Unit-4 Introduction to GSM

Que 1. Draw the block diagram and explain GSM architecture in details indicating all the interfaces.

The GSM architecture consists of three major interconnected subsystems that interact with themselves and with users through certain network interface. The subsystems are Base Station Subsystem (BSS), Network Switching Subsystem (NSS) and Operational Support Subsystem (OSS). Mobile Station (MS) is also a subsystem but it is considered as a part of BSS.

1. Mobile Station (MS):

Mobile Station is made up of two entities.

A. Mobile equipment (ME):

• It is a portable, vehicle mounted, hand held device.
• It is uniquely identified by an IMEI number.
• It is used for voice and data transmission. It also monitors power and signal quality of surrounding cells for optimum handover. 160 characters long SMS can also be sent using Mobile Equipment.

B. Subscriber Identity module (SIM):

• It is a smart card that contains the International Mobile Subscriber Identity (IMSI) number.
• It allows users to send and receive calls and receive other subscriber services. – It is protected by password or PIN.
• It contains encoded network identification details. it has key information to activate the phone.
• It can be moved from one mobile to another

2. Base Station Subsystem (BSS):

It is also known as radio subsystem, provides and manages radio transmission paths between the mobile station and the Mobile Switching Centre (MSC). BSS also manages interface between the mobile station and all other subsystems of GSM. It consists of two parts.

A. Base Transceiver Station (BTS):

• It encodes, encrypts, multiplexes, modulates and feeds the RF signal to the antenna.
• It consists of transceiver units.
• It communicates with mobile stations via radio air interface and also communicates with BSC via Abis interface.

B. Base Station Controller (BSC):

• It manages radio resources for BTS. It assigns frequency and time slots for all mobile stations in its area.
• It handles call set up, transcoding and adaptation functionality handover for each MS radio power control.
• It communicates with MSC via A interface and also with BTS.

3. Network Switching Subsystem (NSS):

It manages the switching functions of the system and allows MSCs to communicate with other networks such as PSTN and ISDN. It consist of

A. Mobile switching Centre:

• It is a heart of the network. It manages communication between GSM and other networks.
• It manages call set up function, routing and basic switching.
• It performs mobility management including registration, location updating and inter BSS and inter MSC call handoff.
• It provides billing information.
• MSC does gateway function while its customers roam to other network by using HLR/VLR.

B. Home Location Registers (HLR):

• It is a permanent database about mobile subscriber in a large service area. – Its database contains IMSI, IMSISDN, prepaid/post-paid, roaming restrictions, supplementary services.

C. Visitor Location Registers (VLR):

• It is a temporary database which updates whenever new MS enters its area by HLR database. – It controls mobiles roaming in its area. It reduces number of queries to HLR. – Its database contains IMSI, TMSI, IMSISDN, MSRN, location, area authentication key.

D. Authentication Centre:

• It provides protection against intruders in air interface. – It maintains authentication keys and algorithms and provides security triplets (RAND, SRES, Ki).

E. Equipment Identity Registry (EIR):

• It is a database that is used to track handset using the IMEI number.
• It is made up of three sub classes- the white list, the black list and the gray list.

4. Operational Support Subsystem (OSS):

It supports the operation and maintenance of GSM and allows system engineers to monitor, diagnose and troubleshoot all aspects of GSM system. It supports one or more Operation Maintenance Centres (OMC) which are used to monitor the performance of each MS, Bs, BSC and MSC within a GSM system. It has three main functions:

• To maintain all telecommunication hardware and network operations with a particular market.
• To manage all charging and billing procedures
• To manage all mobile equipment in the system.

Interfaces used for GSM network : (ref fig 2)
1)UM Interface –Used to communicate between BTS with MS
2)Abis Interface— Used to communicate BSC TO BTS
3)A Interface— Used to communicate BSC and MSC
4) Singling protocol (SS 7)- Used to communicate MSC with other network .

Que 2. Explain Traffic & Control GSM channels along with its sub types and characteristics.

There are mainly two types of GSM logical channels:
(i) Traffic channels (TCHs). (ii) Control channels (CCHs).
Traffic channels carry digitally encoded user voice or user data and have identical formats of both forward link and reverse link. Control channels carry signal and synchronization commands between the base station and mobile station. Other control channels are used only for forward and reverse link.

(i) GSM Traffic channels (TCHs).

GSM traffic channel carry digital voice and user data either at half rate or at full rate. When signal is transmitted at full rate, user data is contained within one time slot per frame. When signal is transmitted at half rate, user data is mapped on to the same time slots, but it is sent in alternative frames.

Full Rate Traffic Channels (TCH):

(i) Full-rate speech channel (TCH/FS): This channel carries user speech in digitized form at a raw data rate of 13 kbps. GSM channel coding is added to digitized speech then the full rate speech channel carries 22.8 kbps.

(ii) Full-rate data channel for 9600 bps (TCH/F9.6): This channel carries raw user data which is transfers at 9600 bps with additional forward error correction applied by GSM, the 9600 bps data sent at 22.8 kbps.

(iii) Full-rate data channels for 4800 bps (TCH/F4.8): This channel carries raw user data which is transferred at 4800 bps with additional forward error correction applied by GSM, the 4800 bps is sent at 22.8 kbps.

(iv) Full rate data channel for 2400 bps (TCH/F2.4): This channel carries raw user data which is transferred at 2400 bps with additional forward error correction coding by GSM, the 2400 bps is sent at 22.8 bps.

Half-rate Traffic Channels:

(i) Half-rate speech channel (TCH/HS): This channel carries digitized speech which is sampled at a half rate then the full rate GSM channel coding added to digitized speech and half rate speech channel carry 11.4 kbps.

(ii) Half-rate data channels for 4800 bps (TCH/H4.8): This channel carries raw user data which is to be transferred at 4800 bps. With additional forward error correction applied by GSM, the 4800 bps data sent at 11.4 kbps.

(iii) Half-rate data channels for 2400 bps (TCH/H2.4): This channel carries raw user data which is to be transferred at 2400 bps with additional forward error correction by the GSM, the 2400 bp data sent at 11.4 kbps.

(ii) GSM Control Channels (CCH):

There are three control channels in GSM:

1. Broadcast control channels.
2. Common control channels.
3. Dedicated control channels.

1. Broadcast control channels (BCH) : The BTS uses this channel to give information to all MSs within a cell. Information uses by this channel is cell and network identity, current control channel structure, channel availability and congestion. The broadcast control channel also sends the list of channels that are currently used within cell.

(a) Frequency Correction Channel (FCCH): The BTS sends information for frequency correction via the Frequency Correction Channel (FCCH). The FCCH is special data burst, which occupies first frame (i.e. frame 0) and repeated after every ten frames in control channel multi-frame.

(b) Synchronization Channel (SCH): BTS broadcast information about time synchronization to all MSS via synchronization channel (SCH). If the mobile station is 30 km away from serving base station, it is often necessary to adjust the timing of particular mobile user. The SCH is transmitted once after every ten frames within the control channel multiframe.

2. Common Control Channels (CCCH):

All the information regarding setting up a connection between MS and BS is exchanged via the CCCH. The common control channel occupies TSO (frame) of GSM frame and that is not used by BCH and ideal channels.

(a) Paging Channel (PCH): The PCH gives paging signal from the base station to all mobile stations within cell. It also notify particular mobile for an incoming call from PSTN. Alternatively, the PCH is used to provide cell broadcast ASCII text message to all subscriber, as a GSM SMS features.

(b) Random Access Channel (RACH): If MS wants to setup a call, it uses Random Access Channel (RACH) to send data to BTS. All mobile must request access or respond to a PCH with TSO of GSM frame. At BTS, every frame will accept RACH transmission from mobile during TSO.

(c) Access Grant Channel (AGCH): The AGCH channel is used by base station to provide forward link communication to mobile station and carries instructional data which tells mobile to operate in particular physical channel with particular control channel. The AGCH is the final common control channel message sent by the base station before subscriber is roaming or moving off the control channel.

3. Dedicated Control Channels (DCCH):

There are mainly three types of dedicated control channels in GSM, same as traffic channel, they are bidirectional. They have same format and function on both forward and reverse links.

(a) Stand-alone Dedicated Control Channels (SDCCH): SDCCH carries signaling data which follows the connection of mobile with base station. The SDCCH ensures that the mobile and base station connection remains constant while the base station and MSC verify the subscriber unit and resource allocation to mobile. The SDCCH is also used to send authentication and alert messages but not speech.

(b) Slow Associated Control Channel (SACCH): The SACCH is always associate with traffic channel or SDCCH, the SACCH carries general information between the MS and BTS. On the forward link, the SACCH is used to send slow but regularly changing control information to the mobile, such as power level instruction, and specific timing advance instruction for each user. The reverse SACCH carries information about the received signal strength and
quality of traffic channel as well as BCH measurement result from neighbouring cell.

(c) Fast Associated Control Channels (FACCH): FACCH carries urgent messages, and contain the same type of information as SDCCH. A FACCH is assigned to a particular user when SDCCH has not been dedicated to particular user. The FACCH access the time slots by taking frame from traffic channel, this is done by using two special bits, called stealing bits, in TCH channel.

Que 3. Explain GSM frame structure with neat diagram.

Each user transmits a burst of data during the time slot assigned to it. These data bursts may have one of five specific formats. Normal bursts are used for TCH and DCCH transmissions on both the forward and reverse link. FCCH and SCH bursts are used in TS0 of specific frames to broadcast the frequency and time synchronization control messages on the forward link. The RACH burst is used by all mobiles to access service from any base
station, and the dummy burst is used as filler information for unused time slots on the forward link.

A frame is one where no time slot is repeated. A frame contains eight time slots TS0 to TS7.
One time slot duration= 576.92 μsec
Number of bits transmitted during 1 time slot= 156.25 bits

Thus, duration of one frame = 576.92μsec×8=4.6153 msec
Each bit duration= 576.92 μsec / 156.25 bits=3.6922 μsec
Transmission rate = 156.25 bits / 576.92 μsec=270.833 kbps
One user’s transmission rate = 270.833 kbps / 8 = 33.854 kbps

One single time slot in frame has eight fields as follows:

• Tale bits: They are present at the start and end of every time slot to distinguish one time slot from another. It identifies beginning and end of the burst.
• Coded data: This is the actual information to be transmitted. Out of 156.25 bits 114 are information bearing bits that are transmitted as two 57 bits sequences close to beginning and end of the time slot.
• Stealing flag: This bit helps base station to distinguish whether the coded data is control or actual information. At the time of urgency control its are also sent using voice channel at that time stealing bit becomes ‘0’ indicating coded data carries control information.
• Midamble: This consists of 26 bits training sequence. It helps the adaptive equalizer in mobile or base station receiver to analyze radio channel characteristics before coding. It gives the amount of fading the channel provides helping to decide which equalizer to be used.
• Guard period: A guard time of 8.25 bits is provided at the end of every time slot to prevent overlapping with next time slot preventing actual information from getting tampered.

During a frame a GSM subscriber uses on one time slot to transmit and one time slot to receive and may use spare five time slots to measure signal strength on adjacent five base station as well as its own base station.

Each normal speech frames are grouped into larger structures called multi-frames which in turn are grouped into super-frames and hyper-frames.

One Multiframe contains 26 TDMA frames, one superframe contains 51 multiframes or 1326 TDMA frames and one hyperframe contains 2048 superframes or 2715648 TDMA frames. One hyperframe is transmitted after every 3.4815 hours.

Hyperframe is important in GSM because the encryption algorithms rely on particular frame number and sufficient security can only be obtained by using a large number of frames as provided by hyperframe.

Que 4. Define Handoff and explain different handoffs in GSM network.

1. When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base station. This procedure is called handoff.
2. Depending on different purpose the handoff is classified are as below

A. Depending on nature of handoffs the classification is as

1. Hard Handoff
2. Soft Handoff

1. Hard Handoff:

• It is also known as “Break before make” connection.
• In this type of handoff the link to the old base station is terminated before the mobile station establishes a link with the new base station.
• It is used in FDMA and TDMA based mobile system.
• Figure shows the mechanism of hard handoff.

2. Soft Handoff:

• It is also called as Mobile Directed handoff or make before Break Connection.
• In these types of handoff the link to the old base station is not terminated before the mobile station established a link with the new base station.
• Once the link is established the connection to old BS is terminated.
• It is used in UMTS to improve the signal quality
• It is more seamless handover.
• Figure shows the mechanisms of soft handoff.

B. Depending on purpose of Handoff classification are as

a. Intra cell handoff
b. Inter cell handoff
c. Inter System Handoff

a. Intra-cell handover

Such a kind of handover is performed to optimize the traffic load in the cell or to improve
quality of a connection by changing carrier frequency.

b. Inter-cell handover

It is also known as Intra-BSC handover.

Here the mobile moves from one cell to another but remains within the same BSC (Base station controller).

Here the BSC handles the handover process

c. Inter System Handoff

i) Inter-BSC handover

• It is also called as Intra-MSC handover.
• As BSC can control only a limited number of cells, we might usually need to transfer a mobile from one BSC to another BSC.
• Here the MSC handles the handover process.

ii) Inter-MSC handover

• It occurs when a mobile moves from one MSC region to another MSC.
• MSC cover a large area. It can be imagined as a handover from Maharashtra MSC to Gujarat MSC while travelling.

C. Handoff schemes based on algorithms of handoff (handoff protocols)

The mobile unit and the BS are connected via radio links which carry data as well as signalling information. There are three different handoff strategies based on algorithms of handoff, which have been proposed for transferring the connection to a new BS.

1. MCHO (mobile-controlled handoff)
2. NCHO (network-controlled handoff)
3. MAHO (mobile-assisted handoff)

Since the number of handoffs increases with decreasing cell size, it will be an almost impossible task to make a handoff decision for every mobile by one central switch (centralized). Moreover, in microcells the connection between MS and BS can deteriorate very quickly. Fast handoff decisions required in such situations can be achieved more readily by decentralizing the handoff decision process.

Network-controlled handoff (NCHO)

1. NCHO is a centralized handoff protocol. In this type of handoff the network (surrounding BS, the MSC or both) makes a handoff decision based on measurements of the RSS (received signal strength) of mobile and the interferences from different BS.
2. The signal-to-interference ratio (SIR) is measured by means of a supervisory audio tone
   (SAT).
3. If the mobile is measured to have a weaker signal in its old cell, while a stronger signal in a neighboring cell, then a handoff decision could be made by the network to switch BS from the old cell to the new cell. Such a type of handoff in general takes 100–200 ms and produces a noticeable “interruption” in the conversation.
4. However, overall delay of such a type of handoff is in general in the range of 5–10 s. Thus, this type of handoff is not suitable to a rapidly changing environment and to a high density of users due to the associated delay.
5. The NCHO is widely used in the first-generation cellular systems, such as AMPS, Total Accesses Communications System (TACS) and Nordic Mobile Telephone (NMT). In NCHO, the MSC is solely in charge of the handoff process and the MSs are completely passive.

Mobile-assisted handoff (MAHO)

1. MAHO is a variant of NCHO strategy. To improve the handoff reaction time and to reduce the handoff administration load of the MSC, the handoff decisions should be distributed towards the mobile phones.
2. One way to achieve this could be to let the mobile phones make the measurements and the MSC make the decisions.
3. In the MAHO strategy, the network (BS and/or MSC) directs the mobile to measure the signal strengths from the surrounding BSs and to report those measurements back to the network.
4. The network then uses these measurements to determine where a handoff is required with which channel.
5. The delay in this protocol starting from the handoff initiation till the handoff execution is around 1 s. This time may still be too long to avoid dropping a call due to street corner effect.
6. Some examples of present cellular networks which implement MAHO are the GSM system and the IS-95 system.

Mobile-controlled handoff (MCHO)

1. In this case, the mobile phone is the only entity which measures the handoff criteria and makes a decision based on them.
2. The MSC is not involved in the handoff process resulting in reduced burden on the MSC. The mobile has to choose the optimum BS based on the measurements.
3. Since the handoff process is implemented in the mobile itself, the delay is usually smaller with a typical value of 0.1 s and is suitable for microcellular systems.
4. In this strategy, the mobile continuously monitors the radio signal strengths and quality of surrounding BSs.
5. A handoff can be initiated if the signal strength of the serving BS is lower than that of another BS by a certain threshold. Then the mobile requests the target BS for a channel with the lowest interference and handoff mechanism will take place.
6. In such a case, the MS does not have any information about the signal quality of other users, but handoff must not cause interference to other users.
7. MCHO is the highest degree of handoff decentralization. Some of the advantages of handoff decentralization are as follows:

1. Handoff decisions can be made fast.

2. MSC does not have to make handoff decisions for every mobile, which is a very difficult task for the MSC of high-capacity microcellular systems (radius < 1 km).

An example of a MCHO-based handoff control network is the standard for cordless
phones in Europe – digital European cordless telephone (DECT).

Que 5. Explain the features of GSM.

The features of GSM are:

1. Call Waiting – Notification of an incoming call while on the handset
2. Call Hold– Put a caller on hold to take another call
3. Call Barring – All calls, outgoing calls, or incoming calls
4. Call Forwarding– Calls can be sent to various numbers defined by the user
5. Multi Party Call Conferencing– Link multiple calls together
6. Calling Line ID – Incoming telephone number displayed
7. Alternate Line Service a.) One for personal calls b.) One for business calls
8. Closed User Group – call by dialing last for numbers
9. Advice of Charge – Tally of actual costs of phone calls
10. Fax & Data – Virtual Office / Professional Office
11. Roaming– services and features can follow customer from market to market.

Que 6. State the various services offered by GSM system.

The three services offered by GSM systems are:

1. Telephone services
2. Bearer services
3. Supplementary ISDN services

Telephone Services:
Teleservices include Standard mobile telephone Mobile-originated Base-originated traffic. emergency calling Fax Videotext Tele text, SMS MMS.

Bearer services: The data services include the communication between computers and packet switched traffic. These services are limited to the first three layers of the OSI reference model. Data may be transmitted using either a Transparent Mode or NonTransparent Mode. Transparent Mode:-Where GSM provides standard channel coding for user data Non-Transparent Mode: – Where GSM offers special coding efficiencies based on the particular data interface.

Supplementary ISDN services:
This service are digital in nature and include

Call diversion
Caller line ID
Closed user group
Call barring
Call waiting
Call hold
Connected line ID
Multiparty (Teleconferencing)
Call charge advice

This service also include the Short Messaging Service (SMS) which allow GSM subscriber and BS to transmit alphanumeric pages of limited length (160 -7 ASCII characters) while simultaneously carrying normal voice traffic.

Also you can refer : Basic Cellular Concept