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Most of what can be achieved via FEEDNET is courtesy of open source software. The sheer hard work, altruism and inventiveness of all open source authors is gratefully acknowledged here.




PTT, VoIP and RoIP. Confused ?


Over the last few weeks we have been asked on more than one occasion about our use of VoIP, RoIP and PTT technology.  We have also been asked about the  misleading advertising by some companies involved in selling these technologies.

Let us try and distil the essence of what these terms mean, particularly in the context of FEEDNET.  We will do it by providing some broad definitions of the terms and then by reference to specific products and technologies together with their respective strengths and weaknesses.

Firstly some definitions:

IP = Internet Protocol.

IP describes the way that computers talk to each other via a network.  Old fashioned, or “regular”  analogue telephones  use analogue representations of the audio to pass along a wire pair to the local telephone exchange.  Modern IP telephones use computer protocols such as IP to talk to each other. You don’t need to know how IP works to use it. If you are looking at this web page it came to you via the magic of IP !

VoIP = Voice over Internet Protocol.

The concept of using computer communication techniques to link telephones together is such an important one that it has been given it’s own name – VoIP, or Voice over Internet Protocol.  The idea is to make a telephone that converts sounds  into digital representations, send them over a computer network then have a telephone at the other end convert them back to faithful reproductions of the original sound. 

SIP = Session Initiation Protocol

Anyone who  makes use of VoIP for communications will come across the acronym SIP at some point. SIP is a kind of signaling method that telephones and computers use to create  pathways between them.  There are other methods but SIP is one of the most important. SIP is not really something that users need to get involved with, but it's useful to know what it actually is since the term is used so much.

RoIP = Radio over Internet Protocol.

Before talking of RoIP it is worth a small diversion to look at two terms that are vital to the understanding of it.   Full-duplex and half-duplex.  When you make an ordinary home telephone call both parties can talk at the same time and be heard – they can interrupt each other.  This is full-duplex.   On a typical handheld “walkie talkie” or “two way radio” one of the parties has to press a button and then can speak. The other party must wait for them to finish before being able to answer.  It is not possible to interrupt, and only one person can speak at a time. This is half-duplex.

Radio over IP is the name given to the general concept of using VoIP techniques for transferring the sound information from one end to the other,   but  in place of using an ordinary two way telephone, a half duplex radio device is used at one or both ends.  i.e. when two or more people are conversing together, only one person can speak at one time. VoIP usually implies full duplex communications over IP.  RoIP is usually half duplex.  Aside from this they are almost identical and tend to use the same or similar  technologies and protocols (e.g. SIP).

PTT = Push to Talk

“Push to talk” often just refers to the button on a two way radio that one presses in order to be able to speak and be heard. But the term also means something a little bigger – it is used to describe a particular type of communications system. This is what we will focus on here.

A PTT system is one where a user is able to listen to others but cannot speak until the channel is free when the user will take control of it by  pressing a PTT button.  On pressing a button they can speak and be heard.  Sometimes they are heard by only a single user or it is possible to have hundreds all joined to the communication channel.

An area where PTT is commonly employed is in RoIP.  Here a traditional radio link is used somewhere in the route between persons talking to each other.   PTT can also be used  where it is a  plain and simple wired data network, as is the case where VoIP is employed.   In the latter case, every part of the route, is carried over the internet.  The PTT is used to give a two way radio “feel” to the conversation even though no radio is in use.

Why use PTT ? Well, for those already well versed in the use of radio networks, it’s a comforting, intuitive and disciplined way of communicating.    PTT often might have hundreds of individuals in the “net”  monitoring for long periods of time.  Normal telephones are good for short but intense conversations. PTT is good for just monitoring for the occasional message.    PTT users manage their communications simply, efficiently and in an ordered fashion.






Simple analogue telephones

For over one hundred years this has been a core means of having a two way simultaneous conversation between two geographically remote locations.

Simple, cheap.

Works without having local power (powered from the telephone line itself).

Limited in scope and facilities,

Slow to deploy.

Telephone can’t easily be moved.


Now the de-facto standard for voice telephone communications. Even if we still put analogue telephones in our homes they get converted to digital at the exchange.

Very few businesses will install analogue nowadays – VoIP is used exclusively.

Portability – you can take your telephone with you. You just need a data connection to make it work.

Extensive range of services – simple conferencing, paging, messaging, etc.

High quality sound.

Low cost. Local calls can cost the same as long distance calls because both just look the same from a technical perspective – data over the network.

Android and Apple mobile telephones, Ipads, Tablets, and PCs can all be used  for VoIP, often at no additional cost . They make use of the existing data connection.

Calls are usually free from VoIP to VoIP numbers.

VoIP equipment is  dependent on local power supply.

It can be a complex  process to configure the equipment.


Used where one end of the link is highly mobile, or far from regular public telephone/internet infrastructure.

Imagine a mountain search and rescue team working in a remote area. With ordinary two way radio the team can talk between themselves and perhaps with a distant remote base. Being out of the range of mobile telephones their options are limited.

With an RoIP link as part of their two way radio network, they can make a phone call from a two way radio handset – to anywhere in the world.. Or perhaps they can temporarily link to another team working  in another part of the mountain range.

RoIP usually makes use of PTT although VOX is a possibility too.

RoIP can add effective two way radio coverage to a telephone system

RoIP can link together disparate parts of a two way radio network via the internet. Or see it as linking different two way radio networks to make one larger single network.

When we use the term "radio" we don't mean wifi or 3G cellular. Although these technically use radio waves of course, we tend to think of them differently.



On the one hand it is a strength that RoIP generally only allows half duplex operation but compared with full duplex this can be seen as a restriction.

Calling ordinary telephones when one end of the link is half duplex can be a little frustrating as one of the parties may not be trained in the more disciplined type of operation that  half-duplex requires.

RoIP can be expensive (but need not be). RoIP systems are often known as Dispatch systems.  This is because a typical Dispatch Controller will talk to radio users (taxis, ambulances etc) but will not use a normal hand held radio to do so.  Instead they will use dispatch software on a PC that keeps records of who is where for example.


RoIP (see above) makes use of PTT, but here we refer to PTT being used over an exclusively  fixed or 3G data network.

For those familiar with conference calls, PTT is like having a lot of folk in one big conference call and all of them have their microphones on mute.  When someone speaks, they release their Mute button, speak and then operate it again. Everyone in the conference hears what they said.

In the PTT world instead of a Mute button there is a PTT button which actually does the opposite. Press to speak, release to go back to listening.

 Ideal for groups of people to share a “channel” when they are all connected to the internet.

 PTT over internet  has nothing to do with RoIP and is totally dependent on the users own data connection.. Examples are the products from Zello, Broadnet Systems etc.

Provides a standard two way radio look and feel to what is essentially telephone type communication.

Simple to understand, simple to use.

Can operate from Apple phones, Android Phones, tables, PCs etc at no additional cost although there are usually charges made by the service provider for access to the server.



Depends on good quality data connection to a service provider who operates a computer server which provides the service.

Can be slow – with delays before messages are heard.  Not too much of a problem with a home or business broadband connection, but often very poor using mobile 3G technology.

Doesn't work at all well in 2G mobile areas. Normally inoperable under these conditions.

PoC (PTT over Cellular)

This is very similar to the many PTT offerings made by Zello, Broadnet etc but the difference is that PoC is an integral part of a mobile telephone company infrastructure and it works much better for that reason.


PoC works well over 2G /3G/4G mobile links and is usually advertised as being of carrier grade quality.

PoC is part of a mobile telephone company's offering, it is not a bolt on by third party providers.

PoC was always much bigger in the USA. There have been offerings from O2 and Orange in the UK and they were of very high quality with a true reach identical to that of the  regular mobile phones.




Expensive and not taken up  in large numbers. There are no current providers in the UK.

Watch out for misleading Claims.

Be careful of extravagant claims concerning PTT products both in relation to network provision and in comparisons with Tetra/Airwave. 

Tetra/Airwave is a PTT radio network that employs over 5,000 dedicated base stations providing a RoIP service.    

A PTT handset designed for use only on 3G/WiFi may look like a Tetra/Airwave handset but it will not operate on a network comparable to Tetra/Airwave and is dependent on your 3G or broadband internet connection. 

Everyone is, of course, free to compare anything with anything -  a Boeing 737 can be compared with a child's scooter - they're both means of transport after all. But for practical purposes we would normally say that there is no comparison between them and likewise we would say there is no comparison between Tetra/Airwave and an internet accessible PTT computer server such as provided by Broadnet. 

Broadnet advertises that it “Operates the UK's largest push to talk & dispatch network covering 99% of the UK." which it compares to Tetra/Airwave.   Very misleading.  We can't find one UK provider of PTT handsets that actually operates a country-wide network**.   Broadnet and ITC utilise  computers which you access by means of the internet. 

Not even Tetra/Airwave claims to cover 99% of the UK so we would advise you to be very wary of a small business selling handsets that claim this kind of coverage.  

To clarify, if you are happy with your internet access (3G or Broadband) then you can indeed use one of the many PTT Internet servers provided by companies like Zello, Intouch, Radphone, PushtoTalk Phones Ltd., Phonetrader,  Broadnet, ITC Global Communications etc.  All of these can be useful services in their own right but be clear that you understand their limitations.   In Scotland there are many large areas not covered by 3G telephones and most SIMs work with only with one operator, be it O2, Vodafone or others.  This might leave you without 3G coverage.    Resilience planners will also be very sensitive to the dangers of relying on mobile telephone technology during incidents. 

We stress these points because questions about claimed coverage come up very frequently in conversations with user services.

One final point, we've been asked if it is true that you can make a phone call to the public networks from one of these PTT Handsets. 

Yes,  a PTT handset is nothing more than a mobile telephone running an internet based app.   So if you put a SIM in it then you of course can make a phone call! 

If you already own a mobile telephone you will already have a telephone number for it. When you buy or rent a SIM for your PTT handset you will be carrying a second  mobile telephone with another mobile number.

We install our Teamspeak PTT solution onto our existing mobile telephones. These telephones are also used for our FEEDNET Voip Network running one of the many VoIP apps, but in our case we rather like Zoiper. 


Some Examples of VoIP/RoIP/PTT Services:



Other Info

Sipgate VoIP

Sipgate provide access to the public telephone network using local geographic and non-geographic telephone numbers. Residential and business VoIP and mobile phone services are provided on a prepaid, bring Your Own Device basis.

To use Sipgate you need to sign up (you can use some facilities free - go to their website and try them)

When you sign up you get a telephone number and a password. Program your VoIP phone with their details and your passwords. Now you can make and receive calls using your internet connection. Everything passes over the internet in a digital format  using the SIP VoIP protocol.

FEEDNET makes use of Sipgate to provide an outside dialing facility for FEEDNET VoIP users.

Public dialing obviously cannot be depended on as a means of communication independent of public services so we do not rely on it, preferring to build extensive repeater networks to cover our region. 


Not a service, but a piece of software. It is such an amazing one that it is well worth mentioning here !

Asterisk is a complete VoIP/RoIP server that is open source and in use in thousands of  locations across the world.

Easy to set up by those who are IT literatem Asterisk can be configured via GUIs supplied by third parties. We prefer not to use them since we do some unusual things with our FEEDNET installation. Interfacing Asterisk to a repeater system cannot be done with GUIs, you will need to modify config files directly to do this. 

Asterisk is a core component of FEEDNET’s systems.

 We use it for general telephony, and we use it for conference calling, voicemail, messaging and  paging

 We have repeaters and radio access points that connect wirelessly to FEEDNET and can provide links from handhelds to outside callers

 Our asterisk servers are located at different points on the network and they pass calls between them.

 Asterisk can cope with almost all codecs and we prefer G729 for its low bandwidth and low cost.

 We often use Cisco phones which are available for £15 second hand and that have G729 built in.


Broadnet provides a PTT app for Android and it operates servers to provide a PTT VoIP service.

Computer aided dispatch also available.

A mobile telephone handset is available with a separate PTT button. Saves you tapping the screen PTT button.

Remember that this is an internet based system.  If there is no internet access, then it doesn’t work.

FEEDNET users could technically make use of Broadnet through the FEEDNET Internet Gateway but we do not want to rely on public services during an incident.  We also  prefer to be wary of giving credence to some of the claims we have seen about network coverage.

We use Teamspeak as an alternative. See below.



Zello sell a PTT Android APP and a hosted server.  Actually you can try it as a free service.   It is just like Broadnet but based in the US . UK users have noticed bad latency problems. i.e. long delays from speaking to being heard.

Remember that this is an internet based system.  If no internet access, then it doesn’t work.

We do not see any real gain from connecting FEEDNET to Zello even though it is technically possible.


Teamspeak is a long established  proprietary VoIP system.  The Apps and the server are  available to non-profit organizations freely.

Teamspeak provides PTT functionality and is noted for being a well designed and effective system with widespread usage in business and in the gaming community where low latency, high speed communications are of the essence.

Teamspeak has been around since 2001 and it does not make unrealistic claims about its service.

Teamspeak is the current FEEDNET choice for a PTT application.   We believe Teamspeak to be ideal for FEEDNET because a free licence is available and the server can be operated in-house at FEEDNET node locations.  The team can make use of  all the PTT functionality of Broadnet and Zello, but unlike with those systems, we can use Teamspeak when the main public services are unavailable.

 Also there are many more facilities than the other PTT apps. We can use file transfers and other workgroup applications.

 Many of our operators and users prefer to carry Apple devices rather than Android. The Teamspeak mobile apps for both Android and Apple are second to none. Echo cancellation on full duplex, and noise reduction all work very well in Teamspeak. You can program one of your telephone's physical buttons to be the PTT or you can use the soft-button on the screen.


Mumble is an open source VoIP application that makes use of PTT like  more or less like Teamspeak.

Mumble is popular amongst home users and was first released in 2005.

Mumble is not as slick as the commercial offerings but many say it has the lowest latency (delay) of any of the PTT VoIP systems.

Android Apps are available but many believe they are unreliable and many do not recommend it for that reason.

FEENET can make use of Mumble and it works well with PC softphones.

 We haven't found the Android softare to be reliable enough for us so we don't use it for FEEEDNET at this time.



**  To clarify:  A network is an interconnected collection of nodes whose purpose is to provide communication between them. A company that operates a network will either own the nodes themselves or they will lease them, maybe on a shared basis.   FEEDNET for example has a very small network comprising some tens of nodes. It does have an internet gateway so is accessible from any one of billions of internet access points across the globe.  It can't claim to operate a network with billions of nodes though - this would be very faulty and misleading logic !!




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