GSM Basic 3
GSM Basic 3
GSM Basic 3
Air Interface
• Radio Transmission Techniques
– FDMA
– TDMA
– CDMA
• Channels
– Physical channels
– Logical channels
FDMA
Frequency
Channel
Time
TDMA
Channel
Time
0 1 2 3 4 5 6 7
CDMA
Frequency
Code
Time
Code 1
Code 2
Code 3
Channels
• Physical Channels
– Associated with frequency bands, time slots, codes
– Physical channels transfer bits from one network element
to another
• Logical Channels
– Distinguished by the nature of carried information and the
way to assemble bits into data units
– Three types
• one-to-one: traffic channels between a BTS and a MS
• one-to-many: synchronization signals from BTS to MSs in a cell
• many-to-one: from MSs to the same BTS
Air Interface Layers
Layer 3
Messages Messages
Messages
Store info
Location-Updating Procedure
Mobile Base Category Logical
station station Channel
Carrier sine wave
FCCH
SYNC CHANNEL INFORMATION SCH
RRM
SYSTEM INFORMATION TYPE3 BCCH
RRM
CHANNEL REQUEST
RRM RACH
IMMEDIATE ASSIGNMENT
RRM AGCH
LOCATION UPDATING REQUEST
MM SDCCH
AUTHENTICATION REQUEST
MM SDCCH
AUTHENTICATION RESPONSE MM SDCCH
CIPHERING MODE COMMAND SDCCH
RRM
CIPHERING MODE ACK RRM SDCCH
LOCATION UPDATING ACCEPT
MM SDCCH
TMSI ALLOCATION COMPLETE
MM SDCCH
CHANNEL RELEASE
RRM SDCCH
Call Deliver to MH
Mobile Base Category Logical
station station Channel
Carrier sine wave FCCH
SYNC CHANNEL INFORMATION RRM SCH
SYSTEM INFORMATION RRM BCCH
PAGING REQUEST RRM PCH
CHANNEL REQUEST RRM RACH
IMMEDIATE ASSIGNMENT RRM AGCH
PAGING RESPONSE RRM SDCCH
AUTHENTICATION REQUEST MM SDCCH
AUTHENTICATION RESPONSE MM SDCCH
CIPHERING MODE RRM SDCCH
CIPHERING MODE ACK RRM SDCCH
SETUP CMCM SDCCH
CALL CONFIRMED CM SDCCH
ALERTING CM SDCCH
CONNECT RRM SDCCH
ASSIGNMENT COMMAND RRM SDCCH
ASSIGNMENT ACK CM SDCCH
CONNECT ACK FACCH
conversation TCH
conversation TCH
conversation CM FACCH
DISCONNECT CM FACCH
RELEASE CM FACCH
RELEASE COMPLETE RRM FACCH
CHANNEL RELEASE
Mobile-Assisted Handover
Mobile Base Logical
station station Channel
conversation TCH
conversation TCH
MEASUREMENT REPORT SACCH
conversation TCH
conversation TCH
conversation TCH
MEASUREMENT REPORT SACCH
conversation TCH
HANDOVER COMMAND FACCH
HANDOVER ACCESS new TCH
HANDOVER ACCESS TCH
HANDOVER ACCESS TCH
PHYSICAL INFORMATION TCH
HANDOVER COMPLETE
conversation FACCH
TCH
conversation
TCH
MEASUREMENT REPORT
SACCH
conversation TCH
conversation TCH
conversation TCH
MEASUREMENT REPORT SACCH
conversation TCH
Broadcast Channels (BCH)
• To help the MH (Mobile Handset) measures
– to turn to a BTS
– to listen for the cell information
• to start roaming, waiting for calls to arrive, making
calls
– Because BTSs are not synchronized with each
other, every time a MH decides to camp to
another cell, its FCCH, SCH, and BCCH must
be read.
Frequency Correction Channel
(FCCH)
• Provide MH with the frequency reference of
the system
– To enable the Mobile Handset (MH) to
synchronize with the frequency
• Transmission properties
– Transmit on the down-link
– Point to multi-point.
Synchronization Channel (SCH)
• MH synchronize with the structure within the
locative cell
– MH can receive information from the proper time slots
on the TDMA structure
• To ensure a GSM BTS is chose
– The Base Station Identity Code (BSIC) can only be
decoded by a GSM BTS
• Transmission properties
– Transmit on down-link
– Point to multi-point.
Broadcast Control Channel
(BCCH)
• BTS broadcast cell information to MH
– LAI ( Location Area Identity), to start roaming,
waiting for calls to arrive, making calls
– maximum output power allowed in the cell
– information about BCCH carriers for the
neighboring cells
• MH will perform measurement to BTS
• Transmission properties
– Transmit on down-link
– Point to multi-point
Common Control Channels
(CCCH)
• CCCH support the establishment of a
dedicated communication path (dedicated
channel) between the MH and the BTS
• Three types of CCCH
– Paging Channel (PCH)
– Random Access Channel (RACH)
– Access Grant Channel (AGCH)
Paging Channel (PCH)
• Used by BTS to page particular MH in the cell
– MH actively listen to PCH to check contact info
within certain time
– Contact could be incoming call or short message
• Contact info on PCH include
– IMSI (MH’s identity number), or
– TMSI (temporary number)
• Transmission properties
– Transmit on down-link
– point to point
Random Access Channel
(RACH)
• Used by MH to request a dedicated channel
for call setup
– Shared by any MH attempts to access the
network
– Channel request message contains the reason
for the access attempt
• Transmission properties
– Transmit on up-link
– Point to pint.
Access Grant Channel (AGCH)
• The network assigns a signaling channel via
AGCH
– A Stand alone Dedicated Control Channel
(SDCCH) is assigned
• Transmission properties
– Transmit on down-link
– Point to point
Dedicated Control Channels
(DCCH)
• DCCH are used for transferring nonuser
information between the network and the MH
– Messages on DCCH Including
• channel maintenance
• mobility management
• radio resource management
• Four kinds of DCCH
– Stand alone Dedicated Control Channel (SDCCH)
– Cell Broadcast Channel (CBCH)
– Slow Associated Control Channel (SACCH)
– Fast Associated Control Channel (FACCH)
Stand alone Dedicated Control
Channel (SDCCH)
• Transfer signaling information between the
BTS and the MH
• Typically used for location updating prior to
use of a traffic channel
• Transmission properties
– Bidirectional channel, transmit on both up and
down-link
– Point to point.
Cell Broadcast Channel (CBCH)
• To carry Short Message Service Cell
Broadcast (SMSCB)
• Use the same physical channel as SDCCH
• Transmission properties
– Transmit on down-link
– Point to multi-point
Slow Associated Control
Channel (SACCH)
• Carries control and measurement parameters along
with routine data necessary to maintain a radio
link between the MH and the BTS
– On the uplink, MS sends averaged measurements
(signal strength and quality) of current and neighboring
BCCH
– On downlink, MS receives information about
transmitting power to use and an instruction with time
advance/retard
• Transmission properties
– Bidirection channel, transmit on both up and down link
– Point to point
Fast Associated Control Channel
(FACCH)
• An FACCH is used over a TCH where it
steals time slots from a TCH
– a 20 ms segment of speech is stolen to carry
handover signaling information
• Appears on demand
Traffic Channels (TCH)
0 1 ... 24 25 0 1 ... 49 50
Frequency correction TB TB GP
burst (FB) Fixed bits 142
3 3 8.25
Synchronization TB TB GP
Encrypted bits 39 Synchronization sequence 64 Encrypted bits 39
burst (SB) 3 3 8.25
TB Encrypted bits 36
Access burst (AB) Synchronization sequence TB GP 68.25
3
41 3
BCCH √ √ √
FCCH √ √ √
SCH √ √ √
RACH √ √ √
PCH √
√ √
AGCH √
SDCCH √ √ √ √
√
SACCH √ √
√
FACCH √ √
TCH √
√ √ √
GSM Architecture
GSM System Architecture
HLR/
UM VLR
Handset
A-bis A SS7
BSC MSC
PSTN
X.25
BTS
OMC
(Operation & Maintenance
Center)
network. cell
Cell Characteristics
• The Basic Union In The System
– defined as the area where radio coverage is
given by one base station.
• Addressed by Cell Global Identity (CGI)
• A cell has one or several frequencies,
depending on traffic load.
– Frequencies are reused, but not used in
neighboring cells due to interference.
Cell Planning
• Reusing frequencies in
different geographic
7-cell reuse
areas: pattern
– 7 cell repeat pattern is A1
A3
typical, but other G1 A2 B1
A1 G3 B3
patterns are also A3
A2 B1 G2 C1 B2
G1 B3 C3
common G3 B2
F1 D1 C2
G2 C1 F3 D3
C3
C2 D1 F2 E3 E1 D2
F1 D3
F3 D2 E2
F2 E1
E3 Frequency
E2
reuse
Cell Patterns
4 1
3 5 2
1 3
2 6
7
K=7 K=3
Frequency Utilization v.s. K
• Frequency can’t be reused in the same pattern
• Larger K
– More channels in a pattern
– Higher subscriber number
– Worse frequency reusability
• Smaller K
– Less channels in a pattern, high channel distortion
– Smaller subscriber number
– Better frequency reusability
Numbering & Routing
Numbering
• Mobile Subscriber ISDN Number (MSISDN)
– Mapping to Mobile Station Roaming Number (MSRN)
by HLR
• International Mobile Subscriber Identify (IMSI)
– Stored in SIM/HLR
• Temporary Mobile Subscriber Identity (TMSI)
– Assigned by VLR, Stored in VLR
• International Mobile Equipment Identity (IMEI)
– Unique ID to handset, used by air interface
ID-Numbers
• MSISDN = CC + NDC + SN
– MSISDN: Mobile Station ISDN Number
– CC Country Code
– NDC National Destination Code
– SN Subscriber Number
• IMSI = MCC + MNC + MSIN
– IMSI International Mobile Subscriber Identity
– MCC Mobile Country Code
– MNC Mobile Network Code
– MSIN Mobile Station Identification Number
ID-Numbers (cont.)
• IMEI = TAC + FAC + SNR + spare
– IMEI Internal Mobile Equipment Identity
– TAC Type Approval Code, determined by a central
GSM/PCS body
– FAC Final Assembly Code, identifies the manufacturer
– SNR Serial Number, uniquely identifies all
equipment within each TAC and FAC
• IMEISV = TAC + FAC + SNR + SVN
– IMEISV International Mobile Equipment Identity and
Software Version Number
– SVN Software Version Number
ID-Numbers (cont.)
• MSRN = CC + NDC + SN
– MSRN Mobile Station Roaming Number
– CC Country Code
– NDC National Destination Code
– SN Subscriber Number. In this case, the address to
the serving MSC
• LAI = MCC + MNC + LAC
– MCC Mobile Country Code
– MNC Mobile Network Code
– LAC Location Area Code
ID-Numbers (cont.)
• CGI = MCC + MNC + LAC + CI
– CGI Cell Global Identity
– CI Cell Identity
• BSIC = NCC + BCC
– BSIC Base Station Identity Code
– NCC Network Color Code (3bits)
– BCC Base Station Color Code (3bits)
• LN = CC + NCD + LSP
– LN Location Number
– CC Country Code
– NCD National Destination Code
– LSP Locally Significant Part
ID -Numbers (cont.)
• LN = CC + NCD + LSP
– LN Location Number
– CC Country Code
– NCD National Destination Code
– LSP Locally Significant Part
• RSZI = CC + NDC + ZC
– RSZI Regional Subscription Zone Identity
– CC Country Code
– NDC National Destination Code
– ZC length of the Zone code (2 octets)
Routing
• Information resident in MS & SIM
– IMSI, TMSI, IMEI, and misc. information
• Routing Information used by Network
– MSISDN, MSRN
Location Registry
Location Registry
• Registration Message Flow
– Inter-LA movement
• In the same MSC
– Inter-MSC movement
• In the same GSM Operator
– Inter-VLR movement
Registration Message Flow
-- Inter-LA Movement
HLR
3 VLR1 VLR2
2
MSC1 MSC2
4
1
LA1 LA2
Registration Message Flow
-- Inter-MSC Movement
HLR
4
3
VLR2 VLR1
5
2
MSC1 MSC2
6
1
LA1 LA2
Registration Message Flow
-- Inter-VLR Movement
HLR
7 5
8 4
VLR1 3 VLR2
2
MSC1 MSC2 1 6
LA1 LA2
Message sequence and system operations for registration
of a terminal in a visited service area
Prior Prior Serving Serving Serving
MSC VLR HLR VLR MSC Base
Registration info
Store info
REGISTRATION NOTIFICATION INVOKE
Store info
REGISTRATION NOTIFICATION INVOKE
Change info
REGISTRATION NOTIFICATION RESULT
Store info
Call Setup
Mobile-originated call, mobile to land call flow (part 1 of 2)
AUThentication REQuest
AUThentication RESponse
Assignment Request
ASSignment CoMmanD
Assignment Complete
Assignment Complete
Initial Address Message (IAM)
Connect Acknowledge
Call setup to a roaming subscriber
7.
BSC
8.
8. 9.
alerting
Address complete message
connect
Call Call
waiting forwarding
Authentication Resp,
Update location
Insert subscriber data
Subs. Data Ins. Acc.
Loc.n update acc
MSISDN HLR
2
MSISDN MSRN
PSTN GMSC 5
1 3 4
IMSI MSRN
MSRN 6
VLR
MSC
IMSI MSRN