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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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Architecture of the GSM network

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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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Frequency reuse

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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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Traffic frames & control frames

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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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Traffic channels categories

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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

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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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GSM control channels categories

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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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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

• 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