Mobile Networks Architecture E-Book

Andre Perez

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 295

Veröffentlichungsjahr: 2013

Beschreibung

This book explains the evolutions of architecture for mobiles and summarizes the different technologies: - 2G: the GSM (Global System for Mobile) network, the GPRS (General Packet Radio Service) network and the EDGE (Enhanced Data for Global Evolution) evolution; - 3G: the UMTS (Universal Mobile Telecommunications System) network and the HSPA (High Speed Packet Access) evolutions: - HSDPA (High Speed Downlink Packet Access), - HSUPA (High Speed Uplink Packet Access), - HSPA+; - 4G: the EPS (Evolved Packet System) network. The telephone service and data transmission are the two main services provided by these networks. The evolutions are fundamentally dictated by the increase in the rate of data transmission across the radio interface between the network and mobiles. This book is intended as a readily understandable support to help students and professionals wishing to quickly acquire the main concepts of networks for mobiles understand the technologies deployed.

Leseprobe

Table of Contents

Preface

Chapter 1. The GSM Network

1.1. Services

1.2. The architecture of the network

1.3. The radio interface

1.4. Communication management

Chapter 2. The GPRS Network

2.1. Services

2.2. Network architecture

2.3. Radio interface

2.4. Communication management

2.5. The EDGE evolution

Chapter 3. The UMTS Network

3.1. The services

3.2. The architecture of the network

3.3. Radio interface

3.4. Communication management

3.5. HSPA evolutions

Chapter 4. The NGN

4.1. Network architecture

4.2. Communication management

Chapter 5. The EPS Network

5.1. Network architecture

5.2. The radio interface

5.3. Communication management

Chapter 6. The IMS Network

6.1. The SIP

6.2. The IMS architecture

6.3. Communication management

List of Abbreviations

Bibliography

Index

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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The rights of André Pérez to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Cataloging-in-Publication Data

Pérez, André.

Network architecture for mobiles/André Pérez.

p. cm.

Includes bibliographical references and index.

ISBN 978-1-84821-333-3

1. Mobile communication systems--Standards. 2. Cell phone systems--Standards. 3. Computer network architectures. I. Title.

TK5103.2.P447 2012

621.3845′6--dc23

2011045484

British Library Cataloguing-in-Publication Data

A CIP record for this book is available from the British Library

ISBN: 978-1-84821-333-3

Preface

This work explains the evolutions of architecture for mobiles and summarizes the different technologies:

– 2G: the GSM (Global System for Mobile) network, the GPRS (General Packet Radio Service) network and the EDGE (Enhanced Data for Global Evolution) evolution;

– 3G: the UMTS (Universal Mobile Telecommunications System) network and the HSPA (High Speed Packet Access) evolutions:

- HSDPA (High Speed Downlink Packet Access);

- HSUPA (High Speed Uplink Packet Access);

- HSPA+;

– 4G: the EPS (Evolved Packet System) network.

The telephone service and data transmission are the two main services provided by these networks. The evolutions are fundamentally dictated by the increase in the rate of data transmission across the radio interface between the network and mobiles.

The services are implemented according to two modes:

– the CS (Circuit Service) mode. This mode is characterized by the allocation of a resource dedicated to a flow. This mode provides both types of service;

– the PS (Packet Service) mode. This mode is characterized by the allocation of a resource shared by several flows. This mode is solely used for data transmission.

Table 1.Mobile networks – rates

The network architecture for mobiles reveals two subsystems:

– the AN (Access Network). This sub-system can be used to allocate the radio resource to the mobile, so that it can either be dedicated or shared. It is significantly affected by successive evolutions;

– the CN (Core Network). This sub-system connects the access networks and third-party networks:

- the PSTN (Public Switched Telephone Network);

- the PDN (Packet Data Network);

- the PLMN (Public Land Mobile Network).

The same sub-system is used for the 2G and 3G core networks and consists of two entities:

– the NSS (Network Sub-System) entity provides services in CS mode;

– the GSS (GPRS Sub-System) entity provides services in PS mode.

The 2G and 3G mobile networks mainly differentiate by the type of access network deployed:

– the BSS (Base Station Sub-system) entity for 2G networks;

– the UTRAN (UMTS TRAnsport Network) entity for 3G networks.

The 4G mobile network consists of two entities:

– the access network E-UTRAN (Evolved UTRAN);

– the EPC (Evolved Packet Core) network. It functions in PS mode and solely provides the data transmission service.

Table 2.Mobile networks – architecture

The NSS entity has evolved into an NGN (Next Generation Network) architecture that is used to separate the functions of telephone traffic transport and signaling processing. Interconnection of the NSS entity machines is ensured by an SDH (Synchronous Digital Hierarchy) transmission network. That of the NGN equipment is implemented by an IP (Internet Protocol) network identical to that deployed for the GSS network.

The 4G network ensures that telephone traffic transport is treated as a source of data. The signaling processing that administers the telephone service is provided by the IMS (IP Multimedia Sub-system), which is an entity external to the mobile network. This IMS entity is independent of the network involved in data transport. It can also be associated with the UMTS network functioning in PS mode.

Figure 1.Mobile networks – the architecture

The following table summarizes the points covered in the different chapters of this work.

Table 3.Mobile networks – the structure of this book

Chapter 1

The GSM Network

Section 1.1 in this chapter explains the services provided by the GSM (Global System for Mobile) network, the main ones being the telephone service and the data transmission service. These services are implemented in CS (Circuit Service) mode, for which a resource dedicated to a flow is reserved.

Section 1.2 explains the architecture of a GSM network consisting of two subsystems – the BSS (Base Station Sub-system) access network and the NSS (Network Sub-System) core network – and the MS (Mobile Station).

It also describes the protocol architecture concerning the signaling data enabling the allocation of the resource dedicated to establishment of communications between two mobiles using the same PLMN (Public Land Mobile Network), two mobiles using two different PLMN networks or a mobile and a PSTN (Public Switched Telephone Network) fixed terminal.

Section 1.3 explains the radio interface between the mobile and the mobile network. The description of the transmission channel is essentially concerned with source coding, channel coding, time-division multiplexing of logical channels, modulation and the frequency plan.

Section 1.4 describes the procedures concerning the establishment of an incoming or outgoing call, location (roaming) management and the control of its access to mobile networks.

1.1. Services

The telecommunication services offered by the GSM network are divided into two categories:

– bearer services, which provide the ability to transmit between access points;

– teleservices, which are communication services between users.

Bearer services are used to transfer two types of signal:

– the audio frequency signal (300–3,400 Hz) used for the transfer of speech or data at a rate ≤14.4 kbps;

– the digital UDI (Unrestricted Digital Information) signal at a rate ≤14.4 kbps.

Two modes of digital transmission are defined:

– transparent mode. The network carries out the transfer a bit at a time between the extremities of the mobile network;

– non-transparent mode. The RLP (Radio Link Protocol) is used for the reliable transfer of data.

The data transmission service consists of establishing a circuit between two users or in accessing a data network. The transmission is carried out in synchronous or asynchronous mode.

The telephony teleservice is used for the transmission of speech. The network must also ensure transmission of the following specific signals:

– the tones used in the fixed network;

– the DTMF (Dual-Tone Multi-Frequency) tones in the mobile-to-fixed direction (for example to control voice mail) throughout an established call.

Table 1.1 contains the list of additional services that complement the telephony teleservice.

The VBS (Voice Broadcast Service) allows a user to broadcast a voice message to several other users within a certain geographical area. The user who makes the call is the speaker and the other users are only listeners.

The VGCS (Voice Group Call Service) defines a closed group of users who can speak using a push-to-talk mechanism.

The SMS (Short Message Service) teleservice is used to transmit a text message between a MS and a SMS SC (Service Center). The service center is functionally separated from the GSM network.

The facsimile teleservice is used for Group 3 fax transmission at 9,600 bps using two modes:

– manual mode is used to pass back and forth between speech and fax;

– automatic mode is used to generate or receive a fax call without going via speech.

Table 1.1.List of additional services

Type

Abbreviation

Description

Priority call

MLPP

Multi-Level Precedence and Pre-emption service

Number identification

CLIP

Calling Line Identification Presentation

CLIR

Calling Line Identification Restriction

COLP

COnnected Line identification Presentation

COLR

COnnected Line identification Restriction

Call forwarding

CFU

Call Forwarding Unconditional

CFB

Call Forwarding on mobile subscriber Busy

CFNRy

Call Forwarding on No Reply

CFNRc

Call Forwarding on Mobile Not Reachable

Call waiting

CW

Call Waiting

HOLD

Call is on hold

Call barring

BAOC

Barring of All Outgoing Calls

BOIC

Barring of Outgoing International Calls

BOIC – exHC

BOIC except those directed to the Home PLMN (Public Land Mobile Network) Country

BAIC

Barring of All Incoming Calls

1.2. The architecture of the network

1.2.1. Network components

The GSM network consists of two sub-systems (Figure 1.1):

– The BSS radio sub-system: this ensures radio transmission of the mobile, manages the radio resources (RRs) and allows for mobile mobility. The BSS sub-system consists of BTS (Base Transceiver Station) radio stations, BSC (Base Station Controller) radio station controllers and TRAU (Transcoder/Rate Adaptor Unit) transcoding equipment.

– The NSS is used for call processing in the establishment of communication as well as for roaming and mobility management (MM). The NSS consists of the telephone switches MSC (Mobile-services Switching Center), GMSC (Gateway MSC), TSC (Tandem Switching Center), and the databases HLR (Home Location Register), VLR (Visitor Location Register), AuC (Authentication Center) and EIR (Equipment Identity Register).

Figure 1.1.Architecture of the GSM network

Different types of equipment in the mobile network are able to interface at a rate of 2,048 kbps. This rate is the result of time-division multiplexing of 32 channels or time-slots at 64 kbps, numbered 0 to 31. The first time-slot is used for synchronization of the digital frame and for monitoring the quality of the timedivision multiplex. The other time-slots are appointed to signaling or traffic channels. The link between the different pieces of network equipment is ensured by a transmission network.

The user can move about within the territory covered by the GSM network. He or she must therefore be able to call and be called, and this network must register the LAI (Location Area Identification) where the mobile is situated: this is the notion of roaming. The mobile is in contact with a BTS that covers an area called a cell. The connection between the mobile and the network must be maintained while moving, which entails a change of cell; this is the notion of mobility or handover.

1.2.2. The mobile

The MS is the piece of equipment given to the user to establish the connections to carry speech and data in order to exchange SMS messages. The MS contains terminal equipment and a SIM (Subscriber Identity Module) card.

Each piece of terminal equipment is uniquely identified by an IMEI (International Mobile Equipment Identity) number allocated by the manufacturer. The SIM card identifies the user and is used to establish the communication. The portability of the SIM card means that all types of terminal can be used to establish a communication. The SIM card contains the IMSI (International Mobile Subscriber Identity) number that identifies the subscriber, and this is only known within the GSM network.

1.2.3. The radio sub-system

1.2.3.1. The physical architecture

The BTS is a set of transmitters and receivers in charge of radio transmission with mobiles (modulation, error-correcting code, time-division and frequency-division multiplexing). It is used to access the physical layer. It provides:

– access to a frequency band in FDMA (Frequency Division Multiple Access) mode;

– access to a time-slot in TDMA (Time Division Multiple Access) mode.

It carries out all radio measures that will be used to verify the correct execution of a communication.

The type and location of the BTS determines the surface of the cells. In a rural area, which has low-density traffic, the BTS can be restricted to a single carrier wave coupled with an omnidirectional antenna, thus covering macrocells whose range can reach 30 km (Figure 1.2).

Figure 1.2.The physical architecture of the radio sub-system

In an urban area, which has high-density traffic, a BTS generally has several carriers coupled on sector antennas that are capable of transmitting at an angle. When the traffic density increases, microBTS – which are in charge of highly restricted or microcell areas and whose range is limited to 300 meters – are used (Figure 1.2).

The BSC controls the BTS in the sense that it manages the allocation of the RR (the frequency band and time-slot allocated to the mobile). It uses radio measurements carried out by the BTS to control the power emitted by the mobile. It decides whether to carry out a handover while the mobile changes cells.

The TRAU is a device that deals with speech transcoding. It is generally placed within proximity of the MSC, although functionally it is a part of the BSS. It is used to convert the G.711 format at 64 kbps into the format used in the radio subnetwork:

– FR (full rate) at 13 kbps;

– EFR (enhanced full rate) at 12.2 kbps;

– HR (half rate) at 5.6 kbps.

The TRAU conducts the rate conversion <14,400 kbits) of the data service to adapt it to the circuit at 64 kbps switched by the NSS.

1.2.3.2. Protocol architecture

Protocol architecture signals transport between the different pieces of equipment in the BSS network. This corresponds to the exchange of messages based on various communication protocols (Figure 1.3).

The function of the RR is to establish, maintain and release a channel between the MS and the BSC. Message exchange concerns the establishment and release of the radio channel, the handover, encryption activation, system information (frequency hopping, emission power control and cut-off during silences) and the indication of an incoming call (paging). The exchanges take place between the mobile and the BTS or the BSC (Figure 1.3).

Table 1.2 contains the list of RR messages.

Table 1.2.The RR messages

Message type

RR messages

Radio channel establishment

ADDITIONAL ASSIGNMENTIMMEDIATE ASSIGNMENTIMMEDIATE ASSIGNMENT EXTENDEDIMMEDIATE ASSIGNMENT REJECT

Encryption

CIPHERING MODE COMMANDCIPHERING MODE COMPLETE

Handover

ASSIGNMENT COMMANDASSIGNMENT COMPLETEASSIGNMENT FAILUREHANDOVER COMMANDHANDOVER COMPLETEHANDOVER FAILUREPHYSICAL INFORMATION

Radio channel release

CHANNEL RELEASEPARTIAL RELEASEPARTIAL RELEASE COMPLETE

Paging

PAGING REQUESTPAGING RESPONSE

System information

SYSTEM INFORMATION TYPES 1 to 8

Various

CHANNEL MODE MODIFYRR STATUSCHANNEL MODE MODIFY ACKNOWLEDGEFREQUENCY REDEFINITIONMEASUREMENT REPORTCLASSMARK CHANGECLASSMARK ENQUIRY

The MM function is in charge of mobile location, its authentication and the allocation of a TMSI (Temporary Mobile Subscriber Identity) to replace the

IMSI identity. The exchanges take place between the mobile and the MSC (Figure 1.3).

Table 1.3 contains the list of MM messages.

Table 1.3.MM messages

Type of message

MM messages

Location registration

IMSI DETACH INDICATIONLOCATION UPDATING REQUESTLOCATION UPDATING ACCEPTLOCATION UPDATING REJECT

Security

AUTHENTICATION REQUESTAUTHENTICATION RESPONSEAUTHENTICATION REJECTIDENTITY REQUESTIDENTITY RESPONSETMSI REALLOCATION COMMANDTMSI REALLOCATION COMPLETE

Connection management (CM)

CM SERVICE ACCEPTCM SERVICE REQUESTCM SERVICE REJECTCM SERVICE ABORTCM REESTABLISHMENT REQUESTABORT

Various

MM STATUS

The CM (Communication Management) function is in charge of the establishment, maintenance and release of the call, the management of additional services such as call waiting, call transfer and SMS transmission indication. The exchanges take place between the mobile and the MSC (Figure 1.3).

Table 1.4 contains the list of CM messages.

Table 1.4.The CM messages

Type of message

CM messages

Call establishment

ALERTINGCALL PROCEEDINGCALL CONFIRMEDCONNECTCONNECT ACKNOWLEDGEEMERGENCY SETUPPROGRESSSETUP

Call information

MODIFYMODIFY COMPLETEMODIFY REJECTUSER INFORMATIONHOLDHOLD ACKNOWLEDGEHOLD REJECTRETRIEVERETRIEVE ACKNOWLEDGERETRIEVE REJECT

Call release

DISCONNECTRELEASERELEASE COMPLETE

Various

CONGESTION CONTROLNOTIFYSTATUSSTATUS ENQUIRYSTART DTMFSTOP DTMFSTOP DTMF ACKNOWLEDGESTART DTMF REJECTFACILITY

Figure 1.3.Protocol architecture of the BSS

At BTS level, the distribution layer specifies two types of message:

– the transparent messages that contain CM, MM and RR signaling exchanged between the mobile and the BSC or the MSC, for which the BTS acts solely as a relay;

– BTSM (BTS Management) messages corresponding to exchanges between the BTS and the BSC.

Transparent messages contain the following fields:

– the type of message, specifying whether transmission on the radio interface is in connected mode or not;

– the number of the time-slot (in the TDMA frame and eventually in multiframe) used on the radio interface;

– the connection identifier giving the type of logical channel on which the message must be transmitted and the type of message, in order to position the SAPI (Service Access Point Identifier) of the LAPDm (Link Access Protocol – Channel D mobile) protocol.

The BTSM function defines the dialog between the BSC and the BTS (Figure 1.3). The main messages exchanged concern call procedures from the mobile or network, activation and deactivation of the RR and the change of mobile cells. This function is also used to recover the radio measurements made and to control the levels of power emitted by the BTS.

At BSC level, there are two types of BSSAP (BSS Application Part) message:

– the messages interpreted by the BSC relating to RR management, supported by the BSSMAP (BSS Management Application Part) sub-layer;

– the (CM and MM) messages supported by the DTAP (Direct Transfer Application Part) sub-layer for which the BSC ensures a relay function.

To differentiate between the BSSMAP and DTAP sub-layers used at BSSAP protocol level, there is a distribution sub-layer that acts as a protocol discriminator.

The BSSMAP function defines the relationship between the MSC and BSC (Figure 1.3). The main messages exchanged concern the availability of RR queries, the broadcast request for a mobile call within a location area, the request for the establishment or release of a radio channel, the execution of a handover and transmission in encrypted mode.

The DTAP protocol sends the CM and MM messages that are received without interpreting the contents. It contains a DLCI (Data Link Connection Identification) identifier which gives the SAPI used on the radio channel.

LAPD (Link Access Protocol – Channel D) is a data-link protocol running in asynchronous mode balanced between the BTS and the BSC (Figure 1.3). This mode is balanced because there is no master-slave relationship. Each station can initialize, control and correct errors and send frames at any time. The LAPD has three types of frame:

– the information frame used for acknowledgement and dataflow control;

– the supervision frame used for acknowledgement and flow control in the absence of traffic;

– the unnumbered frame used for information transfer without acknowledgement and without control or to open or close the connection.

The LAPDm is an adaptation of the LAPD running between the BTS and the mobile (Figure 1.3). It is characterized by a frame of fixed length. The LAPD addresses contain the TEI (Terminal End point Identifier) which identifies the pair of BTS and SAPI radio transmitter/receivers indicating the type of data encapsulated. The LAPDm protocol only retains the SAPI.

1.2.4. The network sub-system

1.2.4.1. The physical architecture

The MSC carries out the time division switch of circuits at 64 kbps. It manages to establish communication thanks to signaling messages exchanged between the MS and the entities of the NSS. It transfers SMS text messages and executes the handover when necessary.

The GMSC is a particular type of MSC that conducts the interface either with the PSTN fixed-line telephone network or with another PLMN mobile network when this cannot query the HLR. It is used to establish a call being received by the MSC to which the mobile is connected.

The TSC carries out the time-division switch of circuits at 64 kbps involving the transmission between two MSC switches. It introduces a hierarchy to the establishment of channels at 64 kbps with the purpose of optimizing the number of links with the MSC.

The VLR is a database that memorizes the data of the user present in the geographical area covered by one or more MSCs. The data stored by the VLR come partly from the HLR. It is completed by the TMSI, the MSRN (Mobile Station Roaming Number) allocated to the user in the network sub-system and by information regarding the mobile’s location (LAI).

Within an area managed by a VLR, a subscriber has a temporary identity, the TSMI, allocated by the VLR to the MS. To avoid an intruder intercepting the IMSI, this is only transmitted when the device is switched on. Subsequently, only the TMSI is transmitted on the radio link. The allocation of a new TMSI occurs, at a minimum, each time the VLR changes, and possibly at the request of the mobile.

The HLR is a database that manages the details of each subscriber:

– the subscriber’s IMSI used by the network;

– the subscriber’s directory number (his or her MSISDN or Mobile Station ISDN Number), known outside the network;

– the subscription profile, such as additional services or the authorization of an international call.

The HLR is also a location database. It records the number of the VLR where the mobile was recorded, even when the mobile is connected to a foreign network. Each subscriber’s data are stored in a single HLR, independent of its location.

A GSM network can contain one or more HLRs, depending on the number of subscribers, the capacity of the HLR and the organization of the network. To identify an HLR in the case of call processing, the MSC uses the MSISDN or the IMSI to consult the virtual HLR where the correspondence between the identity of the subscriber and his or her HLR is recorded.

The AuC is a database that memorizes a secret key used to authorize the user and to encrypt communications for each subscriber. It is generally connected with the HLR and all can be integrated into the same device.

The EIR is a database that contains the IMEI of the terminal piece of equipment. It is consulted when there are connection requests from a user. It can contain a white list of all approval numbers shared by all terminal numbers in the same series, a black list of stolen or prohibited terminals, and a grey list of terminals that have insufficient malfunctions to justify a complete interdiction.

1.2.4.2. The protocol architecture

The signaling network Signaling System 7 is a data network that transports messages exchanged between network sub-system equipment and uses the ISUP (ISDN User Part) protocol for call processing, the MAP (Mobile Application Part) protocol for roaming and MM and the INAP (Intelligent Network Application Part) protocol for querying a service control point.

The Signaling System 7 network consists of the signaling points hosted by the network sub-system’s equipment. These signaling points can communicate via the signaling transfer points, which carry out packet forwarding.

1.2.4.2.1. The ISUP protocol

The ISUP protocol defines the procedures used to configure, manage and release the circuits that transport vocal signals and data. The ISUP message data are encapsulated by the MTP3 header or the SCCP header (Figure 1.4).

Figure 1.4.The ISUP protocol

The main ISUP messages are used to establish and release communication:

– the Initial Address Message is transmitted from switch to switch to communicate the request to establish a communication. It contains the telephone numbers of the user making the request and the user being requested, as well as the service type (voice, data);

– the address complete message is returned by the incoming switch to indicate that the alert has been activated;

– the answer message is transmitted by the incoming switch to indicate that the user being requested has picked up his or her phone;

– the release message is sent to release resources when a user hangs up his or her telephone;

– the release Complete message is transmitted to acknowledge the release message.

1.2.4.2.2. The MAP protocol

The MAP protocol defines the procedures for the authorization of users, the identification of devices, and roaming management. MAP message data are encapsulated by a TCAP header (Figure 1.5).

Figure 1.5.The MAP protocol

The MAP messages are used to create a dialog between the MSC and the HLR/AuC to fulfill the following functions:

– the recovery of mobile authentication data;

– the recovery of the subscriber’s profile data;

– to find the location of the mobile (VLR number);

– to relay information between the GMSC and the MSC during an incoming call.

Table 1.5.Interfaces using the MAP protocol

Interface

Links

Use

A

MSC – BSC

Incoming and outgoing call and signaling exchanges

B

MSC – VLR

Incoming and outgoing call exchanges

C

GMSC – HLR

Query of the HLR during the incoming mobile call

D

VLR – HLR

Management of user data concerning security and location

E

MSC – MSC

Handover of inter-MSC exchanges

F

MSC – EIR

Verification of terminal identity

G

VLR – VLR

Handover of inter-MSC exchanges

H

HLR – AuC

Authorization data exchanges

1.2.4.2.3. The INAP protocol

The INAP protocol defines the messages exchanged between the components of the intelligent network (Figure 1.6):

– SSP (Service Switching Point). This function is integrated into the telephone switch. It is the trigger point for invoked services;

– SCP (Service Control Point). This function contains the service logic and is located in a service platform that is invoked by the SSP function.

Figure 1.6.The INAP protocol

The extension of the INAP protocol to mobile networks is called CAMEL (Common Architecture for Enhanced Mobile Logic). The CAP (CAMEL Application Part) protocol is a sub-assembly of the INAP protocol. It is used to offer services in prepaid mode, short number services such as calling voicemail, and Virtual Private Network services.

1.2.4.2.4. The TCAP protocol

The TCAP (Transaction Capabilities Application Part) protocol divides into two distinct layers: the transactional part and the component part.

The transactional part manages the states of the dialog and the connection between the two signaling points. The structured dialog is used to start up a dialog, exchange the components within this dialog, terminate a dialog or abandon it. No triggering is associated with an unstructured dialog.

The component part, encapsulated by the transactional part, manages the type of information exchanged. An operation invocation requests that an action be carried out by the remote extremity. The response indicates whether the execution of the operation has succeeded or failed.

1.2.4.2.5. The SCCP protocol

The SCCP (Signaling Connection Control Part) protocol provides the control functions from start to finish between two signaling points. Several modes are defined:

– connectionless mode;

– connectionless mode with resequencing;

– connection mode;

– connection mode and flow control.

The SCCP protocol also provides an address translation function called Global Title. The SCCP gateway translates this Global Title into a code point (MTP3 address) and a Sub-System Number that identifies the application protocol.

1.2.4.2.6. The MTP protocol

The MTP (Message Transfer Part) protocol carries out message transfer. It consists of three layers MTP1, MTP2 and MTP3 corresponding to physical, data link and network layers, respectively.

The MTP3 layer is used for routing messages at the signaling transfer point (STP in Figure 1.7). It is in charge of re-routing the signaling channel when the physical link is cut or when there is congestion in the network.

Figure 1.7.MTP3 routing

The MTP2 layer ensures the reliable transfer of the MTP3 packet between two adjacent nodes. It includes flow control, message sequence validation and error detection. If there is an error, the message is retransmitted.

The MTP1 layer corresponds to circuits at 64 kbps that are exchanged between the equipment and the signaling transfer points. These circuits are multiplexed in a link at 2 Mbps, and transported by the transmission network.

1.3. The radio interface

1.3.1. The transmission chain

The transmission chain includes all operations carried out on speech, data and signaling signals (Figure 1.8):

– source coding for speech or adaptation of the bit rate for data1;

– channel coding for all information to be transmitted;

– constitution of the time-slot, time-division multiplexing entity;

– encryption;

– modulation.

Figure 1.8.The transmission chain

1.3.2. Source coding

1.3.2.1. Codecs

The first stage of speech digitization consists of signal sampling at 8 kHz, which is used to obtain a sample every 125 μs. Each sample is then quantified on 13 bits to obtain a digital signal at 104 kbps. The codec’s role is to reduce the source’s bit rate. It functions on blocks of 20 ms, equating to 160 samples per block.

The FR coder transforms the block of 160 samples into a block of 260 bits, which corresponds to a bit rate of 13 kbps.

The EFR coder transforms the block of 160 samples into a block of 244 bits, which corresponds to a bit rate of 12.2 kbps.

The HR coder transforms the block of 160 samples into a block of 112 bits, which corresponds to a bit rate of 5.6 kbps.

1.3.2.2. Discontinuous transmission

For voice communications, it is rare that the two individuals will speak simultaneously. Furthermore, speech characteristics reveal very short silences between words. In fact, on average, speech constitutes only 40% of the length of the communication. Two modes of transmission can be distinguished:

– speech transmission when the user is speaking;

– the transmission of comfort noises during silences.

DTX (Discontinuous Transmission) consists of interrupting the transmission during silences and has the following advantages:

– a reduction in battery consumption of a mobile, which means that there is an increase in autonomy;

– a decrease in the average level of interference generated, which means that frequencies are reused more efficiently.

When DTX mode is used, it is necessary to distinguish the noise signal. This is the role of the Voice Activity Detector, which calculates certain signal parameters (energy, frequency) every 20 ms and compares them with thresholds in order to make a decision.

1.3.3. Channel coding

1.3.3.1. Error correcting codes

The size of the speech block delivered by the FR codec is 260 bits over 20 ms. The channel coding applied depends on the type of bits used (Figure 1.9):

– Class II 78 bits are not protected. If an error occurs in the field, their loss does not induce a significant deterioration in speech quality.

– Class Ia 50 bits are the most important bits. They are subject to dual protection, by a cyclic code and a convolutional code.

– Class Ib 132 bits are subject to protection by the same convolutional code as the class Ia bits.

Figure 1.9.Channel coding – the FR codec

Table 1.6 summarizes the error-correcting codes used for different logical channels. The TCHs (traffic channels) are used to transport traffic (speech or data). The other channels transport signaling messages.

Table 1.6.The coding of different logical channels

1.3.3.2. Interleaving

Interleaving is a technique used to protect the receiver against surges in errors. It consists of spreading bits before their transmission on the radio channel by fragmenting error packets and allowing a greater efficiency of correction by the convolutional code. The main disadvantage of interleaving is the resulting delay that lowers the speech quality.

Figure 1.10.Interleaving

The block of speech bits (either FR or EFR) provided by the convolutional code has a length of 456 bits. The bits are returned line-by-line in a table of 57 lines and eight columns. The block is then read column-by-column and each column of 57 bits corresponds to a half-burst. The initial block is therefore cut into eight half-bursts (Figure 1.10).

Each half-burst of a block is therefore associated with the half-bursts of the preceding block (for 0, 1, 2 and 3 half-bursts) or with the half-bursts of the following block (for 4, 5, 6 and 7 half-bursts). A time-slot contains a half-burst of one block and a half-burst from the block preceding or following it (Figure 1.10).

The last stage consists of interleaving the half-bursts of the two different blocks inside the slot at the bit level. Thus the paired bits of the burst correspond to the most recent block and the odd bits to the preceding block (Figure 1.10).

1.3.4. Time-division multiplexing

1.3.4.1. The structure of multiplexing

The TDMA frame is formed from eight slots. On a frequency pair, a particular slot or half slot is appointed to a mobile link, according to the source coding used. This pair of slots constitutes a physical channel that supports various logical channels. The logical channel is therefore defined as the assignment of part of the physical channel to a particular function:

– a traffic function for the transfer of speech or data channels;

– a signaling function for call establishment and the processing of additional services;

– a time and logic synchronization function between the mobile and the BTS;

– a function of parameter measurements linked to radio propagation;

– a function of network access control.

The physical channel consists of eight time-slots or slots numbered from 0 to 7. A set of slots with the same number is called a multi-frame. There are two types of multi-frame: the 26-slot multi-frame and the 51-slot multi-frame. Each multi-frame transports several logical channels. A logical channel is identified by one or more slot numbers in the multi-frame (Figure 1.11).

The super-frame is a structure common to two types of multi-frame. It consists of 26 multi-frames of 51 slots, or 51 multi-frames of 26 slots. The super-frame has no particular significance (Figure 1.11).

The hyper-frame is an assembly of 2,048 super-frames, that is to say 2,715,648 slots. Each of the TDMA frame’s slots can be found by a FN (Frame Number) frame counter. The BTS regularly sends the FN counter to the mobile, allowing it to locate itself in the hyper-frame. This counter is used for encryption of the link and for tracking system information (Figure 1.11).

Figure 1.11.The structure of time-division multiplexing

1.3.4.2. The structure of bursts

A burst is transmitted to the interior of a slot. There are four types of burst:

– the normal burst, used for the following logical channels:

TCH (Traffic CHannel),

FACCH (Fast Associated Control CHannel),

SACCH (Slow Assisted Control CHannel),

SDCCH (Stand-alone Dedicated Control CHannel),

BCCH (Broadcast Control CHannel),

PCH (Paging CHannel) and

AGCH (Access Grant CHannel);

– the access burst used for the logical channels RACH (Random Access CHannel) and FACCH during a handover operation;

– the frequency correction burst used for the logical FCCH (Frequency Correction CHannel);

– the synchronization burst used for the logical SCH (Synchronization CHannel).

Each burst contains the following fields:

– a header and a tail-end: bits at zero at the beginning and end of the burst used to avoid the loss of synchronization;

– control of traffic data bits;

– a training sequence to model the transmission channel.

The training sequence fulfills the following functions:

– determines the position of the desired signal in the burst;

– evaluates transmission channel distortion;

– distinguishes between two cells using the same frequency.

1.3.4.2.1. The structure of the normal burst

A normal burst contains the following fields (Figure 1.12):

– a header and a tail-end, each consisting of 3 bits, used to avoid synchronization loss;

– information consisting of two sequences of 57 bits, containing traffic or signaling data;

– the 2-bit flag, indicating whether the information is regarding traffic or signaling;

– a training sequence of 26 bits, containing a sequence known to the mobile used by the receiver to equalize the radio transmission channel;

– a guard time of 8.25 bits, which stops adjacent slots received via the BTS from overlapping.

Figure 1.12.The structure of the normal burst

1.3.4.2.2. The structure of the access burst

The access burst has the following characteristics (Figure 1.13):