gsm基本概念

  

ACCH [GSM 05.01, 05.02]    Associated control channel. Two types are defined: slow associated control channel (SACCH) and fast associated control channel (FACCH). An ACCH is assigned for traffic channels (TCHs) as well as for SDCCHs. 

AGCH [GSM 05.01, 05.02]    Access grant channel. A common control channel (CCCH) that is used only in the downlink direction of even-numbered time slots (typically solely in time slot 0) of the BCCH-TRX. It is used to
assign a SDCCH to the MS, and it transports the IMM_ASS (IMMediate ASSign) message. Depending on the chosen channel configuration, the AGCH shares the available downlink CCCHs with the paging channel (PCH) and the SDCCH. The transmission rate per AGCH block is 782 bps.

ARFCN [GSM 05.01]    Absolute radio frequency channel number. An identifier or number of a channel used on the Air-interface. From the ARFCN, it is possible to calculate the frequency of the uplink and the downlink that the channel uses. How to perform this calculation is shown under downlink.

BCC [GSM 03.03]     Base station color code. A 3-bit-long parameter that is part of the BSIC. Used to distinguish among the eight different training sequence codes (TSCs) that one BTS may use on the CCCHs and to distinguish between neighbor BTSs without the need for the MS to register on any other BTS.

BCCH [GSM 04.08, 05.01, 05.02]     Broadcast common control channel. The “beacon” of every BTS. Per BTS, there is always exactly one BCCH, which is transmitted in time slot 0 of the BCCH frequency. The transmission rate is 782 bps.

 

BFI [GSM 05.05, 06.31, 08.60]       Bad frame indicator. A parameter within the TRAU frame. The value of the BFI indicates to the voice decoder if a TRAU frame contains valid data (BFI = 0) or not (BFI = 1). Depending on that information, the voice decoder uses or discards a TRAU frame. Note: For FACCH frames, BFI always equals 1, because they contain signaling data.

 

BS_AG_BLKS_RES [GSM 05.02] Parameter transmitted with the BCCH

SYS_INFO 3 message. BS_AG_BLKS_RES is 3 bits long and hence can take

on the values 0 through 7. The value of this parameter indicates to all mobile

stations in a cell how many of the CCCH blocks of a 51 multiframe on a

BCCH-TS 0 are reserved for access grant channels (AGCHs). The number of

available paging channels (PCHs) is reduced accordingly. Note that during

operation in the combined mode of SDCCH and CCCH the number of

CCCH blocks per time slot is four rather than eight, compared to the noncombined

mode. The complete picture is illustrated in Figure G.7. (See also

CCCH_CONF).

BS_PA_MFRMS [GSM 05.02] Mobile stations are organized into paging

groups based on their IMSI. A mobile station that belongs to a certain

paging group needs to check for a paging message only once in a number of

51 multiframes. In between, the mobile station may switch over to an energysaving

mode, discontinuous reception (DRX).

The 3-bit-wide parameter BS_PA_MFRMS is part of the BCCH

SYS_INFO 3 and tells the mobile station after how many multiframes the content

of the paging channel (PCH) has to be analyzed by the MS. In other

words, this parameter indicates how often a particular paging group is repeated.

Figure G.8 provides an example of how this parameter is used.

BS_CC_CHANS [GSM 05.02] Parameter that indicates how many time

slots on the BCCH frequency are reserved for common control channels

(CCCHs). This parameter is not transmitted but is derived from another

parameter, CCCH_CONF.

BS_CCCH_SDCCH_COMB [GSM 05.02] Parameter that indicates

whether the dedicated control channels (SDCCHs) and the common control

channels (CCCHs) share a given time slot. Such a combined configuration is

described in Chapter 7. This parameter is not transmitted but is derived from

another parameter, CCCH_CONF.

BSIC [GSM 03.03] Base station identity code. An identifier for a BTS,

although the BSIC does not uniquely identify a single BTS, since it has to be

reused several times per PLMN. The purpose of the BSIC is to allow the

mobile station to identify and distinguish among neighbor cells, even when

neighbor cells use the same BCCH frequency. Because the BSIC is broadcast

within the synchronization channel (SCH) of a BTS, the mobile station

does not even have to establish a connection to a BTS to retrieve the BSIC.

Figure G.9 shows the format of the BSIC. It consists of the network color code

(NCC), which identifies the PLMN, and the base station color code (BCC).

Burst [GSM 05.01, 05.02] The nature of TDMA transmission is that radio

energy is emitted in a pulsed manner rather than continuously. Mobile stations

and BTSs send bursts periodically. Figure G.10 illustrates this for a GSM system

in a power-over-time presentation. The actual data transmission is happening

during the time period represented in Figure G.10 as a horizontal line. This

time period is 148 bits, or 542.8 ms, long. Because GMSK—at least in theory

—does not contain an amplitude modulated signal, the effective transmission

power is constant over the entire transmission period. Figure G.10 also

shows the specified corridor for the allowed power level of the signal over time.

In total, a burst has a window of 577 ms, or 156.25 bit, before the next time slot

starts. Physically speaking, the power level has to be reduced by 70 dB after

577 ms. These restrictions apply to the uplink as well as the downlink and

determine the maximum number of bits an MS can send or receive at one time.

The net bit rate is only 114 bits per burst, not 156.25. This reduced number of

bits results from the mapping of a physical burst to a logical burst. The physical

burst needs bits for administrative purposes that reduce the space available for

signaling or user data. Note that all burst types specified for GSM follow a

similar pattern:

· Each burst always begins with tail bits, which are necessary to synchronize

the recipient. Tail bits are, except for the access burst, always

coded as ‘000’.

· The tail bits are followed by 148 data bits, which differ in format for

the various burst types.

· Each burst is terminated by another set of tail bits and the so-called

guard period. This guard period is required for the sender to physically

reduce the transmission power. The guard period is particularly long

for the access burst, to allow mobile stations that are far from a

BTS and hence experience propagation delays to also access the BTS

(see TA).

The functional differences between the five logical bursts, defined for GSM, are

as follows:

· Normal burst. The normal burst is used for almost every kind of

data transmission on all channel types. The only exceptions to that

rule are the initial channel request from the mobile station

(CHAN_REQ/HND_ACC) sent in an access burst and the transmission

of the synchronization data of a BTS that is done via the synchronization

burst. All other data transfer on all traffic channels, dedicated

control channels (DCCHs) and common control channels (CCHs) in

uplink and downlink directions are done in normal bursts.

Every normal burst contains 114 bits of useful data that are sent

in two packets of 57 bits each. The so-called training sequence (TSC)

is placed between the two packets. Note that the term useful data is not

entirely accurate in this context, since the 114 bits are already channel

coded and therefore contain some overhead (channel coding). Last but

not least, there is a stealing flag between the training sequence and each

data packet, which indicates to the recipient whether a 57-bit packet

actually contains user data or FCCH information.

· Synchronization burst. The synchronization burst is used to transmit

synchronization channel information (SCH) The synchronization

burst uses a format similar to that of the normal burst (Figure G.12).

In both cases, there are two data packets, left and right, from the training

sequence. However, for the synchronization burst, each packet

contains only a 39-bit payload, because the training sequence is 64 bits

long. Note that the training sequence for the synchronization channel

is identical for all BTSs and therefore allows a mobile station to easily

distinguish an accessible GSM-BTS from any other radio system that

accidentally works at the same frequency. Therefore, the training

sequence in the synchronization channel serves two purposes: (1) It

allows the mobile station to determine if there might have been transmission

errors, and (2) it allows the mobile station to distinguish a

GSM source from other transmission systems on the same frequency.

· Access burst. In contrast to the bursts described so far, the access

burst comes in a rather unique format because of its special tasks

(Figure G.12). A mobile station uses the access burst only for the initial

access to a BTS, which applies in two cases: (1) for a connection

setup starting from the idle state and (2) for handover (see under synchronized

handover). In the first case, the MS sends the CHAN_REQ

message in an access burst to the BTS. In the second case, the MS

sends HND_ACC messages that also are mapped on access bursts.

In both cases the MS does not know the current distance to

the BTS and, hence, the propagation delay for the signal (see TA). As

long as the propagation delay is not known to the MS, the MS assumes

it is zero. Therefore, it generally is uncertain if the access burst arrives

within the receiver window of a BTS and how big the overlap is

(Figure G.11). That is the reason for the lesser length of an access burst

and the longer duration of the guard period. To ensure that an access

burst arrives at the BTS during the proper time period the number of

bits for the access burst was set to only 88 bits. The maximum distance

between BTS and MS is, with this timing, about 35 km.

The normal burst would not fit into the receiver window if the

unknown propagation delay was greater than zero. That is the reason

why the normal burst is used only after the distance of the MS from

the BTS is determined, and the MS is able to adjust its transmission

accordingly. The adjustment parameter is called offset time and is

calculated fairly simply. The BTS knows format and length of an

access burst and is able to determine the actual propagation delay from

when the signal arrives back at the BTS after being relayed by the MS.

That also allows calculation of the distance of an MS from the BTS.

The BTS provides the offset time to the MS, which in turn transmits

its signal earlier, exactly by that time period (see TA).

The format of an access burst is also different from the other

bursts. The access burst begins with 8 tail bits, rather than 3 as in the

case of the other bursts, and the access burst always starts with the bit

sequence 0011 1010bin. The tail bits, together with the following 41-

bit synchronization sequence which also always carries the same value,

allows the BTS to distinguish the access burst from error signals or

interfering signals. Hence, the access burst serves on the uplink a similar

purpose as the synchronization burst does on the downlink. Nevertheless,

in practice, it is common that the BTS determines background

noise to be a CHAN_RQD message, as presented in Chapter 6.

The data field of an access burst is only 36 bits long and contains

either a CHAN_RQD or an HND_ACC message. Note that both

messages actually contain only 8 bits of “useful data.”

· Frequency correction burst. The most simple format of all the bursts

is used for the frequency correction burst, which is transmitted

only in the frequency correction channel (FCCH) (Figure G.12). All

148 bits (142 bits + 6 tail bits) are coded with 0. A sequence of zeros at

the input of a GMSK modulator produces, because of the peculiarities

of the GMSK modulation, a constant transmitter frequency which is

exactly 67.7 kHz above the BCCH median frequency. Therefore, the

frequency of the FCCH is always 67.7 kHz above the frequency that is

advertised as the downlink frequency. This constant transmission

frequency allows an MS to fine-tune its frequency to the BCCH frequency,

to subsequently be able to read the data within the synchronization

burst.

· Dummy burst. When the MS powers up, it checks the power level of

the BCCH frequencies of the cells (BTSs) nearby to determine which

BTS to use as a serving cell (Figure G.12). Similarly, when the MS is

active, that is, involved in a call, the power level of the BCCH frequencies

of the neighbor cells serve as basis for a possible handover decision.

To be useful as a reference, the BCCH frequency has to be transmitted

with a constant power level. Thus, all time slots have to be occupied, and it is

not allowed to apply power control on the downlink. For this purpose, the

dummy burst was defined. These dummy bursts are inserted into otherwise

empty time slots on the BCCH frequency. To prevent accidental confusion

with frequency correction bursts, the dummy burst is coded with a pseudorandom

bit sequence predefined by GSM.

CBCH [GSM 03.41, 07.05] Cell broadcast channel. Used to transmit

broadcast messages to mobile stations. The transmission rate of this optional

channel is 782 bps. Network operators may choose to equip a CBCH instead of

a SDCCH

CCCH [GSM 05.01/05.02] Common control channel. Generic term for all

point-to-multipoint channels on theAir-interface. CCCHs are in the downlink

direction, in particular the BCCH, the PCH, the CBCH , the AGCH . The

only CCCH in the uplink direction is the random access channel (RACH).

Network operators may configure the BCCH frequency to carry CCCHs in all

even-numbered time slots (0, 2, 4, 6).

DTX [GSM 05.08, 06.12, 06.31, 06.32, 08.60] Discontinuous transmission.

During a telephone conversation, typically only one party speaks at a time. At

times, no one speaks. It is practical to switch off the Air-interface partly or completely

during those silent times until the conversation resumes. One problem

to avoid is clipping, that is, the situation when beginnings and ends of words

are cut off because the volume of the speech is below a threshold and considered

to be silent time. Setting the volume threshold is difficult because different

languages have different dynamics, and what appears to be good enough for

English may be poor quality for spoken Chinese. The process of detecting silent

time and cease transmission is called discontinuous transmission. DTX needs

to be distinguished from DRX (discontinuous reception); both methods are

independent of each other.

DTX can be activated separately for uplink and downlink. The advantage

of using DTX in the uplink direction is the power savings potential within the

MS and for both uplink and downlink to reduce interference. One potential

problem of DTX is related to background noise. People are so used to it that if

it is not there, they assume the connection has been lost, particularly in a

mobile conversation. DTX eliminates background noise, so to avoid the

impression of a lost connection, artificial noise (called comfort noise) is generated

when DTX is active.

With DTX enabled, the BTS or the MS sends only one block of data

(456 bits according to channel coding) every 480 ms, which, because of interleaving

and depending on the channel type, is transmitted with a variable

number of bursts. That allows both sides to still measure the quality of the connection

and to adjust the comfort noise if necessary.

DRX [GSM 03.13/05.02] Discontinuous reception. Used like the DTX as a

power saver for the mobile station and to save radio resources. By separating

mobile stations into paging groups, a particular mobile station needs to listen to

the paging channels (PCH) only in certain multiframes. The transmitter can be

switched off in the meantime, in what constitutes the power saving.

FACCH [GSM 04.04, 05.01, 05.02] Fast associated control channel. An

in-band signaling channel, just like the SACCH, that is associated with an

active connection between the MS and the BTS. In contrast to the SACCH,

which is sent once per multiframe, the FACCH is used only when no delay is

acceptable, that is, if it is not possible to wait for the next SACCH. Then the

FACCH is inserted instead of user data. The stealing flag serves to distinguish

user data from signaling within a burst. The FACCH can transport 9200 bps in

a fullrate channel and 4600 bps in a halfrate channel. (see N201, Burst.)

FCCH [GSM 05.01, 05.02] The BTS sends the frequency correction channel

(FCCH) on time slot 0 of a BCCH-TRX in frequency bursts (see Burst). All

142 data bits are set to zero. Exactly five FCCHs are sent per 51-multiframe.

The FCCH allows an MS to identify the frequency of a BTS in GSM. After sending

an FCCH, an SCH has to be sent.

FN [GSM 05.01/05.02] Frame number. Internal clock of a BTS, to which

every MS has to synchronize before the MS can start communicating with the

BTS. For that purpose, the BTS broadcasts the current frame number five

times for every 51-multiframe over the synchronization channel (see SCH ).

The FN can take on values between 0 and 2,715,647, where each FN

identifies exactly one TDMA frame within a hyperframe. The value 2,715,647

represents the possible number of frames, where 2,715,647 = (26 × 51 × 2048) - 1.

The - 1 is necessary, since the count starts with zero. The equation represents

the composition of a hyperframe. It consist of 2,048 superframes, each superframe

consists of 26 multiframes with 51 TDMA frames or 51 multiframes

with 26 TDMA frames. What is transmitted, however, is not the absolute value

of the FN, but the relative position of an FN in the frame hierarchy, consisting

of 51-multiframe, superframe, and hyperframe. (See also Chapter 7.)

This method of addressing the FN is similar to the way two people tell the

time of day. Compare, for example, “The time is 54.900 seconds,” and “The

time is 3.15 p.m.” In practice, the FN is sent as a combination of the parameters

T1, T2, and T3, what could be brought in the analogy the example hours

(T1), minutes (T2), and seconds (T3’) of a clock. The rule is the following:

· T1 (11 bit): Number of the superframe in the hyperframe

{0¼2,047}; T1 = FN div 1326, where 1,326 = 51 ´ 26

· T2 (5 bit): Number of the 51-multiframe in the superframe {0¼25};

T2 = FN mod 26

· T3: (6 bit): Number of the TDMA frame in the 51-multiframe

{0¼50}; T3 = FN mod 51

· T3’ (3 bit): (T3 - 1) div 10 out of {0¼4}

For T3, only the value of the decade has to be sent, since the synchronization

channel is sent exactly five times per 51-multiframe, in fact, always in the

position FN = 1, 11, 21, 31, 41 (compare Figure G.31). The single digit value,

therefore, is redundant and there is no need for its transmission. The value T3’

{0 ¼ 4} tells the MS exactly which FN in a 51-multiframe is meant and can

easily calculate the FN of a 26-multiframe or the absolute value of FN.

Note that this rule applies only to the transmission of FN on the synchronization

channel. When the CHAN_RQD message is being transmitted, the

entire value of T3 has to be sent. That allows the number of the superframe

(T1) to be truncated. Indeed, T1’ is used in this case, rather than T1, with

T1’ = T1 mod 32. T1’ represents the last five bits of T1. The reason for that is

obvious:

· First of all, depending on the channel configuration, it is possible to

send a RACH, practically anywhere within a 51-multiframe. Thus, T3

cannot be truncated.

· Furthermore, the BSC needs to respond to a CHAN_RQD within

seconds. It is therefore not necessary to know the absolute number of

the superframe. Knowing only the least significant five bits of the

superframe number enables the BSS to uniquely identify and address a

single CHAN_RQD message within a time period of (25 - 1) × 6.12s =

189.72s (a superframe has a cycle time of 6.12s), which is more than

sufficient.

GSM refers to this type of frame number as starting time where starting

time = FN mod 42432

MEAS_RES and MEAS_REP [GSM 04.08, 05.08, 08.58] BTS and MS

measure the signal strength and quality of the received signal during an active

connection. The MS periodically sends the measurements in a MEAS_REP

message to the BTS (in a SDCCH/SACCH every 470.8 ms; on a

TCH/SACCH every 480 ms). The BTS adds the measurements received from

the MS to its own measurements and sends the result in a MEAS_RES message

to the BSC. These measurements serve as input data for the BSC to perform

the power control function and handover decision.

Power control [GSM 05.05, 05.08] GSM requires that every mobile station

is subject to power control. For the BTS, on the other hand, power control is

optional. Depending on the quality of a connection, the BSC will request the

BTS and the mobile station to adjust their output power. The purpose of

power control is to minimize interference with other channels and to increase

the working time of the battery.

The BSC informs the BTS via the Abis-interface within a

BS_POWER_CON message of the output power to be used (Figure G.47).

Only if necessary, the BSC will send an MS_POWER_CON message to the

BTS to initiate an adjustment of the output power of the mobile station. This

new output power level is forwarded to the mobile station within the Layer 1

header of the next SACCH to be sent. Note that one SACCH is sent to the

mobile station every 480 ms, always telling the mobile station the current output

power.

The maximum power is called Pn. Starting from there, the output power

may be reduced in steps of 2 dB. Power control on the BTS side allows

reduction of the output power by 30 dB in 15 steps, while the output power of

the MS can be reduced between 20 dB and 30 dB, depending on the standard

(GSM, DCS1800) and the power class of the MS. Figure G.47 is an example

of the two messages that are used for power control. While the

MS_POWER_CON message always uses an absolute value, the

BS_POWER_CON message always uses a relative value (Pn - X).

Note that all downlink channels of the BCCH-TRX have to permanently

use the maximum output power Pn, since the BCCH is serving as a beacon and

reference for the neighbor cell measurements of the mobile stations.

RACH [GSM 05.01, 05.02] Random access channel (RACH) is an uplink

common control channel (CCCH) that the MS uses to send a connection

request to the BTS. The access burst (see burst) is always used for the transmission

of the RACH. The only two messages that are sent on the RACH are

CHAN_REQ and HND_ACC, with a net data length of 8 bits and a transmission

rate of 34 bps.

SACCH [GSM 04.04, 05.01, 05.02] Slow associated control channel (see

also FACCH ). The inband control channel assigned to the TCH or the

SDCCH. Every 26th burst of a TCH or every 51st burst of an SDCCH is an

SACCH. The consequence is that exactly one SACCH is sent per multiframe.

Figure G.53 illustrates the format of the SACCH for uplink and downlink.

The transmission rate is 391 bps when the SACCH is assigned to the SDCCH.

When assigned to the TCH, the transmission rate is 383 bps.

SCH [GSM 04.08, 05.01, 05.02] Every BTS broadcasts the synchronization

channel (SCH) in time slot zero of the BCCH-TRX. The SCH contains

the absolute value of the frame number (see FN ) of a BTS, which is time

dependent, and the base station identity code (see BSIC ) for an initial rough

identification of the cell. The SCH has a length of 25 bits and is sent in the synchronization

burst.

SDCCH/4 and SDCCH/8 [GSM 05.01, 05.02] Standalone dedicated control

channel. Used for uplink and downlink of the Air-interface to transmit signaling

data for connection setup and location update (LU). The transmission

rate is 779 bps. The distinction between SDCCH/8 and SDCCH/4 refers to

the channel configuration on the Air-interface. An SDCCH/8 channel configuration

can never be realized on TS 0 of the BCCH-TRX. The available

bandwidth is, because of the BCCH that occupies part of the bandwidth there,

not sufficient to allow for that. TS 0 of the BCCH-TRX can be used for,

at maximum, one SDCCH/4 channel configuration with four SDCCH subchannels.

Stealing flag [GSM 05.03] The stealing flag is 1 bit long and is part of the

normal burst. Stealing flags embrace the training sequence (see TSC ). They are

used in the uplink and downlink direction to indicate whether and which bits

of a traffic channel are used (stolen) to carry signaling information (see

FACCH ). Stealing of bits to send signaling information on the traffic channel

(see TCH) may become necessary when the MS or the BTS have to immediately

send signaling data but the SACCH is not available.

Examples of control data are the HND_CMD, CON, and the DISC

messages. Note that 456 bits (four bursts) are necessary to transfer such a message,

completely. The stealing flag was introduced in order not to lose four consecutive

bursts for traffic data. Actually, a signaling message is divided into

eight packets with 57 bits each and then transmitted in eight consecutive

bursts. Only the even-numbered bits are used to carry signaling data in the first

four bursts, while only the odd-numbered bits are used for signaling in the last

four bursts (i.e., 5 through 8). That allows use of the remaining 57 bits per

burst to carry traffic data

TA Timing advance. The agreement in a GSM system is for the MS to send

its data three time slots after it received the data from the BTS. The BTS then

expects the bursts from the MS in a well-defined time frame. This prevents

collision with data from other mobile stations. The mechanism works fine,

as long as the distance between MS and BTS is rather small. Increasing distance

requires taking into account the propagation delay of downlink bursts

and uplink bursts. Consequently, the mobile station needs to transmit earlier

than defined by the “three time slots delay” rule. The information about how

much earlier a burst has to be sent is conveyed to the mobile station by the TA.

The TA is dynamic and changes in time. Its current value is sent to the mobile

station within the layer 1 header of each SACCH. In the opposite direction, the

BTS sends the current value for TA within the MEAS_RES messages to

the BSC (e.g., for handover consideration). The farther the MS is away

from the BTS, the larger is the required TA. Figure G.62 illustrates the relation

between distance and TA.

Using the TA allows the BTS to receive the bursts from a particular MS

in the proper receiver window. The BTS calculates the first TA when receiving

a RACH and reports the value to the BSC. TA can take any value between 0

and 63, which relates to a distance between 0 km and 35 km. The steps are

about 550 m (35 km/63 » 550 m). With respect to time, the different values

of TA refer to the interval 0 ms through 232 ms, in steps of 48/13 ms. It is

important to note that this value of TA represents twice the propagation delay.

Figure G.63 illustrates the effect of TA by an example in which a connection is

active on TS 1.

Interleaving [GSM 03.05, 03.50, 05.03] Procedure to distribute or interlace

the bits of a channel-coded block (see channel coding) onto several bursts. Since

channel coding is designed to detect and correct errors on only a relatively few

bits, it is the goal of interleaving to prevent complete loss of the information

when a whole burst is corrupted. If, for example, a complete burst is lost, but all

the others are transmitted without error, only one bit of a larger piece of information

is missing and can be restored by the Viterby decoder.

The likelihood of group errors on a radio interface is naturally much

higher than errors on single bits. The reason is the effect of fading, which typically

is slower than the 270-Kbps transmission rate of the Air-interface.

For transmission of data, the bits are distributed even more than in the

case of speech. For data transmission, it is even more important not to lose a

single bit, since that could render a complete transmission useless. Speech is not

very sensitive to single-bit errors. Propagation delay, on the other hand, is crucial

for speech and does not have a very high priority for data connections. The

more the bits of one sample are spread over time, the longer the receiver has to

wait until all bits for a certain sample have arrived. For data services, that essentially

affects only timers of the protocol. This affects the RLP protocol for nontransparent

data and the end-to-end protocols of terminal applications for

transparent data (GSM 03.05, 03.50).

In a fullrate speech channel, interleaving accounts for a maximum delay is

37.5 ms, while the maximum delay caused by the more intense interleaving in

case of a fullrate data channel is 106.8 ms. Only RACH and SCH are transmitted

without interleaving. Figure G.38 illustrates interleaving for a fullrate

speech channel. The 456 channel-coded bits of block n are divided into 8 subblocks

with 57 bits each and then rearranged. Subblocks 0 through 3 of block n

are then interleaved with subblocks 4 through 7 of block n - 1, while subblocks

4 through 7 of block n are interleaved with subblocks 0 through 3 of block

n + 1. Initially, subblocks 0 through 3 form the upper half of a burst, while subblocks

4 through 7 form the lower half of a burst. During the subsequent formation

of the burst, the bits of the upper half alternatingly join with the bits of

the lower half. Stealing flags are inserted in the middle of a burst.