Can the Cingular 2125 be pinged?

[Guest Brcobrem]

Hi,

I am having a problem getting the Cingular 2125 to be visible on the internet. I am testing it's capabilities as a "modem".

If I use the USB cable and attach it to Computer A and then go to Computer B and try to use RDP (ie. Terminal Services Client / Remote Desktop) to remote control computer , it fails.

I have the same problem when I tried to test a reverse connection helpdesk software. With that kind of software, you do not need to open ports on the customers firewall. Instead you give the end-user a little exe that allows them to initiate the remote control session. They go out through their firewall (which they can always do) and connect back to my desk.

I can go to something like myipaddress.com, see my current public IP, but I cannot ping the 2125 at that address. I had someone outside of my office try to ping the the 2125 and they couldn't either.

Today I tried a trace route on my 2125 current public IP. It made about 15 hops from my office and around the country, and then stopped and timed out at one of Cingular's routers (easily identified).

I called Cingulars data support desk, explained this, and then they kept me on hold for about 20 minutes, then my call was put into an automated answering machine. I really didnt expect the first level tech to know how to deal with this, but I didnt expect to be dumped on either. Not surprising, its been 6 hours and Ive had no call back.

One of my associates says he has had no similar problems with Verizons wireless network. Any ideas whats up with Cingular and this visibility issue?

Regards,

Brcobrem

Edited April 21, 2006 by Brcobrem

[Guest kam_]

Thats strange your collegue's works on another network - typically over GPRS the ip address is NAT'd to the external ip on the GGSN. Incoming connections are not usually possible to phones using GPRS. This is exactly why Microsoft push email works as it does. The phone must initiate an outgoing TCP connection and a heartbeat checks this is still alive.

You're basically right that you will need a client on the phone that initiates an outbound connection back to you. This connection will also need to periodically check that it hasnt dropped.

It may be possible that your collegues network is giving out public addresses, but i think thats very unlikely. Regulation in europe is forcing companies to use NAT where they can to preserve the IPv4 pool. It may be possible your collegue's network has a huge pool of public addresses (maybe they had a dial up service before?) and so are using those.

Can you check with him that he's definately making an incoming connection to ther phone?

Oh and i assume your talking about GPRS and not CSD? With CSD you usually do get a public address, and thats one of the reasons why its still heavily used even in GPRS areas.

[Guest Brcobrem]

I have a call and an email into my associate about his Verizon setup. I need to make it clear that he does not have a HTC 2125 with Verizon. I believe he is just using Verizon's service and an "Edge" PCMCIA card in his laptop. Let you know more on this when I hear back from him.

I hope you will not mind too much if I ask a couple related newbie questions. I will appreciate your assistance:

1) Do I have CSD capability with Cingular now?

2) Is CSD better, worse. totally diffrent, or the same as GPRS?

3) You mentioned, "...typically over GPRS the ip address is NAT'd to the external ip on the GGSN." What is GGSN?

I look forward to any an all replies. . .

P.S for others who find this post/threads of interest, as a 2125 newbie, I found the descriptions and comparisons of GSM's EDGE, GPRS and 3GSM at http://www.gsmworld.com/technology/index.shtml to be very helpful. See the "GSM Family" drop down for selections.

P.S. I also found this discussion of Cingular's "CSD vs GPRS" very informative:

http://groups.google.com/group/alt.cellula...80af90ee00a43b6

[Guest kam_]

1) Do I have CSD capability with Cingular now?

2) Is CSD better, worse. totally diffrent, or the same as GPRS?

3) You mentioned, "...typically over GPRS the ip address is NAT'd to the external ip on the GGSN." What is GGSN?

Ok let me explain a little bit more about it.

Circuit Switched Data or CSD is where you use your GSM phone to make a data call. If you think of your GSM phone as a landline phone, then CSD is equivalent to making a modem call. However this analogy isnt completely correct.

To explain fully about CSD and GPRS let me give you some background on how GSM works.

GSM phones use radio to transmit information to a tower. The data is encoded digitally and so each radio frequency can send alot of data. To enable multiple phones to use one frequency, the bandwith is split up into time slots.

Esentially, phone 1 transmits, then phone 2 ... then phone 8, then phone 1 again etc.

A broadcast control channel handles syncronisation so that each mobile transmits at the correct time - i.e. during the correct timeslot.

On any one frequency there are 8 timeslots (8 uplink + 8 downlink).

When you make a GSM call, your phone uses the control channel to request a free timeslot. The network sends back a frequency and a timeslot on that frequency that is free.

Your phone and the network then initiate a GSM call, digitise the audio speech and transmit it on that timeslot and frequency. The raw data is transmitted at 28.8 kbps. However alot of this consists of error checking and redundancy in case any one bit of data is lost on the radio layer. The actual voice data only accounts for 9.6 kbps.

So what's CSD exactly?

Well normally your voice data is lossy compressed (which means alot of the frequencies are discarded) - which isnt much of a problem for voice. However if you tried to make a data call by modulating your data to audio - like a modem does - the compression would end up throwing away alot of your data.

Also it would be a bit silly to do this:

Your data ----> modem converts to audio ---> phone converts to data and sends to network.

Instead the data is inserted directly into the time slot and you skip the modem part. Once it gets to the network, the data is converted to audio using a modem and sent to the destination.

Your data ----> data put directly into timeslot ----> gets to the network and converted to audio - modem compatible tones---> sent on normal PSTN phone network to destination

Basically you make a phone call to your destination and turn on the data flag on the radio layer. Actually the phone does this when you make a 'dial up networking' connection from it.

In order to get onto the internet you would need an ISP like AOL (spit!). Also you would be charged per minute for the data call.

In essence the phone does the function of a modem.

With HSCSD (High Speed CSD) the error chekcing in the timeslot is not used and so the data rate goes upto 28.8 kbps, but not many networks and phones support HSCSD yet.

Specific to your problem tho, because CSD is like a normal dialup to an ISP, you would get a public ip address when you connect.

The downside of CSD is that it's like a phone call, you can't make voice calls at the same time.

So what's GPRS?

GPRS is a new system where you can use more than 1 timeslot to send data.

The actual number of timeslots you use is dynamiclly controlled by the network, but the maximum is 8 because that is the actual number of timeslots on any one frequency.

Your phone can't lock onto more than 1 frequency at a time (like a normal FM radio for example) so the max limit is 8.

Your total transfer rate is then 9600 * number of timeslots.

Also your phone may not actually be able to use all 8 timeslots due to hardware limitations.

Again with GPRS its posible to discard the error checking and get upto 28800 kbps PER timeslot. (The error checking is handled on the IP layer instead - the pc asks for a retransmit if the data was corrupted).

In fact by compressing the data you can go even faster - but thats a whole nother story.

Anyway the Class of your mobile phone tells you the maximum number of timeslots and the maximum data rate (with GPRS the downlink timeslots don't have to be the same as uplink timeslots - i.e. your upload rate can be lower than your download rate). See here for multislot classes and meanings:

http://www.gsmworld.com/technology/gprs/class.shtml

Class A,B or C tells you if your phone is capable of making phone calls at the same time as using GPRS.

The qtek 8310/i-mate sp5 is Multi-slot class 10 and GPRS class B which means its configurable between:

One Upload slot, Four Download slots = 8-12Kbps Send - 32-48Kbps Receive

OR

Two Upload slots, Three Download slots = 16-24Kbps Send - 24-36Kbps Receive

Class B = mobile phones can be attached to both GPRS and GSM services, using one service at a time. Class B enables making or receiving a voice call, or sending/receiving an SMS during a GPRS connection. During voice calls or SMS, GPRS services are suspended and then resumed automatically after the call or SMS session has ended.

Now the other difference with GPRS is how the data is handled when it gets to the network. In essencee you don't need a ISP and your phone network is actually your ISP.

Your network asigns you an IP address. You may recall from your GPRS settings you need an APN, username and password. The APN tells the network which GGSN to send your data to (more on this later). The username and password authenticate who you are of course.

You can think of a GGSN like a network router. On one side is you, and on the other are a bunch of network interfaces. Each interface is connected to a network.

Your company may for example link up with your operator and connect their corporate network to one GGSN. The APN identifying that network could be WORK_APN.com, You would be given access to that APN and after filling in your details on your phone, that GGSN would become like your default router.

Your packets would end up going into your work network.

Typically tho the GGSN is connected to the internet and there is one generic APN like Verizon_internet.com. You fill Verizon_internet.com into your phone as the APN and probably no password or a generic one, since they don't care about protecting access - if you don't pay for GPRS they will block the access on the GSM network side instead.

All your packets are routed out through one GGSN connected to the internet.

With GPRS tho, the ip address you're normally given is a private address, and the GGSN NAT's this to its internet address. Just like your linksys router does at home when you want to share internet behind it.

This is fine for outgoing connections, or where you form an outgoing TCP connection and the destination responds on that socket, but its useless for incoming connections. There is no DMZ that you can setup - because you can't access the config of the GGSN and even if you could you have a floating ip, which changes every time u attach to the GPRS network.

Typically incoming connections to GPRS devices is not possible. This is exactly why the blackberry email service needs a special server at your operators premesis, which has an internal private ip address.

This server polls your internet email, and forms a connection to your mobile's private address from inside the network.

Microsoft's push email doesn't need a server at the operators premesis and is 'operator independant'. For them to do this the phone forms an outgoing TCP connection, and a heartbeat packet is sent back and forth to check if the connection is still alive. Of course this costs u for the data sent.

I think that should answer your questions, but more specifically:

1) Do I have CSD capability with Cingular now?

Well they do need to enable data calls on your account, but these days its on by default.

2) Is CSD better, worse. totally diffrent, or the same as GPRS?

CSD is different. Its worse in most cases because data rate is limited to 9600 kbps and you're charged per second for the call. However it does give you a public ip address and thats why its still used alot today.

3) You mentioned, "...typically over GPRS the ip address is NAT'd to the external ip on the GGSN." What is GGSN?

GGSN stands for 'Gateway GPRS Support Node' and you can think of it as your 'default router' to your destination network - typically, but not always, the internet.

Your phone's ip address will be private and so NAT (Network Address Translation) is used to route your packets and make them look like they're coming from the GGSN's public address.

It may be that your collegues network is giving their mobiles public addresses, but i've never seens this before. I installed and VO'd T-Mobiles GPRS network in the uk a few years ago and from my understanding some non trivial firewalling would be needed to get that to work. I for sure know the GPRS standard wasn't designed to do that.

I hope that helps, let me know if you have any questions.

Edited March 25, 2006 by kam_

[Guest Brcobrem]

Hi Enthusiast,

I am so sorry that I have not replied to you until now. Somehow I mised the notice of your reply. Please accept my sincere appology.

Your reply was a wonderful. It was exrtremely informative and provided a clear insight into the difference between CSD and GSM. Execatly what I neeed, plus a lot more !

Oh yes, I will be saving this for future reference.

Thank you very, very much.

Regards,

Brcobrem

Edited April 21, 2006 by Brcobrem