Transport Systems

At today's executive-level meetings, boardroom discussions and engineer desks, futuristic topics such as third generation and wireless/wireline integration have come front and center. But before you dream of innovative technologies that will take you into the next generation, there are more current issues that you should address to optimize your network.

How do you efficiently connect multiple base stations and switching centers to each other? How do you build an infrastructure that is flexible and scaleable for future service offerings? How will that system communicate with the rest of the world?

Before you move to the next generation, you should take a look at the basics of transport systems used in wireless networks and how the revolution in optical networking will have an effect on how wireless networks are designed and constructed. Simple changes in the way the transport network is designed can leverage optical networking and provide you with improved reliability and cost efficiency.

Every cell site or base station needs to be connected to the MSC. Typically, the wireless transport network planner determines how many locations (cell sites and switches) need to be connected and how much traffic is expected in and out of each location. Then, the planner works with a wholesale service provider to provide appropriate bandwidth connecting all locations. Usually, carriers do not buy an entire physical connection between locations. Instead, they buy a certain level of bandwidth, which could be shared with other users.

The basic unit of bandwidth is measured in terms of a DS0 circuit, the bandwidth capacity of a single telephone call. A T1 or DS1 circuit contains the equivalent of 24 DS0 circuits, although the use of compression algorithms means that a T1 in a wireless backhaul network probably carries more than 24 conversations. If you need greater capacity, a DS3 or T3 circuit provides the equivalent bandwidth of 672 DS0 circuits.

Although it is possible to buy direct connections from each cell back to the switch, you would have to purchase a lot of unused capacity. For example, you may need only half the capacity of a T1 to serve a particular cell site. Even in cases where fractional T1 service is available, the per-DS0 cost is more expensive than purchasing full T1 service. Similarly, T3 service is less expensive than multiple T1s. To build an efficient network, you must "groom and fill" T1 facilities. By using a narrowband digital cross-connect system, you purchase T1 service connecting several cells to each other converging at a hub cell. Then, traffic is aggregated and sent to the MSC over fewer T1s or, if traffic warrants, over T3 service. Instead of paying for a lot of partially used T1s all of the way back to the MSC, you pay for fewer fully packed facilities.

The transmission media is the physical copper or fiber-optic circuits that connect cells to switches and switches to the outside world. Increasingly, the physical connections are carried over fiber-optic facilities. Fiber has the advantage of greater call-carrying capacity and can provide that capacity at a higher reliability and lower error rates. Generally, wireless carriers are insulated from the fiber vs. copper selection: They purchase bandwidth, not physical media. If a wireless service provider needs three T1 lines between two locations, the wholesale service provider usually determines the media itself. But because fiber is becoming common, there is a likelihood that connections in the wireless network are provided over fiber-optic facilities.

The transmission equipment has two roles at the MSC. First, it is there to sort out all of the incoming traffic and pass it to the switching system. Once the switching system performs its function, the traffic will need to be passed back to the cell site (in the case of mobile-to-mobile calls), or to local or long-distance companies, which will connect the call at the far end. Depending on the traffic load into and out of the MSC, a narrowband or broadband digital cross-connect is used. Generally, the narrowband cross-connect, like those used in hub cell locations to optimize backhaul traffic, manages traffic at the DS0 and DS1 rates. A wideband digital cross-connect is used for higher speeds, such as DS3, and the optical carrier rates of OC-3 and OC-12. Because substantial traffic will flow to the LEC or long-distance carrier, a wideband cross-connect probably will handle traffic most efficiently.

SONET & FIBER RINGS Based on growth demand, competitive pressures and cost reduction in optical technology, carriers increasingly are deploying SONET and fiber in their networks. RBOCs have started marketing SONET services directly to customers, including large businesses and other carriers. Wireless carriers are adapting their networks to take advantage of this fiber backbone.

Although the need to connect cells to the MSC and the MSC to the local or long-distance network is conceptually a point-to-point connection, there should be multiple paths between each of those point-to-point connections for true reliability. With increased competition, cell-site outages could be enough to drive customers to another provider. Fortunately, the industry has developed a standard by which optical networks can be set up with multiple diverse paths between locations. SONET ring technology provides the ability to route traffic over diverse paths so that a cable cut doesn't interrupt traffic. Although two separate paths between two locations may be considered a SONET ring, in actuality the ring will have many stops along the way, with some traffic originating and some traffic terminating at each location. The key point is that because of the ring topology, when the connection is broken at any point, there still will be another path between any two points, and traffic will reach its destination.

Today's wideband digital cross-connect systems interact with these SONET rings to support rapid facility restoration, provide flexible network administration and permit the reshaping of the transport network with a few keystrokes from a centralized location such as the MSC.

Today's SONET digital cross-connect systems provide the real-time performance-monitoring capabilities to monitor near- and far-end data-handling performance, which can provide your staff with advanced warning of possible circuit failures. The ability to collect and report instantaneous performance statistics gives wireless carriers the ability to reconfigure circuits, swap facilities, map circuits to specific facilities and facilitate the cutover of new switching and cell-site equipment. Even if the facilities are leased rather than owned, the performance-monitoring capabilities help to ensure that the wholesale facility provider is living up to maintenance and grade-of-service commitments.

OPTICAL TRANSPORT TRENDS Taking advantage of SONET network technology does introduce a certain level of complexity. For example, when SONET networks first were deployed, carriers needed a SONET add/drop multiplexer (ADM). The network needed an easy way to add or subtract traffic at any given point on the ring; otherwise, all traffic would have to be remapped at every stop.

Using an ADM, however, adds a new layer of network complexity. A traditional OC-12 interface to a SONET ring requires a fiber interface or LGX, an OC-12 ADM terminal to convert the fiber-optic connection to coax, a coax patch panel (with multiple runs of coaxial cable) and coaxial cable interface cards in the digital cross-connect system. The SONET ADM unit itself can cost tens of thousands of dollars. Each coax connection has to be installed, checked and maintained. In addition, each connection represents a potential point of failure.

One method to address this complexity is to incorporate SONET ADM functionality into the cross-connect. With an integrated SONET ring optical interface technology, there is no need for an ADM terminal.

What are the benefits of integrated optics? First, a duplex interface to SONET rings increases reliability over a coax connection. In the OC-12 example, 96 separate connections exist among the OC-12 terminal, DSX-3 and digital cross-connect -- each with the potential for failure. By reducing the number of physical connections and interfaces, you reduce the chances of failure.

Integrated optics also streamline system administration. With fewer network elements, the number of network alarms to monitor is reduced. This reduces software maintenance and upgrade requirements. Integrated fiber optics also decrease the number of network-timing supplies needed while reducing network-management and operational-support-system connections.

Cost benefits from the integrated ring optics capability go beyond up-front costs. Although the initial investment in the digital cross-connect hardware probably will be higher than your current method of operation, those costs will be offset by the elimination of the ADM terminal and associated installation and wiring costs. In the long run, easier administration capabilities will result in significant cost savings. Fewer product upgrades, conservation of power and office floor space, timing supplies, alarm, network management, and support systems all save dollars.

The benefits will multiply when you connect dual cross-connect systems, a situation you invariably will face if you have multiple MSCs in a market. You can connect two MCSs through respective digital cross-connect systems completely over fiber. No electrical connections are required.

The increased deployment of fiber optics opens doors of opportunities for wireless service providers. Carriers that take advantage of fiber will recognize significant quality and reliability benefits. The new optical capabilities built into today's digital cross-connect systems will offer significant cost benefits.