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43-557: A broadcast control channel (BCCH) is a point to multipoint, unidirectional (downlink) channel used in the Um interface of the GSM cellular standard. The BCCH carries a repeating pattern of system information messages that describe the identity, configuration and available features of the base transceiver station (BTS). These messages also provide a list of absolute radio-frequency channel numbers (ARFCNs) used by neighboring BTSs. This message pattern

86-522: A channel spacing of 200 kHz. Since adjacent channels overlap, the standard does not allow adjacent channels to be used in the same cell. The standard defines several bands ranging from 400 MHz to 1990 MHz. Uplink and downlink bands are generally separated by 45 or 50 MHz (at the low-frequency end of the GSM spectrum) and 85 or 90 MHz (at the high-frequency end of the GSM spectrum). Uplink/downlink channel pairs are identified by an index called

129-567: A connection in each sublayer before accessing the next higher sublayer. The access order is RR, MM, CC. The release order is the reverse of that. Note that none of these sublayers terminate in the BTS itself. The standard GSM BTS operates only in layers 1 and 2. Um logical channel types are outlined in GSM 04.03. Broadly speaking, non-GPRS Um logical channels fall into three categories: traffic channels, dedicated control channels and non-dedicated control channels. These point-to-point channels correspond to

172-466: A crystal oscillator with a GPS time signal, creating a GPS-disciplined oscillator ( GPSDO ). Using a GPS receiver that can generate stable time signals (down to within ~30 ns of UTC ), a GPSDO can maintain oscillation stability of 10 for extended periods of time. Crystal ovens are also used in optics. In crystals used for nonlinear optics , phase matching is also sensitive to temperature, and thus they require temperature stabilization, especially as

215-403: A crystal. The short-term frequency stability of OCXOs is typically 1 × 10 over a few seconds, while the long-term stability is limited to around 1 × 10 (10  ppb ) per year by aging of the crystal. Achieving better stability requires switching to an atomic frequency standard , such as a rubidium standard , caesium standard , or hydrogen maser . Another cheaper alternative is to discipline

258-867: A general rule, each GSM channel uses a block parity code (usually a Fire code), a rate-1/2, 4th-order convolutional code and a 4-burst or 8-burst interleaver . Notable exceptions are the synchronization channel (SCH) and random access channel (RACH) that use single-burst transmissions and thus have no interleavers. For speech channels, vocoder bits are sorted into importance classes with different degrees of encoding protection applied to each class (GSM 05.03). Both 260-bit vocoder frames and 184-bit L2 control frames are coded into 456 bit L1 frames. On channels with 4-burst interleaving (BCCH, CCCH, SDCCH, SACCH), these 456 bits are interleaved into 4 radio bursts with 114 payload bits per burst. On channels with 8-burst interleaving (TCH, FACCH), these 456 bits are interleaved over 8 radio bursts so that each radio burst carries 57 bits from

301-751: A physical channel. The allowed combinations for single-slot systems are listed in GSM 05.02 Section 6.4.1. Additionally, only certain of these combinations are allowed on certain timeslots or carriers and only certain sets of combinations can coexist in a given BTS. These restrictions are intended to exclude non-sensical BTS configurations and are described in GSM 05.02 Section 6.5. The most common combinations are: Basic speech service in GSM requires five transactions: radio channel establishment, location update, mobile-originating call establishment, mobile-terminating call establishment and call clearing. All of these transactions are described in GSM 04.08 Sections 3-7. Unlike ISDN's U channel, Um channels are not hard-wired, so

344-450: A single physical channel. The SDCCH uses 4-burst block interleaving in a 51-multiframe. The FACCH is always paired with a traffic channel. The FACCH is a blank-and-burst channel that operates by stealing bursts from its associated traffic channel. Bursts that carry FACCH data are distinguished from traffic bursts by stealing bits at each end of the midamble. The FACCH is used for in-call signaling, including call disconnect, handover and

387-470: A tone on the radio channel that is used by the mobile station to discipline its local oscillator. FCCH will repeat on every 0th, 10th, 20th, 30th and 40th frames of the 51 frame multiframe. So there are 5 FCCH frames in a 51 frame multiframe. The PCH carries service notifications (pages) to specific mobiles sent by the network. A mobile station that is camped to a BTS monitors the PCH for these notifications sent by

430-429: A traffic channel is dependent on the traffic or vocoder type employed, with most coders capable of overcoming single-burst losses. All traffic channels use a 26-multiframe TDMA structure. A GSM full rate channel uses 24 frames out of a 26-multiframe. The channel bit rate of a full-rate GSM channel is 22.7 kbit/s, although the actual payload data rate is 9.6-14 kbit/s, depending on the channel coding. This channel

473-424: A transfer and an acknowledgment on L(n-1). Only L1-L4 are visible on Um. The transaction steps for MO-SMS are defined in GSM 04.11 Sections 5, 6 and Annex B. In the simplest case, error-free delivery outside of an established call, the transaction sequence is: The transaction steps for MT-SMS are defined in GSM 04.11 Sections 5, 6 and Annex B. In the simplest case, error-free delivery outside of an established call,

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516-507: Is a stub . You can help Misplaced Pages by expanding it . Um interface The layers of GSM are initially defined in GSM 04.01 Section 7 and roughly follow the OSI model . Um is defined in the lower three layers of the model. The Um physical layer is defined in the GSM 05.xx series of specifications, with the introduction and overview in GSM 05.01. For most channels, Um L1 transmits and receives 184-bit control frames or 260-bit vocoder frames over

559-408: Is a small but non-zero probability that two MSs send identical RACH bursts at the same time in step 2. If these RACH bursts arrive at the BTS with comparable power, the resulting sum of radio signals will not be demodulable and both MSs will move to step 4. However, if there is a sufficient difference in power, the BTS will see and answer the more powerful RACH burst. Both MSs will receive and respond to

602-409: Is a temperature-controlled chamber used to maintain the quartz crystal in electronic crystal oscillators at a constant temperature, in order to prevent changes in the frequency due to variations in ambient temperature. An oscillator of this type is known as an oven-controlled crystal oscillator ( OCXO , where "XO" is an old abbreviation for "crystal oscillator"). This type of oscillator achieves

645-425: Is normally used with the GSM 06.10 Full Rate , GSM 06.60 Enhanced Full Rate or GSM 06.90 Adaptive Multi-Rate speech codec . It can also be used for fax and Circuit Switched Data . A GSM half rate channel uses 12 frames out of a 26-multiframe. The channel bit rate of a half-rate GSM channel is 11.4 kbit/s, although the actual data capacity is 4.8-7 kbit/s, depending on the channel coding. This channel

688-458: Is normally used with the GSM 06.20 Half Rate or GSM 06.90 Adaptive Multi-Rate speech codec. These point-to-point channels correspond to the ISDN D channel and are referred to as Dm channels . The SDCCH is used for most short transactions, including initial call setup step, registration and SMS transfer. It has a payload data rate of 0.8 kbit/s. Up to eight SDCCHs can be time-multiplexed onto

731-477: Is performed when the MS powers up or enters a new Location area but may also be performed at other times as described in the specifications. In its minimal form, the steps of the transaction are: There are many possible elaborations on this transaction, including: This is the transaction for an outgoing call from the MS, defined in GSM 04.08 Sections 5.2.1 and 7.3.2 but taken largely from ISDN Q.931. In its simplest form,

774-411: Is synchronized to the BTS frame clock. The minimum BCCH message set is system information messages 2–4, although other messages are normally present. The messages themselves are described in 3GPP Technical Specification 44.018. Any GSM ARFCN that includes a BCCH is designated as a beacon channel and is required to transmit continuously at full power. This article related to telecommunications

817-454: Is time-multiplexed into multiple logical channels according to the rules of GSM 05.02. One logical channel constitute of 8 burst periods (or physical channels) which is called a Frame . Traffic channel multiplexing follows a 26-frame (0.12 second) cycle called a "multiframe". Control channels follow a 51-frame multiframe cycle. The C0T0 physical channel carries the SCH, which encodes the timing state of

860-468: The ARFCN . Within the BTS, these ARFCNs are given arbitrary carrier indexes C0..Cn-1, with C0 designated as a Beacon Channel and always operated at constant power. GSM has physical and logical channels. The logical channel is time-multiplexed into 8 timeslots, with each timeslot lasting for 0.577ms and having 156.25 symbol periods. These 8 timeslots form a frame of 1,250 symbol periods. Channels are defined by

903-497: The ISDN B channel and are referred to as Bm channels . Traffic channels use 8-burst(Break) diagonal interleaving with a new block starting on every fourth burst and any given burst containing bits from two different traffic frames. This interleaving pattern makes the TCH robust against single-burst fades since the loss of a single burst destroys only 1/8 of the frame's channel bits. The coding of

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946-515: The BTS to facilitate synchronization to the TDMA pattern. GSM timing is driven by the serving BTS through the SCH and FCCH. All clocks in the handset, including the symbol clock and local oscillator, are slaved to signals received from the BTS, as described in GSM 05.10. BTSs in the GSM network can be asynchronous and all timing requirements in the GSM standard can be derived from a stratum-3 OCXO . The coding sublayer provides forward error correction . As

989-657: The SIM and in the Authentication Center (AuC), a component of the HLR. Ki is never transmitted across Um. An important and well-known shortcoming of GSM security is that it does not provide a means for subscribers to authenticate the network. This oversight allows for false basestation attacks , such as those implemented in an IMSI catcher . The Um authentication procedure is detailed in GSM 04.08 Section 4.3.2 and GSM 03.20 Section 3.3.1 and summarized here: OCXO A crystal oven

1032-568: The Um interface requires a mechanism for establishing and assigning a dedicated channel prior to any other transaction. The Um radio resource establishment procedure is defined in GSM 04.08 Section 3.3 and this is the basic medium access procedure for Um. This procedure uses the CCCH (PCH and AGCH) as a unicast downlink and the RACH as a shared uplink. In the simplest form, the steps of the transaction are: Note that there

1075-590: The burst type but the total burst length is 156.25 symbol periods. The most commonly used burst is the Normal Burst (NB). The fields of the NB are: There are several other burst formats, though. Bursts that require higher processing gain for signal acquisition have longer midambles. The random access burst (RACH) has an extended guard period to allow it to be transmitted with incomplete timing acquisition. Burst formats are described in GSM 05.02 Section 5.2. Each physical channel

1118-444: The channel with which it is associated. The SACCH uses 4-burst block interleaving and the same multiframe type as its host TCH or SDCCH. These are unicast and broadcast channels that do not have analogs in ISDN. These channels are used almost exclusively for radio resource management. The AGCH and RACH together form the medium access mechanism for Um. The BCCH carries a repeating pattern of system information messages that describe

1161-499: The circuit are also vulnerable to temperature drift, usually the entire oscillator circuit is enclosed in the oven. A thermistor temperature sensor in a closed-loop control circuit is used to control the power to the heater and ensure that the oven is maintained at the precise temperature desired. Because the oven operates above ambient temperature, the oscillator usually requires a warm-up period after power has been applied to reach its operating temperature. During this warm-up period,

1204-429: The current L1 frame and 57 bits from the previous L1 frame. Interleaving algorithms for the most common traffic and control channels are described in GSM 05.03 Sections 3.1.3, 3.2.3 and 4.1.4. The Um data link layer , LAPDm , is defined in GSM 04.05 and 04.06. LAPDm is the mobile analog to ISDN's LAPD. The Um network layer is defined in GSM 04.07 and 04.08 and has three sublayers. A subscriber terminal must establish

1247-431: The frequency will not have the full rated stability. The temperature selected for the oven is that at which the slope of the crystal's frequency vs. temperature curve is zero, further improving stability. AT- or SC-cut (stress-compensated) crystals are used. The SC-cut has a wider temperature range over which near-zero temperature coefficient is achieved and thus reduces warmup time. Power transistors are usually used for

1290-493: The heaters instead of resistance heating elements . Their power output is proportional to the current, rather than the square of the current, which linearizes the gain of the control loop. A common temperature for a crystal oven is 75 °C , but the temperature may vary between 30 and 80 °C depending on setup. Most standard commercial crystals are specified to an environmental temperature of 0–70 °C , industrial versions are usually specified to −40 to +85 °C . Because of

1333-469: The highest frequency stability possible with a crystal. They are typically used to control the frequency of radio transmitters , cellular base stations , military communications equipment, and for precision frequency measurement. Quartz crystals are widely used in electronic oscillators to precisely control the frequency produced. The frequency at which a quartz crystal resonator vibrates depends on its physical dimensions. A change in temperature causes

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1376-482: The identity, configuration and available features of the BTS. BCCH brings the measurement reports it bring the information about LAI And CGI BCCH frequency are fixed in BTS The SCH transmits a Base station identity code and the current value of the TDMA clock. SCH repeats on every 1st, 11th, 21st, 31st and 41st frames of the 51 frame multi frame. So there are 5 SCH frames in a 51 frame multiframe. The FCCH generates

1419-447: The later stages of call setup. It has a payload data rate of 9.2 kbit/s when paired with a full-rate channel (FACCH/F) and 4.6 kbit/s when paired with a half-rate channel (FACCH/H). The FACCH uses the same interleaving and multiframe structure as its host TCH. Every SDCCH or FACCH also has an associated SACCH. Its normal function is to carry system information messages 5 and 6 on the downlink, carry receiver measurement reports on

1462-513: The network. The AGCH carries BTS responses to channel requests sent by mobile stations via the Random Access Channel. The RACH is the uplink counterpart to the AGCH. The RACH is a shared channel on which the mobile stations transmit random access bursts to request channel assignments from the BTS. The multiplexing rules of GSM 05.02 allow only certain combinations of logical channels to share

1505-424: The number and position of their corresponding burst period. The capacity associated with a single timeslot on a single ARFCN is called a physical channel (PCH) and referred to as "CnTm" where n is a carrier index and m is a timeslot index (0-7). Each timeslot is occupied by a radio burst with a guard interval, two payload fields, tail bits, and a midamble (or training sequence ). The lengths of these fields vary with

1548-421: The power required to run the heater, OCXOs require more power than oscillators that run at ambient temperature, and the requirement for the heater, thermal mass, and thermal insulation means that they are physically larger. Therefore, they are not used in battery-powered or miniature applications, such as watches . However, in return, the oven-controlled oscillator achieves the best frequency stability possible from

1591-442: The quartz to expand or contract due to thermal expansion , changing the frequency of the signal produced by the oscillator. Although quartz has a very low coefficient of thermal expansion , temperature changes are still the major cause of frequency variation in crystal oscillators. The oven is a thermally insulated enclosure containing the crystal and one or more electrical heating elements . Since other electronic components in

1634-411: The radio interface in 148-bit bursts with one burst per timeslot. There are three sublayers: Um on the physical channel has 26 TDMA frames each frame consisting of 114 info bits each. The length of 26 TDMA frame also called Multi-frame is 120 ms apart. GSM uses GMSK or 8PSK modulation with 1 bit per symbol which produces a 13/48 MHz (270.833 kHz or 270.833 K symbols/second) symbol rate and

1677-456: The resulting channel assignment in step 3. To ensure recovery from this condition, Um uses a "contention resolution procedure" in L2, described in GSM 04.06 5.4.1.4 in which the first L3 message frame from the MS, which always contains some form of mobile ID, is echoed back to the MS for verification. The location updating procedure is defined in GSM 04.08 Sections 4.4.1 and 7.3.1. This procedure normally

1720-497: The steps of the transaction are: The TCH+FACCH assignment can occur at any time during the transaction, depending on the configuration of the network. There are three common approaches: This is the transaction for an incoming call to the MS, defined in GSM 04.08 Sections 5.2.2 and 7.3.3, but taken largely from ISDN Q.931. As in the MOC, the TCH+FACCH assignment can happen at any time, with

1763-482: The three common techniques being early, late and very early assignment. The transaction for clearing a call is defined in GSM 04.08 Sections 5.4 and 7.3.4. This transaction is the same whether initiated by the MS or the network, the only difference being a reversal of roles. This transaction is taken from Q.931. GSM 04.11 and 03.40 define SMS in five layers: As a general rule, every message transferred in L(n) requires both

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1806-428: The transaction sequence is: GSM 02.09 defines the following security features on Um: Um also supports frequency hopping (GSM 05.01 Section 6), which is not specifically intended as a security feature but has the practical effect of adding significant complexity to passive interception of the Um link. Authentication and encryption both rely on a secret key, Ki, that is unique to the subscriber. Copies of Ki are held in

1849-432: The uplink and to perform closed-loop power and timing control. Closed loop timing and power control are performed with a physical header at the start of each L1 frame. This 16-bit physical header carries actual power and timing advance settings in the uplink and ordered power and timing values in the downlink. The SACCH can also be used for in-call delivery of SMS. It has a payload data rate of 0.2-0.4 kbit/s, depending on

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