IMS vs. Naked SIP

Everyday we get a bit closer to all IP wireless networks in which operators are hard pressed to present a voice over IP solution. Today two approaches are on the horizon: ‘Naked SIP’, already implemented in some 3G phones such as Nokia N-Series and E-Series S60 phones. And then there is the IP Multimedia Subsystem (IMS), based on SIP but with lots of additional specification put around it. So what does IMS do that SIP doesn’t? I came up with the following list of things which are laking in naked SIP today which are dealt with in IMS:

  • General SIP implementations are network agnostic and can not signal their quality of service requirements to a wireless access network. Thus, voice over IP data packets can not be preferred by the system in times of congestion.
  • Handling of transmission errors on the air interface can not be optimized for SIP calls. While web browsing and similar applications benefit from automatic retransmissions in case of transmission errors, VoIP connections would prefer erroneous packets to be dropped rather than be repeated at a later time since such packets are likely to come too late.
  • SIP VoIP calls can not be handed over to the 2G network in case the user roams out of the coverage area of B3G networks.
  • SIP does not work in 2G networks.
  • Most SIP implementations today use the 64 kbit/s PCM codec for VoIP calls. Compared to optimized GSM and UMTS codecs, which only require about 12 kbit/s, this significantly decreases the number of VoIP calls that can be delivered via a base station. Furthermore, mobile network optimized voice codecs have built in functionality to deal with missing or erroneous data packets. While this is not required for fixed networks due to the lower error rates it is very beneficial for connections over wireless networks.
  • Emergency Calls (112, 911) can not be routed to the correct emergency center since the subscriber could be anywhere in the world.
  • No billing flexibility. Since SIP implementations are mostly used for voice sessions, billing is usually built into the SIP proxy and no standardized interfaces exist to collect billing data for online and offline charging.
  • Additional applications such video calls, presence, instant messaging, etc. are usually not integrated in SIP clients and networks.
  • It is difficult to add new features and applications since no standardized interfaces exist to add these to a SIP implementation. Thus, adding new features to User Agents and the SIP network such as a video mailbox, picture sharing, adding a video session to an ongoing voice session, push to talk functionality, transferring a session to another device with different properties, etc. is proprietary on both the terminal and the network components. This is costly and the use of these functionalities between subscribers of different SIP networks is not assured.
  • Insufficient security: Voice data is usually sent unencrypted from end to end which makes it easy to eavesdrop on a connection. Signaling can be intercepted since it is not encrypted. Man in the middle attacks are possible. No standards exist of how to securely and confidentially store user data (e.g. username/password) on a mobile device.
  • Scalability: Mobile networks today can easily have 50 million subscribers or more. This is very challenging in terms of scalability since a single SIP proxy in a network can not handle such a high number of subscribers. A SIP network handling such a high number of subscribers must be distributed over many SIP proxies/registrars.
  • There is no standardized way to store user profiles in the network today. Also, no standardized means exist to distribute user data over several databases which is required in large networks (see scalability above).

The list is quite long I have to admit. But there is one thing the list does not say: While naked SIP is available today I have yet to see an IMS capable terminal in the wild. I wonder how long it will still take?

As always, comments are welcome.

What next for mobile telephony?

… asks Moray Rumney, Lead Technologist over at Agilent in the latest edition of the Agilent Measurement Journal (3/2007). In his article, Moray takes a look at which factors contribute to the ever increasing wireless transmission speeds and explains where the limits are and why the 300+ MBit/s promised by LTE and other technologies in a 20MHz channel will remain a theoretical promise rather then becoming a practical reality. He then goes on to describe what is possible with the given physical limits and presents his thoughts about how to address capacity issues in the future. An absolute must read!

The journal is available here and the article can be found on page 32.

International MMS: A Case Study of Failure

It’s a strange situation: Most mobile operators today would like to retain control over the application layer and rollout new services themselves instead of letting Internet economics do the job. In practice however, they do not spend a lot of effort to making even the few advanced services they have universally usable. MMS is a prime example as I had to discover recently.

Situation 1: I am in France, I have a French SIM card and wanted to send an MMS to a prepaid subscriber of another French operator, Bouygues. Instead of receiving the MMS, only a text message arrives at the other end with a web link. The reason is that the other end did not have a GPRS subscription. 5 years after the introduction of MMS!? It leaves me puzzled.

Situation 2: O.k. so I can’t send my French friend an MMS but maybe I can send one to a friend in Germany. Message sent, I’ve been charged for it but the MMS never arrived. How nice.

Situation 3: Some days later I was in Spain and repeated the international MMS scenario with a Spanish SIM card. Again, the MMS to a German SIM card was not delivered.

To me it looks like even 5 years after the introduction of MMS, there are still no international agreements in place to forward MMS between operators. Could you imagine eMail not being delivered because the recipient lives in another country? No, probably not. That’s because no international agreements for applications have to be in place to forward eMail. And if there had to be, just imagine how the Internet would look like today and how many people would use it.

Some might say, the difficutlies stem from the fact that telephone numbers are used instead of eMail addresses for MMS messages. True, but international SMS messages which also use telephone numbers work just fine these days. But maybe 5 years is too short a time to make it work? One should not think so.

Light Reading Webinar on Mobile Backhaul Evolution

With mobile networks getting faster and faster a growing pain for network operators is the backhaul connection between the base station sites and the next element in the network. Today, T-1 or E-1 connections are used with a line rate of 1.5 and 2 MBit/s. With HSDPA being put in place today,  backhaul capacity requirements of 3G base stations now reach 10 MBit/s or more. This means putting additional T-1 or E-1 lines in place. While this might still work today for HSDPA speeds despite the associated rising costs it certainly won’t work tomorrow for WiMAX, LTE and other Beyond 3G technologies that require backhaul capacities of 60 MBit/s per base station and more.

So the big question is what comes after T-1/E-1 connections over copper, fiber or microwave!? The common answer these days seems to be:

IP over Ethernet with the capability to carry legacy GSM (TDM) and UMTS/HSDPA (ATM) links in IP pseudo-wires alongside native IP traffic generated by native WiMAX and LTE base stations.

But how do you connect the base station sites to Carrier Ethernet Networks? Can the last mile be done over copper, is fiber required or is next generation microwave an alternative? Questions over Questions 🙂

I found some answers in a recent one hour Light Reading Webinar on the topic which is available for free at this link. If you are interested in the topic take a look.

Testing a Cat-7 HSDPA Card In a Live Network

Tpunkt_fdh I recently got an invitation from T-Mobile’s press center to check out T-Mobile’s HSDPA network in Friedrichshafen, which has become Deutsche Telecoms open air test bed  ‘T-City’. There, the invitation said the HSDPA network was upgraded for maximum performance with category 7 HSDPA cards. In theory (no interference, no neighboring cells, single user, close to the antenna), such HSDPA cards are capable of speeds up to 7.2 MBit/s. Interesting stuff, so I went there to check it out.

In the local Telecom shop they let me check things out first hand with a notebook and an Option Globetrotter GT max HSDPA card which they said they had updated with the latest software. You won’t see a big difference when web browsing once speeds are higher than about 1 MBit/s so I decided to download a large file from a high capacity server on the Internet. The average download speed was around 4.2 MBit/s. Quite impressive even when compared to my already fast category 6 Motorola V3xx HSDPA mobile with which I have reached 2.5 MBit/s in the past.

For more on HSDPA on my blog take a look here.

Jumping from EDGE to EDGE on the Train with SFR

I had an interesting wireless experience recently when I took the TGV from Stuttgart to Paris. There isn’t a lot of 3G network coverage along the railway track so while in France I used SFR’s network with a Vodafone Websession from Strasbourg to Paris. So far I thought that  SFR did not have EDGE in its network. Looks like that assumption was not quite correct.

No_edge_lgv_est_sfr_3
Every now and then one of the cells along the railway track was EDGE activated and data rates suddenly jumped from a meager 60 kbit/s (typical GPRS) up to 200 kbit/s, even at 300 km/h. The bad thing is just that EDGE was only available in few places so it’s not really worthwhile to go online and download eMails and do some web browsing. I can only speculate that SFR tries to cover some towns along the railway track but they certainly don’t try to do anything in terms of higher rate data for the railway track in particular. What a shame…

Edge_lgv_est_sfr_2
At first I thought it might be a mobile device problem. So I tried with a second mobile which has an engineering mode which confirmed that EDGE is only sporadically activated. The pictures on the left shows the throughput of a file download in a standard GPRS cell and with EDGE when it was available.

Have to try with Orange next time.

Nokia’s Mobile Web Server Gets A Blog

In case you haven’t heard of Nokia’s Mobile Web Server yet, go check out my blog entry on it. In short the mobile web server is a port of the Apache web server to the S60 platform with a front end to access your mobile phone via the Internet or via a local Wifi network. Not a main stream application yet but with a lot of potential for the future. For those who want to stay informed what’s going on with the project check out the mobile web server blog which has just been created over at S60.

Yesss – 50 Euros For One Year of Prepaid 3G Internet Access

2007 is definitely the year Prepaid Mobile Internet Access took off in Europe. In many countries especially, tourists, business travelers and most importantly local young people and students can now get affordable wireless 3G access to the Internet with prepaid SIM cards. Mobile Virtual Network Operator Yesss in Austria now takes the concept to the next level.

For 50 euros they are selling prepaid SIMs in supermarkets which come with 3GB worth of data volume to be used over ONE’s HSDPA network within 12 months. A 1GB extension afterwards, again valid for up to 12 months, costs 20 Euros. The data rate is ‘limited’ to 1 MBit/s but quite frankly that’s a limit that is not too difficult to live with.

Need an HSDPA data card with it? No problem, they’ll give you a Option 1.8 MBit/s HSDPA PCMCIA card for 39 Euros or a Huawei E220 USB modem for 149 Euros. The offer seems to be a rampant success, their online shop is currently sold out. In ‘Hofer’ supermarkets, and there seems to be one around every corner in Austria, the SIM cards are still available but they also have run short on the hardware.

Since I am in Austria every now and then I recently got myself a SIM card from one of the Hofer shops. Plug and play, no registration, nothing. Just buy the card, put it into your UMTS or HSDPA mobile/modem and off you go.

As always there is at least one catch 🙂 The SIM can only be used for Internet access, no voice or SMS allowed. As a consequence the SIM is only good for Internet access with Notebooks and other ‘non voice’ devices (Note: The SIM works fine in a mobile phone used as a modem for the notebook but voice calls are rejected). Makes sense from an operator point of view I guess. Otherwise nobody would buy pocket Internet access offers for 10 euros a month for a couple of megabytes anymore.

The other slight catch is that the ONE network used by Yesss does not have EDGE in areas where they have no 3G coverage. Therefore it’s kind of all or nothing.

For the details check out the Prepaid Wireless Internet Access Wiki.

DSL Gets ‘Base Stations’

While doing some research on how capacity will grow in fixed line and wireless networks in the future I stumbled over the following thing right in my neighborhood: Future bandwidth increases on the last
mile to the subscriber come with an additional cost in comparison with today’s
standard ADSL or ADSL2+ deployments because of extra hardware that has to be installed close the the subscriber.

Vdslpic
For ADSL2+ the DSLAM is usually installed
in the telephone exchange and the cable length to the subscriber can be up to 8
km for a 1 MBit/s service. For VDSL, which offers data rates of 50 MBit/s
in downlink, the cable length must not exceed 500m. Thus, DSLAMs can no longer
be only installed in central telephone exchanges but equipment has to be
installed in cabinets on the street. The cabinets themselves are quite large (about half the size of a GSM or UMTS base station),
require power, active cooling, and create noise. For the installation
of the cabinets earthworks are necessary to lay the additional fiber
and power
cables required to backhaul the data traffic. The picture on the left shows a VDSL DSLAM cabinet that
has been installed alongside a ‘legacy’ small telecom cabinet as part of the current VDSL build out in my region.

To connect a
new subscriber a technician is required to manually rewire the customer’s line
to one of the ports. Different sources currently
specify the maximum capacity of such cabinets from about 50 to 120 VDSL
ports. To support 500 VDSL connections per km², several cabinets are thus
required. I wonder what happens when 5 different companies put such DSL ‘base stations’ in place!?

What are Secondary PDP Contexts Good For?

I often wondered in the past what ‘Secondary PDP Contexts’ are good for in UMTS networks. I had a vague idea but never had the time to look up the details. These days I had and here’s a short explanation:

‘Secondary PDP contexts’ can be used to separate the real time data traffic from background or signaling traffic into different streams on the air interface while keeping a single IP address on the mobile device. This is done by an application providing the network with a list of IP addresses in a Traffic Flow Template. The mobile device and gateway router (GGSN) in the network will then screen all incoming packets and handle packets with the specified IP addresses differently, like not repeating them on the RLC layer after an air interface transmission error. This is transparent to the IP stack and the applications on both ends of the connection.

External providers of speech services such as Skype, however, do not have access to this functionality. A big advantage for operator controlled IMS services when things get wild on the air interface!?

Resources:

  • Secondary PDP Context Activation: 3GPP TS 23.060, Chapter 9.2.2.1.1 (Rel 6)
  • Traffic Flow Template Description: 3GPP TS 24.008, Chapter 10.5.6.12 (Rel 6)