GSM Over Satellite

Application Note:

GSM OVER SATELLITE

Presented by SES NEW SKIES B.V.

September, 2006

v.2 SES NEW SKIES Proprietary and Confidential Page i

S E S N E W S K I E S

Proprietary and Confidential

SES NEW SKIES provides this information in good faith, and has taken all reasonable steps to ensure its accuracy and completeness as of the date hereof. However, SES NEW SKIES expressly disclaims any and all liability which may be based on the use of such information, errors therein, changes and omissions thereto. SES NEW SKIES shall not be liable for damages of any kind, including special, incidental or consequential damages or consequential damages, loss of goodwill or loss of prospective profits, on account of omissions or errors contained herein. NEWSKIES® and IPSYS® are registered trademarks of SES NEW SKIES B.V. Other product names, company names, brand names, and trade names mentioned within the corporate information center may be the trademarks of their respective holders. All rights in the service marks, company names, trade names, and logos used for the products or services of SES NEW SKIES or of third parties belong exclusively to SES NEW SKIES or their respective owners, and are protected from reproduction, imitation, dilution, or confusing or misleading uses under national and international trademark and copyright laws. 2006 SES NEW SKIES B.V. All rights reserved. Rooseveltplantsoen 4 2517 KR The Hague The Netherlands Phone +31 (0) 70 306 4100 • Fax +31 (0) 70 306 4101


v.2 SES NEW SKIES Proprietary and Confidential Page ii

Table of Contents 1 Introduction 1 2 GSM Overview 2 2.1 GSM Architecture 2 2.2 GSM Interfaces 3 2.2.1 The E Interface 4 2.2.2 The A Interface 4 2.2.3 The Abis Interface 5 2.2.4 The Ater Interface 6 2.3 GSM Interface Summary 7 3 Satellite Applications 8 3.1 Bandwidth Reduction Techniques 8 3.1.1 Voice Compression 8 3.1.2 Time Slot Elimination 8 3.1.3 Signaling Channel Optimization 9 3.2 E Interface over Satellite 10 3.3 A Interface over Satellite 12 3.4 Ater Interface over Satellite 12 3.5 Abis Interface over Satellite 13 3.6 Satellite Link Bandwidth 15 4 Conclusions 17 ABOUT SES NEW SKIES 18 CONTACT NEW SKIES 18 1 IntroductionError! Bookmark not defined. 2 GSM Overview Error! Bookmark not

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ABOUT SES NEW SKIES 18 CONTACT NEW SKIES 18



1 Introduction The worldwide popularity of GSM has driven operators to deploy services in most metropolitan areas and, increasingly, into smaller and more remote areas. Often the terrestrial infrastructure is not sufficient to reach these locations and satellite is the only viable way to extend the service into these regions. The implementation of GSM over satellite is in common use in many regions of the world.

The GSM network technology is normally implemented using fiber, cable or microwave transmission where bandwidth is not normally a concern. Once a fiber is installed, there is little motivation in using one E1 versus eight E1s. Satellite transmission however, is very bandwidth sensitive since every kHz of satellite bandwidth must be leased and incurs an additional cost. When implementing GSM links over satellite it is important to minimize the required link bandwidth in order to reduce operating costs.

While GSM is inherently satellite friendly and is relatively easy to implement in a brute force way, there are more elegant approaches which can lead to significant bandwidth, and therefore cost, reductions. This paper attempts to show how to implement GSM links over satellite links in an efficient and cost-effective manner.



2 GSM Overview

2.1 GSM Architecture A typical GSM network is shown in Figure 1. Since this paper focuses on the transmission of the voice and signaling information, the network has been simplified to focus on only the components of interest. For our purposes, the GSM network can be viewed as consisting of three major parts: the Mobile Switching Center (MSC), the Base Station Controller (BSC) and the Base Transceiver Station (BTS). The Home and Visitor Location Registers (HLR and VLR) and other “back-office” subsystems are considered to be part of the MSC since these links would not normally be routed via satellite.

MSC BSC

BSC

BSC

PSTN

HLR VLR

BTS

BTS

BTSBTS

BTS

BTSBTS

BTS

BTS

Figure 1 – GSM Network Architecture

In a typical network, there is a single MSC, a few BSCs and many BTSs. The equipment cost also decreases from MSC to BSC to BTS. This distribution is important when considering where to put the satellite link.



The main functions for the major network components are:

Mobile Switching Center (MSC) – Controls the call set up for incoming and outgoing calls. Interfaces to the PSTN and other mobile networks. Usually there is one in a network or possibly one in each major city. All calls must go through the MSC.

Base Station Controller (BSC) – Allocates radio channels to individual calls. Performs hand-offs between BTSs located within the same BSC. The BSC subsystem also normally performs the GSM specific voice compression. A single BSC can support many BTSs for coverage of a larger geographic area.

Base-station Transceiver Station (BTS) – Performs the actual transmission over the air to the mobile subscribers. The BTSs are located at the cellular towers throughout the coverage area. The BTS can contain one or more GSM radios, each of which supports eight GSM voice calls.

The most frequent scenario is to locate a single BTS in the remote location. As the use population grows and traffic increases, multiple BTS can be deployed either with or without a BSC.

2.2 GSM Interfaces All of the interfaces between the various components are carried using standard E1 bearer trunks to allow easier transmission over microwave, fiber or satellite. Figure 2 shows a diagram of the different interfaces.

MSC BTSBSCPSTN

E A Abis Um

Figure 2 – GSM Interfaces

The satellite link may be used to support any of these interfaces. While each of these interfaces is carried over a physical E1 link, the format of the data varies for each of the interface types. The selection of each of these interfaces for the satellite link provides different advantages and disadvantages.

Since each these interfaces are compatible with satellite transmission, the exercise then becomes one of how efficiently the traffic can transmitted via satellite in order to minimize the space segment required for the transmission.



2.2.1 The E Interface The E interface is used to interface between the MSC and the PSTN, or between MSCs. The E interface is a “standard” E1 which is in common use in the PSTN for carrying telephone and data traffic.

MSCPSTN

E

Figure 3 – E Interface

The E Interface consists of a 2.048 Mbps carrier with 32 timeslots for 30 voice channels @ 64 kbps, an SS7 signaling channel and one timeslot for framing and alarms.

Timeslot Usage Timeslot Usage 0 Framing 16 SS7 Signalling 1 Voice Ch. 1 17 Voice Ch. 16 2 Voice Ch. 2 18 Voice Ch. 17 3 Voice Ch. 3 19 Voice Ch. 18 4 Voice Ch. 4 20 Voice Ch. 19 5 Voice Ch. 5 21 Voice Ch. 20 6 Voice Ch. 6 22 Voice Ch. 21 7 Voice Ch. 7 23 Voice Ch. 22 8 Voice Ch. 8 24 Voice Ch. 23 9 Voice Ch. 9 25 Voice Ch. 24

10 Voice Ch. 10 26 Voice Ch. 25 11 Voice Ch. 11 27 Voice Ch. 26 12 Voice Ch. 12 28 Voice Ch. 27 13 Voice Ch. 13 29 Voice Ch. 28 14 Voice Ch. 14 30 Voice Ch. 29 15 Voice Ch. 15 31 Voice Ch. 30

Figure 4 – E Interface Format

Since this interface is in common usage, there are many industry standard options for compression which can be implemented.

2.2.2 The A Interface The A Interface is used to connect the MSC and BSC. It contains a maximum of 30 uncompressed voice channels plus a signaling channel for GSM call setup messages for the



BSC and Mobile Subscribers. The E1 format is identical to the E interface and the same transmission and compression options apply to both the A and E interfaces.

2.2.3 The Abis Interface The Abis interface is used to connect a BSC and a BTS. Since there are more BTSs in the network than other components, the Abis interface is the most common interface in a GSM network and is often implemented via satellite.

BTSBSC

Abis

Figure 5 – Abis Interface

The Abis interface contains compressed voice and GSM information. The Abis Interfaces format is shown in Figure 6. A single Abis interface can be used to support up to eight GSM radio access channels. Each radio channel supports eight GSM voice channels. Radio channels can be used either for multiple cells to increase geographic coverage or for multiple frequencies within a single cell to increase the traffic handling capacity.

Timeslot Usage Timeslot Usage 0 Framing 17 Unused 1 Radio Ch. 1 18 Unused 2 8 x voice 19 O & M 3 Radio Ch. 5 20 Unused 4 8 x voice 21 O & M 5 Radio Ch. 2 22 Radio 8 Signaling 6 8 x voice 23 Radio 7 Signaling 7 Radio Ch. 6 24 Radio 6 Signaling 8 8 x voice 25 Radio 5 Signaling 9 Radio Ch. 3 26 Unused

10 8 x voice 27 Radio 4 Signaling 11 Radio Ch. 7 28 Radio 3 Signaling 12 8 x voice 29 Radio 2 Signaling 13 Radio Ch. 4 30 Radio 1 Signaling 14 8 x voice 31 Unused

15 Radio Ch. 8 16 8 x voice

Figure 6 – Abis Interface Format



The voice channels on the Abis interface have been compressed by the BSC. Four GSM voice streams are then placed in a 64 kbps timeslot and two timeslots are used for each 8 channel GSM radio. A single Abis interface can be used for up to eight radio channels. If fewer radios are supported then the unused timeslots are left empty.

There are five timeslots which are always unused, even on a fully loaded Abis interface. In addition, if fewer than eight radios are deployed, there are three timeslots that are unused for each radio that is not deployed. The result is many unused timeslots that should not be transmitted over the satellite.

2.2.4 The Ater Interface The voice compression is normally performed in the BSC by a subsystem called a Transcoder Rate Adapter Unit (TRAU). Sometimes the TRAU function is moved from the BSC and located at the MSC to provide voice compression on the link and reduce the required bandwidth. This scenario is shown in Figure 7.

MSC BTSTRAU

A Ater

Figure 7 – TRAU co-located at MSC

The Ater Interface consists of one timeslot for framing, one timeslot for signaling messages and 15 pairs of timeslots, each containing 8 compressed GSM voice channels. This format is shown in Figure 8.

The Ater Interface can support up to 120 GSM voice channels. This represents a compression rate of 4:1 over a standard A interface. A fully loaded Ater interface is very efficient in its transmission, however, most implementations use fewer than the maximum 120 channel configuration so there are often empty timeslots.



Timeslot Usage Timeslot Usage 0 Framing 17 Radio Ch. 9 1 Radio Ch. 1 18 8 x voice 2 8 x voice 19 Radio Ch. 10 3 Radio Ch. 2 20 8 x voice 4 8 x voice 21 Radio Ch. 11 5 Radio Ch. 3 22 8 x voice 6 8 x voice 23 Radio Ch. 12 7 Radio Ch. 4 24 8 x voice 8 8 x voice 25 Radio Ch. 13 9 Radio Ch. 5 26 8 x voice

10 8 x voice 27 Radio Ch. 14 11 Radio Ch. 6 28 8 x voice 12 8 x voice 29 Radio Ch. 15 13 Radio Ch. 7 30 8 x voice 14 8 x voice 31 SS7 Signalling 15 Radio Ch. 8 16 8 x voice

Figure 8 – Ater Interface Format

2.3 GSM Interface Summary While the E, A, Abis and Ater interfaces are all carried on E1 bearers, there are considerable differences between them, especially in terms of voice channel capacity. Table 1 shows the voice channels carried by a fully loaded E1 for each of the interface types.

Table 1 – GSM Interface Comparison

Interface Voice Channels

E 30

A 30

Abis 64

Ater 120

Since each of these interfaces are carried on an E1 bearer they all run at a rate of 2.048 Mbps. Each interface presents different opportunities for optimization. While the Ater interface is quite efficient from a compression standpoint, it is often not fully populated and therefore contains unused timeslots. Conversely, the A and E interfaces are often fully utilized but the uncompressed voice traffic makes it highly desirable to use compression equipment to improve the link utilization.



3 Satellite Applications It is relatively easy to take a brute force approach and simply transmit the E1 interfaces over the satellite, and there are many networks operating in exactly this manner. However, the transmission of the different GSM interfaces over satellite presents good opportunities for careful system design and implementation to yield significant operational cost savings.

The principal techniques for efficient transmission are voice compression, the elimination of unused timeslots and statistically multiplexing the signaling channels.

3.1 Bandwidth Reduction Techniques

3.1.1 Voice Compression Voice is normally transmitted on E1 bearers using 64 kbps PCM. This uncompressed format requires a full 64 kbps timeslot for each voice call. The most widely used voice compression today is the G.729 family which provides near toll quality compressed voice at 8 kbps. Other algorithms are available with higher compression but are not widely used in public network applications due to lower quality. The G.729 codecs provide near toll quality at reasonably low bit rates. This standard is widely deployed and is available in a number of products to provide E1 voice compression.

A related issue is silence suppression which suppresses the transmission of silent frames when a user is not speaking. This technique is sometimes referred to as Digital Speech Interpolation (DSI). In large trunk groups it is possible to achieve bandwidth savings of 60%, however this is a statistical process and is not applicable for small trunk groups.

In this paper it is assumed that when compression is applied to a link, G.729 voice compression is used but silence suppression is not. For larger trunk groups it is possible to reduce the required bandwidth by utilizing equipment that supports this feature.

The GSM network utilizes voice compression for the air interface. Voice compression is performed by the BSC or TRAU subsystems. The GSM full rate codec operates at 13 kbps. Half Rate (5.6 kbps) and Enhanced Full Rate (12.2 kbps) codecs are also used in GSM networks. The Abis and Ater interfaces carry four voice channels in one 64 kbps timeslot.

3.1.2 Time Slot Elimination The E1 interfaces often have unused timeslots, particularly when the link is not used to full capacity. Significant gains can be made simply by not transmitting these timeslots over the



satellite. This process is often referred to as drop-and-insert. Empty timeslots are dropped at the transmitting side and are inserted at the receiving side. This configuration is shown in Figure 9.

Drop andInsertMux

Drop andInsertMux

E1 with 3 timeslots E1 with 3 timeslotsEmpty

TimeslotsDropped

EmptyTimeslotsInserted

Three Timeslots transmittedover Satellite

Figure 9 – Drop and Insert Operation

Drop and Insert multiplexers are normally integrated into satellite modems that are IDR/IBS compatible to allow transmission of fractional E1 links. The interface to the modem is a 2.048 Mbps E1. The IDR channel unit performs drop and insert to select which timeslots are transmitted over the satellite.

3.1.3 Signaling Channel Optimization The signaling and O&M channels carry packetized messages for call setup, monitor and control. The channels are not fully utilized and the unused portion of the channel is normally filled with idle frames to buffer the channel up to 64 kbps. It is possible to reduce the bandwidth of these channels without the loss of any information by simply discarding the idle frames and transmitting only the actual messages over the satellite. This process is called statistical multiplexing and is often used to combine several packetized channels into one bearer channel. These channels can typically be reduced to 10 – 20% of their original size. Note that these signaling channels are unique to GSM and are sometimes unique to the different vendors of GSM equipment so the equipment must be designed specifically for this application.



3.2 E Interface over Satellite The E interface can be transmitted via satellite for two main applications:

Connection of a GSM network with the PSTN, especially for international long distance calls.

Interconnection of two GSM networks to allow mobile to mobile calls between GSM networks or within a GSM network with multiple MSCs.

A block diagram showing an example of this configuration is shown in Figure 10.

MSC BTSBSCPSTN

E

Figure 10 – E Interface over Satellite

Since the GSM network on the right side of Figure 10 includes an MSC, BSC and BTS it functions as an autonomous network. Calls can be completed between two mobile subscribers or between a mobile subscriber and the local PSTN. Roaming is simplified since all of the required messages can be processed locally. This configuration also allows single hop mesh connections in a multi-site network. This configuration is shown in Figure 11. In this example any of the sites can complete calls directly to any of the other sites. This is important since it prevents double-hop connections which have poor voice quality due to long round trip delays.

The main disadvantage to this configuration is that a significant amount of equipment is required at the local site. This is often not practical for a small number of mobile subscribers. This can sometimes be mitigated by using an integrated product which includes the MSC, BSC and BTS functionality in a single integrated package.



MSC BSC

BTS

BTS

BTS

MSC BSC

BTS

BTS

BTS

MSC BSC

BTS

BTS

BTS

Figure 11 – Fully Meshed GSM over Satellite Network

Since the E interface is a standard E1, it is relatively easy to transmit over satellite and is compatible with a wide variety compression equipment including multiplexers, FRADs, VoIP gateways and DCME. Compression gains of 8:1 and higher are achievable and the cost of the equipment can be quickly recovered.

An example configuration is shown below in Figure 12. For clarity, only one end of the link is shown. The other end of the link would have an identical configuration. In this example a fully loaded A interface with 30 channels is to be transmitted over the satellite. A FRAD is used to provide voice compression at 8 kbps. The 2.048 Mbps E1 with 30 voice channels is compressed to 304 kbps. The duplex link requires 600 kHz of satellite bandwidth compared with 3.7 MHz for the transparent E1 resulting in a savings of 83%.

FRAD withVoice

Compression

SatelliteModem RF Transciever

2.048 MbpsE1 with 30 Ch

304 kbps Link withcompressed Voice

600 kHz

To/FromMSC or PSTN

Figure 12 – E Interface Typical Equipment Configuration



In this example a FRAD was used to provide voice compression. Another option would be to use a VoIP gateway at each end of the link.

3.3 A Interface over Satellite The A interface would be carried over satellite in applications where a BSC needed to be remotely located from the MSC. Since the MSC is a relatively costly component it is often practical to have only one in a GSM network. This configuration is shown in Figure 13. A central MSC may be linked via satellite to multiple BSCs located in different cities.

MSC BSCPSTN

ABTS

Figure 13 – A Interface Over Satellite

Since the BSC in the remote location can support multiple BTSs, it is possible to cover a wide geographic area. Roaming between the BTSs is also supported locally by the BSC, minimizing the number of messages that are transmitted over the satellite link.

The main disadvantage is that subscriber to subscriber calls must go through the MSC which requires a double satellite hop.

The A interface carries uncompressed voice so voice compression equipment should be used to reduce the required satellite bandwidth.

The equipment configuration would be similar to that shown for the E interface example in Figure 12.

3.4 Ater Interface over Satellite The Ater interface is simply a variation of the A interface, so the same design considerations stated above apply.

The Ater link employs GSM voice compression but there are normally unused timeslots which provide a good opportunity to reduce the link speed by using drop and insert to prevent transmitting empty timeslots over the satellite.

An example configuration is shown in Figure 14. For clarity, only one end of the link is shown. The other end of the link would have an identical configuration. In this example an



Ater interface with 64 voice channels is to be transmitted over the satellite. A drop and insert mux is used to drop the empty timeslots and reduce the link bandwidth. Since the Ater interface already contains compressed voice, no compression equipment is used. The original 2.048 Mbps E1 with 64 voice channels is reduced to 1032 kbps.

IDR ChannelUnit with

Drop and Insert

Modulator/Demodulator RF Transciever


1032 kbpsData stream

1.9 MHz

To/FromMSC or BSC

IDR / IBS Complaint Modem

Figure 14 – Ater over Satellite Equipment Configuration

The duplex link requires 1.9 MHz of satellite bandwidth compared with 3.7 MHz for the transparent E1 resulting in a savings of 48%.

3.5 Abis Interface over Satellite Transmitting the Abis interface via satellite is the most common implementation and is often used to extend service to new locations with minimal infrastructure costs. This configuration is shown in Figure 15. As the GSM traffic grows at the remote site, additional BTSs or a BSC may be deployed to support higher traffic loads and/or a larger geographic area.

This configuration has the advantage that there is minimal expense required to deploy the service. An existing MSC and BSC can be used, which could possibly support satellite connections to several remote locations.

The main disadvantage is that the remote location relies heavily on the equipment located at the hub site so hand-offs and subscriber-to-subscriber calls must go over the satellite link.



MSC BSCPSTN

Abis BTS

Figure 15 – Abis Interface over Satellite

An example configuration is shown in Figure 16. For clarity, only one end of the link is shown. The other end of the link would have an identical configuration. In this example an Abis interface with 32 voice channels is to be transmitted over the satellite. A drop and insert mux is used to drop the empty timeslots and reduce the link bandwidth. Since the Abis interface already contains compressed voice, no compression equipment is needed. The original 2.048 Mbps E1 with 32 GSM voice channels is reduced to 560 kbps.

IDR ChannelUnit with

Drop and Insert

Modulator/Demodulator RF Transciever


560 kbpsData stream

1.1 MHz

To/FromBSC or BTS

IDR / IBS Complaint Modem

Figure 16 – Abis over Satellite Typical Equipment Configuration

The duplex link requires 1.1 MHz of satellite bandwidth compared with 3.7 MHz for the transparent E1 resulting in a savings of 70%.



3.6 Satellite Link Bandwidth The primary motivation for implementing good system design techniques for the satellite link is to reduce the link bandwidth, and therefore, the operational costs. This section presents some sample bandwidth calculations for the different interfaces. Three cases are considered for each interface: using a transparent E1 bearer, using drop and insert, and using voice and/or signaling compression. The calculations are based on increments of 8-channels to correspond to an 8-channel GSM radio.

An indication is also given for each case to show the number of mobile subscribers that can be supported depending on the traffic profile (Erlangs per subscriber).

It is assumed that the satellite links are implemented using QPSK modulation with Rate ¾ FEC, with either Turbo or convolutional coding.

The link bandwidth summary for the A and E Interfaces is shown in Table 2.

The link bandwidth summary for the Abis Interface is shown in Table 3. Table 4 shows the link bandwidth summary for the Ater Interface.

Table 2 – Link Bandwidth for the A and E Interfaces

Voice Link data rate (kbps) Duplex Bandwidth (MHz)Channels Transparent Drop & Ins Compress Transparent Drop & Ins Compress

8 2048 576 128 3.7 1.1 0.316 2048 1088 192 3.7 2.0 0.424 2048 1600 256 3.7 2.9 0.530 2048 1984 304 3.7 3.6 0.6

Notes:

Drop and Ins = Use of Drop and Insert mux. Eliminates empty timeslots but does not use compression.Compressed Bandwidth assumes 8 kbps voice compression but no silence suppression.Futher compression is possible by using silence supression. Approx 10 - 30% savings is possible.Data rate is the rate of a simplex channel. The Bandwidth is calculated for a duplex link.

The A and E interfaces are excellent applications for compression equipment. As the table shows, significant bandwidth reduction is achieved over the transparent case.



Table 3 – Link Bandwidth for the Abis Interface

GSM RF Voice Abis data rate (kbps) Duplex Bandwidth (MHz)Channels Timeslots Transparent Drop & Ins Compress Transparent Drop & Ins Compress

1 8 2048 320 152 3.7 0.6 0.32 16 2048 512 288 3.7 1.0 0.63 24 2048 704 424 3.7 1.3 0.84 32 2048 896 560 3.7 1.7 1.15 40 2048 1088 696 3.7 2.0 1.36 48 2048 1280 832 3.7 2.4 1.57 56 2048 1472 968 3.7 2.7 1.88 64 2048 1664 1104 3.7 3.0 2.0

Notes:Drop & Ins = Use of Drop and Insert mux. Eliminates empty timeslots but no optimization of signaling channels.Compressed Bandwidth assumes statistical muxing on signalling channels but no voice silence suppression.Futher compression is possible by using silence supression. Approx 10 - 30% savings is possible.Abis data rate is the rate of a simplex channel. The Bandwidth is calculated for a duplex link.

The table shows that for small numbers of channels simply implementing Drop and Insert produces major improvements. Compression also improves results by reducing the signaling bandwidth.

Table 4 – Link Bandwidth for the Ater Interface

GSM RF Voice Ater data rate (kbps) Duplex Bandwidth (MHz)Channels Timeslots Transparent Drop & Ins Compress Transparent Drop & Ins Compress

1 8 2048 192 136 3.7 0.4 0.32 16 2048 320 264 3.7 0.6 0.53 24 2048 448 392 3.7 0.9 0.84 32 2048 576 520 3.7 1.1 1.05 40 2048 704 648 3.7 1.3 1.26 48 2048 832 776 3.7 1.5 1.47 56 2048 960 904 3.7 1.8 1.78 64 2048 1088 1032 3.7 2.0 1.99 72 2048 1216 1160 3.7 2.2 2.110 80 2048 1344 1288 3.7 2.5 2.411 88 2048 1472 1416 3.7 2.7 2.612 96 2048 1600 1544 3.7 2.9 2.813 104 2048 1728 1672 3.7 3.2 3.114 112 2048 1856 1800 3.7 3.4 3.315 120 2048 1984 1928 3.7 3.6 3.5

Notes:Drop & Ins = Use of Drop and Insert mux. Eliminates empty timeslots but no optimization of signaling channels.Compressed Bandwidth assumes statistical muxing on signalling channels but no voice silence suppression.Futher compression is possible by using silence supression. Approx 10 - 30% savings is possible.Data rate is the rate of a simplex channel. The Bandwidth is calculated for a duplex link.

Since the Ater interface has voice compression and there is only one signaling channel, compression equipment produces only marginal improvements over drop and insert.

The transmission of the Abis and Ater interfaces can be significantly improved by using GSM specific compression/optimization equipment. This type of proprietary equipment uses advanced signaling channel optimization and silence suppression to produce bandwidth savings of up to 50% of the values shown in the tables.



4 Conclusions GSM over satellite is a relatively common application in many regions of the world. Depending on the network topology and the specific requirements of the operator, the MSC, BSC and BTS may be located either at the central site or at the remote location. This flexibility in network planning allows the operator to tailor the satellite solution for the particular needs of each remote site in the mobile network.

It is also clear that while transparent E1s may be used to transmit the E, A, Ater and Abis interfaces over the satellite link, there are significant gains to be made by employing either compression or drop and insert equipment to reduce the satellite bandwidth requirements.

SES NEW SKIES firmly believes that GSM over satellite is an important application today and will experience significant growth in the future as GSM networks are pushed into more rural areas. By understanding the technology and efficient techniques, more GSM operators will be able to take advantage of satellite as a means to extend their networks.



ABOUT SES NEW SKIES

SES NEW SKIES is an SES GLOBAL company (Euronext Paris and Luxembourg Stock Exchange: SESG) offering satellite communication services to a range of customers including telecommunications providers, broadcasters, corporations and governments around the world. SES NEW SKIES headquartered in The Hague, The Netherlands and has offices in Singapore, Washington D.C, Sydney and São Paulo.

SES NEW SKIES Network Solutions uses satellite technology to bridge the gap between dispersed areas, enabling GSM operators to extend their networks efficiently and economically, irrespective of distance, geographic barriers or terrestrial infrastructure.

Further information is available at www.ses-newskies.com/gsmnetwork.htm

CONTACT NEW SKIES

Head Office SES NEW SKIES B.V Rooseveltplantsoen 4 2517 KR The Hague The Netherlands T: +31 70 306 4100 F: +31 70 306 4101 South Africa Office Regus House Country Club Estate Woodlands Drive Woodmead, Johannesburg South Africa T: +27 (0) 11 258 8750 F: +27 (0) 11 258 8511 Americas Sales Office SES NEW SKIES. 2001 L Street NW Suite 800 Washington D.C. 20036 T: + 1 202 478 7100 F: +1 202 478 7101 Asia-Pacific Sales Office Singapore Office #31-05 Centennial Tower 3 Temasek Avenue

Singapore 039190 T: +65 623 804 00 F: +65 623 811 41 Australia Sales Office SES NEW SKIES Australia and South Pacific Level 31, ABN AMRO Building 88 Philip Street Sydney NSW 2000 Australia T: +61 2 8211 2744 F: +61 2 8211 0555

Europe & IMEA Sales Office SES NEW SKIES B.V. Rooseveltplantsoen 4 2517 KR The Hague The Netherlands T: +31 70 306 4100 F: +31 70 306 4101 Brazil Sales Office Sao Paulo Office Av Nacoes Unidas 12551 9 Andar Sao Paulo-SP CP 04548-903

Brazil T:1+55 113 443 7452 F: +55 113 443 7474

GSM Over Satellite

Documents

larger geographic area

base station controller

idr channel unit

mobile switching center

transparent e1 resulting

voice radio ch

link bandwidth summary

eliminates empty timeslots