How efficient is your backhaul link? Local loop traffic aggregation, through the use of ATM switches, can significantly improve the business case for delivering high-speed internet access over ADSL networks - The Unbundled Network

Telecommunications, Nov, 2001 by Jim Guillet

Deciding which is the best network architecture for aggregating local loop traffic will depend on the applications themselves. These can range from voice to multimedia applications (such as mobile, PBX, frame relay and internet traffic). And as each application has a varying quality of service requirements, traffic aggregation needs to support a multi-service environment.

Traffic aggregation typically occurs on the backhaul link (ie, between the central office or base station and the operator's point of presence on the core network). How this backhaul link is designed and managed will be crucial to the service provider's business case, since it will largely determine the costs and levels of customer service it can offer.

One of the key applications is, of course, high-speed internet access (HSI). And without an optimised aggregation and backhaul strategy, the business case for providing HSI weakens. Central to achieving the optimum network architecture for the delivery of HSI are network edge concentration and flow-through service provisioning.

HSI basics

To appreciate the business case for multi-service aggregation generally -- and HSI aggregation specifically -- a quick review of HSI is in order. The principles behind HSI traffic aggregation are the same whether the fixed local loop access is wireline or wireless. For the purposes of this article, however, the focus is on wireline and ADSL (asynchronous digital subscriber line) technology since it is this transmission medium which currently supports the mast HSI traffic.

The basics of HSI via ADSL, as illustrated in Figure 1, are as follows:

* An ADSL modem on the customer premises provides the interface between ethernet and the ADSL network termination. The modem provides an ethernet bridging function, performing ATM segmentation and reassembly of the customers' IP traffic in and out of an ATM virtual circuit (ATM VC).

* A digital subscriber line access multi-plexer (DSLAM) provides the ADSL line termination and aggregates the ATM virtual circuits from all subscriber loops onto a DS-3, OC-3 or STM-1 ATM uplink.

* DSLAM uplinks feed into an ATM backbone, across which each customer's ATM VC is backhauled to the appropriate ISP where it is terminated on a broadband access server (BAS). The BAS provides IP-level session management, including authentication, access control and administration. Behind the BAS are the routers, switches and servers that comprise the internet.

Backhaul utilisation

Initial residential deployments for HSI over ADSL typically impose a maximum downstream speed of 500Kbps and an upstream speed cap of 128Kbps. Business deployments for HSI, which command a higher price, allow higher speeds depending on local loop length.

Despite these bit rates, the peak, simultaneous usage of residential customers is in the order of 15Kbps. With the vast majority of DSLAM deployments supporting 200 to 500 subscribers (a few large sites support up to 2,000), the corresponding peak load on the DSLAM's uplink varies from 3 to 7.5Mbps. Depending on the uplink speed -- 45Mbps or 155Mbps -- the peak, simultaneous uplink utilisation for residential HSI is 2 to 20 per cent.

Taking into account that not all subscribers will be using service simultaneously, over-subscription of data services is usually engineered at four to eight times (ie, an operator sells more capacity to individual subscribers than is actually available on the backhaul link). With residential HSI, however, over-subscription can be as much as 50 times although, in practice, approximately ten times over-subscription is the norm. This partly explains why an ATM backbone is so often used to aggregate HSI traffic at the network edge. Aggregating ATM virtual circuits (VC) from multiple DSLAMs provides a significantly higher utilisation of backbone facilities, compared to having the DSLAM interconnect directly with the BAS over a leased line.

Even though the DSLAM itself aggregates residential HSI traffic from hundreds of ADSL subscribers, backhaul utilisation can be improved by at least another factor of ten using ATM switches.

Capex optimisation

Another consideration in the business case for HSI aggregation is the optimal mix of IP session managers, commonly referred to as broadband access servers (BAS) and ATM switches. The economic question is: since the BAS supports direct termination of ATM interfaces (as well as IP session management), why not co-locate a BAS with each DSLAM, ignoring the ATM network entirely?

Not surprisingly, the answer is cost. It's an old maxim in the networking trade that the price per port generally increases as its location in the network moves toward the core. In addition, the price increases with the platform's intelligence. Both apply to the BAS when compared to the relatively inexpensive ATM edge switch.

By allowing the ATM network to aggregate HSI traffic from multiple DSLAMs and backhauling it to a few centralised BAS locations, net capex is reduced. For example, the market price for a BAS' ATM interface is at least triple that of an ATM switch. From the backhaul utilisation discussion above, aggregating the traffic from ten DSLAMs onto an equivalent sized pipe through an ATM switch, and passing this highly utilised pipe into a single interface on a BAS reduces capex by more than 50 per cent.