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GSM transport evolution   Per Ola Andersson, Håkan Asp, Aldo Bolle, Harry Leino, Peter Seybolt and Richard SwardhThe succe...
WCDMA RBSs, and other equipment thatconnects via IP or Ethernet.  The packet gateway used for Abis Optimi-zation can simul...
is not sensitive to packet loss or long peri-                                                                             ...
semi-public IP networks as a means of re-   ducing their costs.Some operators may also have a corporatestrategy that manda...
tion in costs in an IP or Ethernet scenario.                                                                              ...
erator TCO. Ericsson has thus introduced a       Conclusion                                        to connect RBSs to the ...
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5 gsm transport_web


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5 gsm transport_web

  1. 1. GSM transport evolution Per Ola Andersson, Håkan Asp, Aldo Bolle, Harry Leino, Peter Seybolt and Richard SwardhThe success of GSM to date has been nothing short of sensational. What previously used to maintain a constant bitis more, its future continues to hold great promise. However, two important rate to fit the PDH channel format) are nochallenges must yet be resolved to make GSM the mobile communica- longer inserted.tions system for the “next billion users” and to guarantee the commercial Ericsson has integrated the Abis Optimizationsuccess of its mobile data services: focus on low total cost of ownership solution into its base station subsystem (BSS).(TCO), and deployment of enhanced GPRS (EGPRS) in every network. In The Abis Optimization feature is based on aaddressing these challenges, GSM transport or backhaul constitutes one software upgrade of the RBS, and the intro-very interesting and dynamic area of development. duction of a packet gateway (PGW) in the BSC, to terminate the Packet Abis protocol. To save even more bandwidth, one may add aBackground Technical overview, Packet site integration unit (SIU) at RBS sites to ex- ploit statistical multiplexing gains betweenTransmission to radio base station (RBS) sites Abis RBSs.accounts for a large part of the cost of operat- Packet Abis, Ericsson’s collective solution foring radio access networks. Where operators these developments (Figure 2), includes a Dimensioning example, Abis Optimizationhave built their own transport networks, new protocol for transferring data over the The number of E1/T1 links required per sitetransmission also represents substantial in- Abis link. Simply put, from the air inter- with Abis Optimization depends on site con-vestment (Figure 1). face the solution sends all traffic (voice, data figurations and traffic mix. In T1 markets, Traditional transmission from RBSs to and signaling) in packets over Abis with a one RBS 2106/2206 with up to 12 TRXs cana base station controller (BSC) is realized minimum of repacking and reformatting. cover three sectors with only one T1. Withusing static time-division multiplexing The solution can be used over TDM trans- “classic” Abis, two T1s would be required for(TDM) circuits. This technology has not port networks as well as with IP or Ethernet this configuration.been optimized for efficient transmission of transport services. Ericsson’s Packet Abis so- In E1 markets, one E1 can support twobursty data traffic, however. Nor is it easily lution is built around two main features: RBSs with up to 18 TRXs. For standard baseadapted to the low-cost transport technolo- • Abis Optimization; and station configurations, this could translategies, such as IP and Ethernet, which are be- • Abis over IP. into a savings of up to rolled out over copper, fiber, and micro-wave networks. Low TCO using TDM transport Low TCO with Packet Abis using IP or The good news is that recent develop- networks Ethernet transport servicesments in GSM transport and switching • Abis Optimization, which is Ericsson’s solu- The Abis over IP solution enables operatorstechnologies promise to overcome these is- tion for delivering Packet Abis over TDM to use IP and Ethernet transport networkssues, making GSM suitable to provide ser- transport networks, saves bandwidth by to connect RBSs to the BSC and therebyvices both to low-revenue users and to users solely transferring bits that contain data. benefit from the lower costs of IP- andof high-speed data services. All other bits (for example, those that were Ethernet-based transport services. The solu- tion also opens the door to shared transport with WCDMA and to integrated transportFigure 1 solutions for RBS sites.Breakdown of the costs of operating radio access networks. The solution is fully interoperable with Ericsson’s other products. At an RBS site, all IP features are implemented in the new STN function, connecting the site to IP or Ethernet transport networks. The STN is a software function that can be implemented in Ericssons base stations. The site transport node is implemented on a family of platforms that have been op- timized to fit pico, micro, and macro RBSs. The packet-switched termination unit (PSTU) platform for micro RBSs, for in- stance, has been completely integrated into Ericsson’s micro RBS. In macro RBSs, Ericsson has integrated the STN into the site integration unit (SIU), which is designed to serve as a single point of connection that supports common, inte- grated transport to the RBS site. With the SIU (Figure 3), one can efficiently manage and share backhaul transport for GSM RBSs,26 Ericsson Review No. 1, 2007
  2. 2. WCDMA RBSs, and other equipment thatconnects via IP or Ethernet. The packet gateway used for Abis Optimi-zation can simultaneously handle IP trafficand traffic from the TDM transport network,and terminate Abis over IP at the BSC site.Quadrupling the performance of EDGEwithout extra transmissionMany operators who deploy EDGE havedeemed it necessary to constrain throughputto match the limitations of available TDMtransmission capacity. The classic TDM-based Abis interface has a fixed allocation oftransmission timeslots to basic radio channels.Therefore, the subordinate 16kbps timesloton the Abis interface permanently allocatedto a traffic channel (TCH) for voice servicewill never be available to carry EDGE data.With packet Abis the transport resourcesmake up a common pool that is used by thetraffic offered at each moment in time. The new Packet Abis, however, has nosuch permanent connection. Therefore, E1 orT1 transmission can now offer a multilinkpoint-to-point protocol (MLPPP) bit pipethat may be freely used by every service of-fered by the RBS until it reaches saturation Figure 2(congestion). At that point, the BSC dynami- Packet Abis system solution.cally reduces the generated load through itscontrol of mobile terminals. In many locations, operators have also lim- ernet, for example, does not currently sup- data. This solution, which is very accurate,ited EGPRS to one timeslot, giving subscrib- port layer-1 timing for RBS synchronization. is also used to synchronize radio networks.ers a peak performance of at most 59.2kbps. Other means must thus be used to synchro- Notwithstanding, it requires a clear line ofHowever, by deploying Packet Abis, opera- nize RBSs. Operators who employ Abis over sight from the GPS antenna to the satellitestors can quadruple the speed at most times of IP can synchronize their radio base stations in the sky. In many installations this is im-the day without adding transmission capac- using timing information from possible to achieve, which eliminates GPSity, thereby giving users the ability to, say, • a GPS receiver at the RBS site; or as the synchronization solution of choice forsurf the internet at more than 230kbps. • the IP network. such sites.Synchronization GPS receiver at RBS site At an RBS site equipped with a GPS receiver, RBS synchronization with Abis over IP In Ericsson’s solution, the STN providesTo comply with GSM specifications and one can synchronize RBSs using GPS timing RBSs with the timing they need to fulfill theguarantee proper GSM network function,the RBSs must maintain a stable and con-trolled radio frequency over the air interface.Depending on the type of RBS, the stipu- Figure 3lated level of accuracy is 50 or 100ppb. The site integration unit (SIU). RBSs with traditional TDM-based Abisare synchronized to timing provided by thetransport network layer-1 clock rate. TDMnetworks are typically synchronized to an ac-curacy of 16ppb, which with added wanderand a holdover budget, is well within the re-quirements of the air interface. With Abis over IP, however, one cannotassume that the transport network carriestiming for synchronization on layer-1. Eth-Ericsson Review No. 1, 2007 27
  3. 3. is not sensitive to packet loss or long peri- ods (months) of interruption toward the time server. The supported network may be a switched or routed network that includes xDSL- through-DSLAM access or a satellite hop. The WAN interface of the STN can be Ethernet 10/100/1000BASE or E1/T1. Due to traffic-load-dependent processing and scheduling delays in network nodes, the IP packet transfer delay (IPTD) can accommo- date wander components with a periodicity of days. Calibration may continue for up to 15 days on IP networks with large IPTD wander. The Ericsson Abis over IP solution to RBS synchronization can be employed over IP networks with existing switching and rout- ing equipment in intermediate nodes. The solution can handle the packet delay varia- tion added by these intermediate nodes. All existing intermediate nodes support the mes- sage data format used by the timing packets. The highly stable OCXO keeps startup time independent of packet delay variation in the IP network. In the future, the IEEE 1588 protocol might be implemented in some telecommu-Figure 4 nications networks. However, this will notRBS synchronization with Abis over IP: time-stamping example. improve the performance of synchronization or calibration through existing networks un- less every network node is replaced with afrequency requirements of the air interface. in NTP is used for transferring timing pack- node that supports an IEEE 1588 boundaryThe RBSs can use existing synchronization ets with time-stamp information. clock or transparent clock. With IEEE 1588functionality. The basis for this solution is a Available NTP time servers with Stra­­­­­tum- techniques implemented in every node, thehighly stable oscillator (OCXO) built into the 1 accuracy (for instance, NTP time servers clients will experience less packet delay vari-Pico, PSTU (later version), and SIU hardware. which are synchronized to GPS and which ation, which in turn, means that less stableTo compensate for aging and to avoid site vis- provide a time-stamping accuracy of better oscillators may be used in client nodes. Oneits to recalibrate the oscillator, the OCXO is than 100μs) can be used and shared with consequence of using a less stable oscillatorfurther calibrated over the packet network. other applications. Thanks to the very low is that startup time becomes dependent on With this solution, the startup time is kept timing packet rate, one time server can serve packet delay variation.independent of IP network performance. Five a large number of clients. The robustness ofminutes after power-on, the OCXO can pro- the total synchronization functionality hasvide an accurate frequency that can be used made it possible to relax the requirements Securityas a timing source for RBS synchronization put on time server availability Connecting RBSs over IP networks insteadfunctionality. An STN can provide accurate The STN calibration application analyzes of over TDM-based networks sometimestiming for several months from initial start- and filters received timing packets using an raises security concerns. Although theseup without timing or calibration support algorithm developed by Ericsson. The cali- concerns are not generally motivated by anyfrom the network. bration application can work over every IP technical property of IP, a solution is never- When put into operation, a calibration ap- network path that can be used for Abis over IP theless often required. There are two mainplication in the STN compensates for OCXO traffic. The calibration application thus sup- reasons for this:aging. The STN serves as a client to a time ports IP network paths with a packet delay • the IP network provides connectivity fromserver, regularly sending time-stamped tim- variation of up to 50ms. This is comparable RBSs to the BSC, OSS, and time packets at a very low packet rate. The to the quality of service (QoS) class-1 defi- Depending on how it has been configured,time server returns packets with receive and nition in ITU-T Y.1541, which is intended a network that is shared with other sys-transmit time stamps. On reception of these for voice over IP (VoIP) using network tech- tems might offer widespread connectivity;packets, the STN adds a receive time stamp niques with less constrained requirements on and(Figure 4). The message data format defined routing and distances. The robust algorithm • operators often consider using public or28 Ericsson Review No. 1, 2007
  4. 4. semi-public IP networks as a means of re- ducing their costs.Some operators may also have a corporatestrategy that mandates specific security solu-tions when a generic IP network is used.Limiting access and potential intrusionEricsson’s Abis over IP solution incorporatesa number of measures to improve security(Figure 5). For example, it employs• access control lists in BSC and RBS IP in- terfaces;• optional firewalls in front of sensitive nodes, such as BSC and OSS, or at inter- connection points between the Abis and core IP networks; and• IPsec tunnels from the RBS site to BSC and OSS sites or to a secure IP backbone.IP is implemented natively in all RBS sitetransport nodes. IPsec termination at BSCand OSS sites is implemented using specificfirewalls, and IPsec tunnels are managed Figure 5from the OSS. Security in Ericsson’s Abis over IP solution.O&M securityBecause O&M traffic is particularly sensitivefrom a security viewpoint, all O&M traffic coverage and services to small or medium- nection that provides point-to-point connec-that runs over IP is realized using secure sized enterprises, which benefit from dedi- tivity. Transmission for these sites is expen-socket shell (SSH) and secured FTP (SFTP). cated GSM telephony. In this scenario, op- sive because the satellite connection requiresLikewise, user authentication is mandatory erators could even offer special tariffs when the full SCPC bandwidth even when there isfor all O&M connections. The system logs an users are in the office. This solution repre- no traffic from the site.audit trail of operator activity and periodi- sents a replacement to fixed phones. Previ- Today, however, the introduction of Abiscally uploads it to the OSS. ously, the backhaul cost of an E1 prohibited over IP and advances in satellite modem these types of solutions, but with Abis over IP technology make it possible to reduce trans-Protection of insecure transmission and a cheaper DSL type of backhaul (or even mission expenses for remote sites. AdvancedIf the access IP network is untrusted – that the corporate LAN itself), the Pico solution modulation techniques increase the bit rateis, if it is shared or public – then IPsec should becomes economically feasible. per hertz, and bandwidth-sharing technolo-always be used to prevent wiretapping and Femto cell solutions give operators similar gy allows sites to “share” satellite bandwidthtraffic manipulation. A typical example opportunities to go after the home market by means of point-to-multipoint connectionswould be pico base stations, such as the RBS with an offer of inexpensive (or free) calls at and a resources-in-pool concept.2409 (Figure 6), which target low-power fre- home using any standard GSM or WCDMA With current satellite bandwidth pric-quency licenses for the enterprise market. phone. The Femto runs Abis over IP to the BSC/RNC using the home broadband IPPico and Femto base connection for backhaul. Special optimiza- Figure 6 tions handle low-cost, low-bandwidth ADSLstations links. Abis over IP thus creates a completely Pico base station, RBS 2409.In addition, the new architecture opens up new business opportunity by giving opera-completely new business segments. Abis over tors an entirely new way of building GSMIP yields cost-effective transport for base sta- networks – where end-users introduce newtions with low traffic, enabling operators to cells into the network.address Pico and Femto-types of solutions(small base stations that require data ratesof up to a few hundred kilobits per second Satellitesbetween the RBS and BSC). Satellites have been used for a number of The Pico cell solution, for example, is a years to provide connectivity with basecomplete RBS 2000 base station with one station sites in remote locations. Single-TRX. Among other things, operators can channel-per-carrier (SCPC) connections haveuse this solution to offer dedicated capacity, traditionally served as a leased E1/T1 con-Ericsson Review No. 1, 2007 29
  5. 5. tion in costs in an IP or Ethernet scenario. In a network deployment, on the other hand, the aggregation and statistical multiplexing gains result in more efficient handling of traffic capacity. Therefore substantial gains may also be achieved by moving to Packet Abis over traditional microwave links that support Ethernet over TDM. In addition, new-generation microwave links with na- tive Ethernet transport, QoS support, and adaptive modulation microwave links offer a unique way of providing best-effort traffic transport in combination with guaranteed transport for TDM traffic. This further sup- ports the evolution of the packet-based trans- port in the radio access network. Adaptive modulation microwave links en- able operators to transport best-effort traffic alongside guaranteed TDM traffic. The idea is to plan the microwave links for a target availability, say 99.999%, when operating at a given modulation, for example, 4QAM. InFigure 7 practice, the links usually operate far aboveAvailability of a microwave link running at different modulation rates. the receiver threshold and only approach the receiver limit during very adverse fad- ing conditions. As a consequence, Ericsson plans to introduce microwave systems thating (wholesale average USD 1.2 million per use, allowing for rapid, small-scale deploy- adapt modulation schemes to fading condi-36MHz transponder per year), it is often un- ments. As demand grows, operators can tions. These systems will deliver differentprofitable to provide GSM services to remote easily adjust satellite transport as needed. transport bit rates for different atmosphericsites with satellite transmission. To lower the If demand justifies terrestrial transport, one cost of ownership, one must make more can redeploy satellite equipment and band- Figure 7 shows availability when a mi-efficient use of available bandwidth. To this width, since it is not dependent on location crowave link runs at different modulationend, Ericsson’s integrated IP solution moni- (ordinarily, one satellite can cover an entire rates. A link that uses 4QAM, for example,tors the transport network and IP transport continent). can deliver 34Mbps with 99.999% availabil-quality of service to make it “bandwidth However, to take full advantage of these ity. When it uses 256QAM, this same linkaware” and to give a better end-user experi- advances in technology, one must carefully has 99.999% availability and can deliverence. engineer the system end-to-end. With its 196Mbps. This capacity could be employed These enhancements reduce the demand for combined knowledge of radio access, IP to provide 34Mbps guaranteed traffic withbandwidth, which lowers TCO. Low TCO is backbone and satellite technology, Ericsson 99.999% availability and best-effort capac-particularly relevant where ARPU is low, for is uniquely positioned to deliver the lowest ity of approximately 150Mbps, also withexample, in emerging markets. When used TCO for BTSs connected via satellite. 99.999% availability. Links of this kind canin conjunction with modern satellite technol- provide a competitive, cost-effective alterna-ogy, Ericsson’s IP-BTS enables operators to tive for a capacity-growth scenario in whichprofitably bring communications to people Microwave the bulk of traffic is mobile data with best-and places that could not otherwise have it. Microwave links are used extensively for effort characteristics.With non-optimized transmission based on base station backhaul – at present, moretraditional E1 SCPC, for instance, one can fit than 60% of all base stations are connected Future Packet Abis35 BTSs with 6 TRXs into a 36MHz satellite via microwave links. The majority of op-transponder. By contrast, with careful end- erators seek to minimize their operating ex- developmentto-end engineering using Abis over IP and penses (OPEX) by owning their own trans- The site-transmission-node function and theoptimization together with advanced satel- port networks instead of leasing capacity. In Abis over IP feature also constitute the cor-lite modem technology one can fit 75 BTSs operator-owned transport networks, up to nerstone of Ericsson’s evolving Mobile Soft-into the same available bandwidth. Further- 95% of associated base stations are connected switch concept, which reaches all the way tomore, with local switching and PSTN inter- via microwave links. the RBS. Given that the majority of calls areconnect, one can fit more than 200 BTSs into Although traditional microwave links local, the ability to keep payload within theone satellite transponder. might provide bridged Ethernet capability, RBS and route it directly to the RBS that Satellite transport is flexible and easy to this does not automatically imply a reduc- serves B-subscribers will further reduce op-30 Ericsson Review No. 1, 2007
  6. 6. erator TCO. Ericsson has thus introduced a Conclusion to connect RBSs to the BSC and therebynew IP payload interface between RBSs and benefit from the lower costs of IP- andM-MGWs, to enable direct transfer of pay- Traditional transmission from RBSs to a Ethernet-based transport services. The solu-load from a mobile subscriber to a local fixed BSC has not been optimized for efficient tion also opens the door to shared transportexchange. transmission of bursty data traffic, nor is it with WCDMA and to integrated transport RBS local switching and direct transfer easily adapted to the low-cost transport tech- solutions for RBS sites.of payload are optional features deployed on nologies, such as IP and Ethernet. However, To improve security, Ericsson’s Abis overexisting GSM network infrastructure from recent developments in GSM transport and IP solution incorporates access control listsEricsson. No additional hardware is required switching technologies promise to overcome in BSC and RBS IP interfaces, optionalto deploy RBS local switching apart from the these issues, making GSM suitable to pro- firewalls in front of sensitive nodes or at in-Abis over IP feature, with its SIU hardware vide services both to low-revenue users and terconnection points between the Abis andat the RBS and PGW hardware in the BSC. to users of high-speed data services. core IP networks, and IPsec tunnels from theInstead, one can realize RBS local switching Packet Abis, Ericsson’s collective solution RBS site to BSC and OSS sites or to a secureand direct transfer of payload to M-MGWs for these developments, includes a new pro- IP backbone. All O&M traffic that runs overby means of new functionality in the BSS tocol for transferring data over the Abis link. IP is realized using SSH and SFTP.MAP signaling protocol between the MSC-S From the air interface, the solution sends all The new architecture opens up completelyand BSC. Ericsson’s BSS MAP interface ex- traffic in packets over Abis with a minimum new business segments. Abis over IP yieldschanges information between nodes, making of repacking and reformatting. The solution, cost-effective transport for base stations withit possible to analyze the codec capabilities which is built on Abis Optimization and Abis low traffic, enabling operators to address Picoof involved terminals, and to select the best over IP features, can be used over TDM trans- and Femto types of solutions.common denominator. This eliminates the port networks as well as with IP or Ethernet What is more, the introduction of Abis overneed for intermediate transcoding, effectively transport services. IP and advances in satellite modem technol-removing two transcoding steps, while render- Abis Optimization delivers Packet Abis over ogy makes it possible to reduce transmissioning better voice quality and reducing delay. TDM transport networks, saving bandwidth expenses for remote sites. The same basic AXE functionality used to by solely transferring bits that contain infor- Finally, operators who move to Packettransform the MSC into an MSC-S has been mative data. Ericsson has integrated the Abis Abis over traditional microwave links thatapplied to the BSC. Likewise, the signaling Optimization solution into its BSS. support Ethernet over TDM may enjoyprotocol between the BSC PGW and the The Abis over IP feature enables operators substantial gains in the handling of trafficSTN has been enhanced to facilitate soft- to use IP and Ethernet transport networks capacity.switch functionality in the STN. To transfer payload directly from RBSs toM-MGWs, the GCP protocol has been en- TERMS AND ABBREVIATIONhanced and the M-MGW has been comple-mented to support every codec used in GSM. ADSL Asymmetrical DSL MSC-S MSC system With these additions of functionality on ARPU Average revenue per user NTP Network time protocoltop of the packet-switched Abis over IP solu- BSC Base station controller O&M Operation and maintenancetion, Ericsson has evolved the GSM architec- BSS Base station subsystem OPEX Operating expenses BTS Base station transceiver OSS Operations support systemture from its original hierarchical structure CS Circuit switched OCXO Oven compensated crystal to a completely flat architecture. Ericsson’s DSL Digital subscriber line oscillatorGSM system is thus poised for the final DSLAM DSL access multiplexer PDH Plesiochronous digital hierarchyphase of building GSM coverage for every- DTX Discontinuous transmission PGW Packet gatewayone, everywhere. The flat architecture elimi- E1 2.048Mbps data circuit (Europe) PSTU Packet-switched termination unit EDGE Enhanced data for GSM evolution QAM Quadrature amplitude multiplexingnates the problem of tromboning local traffic EGPRS Enhanced GPRS RAN Radio access networkin remote locations which are served by few FTP File transfer protocol RBS Radio base stationRBSs and which cannot carry the cost of a GCP Gateway control protocol SCPC Single channel per carrierremotely located BSC. GFP Generic framing procedure SFTP Secured FTP GPRS General packet radio service SDH Synchronous digital hierarchy By reducing costs, the new architecture GPS Global positioning system SIU Site integration unitenables operators to profitably build cover- GRE Generic routing encapsulation SONET Synchronous optical networkage in heretofore “unfeasable” locations. In GSM Global system for mobile SSH Secure socket shelladdition, large areas covered by sites con- communications STM Synchronous transfer modenected via satellite benefit from reduced IP Internet protocol STN Site transport node IPsec Secure IP T1 1.544Mbps data circuitOPEX. This, in turn, lowers optimal sub- IPTD IP packet transfer delay TCH Traffic channel on the air interfacescriber tariffs and makes service available to LAN Local area network TCO Total cost of ownershipan even larger group of users. The flat GSM MAP Mobile application part TDM Time-division multiplexingarchitecture will also create new business MGW Media gateway TRX Transceiver MLPPP Multilink point-to-point protocol VoIP Voice over IPopportunities where there are large volumes M-MGW Mobile MGW WAN Wide area networkof local calls, for example, at corporate cam- MPLS Multiprotocol label switching WCDMA Wideband code-division puses. MSC Mobile switching/service center multiple acceEricsson Review No. 1, 2007 31