Interleaving
The decoding result of the convolutional code strongly depends on the frequency and grouping of bit errors that occur during transmission. Especially burst errors during long and deep fading periods, i.e. a series of erroneous sequential bits, have negative impact on error correction. In such cases, the channel is not a binary channel without memory, rather the single-bit errors have statistical dependence, which diminishes the result of the error correction procedure of the convolutional code....
Source Coding and Speech Processing
Source coding reduces redundancy in the speech signal and thus results in signal compression, which means that a significantly lower bit rate is achieved than needed by the original speech signal. The speech coder decoder is the central part of the GSM speech processing function, both at the transmitter Figure 6.2 as well as at the receiver Figure 6.3 . The functions of the GSM speech coder and decoder are usually combined in one building block called the codec COder DECoder . Figure 6.2...
Basic Packet Mode
Two variants of PSPDN access can be realized with the protocol model in Figure 9.14 PSPDN access according to ITU-T X.32 Access to PSPDN packet handlers according to ITU-T X.31 Case A basic packet mode . PSPDN access according to X.32 has not met much acceptance with the applications the X.31 procedure is used more often. PSPDN access according to X.32 is the simpler variant. The X.25 packets can be trans Figure 9.14 X.25 access at the ISDN S interface Figure 9.14 X.25 access at the ISDN S...
External Error Protection Block Coding
The block coding stage in GSM has the purpose of generating parity bits for a block of data, which allow the detection of errors in this block. In addition, these blocks are supplemented by fill bits tail bits to a block length suitable for further processing. Since block coding is the first or external stage of channel coding, the block code is also known as external protection. Figure 6.7 gives a brief overview showing which codes are used for which channels. In principle, only two kinds of...
Channel Measurement
The task of Radio Subsystem Link Control in the mobile station includes identification of the reachable base stations and measurement of their respective received signal level and channel quality quality monitoring task . In idle mode, these measurements serve to select the current base station, whose PCH is then periodically examined and on whose RACH desired connections can be requested. During a connection, i.e. on a TCH or SDCCH with respective SACCH FACCH, this measurement data is...
Short Message Service SMS
Another teleservice which was assigned high priority in the service implementation strategy - and which is now very successful - is the capability to receive or send short messages at the mobile station Short 'Message Service SMS , TS21 and TS22. This service was supposed to be offered in the third phase E3 at the latest from 1996 on all GSM networks. TS21 is the point-to-point version of the SMS, which allows a single station to be sent a message of up to 160 characters. Conversely, TS22 has...
Channel Coding
The heavily varying properties of the mobile radio channel see Section 2.1 result in an often very high bit error ratio, on the order of 10 3 to 10_1. The highly compressed, redundancy-reduced source coding makes speech communication with acceptable quality almost impossible moreover, it makes reasonable data communication impossible. Suitable error correction procedures are therefore necessary to reduce the bit error probability into an acceptable range of about 10 5 to 10 6. Channel coding,...
Signaling at the User Interface
Another often neglected but nevertheless very important interface in a mobile system is the user interface of the mobile station equipment. This Man-Machine Interface MMI can be realized freely and therefore in many different ways by the mobile equipment manufacturers. In order to keep a set of standardized service control functions in spite of this variety, the MMI commands have been introduced. These MMI commands define procedures mainly for the control of basic and supplementary services....
Subscriber Data in GSM
Besides data of the address type, which is the most important subscriber data of any communication network, a whole series of other service- and contract-specific data exists in GSM networks. Addresses serve to identify, authenticate, and localize subscribers, or switch connections to subscribers. Service-specific data is used to parameterize and personalize supplementary services. Finally, contracts with subscribers can define different service levels, e.g. booking of special supplementary...
Appendix A GSM Standards
1 GSM 01.02, General Description of a GSM PLMN 2 GSM 01.04, Abbreviations and Acronyms 3 GSM 02.02, Bearer services BS supported by a GSM PLMN 4 GSM 02.03, Teleservices supported by a GSM PLMN 5 GSM 02.04, General on supplementary services 6 GSM 02.09, Security aspects 7 GSM 02.16, International MS Equipment Identities 8 GSM 02.17, Subscriber Identity Modules - Functional Characteristics 9 GSM 02.22, Personalisation of GSM Mobile Equipment ME Mobile functionality specification 10 GSM 02.30,...
Coding Authentication and Ciphering
The previous chapter explained the basic functions of the physical layer at the air interface, e.g. the definition of logical and physical channels, modulation, multiple access techniques, duplexing, and the definition of bursts. In this chapter, we discuss several additional functions that are performed to transmit the data in an efficient, reliable, and secure way over the radio channel source coding and speech processing Section 6.1 , channel coding and burst mapping Section 6.2 , and...
Protocol Architecture Planes
The various physical aspects of radio transmission across the GSM air interface and the realization of physical and logical channels were explained in Chapter 5. According to the terminology of the OSI Reference Model, these logical channels are at the Service Access Point of Layer 1 physical layer , where they are visible to the upper layers as transmission channels of the physical layer. The physical layer also includes the forward error correction and the encryption of user data. The...
Protocol Architecture 1141 Transmission Plane
Figure 11.7 illustrates the protocol architecture of the GPRS transmission plane. The protocols offer transmission of user data and its associated signaling e.g. for flow control, error detection, and error correction . An application running in the GPRS-MS e.g. a browser uses IP or X.25, respectively, in the network layer. As mentioned earlier in this chapter, IP and X.25 packets are transmitted encapsulated within the GPRS backbone network. This is done using the GPRS Tunneling Protocol GTP ,...
Advanced Speech Call Items ASCI
GSM systems of Phase 2 offer inadequate features for group communications. For example, group call or push-to-talk services with fast connection setup as known from private radio or digital trunked radio systems e.g. TETRA , are not offered. However, such services are indispensable for most closed user groups e.g. police, airport staff, railroad or taxi companies . In particular railroad operators had a strong request for such features. In 1992, their international organization, the Union...
Modulation
The modulation technique used on the radio channel is Gaussian Minimum Shift Keying GMSK . GMSK belongs to a family of continuous-phase modulation procedures, which have the special advantages of a narrow transmitter power spectrum with low adjacent channel interference on the one hand and a constant amplitude envelope on the other hand, which allows use of simple amplifiers in the transmitters without special linearity requirements class C amplifiers . Such amplifiers are especially...
Cell Selection and Operation in Power Conservation Mode 5541 Cell Selection and
A mobile station in idle mode must periodically measure the receivable BCCH carriers of the base stations in the area and calculate mean values RXLEV n from this data see Section 5.5.1.1 . Based on these measurements, the mobile station selects a cell, namely the one with the best reception, i.e. the mobile station is committed to this cell. This is called camping on this cell. In this state, accessing a service becomes possible, and the mobile station listens periodically to the PCH. Two...
Dedicated Packet Mode
As in the case of asynchronous PAD access to packet data networks see Section 9.5.3 , the synchronous case of the X.25 access protocol also offers an alternative, which allows the most immediate transition to the PSPDN, even if the mobile station is in a foreign network international roaming . For this purpose, a dedicated mode is also defined with each PLMN having its own packet handler. Figure 9.15 shows the principle of this Dedicated Packet Mode. It essentially includes the functions of the...
Protocol Architecture
The WAP protocol architecture is shown in Figure 12.12. As mentioned before, WAP is based on the WWW protocol stack and adjusts those protocols to the requirements of wireless transmission and small portable devices. For applications, a uniform microbrowser environment has been specified the Wireless Application Environment WAE . It comprises the following functionality and formats the Wireless Markup Language WML , a simple script language WMLScript, which is based on JavaScript, programming...
Channel Coding 1
Figure 11.17 shows how a block of the RLC MAC layer compare with Figure 11.8 is encoded and mapped onto four bursts. Channel coding is used to protect the transmitted data packets against errors and perform Forward Error Correction FEC . The channel coding technique in GPRS is quite similar to the one employed in conventional GSM. An outer block coding, an inner convolutional coding, and an interleaving scheme is used Figure 6.6 . RLC MAC block Header Information bits BC SI Length depends an...
System Architecture 1
Figure 12.14 gives a schematic illustration of a typical WAP system architecture. The principle how content is stored in a distributed way within the network and finally offered to the user is similar to the principle of the WWW. Servers store content directly as a WML document or content is generated with scripts. Mobile stations download these contents from the server to their microbrowser, which then presents them to the user. In theory, it is possible to store content in HTML and...
Addresses and Identifiers
GSM distinguishes explicitly between user and equipment and deals with them separately. According to this concept, which was introduced with digital mobile networks, mobile equipment and users each receive their own internationally unique identifiers. The user identity is associated with a mobile station by means of a personal chip card, the Subscriber Identity Module SIM . This SIM usually comes in the form of a chip card, which is transferable between mobile stations. It allows to distinguish...
Teleservices
On top of the bearer services, which can be used by themselves, a number of teleservices have been defined. The most important categories are Table 4.3 speech, SMS, access to Message Handling Systems MHSs and to videotext, teletext, and facsimile transfer. Voice services had to be implemented by each operator in the start-up phase E1 by 1991. In this category, two teleservices were distinguished regular telephone service TS11 and emergency service TS12 . For transmission of the digitally coded...
Radio Resource Management
The procedures for Radio Resource Management RR are the basic signaling and control procedures at the air interface. They handle the assignment, allocation and administration Figure 7.21 Format of a Um signaling message Layer 3 Figure 7.21 Format of a Um signaling message Layer 3 of radio resources, the acquisition of system information from broadcast channels BCCH and the selection of the cell with the best signal reception see cell selection in Section 5.5.4.1 . Accordingly, the RR procedures...
Multiple Access Duplexing and Bursts
On the physical layer OSI Layer 1 , GSM uses a combination of FDMA and TDMA for multiple access. Two frequency bands 45 MHz apart have been reserved for GSM operation Figure 5.5 890-915 MHz for transmission from the mobile station, i.e. uplink, and 935-960 MHz for transmission from the base station, i.e. downlink. Each of these bands of 25 MHz width is divided into 124 single carrier channels of 200 kHz width. This variant of FDMA is also called Multi-Carrier MC . In each of the uplink downlink...
Speech Transmission
The digital, source-coded speech signal of the mobile station is transmitted across the air interface in error-protected and encrypted form. The signal is then deciphered in the BTS, and the error protection is removed before the signal is passed on. This specially protected speech transmission occurs transparently between mobile station and a Transcoding and Rate Adaptation Unit TRAU which serves to transform the GSM speech-coded signals to the ISDN standard format ITU-T A-law . A possible...
New Data Services and Higher Data Rates HSCSD GPRS and EDGE
Development also continues with data services. The maximal data rate of 9600 bit s for data services in conventional GSM is rather low compared to fixed networks. The desire for higher data rates in GSM networks is therefore quite obvious. Two prominent trends can be recognized integration of packet services into GSM networks and high-bit-rate bearer services with data transmission rates up to some 10 kbit s. Accordingly, one of the GSM standardization groups specified the High Speed Circuit...
Adaptive Frame Synchronization
The mobile station can be anywhere within a cell, which means the distance between mobile and base station may vary. Thus the signal propagation times between mobile and base station vary. Due to the mobility of the subscribers, the bursts received at the base would be offset. The TDMA procedure cannot tolerate such time shifts, since it is based on the exact synchronization of transmitted and received data bursts. Bursts transmitted by different mobile stations in adjacent time slots must not...
Transparent Data Transmission
The digital mobile radio channel is subject to severe quality variations and generates burst errors, which one tries to correct through interleaving and convolutional codes see Section 6.2 . However, if the signal quality is too low due to fading breaks or interference, the resulting errors cannot be corrected. For data transmission across the air interface Um, a residual bit error ratio varying between 10 2 and 10 according to channel conditions can be observed 58 . This kind of variable...
Disconnection due to Radio Channel Failure
The quality of a radio channel can vary considerably during an existing connection, or it can even fail in the case of shadowing. This should not lead to immediate disconnection, since such failures are often of short duration. For this reason GSM has a special algorithm within the Radio Subsystem Link Control which continuously checks for connectivity. It consists of recognizing a radio link failure by the inability to decode signaling information on the SACCH. This connectivity check is done...
System Architecture
A GSM system has two major components the fixed installed infrastructure the network in the proper sense and the mobile subscribers, which use the services of the network and communicate over the radio interface air interface . The fixed installed GSM network can again be subdivided into three subnetworks the radio network, the mobile switching network, and the management network 21 . These subnetworks are called subsystems in the GSM standard. The respective three subsystems are the Base...
Effect of the MSRN Assignment on Routing
There are two ways to obtain the MSRN obtaining the MSRN at location update obtaining the MSRN on a per call basis For the first variant, an MSRN for the mobile station is assigned at the time of each location update which is stored in the HLR. This way the HLR is in a position to supply immediately the routing information needed to switch a call through to the local MSC. The second variant requires that the HLR has at least an identification for the currently responsible VLR. In this case,...
Handover 841 Overview
Handover is the transfer of an existing voice connection to a new base station. There are different reasons for the handover to become necessary. In GSM, a handover decision is made by the network, not the mobile station, and it is based on BSS criteria received signal level, channel quality, distance between MS and BTS and on network operation criteria e.g. current traffic load of the cell and ongoing maintenance work . The functions for preparation of handover are part of the Radio Subsystem...
Decision Algorithm for Handover Timing
The basis for processing a successful handover is a decision algorithm which uses measurement results from mobile and base station to identify possible other base stations as targets for handovers and which determines the optimal moment to execute the handover. The objective is to keep the number of handovers per cell change as small as possible. Ideally, there should not be more than one handover per cell change. In reality, this is often not achievable. When a mobile station leaves the radio...
Multiple Access and Radio Resource Management
On the physical layer, GPRS uses the GSM combination of FDMA and TDMA with 8 time slots per TDMA frame as explained in Section 5.2.2 . However, several new methods are used for channel allocation and multiple access. They have significant impact on the performance of GPRS. In circuit switched GSM, a physical channel i.e. one time slot of successive TDMA Figure ll.l3 GPRS-Internet interconnection frames is permanently allocated for a particular MS during the entire call period no matter whether...
Signaling at the A and Abis Interfaces
Whereas the transport of user data between MSC and BSC occurs across standard connections of the fixed network with 64 kbit s or 2048 kbit s or 1544 kbit s , the transport of signaling messages between MSC and BSC runs over the SS 7 network. The MTP and SCCP parts of SS 7 are used for this purpose. A protocol function using the services of the SCCP is defined at the A interface. This is the Base Station Application Part BSSAP , which is further subdivided into Direct Transfer Application Part...
CAMEL GSM and Intelligent Networks
Essentially, CAMEL represents a convergence of GSM and Intelligent Network IN technologies. The fundamental concept of IN is to enable flexible implementation, introduction and control of services in public networks and to use the idea of dividing the switching functionality into basic switching functionality, residing in Service Switching Points SSPs and centralized service control functionality, residing in Service Control Points SCPs . Both network components communicate with each other over...
PLMN Configurations and Interfaces
The fixed connections for transport of signaling and user data in a GSM PLMN Figure 3.8 are standard transmission lines. Within the SMSS, lines with a transmission rate of 2 Mbit s or 1.544 Mbit s in North America are typically used fixed lines, mostly microwave links or leased lines . The BSS uses mostly 64 kbit s lines. Signaling has two fundamentally different parts GSM-specific signaling within the BSS, including the air interface, and signaling within the SMSS and with other PLMN in...
Improved Codecs for Speech Services HalfRate Codec EFR Codec and AMR Codec
One of the most important services in GSM is of course voice service. Thus it is obvious, that voice service has to be further improved. In first place is the development of new speech codecs with two competing objectives better utilization of the frequency bands assigned to GSM and improvement of speech quality in the direction of the quality offered by ISDN networks, which is primarily requested by professional users. Half-Rate codec - The reason for improved bandwidth utilization is to...
Mapping of Logical Channels onto Physical Channels
The mapping of logical channels onto physical channels has two components mapping in frequency and mapping in time. The mapping of a logical channel onto a physical channel in the frequency domain is based on the TDMA frame number FN , the frequencies allocated to base and mobile stations - CA and MA - and the rules for the optional frequency hopping see Section 5.2.3 . In the time domain, logical channels are transported in the corresponding time slots of the physical channel. They are mapped...
Basic Handover between two MSCs
The principal sequence of operations for a basic handover between two MSCs is shown in Figure 8.20. The MS has indicated the conditions for the handover, and the BSS requests the handover from MSC-A handover required . MSC-A decides positively for a handover and sends a message perform handover to MSC-B. This message contains the necessary data to enable MSC-B to reserve a radio channel for the MS. Above all, it identifies the BSS which is to receive the connection. MSC-B assigns a handover...
Services and Applications
The first specification of WAP has been released by the WAP Forum in April 1998. Version 1.1 followed in June 1999 and Version 1.2 in December 1999. WAP terminals have been introduced around February 1999 for the first time, and today there exists a broad variety of WAP products mobile equipment, gateways, development tools, WML browsers and editors. Besides the technical implementation in the network and the development of new WAP-capable mobile equipment, innovative WAP services are in...
Mobility Management
The main task of 'Mobility Management MM is to support the mobility of the mobile station for example, by reporting the current location to the network or verifying the subscriber identity. Another task of the MM sublayer is to offer MM connections and associated services to the CM sublayer above. The message format for MM messages is the uniform Layer 3 signaling message format Figure 7.21 . MM has its own protocol discriminator, and the MM messages are marked with a type code MT, Table 7.5 ....
Mapping of packet data logical channels onto physical channels
From Section 5.4 we know that the mapping of logical GSM channels onto physical channels has two components mapping in frequency and mapping in time. The mapping in frequency is based on the TDMA frame number and the frequencies allocated to the BTS and the mobile station. The mapping in time is based on the definition of complex multiframe structures on top of the TDMA frames. A multiframe structure for PDCHs consisting of 52 TDMA frames each with 8 time slots is shown in Figure 11.16. The...
Signaling Procedures for Supplementary Services
As can be seen in Figure 7.21, signaling messages to control Supplementary Services SS are coded with special protocol discriminators 0011 for call related 1011 for noncall related. A special set of signaling messages has been defined for their control Table 7.7 . The category CC Messages Table 7.6 consists of the subcategories Call Information Phase message type MT 0x01tttt and Miscellaneous message type MT 0x11tttt . These two message categories are used in two categories of SS procedures the...
Teleservices Fax
In the following, we briefly explain the realization of the GSM fax service. The GSM standard considers the connection of a regular Group 3 fax terminal with its two-wire interface to an appropriately equipped mobile station as the standard configuration of a mobile fax application. The GSM fax service is supposed to enable this configuration to conduct fax transmissions with standard Group 3 fax terminals over mobile connections. This requires mapping the fax protocol of the analog two-wire...
Bearer Services
The basic services of a GSM network are the foundation for data transmission, i.e. a basic service provides the fundamental technical facilities at the end terminal interface reference point R to transport user payloads. The basic services are called transport services 7 or bearer services in ISDN - and therefore in GSM, too. The GSM bearer services offer asynchronous or synchronous data transport capabilities with circuit-switched or packet-switched data rates of 300-9600 bit s, with a 13 kbit...
Session Management and PDP Context
To exchange data packets with external PDNs after a successful GPRS attach, a mobile station must apply for an address used in the PDN. In general, this address is called PDP address Packet Data Protocol address . In case the PDN is an IP network, this will be an IP address. For each session, a so-called PDP context is created, which describes the characteristics of the session. It contains the PDP type e.g. IPv4 , the PDP address assigned to the mobile station e.g. an IP address , the...
Telecommunication Management Network TMN
TMN was standardized within ITU-T ETSI CEPT almost simultaneously with the pan-European mobile radio system GSM. The guidelines of the M. series of the ITU-T M.20, M.30 serve as a framework. TMN defines an open system with standardized interfaces. This standardization enables a platform-independent multivendor environment for management of all components of a telecommunication network. Essentially it realizes the communication of a management system with network elements it administers, which...
TMN Realization in GSM Networks
TMN and GSM were standardized approximately at the same time, so that there was a good opportunity to apply TMN principles and methods in a complete TMN system for network management in GSM from the beginning and from ground up. For this purpose, specific working groups were founded for the five TMN categories Figure 10.1 as well as for architecture and protocol questions which were supposed to develop as much as possible of the TMN system and its services, while following the top-down...
lll System Architecture
In order to integrate GPRS into the existing GSM architecture see e.g. Figure 3.9 , a new class of network nodes, called GPRS Support Nodes GSNs , has been introduced. GSNs are responsible for the delivery and routing of data packets between the mobile stations and external packet data networks PDNs . Figure 11.1 illustrates the resulting system architecture. Figure 11.1 GPRS system architecture and interfaces Figure 11.1 GPRS system architecture and interfaces A Serving GPRS Support Node SGSN...