GSM_Channels

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NU TEK (I) LIMITED:

NU TEK India Limited NU TEK (I) LIMITED 1505-06, 15 th Floor, Global Business Park, Tower – D, M.G. Road, GURGAON, INDIA TEL NO. 0124- 4171600 FAX NO. 0124-4011275

CHANNELS:

NU TEK India Limited CHANNELS

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Channels in GSM Air interface:

NU TEK India Limited Channels in GSM Air interface Logical Channels : ARFCN –x, Timeslot – y, Frame Number - N Logical Channels Control Channels Traffic Channels

Logical channel:

NU TEK India Limited Logical channel One or more logical channel can be transmitted on a physical channel. Different types of logical channel are – Traffic Channels (Speech & Data) Broad cast channel (Signalling) Common control channel (Signalling) Dedicated control channel (Signalling)

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TRAFFIC CHANNEL:

NU TEK India Limited TRAFFIC CHANNEL

Traffic Channels:

NU TEK India Limited Traffic Channels Traffic Channels TCH - F TCH - H

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NU TEK India Limited Traffic Channel carries the voice data Two blocks of 57 bits contain voice data One TCH is allocated for every active call. While call is in progress if there is degradation in quality of current channel, BTS may shift the communication to another TCH on a different Carrier and/or Timeslot A full rate TCH carries 13 kbps voice data, and Half rate TCH carries a 6.5 kbps voice data

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Traffic Channels (TCH) :

NU TEK India Limited Traffic Channels (TCH) One physical channel (1 timeslot) can support: Traffic channels are used to send Speech or Data services. 1 TCH/F or 2 TCH/H • TCH/F: 13 kb/s voice or 9.6 kb/s data (22.8 Kbps after channel coding) • TCH/H: 6.5 kb/s voice or 4.8 kb/s data (11.4 kbps after channel coding) Both can be used to carry signalling information. Uplink / Downlink Synchronisation The MS transmit burst is delayed by 3 timeslots after the BTS burst.. This delay allows enables: Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous Tx/Rx requirement

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NU TEK India Limited Speech channels are supported by two different methods of coding known as Full Rate (FR) and Enhanced Full Rate (EFR). Enhanced Full Rate coding provides a speech service that has improved voice quality from the original Full Rate speech coding, whilst using the same air interface bandwidth. EFR employs a new speech coding algorithm and additions to the full rate channel coding algorithm to accomplish this improved speech service,

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CONTROL CHANNEL:

NU TEK India Limited CONTROL CHANNEL

Channels in GSM Air interface:

NU TEK India Limited Channels in GSM Air interface Logical Channels : ARFCN –x, Timeslot – y, Frame Number - N Logical Channels Control Channels Traffic Channels

Concepts of Channels in GSM:

NU TEK India Limited Concepts of Channels in GSM A company vehicle is used for several purposes in a day Similarly in GSM, the timeslots are used for different purposes at different times

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NU TEK India Limited These control channels are used by the MS to establish communication with the network in the idle mode and also in initiating calls to enter the dedicated mode. Timeslot 0 is grouped into structures of 51 frames referred to as Control Channel Multiframes The control channels are grouped as Broadcast Control Channels (BCCH) Common Control Channels (CCCH) and Dedicated Control Channel (DCCH).

Broadcast Channels (BCCH):

NU TEK India Limited Broadcast Channels (BCCH)

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BCH (Broadcast Channel):

NU TEK India Limited BCH (Broadcast Channel) One ARFCN, On all the time, in every cell Uses Timeslot 0 on a channel, in Downlink FCH – Frequency correction channel (allows mobile to tune to BTS frequency. This channel carries a 142 bit zero sequence and repeats once in every 10 frames on BCH) SCH – Synchronization channel (carries the Frame number and BSIC). A midamble of 64 bit helps mobiles to synchronize. It also repeats once in 10 Frames

BCH (Broadcast Channel):

NU TEK India Limited BCCH – Broadcast Control Channel. It has information encoded on it which identifies the network (LAC). It also carries list of the channels in use in the cell (CA tables) BCH (Broadcast Channel)

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NU TEK India Limited The Broadcast Control Channel is transmitted by the BTS at all times. The RF carrier used to transmit the BCCH is referred to as the BCCH carrier. The information carried on the BCCH is monitored by the MS periodically (at least every 30 secs), when it is switched on and not in a call. Each cell has one carrier designated as a BCH carrier. The BCH carrier has all 8 timeslots continuously on, either with traffic or dummy bursts. Timeslot 0 of the BCH carrier contains logical control channels. Broadcast Control Channel (BCCH) – Carries the following information (this is only a partial list): Location Area Identity (LAI). List of neighbouring cells which should be monitored by the MS. List of frequencies used in the cell. Cell identity. Power control indicator. DTX permitted. Access control (for example, emergency calls, call barring).

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NU TEK India Limited Information distributed over Broadcast channel helps the MS to orient themselves in the mobile radio N/W .BCH channels are all downlink Pt. to Multi pt. and are allocated to timeslot zero. FCCH: Frequency correction channel This is transmitted frequently on the BCCH timeslot and allows the mobile to synchronize its own frequency to that of the transmitting base site. The FCCH may only be sent during timeslot 0 on the BCCH carrier frequency and therefore it acts as a flag to the mobile to identify Timeslot 0. SCH: (Synchronisation channel ) The SCH carries the information to enable the MS to synchronize to the TDMA frame structure and know the timing of the individual timeslots. The following parameters are sent: – Frame number. – Base Site Identity Code (BSIC). The MS will monitor BCCH information from surrounding cells and store the information from the best six cells. The SCH information on these cells is also stored so that the MS may quickly resynchronize when it enters a new cell.

Common Control Channels (CCCH) :

NU TEK India Limited Common Control Channels (CCCH)

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Common Control Channel:

NU TEK India Limited Common Control Channel CCCH shares Timeslot 0 with BCH on a Multiframe CCCH consists of PCH, RACH and AGCH PCH – Paging channel is used to alert mobiles on incoming calls. PCH carries IMSI to page for Mobiles in the cell. PCH is Downlink channel. RACH – Random Access Channel is a short burst sent by mobile to BTS, to initiate a call request. RACH uses timeslot 0 on reverse BCH channel on uplink

Common Control Channels (CCCH) :

NU TEK India Limited Common Control Channels (CCCH) CCCH contains all point to multi-point downlink channels (BTS to several MSs) and the uplink Random Access Channel: RACH: (UL) Random Access Channel is sent by the MS to request a resources from the network e.g. an SDCCH channel for call setup. • AGCH (DL) Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobile. • PCH ( DL) Paging Channel sends paging signal to inform mobile of a call. Active MSs must frequently monitor both BCCH and CCCH. The CCCH will be transmitted on the RF carrier with the BCCH.

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Dedicated Control Channels (DCCH) :

NU TEK India Limited Dedicated Control Channels (DCCH) DCCH comprise the following bi-directional (uplink / downlink) point to point control channels . Used for signalling between MS & BTS : • SDCCH: S tandalone D edicated control Ch annel is used for call set up, location updating and also SMS.It carries all signalling B/W BTS & MS when no TCH is allocated. Which supports the transfer of Data to and from the MS during call setup and validation. Summary —Used for services request, SMS,Location update ,subscriber Authentication,Equipment validation. (Afterwards TCH Comes into picture) • SACCH: S low A ssociated C ontrol Ch annel is used for link measurements and signalling during a call .Used for non urgent procedures. Summary —transmission of Signalling Data ,Radio Link Supervision measurement ,Transmit power control & Timing advance data

FACCH:

NU TEK India Limited FACCH • FACCH: F ast A ssociated C ontrol CH annel is used (when needed) for signalling during a call, mainly for delivering handover messages and for acknowledgement when a TCH is assigned.Used mainly during handovers when SACCH Signalling is not fast enough.FACCH data is transmitted over TCH, this is marked by Flag called “Stealling Flag”

RACH – Random Access Channel:

NU TEK India Limited RACH – Random Access Channel 8 start bits 41 Synchronization bits 36 Encrypted bits 3 stop bits 68.25 Extended Guard Period 88 bits Used by mobile to get attention from Base Station Several mobiles might originate RACH simultaneously Mobile doesn’t know path delay So RACH has to be a special short burst Mobile sends normal burst after getting timing advance on downlink SACCH

DCCH – Dedicated Control Channels:

NU TEK India Limited DCCH – Dedicated Control Channels SDCCH – Standalone Dedicated Common Control Channel. It is used as a interim channel before final assignment of TCH. SDCCH is used for signaling and Authentication message transfers. It can be called the stepping stone between BSC and TCH The SDCCH, by using less of the cells resource of physical channels, improves efficiency, and provides a useful holding channel for the mobile until speech data needs to be exchanged.

Dedicated Control Channels (cont..):

NU TEK India Limited Dedicated Control Channels (cont..) FACCH – Fast Associated Control Channel Interrupts TCH on uplink and downlink Rapid message exchange for handovers Control bits on either side of midamble – 0 indicates TCH and 1 indicates FACCH Both ways – uplink and downlink

SACCH – Slow associated Control Channel:

NU TEK India Limited SACCH – Slow associated Control Channel Downlink (BTS – MS) Mobile Tx Power Commands Mobile Timing Advance Cell’s Channel Configuration Uplink (MS - BTS) Received signal quality report (RXQual) Received signal level report (RXLev) Adjacent BCH power measurement

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Dedicated Control Channels (DCCH) The DCCH is a single timeslot on an RF carrier which is used to convey eight Stand-alone Dedicated Control Channels (SDCCH). A SDCCH is used by a single MS for call setup, authentication, location updating and SMS point to point. As we will see later, SDCCH can also be found on a BCCH/CCCH timeslot, this configuration only allows four SDCCHs. Associated Control Channels (ACCH) These channels can be associated with either an SDCCH or a TCH. They are used for carrying information associated with the process being carried out on either the SDCCH or the TCH. :

NU TEK India Limited Dedicated Control Channels (DCCH) The DCCH is a single timeslot on an RF carrier which is used to convey eight Stand-alone Dedicated Control Channels (SDCCH). A SDCCH is used by a single MS for call setup, authentication, location updating and SMS point to point. As we will see later, SDCCH can also be found on a BCCH/CCCH timeslot, this configuration only allows four SDCCHs. Associated Control Channels (ACCH) These channels can be associated with either an SDCCH or a TCH. They are used for carrying information associated with the process being carried out on either the SDCCH or the TCH.

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NU TEK India Limited CONTINUE Slow Associated Control Channel (SACCH) Conveys power control and timing information in the downlink direction (towards the MS) and Receive Signal Strength Indicator (RSSI), and link quality reports in the uplink direction. Fast Associated Control Channel (FACCH) The FACCH is transmitted instead of a TCH. The FACCH ‘‘steals” the TCH burst\ and inserts its own information. The FACCH is used to carry out user authentication, handovers and immediate assignment. All of the control channels are required for system operation, however, in the same way that we allow different users to share the radio channel by using different timeslots to carry the conversation data, the control channels share timeslots on the radio channel at different times. This allows efficient passing of control information without wasting capacity which could be used for call traffic. To do this we must organize the timeslots between those which will be used for traffic and those which will carry control signalling.

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