AllStar is a somewhat obscure niche within ham radio that many hams surprisingly have never heard of. Very few ham stores carry AllStar products and no big manufacturers make anything that directly supports it. AllStar has evolved over ~20 years from a complicated system originally requiring expensive PCI cards to then being ported to RPi's and some very "old-school" node designs, to now becoming increasingly more popular, with a talented and hard-working development team now behind ASL3 bringing it fully up-to-date.
AllStar is not just a simple gadget you buy and plug into the wall or an app you download in 2 minutes and run on Windows or a smartphone. (There are some phone apps but they have a lot of limitations and don't work nearly as well as a real node.) It does many different things and is part of Asterisk and Debian Linux - each very powerful platforms. Many newcomers to AllStar don't fully understand its history, how it evolved, and the many ways it can be used, and as result many common misconceptions result.
Going back to the beginning, the whole idea of AllStar was to link repeater systems. To leverage IP and VOIP technology to extend the range of repeaters, and transcend the limitations of RF. There are two things in ham radio: hams and radio. Which is more important? Obviously the hams. Being a ham is not just about using radios, it's about communicating, whether over RF, IP, email, telephone, blog posts, articles, social media, youtube, hamfests, etc. Ham radio is more than just radio, or technology. It's a culture and a global phenomenon.
Two tin cans and a string are also a way of communicating, as are smartphones, $15 Chinese HTs, or DMR radios or QRP rigs. Repeaters are a little different than all these things though, in some important ways. Repeaters have been around for 60+ years, usually have wide coverage with consistent RF performance, very little dependence on band conditions and excellent audio quality. And they support full-duplex, enabling a few Watts from an HT 10 or 20 miles away to be amplified and simultaneously rebroadcast from a mountain top - thereby expanding the coverage to 50-100+ miles. They do all this for free with no cell phone monthly plan required, no apps to download, and no dependence on the grid, while often being associated with ham radio clubs that meet for lunches, pizza & beer, field days, community service events, and various projects.
Repeaters are unique within ham radio as being one of the few types of systems that are full-duplex, meaning that they receive and transmit at the same time. As a result, AllStar also had to be full-duplex. Or more precisely, it could have been half-duplex, and most users in fact only have half-duplex nodes - but there's a problem with that: Half-duplex fails to properly implement the original intent of the system. Let's say you have a repeater on a mountain near you that you talk on with your friends. You have nets and long QSO's and can talk from anywhere in your town, city or county. It works great, and it's intuitive and simple to use. When someone unkeys, you wait for the courtesy tone, key up and you're on the air. If it's a busy net maybe you unkey for half a second just to make sure no one doubled with you. Or to make a quick comment, just key up before the courtesy tone. This all works well, and has been working like that for 60+ years.
A properly set up full-duplex AllStar node preserves all the above capabilities. If done correctly it is completely indistinguishable from the actual repeater system. Let's say you have to go out of town somewhere and are out of range of your favorite repeater but want to stay in contact with everyone. With a full-duplex node you can simply connect to the repeater through AllStar and talk on the repeater in the exact same way you would when at home. No limitations and no difference in audio quality. Just key up your HT or mic and you're talking on that repeater exactly like you would be at home in your back yard.
This is why full-duplex is an important feature to have in an AllStar node, yet most of the nodes people are using today are not full-duplex. As a result these users can't talk on a connected repeater until all users unkey and the carrier drops. If it's a busy net and people do what they normally do - key up after the courtesy tone but before the subsequent hang time and carrier drop - people on half-duplex nodes can't key up the system. Their own nodes aren't listening to them. Why would anyone want a node like that?
Full-duplex nodes don't have that issue. They are always receiving and always able to instantly respond when you transmit, no matter what else is happening on AllStar. And if you key up on a busy net and want to be sure no one doubled with you, even if your HT is only half-duplex, you can just unkey for half a second and you'll hear what's happening on the remote system. Thus full-duplex nodes not only always listen, but they also always transmit when you're keyed up so that you can easily hear if someone else keys up or doubles. This works perfectly on radio-less interfaces such as the UCI120, works well on the rare vintage HT models that support cross-band full-duplex (aka Frequency Division Duplexing - FDD) eg. Alinco DJ-G7T or Yaesu FT-51R or FT-530, works great on higher end mobile radios that support FDD, and also works on all half-duplex radios with the caveat that you have to briefly unkey to hear what's on the channel. In essence, full-duplex nodes make your life easier, and make it easier to do what hams do - communicate.
A couple days ago I posted on a couple FB groups and posted a demo video on youtube about the latest generation of the AllScan ANF101 Full-Duplex AllStar node - which now enables very high-quality nodes to be built far more quickly, easily and cost-effectively than ever before. These posts and the demo video were just quick and simple with the basic information and a link to allscan.info which has tons of additional info as well as detailed How-To Guides on how to build your own HT, mobile-radio, or radio-less node. People don't usually follow links and read all that information however and so there are occasional questions from people who don't really understand what it is they're looking it or what "Full-Duplex" actually means or why it might be useful.
With this being the case I thought I would make a post specifically about what Full-Duplex is, and why I consider it essential in any node. A few quotes from my ANF101 How-To-Guide (AllScan - How To - Build a High-Quality Full-Duplex AllStar Node for Under $150) -
"Full-duplex nodes enable more interactive and more efficient communications, prevent doubles, and improve situational awareness, even if your other radios are only half-duplex. Full-duplex smartphone VOIP/IAX apps and SIP phones can also be used."
"AllStar works perfectly with Full-Duplex radios because it does not echo your transmit audio back to your receive audio. Thus you only hear what's on the remote node/repeater you're connected to (minus your outgoing audio), just like talking on a speakerphone – true full-duplex with no echoes, delays or feedback. This uses the duplex=3 setting in rpt.conf – which is not a repeater and does not echo your outgoing audio back to you."
"...it's important to understand why Full-Duplex nodes are better than simplex nodes and why a little extra time and effort to support Full-Duplex is a good investment. This is discussed in detail in the Full-Duplex Communication Benefits section below (AllScan - How To - Build a High-Quality Full-Duplex AllStar Node for Under $150). Despite these clear advantages, most hams don't really "get" what a full-duplex node is, and even AllStar "experts" often have no familiarity with the concept or jump to the conclusion that you're talking about a repeater, but to be clear, a full-duplex personal node is not a repeater and does not repeat Rx audio into Tx audio. The purpose of full-duplex in a personal node is to enable you to hear what's happening on other nodes you're connected to at all times – not to echo your own audio back to you. ie. you can always hear anyone else who keys up, and your node can always hear you."
"This node design is intended for personal use, not as a repeater or for commercial use. There are big differences between cross-band full-duplex, full-duplex (on a single band), and a repeater. Cross-band full-duplex is easy to do with HTs going as far back as the Yaesu FT-727, which I believe was the first ham HT that supported cross-band full-duplex (I bought one new in 1987)..."
"From FAQs:
Q: I like the idea of Full-Duplex but I have to wonder wouldn't I get a lot of feedback? When I use Full Duplex on satellites I need earbuds.
A: The great thing about Full-duplex on AllStar (with duplex=3 in rpt.conf, not 2 or 4 which is a repeater), is that it does NOT repeat your Tx audio into the Rx audio that goes back to you. Thus it's no different than talking on a speakerphone – you hear the other people on the call but it doesn't feed back your own audio. Because of this, Full-Duplex is actually much easier to use on AllStar than it is on satellites or non-ASL-linked repeaters."
Full-Duplex Communication Benefits
In the beginning...there was CW. Most HF/CW/All-mode transceivers support full or semi break-in keying, meaning that the receiver is on between symbols/letters/words sent. Thus if someone is transmitting on a decent radio and making a reasonable attempt to monitor the frequency between characters or words, anyone can easily break in. Thus CW supports interactive, fluid conversation, and allows others to join a QSO or reply to a CQ at any time. CW can be considered semi-duplex because each dit and dash is only a couple 100mS and good radios can switch between Tx and Rx very quickly. Thus the receiver is never off for a significant time and it's easy to hear responses and other activity on the frequency.
And then there was AM, and soon after SSB which is much more energy-efficient and more widely used. Because SSB has no carrier there is no energy transmitted (or very little) during quiet parts ie. between words or during pauses. Experienced hams take advantage of this by briefly unkeying here and there, during pauses or maybe every other sentence, for enhanced awareness of the channel. This is a subtle technique that many aren't aware of. Some operators will just key up and talk for minutes straight, whereas more experienced OMs tend to keep things shorter and more interactive – just like in-person conversations. When you talk to someone in person you don't just talk for 3+ minutes straight at a time without ever pausing to allow someone to comment, acknowledge, reply, etc. And there's usually no good reason to do this on the ham bands, with some exceptions such as on nets. Thus while SSB is not generally full-duplex, experienced hams use it in an interactive way where they pay attention to what's happening on the frequency (as well as plus or minus a few KHz where there might be QRM or QRN that needs to be worked around), and as a result QSOs tend to be more dynamic and interesting.
And Then There Was FM
Next there was FM. FM sounds better than AM because of its better noise resistance, but it is less energy efficient than SSB due to having a constant power regardless of modulation level. And unlike SSB where you can unkey briefly with no audible effect, if you unkey on FM it results in squelch drops for listeners (unless their radio has unusually good squelch or they have tone squelch properly set up), thus there is a slight disincentive on FM to pay attention to the channel while you're transmitting.
Due to the FM capture effect, multiple people can't transmit at the same time or the signals interfere destructively or one covers up the other. With SSB that's not an issue and multiple signals combine linearly. One might be louder than the other or you might not be able to follow what either is saying very well but at least you'll be able to tell that there are 2 people talking, vs. FM doubling which can sound like a train wreck. Thus when FM first started being used on repeaters 60+ years ago it sounded great but also had some limitations that earlier modes did not.
As a side note, repeaters don't need to only use FM (or PSK digital equivalents). They could also support SSB without a huge amount of effort. Squelch would be a little more complicated because you don't have a constant carrier to reference, but DSP, signal analysis, noise reduction and AGC techniques have come a long way since the days of the first FM repeaters. It would not be hard to set up a repeater with separate FM and SSB inputs and repeat the audio on separate FM and SSB outputs. This would for the first time (that I know of) enable the advantages of SSB to be achieved on a repeater, namely SSB's inherent resistance to destructive interference and thus the ability to support multiple signals on a single frequency, and SSB's greater power efficiency that would reduce battery use for portable users or give more range with the same average power.
Before the Above There Was The Telephone
Originally repeaters and radios only supported half-duplex (from the user perspective). If you receive at the same time you're transmitting, from a repeater that has only one FM receiver, you're generally just going to hear yourself, or maybe a bunch of noise if someone doubles with you. Technically a repeater is full-duplex because it can transmit at the same time it's receiving, but that doesn't do you a lot of good (beyond the basic advantages that repeaters provide) if no more than 1 repeater user can transmit at the same time. Prior to ASL, only rarely did repeaters actually support true multi-user full-duplex communications. This is in contrast to phones which are multi-user full-duplex and always have been, which is much more natural and intuitive. Even the very oldest telephones from the 1800's were full-duplex. Apparently Thomas Edison and whoever else contributed to the early development of telephones figured out that it would be easier to just let people talk whenever they want with no limitations on how many people can talk at a time. This is easier to do on a pair of wires than over RF, but can be easily done on RF by using multiple bands/frequencies.
Multi-User Full-Duplex Repeaters
Most repeaters have only one FM receiver, which due to the FM capture effect can only properly receive one signal at a time, thus there was really no such thing as a repeater where any number of people could talk at any time while also hearing everyone else. To support that without ASL currently requires a repeater to have multiple receivers. Such systems do exist and work well for trivia nets or other scenarios with quick-witted hams jumping in with quick comments. San Diego's W6ZN repeater does this and there are numerous interlinked repeater systems with multiple inputs such as the SoCal DARN or PAPA systems where you can listen to one repeater on one mountain and transmit into a linked repeater on another mountain. ASL now makes it much easier for repeaters to support true multi-user full-duplex, as connected (radio-based) user nodes in fact extend the repeater with additional receivers and user-specific link monitoring transmitters (ie. that do not repeat local Rx audio).
And Then There Was The Internet
The Internet is essentially an extension of public telephone systems that had existed for a century before, with the major difference being packet-switching vs. circuit-switching. They both work for getting audio from one place to another regardless of if it's half-duplex or multi-user full-duplex. The internet does this in a general way with no limitations on the application layer which is a big improvement over the closed and proprietary phone system networks that have now become mostly obsolete.
And Finally There Was Asterisk and Then AllStarLink
Because ASL is based on Asterisk – a phone system, it supports full-duplex perfectly and makes it very simple. Even repeaters that have only one RF input, that would not ordinarily support multi-user full-duplex, if ASL-linked now support this capability by default for all AllStar users. I have tested this on many repeaters and it works just like a full-duplex phone call on a speakerphone. You talk just like you would on a simplex node but if someone else keys up you hear them and if someone doubles you hear it right away. Because you always hear the remote system, and if someone starts to double or there's echoes, timeouts, interference, etc. you can simply unkey until the repeater (or node / hub / bridge / etc.) is clear. This is a powerful feature that greatly improves the interactivity, efficiency and flexibility of amateur radio repeater communications. And it's an essential feature in emergency comms scenarios.
For those who have not used a full-duplex radio or node, or who do not intuitively understand how much of a difference it makes, a simple analogy is to consider what it would be like if the phone system was only half-duplex. Imagine that when calling a friend, relative, or business if only one person could talk at a time, no one could speak until the other person or people on the call unkeyed, and if when you then did key up to speak you couldn't be sure someone else hadn't keyed up at the same time. This would result in awkward and inefficient conversation because it would no longer be like real in-person communication. In contrast, full-duplex enables natural, smooth, effortless communication just like when you're standing right next to someone.
Multi-User Full-Duplex
Although most people may not know it (even repeater owners), ASL-linked repeaters by default do support true Multi-User Full-Duplex. This is a subtle detail that can be hard to appreciate until you've tried it yourself. The repeater itself on analog will not support more than one analog RF input at a time if the repeater has only one RF receiver, but generally will support any number of simultaneous ASL full-duplex users and at least one analog RF input.
By default when a repeater links to ASL it will be with a node whose audio output gets mixed with the RF receiver audio input(s). Some repeaters may prioritize one over the other and only allow one audio channel at a time, or ASL may be linked through some intermediary interface, but even then multiple connected AllStar users can still be full-duplex and talk and hear each other fine, as that's a core feature of Asterisk's architecture. Audio streams from each connected node are always routed to all other connected nodes, no different than a VOIP conference call.
Some repeater system admins may opt to give RF Rx audio priority over AllStar Rx audio but there is no advantage to this and it only causes issues. For example if an RF user and an AllStar user key up at the same time the RF users on the repeater will hear only the RF user while AllStar users hear only the AllStar user, thus resulting in 2 separate conversations, which makes for quite a dysfunctional system. Such a phenomenon could be called "halfing" rather than "doubling", as it results in the communications topology being split into separate halves while both users are keyed up. Fortunately only a small number of repeaters seem to have this issue. If you should encounter such a system I suggest letting the admins know there is a better approach, ie. combine (mix) the Rx audio inputs rather than prioritize one over another.
Amateur vs. Professional
Go on any repeater net that has more than a half dozen or so users and you'll frequently hear doubles, quick-keying, and other issues, which can waste minutes of everyone's time, result in inefficient communication, and could even be described as "amateurish". Full-duplex solves these issues and gives you full situational awareness. This goes back to the definitions of "amateur" vs. "professional". Amateur just means it's not for-profit – not that we can't communicate well. Full-duplex enables communicating as if you were standing right next to everyone in the QSO, and AllStar makes this so easy to do that there's no reason not to.
Situational Awareness
Another nice detail about full-duplex is that once you start transmitting from your cross-band full-duplex HT it's nice to see your node transmitting back to you right away. I can be over a mile away on an HT and start transmitting, its red Tx LED lights up, and then ~25mS later you see the green Rx LED light up which confirms that you're in range, the node is receiving you and transmitting back to you, no one is doubling with you, and you hear if there is any path noise. This is another example of how Full-Duplex improves overall situational awareness.
Radio Compatibility
Cross-band full-duplex nodes are fully compatible with half-duplex radios if they are dual-band dual-receive. For example a Kenwood TH-D74 I used to have won't do full-duplex but I had the node's 70cm Rx frequency on one memory channel and the 2m Tx frequency on the next memory channel, with dual-watch on and when I transmit the D74 will not receive at the same time but once you unkey you hear the node transmitter just fine. Thus any dual-band dual-receive radio can be used, which pretty much all modern HTs are.
Full-duplex nodes provide powerful benefits even if your other radios are only half-duplex. You can key up any time without having to wait for the node to unkey first, and if during a transmit you quickly unkey on a half-duplex radio you'll immediately hear what's on the remote system and can confirm that the channel is clear and no one is doubling with you then key up again. This is essentially the same technique mentioned above used by savvier hams on CW and SSB since the dawn of those modes, and makes checking into larger nets more efficient and enables large QSOs and nets to run more smoothly and interactively.
Some Chinese HTs may also support cross-band full-duplex, most likely with the limitation that they can only Tx on 70cm and Rx on 2m, so their 70cm Rx doesn't get de-sensed/overloaded by the nearby 3rd harmonic of their 2m Tx, but I have not been able to find any currently available models that do this and work well. For example the TYT 8000 supports cross-band repeat but it does not support cross-band full-duplex. Fortunately there are plenty of older Japanese radios that fully support CBFD, and maybe some new Chinese radios will support it at some point. For the purposes of a node radio however, as a pair of RT85's is only $50 it is unlikely that any old or new CBFD radio will be available at a lower price. As for mobile radios the Alinco DR-735T does CBFD and has a MiniDIN6 jack thus there is now at least one current production mobile that can work well, though there do not appear to be any Chinese mobile radios that provide these features and have good reviews.
Full-Duplex Nodes vs. Repeaters
A Cross-Band Full-Duplex personal node (ie. that has duplex=3 in rpt.conf) does not repeat receive audio into the transmit audio. This is an important distinction and I do not recommend that this node design be used as a repeater or in a commercial application. A common misconception is that a full-duplex node is a repeater. This is not a valid assumption, and building a repeater is not the subject of this guide. ASL was used more for repeaters than personal nodes in the beginning because things like RPi's and MiniPC's did not yet exist or were not powerful enough to run Linux and Asterisk, and FCC-certified ~$20 HTs did not yet exist. But things have changed and there are now many ways to build a node, which if properly done can deliver professional-grade audio quality and long-term reliability.
A full-duplex personal node can be used as a repeater (by setting duplex to 2 or 4 in rpt.conf), but that defeats one of the main benefits of a full-duplex personal AllStar node – the ability to hear audio from other connected nodes at all times without your own outgoing audio being repeated back, and thus no need to use a headset or headphones. This gives much better situational awareness and flexibility than a simplex node because if someone else keys up while you're talking you hear it right away and can then unkey thereby preventing a double, or you can both continue talking. In that case the conversation can work just like a good quality speakerphone. ASL is based on Asterisk – an enterprise-grade phone switch system that supports full-duplex by default for all users, thus anyone can key up with a quick comment or break any time. There are applications where a small low-cost portable repeater is useful though, such as if traveling with other hams to a remote area or field day site. For occasional repeater use this node design works well and provides the majority of ASL's features at a very low cost. You can even add additional receivers (eg. $25 HTs) along with a simple audio mixer circuit to provide true multi-user full-duplex functionality.
Because the Tx audio is not repeated a personal node is generally limited to one RF user, in contrast to a repeater which can be shared by numerous users, but a personal node can be used by multiple RF users if they are all within simplex range of each other and thus can all hear each other's transmit audio. Cross-Band Full-Duplex (FDD) radios also support dual-receive, thus when receiving you can monitor the node input and output frequencies at the same time. Thus if you have 10 ham friends within a few miles who all have FDD radios, you can all use one FDD personal node and hear all local RF users on the node input frequency and all ASL users on the node output frequency.
(Ideally a radio supporting FDD could be set up with echo-cancellation DSP functionality, in which case any number of users could share a FDD repeater node and each radio could subtract its transmit audio from the receive audio. Echo-cancellation is used in telephone and VOIP systems to help prevent echoing/feedback. No such radios currently exist, but it wouldn't be hard to build one. Maybe someday an open-source Linux-based HT product will be created that supports FDD and runs GNU Radio or other SDR programs enabling DSP capabilities. This could be a very popular product as it could then also support other analog and digital modes and anyone could create their own software, DSP algorithms, codecs, etc. or even whole new analog, digital or hybrid modes.)
Repeaters are therefore full-duplex in a different way than full-duplex personal nodes – a Full-Duplex personal node supports one or a small number of nearby users, whereas a Repeater supports multiple users over a much larger area who usually have only half-duplex radios. A full-duplex radio (or 2 half-duplex radios) are commonly used with repeaters, particularly by net controllers, however in that case because the Tx audio is echoed back a headset or headphones usually need to be used to prevent feedback/echoing, and there also can be a delay in the repeated audio on some linked repeater systems which makes the full-duplex aspect from a user perspective not as user friendly as full-duplex on a speakerphone or full-duplex personal node.
Building a half-duplex node is slightly simpler and less expensive, but even if you do not yet fully understand or appreciate the advantages of full-duplex there is no disadvantage to supporting full-duplex and you will then have a higher-quality node that will be a better long-term investment.
Summary
There are no disadvantages to full-duplex other than that it slightly increases the cost of the node, which is a small price for all the above advantages.
For more info see http://allscan.info/
