I have to control a modem Cinterion ELS61 with a host application. Can anyone tell me in simple words which is the real difference among at+creg?, at+cgreg? and at+cereg? commands?
I know that cgreg is related to 2G and 3G radio access technology and cereg with 4G/LTE one, but the actual differences are not so clear and neither the relation with the creg command results.
It follows an example from a real case:
AT+CREG?
+CREG: 2,5,"38AD","02531900",7
OK
AT+CGREG?
+CGREG: 2,4
OK
AT+CEREG?
+CEREG: 2,5,"2AF2","02531900",7
OK
What is the difference among the answers to +creg and +cereg? Aren't they quite the same? And by the way, what does it mean that the same cell (02531900) has 2 difference location area codes (i.e. 38AD and 2AF2)?
Thanks for the attention
All the +C*REG commands, as you probably know, query the status of the registration to some network.
The first step is the following categorization:
+CREG, that queries the registration to the circuit switched network.
It is basically the GSM "classic" network, on which voice/data calls and SMSs are based
+C*REG commands, that query the registration to the packet networks.
They are the networks on which the packed data traffic is based. The network allowing the access to internet, to be clear.
+C*REG commands are technology specific commands:
+CGREG queries the registration to GPRS network
+CEREG queries the registration to LTE data packet network, and in some cases is mandatory also to perform tasks that were usually based on GSM: voice calls (if VoLTE, Voice Over LTE, is supported) or to sent SMSs (IMS protocol).
LTE technology, basically, moves almost everithing on packet traffic
The reason why different commands are available (even in the same cellular module) is that different registration processes are required for each sub-network.
Related
I'm doing a project where I have 2 raspberry pi zeros connected and a psoc 5 connected to one of the rpi's.
The connection between them are all i2c.
One of the rpi's are considered the "master" and from my android phone I will use ssh to remote control the master.
[Phone] = SSH = [RPi Master] = I2C = [RPi Slave]
||
[PSOC 5]
My teacher told me to consider tcp/ip, but I will still have the 2 rpi's and the psoc connected by wires.
My question therefore is - are there any benefits in doing tcp in my project?
There is no clear way to answer whether TCP is better than I2C without more info about your project, as that is an application-specific question. This question would be easier to answer with a little more detail on your project, including requirements, data rates, etc., but I will try to give you some info based on personal experience.
The only reason to consider TCP/IP between the devices is if the data rate needed is higher than I2C can support, or if it would be useful to keep them separated beyond I2C range.
Creating a program that utilizes TCP to run on your phone and the master would make the system a little more complete, as you would have a program running on the master constantly to automatically accept information from the user and run the commands, rather than having to control it from the command line. This would also force you to create some sort of user-facing program for the phone. From a design perspective for a class project, this could be preferable.
Without knowing why your teacher suggested that it is hard to give you a straight answer; it is possible the suggestion was given not for any real performance reasons but for you to learn about networking, TCP/IP protocols, or to be able to say you have "IoT experience".
I just spoke with a phone company in Denmark, that offer "Free 3G/4G", on the phone, but they have a limitation if you create a hotspot and share the connection, this limitation is on 50GB.
How do they detect if your mobile device is sharing the connection? Do they intercept some special headers that the computer/other phones send over the connection?
MAC addresses are used at a lower layer than IP and relate to each hop or leg of an end to end communication so they don't really help with this issue.
Detecting tethered devices is quite a complicated task and there are special solutions to do exactly this. They tend to look at multiple things to try to determine if other devices are using the mobiles connection, even if the devices are spoofing or manipulating headers etc.
Examples of things a solution will look at:
number of simultaneous sessions
HTTP user-agent headers
device type
device screen size
TCP Timestamp
TCP Source Port
TCP Sequence Number
Application-based correlation to TCP flows
Node-pair correlation to TCP flows
See here for an example solution and more details of the above (this is just one example solution):
https://www.sandvine.com/downloads/general/sandvine-technology-showcases/policy-control-for-connected-and-tethered-devices.pdf
We put the question to Three, which not too surprisingly declined to
answer. However, if you think about it logically, it should be fairly
simple to detect tethering. Every device with a network connection
(including Wi-Fi) has a unique hardware identifier called a MAC
address. Assuming that the operator can trace the final destination of
the data packets, it should be able to determine that the final MAC
doesn't match your phone's.
source
Although the post is about UK operators, it should apply in Denmark.
I build a script with scapy to capture probe requests in a monitornig wi-fi interface.
I successfully capture the requests, and some of the SSIDs contained in them. But most of the networks stored in the phone don't get broadcasted.
And there isn't a clear pattern of why this happens. Some phones don't broadcast ssids at all.
I'm trying to find an explanation for the reasoning behind this behaviour, but haven't found any, apart that the hidden networks should be broadcasted in order for the phone to connect to them, but even that is not true, and most of the broadcasted ones are visible.
Another behaviour is the iPhones, that only seem to broadcast the network that they are connected to, and nothing else. (no network -> no SSIDs).
I have tried putting the interface in various channels, and results vary on the broadcasted networks, but the great majority of the saved ones in the device still aren't broadcasted.
Is there a reason behind this? Or a way to force the device to broadcast them all?
You seem to assume that the phone would do a probe request for each and every known network, permanently.
This is not the case - and not just for phone, but in general. Quoting the Wi-Fi Alliance[*]:
What are passive and active scanning?
The reason for client scanning is to determine a suitable AP to which
the client may [emphasis mine] need to roam now or in the future. A client can use two
scanning methods: active and passive. During an active scan, the
client radio transmits a probe request and listens for a probe
response from an AP [emphasis mine]. With a passive scan, the
client radio listens on each channel for beacons[emphasis mine again]
sent periodically by an AP. A passive scan generally takes more time,
since the client must listen and wait for a beacon versus actively
probing to find an AP. Another limitation with a passive scan is that
if the client does not wait long enough on a channel, then the client
may miss an AP beacon.
So this is entirely application/OS dependent if
the phone STA do an active scan, sending probe requests,
or just seat there listening for beacons (or doing nothing at all).
In my remembering - it's been a few years I didn't worked/looked at Android code, so it may have change - Android will not do an active scan, and thus will not send probe request to known SSID, unless you're in the Wi-Fi networks setting screen. It will just listen to beacons.
There are some Wi-Fi 802.11 design rationale behind this:
STA are supposed to be mobile. After all, if you're not moving from
time to time, there's not much point in using Wi-Fi (except marketing
or laziness, and of course smartphones changed that), you might as
well get wired.
...if you're mobile, it's reasonable to think you're running on a
battery,
And so you want to save battery life: so you'll rather do passive
scans listening to beacons rather than active scan sending probe
request, because this uses less power.
This idea of power saving alternative capabilities is spread all other the place in 802.11 design, hidden under carpet, when you're a STA.
So it is fully OS stack/application dependent from the STA if it 1/ just listen to beacons /2 actively send probe-request for every know AP 3/ send a broadcast probe-request, and also if it do so in a continuous manner, or periodically, or depending if it's in a know state (ex screen ON, and user going to the Wi-Fi networks setting screen).
Now there may be some other considerations, like some regional regulations that mandate that you first listen to beacons to decide if you can or cannot use some channels. But the main point is above.
*:
http://www.wi-fi.org/knowledge-center/faq/what-are-passive-and-active-scanning
EDIT:
On the programming side:
1/ What you seem to have is an IOP (interoperability) problem, because you expect a specific behavior from STA regarding scanning active vs passive and the involved probe-requests, and this is not how it works in the real world. Depending on your application final main goal, this may be a flawn in the design - or just a minor nuisance. You may want to restrict yourself to some specific device's brand, or try to cover all cases, which has a development cost.
2/ ...OR you were just surprised by your observations, and look for an explanation. In such case of surprising results, it goes without saying: go straight to wireshark to check your program observations (if your program is a packet sniffer) or behavior (if your program is a client/server/layer XYZ protocol implementation).
On the 802.11 strategies regarding active vs passive scan and power saving:
From "802.11 Wireless Networks: The Definitive Guide, 2nd Edition", by Matthew S. Gast ("member of the IEEE 802.11 working group, and serves as chair of 802.11 Task Group M. As chair of the Wi-Fi Alliance's Wireless Network Management marketing task group, he is leading the investigation of certification requirements for power saving, performance optimization, and location and timing services" - from his publisher bio). A book i can highly recommend.
p. 171:
ScanType (active or passive)
Active scanning uses the transmission of Probe Request frames to
identify networks in the area. Passive scanning saves battery power by
listening for Beacon frames.
p. 172:
Passive Scanning
Passive scanning saves battery power because it does not require
transmitting. In passive scanning, a station moves to each channel on
the channel list and waits for Beacon frames.
Also, a bit old (2003), but these guys know their stuff about networking. About scanning strategies:
From Cisco "802.11 Wireless LAN Fundamentals", chapter 5 "mobility".
Page 153:
Roaming Algorithms
The mechanism to determine when to roam is not defined by the IEEE
802.11 specification and is, therefore, left to vendors to implement. [...] The fact that the algorithms are left to vendor implementation
provide vendors an opportunity to differentiate themselves by creating
new and better performing algorithms than their competitors. Roaming
algorithms become a vendor’s “secret sauce,” and as a result are kept
confidential.
Page 154 "Determining Where to Roam":
There is no ideal technique for scanning. Passive scanning has the
benefit of not requiring the client to transmit probe requests but
runs the risk of potentially missing an AP because it might not
receive a beacon during the scanning duration. Active scanning has the
benefit of actively seeking out APs to associate to but requires the
client to actively transmit probes. Depending on the implementation
for the 802.11 client, one might be better suited than the other. For
example, many embedded systems use passive scanning as the preferred
method [emphasis mine] [...]
Other interesting stuff on page 155, "Preemptive AP Discovery".
I am working on Windows. Lets say I am running a Twitter app. I want to intercept all the network packets that this app is sending or receiving at runtime. I wish to look for certain packet (segment) features like TCP destination port, windows size etc. and based on this information I wish to perform certain actions. Basically I want all the information that is available to a traffic analyzer like Wireshark or MS Network Monitor. How can I accomplish this on Windows?
I'm not sure to understand completely what you're aiming at - from what I understand, you could start Wireshark, log the relevant info, and then perform "certain actions" based on the data you could retrieve from this log - even automatically with some scripting.
I would like to create a wireless network from a laptop. If laptops come within range, I would like it to send them a welcome message and send them a goodbye message when they leave the wifi range. Is it possible to do this in C?
Please help me out with this.
It is possible, but it is a very complex task and I don't think that programming language choice is the first thing to look into.
As a start, you can read up on Wikipedia on Wireless ad-hoc networks.
How should your messages be received and displayed on the remote side? If you want to use some existing protocol over TCP/IP, or create your own (deploying custom applications on the remote machines), you will need to mess with networks and this is not always possible as one machine can be a part of only one network. So the machines need to be not connected to anything and somehow allow you to connect to them, it involves changing network settings on all that machines (for example, setting them to join the ad-hoc network with predefined name).
If all machines automatically join the existing network, this question has nothing to do with wireless (physical layer) but with Avahi, Netbios or whatever other services allowing you to get notifications and/or enumerate devices in the network.