November 19, 2013 at 09:32What happens to the phone when driving - mobility procedures in cellular networks
Quite often they ask questions: “ What happens when the phone switches to another base station? "," And how is the transition? "," And what does the phone do when it does nothing? " etc. Answers to these questions exist, but they are somewhat beyond the scope of ordinary popular science reviews. Some of the questions were described in detail a little earlier, in the topic of my colleague about the radiation of telephones.
To illuminate this topic from the point of view of the operator’s network elements, you need to dive deep enough into the technology and signaling of mobile communications. First you need to agree on the terminology that is used to describe the transition processes between base stations and cellular standards.
The main nodes in the mobile networks GSM (2G) / UMTS (3G)
The procedures associated with mobility for data services are much more complicated than voice, but they are approximately similar in essence, so we will only consider voice.
The procedure for switching a subscriber between base stations in the standby state (Idle) - Reselection. This procedure takes place in a fully automatic mode in any mobile communication standard. By the way, therefore, those who know the theory even a little do not respond to calls by intruders with stories of the form: “We switch your phone to another base station, and to do this, dial the following combination on the phone ....”.
Situation: our subscriber moves from 2G BTS 1 to 2G BTS 2. The BSC controller sends a list of neighboring frequencies that need to be measured to the phone. The mobile phone measures the signal level of the base stations from this list approximately twice per second and periodically sends reports with the measurement results to the serving base station. Further these reports are collected on the BSC controller. If at some point in time the power or quality of the signal from the serving base station becomes worse than that fixed in the BSC settings, and the signal level from the neighboring base station satisfies the reselection criteria, a change in the serving frequency is initiated. The cell selection criterion is described by the formula:
Where RLA_C is the measured average radio signal power, dBm;
RXLEV_ACCESS_MIN - the minimum value of the received signal power for access to the cell, dBm;
MS_TXPWR_MAX_CCH - the maximum allowed output power of the UE when accessing the network, dBm;
P - maximum output power of UE (depends on the class of UE), dBm;
Traditionally, such procedures are described as a signaling message flow between nodes of the Message Flow network.
After changing the frequency and the serving base station, the Location Area and (if necessary) Routing Area are changed. At this point, it is worth paying attention, since this is what determines the change in the charging of the UE. Billing systems (servers that provide online billing) use the Location Area Code most often as a trigger for choosing a particular tariff plan, although, if necessary, the zone can be detailed to the Cell Global Identity.
To roughly imagine what Location Area is, you can give such a simple example of LAC distribution:
Distribution of Location Area Code
All that we described is the simplest case of a transition between two GSM base stations in the absence of services mode.
We describe the second option: the transition between GSM and UMTS base stations, while the UE must provide voice service first in GSM, and then (without affecting the perception of the subscriber) in UMTS. This is an intersystem voice handover procedure.
Initial state - the subscriber with the phone made a call while in the service area of 2G BTS 2, and moves towards 3G NodeB until the radio conditions change to such a level that you need to switch the conversation to 3G NodeB to save the conversation.
The approach to assessing the need to change the serving base station remains the same as before: the UE constantly measures the level and quality of the signal from neighboring 2G / 3G base stations and sends the measurements to the BSC. In case of degradation of the power or signal quality of the serving base below a specified criterion, the BSC controller initiates a change in the base station with preliminary allocation of the necessary resources at all nodes of the mobile network involved in this procedure.
Handover from a GSM (2G) network to a UMTS (3G) network
Of course, this is a very conditional Message Flow, in which many messages are missed and the option with the minimum possible number of network elements involved is selected. However, even here you can understand how complicated this procedure is and how many settings must be correctly performed in the operator’s network for the successful completion of such an operation familiar to the subscriber. If someone wants to plunge into the world of signaling, I can recommend the technical specification 3GPP TS 23.009 “Handover procedures”, which considers many complex options for moving a subscriber.
- How do base stations transmit a subscriber to each other?
- How does charging work when transferring between base stations, for example, between a home Femto AP and a standard base station?
The UE is charged according to its geographical location (Location Area Code) or, if necessary, more accurate positioning - according to the cell identifier - Cell Global Identity. Change of Location Area is carried out using the special procedure “Location Area Update”, and this procedure is performed in Idle mode. Which means: if you started a conversation at one base station (Femto at your home) with a specific LAC and moved to the coverage area of another base with a different LAC (left the apartment), then the tariff will be charged according to the tariff of the base station at which the conversation started;
- How is the tariffication going if we, without interrupting the conversation, move a considerable distance, for example, to another city?
As described above, tariffing until the call is completed will be carried out at the tariff corresponding to the base station at which the call was started. The only restriction prescribed in the standard is: Handover between base stations located in different countries is prohibited, therefore, when crossing the border, the call will cut off in any case, and the phone will register in the new network with new tariffs;
- How does the subscriber switch to the “big cell” in the event of the sudden disappearance of the “small cell” (power off at the micro base station on the pole, or Femto base at your home)?
You cannot save such a call and quickly switch it to another base, since not a single signal message described above was sent to the system, and the network “does not know” that this phone needs to allocate radio resources somewhere else. So in this case the conversation will be interrupted, the phone will perform the Reselect procedure in Idle mode, and there it will be possible to make a call again;
- What will happen if the neighboring base is not registered or is registered incorrectly?
This is the case when a connection breakdown so unloved by everyone happens (in the operator’s terminology - Call Drop). The phone will keep the conversation on the old base station as long as possible, until the radio parameters are sufficient to save the conversation, after which the conversation will be interrupted and the phone will make Reselect;
- What will happen if we leave the coverage area of the home network while calling the rescue service at number 112?
You can call 112 even if there is no coverage from your operator, but there is coverage from any other operator. However, to make a Handover if the coverage from your operator to the base station of another operator disappears, it will almost certainly fail. Indeed, for Handover to be possible, we must explicitly register all the base stations allowed for this procedure, and only our own base stations are considered candidates for voice switching in the operator’s network. Therefore, the conversation even to number 112 will be interrupted, but then you can dial the number again, even if there is no coverage from the "home" operator.
To illuminate this topic from the point of view of the operator’s network elements, you need to dive deep enough into the technology and signaling of mobile communications. First you need to agree on the terminology that is used to describe the transition processes between base stations and cellular standards.
The main nodes in the mobile networks GSM (2G) / UMTS (3G)
Knots
- Base stations: BTS - GSM standard, NodeB - UMTS standard;
- Controllers: BSC - GSM standard; RNC - UMTS Radio Networks;
- MSC - 2G / 3G voice call switch;
- SGSN - 2G / 3G packet service node;
- UE - terminal, for example, a mobile phone. From now on I will call the UE “telephone” for simplicity;
- Um, Abis, Gb, A - 2G logical interfaces described in 3GPP TS 23.002 “Network architecture” standard;
- Uu, Iub, IuCS, IuPS - 3G logical interfaces described in 3GPP TS 23.002 “Network architecture” standard.
States
- Idle Mode - the state of the user equipment when it is not in use (voice or other user traffic is not processed);
- Busy State - the state of the phone or other UE at the time of work with the voice service;
- Reselect - the process of changing the frequency, code or communication standard of the phone in the Idle Mode state. Switching the phone between base stations when it is in your pocket is a special case of the Reselect process;
- CS Handover - the process of changing the frequency, code or standard of user equipment in the Busy State state. Switching your phone between base stations when you are talking is a special case of the CS Handover process.
The procedures associated with mobility for data services are much more complicated than voice, but they are approximately similar in essence, so we will only consider voice.
Switch between base stations
The procedure for switching a subscriber between base stations in the standby state (Idle) - Reselection. This procedure takes place in a fully automatic mode in any mobile communication standard. By the way, therefore, those who know the theory even a little do not respond to calls by intruders with stories of the form: “We switch your phone to another base station, and to do this, dial the following combination on the phone ....”.
Situation: our subscriber moves from 2G BTS 1 to 2G BTS 2. The BSC controller sends a list of neighboring frequencies that need to be measured to the phone. The mobile phone measures the signal level of the base stations from this list approximately twice per second and periodically sends reports with the measurement results to the serving base station. Further these reports are collected on the BSC controller. If at some point in time the power or quality of the signal from the serving base station becomes worse than that fixed in the BSC settings, and the signal level from the neighboring base station satisfies the reselection criteria, a change in the serving frequency is initiated. The cell selection criterion is described by the formula:
C1 = RLA_C – RXLEV_ACCES_MIN – max (B,0) >0, где B = MS_TXPWR_MAX_CCH - PWhere RLA_C is the measured average radio signal power, dBm;
RXLEV_ACCESS_MIN - the minimum value of the received signal power for access to the cell, dBm;
MS_TXPWR_MAX_CCH - the maximum allowed output power of the UE when accessing the network, dBm;
P - maximum output power of UE (depends on the class of UE), dBm;
Traditionally, such procedures are described as a signaling message flow between nodes of the Message Flow network.
Reselection procedure in a GSM network
After changing the frequency and the serving base station, the Location Area and (if necessary) Routing Area are changed. At this point, it is worth paying attention, since this is what determines the change in the charging of the UE. Billing systems (servers that provide online billing) use the Location Area Code most often as a trigger for choosing a particular tariff plan, although, if necessary, the zone can be detailed to the Cell Global Identity.
To roughly imagine what Location Area is, you can give such a simple example of LAC distribution:
Distribution of Location Area Code
All that we described is the simplest case of a transition between two GSM base stations in the absence of services mode.
We describe the second option: the transition between GSM and UMTS base stations, while the UE must provide voice service first in GSM, and then (without affecting the perception of the subscriber) in UMTS. This is an intersystem voice handover procedure.
Initial state - the subscriber with the phone made a call while in the service area of 2G BTS 2, and moves towards 3G NodeB until the radio conditions change to such a level that you need to switch the conversation to 3G NodeB to save the conversation.
The approach to assessing the need to change the serving base station remains the same as before: the UE constantly measures the level and quality of the signal from neighboring 2G / 3G base stations and sends the measurements to the BSC. In case of degradation of the power or signal quality of the serving base below a specified criterion, the BSC controller initiates a change in the base station with preliminary allocation of the necessary resources at all nodes of the mobile network involved in this procedure.
Handover from a GSM (2G) network to a UMTS (3G) network
Of course, this is a very conditional Message Flow, in which many messages are missed and the option with the minimum possible number of network elements involved is selected. However, even here you can understand how complicated this procedure is and how many settings must be correctly performed in the operator’s network for the successful completion of such an operation familiar to the subscriber. If someone wants to plunge into the world of signaling, I can recommend the technical specification 3GPP TS 23.009 “Handover procedures”, which considers many complex options for moving a subscriber.
Returning to the questions that were asked and because of which, in fact, this whole theory was raised
- How do base stations transmit a subscriber to each other?
- In Idle mode (that is, without providing services) - based on measurements of the power and quality of the radio signal by simply reassigning the radio channel to the new base station;
- in Busy mode (with the provision of voice service) - based on the same measurements, but with a preliminary reservation of all necessary resources in the mobile network;
- How does charging work when transferring between base stations, for example, between a home Femto AP and a standard base station?
The UE is charged according to its geographical location (Location Area Code) or, if necessary, more accurate positioning - according to the cell identifier - Cell Global Identity. Change of Location Area is carried out using the special procedure “Location Area Update”, and this procedure is performed in Idle mode. Which means: if you started a conversation at one base station (Femto at your home) with a specific LAC and moved to the coverage area of another base with a different LAC (left the apartment), then the tariff will be charged according to the tariff of the base station at which the conversation started;
- How is the tariffication going if we, without interrupting the conversation, move a considerable distance, for example, to another city?
As described above, tariffing until the call is completed will be carried out at the tariff corresponding to the base station at which the call was started. The only restriction prescribed in the standard is: Handover between base stations located in different countries is prohibited, therefore, when crossing the border, the call will cut off in any case, and the phone will register in the new network with new tariffs;
- How does the subscriber switch to the “big cell” in the event of the sudden disappearance of the “small cell” (power off at the micro base station on the pole, or Femto base at your home)?
You cannot save such a call and quickly switch it to another base, since not a single signal message described above was sent to the system, and the network “does not know” that this phone needs to allocate radio resources somewhere else. So in this case the conversation will be interrupted, the phone will perform the Reselect procedure in Idle mode, and there it will be possible to make a call again;
- What will happen if the neighboring base is not registered or is registered incorrectly?
This is the case when a connection breakdown so unloved by everyone happens (in the operator’s terminology - Call Drop). The phone will keep the conversation on the old base station as long as possible, until the radio parameters are sufficient to save the conversation, after which the conversation will be interrupted and the phone will make Reselect;
- What will happen if we leave the coverage area of the home network while calling the rescue service at number 112?
You can call 112 even if there is no coverage from your operator, but there is coverage from any other operator. However, to make a Handover if the coverage from your operator to the base station of another operator disappears, it will almost certainly fail. Indeed, for Handover to be possible, we must explicitly register all the base stations allowed for this procedure, and only our own base stations are considered candidates for voice switching in the operator’s network. Therefore, the conversation even to number 112 will be interrupted, but then you can dial the number again, even if there is no coverage from the "home" operator.