A faster, cheaper route to broadband?

Rapidly growing numbers of users and their unrelenting hunger for bandwidth are precipitating a revolution at the network edge. Users are increasingly looking to upgrade their low- and medium-speed access connections to T-1 speeds and higher. Service providers must address this demand by greatly increasing the capacity of their points of presence.

To date, broadband connections at DS-3 (45 Mb/s) speeds and higher have been used primarily within the networks of major carriers and to build the backbone networks for competitive local exchange carriers, Internet service providers and large enterprise networks. But in the next few years, use of broadband connections at the network edge will increase dramatically. Analysts estimate that within three years, the number of DS-3s in use outside the networks of primary carriers will increase by a factor of five, and that CLECs, ISPs, microwave and mobile operators will be the biggest consumers.

Today, those types of carriers typically use T-1 circuits to connect most of their POPs. Although they could address increased capacity requirements by installing additional T-1 lines, this approach not only adds complexity, but it also fails to take advantage of the economic value of DS-3 connections. DS-3s are usually available for only four to eight times the cost of a T-1-only 10% to 20% of the cost per unit bandwidth.

Although the case for service providers to push DS-3 to the edge of their networks is economically compelling, this migration brings its own set of challenges. Equipment currently installed in the POP typically does not support DS-3 directly, and in those few cases in which DS-3 interfaces are available, they can be extremely expensive. And DS-3 interfaces sometimes suffer from throughput limitations: The economic benefits of a DS-3 connection quickly disappear if the equipment attached to it cannot handle the throughput.

On the other hand, existing telco DS-3 multiplexers cannot meet the need of geographically distributed POPs. They are expensive to buy and operate, and require an army of technicians to install, operate and maintain. Although these limitations may be manageable in the limited context of a core network, they are not acceptable in a more extensive implementation.

The utility challenge A critical part of the challenge that CLECs and ISPs face is to match the utility-class service expected of telephone companies without matching the enormous expenditures in people and infrastructure that telephone companies carry. To meet this challenge, those carriers need to exploit the latest developments in networking technology.

As customers increase the bandwidth they consume and deepen their commitments to their carriers, the exposure from any service outage also increases. Every new component introduced into the network must have the potential to offer non-disruptive service. Because of the widespread impact of any failure, this need is particularly acute for DS-3 equipment.

Before examining solutions for upgrading POPs to DS-3, we will look at two application examples. The first relates to an ISP and the second to a CLEC. Similar requirements are emerging in other carrier categories and in large enterprise networks.

To deliver T-1 service economically to their customers, ISPs need to run DS-3 circuits to each POP, then install a DS-3 multiplexer (known as an M13 multiplexer) in the POP to interface to the DS-3 and subdivide it into T-1 links. Some of the T-1s can be used for customer access, while others are used to support lower-speed access from the POP using the same equipment that is in use today (Figure 1).

CLECs are increasingly installing Sonet rings in the business centers they serve. To provide T-1 service from the Sonet ring, an M13 multiplexer is used to fan out the DS-3 connection to the Sonet ring into multiple T-1 connections. The M13 multiplexer can be located in a POP connected to the Sonet ring by a DS-3. It can also be collocated with the ring, providing T-1 ports at a much lower cost than adding T-1 ports to the Sonet add/drop multiplexer (Figure 2).

Traditional multiplexers don't fit For many years, traditional telco equipment vendors have supplied major telcos with a range of DS-3 multiplexing equipment. This equipment has provided M13 multiplexer functionality with the high reliability required and has met the exacting mechanical requirements for telco installation. But these multiplexers have a number of characteristics that make them unsuitable for higher volume applications:

* They are too expensive.

* Their physical dimensions are too large for the limited space available in many POPs.

* Installation and support are too complex and labor-intensive for volume implementations.

* Network management support is inappropriate for geographically distributed equipment and inconsistent with other equipment installed in the POP.

But advances in broadband technology have made possible a new generation of M13 multiplexers designed specifically for network edge applications.

To provide all the benefits of the traditional telco M13 multiplexer, the new generation must deliver the same highly reliable service and meet the same exacting requirements for mechanical and thermal design, standards compliance and non-stop operation.

The new M13 multiplexer must also provide the low cost, small size and easy operation critical to network edge applications. It is this combination of capabilities that will enable CLECs and ISPs to integrate DS-3s in their infrastructure as easily as any of the services they currently use.

The core functionality of an M13 multiplexer is straightforward. On the network side of the multiplexer is a 45 Mb/s DS-3 circuit, and on the other side are 28 DS-1/T-1 circuits. The multiplexer maps the 28 T-1s onto the DS-3 using the framing conventions of the digital hierarchy, ensuring end-to-end compatibility with other devices elsewhere in the network.

One issue that can arise is that DS-3s are provided in two formats, known as c-bit and p-bit. The c-bit format identifies a newer implementation that provides additional alarming and remote provisioning to the telco. Because the customer often has no choice over which type is provisioned, it is vital that the M13 multiplexer supports both.

The most important element of M13 multiplexer functionality is fault tolerance. Although hardware has become extremely reliable, today's exacting service level agreements often demand that service disruption caused by hardware failure be eliminated. With the enormous quantities of data carried on a DS-3, the ability to provide fault tolerance as a standard or optional feature can be critical.

But automatic fault tolerance is only part of the requirement. Once a failure has occurred, it must be possible to service the unit without taking it-and the customers it supports-out of service. Even a short scheduled service disruption typically requires customer notification, the logistics of which make it an expensive undertaking to be avoided where possible.

Other considerations One of the greatest concerns for many POPs is physical space. Unlike traditional M13 multiplexers, this new generation needs to be highly compact and stackable, resulting in a high port density.

Reflecting the range of environments in which POPs are installed, the new M13 multiplexer should be suitable for mounting in a commercial cabinet or a telco frame, as well as equipped for wall mounting in a utility closet. Power consumption should also be low, with 48 VDC power standard.

For CLECs and ISPs that operate POPs on LEC premises, another consideration is compliance with Bellcore NEBS, which many LECs require for any equipment installed in their central offices. NEBS are strenuous, and in general cannot be met unless designed into the product. And in today's heightened competitive environment, many LECs are becoming less flexible in allowing exceptions to NEBS compliance.

Over the life cycle of a network, far more resources go into operating it than procuring equipment. When planning a network, one of the most important considerations is minimizing operating expenses.

Some new M13 multiplexers are designed with many features to minimize operating costs, such as a comprehensive range of remote support capabilities. Although remote support is often taken for granted in equipment such as routers, this capability is limited among M13 multiplexers.

The remote support capabilities should start with an embedded simple network management protocol (SNMP) agent that supports the industry-standard DS-1 and DS-3 management information bases, using them to provide a complete array of status and event information to an SNMP manager. Diagnostic tools should include a complete array of alarm conditions, and local and remote loopbacks.

Training costs can also make a significant contribution to life cycle costs. Some of the new M13 multiplexers are designed to be installed and serviced by a technician with no specialized training. On-site configuration should not be required during installation, and unlike traditional M13 multiplexers, T-1 ports should be connectorized rather than wire wrapped to speed installation and eliminate error.

Replacement following hardware failure is simple and non-intrusive with some M13 multiplexers. One features a fault-tolerant unit with just two identical cards, each of which houses all of the active components for the system to operate. If there is a failure on one card, service is automatically switched to the backup. The service technician replaces the faulty card, which is clearly indicated by a fault light and is accessible from the front of the unit with no cable removal. Management of spares is simple: There is only one field-replaceable unit to stock, so the service engineer can be sure he is carrying the right component.

With soaring demand for their services, CLECs and ISPs will increasingly need to boost the bandwidth to their POPs. In most cases, economic considerations will push them toward DS-3s for this purpose. The POPs must be upgraded to support DS-3 connectivity, and a key component of this upgrade is an M13 multiplexer to divide the DS-3 into its constituent T-1s. But traditional M13 multiplexers cannot meet the requirements of this application.

To respond to this need, a new class of M13 multiplexers has emerged for the network edge. They provide the full functionality of an M13 multiplexer with the quality and reliability of a telco class product, but at price levels and with physical and operational characteristics in line with the needs of a high-volume, geographically dispersed and cost-sensitive implementation.

This latest development in multiplexing technology enables ISPs and CLECs to reap the benefits of attractive DS-3 tariffs without introducing major economic and operational headaches. Now, those service providers can be big winners in the revolution at the network's edge.