BEVERLY — The city of Salem didn't have to go far to install wireless surveillance cameras for Halloween.

Help was just over the bridge.

SideBand Systems Inc. of Beverly, a wireless communications firm on Rantoul Street, did the high-tech engineering that allowed Salem police to watch the crowd of 100,000 from monitors inside police headquarters.

Another company made the cameras, but SideBand did the design, installation and oversight of a system that now provides police with eight sets of eyes all the way from Salem Common to Riley Plaza.

Asked to explain his business, Jack Davis, the company's CEO and president, said: "We put together the infrastructure, the plumbing."

Since 1992, SideBand has done a lot of "plumbing."

This little company with a dozen employees working out of a converted, 1906 firehouse has found a niche moving data, images and sound through the air. It has put in 2,000 wireless systems throughout New England.

Although video surveillance is grabbing the headlines, it is only a small part of SideBand's operation.

"We do a lot of business with virtually every major hospital chain in the New England area," Davis said.

SideBand designed the wireless system that allows doctors and nurses at several area hospitals to get patient information over laptop computers while sitting at a hospital bedside.

The company also has a foothold in the world of law enforcement. It maintains the wireless communication system for the Rhode Island State Police.

Last week, SideBand held a seminar at the Hawthorne Hotel for North Shore law enforcement agencies and municipal officials interested in wireless communication and video surveillance — a relatively new and growing business.

"I think a new chapter in this (business) has been motivated by 9/11," Davis said.

Lt. Mary Butler and other Salem officials demonstrated their system in a basement meeting room at the hotel.

The system installed in Salem over the past three years is much more sophisticated than the old wall-mounted bank cameras. These new cameras have no wires and can be controlled by someone far away.

There are eight cameras in the downtown — three on Salem Common and others on the Essex Street pedestrian mall, Derby Square, Lafayette Street, Townhouse Square and Riley Plaza. The cameras on Salem Common have been in use for several years.

Police officers back at headquarters on Margin Street not only can watch the action on a 52-inch plasma screen, but can control the camera's movement as someone walks down the street or zoom in on a subject. The cameras also can be preset to move quickly from one security point to another.

Although police usually watch the video on the TV monitor, it can be viewed on desktop computers by users with security access. In the future, it is expected to be available inside cruisers.

In addition to live action, officials can view something that happened months ago — a useful tool in investigations.

Although the system is not cheap — cameras and the wireless setup cost around $12,000 each — Salem is considering expanding the surveillance network when funds are available, Butler said. Possible future sites are the power plant, the LNG tank on the waterfront and Winter Island.

Salem is one of the few small cities on the North Shore that has entered this high-tech world of crime fighting, but probably won't be the last.

"The bad guys have it," Davis said at last week's police seminar, "so you have to level off the playing field."