GSM MAC
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Architecture
Frequency bands
Spectrum efficiency
Traffic and control frames
Speech coding and data rates
Control channels
Signaling
Original by: Ahmed Ibrahim, Chang Wu Ma,
Danny Mangra, EL604, Fall 2001;
Modified by Prof. M. Veeraraghavan
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Architecture of the GSM network
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Frequency bands
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Frequency band: Uplink: 890-915 MHz, Downlink: 935-960 MHz
Frequency range: 50 MHz (25 MHz Up, 25 MHz Down)
Carrier spacing: 200 KHz (but time shared bet. 8 subscribers)
Duplex distance: 45 MHz (FDD)
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Frequency bands
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Number of carriers: 25 MHz/200KHz =124
Users/carrier: 8
The reverse channel is retarded by 3 time slots relative to the
forward
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Frequency bands
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One or more carrier frequencies are assigned to each BS
Eight time slots are grouped into a TDMA frame (120/26 ms, or approx.
4.62 ms; 120 frames in a multiframe that is 26ms in duration)
Time slot = 4.62/ 8 ms (or approx. 0.577 ms)
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One physical channel is one time slot per TDMA frame.
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Conventional carriers
• The conventional carrier is a sine wave at a single frequency
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Slow frequency hopped carriers
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Each TDMA frame in a given channel is carried on a different carrier frequency
The purpose is to reduce co-channel interference between signals in nearby cells
Frequency hopping adds a new dimension of complexity to cellular reuse planning
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Frequency reuse
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The Cellular Concept
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Hexagonally tiled cells
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Cannot reuse frequencies in six
surrounding cells
Minimum of seven frequency sets
is required if N=7
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Repeat tiling of seven-cell array
Distance between like cells must
be far enough to avoid
interference
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Smaller cells lead to better frequency
reuse
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More calls per unit area
Transmitted power must be smaller to
avoid interference
Requires careful power management
Requires
larger number of
base stations
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Spectrum efficiency
• Assume N =3 (depends on environment)
• Country side: N = 2 or 3;
• Metropolitan areas: N > 3 or higher
Carriers: 124; each carrier has 8 channels:
No. of physical channels = 124*8 = 992
Total frequency band: 25(uplink)+25(downlink)=50MHz
Efficiency = 992/(3*50MHz)
= 6.61 conversations/ cell/ MHz
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Traffic frames & control frames
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Traffic channels
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A traffic channel (TCH) is used to carry speech and data traffic.
TCHs are defined using a 26-frame multiframe (a group of 26 TDMA frames)
The length of a 26-frame traffic multiframe is 120 ms
Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow
Associated Control Channel (SACCH) (12 and 25 on alternate multiframes) and
1 is currently unused.
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Traffic channels categories
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GSM transmission rate
26 frames / multiframe
 8slots / frame 156.25b / slot  270.8333kb / s
120ms / multiframe
• A full-rate traffic channel has a bit rate of
24slots / multiframe114bits / slot
 22.8kb / s
120ms / multiframe
Within each time slot, some bits reserved for control; only
114 bits of 156.25 bits are for voice data
• Contrast this to IS136 where transmission rate is 48.6kb/s
and to the full-rate channel bit rate of 16.2kbps

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Speech coding
• new: Enhanced Full-Rate (EFR) coding
• original: Linear Prediction Coding with
Regular Pulse Excitation (LPC-RPE)
– coder: 13kbps
– with channel coding, rate becomes 22.8kbps –
for a full-rate channel
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Control channels
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Common channels can be accessed both by idle mode and dedicated
mode mobiles.
• The common channels are used by idle mode mobiles to exchange
signaling information required to change to dedicated mode.
• Mobiles already in dedicated mode monitor the surrounding base
stations for handover and other information.
• The common channels are defined within a 51-frame multiframe, so
that dedicated mobiles using the 26-frame multiframe TCH
structure can still monitor control channels.
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GSM control channels categories
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Reverse control channel access protocol
begin
Send message
Slotted Aloha
Scheme
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Other
transmissions
in this slot?
yes
yes
no
Base detects
message ?
no
Another message
with same 5-bit
code?
yes
no
Max
attempts?
Access
Access
Access
Random
Succeeds
Conflict
Fails
time delay
terminal is assigned an SDCCH
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GSM signaling protocol architecture
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Messaging
• GSM specifies the communication protocols employed
on ALL the Network Interfaces
• All of the signaling channels (except FCCH, SCH &
RACH) transmit information in LAPDm format.
• PHY carries these messages in 184-bit segments
Address Control
LI
Data
Fill
184 bits
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Functions
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Radio Resources Management (RRM)
– Controls the setup, maintenance, and termination of
channels, including handovers.
Mobility Management (MM)
– Manages the location updating and registration
procedures, as well as security and authentication.
Call Control Management (CCM)
– Handles general call control, similar to CCITT Recommendation
Q.931, and manages Supplementary Services and the Short
Message Service.
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Delivery of a call to a GSM mobile station
Mobile Station
Base Station
Initial Procedure (see slide 33)
RACH: “Channel request”
AGCH: “Immediate assignment”
I am here.
SDCCH: “Paging response”
MSC gets
MS’s
location
SDCCH message exchange (see
Slide 34)
SDCCH: “Assignment ACK”
FACCH: “Connect ACK”
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Conversation
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Initial procedure in delivery of a call to a GSM
mobile station
Mobile Station
SCH: “Sync Channel Information”
Base Station
BCCH: “System Information”
PCH: “Paging Request”
RACH: “Channel request”
PCH: Paging Channel
Purpose: To notify terminals of arriving calls.
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SDCCH message exchange in delivery of a call to a
GSM mobile station
Base Station
Mobile Station
SDCCH: Standalone dedicated control channel
Authentication request
Authentication response
CIPHERING MODE
Ciphering Mode ACK
setup
Call Confirmed
ALERTING
CONNECT
Assignment Command
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Termination of the call (by MS)
Mobile Station
How is the call terminated at MS?
Base Station
Conversation
FACCH: “Disconnect”
FACCH: “Release”
FACCH: “Release complete”
FACCH: “Channel release”
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References
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Wireless Personal Communication Systems, David J. Goodman
Overview of the Global System for Mobile Communications, John
Scourias, University of Waterloo
• GSM Cellular Standards: A look at the world’s most common digital
cellular system, Kevin Bolding, Electrical Engineering, Seattle Pacific
University
• Wireless and Mobile Network Architectures, Yi Bin Lin, Imrich
Chlamtac
• Mobile Radio Networking, Networking and Protocols, Bernhard H.
Walke
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