The Truth About 4.9 GHz for Citywide CCTV Systems

So you are going to use 4.9GHz huh!

I often ask myself this question when designing a citywide surveillance solution. My experience working with 4.9 in various deployments has not been the most pleasant. The obvious answer for me is NO, and Sometimes. Let me explain what I mean by this statement.

In comparison 4.9 GHz as well as 5 GHz frequencies have the least amount of utilization than 2.4GHz. Most private homes and small businesses use 2.4Ghz predominantly because hardware for 4.9 and 5.0Ghz was reserved for enterprise specialized hardware and applications. Over the last 5 years I have seen an increase in the use of 5 GHz and 4.9, mostly because the hardware has become more accessible. 4.9 GHz is reserved for public safety, fire and.gov (dot gov) applications so the hardware for this frequency is still out of reach of the general public.

So if the hardware is not readily accessible to everyone then shouldn’t 4.9 be pretty much open? The short answer to this question is yes, it should be but it’s not. Here is the problem with 4.9 GHz. Before I get into my explanation I need to explain the structure of the frequency. I will start with the 5 GHz first.

Lest Start with the easy stuff 5 GHz

I am going to attempt to keep this topic very modest. A frequency like 5 GHz is actually referring to a group of numbers that are in the five thousand megahertz (5000Mhz) range of the radio frequency spectrum. These numbers are known as channels. Channels on a wireless signal work pretty much the same as the general concept of channels on a TV. For example on your TV each channel takes you to a different program. You don’t know what’s on the other channels until you turn to it. It is total isolated from other channels. In Wireless the concept is the same. Channels represent a unique frequency, but just like a TV everyone has access to the same available channels.

The 5 GHz frequency band is comprised of four bands. These bands are 5.1 GHz, 5.3 GHz, 5.4 GHz and 5.8 GHz. Combined the bands have a total of 24 channels with each channel at 20 MHz bandwidth. This means that 24 channels on the 5 GHz frequency band has a bandwidth (Big Pipe) of 20MHz each. This makes the 5 GHz band very suitable for video over wireless.

But 4.9 GHz Also has 20 MHz, right?

On the other hand 4.9 GHz has very little similarities to 5 GHz. First difference is 4.9 GHz is the public safety band. Meaning it is a license band and only to be utilize by public safety agencies. You must apply on the FCC website if you are not public safety but wish to use the frequency. Here is what really confines the usability of 4.9 GHz for building an entire video wireless network. You may not understand the hertz and bertz and so forth just pay close attention to the number. Numbers don’t lie. The 4.9 GHz band is limited to 50 MHz with only 2 standard, independent channels of 20 MHz. Let’s compare this to the 5 GHz that has 24 channels at 20 MHz each. See the similarities, I think not.

So how do you deploy a citywide CCTV system using 4.9 GHz when it is limited to only 2 channels at 20 MHz? The only option is alternating your frequency, which translates to a very short game of Tic, Tac, Toe, to avoid causing yourself a complicated troubleshooting session. Eventually you will step on yourself, which is a very technical term meaning create your own interference.

OK, so should I use 4.9 in my design?

The reason 4.9 GHz, in my opinion, is not suitable for a citywide CCTV system is the lack of independent 20 MHz channels. What the FCC has done to the 4.9 GHz band is to stretch it out to allow more than two systems to utilize it. The band is fragmented in to smaller pieces. You can select to use 20 MHz (Default), 10 MHz or 5 MHz channels. The 10 MHz band allows for four (4) independent channels and the 5 MHz band allows for 10 independent channels. So what’s the problem, it appears we have plenty of channels to work with! Wrong. All it takes is for someone else, not you, to be utilizing the 20 MHz channel. Which if you recall is the full spectrum of the 10 MHz and 5 MHz channels. 4.9 GHz uses a smoking mirror technique to appear to have more available channels to operate on than it really has. The technique is referred to as channel fragmentation.

In a citywide CCTV deployment 4.9 GHz has its place. An experienced consultant, designer or integrator should have a strong background in wireless deployments and building networks. Only with experience will they know to best utilize the 4.9 GHz band.