Unit - 4

Second Generation Mobile Networks-GSM

Q1) Explain GSM services and features?

A1) GSM Services & Features

The system will provide service portability that is mobile stations (MSs) or mobile phone can be used in all participating countries. The system will offer services that exist in the wire line network as well as service specifies to mobile communication.

Basic Services

1 Voice services

2 Data Services

3 Short Message Services

GSM voice service started as a full rate voice service that allowed 8 users per GSM radio channel. The original design allowed for the use of a half rate voice service (lower quality audio) to increase the number of simultaneous GSM voice users to 16 per radio channel.

GSM Data services support two data service groups short message services & bearer services.

The GSM short message service (SMS) provides a connectionless transfer of message with low-capacity & low-time performance. Every GSM short message can contain up to 140 octets or 160 characters of GSM default alphabet as defined in GSM.

The GSM barer services are similar ISDN services except that the maximum data rate is limited to 9.6Kbps. These services can be employed on notebook PCs or PDAs connected to the MS. Most mobile operator offer short message service as part of the basic subscription package.

Additional Services

GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features.

1. Call forwarding—This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.

2. Barring of outgoing calls—This service makes it possible for a mobile subscriber to prevent all outgoing calls.

3. Barring of incoming calls—This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.

4. Advice of charge (AoC)—The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.

5. Call hold—This service enables the subscriber to interrupt an ongoing call and then subsequently re-establish the call. The call hold service is only applicable to normal telephony.

6. Call waiting—This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.

7. Multiparty service—The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.

calling line identification presentation/restriction—These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.

8. Closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

Q2) Mention all the GSM specifications?

A2) GSM Specifications

Before looking at the GSM specifications, it is important to understand the

following basic terms:

bandwidth—the range of a channel's limits; the broader the

bandwidth, the faster data can be sent

• bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to one byte

• frequency—the number of cycles per unit of time; frequency is measured in hertz (Hz)

• kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits per second

• megahertz (MHz)—1,000,000 hertz (cycles per second)

• milliseconds (ms)—one-thousandth of a second

• watt (W)—a measure of power of a transmitter

Q3) Explain architecture of GSM?

A3)

Fig.1 GSM Architecture

In this architecture, a mobile station (MS) communicates with a base station system (BSS) through the radio interface. The BSS is connected to the network and switching subsystem (NSS) by communicating with a mobile switching centre (MSC).

The Switching System

The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units:

• Home location register (HLR)—The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.

• mobile services switching center (MSC)—The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.

• visitor location register (VLR)—The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

• authentication centre (AUC)—A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

• equipment identity register (EIR)—The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations.

The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The Base Station System (BSS)

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

• BSC—The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A

number of BSCs are served by an MSC.

• BTS—The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC

-Mobile station (MS) – The MS consist of two parts the SIM (subscriber identity module) and the ME (mobile equipment)

An SIM can be

-          A smart card, usually size of credit card

-          A smaller sized ‘plug-in SIM” that can be broken out of it

The SIM is protected by a personal identity number (PIN) between four to eight digits in length. The PIN is loaded by a network operator at subscription time.

A SIM contain the subscriber-related information, including the PIN & PUK (PIN unblocking key) codes.

The ME contain the noncustomer-related hardware & software specific to the radio interface. When the SIM is removed from an MS the remaining ME cannot be used for reaching the services, except from emergency call.

This SIM-ME design supports portability as well as enhancing security. Usually, the ME is the property of subscriber.

Q4) What basic access technologies can be used in GSM?

A4) Listed below is a description of the specifications and characteristics for GSM.

 • frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).

 • duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel

has two frequencies, 80 MHz apart.

• channel separation—The separation between adjacent carrier

frequencies. In GSM, this is 200 kHz.

• modulation—Modulation is the process of sending a signal by

changing the characteristics of a carrier frequency. This is done in GSM

via Gaussian minimum shift keying (GMSK).

• transmission rate—GSM is a digital system with an over-the-air bit

rate of 270 kbps.

• access method—GSM utilizes the time division multiple access

(TDMA) concept. TDMA is a technique in which several different calls

may share the same carrier. Each call is assigned a particular time slot.

• speech coder—GSM uses linear predictive coding (LPC). The purpose

of LPC is to reduce the bit rate. The LPC provides parameters for a

filter that mimics the vocal tract. The signal passes through this filter,

leaving behind a residual signal. Speech is encoded at 13 kbps.

Q5) How a call is set up in GSM explain?

A5) The operation of the GSM system can be understood by studying the sequence of events that takes place when a call is initiated from the Mobile Station.

Call from Mobile Phone to PSTN:

When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place:

Call from PSTN to Mobile Phone:

When a PSTN subscriber calls a mobile station, the sequence of events is as follows:

Location tracking and call setup

Fig 2 The MS Registration Process

Step 1. The MS periodically listens to the BCCH broadcast from the BSS.

Step 2. The new VLR communicates with the old VLR to find the HLR of the MS.

Step 3. The new VLR sends a registration message to the HLR.

Step 4. The new VLR informs the MS of the successful registration.

Step 5. The HLR sends a deregistration message to the old VLR.

Call Setup

The radio network consists of a large number of BTSs. Each BTS is given an identity. These BTSs are grouped according to location area, also given an identity. Each MSC/VLR (Mobile Services Switching Center/Visitor Location Register) serves the BTSs in a number of location areas. The GSM phones reports to the network (VLR) when it moves from a BTS in oneLocation Area to a BTS in another location area

The VLR always knows in which location area the GSM subscriber is located in at any given moment. Accordingly, the HLR always knows which MSC/VLR the GSM subscriber is at as well. Then, the GSM subscriber’s telephone number tells the network which HLR the actual GSM subscriber belongs to.

How a Call Gets to a GSM Mobile Phone

The call is routed through the telephone network to the closest MSC to the called GSM subscriber. Next, Gateway MSC checks with HLR, asking “Where is the GSM subscriber?” After that, the call is established to the actual MSC/VLR (Visiting MSC) either directly or through the fixed or international telephone network. Finally, the request for a mobile call is transmitted over all BTSs in the actual location area of the called GSM subscriber. Mobile recognizes its own identity, and the call begins.

Fig 3 Call Set Up

Q6) What are features of HSCSD?

A6)

Features

1. High speed circuit switched data technology gives four times faster compare to typical GSM networks.

2. 38.8 kbps speed for all type of non-voice application.

3. It works with multiple slot which makes allow for data transmission at higher rate.

4. With all standard conditions (UL, DL) user can attain 40 to 57.4 kbps in specific area.

Q7) Explain GPRS network?

A7)

Fig 4 GPRS network (1)

Fig 5: GPRS Network (2) (by Peter Rysavy)

Q8) Explain working of GPRS?

A8) Working of GPRS

 It manages to send and receive of packet data to and from MS. It is useful for keeping track of mobile devices within scope of service area. It works in mobility management subscribed user verification and provides data required for billing.

 It has vital role of routing information whichever is necessary for tunneling Protocol Data Unit (PDUs). To SGSN to Serve Distinct MS. It is on interface for external PDNs. i.e., public data units like internet and X.25.

Charging gateway

GTP

GPRS tunneling protocol uses to encapsulate IP or X.25 packet which are transferred among SGSN and GGSN.

Q9) List the applications of GPRS?

Application of GPRS

GPRS provides many functions among several applications. These are listed below:

Q10) What is EDGE technology?

A10) Enhanced Data rates for Global Evolution (EDGE) introduces a new modulation technique, as well as protocol enhancements for transmitting packets over the radio.

The use of the new modulation and the protocol enhancements, result in dramatically increased throughput and capacity gains enabling 3G services in the existing GSM/GPRS networks. No changes are needed to the existing core network infrastructure to support EDGE. This emphasizes the fact that EDGE is only an “add-on” for BSS.

For EDGE, nine Modulation and Coding Schemes (MCS) are introduced (MCS1 to MCS9) and optimized for different radio environment. Four EDGE coding schemes are using GMSK and five are using 8 PSK Modulation.

Up gradation to EDGE

Benefits of EDGE

What EDGE Would Mean to Subscribers

Q11) Compare the 2.5G technologies?

A11)

Q12) Explain the concept of data communication in GPRS?

A12) GPRS architecture works on the same procedure like GSM network, but, has additional entities that allow packet data transmission. This data network overlaps a second-generation GSM network providing packet data transport at the rates from 9.6 to 171 kbps. Along with the packet data transport the GSM network accommodates multiple users to share the same air interface resources concurrently.

Following is the GPRS Architecture diagram:

Fig 6 GPRS Architecture

GPRS attempts to reuse the existing GSM network elements as much as possible, but to effectively build a packet-based mobile cellular network, some new network elements, interfaces, and protocols for handling packet traffic are required.

GPRS Mobile Stations

New Mobile Stations (MS) are required to use GPRS services because existing GSM phones do not handle the enhanced air interface or packet data. A variety of MS can exist, including a high-speed version of current phones to support high-speed data access, a new PDA device with an embedded GSM phone, and PC cards for laptop computers. These mobile stations are backward compatible for making voice calls using GSM.

GPRS Base Station Subsystem

Each BSC requires the installation of one or more Packet Control Units (PCUs) and a software upgrade. The PCU provides a physical and logical data interface to the Base Station Subsystem (BSS) for packet data traffic. The BTS can also require a software upgrade but typically does not require hardware enhancements.

When either voice or data traffic is originated at the subscriber mobile, it is transported over the air interface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call. However, at the output of the BSC, the traffic is separated; voice is sent to the Mobile Switching Center (MSC) per standard GSM, and data is sent to a new device called the SGSN via the PCU over a Frame Relay interface.

GPRS Support Nodes

Following two new components, called Gateway GPRS Support Nodes (GSNs) and, Serving GPRS Support Node (SGSN) are added:

Gateway GPRS Support Node (GGSN)

The Gateway GPRS Support Node acts as an interface and a router to external networks. It contains routing information for GPRS mobiles, which is used to tunnel packets through the IP based internal backbone to the correct Serving GPRS Support Node. The GGSN also collects charging information connected to the use of the external data networks and can act as a packet filter for incoming traffic.

Serving GPRS Support Node (SGSN)

The Serving GPRS Support Node is responsible for authentication of GPRS mobiles, registration of mobiles in the network, mobility management, and collecting information on charging for the use of the air interface.

Internal Backbone

The internal backbone is an IP based network used to carry packets between different GSNs. Tunnelling is used between SGSNs and GGSNs, so the internal backbone does not need any information about domains outside the GPRS network. Signalling from a GSN to a MSC, HLR or EIR is done using SS7.

Routing Area

GPRS introduces the concept of a Routing Area. This concept is similar to Location Area in GSM, except that it generally contains fewer cells. Because routing areas are smaller than location areas, less radio resources are used While broadcasting a page message.

Q13) Compare GSM and GPRS?

A13)

Q14) How is data transferred though GPRS explain?

A14) Data routing or routing of data packets to and fro from a mobile user, is one of the pivot requisites in the GPRS network. The requirement can be divided into two areas:

Data Packet Routing

The important roles of GGSN involve synergy with the external data network. The GGSN updates the location directory using routing information supplied by the SGSNs about the location of an MS. It routes the external data network protocol packet encapsulated over the GPRS backbone to the SGSN currently serving the MS. It also decapsulates and forwards external data network packets to the appropriate data network and collects charging data that is forwarded to a charging gateway (CG).

There are three important routing schemes:

The GPRS network encapsulates all data network protocols into its own encapsulation protocol called the GPRS tunnelling protocol (GTP). The GTP ensures security in the backbone network and simplifies the routing mechanism and the delivery of data over the GPRS network.

Mobility Management

The operation of the GPRS is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions to increase efficiency and to make optimum use of free GSM resources (such as unallocated time slots).

An MS can be in any of the following three states in the GPRS system. The three-state model is unique to packet radio. GSM uses a two-state model either idle or active.

Active State

Data is transmitted between an MS and the GPRS network only when the MS is in the active state. In the active state, the SGSN knows the cell location of the MS.

Packet transmission to an active MS is initiated by packet paging to notify the MS of an incoming data packet. The data transmission proceeds immediately after packet paging through the channel indicated by the paging message. The purpose of the paging message is to simplify the process of receiving packets. The MS listens to only the paging messages instead of to all the data packets in the downlink channels. This reduces battery usage significantly.

When an MS has a packet to transmit, it must access the uplink channel (i.e., the channel to the packet data network where services reside). The uplink channel is shared by a number of MSs, and its use is allocated by a BSS. The MS requests use of the channel in a random access message. The BSS allocates an unused channel to the MS and sends an access grant message in reply to the random access message.

Standby State

In the standby state, only the routing area of the MS is known. (The routing area can consist of one or more cells within a GSM location area).

When the SGSN sends a packet to an MS that is in the standby state, the MS must be paged. Because the SGSN knows the routing area of the MS, a packet paging message is sent to the routing area. On receiving the packet paging message, the MS relays its cell location to the SGSN to establish the active state.

Idle State

In the idle state, the MS does not have a logical GPRS context activated or any Packet-Switched Public Data Network (PSPDN) addresses allocated. In this state, the MS can receive only those multicast messages that can be received by any GPRS MS. Because the GPRS network infrastructure does not know the location of the MS, it is not possible to send messages to the MS from external data networks.

Routing Updates

When an MS that is in an active or a standby state moves from one routing area to another within the service area of one SGSN, it must perform a routing update. The routing area information in the SGSN is updated, and the success of the procedure is indicated in the response message.

A cell-based routing update procedure is invoked when an active MS enters a new cell. The MS sends a short message containing the identity of the MS and its new location through GPRS channels to its current SGSN. This procedure is used only when the MS is in the active state.

The inter-SGSN routing update is the most complicated routing update. The MS changes from one SGSN area to another, and it must establish a new connection to a new SGSN. This means creating a new logical link context between the MS and the new SGSN and informing the GGSN about the new location of the MS

Q15) Explain 4G technology?

A15)

Motivation for 4G Research Before 3G Has Not Been Deployed?

Reasons to Have 4G

What's New in 4G?

Entirely packet-switched networks.

Q16) Explain the need of 4G?

A16) Need of 4G: -

Q17) Explain the applications, advantage and disadvantages of 4G?

A17) Applications: -

Advantages and Disadvantages of 4G:

Advantages:

Disadvantages: