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International Issue: September 2006
Feature
Overhaul the backhaul
The emergence of HSDPA will
force mobile operators to
rethink their cellular backhaul
strategies
by Gaby Junowicz
The introduction of high-speed downlink protocol access (HSDPA) is expected to enable mobile operators to offer broadband services at competitive costs. With a maximum theoretical peak throughput of 14.4Mbps per user, HSDPA can significantly speed the download of rich-media files to an HSDPA handset.
As an example, downloading one minute of audio from an MP3 music file is calculated to take 132 seconds with GPRS, 22.4 seconds with UMTS and only 4.1 seconds with HSDPA. The result is an 82 per cent saving in airtime. That means it’s possible to serve more mobile subscribers using the same infrastructure.
Such high capacity performance, however, comes at a price: an exponential increase in the bandwidth required to backhaul cellular traffic from Node Bs to the radio network controller (RNC).
In the mobile space, backhaul is the means for connecting base stations or cellular sites and the operator’s core network over a variety of transport media. It is one of the major costs of building and running a mobile network.
Industry consensus indicates that transport equipment accounts for 25 per cent of the costs of private cellular backhaul infrastructure. Transport outlays, moreover, vary between 40-60 per cent of the total cost of leased lines, with backhaul contributing 75 per cent of that sum. This reality obliges cellular providers to reconsider their cellular backhaul strategy.
Alternatives to E1/T1 lines
Today the majority of cellular networks rely on SDH/SONET or ATM transmission services with E1/T1 access lines. Although 3G data traffic is still only a relatively small overall portion of mobile transmission, this is expected to change as UMTS operators take their HSDPA networks on-stream over the next few years. Assuming that additional E1/T1 lines are readily available from the landline operator, the amount of backhaul traffic is predicted to grow faster than the expected average revenue per user (ARPU).
The challenge is that the explosive increase in backhaul that is required must be cost-managed to ensure mobile service provider profitability.
For purposes of comparison, while one or two E1/T1 lines comprising the transport network would be sufficient to handle the average number of links connected to 2G cellular base stations, HSDPA may increase this to between eight and 16 E1/T1s per 3G cellular site. Without mechanisms in place to control backhaul operating and capital expenses, the costs may not justify the business case for broadband mobile.
Supporting divergent demands
Complicating the equation is the need to support simultaneously the divergent technological demands and applications of existing 2G/2.5G networks and emerging 3G operations. The transition from TDM circuitswitched networks to ATM and, eventually, Gigabit Ethernet/IP/MPLS packet-switched networks raises new challenges. This is particularly true regarding the cost and suitability of the access platform to manage increased bandwidth capacity and the complexities of voice and data in a converged network.
Mobile operators are saddled with a bewildering choice of backhaul technologies and network interfaces as they try to anticipate which access infrastructure will best serve their needs.
The easiest decision would be to build out parallel networks, using a dedicated transport network for each different mobile generation. However, this is not as efficient or potentially cost-effective as integrating diverse traffic streams over a single backhaul link.
Given the drawbacks, does a converged backhaul access network solution exist that is technologically feasible, economically sound and readily available?
Aggregation and statistical multiplexing
One familiar method to reduce backhaul costs, traditionally implemented in high-density segments of the core network, is to aggregate several E1/T1s together and utilise statistical multiplexing to transport them over STM-1 lines.
Aggregation allows for more efficient use of the transport bandwidth and simplifies network management. Statistical multiplexing, moreover, is quite appropriate for the new types of data services that 3G will introduce.
With the introduction of 3G, the mobile world is evolving into a real multimedia environment. The range of new services embraces delay-sensitive and high quality services like video streaming, which require a reserve backhaul bandwidth (constant rate) to best effort-type services like internet surfing, back office services, mailing and data downloads. These services, by nature, are statistical also in terms of the air interface and backhaul bandwidth usage. They allow the operator to design its transport network so as to maximise efficiency by employing statistical techniques.
This new era of 3G and HSDPA services presents additional challenges to network designers. Aggregation, therefore, has now become an essential building block in the radio access and transport networks. In other words, it’s now implemented even at cell sites.
Fixed and mobile convergence
While all of this presents a significant challenge, there is also the convergence of fixed and mobile networks to consider. This trend is now being pioneered by world-class providers, such as BT and France Telecom, which already have announced the convergence of cellular and other services.
BT’s 21CN, for example, is intended to become ‘an advanced communications network for the future’ in which ‘end-users will be able to access voice messages, data or video at any time on any device’ and ‘share personal contact directory across their home phone, PC, mobile and PDA’.
Meanwhile, across the Channel, Orange already has become the single brand for France Telecom’s internet, television and mobile offerings.
The test here for network architects is that to succeed, convergence between fixed line and mobile services will have to provide the same look and feel. For that reason, services will have to be agnostic both to the access method and the devices that are being connected.
From the user’s standpoint, that means that e-mail or internet surfing must be indifferent to whether they’re being provided over a WiFi home connection or a 3G mobile handset. This requires a unified transport network, probably based on IP technology. In fact, the various standards bodies already are defining this unified IP-based transport network. IMS (IP multimedia subsystem) is one such technology enabler for this convergence. On the other side of the equation, 3GPP, the 3rd generation partnership project, has defined an ‘all-IP’ approach in all its standards ever since Rev 5.
Packet-based backhaul networks
The task ahead, therefore, will be to provide an effective solution to connect the installed base of mobile infrastructure — which has both GSM TDM-based and UMTS ATMbased network elements — over IP. This requires the use of pseudo-wire technology, which transports TDM/ATM circuits transparently across ethernet, IP or MPLS packet switched networks (PSNs).
Pseudo wire solutions are particularly suited to cellular backhaul because they’re transparent to the underlying traffic. Unlike VoIP, which requires translation of signalling, pseudo wires provide a transport tunnel across the statistical packet network without distortion.
As a rule, mobile networks require a high degree of synchronisation to maintain a proper service quality because cellular traffic is extremely sensitive to latency and packet loss. This is achieved by distribution of a common clock to serve as a point of reference among the numerous base stations spanning the network. Packet networks, however, are statistical-based by nature and do not provide inherent timing information whatsoever.
Matters are made worse in a PSN as a result of packet loss (PL), when packets do not arrive at their destination, and packet delay variation (PDV), when packets arrive with random, unpredictable delay. Sophisticated clock recovery mechanisms are required to reconstruct timing and achieve the desired timing accuracy in the presence of packet delay variation and packet loss. This kind of clock recovery mechanism results in a process that negates the effect of the random PDV and captures the average rate of transmission of the original bit stream.
By applying pseudo wire technologies, mobile operators will be able to speedily deploy high capacity W-CDMA services and keep HSDPA operating costs to a minimum while increasing their revenues and profitability from media-rich 3G content. As an interim solution, those mobile operators may consider a hybrid solution that will run all their existing and delay-sensitive traffic over the deployed TDM links, while only the aggregation HSDPA traffic will be connected employing pseudo-wire technologies and packet transport networks.
Gaby Junowicz, director, business development, RAD Data Communications
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