Evolution Data Service In Gsm

Evolution Data service in GSM Ali Nabeel Ibraheem Email: [email protected] ABSTRACT

INTRODUCTION

Enabling wireless internet access at data rates comparable to wired network, is a growing concern in recent years. As technology improved, it become very costly for the service providers to replace the whole infrastructure. Therefore, incremental improvement to existing networks provided an interim step with significant benefit to users. Global System for Mobile (GSM) is a second generation cellular system standard that was developed to solve the fragmentation problems of the first cellular systems in Europe. GSM services follow ISDN guidelines and are classified as either teleservices or data services. But the GSM are circuit switched- at air interface, a complete traffic channel is allocated to a user for the entire duration of a call. It will remain idle, in case there is no data to be transmitted in certain intervals during call. This limits both the data rates and the number oh users that can be supported by circuit switched systems. Also the connection setup can take up to several seconds, and data rates are restricted to 9.6 Kbit/s. the search for a more efficient handling of the air-interface leads us to GPRS. GPRS provided a data delivery mechanism on the GSM network with higher bandwidth. Regarded as a subsystem within the GSM standard, GPRS has introduced packet-switched data into GSM networks. Many new protocols and new nodes have been introduced to make this possible. GPRS allows data rates of 115 kbps and, theoretically, of up to 160 kbps on the physical layer. Enhanced Data-rate for Global Evolution, (EDGE) is the next step in the evolution of GSM and IS- 136. The objective of the new technology is to increase data transmission rates and spectrum efficiency and to facilitate new applications and increased capacity for mobile use. With the introduction of EDGE in GSM phase 2+, existing services such as GPRS and high-speed circuit switched data (HSCSD) are enhanced by offering a new physical layer. EDGE is introduced within existing specifications and descriptions rather than by creating new ones. EDGE is a method to increase the data rates on the radio link for GSM. Basically, EDGE only introduces a new modulation technique and new channel coding that can be used to transmit both packetswitched and circuit-switched voice and data services. In this project will focuses on the packet-switched GPRS and introduce the new technique that used in enhanced GPRS which called EGPRS. KEYWORDS

Commercial mobile network were launched in the mid-1980s. Since then the mobile communication world has been witnessing rapid changes marked by significant improvement in the services being offered. Satisfying consumer demands for better and improved services, and generating more revenue for the operator have been the area focus. On the other hand, the communication world has witness an equally significant growth in the internet arena. Internet popularity has grown manifold over the same period, and therefore it comes as no surprise that these technological marvels-the internet and mobile network-are witnessing a merger of sorts. Evolution of GSM network to GSM/GPRS network is a means to provide mobile communication network. The General Packet Radio service (GPRS) network does not exist in isolation but conjunction with a GSM network. While the GSM network provide the conventional circuit-switch services (voice and circuit switched data services), GPRS network provides an efficient means to support packet-data services. The GPRS network can therefore be used to provide an existing GSM subscriber an efficient mechanism to access the internet. Since the GPRS network is built over an existing GSM network, understanding how GSM network operates really help the interpret the deeper principle associated a GPRS network Enhanced Data Rates for Global evolution (EDGE) is a highspeed mobile data standard that can be introduced in the GSM and the GPRS system. EDGE is considered as a 2.5 G standard, a transition between 2G and 3G. The evolution of GPRS toward EDGE is known as Enhanced GPRS (EGPRS). It is also known as EDGE classic. GPRS allows data rates of 115 kbps and, theoretically, of up to 160 kbps on the physical layer. EGPRS is capable of offering data rates of 384 kbps and, theoretically, of up to 473.6 kbps. A new modulation technique and error-tolerant transmission methods, combined with improved link adaptation mechanisms, make these EGPRS rates possible. This is the key to increased spectrum efficiency and enhanced applications, such as wireless Internet access, email and file transfers. EDGE is therefore an add-on to GPRS and cannot work alone. GSM The GSM standard was developed by the Group Special Mobile, which was an initiative of the Conference of European Post and Telecommunications (CEPT) administrations. The underlying aim was to design a uniform pan-European mobile system to replace the existing incompatible analog systems. Work on standard was started in 1982, and the first full set of specifications (phase 1) became available in 1990. The responsibility for GSM standardization now resides with

ETSI, GMSK, GSM, GSN , GPRS, SGSN, BSC, BSS, BTS, MS, PCU, HLR,VLR,MSC,AUC, USF, TBF,MAC, RLC, LLC

special mobile group (SMG) under the European telecommunication standards institute (ETSI), and revisions/enhancements to various aspects of GSM standard are being carried out in SMG technical subcommittees. The characteristics of the initial GSM standard include the following:  Fully digital system utilizing the 900 MHZ frequency band  TDMA over radio carrier (200 KHZ carrier spacing)  8 full-rate or 16 half-rate TDMA channel per carrier  User/terminal authentication for fraud control  Encryption of speech and data transmissions over the radio path  Full international roaming capability  Low speed data services (up to 9.6 Kb/s)  Compatibility with ISDN for supplementary services  Support of short message service (SMS) GSM Architecture A GSM system consists of a fixed installation infrastructure and the mobile subscribers. The fixed installation GSM network can be subdivided into three subsystems. The three subsystems are the Base Station Subsystem, (BSS), the Network and Switching Subsystem, (NSS), and the Operation and Support System, (OSS). The base station subsystem (BSS) is made up of the base station controller (BSC) and the base transceiver station (BTS). The base transceiver station (BTS): GSM uses a series of radio transmitters called BTSs to connect the mobiles to a cellular network. Their tasks include channel coding/decoding and encryption/decryption. A BTS is comprised of radio transmitters and receivers, antennas, the interface to the PCM facility, etc. The BTS may contain one or more transceivers to provide the required call handling capacity. A cell site may be omnidirectional or split into typically three directional cells. . The base station controller (BSC): A group of BTSs are connected to a particular BSC which manages the radio resources for them. Today's new and intelligent BTSs have taken over many tasks that were previously handled by the BSCs. The primary function of the BSC is call maintenance. The mobile stations normally send a report of their received signal strength to the BSC every 480 ms. With this information the BSC decides to initiate handovers to other cells, change the BTS transmitter power, etc. Network Subsystem which consist The mobile switching center (MSC): An act like a standard exchange in a fixed network and additionally provides all the functionality needed to handle a mobile subscriber. The main functions are registration, authentication, location updating, and handovers and call routing to a roaming subscriber. The signaling between functional entities (registers) in the network subsystem uses Signaling System 7 (SS7). If the MSC also has a gateway function for communicating with other networks, it is called Gateway MSC (GMSC).

The home location registers (HLR): A database used for management of mobile subscribers. It stores the international mobile subscriber identity (IMSI), mobile station ISDN number (MSISDN) and current visitor location register (VLR) address. The main information stored there concerns the location of each mobile station in order to be able to route calls to the mobile subscribers managed by each HLR. The HLR also maintains the services associated with each MS. One HLR can serve several MSCs. The visitor location register (VLR): Contains the current location of the MS and selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. A VLR is connected to one MSC and is normally integrated into the MSC's hardware. The authentication center (AUC): A protected database that holds a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel. The AUC provides additional security against fraud. It is normally located close to each HLR within a GSM network. The equipment identity register (EIR): The EIR is a database that contains a list of all valid mobile station equipment within the network, where each mobile station is identified by its international mobile equipment identity (IMEI). The EIR has three databases: White list: for all known, good IMEIs Black list: for bad or stolen handsets Grey list: for handsets/IMEIs that are uncertain. The Operation and Maintenance Subsystem (OMS) is a management system that oversees the GSM functional blocks. The OMC assists the network operator in maintaining satisfactory operation of the GSM network. Hardware redundancy and intelligent error detection mechanisms help prevent network down-time. The OMC is responsible for controlling and maintaining the MSC, BSC and BTS. It can be in charge of an entire public land mobile network (PLMN) or just some parts of the PLMN.

MODULATION AND CODING in GSM

For

The figure below depicted the sequence of operations from speech to radio waves and from radio waves to speech.

Where Q(t) is Q-function

Bb is the bandwidth of the low pass filter having a Gaussian shaped spectrum. T is the bit period.

The full rate speech codec in GSM is described as Regular Pulse Excitation with Long Term Prediction (GSM RPE-LTP). Basically, the encoder divides the speech into short-term predictable parts, long-term predictable part and the remaining residual pulse. Then, it encodes that pulse and parameters for the two predictors. The decoder reconstructs the speech by passing the residual pulse _first through the long-term prediction filter, and then through the short-term predictor. then will follow to Channel coding which adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission. An interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors. Being the errors less concentrated, it is then easier to correct them. The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK).it is derived from MSK. Minimum Shift Keying (MSK) is derived from OQPSK by replacing the rectangular pulse in amplitude with a half-cycle sinusoidal pulse Because of the effect of the linear phase change, the power spectral density has low side lobes that help to control adjacent-channel interference. However the main lobe becomes wider than the quadrature shift keying. Obviously other pulse shapes are possible. A Gaussian-shaped impulse response filter generates a signal with low side lobes and narrower main lobe than the rectangular pulse. The Gaussian low-pass filter has an impulse response given by the following equation

The relationship between the premodulation filter bandwidth, B and the bit period, T defines the bandwidth of the system. GSM designers used a BT = 0.3 with a channel data rate of 270.8 kbs. This compromises between a bit error rate and an out-ofband interference since the narrow filter increases the intersymbol interference and reduces the signal power. GPRS The General Packet Radio Service (GPRS), like the name suggests is a mobile communication standard based on packet switched radio transmission. The main feature that gives it an edge over the existing circuit switched technology like GSM is its handling of the radio resource. Traffic channels is allotted only when needed and is released immediately after the transmission of packet is over. GPRS also allow for a user to be allotted multiple channels, leading to higher data rates. The data rate up to 180 K bit/s can thus achieved. GPRS call set up times are typically less than a second. Billing is based on the volume of traffic rather than connectivity and the users can thus be connected throughout. GPRS Architecture GPRS maintain the core GSM radio access technology and provides packet data services by introducing two new network elements called GPRS support node (GSN) and gateway GPRS support node (GGSN). In addition, the GPRS register, which may be integrated with the GSM HLR, maintain the GPRS subscriber data and routing information The SGSN is responsible for keeping track of a MS in the system. Therefore it also handles packet routing inside and between PLMNs (Public Land Mobile System, i.e. one operator‟s GSM system is a PLMN). Security functions are also implemented here. The GGSN provides an interface with the PDN. It converts the GPRS packets received from SGSN into the appropriate format of the external network (typically internet protocol IP network). In the reverse path, the GGSN converts the incoming packet to the GPRS packet and delivers it to the destined MS using PDP context stored by it. A GPRS cell phone will transmit data in a

packet switched mode, but voice will be transmitted as circuit switched calls. This means that there has to be a network element that distinguishes between the two different kinds of transfer and diverts each to the correct core network, i.e. towards the MSC or towards the SGSN. This is one of the main functions of the packet control unit (PCU). Other PCU functions include: Packet segmentation and reassembly, on the downlink and uplink respectively, Access control, scheduling for all active transmissions including radio channel management, Transmission control (checking, buffering, retransmission). The PCU can be placed at various different positions in the network: at the BTS site, in the BSC or right before the switch, the preferred location being at the BSC. the PCU is located in the Packet Control Unit Support Node (PCUSN), which resides between the BSC and the Serving GPRS Support Node (SGSN).When implementing GPRS in an existing GSM network, every BTS requires a software upgrade so as to be capable of performing GPRS specific functions such as the use of CS-1 to CS-4, support of the GPRS radio channel measurements. This software upgrade is specified as the channel codec unit (CCU) and is always located in the BTS. It can be necessary that the CCU takes over some of the PCU radio management functions if the latter is not located in the BTS itself.

GPRS Procedures Before data can be transferred between the MS and the external data network, some preparation is necessary to enable the transfer of IP packets through the GPRS network to take place. There are three important steps involved : 1. The MS must be attached in the GPRS network. The procedure for this is called „GPRS attach‟. This is a logical procedure between the MS and the SGSN which takes note of the position (i.e. the „routing area‟) of the MS. Storing and updating the position of the MS is particularly important for DL transmissions because this information enables the GPRS network to locate the MS.

2. How is the right path found for the IP packets inside the GPRS network? A connection between the MS and the GGSN must be set up, We speak of the activation of a PDP context. After that procedure each node in the GPRS network knows how it has to forward the IP packets of this MS. 3. The path between the MS and the external data network is prepared, so IP packets can be sent through the GPRS network towards the destination address.

The GGSN connect with SGSN through IP backbone over which the packets are tunneled. Using GPRS tunneling protocol (GTP). The user payload carried as an IP packet is encapsulated within the GPRS network. In other word, the source and destination address of the user IP packet is not relevant for packet routing. The IP packet is just like an application payload and is carried transparently between the Ms and GGSN. frame relay is used as network service layer to carry RLC/MAC between the PCU and the SGSN. Over the network service layer is the BSS GPRS protocol (BSSGP) that uses the frame relay to provide virtual connections between the BSS and the SGSN. The BSSGP also convey routing and Qos-related information between the BSS and the SGSN.

The messages concerning the GPRS attach procedure between the MS and the SGSN belong to the protocol GPRS Mobility Management GMM. Other examples of GMM are the update of the MS‟s position (routing area update), switching off of the mobile device (GPRS detach), authentication, and GPRS paging. Another important protocol between the MS and the SGSN is the Logical Link Control LLC which provides a logical connection between the MS and the SGSN. It is defined by the Data Link Connection Identifier DLCI which consists of the Temporary Logical ink Identifier TLLI and the Service Access Point Identifier (SAPI). The TLLI identifies the specific MS and the SAPI identifies the service access point (SNDCP, GMM/SM or SMS) – i.e. whether the data in the packet‟s payload concerns user data, signaling or SMS. In the case of a

GPRS attach procedure the SAPI shows that it concerns GMM/SM (SM = Session Management). A subscriber requests the activation of a PDP context (PDP = Packet Data Protocol) when he wants to start a GPRS service. Using a play on the PDP context activation, a mobile station has attached itself to a SGSN in the GPRS Public Land Mobile Network (PLMN). The mobile station has been assigned a TLLI that the wireless network knows. However, the external network nodes (IP or X.25) do not yet know of the mobile station. Therefore, the mobile station must initiate a PDP context with the GGSN. Both the SGSN and the GGSN are identified by IP addresses. A many-to- many relationship exists between the SGSN and the GGSN. Multiple tunnels (used for secure data transfer between the SGSN and the GGSN) may exist between a pair of GGSNs, each with a specific tunnel identifier (TID). Four steps are involved in the activation process: 1. The mobile station first sends an Activate PDP Context Request message that contains the following: NSAPI PDP type PDP address, whether it is a static or dynamic address (IP address) Requested QoS (best effort is all that is currently available, but will get to specific QoS in the future) Access Point Name (APN) (optional) to select a certain GGSN, either the IP address or logical name is used PDP configuration options. 2. The SGSN chooses the GGSN based on information provided by the mobile station and other configurations and requests the GGSN to create a context for the mobile station. The SGSN will select a GGSN that serves the particular type of context needed (such as one for IP network access and one for X.25 access). 3. The GGSN replies to the SGSN with the TID information. It also updates its tables wherein it maps the TID and the SGSN IP addresses with the particular mobile associated with them. 4. The SGSN sends a message to the mobile station informing it that a context has been activated for that particular mobile. The SGSN also updates its tables with the TID and the GGSN IP address with which it has established the tunnel for the mobile. RLC/MAC Block Network Layer Protocol Data Units (NL-PDU) are transmitted over the air interface by using the Logical Link Control (LLC) and the RLC/MAC protocols. The Sub-network-Dependent Convergence Protocol (SNDPC) transforms packets into LLC frames. LLC frames (currently variable up to a maximum of 1,600 octets) are then segmented into RLC data blocks (or RLC/MAC control blocks), which are formatted by the physical layer into blocks of four successive time slots on the same physical channel (one per frame). The rate of RLC/MAC data blocks is one block every 20 ms. The MAC layer provides capability for multiple mobile stations to share a common transmission medium. It interfaces directly with the physical layer. The MAC layer uses several identifiers

to transfer data. A brief description of two of them is listed in the following section. Temporary Block Flow Used to identify a series of RLC/MAC blocks to/from a specific mobile station. The TBF is unique for a given direction (uplink/downlink). Each mobile station occupying a radio resource is assigned a TBF for the duration of the data transfer. Because data transfers are typically bursts of data followed by idle time, the TBF is temporary; it only lasts until all RLC/MAC blocks have been transferred and acknowledged. Temporary Flow Identity Uniquely identifies each TBF for a given direction. The TBF, TFI, and direction uniquely identify a RLC data block. The message type together with the TBF, TFI, and the direction designates the RLC/MAC control message. Data Burst 1 The MAC layer in the mobile station receives an LLC frame that is ready for transfer. The mobile station communicates with the network and ultimately receives a TFI that will be used to identify all consecutive (that is, one data burst) RLC blocks that are transferred. The MAC layer then segments the LLC frame and encapsulates it with an RLC header containing the TFI. Once all RLC blocks have been transferred and acknowledged (in the ACK mode), the TFI is released. At this point, the radio resources are not required; the TBF no longer exists. Idle The mobile station has no data to transfer even though GPRS data services remain active. Data Burst 2 The mobile station has additional data to transfer. It notifies the network in order to establish another TBF. The TFI corresponding to this TBF will most likely be different from that corresponding to the first TBF. Again, once all RLC blocks have been transferred and acknowledged, the TFI is released and the TBF disappears. The RLC layer is primarily responsible for segmenting and reassembling data sent over the air interface. The frames used in the LLC layer are much too big to send over the air. Thus, the RLC layer segments or breaks the LLC frame into blocks, and encapsulates each block forming an RLC block. A BSN designates each RLC block. The BSN is contained in a field of the block header. Upon receipt of an RLC block, the RLC layer reverses the action required to send the data. First, the BSN is used to arrange the RLC block in sequential order. Then the header is stripped off the block and the blocks are reassembled into LLC frames. This layer supports two modes of operation: acknowledged and unacknowledged. The acknowledged mode enables selective retransmission. In this mode, the BSN is also used to request the retransmission of a missing or undelivered block. The unacknowledged mode of operation does not guarantee the arrival of the transmitted RLC blocks. This mode is important to applications that require a constant delay. The RLC layer increases the reliability of the air interface by providing BEC, which enables selective transmission. The RLC data block consists of the RLC header, RLC data field, and spare bits. Each RLC data block may be encoded using any of the available coding schemes (CS-1, CS-2, CS-3, or CS-4) and will affect the degree of segmentation and reassembly released.

Enhanced Data Rates for GSM Evolution EDGE is the next step in the evolution of GSM and IS136. The objective of the new technology is to increase data transmission rates and spectrum efficiency and to facilitate new applications and increased capacity for mobile use. With the introduction of EDGE in GSM phase 2+, existing services such as GPRS and high-speed circuit switched data (HSCSD) are enhanced by offering a new physical layer. EGPRS is capable of offering data rates of 384 kbps and, theoretically, of up to 473.6 kbps. A new modulation technique and error-tolerant transmission methods, combined with improved link adaptation mechanisms, make these EGPRS rates possible. This is the key to increased spectrum efficiency and enhanced applications, such as wireless Internet access, email and file transfers. EDGE leverages the knowledge gained through use of the existing GPRS standard to deliver significant technical improvements.

The modulation type that is used in GSM is the Gaussian minimum shift keying (GMSK), which is a kind of phase modulation. This can be visualized in an I/Q diagram that shows the real (I) and imaginary (Q) components of the transmitted signal as shown in figure below. Transmitting a zero bit or one bit is then represented by changing the phase by increments of + _ p. Every symbol that is transmitted represents one bit; that is, each shift in the phase represents one bit.

To achieve higher bit rates per time slot than those available in GSM/GPRS, the modulation method requires change. EDGE is specified to reuse the channel structure, channel width, channel coding and the existing mechanisms and functionality of GPRS and HSCSD. The modulation standard selected for EDGE, 8phase shift keying (8PSK), fulfills all of those requirements. 8PSK modulation has the same qualities in terms of generating interference on adjacent channels as GMSK.

Link Quality Control (LQC) is a term used for techniques to adapt the channel coding of the radio link to the varying channel quality. Different modulation and coding schemes are optimal during different situations, depending on the link quality. The LQC used for EDGE is performed through the techniques of 1. Link Adaptation (LA) 2.Incremental Redundancy (IR) LA provides a dynamic switching between coding and modulation schemes, so that the highest throughput (e.g., maximum user bit rate) according to the time-varying link quality (e.g. C/I) is achieved. With IR, information is first sent with very little coding. If decoding is successful, this will yield a very high user bit rate or throughput. However, if decoding is unsuccessful, then additional coded bits are sent until decoding is successful. The more coding bits that have to be sent, the less the resulting bit rate and the higher the delay. As a result of the link quality control, a low radio link quality will not cause a dropped data transfer, but only give a reduced bit rate for the user. CONCLUSION GPRS will promote and seamless advanced services and enable mobile operators to combine wireless networks with public and private/corporate networks. GPRS paves the way for migration to 3G that will enable high speed, universal communication services regardless of the terminal, network, or location .GPRS maintain the core GSM radio access technology and provides packet data services by introducing two new network elements called GPRS support node (GSN) and gateway GPRS support node (GGSN). In addition, the GPRS register, which may be integrated with the GSM HLR, maintain the GPRS subscriber data and routing information. GPRS will operates at transmission data rates from 14.4 to 115 kb/s by using from one to eight time slots in GSM TDMA structure. Enhanced Data rates for GSM Evolution (EDGE) is a further development of the GSM data services High-Speed CircuitSwitched Data (HSCSD) and GPRS and is suitable for circuitand packet-switched services. Applying modified modulation and coding schemes EDGE reaches very high raw bit rates of up to 69 kbit/s per GSM physical channel. The modifications mostly concern the RLC/MAC layer and the physical layer. Since these protocols are implemented in the MS and the Base Station (BS), both have to be modified. REFERENCES [1] PAJ PANDYA, “Mobile and personal communication systems and services” [2] Vijay K. Garg, Joseph E. Wilkes, “Principles & Applications of GSM” [3] Wayne Tomasi , “Electronic Communications Systems” [4] T.S. Rappaport, Wireless Communications : Principles and Practice , 2nd Edition, Prentice Hall [5] Timo Halonen,Javier Romero and Juan Melero, GSM, GPRS, AND EDGE Performance [6] REGIS J.”BUD” BATES, GPRS [7] Sumit Kasera, Nishit, A P Priyanka.”2.5G Mobile Networks: GPRS and EDGE”