User Description, Abis Optimization

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© Ericsson AB 2007 - All Rights Reserved

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Contents

1 Introduction

2 Capabilities 2.1 Bandwidth Savings 2.2 BSC Hardware 2.3 Delay and Speech Quality 2.4 GRPS/EGPRS Performance 2.5 Transmission Requirements 2.6 Migration 2.7 Start of RBS 2.8 Compatibility Information

3 Technical Description 3.1 Architecture 3.2 Bandwidth optimization 3.3 Abis Load Regulation and Overload

Handling 3.4 Supervision of Super Channel 3.5 Related Statistics 3.6 Main changes in Ericsson BSS 07B

4 Engineering Guidelines 4.1 Introduction 4.2 Dimensioning Strategy 4.3 SC Dimensioning 4.4 Link Quality

5 Parameters 5.1 Main Controlling Parameters 5.2 Parameters for Special Adjustments 5.3 Value Ranges and Defaults Values

6 Concepts

Glossary

Reference List

1   Introduction

Abis Optimization is a feature to achieve bandwidth savings on the Abis interface. Bandwidth savings are achieved by removal of redundant information and packing of frames in both uplink and downlink.

Bandwidth savings are also accomplished by introducing the super channel concept. A super channel is one E1 or one T1 link, or a fraction of one E1 or one T1, where 64 kbit/s consecutive Abis timeslots can be used as a wideband connection for sending signalling and payload as LAPD frames between BSC and BTS. As all traffic and signalling share the same wideband connection, statistical multiplexing gains are achieved.

2   Capabilities

2.1   Bandwidth Savings

Abis Optimization gives significant bandwidth savings on the Abis interface. The amount of savings are depending on the traffic mix and voice activity. The operator can then reduce the amount of transmission to the base stations or use the existing transmission more efficiently by expanding the number of transceivers. It is also possible for the operator to monitor the utilization of the Abis links to each base station in order to determine when new Abis resources needs to be added or when resources can be removed.

Abis Optimization takes benefit of statistical multiplexing and removes redundant information on the Abis interface to optimize the bandwidth usage. This is achieved by, for example, removal of TRAU frames during silence speech periods and removal of spare bits in CS and PS frames.

2.2   BSC Hardware

The Packet Gateway (PGW) based on RP HW, is needed in the BSC to support Abis Optimization. One PGW can support up to 100 TRXs, depending on traffic load, or 1000 channels. The amount of channels is a hard limit in the PGW and not dependent on traffic load. The amount of PGW boards needed and how many that can be placed in each magazine is described in Reference [4].

Automatic PGW redundancy is supported, see Reference [9].

The Gigabit Ethernet Switch Board (GESB) are used to connect ethernet of multiple GEM magazines of packet gateways and multiple magazines of PCU on GPH RP. The GESB are used for BSC internal ethernet communication. In addition, SCB-RP/3 are needed for magazine ethernet switching.

2.3   Delay and Speech Quality

The additional average delay within BSS is approximately 20 ms one way for CS speech and CS data. This impacts the speech quality slightly.

Speech quality may also be impacted at high load on the super channels due to discarded frames. The amount of discarded frames at traffic peaks are less than 1x10-4.

2.4   GRPS/EGPRS Performance

Ping performance will be approximately the same as without Abis Optimization.

Since a checksum is added on each frame sent on the super channel, the GPRS/EGPRS packet data throughput will be improved. A bit error detected by the checksum will result in that the frame is discarded and a retransmission will be performed by the RLC protocol. This will give better performance compared if the bit error was detected by higher protocol layers like TCP.

2.5   Transmission Requirements

The BSC and the BTS supports both E1 and T1 transmission. The BSC also supports ET155 (STM-1 and OC-3).

Abis Optimization requires that time slot integrity is provided by the transport network between the BSC and the base station. The operators have to put this requirement on their transport network providers. Ericsson's DXX solution provides time slot integrity as well as their MINI-link products.

2.6   Migration

To change a TG in operation to support Abis Optimization, the TG must be taken out of service. At migration, inter cell handovers is attempted to minimize impact on ongoing traffic. The time to change to Abis Optimization depends on the size of the TG and is approximately 6 minutes for a TG with 12 TRXs.

2.7   Start of RBS

During start of an RBS (power on) configured for Abis Optimization, OML Fault alarm on the CF and TRX will be raised. The alarm will be ceased 8 to 10 minutes after power on.

2.8   Compatibility Information

2.8.1   Supported RBS Hardware Configurations

Base stations with DXU-21/IXU-21 and only PPC based TRUs, that is RBS2106, 2107, 2109, 2112, 2206, 2207, 2308, 2309 are supported together with Abis Optimization. Mixed Micro and Mixed HW configurations are not supported together with Abis Optimization.

2.8.2   Supported BSC Hardware Configurations

BSC configurations with AXE810 hardware and BYB 501 with NNRP-4 and NNRP-5 are supported.

2.8.3   DXX Support Function

The BTS feature DXX Support Function, which means that DXX supervision is integrated in the BTS software, is supported. One 64 kbit/s timeslot is used for this purpose and is not a part of the super channel. The OMT is used to define which time slot to use. A timeslot chosen by OMT must avoid breaking the contiguity of the SC devices.

2.8.4   Terrestrial Link Supervision

Terrestrial link supervision [at 16 kbit/s] is replaced by supervision of super channels, see Section 3.4.2.

2.8.5   Interface over Satellite

The feature Interface over Satellite is supported together with Abis Optimization (with the same limitations as without Abis Optimization).

2.8.6   Abis Triggered HR Allocation

Abis Triggered HR Allocation is supported. This is similar functionality as described in Reference [2] but other parameter names are used to trigger the functionality, see Section 5.1.

2.8.7   Fullrate AMR on 8 kbps Abis

Fullrate AMR on 8 kbps Abis is supported. This is similar functionality as described in Reference [2] but note that 8 kbps Abis timeslots are not used for Abis Optimization. Instead, this feature reduce codec set to maximum 7.4 kbps, which will reduce the required bandwidth on Abis. A separate parameter is used to trigger this feature with Abis Optimization, see Section 5.1.

2.8.8   Cascaded Sites

Base stations in cascade is supported to the same level as previously. This means that timeslots not being part of the super channel are sent transparently through the BTS as a cascaded path. The transmission in each TG in the cascaded chain has its own SC defining the portion of the E1/T1 that they use.

A BTS using Abis Optimization can only be cascaded through a DXU-21/IXU-21 based BTS due to the requirements on timeslot integrity. A BTS without Abis Optimization can be cascaded through a BTS using Abis Optimization.

2.8.9   DIP Supervision

DIP supervision is supported.

2.8.10   Flexible Abis

Flexible Abis can not be used together with Abis Optimization for the same TG.

2.8.11   LAPD Concentration and LAPD Multiplexing

LAPD Concentration and LAPD Multiplexing is not possible to combine with Abis Optimization for the same TG and is not needed.

2.8.12   Semipermanent Connected Transcoders

Semipermanent connected transcoders are not supported together with Abis Optimization.

2.8.13   OMT

The Remote OMT over IP (ROMT/IP) has the same functionality as the locally connected OMT and is supported.

The Remote OMT feature using a dedicated PCM timeslot on the Abis interface is not supported for interfaces where Abis Optimization is in use.

2.8.14   Dedicated Packet Data Channels

With Abis Optimization, all traffic and signalling share the same super channel between the BSC and the BTS. This means that there is no dedicated resource for a packet data channel on the Abis interface. Therefor, there is no guarantee that a dedicated or semi-dedicated Packet Data Channel gets an Abis resource at Abis overload.

3   Technical Description

3.1   Architecture

Figure 1   Architecture for Abis Optimization using LAPD based transport on E1/T1 transport network.

Note:   1) 1 to 4 E1/T1 links per TG

The BSS solution for Abis Optimization is based on that traffic and signalling on the Abis interface are sent as LAPD frames on super channels instead of using dedicated timeslots. As normal E1/T1 links are used, synchronization of the radio base stations is kept.

In the uplink direction, the TRX handles removal of redundant information in order to save bandwidth. Each "compressed" traffic frame is sent as one LAPD frame to the DXU every 20 ms. The DXU is configured so that a number of TRXs are dedicated to a specific Abis link or part of an Abis link (super channel). In the downlink direction, jitter buffers exists in each TRX due to that frames sent from the BSC are sent in series and can arrive in the TRX at different times depending on the load on the super channel.

The Packet Gateway (PGW) in the BSC handles the LAPD frames in the same way as the base station. Up to 31 timeslots for E1 and up to 24 timeslots for T1 can be used. The minimum super channel size is 8 timeslots. The LAPD frames are sent over E1/T1 using ETCs. The ETC supports the super channel concept since TS0 (for E1) and F-bits (for T1) for alignment and supervision are preserved.

The operator has to define which TRXs that shall use a specific super channel. The CF link is automatically configured to the first super channel in a TG. The CF link can be automatically moved to another super channel (if available) in case of a fault while the TRXs are statically defined, see Section 3.4.1.

3.2   Bandwidth optimization

The transmission gain with Abis Optimization is obtained due to the following facts:

Discontinuous transmission (DTX) is a mechanism that allows the radio transmitter to be switched off during speech pauses. During a normal conversation, the participants alternate so that each is silent for about 50% of the time. The silence and speech periods on different calls are uncorrelated and virtually no information needs to be transferred during silence periods. By this fact and by using statistical multiplexing on the super channel transmission bandwidth can be saved in both uplink and downlink. Discontinuous transmission needs to be activated in both UL and DL to be able achieve bandwidth savings, see Reference [8]

Erlang aggregation gain is achieved from sector cells. If, for example, a three sector site is using the same super channel, there is a Erlang aggregation gain from the sectors. This is due to that there is a small risk that all cells have congestion at the same time. The super channel can be dimensioned with lower blocking than each sector cell but still saving in transmission can be obtained. This gain is depending on the type of area the sectors are covering and grade of service in cells but measurements have shown a gain between 10-20%.

No static allocation of transmission for GPRS/EGPRS is needed. Abis time slots allocated for GPRS/EGPRS only use bandwidth when there is GPRS/EGPRS traffic on the air TS. Air TS capable of EGPRS and CS3/CS4 does not need any fixed allocated bandwidth. Instead GPRS/EGPRS will use available transmission on the super channel.

Redundant information is removed from GPRS/EGPRS frames. With Abis Optimization, an EGPRS MCS-1 frame will take much less Abis resources compared to an MCS-9 frame. Without Abis Optimization, MCS-1 frames uses as much Abis transmission as an MCS-9 frame.

Redundant information is removed from AMR frames. With Abis Optimization, an AMR speech frame with codec rate 4.75 kbps will take much less Abis resources compared to an AMR speech frame with codec rate 12.2 kbps. Without Abis Optimization, the same amount of Abis transmission is needed independent of AMR codec rate.

The LAPD RSL and OML signaling is more efficiently used on Abis. The signaling needs is different between TRXs, and without Abis Optimization at least a 16kbps Abis resource is needed for each TRX. Compared with LAPD concentration, Abis Optimization only needs approximately half of the bandwidth for RSL and OML signaling on average.

3.3   Abis Load Regulation and Overload Handling

3.3.1   Functionality to reduce the Abis load

The PGW measures the Abis load per super channel. When the transmission load to a super channel has increased above one of three operator specified thresholds, the following high load measures are applied:

Allocate half rate speech calls by triggering the feature "Abis triggered HR Allocation". This is applicable to half rate capable terminals. Quality triggers are used to determine which connections are suitable to allocate to half rate. The threshold is set per TG. Note that the this feature needs to be activated for each cell in the TG, see Reference [2].

Move full rate speech calls to half rate by triggering the feature "Abis triggered HR Allocation". This is applicable to half rate capable terminals. Quality triggers are used to determine which connections are suitable to move to half rate. If the SC load decreases, the half rate calls may be moved back to full rate again. The thresholds are set per TG. Note that the this feature needs to be activated for each cell in the TG, see Reference [2].

Trigger the feature "Fullrate AMR on 8 kbps Abis". This feature allocates full rate AMR calls with codecs restricted to a maximum of 7.4 kbps. Abis transmission resources are saved, by limiting the codec set while keeping FR AMR in the air. The threshold is set per TG. Note that the this feature needs to be activated for each cell in the TG, see Reference [7].

A filtering time is used to avoid that the features are triggered for short traffic peaks.

In case of several super channels to one TG, high load on one super channel will trigger the features above. It is possible to trigger the features above at different load levels, or in different order, for different subscriber priority levels. Then the feature Speech Quality Priority shall be used, see further Reference [2].

There is still a risk of 100% transmission load to a TG (that is, buffer overflow), due to that the actions described above are not sufficient. If an overload situation occurs in UL, the BTS discard speech and CS&PS data as a first choice. These frames are discarded if older than 20 ms (that is, before next batch of frames arrives). In the DL direction, PS frames (except CS-1 frames) are discarded before CS frames by the BSC. Signalling frames are prioritized in both directions.

In case transmission on a super channel is suffering from continuous overload, the BSC reduces the load by limiting the allowed number of simultaneous ongoing connections. In case of a continuously overloaded SC, the traffic level where new connections are allowed is further decreased. As a result the traffic level is decreased step by step if the overload remains. When the overload has disappeared, new connections are allowed but only a portion at a time to reduce the risk for dropped frames. During the overload, no new allocation of PDCH channels, channels for DTM or channels for HSCSD is made.

3.3.2   Alarms and Printouts due to Overload

The operator is informed about an Abis overload situation. The alarm is triggered when BSC or BTS has started to discard frames due to buffer overflow. The BSC calculates frames discarded by the BTS based on sequence numbers and the load situation. The alarm is seized when the overload situation has disappeared. The super channel are considered overloaded as long as new connections are rejected for the TG. An alarm filtering time is used

to avoid any ping-pong effect. The default alarm class is A2. The alarm is issued per super channel.

It is possible to print the load level per super channel. The load level is in percent of total available bandwidth and shows the load level in 5 second intervals the last minute.

3.4   Supervision of Super Channel

3.4.1   Quality Supervision

A mechanism exists for quality supervision of each super channel. The CF link is not dedicated to a specific super channel, which means that when a faulty or severely degraded super channel is taken out of service, the CF link is moved to another super channel (if available for that TG) within 20 seconds. The TRXs are however always dedicated to a specific super channel, so they will not be moved if a fault occurs.

3.4.2   Fault Supervision

In case of a failure, the BSC triggers an alarm with information about the faulty super channel. The default alarm class is A2.

3.5   Related Statistics

3.5.1   Impact on Legacy Counters

Depending on dimensioning of the Abis link, the following STS counters might be impacted.

GPRS/EGPRS throughput counters

GPRS/EGPRS throughput counters might indicate increased throughput compared to Flexible Abis due to increased possibility to get Abis resources.

GPRS/EGPRS throughput counters might indicate reduced throughput due to that GPRS/EGPRS frames are dropped during high Abis load. The amount of dropped frames depends on traffic variations and dimensioning of the super channels.

Cell congestion counters

In case of continuously Abis overload, new connections in the cell will be limited. This will result in increased value of cell congestion counters.

3.5.2   Statistics for Performance Management

The operator can monitor the utilization of the Abis links to each base station in order to determine when new Abis resources need to be added or when resources can be removed. STS counters below, marked in bold, are used with Abis Optimization. All counters are per super channel.

KBSENT: Number of kbytes sent DL by PGW during last recording period.

KBREC: Number of kbytes received UL by PGW during last recording period.

KBSCAN: Number of scans for number of kbytes sent and received by the PGW.

To be able to detect any traffic peaks, the following counters are used with Abis Optimization:

KBMAXSENT: Maximum number of kbytes per second sent DL by the PGW during the last 15-minute interval.

KBMAXREC: Maximum number of kbytes per second received UL by the PGW during the last 15-minute interval.

The operator can calculate the average and maximum load per super channel by comparing the figures to the number of Abis devices defined for that super channel.

A report is available in OSS that shows Abis link utilization per super channel. The report shows the throughput in percent in both UL and DL direction in relation to defined bandwidth. Both average and maximum throughput are shown.

Counters are also used to measure lost frames (due to overload or due to bad transport network). Values are calculated in the BSC both for DL and UL.

THRULPACK: Number of discarded CS frames in the UL by the DXU due to Abis overload during last recording period (normally 15 minutes). The number of discarded frames are estimated by the BSC during high super channel load and includes only TRA frames.

THRDLPACK: Number of discarded frames in the DL by the PGW due to Abis overload during last recording period (normally 15 minutes). The counter is stepped for discarded speech and data frames.

LOSTULPACK: Number of lost frames on the UL during last recording period (normally 15 minutes). This includes all speech and data frames that are missing in the PGW, that is frames that were corrupted in the transmission network as well as frames that were not sent by the BTS due to super channel overload.

LOSTDLPACK: Number of lost frames on the DL during last recording period (normally 15 minutes). This includes all speech and data frames that are missing in the BTS, that is frames that were corrupted in the transmission network as well as frames that were not sent by the PGW due to super channel overload.

3.6   Main changes in Ericsson BSS 07B

4   Engineering Guidelines

4.1   Introduction

The objective with this chapter is to dimension the Abis Optimization transmission for a certain BTS site/TG using the transmission as effective as possible. The output from the dimensioning is the number of Abis E1/T1 time slots needed for the Super Channels. The dimensioning is performed with an aim to save as much bandwidth as possible while maintaining close to as good speech quality as without Abis Optimization.

4.2   Dimensioning Strategy

To maintain good speech quality the packet drop rate caused by Abis Optimization must be kept below a certain limit. This means that the average load on the super channel must be kept below a certain percentage limit. This load limit is depending on the number of calls handled on the super channel. A higher number of calls mean that a higher average load can be tolerated while maintaining good speech quality. The graph below shows how much the link can be loaded depending on number of calls while still maintaining packet drop rate below 1x10-4.

Figure 2   Max load on a SC dimensioned for n calls.

If the SC load limit above are exceeded, the Abis Optimization load regulation function in the BSC is triggered and no more packet data channels are allowed to be allocated on that super channel. Also the number of speech calls will be regulated keeping the super channel load below the load threshold. This will automatically make sure that good speech quality is maintained for the Abis Optimization feature even at super channel overload situations.

To keep the super channel load below the SC load limit, the features: "Fullrate AMR on 8 kbits/s" and "Abis triggered HR Allocation" can be triggered. This means that the thresholds for these features should be set to a value below the SC load limit, see figure below.

The thresholds for triggering the "Fullrate AMR on 8 kbits/s" and "Abis triggered HR Allocation" should be set to a level guaranteeing that packet data will get the required bandwidth. A good strategy is to set the difference in load regulation threshold and the HR thresholds so that GPRS/EGPRS get a margin to handle fast increase of packet data load. For example start of a streaming session. The figures below explains this scenario.

Figure 3   Bandwidth utilization of the super channel.

On the DL the EGPRS and GPRS traffic can be dimensioned to use the remaining bandwidth on the super channel. This since the packet data on the DL has lower priority than the speech and signaling. Also as shown in the figure above, the load from speech decreases when the packet data volume increases due to start of allocating more HR channels. In this way the speech quality can remain high when there is low packet data and be slightly reduced by allocating HR or reduce the AMR codec rate at packet data peaks.

In this way the packet data can increase the utilization of the super channel with maintained speech quality. Packet data is not very sensitive for packet losses. The packet data can maintain good throughput even at high packet drop rate due to retransmissions on the RLC protocol layer.

The DL direction will have higher peaks of data flow than the UL due to the MS capabilities in terms of supported time slots UL and DL. In most cases the number of supported timeslots in DL is higher than UL. Even if the average data flow is symmetrical in average during busy hour the DL will momentarily generate higher packet data rates.

Due to the burstiness of packet data a margin for packet data is used in the dimensioning. In this way the load impact from fast ramping up of packet data is minimized. This is achieved by setting the thresholds for HR allocation, Dynamic FR/HR Mode Adaptation and Fullrate AMR on 8kbps Abis to proper values in accordance with the dimensioning tool, see Reference [3]. HR Allocation and Fullrate AMR on 8kbps Abis are set to give a margin for packet data of 200 kbit/s. Dynamic FR/HR Mode Adaptation is set to trigger at higher super channel load and will give a margin for packet data of 100 kbit/s.

4.3   SC Dimensioning

4.3.1   Required Input

The formulas for dimensioning of the SC are complex and involve many steps so this part of the dimensioning has been implemented in an Excel Dimensioning Guideline (see Reference [3]). The needed input for this dimensioning is:

BTS Site Configuration

TRXs

This is the number of TRXs per sector in the TG. Note that all TRXs in all sectors must belong to the same TG (max 12 TRXs per TG).

E-TCHs

This is the number of EDGE capable TCHs in the sector.

Fixed PDCHs

This parameter defines the number of fixed configured GPRS/EDGE channels in the sector.

Control channels

This is the number of defined control channels on the air interface in the sector. For example: total number of BCCH, SDCCH8 or CBCH in the sector.

Traffic Model

VAF (%)

VAF (Voice Activity Factor) defines the average level of speech that is detected by the voice activity detection algorithm in TRA or MS. A typical value is 60% but this can vary between networks and cells. In the UL direction, VAF can be estimated by using the following STS counters and formulas:

o VAF [%] for AMR FR = 100 * (TFV3TFCMx / TFV3CMxUL) where x is codec mode 1-4

o VAF [%] for AMR HR = 100 * (THV3TFCMx / THV3CMxUL) where x is codec mode 1-4

o VAF [%] for FR = 100 * (TFV1FERTF / (50*MP*TFV1TRALACC / TFV1NSCAN))

o VAF [%] for EFR = 100 * (TFV2FERTF / (50*MP*TFV2TRALACC / TFV2NSCAN))

o VAF [%] for HR= 100 * (THV1FERTF / (50*MP*THV1TRALACC / THV1NSCAN))

where MP is the measuring period in seconds, normally 900 (15 minutes) or 3600 (1 hour).

Max % HR in TG

This parameters describes the expected maximum share of HR and AMR HR to support the traffic in the TG.

% AMR in TG

This is the percentage share of the calls in the TG that are using AMR codec.

Erlang Aggregation Gain

This parameter is automatically generated by the dimensioning tool. It defines the percentages gain from Erlang aggregation from sector cells in the same TG. There must be at least two cells on one SC to get any Erlang aggregation gain. A typical figure for a medium size TG with three cells with 2% TCH blocking is 10-20%. For calculation, Grade of Service (congestion) in the cell and on Abis must be inserted. Typical values for Grade of Service are 1-2%.

Abis Opt Configuration

Full Rate AMR on 8kbps Abis

This parameter shall be selected if AMR FR calls with codec rate restricted to 7.4 kbps shall be triggered at high Abis load.

Dynamic HR Allocation

This parameter shall be selected if Dynamic HR Allocation shall be triggered at high Abis load.

Dynamic FR/HR Mode Adaptation

This parameter shall be selected if Dynamic FR/HR Mode Adaptation shall be triggered at high Abis load.

GPRS Bandwidth [kbit/s]

This parameter defines the bandwidth reserved for GPRS/EGPRS traffic on the super channel during CS busy hour. The average available Abis bandwidth for GPRS/EGPRS is higher.

4.3.2   Delivered Output

The output from the dimensioning tool is:

Size of the SC

The number of E1/T1 PCM time slots required on the super channel. If this value is more than 31/24 for E1/T1, additional E1/T1 transmission links needs to be allocated to the TG. The dimensioning must then be performed again with fewer TRXs on each super channel.

The recommended settings for the parameters for triggering "Full Rate AMR on 8 kbps Abis" and "Abis triggered HR Allocation".

The number of required E1/T1 time slots without Abis Optimization. This is only a help for comparing the gain when using Abis Optimization for the BTS dimensioned. LAPD concentration is assumed here but no flexible Abis when making the comparison.

4.3.3   Dimensioning Example

In this example we have a 12 TRU BTS site in one TG. The setup is shown in Figure 4. The TRUs are divided equally in three cell sectors. For this site it would be beneficial to use only one SC since this will give the maximum utilization of the transmission. So the first step would be to try that scenario. The first step in the dimensioning is set values for the input to the dimensioning tool:

TRXs: In this example there is 12 TRXs on the SC, 4 TRXs per each sector.

E-TCHs: 4 EDGE capable TCHs per sector.

Fixed PDCHs: 3 fixed E-PDCHs are used, one per sector.

Control channels: 3 control channels per sector (1 BCCH and 2 SDCCH/8).

VAF (%): 60%. The lowest share of VAF UL or DL in the site. 60% is a typical value measured from several markets.

Max % HR in TG: 50 % half rate is the maximum expected HR in this site.

% AMR in TG:80% AMR usage in traffic peak.

GOS in TG and cell: 1% congestion is used for both the cell and the TG.

GPRS Bandwidth [kbit/s]: 256 kbit/s allocated for GPRS/EGPRS

Figure 4   BTS site example with one super channel handling 12 TRXs.

Using this parameters in the Excel Dimensioning Guideline (see Reference [3]), we get 19 required PCM time slots on the super channel. This means that we can handle the traffic with one E1 or one T1. In case more than 24 (T1) or 31 (E1) PCM time slots is required, the TG has to be split into two SCs. The most beneficial is this case is then to split the BTS site into two equally large SCs. Then the dimensioning shall be performed again using half the number of TRXs per each of the two SC according to Figure 5. The other parameters must then also be split between the two SC. The Erlang aggregation gain will be much less in this case.

Figure 5   BTS site example with two super channels handle 12 TRXs

4.3.4   Bandwidth saving examples

By using the Excel Dimensioning Guideline, the super channel size can be calculated. In the table below, bandwidth savings are calculated for some RBS configurations. Input parameters are the same as in example 1. Please use the dimensioning tool for any other traffic mix or configuration.

Table 1    Bandwidth savings examples with traffic mix and cell configuration as in example 1

RBS Configuration

Number of TRXs in cell A+B+C

Number of Abis TS required without Abis Optimization (LAPD Conc and no Flexible Abis)

Number of Abis TS required with Abis Optimization

Bandwidth saving for the RBS

4+4+4 34 19 44%

3+3+3 28 15 46%

2+2+2 21 10 52%

12+0+0 30 22 26%

8+4+0 32 20 37%

As can be seen in the table, the largest savings are achieved for 3 sector sites.

For T1 markets, this example shows that one 4+4+4 RBS (or one 3+3+3 RBS) is possible to handle with one T1 while 2 T1s are required with "classic" Abis. Two 2+2+2 RBSs can be connected in cascade and can share the same T1 as they together require 20 Abis timeslots. Two super channels with 10 time slot each is to be configured on one T1.

For E1 markets, one 4+4+4 RBS and one 2+2+2 RBS can share one E1 while 2 E1s are required with "classic" Abis. The same applies for two 3+3+3 RBSs.

Note that these are examples and using the Excel Dimensioning Guideline (see Reference [3]) are required for correct dimensioning.

4.3.5   How to detect when to change super channel size

The super channel is dimensioned based on the number of TRX's and the traffic mix. If the super channel size is dimensioned correctly and this input does not change, there is no need to change the super channel size. If the number of TRXs are changed or the traffic mix changes (such as increased GPRS/EGPRS traffic during busy hour), the super channel might need to be changed accordingly. The following counters can be used to detect if the super channel needs to be increased:

If the Abis link is continously overloaded new call set up will be rejected. This can be seen by the congestion counters THTCONGS and TFTCONGS. Note that the counters are triggered by both cell congestion and Abis congestion.

Abis overload can also be seen on the counters THRULPACK and THRDLPACK. If these counters indicates a frame loss of more than 1*10-4, this might impact speech quality. If THRULPACK / (KBREC * 1000 / 35) or THRDLPACK / (KBSENT * 1000 / 35) > 1*10-4 then consider increase the super channel size.

4.4   Link Quality

4.4.1   General

For required E1/T1 link quality, see also Reference [5] and Reference [6].

4.4.2   BER Performance for the Abis Interface

For voice traffic:

at a constant BER of 1x10-4 the system is working but the speech quality will be bad.

at a constant BER of 1x10-5 the system is working, the speech quality will be good and this BER level is sufficient for normal operation.

at a constant BER of 1x10-6 the system works satisfactory.

For GPRS traffic:

at a constant BER of 1x10-4 the system is working and the throughput is good.

4.4.3   Round Trip Delay for the transport network

The maximum recommended round trip delay for the transport network is 15 ms. This should be considered when dimensioning the transport network between the BSC and the BTS. In case of exceeding this limit, there is a risk for decreased GPRS performance and decreased LAPD signalling capacity. At excessive round trip delay such as satellite transmission, certain parameters need to be changed.

5   Parameters

5.1   Main Controlling Parameters

NUMDEV

This parameter defines the number of subsequent PGW devices and RBLT devices that constitute the super channel.

PACKALG

This parameter sets the packing algorithm used in TG. For Abis Optimization, this parameter shall always be set to 1 to achieve optimal bandwidth savings.

SC

This parameter defines the super channel number within a super channel group.

SCGR

This parameter defines the group of super channels that terminate in the same Transceiver Group (TG).

SDAMRREDABISTHR

This parameter defines a threshold value for starting and stopping the allocation of AMR FR channels with reduced codec set, in case of high transmission load on a super channel. The allocation is stopped when the threshold value is set to 100%. The parameter is set per TG. Note that this functionality is activated by the parameter DAMRCRABIS per cell, see Reference [2]. If the feature Speech Quality Priority is activated then the parameter can be set to different values for different subscriber priority levels by using the parameter AHPRL, described in Reference [2].

SDFRMAABISTHR

This parameter defines a threshold value for starting and stopping of move from FR to HR channels, including AMR, in case of high transmission load on a super channel. The moving of channels is stopped when the threshold value is set to 100%. The parameter is set

per TG. Note that this functionality is activated by the parameter ATHABIS per cell, see Reference [2].

SDHRAABISTHR

This parameter defines a threshold value for starting and stopping the allocation of AMR HR and HR channels, in case of high transmission load on a super channel. The allocation is stopped when the threshold value is set to 100%. The parameter is set per TG. Note that this functionality is activated by the parameter ATHABIS per cell, see Reference [2]. If the feature Speech Quality Priority is activated then the parameter can be set to different values for different subscriber priority levels by using the parameter AHPRL, described in Reference [2].

SDHRMAABISTHR

This parameter defines a threshold value for starting and stopping of move from HR to FR channels, including AMR, in case of low transmission load on a super channel. The moving of channels is stopped when the threshold value is set to 0%. The parameter is set per TG. Note that this functionality is activated by the parameter ATHABIS per cell, see Reference [2].

TMODE

This parameter sets the TG Transmission Mode.

5.2   Parameters for Special Adjustments

PTA

This parameter indicates a factor for counting the timing advance for P-GSL when TG is in SCM transmission mode. It is not recommended to set this parameter to less than the default value as it can impact the GPRS/EGPRS performance.

5.3   Value Ranges and Defaults Values

Table 2    Parameters

Parameter Name Default Value

Recommended Value

Value Range Unit

DCP - - If a superior TG is in the transmission mode SCM: Numeral 256 - 1023 A total number of 232 DCPs must be given.

-

The given DCPs must be consecutive.

NUMDEV - - 8-31 (E1)

8-24 (T1)

Number of device on one super channel

PACKALG 1 1 0-1 1 = packing algorithm No. 1

0 = switch off packing

PTA 7 7 0-63 P-GSL Timing Advance

SC - - 0-3 -

SCGR - - 0-511 -

SDAMRREDABISTHR 100 See Reference [3] 1-100 %

SDFRMAABISTHR 100 See Reference [3] 1-100 %

SDHRAABISTHR 100 See Reference [3] 1-100 %

SDHRMAABISTHR 0 See Reference [3] 0-100  

TMODE - - TDM, SCM TDM = TG in Time Division Multiplexing mode

SCM = TG in Super Channel mode

6   Concepts

Abis device  A 64 kbit/s time slot on an Abis link.Abis link  One E1 (2048 kbit/s) link or one T1 (1544 kbit/s) link.E-TCH  A traffic channel, which in the packet switched domain is capable of

carrying EGPRS MCS-1 to MCS-9 and GPRS CS-1 to CS-4 packet data.G-TCH  A traffic channel, which in the packet switched domain is capable of

carrying GPRS CS-1 to CS-4 packet data.LAPD frame  A LAPD frame is a frame with variable length. Start and stop of the frame

are indicated with Flags = `01111110'. The frame also contains address and data information as well as check sum etcetera. Note that for Abis Optimization, the LAPD format used for traffic is the same as used for existing LAPD UI (Unnumbered Information) signalling frames. The signalling frames can be acknowledged (I and S frames) or un-acknowledged (U and UI frames).

Structured E1/T1 interface  TS0 is kept in the E1 interface for frame alignment signal, CRC etc. In the

T1 interface, the framing bit in every frame (the F-bits) is kept for frame alignment, CRC etc. That is, one F-bit and 8*24=192 bits for data are used in a single frame.

Super Channel  One E1 or one T1 link or a fraction of one E1 or one T1, where 64 kbit/s

consecutive Abis timeslots are used as a wideband connection for sending LAPD frames. Up to 31 for E1 and up to 24 for T1 can be used, but it is not possible to have more than 1 SC per port in a TG. One TG can communicate via one up to four super channels.

Time slot integrity  All time slots have the same delay. If the delay difference is 125 micro

seconds or more, time slot integrity is not fulfilled.

Glossary

AbisInterface between BSC and BTS (Abis-interface) AMRAdaptive Multirate BERBit Error Rate BSCBase Station Controller BSSBase Station Subsystem BTSBase Transceiver Station CFCentral Function CONConcentrator CPCentral Processor CRCCyclic Redundancy Check CSCircuit Switched 

CSnCoding Scheme n (where n is 1 to 4) DCPDigital Connection Point DIPDigital Path DLDownlink DTMDual Transfer Mode DTXDiscontinuous Transmission DXUDistribution Switch Unit DXXDigital Cross Connect system E1European digital transmission format 1 for PCM connections [2.048 Mbits/s, 32 independent 64kbps channels, time slots 0-31] EGPRSEnhanced General Packet Radio Service ETExchange Terminal ETCExchange Terminal Circuit FRFull Rate GEMGeneric Ericsson Magazine GESBGigabit Ethernet Switch Board GPHGPRS Packet Handler 

GPRSGeneral Packet Radio Service GSGroup Switch GSMGlobal System for Mobile communications HRHalf Rate HWHardware IPInternet Protocol LAPDLink Access Procedure on the D-channel MCS-nModulation and Coding Scheme n (where n is 1 to 9) MSMobile Station NOM1Network Operation Mode 1 NNRP-5Network Node Renewal Process 5 OC-3Optical Carrier level 3 OMLOperation and Maintenance Link OMTOperation and Maintenance Terminal OSSOperation and Support System P-GSLPacket GPRS Signalling Link PCM

Pulse Code Modulation PCUPacket Control Unit PGWPacket Gate Way PSPacket Switch RBLTRTS Abis Interface Line Terminal RBSRadio Base Station RLCRadio Link Control ROMTRemote Operation and Maintenance Terminal RPRegional Processor RSLRadio Signalling Link SCSuper Channel SCGRSuper Channel Group SCMSuper Channel Mode STMSynchronous Transport Module STSStatistics and Traffic Measurement Subsystem T1American standard for 1.544Mbps PCM connections carrying 24 independent 64kbps channels (DS0) numbered 1-24 TCH

Traffic Channel TCPTransmission Control Protocol TGTransceiver Group TRATranscoder Rate Adapter TRAUTranscoder and Rate Adapter Unit TRHTransceiver Handler TRXTransceiver TSTime Slot ULUplink VAFVoice Activity Factor