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8/6/2019 BSNL Nokia BSS Implementation
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Fundamentals of the Nokia BSSImplementation
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Contents
1 Transcoding and the TCSM2E.............................................................. 5
1.1 TCSM2E functions................................................................................... 51.1.1 Transmission between MSC and BSC .................................................... 51.2 Transcoding............................................................................................. 71.2.1 TRAU frame............................................................................................. 91.2.2 Submultiplexing ..................................................................................... 121.3 Timeslot allocation................................................................................. 121.4 TCSM2E units and configurations ......................................................... 141.5 TRAU operating modes......................................................................... 151.5.1 Discontinuous transmission................................................................... 161.5.2 Fixed and adaptive gain adjustment...................................................... 181.5.3 Acoustic echo cancellation .................................................................... 181.6 Operation of TCSM2E ........................................................................... 19
1.6.1 Supervision by the BSC......................................................................... 191.6.2 Alarms ................................................................................................... 191.6.3 Reliability ............................................................................................... 211.6.4 State administration and reconfiguration............................................... 211.7 BSC functions........................................................................................ 231.7.1 Management of radio channels ............................................................. 241.7.1.1 Channel types........................................................................................ 251.8 Management of frequency hopping....................................................... 271.8.1 Handover management ......................................................................... 281.8.2 Signalling management ......................................................................... 311.8.3 Management of terrestrial channels ...................................................... 341.8.4 Interfaces............................................................................................... 351.8.4.1 A Interface to the MSC .......................................................................... 351.8.4.2 Abis interface (BSC-BTS)...................................................................... 351.8.4.3 Q3.......................................................................................................... 351.8.5 Operation and maintenance.................................................................. 361.8.6 Measurements and observations .......................................................... 371.8.7 Support of call control functions ............................................................ 381.8.8 Ciphering management ......................................................................... 381.9 Cellular network functions ..................................................................... 391.9.1 Data services......................................................................................... 391.9.2 Support for multiple speech codecs ...................................................... 391.9.3 Dual band GSM/DCS ............................................................................ 40
1.9.4 Intelligent underlay overlay.................................................................... 401.9.5 Directed retry ......................................................................................... 421.10 Architecture of DX 200 BSC.................................................................. 421.10.1 General design ...................................................................................... 421.10.2 Redundancy principles .......................................................................... 451.10.3 Reliability ............................................................................................... 45
2 Understanding the Nokia BTS............................................................ 462.1 Introduction............................................................................................ 462.2 Problems and solutions for the air interface .......................................... 46
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Contents
2.2.1 Multipath propagation.............................................................................482.2.2 Flat fading ..............................................................................................48
2.2.2.2 Channel coding ......................................................................................512.2.2.1 Frequency hopping ................................................................................50
2.2.2.3 Interleaving.............................................................................................52 2.2.2.4 Antenna receiver diversity......................................................................532.2.3 Selective fading......................................................................................542.2.4 Modulation..............................................................................................54 2.2.5 Shadowing .............................................................................................562.2.6 Propagation delay ..................................................................................572.2.6.1 Frequency correction and synchronisation.............................................592.2.6.2 Transmission of BCCH information........................................................602.2.7 Ciphering................................................................................................60 2.2.8 Signalling................................................................................................60 2.2.9 Measurements .......................................................................................622.3 Implementation.......................................................................................63 2.3.1.1 Transmission unit ...................................................................................642.4 Control functions ....................................................................................642.4.1.1 Operation and maintenance...................................................................642.4.1.2 External alarms and controls..................................................................642.4.1.3 Master clock ...........................................................................................642.4.1.4 Frequency hopping control.....................................................................652.5 Transceiver (TRX)..................................................................................652.5.1 Combiner/coupler (antenna filter)...........................................................66
3 The Traffic Channels in the BSS.........................................................67
4 A Interface.............................................................................................69
5 Ater Interface ........................................................................................71
6 Base Station Controller, BSC..............................................................73
7 The Abis Interface ................................................................................75
8 The Base Transceiver Station, BTS....................................................77
9 Air Interface ..........................................................................................79
10 Traffic Channels in Different Interfaces.............................................80
11 Signalling in BSS..................................................................................82
12 Signalling Layers in BSS.....................................................................8312.1 Signalling Model for GSM.......................................................................8312.1.1 The Physical Layer.................................................................................8512.1.2 The Link Layer .......................................................................................8512.1.3 The Network Layer.................................................................................85
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12.1.4 The Applications.................................................................................... 85
13 Signalling Protocols............................................................................ 8713.1 CCS7..................................................................................................... 8713.1.1 Message Transfer Part, MTP ................................................................ 8913.1.1 Signalling Connection Control Part, SCCP............................................ 9313.2 Link Access Protocol on D-channel, LapD ............................................ 9613.2.1 LapD Frame Format .............................................................................. 9613.2.2 LapDm................................................................................................. 100
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the 2 Mbit/s links between the MSC and the BSC. Up to four E1 interfaces
can be submultiplexed into one 2 Mbit/s link.
MSC
Transcoder, TCSM2E
30 TCH / 64 kbit/s
CCS7 orX.25 / 64 kbit/s
BS
A interface Ater interface
2 Mbit/s2 Mbit/s
DSP90
DSP1
90 TCH / 16 kbit/s
3 x CCS7 or X.25 / 64 kbit/s3 x
2 Mbit/s
2 Mbit/s
2 Mbit/s
Figure 2. Transcoder TCSM2E and A-/A-ter interfaces
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1.2 Transcoding
The Transcoder converts 160 A-law PCM samples of an 8 bit speech channel
(20 ms) into a vocoder block of 260 bits in the downlink direction and vice
versa in the uplink direction. Regular Pulse Excited-Long Term Prediction,
RPE-LTP, vocoder is used in the Transcoder. For each speech channel there is
a Digital Signal Processor, DSP, providing the transcoding / de-transcoding.
In the downlink direction, 20ms samples are taken from the incoming 2
Mbit/s signal from the MSC (20 ms = 160 x 2 Mbit/s frames). 160 x 8 bit
timeslots are then brought into a Digital Signal Processor, DSP, where the
RPE-LTP vocoding is done (160 x 8 bits => 260 bit vocoder block). The
Transcoder creates a TRAU frame that is then packed into the traffic channel
sub-timeslot in the 2 Mbit/s frame in the outgoing direction, in the Aterinterface. The TRAU frame is used between two DSPs, one in the Transcoder
and another one in the TRX of the BTS. One TRAU frame consists of 260 bits
speech / data information together with 60 bits that are used for creating the
TRAU frame structure (inband signalling between the two DSPs).
Transcoder software, i.e. TRAU SW, is capable of handling both full rate (FR
and enhanced full rate speech coding and FR data) and half rate (HR speech
coding and HR data) traffic channels. It is possible to configure the TCSM2
equipment with O&M commands to be one of three types: full rate, half rate
or dual rate (DR). DR refers to the ability of the TRAU to change between FR
and HR traffic channels in real time according to the control information
received from the base station. (In this chapter, FR refers to either FR codingor enhanced full rate (EFR) coding. HR refers to HR coding).
Full rate transcodingmeans that the subchannel handled by the TRAUis a 16 kbit/s channel, containing 16 kbit/s FR or EFR traffic.
Half rate transcodingmeans that the subchannel handled by the TRAUis an 8 kbit/s channel, containing 8 kbit/s HR traffic.
In dual rate transcoding, the subchannel handled by the TRAU is a16 kbit/s subchannel, containing either 16 kbit/s FR or EFR traffic or
8 kbit/s HR traffic.
There are always two HR traffic channels or one FR traffic channel in one
16 kbit/s subchannel between the BSC (Base Station Controller) and the BTS(Base Transceiver Station), i.e. in the Abis interface. In case of DR mode,
however, the TRAU handles only one HR traffic channel or one FR traffic
channel in one 16 kbit/s subchannel in the Ater interface. This is shown in
Figure 3. (Please note that this picture is only an example and that it does not
show all possible configurations).
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Figure 3. The location of the TRAU in the BSS and the use of FR andHR traffic channels
The EFR compression method used is ACELP (Algebraic Code Excited
Linear Prediction). The HR speech compression method is also a CELP-type
(Code Excited Linear Prediction) VSELP-coding (Vector Sum Excited Linear
Prediction), which uses the analysis by synthesis method with fixed code
books and 10th order LPC for short term prediction and adaptive code bookwith fractional lags for long-term prediction, resulting in a bit rate of
5.6 kbit/s.
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160 159 158157 156 157 154 7 6 5 4 3 2 1 160 159 158157 156 157 1547 6 5 4 3 2 1
031 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
B1 B2 B3 B4 B5 B6 B7 B8
2 Mbit/s Frame, 125 us
20 ms sample, 160 x 2 Mbit/s Frames
64 kbit/s Timeslot, 8 bits
DSP for TS 1160 x Timeslot 1 from the 160 x 2 Mbit/s Frames
2 Mbit/s frames from the MSC to Transcoder
160 159 158157 156 157 154 7 6 5 4 3 2 1 160 159 158157 156 157 1547 6 5 4 3 2 1
031 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
2 Mbit/s Frame, 125 us
20 ms sample, 160 2 Mbit/s Frames
64 kbit/s Timeslot where only first two bits are used for traffic channel
160 x 2 bits from Timeslot
2 Mbit/s frames from the Transcoder to the BSC (SM2M)
DSP for TS 31
B1 B2 1 1 1 1 1 1
Figure 4. Transcoding
1.2.1 TRAU frame
The Transcoder creates the TRAU frame , which is used between the DSP in
the Transcoder and the DSP in the TRX of the BTS for carrying the vocoder
traffic channel blocks as well as inband signalling when the call is connected.
The inband signalling thus gives the transcoder the necessary information
about the type of speech coding algorithm, information as to the nature of the
call (i.e. speech or data) information regarding the use of DTX transmission
etc.. The TRAU frame is packed into the 2 Mbit/s frame. That is, the inband
signalling as well as the 260 bit vocoder block are transmitted within the 16kbit/s subchannel. 260 bits carry the vocoder block (13 kbit/s) and 60 bits (3
kbit/s) are used for TRAU frame alignment word (TS0 and 1), Frame type,
(C1-C4), Channel type (C5), Time Alignment info (C6-C11), Frame indicator
(C12-C16), DTX ON/OFF (C17), Spare Bits (C18-C21) and Time Alignment
(T1-T4).
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0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 01 C1 C2
Bit number
1 2 3 4 5 6 7 8
C3 C4 C5 C6 C7
C8
1D8
1
D23
1
D38
1D53
1
D681
D83
1
D98
1D113
C9 C10 C11 C12 C13 C14 C15D1 D7
D112
D114 D115 D116 D117 D118 D119 D120
D106
Octet no.
1
23
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
2728
29
30
31
32
33
34
35
36
37
38
3940
D37
D39 D40 D41 D42 D43 D44 D45
D46 D52
D2 D3 D5 D6
D9 D10 D11 D12 D13 D14 D15
D4
D16 D17 D18 D19 D20 D21
D24 D25 D26 D27 D28 D29 D30D31 D32 D33 D34 D35 D36
D22
D54
D47 D48 D50 D51D49D55 D56 D57 D58 D59 D60
D61 D62 D63 D64 D65 D66 D67
D69D70
D71 D72D73 D74 D75
D76 D77 D78 D79 D80 D81 D82
D84
D91D99
D90D97D105
D85
D92
D100D107
D86 D87 D88 D89
D93
D101D108
D94 D95 D96D104
D111D103D102
D109 D110
1
D1281
1
1
1
1
1
1
1
1
D143
D158
D173
D188
D203
D218
D233
D248
C18
D121 D122 D123 D124 D125 D126 D127
D129 D130 D131 D132 D133 D134 D135
D256 D257 D258 D259 D260 C16 C17C19 C20 C21 T1 T2 T3 T4
D136 D137 D138 D139 D140 D141 D142D144 D145 D146 D147 D148 D149 D150D151 D152 D153 D154 D155 D156 D157
D159 D160 D161 D162 D163 D164 D165
D166 D167 D168 D169 D170 D171 D172D174 D175 D176 D177 D178 D179 D180D181 D182 D183 D184 D185 D186 D187D188 D189 D190 D191 D192 D193 D194D195 D196 D197 D198 D199 D200 D201
D204 D205 D206 D207 D208 D209 D210
D211 D212 D213 D214 D215 D216 D217
D219 D220 D221 D222 D223 D224 D225
D226 D227 D228 D229 D230 D231 D232D234 D235 D236 D237 D238 D239 D240D241 D242 D243 D244 D245 D246 D247D249 D250 D251 D252 D253 D254 D255
Table 1. TRAU frame structure
The TRAU frame is disassembled in the TRX of the BTS. This is done by the
DSP , which then performs block coding, convolutional coding, interleaving
and ciphering for the 260 bits. The DSP also formats the burst. The burst is
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then sent to the Carrier Unit/Tx part of the TRX, which performs GMSK
modulation and up-conversion to the RF in the downlink direction. The RF
signal is then brought to the antenna and finally transmitted into the air. In the
Mobile Station, MS, all decoding is carried out and digital speech is converted
into analogue format. In the uplink direction, the MS does the transcoding, block coding, convolutional coding, interleaving, ciphering and formats the
burst. This is then sent to the BTS through the air. In the BTS, the DSP
creates the TRAU frame and sends the vocoded block to the Transcoder
where the de-transcoding is done.
The Transcoder is responsible for vocoded block timing, adjusting the phase
of the blocks in the downlink direction, to achieve a minimum delay.
DSP1
DSP90
TCSM2E
BSC
BTSMS
DSP
TRX(x)
DSP
A-ter Abis Air A-Interface
64 kbit/s 16 kbit/s
Figure 5. Traffic channels between TCSM2E and MS
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1.2.2 Submultiplexing
The submultiplexer is integrated in the Transcoder, TCSM2E. Thesubmultiplexing function provides the possibility to reduce the number of 2
Mbit/s links on Ater-interface. This gives a much more efficient use of the 2
Mbit/s PCM line. Normally, the Transcoder is located at the MSC site even
though it is controlled by the BSC. The TCSM2E can submultiplex up to four
transcoded full rate traffic channel 2 Mbit/s links into one 2 Mbit/s link. An
example of a full rate timeslot is shown in table 2. In the case of half rate
traffic channels seven 2 Mbit/s links can be submultiplexed into one Ater
interface link.
1.3 Timeslot allocation
The routing of signals between the BSC side and MSC side trunk interfaces is
controlled by the chosen timeslot allocation. Different allocations can be
programmed and loaded into the TRCO, the master unit within the TCSM2.
The selected allocation type must also be supported by the BSC. Different
TCSM2Es of the same BSS may use different allocations. Though timeslot
allocations will be described in a later section, a typical full rate allocation is
shown here:
Table 2. Timeslot allocation for 16 kbit/s bit rate channels (typically fullrate or enhanced full rate) on the Ater 2 Mbit/s interface withthe TCSM2E
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BIT-> 1 2 3 4 5 6 7 8
TS-> 0 LINK MANAGEMENT
1 LAPD TCH.1 TCH.2 TCH.3 A-PCM 1
2 TCH.4 TCH.5 TCH.6 TCH.7
3 TCH.8 TCH.9 TCH.10 TCH.11
4 TCH.12 TCH.13 TCH.14 TCH.15
5 TCH.16 TCH.17 TCH.18 TCH.19
6 TCH.20 TCH.21 TCH.22 TCH.23
7 TCH.24 TCH.25 TCH.26 TCH.27
8 TCH.28 TCH.29 TCH.30 TCH.31
9 x TCH.1 TCH.2 TCH.3 A-PCM 2
10 TCH.4 TCH.5 TCH.6 TCH.7
11 TCH.8 TCH.9 TCH.10 TCH.11
12 TCH.12 TCH.13 TCH.14 TCH.15
13 TCH.16 TCH.17 TCH.18 TCH.19
14 TCH.20 TCH.21 TCH.22 TCH.23
15 TCH.24 TCH.25 TCH.26 TCH.27
16 TCH.28 TCH.29 TCH.30 TCH.31
17 x TCH.1 TCH.2 TCH.3 A-PCM 3
18 TCH.4 TCH.5 TCH.6 TCH.7
19 TCH.8 TCH.9 TCH.10 TCH.11
20 TCH.12 TCH.13 TCH.14 TCH.15
21 TCH.16 TCH.17 TCH.18 TCH.19
22 TCH.20 TCH.21 TCH.22 TCH.23
23 TCH.24 TCH.25 TCH.26 TCH.27
24 TCH.28 TCH.29 TCH.30 TCH.31
25 x TCH.1 TCH.2 TCH.3 A-PCM 4
26 TCH.4 TCH.5 TCH.6 TCH.7
27 TCH.8 TCH.9 TCH.10 TCH.11
28 TCH.12 TCH.13 TCH.14 TCH.15
29 TCH.16 TCH.17 TCH.18 TCH.19
30 TCH.20 TCH.21 TCH.22 TCH.23
31 TCH.24 TCH.25 TCH.26 TCH.27
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Note:
It is recommended to add CCS7 links starting from TSL 31, (i.e.
31,30,29) to maximise on the number of traffic channels.
1.4 TCSM2E units and configurations
The TCSM2E contains 4 different types of plug-in units: TRCO, TR16,
ET2Es as well as a power supply. Basic functions are as follows:
TRCO: master unit, internal supervision of the other plug-in units,synchronisation, operation and maintenance
TR16: contain the DSPs (16 per card) and therefore perform the speech
coding and other transcoding functions
ET2E: provide the external 2Mbit/s interface to the BSC and MSC
ET2EET2E
TRCOTR16
TR16
TR16
TR16
TR16
TR16
ET
ET
TCSM2E BSC
ET2E
ET2ETRCOTR16
TR16
TR16
TR16
TR16
TR16
to MSC
Ater
A
Figure 6. TCSM2E units and configurations
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The actual plug-in unit configuration depends on the desired traffic channel
capacity. Additional capacity can be added by adding additional TCSM2E
units. The total capacity of the Transcoder rack is 8 transcoder units or a
maximum TCH rack capacity of 8 X 120 TCH or 960 TCH. However, each
TCSM2E unit functions independently and is dependant almost solely on theexternal connection to the BSC and the MSC.
Figure 7. Transcoder rack
1.5 TRAU operating modes
TRAU software is capable of handling both full rate (FR) and half rate (HR)
speed traffic channels, i.e. 16 kbit/s and 8 kbit/s TRAU frames. It is possible
to configure the TCSM2E equipment to be one of three types: full rate, half
rate or dual rate (DR). DR refers to the ability of the TRAU to change
between FR coding, Enhanced Full Rate (EFR) coding traffic channels or HR
coding traffic channels in real time, according to the control information
received from the Base Station (BTS).
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The Transcoder performs the speech activity detection in the downlink
direction. If no speech is detected in the TC, SID-frames (Silence Descriptor),
which include the characteristics of the background noise, are sent to the MS
where it is able to generate comfort noise. In the downlink direction, DTX is
used to reduce the interference in the Air interface.
If no speech has been detected at the Mobile Station, a similar kind of
function takes place. The parameters of the background noise are sent to the
Transcoder , which is then able to generate comfort noise.
In the uplink direction, DTX is used to save the batteries of the MS and to
reduce interference between the TDMA frame timeslots in the Air interface.
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1.5.2 Fixed and adaptive gain adjustment
By means of O&M messages, it is possible to choose a fixed level adjustmentin the TRAU software in the uplink and downlink directions between +6dB
and -6dB, in 1 dB intervals.
In TRAU software, it is also possible to choose an adaptive gain control in the
downlink direction. In order to have an overall solution to the problems
caused by a speech volume that is too low in GSM phones and a variable
volume level in the PSTN, an adaptive gain has been implemented in the
downlink direction to the TRAU. This guarantees sufficient volume level in
the Mobile Station. The gain is attenuated in steps of 1 dB. In adaptive gain
control, it is also possible to select the minimum and maximum values within
which the gain can vary.
1.5.3 Acoustic echo cancellation
On the MS side, the voice coming from the earpiece of MS will also be picked
up by the microphone. There will be an acoustic echo travelling by air and
along the body of the MS. The GSM recommendation 3.50 states that a
handset and a hands-free MS should perform acoustic echo cancellation. In
other words, the mobile should have a built-in echo suppressor or cancellor
and no acoustic echo cancellation should be needed on the network side.
However, it seems that some mobiles are not capable to remove the acoustic
echo sufficiently and the subscriber may sometimes hear the mobileoriginated echo.
BTS BSC TCSM MSC
ECU
Downlink
Uplink
BSS
AEC
Echo No echo
Acousticecho
Figure 9. Acoustic echo cancellation
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1.6 Operation of TCSM2E
Operation of the TCSM2E is facilitated by the integrated operation and
maintenance offered by the BSC and the OMC. All TCSM2E functionsrelated to operation and maintenance can be carried out
from the BSC or the OMC by a remote session;
by using a local terminal.
The O&M link of the TCSM2E towards the BSC uses a 16 kbit/s LAPD
channel.
The configuration data including the internal settings of the TCSM2E as well
as the hardware configuration data are stored in the non-volatile memory of
the TRCO. The hardware layout data of the rack as well as the PCM
configurations are stored in the BSC. This means that, from a practical point
of view, the TCSM2E is seen almost as a unit within the BSC.
1.6.1 Supervision by the BSC
The BSC is responsible for supervising that the state administration program
in the TCSM2E works as intended. This is achieved by way of comparing the
TCSM2E state information as supposed by the BSC to the state reported by
the TCSM2E. Internally, the master unit of the TCSM2E, the TRCO,
supervises the other unit.
1.6.2 Alarms
Alarms are transferred over the O&M link to the BSC. Current alarms can,
however, be viewed on a local MMI terminal. The BSC assigns to each alarm
an identity, time stamp, alarm class, text string and physical location. The
BSC holds information on the physical location of each TCSM2E equipment.
It converts the logical location information of the alarms received from the
TCSM2E into a physical address.
Alarms which indicate that traffic channels, trunks or other portions of the
traffic capacity are lost, lead to blocking of the respective elements by the
BSC. The TCSM2E is not itself aware of the blocking measures taken by theBSC. More common alarms received by the Transcoder would be either
transmission related or related to individual channel failures.
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WELCOME TO THE TCSM LOCAL TERMINAL DIALOGUE
TCSM_034:LUC> ZAI
/*** ALARMS CURRENTLY ON ***/
2910 FRAMING ERROR
AF 00 01 00 11
2915 FAULT RATE MONITORING
AF 00 01 00 00
/*** COMMAND EXECUTED ***/
DX 200 BSC1-KUTOJA 1998-06-26 10:49:08
ALARMS CURRENTLY ON
BSC1-KUTOJA OMU TRANSM 1998-06-26 10:34:44.11
*** ALARM TCSM-34 1C087-01 PXRECE *RECOV* (0040)2915 FAULT RATE MONITORIN
TCSM 1d 00
BSC1-KUTOJA OMU TRANSM 1998-06-2610:34:07.11
* ALARM TCSM-34 1C087-01 PXRECE
(0039) 2910 FRAMING ERROR
TCSM 1d 11
END OF ALARMS CURRENTLY ON
Figure 10. Alarms viewed locally and in the BSC
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1.6.3 Reliability
The design objectives have been adopted to ensure that the downtime of theTCSM2E is very low. The figures are calculated on the basis of an average
repair time of 4.5 h. The objective for TCSM2E fault intensity is max. 1 fault
every 17000 hours.
To achieve the highest availability level, a minimum of two TCSM2E units
(possibly only partly equipped) per BSC is recommended, even if the traffic
dimensioning requires only one.
The following objectives have been established for the servicing of the
TCSM2E:
Mean active repair time per fault: less than half an hour
Fault localisation accurate to one plug-in unit: 95 %
Fault localisation accurate to one tcsm2e unit: 100%
1.6.4 State administration and reconfiguration
The TCSM2E is a functional unit from the point of view of the BSC, and as a
functional unit it has the following states:
WO-EX: working, executing;
WO-RE: working, restarting; TE-EX: test execution;
BL-EX: blocked, executing;
BL-ID: blocked, idle.
The user can invoke a restart command by MMI from the BSC or locally
from the TCSM2E in a situation where he/she supposes that the TCSM2E
is not working in a normal mode. The BSC also commands a restart when
downloading a new configuration to the TCSM2E.
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Nokia DX 200 BSC
The Base Station Controller, DX 200 BSC, is a modern digital switching
product for GSM/DCS networks. The DX 200 Base Station Controller, BSC,
belongs to DX 200 Switching System Product Family. The DX 200 SwitchingSystem Product Family is based on a Fault Tolerant Computing Platform ,
which constitutes a base for a Switching Platform.
Figure 11. DX200 platform
The main function of the BSC is to control and manage the Base Station
Subsystem (BSS) and the radio channels. Based on Nokia's long experience in
cellular networks, the BSC is designed for efficient use of radio resources,
easy operability and maintainability and comprehensive information about
quality of service. The Nokia BSC is a stable, mature product, which has field
proven high reliability. One major feature of the BSC is also field proven
multivendor functionality.
Together with functionally distributed modular architecture of the DX 200
Switching System Product Family and the latest commercially available
industry standard hardware components, the DX 200 BSC is easilyexpandable and cost-efficient and has high capacity.
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1.7 BSC functions
The BSC manages a variety of tasks ranging from channel administration to
short message service. The tasks are explained in brief below.
Figure 12. Base station controller in the GSM / DCS 1800 network
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1.7.1 Management of radio channels
Figure 13. Configuration and management of the radio resources
Configuration and management of Radio Resources is one of the primary
functions of the BSC. The actual channel configuration, i.e. how many trafficchannels and signalling channels can be used in the BSS, is done in
connection with radio network planning during integration (or
reconfiguration) and initial configuration.
Management of traffic channels (TCH) and stand-alone dedicated control
channels (SDCCH) can be further subdivided into following tasks:
Resource management
Channel allocation
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Link supervision
Channel release
Power control
Management of broadcast control channels (BCCH) and common control
channels (CCCH) be subdivided into the following tasks:
Channel management
Random access
Access grant
Paging
1.7.1.1 Channel types
Physical channels can be defined as one timeslot in a TDMA-frame, whereas
Logical channels define what the actual resource is used for. The following is
a definition of the uses of the different logical channels:
Broadcast channels, BCH
Frequency Correction Channel, FCCH
Carries frequency correction information for MS
Downlink, point-to-multipoint
Synchronisation Control Channel, SCH
Carries frame synchronisation information, e.g. TDMA frame number
and BTS identification, BSIC
Downlink, point-to-multipoint
Broadcast Control Channel, BCCH
Carries general data about the BTS (cell specific)
Downlink, point-to-multipoint
Common Control Channels, CCCH
Paging Channel, PCH
Used for paging the specific Mobile Station
Downlink, point-to-point
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The mobile then forwards these measurements to the BTS , which in turn
averages them and forwards them to the BSC for processing. At the same
time, the BTS is measuring for processing by the BSC the following:
Uplink quality Uplink Rx level
MS distance
MS speed
Idle channel quality
Note:
MS speed detection is an optional BSC feature
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1.8.2 Signalling management
Figure 18. BSS signalling
GSM signalling is based on the lower three layers of the Open System
Interconnection model: physical, link and network.
OSI Layer 1 represents the physical layer. It is a digital interface at
2048 kbit/s, based on the ITU-T Recommendation G.703. This
interface is normally used as an A interface between the MSC and the
BSS providing a 64 kbit/s transmission rate on each channel. In the
Abis interfaces signalling links may have either a 16, 32 or 64 kbit/s
rate of transmission. In the air interface this function is fulfilled by the
radio path.
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OSI Layer 2 is based on the MTP of the Signalling System No.7. The
MTP provides a mechanism for reliable transfer of signalling messages
between the MSC and the BSS. At least two signalling links are
normally provided between the BSS and the MSC for both capacity and
reliability reasons. In the Abis, this function is performed by the LAPD.In the air interface, this function is performed by a modified form of
LAPD - LAPDm
OSI Layer 3 of the signalling network includes the SCCP and a part of
the MTP. Together, the MTP and the SCCP functions provide both
connection-less and connection-oriented network services. These
services are used to transfer circuit-related and non-circuit-related
signalling information and other types of information between the MSC
and the BSS.
The user part protocol used between the MSC and BSC is the BSS application
part (BSSAP) , which can be divided into two sublayers:
1. Direct transfer part, DTAP : used for direct communication between the
MSC and MS
2. BSS Management Application Part, (BSSMAP): used for
communication between the MSC and BSC
BSC
ET
GSW
BTS
MSOMU
BCSU
OMUSIG
TRXSIG
FU
BIEET
Figure 19. LAPD signalling
The DX 200 BSC can handle LAPD signalling links with the bit rates of 16
kbit/s, 32 kbit/s and 64 kbit/s. Each site will have one BCF/OMU signalling
link allocated to it as well as one TRX signalling link per TRX.
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1.8.3 Management of terrestrial channels
Figure 20. BSS trafic channels
Though part of the MSC's function is to hunt free circuits toward a BSC, the
BSC's role is to provide and indication of blocking on the channels between
the BSC and the MSC.
In the Abis interface, the allocation of the traffic channels between the BSC
and the BTS is performed according to the allocation of a circuit in the Air
Interface. That is, a circuit in the Air interface corresponds directly to an Abis
circuit.
The BSC also manages both the LAPD signalling links in the Abis interface
as well as the CCS7 links in the A interface.
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1.8.4 Interfaces
Figure 21. BSS Interfaces
1.8.4.1 A Interface to the MSC
The A interface between the MSC and the DX 200 BSC is implemented
according to the GSM standards.
1.8.4.2 Abis interface (BSC-BTS)
The Abis interface telecommunication part between the DX 200 BSC2E and
the BTS is implemented according to the 08.5X Series of GSM
Recommendations. The Abis O&M part is Nokia property supporting
additional features like the Site Test Monitoring unit, alarm consistency,
remote transmission equipment management and BTS database management.
1.8.4.3 Q3
The X.25 protocol is used on the interface between the DX 200 BSC and the
Nokia NMS/2000. Either the PSPDN or the PCM-time-slot-based connection
or LAN connection can be used.
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2 Understanding the Nokia BTS
2.1 Introduction
To say that a BTS only transmits and receives information between the mobile
station and the BSC might perhaps be an over-simplification. The BTS
performs the radio function for the Base Station System (BSS) and is
connected to the Base Station Controller (BSC) via the Abis interface and to
the Mobile Stations (MS) via the Air interface. The BSC is further connected
to the Mobile Switching Centre (MSC) and the Network Management System
(NMS). The BTS provides the Air interface, creates TDMA frames (Time
Division Multiple Access) and the bursts that are used for carrying speech /
data and signalling information between the BTS and MS.
BSC BTS
OMC
MS
To MSC
A-if Abis-if Air-if
Figure 27. Base station sub-system
2.2 Problems and solutions for the air interface
The radio link is the most vulnerable part of the GSM connection. That being
said, there are three major sources of problems in the air interface , which can
lead to loss of data. These are:
Multipath propagation
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Shadowing
Propagation delay
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Since the frequency in GSM/DCS 1800 is of the order 900/1800 MHz, the
dips will occur at approximately every 17/8.5cms.
If the dip is severe enough. the strength of the received signal may go below
that of the receiver sensitivity, resulting in loss of signal.The speed that you travel through the radiating field is also a factor, since the
faster you travel the less time is spent in each potential dip, and the less
information is lost.
approx 17 cm
Rx sensitivity
Fading dips
Figure 29. Flat fading
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In order to overcome the problem of flat fading the following methods are
employed:
1. Frequency hopping
2. Channel coding
3. Interleaving
4. Antenna/receiver diversity
2.2.2.1 Frequency hopping
The BSC, as was mentioned in the previous section, manages frequency
hopping. That is, the BSC provides the Frequency Hopping parameters.
However, the BTS performs it. The frequency on which the information is
sent is changed for every burst. The main benefit is in the ability to average
out the effect of fading dips as well as the effects of interference, thereby
improving the overall quality of the network.
The type of frequency hopping , which can be implemented in the BTS
depends on the type and configuration of BTS and can be either synthesised
(RF hopping) or Base-band hopping.
0 1 2 3 4 5 6 7
TDMA frame n
7FU1
CU 1
FU3
FU3
CU1
CU2
CU3
FHU
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 77
CU 20 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 77
CU 30 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
BTS
TDMA frame n+1 TDMA frame n+2
Figure 30. Frequency hopping in the BTS
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2.2.2.3 Interleaving
Interleaving means spreading of the coded speech into many bursts. As the
transmission of a 20ms block of speech is spread over 8 separate bursts, the
system will be able to recover the data even if up to one burst is lost.
2, 10 ... 4501, 9, 17, 25 ... 449
3, 11 ... 451 4, 12 ... 452
5, 13 ... 453 6, 14 ... 454
7, 15 ... 455 8, 16 ... 456
Figure 32. First level of interleaving
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2.2.2.4 Antenna receiver diversity
In this case, there are two antennas used for receiving the signal. This helps in
such a way that if a fading dip occurs at the position of one antenna, the other
antenna will still be able to receive the signal. Since the distance between two
antennas is a few metres, it can only be implemented at the base transceiver
station. Theoretically, the optimum value for this distance is 20 (where is
the wavelength of the carrier). Figure 33 shows examples of frequency
hopping and antenna receiver diversity.
Received signalf1
f2
f3
f4
Rx Rx
DSPUBTS
Frequency Hopping
Antenna Receiver Diversity
Figure 33. Frequency hopping and antenna receiver diversity
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2.2.3 Selective fading
Selective fading is like flat fading caused by reflections, but unlike flat fading,the reflected signal comes from objects that are some distance away from the
receiver. This distance is of the order 1-5 kms. This sort of problem is
particularly noticeable in areas of mountainous terrain or areas with large
expanses of water, or worse both.
Since the bit rate in GSM/DCS 1800 is 270 Kbits / sec, this means that the
time between bits is 3.7uS. This time corresponds to 1.1 Km. Now if signals
are reflected from objects , which cause the reflected path to be of a length of
this, or greater, then the individual bits arriving at the receiver will be
coincident, and result in difficulty interpreting them. This problem is also
referred to as ISI, Inter Symbol Interference.
When user information is transmitted by either the base transceiver station or
mobile station, the information contained in the burst is not all user data.
There are certain bits , which are called training sequence.
T B
3
E n c ry p te d b i ts
5 8
T r a i n . s e q .
2 6
E n c ry p te d b i ts
5 8
T B
3
G P
8 . 2 5
TB: Tail bitsGP: guard periodNormal burst (NB)
(1 bit duration 48/13 3.69 us)
Figure 34. Normal burst
These bits are known to both mobile station and base transceiver station. By
analysing the effect of the air on these training bits, the air interface is
modelled as a filter. Using this mathematical model of the air, the transmitted
bits are estimated based on the received bits. The algorithm used for this
purpose is called Viterbi equalisation.
2.2.4 Modulation
GSM is a digital mobile standard. However, radio frequencies are analogue.
So, the question is how do we transmit digital information in an analogue
signal.
This looks difficult, but if you think about the values that must be transmitted
(0 and 1), an easy solution can be found. Suppose that the frequency varies
between two values, one representing 0 and the other representing 1. By
altering the value of a certain characteristic of frequency at every specified
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interval (the bit duration), we can translate an analogue signal into a bit stream
in the frequency domain. This technique is called modulation. The
characteristics that can be varied are the frequency itself, the amplitude or the
relative phase shift. Figure 35 illustrates this.
Frequency Modulation Amplitude Modulation
Figure 35. Modulation examples
The modulation technique used in GSM is the Gaussian Minimum Shift
Keying (GMSK). This is a phase modulation. In order to understand what it
means, let us take a simple example.
In GSM transmission in the air, the bit rate is approximately 270 kilo bits per
second, which means that the duration of one bit is 3.69 s, i.e. the value ofthe bit requires 3.69 s of transmission time. If we measure the phase
variation of the carrier wave every 3.69 s, we can determine the value of the
bit. If the phase shift is +90, the bit isset to1. If the variation is 0, the valueof the bit is set to 0.
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BTS
Figure 37. Shadowing effect
Shadowing is generally a problem in the uplink direction, because a base
transceiver station transmits information at a much higher power compared to
that from the mobile station. The solution adopted to overcome this problem
is known as adaptive power control. Based on quality and strength of the
received signal, the base station informs the mobile station to increase or
decrease the power as required. This information is sent to the mobile station
on the SACCH, the slow associated control channel.
2.2.6 Propagation delay
Information is sent in burst from the mobile station to the Base Transceiver
Station (BTS). These bursts have to arrive at the base transceiver station such
that they have to map exactly onto their allocated timeslots. But if the distance
between the mobile station and the base station is several kilometres, then the
time taken by the signal (burst) to travel from the mobile station to base
station (and vice versa) is not instantaneous. There is finite time delay. This
means that if the mobile station or base station transmits the burst only when
the timeslot appears, then when the burst arrives at the other end, it smears
onto the region of the next timeslot, corrupting data from both sources.
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2.2.6.2 Transmission of BCCH information
A mobile station needs a certain amount of information in order have access
to the network. The BTS transmits information from the BSC regarding the
local network environment. Each cell/sector would have one BCCH TRX ,
which would transmit information to all mobiles camping under that particular
cell:
Cell identity
Location area
Neighbour cell descriptions
Cell selection parameters
Control channel description
PLMNs permitted
RACH control parameters
2.2.7 Ciphering
The air interface can be ciphered to provide additional security Ciphering and
deciphering of the Air interface traffic and signalling are supported as defined
in the GSM recommendations. The BTS supports several ciphering algorithms
optional as defined in the GSM recommendations.
A5/0 no ciphering
A5/1 A5/2
The ciphering key is calculated in the HLR, forwarded to the VLR and BSC
and then to the BTS , which performs the actual ciphering.
2.2.8 Signalling
In order for the system to function correctly, measurements, fault reports as
well as telecom signalling must be transferred between the BSC, BTS and
MS.LAPD signalling is used between the BSC and BTS. There are two types of
signalling used in the Abis interface: OMUSIG/ BCFSIG and TRXSIG.
LAPDm signalling is used between the BTS and MS, which means that
messages destined for a mobile from the network are restructured into LAPD
format in the BTS.
TRXSIG signalling is used for carrying signalling information between MSC
- MS, BSC - MS and BSC - BTS. The TRXSIG is used for:
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Location updates
Call control
Handover
Paging
Up/downlink measurement results/ controls
Radio link management procedures
Dedicated channel management procedures
Common channel management procedures
TRX management procedures
The BCFSIG is used for control of a particular BTS site. Information carried
on the BCFSIG link can be relative to:
Fault management
Fault reporting
BTS recovery
BTS test handling
Configuration management
BSC
ET
GSW
BTS
MSOMU
BCSU
OMUSIG
TRXSIG
FU
BIEET
Figure 42. Abis signalling links
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2.3.1.1 Transmission unit
The task of the transmission unit is to connect the BTS to the Abis interface
and, in doing so, create different types of transmission configuration
possibilities. All of the Nokia BTSs have integrated transmission units.
Certain Talk-family models offer additional integrated radio relay links.
Transmission units are monitored by the operation and maintenance unit by
means of an internal Q1 bus.
2.4 Control functions
Control functions can be split into four individual functions:
1. Operation and maintenance
2. Master clock function
3. Frequency hopping control
4. External alarms and controls
That being said, depending on the type of BTS this could mean from one
integrated unit to up to four individual plug-in units.
2.4.1.1 Operation and maintenance
The O&M processor controls and supervises the operation of all BTS units
alone or in co-operation with other processors. It is the main interface forlocal O&M and controls and supervises the other units as well as delivers all
status information to the BSC by means of the O&M signalling link , which it
manages. It stores SW as well as downloads SW to the other units. It also
downloads the software and configuration information received from the BSC
or the MMI to other processors.
2.4.1.2 External alarms and controls
External alarms and controls are programmable interfaces to other devices in
the BTS , which can be used to monitor environmental conditions at the BTS
site as well as monitor the state of units, which do not have a processor of
their own. An example of external alarm might be an intruder alarm or asmoke detector.
2.4.1.3 Master clock
The master clock generates the accurate 13MHz time base for the BTS. Most
of the time the BTS would operate in hierarchical mode and would tune itself
to the clock signal from the 2Mbit/s link, though in many cases it can also
function in plesiochronous mode.
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The Traffic Channels in the BSS
3 The Traffic Channels in the BSSThe traffic channels are described in the down link direction (from MSC to
MS) in different interfaces and network elements.
MSC
TCSM2E
BSC
BTSET
ET
ET
ET
ET
ETFXC
A I/F Ater Abis Air I/F
A B C D E F G H I
MS
Fig. 1.1 The traffic channels in the BSS network.
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A Interface
4 A InterfaceThe A interface is based on the CCITT recommendation G.703 (electrically)
and G.704 (frame structure). The traffic channels baud rate in the A interface
is 64 kbit/s and they are located in the time slots 1 - 15 and 17 - 31.
TS 16 in A-interface is normally used for the CCS7 signalling and its baud
rate is 64 kbit/s.
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Ater Interface
5 Ater InterfaceThe A-ter interface is based on the CCITT recommendation G.703
(electrically) and G.704 (frame structure). The traffic channels and signalling
channels coming from three different PCMs from the MSC are reallocated in
the transcoder.
MSC
TCSM2E
BSC
BTSET
ET
ETET
ET
FXC
Ater
C
MS
Fig 4.1 Ater interface.
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1 TCH0 TCH1 TCH2 TCH3
2 TCH4 TCH5 TCH6 TCH7
3 TCH0 TCH1 TCH2 TCH3
4 TCH4 TCH5 TCH6 TCH7
5 TCH0 TCH1 TCH2 TCH36 TCH4 TCH5 TCH6 TCH7
7 TCH0 TCH1 TCH2 TCH3
8 TCH4 TCH5 TCH6 TCH7
9 TCH0 TCH1 TCH2 TCH3
10 TCH4 TCH5 TCH6 TCH7
11 TCH0 TCH1 TCH2 TCH3
12 TCH4 TCH5 TCH6 TCH7
13 TCH0 TCH1 TCH2 TCH3
14 TCH4 TCH5 TCH6 TCH7
15 TCH0 TCH1 TCH2 TCH3
16 TCH4 TCH5 TCH6 TCH7
17 TCH0 TCH1 TCH2 TCH3
18 TCH4 TCH5 TCH6 TCH7
19 TCH0 TCH1 TCH2 TCH3
20 TCH4 TCH5 TCH6 TCH7
21 TCH0 TCH1 TCH2 TCH3
22 TCH4 TCH5 TCH6 TCH7
23 TCH0 TCH1 TCH2 TCH3
24 TCH4 TCH5 TCH6 TCH725 TRX1 OMU1 TRX2 OMU2
26 TRX3 OMU3 TRX4 OMU4
27 TRX5 OMU5 TRX6 OMU6
28 TRX7 OMU7 TRX8 OMU8
29 TRX9 OMU9 TRX10 OMU10
30 TRX11 OMU11 TRX12 OMU12
31 XX XX XX XX
TS 1 2 3 4 5 6 7 8
0
Fig. 6.2 The traffic and signalling channels in the Abis interface.
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The baseband part of the transceiver unit, TRX, is responsible for:
(in down link direction)
the block coding
the convolutional coding
the interleaving
the encryption
the TDMA formatting.
The Transmitter part, TX, of the TRX is responsible for:
(in down link direction)
GMSK modulation
Up conversion power amplification.
The signal from the TX part is connected to the Antenna Filter Unit, AFE.
In the up link direction, the signal from the received antenna is connected first
to AFE and then to the RX part of the TRX.
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Air Interface
9 Air InterfaceThe Air interface is located between the BTS and the MS.The traffic channels
in the Air interface are allocated onto a TDMA frame. The TDMA frame
consists of 8 time slots. Generally, all time slots are used for traffic channels.
Time slot 0 and sometimes also time slot 1 can be used for the signalling
between the BTS (BSC, MSC) and the MS.
MSC
TCSM2E
BSC
BTSET
ET
ET
ET
ET
ET
TRU
0 1 2 3 4 5 6 7
TDMA frame = 8 time slots
0 1 2 3 4 5 6 70 1 2 3 4 5 6 7
Air Interface
H
MS
Fig. 9.1 The air interface.
Fig. 9.2 The TDMA frame.
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Fundamentals of the Nokia BSS Implementation
BSSAP BSS Application Part
BSSMAP BSS Management Application Part
DTAP Direct Transfer Application Part
MM Mobility Management
CM Connection ManagementCC Call Control
SS Supplementary Services
SMS Short Message Services
SCCP Signalling Connection Control Part
MTP Message Transfer Part
RR Radio Resources
BTSM BTS Management
LapD Link Access Protocol on the D channel
LapDm Link Access Protocol on the D channel modified
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Fundamentals of the Nokia BSS Implementation
Messages transferred between the BSC and MSC use the BSS management
application part (BSSMAP). Radio resources (RR) and BTS management
(BTSM) are used to transfer messages between the BSC and the BTS.
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Fundamentals of the Nokia BSS Implementation
Only the lowest level in the protocol stack has the actual connection to the
physical medium between the network nodes, and all data transfer must go
through the lowest three levels. Network elements which are to communicate
with each other must have the same protocols in each level.
The Message Transfer Part contains the functions corresponding with OSI
layers 1 to 3. It is responsible for delivering signalling messages reliably from
one node to another. The User Parts use the services provided by the MTP.
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Signalling Protocols
Level 3 is the Signalling Network level. Its functions are divided into two
parts:
5. message handling, which includes routing of outgoing and transfer
messages to a neighbouring node, and the distribution of incomingmessages to the respective user part of it's own node; and
6. network management, which provides all necessary procedures for
using the signalling network in an optimised and fault tolerant way.
Control of message routing and reconfigurations is performed in order
to preserve or restore the normal message transfer capability.
As far as the message handling is concerned, Level 3 matches with OSI layer
3. The network management functions go beyond OSI layer 3 functionality.
As mentioned earlier, MTP is just a transmission service for the User Parts.
So it does not normally send its own messages but rather delivers User Part
messages through the network. In order to perform its task of network
management, however, an exception has been made: MTP sends Signalling
Network Testing (SNT) and Signalling Network Management (SNM)
messages to the MTPs of other network nodes without distributing them
further to any User Part.
In order to route and distribute outgoing, transferred or incoming messages
MTP needs two types of addresses: one address for the network node as a
whole and another for MTPs neighbours in the protocol stack. The former is
called Signalling Point Code (SPC) and the latter Service Information
Octet (SIO).
Both addresses are included in the signalling message. In cases where the
identification of the receiver of an incoming message is the same as the
network nodes own SPC, the message is distributed to SCCP or one of the
User Parts. This is done by means of the SIO. If it's own SPC is different, then
the message is forwarded to the most suitable of the neighbouring network
nodes.
Since MTP is fundamental to the whole signalling system, it has to be present
in every network element that participates in CCS7 signalling. Such network
nodes are called Signalling Points (SP).
The following diagram shows some examples of CCS7 signalling on the A
interface as recorded by Nethawk, a GSM protocol analyser.
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Fundamentals of the Nokia BSS Implementation
Conn:1 Card:1 TS:25 Subch:0 2016 19:35:50.59
MSU
- BSN: 126 (7Eh) BIB: 1 FSN: 7 (07h) FIB: 1
- Signalling Network Test & Maintenance regular
- network indicator : national network
- data, length : 14, (0Eh)
SLTM - SIGNALLING LINK TEST MESSAGE
- DPC : 432 (01B0h) OPC : 12600 (3138h)
- SLC : 0 (00h)
- test pattern length : 8 (8h) 14 15 16 17 18 19 1A 1B
Conn :1 Card:1 TS:25 Subch:0 2017 19:35:50.603
MSU
- BSN: 7 (07h) BIB: 1 FSN: 127 (7Fh) FIB: 1
- Signalling Network Test & Maintenance regular
- network indicator : national network
- data, length : 14, (0Eh)
SLTA - SIGNALLING LINK TEST ACKNOWLEDGE
- DPC : 12600 (3138h) OPC : 432 (01B0h)
- SLC : 0 (00h)
- test pattern length : 8 (8h) 14 15 16 17 18 19 1A 1B
Conn:1 Card:1 TS:25 Subch:0 2038 19:36:07.732
MSU
- BSN: 7 (07h) BIB: 1 FSN: 0 (00h) FIB: 1
- SCCP
- network indicator : national network
- data, length : 50, (32h)
Conn:1 Card:1 TS:25 Subch:0 2039 19:36:07.741
MSU
- BSN: 0 (00h) BIB: 1 FSN: 8 (08h) FIB: 1
- SCCP
- network indicator : national network
- data, length : 13, (0Dh)
SLTM - SIGNALLING LINK TES
- DPC : 432 (01B0h) OPC : 1- SLC : 0 (00h)- test pattern length : 8 (8h)
Service Indicator :- indicates what typprovided by the hig
layer messages- eg. Sign NW testin
MTPL2
MTPL3
Figure 13.3 MTP Layer 3 carried by MTP Layer 2
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The SAPI, Service Access Point Identifier is used to identify BCFSIG (= 62),
TRXSIG (= 0) and SMS (= 3)
The address field can be 8 or 16 bits.
6 1 1
SAPI C/R
TEI
0
1
The C/R bit indicates whether the frame is a command or a response frame.
The TEI, Terminal End Point Identifier identifies a specific connection
endpoint. In the case of a BTS, each TRX has a different TEI depending on the
logical ID of the TRX, NOT the physical address. The TEI for BCFSIG = 1.
Control
The control field is 16 bits and the contents change depending on the purpose of
the frame which can be information, supervisory or unnumbered.
The control field of an information frame is shown below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
N(S) 0 N(R) P/F
N(S) is the number of the frame in a sequence of frames, and N(R) is the
number of the next expected frame.
The P/F (Poll/Final) bit indicates whether a response is required. If the bit is set
(= 1) then the frame requires a response, and the response to this frame must
also have its P?F bit set.
The control field of a supervisory frame is shown below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 0 S Not used P/F N(R)
The supervisory frames are used for flow control. The S bits are used to
indicate:
RR Receiver Ready acknowledgement to indicate that the receiver is
ready for the next frame. If the link is idle, this frame is sent every 10
seconds (timer T203).
RNR Receiver Not Ready.
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Conn:2 Card:2 TS:3 Subch:1 2242 19:36:43.071
INFO
SAPI: 00H TEI: 01H C/R: 1 P/F: 0
N(R): 19H N(S): 52H: 08 2F 01 0E 0D 02
Conn:2 Card:2 TS:3 Subch:1 2243 19:36:43.078RECEIVER READY
SAPI: 00H TEI: 01H C/R: 1 P/F: 0
N(R): 53H
Conn:2 Card:2 TS:3 Subch:1 2245 19:36:43.266
INFO
SAPI: 00H TEI: 01H C/R: 1 P/F: 0
N(R): 19H N(S): 53H
03 01 01 0E 02 00 0B 00 09 83 25 02 E0 90 1E
02 E0 88
Conn:2 Card:2 TS:3 Subch:1 2246 19:36:43.273
RECEIVER READY
SAPI: 00H TEI: 01H C/R: 1 P/F: 0
N(R): 54H
Conn:2 Card:2 TS:3 Subch:1 2247 19:36:43.476UNNUMBERED INFO
SAPI: 00H TEI: 01H C/R: 0 P/F: 0
08 28 01 0E 1B 33 19 03 2A 29 00 04 0E 0A 08
00 0B 00 12 06 15 18 16 10 00 00 00 00 00 00
00 00 00 00 00 00 00
LapD frame used for
sending higher levelmessages
Figure 13.6 Example of LapD communication taken from Nethawk
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Fundamentals of the Nokia BSS Implementation
13.2.2 LapDm
LapDm is LapD modified for use on the air interface between the BTS and theMS. The modifications are imposed by the nature of the air interface and this is
discussed below.
The frame check sequence is not required in the LapDm frame format as error
detection is already carried out on the air interface by the transmission coding of
the physical channel.
The use of flags to delimit the start and end of frames is not necessary due to the
ready-made blocks of the physical layer. So the frame format becomes:
8 bits 8 bits 8 bits 21 or 23 octets
Address Control Length Information
The address field is as follows, where the SAPI is 0 for Mobility Management,
Call Control and Radio Resource messages, and 3 for SMS.
0 0 SAPI C/R EA
The control field is formatted as illustrated below, where the abbreviations are
the same as defined for LapD earlier.
Information N(R) P/F N(S) 0
Supervisory N(R) P/F S S 0 1
Unnumbered 0 M M P/F M M 1 1
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Each frame is designed to fit into a single physical block which is 23 octets long
on the TCH. The frame is only 21 octets long on the SACCH block as 2 octets
are needed for power control and timing advance.
Since the effective information length may be less than the specified number ofoctets, a length indicator is included in each frame. Unused octets are filled with
the default pattern (00101011).
The number of octets available for messages on the LapDm (21 or 23) are not
sufficient for most signalling needs, and so a segmentation and reassembly
facility is defined. It involves the use of a "more" bit in the message header.
When the value of this bit is 1, it is indicating that this frame is not the last to
contain message information. A 0 indicates the final frame.
Message
1
1
0
1
T
T
T
F T
More bits
T = Tail Bits
F = Fill Bits
Figure 13.7 Segmentation of messages in LapDm