What is the internal structure of a mobile phone call packet/datagram?

Question

Coming from a Networking (IP/TCP/etc) background, I understand that a typical network packet looks something like the following:

I further understand that VoIP packets typically do use the same format and encapsulate all of the calling data to route it to the machine itself before handing it up the OSI model until the call is picked up by the application.

What I'm trying to understand, is how a telephone or mobile phone packet is physically laid out. Simple PSTN lines are an analog signal transmitted over copper wires that lay out a few rules and essentially only provide status updates. What I'm trying to understand is how a mobile phone call packet/datagram is constructed and what information is actually provided to the handset.

In my simplified understanding of how a cell phone tower works, I think of it like a WiFi router with all of the subscribed cell phones attempting to speak with it and following a lot of the same rules. As well, my phone needs to be able to know when it's being directly communicated with instead of picking up packets aimed at another subscriber. How is this addressed? (IMEI?)

When a phone call is made, what packet information is transmitted to my phone? Will my phone know the destination IMEI/whatever address of the calling phone or will it only know it's next hop? Caller ID (to me) seems to something of a higher layer and less dependent on actual routing.

If we're specifically speaking about GSM Networks (Older/Common), I have looked through the ETSI standards from a high level here but I still can't quite wrap my mind around it.

Answer 1

I think one of the issues is the assumption that it looks exactly like a standard IP Packet. Most phones at this time (using standard GSM or CDMA technology) don't use an IP network but older phone technology.

The phone itself will not know anything about the end device. It will send some basic information to the carriers voice switch. That switch will handle all of the routing using a combination of SIP for calls to other phones on its network, or SS7 for calls that go to the PSTN (Public Switching Telephone Network).

Verizon Wireless, when HD calling is enabled, does do VoIP (SIP) from end to end. When these HD calling phones try to go to a non HD calling phone, they have a device that will transcode the call and do signaling for the PSTN to connect the call.

SS7 does not use IPs, it uses point codes for routing (in the format xxx-xxx-xxx). While it doesn't use IPs, it is still data and not analog. It is transferring bits across the signaling path about how the call is going to be setup. Most devices in this path will only know the next hop.

Answer 2

In mobile networks, the voice call data is Channel Switched and there is no packets.In Packet Switched data networks information is transferred in packets.

Also there are different protocols and technologies in the different interfaces in the mobile network. Interfaces like the ones between the user equipment and the basestation, and other protocols an technologies between the basestation and the core network.

If you are asking about the GSM Networks, the radio interface each frequency is divided to time-slots and each of them can contain several types of logical channels for signaling or voice traffic. This is TDMA (Time Division Multiple Access). Each subscriber is using the radio channel within specific time interval is a cycle. Basically all the subscribers in the cell are using the channel in a cyclic order.

The channel carrying speech is called traffic channel (TCH). It carries digital speech data.

The process of forming this data is roughly the following:

1. Speech
2. Analog to digital conversion
3. Grouping the digital audio signals into 20 msec speech frames
4. Using codec to compress the voice data
5. Adding protection bits (checksum) and some service information
6. The formed block is transmitted within one timeslot of the TCH

The whole process is more complex and it is difficult to be described in a short answer.

2G is using TDMA and 3G is using CDMA methond for accessing the radio resource.