Data communications


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  • Data communications

    1. 1. Data two types Analog  Form of sinusoidal wave pattern… changing states Digital  Form of ON/OFF pattern… pulses
    2. 2. Communication Analog data transmission  States changes (follows SIN wave)  Maintenance cost is low  But.. Cost of transmission is high  Effect of environment is very high  Amplifier circuit loses data
    3. 3.  Digital data transmission  Follow ON/OFF pattern  Mainainance cost is high  Transmission cost is low  Environment factor… low  No loss while using amplifier/reapeter
    4. 4. Channel Characteristic Ideal channel  Should convey the maximum from sender to receiver  Should not ALTER…(additional noise)  No distance restriction  Convey cost should be maintain Type  Analog  digital
    5. 5. Close look to the digital channel Channel carries BITS Measurement… how many BIT/ second called bit rate (bps) The bps is the rate at which the channel can carry BITS (digital data) Distribution of bits determine bit rate shorter the duration greater bps
    6. 6. TRANSMISSION MODES Simplex DATA Sender receiver  Only one way communication … unidirectional flow  Interactive part is absent… so no ackw  Examples… keyboard/printer.. Radio, TV  Cheapest… but low efficient
    7. 7.  Half duplex Sender/ DATA Receiver Receiver Sender  Both communication thru same medium  Only one is active at a time … no one at the same time  Either send or receive at a time  Examples HDD, RAM
    8. 8.  Full Duplex Data Sender Receiver Receiver Sender  Simultaneous transmission in both direction  Full interactive communication  Examples… telephone/mobile  costly
    9. 9. Asynchronous Mode Transmission Referred as ON/OFF (Start/stop) transmission Transmission takes place character by character Character sequence Irregular time interval
    10. 10.  Each channel is started by ‘start’ bit and ended by ‘stop’ bit Channel remain unused between the two character… hence at each character start and stop bit is required to notify the receiver Summery :- Data is transmitted character by character at irregular time interval
    11. 11. Synchronous Mode Transmission Character are grouped as a block Series of such blocks are transmitted Each block is started by HEADER and ended with TRAILER information and each block may contain hundreds of characters Indefinite time interval between blocks
    12. 12.  Summary  Synch  Entire blocks of characters are framed and transmitted  Expensive  Efficient  Need for BUFFER and accurate synch and is required.
    13. 13.  Asynchronous  Data is transmitted character by character.  Less costly… but not efficient  No buffer is required… but channel will remain unused
    14. 14. Type’s Of Media Guided media  Signals are guided thru solid media (like copper wire)  Thru cables Unguided media  Signals are not guided… not thru solid medium (use of air)  Usage of radio waves
    15. 15. Guided Media Twisted pair cable  Two insulated wires (1mm thickness) With each other  Less expensive  FD transmission  Can be used for analog and digital  Flow efficient is carries the signal depends on thickness and distance  Very efficient for short distance (less then 100meters)
    16. 16.  More noise friendly Normally Used in LAN If more twisted per centimeter… results less noise effect and better will be the quality Easy to maintain If used less then 100 meters can give up to 9600bps
    17. 17.  CO-AXIAL CABLE  Better shielding … higher data bps @longer distances …several tens of bps at distances up to thousands feet  Used for analog (75 ohm cable) and digital (50 ohm cable) communication  Costlier then twisted pair Wire mesh conductor ProtectiveCopper wire plastic covering Insulating material
    18. 18.  OPTICAL FIBER  Inner core… glass/plastic… conducts the light…size is in microns(1/25000 inch)  Cladding … reflects the light  Most expensive  Data rate up to 100 mbps to 2Gbps  No effect of EM noise  Mainly for digital  Half duplex (FD cause interference)
    19. 19. claddingFiber corer jacketElectrical signal Light signal To To Electrical signalLight converter Converter Fiber optic
    20. 20. UNGUIDED MEDIA  Radio wave can travel ideally with the speed of light (in vacuum) – cover long distance  RF are omni directional  RF is subjected to interference at any frequency Visible Micro light Radio wave infrared UV X Ray 4 5 6 7 8 9 10 11 16Hz
    21. 21.  TERRESTRIAL MICROWAVE TRAN.  4 TO 6 GHz and 21 to 23 GHz  Cheaper then fiber optic  1 to 4 mbps travel in straight line (hence line of sight is required)  Cost depend on distance  Long distance telephone, cellular, TV, link to cities etc..
    22. 22.  SATELLITE MICROWAVE TRANS.  One antenna is on a satellite  4 -6 GHz and 11-14GHz  Use of satellite- cost  Normally uplink is 6 KHz and downlink is 4 KHz  Earth based station required careful adjustment  Can reach most remotes places on earth
    23. 23.  INFRAERED  Used for short distance communication  Do not pass thru solid object  Generally cheaper  Used for wireless LAN, remote controls etc..  licensing is not required.
    24. 24. MODEMS Types  A) Landline :- connected to PSTN… having jacks RJ11  Internal – inside computer  External –separate device… outside the computer… connected to serial port  PCMCIA – small size normally used for laptops.  Personal computer memory card (designed by) international associatioan.
    25. 25.  B) Wireless  Radio transmitters/receiver generally used for mobile device.  RJ11 is not there instead they can access thru radio waves.  If it is out of range – no use. C) LAN  Allow shared remote access to LAN.
    26. 26.  Standards  Bell Modem :- designed by bell lab. There are 103/113 series,202 series, 212 series, 201 series, and 208/9 series.  ITU- T modem- V.22,V.26,V.29  V.26 from 1200-2400 bps… user phase shift keying.  V.22 bit- 600 baud line… during each signal period (baud) the modem conveys 4 data bit 600*4= 2400 bps.  V.29 operating at 2400 baud *4 =9600 bps.
    27. 27. Encoding Techniques Analog data to Analog signal Digital data to Digital signal Digital data to Analog signal Analog data to Digital signal
    28. 28. Analog data to Analog signal Types of modulation  1) Amplitude Carrier
    29. 29. ModulatedDouble sidebandTransmittedcarrier Amplitude Modulation
    30. 30.  Modulation  To modulate to mix the signal with the carrier.  Process of encoding signals (information) for the transmission  Translate the source signal, base band to a band pass signal (high frequency compared to the source frequency).  Source signal – MODULATING signal.  Band pass signal – MODULATED signal.  MODULATION is done by varying the amplitude or frequency of high frequency carrier according to the modulating signal.
    31. 31.  Amplitude Modulation  Amplitude of high frequency carrier signal is varied accordance to the instantaneous amplitude of the modulating signal.  Easy  Environment friendly.  Strength decrease with distance.
    32. 32.  Frequency Modulation  FM signal constant MODULATED but frequency VARIES IN ACCORDING TO THE SIGNAL to be transmitted.  Mixing of two frequency high frequency (carrier) with the signal (low frequency) compound frequency varying according to signal.  Least affected by noise.  Requires high bandwidth than AM.
    33. 33. Frequency Modulation carrier
    34. 34.  Phase Modulation  The shape of the carrier signal (phase) is made to change at given pint of time.  The difference between two sine signals is a phase angle… normally 180 out of phase.  USES:- medium speed modems use phase modulation to convert digital signals into phase modulated signals. This process of phase shifting keying (PSK) allows modem to modulate and demodulate
    35. 35. Phase Modulation carrier
    36. 36. Digital data to Digital signal Digital data to digital signal conversion. Equipment less complex and expensive then digital data to analog modulation equipment. One logic state represented by positive the other by negative voltage Data rate  Rate of data transmission in bps.
    37. 37. Schemes of D to D 1)Non return to zero –level (NRZ-L)  To different voltage for 0 & 1 bits  Voltage constant during bit interval  No transmission i.e. return to 0 voltage  E.g. absence of voltage for 0, constant positive for 1  More often negative voltage for 1 value and positive for the other
    38. 38. NRZ-L0 1 0 0 1 1 0 0 0 1
    39. 39.  2) non return to 0 interval  Constant voltage pulse for duration of bit  Data encoded else presence or absence of signal transmission at beginning of bit time  Transition (low to high or high to low) denotes binary 1  No transition denotes binary 0
    40. 40. NRZ-I0 1 0 0 1 1 0 0 0 1
    41. 41.  3)Manchester  Transition in middle of each bit period  Transition serves as clock and time  Low to high represent 1  High to low represent 0
    42. 42. Manchester0 1 0 0 1 1 0 0 0 1
    43. 43.  4)Defrential Manchester  Mid bit transition is clocking only  Transition at start of a bit period represent 0  No transition at start of a bit period represent 1
    44. 44. Differential Manchester0 1 0 0 1 1 0 0 0 1
    45. 45. Digital Data to Analog Signal Public telephone system  300 Hz to 3400 Hz. Amplitude Shift Keying (ASK). Frequency Shift Keying (FSK). Phase Shift Keying (PSK).
    46. 46.  ASK (Amplitude Shift Keying)  Values represented different amplitude of carries.  Usually one amplitude as 0.  i.e. presence or absence of carrier is used.  Susceptible to sudden gain changes.  Inefficient.  Up to 1200 bps on voice grade line.  Used over optical fiber.
    47. 47. ASK0 0 1 1 0 1 0 0 0 1 0
    48. 48.  Binary Frequency Shift Keying  Most common form is Binary FSK.  Two binary values represented by 2 different frequencies (near carrier).  Less susceptible to error than ASK.  Up to 1200 bps on voice grade lines.  High frequency radio .  Even higher frequency on LANs using co-ax.
    49. 49. BFSK0 0 1 1 0 1 0 0 0 1 0
    50. 50.  Binary phase shift keying (BPSK)  Phase of carrier signal is shifted to represent data.  Binary PSK  Two phases represent two binary digits.  Differential PSK  Phase shifted relative to previous transmission rather than some reference signal.
    51. 51. 0 0 1 1 0 1 0 0 0 1 0
    52. 52. Analog Data To Digital Signal Digitization  Conversion of analog data in to digital data.  Digital data can then be transmitted using NRZ-L.  Digital data can then be converted to analog signal.  Analog to digital conversion done using a codec.  Pulse code modulation conversion of analog data in to digital data.
    53. 53. Digitizer ModulatorAnalog data Digital Data (voice)
    54. 54.  Pulse code modulation  It’s a digitizing process in which analog is represented in digital form.  The sound are transformed in to pulse by codec…sampling of the amplitude of the analog signals at very short interval of time… the sampled valued converted in to digital number of 0’s and 1’s… and finally it is transmitted.
    55. 55.  At the receiving, the original A/D is reversed… voltage values are converted read and production of the exact signal will be achieved. If a signal is sampled at regular interval at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal. Voice data limited to below 4000Hz. Required 8000 samples per second. Each sample assigned digital value.
    56. 56.  CODEC (Compressor/DECompressor)  Its an electronic circuit that convert analog to digital.  Converts human voice in to digital code using pulse code modulation.  The resulting digital signal can travel through all digital communication equipment… provides more reliable and less costly compared to analog.  Its also converting back to voice.  CODEC electronics used in digital phone.
    57. 57. Multiplexing Multiplexing is a set of techniques that allows the simultaneous transmission of multiple signal across a single data link. Whenever the transmission capacity of a medium linking two devices is greater then the transmission needs of the devices, the link can be shared in order to maintained the utilization of the link, much at one cable can carry a hundreds of TV channel.
    58. 58. MUXFDM TDM Synch Asynch
    59. 59.  Frequency division mux (FDM)  In FDM signal generated by each sending device modulated different carrier frequencies. These modulated signals are then combined in to a single composite signal that can be transported by the link. The carrier frequencies have to be different enough to accommodate the modulation and demodulation signals.  (refer fig.) The first PC terminal is sending “1010” where as second terminal is sending “0110”. The multiplexing process starts by applying amplitude modulation in to each signal by using different carrier frequencies as f1 and f2
    60. 60. FDM mux process Amplitude Modulation101 0 With Carrier f1 Modulated signal + Amplitude0 11 0 Modulation With Carrier f2 Signal connected
    61. 61.  In demux process, we use filters to decompose. The multiple signal in to its constitute signals. Then each signal is passed to a amplitude demodulation process to separate the carrier signal from the message signal. Then the message signal is sent to the waiting receiver.
    62. 62. Signal with Carrier f1Bandwidth f1 Amplitude 101 0 Filter Filter Amplitude 0 11 0Bandwidth f2 Signal with Carrier f2
    63. 63.  Time Division mux (TDM)  In the TDM multiple transmission can occupy a single link by subdividing them and interleaving the portion. We say that TDM is a round robin use of a frequency.
    64. 64.  Synch TDM  The mux allocate exactly the same time slot each device at all times, whether or not a device has any thing to transmit. Time slot 1 ,for example is assigned to device 1 alone and can not be used by any other device.  FRAME: In synch TDM, a frame consist of one complete cycle of time slots. Thus the number of slots in frame is equal to the number of inputs.
    65. 65. Synch TDM: mux process12 4321 4321 4321 4321 MUX34
    66. 66. Synch TDM: mux process1AAAA2 BB A D A D BA DCBA MUX3 C4 DDD
    67. 67. Synch TDM: demux process AAAA D BBA D A D BA DCBA E M U X C DDD
    68. 68.  Asynch TDM  In asynch TDM each slot in a frame is not dedicated to the fix device. Each slot contain an index of the device to be sent to and a message. Thus the number of slots in a frame is not necessary to be equal to the number of inputs devices. More than one slots in a frame can be allocated for an input device. Asynch TDM allows maximization the link. It allows a number of lower speed input lines to be multiplexed to a single higher speed line.
    69. 69. Synch TDM: mux process1AAAA BB A D A D BA DCBA MUX CDDD
    70. 70.  FRAME: In asynch TDM, a frame contain a fix number of time slots. Each slot has an index of which device to receive.
    72. 72. MULTIPLE ACCESS TECHNOLOGIES FOR WIRELESS COMMUNICATION COMMUNICATION : Fixed BAND of Frequency Spectrum. Multiple Access Methods - WHY ?  SHARE THE FREQUENCY SPECTRUM.  Differentiates the signals from different sources , without degrading the Quality.  Different techniques of SHARING … called Multiple Access Methods / Techniques / Schemes / Technologies.
    74. 74. FDMA (Frequency Division Multiple Access) Channel 2 Channel 3 Channel 5 Channel 4 Channel 1 User B User E User A User D User C Frequency F1 F2 F3 F4 F5 Users SHARE the available spectrum in the FREQUENCY domain. Assigns the individual CHANNEL ( Unique Frequency) to users - Allocated band is called TRAFFIC CHANNEL. Hence .. Different Users …..Different Traffic Channels.
    75. 75. FDMA (Frequency Division Multiple Access) If User A is in USE .. Channel 1 will not be allotted to others. Disadvantage -> When Channel is not in Use … can not be used by others .. Wastage of Resource. Each Channel has Very LOW Bandwidth …. Hence Implemented normally in Narrow band Systems. Requires TIGHT filtering to reduce the Channel Interference . Channel ID = Frequency Slot ID.
    76. 76. TDMA (Time Division Multiple Access)Time T3 User C User F User I T2 User B User E User H T1 User A User D User G Frequency F1 F2 F3 Spectrum is divided in narrow frequency bands (Like FDMA) and further divided into a number of time slots. Each User is allotted a Time Slot that permit access to the frequency channel for that duration of the time slot.
    77. 77. TDMA (Time Division Multiple Access) Traffic Channel ID = Frequency Slot ID + Time Slot ID Periodic train of time slots … make a FRAME. Each User shares a frequency with several users. Transmission for any user is non continuous. Allocation of different numbers of Time Slots per frame to users … Better Utilization of Spectrum… Analog Systems used FDMA .. Digital Systems used TDMA.
    78. 78. Spread Spectrum Multiple Access PN Code - pseudo-noise code … random binary Sequence / Code. SSMA - a) Frequency Hopped Multiple Access (FHMA) & b) Direct Sequence Multiple Access(DSMA).  FHMA :- Carrier Frequencies of individual user are VARIED in a pseudo random way.  Based on the PN code of the user .. Each user occupy the narrow band channel at one particular time.  Because of the PN … Signals changes channels rapidly.  Difference between FHMA & FDMA is that the FHMA signal changes channels at rapid interval.
    79. 79. CDMA (Code Division Multiple Access)PN codesCode 3 User CCode 2 User BCode 1 User A Frequency F1  DSMA is also called CDMA.  Unique PN code is assigned to unique user.  Users share the Block of frequency spectrum on the basis of PN code.
    80. 80. CDMA (Code Division Multiple Access) Channel ID = PN Code ID Utilizes the entire spectrum of allotted spectrum -  All the PN code modulated signals from the users are transmitted over the entire spectrum. And at the receiving end the signals classified as per the copy of PN sequence . Unlike FDMA - TDMA … There is no LIMIT of number of users … but increase in users degrades the quality. Each user operate independently with NO knowledge of other users.
    81. 81. CORDLESS TELEPHONE SYSTEMSPublicTelephone Fixed Station -Network ..(DoT) Base Station Handset  Cordless Telephone System - provide the user limited range and mobility. Coverage rang is few Tens of Meters to Few hundred Meters.
    82. 82. CELLULAR TELEPHONE SYSTEMS The concept was developed in early 70’s by Bell Laboratories Extension of your wireless connection to the public telephone network for any user location within the range of the system. The principle of cellular system…To divide a large geographic area into cells. Each adjacent Cell Transmitters operate on different frequencies to avoid interference.
    83. 83. CELLULAR TELEPHONE SYSTEMS Transmitted power and height of antenna of each CELL is low so that the same set of frequency can be used for different cells far apart. Hence theoretical coverage range and capacity of a cellular system are therefore UNLIMITED. Each Cell is represented by HEXAGONE.
    84. 84. A cellular System – An Overview. Public Telephone Network Mobile Switching Center To Other MSC MSC Basic Cellular System - Mobile Stations+Base Stations+Mobile Switching Center.
    85. 85. A cellular System - An Overview. Mobile Station - Contains a transceiver+ antenna+Control Circuitry Base Station - Bridge between MS and MSC MSC - Coordinated the activities of all BS and connect them to PTN. Plus Billing & System Maintenance. The Channel used for VOICE transmission from BS to MS …called Forward Voice Channel (FVC). The Channel used for VOICE transmission from MS to BS …called Reverse Voice Channel (RVC).
    86. 86. How it WORKS ? When MS is turned ON … Searches for the strongest FVC. When a Call is made for MS.. MSC dispatches the request to all BS. The Mobile Number is broadcasted as a paging message. MS acknowledges the Paging message. BS relays this ACK to MSC . MSC instructs the BS to select particular frequency Channel for communication .
    87. 87. How it WORKS ? BS TO RING sends DATA message in FVC TO RING the MS. During CALL .. MSC handles the transmitted power and controls the channel between BS and MS in order to maintain the Quality ( as MS is likely in MOBILE mode) When MS goes out of range of BS … called HANDOFF … Two other Channels are also Used besides FVC & RVC … a) FCC & b) RCC
    88. 88. How it WORKS ? Call fromMSC DoT. Sends Number to all BS Paging Message for FCC MS RCCBS FVC RCC Receives FCC Paging Message RVCMS FVC RVC TIME
    89. 89. How it WORKS ? When MS Originates Call… sends all information to BS. BS passes information to MSC MSC Validates .. And If required help from Public Telephone Network requested. And the two way PATH will be maintained till the Call Lasts.
    90. 90. Handoff During the ongoing call if BS senses the LOW power Quality from MS , it requests neighboring BS to check the signal level . ( This happens when MS moves to different cell while in USE) If the signal is BETTER , current BS signals the MS to switch over to new BS and inform the new BS to take over. This change of SPEECH channel is called “Handoff” This changeover will not be noticed / experienced by the user.
    91. 91. Frequency REUSE E E F C F C A G B A B D G D BS in adjacent cells are assigned channel groups … totally different from the neighboring cells. BS antennas are designed to cover the particular cell.
    92. 92. Wireless Systems Standards
    93. 93. 1G Cellular Systems Based on Analog Cellular Systems Concept Depends on Frequency Band , Channel Spacing and channel coding Individual calls use different channels and the Spectrum is shared on the basis of FDMA Uses Analog FM for speech transmission Normally uses 7 Cell reuse pattern – provision for Cell splitting.
    94. 94. 2G Cellular System Completely DIGITAL Cellular System  Increased in Capacity ( 3 to 10 times)  MS Terminal Size Reduction  Reduces the Power requirements … Increases the battery life  Improved Reception  Highly Secured … Interference prone environment.  Cell Splitting … Better  Wide Area Roaming  More Popular
    95. 95. 2G Cellular System Spectrum Sharing in the digital environment can be based on ;  TDMA : Each Radio Channel is partitioned in to number of time slots - each user is assigned a frequency/time slot COMBINATION  CDMA : A radio Channel is used SIMULTANEOUSLY by multiple mobile users , and the signals from different users are distinguished by SPREADING them on the basis of PN code.
    96. 96. Global System for Mobile (GSM) Introduced in Europe in 1990. World’s most popular standard now. A memory device that stores the subscriber Id , Networks, Countries where he’s entitled to get services , personal information is inserted into GSM phones . (Subscriber Identity Module - SIM). Example TOI dated 18th March. Without SIM – non operational. Encryption is possible … More secured
    97. 97. GSM ArchitectureHLR MSC Other MSCVLR PSTNAUC BSC BTS
    98. 98. GSM Architecture MS (Mobile Station) • Low Power Requirement…0.8-8.0w • SIM is Required BSS (Base Station System) • BSC+BTS (Base Transceiver Station) • Responsible For Radio Channel Allocation/ Monitoring (BSC) • Power Control (BSC) • Handoff Management (BSC) – Reduce The Burden of MSC • Digital Signal Processing (BTS)
    99. 99. GSM – System Architecture MSC  Doesn’t contain Info regarding MS .  Call Setup, Supervision & End / Routing  BILLING  MOBILITY Management  Management with Other MSCs , PSTN . Home Location Register – HLR  Centralized Database of MS falling under MSC  Refer for every Incoming Call
    100. 100. GSM – System Architecture VLR – Visitor Location Register  Temporarily stores the MS … Each roaming MS visiting MSC. AUC – Authentication Center  Strongly protected database which handles the authentication and encryption keys of every MSInterfaces : Between BSC & MSC ::: A Interface Between BSC & BTS ::: Abis Interface Between BTS & MS ::: GSM Radio Air Interface SS7 Protocol ::: Signal Correction control part
    101. 101. GSM Specifications RC :: 890 – 915 MHz FC :: 935 – 960 MHz Separation ::: 45 MHz Channel Spacing ::: 200 kHz Each Channel is TIME SHARED between 8 subscribers using TDMA Total number of channel ::: 125 (25MHz bandwidth) * 08 = 1000 approximately
    102. 102. GSM Specifications  Channel Frame :156.25bits576.92μs TS0 TS1 TS2 TS3 TS4 TS5 TS6 TS7 4.615ms
    103. 103. GSM Traffic Channels Traffic Channels (TCHs)  Carry digitally encoded user SPEECH or DATA Control Channels (CCHs)  Carry signaling and synchronizing commands between BS & MS Full Rate :  User Speech / Data … one TS per Frame Half Rate :  Same time slot but sent in alternate frames  Two half rate channel users would share the same time slot but would alternately transmit during every other frame
    104. 104. TCHs - Types Full Rate TCH  TCH/FS … Full rate speech channel carries @13kbps  TCH/F9.6 … Full rate DATA channel @9600bps  TCH/F4.8 … Full rate DATA channel @4800bps  TCH/F2.4 … Full rate DATA channel @2400bps Half Rate TCH  TCH/HS … half rate of the full rate channel … 6.5kbps  TCH/H4.8 …half rate DATA @4800bps  TCH/H2.4 …half rate DATA @2400bps
    105. 105. CCH - Types Three Main Control Channels – Broadcast Channel (BCH), Common Control Channel (CCCH) & DEDICATED Control Channel (DCCH) BCH – Operates only on Forward link … Synchronization for all MS  Broadcast Control Channel – BCCH … used to broadcast info. Such as cell & network identity . Plus … Channel structure , channel availability and congestion parameters.  Frequency Correction Channel – FCCH … allows each MS to synchronize its internal frequency as of BS  Synchronization Channel – SCH … used to identify the serving BTS
    106. 106. CCH - Types Common Control Channel – CCCH – used to page specific MS , assign signaling signals to specific MS and receive requests for service from MS  Paging Channel – PCH … provides paging signals from BSC to all MS in the cell … used to provide cell broadcast ASCII text messages to all MS – SMS feature.  Random Access Channel – RACH … reverse link used by MS .. Used by MS to originate calls  Access Grant Channel – AGCH … used by the BSC to provide forward link communication to the MS and carries signals which instructs
    107. 107. CCH - Types Dedicated Control Channels (DCCH) – bi- directional in nature like traffic channels …  Stand-alone Dedicated Control Channel – SDCCH …ensures that MS will remain connected with BSC while MSC verify the MS …  Slow Associated Control Channel – SACCH … carries general info. Between the BTS and MS… on the forward , regular signals to MS like transmitted power … in reverse , it carries received signal strength , quality of TCH info.  Fast Associated Control Channel – FACCH … carries urgent messages same as of SDCCH … urgent message like handoff request .