Global System of Mobile Communication (2G)

2. Global System of Mobile Communication (2G) In 1982, the Conference of European Posts and Telegraphs (CEPT) nominated a group called the GroupeSpécial Mobile (GSM) to develop a public land mobile system that could operate across Europe with the objectives of:

3. Low mobile device and service cost • Good speech quality • International roaming capability • Ability to support handheld mobile devices • Extensibility for adding new services and facilities • Compatibility with the ISDN

4. The GSM system supports a variety of data services at rates upto 9600 bps. The GSM network is also capable of supporting call forward (such as call forwarding when the mobile subscriber is unreachable by the network), call barring of outgoing or incoming calls, caller identification, call waiting, and multi-party conversations. Fig.15.4 shows the layout of a generic GSM network. A GSM network consists of three major subsystems:

6. The mobile Station: • In the GSM network the Mobile Station (MS) consists of the equipment, also often referred to as the terminal, and a removable Subscriber Identity Module (SIM) in the form of a smart card. The SIM card offers personal or identity mobility. • In the GSM network the mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI) assigned at the time of manufacturing.

7. The SIM card identity is independent of the IMEI. It uses the International Mobile Subscriber Identity (IMSI) for identifying the subscriber to the system, a secret key for authentication and other information. The independence of IMEI and the IMSI and the use of IMSI alone to identify the subscriber on the GSM network provide personal mobility with regards to the mobile equipment. The SIM card also has a provision for protection against unauthorized use by use of a password or Personal Identity Number (PIN).

8. The Base Station Subsystem • The base station subsystem is made up of two important components, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). • Base transceiver station is typically a radio transceiver that operates within a cell defined by the power and footprint of the antenna used. It deploys and communicates with the mobile station through radio link protocols.

9. Large and dense cellular networks may deploy a large number of BTSs, thus the requirements for BTS are ruggedness, reliability, portability, and minimum cost. One or more of base transceiver stations operating in a cell, are controlled by a base station controller. It manages the radio resources for the BTS, radio-channel setup, frequency hopping, and handovers. On the other hand, the BSC is connected to the Mobile service Switching Center (MSC).

10. The Network Subsystem: • The MSC forms the core of the network subsystem. It works like any ISDN or PSTN switching centre and performs the switching of calls between the mobile users, and between mobile and fixed network users. In addition to the normal call switching functions, it also handles mobility management. The information on the registration; authentication; location; call handovers; routing, in case of roaming users, are all handled by the MSC uses four databases, viz., home location register, visitor location register, authorization, and equipment identity register.

11. The Home Location Register (HLR) maintains registration and the required administrative information for all subscribers registered in the GSM network along with the current location of the mobile. The location of the mobile device is typically stored as the signaling address used by the Visiting Location Register (VLR) associated with the mobile station. The home location register, along with the current location and other information of the VLR, is used for managing roaming and call routing.

12. Spectral Allocation The GSM uses 25 MHz for the mobile device to base station transmission (uplink) and an additional 25 MHz for the base station to the mobile device (downlink) transmission. The International Union (ITU), the managing body for the international allocation of radio spectrum, allocated the bands 890 -915 MHz for uplink and 350 -960 MHz for downlink transmission for mobile network in Europe.

13. The allocation of 25 MHz for the analog system had reserved 10 MHz for future use. GSM networks were initially built using this 10 MHz and later expanded to full spectrum.

14. Multiple Access GSM networks use a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). The 25 MHz of limited radio spectrum allocated for the use in GSM networks is shared by all users by dividing the bandwidth among as many users as possible. GSM networks divide up the 25 MHz radio spectrum in 124 carrier frequency channels that are allotted 200 KHz each. Each base station is allotted at least one or more carrier frequencies. Each base station uses TDMA by dividing the carrier channel in to time slots.

15. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 micro second (or approximately 0.577 micro seconds). Eight burst periods are grouped into a TDMA frame (120/26 micro second, or approximately 40615 micro second), which forms the basic unit for the definition of logical channels.

16. One physical channel is one burst period per TDMA frame. Channels are defined by the number and position of their corresponding burst periods. All these definitions are cyclic, and the entire pattern repeats approximately every 3 hours. Channels can be divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode.

17. 3G Networks The enhanced data rates offered by EDGE through the evolution of second generation (2G) GSM and TDMA networks were still not fast enough for many multimedia mobile applications. The wireless network technology offered the next generation (3G) of solutions that provides high speed bandwidth to handheld devices.

18. Third generation (3G) networks are derived from the Universal Mobile Telecommunications Services (UMTS) for high speed networks that enable a variety of data intensive applications. • CDMA200 A third generation solution for mobile networking that evolved from existing wireless standard is CDMA it is also known as IMT IS-95. It supports 3G services as defined by the International Telecommunications Union (ITU) for IMT-2000.

19. W-CDMA Wideband Code – Division Multiple Access is a standard defined by the ITU standard and is derived from Code – Division Multiple Access (CDMA) standard. The standard is officially called IMT-2000 direct spread. It is a 3G mobile wireless technology that supports high speed transfers to mobile and portable wireless devices.

20. In the local area access mode it supports data rates of 2 Mbps for transferring multimedia information. In WCDMA the signal is coded and transmitted in spread – spectrum mode over a 5 MHz carrier band compared to 200 KHz carrier band used for CDMA.

21. In addition to these important widely adopted standards there are several variants that are also in use. The high data transfer rates offered by 3G networks is capable of running multimedia services that combines voice and data. The following data rates are supported by 3G wireless networks: • 2.05 Mb per second to stationary devices. • 384 Kb per second for slowly moving devices, such as a handset carried by a walking user. • 128 Kb per second for fast moving devices, such as handsets in moving vehicles.

22. These data rates are highest achievable under exclusive use conditions. This means that in case of delivery to a stationary device, the 2.05Mb per second rate is achieved when one user occupies the entire capacity of the base station. Thus the normal work load environment data rates attained are lower if there is any other traffic. The actual data rates achieved by a user in practice depend upon the number of calls and other traffic in progress.

23. 3G Standard • The International telecommunications Union (ITU) has worked out certain standards for 3G networks. CDMA has emerged as the leading mechanism for 3G. The five ITU approved 3G standards are as follows: • CDMA 2000 • WCDMA • TD-SCDMA • FDMA/TDMA • TDMA-SC (EDGE)

24. CDMA uses a spread spectrum mechanism. In the spread spectrum, a message consisting of Y bits per second is converted into a longer message of kY bits and then transmitted at a higher rate. The k is called the spreading factor. The spreading of messages seems counter intuitive for attaining higher rates. The spread spectrum has been used in military communication as it provided immunity from jamming signals.

25. In CDMA each transmitting entity uses a unique code assigned to it. The coding scheme uses the user code for transmitting 1 and its complement for transmitting a 0. The data bit stream is converted into a coded bit stream and transmitted using the full frequency spectrum rather than a limited frequency slot, as in FDMA, or time slot, as in TDMA.

26. Some of the important features of 3G networks are: • The new ratio spectrum relieves the overcrowding in existing systems. • It provides more bandwidth because the same frequencies can be used by more than one pair of users. • The adoption of 3G network based on IP packets offers better interoperability between service providers. • The standard supports fixed and variable data rates.

27. The 3G networks have devices that are backward compatible with those of existing networks. • It offers support to always-on devices as it provides packet-based services using internet protocol packets. • The high data transfer rates support the smooth functioning of multimedia services. • Although some degree of backward compatibility is supported, the cost of upgrading base stations and cellular infrastructure to 3G is very high.

28. Handsets that can use 3G services are complex products. The higher power talks time and larger batteries. Thus, through miniaturization of technology will alleviate the problem, handsets and exited higher cost. • Base stations need to be closer to each other, which implies that service providers will incur more cost.

29. Wireless Access Protocol (WAP) The WAP protocol is the leading standard for information services on wireless terminals like digital mobile phones. WML is the language used to create the pages displayed in a WAP browser. The wireless application protocol (WAP) is the bridge that assists in developing technology independent access to the Internet and telephony services from wireless devices. It provides a mobile device user with the ability to access the same set of information available on the Internet, Intranets, or through the World Wide Web. That they could access through their desktops.

30. Since earlier attempts to provide internet access from wireless devices used proprietary protocols and technology, they were limited by the capability of wireless networks and handheld devices. WAP addresses these issues by developing a standard architecture for wireless access to net by utilizing the Internet standard protocols with suitable modifications. The wireless environment faces distinct constraints of lower connection, stability, higher latency and lower available bandwidth.

32. The architecture for building systems with wireless application protocol utilizes Wireless Markup Language (WML) and WML Script to produce content suitable for WAP enabled devices that makes optimal use of small displays and makes one hand navigation possible. WAP is a lightweight protocol requiring only the minimal resources available on the devices to produce scalable content offering deftly adaptively from one two line text displays available on basic devices to graphic screens available on palmtops and newer phone devices.

33. The client, i.e., mobile devices, uses the lightweight WAP stack to communicate with the WAP gateway for sending the URL through the wireless system operator’s network to the WAP gateway and a WAP browser that can interpret the binary codes of compact WML and the WML script content delivered to it. The WAP gateway is the interface that interconnects the wireless service operators’ network with the internet. The requests received from mobile devices are transformed to Hyper Text Transfer Protocol (HTTP) and submitted to the Internet hosts.

34. WAP is a layered protocol consists of following layers: • Wireless Application Environment (WAE) • Wireless datagram Protocol (WDP) • Wireless transaction Protocol WTP) • Wireless transport Layer security (WTLS) • Wireless Session Protocol (WSP), and • Bearer networks

35. The Wireless Access protocol operates over a variety of wireless bearer mechanisms, such as GSM’s GPRS and EDGE, CDMA, CDPD, IS-136, and iDEN. The WAP works on a variety of bearer networks which may support the packet, or connection oriented services. Users of WAP are shielded from the details of the bearer network. The various protocol layers and the application environment of WAP that offer bearer network transparency to applications are described as follows:

37. Wireless Datagram Protocol (WDP) The WDP has to directly deal with the heterogeneous bearer network environment. One of the important functions WDP has to perform is to offer the higher layers of the protocol a consistent interface irrespective of the underlying bearer. the bearer may or may not support the Internet Protocol (IP) services. In case of bearers with IP support it uses the User Datagram Protocol (UDP).

38. In case IP less bearers such as GSM, it follows the WAP specification to carry out the function. Thus WDP provides operational transparency over one of the available bearer services, thereby making the upper layers of the WAP stack independent of the bearer. WDP accomplishes operational transparency over the widely varying services offered by the bearer through the adaptation sub layer. The adaptation layers map WDP functions to services offered by different bearers. In case where the bearer is IP capable, WDP functions in exactly in the same manner as the standard User Datagram Protocol (UDP) of the Internet.

39. Wireless Transaction Layer Security (WTLS) • This is operational layer implemented over WDP, offers a secure transport service interface to higher layers in order to preserve the transport service interface of WDP. the WTLS layer provides end-to-end security features, which includes: • Confidentiality using data encryption algorithms. • Data integrity using message authentication codes. • Authentication through digital certificates • Non-repudiation also through digital certificates and message authentication codes.

40. WTLS is derived from the Internet standard TLS protocol. It offers standard connection security and also optimizations through on-the-fly payload compression to increase the effectiveness of datagram service running on a low-bandwidth network.

41. Wireless Transaction Layer (WTP) In the WTP layer context a transaction is defined as a request/response. The responsibility of the layer is to offer an efficient transaction service over the secure as well as insecure datagram service. It is a lightweight transaction service that supports a request/response service. The transaction services offered by the WTP can be put in the following three class of services:

42. Class 0: Unreliable push service • Class 1: reliable push service • Class 2: reliable transaction service • Unreachable push service is a one-way communication service that does not bother to resend the request in case it is lost in transaction. Reliable push service, on the other hand, waits for acknowledgement from the receiver and in case of lost request/ timeout way service in which a data request is sent and the sending stack waits for the result.

43. On receiving the result of the request the acknowledgement is sent. Reliable service at is accomplished this layer by selective retransmission and duplicate removal. Additionally, like the TCP in the internet protocol stack, WTP is also responsible for taking care of segmentation / reassembly of larger packets port number addressing,

44. user-to-user reliability in addition to protocol acknowledgement, asynchronous transactions, optimal out-of-band information, delayed acknowledgements, and message concatenation to improve over-the-air efficiency. WTP is massage oriented protocol, which makes it suitable for interactive browsing applications.

45. Wireless Session Protocol (WSP) The WSP layer is a stripped down version of the Internet standard, Hyper Text transfer Protocol (HTTP / 1.1). One of the important features of this protocol is to support the suspension and resumption of a session. In an unstable connection situation that is prevalent in the mobile environment, users who may be disconnected can continue the operation from exactly the same point where the device had been disconnected.

46. Context encoding, for efficiently transferring the contents in a low bandwidth environment, is also addressed by the layer. the following functionalities are offered and addressed by this layer: • protocol feature negotiation (capability negotiation) • compact encoding of data • session suspend / resume • long lived session states • asynchronous requests • common facility for confirmed data push.

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