mobile gsm modem

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dan - 27 May 2006 20:41 GMT

Hi,

According to what I'm reading, gsm supports a max of 9.6Kbps rates? Is there
some way to achieve bigger rates? The modem docs state the modem supports
higher speed, such as 28.8K. Is this true, or in practice most networks
don't support rates higher than 9.6? An option is hscsd, but is this
supported, or noone is considering this in favour of 3g?

Thanx!

Reply to this Message

Me - 28 May 2006 06:42 GMT

> Hi,
>
[quoted text clipped - 5 lines]
>
> Thanx!

What is your application? Do you need circuit switched data or would GPRS be
even better?

GSM can do 14.4 CS data on a single slot. Maximum 64 kbit/s but in practise
often 3*14.4 kbit/s (43.2) or at most 4*14.4 kbit/s (57.6). This is for the
downlink, uplink normally 14.4 or 28.8 (you can have 28.8 kbit/s symmetric
data rate).

The maximum CS data rate in UMTS is also 64 kbit/s but this is normally
supported symmetrically (mainly to allow video calls).

Not all networks support HSCSD, you can of course always pick up a phone
that does if you need this service.

If you can us packet connectivity, GPRS can reach max 4*59.2 (today, 5*59.2
coming) where EGPRS is supported. This again for the downlink, uplink
normally 2*59.2. 3*59.2 is getting more popular, some specific data devices
claim 4*59.2 uplink speeds already today (but more than 2 uplink slots needs
specific support from the networks and this is just starting to appear).

UMTS can normally do packet data at 384 kbit/s for the downlink, not a big
difference to EGPRS (236.8 or 296.0) but the UMTS downlink data rate is less
dependent on the mobile distance to the base station.

Be careful with UMTS phones, some only support 64 kbit/s on the uplink, even
for PS data. More modern ones support 128 kbit/s (compared to but the UMTS
uplink is equally dependent on the mobile distance from the base station.
This is comparable to the EGPRS 2 slot UL (118.4).

HSDPA is improving the UMTS downlink but is not available widely and not yet
common on mobile devices.

Reply to this Message

dan - 28 May 2006 18:32 GMT

Thank you for your detailed response. Some questions below...

>> According to what I'm reading, gsm supports a max of 9.6Kbps rates? Is
>> there some way to achieve bigger rates? The modem docs state the modem
[quoted text clipped - 4 lines]
> What is your application? Do you need circuit switched data or would GPRS
> be even better?

No, I need CSD only. I just need to connect to a remote modem. The problem
is that the highest speed I get is 9.6K, and I'd really like something a bit
faster if at all possible.

> GSM can do 14.4 CS data on a single slot. Maximum 64 kbit/s but in
> practise often 3*14.4 kbit/s (43.2) or at most 4*14.4 kbit/s (57.6). This
> is for the downlink, uplink normally 14.4 or 28.8 (you can have 28.8
> kbit/s symmetric data rate).

No problem with the uplink, I'm more interested in the downlink. The 14.4
value you mention is what you generally get, and the 64K is the theoretical
limit?

> The maximum CS data rate in UMTS is also 64 kbit/s but this is normally
> supported symmetrically (mainly to allow video calls).

I'm using a 3g device. The problem is that for making this call, I can't do
it in 3g, I get an error. Could this be changed? I really don't know the
reason of the error.. What I wonder is what it is used when doing a voice
call, which is also data but looks much faster. Maybe by means of some at
command?

> Not all networks support HSCSD, you can of course always pick up a phone
> that does if you need this service.

Yes, I found out that these phones don't support it either, so this is
almost discarded.

Thanks again for any pointer.

Reply to this Message

Me - 28 May 2006 19:30 GMT

> Thank you for your detailed response. Some questions below...
>
[quoted text clipped - 19 lines]
> value you mention is what you generally get, and the 64K is the
> theoretical limit?

64k is the theoretical limit, the core network is not designed to support
higher CS data rates (applies for GSM and UMTS).
I'm used to Nokia phones, those are limited to 3 downlink CS slots even if
the model supported more slots with GPRS. When downlink speed is of
interest, I get max 3*14.4 or 43.2 kbit/s.

>> The maximum CS data rate in UMTS is also 64 kbit/s but this is normally
>> supported symmetrically (mainly to allow video calls).
[quoted text clipped - 4 lines]
> voice call, which is also data but looks much faster. Maybe by means of
> some at command?

I have no personal experience about 3G CS data. Perhaps your operator has no
interworking unit implemented for 3G, you may be limited to CS video or
perhaps ISDN data call would be supported. But I understood that you need a
connection to a "real" analog modem (the other party has no ISDN
capability), then CS data is the only option (with the help from the analog
modem at the IWU). Perhaps you should ask your service provider. Could be
that your operator assumes nobody needs CS data anymore.

Perhaps you would get a better indication of the error if you open a
terminal window on your PC and see which response you get from the 3G
network when trying to dial to the other modem.

Voice is coded at max 12 kbit/s for UMTS (NB AMR) but the error protection
for voice is not as good as for CS data (GSM and UMTS). Anyway there is no
way to make use of that data stream for a modem connection. You need a modem
at the network side, the cellular data connection is terminated at the IWU
and carried further via an ordinary analog modem which again connects
through an ordinary phone line to the other modem that you wanted to connect
to. The data transfer over air interface is not that different for speech
and modem data, only that error protection is more optimised for the voice
coded in the case of a voice call.

>> Not all networks support HSCSD, you can of course always pick up a phone
>> that does if you need this service.
>
> Yes, I found out that these phones don't support it either, so this is
> almost discarded.

Which phone do you have? Many Nokia devices seem to support HSCSD even if
www.nokia.com does not mention it.

In any case, 43.2 kbit/s downlink should be straightforward if your operator
(and your phone) supports HSCSD, some networks support 14.4 on a single slot
even if they do not support HSCSD (actually even single slot 14.4 is HSCSD
according to the spec). Which operator are you using? By the way, HSCSD
needs a separate service subscription from CSD. In case your operator does
support HSCSD, you had better check they have enabled that service for your
account.

> Thanks again for any pointer.

Reply to this Message

dan - 29 May 2006 01:06 GMT

Me,

Once again, thanks for your input. Very much appreciated.

> 64k is the theoretical limit, the core network is not designed to support
> higher CS data rates (applies for GSM and UMTS).
> I'm used to Nokia phones, those are limited to 3 downlink CS slots even if
> the model supported more slots with GPRS. When downlink speed is of
> interest, I get max 3*14.4 or 43.2 kbit/s.

Ok, but what I'm confused is that AFAIK gsm csd supports only 1 time slot,
so we could get 14.4K at most right? Packet switched is not an option in
this scenario.

>>> The maximum CS data rate in UMTS is also 64 kbit/s but this is normally
>>> supported symmetrically (mainly to allow video calls).
[quoted text clipped - 11 lines]
> analog modem at the IWU). Perhaps you should ask your service provider.
> Could be that your operator assumes nobody needs CS data anymore.

And how cs video differ from an ordinary csd call? Is there some specific at
command send to it? Coudl I make the csd look like a cs video?
Yes, the problem I have is that I need to connect to a remote end, which
basically is a ppp interface allowing me to route tcp.
The connection works fine, but it's not as fast as I'd like.

> Perhaps you would get a better indication of the error if you open a
> terminal window on your PC and see which response you get from the 3G
> network when trying to dial to the other modem.

Yes, this is what I'm doing in my tests. When connecting, the terminal shows
me a CONNECT 9600 , and I'd really like something faster.
Maybe I'm focusing this from the wrong side. In current situation, would
there be something better to try in this scenario? The only constrain is
that it must be a cs link. From what I see, with csd I won't get more than
9.6K in this case, so what alternatives would be the more easy to try? (by
easy I mean, which are supported by current networks without pain)

> Voice is coded at max 12 kbit/s for UMTS (NB AMR) but the error protection
> for voice is not as good as for CS data (GSM and UMTS). Anyway there is no
[quoted text clipped - 5 lines]
> for speech and modem data, only that error protection is more optimised
> for the voice coded in the case of a voice call.

Ok, but I don't think I could ignore this, as it's also important to make
sure the data arrives ok :)

>>> Not all networks support HSCSD, you can of course always pick up a phone
>>> that does if you need this service.
[quoted text clipped - 4 lines]
> Which phone do you have? Many Nokia devices seem to support HSCSD even if
> www.nokia.com does not mention it.

The one I'm doing my tests with is a n.70 but I'm also trying with a 6670.
But when using the pc, how to force making a hscsd connection instead of an
ordinary csd?

> In any case, 43.2 kbit/s downlink should be straightforward if your
> operator (and your phone) supports HSCSD, some networks support 14.4 on a
[quoted text clipped - 3 lines]
> operator does support HSCSD, you had better check they have enabled that
> service for your account.

Ah, I see, I'll talk to them. I have vodafone, and at least didn't need to
enable csd. Maybe with hs this is not the case.

Thanks.

Reply to this Message

Me - 29 May 2006 06:54 GMT

***

> The one I'm doing my tests with is a n.70 but I'm also trying with a 6670.
> But when using the pc, how to force making a hscsd connection instead of
> an ordinary csd?

Try AT-command: AT+CHSD
This one should show your device parameters. If you get
+CHSD: 6,3,2,4,12
it should equal to <mclass>, <maxRx>, <maxTx>, <sum>, <codings> where
multislot class 6 indicates HSCSD support for 3 downlink slots (and 2 uplink
slots, with sum 4).

***

> Ah, I see, I'll talk to them. I have vodafone, and at least didn't need to
> enable csd. Maybe with hs this is not the case.

I'm not aware of a Vodafone network that supports HSCSD. Orange in UK does
but even they do not support more than 2 slots:
http://www2.orange.co.uk/servlet/Satellite?pagename=OUKPersonal&c=OUKService&cid
=1096023564525

> Thanks.

Reply to this Message

dan - 29 May 2006 10:40 GMT

>> But when using the pc, how to force making a hscsd connection instead of
>> an ordinary csd?
[quoted text clipped - 4 lines]
> multislot class 6 indicates HSCSD support for 3 downlink slots (and 2
> uplink slots, with sum 4).

Yes, I'm getting exactly that answer from the modem. So then I have to
assume the network doesn't support hs, right?
So given that I'm stuck with csd, could there be any way to improve the
current 9.6K I'm getting? (disabling some checksums, etC?)

Thanks.

Reply to this Message

Me - 31 May 2006 14:26 GMT

>>> But when using the pc, how to force making a hscsd connection instead of
>>> an ordinary csd?
[quoted text clipped - 11 lines]
>
> Thanks.

Unfortunately I do not see any options for the user to increase the user
data rate for a GSM 9.6 kbit/s CS data service, none of the checksums etc.
are user controllable and the network would anyway use the modem at the IWU
at 9.6 kbit/s mode.

The only option would be to use HSCSD, but it seems that your network does
not support it.

From an earlier post I understood your application ended up using TCP, can
you explain then why GPRS is not an option for you?

Reply to this Message

matt weber - 29 May 2006 00:34 GMT

>> Hi,
>>
[quoted text clipped - 5 lines]
>>
>> Thanx!

The standard GSM modulation scheme provides 14.4kb/sec per time slot.
GPRS allows you to use more than one time slot, in theory you could
have 7 out of 8, or about 100kb/sec. Depending upon how the operator
manages it, the 14.4kb can be 9.6kb. The actual payload is 9.6kb, the
rest is overhead including error correction/detection. Since the data
is checksummed, the error management is redundant, so it may be
disabled, giving you the 14.4kb...

If you change modulation scheme from the standard GMSK to EDGE, you
can get much higher data rates, so while calling GPRS broadband is a
bit of stretch, EDGE is broadband. You can get several hundred
kilobits per second.

>What is your application? Do you need circuit switched data or would GPRS be
>even better?
[quoted text clipped - 27 lines]
>HSDPA is improving the UMTS downlink but is not available widely and not yet
>common on mobile devices.

Reply to this Message

Me - 29 May 2006 06:02 GMT

>>> Hi,
>>>
[quoted text clipped - 13 lines]
> is checksummed, the error management is redundant, so it may be
> disabled, giving you the 14.4kb...

Matt, I think you are normally very accurate but it is not only GPRS that is
capable of using multiple slots (one could read your comment as if it was),
multislot is possible for CS data too (depending on the phone and the
network capabilities). If your refer to GMSK with "standard modulation
scheme", it does allow several options. 9.6 for CS data is actually 12k at
the air interface and provides true 9.6k user payload (both in transparent
and NT mode). See e.g. 3GPP TS 45.001 and 3GPP TS 44.021. 14.4 for CS data
is actually 14.5 at the air interface but still provides a true net bitrate
of 14.4 kbit/s (these all for a single slot).

GPRS (PS connection, to clarify for the OP) is using "9.6 and 14.4 -like"
coding, but actually a bit different, CS-1 and CS-2 where the net data rates
are a bit different from those of similar CS data.

The standard does support 8-slot capable mobiles (type 2) but such mobiles
do not exist. And networks normally are limited to CS-1 and CS-2 even if the
standard (and the mobiles) support CS-3 and CS-4 too.

But since the OP was restricted to CS data (I didn't understand why though
as he ended up using TCP), this is less relevant.

> If you change modulation scheme from the standard GMSK to EDGE, you
> can get much higher data rates, so while calling GPRS broadband is a
> bit of stretch, EDGE is broadband. You can get several hundred
> kilobits per second.

Reply to this Message

dan - 29 May 2006 10:44 GMT

Thanks Matt, Me.

One other question from a comment you made, saying that some networks
restrict only video calls in 3g. How are these calls separated from ordinary
calls? How are they done? (some special at command?) If it were possible,
could I use that link to send ordinary data instead of video?

>>>> According to what I'm reading, gsm supports a max of 9.6Kbps rates? Is
>>>> there some way to achieve bigger rates? The modem docs state the modem
[quoted text clipped - 38 lines]
>> bit of stretch, EDGE is broadband. You can get several hundred
>> kilobits per second.

Reply to this Message

Me - 31 May 2006 14:39 GMT

> Thanks Matt, Me.
>
> One other question from a comment you made, saying that some networks
> restrict only video calls in 3g. How are these calls separated from
> ordinary calls? How are they done? (some special at command?) If it were
> possible, could I use that link to send ordinary data instead of video?

3G video works on top of an ordinary transparent CS data connection. One
would need to hack the phone to accept data to/from an external interface
instead of the internal video codec application. This is probably
impossible. But "the other end" is even more difficult because you would
need to use a 64kbit/s ISDN connection at the other end while I understood
that you want to connect to an ordinary analog PSTN modem. You cannot "fool"
the network to pass the video data, or part of it, to a modem at the network
(which probably does not exist in the network in the first place, in case of
your operator) that talks to your modem at the other end.

I'm not sure if some phones would support a video call established through
AT commands and then allowing and external video codec application (also
some specific initial signalling would need to be supported). The other end
would still be a problem unless you can use a 3G phone there too.

>>>>> According to what I'm reading, gsm supports a max of 9.6Kbps rates? Is
>>>>> there some way to achieve bigger rates? The modem docs state the modem
[quoted text clipped - 39 lines]
>>> bit of stretch, EDGE is broadband. You can get several hundred
>>> kilobits per second.

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