• Antenna Installation and Downtilting

When we talk about antenna then we need to understand about antenna installation and specially about antenna downtilting.lets assume that antenna installed then how you can change its position.

Its two types:

1.  left-right = its called azimuth change
2. up-down = its called tilt change.

Lets understand from start.
ANTENNA INSTALLATION

• Antenna installation configurations depend on the operators preferences.

• GSM Interface and Channel Usage

Objectives

On completion of this module you will be able to ...

• comprehend how the various types of information like speech and data are transmitted from the GSM network to the customer's mobile station.
• list and describe the technological details of the terrestrial interfaces in the Base Station Subsystem.
• explain the basics of radio transmission.
• explain and describe the particular importance of the GSM air interface from a technological point of view.
• define the functions of the different radio channels.
• understand and describe the effects of technologies like channel coding and DTX on speech and noise quality in GSM. 

Content

4.1	The BSS Interfaces
4.2	The A Interface
4.3	The A-ter Interface
4.3.1	Fullrate Speech Codec
4.3.2	Discontinuous Transmission
4.4	The A-bis Interface
4.5	The Terrestrial Interfaces - Summary
4.6	The Air Interface Um
4.6.1	Basic Principles of Transmission
4.6.2	The Physical Channels
4.6.3	The Logical Channels
4.7	Channel Coding

4.1 The BSS Interfaces

Within the BSS, the user- and signalling data is transported over a series of interfaces. The A interface connects the Mobile Services Switching Center (MSC) with the Transcoder TC.

The A-ter interface connects the Transcoder with the Base Station Controller (BSC). The A-bis interface connects the BSC with the Base Transceiver Station (BTS). Finally, the data is transmitted to the mobile station via the air interface Um.

Let's consider the PCM30 configuration as an example for the frame structure of data transmission between the MSC and the mobile station, to understand the dataflow at the A interface, the A-ter, A-bis and Um interfaces.

We see that the 4 A-links are mapped onto one A-ter link. 4 A-channels of 64 kbps each are mapped onto an A-ter channel consisting of 4 subchannels of 16 kbps each. In total, the 128 channels of 4 A-links are reduced to the 32 channels of one A-ter link, which are numbered consecutively from 0 to 31. The SS7 signalling, which in our example is to be found in timeslot No 16, is transmitted from A to A-ter transparently, i.e. unchanged.

The frame structure consisting of 32 channels is also found at the A-bis interface. Channel 0 is used for synchronization, the remaining 31 channels transmit warning information for operation and maintenance of the BTS, known as O&M alarms, as well as signalling and voice data. Finally, the information from A-bis is transmitted to the air interface Um via the TRXs, the radio transceivers of the BTS. Two A-bis channels of 4 subchannels each correspond exactly to the eight timeslots of a TDMA frame, which carries the data to the mobile station. A TDMA frame, which we will discuss in more detail later in the course, portions the stream of physical channels or timeslots on a particular carrier frequency into periods.

Its timeslots are numbered consecutively from 0 to 7, and can be assigned to one TRX.

4.2 The A Interface

The A-interface transmits user and signalling data between the MSC and the transcoder. It's the second completely standardized interface in GSM after the air interface. As an open interface it is not tied to a specific producer.

The A-interface is an ISDN-S2M interface that has been adjusted to GSM with a data rate of 64 kbps per timeslot. In the PCM30 configuration, the A interface contains 30 traffic channels. Timeslot number 0 takes over synchronization tasks, and timeslot number 16 contains signalling information in the No 7 signalling system format, or SS7. Thus the air interface has an overall bit rate of 2048 kbps.

The PCM24 configuration, which is generally used in the USA, uses 24 traffic channels. In both configurations, each frame has clearly defined channels for signalling and synchronisation information.

4.3 The A-ter Interface

4 traffic channels of the A interface are bundled into four A-ter channels of 16 kbps each, which are subsequently transmittted to the BSC in a 64 kbps physical A-ter timeslot.

Conversely, signals coming from the BSC are transcoded from 16 to 64 kbps, which is the bit rate typically used in fixed networks. Signalling channels are not transcoded. At the A-ter interface, 120 speech channels of 16 kbps each form a 2 Mbit/s multiplex connection. Four times as many A links as A-ter links are necessary to transmit the same amount of voice data.

4.3.1 Full rate Speech Codec

Now let's turn to a procedure which takes the original speech, and generates the speech description parameters in the TC.

During the first phase of GSM, which lasted until 1995, a speech codec in the MS and in the transcoder was specified as the Full-Rate Codec. The basic characteristics of speech, that is the volume, the base frequency, and the tone, are extracted in 20 ms segments from the 64 kbps signal so that descriptive parameters in 16 kbps signals are generated. The prediction algorithms, that is to say the calculability of speech, make the data less sensitive to the interference a signal meets on its way from and to the mobile station at the air interface.

4.3.2 Discontinuous Transmission

In GSM, all voice signals are transmitted the same way and in a continuous data stream. The channel is occupied even during silence intervals. This has two fundamental disadvantages:

1. Since the mobile station must send for the whole duration of the call, transmitting power is used even in silence intervals, i.e. when the subscriber is only listening. This wastes the mobile station's battery power.
2. Other subscribers using the same frequency in distant cells could be disturbed more than necessary.

Therefore it is logical to switch off the sender whenever the subscriber is not actively transmitting information. Considering the pauses in the dialogue, and also the pauses between and within the sentences, we will find that the average occupation of the radio link is less than 40%.

Discontinuous Transmission (DTX) is a remedy to this problem..

In DTX, a function known as voice activity detection switches off the sender of a mobile station whenever there is no data to be transmitted.

During speech pauses, a "stopgap" in the receiver, which in the uplink is the corresponding transcoder element in the TC, must simulate a functioning channel for the user. In GSM this is called "comfort noise". It is the background noise analysed before the MS is switched off, re-generated by the TC. The comfort noise is even updated during a speech pause, by the mobile station transmitting relevant information to the TC.

4.4 The A-bis Interface

The A-bis interface connects the Base Transceiver Station (BTS) with the Base Station Controller (BSC). In the PCM30 configuration, the data at this interface is transmitted via cable or via microwave transmission at a bit rate of 2 Mbit/s. A cable connection is more resistent to interference, but a network operator must lease it from a fixed network operator.

The microwave links can be operated independently, and are easily configured by the network operator, but they are more sensitive to interference. 4 types of information can be transmitted over the A-bis interface: user information, synchronisation data, signalling information, and data for the operation and maintenance of the BTS, known as O&M alarms.

In the basic configuration, the channels of the A-bis interface are directly connected to the timeslots of the radio transmission at the air interface. The physical data rate is 64 kbps. In PCM30, timeslot 0 of the A-bis interface is used for synchronization. The remaining 31 timeslots of the PCM30 configuration carry data from and to the transceivers of the BTS, as well as signalling information and O&M alarms.

In the uplink, 4 traffic channels of 16 kbps each are sub-multiplexed and transmitted from the BTS to the BSC in a physical A-bis time slot. The same happens in the downlink, only in the opposite direction, i.e. from the BSC to the transceivers of the BTS.

Today's BSC - BTS connection can also be configured as a dynamic link with variable signaling and traffic time slots, according to the current traffic situation.

Two PCM30 channels can be assigned to one TRX. These channels consist of 4 sub-timeslots each. Each PCM30-subtimeslot corresponds to a timeslot in the TRX. Thus, by mapping 8 PCM30 sub-timeslots onto one TDMA frame consisting of timeslots 0 to 7, the entire TDMA frame of the TRX would theoretically be available for the transmission of payload data. But then there wouldn't be enough space left for the necessary signalling traffic from and to the mobile stations. According to a fixed, producer-, and configuration-specific pattern, the signalling information is carried in specific A-bis timeslots of 64 kbps each, or in 16 kbps sub-timeslots, to at least 1 TRX per cell, where it uses timeslot 0 to be transmitted over the air interface.

Special timeslots carry the O&M alarm traffic between the OMC and the BTS over the BSC. The information is, of course, not transmitted over the air interface. As we could see at the A-ter interface, each 16 kbps of a traffic channel consist of 13 kbps of payload and 3 of inband signalling between the BTS and the transcoder.

Only the 13 kbps of payload data may be transmitted over the air interface.

Depending on the producer, and on the configuration, each A-bis connection in the PCM30 configuration may transport user information, signalling information, and O&M information from and to up to 15 transceivers.

In the PCM24 configuration, 24 channels achieve an overall bit rate of 1536 kbps at the A-bis interface. Up to 10 transceivers can be assigned to a connection.

4.5 The Terrestrial Interfaces – Summary

Let's summarize what we have learned about the three terrestrial interfaces A, A-ter and A-bis:

Each of these three interfaces transmits information for the synchronization of the individual network elements point-to-point, at a data rate of 64 kbps, and using timeslot 0.

The transcoder merely forwards the SS7 signalling between the MSC and the BSC. This is done transparently, at a bit rate of 64 kbps, both over the A and over the A-ter interface, for example in timeslot 16. The TRX-related signalling between the BSC and the BTS is transmitted over the A-bis interface at 16, 32 or 64 kbps, depending on the producer. O&M alarms from the transcoder are transmitted to the BSC over the A-ter interface at 16 kbps, or as inband signals through a normal traffic channel. O&M alarms from the BTS are transmitted to the BSC, which is also the O&M master for the entire BSS, over the A-bis interface at 16 or at 64 kbps. If the BSC is unable to correct the errors that caused the alarms, or if it detects an error within itself, it informs the OMC directly, or forwards the alarms from the BTS or TC to it.

Let's consider the transmission of speech and user data, which is transmitted at a data rate of 64 kbps over the A interface, at 16 kbps over the A-ter interface - after being turned into transcoded speech or rate adapted data - and also at 16 kbps per subchannel over the A-bis interface. SMS messages are transmitted via signalling channels. The number of physical timeslots that's available for the transmission of signalling information over the air interface depends on the configuration, and is up to the manufacturer or to the operator.

4.6 The Air Interface Um

Within mobile radio networks, data is transmitted over PCM lines at a bit rate of 2 Mbit/s. Air transmission is used between the mobile station and the BTS, and the information transmitted over the air interface must be adjusted to the PCM lines so it can pass through the rest of the network. The air interface, or Um, is the weakest part of a radio link. In GSM, a lot is done to ensure high quality, security, and reliability. 

At the air interface, the frequencies are arranged in pairs. Each uplink frequency has a downlink frequency permanently assigned to it. The uplink signal goes from the mobile station to the base station, and the downlink signal goes in the opposite direction - from the base station to the mobile. The arrangement in pairs is what actually enables simultaneous communication. The difference between the frequency pair is fixed and is called "duplex frequency". In GSM 900, the duplex frequency is 45 MHz. Accordingly, the uplink frequency range 890 to 915 MHz, is assigned to a frequency range of 935 to 960 MHz in the downlink. In GSM 1800, the duplex frequency is 95 MHz. The uplink frequency range lies between 1710 and 1785 MHz, the downlink frequency range between 1805 and 1880 MHz. In GSM 1900, the duplex frequency is 80 MHz. The uplink frequency lies between 1850 and 1910 MHz, and the downlink frequency between 1930 and 1990 MHz.

4.6.1 Basic Principles of Transmission

The BTS elements which send and receive radio signals in the downlink and uplink channels, are known as transmitter & receivers, or transceivers (TRX) for short. In GSM networks, the transmission over the air interface is digital. Digital transmission in GSM is based on a combination of the FDMA- and the TDMA methods, which already have been introduced. In Frequency Division Multiple Access - or FDMA - different frequency channels are assigned to each BTS. Mobile phones in neighbouring cells - or within the same cell - can be used simultaneously, but occupy different frequencies. The FDMA method uses different carrier frequencies - 124 in GSM 900, 374 in GSM 1800, and 299 in GSM 1900.

4.6.1 Basic Principles of Transmission

Time Division Multiple Access, or TDMA, is a method where several subscribers share one frequency - each subscriber is assigned its own time unit, which is known as a timeslot. In analog mobile systems, on the other hand, a frequency is occupied by one subscriber for the duration of the call. In TDMA systems, each mobile station sends and receives information only on the timeslot it has been assigned. These timeslots are either used to transmit voice data, or information on signalling and synchronization

To send digital information over the air interface, the analog radio signals must be interpreted as bit signals. This process - the transmission of digital information to the air interface - is called modulation. Modulation takes advantage of the physical characteristics of analog signals, and changes them in a certain way, depending whether the digital value to be transmitted is 1 or 0. Signals can be modulated on the basis of their amplitude, their frequency, or their phase. GSM uses a specific phase modulation known as the Gaussian Minimum Shift Keying, or GMSK.

Time Division Multiple Access, or TDMA, splits a radio frequency into consecutive periods known as TDMA frames. A TDMA frame, in turn, consists of 8 short time units, which are referred to as time slots. These time slots represent the physical basis for data transmission. Therefore they are also called physical channels. The radio signal between the mobile station and the BTS consists of a continuous stream of time slots, organized in TDMA frames. Each connection is always assigned one timeslot.

Thus, the physical channels provide the resources used to transmit specific types of information. The types of information and the functions define the logical channels. The logical channels differ according to the function they fulfil in data transmission.

To organize the radio transmission, various frame types consisting of numbered timeslots are specified in GSM. The numbered timeslots are continuously numbered off by the mobile station.

A simple TDMA frame consists of eight physical channels, or timeslots. A timeslot is 0.557 ms long. Thus a simple TDMA frame is 4.62 ms long. The length of a timeslot is also referred to as the burst period. A burst is the content of a physical channel.

Information is transmitted as bursts each TDMA frame period. Traffic channels, i.e. time slots 0 to 7 in a basic TRX configuration, contain their information organised in 26 TDMA periods of time known as a multi-frame. This is 26 x 4.62 ms = 120 ms long. Signaling information, normally provided in time slot 0, is organised in 51 TDMA periods of 4.62 ms each, which makes 235 ms altogether. 26 of these "long" 51-multiframes, or 51 of the "short" 26-multiframes form a superframe, which is 6.12 seconds.

The largest transmission unit defined is the hyperframe, which contains 2,048 superframes and is 3 hours, 28 minutes, 53 seconds, and 760 ms long. TDMA frames, multiframes, superframes and the hyperframe can be considered as counters to organize user and signalling information within the TRX, and to support cyphering at the air interface.

4.6.2 The Physical Channels

The information which is physically transmitted over the air interface Um via the physical channels must be converted into a 16 kbps signal within a 2 Mbit/s Frame, which connects the BTS and the BSC as the A-bis interface. It is very important that all mobile stations within a cell send their digital information at the right moment, in order to avoid collisions at the timeslots of the air interface, which would destroy the transmitted information. Therefore, each mobile station sends its digital voice data at regular periodic intervals, using a different timeslot to the other mobile stations within the same cell. The medium for this transmission process is the timeslots, or physical channels. The content of such a channel is also known as a burst. Bursts consist of different data blocks containing payload- as well as security information, to guarantee high data reliability and transmission quality.

4.6.3 The Logical Channels

In GSM, there are two types of logical channels: the dedicated channels, and the common channels. Let's explain the difference between the two with a metaphor from gardening. If we want to water a whole area, and not a particular plant in it, we use a watering can.

This metaphor describes the common channels. These supply their data according to the principle of "equal shares for all", and are not directed to a specific target. They are used to broadcast information area-wide to all the mobile stations within the service area of a BTS. This is general signaling information, for example to log onto the network and cell-broadcast SMS.

If, on the other hand, we only want to water a specific plant and deliberately leave out the neighbouring ones, we use a jet of water. This metaphor corresponds to the Dedicated Channels. These are always directed to a particular addressee. Various types of signalling channels, known as the dedicated control channels, facilitate communication between the mobile station and the mobile radio network. And, of course, traffic channels that carry user speech and data also belong to this category. To understand the tasks of the individual logical channels, we will now look at how a mobile station logs on to the network.

After the subscriber has switched on his mobile station and typed in his PIN code, the mobile station searches for a network. But how does it log on to the network the subscriber is registered with? For this purpose, the BTS sends out the Frequency Correction Channel (FCCH) at short regular intervals, to help the mobile station find a frequency for downlink reception and adjust its frequency oscillator for the uplink transmission. To do so, it picks out the strongest received signal. The Synchronization Channel (SCH) then helps the mobile station to synchronize itself to timeslot 0 sent out by the BTS. This means the mobile station must adjust to the rhythm given by the BTS.

The SCH contains the TDMA frame number as well as the Base Station Identity Code, containing basic information about the network operator that can be compared with the info stored on the SIM card. After this step, the mobile is able to decide whether it has chosen the proper network. If not, it starts the same procedure again trying with the second strongest FCCH received.

While the mobile station uses the FCCH to adjust its frequency, and the SCH for synchronization and network identification, the Broadcast Control Channel (BCCH), which is also sent by the BTS, supplies the mobile station with additional information about the selected cell, for example for ciphering. For some Value Added Services, for example location-dependent services, additional information has to be transmitted from the BTS to the mobile. The Cell Broadcast Channel CBCH is used for this purpose to transmit geographical parameters, for example Gauss-Krueger-Coordinates of the BTS, to the mobile. The FCCH, SCH, BCCH and CBCH are Broadcast Channels, and exist only in the downlink. They are the first logical channels belonging to the Common Channels.

The mobile station has now adjusted its frequency and synchronized its TDMAs, and has picked out the best cell available. But before it can be reached by other subscribers, and before it can initiate calls, a Location Update and authentication procedure are necessary. Only after that is the mobile station logged on to the network and has radio coverage. It can now be reached by other mobile stations, or initiate a call. For this purpose, Common Control Channels are required. Common Control Channels are "point-to-multipoint" channels, which exist either only in the uplink, or only in the downlink.

When a subscriber is called, the Paging Channel (PCH) is broadcast in the downlink by all base stations within a Location Area, so that the mobile station concerned can react. To initiate a call, the mobile station sends out a Random Access Channel (RACH), which carries its identification and request, for example for registration, to the network. This channel only exists in the uplink. In return, the network sends the Access Grant Channel (AGCH) in the downlink direction, to assign resources to the mobile station, by granting it a Stand-Alone Dedicated Control Channel, SDCCH. The PCH, RACH and AGCH form the group of the Common Control Channels belonging also to the Common Channels.

A Stand-alone Dedicated Control Channel (SDCCH) has to be assigned to the mobile station to exchange the requested signaling with the network, for example authentication, ciphering or call set-up. Also, it assigns a traffic channel, and it transmits short messages.

The SACCH is always linked with an SDCCH or a traffic channel. It sends measurement reports to the network, and is used for power control and to handle the exact temporal alignment of the channels, the so-called Timing Advance.

If the subscriber moves into the service area of another BTS, the handover command needed is transmitted over the FACCH. This channel is also used for every call release. During the call, FACCH data is transported over the Traffic Channel assigned.

The Dedicated Control Channels are bidirectional point-to-point channels and belong to the group of Dedicated Channels. 

User speech and data are transmitted over the traffic channels we have already spoken about. Traffic channels are bidirectional, and also belong to the group of dedicated channels.

There are two different channel types supporting different gross bit rates. The Traffic Channel Full rate (TCH/F) has a gross bit rate of 22.8 kbps. It is used for speech encoded by a Full Rate or Enhanced Full Rate codec as well as for user data encapsulating a net bit rate of 9.6 kbps for standard bearer services, 14.4 kbps per timeslot in the case of HSCSD, or up to 21.4 kbps with GPRS. The Traffic Channel Half rate (TCH/H) supports 11.4 kbps and is only used for Half Rate codec speech.

Let us sum up what we just learned about the classification of logical channels. Common channels include FCCH, SCH, BCCH, PCH, RACH, AGCH and, finally, CBCH. All contain point-to-multipoint signaling information.

Dedicated Channels contain point-to-point signalling, such as SDCCH, SACCH and FACCH, or traffic, such as TCH/F and TCH/H.

4.7 Channel Coding 

To be able to detect and correct bit errors at the air interface, GSM performs channel coding. This procedure is organized in two consecutive processes: block coding and convolutional coding.

In block coding, the parameters describing the speech data are first subdivided into three classes, which define if the data is important, required or unimportant for speech intelligibility. With convolutional coding, the information relevant to speech intelligibility is doubled with an arithmetical operation. That means a copy of the data is made so the data can be restored if necessary. This procedure allows to fully compensate bit error rates of up to 12.5 % in the secured relevant data. Channel coding increases the bit rate necessary at the air interface from 13 to 22.8 kbps.

• Signal 2

Signal 2 is a utility created by iPhone Dev Team member planetbeing that displays information about the cell tower your iPhone is connected to as well as neighbouring cells. It detects and maps the location of all nearby cell towers so that the user can easily get a good idea of their cellular coverage or where to go to get the best cellphone reception. On an unlocked phone, a user can swap SIMs and determine which carrier is best for him or her based on cellular coverage. In addition the map, other useful technical data is displayed including RSCP (signal strength) in dBm and Ec/N0 (signal quality) to all neighbours, the PSC of the cells, as well as the frequency each cell is broadcasting in.

Signal 2 is exclusively available from the Cydia Store on jailbroken iOS devices with cellular radios running iOS 5 and above.

You can download from some buy it or search from some hack source such as:iphone.gsm.vn or appt.xellize.com.

• Flex Abis Mode Configuration Huawei

Description :
This parameter indicates the ways of assigning Abis timeslots for sites and cascaded sites.
Fixed: The Abis timeslots and Um timeslots are assigned fixedly.
Flex: Different services in a site share Abis timeslots, and different sites share Abis timeslots. The Abis timeslots are assigned dynamically according to actual commands.
SemiSolid: The Abis timeslots are fixedly assigned to the local BTS, and dynamically assigned to the upper-level BTS.

If the Flex Abis Mode of a site is set to be Flex, the Flex Abis Mode of the level-1 BTS that is cascaded with the site is set to be Flex.
If the Flex Abis Mode of a site is set to be SemiSolid, the Flex Abis Mode of the level-1 BTS that is cascaded with the site is set to be SemiSolid.
Speed Mode
Description :
This parameter determines the multiplexing mode of the timeslots on the Abis interface for the BTS.
There are two multiplexing modes: 64 kbit/s statistical multiplexing mode and 16 kbit/s physical multiplexing mode.
The 64 kbit/s statistical multiplexing mode is of the following types:

1:1 64 kbit/s statistical multiplexing mode
2:1 64 kbit/s statistical multiplexing mode
3:1 64 kbit/s statistical multiplexing mode
4:1 64 kbit/s statistical multiplexing mode
5:1 64 kbit/s statistical multiplexing mode
6:1 64 kbit/s statistical multiplexing mode
The six timeslot objects of the BTS are as follows:

OML: Operation and maintenance link of the BTS.

One BTS is configured with one OML.
In 64 kbit/s statistical multiplexing mode, one 64 kbit/s OML can be multiplexed with only the RSL of the same-level BTS.
In 16 kbit/s physical multiplexing mode, the 16 kbit/s OML occupies one 16 kbit/s timeslot.
ESL: Extend signaling link of the BTS.

When the BTS supports Flex Abis and uses the 64 kbit/s statistical multiplexing mode, the 64 kbit/s timeslot should be assigned, and the ESL is fixedly multiplexed with the OML onto one 64 kbit/s timeslot.
In 16 kbit/s physical multiplexing mode, timeslot resources are not assigned, and the ESL and OML use the same timeslot resources.
RSL: Signaling link of a TRX.

Each TRX is configured with one RSL.
In 64 kbit/s statistical multiplexing mode, one 64 kbit/s RSL can be multiplexed only with the OML of the same-level BTS or with the RSL of the same cabinet group.
In 16 kbit/s physical multiplexing mode, the 16 kbit/s RSL occupies one 16 kbit/s timeslot.
TCH: Traffic Channel of the TRX with a data rate of 16 kbit/s.

Idle: Idle timeslot of the BTS with a data rate of 16 kbit/s. The idle timeslot can be multiplexed only with the TCH of the same cabinet group onto a 64 kbit/s timeslot.

Semi: Monitoring timeslot of the BTS with the data rate of 8 kbit/s, 16 kbit/s, 32 kbit/s, and 64 kbit/s. The monitoring timeslot can be multiplexed with only the semipermanent link.

An E1 (2.048 Mbit/s) can be divided into thirty-two 64 kbit/s timeslots, and timeslot 0 is used for synchronization.

In statistical multiplexing mode, signaling timeslots (OML and RSL, RSL and RSL) are multiplexed onto one E1 timeslot.For example, in 4:1 multiplexing mode, three RSLs and one OML (or four RSLs) of one BTS can be multiplexed onto one 64 kbit/s timeslot.

When this parameter is set to 5:1 or 6:1, the Flex Abis function must be enabled.

In Flex Abis mode, a maximum of 1OML+2RSLs+1ESL can be multiplexed onto one 64 kbit/s timeslot.

When this parameter is set to 5:1, five RSLs are multiplexed onto one 64 kbit/s timeslot.

When this parameter is set to 6:1, six RSLs are multiplexed onto one 64 kbit/s timeslot.

After the Flex Abis function is enabled, an ESL is required to transmit the Abis timeslot dynamic connection message on the Abis interface. In 64 kbit/s statistical multiplexing mode, the ESLs and OMLs must be physically multiplexed onto the 64 kbit/s timeslot of the same E1. Therefore, the maximum number of RSLs multiplexed with the OMLs must be adjusted so that a maximum of two RSLs (three RSLs previously) can be multiplexed.

In 4:1 multiplexing mode, if there is one or two RSLs left, configure one or two 64 kbit/s timeslots with the 5:1 or 6:1 multiplexing mode (five or six RSLs multiplexed onto one 64 kbit/s timeslot) to avoid the usage of one extra 64 kbit/s timeslot on the Abis interface.

• PS Network Optimization Parameters

The TBF drop rate involves the uplink and downlink of the GPRS and EGPRS services. At the same time, the drop rate can be classified into the N3101, N3103, and N3105 overflow drops according to the timer.

Majority of the TBF drops is generated because the CS service preempts the dynamic channels.

1. When the TBF drop rate is degrading, check the GPRS or the EGPRS service according to the service type. If a service is faulty, the setting or the related parameter is incorrect.
2. If the problem occurs in certain cells, check whether the cell is configured with the dynamic PDCH. To solve the problem, disable the dynamic PDCH temporarily (or change the dynamic PDCH into the static PDCH).
3. If the problem is not caused by the incorrect setting, check whether the accessibility of each cell is of the same level. Then, determine whether the radio quality is affected according to the drop rate in the CS domain.

For how to check and process the TBF drop rate, refer to the GSM BSS Network KPI (TBF Drop Rate) Optimization Manual.
Owing to the unbalanced development of the GPRS service, the KPI in the PS domain of certain cells in the network cannot reach the standard, especially when the handover is not enabled in the PS service.As a result, the drop rate is high in certain cells and the problem cannot be processed.In this case, provides the carrier with a written report according to the actual condition. You need not process the problem.

Maximum Value of N3101
Value Range: 8-30
Unit: None
Default Value: 20
Description: Maximum value of N3101.

In uplink dynamic assignment mode, multiple MSs can share one uplink channel if the downlink data blocks carry the USF value.
After the network starts to assign a USF value to the uplink TBF (uplink TBF is established), N3101 is started.
The network waits for the uplink data sent by the MS on the reserved uplink RLC block corresponding to the USF.
If the MS sends a valid uplink RLC block, N3101 is reset.
Otherwise, N3101 is added on the network side.
When this counter overflows, the current uplink TBF is released abnormally.
Remarks

If this parameter is set to a lower value, the tolerance of the network to uplink errors decreases and the probability of abnormal TBF releases increases.
If this parameter is set to a higher value, the network still assigns uplink resources to an MS even though it does not receive correct MS data blocks because of MS activities. Therefore, network resources are wasted.
Maximum Value of N3103
Value Range: 2-5
Unit: None
Default Value: 3
Description: Maximum value of N3103.

If the network receives the last RLC data block when the uplink transmission is complete, it sends the MS a Packet Uplink Ack/Nack message with FAI=1 and starts N3103.
If the network does not receive a packet control acknowledgment message within scheduled time, N3103 is added on the network side and the network resends the Packet Uplink Ack/Nack message.
When this counter overflows, the network starts T3169.
When T3169 expires, the current TBF is released abnormally.
Remarks

If this parameter is set to a modest value, the tolerance of the network to uplink errors decreases and the probability of abnormal uplink TBF releases increases.
If this parameter is set to a higher value, the network still assigns radio resources to an MS even though it does not receive a correct packet control acknowledgment message because of MS activities.
Therefore, network resources are wasted.

Maximum Value of N3105
Value Range: 3-10
Unit: None
Default Value: 10
Description: Maximum value of N3105.

After the a downlink TBF is established, the network starts N3105.
After the network set the RRBP on the downlink RLC data block, it resets N3105 when receiving a valid packet acknowledgment message on the uplink RLC data block corresponding to the RRBP.
Otherwise, N3105 is added on the network side and the network resends the downlink RLC data block where the RRBP is set.
When N3105 overflows, the network starts T3195.
When T3195 expires, the current TBF is released abnormally.
Remarks

If this parameter is set to a lower value, the tolerance of the network to downlink errors decreases and the probability of abnormal downlink TBF releases increases.
If this parameter is set to a higher value, the network still assigns radio resources to an MS even though it does not receive a correct packet control acknowledgment message because of MS activities. Therefore, network resources are wasted.

• BTS Multiplexing Mode Huawei BSC6000

This describes how to change the BTS multiplexing mode to improve the utilization of the Abis timeslot resources. If the BTS uses the TDM transmission mode, the requirement for BTS timeslots changes when the BTS traffic increases. In this case, the BTS multiplexing mode should be changed.

BTS multiplexing mode indicates the Abis timeslot multiplexing mode of the BTS. It consists of the following seven types:

• 1:1 64 kbit/s statistic multiplexing mode
• 2:1 64 kbit/s statistic multiplexing mode
• 3:1 64 kbit/s statistic multiplexing mode
• 4:1 64 kbit/s statistic multiplexing mode
• 5:1 64 kbit/s statistic multiplexing mode
• 6:1 64 kbit/s statistic multiplexing mode

Physical 16 kbit/s multiplexing mode
The bandwidth of each E1 is 2.048 Mbit/s and is divided into 32 timeslots. The transmission rate on each timeslot is 64 kbit/s. The bandwidth of each T1 cable is 1.544 Mbit/s and is divided into 24 timeslots. There are six types of Abis timeslot objects:

Operation and Maintenance Link (OML). One BTS has one 64 kbit/s OML. The OML cannot be multiplexed with the RSL of another BTS.
Radio Signaling Link (RSL)Each TRX has one 64 kbit/s RSL. The RSL cannot be multiplexed with the OML or RSL of another BTS.
Extend Signaling Link (ESL)If the Abis timeslot assignment mode of the BTS is set to Flex, one BTS requires one 64 kbit/s ESL to transfer the Abis timeslot dynamic connection message. In 64 kbit/s statistic multiplexing mode and physical 16 kbit/s multiplexing mode, the ESL can be multiplexed only with the OML of the same BTS onto a 64 kbit/s timeslot on the same E1.
Traffic Channel (TCH)The transmission rate on the TCH is 16 kbit/s.
IdleIdle timeslots of the BTS. The rate of the idle timeslots is 16 kbit/s. The idle timeslots can be multiplexed only with the TCH in the same cabinet group.
SemiMonitoring timeslots of the BTS. The rates of monitoring timeslots are 8 kbit/s, 16 kbit/s, 32 kbit/s, and 64 kbit/s. Monitoring timeslots cannot be multiplexed with other types of timeslot objects.
When changing the BTS multiplexing mode, pay attention to the following:

If the BTS multiplexing mode is 5:1 or 6:1, the Abis timeslot assignment mode must be set to Flex Abis.
If the Abis timeslot assignment mode of a BTS must be set to Flex Abis, you should add an ESL for the BTS, and change the maximum number of RSLs that can be multiplexed with the OML.
When the BTS multiplexing mode is 4:1, if the Abis timeslot assignment mode is Fix Abis, a maximum of three RSLs can be multiplexed; if the Abis timeslot assignment mode is Flex Abis, a maximum of two RSLs can be multiplexed.
When the BTS multiplexing mode is 4:1, if the Abis timeslot assignment mode is modified from Fix Abis to Flex Abis, a redundant RSL will occupy a 64 kbit/s timeslot, thus reducing the utilization of resources. In this case, you can change the BTS multiplexing mode to 5:1 or 6:1. Therefore, more RSLs can be multiplexed onto one 64 kbit/s timeslot. This saves the system resources.
If the Abis timeslot assignment mode is Fix Abis or SemiSolid, the BTS multiplexing mode cannot be set to 5:1 or 6:1.
On the BTS cascading main link, the BTS in physical 16 kbit/s multiplexing mode cannot coexist with the BTS in other multiplexing mode.
When you change the multiplexing mode of a BTS from non-4:1 to 4:1 or from 4:1 to non-4:1, the related cell parameters are modified automatically.
When you change the multiplexing mode of a BTS from non-4:1 to 4:1, the measurement report preprocessing parameter of all the cells under this BTS is set to Yes and the sent frequency of preprocessed measurement report is set to once every second automatically.
When you change the multiplexing mode of a BTS from 4:1 to non-4:1, the measurement report preprocessing parameter of all the cells under this BTS is set to No and the sent frequency of preprocessed measurement report is set to twice every second automatically.
Multiplexing Mode :

BTS multiplexing mode indicates the Abis timeslot multiplexing mode of the BTS. The OML and RSL are signaling links. Only signaling links can be multiplexed together. Statistic multiplexing mode means that the OML and RSL use one E1 timeslot through Time Division Multiplexing (TDM). For example, 4:1 multiplexing mode means that three RSLs and one OML are multiplexed onto one 64 kbit/s timeslot.

If the BTS multiplexing mode is set to 5:1, one OML, one ESL, and two RSLs are multiplexed onto one 64 kbit/s timeslot, or five RSLs are multiplexed onto one 64 kbit/s timeslot.

If the BTS multiplexing mode is set to 6:1, one OML, one ESL, and two RSLs are multiplexed onto one 64 kbit/s timeslot, or six RSLs are multiplexed onto one 64 kbit/s timeslot.

If the BTS multiplexing mode is set to Physical 16 kbit/s, one OML (ESL) and one RSL occupy one 16 kbit/s timeslot respectively.

• Network Architecture

Objectives

On completion of this module you will be able to ...

• Recognize the different GSM network subsystems and describe their functions.
• List the different GSM network elements.
• Explain the tasks and functions of the GSM network elements.
• Describe the structure of a GSM network.

Content

3.1	Network Elements and their Basic Functions
3.1.1	Base Station Subsystem (BSS)
3.1.2	Network Subsystem (NSS)
3.1.3	Operation & Maintenance Subsystem (OMS)
3.1.4	Additional GSM Components
3.2	GSM Network Topology

3.1 Network Elements and their Basic Functions

For the subscriber, a mobile telephone call is a simple process. In reality, though, this call is only possible thanks to a complex network architecture consisting of various different network elements. In this lesson, you' ll get to know the individual elements of the GSM network and their basic functions.

The Base Station Subsystem BSS provides the connection between the mobile stations and the Network Subsystem NSS. The NSS forwards user signals to other mobiles via the BSS or subscribers in the Public Switched Telephone Network (PSTN), and provides necessary customer data. The Operation & Maintenance Subsystem (OMS) monitors BSS and NSS performance, and remotely debugs occurring faults in the network elements.

Additional components such as interface elements to data networks, the Short Message Service Center or the Voice Mail System complete the GSM system architecture.

3.1.1 Base Station Subsystem

The Base Station Subsystem ensures as complete a network coverage as possible and includes a large number of structurally organized radio cells. It consists of the following elements:

• The Base Transceiver Station
• The Base Station Controller

and

• The Transcoder.

The central element of one cell of this kind is a transmitting and receiving unit known as a Base Transceiver Station (BTS). This makes the connection to the mobile station via the air interface and controls the transceiver (TRX). The transceiver, the central functional unit of the BTS, maintains calls to a maximum of 8 mobile stations via one frequency pair each. The BTS is also responsible for the monitoring of the signal quality and the encoding and modulation of useful signals. Via the A-bis interface, it forwards calls, signals and control information destined for the OMS and the NSS to the Base Station Controller (BSC).

Several BTSs are controlled by the Base Station Controller, or BSC.

This assigns free radio channels in the TRX for the link to the mobile station. It controls the necessary output power for mobile station and TRX. It monitors the existing radio link to and from the mobile station and controls handover between neighboring radio cells if they are under its control. During an existing radio connection, the BSC monitors its quality and controls disconnection of the radio link when the call is over. The BSC communicates with the transcoder (TC) via the A-ter interface.

The transcoder is the third element in the BSS and is needed to convert 64 kbps original speech into a 16 kbps signal of speech description parameters to ensure a spectrum-efficient modulation on the air interface. BTS, BSC and TC together form the Base Station Subsystem (BSS).

3.1.2 Network Subsystem

The Base Station Subsystem forwards the signals to the Network Subsystem (NSS) where speech and circuit-switched data are controlled and forwarded to other networks if necessary. The NSS provides data relevant to security and mobility.

The speech signals processed by the transcoder reach the Mobile Services Switching Center (MSC) via the A interface. The MSC serves as a digital exchange for the forwarding of messages, connecting mobile subscribers with each other or with subscribers in other networks such as the Public Switched Telephone Network, the ISDN network, or data networks.

The MSC is responsible for the following functions:

It forwards incoming and outgoing calls.

It makes a connection to other MSCs in the same mobile radio network and makes connections with other mobile radio networks and to fixed networks. It monitors and controls the calls.

It is responsible for call data acquisition and the forwarding of signalling information to connected registers or data bases. In order to monitor, route and control mobile telephone calls in GSM networks, several registers are connected to the MSC.

One of these registers is the Visitor Location Register (VLR), which is usually to be found in the MSC, but is a functional unit in its own right. It is designed as a dynamic subscriber file with dedicated geographical areas of responsibility, the so-called Location Areas. The VLR acquires the data of all GSM customers in its areas and is always well informed of their whereabouts. It assists the MSC in the acquisition of charge-relevant data with subscriber information. The bills are prepared from these data in the Billing Center. But where does the VLR get the GSM customer data from?

For GSM customer data acquisition, there is a register, the so-called Home Location Register (HLR), in which each network operator registers the customer data necessary for dealing with traffic. The HLR supplies these data to all VLRs in which the GSM customers involved are to be found at any given moment. Inversely, the VLR in question informs the HLR of the location area of the customer, and is thus able to give routing information when calls come in. The HLR data contain information on access rights with regard to roaming, service rights with regard to voice, fax and data services, and additional subscribed services.

The Authentication Center (AuC) contains the customer data necessary to protect connections against unauthorised access, and is mostly integral to the HLR. The AUC checks the information stored in the Subscriber Identity Module, that is the SIM card, for correspondence with its own register. If the data proves to be identical, the authentication of the subscriber is successful, and he is given permission to enter the network. If the SIM card is stolen, authorisation to access the network is disabled very easily via the AUC. Additionally, the AUC provides necessary information to cipher the air interface.

The Equipment Identity Register (EIR) can be implemented as an option by the network operator. The EIR permits the detection of stolen terminal equipment used in GSM networks by checking the IMEI (International Mobile Equipment Identity) against the data stored in the EIR. This check is carried out independently of the SIM card, and only applies to the mobile station in question.

All the components which control and forward the call, and are responsible for security and mobility management, that is the MSC, HLR, VLR, AUC and EIR, form the Network Subsystem (NSS).

3.1.3 Operation & Maintenance Subsystem (OMS)

The GSM network is monitored and controlled from a central point. This is the Operation and Maintenance Center (OMC).

The OMC has the following tasks:

1. The Fault Management system analyses alarms from the BSS elements. When faults occur, they are eliminated when necessary via software command or in situ by technicians.

2. The Configuration Management function installs the software when new BSS network elements are implemented, manages hardware inventory lists, and changes operation parameters, for example for radio frequencies of a BTS.

3. The Software Management system feeds in new software or updates and manages the software inventory lists.

The Network Management Center (NMC) assumes special functions in the context of OMS which are not defined in the GSM standard but are based on definitions of the International Standardization Organization (ISO), and on recommendations of the International Telecommunication Union (ITU).

An NMC carries out functions of Performance Management

• Alarms and fault elimination times are evaluated statistically.
• Capacity bottlenecks in the network are detected.

and

• The service quality is monitored, for example the Dropped Call Rate in percent. Depending on the network operator, the NMC functions are carried out in a centralised or decentralised way in the geographical areas.

All NMC and OMC of a certain defined geographical area form the third subsystem, the Operation and Maintenance Subsystem, or OMS.

The three subsystems BSS, NSS and OMS are vital for the operation of a GSM network. The interfaces within and between the subsystems are mostly specified by the ETSI.

3.1.4 Additional GSM Components

For dealing with customer support and supplying certain services, GSM includes a number of additional components. The Administration & Billing Center ABC transfers customer data to the appropriate registers of the NSS and into the AUC and the HLR. The Administration Center is connected to the Personalization Center for SIM Cards (PCS) via an interface. This makes it possible to disable the SIM card if necessary and protect it from abuse. The so-called Call Detail Records are used in the Billing Center for bill preparation.

The Voice Mail System (VMS) is a memory system for voice, data and fax messages spread over the network, i.e. a large-scale answering machine. If a subscriber has switched off his mobile station or can't be reached for other reasons, the messages are not sent to his mobile station but are fed directly into the VMS and stored there. The subscriber can either request them from the VMS or he is notified via SMS. The VMS can have interfaces to several MSCs and to the Short Message Service Center.

Via the Short Message Service Center (SMS-C), network operators, service providers and private customers can send short messages directly onto the mobile station of any subscriber. In the SMS-C, the short messages are stored temporarily and forwarded to the recipient.

Point-to-point short messages are alphanumerical messages with a maximum basic length of 160 characters, which are entered directly via the keyboard of the mobile phone. Compression and concatenating techniques increase the number of transmitted characters. The Cell Broadcast SMS, i.e. the service offering point-to-multipoint short messages, is a "one-way" communication from the network to all mobile phones in certain geographical areas. The messages with a basic length of 93 alphanumerical characters are entered in the OMC, fed centrally into the BSC, and transmitted to the mobile stations via all connected BTSs at regular intervals.

In order that data can be fed into the GSM network from packet-switched networks such as the Internet or company Intranets, a so-called Interworking Function (IWF) is required. This is an external data server connected to the different data networks. The IWF translates the unstructured incoming packet-switched data into circuit-switched signals which can be understood by GSM. A firewall upstream of the IWF protects the GSM network from unauthorised access by hackers.

In GSM Phase 2, only circuit-switched data services are supported. The Interworking Function (IWF), integral to the MSC, connects the circuit-switched GSM data traffic to the existing packet-oriented networks, in other words, the Internet, corporate networks, public data networks and WAP servers. It converts protocols and adapts the data rate for the BSS.

3.2 GSM Network Topology

In GSM, the Public Land Mobile Network (PLMN) is a cellular network with a hierarchical structure.

The smallest unit is the radio cell, which the BTS supplies with frequencies, or, in other words, radio channels. It provides the network coverage. Several radio cells are put together to form administrative areas controlled by a BSC. Various areas controlled by one BSC each form a location area controlled by a VLR. It is also possible for a Location Area to cover one BSC only, or even one cell, if reasonable. If a mobile phone subscriber changes to a new Location Area, a Location Update takes place automatically, so the location of the subscriber is known to the network via a VLR linked to the MSC.

If a BTS is in the centre of exactly one cell, we speak of an Omni directional radio cell. The BTS transmits its frequencies with Omni directional characteristics and a high output.

Omni directional radio cells are used particularly in relatively sparsely populated rural areas. In densely populated areas, though, the network must supply higher capacities. One way of doing this is the sectorization of radio cells. With a sectored radio cell, the BTS can supply up to three radio cells in 3 times 120 degrees with several frequencies each.

On motorways, Base Transceiver Stations are preferentially configured in 2 sectors. For example, the BTS transmits frequencies in two times 180 degrees. The cell is aligned along the course of the road to be covered.

In densely populated cities, we often find a combination of Omni directional cells and sector cells. This is because there can often be zones of missing coverage between sector cells. A superordinated Omni directional umbrella cell takes over the radio supply for scattered individual mobile stations located in these locally occurring receptionless zones and for rapidly moving mobile stations used on motorways and in high-speed trains. Rapidly moving mobile stations in particular are supplied via the larger umbrella zones, in order to avoid as far as possible handovers taking place in rapid succession.

In order to supply areas with a large number of mobile phone users, so-called microcells are used.

Thus, for example, BTS with a low output are used in underground stations. These take over the radio supply on the platform or, with special antennae, in the subway tunnels.