TELE COMMUNICATIONS Objective Introduction Global System for Mobile Communication (GSM):

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1 TELE COMMUNICATIONS Objective This unit discusses the telecommunications systems which includes the GSM, the GPRS, DECT. The basics of the Satellite networks, the Parameters and Configuration and Capacity Allocation, the FAMA and DAMA. The Broadcast Systems like Digital Audio Broadcasting and Digital Video Broadcasting are also being discussed. Introduction Digital cellular networks are the segment of the market for mobile and wireless devices which are growing most rapidly. They are the wireless extensions of traditional PSTN or ISDN networks and allow for seamless roaming with the same mobile phone nation or even worldwide. Today, these systems are mainly used for voice traffic. However, data traffic is continuously growing and, therefore, this chapter presents several technologies for wireless data transmission using cellular systems. Global System for Mobile Communication (GSM): Rapid increase in the demand for data services, is the primary thrust for the mobile industry to advance. However the radio access platforms that are different by different service provides has been the inhibitor for the growth as there are a number of different access 2 G technologies. International Telecommunications Union (ITU ) has specified the International Mobile Telecommunication 2000 (IMT 2000). IMT 2000 is the Radio and Network access specifications defining several methods or technology platforms that meet the overall goals of the final specifications. It is meant to be unifying specifications enabling mobile and fixed high speed data services to use one or several radio channels with fixed network platforms for delivering the services envisioned. Some IMT specifications are: Global standard. Compatibility of service within IMT 2000 and other fixed networks. High quality Worldwide common frequency band Small terminals for worldwide use Worldwide roaming capability Multimedia application services and terminals 1

2 Improved spectrum efficiency Flexibility for evolution to the next generation of wireless systems. High speed data rates. 2 Mbps for fixed environments 384 kbps for pedestrian 144 kbps for vehicular traffic IN order to provide transition to 3 G technology different technology provider were moving following different path so as to cater to the current 2G infrastructure and the changes that need to be incorporated gradually to reach 3 G technology. Most of these technologists have introduced a middle technology called 2. 5 G technology. These are shown in the following diagram GSM (800,1900) GPRS (800,1900) WCDMA TD-SCDMA (CHINA) EDGE (800,1900) TD-CDMA (JAPAN) CDMA x RTT IS 95 CDMA MIGRATION PATH WiDEN i DEN CDMA 2000 DO/DV General Packet Radio Service: Provides higher data rate service Same air interface as that of GSM is used. GPRS defines a number of channel coding scheme Coding Air Interface Data Rate Usable data rate scheme CS 1 Kbps

3 CS 2 CS-3 CS If a single user access a multiple time slots, then speeds such as 40.2, 53.6 kbps becomes available. To the user CS-2 has robust error correction over the air interface. CS -, 4 may have higher data output rate, but have no error correction schemes which may require retransmission leading wastage of transmission. Biggest advantage of GPRS is not that it provides higher data rate, but it facilitates packet switching wherein the network is connected as per requirement rather than like in circuit switched for the full call duration. When no data is transferred, channel is used by the other users. GPRS User devices: The devices can transreceive both voice and data transmission. User may need such service either separately or in combination. Accordingly, user can be grouped into following classes. a) Class A : Supports simultaneous use of voice and data services. That is while holding voice conversation, can download data at the same time. b) Class B : Supports simultaneous GPRS attach and GSM attach but not the simultaneous use of both services. That is while downloading data, if a call a received, the data download is put on hold and the voice call is attended. Later down is resumed. c) Class C : Can attach eith GSM or GPRS but cannot attach both simultaneously. That is at a given instant a class C device is either a GSM device or a GPRS device. Other aspects of device are multislot capability of the device which directly affects the supported data rate. Some device support three slot, others only two. As the data down load requirement is asymmetric high data rate in the down link 3

4 and lower data rate in the uplink, some devices may have different slot for both up and down link. GPRS Air Interface: Air interface is similar to GSM air interface. It is 200 KHz RF carrier and the same eight slots per carrier. This allows both GSM and GPRS to share the same RF resources. That is some slot may be carrying voice while some slot may be carrying data. Although GPRS makes use of the same basic infrastructure as GSM, it introduces a number of logical channel types and new channel coding scheme. When the GPRS related data traffic is handled, then it is known as Packet Data Channel (PDCH) which has 52 frames per multiframe. GPRS offers a point to point (PTP) packet service known as PTP CONS (Point to Point Connection Oriented Network Service.) This is the ability of GPRS to maintain the virtual circuit upon change of the cell with the GSM network. This type or service corresponds to X.25 The other service PTP CLNS is the point to point connection less network service that supports applications based on the internet protocol IP. GPRS Network Architecture: 4

5 Details regarding various subsystems are given below: PACKET CONTROL UNIT (PCF): Responsible for a number of GPRS related functions such as air interface access control, packet scheduling on the air interface and packet assembly and re assembly. Located at BTS /BSC / SGSN Serving GPRS Support Node (SGSN) : Analogues to MSC / VLR of GSM. Performs MSC equivalent function in packet switched domain mobility management, security and access control etc. Service area is divided into routing area similar to location area. A SGSN may serve multiple BSC, but each BSC will have only one SGSN. Various interfaces are shown in the above diagram. Gb interface SGSN PCU : Frame relay based interface that uses Base Station System GPRS Protocol (BSSGP) used to pass signaling, control information and data packet to and fro from SGSN. Gr interface between SGSN HLR : 5

6 It uses Mobile Application Part (MAP) protocol. (similar to D interface between a VLR and HLR). It is used to provide equipment identification, user authentication and roaming and location updates to the HLR for GPRS subscribers. SGSN may also retrieve the same information from HLR. Gs interface between SGSN SS7 HLR : based interface that used signaling, connection control part (SCCP). Supports both circuit and packet switched services. This enables the coordination between SGSN and MSC/VLR for subscribers of GPRS. Gd interface between SGSN SMSC: Enables tran reception of SMS over GPRS. Uses SS 7 MAP protocol Gn interface between SGSN ---- GGSN / SGSN: It is internet protocol based interface carries signal and user data. This uses the GPRS Tunneling Protocol GTP, which enables user data through the IP backbone network between the SGSN and the GGSN. Gateway GPRS Support Node (GGSN) : It is a point of interface with external packet data network (such as internet). Entry into and out of PLMN (Public Land Mobile Network) is through GGSN via Gn interface. Gn is internet protocol based interface carries signaling and user data. Gn interface uses GPRSs Tunneling Protocol (GTP), which tunnels the data through IP backbone. Gc interface between GGSN HLR : It uses MAP (Mobile Application Part ) over SS7.It is used when GGSN wants to know the serving SGSN for any subscriber. GPRS Transmission Plane Protocol Reference Model: 6

7 At the MS, there is RF interface, above which are Radio Link Control and Medium Access Control functions. Above these are Logical Link Control (LLC) which provides a logical link and framing structure for communication between the MS and the SGSN. Any data between MS and SGSN is sent as Logical Link Protocol Data Units (LLPDU ). LLC supports management of this transfer, including mechanism for the detection and recovery from, lost or corrupted LL-PDU, ciphering and flow control. Above LLC, is Sub Network Dependent Convergence Protocol (SNDCP) which resides between LLC and the network layer (such as IP and X.25). The purpose of SNDCP is to enable support for multiple network protocols without having to change the lower layers such as LLC. It also help to multiplex several packet streams into a single logical channel between MS and SGSN. At the BSS, a relay function relays LL_PDU from Gb interface to the air interface ( the Um interface). Similarly, at SSGN, a relay function relays PDP PDUs between the Gb interface and the Gn interface. GTP- GPRS Tunneling Protocol: All data within GPRS backbone is transferred GTP. It can use different transport protocols either the reliable TCP (needed for reliable transfer of X.25 packets) or the non reliable UDP used for packet IP. Tunneling is the procedure of wrapping up the connection and its associated packets in a wrapper for transmission through the IP network between GGSN and SGSN. In this case, the IP network nodes (routers) between the SGSN and GGSN consider the GTP packets to be application and those routers do not examine the contents of the GTP layer. At the 7

8 SGSN, the wrapper is removed and the packet is passed to the MS using SNDCP, LLC and the lower layers. For the packets from MS to external network- such as internet, the GGSN removes the wrapper and forwards the IP packets. Digital Enhanced Cordless Telecommunication (DECT) It is specified by ETSI in DECT is mainly used in offices, on campus, at trade centers, or at home. Further access points to PSTN can be established within eg at railway station, large government buildings and hospitals offering much cheaper telephone services compared to a GSM DECT may also provide last mile connectivity: A big difference between the DECT and GSM is in terms of Cell diameter and cell Capacity. While GSM can be used upto 70 KM, DECT is limited to 300 Meters from the base station. DECT along with mobile station are cheap compared to that of GSM. The frequency range of DECT is MHz offering 120 full duplex channels. TDD is applied using 10 ms frames. The frequency range is divided into 10 carrier frequencies using FDMA each frame being divided into 24 slots using TDMA. For the TDD mechanism, 12 slots are used for up link and 12 for down link. The digital modulation technique used to GMSK. Each station has an average power of only 10 mw with a maximum of 250 mw. System Architecture: 8

9 Different DECT entities can be integrated into one physical unit or entities can be distribute, replicated etc. However, the basic reference model is shown above. In this a global network connects the local communication network to the outside world and offers its services via a interface D. Global network could be ISDN, PSTN, PLMN eg GSM or Packet Switched Public Data Network (PSPDN). The service offered by the networks include transportation of data on one hand and the transition of address and routing of data between the local networks on the other. Local networks provides simple service to everything such as switching to call forwarding, address translation etc. HDB Home Data Base and VDB visitor data base are also located at the local network. The DECT core network consists of the fixed radio termination (FT) and the portable Radio Termination PT and basically only provides multiplexing service. Protocol Architecture: DECT protocol reference architecture follows the OSI reference mode. Following figures shows the layers covered by this standard. 9

10 Physical Layer The physical layer, medium access control, and data link control for both the control place (C Plane) and the user plane (U-Plane). Only for the C place, an additional network layer has been specified so that the user data from layer two is directly forwarded top the U plane. A management plane covers all lower layers of a DECT system. Physical layer comprises all functions for modulation / demodulation, incoming signal detection,, sender / receiver synchronization, and collection of status information for the management plane. On request from MAC layer, the physical layer assigns a channel for data transmission. Following diagram shows the TDMA frame structure used in DECT: 10

11 Each frame has a duration of 10 ms and contains 12 slots for down link and 12 slots for uplink. In basic connection mode. If a mobile receives data in slot n, it returns data in n+ 12 slot. After leaving for sync bits and guard space, 388 bits are used for network control (A field) and user data(b field). With a data rate of 6.4 kbps, (64 bits per 10 ms), the user data rate depends on additional error correction mechanism. Simplex bearer provides a data rate of 32 kbps in unprotected mode while using 16 bit CRC checksum for data block of 64 bits in the protected mode. This reduces the data rate to 25.6 kbps. Medium Access Control Layer: It is intended to establish, maintain and release channels for higher layers by activating and deactivating physical channels. It multiplexes several logical channels onto physical channels. Logical channel exists for signalling network control, user data transmission, paging or sending broadcast messages. Additional services offered include segmentation / re assembly of packets and error control / error correction. Data Link Control Layer: The purpose of the DLC is to create and maintain reliable connections between the mobile and the base station. There are two services called C Plane (control plane)and U plane (user plane). C Plane: It has Connectionless broadcast service for paging called Lb and point to point protocol similar to LAPD (Link Access Procedure for D Channel) in ISDN but adopted to the underlying MAC as LAPC +Lc U plane: In case of User plane, several services exists. Transparent unprotected service. As forward error correction service, rate adaptation service, services for future enhancement. Network Layer: 11

12 Network layer exist only in C plane. This layer provides services to request, check, reserve control and release resources at the fixed station and the mobile terminal Mobility Management: Mobility Management within the network layer is responsible for identity management, authentication and management of the location data bases. Call Control (CC): Handles connection set up, release, and negotiation. Two message services, connection oriented message service and connectionless message service transfers data to and from the internetworking unit that connects the DECT system with the outside Data Link control Medium Access Control Time Division Multiple Access: Physical Data Link control Medium Access Control Physical TDMA radio access, with its low radio interference characteristics, provides high system capacity to handle up to 100'000 users per km² floor space in an office environment. Adaptive Differential Pulse Code Modulation: ADPCM (Adaptive Differential Pulse Code Modulation) speech encoding ensures a DECT cordless phone very high speech quality, comparable to wire line telephony. SATELLITE SYSTEMS 12 Mobility Management DECT Connection Oriented Message Service Connectionless Message Service Supplementary Services Call control Mobility Management Connection Oriented Message Service message Connectionless Message Service Supplementary Services Call control world.

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26 Summary In this unit, GSM is presented as the most successful second generation digital cellular network. Although GSM was primarily designed for voice transmission, it showed the evolution of the data oriented transfer via HSCSD and GPRS. This evolution also includes the transition from a circuit switched network to a packet-switched system that comes closer to the internet model. Other systems presented include DECT, the digital standard for cordless phones and TETRA, a trunked radio system. DECT can be used for wireless data transmission on a campus or indoors, but also for wireless local loops. Also, an overview of the current and the future third generation systems were discussed. The Broadcasting systems and their technologies were discussed in the later part of the unit. DAB and DVB likely the successors of the traditional radio and television, were presented. Key Terms GPRS-General Packet Radio Service 26

27 DECT- Digital Enhanced Cordless Telecommunications TETRA- Terrestrial Trunked Radio DAB Digital audio broadcasting DVB Digital video broadcasting HSCSD -High Speed Circuit Switched Data Multiple Choice Questions introduced a packet-oriented service and higher data rates to GSM. a) GPRS b)dect c)tetra d)umts replaces older analog cordless phone systems such as CT1 and CT1+. a) GPRS b)dect c)tetra d)umts use many different radio carriers but only assign a specific carrier to a certain user for a short period of time according to demand. a) GPRS b)dect c)tetra d)umts 4. The system does not only offer sound in a CD-like quality, it is also practically immune to interference and multi-path propagation effects. a) DAB b)dvb c)tetra d)umts can be also used for high bandwidth, asymmetrical Internet access. a) DAB b)dvb c)tetra d)umts Question & Answers Part A Questions 1. What is Network and Switching subsystem? The heart of the GSM is formed by the Network and Switching System (NSS). NSS consists of the following switches and databases: Mobile Services switching center(msc) Home Location register (HLR) Visitor Location Register (VLR) 2. What is authentication centre? As the radio interface and mobile stations are particularly vulnerable a separate AuC has been defined to protect user identity and data transmission. The AuC contains the algorithms for authentication as well as the keys for encryption and generates the values needed for user authentication in the HLR. The AuC may, in fact, be situated in a special protected part of the HLR. 3. What is called burst and normal burst? Data is transmitted in small portions called bursts, normal burst are used for data transmission inside a slot (user and signaling data). 4. What are the basic groups of logical channels? GSM specifies 2 basic groups of logical channels, Traffic channels 27

28 Control channels 5. Define traffic multiframe and control multiframe? The periodic pattern of 26 slots occurs in all TDMA frames with a TCH. The combination of these frames is called traffic multiframe. TDMA frames containing data for the other logical channels are combined to a control multiframe. 6. What is OVSF? Using orthogonal codes separates the different data streams of a sender UMTS uses so-called Orthogonal Variable Spreading factor codes (OVSF). 7. Specify the steps perform during the search for a cell after power on? Primary synchronization Secondary synchronization Identification of the scrambling code 8. Explain about transparent mode? The transparent mode transfer simply forwards MAC data without any further processing. The system then has to rely on the FEC which is always used in the radio layer. 9. What are the basic classes of handovers in UMTS? UMTS has 2 basic classes of handovers, Hard handover Soft handover 10. Specify the security services offered by GSM. There are three security services offered by GSM.They are: Bearer Services. Tele Services. Supplementary Services. 11. What is the frequency range of uplink and downlink in GSM network? The frequency range of uplink in GSM network is MHz. The frequency range of downlink in GSM network is MHz. 12. What are the two basic groups of logical channels in GSM? The two basics groups of logical channels in GSM are: Traffic channels (TCH). Control Channels (CCH). 13. What are the control channel groups in GSM? The control channel groups in GSM are: Broadcast control channel (BCCH). Common control channel (CCCH). Dedicated control channel (DCCH). 14. List out the numbers needed to locate an MS and to address the MS. The numbers needed to locate an MS and to address the MS are: Mobile station international ISDN number (MSISDN). International mobile subscriber identity (IMSI). Temporary mobile subscriber identity (TMSI). Mobile station roaming number (MSRN). 15. What are the four possible handover scenarios in GSM? The four possible handover scenarios in GSM are: Intra-cell handover. Inter-cell, intra-bsc handover. 28

29 Inter-BSC, intra-msc handover. Inter MSC handover. 16. What are the security services offered by GSM? The security services offered by GSM are: Access control and authentication. Confidentiality. Anonymity. 17. What is meant by GGSN? GGSN is Gateway GPRS Support Node. It is the inter-working unit between the GPRS network and external packet data networks. The GGSN is connected to external networks via the Gi interface and transfers packets to the SGSN via an IP-based GPRS backbone network. 18. What is meant by SGSN? SGSN is Serving GPRS Support Node. It supports the MS via the Gb interface. The SGSN is connected to a BSC via frame relay. 19. What is meant by BSSGP? BSSGP is Base Station Subsystem GPRS Protocol. It is used to convey routing and QoS-related information between the BSS and SGSN.BSSGP does not perform error correction and works on top of a frame relay network. 20. Define the protocol architecture of DECT. The protocol architecture of DECT consists of three layers. Theyare: Physical Layer. Medium Access Layer. Data Link Control Layer. Network Layer. The first three layers are common for both Control Plane (C-Plane) and User Plane (U-Plane). The network layer has been specified only for U-Plane, so that user data from layer two is directly forwarded to the U-Plane. 21. Specify the standards offered by TETRA. TETRA offers two standards. They are: Voice+Data (V+D). Packet Data Optimized (PDO). 22. How many ITU standardized groups of 3G radio access technologies are there in IMT-2000? There are five groups. They are: IMT-DS IMT-TC IMT-MC IMT-SC IMT-FT 23. What are the steps perform during the search for a cell after power on? The steps perform during the search for a cell after power on is: Primary Synchronization. Secondary Synchronization. Identification of the scrambling code. 24. What are the two basic classes of handover? The two basic classes of handover are: Hard Handover. Soft Handover. 29

30 25. What are the two basic transport mechanisms used by DAB? The two basic transport mechanisms used by DAB are: Main Service Channel (MSC). Fast Information Channel (FIC). 26. What are the two transport modes defined for MSC? The two transport modes defined for MSC are: Stream Mode Packet Mode. 27. Define the terms: i. Earth Station. ii. Uplink. iii. Downlink. Earth Station:The antenna systems on or near the earth are referred to as Earth Station. Uplink:A transmission from an earth station to the satellite is referred to as Uplink. Downlink:A transmission from the satellite to the earth station is referred to as Downlink. 28. Define Elevation Angle. The Elevation angle is the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directlyat the satellite. 29. What are the factors limited the number of sub channels provided within the satellite channel? There are three factors limited the number of sub channels provided within the satellite channel. They are: Thermal Noise. Intermodulation Noise. Crosstalk. Part B Questions 1. Explain GSM architecture? Mobile services Bearer services Tele services Supplementary services System architecture Radio subsystem Network and switching subsystem Operation subsystem 2. Explain Protocol architecture? Protocol architecture Physical Layer Medium access control Layer Data link control layer 3. Explain briefly about TETRA? The 2 standards are, Voice Data Packet data optimized (PDO) 30

31 The system architecture of TETRA is similar to GSM. TETRA offers 2 channels similar to GSM as, Traffic channels control channels 4.Write brief about UMTS and IMT-2000? UMTS releases and standardization UMTS system architecture UMTS radio interface 5.Explain about UTRAN? Radio network controller User equipment Core network Hand over 6. Explain Satellite networks in detail. Basic concepts Parameters and configurations Capacity Allocation-FDMA,TDMA FAMA DAMA 7. Write short notes on DAB. MSC FIC DAB Frame Structure Components of DAB sender Multimedia Object Transfer Protocol 8. Write short notes on DVB. DVB data broadcasting DVB for high-speed internet access 9. Explain DECT. DECT definition System architecture Protocol architecture 10. Explain about GSM 31

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34 References