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1 Polytechnic University
GSM MAC
Original by: Ahmed Ibrahim, Chang Wu Ma, Danny Mangra, EL604, Fall 2001;Modified by Prof. M. Veeraraghavan
• Architecture• Frequency bands• Spectrum efficiency• Traffic and control frames• Speech coding and data rates• Control channels• Signaling
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Frequency bands
• 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
• 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
• 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)
• 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
• 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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The Cellular Concept
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• Repeat tiling of seven-cell array• Distance between like cells must
be far enough to avoid interference
• Smaller cells lead to better frequency reuse
• More calls per unit area• Transmitted power must be smaller to
avoid interference• Requires careful power management• Requires larger number of base stations
• Hexagonally tiled cells• Cannot reuse frequencies in six
surrounding cells• Minimum of seven frequency sets
is required if N=7
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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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• 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.
Traffic channels
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GSM transmission rate
• A full-rate traffic channel has a bit rate of
skbslotbframeslotsmultiframems
multiframeframes/8333.270/25.156/8
/120
/26
skbmultiframems
slotbitsmultiframeslots/8.22
/120
/114/24
Within each time slot, some bits reserved for control; only114 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
16 Polytechnic University
Control channels
• 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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Send message
yes
yes
yes
no
nono
no
Access
Fails
Access
ConflictAccess
Succeeds
begin
Other transmissions in this slot?
Base detects
message ?
Max attempts?
Random
time delay
Another message with same 5-bit code?
Slotted Aloha Scheme
Reverse control channel access protocol
terminal is assigned an SDCCH
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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
• 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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Mobile Station Base Station
RACH: “Channel request”
AGCH: “Immediate assignment”
SDCCH: “Paging response”
FACCH: “Connect ACK”
SDCCH: “Assignment ACK”
Conversation
SDCCH message exchange (see Slide 34)
Initial Procedure (see slide 33)
Delivery of a call to a GSM mobile station
MSC gets MS’s
locationI am here.
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SCH: “Sync Channel Information”
BCCH: “System Information”
PCH: “Paging Request”
Initial procedure in delivery of a call to a GSM mobile station
PCH: Paging Channel
Purpose: To notify terminals of arriving calls.
Mobile Station Base Station
RACH: “Channel request”
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SDCCH: Standalone dedicated control channel
Authentication request
Authentication response
CIPHERING MODE
Ciphering Mode ACK
setup
Call Confirmed
ALERTING
CONNECT
Assignment Command
SDCCH message exchange in delivery of a call to a GSM mobile station Base StationMobile Station
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How is the call terminated at MS?
FACCH: “Release complete”
FACCH: “Release”
FACCH: “Disconnect”
FACCH: “Channel release”
Conversation
Termination of the call (by MS)
Mobile Station Base Station
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References
• 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