Cellular networks today are grow ing and expanding rapidly and cellular service companies are looking for more cost-effective high capacity backbone network solutions. Prior to this, cellular operators didn’t pay much attention to building their own backbones, and instead leased lines from existing wireline operators. Cost and efficiency weren’t the issues; the name of the game was the number of subscribers.

In today’s telecom market, the key to success is profitability. Cellular companies today cannot afford to rent their backbones, and are suddenly aware of the great cost that backbones entail, within the framework of their overall network expenses. That cost is not only a heavy burden on today’s cellular network, but is expected to rise due to the added capacity required by next-generation services, such as 2.5G (GPRS) and 3G (UMTS) cellular networks.

While being challenged with limited cash flow and the need to show profitability, cellular operators need to invest in a higher capacity backbone capable of supporting next-generation technologies that will enable them to remain competitive in the cellular market. Another factor challenging cellular operators is that the outlook for future technologies is quite vague, as are typical applications and services that will be offered in the future. Caution is now a key factor in planning a cost-effective high capacity backbone, and it is this caution that’s saving today, and will save in the future, network development expenses.

This article will overview two different network solutions, that can be implemented by cellular operators who utilize high-capacity wireless systems to replace "leased lines." It will also outline a suggested topology solution that is both cost effective and provides the flexibility and modularity required to cope with future developments.

So What’s It all about?
Let’s consider the cellular network. We all see people walking around with their mobile phones, talking into the microphone. The voice signals from the microphone are digitized and transmitted to a cellular base station (BTS). Base stations are located in various places in order to cover the entire area. The voice call from the mobile phone is transported from the base station, over an E1 connection (2.048 Mbps) to a large hub site that collects the information from several BTS sites. From the hub site, the call is then transported to the main switch, and from there to the telephone or data network.

In a cellular network, the hub site is in the middle, and many BTS sites are located around it. In order to make things simple, we will assume that each BTS requires a capacity of up to 4 x E1. The aim of the backbone network is to channel all BTS traffic to the main hub site.

Backbone Network Topologies
All networks operate in one (or a combination) of several network topologies, which are interconnection schemes for data distribution. The most common topologies used for wireless networks are the star, and the ring. In a star topology, a core station is connected directly to smaller stations, which in turn are connected directly to other small stations. In a ring topology, the core station is connected to other stations, some directly and some indirectly, in a ring formation.

Traffic can be delivered from BTS sites to the main hub site.
In a star topology solution, standard PDH radios are used to deliver the traffic from the BTS site to the hub sites. Since several BTS sites are ‘cascaded’ in a single chain connection, a failure in one of the links close to the hub site, will cause a failure in the whole chain. Therefore, in this configuration, most of the radios in the chain (except the first one) are doubled up for connection redundancy.

In a ring topology solution, the synchronous digital hierarchy (SDH) ring is capable of providing the same capacity as the PDH star topology, and while PDH stars can only ensure equipment protection, SDH rings can ensure both equipment and traffic protection. A common method of traffic protection in the ring is known as path protection. Path protection means that a problem arises in a specific path (heavy rain, faulty antenna, or an unexpected obstruction), traffic will be transported from another path. When two alternative paths are defined in the system, if a radio link disconnection or failure occurs, a switch to the protected path will occur in less than 50 milliseconds.

In addition to the advantages mentioned above, while the initial deployment cost of wireless SDH rings is similar to that of PDH star networks, when network expansion is required, the cost for SDH ring expansion is significantly lower than that for PDH star networks. This is due to the fact that for SDH rings, less equipment is required. That, coupled with the fact that SDH rings allow more flexible bandwidth allocation compared with PDH star systems, make SDH rings the most beneficial way to connect cellular communication sites.

Do Rings Cost the Same?
Building a ring traditionally required at each site two STM-1 radios with an external box called an Add-Drop-Multiplexer (ADM). The idea of ADM is to allow new signals to enter the unit and existing signals to be dropped from a carrier channel by passing through the multiplexer. The goal of ADM is to add and drop signals without disrupting the onward transmission of other signals. The ADM is also responsible for the ring protection feature. This solution has many benefits in comparison with PDH stars, but also costs significantly more. The bottom line, profitability, is why cellular operators chose the PDH star topology, even though rings provide a significantly better solution.

The above was true till now. Recent technological innovation in the high-capacity wireless industry has led to the development of an STM-1 wireless system with built-in ADM (Add-Drop Multiplexer) functionality. The cost of STM-1 radio systems has been reduced in the past few years. Today, the cost of an STM-1 radio with a built in ADM, is similar to the cost of a redundant PDH radios required by the star solution.

Let’s take the example of a radio with a built-in ADM. A typical new generation ring-based cellular backbone can use these radios.

With built-in ADMs, wireless SDH equipment can integrate easily and more cost-effectively in cellular networks. This enables operators to enjoy the benefits of a ring-based backbone without adding too much to their network costs.

Flexibility and Future Upgrades
SDH ring equipment enables more flexible expansion opportunities for the network. At sites that have grown and require more capacity, the SDH ring can be adjusted quickly and cost-effectively to produce higher capacity network connections.

For example, if a cellular base station needs to have more 8 E1 links available at the site, for PDH networks one of two methods can be employed:

n Install an additional 8 E1 (1+1) link, in parallel to the existing link.
n Replace all E1 links with STM-1 radios and ADMs.

For SDH rings, the possibilities are as follows:

• While PDH equipment operates with fixed capacities, SDH ring equipment is more flexible and can generally be programmed to deliver higher capacities per site, as long as the ring capacity is not exceeded.
• SDH rings can be split to provide more capacity by adding just two additional links.

As an example of the second SDH method mentioned above, in a PDH star topology, three additional 8 E1 links can been added in the cellular network to provide more capacity.

The same cellular network can also have an SDH ring topology, whereby the single STM-1 ring was split using just two additional links, providing more additional capacity for the base station sites than the PDH star network.

Thus, simply by turning one STM-1 ring into two STM-1 rings, adding just two additional radio links, the SDH ring can deliver more capacity than PDH star networks. The obvious advantage is therefore reduced network cost due to less radio equipment, and faster and easier deployment.

Things to look for
Modern wireless transmission equipment comes in different types and sizes. The latest models include built-in ADMs and interfaces for a wide variety of data transmission protocols, such as Fast Ethernet, E1/T1, and E3/DS3. Different modulation schemes (16, 32, and 128 QAM) offer high system gain and high spectral efficiency.

When it’s time to consider equipment for your network, compare prices and find the equipment with the most features for the least cost, or at least for the most reasonable cost as compared with other similar equipment.

Look for field-proven equipment that has been deployed successfully, and can be easily integrated in networks with other existing equipment and management platforms.

Always estimate future network growth and what it would take to increase the capacity at some or all of the sites you intend to install.

For cellular networks, whichever equipment you choose, you should seriously consider the SDH ring topology before all other solutions.

And It All Comes down to…
Ideal network planning consists of a low budget and top quality. When considering the ideal solution for cellular networks, what’s needed is not only a relatively small initial investment for good wireless equipment, but also a peek into the future to plan ahead for the eventual growth of the network.

When you go looking for a cellular network solution that will do it all, for less cost, keep in mind that the SDH ring can satisfy your network requirements in the best possible way.

As pointed out in this article, in the search for an ideal cellular network solution, the ring topology is closer to the ideal solution than the star topology. With its ability to provide quality radio transmission of voice, video, and other data at higher capacities and less cost, SDH rings are the best possible way to connect cellular service sites. No other topology available today can match SDH ring expansion flexibility, and no other topology can compete with its ease of deployment and efficiency.

Shmuel Wasserman, Director (Product Management) Ceragon