Hamburger Helper and OC-12: stretching SONET rings on a budget

Sept. 1, 2000

At the University of Texas, upgrading the network with a WDM access solution allowed SONET and Gigabit Ethernet to coexist on the existing OC-12 ring.

Paul Zalloua

The communications business faced the possibility of a widespread bandwidth famine a few years ago, as customers' capacity requirements threatened to exhaust the deployed fiber in long-haul and metropolitan networks. The advent of WDM has alleviated this threat in all but a few congested markets, however. While optical solutions can meet most demands, as bandwidth needs continue to escalate, particularly in access networks, it is evident that an optimal access solution remains unclear.

Legacy access networks usually contain some level of SONET/SDH, a technology designed and perfected for voice traffic. As data traffic begins to exceed voice, upgrading these SONET/SDH networks to transport higher volumes of traffic-and a greater portion of bursty data traffic-is becoming increasingly important.

Superficially, this situation looks very similar to the fiber-exhaust problem. The capacity of legacy fiber networks can be increased in several ways, however. Network operators can upgrade the speed of an OC-12 (622-Mbit/sec) ring to OC-48 (2.5 Gbits/sec) or OC-192 (10 Gbits/sec). Another approach is to graft metropolitan DWDM equipment onto the ring.

Wayne Wedemeyer, the manager of fiber infrastructure at the University of Texas/Austin faced this dilemma earlier this year. When users of the university's information-technology (IT) department demanded native Gigabit Ethernet (GbE) throughput from one end of its research and development facilities to the other, Wedemeyer needed to boost a significant portion of the OC-3/OC-12 SONET network to the next performance level.

During construction of the fiber plant several years ago, the university elected to install only six fiber strands throughout much of the campus. One of the largest campus environments in the world, the University of Texas's existing wide area network resembled a modestly sized carrier's network. It was dominated by SONET technology and consists primarily of OC-3, OC-12, and OC-48 rings. The existing fiber was almost completely saturated with SONET traffic, which was forcing Wedemeyer to either find a solution to the fiber-exhaust problem within one calendar quarter or face the strong likelihood of network performance degradation.
The University of Texas's network uses wavelengths to carry T1, 10/100Base-T Ethernet, Gigabit Ethernet, and OC-12 SONET traffic. The voice traffic continues to be transported on the legacy SONET ring.

Wedemeyer describes his problem in terms that resonate with managers operating similar networks: "I had to meet three requirements on this production network: Get the most out of our nearly exhausted fiber plant; support the explosive growth in bandwidth across all applications; and start upgrading to dynamic wavelength service delivery in a way that would scale across all nodes."

It was not practical or cost-effective to pull more fiber. Instead of upgrading the existing OC-12 ring to an OC-48 ring, a process that could have easily cost more than $1 million, Wedemeyer opted to increase the bandwidth between the university's Network Operations Center and the Balcones Research Center by installing WDM access systems. But employing the type of DWDM equipment currently designed for metropolitan networks appeared to be a costly proposition.

Instead, Wedemeyer decided to work with LuxN, a provider of physical-layer, optical access systems, to devise an overlay solution that could meet the network's needs. The optical transport equipment was designed to coexist transparently with SONET and SDH rings and provide single-wavelength or multiwavelength optical services side-by-side with bidirectional line-switched ring (BLSR) OC-n channels or ATM circuits.

When the network design work began, only limited "wavelength plus SONET/ATM" network configurations had been implemented on the chosen platform. The University of Texas's network was the first large-scale network to implement this concept.

The solution was designed so that two 16-channel WDM access products (LuxN WavStations) could be inserted in a cut in the SONET ring. The SONET ring would then be reconnected through the WDM equipment and subsequently brought back up. There would be no disruption in the legacy applications during the equipment configuration process because the ring was protected. Once the ring was re-established, the add/drop multiplexers that bring traffic in and out of the ring would be unable to detect the presence of the additional equipment.

With the optical WDM access equipment in place, it was a relatively simple matter to install additional wavelengths to carry GbE, T1, and 10/100Base-T Ethernet traffic on the existing fiber of the OC-12 ring. In effect, the new equipment would provide the SONET equivalent of Hamburger Helper by taking the basic capacity of a SONET system and allowing it to be stretched, without taking away the core ingredient or adding significantly to the cost structure.

The installation of the new equipment on the university's SONET ring began with a site survey-well in advance of the actual equipment configuration. The survey reviewed the customer-premises equipment's optical interface power requirements and measured the optical losses on all site-to-site links, a process also known as "shooting the glass." The physical installation required bringing down the OC-12 SONET ring just long enough to re-route the traffic through a WDM access system (WavStation) at the Network Operations Center and at the Balcones Research Center.

Each WavStation was then configured to support four wavelengths (lambdas). The first lambda was assigned to support OC-12 ring traffic and restore SONET services. The second lambda was configured to provide GbE services. The third lambda was configured to support four T1s and one 10/100Base-T Ethernet link. Because all optical protocols (OC-3, OC-12, OC-48, GbE, and Fibre Channel) are supported with the same card, each WavStation is equipped with just one redundant module. These modules are currently assigned to carry additional GbE services. The entire configuration process took less than 15 minutes.

The university's network now uses wavelengths to carry T1, 10/100Base-T Ethernet, GbE, and OC-12 SONET traffic. Meanwhile, the voice traffic is still transported on the legacy SONET ring (see Figure). Says Wedemeyer, "I did not initially intend to deploy WDM in this network, but the solution has proven to be affordable and met my top requirements."

The upgraded communications network is designed to scale to meet increasing bandwidth demand.

Wedemeyer plans to utilize the same WDM access system technology in situations where higher bandwidth is required for video applications; for instance, to support the university's expanding distance-learning program. The university currently provides 900 hours of distance learning per month. Future applications for the university include upgrading a four-node OC-12 network to increase the bandwidth for campus users, state government offices, and local school district sites.

The University of Texas effectively deployed LuxN WavSystems to enhance the bandwidth of legacy SONET rings. However, the flexibility and range of the native transmission protocol LuxN supports offers more than a simple extension of bandwidth. One of the more compelling applications of technology, not lost on Wedemeyer and his staff, is the ability to reduce network capital costs. The WDM access architecture can extend the reach of the LAN up to 100 km, thus the cost of equipping larger multiple building networks can be greatly reduced. This savings is associated with the ability to reduce traditional network and WAN technology for LAN segments that extend more than 100 m. When combined with operational savings realized by reducing or eliminating campus T1 and DS-3 circuits, the return on investment is significant.

Paul Zalloua is director of product management at LuxN (Sunnyvale, CA).