03r Radio Channel Configuration

BSSDIM

Radio Channel ConfigurationsTraining DocumentBSSPAR

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The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This document is intended for the use of Nokia Networks' customers only for the purposes of the agreement under which the document is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia Networks. The document has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Networks welcomes customer comments as part of the process of continuous development and improvement of the documentation.

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Radio Channel Configurations

Contents

1 Module Objectives........................................................4

2 GSM Air Interface..........................................................5

3 Logical Channels..........................................................7

4 Channel Configuration...............................................104.1 Signalling Channel Configuration..................................114.1.1 Combined Channel Structure for BCCH/SDCCH.........114.1.2 Separated Channel Structure BCCH + SDCCH/8........124.1.3 Hybrid Channel Structure BCCH + SDCCH/4 +

SDCCH/8......................................................................134.2 Traffic Channel Configuration.......................................14

5 Channel Parameters...................................................155.1 Paging Channel (PCH) Parameters..............................155.2 Access Grant Channel (AGCH) Parameters.................175.3 Random Access Channel (RACH) Parameters............175.4 CCCH/PBCCH Parameters..........................................185.5 GPRS Non-DRX Mode On CCCH, DRX-Timer-Max In

BSC...............................................................................19

6 Capacity (SDCCH, PCH, AGCH).................................20

7 Dynamic SDCCH Allocation.......................................25

8 CCCH Improvements..................................................27

9 Key Learning Points...................................................28

10 Review Questions.......................................................30

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1 Module ObjectivesAt the end of the module, the participant will be able to:

Discuss the function of Paging Channel (PCH), Random Access Channel (RACH), and Access Grant Channel (AGCH)

State the purpose of the following BSS parameters:

noOfMultiframesBetweenPaging (MFR)(BTS)(2..9) maxNumberOfRetransmission (RET)(BTS)(1, 2, 4 or 7) numberOfSlotsSpreadTrans (SLO)(BTS)(3..12, 14, 16, 20, 25, 32, 50)newEstabCausesSupport (NECI)(BTS)(Yes/No) and numberOfBlocksForAccessGrant (AG)(BTS)(0..7)(1..7)(0..2)(1)

List the ways in which Physical channels TS0 and TS1 can be used by logical channels for signalling purposes

Explain the need for Dynamic SDCCH allocation

Name two mechanisms available for CCCH usage improvements

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2 GSM Air InterfaceGSM is based on Time Division Multiple Access (TDMA) technology, which means that channels are separated from each other by time. This means that in radio path between the antennas of a Mobile Station (MS) and a Base Station (BTS), every channel has a specific time on each frequency during which it can act. The basic division is that one frequency is divided into eight Time Slots or Bursts and each of these Bursts is an individual channel. More precisely, each frequency has eight channels, either traffic channels or signalling channels. These eight channels have their own "time slots" related to the time for transmitting or receiving data. Therefore, every channel has a 'right' to act every eighth time slot.

Each burst lasts 0.577 ms (exactly 15/26 ms) and thus eight bursts last 4.615 ms. There are a couple of different kinds of bursts for different purposes. The contents of the burst can vary, but the time duration of each burst is always the same. The structure of the eight bursts is called TDMA frame and the duration of a TDMA frame is called the Burst Period. The TDMA frame is the smallest and the basic unit of a TDMA frame structure.

0 7

TDMA frame = 4.615 ms

= BURST PERIOD

0

0 0

f s

Figure 1. Burst Period

The whole TDMA structure is based on TDMA frames, which are placed continuously after each other's as in Figure 2.

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Figure 2. TDMA frame structure

As we can see, the TDMA frame is cyclically repeating itself time after time. Now, other higher-level frames are needed for the GSM channel structure. In Figure 2, two different kinds of superframes can be seen, repeated time after time: the 26 x 51 Superframe and the 51 x 26 Superframe. These Superframes have been used so that the 51 x 26 Superframe is used for time slots with traffic channel configuration, and 26 x 51 Superframe is used for time slots with signalling channel configuration. Finally, these Superframes are repeated so that the result is a Hyperframe, which is the highest level of the frames in the GSM.

As mentioned above, there are two main types of channels: traffic channels and signalling channels. Traffic channels are used for sending data such as speech or data service fax, etc. and signalling channels are used for negotiations between a Mobile Station and the Network, in order to handle the management of the network. A Mobile Station and the Network are sending different kinds of messages between each other through signalling channels.

The other division between channels is between full rate and half rate. In a full rate channel, speech has been coded at a rate of 13 kbit/s, and in half rate, around 7 kbit/s. In both rates, data can be sent at the rate of 3.6 or 6.0 kbit/s and in full rate also 12 kbit/s. In the whole material, the full rate will be discussed, but if needed, also half rate has been mentioned. All these channels (traffic and signalling, full and half rate) have a common name: Logical channels.

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3 Logical ChannelsQuite a lot of information is sent between a Mobile Station and a Base Station, hence different kinds of signalling channels are needed to fulfil all these needs. In the GSM standard, the concept of a logical channel is used for different signalling channels that may be transmitted on the same physical channel, which is usually Time Slot 0 and/or 1 in every TDMA Frame.

There are twelve different types of Logical Channels, which are mapped into Physical Channels in the radio path. Logical channels comprise of Common Channels and Dedicated Channels. Common Channels are those that are used for broadcasting different information to mobile stations and setting up of signalling channels between the MSC/VLR and the mobile station. Common control channels are divided into Broadcast Channels (BCH) and Common Control Channels (CCCH).

Traffic channels are also Dedicated Channels as each channel is dedicated to only one user to carry speech or data.

Figure 3. GSM Logical Channels

Broadcast Channels

Base Stations can use several TRXs but there is always only one TRX, which can carry Common Channels. Broadcast channels are downlink point-to-multipoint channels. They contain general information about the network and the broadcasting cell. There are three types of broadcast channels:

1. Frequency Correction Channel (FCCH)

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COMMONCHANNELSCOMMON

CHANNELS

BROADCASTCHANNELS

BROADCASTCHANNELS

COMMONCONTROL

CHANNELS

COMMONCONTROL

CHANNELS

DEDICATEDCONTROL

CHANNELS

DEDICATEDCONTROL

CHANNELS

TRAFFICCHANNELSTRAFFIC

CHANNELS

FCCHFCCH SCHSCH BCCHBCCH SDCCHSDCCH SACCHSACCH FACCHFACCH

PCHPCH RACHRACH AGCHAGCH TCH/FTCH/F TCH/HTCH/H TCH/EFRTCH/EFR

DEDICATEDCHANNELS

DEDICATEDCHANNELS

LOGICALCHANNELSLOGICAL

CHANNELS

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FCCH bursts acts like a flag for the MS, which enables them to find the TRX that contains the broadcast transmission. The MS scans for this signal after it has been switched on since it has no information as to which frequency to use.

2. Synchronisation Channel (SCH)

The SCH contains the Base Station Identity Code (BSIC) and a reduced TDMA frame number. The BSIC is needed to identify that the frequency strength being measured by the mobile station is coming from a particular base station. In some cases, a distant base station broadcasting the same frequency can also be detected by the mobile station. The TDMA frame number is required for speech encryption purpose.

3. Broadcast Control Channel (BCCH)

The BCCH contains detailed network and cell specific information such as:

Frequencies used in the particular cell and neighbouring cells.

Frequency hopping sequence. This is designed to reduce the negative effects of the air interface, which sometimes results in the loss of information transmitted. The mobile station may transmit information on different frequencies within one cell. The order in which the mobile station should change the frequencies is called the "frequency hopping sequence". However, implementing Frequency Hopping in a cell is optional.

Channel combination. As we mentioned previously, there are twelve logical channels. All the logical channels except Traffic Channels are mapped into Timeslot 0 or Timeslot 1 of the broadcasting TRX. Channel combination informs the mobile station about the mapping method used in the particular cell.

Paging groups. Normally in one cell, there is more than one paging channel (described later). To prevent a mobile from listening to all the paging channels for a paging message, the paging channels are divided in such a way that only a group of mobile stations listen to a particular paging channel. These are referred to as paging groups.

Information on surrounding cells. A mobile station has to know what are the cells surrounding the present cell and what frequencies are being broadcast on them. This is necessary if, for example, the user initiates a conversation in the current cell, and then decides to move on. The mobile station has to measure the signal strength and quality of the surrounding cells and report this information to the base station controller.

Common Control Channels

Common Control Channels comprise the second set of logical channels. They are used to set up a point-to-point connection. There are three types of common control channels:

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1. Paging Channel (PCH)

The PCH is a downlink channel, which is broadcast by all the BTSs of a Location Area in the case of a mobile terminated call.

2. Random Access Channel (RACH)

The RACH is the only uplink and the first point-to-point channel in the common control channels. It is used by the mobile station in order to initiate a transaction, or as a response to a PCH.

3. Access Grant Channel (AGCH)

The AGCH is the answer to the RACH. It is used to assign a mobile a Stand-alone Dedicated Control Channel (SDCCH). It is a downlink, point-to-point channel.

Dedicated Control Channels

Dedicated Control Channels compose the third group of channels. Again, there are three dedicated channels. They are used for call set-up, sending measurement reports and handover. They are all bi-directional and point-to-point channels. There are three dedicated control channels:

1. Stand Alone Dedicated Control Channel (SDCCH)

The SDCCH is used for system signalling: call set-up, authentication, location update, and assignment of traffic channels and transmission of short messages.

2. Slow Associated Control Channel (SACCH)

An SACCH is associated with each SDCCH and Traffic Channel (TCH). It transmits measurement reports and is also used for power control, time alignment and in some cases to transmit short messages.

3. Fast Associated Control Channel (FACCH)

The FACCH is used when a handover is required. It is mapped onto a TCH, and it replaces 20 ms of speech and therefore it is said to work in "frame stealing" mode.

Traffic Channels (TCH)

Traffic Channels are logical channels that transfer user speech or data, which can be either in the form of Half Rate traffic (6.5 kbits/s) or Full Rate traffic (13 kbits/s). Another form of traffic channel is the Enhanced Full Rate (EFR) Traffic Channel. The speech coding in EFR is done at 12.2 kbits/s, and the coding mechanism is different to that used for normal full rate traffic. EFR coding gives better speech quality at about the same bit rate as normal full rate. Traffic channels can transmit both speech and data and are bi-directional channels.

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4 Channel ConfigurationTimeslots 0 and 1 in each TRX are usually needed for the use of signalling channels. For capacity reasons, there are two main configurations for these channels.

0 7

Non-combined Configuration

Combined Configuration

0 7

ts0=bch / pch / agch ts1=sdcch/8

ts0=bch / sdcch/4 / pch / agch

1

Figure 4. Combined vs. Non-combined configuration

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4.1 Signalling Channel Configuration

4.1.1 Combined Channel Structure for BCCH/SDCCH

This channel structure is used when there are up to a maximum of 2 TRXs per cell as shown in Figure 5. BCCH, CCCH/3 and SDCCH/4 are transmitted in the same timeslot TS0 in both directions (uplink, downlink).

f s bb bbc fc fc scc cc cc cc fc fs t t t t tt t t f ft t t t tt t t fs fs ss s ss s ss

t t tt r r s fs ss ss sr r rr r r rs fr r r r r rr r r r fr r r r tr t t tr ft t t r tr t tt

Downlink

Uplink

CHANNELS f = FCCH b = BCCH r = RACH i=idle s = SCH/SACCH c = CCCH t = SDCCH/4

51 TDMA frames = 235 ms

t

0 50

1. 2. 3. 4.

i

Figure 5. Combined BCCH and SDCCH/4 channel structure

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4.1.2 Separated Channel Structure BCCH + SDCCH/8

In this configuration, TS0 is used to carry BCCH and CCCH/9, and TS1 is used for all SDCCH/8s in the uplink and downlink directions as shown in Figure 6 and Figure 7. This configuration requires 3-4 TRXs/Cell.

f s bb bbc fc fc sc cc c cc c c fc fsc ccc cc cc f fc ccc cc c c fs fc c cc cc c cs

r r rr r r r fr rr r r r r r rr r r rr fr r r r r rr r r r fr r r r rr r r rr fr r r r rr r rr

Downlink

Uplink

CHANNELS f = FCCH b = BCCH r = RACH i=idle s = SCH c = CCCH = PCH/AGCH


r

0 50

i

Figure 6. BCCH/CCCH multiframe

t t tt t t t ft tt t t t t t tt t t tt ft t t t t tt t t t fs s s s ss s ss fss ss ss

ss ss s ss fs ss s t tt t t ts ft t t t t tt t t t ft t t t tt t t tt ft t t t tt t tt

Downlink

Uplink

CHANNELS t = SDCCH/8 s = SACCH/8 i=idle

t

t


s

1. 2. 3. 5. 6. 7. 8.4.

0 50

iii

iii

Figure 7. SDCCH/8 Multiframe

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4.1.3 Hybrid Channel Structure BCCH + SDCCH/4 + SDCCH/8

This configuration requires 3-4 TRXs per cell and uses a combination of the configurations shown in Figure 5 and Figure 7. TS0 is used for BCCH, CCCH/3 and SDCCH/4 (uplink, downlink) and TS1 is used for SDCCH/8 (uplink, downlink).

This configuration gives more SDCCH capacity for call set-ups and location updates but less for paging and channel assignment (access grant AGCH).

In general, 1-2 time slots are needed for signalling. Finally, the signalling capacity and the need of signalling channels depends on paging (PCH) and the need of SDCCH. Examples of these channel capacities are presented later.

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4.2 Traffic Channel Configuration

Traffic channels use the 51 x 26 Superframe, which means that the structure of the 26-frame Multiframe is always the same as in Figure 8.

t t tt t t t ft tt t t tt t t tt ft t t tt

Downlink and Uplink

CHANNELS t = TCH s = SACCH i=idle

ts


0 25

i

Figure 8. Full Rate Traffic Channel (TCH) configuration

Downlink and Uplink



0 25

Downlink and Uplink



0 25

t t t t t t t t t t ttt t t t t t t tt t tt

s i

Figure 9. Half Rate Traffic Channel (TCH) Multiframe

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5 Channel ParametersCommon Control Channels (CCCH) can be configured with different parameters. The parameters are directly related to PCH, AGCH, FACCH and RACH.

5.1 Paging Channel (PCH) Parameters

Paging is performed when an incoming call or short message is directed to an MS. The MSC sends a paging query to all the BSCs inside the location area where the MS is registered. The paging message contains the subscriber identity (IMSI/TMSI number). The mobile recognises an incoming call or short message by this number. There are counters in the VLR for both successful and failed paging messages, which can be read by traffic measurements.

Parameter noOfMultiframesBetweenPaging (MFR)(BTS)(2..9) tells how often paging messages are sent to Mobile Stations. There is a direct influence on the battery saving of a Mobile Station. The Mobile Station will only need to listen the paging sub-group it belongs to (Discontinuous Reception, DRX), which will make the mobile spend less power. However, this makes the call assignment time longer.

• Paging messages sent on PCH which MS continually listens to (TSL0 on BCCHTRX)

• Limited to geographical sub-group of BSCs using LACs• Paging sub-groups are used to save MS battery-life since MS only needs to listen to

its own sub-channel and not entire PCH• MS can be paged using IMSI or TMSI (determines number of MSs that can be

paged per message (IMSI = 2 MSs : TMSI = 4 MSs)• Split between PCH and AGCH determined by numberOfBlocksForAccessGrant

and the type of channel configuration used (i.e. combined or non-combined)

Blocks for PCH & AGCHNo. Blocks Res AGCH

No. Blocks Res PCH

Combined NonCombined

3 90-2 0-73-1 9-2

• PCH can be used for AGCH ifno paging messages are to besent, but AGCH can not beused for PCH• Three types of pagingmessages (type 1, 2 and 3)depending on no. of MSs paged

Figure 10. Paging Procedure

The mobile unit listens for a possible incoming paging message once every noOfMultiframesBetweenPaging (MFR)(BTS). The range of values for this

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parameter is between 2 (i.e. every 0.47 seconds) and maximum of 9 (i.e. every 2.1 seconds). This means that if on average it takes 2 paging messages to page a mobile station, it’ll take from 1 to 4 seconds for the transaction to be completed.

Experimental results from live networks show that more than 3 paging attempts are usually unnecessary to ensure that the paging message reaches the MS. Thus, the paging messages are sent several times. The repetition procedure is defined using the MSC parameters:

Repaging Interval (INT)(MSC)(50..100): Time between paging attempts and

Repaging Attempts (AT)(MSC)(0..5).

The parameters are defined in a per location area basis. The repaging_internal must be configured so that there is enough time between consecutive paging messages. This is to avoid overlap of messages that are sent over the same channel in the air interface (paging block). Average page time information for a certain cell can be collected in the traffic measurement report (in the MSC).

During the paging and call establishment procedure, if no SDCCH channels are available, the BSC will command the MS to stay in the idle state for a certain period (wait indication). During that time the MS will not send any channel request message or answer to any paging messages. The parameters should be defined so that no repaging attempts are lost during this period (i.e. the repaging interval in the MSC should a few seconds longer than the wait indication time in the BSC).

• One Block Reserved for AGCH => 2 Blocks for PCH• Paged MS per Paging_Request Message : From 2 to 4

(average 3)• Average 2 Pages per MS• 3 Pages/Blocks * 2 Blocks = 6 Pages every 51-frame

Multiframe ( 235 ms. )• 2 Pages / Paged MS => 3 Paged MS every 235 ms.• ( ( 3600 * 1000 ) / 235 ) * 3 = 45.957 Paged MS per Hour.

• In the worst case all Transactions are Mobile Terminating• All Cells in Location Area get the same PCH Load• Capacity

• With one TRX 2,95 Erl.• 25 mErl. / subs. => 120 subscribers

BTS MS

Paging_Request

BTS 3MS

2 Paging_Request

Capacity of PCH calculated for a Location Area

Figure 11. Paging Capacity Example

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5.2 Access Grant Channel (AGCH) Parameters

numberOfBlocksForAccessGrant (AG)(BTS)(0..7) is a parameter for reserving the number of CCCH blocks for AGCH (Figure 12). CCCH blocks are used either for PCH or for AGCH.

CHANNELS f = FCCH b = BCCH t = SDCCH/4 s = SCH/SACCH c = CCCH = PCH/AGCH

f s bb bb c fc fc sc c c c cc c c fc fs t t t t tt t t f ft t t t tt t t fs fs s s s ss s ss

1 2 3

f s bb bb c fc fc sc c c c cc c c fc fs c c c c cc c c f fc c c c cc c c fs fc c c c cc c cs

BCCH/CCCH 51 TDMA frames = 235 ms

1 2 3 4 5 6 7 8 9

BCCH/SDCCH/4

Figure 12. Non-Combined and Combined Multiframes

5.3 Random Access Channel (RACH) Parameters

The configuration of RACH takes two parameters;

maxNumberOfRetransmission (RET)(BTS)(1, 2, 4 or 7) and

numberOfSlotsSpreadTrans (SLO)(BTS)(3..12, 14, 16, 20, 25, 32, 50).

numberOfSlotsSpreadTrans (SLO)(BTS) describes a window during which an MS tries to send random access to a Base Station.

maxNumberOfRetransmission (RET)(BTS)(1, 2, 4 or 7) describes the maximum number of RACH retransmissions the MS can send to the Base Station. If maxNumberOfRetransmission (RET)(BTS) is set to "2", the MS will try a first time to send the message within the window defined within a first 51-TDMA RACH multiframe. If no reply comes from the network, the MS will try a second time, or as many times as needed until the maximum as specified in the parameter maxNumberOfRetransmission (RET)(BTS), within a window of another 51-TDMA RACH multiframe.

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Number of retransmission = maxNumberRetransmission (1, 2, 4, 7)

window = numberOfSlotsSpreadTrans (3 ... 12, 14, 16, 20, 25, 32, 50)

0 0

1 RACH (Re)transmission during the window

=> Total time for RACH = maxNumberRetransmission * numberOfSLotsSpreadTrans +...

TDMA-frames

Figure 13. RACH Controlling

All the above mentioned parameters belong to the GSM phase 1. In GSM Phase 2, newEstabCausesSupport (NECI)(BTS)(Yes/No)(N) and (EPF)(ERF)(EEF)(EOF) are used to indicate the possibility to use FACCH in call assignment procedure.

5.4 CCCH/PBCCH Parameters

Common Control Channel capacities are configured as follows:

CCCH Configuration –

• Pages always used to have priority in CCCH blocks no matter what the numberOfBlocksForAccessGrant setting was.• Now, if numberOfBlocksForAccessGrant =0 then AGCH messages would

have priority over PCH messages.• If numberOfBlocksForAccessGrant <> 0 then PCH messages would have

priority over AGCH messages .

Thus capacity can be dynamically shared between PCH and AGCH =>better throughput for PCH especially for combined-BCCH

Figure 14. numberOfBlocksForAccessGrant Parameter

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• # of 51 TDMA frame multiframes (2..9) between transmissions of Paging_Requestmessages to mobiles of the same paging group.

# of paging groups = (# of CCCH blocks- numberOfBlocksForAccessGrant )*

noOMultiframesBetweenPaging

Page / group every (2 ... 9) * 235 ms = 0.47 ... 2.115 s• Mobile Station calculates its Paging Group based on IMSI and on the Number of

Paging Groups.

• Affects of the # of the Paging Groups• Battery Consumption of the Mobile Station• Speed of Call Setups

Figure 15. Number of Multiframes between paging

The PBCCH feature brings dedicated CCCH capacity for (E)GPRS services and is described in a separate module called BSSPAR: GPRS and EGPRS (EDGE).

5.5 GPRS Non-DRX Mode On CCCH, DRX-Timer-Max In BSC

This feature is described in a separate module called BSSPAR: GPRS and EGPRS (EDGE).

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6 Capacity (SDCCH, PCH, AGCH)Signalling capacity depends mostly on the paging channel (PCH) capacity and on the SDCCH capacity. Both capacities can be calculated very easily. Based on these calculations, the final channel configuration (combined BCCH/SDCCH or separated BCCH and SDCCH) can be chosen.

Paging capacity is related to the number of paging groups, which depends on the frame and channel structure and the following parameters:

noOfMultiframesBetweenPaging (MFR)(BTS)(2..9) and

numberOfBlocksForAccessGrant (AG)(BTS)(0..7)

Paging capacity also gives a very good view of the size of location areas, because pages (from BTS to MS) are sent over the whole location area every time. Examples of the capacities of both types of channel will clarify the situation:

Example of Capacity Computation for SDCCH

Consider 2 TRXs per Cell each carrying 9.01 Erlangs per Cell with 2% blocking probability. Assuming that the call duration is 1.5 min during BH. SDCCH is used for location updates once in 60 min and call assignment (7 sec/call including ciphering and authentication). There is a traffic density of 25 mErl per subscriber, which translates to 360 subscribers per cell.

Example 1 - Call Establishment & Location Updates included

Call Establishment

2 TRXs / Cell ~ 8,11 Erl / Cell (1% Blocking probability)

1,5 min / Subs / BH = 25 mErl. / Subs

8,11 Erl / Cell /25 mErl. / Subs = 325 Subs / Cell

Authentication and Ciphering = 7 sec = 1,94 mErl / Call (SDCCH reservation time)

=> 325 Calls / Cell * 1,94 mErl / Call = 0,6305 Erl / Cell (SDCCH)

Location Update

Location Updates once in 60 minutes - set by the parametertimerPeriodicUpdateMS

325 Subs / Cell

SDCCH reservation time for Location Update = 7 sec = 1,94 mErl

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Call Establishment and Location Update together

0,632 Erl + 0,632 Erl = 1,261 Erl / Cell

With 1% Blocking Probability (Erlang B table) => 5 SDCCH / Cell

Combined BCCH/SDCCH channel structure is not possible in this case!

Separate Configuration (1 BCCH/CCCH and 1 SDCCH/8) is needed!

Call establishment time

SDCCH reservation time 7 sec / 3600 sec = 1.94 mErl /call

=> 360 calls/cell *1.94 mErl/call = 0.699 Erl/Cell (SDCCH)

Location update

Location update once in 60 minutes

=> 360 calls/cell *1.94 mErl/call = 0.699 Erl/Cell (SDCCH)

=> Needed SDCCH capacity 0.699 Erl/Cell + 0.699 Erl/Cell = 1.398 Erl/Cell (SDCCH)

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Chs 1% 2% 3% 5% Chs 1% 2% 3% 5%1 0.01 0.02 0.03 0.05 21 12.80 14.00 14.90 16.202 0.15 0.22 0.28 0.38 22 13.70 14.90 15.80 17.103 0.46 0.60 0.72 0.90 23 14.50 15.80 16.70 18.104 0.87 1.09 1.26 1.52 24 15.30 16.60 17.60 19.005 1.36 1.66 1.88 2.22 25 16.10 17.50 18.50 20.006 1.91 2.28 2.54 2.96 26 17.00 18.40 19.40 20.907 2.50 2.94 3.25 3.75 27 17.80 19.30 20.30 21.908 3.13 3.63 3.99 4.54 28 18.60 20.20 21.20 22.909 3.78 4.34 4.75 5.37 29 19.50 21.00 22.10 23.80

10 4.46 5.08 5.53 6.22 30 20.30 21.90 23.10 24.8011 5.16 5.84 6.33 7.08 31 21.20 22.80 24.00 25.8012 5.88 6.61 7.14 7.95 32 22.00 23.70 24.90 26.7013 6.61 7.40 7.97 8.83 33 22.90 24.60 25.80 27.7014 7.35 8.20 8.80 9.73 34 23.80 25.50 26.80 28.7015 8.11 9.01 9.65 10.60 35 24.60 26.40 27.70 29.7016 8.88 9.83 10.50 11.50 36 25.50 27.30 28.60 30.7017 9.65 10.70 11.40 12.50 37 26.40 28.30 29.60 31.6018 10.40 11.50 12.20 13.40 38 27.30 29.20 30.50 32.6019 11.20 12.30 13.10 14.30 39 28.10 30.10 31.50 33.6020 12.00 13.20 14.00 15.20 40 29.00 31.00 32.40 34.60

SDChannels

Figure 16. Erlang-B Table

Transformation to channels by using Erlang-B table:

Blocking probability 1% (usually set below 1%, for example 0,2%)

= 6 SDCCHs

In this case, result shows that it is not possible to use combined channel structure up to 2 TRXs/Cell. However, if the location update is done only once in six hours then the needed SDCCH capacity is 0,816 Erl/Cell. When the blocking probability for SDCCH is 1%, there is needed 4 SDCCHs/cell. This time the combined channel structure would be possible, but we have to remember to take into consideration also the capacity what is needed for short messages.

Example of the capacity of PCH

Combined BCCH/SDCCH signalling channel configuration in which

1 block is used for AGCH -> 2 blocks for paging

Maximum of 4 paging messages/block, (TMSI) used, 3 in average

In average, we have to send 2 paging messages to page a mobile.

Therefore, on average, we send 3 pages/block, but we have reserved 2 blocks for paging. This gives us in total 6 paging messages in every 51-frame Multiframe, which means 6 paging messages in every 235 ms.

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If we now calculate how many paging messages we can get during busy hour:

3600 sec. / 0.235 sec * 6 paging messages= 91915 paging messages

now we can calculate how many mobiles we can page during busy hour while in average we have to send 2 paging messages to page a mobile:

91915 / 2= 45 957 mobiles/BH.

Example 2 - Call Establishment & Location Updates included

Call Establishment

- 2 TRXs / Cell ~ 8,11 Erl / Cell (1% Blocking probability)

- 1,5 min / Subs / BH = 25 mErl. / Subs

- 8,11 Erl / Cell /25 mErl. / Subs = 325 Subs / Cell

- Authentication and Ciphering = 7 sec = 1,94 mErl / Call (SDCCH reservation time)


Location Update

- Location Updates once in 120 minutes - parameter timerPeriodicUpdateMS

- 325 Subs / Cell

- SDCCH reservation time for Location Update = 7 sec = 1,94 mErl

=> 325 Calls / Cell * 1,94 mErl / Call * 1/2 = 0,31525 Erl / Cell (SDCCH)

Call Establishment and Location Update together

- 0,6305 Erl/Cell + 0,31525 Erl/Cell = 0,94575 Erl/Cell (SDCCH)

- With 1% Blocking Probability (Erlang B) => ~ 5 SDCCH / Cell

Combined BCCH/SDCCH channel structure is not possible in this case !

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Example 3 - Call Establishment, Location Updates & SMS

Call Establishment

- 2 TRXs / Cell ~ 8,11 Erl / Cell (1% Blocking probability)

- 1,5 min / Subs / BH = 25 mErl. / Subs

- 8,11 Erl / Cell /25 mErl. / Subs = 325 Subs / Cell

- Authentication and Ciphering = 7 sec = 1,94 mErl / Call (SDCCH reservation time)


Location Update

- Location Updates once in 120 minutes set using the parameter timerPeriodicUpdateMS

- 325 Subs / Cell

- SDCCH reservation time for Location Update = 7 sec = 1,94 mErl

=> 325 Calls / Cell * 1,94 mErl / Call * 1/2 = 0,31525 Erl / Cell (SDCCH)

SMS

- SMS traffic estimation 1.0 mErl / subscriber

=> 325 Calls / Cell * 1 mErl / Call = 0,325 Erl / Cell (SDCCH)

Call Establishment, Location Update and SMS together

- 0,6305 Erl/Cell + 0,31525 Erl/Cell + 0,325 Erl/Call = 1,27075Erl/Cell (SDCCH)

- With 1% Blocking Probability (Erlang B) => ~ 5 SDCCH / Cell

Separated channel structure is needed in this case

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7 Dynamic SDCCH Allocation Dynamic SDCCH is an optional feature. The BTS should be configured with the minimum static SDCCH capacity that is sufficient to handle the normal SDCCH traffic. Extra SDCCH resources are allocated from free TCH only when SDCCH congestion has been experienced after the last free SDCCH is allocated. Consequently, when the dynamic SDCCH radio resources are not used it is immediately configured back for TCH use. Thus the maximum number of TCHs are always in traffic use depending on the actual need of the SDCCH resources at each moment.

A particular benefit is derived from this feature in traffic cases where the signalling is the only transmission mode during the connection to the network. Short Message service (SMS) traffic as well as location updatings are counted among them. In some special places e.g. airports and ports, the location updatings can produce sudden short time SDCCH traffic peaks which can now be handled without any need to configure extra permanent SDCCH capacity.

Dynamic SDCCH resource can be configured only when SDCCH is allocated for Immediate Assignment. During the SDCCH handover it is not allowed (restriction concerns the BSC). However, channels of the dynamic SDCCH resources can be used in handovers. Placement of the new dynamic SDCCH is dependent on the following factors:

SDCCH resource is configured only to regular TRX. A RTSL of least uplink interference should be selected.

The SDCCH is configured to a TRX, which does not yet have any SDCCH resources or to the TRX, which has least number of SDCCH.

Priority is given to the TRX, which has least working channels.

When in a particular TRX and a different type of TCH resource must be selected, then the preference order is the following: first HR then FR, DR TCH resource.

These requirements must be compromised according to the actual TCH occupation situation in the TRXs.

CBCH carrying SDCCH cannot be configured dynamically.

Principles in radio channel allocation from the SDCCH resources of the BTS are:

SDCCH is always allocated from static SDCCH resource if there is any free channel left.

When SDCCH is allocated from the dynamic SDCCH resources then the one shall be used which has least idle channels left.

These rules are for minimising the consumption of the TCH resources.

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When the feature FACCH call set-up is activated, in situations of SDCCH congestion of the BTS, the MS can be assigned a TCH from the CCCH at the time of Immediate Assignment. This feature can be applied also with the Dynamic SDCCH in some special cases:

The FACCH call set-up is used in true SDCCH congestion when it is not possible to configure any dynamic SDCCH resource in the BTS.

When the last TCH resource of the BTS is going to be taken in use and the connection requires a TCH then it is reasonable to use the FACCH call set-up.

The upper limit for the number of SDCCHs, which are possible to configure in BSC are determined by the number of TRXs connected to the BSC Signalling Unit (BCSU). With maximum TRX configurations the average SDCCH capacity is determined to be 12 SDCCH channels per TRX. For 1-32 TRX BCSU the max number of the SDCCH channels is 384.

Dynamic SDCCH resources can be shared between all TRXs of the BTS. The absolute limit is that the maximum SDCCH number in a TRX must not exceed 16 channels; while this limit value is reached then at least one of the two SDCCH/8 resources must be a dynamic one.

The capacity restriction of the 16 kbit/s telecom signalling link (TRXSIG LAPD) produces additional constraints. The uplink capacity is not sufficient in the worst traffic load cases. The main reason for capacity loss is the increased uplink load due to measurement result reporting.

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8 CCCH ImprovementsThere are two mechanisms available to improve the performance and usage of the CCCH channels:

The CCCH scheduling algorithm

The CCCH scheduling algorithm is used to allow priority for access grant messages over paging messages when the parameternoOfBlocksForAccessGrant(AG)(BTS)(0..7)(1..7)(0..2)(1) equals zero.

For non-zero values, the situation will remain as previously, i.e. paging messages have priority over access grant messages on PCH. This greatly improves the PCH throughput especially for combined-BCCH-CCCH channel structure.

Modified buffering mechanism

For PCH the target is to offer a buffering mechanism in which the paging buffer capacity per paging group is dependent on the CCCH-configuration and on the identity type (IMSI/TMSI) in use, in such a way that there can be configuration-independent maximum paging-delay for a paging message.

Previously, each paging group buffer has a fixed depth (8 A-bis paging messages) regardless of the paging group repetition rate set by the parameter noOfMultiframesBetweenPaging(MFR)(BTS)(2..9)(4).

In the worst case, (when buffers are full and BS_PA_MFRMS = 9 and IMSI used), a page arriving at a BTS may have to wait for 4 paging multiframes (approx. 8.4 seconds). The page is clearly outdated by the time it gets transmitted to air.

Since page repetition is done at the MSC, after some point in time it is better to discard excessive pages rather than store them for very long time. In this new mechanism a page is not deleted because of insufficient buffering space, but because it cannot be transmitted to air within the defined maximum paging delay.

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9 Key Learning Points In GSM the logical channels can be divided into Dedicated Channels (DCH)

and Common Control Channels (CCCH)

Common Control Channels (CCCH) are used to set up point-to-point connections. There are three types of CCCH: Paging Channel (PCH), Random Access Channel (RACH), and Access Grant Channel (AGCH)

Paging Channel (PCH) is a downlink channel, which is broadcast by all the BTSs of a Location Area in the case of a mobile terminated call and SMS

Random Access Channel (RACH) is the only uplink logical channel and the first point-to-point channel in the common control channels. It is used by the mobile station to initiate a transaction, or as a response to a PCH.

Access Grant Channel (AGCH) is the answer to the RACH. It is used to assign to a mobile a Stand-alone Dedicated Control Channel (SDCCH). It is a downlink, point-to-point channel

noOfMultiframesBetweenPaging (MFR)(BTS)(2..9) tells how often paging messages are sent to an MS. This parameter is set so that the MS listens to incoming paging messages between every 0.47 seconds (minimum=2) and 2.1 seconds (maximum=9)

maxNumberOfRetransmission (RET)(BTS)(1, 2, 4 or 7) describes the maximum number of RACH retransmissions the MS can send to the Base Station. If it set to 2, it will try twice.

numberOfSlotsSpreadTrans (SLO)(BTS)(3..12, 14, 16, 20, 25, 32, 50), describes a window during which an MS tries to send random access to a Base Station.

In GSM Phase 2, newEstabCausesSupport (NECI)(BTS)(Yes/No)(N) and (EPF)(ERF)(EEF)(EOF) are used to indicate the possibility to use FACCH in call assignment procedure.

numberOfBlocksForAccessGrant (AG)(BTS)(0..7) is a parameter for reserving the number of CCCH blocks used for AGCH.

Physical channels TS0 and TS1 are available for signalling and they can be used by logical channels in 3 ways:

1. Combined channel structure for BCCH, CCCH and SDCCH on TS0 for uplink and downlink.

2. Separate channel structure for BCCH and CCCH on TS0 and SDCCH on TS1 on uplink and downlink.

3. Hybrid Structure in which BCCH, CCCH and SDCCH/4 are transmitted on TS0 in both directions and SDCCH/8 is transmitted on TS1 for both directions.

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Dynamic SDCCH allocation is an optional feature where extra SDCCH capacity is allocated from free TCH when SDCCH congestion is experienced. These resources revert to TCH usage when SDCCH congestion subsides.

There are two mechanisms available for CCCH usage improvements:

CCCH Scheduling Algorithms : which provides priority for Access Grant messages over Paging Messages.

Modified Buffering Mechanism in which pages are deleted when they cannot be transmitted to air within the defined maximum paging delay.

Figure 17. Parameters related to signalling

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noOfMultiframesBetweenPaging 2 ... 9

numberOfBlocksForAccessGrant 0 ... 7 (if BCCH/CCCH used)(non-combined)

1 ... 7 (if CBCH used on SDCCH/8)0 ... 2 (if combined BCCH/SDCCH used)

maxNumberRetransmission 1, 2, 4, 7

numberOfSlotsSpreadTrans 3 ... 12, 14, 16, 20, 25, 32, 50

pagingAnsOnFacch Y/NemerCallOnFacch Y/NordinaryCallOnFacch Y/NrestablishCallOnFacch Y/N

newEstabCausesSupport Y/N (FACCH setup allowed)

Parameters Value

BSSPAR

10 Review Questions Q1. Which logical channel is used by the BTS to assign to an MS, a traffic

channel for communication?

a) Common Control Channel (CCCH)

b) Paging Channel (PCH)

c) Random Access Channel (RACH)

d) Access Grant Channel (AGCH)

e) Traffic Channel (TCH)

Q2. The parameter numberOfSlotsSpreadTrans(SLO)(BTS) is used to allocated a number of CCCH blocks for …

a) Common Control Channel (CCCH)

b) Paging Channel (PCH)

c) Random Access Channel (RACH)

d) Access Grant Channel (AGCH)

e) Traffic Channel

Q3. Which parameter defines how often paging messages are sent to MS?

a) noOfMultiframesBetweenPaging (MFR)(BTS)(2..9)

b) maxNumberOfRetransmission (RET)(BTS)(1, 2, 4 or 7)

c) numberOfSlotsSpreadTrans (SLO)(BTS)(3..12,14, 16, 20, 25, 32, 50)

d) newEstabCallSupport (NECI)(BTS)(Yes/No) and

e) numberOfBlocksForAccessGrant (AG)(BTS)(0..7)

Q4. Which parameter is associated with AGCH?

a) noOfMultiframesBetweenPaging (MFR)(BTS)(2..9)

b) maxNumberOfRetransmission (RET)(BTS)(1, 2, 4 or 7)

c) numberOfSlotsSpreadTrans (SLO)(BTS)(3..12,14, 16, 20, 25, 32, 50)

d) newEstabCallSupport (NECI)(BTS)(Yes/No) and

e) numberOfBlocksForAccessGrant (AG)(BTS)(0..7)

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Q5. Which of the following are used for signalling on physical channels TS0 and TS1?

a) Combined channel structure for BCCH, CCCH and SDCCH on TS0 for uplink and downlink.

b) Separate channel structure for BCCH and CCCH on TS0 and SDCCH on TS1 on uplink and downlink.

c) Hybrid Structure in which BCCH, CCCH and SDCCH/4 are transmitted on TS0 in both directions and SDCCH/8 is transmitted on TS1 for both directions.

d) All of the above

e) None of the above

Q6. Which of the following is true about dynamic SDCCH Allocation?

a) It is an optional feature.

b) Extra SDCCH capacity is allocated from free TCH when SDCCH congestion is experienced.

c) When SDCCH congestion subsides, allocated TCH revert to TCH usage.

d) All of the above

e) None of the above

Q7. Which of the following mechanism(s) is/are used to improve CCCH usage?

a) CCCH Scheduling Algorithms provide priority for Access Grant messages over Paging Messages.

b) Modified Buffering mechanism in which pages are deleted when they cannot be transmitted to air within the defined maximum paging delay.

c) All of the above

d) None of the above

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Documents

03r Radio Channel Configuration