Category: Wireless VoIP

If you've been following my blog for a while you've probably noticed my support for Voice over LTE via GAN, or VOLGA for short. In case not and you are interested here's a link to a short intro and here a link to a whitepaper which I've recently written on the subject. It looks like the VOLGA forum has not been idle over summer and has continued working on the final stage 3 specification that contains all the nitty gritty details on messages, information elements, etc. Now in September 2009, the final version has been published which means all documents are together now to implement VOLGA. For the details, see here. Well done, I am looking forward to seeing the first implementations!

A main feature that is missing in LTE so far is a viable standardized system for voice calls for the short and medium term. Sure, 3GPP has specified CS fallback and many network operators are hoping for IMS in the long-term. However, in my opinion, both leave LTE very exposed on the voice front at least over the next couple of years.

That's why I am a fan of VOLGA, short for Voice over LTE via GAN as it offers a smooth way of connecting the already existing voice infrastructure to LTE. I've already voiced my thoughts on it here and here, but blog posts can't really go into the technical details. So with the kind support of Kineto Wireless, I've assembled a whitepaper about VOLGA to describe how it works and its advantages and disadvantages compared to other voice over IP systems for wireless networks. So if you have a bit of time and are interested in the details, have a look and enjoy!

In a discussion around VoIP someone recently said to me that he thinks that "non-network operator supplied" VoIP is having a big impact on voice prices, both in fixed and mobile networks. I am not quite sure this is yet the case, however. When looking at prices for fixed and mobile voice calls, it can be observed in most countries where telecommunication was liberalized a decade or so ago, that prices are falling. This has consistently been happening over that time, long before VoIP came up.

From my point of view, falling fixed and mobile voice prices are more a result of competition between incumbent and startup telecom companies, who are offering voice services over circuit switched technology.

Technology has advanced, so in many cases, voice calls are transmitted over IP in the backbone networks of network operators and also internationally, but that's still virtual circuit switching and not 'end to end' VoIP over the Internet. Sure, there are services such as Skype, which are free while the call stays between two users of the same service but all people I know use it as additionally to circuit switched services rather than a replacement.

So why are VoIP services still lacking popularity today? I think it's quality of service and ease of use, in which end-to-end VoIP is still very much behind traditional circuit switched voice. That doesn't mean VoIP isn't catching up, but in my opinion we are not yet at a point where VoIP is a serious threat to circuit switched fixed and wireless voice.

As always, comments are welcome.

It seems I am not the only one thinking quite positivity about Voice over LTE via Generic Access Network (VOLGA). Recently, Ajit Jaokar posted an interesting article in which he mentions that with VOLGA, the traditional circuit switched voice service becomes a bearer aware application, as it can choose between a 2G circuit bearer, a 3G circuit bearer and an IP based bearer over LTE. All seamlessly with handovers during the call with all bells and whistles attached!

An interesting way to look at it even more so as the bearer awareness does not come into play on the mobile device but actually in the network. This is because the controlling entity for the voice call, the mobile switching center (MSC), sits in the network and is informed by the network that a different bearer should be selected. It can then decide to go along, arrange for the network to prepare the handover and then instructs the device to make the jump.

So maybe VOLGA makes voice even more than bearer aware!? So far the term 'bearer aware' has mostly been used for applications being aware what kind of networks are available at a time and then make a choice as to which IP network to use or to stay put in case a network is available but the cost attached to it is too high to make the application feasible.

In the case of voice, however, the service can ensure continuity by jumping from one bearer to another. So terms like 'bearer adaptive' or maybe even 'bearer agnostic'  come to my mind, because that voice call will just work over any kind of network the device supports.

It could even work over the Wi-Fi you have at home if you extend the idea of VOLGA. Not for the moment, as the standard currently focuses on LTE, but in the future, who knows?

If you have a couple of minutes, join me for a tour of the incredible evolution of the Mobile Switching Center for mobile voice calls from a circuit switched elephant to an IP based leopard. As much as possible, I've tried to use general terms and reduce the number of telecom acronyms to have a blog entry that is understandable to a wider audience. The purpose? To explain at the end why the circuit switched and the packet switched IP world should loose the fear from each other.

Only 10 years ago, the mobile world was a very different place. GPRS was only in the standardization phase and GSM networks were purely circuit switched voice networks. The core component was the Mobile Switching Center, the MSC. A single MSC was composed of several long aisles of cabinets which were full of circuit boards and cabling. Voice calls were circuit switched, 30 of them multiplexed over 2 MBit/s E1/T1 links and many of these links connected the MSC to other MSCs and with the GSM radio network. But if you had seen the earlier generation of voice switches that was definitely state of the art!

Fast forward to today: The MSC still exists and to the user it still behaves like it has 10 years ago. However, today's MSCs have not only completely changed the way they look but also how they work. The switching power of aisles of cabinets is now contained in a single cabinet and the physical circuit switching of 64 kbit/s channels has been virtualized into IP packets running through gigabit optical cables. In effect the MSC has become a powerful server that communicates with the outside world over IP and instead of connecting voice circuits physically in a switching matrix they are now virtually connected by routing IP packets from one IP address to another.

Since 3GPP Release 4, the MSC is logically and in many cases also physically separated into two different boxes with a standardized interface in between them:

The MSC-Server: This is the brain of the voice switching system if you want. When a mobile subscriber initiates a call, the mobile exchanges messages with the MSC-Server which then finds the destination party and signals the incoming call to it. If the destination is available the MSC-Server then establishes a voice connection between the two parties and the conversation can begin once the other side accepts the call. With the old MSC architecture, the Signaling System number 7 has been used for this purpose. Today, the MSC-Server is an IP based system and all messages between the MSC-Server and the radio access network pass an SS7 to IP gateway that exchanges the lower layer of the protocol stack so SS-7 messages can be transported over an IP connection. A mobile network usually contains many MSC-Servers at different physical locations and messages between them are directly transported over IP. Interested in how an MSC-Server looks like? Here's a cool 3D example.

The Media Gateway: The MSC-Server only sends messages through the system via IP links, it is not involved with actually setting up the speech path between two phones. This is the responsibility of the Media Gateway. The MSC-Server is connected to the Media Gateway via an IP connection and the protocol for establishing voice connections is called H.248 or Megaco (Media Gateway Control Protocol). Basically the MSC-Server uses these messages to tell the Media Gatway to connect a virtual or physical circuit coming in on one interface with a virtual or physical circuit on another. I call them virtual and physical because there are different kinds of interfaces. To the GSM radio network, a real physical circuit switched channel is used. Thousands of them are usually multiplexed into an optical link. IP is not used on this link. To the UMTS radio network, it's already a bit more virtual, as ATM (Asynchronuous Transfer Mode) links are used, usually again over an optical link. Also, no IP used here. To other Media Gateways in the network IP links are used and the protocol to transport the voice stream is called Real Time Transfer protocol (RTP). Interested how a Media Gateway looks like? Here's a cool 3D example.

Below I will use the term MSC system to describe the combination of MSC-Server and Media Gateway.

SIP

Speaking of the RTP protocol for sending a voice stream through an IP network. It's probably not a coincidence that the same protocol is also used by SIP, which is the protocol used for most of the voice over IP solutions that are in use today. SIP stands for Session Initiation Protocol and if the MSC System is enhanced to understand this protocol, it could also accept and forward voice calls to SIP lines. That's not done in practice today as far as I know but let's keep that in mind for the future.

IP based GSM and UMTS access

Today, the links in most wireless networks to the radio network for the call establishment signaling and the voice calls are not based on IP. However, there are now 3GPP standard enhancements to offer an IP alternative for these interfaces as well for both GSM and UMTS. As a consequence it is likely that over time, network operators will convert these links to IP, too. This could be part of a radio network backhaul convergence, where all data from base stations that support several radio technologies (e.g. GSM, UMTS and LTE) use the same IP based backhaul link instead of several different ones based on different lower layer standards as is mostly still the case today. In short, there is also a trend to replace the current circuit switched based radio network links with IP based connections.

Interconnection between different networks

To allow calls between different networks in the same country and between different countries, MSC-Servers and Media Gateways of different network operators are interconnected. Today, these interconnections are also still based on E-1 circuit switched interfaces and SS-7 signaling. However, as we go forward network operators will start replacing these legacy interfaces with IP links which saves the SS-7 to IP message converters mentioned above reduces cost as E-1 circuit links are much more expensive than using an IP interconnect.

VOLGA (Voice over LTE over Generic Access)

A special case is the LTE radio network. As it is fully IP based system it has no connection to the MSC system today. As reported recently, the VOLGA forum is about to change that as discussed here. Effectively, VOLGA introduces a gateway between the LTE network and the MSC system. To the MSC system, the gateway looks like a GSM BSC or a UMTS RNC. This way, no code has to be changed on the MSC system and the link can either be based on circuit switched or IP technology.

To the LTE network the VOLGA gateway just looks like an IP server and the communication between it and mobile devices flows transparently through the LTE core and radio networks. An already standardized interface is used to announce handovers to the gateway, so the LTE network can inform the VOLGA gateway and the MSC system that an ongoing session is to be handed over to a GSM cell. To the MSC Server, the handover is no different from a GSM/UMTS to GSM handover.

Circuit to Circuit, IP to IP

So when looking at this evolution from a slightly different angle one could say that the MSC looks like to each world what it would like to see. For the circuit switched world, the split MSC-Server and Media Gateway still looks like a circuit switched box. For the IP world, however, the MSC-Server and Media Gateway are just IP based devices which send and receive IP packets. For the voice stream, RTP is used, a protocol well known in the IP world. The call establishment signaling also uses IP packets but the protocol used is originally from the circuit switched world. It doesn't change the principle that it's now IP based but protocols like M3UA, SCCP, TCAP, MAP, etc. are not yet in the toolkit of IP engineers. But fear not, Wireshark and other well known IP tracing tools understand these protocols so they are as easy to trace and analyze as any other IP based protocol. The beauty of the approach is that old and new, circuit and IP can be mixed and matched for each voice call as required.

As we go into the future, more and more links connecting the MSC system will be converted from legacy circuit switched to IP and physical circuits are put into virtual circuits, embedded into RTP IP packets. At some point, some MSCs in the wireless network will only use IP based links between different servers, to the radio access network and even servers in another mobile network. This might still be a couple of years away but the standards are all in place and the changes are well underway.

So from my point of view there is still a lot of life left in the MSC architecture and it will serve us well into the future, possibly even in LTE networks unless a miracle happens and alternatives such as the IMS based system suddenly emerge from hibernation.

Google has been in the press in the past couple of days for resurrecting Grand Central, a web based PBX system that, among other cool features, can forward incoming calls to several virtual and physical phones simultaneously. Some of the reports also say that for the moment Google Voice is only available in the U.S. and not in Europe and other parts of the world, as termination charges to fixed and mobile networks are higher. That’s an interesting twist on the different charging models for calls that are sent from one telephony network to another.

In the U.S., telephony networks use bill and keep (B&K), which means that the originating network can keep whatever it charges the caller and the terminating network routes the call to the destination free of charge for the originator network.

In Europe and elsewhere, calling party pays (CPP) is the standard interconnection charging model. Here, the terminating network charges an interconnection fee to the originating network. For fixed line networks this interconnection fee is around 1-2 cents a minute these days while for mobile networks it’s much more expensive, around 6-10 cents depending on the country.

Some people including me and for example Tomi Ahonen think that CPP is a good idea because:

• Incoming calls on mobile phones are free unlike in the US, where B&K makes the mobile operator charge mobile users for incoming calls.

• This in turn keeps many people from getting a phone, has led to relatively high priced flat rate plans or a high number of included minutes and thus keeps people from communicating more cheaply and getting more than one subscription or mobile device.

• As a result mobile adoption in the US is still far behind that of most other industrialized nations.

• Also, there is no incentive to increase coverage and capacity of networks since the terminating network is not getting paid for incoming calls so it doesn’t matter if they are connected or not. Outgoing calls are included in the flat fee as well so missing or bad coverage does not reduce revenue either.

The Internet Based Voice Centric Service Twist with B&K

So in general, CPP has been quite good for competition and network build outs. But here is a twist with Google Voice and Internet based voice centric services: In the US, while B&K doesn’t generate revenue for Google for incoming calls, it allows them forward calls to mobile networks for free, because the terminator pays for it. In the CPP world (Europe, Asia, etc.) Google would have to pay for those calls. As the users phone number for incoming calls would have to be a fixed line number for the service to be attractive to users, the difference in termination charges they could earn with the fixed line termination fee and the money they would have to spend on the much higher mobile termination fee makes the business model very different.

What this means is that in the US, Google just has to recover its own costs for running the servers and for interconnecting with the telephony network. In the CPP world, Google would in addition have to recover the cost for the difference between the fixed and mobile termination fee, which is anywhere around 4 and 8 cents a minute.

So it looks like this aspect of B&K is quite beneficial for Internet based voice centric service innovation. From a mobile operator point of view, the bottom line does not look as good because when people start using this service their traffic and workload increases while their revenues stays flat due to flatrate pricing. In effect the potential service upsale can easily be made by Internet based companies and mobile operators have no handle to promote their own services, if they had them. Maybe an incentive for them to consider moving to CPP to reduce competition from Internet based voice services? It would be an irony.