NETWORK TOPOLOGY: Rings Get Affordable

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