The business case for femtocells in the Middle East.

Etisalat is the first operator in the Middle East who shows interest in FemtoCells, and therefore they asked us at Ubiquisys to showcase their femtocell on the Etisalat stand at Gitex 2008. They branded the product “Etisalat Home Cell”.

So we took few units and demonstrated the first commercial femtocell ever in the Middle East and I ended up spending a week in Dubai during the Gitex technology week at Gulfcomms 2008.


Etisalat were very surprised at the level of maturity of this technology: first they were amazed how far the femtocell coverage can extend with only 10mW of transmission power! Then the surprise turned to admiration when we showed some of the applications of femtocells that go beyond the traditional voice + mobile broadband. The spare femtocell I had with me kept disappearing from time to time, and then I figured out that Etisalat marketing folks were borrowing it to show to journalists and TV stations covering the event . They also kept bringing VIPs to the stand from various ministries and governmental bodies as well as investors to have a look at the femtocell.

After the news hit the net and newspapers that our femtocell is at Gitex2008,
I had a busy week talking to delegates from almost every operator in the region, all keen to find out more about this interesting product. I even had a number of industry analysts who were keen to see the femtocell in action: they heard a lot about this product but never had the chance to see it in action before.

After talking to CTOs and CMOs of the top operators in the Middle East, it was very obvious why there will be a clear winning business case for 3G femtocells in the region. First of all, all operators without exception are struggling with 3G coverage indoors. One CTO has complained that UMTS technology means that the inter-site distance has to be relatively smaller in comparison to GSM in order to provide adequate indoor coverage even for the taken-for-granted voice telephony. Many operators have expansion plans to beef up their 3G networks and add sites in order to improve the quality of coverage indoors. The budgets allocated for these expansion projects can almost buy them a femtocell for every household in the country! So at least from a cost-effectiveness point of view femtocells are a clear winner

The building material and density of urban structures in the Middle East is particularly challenging to cover with a conventional network. Reinforced concrete is widely used, and towers from 30 stories upwards are springing up every day. Some of the commercial towers have expensive distributed indoor systems which are usually installed during construction. Clearly the femtocell proposition is a more cost effective way of providing indoor coverage for the office and home apartment. In summary, most of the commercial people I met immediately recognised the opportunities that femtocells can bring.

However, there are few things that have to be said about the commercial challenges to bring complete femtocell networks to the region. First of all, innovation and the desire to try new ideas is still not an outlook that you face regularly among the Middle Eastern operators. This however is slowly changing. I was positively surprised at the number of innovative offerings Etisalat was offering during the event.

Second, it did seem to me that decision making is not based on any clear or spelt out processes. For example, one operator explained that they ruled out femtocell in the past because their “3G expert” has suggested that (from a technical point of view) the idea will not fly off the ground. After checking with their3G expert, it turns out his source was a single IEE paper written by opponents of femtocells when they were trying to dilute the discussion at 3GPP (the standard body). The number of people involved in the decision process is fairly limited and not diligent enough.

A major difficulty is also lack of visibility of small and innovative companies involved in the development and making of femtocells. Operators want to deal with big vendors directly, and they get suspicious of vendors trying to become System Integrators of smaller companies. Therefore operators are more willing to work with the likes of Huawei and Alcatel. The big vendors have got the most to lose out of introducing femtocells, and therefore they take any opportunity to badmouth the concept and the technology.

All in all, it was exciting to be able to show a really cool product to a very receptive audience. I have no doubt that femtocells will change the game of Middle Eastern operators. I’ll be there again at the Comms World GSM >3G event in December.

Femto cell Architecture

The business case for indoor femto cells has gained increasing traction in the last few weeks, and lots of news comments about various femto cell development companies. For example, check here, here and here.

Although the femto cell concept is quickly gaining grounds towards complete and operational products, there is still some misunderstanding on how the overall network architecture will look like. I'll try to address this issue in this post.

A femto cell router is a small device, the size of any Wi-Fi router, which is in effect a miniature base station. The radio is a standard based radio such as UMTS/HSPA which the operator will likely require a license to operate on. The router connects to a DSL line. The idea is to enable the subscriber to make and receive mobile calls indoors, with low signal levels which has a number of benefits to the operator:
- Hopefully accelerate fixed line substitution.
- Reduce the cost of building a full macro layer network.
- Lock subscriber in to the operator and reduce the likelihood of churn.
- Provide a viable medium for content distribution (the ugly walled garden paradigm)

It is not my intention here to argue for or against these benefits. I will dedicate a full post on the business case for the femto cell and the potential cons sometime soon. So let us stay on the technology side and try to investigate how a UMTS based femto cell routers will be integrated with the operator's core network.

Basically, there are a number of ways, the first of which is the conventional hierarchy using an RNC . Just like any node-b is connected back to an RNC which is then in turn is connected to the core network, the femtocell routers can be treated like individual node-bs and connect to RNCs. This may appeal to big manufacturers who already have substantial deployments and many RNC on the ground. The downside to this approach is the limited chances of inter-operable devices if the operators chooses to diversify their suppliers. Although the Iub interface (base station to RNC) is standardised, the reality is most implementations are proprietary. Typically operators don't like to put all their eggs in one basket and would prefer to get solutions from various suppliers. The other downside of this approach is that most available RNC solutions are geared towards Macro/Micro type of deployments. In other words they are built in order to support relatively small number of cells with a huge number of subscribers in each cell. They don't scale very well to support Femto cell deployments with almost as many cells as there are subscribers, and a handful of subscribers per cell.

An alternative approach is to use UMA (or now called GAN). UMA was originally conceived to support dual mode cellular/WiFi-over-Internet type of connectivity. When the mobile is detected indoors within the range of pre-determined WiFi coverage, a UMA concentrator does the core network negotiation on behalf of the mobile, e.g. registering, and updating location ... etc. It is thought that a similar procedure can be used for femto cells. When the mobile is detected within the coverage of a femto cell, UMA kind of hand shaking takes place to ensure that: 1. the mobile is allowed to access the network at this particular femto cell, 2. the traffic between the mobile and the core network is forwarded accordingly. Using this method a UMA concentrator is required, so in a way, the solution is still hierarchical and it is certainly one of the criticisms of this method is that it is not a fully flat architecture. Kineto is one of the companies promoting this approach.

Yet another alternative approach is enabling femto cell connectivity through an IMS service. In this approach, the femto cell router talks SIP over the internet back to an IMS service which acts as a bridge between the femto layer and the rest of the network. Although this is considered the "flattest" approach, it is yet to understand how it will work in practice, the delay in working fully featured IMS platforms is one of the concerns of femto cell development companies.

Incidentally, my current clients are hedging their bets by supporting more than one approach and have developed partnerships with various companies in their respective fields to make the Femto architecture as flexible as possible.

The advent of Femto Cells

Home base station solutions, also called Femto cells have recently attracted a lot of attention. The idea is to give the mobile user a small box similar to wireless routers, which provides mobile cellular coverage at home. But why would mobile operators want to do that?

In order to answer this question, one has to look at what mobile operators are competing against. Fixed line carriers and DSL service providers are seriously threatening the mobile business by offering new products centered around Fixed Mobile Convergence, which in essence utilises wireless access technology to capture mobile users when at home, and divert their mobile originated voice and data calls to the fixed line network.

Mobile operators found themselves in a predicament: the traditional Macro cellular networks are not able to compete with what DSL service providers can offer, neither from a cost structure point of view, nor in terms of access speeds. (of course the mobile operators are partly to blame because they picked the wrong battle by trying to compete on the basis of access speeds rather than mobile centric applications).

A home base station or femto cell can help mobile operators in many ways. First they can compete directly with other home access solutions which rely on other types of access technology such as WiFi or WiMax. This is because low cost mobile handsets are widely available, whereas dual mode handsets with WiFi capabilities are pricey and still limited in variety.

The femto cell will also enable the operator to cater for subscribers where the service is most needed. Compare this with traditional macro deployment where the infrastructure is first built in places where subscribers are "likely" to appear and even before subscribers start using the network. A femto cell also circumvents the issue of outdoor-to-indoor propagation which typically kills the capacity of 3G and 3.5G systems.

The deployment costs associated with a femto cell are marginal in comparison with Macro cell deployment. This is because the customer is providing the "real estate" to keep the box, the power, as well as the backhaul (e.g. DSL subscription). I don't assume here that Femto cells will replace the macro layer, nevertheless the extent of investment in the macro layer will be considerably less.

Femto cells will also enable the operator to offer creative and disruptive tariffs to directly compete with fixed line. Home zone offers can also be more effective than traditional home zone offers using macro cells which extend for kilometers sometimes (because they rely on a cell ID parameter).

So where is the catch? I will discuss the downside in a future post.

VoIP Episode 6: To VoIP or not to VoIP, that is the question

Cellular operators seem to be confused about VoIP. On one hand they seem to think that the whole world is going IP, hence VoIP is inevitable. On the other hand, they do not want to undermine their number one profitable application: trunked voice. There is also confusion about benefits of carrying VoIP on cellular radio access networks.

Most cellular operators see VoIP as a dangerous proposition because it cannibalises their stable voice market share. That's why, until recently, any talk about VoIP over cellular was accompanied by a view that VoIP traffic will be controlled and billed by the operators in a very similar fashion to traditional circuit switched voice. Therefore the concepts of a myriad of sophisticated "boxes" were introduced to enable quality controlled VoIP (as opposed to best effort VoIP over the Internet) along with all the added value services associated with the IMS architecture. So the emphasis is on the user experience, and perhaps the vision is to do voice "sessions" in the same way you use your Instant Messenger with the added guarantee of quality. Operators don't want to merely become bit pipes for other content providers, and they dread the prospects of following the footsteps of DSL providers.

The other driver is to have a more integrated core network. This may be true in the long term, because legacy networks will stay for some time (perhaps a generation) before they get obsolete, so the vision of one integrated core still needs sometime to materialise. Incidentally, convergence (I know, an overused word these days) is really about using less boxes to carry different types of traffic, and it is not necessarily about everything becoming IP based.

In a nutshell, the vision is to bring VoIP to cellular radio in a controlled fashion that guarantees the operator's grip on the revenue. UMTS operators can stop users from using their packet access to transmit/receive VoIP traffic. There are network devices that can "sniff" the types of packets generated by users and if you are running a Skype client for example, they can block it for you. Nokia launched a VoIP blocker to help operators stop cheap peer-to-peer VoIP from competing with their own voice services.

So on one extreme, the operator may want to run the business as a "walled garden" and have full control over content and traffic. The other extreme is to run the business following the ISP model: they give the user a bit pipe (could be fat or slim) and the user does whatever he likes with it. Content providers provide their services to the end user independently of the operator in the same way it is done over the Internet.

The cellular business could also be run in a balanced way between these two extremes, perhaps by "cherry picking" content providers that are allowed to promote their service over the cellular network. Perhaps the cellular business model can shift from generating income from the end user to making money from content providers instead. Perhaps they can diversify their business and get into making content themselves to compete in equal footing with everyone else.

3 has recently announced its launch of mobile Internet in the UK. Their model is "all you can eat" flat fee. They are partnering with Skype, eBay and Google. It is yet to be clear how much control they will have over the traffic, and what revenue streams they will have together with their new Internet partners. It seems likely they followed a "we give you the pipe you do the skype!" model. If this turns out to be true, it can be considered as the end of the cellular industry in the UK as we know it. peer-to-peer VoIP does not need any "expensive?" boxes to control it or monitor it. Users should not expect a quality better than VoIP over the Internet though; this is not trunked grade voice. So It is all about extending the Internet experience to the mobile world in this case, full stop. In contrast to this, do you remember the "IMS tax" we talked about before? It is about the quality vs complexity tradeoff again.

3 has got nothing to loose, they have a loss making business and Hutchinson was rumoured to be thinking of selling the business (I hope they will not sell to Orascom, otherwise the egyptians will run it in the same way they run EgyptAir) , therefore it is not surprising to hear that they have decided to try the rather uninspiring ISP model.

On the other hand, incumbents like Vodafone are not doing any better. They have an endemic problem of lack of creativity and ability to introduce new exciting services. The are too frightened of competition from fixed wireless and fixed wired, and feel the threat coming from every corner.

Cellular operators tend to forget that they have two very strong commodities which the others do not have. The first is mobility. The fact that they can support an advanced level of mobility is something competitors can not even get close to, at least not in the short to medium run. Roaming, portability and ubiquity flow naturally from this.

The other strong commodity they have is the location information, which many services and applications can be wrapped around. In their short sighted and rather uncreative vision, they have only looked at things like emergency services and mapping applications, but very little real work on any real creative applications.

Meanwhile, most operators are still stuck in the same corner thinking: shall we VoIP or shall we not?

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VoIP Episode 5: VoIP over UMTS mouth-to-ear delay

I have received some queries about the delay encountered by VoIP calls. Thanks for the interaction and questions, keep them coming.

In this post I will summarise the various reasons that cause delay of VoIP speech over a UMTS network, or over any network for that matter.
I should mention here that I am not trying to show that VoIP over UMTS is any better or worse than conventional UMTS (I will only do that for a high consultancy fee! ;-) ).

Let us first "dissect" a VoIP call:
Before you start a VoIP call (and during the call) signalling messages are exchanged between the two VoIP terminals and possibly with some other network elements. These are carried using the Session Initiation Protocol (SIP). When you talk into your handset speaker, your analogue voice signal is "encoded" as a string of bits. A number of bits are then packed together in one packet and all the necessary headers are added to it. These packets are then transmitted over the network. They propagate through one or more packet switched networks before they reach their destination. At the destination, the received packets are then reordered and a decoder transforms the digital bits to an audible audio signal.

The sources of delay are numerous:

• First, there is the coding delay, which is the time spent on transforming your speech into a digital stream of bits. This can be minimised by powerful DSP/electronics.
• Then there is the time it takes to compile a packet and add headers to it (packetisation delay). For example, if a packet contains X voice samples, then it has experienced a delay from the instant of the first sample to the instant the packet was complete and ready for transmission. Obviously the bigger the packets are, the longer is the packetisation delay.
• There is also the propagation delay. This is the time the packet takes to traverse the various networks before it reaches its destination. This could be the propagation time over the air interface, or over the various packet core elements. This is essentially determined by the speed of the different physical mediums over which VoIP packets are transmitted: wires, optical cables, radio ...etc
• In addition to this, there is the summation of queuing delays encountered by the packet at each node in the network(s). For example, we have seen in a previous post that a VoIP packet may have to queue for a HSDPA radio resource, or the packet may have to queue at some of the routers and switches before it reaches its destination. The various routers and switches along the route of the packet can be loaded differently, so some packets may be delayed more than others.
• When the packets arrive at the destination, they may not arrive in the right order, because they may traverse different routes before arriving at the destination and because some of them are delayed more than others. Therefore, the receiver has to buffer the packets in order to wait for the slower ones and reorder the packets before submitting them to the decoder. This is called "De-jittering". The de-jittering introduces another delay before the packets are decoded into an audible signal again.
• There is also the delay associated with call signalling. You can not start talking until it rings at the other side and the called party answers. Because of the large SIP messages this delay could also be long. The same issues arises when the call ends, which does not really affect the user perceptions as much as it keeps some network resources tied up for a while.

I hope this answered your questions. I will talk about how some of the advanced 3GPP features will address some of the delay issues in a coming post, so stay tuned!.

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VoIP Episode 4: VoIP over UMTS high speed channels

In the first episode we discussed how the various implementation options compare in terms of performance and complexity. We say that there are various necessary changes to the high speed channels to make them an attractive medium to carry VoIP traffic. Some of these changes are also necessary to reach VoIP capacities that exceed conventional circuit switched capacity. In this post I will discuss some of the suggested changes to the high speed channels to make them suitable to carry VoIP.

• Scheduler changes:
  3GPP defined the structures of the high speed shared channels such that they are optimal to carry large chunks of data. To make the system reactive to users' channel conditions, an intelligent scheduling entity that usually resides in the RNC is moved to base station. Thus if a user happens to be in good channel conditions, the scheduler allocates resources and send them to the user before the favorable channel conditions disappear. The problem with VoIP traffic is that the packets have to be delivered to the user pretty frequently in order to maintain the perception of a good quality connection. The packets can not be queued forever waiting until the user has good channel conditions. Therefore the scheduler algorithms, which were originally developed to be optimal to carry discontinuous chunks of data, have to be adapted to work well with VoIP traffic side-by-side. One of the concepts to enable this is called “priority queuing" which as the name implies, gives priority to delay intolerant traffic such as VoIP. Most of what I describe above applies to HSDPA (downlink). In the uplink, 3GPP included enough functionality in the HSUPA requirements to make it versatile and able to cope with VoIP traffic using a scheme called “Non-Scheduled Transmission”.
• HARQ Retransmission Reduction:
  The HSDPA channel uses also a concept called "HARQ retransmission", where lost packets are retransmitted (partially or completely). The idea is to optimize the air interface efficiency, so instead of sending few highly protected bytes that will be definitely received correctly, the transmitter sends a big chunk of bytes and hope for the best. If part of the data is received incorrectly, then the transmitter can retransmit the packet. This approach is one of the enablers of high speeds achieved on this type of shared channels (see note 1 below). It turns out that the HARQ retransmission concept is a challenge to smooth VoIP operation. This is because the VoIP connection is intolerant to delay, and obviously packet retransmission will increase the round-trip delay, which degrades the call quality. Packet retransmissions also increase the severity of packet jitter at the receiving end: one packet may arrive almost immediately and the next packet may be transmitted several times before successful reception happens. If you eliminate re-transmission altogether the channel efficiency drops and you end up with a reduced cell capacity. (Check note 1 below) .
• Reduce the possibility of code limitation (Fractional DPCH):
  Every user connected to the network using a shared channel requires a number of small dedicated channels to carry signaling and control information. These channels carry very little traffic, but they are numerous. If there are many users in a cell using the HSDPA channel, a large number of dedicated channel is required to support them, and it could happen that the system runs out of channelization codes (if you don’t understand the concept of channelization codes, check this). Therefore, 3GPP included a new feature called Fractional DPCH, which essentially multiplexes the control information of multiple users on one physical channel.
• Continuous Packet Connectivity (CPC):
  This is a huge 3GPP work item which includes many aspects that, although not necessary for VoIP to work, but will benefit its operation and increase the cell VoIP capacity. The most noteworthy of these aspects are:
  
  1. DPCCH gating: The control channels have a discontinuous pattern, which reduces the amount of interference and increases battery life.
  
  2. Modified HS-SCCH operation. HS-SCCH is a channel that carries HSDPA signaling in the downlink. This channel typically consumes large amount of power which eats up a part of the cell capacity. Therefore 3GPP came with various suggestions to reduce its effects by restricting it, or eliminate it altogether.
  
  3. CQI reporting reduction: CQI stands for Channel Quality Indicator, which is a message sent by the terminal to the bas station reporting the channel quality so that the base station can decide what to send to the terminal. As we discussed in the scheduler section above, successful VoIP operation is more concerned with the continuity of the traffic instead of mapping transmission to channel conditions, therefore reducing the rate of CQI signaling will reduce overheads without affecting VoIP quality. This means that more bandwidth is available for VoIP packets instead of overhead.
  
  All in all, CPC is expected to increase VoIP cell capacity by anything from 15% to 40%.
  

Note 1: There is an optimal packet loss rate that will cause the system to operate at the maximum possible capacity. On one hand, high packet loss rates mean that many packets are being retransmitted many times and hence the air interface resource is being wasted on these retransmissions. On the other hand, low packet loss rates mean that the transmitter is not taking chances and is protecting data more than necessary. This will lead to a lower cell capacity.

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VoIP Episode 2: Why do operators want VoIP/UMTS?

In my previous post, I looked at some of the reasons why mobile operators think they need VoIP/UMTS. In this post I'd like to investigate these reasons a bit further. The list is not comprehensive so if you think of any other motivations please let me know:

1. Operators think that VoIP can help them utilise their transmission network more efficiently. The idea here is simple: if all traffic is IP based, you can carry all of it on one IP backhaul network. This may be true to a certain degree in the sense that reliance on legacy transmission networks can be slowly phased out. Then the operator eventually can consolidate the traffic on one IP network, and hence achieve cost savings due to economies of scale. Also the network becomes simpler and hence cheaper to manage and run (One box does everything instead of many boxes doing many things). However, when talking about VoIP, the amount of overhead that the network has to cope with reduces the attractiveness of any cost savings. This is the famous "IMS Tax" term that was coined to refer to the additional costs a cellular operator has to absorb to provide VoIP - here's a good example that refers to "IMS tax".
2. They think that VoIP can help them squeeze a bit more out of their radio network. The idea is to offer VoIP calls with low rate codecs using packet channels (dedicated or shared), which has a capacity advantage over circuit switched voice. This, unfortunately, is nothing but wishful thinking. We discussed this in the previous post, so I am not going to delve into it again. In a nutshell, the IP overhead waste some of the air interface resources, and even with header compression schemes, the performance is only partially recovered.
3. They freak out when competing providers using other access technologies (WiMax, EV-DO ..etc) are offering VoIP. UMTS providers feel the urge to support VoIP to compete. This view is rather delusional because some competitors do not have a choice but offer voice over IP. For example, EV-DO can not carry circuit switched voice (DO: stands for data only), therefore VoIP is the only route to offer voice services in this case. WiMax, is also built from the onset to carry packets and packets only, hence VoIP is again the only alternative to offer voice services. For UMTS networks, this is a non-issue really. You can carry voice much better on dedicated circuits. Now some operators may want to run their networks around the Internet model, where they see themselves as a provider of a "fat pipe" and users are free to use it the way they like. Some users may want to run VoIP clients from their terminals (e.g. Skype), and that's fair enough as long as they understand the difference in quality they'll experience. If VoIP allows me to call Venezuela, or the Gabon for a fraction of the cost of trunked calls I'll go for it even if the quality is slightly worse. I will dedicate a post in the future to discuss operators' strategy in light of the recent news about Hutchinson's 3.
4. Convergence with their IMS networks. This was partly covered in point 1 above when we talked about all traffic types being carried over IP. "Convergence" in this case also partly refers to the additional services that can accompany voice such as presence information, contact lists, profiles ...etc . This is a valid point, and operators can perhaps create a value system around such services. Nevertheless, if operators follow the "ISP" model (provide a pipe and let the user fill it), there is no need to offer any of these services as part of the IMS network.

Please feel free to comment on my "rant" above!.

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VoIP Episode 1: VoIP over UMTS

There has been talk in the market about introducing VoIP over UMTS networks. In this posting I investigate the viability of various VoIP over UMTS solutions. The three options presented here are: VoIP gateway, end-to-end VoIP over shared channels and end-to-end VoIP over dedicated channels.

Clarification of terms:
Before starting, let me clarify the term "VoIP over UMTS". I chose it because the prevalent term "VoIP over 3G" tends to be more generic and encompasses all technologies people consider as "3G" e.g. EV-DO. In this case I concentrate on UMTS only. Some people, when they refer to UMTS, they mean the 3GPP release 4 functionalities, and tend to refer to 3GPP R5 as HSDPA system and R6 as the HSUPA system. When I say UMTS here, I mean all the above. I should mention here that all the discussion that follows is primarily from a radio network point of view.


What drives service providers to think of the possibility of carrying VoIP over a UMTS radio network? Some of the reasons that I hear from time to time:

• Operators think that VoIP can help them utilise their transmission network more efficiently.
• They think that VoIP can help them squeeze a bit more out of their radio network.
• They freak out when competing providers using other access technologies (WiMax, EV-DO ..etc) are offering VoIP. UMTS providers feel the urge to support VoIP to compete.
• Convergence with their IMS networks.
  

Putting all the commercial drivers aside (competition, prices, availability ...etc), the four technical factors that influences the VoIP implementation are:

1. Capacity: Can the system carry a reasonable and viable amount of VoIP calls?
2. Complexity: How complex is the proposed solution?
3. Call Quality: How does the VoIP call quality compare with users' experience?
4. Seamless convergence: How easily can the implementation offer introduction of converged multimedia services and how readily is it compatible with existing and future network entities?
   

So what are the various ways to introduce VoIP/UMTS? There are three methods I can think of, which score differently in light of the four factors discussed above:

1. Using a gateway solution:
   In this case, the VoIP traffic is terminated in the core network, and the voice is translated to circuit switched format to be carried over the radio network.
   UMTS was built from the onset to carry Circuit switched voice and packet switched data simultaneously, so by using the the UMTS circuit switched capability to carry voice traffic the call setup and release times are short. The call quality is also more predictable, sustainable, and is maintained at a reasonably high quality. The disadvantage is that the end-to-end VoIP capability is lost, and with it the ability to provide added services such as Contact Lists and Contact Presence Information. (It maybe still possible to offer the latter types of services on a separate bearer with an order of magnitude increase in network complexity.)
   Another advantage is that all the IP overheads are stripped off at the gateway, so there is less overhead to be carried over the radio, which is often considered the most expensive resource.
   
2. Providing end-to-end VoIP over high speed channels:
   In this case, the VoIP call is carried all the way to the user terminal over high speed shared channels in the uplink and downlink (dubbed HSDPA and HSUPA in the 3GPP standard). The motivation is to use the channels originally envisaged for data applications to carry VoIP packets.
   HSDPA was originally conceived to increase air interface throughput by giving more channel resources to users who experience good channel conditions. Unfortunately, in VoIP case, this leads to degraded radio bandwidth utilisation, because users' channel condition are irrelevant. Instead, link continuity and sustained voice quality are more important. Therefore, new scheduling schemes have to be developed to suit VoIP type of traffic, which unfortunately defeats the whole purpose of a shared channel that is allocated to users based on their channel conditions. In a nutshell, the capacity performance of this option does not look very appealing. Another disadvantage is the increased call setup delay and packet round-trip delay. Essentially users' packets have to stand in a queue to get a portion of the shared channel resources. Luckily, this effect is less of an issue in the uplink (HSUPA) because of a standard feature called (Non-Scheduled transmission).
   
   Nevertheless, there are various proposed schemes to improve the shared channels in order to carry more VoIP calls. For example, Robust Header Compression (RoHC) reduces the amount of overhead in the IP packets. Another idea is carrying the call signaling over shared channel too, which reduces call delays. There are also other improvements to the uplink and downlink shared channels that will increase the capacity up by anything from 20% to 50%.
   
   The proponents of this option argue that the E2E attributes of the VoIP call are maintained. They also argue that on the long term the VoIP capacity is much enhanced by introduction of advanced receivers and diversity (transmit div, receive div or both).
   
3. Providing end-to-end VoIP over dedicated channels:
   In this case, the VoIP traffic is carried over dedicated channels. This sounds a more plausible implementation than using high speed channels: The high speed channel model tries to blast high data rates to users in good conditions, which is great for data hungry applications, but is not advantageous to VoIP users who are more interested in quality than high data rate. The dedicated channels on the other hand adjust the power allocated to each channel (power control) , so each user gets just the right share of the radio resource (give or take). There is still the issue of packet overhead, therefore the capacity performance of this option is better than VoIP/high speed channels, but is still less than pure circuit switched voice.
   The E2E benefits are there too, so it is easy to introduce IM types of services.

So what is the verdict? VoIP over dedicated channels seems to offer the best trade-off between capacity and complexity. It also provides for reasonable VoIP call quality while maintaing enough attributes to enable easy multimedia convergence. On the long run, the introduction of enhanced features may cause the VoIP/HS implementation to perform better in terms of capacity and call quality.

I will discuss the various enhancements in a another post soon!.

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