Bsnl Report By Nagi

OCB-283 EXCHANGE INTRODUCTION The telephone network is an interconnection of telephone exchange through transmission links serving the connected subscribers. OCB-283 stations are normally designed on the principle of multiprocessing multitasking computer structured on design of M-68020 and 68030 microprocessors. All the stations have almost bus structure and many of the PCBs are repeated in stations, so no. of types of PCBs is very errorless. Only 35 types of PCBs are in use in whole exchange except CSN. Including CSN's special PCBs, total types of PCBs are only 55. The full abbreviation of OCB is "Organ Control Bersion" retrieved from French exchange, which introduced the technology of OCB-283. In exchange, switch room is named as OCB-283 based on an advanced technology recently introduced in H.P. Only 4 districts in H.P. namely Mandi, Solan, Shimla and Dharamshala have OCB-283 exchanges.

SWITCH ROOM OCB-283 is digital switching system supporting a variety of communication needs like basic telephone, ISDN, interface to mobile/data communication etc. which has been developed by CTT AICATEL of France so has many similarities to its predecessor E-10B (also known as OCB 181 in France). The first OCB-283 exchange of RII version was commissioned in Brest (France) and Beijing (China) in 1991.The first OCB-283 exchange came to India in 1993.The system has been upgraded and current version R-20 was fully validated in Jan. 1994,which are being supplied to India.

SALIENT FEATURES

1



It is digital switching systemwith single 'T' stage switch and a maximum number of 2048 PCMs can be connected.



It supports both analog and digital subscribers.



The system supports all the existing signaling system like decadic, MF (R2), CAS and also CCIII # 7 signaling system.



It provides telephone, ISDN, data communication, cellular radio and other value added service.



The system has automatic recover feature. When a serious fault occurs in a control unit, it gives a message to SMM (O&M unit) which puts this unit out of service, loads software of this unit in a back up unit and brings it into service.



It has a double remoting facility subscribers access unit CSND can be placed at a remote place and connected to main exchange through PCM links. Line connectors can also be placed at a remote location and connected to CSNL or CSND through PCMs meating urban, rural and tribal necessities.



Various units are connected across token rings (IEE 802.5 standard) enabling fast exchange of information, avoiding complicated links and wring between various units.



The subscribers charge accounts are saved in the disc automatically, once in a day avoiding loss of revenue due to total power supply/battery failure.



It can handle 8,00,000 BHCA and 25,000 erlangs of traffic depending on which maximum of 20,000 subscribers or 60,000 circuits can be connected, so it has huge traffic handling capacity.



The exchange can be managed locally or from NMC through 64 Kbps link.



All control units are controlled on same type of hardware called station.



It is made of 35 types of cards excluding cards required for CSN, due to this no. of spare cards for maintenance of cards are drastically reduced.



It has modular structure. The expansion can be easily carried out by adding hardware and software.

2



The SMMs are duplicated with one active and other hot standby. Switch takes over automatically in case of faults. Discs are connected to both SMMs avoiding cables from one system to another.



The hard disc is very small in size, compact and maintenance free. It has huge memory capacity of 1.2Gb.

EXCHANGE HAS THREE BASIC SUB SYSTEMS  Subscriber Access Sub System  Connection And Control Sub System  Operation And Maintenance Sub System

FUNCTIONAL ARCHITECTURE It comprises of following components.

 CONNECTION UNITS These provide facility to connect a subscribers loop or circuit from an external PCM and transfer these speech signals on selected time slots called voice channels on LR link towards switching matrix. These units are: NO. 1. 2. 3.

NAME Subscriber Connection Units Circuit Connection Units Frequency Generator, Sender and receiver and CCS protocol handlers

3

FUNCTIONAL NAME CSNL, CSND, CSEN SMT (URM) SMA (ETA), SMA (PUPE)

 CONTROL UNITS These provide control of calls on basis of stored programmes. They process calls on reception of dialed digits from calling Subs.circuit and take part in call set of handling and release by processing, monitoring, measuring charging of calls and other important functions nedded for working of automatic common control exchange.

NAME OF FUNCTION

NATURE OF JOB

Mr.

MULTIREGISTER

Call handler set up and release of call. Translation of digit Data bank of subs.

Tr.

TRANSLATOR

& chtsin files. Computing charge of a cell, keeping

TX

CHARGER

meter. Message Distribution between common

MQ

MARKER

control and connection units. Process and makes connections in

GX

MATRIX SYSTEM HANDLER

switching matrix on orders from MR

PC

and Or MQ. Manage the CCst network for signaling.

CCS NETWORK

CONTROLLERS

 SWITCHING NETWORK These provide facility for connecting LRs (internal PCMs) coming from connection units and performs switching operations for calling subscribers TS on to called subscribers TS and vice versa for a two way connection per call of telephony.

 O & M UNITS AND MAINTENANCE PERIPHERALS: All the operational and maintenance activities are performed by O & M unit providing assess for man machine dialogues for human operations to interact and command the working of exchange equipments. 4

SYSTEM ARCHITECTURE The following hardware units are:

1. SUBSCRIBER ACCESS UNITS i)

CSNL-(MSU) Subs. Unit.

ii)

CSND-(RSU) Subs. Unit.

iii)

CSED-Remote Access Unit.

2. TRUNKS AND JUNCTION CONNECTION UNITS. i)

SMT-Trunk Control Station

3. SWITCHING MATRIX i)

SMX-Matrix Control Station

4. AUXILLARY EQUIPMENT i)

SMA-Auxillary Equipment Rack

5. CONTROL UNITS i)

SMC-Main Control System

6. COMMUNICATION MULTIPLEXES i)

MTS-Interstation Multiplex

ii)

MAS-Station Access Multiplex

iii)

MAL-Alarm Multiplex

7. TIME BASE GENERATOR 5

i)

STS-Time Switch Control

8. OPERATION AND MAINTENANCE UNIT i)

SMM-Maintenance Station

The subscriber connection units CSN, SMPs and SMAs are connected to switching network through PCM links. The interchange of messages between SMT, SMX, SMA and control units SMCs takes place on 'MAS' token rings. The control units interchange messages with one another and with SMM on 'MIS' token rings. The SMM is the O & M function unit and is duplicated as SMMA and SMMB. These work is pilot/standby mode. The SMCs are the units holding the control functions MR, TR, TX, MQ, PC,

GX these functional units are in

software form and are duplicated except MR which can be more than two. The duplicated functions work in load sharing mode(except PC) hence SMCs can be minimum 2 And maximum 32 as per design. The SMA stations hold the ETA and PUPE functions and these are also minimum 2 to maximum 32.SMT station which is the interface for the external PCMs is made of duplicated hardware and can handle either 32 PCMs if SMTIG or 128 PCMs if 2G. The SMTs hardware is fully duplicated and functions P/R mode. The brief description of these units is as follows:

SWITCHING NETWORK The switching network in OCB-283 is single 'T' stage system. It is made up: a)

Host switching matrix

b)

Branch selection and amplification(SAB) function

6

HOST SWITCHING MATRIX The host switching matrix consists of two identical branches A and B.The host switching matrix is implemented on the hardware units known Matrix control station(SMX). Each having upto 2048 incoming PCM links (LRE) and 256 outgoing links. Out of 2048 incoming links are coming from the other seven SMXs. In full configuration, the host switching matrix is 2048 X 2048 matrix. A matrix control station canm establish between any TS on 2048 LRE and my TS on 256 LRS. Similarly a host switching matrix can establish connections between any TS on 2048 LRE and TS on 2048 LRS. The matrix control station is built around a processor implementing switching machine MLCOM functions, to establish and break connections between time slots caring out two way communications with other units in the system over MAS rings.

THE SWITCHING CONCEPT This time switch comprises of a speech buffer memory, a control memory, an incoming highway of digital speech in parallel bits and an outgoing highway. This is an INPUT

ASSOCIATED CONTROLLED TIME SWITCH In this switch the BUFFER MEMORY AND CONTROL MEMORY are controlled type i.e the writing in it is controlled. The control function writes in the Control memory at the location corresponding to the INCOMING TIME SLOT NUMBER the location where it should be written in the buffer memory. Both these memories are sequential read type. Reading of control memory gives the address in BUFFER MEMORY for writing the INCOMING TS BYTE. And thus reading of buffer memory sequentially the TS will be read from the location given by the control memory. Thus a one way TIME switching has taken place. 7

DUPLICATED SWITCHING The switching is done in OCB-283 in two fully duplicated branches simultaneously. For this purpose from each connection units the LR links originate in two parallel branches towards two parallel sets of switching matrices called the SMX and SMXB and these branches of such network are called A & B and are terminated on the respective connection units. The duplicated branches of switching have been designed to prove highly reliable switching path for DATA SWITCHING, VIDEO CONFERENCING, ISDN ETC.

SAB FUNCTION The connection units have their internal duplicated hardware called control logic, working in pilot/reverse arrangement. The duplicate LRs originate from a function in connection units called SAB-Selection and Amplification of Branches. Its role is to generate two sets of LRs in trans direction with calculation of parity etc. In reverse direction it gets data from both the branches which it checks for parity etc and compares to detect any error in the two branches. In case of error the samples from only the good branch are taken after automatic testing of the quality of transmission of both the branches by the common control and the faulty branch is withdrawn from service. The connection unit's LR links are performed into group of 8 LRs at the factory into cables with both ends terminated with plugs for the convenience of installation. Such groups of LRs are called GLR.

SUBSCRIBER ACCESS UNITS (CSN)

8

Subscriber connection units (CSN) are so designed that they can be equipped with either analogue of digital subscriber or both having different cards for each. CSN can either placed in the exchange switch room or at a remote location. Depending on their location, CSN is known as CSNL or CSND and the subscriber shelf is known as local or remote concentrator, CNL or CNE. The CSNL is connected to switching matrix(SMX) through a minimum of IGLR or a maximum of 2 GLRs(group of 8 LR is called as a GLR and each LR is a PCM link having 32 times slots). The CSND is connected to SMT rack through a minimum of 2 PCM and a maximum of 16 PCMs.CSED of E-10B System can also be connected to an SMT.The message interchanges between CSN and control units take on a common signaling channel using local version of CCS#7 signaling.

ARCHITECTURE OF CSN The CSN can have one BASIC RACK and three EXTENSION RACKS and its architecture can be broadly divided into 2 parts: i)

DIGITAL CONTROL UNIT (UCN)

ii)

CONCENTRATOR (CNL OR CNE)

1.

DIGITAL CONTROL UNIT (DCU)

The digital control unit (UCN) is the interface between concentrators and the exchange. It is in Basic Rack and placed in switch room for CSNL and at a remote location for CSND.It can be further broken down into: a)

CONTROL AND CONNECTION UNIT (UCX)

9

There are two such units which are the controlling logics of the CSN and operate in the pilot standby mode. The active UCX controls the working of CSN and also updates standby UCX.

b)

AUXILLARY EQUIPMENT PROCESSING GROUP (GTA)

This component performs the following functions in case of stand alone operation (i.e. when CSND is isolated from main exchange): i)

Generates tones and recorded announcements for local communication.

ii)

Decodes DTMF dialing

2.

CONCENTRATORS

The shelf which accommodates subscriber line cards is known as concentrator. The concentrators can either be co-located with the digital unit in which case they are known as local concentrators. CNL or at a remote location in which case they are known as remote concentrators CNE.When a remote concentrator is used than to connect its PCMs to digital control unit an interface shelf ICNE is required. The maximum capacity of a concentrator is 256 subscribers. The following type of subscribers can be connected to a concentrator, by equipping suitable kind of card. i)

ANALOGUE SUBSCRIBER

(TABAS CARD)

ii)

2B+D DIGITAL SUBSCRIBER

(TAVAE CARD)

iii)

30B-D DIGITAL ACCESS

(TADP CARD)

When all the concentrators are local, a maximum of 19 concentrators can be equipped in one CSN having 4 rack shelves, which as if all are remote or if atleast 2 are remote, a maximum of 20 concentrators can be equipped in one CSN.

10

SIGNALING BETWEEN CN & UCN AND WITH PUPE The interchange of message between concentrators (local and remote) and digital control unit of CSN takes place through HDLC protocol whereas the signaling between CSN and the exchange is through local version of CITT#7.

TRUNK AND JUNCTION CONNECTION UNIT (SMLT) This is also known as PCM trunk control station and is an interface between PCM junction coming from other exchanges (or CSND, CSEDs) and the switch. The current version of SMT being supplied to India is SMT 2G. In each SMT 2G, there are 8 modules and in module there are 16 PCMs. Thus there are 128 PCMs, in a single SMT 2 G is built around microprocessor 68030.

GENERAL ARCHITECTURE SMT 2G consists of duplicated processing subsystems. SMTA and SMTB which are connected through internal links LISM. Both of them to PCM interfaces as well as to MAS token tings. PCMs are connected to PCM interfaces, which are not duplicated. SMT 2G is connected to SMX A and SMX B through 128 PCMs which are connected to SAB branch A and SAB branch B. Speech samples are sent on both the branches from SMX, but one which is better is selected and connected to the concerned PCMTS by SMT. Out of the two processing logics SMT A and SMT B one remains active and other standby. In case of fault in active logic, automatic switch takes over place providing an uninterrupted service and locavar is activated on the faulty logic and the diagnostic is printed on a terminal.

FUNCTIONS: 11

The software MLURM is loaded on SMT to perform functions of a PCM controller The functions performed on receive side i)

Converts HDB3 code to binary.

ii)

Extracts channel associated signaling.

iii)

Manages CCS 7 messages carried on TS 16.

iv)

Cross connects a channel on PCM to a TS on LR.

The Functions performed on transmit side i)

Converts binary code to HDB-3 (line) code.

ii)

Injects channel associated signaling.

iii)

Manages CCS 7 messages on TS 16.

iv)

Cross connects a TS on LR to a channel on PCM.

AUXILLARY EQUIPMENTS CONTROL STATION (SMA) The SMA contains the following two functional units. i)

ETA

ii)

PUPE

ETA The ETA contains following subcomponents. a)

Frequency receiver/generators.

b)

Conference call circuits.

c)

Tone generators.

12

The frequency receivers/generators recognizes the digits dialed through DTMF instrument and also the MF(R2) signals received on junctions. They also generate the various frequencies required for MF(R2) signaling and testing etc. The conference circuits are used to set up connection between a maximum of 4 subscribers. These 4 subscribers can hold conference on the telephone i.e. they can talk to each other. Tone generators generate various tones required to be connected during call processing. These tones are dial tone, Busy tone, Ring back tone, processing tone etc.

PUPE The PUPE performs level 2 part of level 3 functions for CCITT No.7 signaling. The rest of the level 3 functions performed by PC. The various functions performed by PUPE are as below. TRANSMIT SIDE: i)

It sends 'flag' and 'check bit' in the HDLC frame while transmitting CCS7 messages. It also inserts zeros. When there are more than 5 consecutive AOnes (Is) in the message.

ii)

PUPE sends 'fill in signal units' (FISU) automatically, when there are no messages to be sent.

iii)

PUPE also sends 'link status signal units' (LISU) when commanded.

iv)

It re-transmits a signal unit on receipt of negative acknowledgement.

RECEIVE SIDE: i)

On receipt of CCS 7 signaling messages, it eliminates zeros which were inserted after five consecutive ones(Ls)

13

ii)

It detects the flag and also computes the checksum and compares them with bits. If these two match it sends positive acknowledgement otherwise it sends a negative acknowledgement.

iii)

It eliminates fill in signal units as they do not carry any information.

IMPLEMENTATION OF ETA AND PUPE ON SMA Either ETA or PUPE or both can be implemented on the same SMA. When both are implemented on the same SMA MLPUPE (logic machine PUPE i.e. (PUPE software) is loaded on the principal processor (PUP) and MLETA is loaded on secondary processor (PUS). When only PUPE is implemented on SMA, it is loaded on PUP and when only ETA is implemented, it is loaded on PUS. Only first two ETAs have tone generators CCFs and RGFs are provided as per requirements. The PCB used is common for RGF, CCF and tone generators, only the Software is different when no CCF or tone generator is required an ETA can have a maximum of 96 RGFs. An SMA is connected to SMX by 8 LR links. The following table illustrated the capacity and modularity of SMA.

SMA

UNITS CAPACITY MIN.

EQUIPPED WITH ETA alone PUPE alone ETA and PUPE both

MAX &

96 RGF 2 to 32 64 CCS-7 Channels 2 to 15 64 RGFs/32 CCS-7 2 to 15 Channels

CONTROL UNITS 14

All control units like MR, MQ, TX, TR, ETC and SMA are implemented on common type of hardware architecture, called station which is built around a microprocessor station bus 'BSM'. One or more processors and 1 or more intelligent couplers can be connected to this bus. They exchange data through common memory. The main processor is connected to common memory through a 32 bit private bur, apart from through BSM. All the processors are Motorola 68020 processors and operate at 15.6 MHZ.clock. Multiprocessor station bus BSM is a 16 bit bus operating at 44.8 Mbps. There can be 1 principal processor (PUP) and 4 secondary processor (PUS) in a station. Also, there can be 1 main coupler (CMP) and upto 4 secondary couplers (CMS). A station can function as MR, TR or any unit when a particular software is loaded in the station depending on traffic. A functional unit can be implemented on the principal or secondary processor and on main or secondary coupler. A software called 'SUPERVISOR' is provided in station for communication and loading facilities; 'HYPERVISOR' to permit co-habitation of many software machines on same station. The station is generally called SMC. CONTROL STATION There are 6 common control functions in an OCB-283 which are alloted max. and min. nos.

SR. NO. 1. 2. 3. 4. 5. 6.

NAME OF UNIT MR TR TX MQ PC GX

MINIMUM 2 2 2 2 2 2 15

MAXIMUM 7 2 2 2 2 2

They are called 'LOGICAL MACHINES' in software form. 'ML' is implemented on hardware of SMC (station) by loading suitable software on it. An SMC can support any 1 or many MLs in defined combinations.

SMCs can be min. 2 and max. 32 bit nos. depending upon exchange configurations and traffic requirement. The required no. of SMCs is decided by planners & manufacturers. There is also a 'BACKUP SMC STATION' not loaded with any software. When any SMC becomes faulty, it sends message to SMM which blocks this unit and informs all control units regarding non-availability of this unit. SMM then loads software of all functional units on back up station and brings it into service. So, there is automatic recovery.

FUNCTIONS OF VARIOUS COMMON CONTROL SOFTWARE MLs MULTIREGISTER (MLMR) It establishes and releases calls and takes real time decisions for call processing. It consults TR to find out subs. Entitlements and orders for connections and disconnections of various tones and subs. It also carries out observation functions.

TRANSLATOR (MLTR) The TR stores exchange data base in its memory and tells MR the characteristic and entitlements of subs. and circuits on request. It stores routing and analysis data and converts received digits into equipment no. of the called subs. 16

MARKER (MLMQ) It carries out messages between control functions MLs and connection units for subs./circuits. It also acts as 'gate' for messages while passing from one communication to other. The MQ also supervises semi permanent connections in the network. CHARGING UNIT (MLTX) The TX carries out charging for each communication set up. It keeps charge account of all subs. and prepares and sends detail billing messages to SMM. It also carries out subs. and circuit observation functions. MATRIX SYSTEM HANDLER (MLGX) The GX monitors the connections in the switching network and in case of fault carries out appropriate defence functions. It also monitores internal links in the switching network. CCS 7 CONTROLLER (MLPC) The PC carries out 'routing and traffic' management functions for CCITT No.7 signaling. It also carries out observation functions and defence of PUPE, if a PUPE developes fault, it is automatically blocked, the semi-permanent link is reconfigured and 'standby PUPE, is brought in service.

EXCHANGE CONFIGURATION OCB-283 can have 4 configurations depending upon its size and traffic. One or more MLs can be implemented on one SMC station. Taking advantage of this flexibility the following 4 configurations are designed. a) COMPACT

'C' CONFIGURATION.

b) SMALL

'P' CONFIGURATION.

c) MEDIUM

'M' CONFIGURATION. 17

d) LARGE

'G' CONFIGURATION.

SR.NO.

COMPACT (C)

SMALL (P)

MEDIUM (M)

1. SMC

2

2+1 BACK UP

2 or 3 MR + 2 4 to 7 MR + 2 =5

LARGE (G)

+ 2 SMC for others.

2. SMA

2

2

2 or MORE = 3

2 or MORE

3. SMT

1

1

1 or MORE = 2

1 or MORE

4. SMX

1 (48 LR)

1

1 or MORE =2

1 or MORE

5.(1or SMCs be

2) (1)

MR, TR, (1)

TR,

TX, (2) TR, MQ,

can TX, MQ, GX & TX, MC, GX & MQ, GX & PC GX

loaded PC Software.

with 6.SMA can ETA, be

MR, TR, (2)

PC Software.

PUPE ETA/PUPE

loaded Software.

Software.

Software. ETA, Software.

&

PC

Software. PUPE ETA,

PUPE

Software.

with 7.Performa- 5 nce Data

CA/S

BHCA.

or 36

CA/S

or 100 CA/S or 220 CA/S or

1,30,000

3,60,000

8,00,000

BHCA

BHCA.

BHCA.

18

INTERNATIONAL INTELLIGENT NETWORK (IN) DEFINITION In an intelligent network (IN), the logic for controlling telecommunications services migrates from traditional switching points to computer based, service independent platforms. This provides network operators an open platform provisioned with generic service components that can incorporate with elements from different vendors, based on published, open-interface standards. This platform can be used to develop new and different services.

OVERVIEW The evolution and direction of the IN is based on International Telecommunications Union-Telecommunications

Standardization

Sector

(ITU-T)

and

European

Telecommunications Standards Institute (ETSI) standards development.

DRIVING FORCES BEHIND THE INTELLIGENT NETWORK Within the traditional telecommunications environment, telecommunications companies acted both as network operators and service providers. The network operator is the entity that owns and operates the network infrastructure. A service provider is an entity that offers services to the subscribers. The service provider uses the network infrastructure of a network operator to deliver the service to the subscriber but is responsible for the management and development of the service. Service offerings were more likely driven by technological availability rather than customer need, as much of the network infrastructure has been based on proprietary interfaces with bounded capabilities. This type of environment resulted in long 19

development times and large investments to deploy services. New technological capabilities, privatization and deregulation, and changes in market and customer demand have driven the emergence of INs. The result is an increase in competition that has forced operators and service providers to add new features rapidly to attract and retain customers. The IN can play an important role in providing such new features and services. In an intelligent network, control of call processing is moved out of the switch and into the network. The idea is to give service providers the ability to develop new services quickly, independently and inexpensively; a capability they do not have when new services are implemented on the network switches. With the IN, service providers or their IN vendors develop the intelligence or service logic to provide new services using service creation environments. Then they deploy this intelligence on network control points within the IN. so, service providers can use the facilities of IN to deploy new services to their subscribers without any change in the programming in the network switches. By separating

services

from

switching

equipment,

the

IN

opens

markets

for

telecommunications-service creation and switching-equipment providers.

OVERVIEW OF THE INTELLIGENT NETWORK Within traditional public switched telephone networks, the hierarchy of switching equipment and software must be upgraded each time a new service is added to the network. This is a complex and costly process. Further, network switches could not provide

new

number

translation,

routing

and

charging

capabilities.

As

telecommunications services have evolved, the need to reduce the maintenance and service upgrades or additions. The IN essentially separates these services from switching equipment and organizes a centralized system so that providers need not perform major modifications on multiple 20

switches when they introduce new services. The first step in IN development was to create separate service data in a centralized database outside the switching nodes. The second step was to separate the service programs, or service logic, and to define a protocol that would permit interaction between switching systems and intelligent nodes containing the service logic and data. For service switching points and service control points (intelligent nodes) to work, common channel signaling, or out-of-band was required as opposed to the traditional inband signaling. Relaying on out-of-band signaling, or signaling system 7 (SS7) protocols, provides the mechanism to place various logic and service data into dedicated network elements that can remotely handle call control and connection. SS 7 also enables intelligent applications to communicate with other applications and to access databases located in various parts of the network. Certain network elements can be distinguished in every IN, as shown in the figure below:

IN FUNCTIONS AND FUNCTIONAL RELATIONSHIPS FOR CS-1

21

CCAF

Call control agent function

CCF

Call control function

SCEF

Service

SCF

Service control function

SDF

Service data function

SMAF

Service management access function

SMF

Service management function

SRF

Service resource function

SSF

Service switching function

function

Service switching points (SSPs) are stored program control switches that interface to the SS7 signaling network. The SSP embodies the call control function (CCF) and service switching function (SSF) entities. The SSF recognizes IN service calls and routes the appropriate queries to the service control function (SCF) that resides in a service control point (SCP) via the SS7 network through signaling transfer points (STPs). STPs are high22

capacity, high-reliability packet switches that transport signaling messages, using large routing databases, between the IN nodes. SCP commands are used by the SSP to process calls. The SCP is a fault-tolerant, highcapacity, transaction processing entity that provides call-handling information in response to SSP queries. The service management point (SMP) provides operation, administration and maintenance function for the IN. the intelligent peripheral (IP) provides enhanced services or function under the control of an SCP, possibly relayed by an SSP, such as play announcements and speech recognition. The IN architecture is fundamentally based on SS7 and its protocol architecture. A common signaling transport capability known as the message transfer part (MTP) handles the corresponding open systems interconnections (OSI) physical, data link and network layers. The next level, signaling connection control part (SCCP), augments the MTP by providing both connection-less and connection oriented message transport, as well as enabling addressing capabilities application part (TCAP) provides procedure for real-time transaction control. The final layer, IN application protocol (INAP) defines the operations required between IN network elements, such as SSPs and SCPs.

The figure shows the IN protocol architecture:

IN PROTOCOL STACK

23

An important concept in IN has been the notion of service independence. Here, the primary goal is to identify and create generic sets of reusable service components that could be used to build new services and loaded in SCPs to generate new services rapidly. These service components are also called as service independent building blocks (SIBs). To provide a framework that would lead toward IN engineering standardization, the IN conceptual model (INCM) was developed. The INCM is a tool for describing IN capabilities and characteristics, is composed of four ‘planes’ that represent different aspects of implementing IN services. This model depicts the relationship among services and service features, global service logic, distributed service logic and the physical network entities such as SCP and SSP.

These planes include the service plane, the global functional plane, the distributed functional plane and the physical plane as shown in the figure shown:

24

The service plane describes services from a user perspective, where a service consists of generic blocks or service features that make up part or all of a service (e.g., free phone). The global functional plane deals with service creation and is comprised of the SIBs that will be used to create service features. Global service logic defines how SIBs are linked together to form features and how these SIBs interact with another basic SIB known as the basic call process (BCP). The BCP is the process that optimally supports services that do not require special features and is basic to the processing of all services. The distributed functional plane defines a set of functional entities that perform specific actions. SIBs are implemented through a specific sequence of functional-entity actions performed by those functional entities. Table 1 describes functional-entity components as well as their relationship to IN physical entities.

Table 1. IN Physical and Functional Entities

25

Physical

Distributed functional

component

components

Description

Connects call processing and provides Service Switching

Call Control Function(CCF)

network connection services. Supports IN triggering during call

Service Switching Function

processing and access to IN

(SSF)

functionality Supports the interaction between the

Specialized Resource Function

call processing software on the switch

(SRF)

and the service control function. Supports specialized network

Call Control Agent Function

resources generally associated with

(CCAF)

caller interaction: provides user access

Point(SSP)

to the network. Executes IN service logic and Service Control Point

Service Control Function

influences call processing on the

(SCP)

(SCF)

switch via its interface to the SSF. Manages customer and network data

Service Data Function (SDF)

for real-time access by the SCF in the execution of an IN service. Supports specialized network

Intelligent Peripheral

Specialized Resource Function

resources generally associated with

(IP)

(SRF)

caller interaction. Alloys deployment and provision of

Service Management

Service Management Function

IN services and allows the support of

Point (SMP)

(SMF)

ongoing operation. Provides an interface between service

Service Management Access

managers and SMF (could be

Function (SMAF)

implemented in a separate physical element, the SMAP)

26

Service Creation

Service Creation Environment

Allows services provided in the IN to

Environment Point

Function (SCEF)

be defined, developed, tested and input

SCEP)

to the SMF. Manages customer and network data

Service Data Point

Service Data Function (SDF)

(SDP)

for real-time access by the SCF in the execution of an IN service.

CAPABILITY SETS: STANDARDS FOR INTELLIGENT NETWORKS International standards work for IN began in 1989 within the ITU and ETSI. These standards embodies have been developing IN capability sets that will be upwardly compatible in parallel. The ITU builds its”Q.1200 Recommendation Series for IN architecture”, and ETSI takes these recommendations and modifies them for use by European operators. Capability sets refers to a set of services and service features that can be built using SIBs. All capability sets use the IN conceptual model. Each capability set is associated with a planned phase in the standards process. The first capability set CS-1 was defined by the ITU in 1992 but was found to be too extensive and incomplete by ETSI. ETSI defined the Core INAP standard in 1994 as a subset of the original CS-1. The ITU adopts this work and reissued the standard in 1995. Examples of basic CS-1 information flow include the following: •

Connect

•

Prompt and collect user information

•

Analyze information

•

Play announcement

•

Release call

Using a common IN service, the premium rate service, the interrelationship of services, service features and the SIBs is provided below. The premium rate service provides a 27

business owner with a special premium-rate number. Customers that call this special number are charged at a special rate for the call as well as the information and/or services obtained through the call. The network operator collects the revenue associated with the call and distributes a share to the business owner of the premium-rate number. Examples of premium rate include weather, stock, sports, and other information services. A premium rate service has two core service features: 1. The one number service feature permits the business owner (subscriber) to have two or more terminating line sin a number of locations, using a single premiumrate number. 2. The premium charging service feature permits the subscriber to receive some of the revenue associated with each premium-rate call. There are other optional service features associated with the premium-rate service, such as call distribution, time and origin-dependent routing, recorded announcements and many others. Within the premium charging service feature there are three associated SIBs: the charging SIB, the call log information SIB and the service data management SIB. Within the charge SIB, there are a number of information flows/operations at work. For example, the SCF can issue a Furnish Charging Information flow that carries charging characteristics for particular call to the SSF. Once the SSF receives these characteristics, the SSF generates a billing record. Within the same SIB, the SCF may also issue the Send Charging Information flow that send charging characteristics to an SSF that controls the way that the SSF charges for a call. Other information flows are associated with this SIB, such as Event Notification Charging and Apply Charging that perform related charging function.

INTERNET 28

INTRODUCTION 1. (Lower case "i"nternet) A large network made up of a number of smaller networks. 2. (Upper case "I"nternet) The largest network in the world. It is made up of more than 100 million computers in more than 100 countries covering commercial, academic and government endeavors. Originally developed for the U.S. military, the Internet became widely used for academic and commercial research. Users had access to unpublished data and journals on a variety of subjects. Today, the "Net" has become commercialized into a worldwide information highway, providing data and commentary on every subject and product on earth. Internet, the, international computer network linking together thousands of individual networks at military and government agencies, educational institutions, nonprofit organizations, industrial and financial corporations of all sizes, and commercial enterprises (called gateways or service providers) that enable individuals to access the network. The most popular features of the Internet include electronic mail (e-mail), discussion groups (called newsgroups or bulletin boards, where users can post messages and look for responses on a system called Usenet), on-line conversations (called chats), adventure and role-playing games, information retrieval, and electronic commerce (ecommerce). The public information stored in the multitude of computer networks connected to the Internet forms a huge electronic library, but the enormous quantity of data and number of linked computer networks also make it difficult to find where the desired information resides and then to retrieve it. A number of progressively easier-to-use interfaces and tools have been developed to facilitate searching. Among these are search engines, such as Archie, Gopher, and WAIS (Wide Area Information Server), and a number of commercial indexes, which are programs that use a proprietary algorithm to search a large collection of documents for keywords and return a list of documents containing one or more of the keywords. Telnet is a program that allows users of one computer to connect with another, distant computer in a different network. The File Transfer Protocol 29

(FTP) is used to transfer information between computers in different networks. The greatest impetus to the popularization of the Internet came wit E-Mail Was the Beginning The Internet's surge in growth in the mid 1990s was dramatic, increasing a hundredfold in 1995 and 1996 alone. There were two reasons. Up until then, the major online services (AOL, CompuServe, etc.) provided e-mail, but only to customers of the same service. As they began to connect to the Internet for e-mail exchange, the Internet took on the role of a global switching center. An AOL member could finally send mail to a CompuServe member, and so on. The Internet glued the world together for electronic mail, and today, SMTP, the Internet mail protocol, is the global e-mail standard. The Web Was the Explosion Secondly, with the advent of graphics-based Web browsers such as Mosaic and Netscape Navigator, and soon after, Microsoft's Internet Explorer, the World Wide Web took off. The Web became easily available to users with PCs and Macs rather than only scientists and hackers at Unix workstations. Delphi was the first proprietary online service to offer Web access, and all the rest followed. At the same time, new Internet service providers (ISPs) rose out of the woodwork to offer access to individuals and companies. As a result, the Web grew exponentially, providing an information exchange of unprecedented proportion. The Web has also become "the" storehouse for drivers, updates and demos that are downloaded via the browser as well as a global transport for delivering information by subscription, both free and paid. Newsgroups Although daily news and information is now available on countless Web sites, 30

long before the Web, information on a myriad of subjects was exchanged via Usenet (User Network) newsgroups. Chat Rooms Chat rooms provide another popular Internet service. Internet Relay Chat (IRC) offers multiuser text conferencing on diverse topics. Dozens of IRC servers provide hundreds of channels that anyone can log onto and participate in via the keyboard.

The Original Internet The Internet started in 1969 as the ARPAnet. Funded by the U.S. government, the ARPAnet became a series of high-speed links between major supercomputer sites and educational and research institutions worldwide, although mostly in the U.S. A major part of its backbone was the National Science Foundation's NFSNet. Along the way, it became known as the "Internet" or simply "the Net." By the 1990s, so many networks had become part of it and so much traffic was not educational or pure research that it became obvious that the Internet was on its way to becoming a commercial venture. It Went Commercial in 1995 when, the Internet was turned over to large commercial Internet providers (ISPs), such as MCI, Sprint and UUNET, which took responsibility for the backbones and have increasingly enhanced their capacities ever since. Regional ISPs link into these backbones to provide lines for their subscribers, and smaller ISPs hook either directly into the national backbones or into the regional ISPs. The TCP/IP Protocol Internet computers use the TCP/IP communications protocol. There are more than 100 million hosts on the Internet, a host being a mainframe or medium to highend server that is always online via TCP/IP. The Internet is also connected to nonTCP/IP networks worldwide through gateways that convert TCP/IP into other protocols. 31

History of the Internet The story of the Internet begins in 1969 with the implementation of ARPANET by academic researchers under the sponsorship of the United States Department of Defense Advanced Research Projects Agency (ARPA). Some early research which contributed to the ARPANET included work on decentralized networks, queueing theory, and packet switching. However, ARPANET itself did not interact easily with other computer networks that did not share its own native protocol.

Today's Internet

Some of the most used protocols in the Internet protocol suite are IP, TCP, UDP, DNS, PPP, SLIP, ICMP, POP3, IMAP, SMTP, HTTP, HTTPS, SSH, Telnet, FTP, LDAP, SSL, and TLS.

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Some of the popular services on the Internet that make use of these protocols are e-mail, Usenet newsgroups, file sharing, Instant Messenger, the World Wide Web, Gopher, session access, WAIS, finger, IRC, MUDs, and MUSHs. Of these, e-mail and the World Wide Web are clearly the most used, and many other services are built upon them, such as mailing lists and web logs. The Internet makes it possible to provide real-time services such as web radio and webcasts that can be accessed from anywhere in the world.

The World Wide Web Through keyword-driven Internet research using search engines like Google, millions worldwide have easy, instant access to a vast and diverse amount of online information. Compared to encyclopedias and traditional libraries, the Internet has enabled a sudden and extreme decentralization of information and data.

OSI MODEL INTRODUCTION The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnect initiative. It is also called the OSI seven layer model.

Purpose The OSI model divides the functions of a protocol into a series of layers. Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. A system that implements protocol behavior consisting of a series of these layers is known as a 'protocol stack' or 'stack'. Protocol stacks can be implemented either in hardware or software, or a mixture of both. Typically, only the

33

lower layers are implemented in hardware, with the higher layers being implemented in software. This OSI model is roughly adhered to in the computing and networking industry. Its main feature is in the junction between layers which dictates the specifications on how one layer interacts with another. This means that a layer written by one manufacturer can operate with a layer from another (assuming that the specification is interpreted correctly.) These specifications are typically known as Request for Comments or "RFC"s in the TCP/IP community. They are ISO standards in the OSI community. Usually, the implementation of a protocol is layered in a similar way to the protocol design, with the possible exception of a 'fast path' where the most common transaction allowed by the system may be implemented as a single component encompassing aspects of several layers. This logical separation of layers makes reasoning about the behaviour of protocol stacks much easier, allowing the design of elaborate but highly reliable protocol stacks. Each layer performs services for the next higher layer, and makes requests of the next lower layer. An implementation of several OSI layers is often referred to as a stack The OSI reference model is a hierarchical structure of seven layers that defines the requirements for communications between two computers. The model was defined by the International Standards Organisation. It was conceived to allow interoperability across the various platforms offered by vendors. The model allows all network elements to operate together, regardless of who built them. By the late 1970's, ISO was recommending the implementation of the OSI model as a networking standard, unfortunately, TCP/IP had been in use for years. TCP/IP was fundamental to ARPANET and the other networks that evolved into the Internet. Only a subset of the whole OSI model is used today. It is widely believed that much of the specification is too complicated and its full functionality has taken too long to implement, although there are many people that strongly support the OSI model.

34

Description of layers •

Physic al layer Layer 1: The

.

physical layer defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices. The major functions and services performed by the physical layer are: o

establishment and termination of a connection to a communications medium.

o

participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. 35

o

modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling -- copper and fibre optic, for example. SCSI operates at this level.

•

Data link layer Layer 2: The Data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The addressing scheme is physical which means that the addresses are hard-coded into the network cards at the time of manufacture. The addressing scheme is flat. Note: The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and LLC and Aloha for local area networks. This is the layer at which bridges and switches operate. Connectivity is provided only among locally attached network nodes.

•

Network layer Layer 3: The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions. The router operates at this layer -- sending data throughout the extended network and making the Internet possible, although there are layer 3 (or IP) switches. This is a logical addressing scheme - values are chosen by the network engineer. The addressing scheme is hierarchical.

•

Transport layer Layer 4: The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer. The transport layer controls the reliability of a given link. Some protocols are stateful and connection oriented. This means that the transport layer can keep track of the

36

packets and retransmit those that fail. The best known example of a layer 4 protocol is TCP. •

Session layer Layer 5: The Session layer provides the mechanism for managing the dialogue between end-user application processes. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. This layer is responsible for setting up and tearing down TCP/IP sessions.

•

Presentation layer Layer 6: The Presentation layer relieves the Application layer of concern regarding syntactical differences in data representation within the end-user systems. MIME encoding, encryption and similar manipulation of the presentation of data is done at this layer. An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

•

Application layer Layer 7, the highest layer: This layer interfaces directly to and performs common application services for the application processes. The common application services provide semantic conversion between associated application processes. Examples of common application services include the virtual file, virtual terminal (for example, Telnet), and "Job transfer and Manipulation protocol" (JTM, standard ISO/IEC 8832).

TYPES OF INTERNET CONNECTIONS Dial-up Refers to analog modem service for connecting to the Internet or to the company LAN. For home users, dial-up was the first Internet access service available. It was followed by ISDN, cable and DSL.

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MODEM Modem [modulator/demodulator], an external device or internal electronic circuitry used to transmit and receive digital data over a communications line normally used for analog signals. A modem attached to a computer converts digital data to an analog signal that it uses to modulate a carrier frequency. This frequency is transmitted over a line, frequently as an audio signal over a telecommunications line, to another modem that converts it back into a copy of the original data. Synchronous data transmission uses timing signals in the data stream along with transmitted bits of uniform duration and interval. This permits the receiving modem to ignore spurious signals that do not conform to the anticipated signal. Although most modems are either of the synchronous or asynchronous variety, some employ both methods of communication. Wireless modems send or receive data as a radio signal.

Broadband 1. High-speed transmission. The term is commonly used to refer to Internet access via cable modems or DSL, which is faster than dial-up. For years, "broadband" has referred to a higher-speed connection, but the actual speed threshold has varied. While T1 (1.5 Mbps) has been widely used as the threshold, others have used T3 (45 Mbps) for broadband. For example, after the turn of the century, South Korea leapfrogged the U.S. in Internet access, offering DSL up to 50 Mbps and calling their 1.5 Mbps service "light." In every case, however, it implies transmitting at higher speeds 2. A method of transmitting data, voice and video using frequency division multiplexing (FDM), such as used with cable TV. Modems are required to modulate digital data streams onto the line. Broadband in this context is used in contrast with "base band," which is all digital transmission and uses time division multiplexing (TDM).

Leased line 38

A leased line is a (usually) symmetric telecommunications line connecting two locations together. Unlike traditional PSTN lines they do not have a telephone number, each side of the line being permanently connected to the other. They can be used for telephone, data or Internet services.

INTERNET PROTOCOLS Refers to all the standards that keep the Internet running. The foundation protocol is TCP/IP, which provides the basic communications mechanism as well as ways to copy files (FTP) and send e-mail (SMTP). The Web added the HTTP protocol for downloading Web pages and HTML, XML and XHTML for formatting them. There are many others and many more are expected, as the Internet has become "the" arena for global standards.

Application layer Transport layer Network layer Data link layer Physical layer

Internet protocol suite HTTP, HTTPS, SMTP, FTP, UUCP, NNTP, SSH, IRC, SNMP, SIP, RTP, Telnet ,... TCP, UDP, SCTP, DCCP, ... IPv4, IPv6, ICMP, ARP, IGMP, ... Ethernet, Wi-Fi, Token ring, FDDI, PPP, ... RS-232, EIA-422, RS-449, EIA-485...

The Internet Protocol (IP) is a data-oriented protocol used by source and destination hosts for communicating data across a packet-switched internet work.

IP addressing and routing Perhaps the most complex aspects of IP are addressing and routing. Addressing refers to how end hosts are assigned IP addresses and how subnetworks of IP host addresses are divided and grouped together. IP routing is performed by all hosts, but most importantly by internetwork routers, which typically use either interior gateway protocols (IGPs) or external gateway protocols (EGPs) to help make IP datagram forwarding decisions across IP connected networks. 39

GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATION) INTRODUCTION (Global System for Mobile Communications) A digital cellular phone technology based on TDMA that is the predominant system in Europe, but is also used around the world. Developed in the 1980s, GSM was first deployed in seven European countries in 1992. Operating in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz PCS band in the U.S., GSM defines the entire cellular system, not just the air interface (TDMA, CDMA, etc.). As of 2000, there were more than 250 million GSM users, which is more than half of the world's mobile phone population. GSM phones use a Subscriber Identity Module (SIM) smart card that contains user account information. Any GSM phone becomes immediately programmed after plugging in the SIM card, thus allowing GSM phones to be easily rented or borrowed. SIM cards can be programmed to display custom menus for personalized services. GSM provides a short messaging service (SMS) that enables text messages up to 160 characters in length to be sent to and from a GSM phone. It also supports data transfer at 9.6 Kbps to packet networks, ISDN and POTS users. GSM is a circuit-switched system that divides each 200 kHz channel into eight 25 kHz time slots.

Home Location Register (HLR) A Home Location Register (HLR) is a database that contains semi-permanent mobile subscriber information for a wireless carriers' entire subscriber base. HLR subscriber information includes the International Mobile Subscriber Identity (IMSI), service subscription information, location information (the identity of the currently serving Visitor Location Register (VLR) to enable the routing of mobile-terminated calls), service restrictions and supplementary services information. 40

The HLR handles SS7 transactions with both Mobile Switching Centers (MSCs) and VLR nodes, which either request information from the HLR or update the information contained within the HLR. The HLR also initiates transactions with VLRs to complete incoming calls and to update subscriber data. Traditional wireless network design is based on the utilization of a single Home Location Register (HLR) for each wireless network, but growth considerations are prompting carriers to consider multiple HLR topologies.

Visitor Location Register (VLR) A Visitor Location Register (VLR) is a database which contains temporary information concerning the mobile subscribers that are currently located in a given MSC serving area, but whose Home Location Register (HLR) is elsewhere. When a mobile subscriber roams away from his home location and into a remote location, SS7 messages are used to obtain information about the subscriber from the HLR, and to create a temporary record for the subscriber in the VLR. There is usually one VLR per MSC.

International Mobile Subscriber Identity (IMSI) Number The IMSI is a unique non-dial able number allocated to each mobile subscriber in the GSM system that identifies the subscriber and his or her subscription within the GSM network. The IMSI resides in the Subscriber Identity Module (SIM), which is transportable across Mobile Station Equipment (MSE). The IMSI is made up of three parts (1) the mobile country code (MCC) consisting of three digits, (2) the Mobile Network Code (MNC) consisting of two digits, and (3) the Mobile Subscriber Identity Number (MSIN) with up to 10 digits.

Mobile Subscriber ISDN (MSISDN) Number

41

The MSISDN is the dial able number that callers use to reach a mobile subscriber. Some phones can support multiple MSISDNs - for example, a U.S.-based MSISDN and a Canadian-based MSISDN. Callers dialing either number will reach the subscriber.

Mobile Station Equipment (MSE) Subscription Services GSM carriers typically order Mobile Station Equipment (MSE) (or GSM phones) from their suppliers (Nokia, Motorola, Sony, etc.) in large quantities (e.g. 1000 Units). After receiving an order, the equipment supplier will program the ordered MSE SIMs with a range of IMSI numbers. Example: ABC Communications Inc. orders 1000 MSE Units with the following range of IMSIs. MCC

MNC

MSIN

Unit #1

310

68

4451000

Unit #1000

310

68

4451999

Once the range of IMSI numbers has been determined, the HLR can be populated with the new IMSI records that will be configured and activated at a future date by authorized sales or service subscription representatives. The fact that the HLR can be populated with ranges or blocks of IMSI numbers creates efficiencies in the storage and retrieval of routing information. The wireless carrier distributes the Mobile Station Equipment to Sales Outlets that sell GSM subscription services. When a new subscriber orders a GSM phone at one of the outlets, the service representative will create a Service Order (SO) to enter the new subscriber's service subscription information, including the MSISDN number. The key to the Service Order is the IMSI that is programmed in the SIM. The SO is sent to the HLR, where the IMSI record is created. It can either be set to an active state immediately, allowing the new subscriber to send and receive telephone calls or it can be activated at a future date. 42

Note that the MSISDN numbers are assigned one at a time as each new customer subscribes. The MSISDN numbers are therefore provisioned individually, rather than in blocks, which complicates the MSISDN based routing of messages.

GPRS (General Packet Radio Service) An enhancement to the GSM mobile communications system that supports data packets. GPRS enables continuous flows of IP data packets over the system for such applications as Web browsing and file transfer. GPRS differs from GSM's short messaging service (GSM-SMS) which is limited to messages of 160 bytes in length.

Customized Applications for Mobile networks Enhanced Logic Customised Applications for Mobile networks Enhanced Logic, or CAMEL for short, is a set of GSM standards designed to work on a GSM core network. They allow an operator to define services over and above standard GSM services. The CAMEL architecture is based on the Intelligent Network (IN) standards. Many services can be created using CAMEL, and it is particularly effective in allowing these services to be offered when a subscriber is roaming.

GSM Call Routing Mobile Subscriber Roaming When a mobile subscriber roams into a new location area (new VLR), the VLR automatically determines that it must update the HLR with the new location information, which it does using an SS7 Location Update Request Message. The Location Update Message is routed to the HLR through the SS7 network, based on the global title translation of the IMSI that is stored within the SCCP Called Party Address portion of the 43

message. The HLR responds with a message that informs the VLR whether the subscriber should be provided service in the new location.

Mobile Subscriber ISDN Number (MSISDN) Call Routing When a user dials a GSM mobile subscriber's MSISDN, the PSTN routes the call to the Home MSC based on the dialed telephone number. The MSC must then query the HLR based on the MSISDN, to attain routing information required to route the call to the subscribers' current location. The MSC stores global title translation tables that are used to determine the HLR associated with the MSISDN. When only one HLR exists, the translation tables are trivial. When more than one HLR is used however, the translations become extremely challenging, with one translation record per subscriber (see the example below). Having determined the appropriate HLR address, the MSC sends a Routing Information Request to it. When the HLR receives the Routing Information Request, it maps the MSISDN to the IMSI, and ascertains the subscribers' profile including the current VLR at which the subscriber is registered. The HLR then queries the VLR for a Mobile Station Roaming Number (MSRN). The MSRN is essentially an ISDN telephone number at which the mobile subscriber can currently be reached. The MSRN is a temporary number that is valid only for the duration of a single call. The HLR generates a response message, which includes the MSRN, and sends it back across the SS7 network to the MSC. Finally, the MSC attempts to complete the call using the MSRN provided.

Adding a Second HLR to the GSM Network

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As a GSM wireless carrier's subscriber base grows, it will eventually become necessary to add a second HLR to their network. This requirement might be prompted by a service subscription record storage capacity issue, or perhaps an SS7 message processing performance issue. It might possibly be prompted by a need to increase the overall network reliability. The new HLR can be populated with service subscription records as new subscribers are brought into service or existing service subscription records can be ported from the old HLR to the new HLR to more evenly distribute the SS7 traffic load. Typically, when new subscribers are brought into service, the second HLR will be populated with blocks of IMSI numbers that are allocated when new MSE equipment is ordered. As the following example shows, this grouping of IMSI numbers within a single HLR simplifies the routing translations that are required within the SS7 network for VLR to HLR Location Update Request transactions. Global Title Translation(GTT) tables will contain single translation records that translate an entire range of IMSIs numbers into an HLR address. Even if some individual records are moved between the HLRs, as shown in the example, the treatment of IMSIs as blocks results in a significant simplification of the Global Translation tables. Much more complicated SS7 message routing Global Title Translations are required for Routing Information Request transactions between the MSCs distributed over the entire wireless carrier serving area and the two or more HLRs. MSC Routing Information Requests are routed to the appropriate HLR based on the dialed MSISDN and not the IMSI. Unlike the IMSI numbers, the MSISDN numbers can not easily be arranged in groups to reside within a single HLR and therefore, the MSC must contain an MSISDN to HLR address association record for every mobile subscriber homed on each of the MSCs. As the example illustrates, the MSC routing tables quickly grow much more extensive than the STP tables. The network administration becomes increasingly complex and prone to error. Example: Simple Network with two MSCs and two HLRs 45

The following example illustrates the issues relating to GSM network routing table administration with multiple HLRs. A simple GSM network is shown, with the various routing tables following:

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