UNIT 5
GSM Services
GSM physical layer is nothing but the modules through which speech will pass through before they are transmitted in the air. These modules are depicted in the figure below.
These modules are speech coding, channel coding, interleaving, ciphering, burst assembly, modulation. Speech coding block uses 13kbps RELP (Residually Excited Linear Predictive coder). Channel coding block uses convolution coding of rate 1/2 with constraint length of 5. Interleaving block does diagonal interleaving, after 456 encoded bits in 20ms duration are broken into 57 bits sub-blocks.
There will be about total 8 sub blocks of 57 bits each. Ciphering block uses A3 and A5 encryption algorithms. Encryption is changed call by call to enhance privacy. Burst assembly block frames the burst as required by GSM frame structure. The same is modulated and Gaussian filtered. Modulation block minimizes the occupied BW using GMSK modulation with BT of 0.3.
- Gaussian shape impulses gives response to filter generation at signal with low side lobes and narrow shape main lobe.
- Since filter theoretically has output before input and it can be approximated with delay and shaped impulse response i.e. it’s a Gaussian like shape. This modulation is called Gaussian Minimum Shift Keying (GMSK).
- Two methods for generating GMSK are :
(a) Frequency shift keying modulation.
(b) Quadrature phase shift keying modulation.
Fig. (a) : GMSK implemented by frequency shift keying modulation with FM-VCO
Fig. (b) : GMSK implemented by a quadrature baseband method
- Fig.(a) GMSK with FM-VCO modulator is most suitable to coherent demodulation as it has problem in component tolerance.
- Fig.(b) GMSK with quadrature baseband method gives maintained modulation index exact of 0.5. It is easy to implement compared to FM-VCO.
- Both methods gives same GMSK modulated signal. The GMSK signal m (t) can be written as :
m (t) = sin (2 fc t) I (t) + cos (2 fc t) Q (t)
- GSM provides the well integrated services for both voice and data : GSM provides three different kind of services :
(i) Bearer services
(ii) Tele services
(iii) Supplementary services.
- Bearer services consists of all the services that provides the transparent transmission of data between the various interfaces to the network.
- Teleservices consists the main services as the telephony and other services as the emergency number services.
- Supplementary services provides various services like call forwarding and multi-party communication.
Fig.: Integrated services in GSM system
Bearer Services
- Bearer services are used for the transmission of data and voice-data between two user network interfaces in a mobile network.
- Here Bearer is a set of data which is transmitted from and received by the terminal (TE) in certain specified formats. The Fig. 3.1.1 shows the Bearer services for the data transmission and reception.
- On the basis of transmission and reception through the interfaces bearer services can be either :
(i) Transparent Bearer Services
(ii) Non-transparent Bearer Services
- Bearer services can also be classified in
(a) Synchronous Data Transfer
(b) Asynchronous Data Transfer
(c) Synchronous data packet transfer
(i) Transparent Bearer Services
- Physical layer protocol are use to transmit the data in transparent bearer services. Physical layer is layer 1 in OSI model, which transmits or receives the data after proper formatting or multiplexing.
- If there is no transmission errors occurred during transmission then the constant delay and throughput can be achieved. To improve the quality of transmission Forward Error Correction (FEC) techniques are used. By using FEC techniques redundant bits are inserted into the data steams before transmission. These redundant data helps to receiver to detect the error and reconstruct the original data.
- FEC techniques help to broadcasting without handshaking. As an disadvantage the FEC technique requires higher band width and it also reduces the data rates.
(ii) Non-transparent data transfer
- Non-transparent bearer service uses the protocols of physical layer with the addition of data link layer and flow control protocols. Data link layer provides the functions of framing, error control and flow control.
- A non-transparent service uses the transparent bearer services with addition to the Radio Links Protocol (RLR). This protocol uses the selective reject mechanism for retransmission of the erroneous data.
Other categories of bearer services
(a) Synchronous data transmission
- In this, data is transmitted from transmitter to receiver at a fixed rate.
- The receiver must synchronize the clock rate according to the transmitting or incoming data bits. This technique is fast because there is no waiting period during data transfer.
(b) Asynchronous data transfer
Here the data is transmitted on the variable rates, so there is no constant time interval is maintained between consecutive frames.
(c) Synchronous packet transmission
- This techniques is used after the formation of packets.
- Various packets are transmitted through the different routes, channels and different interface to reach to the destination.
- Original sequence of packets are obtained at the destination side with the help of sequence number.
Some example of bearer services
(i) After a peer-to-peer link Unrestricted Digital Information (UDI) is designed.
(ii) Packet Assembler/Disassembler (PAD) allows kind of asynchronous connection that allows the Public Land Mobile Network (PLMN) subscribers to access a Packet-Switched Public Data Network (PSPDN).
(iii) Provide the facility to switch between voice and data during a call.
Tele Services
The services that are offered by a mobile service network to a caller (TE) are called as Teleservices. Following are some of the tele services provided by the GSM services providers :
(i) Emergency number
(ii) Telephonic-voice at full data rate
(iii) Fax services
(iv) SMS (Short Message Service)
(v) MMS (Multimedia Message Service)
Supplementary Services
GSM provides some supplementary services with addition to the tele and bearer services. Some important services that is provided as supplementary services.
(i) Call forwarding or redirecting
Calls can be sent to other various numbers given by the user.
(ii) Call waiting
Notification of an incoming call while on the another call.
(iii) Call hold
Put a caller on hold to take the another call.
(iv) Call conferencing
Link multiple calls together.
(v) CUG facility
Closed user group facility.
(vi) CLIP and CLIR
Caller Line Identification Presentation and Caller Line Identification Restriction.
(vii) Call barring
Incoming or outgoing or all calls.
- Short Message Service (SMS) is the most basic communications technology for mobile data transfer and is characterized by the exchange of short alphanumeric text messages between digital line and mobile devices. SMS messaging's key influential factor is affordability.
- SMS messages hold up to 140 bytes (1,120 bits) of data, which allows a 160-character alphanumeric message in the default 7-bit alphabet or a 70-character message in a non-Latin language, such as Chinese.
- SMS is also known as text messaging.
SMS streamlines communication on many levels, as follows:
- Quick communication: Brief updates between family and friends
- Alerts: Voicemail, sales lead inquiries, appointments, meetings or deliveries
- Enhanced messaging service (EMS): Facilitates ring tone, image and simple media transfer
- HSCSD stands for high speed circuit switched data. It is enhanced version of circuit switched data, it help to improve error correction in various level of quality of radio links.
- HSCSD provides data rate upto 14.4 kbps by using multiple time slot at same time. It transfer time sensitive images or video at high speed.
- In generate some latency compared to GPRS while in transmission of normal voice and data traffic.
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 work 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.
- General Packet Radio System (GPRS) is a packet based data services for wireless communication.
- A packet radio principle are used to transfer subscriber data packet within GSM mobile station and external packet data network.
- Data is split and transmitted at sender node and reassembled at receiving end. GPRS support IP and X.25, these operate over cellular connection of a GSM.
- According to peter Rysavy, Rysavy Research 1998 GPRS network is as follows :
Fig.: GPRS network (1)
- GPRS system has a base of GSM communication and somewhat of circuit switched phone connection, Short Message Service (SMS).
Fig.: GPRS Network (2) (by Peter Rysavy)
Working of GPRS
- SGSN :
It manage 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.
- GGSN :
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
- It maintain log entries for activities like data being transfer, change in charging terms in peak/off to peak and vice-versa, end of session for GPRS etc.
- It has collection records related to GPRS, usage in buffering of data, storage, transfer of data.
GTP
GPRS tunneling protocol uses to encapsulate IP or X.25 packet which are transferred among SGSN and GGSN.
Application of GPRS
GPRS provides many function among several applications. These are listed below :
- Chat
- Information services as text or graphics.
- Still images.
- Moving images
- Web browsing
- Document sharing and remote collaborative working
- Audio reports
- Job dispatch
- Corporate email
- Vehicle positioning
- File transfer.
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
- Mobile Station (MS) − MS should be EDGE enabled.
- BTS − HW supplied is Edge enabled.
- BSC − Definitions for EDGE timeslots needs to be done in BSC.
- GPRS Support Nodes (GSNs) − Definitions for Edge need to be defined in GSNs.
- Databases (HLR, VLR, etc.) − No definition is required.
Benefits of EDGE
- Short-term benefits − Capacity and performance,
- Easy implementation on a GSM/GPRS network,
- Cost effective,
- Increases the capacity and triples the data rate of GPRS,
- Enables new multimedia services,
- Long-term benefit − Harmonization with WCDMA.
What EDGE Would Mean to Subscribers
- Streaming applications
- Very high speed downloads
- Corporate intranet connections
- Quicker MMS
- Video phone
- Vertical corporate applications - Video conference, Remote presentations.
References
1. Theodore S Rappaport, “Wireless Communications Principles and Practice” Second Edition, Pearson Education
2. John C. Bellamy, “Digital Telephony”, Third Edition; Wiley Publications
3. ThiagarajanVishwanathan, “Telecommunication Switching Systems and Networks”; PHI Publications
4. Wayne Tomasi, “Electronic Communications Systems”; 5th Edition; Pearson Education
5. Vijay K Garg, Joseph E Wilkes, “Principles and Applications of GSM” Pearson Education
6. Vijay K Garg, Joseph E Wilkes, “IS-95CDMA and CDMA 2000 Cellular/PCS Systems Implementation” Pearson Education
7. Mischa Schwartz, “Mobile Wireless Communications”, Cambridge University Press