunication services
according to technical specifications. Also, to meet demands made by operating companies (sometimes
referred to as "operators" and "carriers" in some countries) for flexible network solutions, innovative
charging, service segmentation, service creation, and easy Operation and Maintenance (O&M).
Ericsson has been designing cellular radio systems since the 1970s, and offers network products for all
major cellular standards - such as the Nordic Mobile Telephone (NMT) system, the Total Access Communication System (TACS), Advanced Mobile Phone System (AMPS), Digital AMPS; the Personal Digital
Cellular (PDC) standard (Japan), and the Global System for Mobile Communication (GSM) standard GSM
900, Digital Communication System (DCS) 1800, and the Personal Communications System (PCS) 1900.
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System Nodes and Related Equi pme nt
Refer to Table 2-1 for system frequency ranges.
Table 2-1. Frequency Ranges
System Frequency
GSM 900 890-960 MHz
DCS 1800 1710-1880 MHz
PCS 1900 1850-1990 MHz
GSM 900 is strong in radio coverage. DCS 1800 and PCS 1900 are strong in radio capacity. Additionally,
GSM and DCS is often offered by combined license (PCS 1900 is the GSM standard’s American version).
See Figure 2-2
. CME 20 / CMS 40 consists of the following:
• The Switching System (SS), where most call processing and subscriber related functions are
implemented
• The Base Station System (BSS) where radio related functions are concentrated
• The Mobile Intelligent Network (MIN) which is the provider of customized subscriber services
and service management
• A network management system in the form of the Operation and Maintenance System (OSS), for
centralized network maintenance and operation
• A network planning tool, the Er icsson Engineering Tool (EET) for qu ick, conveni ent network
planning on the radio end
• Gateway products like the Service Order Gateway (SOG) and Billing Gateway (BGW) for centralized subscriber data handling.
• The CME 20 / CMS 40 also has adjunct system parts: for example, it supports Message Centers,
transport networks (Digital Cross Connect (DXX)) and microwave links.
AUC EIR
HLR ILR
MSC/VLR
BSC
Base
Station
System
Other
PLMNs
ISDN
PSTN
PSPDN
GSPDN
OSS
Network
Management
System
EET
Network
Planning
Tool
Network Management Products
Management
Center
Gateway Products
BGW
SOG
MIN Network
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SCP
BTS
GSM800 DCS1900
BTS
BTS
Purchase 1900
Figure 2-1. The CME 20/CMS 40 and its Different Groups
LZB 1 15 2508 R1A Introduc tion to GSM/DCS/PCS Systems
The Switching System (SS)
The Switching System contains the following nodes:
•The Mobile Services Switching Center (MSC) performs telephony sw i tching functions. A spec i al
Gateway MSC (GMSC) function co-ordinates traffic from other networks. Functions for switching, network interfacing, common channel signaling, toll ticketing, etc., are also included. The
GSM Inter W orking Unit (GIWU) for data communication is also implemented in the MSC.
•The Visitor Location Register (VLR) is a database containing information about all Mobile Stations
currently located in the MSC area. The VLR is always integrated with the MSC.
•The Home Location Register (HLR) is a database that stores and manages subscriptions. It contains
permanent subscriber data such as subscription types, subscribed services, and so forth. It also
stores information about a subscriber's location. The HLR can be integrated in the same node as
the MSC, or can be implemented as a separate node.
•The Interworking Location Register (ILR) offers roaming between mobile telephony systems com-
plying with different standards. The possibility to offer intersystem roaming to subscribers gives
operating companies a new, competitive edge. The ILR is a stand-alone node: it will be co-located
only with an AMPS Authentication Center (AUC).
•The Authentication Center (AUC) provides authentication and encryption parameters required to
assure Mobile Station (MS) user identity and to ensure call confidentiality.
•The Equipment Identity Register (EIR) is a database containing information about mobile equip-
ment identities in order to prevent Mobile Station (MS) unauthorized use. (The EIR is often
located in the same node as the AUC, but it can be implemented as a separate node.)
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The Base Station System (BSS)
The Base Station System contains the following nodes:
•The Base St atio n Controller (BSC) is a high capacity switch responsib le for radio functions such as
hand-over, managing radio network resources, and handling cell configuration data. It also controls radio frequency power levels in RBSs (BTSs), and Mobile Stations (or, mobile telephone
handsets).
•The Base Transceiver Station* (BTS) is radio equipment needed to serve one cell (according to
GSM). Radio Base Stations (RBSs), however, are the equipment needed to serve many cells. The
traffic interface between the BSC and the BTS is the Abis interface. Typically, one MSC in the
Switching System supervises one or more BSCs, which in turn can control a number of BTSs.
NOTES
CME 20 and CMS 40 has ETSI (CME 20) 32 channel a-law, and ANSI (CMS 40)
24 channel (-law ("bit-stealing") that serves BTSs at 900, 1800, and 1900 MHz (bit
rates are 2 Mbit/s for CME 20 and 1.5 Mbit/s for CMS 40, respectively).
The MSC/VLR, HLR, ILR, MIN as well as BSC are all AXE applications. The
AXE is a flexibly applied, high capacity switch that is the basic building block in
Ericsson Public Land Mobile Networks (PLMNs), Public Switched Telephone
Exchanges (PSTNs), Public Data Networks (PDNs) and Integrated Services Digital
Networks (ISDNs). Using AXE technology gives network operators a proven telecommuni cation base when setti ng up their network and ensu ring continued techni-
cal development in the system. Having system nodes AXE based also makes
operation and maintenance easier, since the same maintenance skills that apply to
AXE also apply to the MSC, HLR, the ILR, BSC, and SCP.
Mobile Intelligent Network (MIN)
The MIN is a concept consisting of all possible Intelligent Network (IN) nodes. MIN provides intelligent
network services to cellular network users. It is also the foundation for service creation and management in
the network. It lets the operating company to differentiate its services in the market.
The MIN executes service logic and consists, among other nodes, of the Service Control Point , which is a
vital MIN network node that can serve as a stand alone node or be integrated with the Service Switching
Point (SSP), or stand separate from the MIN, altogether.
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Operation and Support S ystem (OS S)
The Operation and Support System (OSS): a network management product and tool based on system node
software making it possible to centralize operation, support, and management of a network. The basic
functions can always be accessed from local operation and maintenance terminals. But higher level network administrative tasks can be performed from one or multiple Operation and Maintenance Centers
(OMC), Network Management Centers (NMCs), or both, through the OSS. The OSS is necessary for efficient network management as the network rapidly grows. The OSS also provides functions for radio network configuration management, as well as network and radio traffic measurements, post processing,
display, and report generation.
Ericsson Engineering Tool (EET)
The Ericsson Engineering Tool (EET): a software-based network management product that engineers use
as a cell planning tool, and that can interface with the OSS. EET allows for planning and managing a radio
network’s resources.
Gateway Products
The Service Order Gateway (SOG): connects an operating company’s Customer Administrative System
(CAS) to the various system node s.
The Billing Gateway (BGW), which connects an operating company’s Customer Billing System to the various system nodes.
Adjunct System Parts
GSM-based systems can access or use the following adjunct parts.
• Message Center: Different Message Systems can be connected to the network. The messaging sys-
tem stores and passes on messages such as voice mail, fax mail, short text messages (GSM/paging), electronic mail, etc.
• Digital Cross Connect (DXX): Ericsson's DXX transport network solution is a highly flexible sys-
tem of modular hardware and management software designed for quickly deploying and broadening public telecom transport networks. DXX is an ideal transmission solution for all mobile
communications networks, whether they carry digital or analogue cellular, PCN or paging services.
• Microwave Links (Mini Link): Mini link (microwave) products are available for use with GSMbased systems. Mini links integrate completely with existing telecom (like wire line) networks,
adding new levels of "short-haul" radio flexibility to networks, or point to point transmission of
voice and data. Microwave links are easy to install, are flexible, and are reliable in all climates.
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Features
The cellular mobile telephone system provides communication to and from Mobile Station subscribers by
providing various basic telecommunication services. Each system release (1.0, 2.0, etc.) witnesses an
increase in new basic and optional functionality. Feature development is driven by customer demands and
feedback. System development also incorporates product enhancements such as new hardware, network
node integration, increased capacity, customer services, continuing staff education and training, and guaranteed system reliability.
System Capabilities
For the User
The system provides its users with an extensive set of services and facilities, such as:
• Roaming, or, a Mobile Station's ability to move freely throughout the entire service area, irrespective of the network operator.
• Integrated voice and data communication. Not only does the system give its users excellent mobile
voice communication, it also enables data communication handling to and from Mobile Station
subscribers.
• Security. Subscriber authentication, transmitted information encryption, and using temporary sub-
scriber identities in the network protects the user against false call charges and ensures total privacy . (Security aspects of the system are catered to via equipment validity checking in the EIR.)
• An extensive set of Supplementary Services for call forwarding, call barring, multiparty conversations, and so on.
•The Short Message Serv ice makes it possible to send a short text (alphanumeric) message to, or
from, Mobile Stations. (The short message service can be viewed as an advanced form of alphanumeric paging.)
• A wide var iety of Bearer Services (BSs) up to 9.6 kbit/s.
In addition to normal call handling functions necessary to support the services and facilities just mentioned, the system also implements generic features like network wide hand-over. In order to ensure the
connection continuity as a Mobile Station moves, an active call can be handed over between:
• Physical channels within one cell
• Cells controlled by the same BSC
• BSCs connected to MSC
• MSCs belonging to the service area.
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For the Operating Company
The system also ensures optimum performance and economy for the network operating company’s staff:
• Operation and maintenance functions inherent in the MSC/VLR, HLR, and the BSC are virtually
identical to the functions in a general AXE telephone exchange. These functions provide continuous exchange performance supervision and statistics collection, as well as automatic reconfiguration during restarts .
• Centralized network management for all system parts is available via the OSS, including dedicated
applicat ions for mana ging a network.
• Remote control and supervision of RBSs from the BSC (including software updates that decrease
the need for RBS site visits) is also provided by the system.
• Submultiplexing and transcoding in the BSC minimizes the transmissions to Radio Base Stations,
and so reduces costs. (Four voice channels are submultiplexed into one PCM channel.)
• Drop/Insert functionality and Digital Cross Connect (DXC) in RBSs makes it possible to connect
numerous RBSs (BTSs) on the same 1.5 or 2.0 Mbit/s PCM path allowing for a very efficient and
redundant transmission network topology.
• Discontinuous Reception (DRX) and Uplink Discontinuous Transmission (DTX). All paging
requests are transmitted at predefined time intervals, allowing the Mobile Station to switch off the
receiver between paging intervals. Additional battery power is saved with a voice activity detector
that suppresses the handset's transmissions during speech pauses.
• Frequency hopping, Downlink Discontinuous T ransmission (DTX), intra cell hand-over, and
dynamic mobile and radio base station power control. These functions maintain cells' call quality
levels as th e number of subscribers grows in a network.
• Mobile Station subscriber activity supervision. A Mobile Station subscriber will automatically be
pointed out and not paged if the subscriber has been inactive (or is in an idle state) or is beyond
coverage for a certain time period. Such call supervision decreases the load on paging channels
and instantly tells other networks the Mobile Station user cannot be reached.
• Charging for both voice and data traffic (including Short Message Services) is done on a per call
basis using toll ticketing.
• Accounting functions collect and administer data needed for remuneration purposes between operating companies (that is, network operators).
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Telecommunication Services
Basic telecommunication services can be divided into two main categories: Teleservices and Bearer Services. These "basic services" are augmented by various Supplementary Services, such as call forwarding,
call transfer, and so on.
A Teleservice lets the MS subscriber communicate with another subscriber (usually voice, fax, and Short
Message Services). A Bearer Service lets the MS subscriber send data (transmitting signals between two
access points that provide an interface to the network).
The system is also designed to enable operating companies to distinguish their services from the competition’s using Ericsson innovative, or customer-defined, services based on Ericsson’s mobile Intelligent Network (IN) techniques. Mobile Intelligent Networks (MINs) let an operating company define personalized
new services for delivery to Mobile Station subscribers in different customer segments (business, residential, etc.).
Teleservices
The system provides the following Teleservices:
• Telephony
• Emergency calls
• Dual T one Multi Frequency
• Short Message Service, mobile originated/point to point
• Short Message Service, mobile terminated/point to point
• Automatic facsimile service Gr.3 in transparent mode
Telephony
All subscribers are able to receive and originate calls independently of where in th e entire service area they
are located. Telephony is normally two-way voice communication between Mobile Station (MS) subscribers, and is the most fundamental service.
Emergency Calls
Emergency calls are initiated by dialing a simple emergency call procedure. An emergency call can be
made without using a Subscriber Identity Module (SIM) card in the MS. If the subscriber uses the emergency function in the MS to set up the call, there is no called number to charge.
DTMF
Dual Tone Multi Frequency (DTMF) tone generation provides means to send digits from a Mobile Station
to a remote end. By sending these digits an MS can request the services provided by the remote end. The
digits are sent through the network in the form of tones. DTMF tones are not generated by the MS, but by
the MSC/VLR due to distortion otherwise introduced by a speech coder.
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Short Message Service
The Short Message Service (SMS) provides a means of sending text messages of limited size (containing
up to 160 alphanumerical (that is, both text and number (characters) to and from Mobile Stations. The SMS
makes use of a Service Center (SC), which acts as a store and forwarding center for short messages.
The SMS comprises two basic services:
• Mobile terminated (from an SMSC to a Mobile Station).
• Mobile originated (from a Mobile Station to an SMSC).
Mobile Terminated SMS
The mobile terminated SMS can transfer a short message from the SMSC to a Mobile Station. It also provides information about the delivery of the message. Information is either a delivery report, which confirms the delivery of the short message to the recipient, or a failure report, which informs the originator that
the short message was not delivered, and the reason why.
Mobile Originated SMS
Mobile originated SMS will transfer a short message submitted by the MS to an SC. It will also provide
information about the delivery of the short message either by a delivery report or a failure report.
Interworking MSC for SMS (SMS-IWMSC) is an MSC capable of receiving a short message from within
the PLMN and submitting it to the recipient SC. SMS-GMSC and SMS-IWMSC are located in all MSC/
VLR nodes.
Cell Broadcast Short Message
Another short message service type is the cell broadcast facility, where a short message is forwarded to all
Mobile Station subscribers in a geographical area. Typical types of messages (93 characters or less) may
include, for example, traffic delay warnings, bad weather ahead, etc., for boaters, surfers, or drivers, and so
on.
Automatic Facsimile Gr.3
The automatic (autocalling/autoanswering) fax Gr.3 in the transparent mode is supported by CME 20 /
CMS 40. This means that fax connections may be established to or from group 3 apparatus.
Bearer Services
With a Bearer Service, the network provides a transmission path between two access points and also a
user-to-network interface. The network will be responsible for delivering in one interface what was
received in the other. Interworking attributes may be defined for the support of Bearer Services over transit
networks.
The system supports both transparent and non-transparent data services. A transpar ent data service is char-
acterized by a fixed delay (no flow control seen or handled by the network and known user traffic throughput) and an error rate that may vary. A non-transparent data service includes the flow control and the
Radio Link Protocol (RLP), which gives a variable delay, but a lower and more constant error rate over the
radio interface.
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