gb edge dimensioning

Gb EDGE Dimensioning

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# Nokia Siemens Networks 1 (38)

BSC3153Nokia GSM/EDGE BSS, Rel. BSS13, BSC andTCSM, Rel. S13, Product Documentation, v.1

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2 (38) # Nokia Siemens Networks DN7032484Issue 4-0 en25/03/2008


Contents

Contents 3

List of tables 4

List of figures 5

Summary of changes 7

1 Gb EDGE dimensioning 91.1 Gb over frame relay 101.2 Gb over IP 14

2 Planning process 17

3 Key strategies for EDGE dimensioning 19

4 Dimensioning process 234.1 Dimensioning of network elements and interfaces 234.2 Gb EDGE dimensioning based on EDAP 274.3 Gb EDGE dimensioning based on traffic figures 294.3.1 Traffic and quality inputs 294.3.2 Network capabilities 334.4 Outputs of Gb EDGE dimensioning 35

5 Gb traffic monitoring principles 37

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Contents

List of tables

Table 1. k-factor: short-term traffic distribution 28

Table 2. Overhead with different applications and protocols 31

Table 3. Gb interface connectivity for different PCU types 34



List of figures

Figure 1. EGPRS traffic multiplexed on the same physical connection as for GSMtraffic on the Ater interface 10

Figure 2. EGPRS traffic multiplexed on the same physical connection as for GSMtraffic on the Ater interface 11

Figure 3. GPRS traffic multiplexed on the same physical connection as for GSMtraffic on the Ater interface 12

Figure 4. GPRS traffic is concentrated and carried in a packet data network over theGb interface 13

Figure 5. GPRS data traffic is carried in dedicated 2 Mbit/s E1 PCM links 14

Figure 6. Available data capacity 20

Figure 7. Required data capacity 21

Figure 8. Available data capacity process 23

Figure 9. Required data capacity process 25

Figure 10. Peak margin correlation to the Gb link size 30

Figure 11. NS-VC load sharing 32

Figure 12. PCU connection 33

Figure 13. Triggers for optimisation 37

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List of figures

Summary of changes

Changes between document issues are cumulative. Therefore, the latestdocument issue contains all changes made to previous issues.

Changes made between issues 4-0 and 3-1

Updated the Gb EDGE dimensioning process with the new computationmethod.


Changes made between issues 3-1 and 3-0 lists the changes made to thedocument after the Nokia GSM/EDGE BSS, Rel. BSS12, SystemDocumentation pilot release. The following changes have been made:

. Table Gb interface connectivity for different PCU types has beenupdated in the Network capabilities section in chapter Inputs for GbEDGE dimensioning.


The document has been restructured for better usability and the focus ismore on the actual dimensioning process. The following changes havebeen made:

. Chapter EDGE dimensioning has been renamed as Planningprocess. The dimensioning strategy information has been moved tochapter Key strategies for EDGE dimensioning and an overview ofthe dimensioning steps has been moved to chapter Dimensioning ofnetwork elements and interface and the content has been updated.

. All steps in the dimensioning process are now under the mainchapter Dimensioning process.

. The impact of the used transport technology (Gb over IP or Gb overframe relay) on PCU output and bandwidth has been added to theTransport technologies section in chapter Gb EDGE dimensioning.

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Summary of changes

. Calculations for trunk line dimensioning has been added to the Gbover IP section in chapter Gb EDGE dimensioning.

. A check for peak throughput has been added to chapter Gb EDGEdimensioning process and to the final step in all examples in chapterExample cases of Gb EDGE dimensioning.

. Gb over IP information has been added to table The capability of theGb interface for different PCU types in chapter Inputs for Gb EDGEdimensioning. In addition, information on related software has beenremoved because their effect on dimensioning has been taken intoconsideration in earlier dimensioning phases.

. Chapter Examples of Gb EDGE dimensioning has been removed. Adimensioning example is now included in the BSC EDGEDimensioning document, in chapter Example of BSS connectivitydimensioning.

. Chapter Traffic monitoring principles has been moved to the EDGEand GPRS Key Performance Indicators document.



1 Gb EDGE dimensioning

These guidelines provide information on dimensioning the Gb interface forEDGE into an existing GSM network.

The aim is to ensure that the Gb link is large enough to handle the shortterm peak traffic of any single EDAP. In addition to this, the target is toestimate that the Gb link is large enough to support simultaneous traffic ofseveral EDAPs. This is highly dependent on the traffic distribution.

The EDGE dimensioning guidelines in the BSS system documentation setcover BTS, Abis, BSC, and Gb dimensioning and some parts of pre-planning. An example of BSS connectivity dimensioning is included in theBSC EDGE Dimensioning document.

Gb dimensioning results in specific outputs that are used as input in thenext dimensioning phase, SGSN EDGE dimensioning.

Transport technologies

In the Gb interface, two different transport technologies can be used: Gbover frame relay or Gb over IP. Gb over IP has a higher overhead than Gbover frame relay. This has an effect on bandwidth usage.

The transmission solution for the Gb interface can be implemented indifferent ways. There is no single correct solution that could be used inevery planning case. The optimum transmission solution is case specificand depends on the availability and cost of alternative transmissionsolutions and on the existing network infrastructure of the operator.

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Gb EDGE dimensioning

1.1 Gb over frame relay

GSM voice traffic is carried between the BTSs and the MSCs through theAbis (BTS to BSC) and Ater (BSC to transcoders) interfaces. The physicallayers of the Abis and Ater interfaces are based on the ITU-Trecommendations G.703/G.704, where traffic is carried in E1 PCM frames.The bit rate of one bearer channel is N x 64 kbps, where N is 1-31 (ETSI)and 1-24 (ANSI).

Voice and data multiplexed

Voice and data traffic can be multiplexed on the same transmission linksthat are used for GSM voice traffic on the Ater interface (see figure EGPRStraffic multiplexed on the same physical connection as for GSM traffic onthe Ater interface). At the BSC, some of the 64 kbps PCM timeslots arepermanently reserved for GPRS traffic and some for GSM traffic. EGPRSand GSM traffic are transferred together to the digital cross-connectiondevice (for example, DN2) residing at the MSC/SGSN site. In the digitalcross-connection device, the EGPRS and GSM traffic are separated sothat the EGPRS traffic is carried in dedicated E1/T1 links to the SGSN.

Figure 1. EGPRS traffic multiplexed on the same physical connection as forGSM traffic on the Ater interface

Gb

Ethernet Switch

GGSN #1GGSN #2

MSC

SGSN

Transcoders

MSC/SGSN

BSC

BSC

BSCAbis

FrameRelayGb-Int.

2 Mbit/s PCMAter + Frame Relay

MUX



Voice and data separated in the transcoder

EGPRS traffic is multiplexed into the same transmission links that are usedfor GSM voice traffic on the Ater interface (see figure EGPRS trafficmultiplexed on the same physical connection as for GSM traffic on the Aterinterface). In the transcoder, the EGPRS and GSM traffic are separated sothat 64 kbps frame relay traffic timeslots are through-connected to thededicated E1 links, which are connected to the SGSN.

Figure 2. EGPRS traffic multiplexed on the same physical connection as forGSM traffic on the Ater interface

Channels going through the transcoders and MSC

EGPRS traffic is multiplexed into the same transmission links that are usedfor GSM voice traffic on the Ater interface. In the transcoder, channels thatgo through the transcoder are created and the EGPRS data traffic isforwarded to the MSC switching matrix. At the MSC, the 64 kbps virtualchannels (VCs) are multiplexed into one or more ET2E cards, which areconnected to the SGSN.

Gb

Ethernet Switch

GGSN #1GGSN #2

MSC

SGSN

Transcoders

MSC/SGSN

BSC

BSC

BSCAbis

FrameRelayGb-Int.

2 Mbit/s PCMAter + Frame Relay

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Figure 3. GPRS traffic multiplexed on the same physical connection as forGSM traffic on the Ater interface

Traffic streams concentrated in the FR switch

To use the capacity more efficiently or cost effectively, it is possible toconcentrate the traffic streams coming from several BSCs and packetcontrol units (PCUs) into one aggregate line towards the SGSN.

This concentrated traffic can be multiplexed into the same physical linkthat is used for GSM traffic on the Ater interface. Alternatively, it can becarried over to the SGSN site in a compatible packet data network (PDN)(see figure GPRS traffic is concentrated and carried in a packet datanetwork over the Gb interface).

There are several solutions that can be used to implement this method.Again, there is no single correct solution that works with each planningcase. However, there are a few basic rules for the implementation anddimensioning. The data network used for transmission does notnecessarily have to be a frame relay network. The frame relay traffic canbe run over different kinds of networks, such as ATM. At either end of theconnection, a frame relay switch or similar equipment is required for theconnection to the packet data network. The switches must be able toconnect to the E1/T1 link coming from the BSC with a physical interface,such as G.703, and to adapt to the PDN access point interface. In addition,the switch must be able to do the correct protocol conversion (for example,convert FR into ATM, and vice versa).

Gb Interface

Ethernet Switch

GGSN #1GGSN #2

MSC

SGSN

Transcoders

MSC/SGSN site

BSC

BSC

BSCAbis

2M PCMFrameRelay



Figure 4. GPRS traffic is concentrated and carried in a packet data networkover the Gb interface

Dedicated 2 Mbit/s E1 PCM links

In this transmission option, one or more (a maximum of eight per BSC) E1/T1 PCM links per BSC are dedicated only for GPRS data traffic (see figureGPRS data traffic is carried in dedicated 2 Mbit/s E1 PCM links). If, forexample, 15 or more 64 kbps Gb interfaces are required for one BSC, it isreasonable to dedicate the needed amount of 2 Mbit/s E1 interfaces onlyfor data traffic. If, for example, 18 PCM timeslots are needed for a BSC,one E1 PCM interface of an ET2E card at the BSC and SGSN could bededicated only for GPRS data traffic.

Gb Interface

Ethernet Switch

GGSN #1GGSN #2

MSC

SGSN

Transcoders

BSC

BSC

BSC

MSC/SGSN siteAbis

FR Switch

FR Switch

Packet DataNetwork

(FR, ATM, etc.)

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Figure 5. GPRS data traffic is carried in dedicated 2 Mbit/s E1 PCM links

For more information on Gb over frame relay, see chapter Nokia GPRS,section Frame Relay and Gb Interface in (E)GPRS System FeatureDescription.

1.2 Gb over IP

With Gb over IP, it is possible to configure the subnetwork of the Gbinterface so that the subnetwork is IP-based and the physical layer isEthernet.

When Gb over IP is used, the data from all PCUs and the data from theelements that use IP traffic in other BSCs can be combined with switchesor routers into one or two trunk lines. The dimensioning of the trunk linecan be based on one of the following calculations:

. The capacity of the trunk line = PCU + the total amount of traffic ofthe other elements

. The capacity of the trunk line = the amount of traffic of the largesttraffic-generating element

Often the capacity of the trunk line is a combination of the above-mentioned calculations.

For more information on Gb over IP, see Gb over IP System FeatureDescription.

Gb Interface

Ethernet Switch

GGSN #1GGSN #2

MSC

SGSN

Transcoders

BSC

BSC

BSC

MSC/SGSN siteAbis

2 M PCM E1 links

Frame Relay



Related topics

. document BTS EDGE Dimensioning

. document Abis EDGE Dimensioning

. document BSC EDGE Dimensioning

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2 Planning process

Dimensioning is the part of network planning that produces a master planindicating the selected network architecture and the number of networknodes and communication links required during the roll-out of the network.

The following phases are included in the network planning process:

. dimensioning

. pre-planning

. detailed planning

. implementation

. optimisation

Network dimensioning is done by creating a traffic model of the networkand selecting the equipment to support it. Dimensioning takes into accountthe available equipment specifications, business plans, site availability andtype, quality of service (QoS) requirements, and charging cases.

The EDGE dimensioning guidelines in the BSS system documentation setcover BTS, Abis, BSC, Gb, and SGSN dimensioning and some parts ofpre-planning.

These guidelines focus on dimensioning. Network optimisation is notincluded in the guidelines.

The dimensioning guidelines consist of both hardware dimensioning andsoftware dimensioning. Hardware dimensioning defines how many traffictype and traffic volume dependent hardware units are needed in the BTS,BSC, and SGSN to support the targeted traffic and service performance.Software dimensioning defines the key system settings associated withtraffic dependent units. You can modify the existing configuration once the

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Planning process

amount of needed traffic dependent hardware and the associated softwaresettings have been defined. If necessary, you can place an order foradditional products and licences, based on the agreed standardconfigurations.

Nokia Siemens Networks has a wide range of services and trainingavailable to support all phases of system planning, deployment, andoptimisation. Contact your local Nokia Siemens Networks representativefor details.



3 Key strategies for EDGE dimensioning

The dimensioning of a network can be based on two different approaches:

. available data capacity

. required data capacity

The dimensioning strategy must be selected before the BTS dimensioningbegins.

Available data capacity

Available data capacity strategy is used when you want to introduce EDGEto an existing network. Dimensioning determines how much traffic isavailable through the current system. The dimensioning input is a pre-defined system configuration. The dimensioning output is the availabletraffic volume with a defined performance level. Alternatively, you cancalculate available capacities for different alternative configurations.

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Key strategies for EDGE dimensioning

Figure 6. Available data capacity

Required data capacity

Required data capacity strategy is used when you want to design anetwork that supports the defined amount of traffic and targetedperformance level. The dimensioning inputs are traffic volume, type, andperformance requirements. The dimensioning output is the neededamount of traffic dependent hardware and the associated softwareconfigurations.

All current resources in a cell

Average voice trafficresource usage

Averageavailableresources

Input information:

Current network configuration

Current equipment’sEDGE capability

Current network’s voiceperformance

Current network’s radioconditions (C/N, C/I)

Planned EDGE data resourcesare used for voice trafficwhen needed


EDGE data



Figure 7. Required data capacity

Input information:

Current network configuration

Current equipment’sEDGE capability

Current network’s voiceperformance

Current network’s radioconditions (C/N, C/I)

Required EDGE capacity

Required EDGE performance

Planned EDGE dataresources may be fully orare at least partiallydedicated to data traffic.Dedicated resources are notused for voice traffic.

All current resources in a cell


Average availableresources


EDGE data

Shared Dedicated

Required EDGE Capacity

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Key strategies for EDGE dimensioning

4 Dimensioning process

4.1 Dimensioning of network elements and interfaces

The dimensioning of GSM EDGE network elements and interfaces isproposed to be done as described in this section. Depending on thedimensioning strategy, you can use either the available capacity strategyor the required capacity strategy. At first, the input for BTS dimensioninghas to be agreed. Once this has been done, the output of each element orinterface serves as the input for the next phase.

Available data capacity strategy

The dimensioning process of the available data strategy is illustrated infigure Available data capacity process.

Figure 8. Available data capacity process

1. Estimate the average available data capacity andthroughput.

2. Use existing TRX hardware capacity.3.-6. Dimension the rest of the elements according to the

available capacity estimate done in step 1.

TSL

TRX

Cell

BTS

PCU

BSC

Basic unit

2G SGSNGbAbis

1

2

3 4 5 6

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Dimensioning process

The available data capacity strategy consists of the following steps:

1. Definition of the input information. Select the data deployment strategy.. Calculate the existing traffic load.. Review the hardware/software capability.. Define the BTS/transceiver (TRX) configuration.. Simulate the coverage and interference performance (carrier-

to-noise ratio (C/N), carrier-to-interference ratio (C/I)).

2. BTS dimensioning. Estimate throughput/ radio timeslot (RTSL).. Calculate the available capacity/number of RTSLs based on

the circuit-switched (CS) traffic needs.. Verify the dimensioning outcome.

The dimensioning process results in throughput/RTSL, territory size/BTS, guaranteed/not guaranteed throughput, RTSL configuration ofTRXs, numbers of TRXs per cell, and the simulation results.

3. Abis dimensioning. Use the output of BTS dimensioning as the input.. Define the EGPRS dynamic Abis pool (EDAP) size.

The dimensioning process results in the size of each EDAP.

4. BSC dimensioning. Use the output of BTS and Abis dimensioning as the input.. Verify the amount of packet control units (PCUs).. Verify the number of BSC signalling units (BCSU) and

Exchange Terminals (ETs).. Verify the Gb requirements for BSC dimensioning.. Define the BSC configuration.. Perform a use check.

The dimensioning process results in the number and type of BSCs,the number and type of PCUs, and the number and size of Gbinterfaces.

5. Gb dimensioning. Use the output of BTS and BSC dimensioning as the input.. Calculate the amount of payload.. Verify the number of network service elements (NSEs) and

BCSUs.. Estimate the need for redundant links.. Evaluate the results.



The dimensioning process results in the number of timeslots,number of payloads, number of network service virtual connections(NS-VCs), and number of frame relay timeslots/data transfercapacity.

6. SGSN dimensioning. Use the output of BTS and Gb dimensioning as the input.. Define the maximum number of attached subscribers and

packet data protocol (PDP) contexts to be expected in therouting area (RA) served by the SGSN.

. Calculate the amount of total data payload (generated usertraffic) during a busy hour.

. Verify the needed basic units/SGSN according to thepreviously calculated generated traffic and the expectedsubscribers served in the area.

. Check all other restrictions, especially the expected mobilityprofiles of the users versus the dynamic capacity of the SGSN.

The dimensioning process results in the number of packetprocessing units (PAPUs) and signalling and mobility managementunits (SMMUs).

Required data capacity strategy

The dimensioning process of the required data strategy is illustrated infigure Required data capacity process.

Figure 9. Required data capacity process

1. Calculate the required TSL count based on required datacapacity and throughput.

2. Calculate the required amount of TRX hardware.3.-6. Dimension the rest of the elements according to the

required capacity calculation done in step 1.

TSL

TRX

Cell

BTS

PCU

BSC

Basic unit

2G SGSNGbAbis

1

2

3 4 5 6

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The required data capacity strategy consists of the following steps:

1. Definition of the input information. Select the data deployment strategy.. Determine the targeted traffic capacity.. Estimate the traffic mix.. Review the hardware/software capability.. Define the BTS/TRX configuration.. Simulate the coverage and interference performance (C/N, C/

I).

2. BTS dimensioning. Calculate the required throughput.. Estimate throughput/RTSL.. Calculate the required number of RTSLs.. Verify the dimensioning outcome.

The dimensioning process results in throughput/RTSL, territory size/BTS, guaranteed/not guaranteed throughput, TSL configuration ofTRXs, number of TRXs/cell, and the simulation results.

3. Abis dimensioning. Use the output of BTS dimensioning as the input.. Define the EDAP size.

The dimensioning process results in the size of each EDAP.

4. BSC dimensioning. Use the output of BTS and Abis dimensioning as the input.. Calculate the needed amount of PCUs.. Calculate the number of BCSUs and ETs.. Calculate the Gb requirements for BSC dimensioning.. Define the BSC configuration.. Perform a use check.

The dimensioning process results in the number and type of BSCs,the number and type of PCUs, and the number and size of Gbinterfaces.

5. Gb dimensioning. Use the output of BTS and BSC dimensioning as the input.. Calculate the amount of payload.. Calculate the required number of NSEs and BCSUs.. Estimate the need for redundant links.. Evaluate the results.



The dimensioning process results in the number of timeslots, thenumber payloads, the number of NS-VCs, and the number of framerelay timeslots/data transfer capacity.

6. SGSN dimensioning. Use the output of BTS and Gb dimensioning as the input.. Define the required number of attached subscribers and PDP

contexts to be expected in the RA served by the SGSN.. Calculate the amount of total data payload (generated user

traffic) during a busy hour.. Calculate the needed basic units/SGSN according to the

previously calculated generated traffic and the expectedsubscribers served in the area.

. Check all other restrictions, especially the expected mobilityprofiles of the users versus the dynamic capacity of the SGSN.

The dimensioning process results in the number of PAPUs andSMMUs.

4.2 Gb EDGE dimensioning based on EDAP

The dimensioning of Gb for EGPRS traffic is a straightforward process.

Each PCU has typically one Gb link towards the SGSN. In case ofredundant Gb, two independent links are needed. The outcome of the Gblink dimensioning process is the average size of the Gb link to carry thedata traffic forecast. This part of the process affects SGSN dimensioningand should be conducted together with PS Core planning. The Gb shouldbe capable of supporting the instantaneous data traffic being carried by allcells connected to a particular PCU. If there is insufficient capacity theeffective user rate at the radio cell will be reduced.

The following equation is used to calculate the average Gb link size (=Frame Relay Bearer Channel capacity).

Average Gb size = k * Average EDAP size for that network area.

The k-factor is based on the estimate of the short term traffic distribution. Ifno specific information about the distribution is available, it isrecommended to use the default values.

The table below gives the k-values. The theoretical minimum k-value(1.25) is assuming that the short term traffic is totally unequal, meaningthat when one EDAP is full of traffic the others within the same PCU haveno traffic. The theoretical maximum k-value is the number of EDAPs

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allocated into one PCU. This assumes that all the EDAPs are heavilyloaded at the same short term period and the Gb link is supposed to carrysuch traffic without additional delays. The %-value in the table indicatesthe portion of traffic in the second most loaded EDAP when the mostloaded EDAP is full of traffic. And in general it indicates the portion of trafficin ith loaded EDAP comaper to (i-1)th loaded EDAP. In reality, some delayis allowed during heavy simultaneous short term traffic bursts and thus it isassumed that k-values greater than 2 are rare.

Table 1. k-factor: short-term traffic distribution

Unequal (low likelihood of heavysimultaneous short term traffic)

Default Equal (high likelihood of heavysimultaneous short term traffic)

30% 50% 70%

1.4 2 3

During the planning phase, when individual EDAPs are associated toPCUs, more accurate values for individual Gb links are calculated takinginto account the usage of individual E1/T1 links. To make it easier toconsider other than recommended k-values some impact calculations isdone. selections.

Upgrading existing Frame Relay based links into IP based links

The Gb link traffic is measured for periods long enough to contain at leastweekly behavior of the mobile users. The highest sum of hourly Gb linktraffic figures over the links, which are going to share the planned WANconnection is taken as a base traffic. This base traffic is corrected usingtraffic growth estimate. The individual WAN link load by the corrected basetraffic and possible other traffic shall not exceed 70%. The measured trafficcontains the Gb protocol overhead and thus the overhead calculation isnot required.

Dimensioning Gb over IP

Typically, there are some estimates available for the total traffic volumesbetween the SGSN and the BSC site. The individual cell level figures aretypically given for cell level busy hour (BH). Due to the fact that the celllevel BHs do not occur at the same time, the sum of cell level BH couldlead to over dimensioning. The theoretical maximum Gb traffic is 2Mbps *the number of logical PCUs. The minimum practical WAN capacity for Gbis 2 * E1/T1 to support redundancy. It is rare to have peak PS traffic on allPCUs at the same time and thus the practical estimate for required WANcapacity for Gb traffic is 10% to 40% of the theoretical maximum, however,at least the 2 * E1/T1.



4.3 Gb EDGE dimensioning based on traffic figures

4.3.1 Traffic and quality inputs

Data volume

The basic dimensioning of the Gb interface depends mainly on EGPRStraffic. Because of very different coding schemes and data rates, it isextremely relevant to know whether the traffic is GPRS or EDGE.Therefore, the main decision needed for Gb dimensioning is the amount ofpayload used, on average, for EGPRS traffic during a busy hour and thedeviation of the traffic between the peak and minimum values (this alsoprovides the difference between the peak and average values).

Data volume per BSC can be calculated (or estimated) as the total datavolume per BSC or based on subscriber information. One option is toestimate the total data volume going through a BSC during a busy hour,based on the available average throughput for EGPRS enabled timeslotsin the BSC. A more accurate option is to use traffic monitoring for theflawless calculation of peak traffic during the busiest moment of a busyhour.

Calculating traffic using subscriber information is more complicated. Firstof all, the total number of subscribers must be known (or the data userpenetration value). Then, a user data amount per busy hour has to beestimated as a total value or based on assumptions of data usage (WWW,FTP, e-mail, and so on).

EGPRS best effort user information (example values, headers included):

. 70% of the data users

. one e-mail (5 kB)

. three WWW pages (30 kB)

. one MMS (30 kB)

. a total of 65 kB per busy hour (BH) = 520 kbit/BH

EGPRS streaming user information:

. 10% of the of the data users

. one e-mail (5 kB)

. two WWW pages (20 kB)

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. three minutes streaming (~ 50 kbps → 9 Mb ~ 1 MB)

. a total of 9.2 Mbit/BH

To make sure that the Gb link is not the bottleneck for EGPRS usage, alsothe peak margin should be taken into consideration. The peak margin ofthe data volume can deviate a lot depending on, for example, the amountof data volume, different coding schemes, throughput rates, and offeredservices. The smaller the size of the Gb link, the bigger its effect on asingle user. In the examples, a 10% peak traffic margin is used. FigurePeak margin correlation to the Gb link size shows an example of how peakmargins correlate to link capacity.

Figure 10. Peak margin correlation to the Gb link size

The safety margin in the Gb link is 25% in all examples. The safety marginis used to avoid reaching 100% of the PCM usage situations that cancause several problems, such as the rejection of service and decreasedquality. The usage percentage operates as a buffer, so that small changesin user penetration or data usage do not require redimensioning of the Gbinterface.

1009080706050403020100

100

300

500

700

900

1100

1300

1500

1700

1900

Peak margin % example

Peakmargin%

GPRSEDGE

Gb link size



Gb overhead

Usually, the Gb traffic per user value is taken during a busy hour. Theaverage packet size of 512 bytes, including the IP header, isrecommended.

In addition to the length of the IP packets also the overheads varyaccording to the different application and protocols. A smaller packet sizeresults in a larger overhead percentage.

Table 2. Overhead with different applications and protocols

Configuration Layers Userdata +headers,minimum

Userdata+ headersmaximum

Min.%

Max.%

Frame relay SNDCP+LLC+BSSGPRel'4+NS+FR

512+3+6+12+4+6 = 543

512+4+40+63+4+6 =629

Min.%

Max. %

Gb over IP(IPv4/IPv6)

SNDC +LLC+BSSGPRel'4+NS+UDP+IP

512+3+6+12+4+8+ 20/40 =565/585

512+4+40+63+4+8+20/40 =656/676

10.4 27.1

USERPACKET = IPHEADER + USERDATA

For the overhead (OH), it is recommended to take the average of theminimum and maximum overhead percentages to obtain a more realisticfigure. An overhead of 14.5% has been used in the examples with Gb overframe relay. An overhead of 18.8% has been used in examples with Gbover IP.

Percentage OH = GbOH/USERPACKETSIZE

[{(OHmax/packetsize) + (OHmin/packetsize)}/2] %

Frame relay: [{(117/512) + (31/512)}/2] % = 14.5%

Gb over IP, IPv4: [{(139/512) + (53/512)}/2] % = 18.8%

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Redundancy and load sharing

The need for redundancy in the link depends on the offered services andthe market needs. When the payload from the BSC exceeds the capacityof transmission (one PCM), it is recommended to perform dimensioning ofthe Gb interface so that it also supports redundancy and load sharing. Thismeans that one network service entity (NSE) is divided into two or morenetwork service virtual connections (NS-VCs): the NS-VCs are divided intoseparate transmissions.

Frame relay load sharing is supported in 2G SGSN. It allows the sendingof traffic above the committed information rate (CIR) on another NS-VC onthe same bearer channel. The pacet control unit (PCU) decides the loadsharing of the NS-VCs in uplink for the time one TBF is allocated. The PCUdecision is based on the temporary logical link identity (TLLI), which in turnis allocated by the SGSN.

Figure 11. NS-VC load sharing

PCU 1NSE 1NSVC 1

BSC

BSC

PCU 1NSE 1NSVC 1NSVC 2

SGSN

NSVC_2

NSE1

NSVC_1

SGSN

NSE1

NSVC_1

Bearer CH. 1

E1 E1

Bearer CH. 1

E1 E1

Bearer CH. 2



If the NS-VC is 128 kbps and both NS-VCs have traffic of 117 kbps, thereis no room for a new subscriber demanding 22 kbps. One big NS-VC of256 kbps and 234 kbps of load can take one more 22 kbps subscriber.One NS-VC of double capacity is more efficient than two small ones.Therefore, it is recommend to keep the NS-VC links as big as possible.

4.3.2 Network capabilities

BSC type and capacity (the number of PCUs)

In Gb dimensioning, the PCU capacity may be the limiting factor in theBSC.

Figure 12. PCU connection

Usually, the performance of different BSC hardware/software releases iscompared by using the maximum values of the transceivers (TRXs)supported by a BSC or throughput (kbps) delivered through a BSC. Themaximum number of TRXs that can be connected to a BSC depends onthe type of the BSC:

. BSCi: 512

. BSC2i: 512

. BSC3i 660: 660

. BSC3i 1000: 1000

. BSC3i 2000: 2000

SGSN

ETs

ETs

ET

PCU

GSWB

Packets in FR

AbisGb

FR: bearer channel + optionalload sharing redundant bearer (2 Mbit/s)

Packets inTRAU frames

4 Mbit/s internal PCM256 channels

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Table Gb interface connectivity for different PCU types shows the PCUcapability for the Gb interface towards the SGSN. The table shows thephysical PCUs. Note that there are two logical PCUs in PCU-B and PCU-2D.

Table 3. Gb interface connectivity for different PCU types

PCU type BSC type Gb over FR

PCU BSCi, BSC2i 32 x 64 kbit/s

PCU-S BSCi, BSC2i 32 x 64 kbit/s

PCU-T BSCi, BSC2i 32 x 64 kbit/s

PCU-B BSC3i 2 x 32 x 64 kbit/s

PCU2-U BSC2i 32 x 64 kbit/s

PCU2-D BSC3i 2 x 32 x 64 kbit/s

Note

The maximum rate of one frame relay bearer channel is 31 x 64k (ETSI)or 24 x 64k (ANSI). If there is more than one bearer in a logical PCU,their maximum summary rate is 32 x 64k. In the ANSI environment, theGb interface must be split between two physical ET ports to support themaximum PCU capacity for Gb over FR.

A PCU can be connected to the SGSN either via the Gb over frame relayor Gb over IP interface but not via both interfaces simultaneously. The IPinterface for a PCU can be IPv4 or IPv6 but not both.

The PCU capacity of the Abis channels, BTS, TRX, and EGPRS dynamicAbis pool (EDAP) under the PCU cannot be exceeded. For moreinformation, see the BSC EDGE Dimensioning document.

SGSN capacity

The packet processing capacity depends on various factors, such as thepacket length ciphering, use of data compression, and the selected LLCmode. Therefore the actual SGSN data rate may vary depending on thefactors mentioned above.

The SGSN should be able to handle all traffic from the Gb interface.



Gb connection type

The selection of the connection type depends on the hardware andsoftware versions in the SGSN and BSC and on the available transmissionalternatives in the backbone network.

The capacity of the Gb interface remains the same in BSC, regardless ofwhether IP or FR is used as the transport technology.

4.4 Outputs of Gb EDGE dimensioning

Gb dimensioning results in specific outputs. These outputs are used asinput in the next dimensioning phase, SGSN EDGE dimensioning.

Gb dimensioning outputs:

. total number of timeslots in the Gb interface

. total number payloads in the Gb interface

. total number of network service virtual connections (NS-VCs)

. total number of needed frame relay timeslots or needed data transfercapacity

The values of these outputs should be analysed. Based on the analysis, adecision about implementing the Gb interface or redimensioning theinterface should be made. As described earlier, the dimensioning is oftenan iterative process, and redoing the dimensioning calculations may beneeded if the output values are not acceptable.

Possible triggers for redimensioning:

. too many timeslots per network service entity (NSE) / packet controlunit (PCU)

. too much Gb traffic per NSE/PCU

. if the payload is very low (for example, less than six timeslots or if afew simultaneous users can overload the Gb link) the peak trafficmargin should perhaps be higher.

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. If the payload is very low (for example, less than six timeslots) theremay be a need for redundancy.

. If the needed capacity is 16 timeslots or higher, it might be better (fortransmission reasons) to implement more NS-VCs so that two NS-VCs belonging to different NSEs are connected to the same PCM(NSE1 NS-VC 1 = timeslots 1-15, NSE 2 NS-VC 1 = timeslots 15-31)to save transmission costs.

The objective of the redimensioning should be to:

. optimise the number of PCUs to each BSC,

. re-estimate the traffic to avoid over dimensioning, and

. optimise the network, based on needs and transmission.



5 Gb traffic monitoring principles

The most important Gb traffic monitoring areas are the following.

. downlink Gb load

. total EGPRS uplink/downlink payload in BSC

. SGSN data (the amount of data passed in the uplink and downlinkdirection in the SGSN, and the resource usage)

. GPRS session management counters (PDP context relatedinformation)

Monitoring these measurements gives the operator an initial idea of howwell the current data traffic reflects the Gb dimensioning (including thethree main capacity restrictions) and whether there is a need toreconfigure the Gb capacity (see figure Triggers for optimisation).

Figure 13. Triggers for optimisation

Gb dimensioning

Triggers for redimensioning:-Too many TSLs per NSE/PCU-Too much Gb traffic per PCU-SGSN capacity-Transmission capacity exceeded

To do in redimensioning:-Optimise the number of PSCs-Re-estimate traffic to avoidover dimensioning-Optimise transmission

Configurations:-Total number and type of Gb-Total number and type of PCUs-Peak payload and services-SGSN limitations

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Gb traffic monitoring principles

For information on EDGE-related KPIs, see EDGE and GPRS KeyPerformance Indicators.



gb edge dimensioning

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