| ntroduction of HSDPA (High Speed Downlink Protocol | | | | nature, are statistical also in terms of the air interface |
| Access) is expected to enable mobile operators to | | | | and backhaul bandwidth usage. These diversified |
| exploit the full potential of 3G technology by offering | | | | services allow the operator to design its transport |
| mobile broadband services at competitive costs. With | | | | network so as to maximize efficiency by employing |
| a maximum theoretical peak throughput of 14.4 Mbps | | | | statistical techniques. |
| peak per user, HSDPA can significantly shorten the | | | | This new era of 3G and HSDPA services presents |
| time required to download rich-media files to an | | | | additional challenges to network designers. |
| HSDPA handset. As an example, downloading one | | | | Aggregation, therefore, which characterizes existing |
| minute of audio from an MP3 music file is calculated to | | | | core networks, now has become an essential building |
| take 132 seconds with GPRS, 22.4 seconds with | | | | block in the radio access and transport networks. In |
| UMTS and only 4.1 seconds with HSDPA. The result is | | | | other words, it is now implemented even at cell sites. |
| an 82% savings in airtime. That means it is possible to | | | | Fixed and Mobile Convergence |
| serve more mobile subscribers using the same | | | | While all of this presents a significant challenge for |
| infrastructure. Such high capacity performance, | | | | mobile system architects, we haven't yet mentioned |
| however, comes at a price: an exponential increase in | | | | the next great challenge looming just above the |
| the bandwidth required to backhaul cellular traffic from | | | | horizon, which is posed by the convergence of fixed |
| Node Bs to the RNC (radio network controller). | | | | and mobile networks. This trend is now being |
| "Backhaul" is a broad term for the provision of | | | | pioneered by world-class providers such as British |
| connectivity in the service provider's core network. In | | | | Telecom (BT) and France Telecom (FT), which |
| the mobile space, backhaul is the means for | | | | already have announced the convergence of cellular |
| connecting base stations or cellular sites and the | | | | and other services. |
| operator's core network over a variety of transport | | | | 21CN, BT’s "21st century programme," for |
| media. Backhaul is one of the major contributors to the | | | | example, is intended to become "an advanced |
| high costs of building out and running a mobile network. | | | | communications network for the future" in which |
| Industry consensus indicates that transport equipment | | | | "end-users will be able to access voice messages, |
| accounts for 25% of the costs of private cellular | | | | data or video at any time on any device" and "share |
| backhaul infrastructure. Transport outlays, moreover, | | | | personal contact directory across their home phone, |
| vary between 40-60% of the total cost of leased lines, | | | | PC, mobile and PDA." Meanwhile, across the English |
| with backhaul contributing 75% of that sum. This reality | | | | Channel, Orange already has become the single brand |
| obliges cellular providers to reconsider cellular backhaul | | | | for France Telecom's Internet, television and mobile |
| strategy. | | | | offerings, the first step towards fully convergent |
| Alternatives to E1/T1 Lines | | | | operations. |
| Today the majority of cellular networks rely on SDH | | | | The test here for network architects is that to |
| SONET or ATM transmission services with E1/T1 | | | | succeed, convergence between fixed line and mobile |
| access lines. Although 3G data traffic is still only a | | | | services will have to provide the same look and feel. |
| relatively small overall portion of mobile transmission, | | | | For that reason, services will have to be agnostic both |
| this situation is expected to change quickly as UMTS | | | | to the access method and the devices that are being |
| operators take their HSDPA networks onstream over | | | | connected. From the user’s standpoint, that |
| the next few years. Assuming that additional E1/T1 | | | | means that e-mail or Internet surfing must be |
| lines are readily available from the landline operator, the | | | | indifferent to whether they’re being provided |
| amount of backhaul traffic is predicted to grow faster | | | | over a WiFi home connection or a 3G mobile handset. |
| than the expected average revenue per user (ARPU). | | | | This requires a unified transport network, which, we |
| The challenge is that the explosive increase in | | | | probably can assume, will be based on IP technology. |
| backhaul waiting on the doorstep must be controlled to | | | | In fact, the various standards bodies already are |
| ensure mobile service provider profitability. For | | | | defining this unified IP-based transport network. IMS (IP |
| purposes of comparison, while one or two E1/T1 lines | | | | Multimedia Subsystem) is one such technology enabler |
| comprising the transport network would be sufficient | | | | for this convergence. On the other side of the |
| to handle the average number of links connected to | | | | equation, 3GPP, the 3rd Generation Partnership |
| 2G cellular base stations, the impending introduction of | | | | Project, has defined an "all-IP" approach in all its |
| HSDPA may require this number to be increased to | | | | standards ever since Rev 5. |
| anywhere from eight to sixteen E1/T1s per 3G cellular | | | | Packet-Based Backhaul Networks |
| site. Without mechanisms in place to control backhaul | | | | The task ahead, therefore, will be to provide an |
| operating and capital expenses (Opex and Capex), the | | | | effective solution to connect the installed base of |
| costs may not justify the business case for broadband | | | | mobile infrastructure –which has both GSM |
| mobile. | | | | TDM-based and UMTS ATM-based network |
| Supporting Divergent Technological Demands and | | | | elements – over IP. This requires the use of |
| Applications | | | | pseudowire technology, which transports TDM/ATM |
| Complicating the equation is the need to support | | | | circuits transparently across Ethernet, IP or MPLS |
| simultaneously the divergent technological demands | | | | packet switched networks (PSNs). Pseudowire |
| and applications of existing 2G/2.5G networks and | | | | solutions are particularly suited to cellular backhaul |
| emerging 3G operations. The transition from TDM | | | | because they are transparent to the underlying traffic. |
| circuit-switched networks to ATM and, eventually, | | | | Unlike VoIP, which requires translation of signaling, |
| Gigabit Ethernet/IP/MPLS packet switched networks | | | | pseudowires provide a transport tunnel across the |
| raises new challenges, particularly regarding the cost | | | | statistical packet network without distortion. |
| and suitability of the access platform to handle and | | | | As a rule, mobile networks require a high degree of |
| manage efficiently increased bandwidth capacity and | | | | synchronization to maintain a proper service quality |
| the complexities of voice and data in a converged | | | | because cellular traffic is extremely sensitive to |
| network. | | | | latency and packet loss. This is achieved by distribution |
| Mobile operators are saddled with a bewildering choice | | | | of a common clock to serve as a point of reference |
| of backhaul technologies and network interfaces as | | | | among the numerous base stations spanning the |
| they try to anticipate which access infrastructure will | | | | network. Packet networks, however, are |
| best serve their current and future requirements. The | | | | statistical-based by nature and do not provide inherent |
| easiest decision would be to build out parallel networks. | | | | timing information whatsoever. Matters are made |
| Using a dedicated transport network for each different | | | | worse in a PSN as a result of Packet Loss (PL), when |
| mobile generation, however, is not as efficient or | | | | packets do not arrive at their destination, and Packet |
| potentially cost-effective as integrating diverse traffic | | | | Delay Variation (PDV), when packets arrive with |
| streams over a single backhaul link. | | | | random, unpredictable delay. Sophisticated clock |
| Given the drawbacks, does a converged backhaul | | | | recovery mechanisms are required to reconstruct |
| access network solution exist that is technologically | | | | timing and achieve the desired timing accuracy in the |
| feasible, economically sound and readily available? | | | | presence of packetdelay variation and packet loss. |
| Aggregation and Statistical Multiplexing | | | | This kind of clock recovery mechanism results in a |
| One familiar method for reducing backhaul costs, | | | | process that negates the effect of the random PDV |
| traditionally implemented in high- density segments of | | | | and captures the average rate of transmission of the |
| the core network, such as the Base Station Controller | | | | original bit stream. |
| (BSC) or the Mobile Switching Center (MSC), is | | | | Deploying HSDPA |
| aggregating several E1/T1s together and utilizing | | | | By applying pseudowire technologies, mobile operators |
| statistical multiplexing to transport them over STM-1 | | | | will be able to speedily deploy high capacity W-CDMA |
| lines. Aggregation is an essential part of existing cellular | | | | services and keep HSDPA operating costs to a |
| network transport design because it allows for more | | | | minimum while increasing their revenues and profitability |
| efficient use of the transport bandwidth and simplifies | | | | from media-rich 3G content. As an interim solution, |
| network management. Statistical multiplexing, | | | | those mobile operators may consider a hybrid solution |
| moreover, is quite appropriate for the new types of | | | | that will run all their existing and delay-sensitive traffic |
| data services that 3G will introduce. | | | | over the deployed TDM links, while only the |
| With the introduction of 3G, the mobile world is evolving | | | | aggregation HSDPA traffic will be connected |
| into a real multimedia environment. Instead of plain | | | | employing pseudowire technologies and packet |
| voice services, a wider range of services is available | | | | transport networks. |
| to subscribers. This range embraces delay-sensitive | | | | Mobile telephony's backhaul challenge opens a door for |
| and high quality services like video streaming, which | | | | new solutions to be incorporated in the transport |
| require a reserve backhaul bandwidth (constant rate) | | | | network, such as packet-based technologies. |
| to best effort-type services like Internet surfing, back | | | | Pseudowire techniques are proven technology |
| office services, mailing, data downloads, etc., which, by | | | | enablers for such a migration. |