Combined GSM/UMTS mobile backhaul network

Deployment of UMTS networks and enhancement of GSM/GPRS with EDGE provides more efficient radio interface, thus enabling higher data speed and more capacity for voice. That evolution requires additional transmission capacity to transport extra traffic through mobile backhaul network from Base stations to Mobile Switching Centers. Moreover, UMTS specifies IP or ATM as a bearer, requiring upgrade of TDM based backhaul network used for GSM. Building separate mobile backhaul networks for UMTS and GSM is inefficient and expensive, especially since mobile operators expect UMTS to cannibalize the use of GSM network over time, leaving GSM backhaul capacity unused. This white paper defines a solution for a single mobile backhaul network that supports UMTS and GSM with GPRS and EDGE. The solution reduces mobile backhaul transmission capacity requirements by using advanced lossless compression method for GSM voice, traffic aggregation, and statistical multiplexing of voice and data traffic generated by both GSM and UMTS networks.

Combined GSM/UMTS mobile backhaul network

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Content 1 Introduction 1 2 Radio Access Network Backhaul 1 3 Backhaul Network Efficiency

Considerations 3 4 Improving Backhaul Network Efficiency 3 5 Further Optimizing GSM Backhaul

Bandwidth 4 6 Backhaul network options 5 7 Conclusion 7

1 INTRODUCTION

GSM mobile operators have already executed plans or have strong commitments to upgrade their GSM network with EDGE (Enhanced Data Rates for GSM Evolution) and/or to evolve to UMTS. The way forward for such cases is to gradually implement UMTS network architecture to build cost effective converged GSM/UMTS network.

Along with growth of subscriber base and increase of data traffic (EDGE and UMTS), ensuring sufficient network capacity is turning into an essential issue for network operators.

Most of mobile service providers lease transmission lines for their backhaul network, which creates major operational expenses.

GSM/GPRS backhaul transmission is based on a TDM circuit-switched technology, whereas UMTS deployment requires a new backhaul network based on IP or ATM.

This whitepaper describes evolution of GSM/GPRS backhaul transmission network to support EDGE and UMTS, and proposes optimized architecture, with a particular accent on lossless compression techniques for GSM backhaul network that provides bandwidth savings for further optimization of GSM mobile transmission network.

2 RADIO ACCESS NETWORK BACKHAUL

Radio Access Network (RAN) is a part of every cellular network that facilitates a connection between the end-user mobile station over air interface and the core network over a landline transmission network usually called – mobile backhaul (Figure 1).

Backhaul section

Cellular Network Architecture

CoreNetworkController

Base StationRAN

MobileStation

Figure 1. Backhaul transmission section in the cellular

network architecture

2.1 GSM/GPRS network

Backhaul transmission in the GSM networks includes transmission lines between GSM Core Network and Base Station System (BSS) network elements: the Base Station Controller (BSC) and the Base Transceiver Station (BTS).

Connection between the BTS and the BSC is denoted as Abis interface, while the connection between the BSC and the Core Network is denoted as A interface (Figure 2).

BSCGSMCore

Network

GSM Network

BTS

BTSBSS

Abis

Abis

A

Figure 2. Abis and A interfaces for backhaul transmission in the GSM network

The GSM evolution to 2,5G network (GSM phase 2+) is facilitated by providing the General Packet Radio Services (GPRS) system upgrade that allows packet-switched mobile data service, where all the data to be sent are broken down into several smaller data packets first. Those packets are then sent individually across the GPRS network and each of those packets

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can take a different route. At the target destination, the packets need to be reassembled. For that reason, new functionality has to be introduced into existing GSM network elements, especially for allowing efficient use of the air interface. Four coding schemes are introduced (CS1 to CS4), with capability that one user can occupy more than one timeslot or more than one user can be on a single timeslot.

Depending on coding scheme and number of concatenated time slots, a maximum theoretical data rate of 171.2 kbit/s can be achieved. The “Real life” experience is around 40 kbit/s, which is acceptable for Internet access and web browsing. To provide GPRS service, upgraded BSS network elements need to be connected with the GPRS backbone system, and that can be performed by allocating distinct time slots on existing A link (“nailed-up” connection) or by separate Gb interface (Figure 3). Both solutions put additional demand on backhaul capacity.

BSC

GSM/GPRS Network

BTS

BTSBSS

Abis

Abis

A

Gb

GSM/GPRSCore

Network

Figure 3. GSM/GPRS network evolution

2.2 Introduction of EDGE

Enhanced Data Rates for GSM Evolution (EDGE) is another step in GSM/GPRS evolution towards 3G mobile systems. EDGE introduces a new modulation technique known as 8-Phase Shift Keying (8-PSK) in order to support higher transmission data rates and increase the network capacity. Similarly like GPRS, these applications use the same GSM carrier bandwidth and timeslot structure. EDGE also shares the GPRS network elements. EGPRS provides packet data services using the GPRS architecture and the new EDGE modulation technique and coding schemes. Enabling it requires some hardware changes, as well as adaptations in the signaling structure on the BSS side. Nine modulation and coding schemes are defined for EDGE: MCS-1 to MCS-9. EGPRS can theoretically offer a maximum data rate of 473.6 kbit/s. Those data rates can provide more advanced services, such as real time video streaming or video conferencing. EDGE traffic connections are provided over Gb interface which need to be allocated on Abis interface, which will call for additional optimization of backhaul lines (Figure 4).

BSC

GSM/GPRS/EDGE Network

BTS

BTS

BSS

Abis/Gb

Abis/Gb

EDGETRX

EDGETRX

GSM/GPRSCore

Network

A

Gb

Figure 4. EDGE enhancement for GSM/GPRS network

2.3 UMTS Radio Access Network

UMTS Terrestrial Radio Access Network (UTRAN) represents an entirely new radio access for 3G networks. Based on the Wideband Code Division Multiple Access (WCDMA) radio access technique, UTRAN provides broader bandwidth and better spectrum efficiency allowing high data rates on the air interface.

UTRAN introduces the Node B, as equivalent to 2G BTS, and the Radio Network Controller (RNC), as equivalent to 2G BSC. The new Iu interface is introduced to connect RNC to the UMTS core network. In addition to UTRAN, GSM/EDGE form a Radio Access Network (GERAN) to provide the same Circuit Switched (CS) and Packet Switched (PS) services as UTRAN. For that purpose GERAN architecture must offer backward compatibility to GSM/GPRS using A and Gb interfaces (Figure 5). Release 99 (R99), which is designed to provide smooth transition from GSM, specifies ATM as a bearer for the Iu interfaces for carrying voice and data traffic, and that will require a whole new backhaul transmission network for UTRAN. Taking into consideration that most of today’s GSM networks will evolve to converged GSM/UMTS, requirements for cost-effective and efficient backhaul will be of greatest importance.

BSC

UMTSCore

Network

BTS

BTSGERAN

AAbis

RNCNode B

UTRANIubNode B

Abis

IubIu

Iu

Gb

Figure 5. GERAN and UTRAN in Release 5

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3 BACKHAUL NETWORK EFFICIENCY CONSIDERATIONS

GSM/GPRS mobile service providers incur extensive facilities and equipment expenses when they design and build networks to provide backhaul transmission for services and support of mobile communication.

In most cases mobile service providers lease multiple E1, E3 or STM-1 lines for backhaul transmission links from the Incumbent Local Exchange Carriers (ILEC), which account for a large part of their operational expenses. Continuous growth of voice and data traffic will result in a need for additional transmission capacity, thus increasing mobile operator’s leased lines costs.

3.1 Network Topology

Majority of mobile networks still use “star topology” architecture in their backhaul, where each BTS is connected directly to the BSC over dedicated E1 lines. Since each E1 line is dedicated to a particular BTS, each Transmitter-Receiver (TRX) of that BTS must be assigned to dedicated channels on a given E1. The Star topology offers easiest deployment and maintenance, especially in initial stages of network development, but in case of extensive growth of subscribers and installed network base this solution, however, increases costs of delivering mobile services. Vast majority of mobile networks are designed and dimensioned to comply with the requirements of worst-case conditions in which busy hour or peek traffic demand can arise simultaneously in all of the backhaul network elements. To meet capacity demands, the TRXs are assigned to each BTS according to busy hour call usage projections. The busier an area is, the more TRXs need to be installed in the BTSs to provide sufficient capacity, and the more backhaul bandwidth is required to transport increased traffic. There is also a coverage demand, so the mobile service providers will add additional BTSs to insure full service coverage for all areas (urban, sub-urban and rural), regardless of usage remaining low in some areas.

This will lead to situation where the traffic channels in one system are rarely simultaneously busy and the result is - inefficient allocation of limited network resources.

3.2 TDM technology as a bearer

Another issue that must be taken into account is inherent inefficiency of using traditional circuit-switched TDM technology for backhaul. Because of the static multiplexing nature of TDM connections, for each and every channel on the air interface, regardless if carrying traffic information or not, the appropriate channel resource must be allocated on the Abis interface. Thus the service providers have no

way of using extra capacity on seldom used lines to carry overflow traffic from busier lines. Consequently, while service providers can lease additional E1 lines for BTSs in high-traffic areas, they must absorb the cost of providing spare backhaul bandwidth for BTSs in low-use areas.

4 IMPROVING BACKHAUL NETWORK EFFICIENCY

It seems apparent that the future step toward 3G should include a combined GSM/EDGE/UMTS network where GSM/EDGE network is to fulfill coverage demands, whereas UMTS network is to ensure voice and high-speed data in densely populated areas. Release 99, as well as Releases 4 and 5, requires IP or ATM as a transport technology for UMTS. Therefore, one option is to build an entirely new backhaul besides the legacy TDM backhaul network. Of course, that option would be inefficient and expensive for mobile service providers. A better solution would include integration of 3G-backhaul elements into the existing 2G-backhaul, thus creating a single common, cost-effective 2G/3G backhaul network.

4.1 Combined 2G/3G ATM-based backhaul network

ATM as a bearer technology offers an integrated solution for voice and data, and guarantied QoS. ATM Adaptation Layer 2 (AAL2) is designed to increase efficiency when transporting delay-sensitive voice over an ATM network, AAL2 enables switches to fill ATM cells more quickly by multiplexing multiple voice calls into the same ATM cell. Data services such as Frame Relay (FR) are efficiently mapped into ATM Adaptation Layer 5 (AAL5), which is compliant with Frame Relay Forum-FRF.5 and FRF.8 specifications, while emulation of legacy TDM circuit switched voice and data services are supported by AAL1 Circuit Emulation Service (CES).

4.2 Inverse Multiplexing over ATM (IMA)

Mobile service providers usually lease E1 or E3 lines for their backhaul transmission. Because of a huge bandwidth gap between E1 (2 Mbit/s) and E3 (34 Mbit/s), operators requiring more bandwidth than a single E1 and less than the much more expensive E3, are faced with very expensive alternatives. IMA addresses this issue and provides a solution where the bundles of parallel physical E1 links are combined into a single logical connection. That offers an aggregate bandwidth. IMA is also a very robust, offering a great immunity to service interruptions. If one of its constituent E1 links fails, transmission of traffic will continue as long as at least one of its constituent links is operating. This will result in decrease of available bandwidth of all connections,

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however, an overall service availability and reliability in the backhaul network will increase.

4.3 Access Aggregation

ATM access devices, such as Lucent PacketStar (PSAX) multiservice media gateway, offers access aggregation for a large number of TDM E1 links to facilitate connection of Abis interfaces, from BTSs to BSCs and further to the MSC. Once Node Bs and RNCs are deployed, Iub and Iu can be connected to the same ATM aggregation devices. PSAX multiservice gateways are capable of utilizing existing TDM interfaces as a physical layer for ATM transmission links. In that way we get a common ATM-based backhaul (Figure 6). Usage of ATM over the same TDM interface produces a single aggregated physical channel that statistically allows use of less bandwidth than the sum of bandwidths necessary for each separate channel. Since it is highly unlikely all allocated channels will be active at the same time, service providers can benefit from ATM’s statistical multiplexing and can overbook their ATM links. This method is usually referred to as Oversubscription, and it considerably reduces a total bandwidth needs for all access interfaces (Abis, A, Gb, Iub, Iu)

BTS BTSNode B Node B

RNCBSC

2GMSC

3GMSC

SGSN

Abis AbisIub Iub

ATM PSAX

PSAX

PSAX

AbisA, Gb

IubIu

A Gb, Iu-ps

Iu-cs

Figure 6. Combined 2G/3G ATM-based backhaul network

4.4 Channel grooming

Further optimizations of the backhaul bandwidth can be performed by selectively transmitting only active E0 channels over a backhaul network. TDM E1 link normally has an amount of active and inactive E0 channels depending on usage hour. Regardless the inactivity of E0 channels, they still reserve backhaul bandwidth. PSAXs are capable of eliminating unused E0 channel from the backhaul transmission, and send only active E0s. This E0 grooming of only active connections can provide diversity effect for GSM operators and reduce the risk of individual E1 link failures, capable of affecting service.

5 FURTHER OPTIMIZING GSM BACKHAUL BANDWIDTH

Even though the GSM speech coding techniques already have low bit rates, there is still some space for further savings in the bandwidth. Another method to additionally save backhaul bandwidth, without compromising voice quality, is to suppress or remove redundant information from speech transmissions, on Abis and Ater interfaces.

5.1 Mapping of channels on Abis /Ater interface

The Abis/Ater interface utilises a TDM connection using standard E1 links with 32 E0 timeslots, (or channels), which are used to transport both traffic and signaling. Those 32 channels have data rate of 64 kbit/s each (E0), thus providing 2.048 Mbps overall throughput over E1 link. The GSM channels in are coded in different capacities where 8 and 16 kbit/s are the most-widely used types. In most cases, there is a 4:1 multiplexing on the Abis/Ater interface where four of 16 kbit/s sub-channels are mapped into one 64 kbit/s E0 channel.

As defined in the GSM standards, channel can carry traffic information or it can be idle. When the information is transferred on the Abis or Ater interface, it is transferred in frames with a fixed length of 320 bits (20 ms, 16 kbit/s). Those frames are called TRAU frames and they carry speech, data, signaling and associated control signals.

I broad terms, we can divide TRAU frames as: Frames for speech services - according to speech codecs used: Frames for Half and Full Rate (HR and FR), Enhanced Full Rate (EFR) and Adaptive Multi-Rate (AMR) speech, O&M Frames, Data Frames and Idle Speech Frames. In the case where FR, EFR or AMR is used for speech coding, 16 kbit/s traffic sub-channels, mapped on the Abis interface, would emerge. For Half Rate speech coding, the 8 kbit/s traffic channels are used. They are regularly matched in pairs to form one 16 kbit/s sub-channel which is then mapped on the Abis interface. (Figure 7)

TS0 TS1 TS2 TS31 ...

TS0 TS1 TS2 TS3

16 kbit/s FR, EFR or AMR channels

64 kbit/s E0 channelsTS2 Idle Channels

(64 kbit/s channels)

TS3

TS2 Idle Speech Frames (16 or 8 kbit/s channels)

Abis Ater

BTS BSC TRAU MSC4 x A

Figure 7. Mapping of TRAU frames and Idle Channels on the Abis

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5.2 GSM voice suppression methods

There are common patterns in Idle Channels and Idle Speech Frames that can be removed from speech transmissions over backhaul network. That will lead to best possible bandwidth reduction and greater trunk utilization, resulting in more operational savings for wireless carriers. Lucent PSAX is the only multiservice media gateway, available in the market, utilizing two suppression techniques:

1. GSM Idle Channel Suppression and

2. GSM Idle Speech Frame Suppression.

5.2.1 GSM Idle Channel Suppression

Whenever no traffic is received from the radio interface (e.g. channel is not allocated to a call, frame stealing applies, layer 2 fill frames are received, etc.), a channel is considered idle. In that case, the “Idle Patterns” or the “Idle Channels” are generated on corresponding channels on the Abis interface. PSAX GSM Idle Channel suppression feature detects “Idle Channels" and removes them from the data stream to be transported over Abis/Ater interface.

Using the PSAX, transmission in the RAN will be handled by the ATM cell technology so the GSM traffic has to be sent over ATM links between PSAX systems. ATM statistical multiplexing extended with suppression feature can ensure substantial backhaul bandwidth savings, which can go up to 80% (in the ideal case) allowing for service providers to minimize the number of leased E1 in the backhaul network.

5.2.2 GSM Idle Speech Frame Suppression

For a regular voice call it is common for one person to speak for 50% of the time, while the rest of 50% of the time there is a silence while listening to what the other person is saying. Speech information can be carried over FR, EFR or AMR speech frames, but if no speech is received (e.g. during periods of silence on calls), the Idle Speech Frames will be generated and transported instead of the FR or EFR speech frames. Those Idle Speech Frames don’t carry useful information but still allocate the same bandwidth as FR or EFR speech frames. PSAX advanced logic detects Idle Speech Frames and suppresses transmission of these frames over the ATM trunk and regenerates them at the far end of the ATM network to assemble Abis interface. This provides an additional transmission bandwidth savings.

5.2.3 Transmission bandwidth reduction considerations

The actual savings realized with these two supression features depend on the network topology, the number of E1 interfaces strapped together, type of base station (dense urban, urban, rural, road) types of

traffic on the network and voice activity of users that influences the percentage of idle patterns.

In addition to lossless compression of Abis interface, GSM Idle channel and Idle Speech frame suppression methods have important “side effect”. Abis traffic is not statically allocated to time slots on TDM interface – it is transported as ATM AAL2 Variable Bit Rate (VBR) over the backhaul network. This enables bandwidth reduction because of statistical multiplexing.

Gains of statistical multiplexing are especially significant in case of traffic aggregation for the whole region. Single region typically has a mix of dense urban, urban, rural and road base stations, with different daily and yearly traffic distribution, busy hour at differet times of a day and different overall base station load.

Practical results for aggregation of regional traffic shows that PSAX achieves reduction of transmission bandwidth of average 40% compared to the best possible optimization with TDM.

6 BACKHAUL NETWORK OPTIONS

GSM mobile operators which have acquired licenses for UMTS have to decide which way to go in evolution of their backhaul network. The decision they make is probably one of the most important ones, second only to the decision to take a part in the 3G game itself. There are several ways to go and failing to chose the right one might cause tremendous damage to their backhaul network efficiency and subsequently to their revenues. There are four main options whose advantages and disadvantages are to be presented:

1. Direct E1 links for Iub

2. Combined GSM/UMTS backhaul with fractional E1s

3. “Standard” ATM combined GSM/UMTS backhaul

4. PSAX combined GSM/UMTS backhaul

6.1 Direct E1 links for Iub

This approach is probably the easiest to deploy but it hides the greatest possible treat called: “GSM Cannibalization”. Newer and more attractive UMTS services will gradually take over GSM services and successively the UMTS backhaul traffic will increase, leaving the GSM backhaul with less traffic and more inefficient utilization of scarce transmission resources.

Moreover, such approach requires much more interface capacity on RNC that incures unnecessary capital expenses. With direct E1 connections, capacity of RNC interface is equal to sum of all capacities of

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NodeB interfaces. Taking into account that typical network load in busy hours is 40% of overall installed Base station capacity (because of different traffic patterns in dense urban, urban, rural and road Base stations), RNC in this scenario has 60% unused interface capacity.

UMTS

DXCBSC

UMTS

UMTS

GSM

GSM

GSM

DXC

RNC

Multiple E1s

nxE1

E1E1

E1

UMTS

GSM

E1

nxE1 nxE1

nxE1

nxE1

Advantages:

� Existing GSM backhaul left intact

� Easy to deploy

Disadvantages:

� Extreme waste of bandwidth

� GSM Cannibalisation; No capacity benefits from customer’s migration from GSM to UMTS - freed capacity in GSM backhaul cannot be re-used for growing UMTS network

� Waste on RNC interface (typically 60%)

6.2 Combined GSM/UMTS backhaul with fractional E1s

This approach introduces ATM switches for carrying both GSM and UMTS traffic over fractional E1 links from the locations with lower traffic utilization. For higher capacity GSM and UMTS base stations, separate E1 links are used.

DXCBSC

GSM

GSM

GSM

DXC

RNC

Multiple E1s

E1E1

E1E1

E1

nxE1

UMTS

GSM

E1

UMTS

UMTS GSM

GSM UMTS

E1

GSM

ATM

STM-1

Fractions of E1 used for GSM

and UMTS

nxE1

Advantages:

� Unused fractions of E1s on GSM connections are provisioned for UMTS traffic

� STM-1 ATM interface on RNC can be used to optimize RNC interface capacity

Disadvantages:

� Not applicable for dense populated areas with high capacity GSM and UMTS base stations

� With growth of UMTS traffic fractional E1 capacity becomes insufficient

� No benefits from statistical multiplexing

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6.3 “Standard” ATM combined GSM/UMTS backhaul

All GSM and UMTS traffic is transported over a single ATM-based backhaul where GSM traffic is transported via AAL1 layer as CES, while UMTS traffic is transported over AAL2 layer.

RNC

nxE1

UMTS

UMTS

UMTS

UMTS

GSM

Multiple E1s, E3, STM-1

GSM

GSM

UMTS

BSC

ATM ATM

STM-1 E1E1

nxE1

nxE1 E1

nxE1

nxE1

nxE1, STM-1

Advantages:

� Single ATM backhaul network serving both GSM and UMTS

� Single backhaul network eases network management and optimizes operating expenses

� Statistical multiplexing gain for UMTS traffic only

Disadvantages:

� GSM channels are transported through AAL1 layer as CES, which consumes around 13% more bandwidth comparing to ordinary TDM channels – additional leased lines capacity for GSM has to be ensured.

� No ATM statistical multiplexing possible among UMTS and GSM traffic channels - bandwidth for GSM permanently dedicated to CES ATM channels

6.4 PSAX combined GSM/UMTS backhaul

Using market unique GSM voice suppression features in PSAX, GSM voice is carried over AAL2. Adding AAL2 for UMTS voice and AAL5 for GSM/UMTS data services, usage of PSAX within combined GSM/UMTS backhaul allows statistical multiplexing gain, ensuring best possible optimisation of backhaul transmission bandwidth.

RNC nxE1, STM-1

nxE1

UMTS nxE1

UMTS

UMTS

UMTS

GSM

nxE1 IMA, E3 or STM-1

GSM

GSM

UMTS

BSC

STM-1 E1E1

nxE1

nxE1 E1

nxE1

PSAX PSAX

Advantages:

� Single ATM backhaul network serving both GSM and UMTS - eases network management and optimizes operating expenses

� Statistical multiplexing of GSM and UMTS channels ensures substantial reduction of required transmission bandwidth

� GSM voice suppression features offer additional transmission bandwidth savings

� Efficiently solves “GSM cannibalisation” problem

� Future proof - IP transport in 3GPP Rel5 architecture

Combined GSM/UMTS mobile backhaul network

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7 CONCLUSION

Service providers about to build UMTS network and committed to upgrading their existing GSM network to GPRS and EDGE, face the necessity for backhaul optimization in order to ensure sufficient network capacity for the increased voice and data traffic.

Implementation of ATM as a transport technology into the existing backhaul network architecture enables building of combined backhaul network for both GSM and UMTS, where the most important building block is represented through Lucent Technologies PSAX multiservice media gateway.

PSAX provides the most comprehensive range of capabilities required to integrate seamlessly into all mobile network types where the large number of different interfaces can be aggregated into one combined transmission network.

Capability of E0 channel grooming to selectively transmit only active GSM channels over backhaul network and IMA over E1 links ensuring robustness and increased immunity to service interruptions, allow further optimization of the backhaul bandwidth.

Market unique, GSM Idle Channel Suppression and GSM Idle Speech Frame suppression algorithms are used to suppress transport of redundant information from speech transmissions on GSM Abis and Ater interfaces. Such methods enable Abis and Ater interface information to be transported as Variable Bit Rate traffic through the ATM network, which is a prerequisite for leveragining ATM statistical multiplexing capability.

Statistical multiplexing of GSM voice, UMTS voice, GPRS/EDGE data and UMTS data provides considerable backhaul bandwidth savings and is the only method that efficiently solves the problem of cannibalization of GSM services by UMTS.

All of these benefits make PSAX the most efficient and flexible tool for optimization of available transmission bandwidth, which provides the existing GSM service providers with smooth and cost effective transition to UMTS.