gsm network and services - personliga hemsidor på kthjohanmon/attic/2g1723/lectures/radio.pdf ·...
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1GSM Network and Services 2G1723 Johan Montelius
GSM Network and Services
Logical and physical layer - how waves turn into reliable bits andbits into logical channels
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2GSM Network and Services 2G1723 Johan Montelius
Logical channels
• Defined at layer 1 and provided for layer 2 – except the frequency correction channel and the
synchronization channel.
• Mapped on the physical channels:– more than one logical channel can be mapped on
one physical channel
– mapped in time and coding
– channels can be up, down or duplex
• Layer two of the signalling layer (LAPDm) can add acknowledgement and retransmission.
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3GSM Network and Services 2G1723 Johan Montelius
The logical channels
• Traffic channels - TCH– Full rate used for 13kbps voice or up to 14.4kbps
data
– Half rate used for 6.5kbps voice or low rate data
• Signalling channels – Broadcast channels - BCH
– Common Control Channels - CCH
– Dedicated Control Channels - DCH
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4GSM Network and Services 2G1723 Johan Montelius
Broadcast Channels
• Frequency Correction Channel (FCCH)– only visible at physical layer, helps the mobile tune
in to the right frequency
• Synchronization Channel (SCH)– Helps the mobile stay synchronized with the base
station, also carries the base station identity code (colour codes)
• Broadcast Control Channel (BCCH)– Information about the cell, neighbouring cells,
location area identifier, structure of the Common Control Channels
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5GSM Network and Services 2G1723 Johan Montelius
Common Control Channels
• Random Access Channel (RACH)– for the mobiles to request access
• Access Grant Channel (AGCH)– for BSC to grant access
• Paging Channel (PCH)– for BSC to page a mobile
• Notification Channel (NCH) – for the BSC to notify a group of mobiles of
Voice Broadcast Services
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6GSM Network and Services 2G1723 Johan Montelius
Dedicated Control Channels
• Stand alone Dedicated Control Channel (SDCCH)– duplex point to point signalling channel
• Associated Control Channels– Always allocated together with a traffic channel or
signalling channel Will control the performance of the channel.
– Slow (SACCH)
– Fast (FACCH), only with TCH, used for handover
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7GSM Network and Services 2G1723 Johan Montelius
Cell Broadcast Channel
• Use to send out SMS broadcast messages, for example the “KTH/Monaco” that shows up in the screen when you connect to the Monaco network.
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8GSM Network and Services 2G1723 Johan Montelius
Mapping of logical channels
• TCH/F + FACCH + SACCH – needs a full physical channel
• TCH/H + FACCH + SACCH – half a physical channel
• SDCCH + SACCH – eight channels in one physical
• BCH (BCCH/FCCH/SCH) + CCCH (RACH/PCH/AGCH/NCH)– one physical channel, the broadcast channel
• A cell always needs a Broadcast channel.
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9GSM Network and Services 2G1723 Johan Montelius
Incoming callMS BSS
paging (PCH)
assigned (AGCH)
channel req (RACH)
paging resp (SDCCH)
auth req and resp (SDCCH)
cipher setup (SDCCH)
call setup (SDCCH)
call connect (FACCH)
talk (TCH)
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10GSM Network and Services 2G1723 Johan Montelius
Radio – bits - channels
• Modulation – how do we modulate our carrier
• Multiple access and duplex – time/frequency division and random access
– frequency/time synchronization
• Logical channels – mapping to the physical channels
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11GSM Network and Services 2G1723 Johan Montelius
GSM radio modulation
differential encoding
carrier modulation
symbol coding
0101101000100101111001010
..111011100110111000101111
� � /2 0 � � /2 -� -� /2 -� -� /2 0 +� /2
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12GSM Network and Services 2G1723 Johan Montelius
Symbol modulation
• GSM uses GMSK – Gaussian Minimum Shift Keying
• Each bit is coded as one symbol so we have two symbols to code. Each symbol is codes as a ±� /2 shift in phase e.g. Not an absolute phase but a increase or decrease in the phase.
• A shift in phase is gradual (Gaussian) and spread over more than one symbol duration.
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13GSM Network and Services 2G1723 Johan Montelius
Symbol coding
1 0 1 1 0 1 0 0 0
� � /2 0 � � /2 - � -� /2 -� - � /2 0 +� /2
resulting phase
� � /2 +� /2 � � /2 -� +� /2 -� +� /2 +� /2 +� /2
phase shift
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14GSM Network and Services 2G1723 Johan Montelius
What if we interpret one symbol 180 degrees ( � radians) wrong
� � /2 +� /2 � � /2 -� - � /2 -� -� /2 0 +� /2
sender
receiver� � /2 0 � � /2 0 -� /2 -� -� /2 0 +� /2
decoding
� � /2 +� /2 � � /2 +� - � /2 -� +� /2 +� /2 +� /2
1 0 1 0 1 1 0 0 0two bit errors!
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15GSM Network and Services 2G1723 Johan Montelius
Differential encoding
1 0 1 1 0 1 0 0 0
0 1 1 0 1 1 1 0 0
+ + + + + + + + + 1
+ + + + + + + + + 1
1 0 1 1 0 1 0 0 0
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16GSM Network and Services 2G1723 Johan Montelius
Differential encoding
0 1 1 0 1 1 1 0 0
0 1 0 1 1 1 1 0 0
sender
(one symbol error results in two bit errors)
+ + + + + + + + + 1
1 0 0 1 0 1 0 0 0decoded we are back at one bit error.
receiver
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17GSM Network and Services 2G1723 Johan Montelius
Carrier modulation
• Each carrier is 200 kHz wide.
• The symbol rate 271 kBaud.
• GMSK can also be seen as a Frequency Shift Keying (FSK) modulation using two frequencies ± 68 kHz from the central frequency.
• Note: 271 kBaud / 4 = 68 kHz, this is a property of “minimum”.
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18GSM Network and Services 2G1723 Johan Montelius
Duplexing and Multiple access
• One duplex channel consist of one up-link and one down-link carrier.
• Distance between the up/down link carrier is always 45MHz (in the 900 band).
• Each carrier is divided into radio frames and each frame is divided into eight time slots.
• A physical channel is a specified time slot in consecutive frames.
• The up link carrier is delayed three time slots. No need for a duplex transceiver!
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19GSM Network and Services 2G1723 Johan Montelius
Duplex and Multiple access
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20GSM Network and Services 2G1723 Johan Montelius
Burst
• Since mobiles take turn sending the will send in bursts. Each burst will fit into a time slot.
• There are five types of bursts:– Normal
– Frequency correction
– Synchronization
– Dummy
– Access
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21GSM Network and Services 2G1723 Johan Montelius
Normal burst
data (57bits)stealing (2bits)
start (000) training (26bits) stop (000)
guard (8.25 bits)
data (57bits)
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22GSM Network and Services 2G1723 Johan Montelius
Normal burst
• 114 bits of user data
• Tow stealing flags for FACCH
• Training sequence used for synchronization and resolve inter-symbol interference. – The training sequence can be changed by layer
three protocols.
• Enough space behind the burst so the sender can be a bit late (far away) and still not interfere with the following time slot.
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23GSM Network and Services 2G1723 Johan Montelius
Frequency correction burst
all zeros (000...)
start (000) stop (000)
guard (8.25 bits)
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24GSM Network and Services 2G1723 Johan Montelius
Frequency correction burst
• A sequence of zeros will after differential encoding and phase modulation result in a constant negative phase shift.
• The constant phase shift will also be visible as a frequency shift to a perfect sine wave 68 kHz above the carrier frequency.
• A mobile can detect this sine wave and tune in to the frequency of a base station.
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25GSM Network and Services 2G1723 Johan Montelius
Synchronization burst
start (000) stop (000)
guard (8.25 bits)
data (39 bits)
training (64 bits)
data (39 bits)
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26GSM Network and Services 2G1723 Johan Montelius
Synchronization burst
• A longer training sequence so that the mobile can do better synchronization.
• The data fields holds information about:– the sequence number off the frame
– the colour codes of the cell (BSIC)
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27GSM Network and Services 2G1723 Johan Montelius
Dummy burst
start (000) stop (000)
guard (8.25 bits)
dummy
training (26 bits)
dummy
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28GSM Network and Services 2G1723 Johan Montelius
Finding the Broadcasting control channel• One carrier of each cell will carry the broadcast
channel (BCCH/SCH/FCCH).
• Idle time slots of the carrier will be filled with dummy burst e.g. the broadcast carrier is always among the strongest carriers.
• The frequency correction burst will allow the mobile to identify and tune in to the carrier.
• The synchronization burst is found and the BSIC and frame number can be identified.
• Ready to listen for the BCCH.
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29GSM Network and Services 2G1723 Johan Montelius
Mapping of traffic channel
TCH TCH SA idle
1 2 13 26
A physical channel is a specified time slot. Following this time slotin consecutive frames we have a sequence. Number this sequence mod 26, the traffic channel will occupy 1-12 and 14-25. The slow associated control channel occupy number 13, frame 26 is idle.The fast associated control channel is implemented using “stealing flags”in the traffic channel.
120 ms
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30GSM Network and Services 2G1723 Johan Montelius
Broadcast control channel
FSBBBBCCCCFSCCCCFSCCCCCCCCFSCCCCCCCCFSCCCCCCCC-
• The broadcast channel group the FCCH/SCH/BCCH and CCCH in a pattern of a 51 frame multiframe
• How does this align with the traffic channel that uses a 26 frame multiframe?
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31GSM Network and Services 2G1723 Johan Montelius
Frame numbering
• All frames are numbered and knowing the frame number you will know the sequence of logical channels in a multiframe.
• The frame number modulo 51 will give us an index in a signalling multiframe.
• The frame number modulo 26 will give us an index in a traffic multi frame
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32GSM Network and Services 2G1723 Johan Montelius
Frame numbering
Hyper frame: 2048 superframes
Superframe: 1326 frames (26x51)
T1 : superframe number [0 – 2047 ]T2 : frame number mod 26T3 : frame number mod 51 FN = T1x26x51 + ((T3 – T2) mod 26 ) x 51 + T3
T3 is sent using only three bits?
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33GSM Network and Services 2G1723 Johan Montelius
Access burst
Start (0000 0000) stop (000)guard (68.25 bits)
data (36 bits)
training (41 bits)
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34GSM Network and Services 2G1723 Johan Montelius
Access burst
• Made shorter to fit into a time slot even if the mobile is far a way (do the math, how many km is 68.24 bits).
• The data fields contains the address of the mobile doing the request.
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35GSM Network and Services 2G1723 Johan Montelius
Access burst
TS1TS2
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36GSM Network and Services 2G1723 Johan Montelius
Adaptive frame synchronization
TS1TS2
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37GSM Network and Services 2G1723 Johan Montelius
Timing advance
• When replying to a random access request the BSS will inform the MS of the timing advance to use.
• Timing advance is coded in six bits (0-63) and each step indicates one bit period earlier transmission.
• An idle mobile need not know its timing advance.
• A mobile that is connected need constant updating of the timing advance value.
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38GSM Network and Services 2G1723 Johan Montelius
Timing advance
• The extra guard of the access burst allows a phone to be aprx 35 km away from a BTS .
• Each bit duration is aprx equivalent to 500m.
• Timing advance is reported using the SACCH when a connection (SDCCH/TCH) is established.