CDMA Networks vs GSM/GPRS Networks

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freitasm

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#6084 27-May-2004 17:43

Good stuff Alasta.

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ajones

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#6088 27-May-2004 18:55

alasta: I agree, it was an inappropriate request on my part and I apologise.

I just wonder what position he has there as he posts little hints and snippets as if he's quite high up (see the meeting post) yet really seems to have only a basic grasp of his subject matter, instead blindly defending Vodafone or sulking (again, see this thread) if he's offered a point of view he doesn't agree with. I can post about 10+ examples, but I think you only have to read this thread to see what I'm talking about.

This is a really interesting thread, thanks to those people that have posted and have taken the time to back up their opinions and thoughts with facts. I've certainly learnt something.
Does anyone have any good links about the 3G protocols?

Andy.

timbo

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Master Geek

#6178 30-May-2004 18:16

This has been an interesting thread - I think I can make some valuable contributions as an ex GPRS network engineer, and will try not to be (too) biased.

In general it has always struck me that GSM stands for 'Global System for Mobile...' and CDMA defines itself simply on the bit between the handset and the cell site. Sure CDMA is better at that bit, but the rest of the 'system' is less mature, i.e. SIM cards, roaming, frequency standardisation. 3GSM will adopt Wideband CDMA on the air-interface because it is better, but it will also carry through all the other benefits of GSM.

Can anyone answer the mystery question of why Telecom CDMA 1X can't even be used in Australia?

Anyway, some other points and corrections to errors I saw.

1. Someone did mention the GPRS network Optimiser (from Bytemobile) but what wan't covered was its effect on cost. If you go to www.toast.net/performance and take the jpeg download test GPRS + Optimiser can return anywhere from 180-300kbps. Obivously this is because the image is heavily compressed (and it suits the Optimiser well) but if the raw speed is say 35kbps (which is about right - I have seen some higher figures in here for GPRS which is flattering but untrue) then that's a compression factor of 5-9 times. Guess what - you are payng 1/5th to 1/9th to get that picture on GSM - as GSM and CDMA data volume prices are roughly equivalent for the same volume of network capacity.

Sure this is a bit of an artificial example but the same principle applies to all web browsing, IMAP/POP and SMTP e-mail (including attachments - which are lossless compression so the factors aren't so high), FTP. VPN traffic is excluded but keep an eye out for Vodafone's equivalent in the Enterprise LAN space. As I say the cost savings are significant especially if you are paying $30.00 per MB while GPRS roaming.

2. GPRS is definately available everywhere GSM is. Some data devices (e.g. Mobile Connect Card) have much worse RF sensitivity than voice devices but the network capability exists and is enabled everywhere. I had heard rumours that CDMA 1X was not enabled on the entire 027 network - can anyone confirm this?

3. Latency of GRPS was quoted by someone as 1.3s. It is usually in the range of 550-900ms. CDMA generally achieves 300-550, so significantly better. Using default ICMP packets in all cases. But what applications specifically notice this difference? The only major one I've seen is Citrix which requires more tuning on GPRS but can be made to work quite well.

4. GPRS definately supports TCP. The issue is that cell-site handovers (when you are moving) cause a latency spike to which TCP reacts very badly by 'windowing down' the packet transmission size, resulting in immediate and large reduction on throughput speed, and slow recovery. The GPRS network optimiser as mentioned above implements a new TCP layer which is GPRS aware and is not sensitive to GPRS latencies.

5. GPRS can use private IP addressing (NAPT) or public IP addressing. It depends what 'APN' you define on dial-up. Private is the default and is fine for most users and of course it also provides a level of security from the Internet for average users. Public IP addressing is required by certain VPN clients (older ones and MS PPTP) and certain multi-streaming protocols like active FTP.

6. GSM air-interface security has not been hacked to my knowledge outside of a lab environment - but it is probably true that CDMA is more secure - if it matters. All legal interception (for any network) is done at the switch once the voice has been turned into 64kbps PCM channel.

Hope this was useful.

Jama

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#6191 31-May-2004 14:05

Hi Timbo

Good post. To answer a couple of your questions:

1. CDMA 1x works in Aussie, just change the login/password on your DUN session to:
user@telstra.internet
telstra
I have tried and it works great.

2. CDMA 1x is enabled on all 800 odd cellsites that Telecom have. So, that is nationwide

3. CDMA does have frequency standardisation - 450MHz, 800Mhz or 1900MHz bands which were also used for AMPS/TDMA/NMT cellular systems.

Also, when you mention issues with cell site hand over is this because GSM/GPRS uses hard handover? CDMA does not experience this issue because it uses soft handover.

I have tried the GPRS optimiser for web surfing but didn't like the experience. The graphics ended up broken and colours washed out, certainly a page wasn't displayed as the web designer intended.

timbo

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Master Geek

#6192 31-May-2004 14:27

Thanks Jama.

Yes the handover in GPRS is 'hard' i.e. the data flow stops, it is buffered back up in the network, the new data path is established, and the data starts flowing again. Unlike 'soft' handovers for voice (and as you say, data with CDMA) where the new data path is established first and there is no disruption when the actual handover occurs.

CDMA 1X in Aus: is this an official service yet? How does billing work - time or volume? How will it compare to GPRS $30.00 per MB. If this is sensitive info. then I appreciate that you may not be able to divulge it.

With the Optimiser, if you do want to original image then you perform a 'source' refresh with the browser to get the original image but you are correct that the image is affected.

Timbo

freitasm

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#6193 31-May-2004 14:33

1. Someone did mention the GPRS network Optimiser (from Bytemobile) but what wan't covered was its effect on cost. If you go to www.toast.net/performance and take the jpeg download test GPRS + Optimiser can return anywhere from 180-300kbps. Obivously this is because the image is heavily compressed (and it suits the Optimiser well) but if the raw speed is say 35kbps (which is about right - I have seen some higher figures in here for GPRS which is flattering but untrue) then that's a compression factor of 5-9 times. Guess what - you are payng 1/5th to 1/9th to get that picture on GSM - as GSM and CDMA data volume prices are roughly equivalent for the same volume of network capacity.

This is probably not 100% correct. I believe the speed 180-300kbps reported is between the source of the image to the terminal being tested. In this case not the mobile terminal itself, but the Bytemobile proxy server sitting on Vodafone's premisses. From this server to the mobile terminal it's still the same 35kbps-42kbps we're used to. Ok, size is reduced, but speed remains the same. Therefore time shortens, but there's no real increase in speed for that matter. And as Jama pointed images get really washed out. Ok for some browsing, but that's it.

Who notices latency? Anyone not using e-mail. E-mail clients in general need only a single connection to the server, and that's fine. Web browsing sometimes uses multiple connect connections, depending on HTTP header used and browser. The GPRS experience can be very frustating at a times, because of this.

Of course I'm writing as an end-user, not an engineer.

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timbo

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Master Geek

#6195 31-May-2004 14:58

From the Optimiser to the terminal is same 'raw' speed of 35-42kbps but the image that is transferred is shrunk considerably. Download the front page of the Herald using the Optimiser, take the main image and look at its size. Then do source refersh. That exact same image through Optimiser will be anwyhere from 1/5 to 1/15 its original size - and it don't look too bad. Of course if you are needing an image for aesthetic reasons then this is no good - otherwise you are saving some big bucks.

And your point about multiple connections with HTTP is very true so in next version (don't know when it will be installed by Vodafone) of Optimiser the HTTP layer is modified so that from the browser to the Optimiser proxy there is only one HTTP connection and the Optimiser fetches all the content via multiple connections to the internet. I haven't seen it working but apparently it makes a huge difference. This will again save cost.

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