gsm channels - physical and logical
TRANSCRIPT
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GSM CHANNELS
By Mohit Kumar
http://mohitkumar.net
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GSM?
Global system for mobile communication
2nd Generation mobile telephony
Duplexing schemes FDD/TDD
Multiple access TDMA
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Nomenclature
MS (Mobile Station) = MT (Mobile Terminal ) +TE (TerminalEquipment)
BSS (Base Station Subsystem) = BTS (Base Transceiver Station)+BSC (Base Station Controller)
NSS (Network Switching Subsystem)
MSC (Mobile Switching Center): telephony switching function andauthentication of user
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ChannelsPhysical Channels
Associated with frequency bands, time slots, codes Physical channels transfer bits from one network element to
anotherLogical ChannelsDistinguished 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
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PHYSICAL CHANNELS
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GSM frequency bands Original GSM radio band is 900 MHz (GSM 900).
Another band that was later assigned termed as DCS was 1800 MHz(DCS 1800).
Can be classified further as
P-GSM, Standard or Primary GSM-900 Band
E-GSM, Extended GSM-900 Band (includes Standard GSM-900band)
R-GSM, Railway GSM-900 Band (includes Standard andExtended GSM-900 band)
T-GSM or TETRA-GSM (TETRA = TErrestrial Trunked RAdio,formerly Trans-European Trunked Radio)
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FDD
Each GSM band is divided into two parts UPLINK and DOWNLINK
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Radio channel GSM radio channel is 0.2 MHz wide. Each channel has a fixed ID
number, called Absolute Radio Frequency Channel Number (ARFCN)
Example: GSM 900
ARFCN 0 represents the 0.2 MHz channel from 890 to 89.2 MHz(usually called 890 MHz channel)
ARFCN 1 to 124 represent 890 + ARFCN * 0.2 MHz channels
Note: ARFCN 0 is reserved as a guard band between GSM bandand its neighboring band.
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GSM represents an ARFCN with a 10-bit number (0 to 1023).
When the network assigns a channel to an MS (mobile station) itidentifies this number
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Radio channels. Each ARFCN channel is a duplex channel and consists of an up and a
down links.
When we say ARFCN 1 we mean uplink 890.2 MHz and its downlink935.2 MHz channels as a duplex.
The uplink and downlink of all ARFCNs maintain a frequency distanceequal to band gap + unidirectional bandwidth.
Eg.-GSM 900 has band gap = 20 + 25 = 45 MHz
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TDMA in GSM Each frequency channel or ARFCN (200 kHz bandwidth) is shared by
multiple users and/or control signal functions one at a time.
For a voice channel every 8th time-slot belongs to the same user.
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TDMA Frame
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TDD One uplink slot and a downlink slot of a duplex GSM frequency (an
ARFCN) forms a pair to provide one voice connection.
These slots carry voice traffic bursts. This is called Traffic Channel(TCH) according to GSM terminology. The slots are like physical carrier, good for any type of data. A duplex
pair of them becomes TCH when they are used (or designated) forvoice traffic.
The timing of uplink and downlink slots maintains a 3-slot distance inorder to ensure that a cell-phone does not require transmission andreception operations simultaneously.
Avoids a number of complexities including the requirement of high peakpower, processor speed and large memory. This also helps simplifytransceiver circuit.
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GSM LOGICALCHANNELS.
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Logical channels One or more logical channels can be mapped to one physical channel.
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Traffic channels Full rate
It transmits full ratespeech (13Kbps).
A full rate TCH occupiesone physical channel
Half rate
It transmits half ratespeech(6.5 Kbps).
Two half rate TCHs canshare one physical
channel to double thecapacity
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Control channels
Can also be termed as signaling channels
These channels are used for non-traffic partthat is used to support the traffic.
Can be divided into three parts
Broadcast channels (BCH)
Common Control Channels (CCCH)
Dedicated Control Channels(DCCH)
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Broadcast Channels Helps the MSs to orient themselves in the mobile radio network.
Point to multi point channels ( BTS to MSs)
Downlink only
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FCCH The logical Channel contains no information (continuous zeros)
pure carrier is transmitted so that the mobile can tune itself and
synchronise with the frequency.
FCCH will be transmitted in the same beacon carrier as BCCH.
DOWNLINK
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BCCH
Through the BCCH logical channel MS is informed about thesystem configuration parameters like
Local Area Identity
Cell Identity
Identity of BCCH carrier frequencies of Neighbor cells. Max trans power of MS to access
Configuration of control channels
BCCH is also known as beacon. Each cell/sector will have one
carrier frequency on which BCCH will be broadcasted.
DOWNLINK
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SCH
This logical channel transmits information about TDMA framestructure in a cell /sector (frame number)
Base Station Identity Code which can be decoded only if the BTSbelongs to the network for which the MS has subscribed
SCH will also be transmitted in the same carrier of FCCH andBCCH.
DOWNLINK
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Common Control Channels support common procedures required to establish a dedicated link with
the network.
are point-to-multipoint channels that are primarily intended to carrysignaling information for access handling functions.
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PCH PAGING CHANNEL
Used by BTS to page particular MS in the cell MS 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 (MSs identity number), or
TMSI (temporary number)
Transmission properties Transmit on down-link
point to point
DOWNLINK
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RACH REQUEST ACCESS
CHANNEL
Used by MH to request a dedicated channel for call setup
Shared by any MS attempts to access the network
Channel request message contains the reason for the accessattempt
Transmission properties
Transmit on up-link
Point to pint
UPLINK
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AGCH ACCESS GRANT
CHANNEL 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
DOWNLINK
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DEDICATED CONTROL
CHANNEL -DCCH
DCCH are used for transferring nonuser information between thenetwork and the MS
Messages on DCCH Including
channel maintenance
mobility management
radio resource management
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Stand alone Dedicated Control
Channel (SDCCH)
Transfer signaling information between the BTS and the MS
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.
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Types of SDCCH
SDCCH/4 sdcch is combined with the
bcch channel
the bcch and sdcch remainactive for almost half of the
time in a multiframe
Transmits only on TS0
SDCCH/8 takes up an entire TCHand keeps trasmitting
on that[alongwithSACCH].
the TCH it is to occupy,can be defined
SDCCH uses 1/8 of time slots.
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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
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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
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Slow Associated Control
Channel (SACCH) Carries control and measurement parameters along with routine data
necessary to maintain a radio link between the MS 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
Bidirectional channel, transmit on both up and down link Point to point
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Channel Combinations Several logical channels can be mapped onto
physical channels There are many combinations that can be
used in case of GSM logical channels
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Combination I
TCH/F + FACCH/F + SACCH.
This combination is used for full rate traffic.
TCH/F FACCH/F SACCH
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TCH/F + FACCH/F + SACCH.
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Combination II
TCH/H + FACCH/H + SACCH.
This combination is used for half rate traffic
when only one channel is needed.
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Combination III
2 TCH/H + 2 FACCH/H + 2 SACCH
This combination is used for half rate traffic.
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Combination II & III
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Combination IV
FCCH + SCH + BCCH + CCCH.
This is the standard combination for mediumand large cells.
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Combination IV
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Combination V
FCCH + SCH + BCCH + CCCH +
4 SDCCH + 4 SACCH.
This is the typical C0T0 combination for small cells, which allows the
BTS to trade unnecessary CCCH capacity for a pool of 4 SDCCHs.
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Combination V
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Combination VI
BCCH + CCCH.
This combination is used to provide additionalCCCH capacity in large cells.
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Combination VI
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Combination VII8 SDCCH + 8 SACCH.
This combination is used to provide additionalSDCCH capacity in medium and large cells.
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Combination VII
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GSM Bursts