February 11, 2010 at 14:54GPRS is not an obstacle to calls
The article is inspired by questions about the ability to make / receive voice services during an open GPRS / EDGE session, as well as indignation on this issue expressed in the article about problems with a certain “oval” operator in Russia :)
In order to dispel some TyMaH moments, we are with you consider the technical prerequisites necessary to realize the ability to receive / make voice calls with an active GPRS / EDGE session.
So, what do we need so that during an active session of packet data transfer, we can receive / make calls or send / receive SMS. And you need, in fact, not so much, but more specifically:
Thus, the first requirement is the presence of an unknown until the G s interface. This is the interface between network elements such as SGSN and MSC / VLR. The interface is optional, i.e. optional in the general architecture of the GPRS / EDGE network and actually allows you to perform the following procedures:
For us, the most important procedures from this list will be paging procedures (CS paging), procedures for updating the location of the subscriber (Combined RA / LA update), as well as the combined attachment of the subscriber in the operator’s network (IMSI attach for a GPRS attached MS, GPRS attach for an IMSI attached MS) that will allow us to connect to the voice services subnet (CS Core Network), as well as “find” the subscriber immediately before the call.
If we draw an analogy with the establishment of a “standard” voice connection and voice connection through SGSN, then you can find many similar points. For example, a comparative diagram of the paging procedure through MSC and SGSN is presented below:
As you can see, the procedures are quite similar, the only difference is that SGSN acts as an intermediate link that allows the “subscriber terminal” (MS) and the switch (MSC / VLR) to “communicate” with each other. The ability to be a transport link provides SGSN with a special protocol stack or, to use the terminology of the General Channel Signaling ( SSN7 / SS7 ), the user subsystem [Application Part] will be more acceptable.
In order to understand some points of using just such a subsystem, let's take a look at the structure of providing access from the radio subnet to the base (Core Network) operator subnet for several generations of architectures.
In the diagram below, several access options from the radio subnet of various mobile technologies to the operator’s base subnet are presented.
I want to draw your attention, but what subsystem is used to connect the switch (MSC) to the base station controller (BSC) is the BSSAP (en) subsystem [Base Station System Application Part] on interface A. A pretty similar subsystem is used between SGSN'om and MSC / VLR'om - BSSAP + on the interface G s , i.e. the use of this subsystem allows SGSN to “pretend” to be a base station controller (BSC) for a switch (MSC / VLR) and, accordingly, makes it possible to carry out / receive voice services for a subscriber through SGSN.
The above subsystem (BSSAP +) is based on the underlying levels of the SS7 stack and, together with the other subsystems in the general scheme of SSN7 / SS7, is presented in the figure to the right.
The NOM parameter is configured on the side of the base station controller (BSC) and basically determines how paging procedures for the subscriber will go through the "standard" GSM channels, or the paging channels of the packet subnet will be activated.
There are several basic Network Operation Modes:
To notify subscriber terminals which of the GPRS modes is currently used on the operator’s network, the GSM Broadcast BCCH [Broadcast Common Control Channel] is used.
You can find out which mode the operator is currently using, using special applications, for example, Netmonitor .
The second important point that allows us to make / receive voice services is the terminal class. Below are the main classes of mobile terminals.
Most of the devices currently in use by users are “representatives” of class B and, accordingly, without a G s interface they will not be able to receive / make voice services. In this situation, the subscriber trying to reach you will receive a reply, “The subscriber is out of the network coverage area ...”, and until you end the GPRS / EDGE session, you will be out of the zone for voice services (including SMS).
So, what did we find out from this article ... For the simultaneous operation of voice services and packet data transfer, several conditions must be met:
If these conditions are not met, it is not possible to call (send SMS / establish CSD communication) to a subscriber who has a GPRS / EDGE session open.
Small helper:
AP - Access Point
BSC - Base Station Controller
BSSAP - Base Station System Application Part
BSSGP - Base Station System GPRS Protocol
BTS - Base Transceiver Station
CAMEL - Customized Applications for Mobile Enhanced Logic
CAP - CAMEL Application Part
CS - Circuit Switched
eUTRAN / E-UTRA - Evolved UTRAN
GAN - Generic Access Network
GANC - GAN Controller
GERAN- GSM EDGE Radio Access Network
LA - Location Area
MAP - Mobile Application Part
MM - Mobility Management
MS - Mobile Station
RA - Routing Area
RANAP - Radio Access Network Application Part
RNS - Radio Network Controller
SCCP - Signaling Connection Control Part
SCP - Service Control Point
TCAP - Transaction Capabilities Application Part
UMA - Unlicensed Mobile Access
UTRAN - UMTS Terrestrial Radio Access Network
Related links:
In order to dispel some TyMaH moments, we are with you consider the technical prerequisites necessary to realize the ability to receive / make voice calls with an active GPRS / EDGE session.
So, what do we need so that during an active session of packet data transfer, we can receive / make calls or send / receive SMS. And you need, in fact, not so much, but more specifically:
- presence of G s interface between SGSN 'ohm and MSC / VLR ' ohm
- Base Station Controller ( BSC ) support Network Operation Mode [NOM] I
- the presence of a mobile terminal of a certain class, allowing the simultaneous use of two services (voice and packet data)
G s interface
Thus, the first requirement is the presence of an unknown until the G s interface. This is the interface between network elements such as SGSN and MSC / VLR. The interface is optional, i.e. optional in the general architecture of the GPRS / EDGE network and actually allows you to perform the following procedures:
- CS paging
- IMSI attach for a GPRS attached MS *
- GPRS attach for an IMSI attached MS
- IMSI detach
- Gprs detach
- Combined RA / LA update
- Non-GPRS alert
- MS information inquiry
- MM information
- Reset (MSC / VLR <-> SGSN).
- MM information
- Tunnelling for non-GSM signalling
- MM information
- Location Information retrieval through the CAMEL Any Time
Interrogation (ATI) operation (SCP <-> HLR <-> MSC / VLR <-> SGSN)* - when the subscriber requests only voice services during GPRS Attach'e
(see the GPRS article from the inside. Part 2 )
For us, the most important procedures from this list will be paging procedures (CS paging), procedures for updating the location of the subscriber (Combined RA / LA update), as well as the combined attachment of the subscriber in the operator’s network (IMSI attach for a GPRS attached MS, GPRS attach for an IMSI attached MS) that will allow us to connect to the voice services subnet (CS Core Network), as well as “find” the subscriber immediately before the call.
If we draw an analogy with the establishment of a “standard” voice connection and voice connection through SGSN, then you can find many similar points. For example, a comparative diagram of the paging procedure through MSC and SGSN is presented below:
As you can see, the procedures are quite similar, the only difference is that SGSN acts as an intermediate link that allows the “subscriber terminal” (MS) and the switch (MSC / VLR) to “communicate” with each other. The ability to be a transport link provides SGSN with a special protocol stack or, to use the terminology of the General Channel Signaling ( SSN7 / SS7 ), the user subsystem [Application Part] will be more acceptable.
In order to understand some points of using just such a subsystem, let's take a look at the structure of providing access from the radio subnet to the base (Core Network) operator subnet for several generations of architectures.
Protocol stack
In the diagram below, several access options from the radio subnet of various mobile technologies to the operator’s base subnet are presented.
UMA / GAN (en)
The most typical application of UMA / GAN technology is the use of a mobile terminal, which allows you to "switch" between the use of wireless LAN and WAN networks. As a LAN, a network based on the so-called unlicensed 802.11 technologies, and WAN networks are usually represented by GPRS / EDGE / UMTS architectures. At the same time, while the subscriber’s terminal is in the coverage area of the wireless LAN network, it will be connected to it through a secure connection, and if the signal quality decreases, the terminal will “switch” to the GPRS / EDGE / UMTS architecture. For the UMA / GAN architecture, instead of the concept of a base station (BTS), the concept of GANC [GAN Controller] is used, which is physically very similar to a Wi-Fi access point. This controller has a direct connection to the base station controller in the GPRS / EDGE architecture.
I want to draw your attention, but what subsystem is used to connect the switch (MSC) to the base station controller (BSC) is the BSSAP (en) subsystem [Base Station System Application Part] on interface A. A pretty similar subsystem is used between SGSN'om and MSC / VLR'om - BSSAP + on the interface G s , i.e. the use of this subsystem allows SGSN to “pretend” to be a base station controller (BSC) for a switch (MSC / VLR) and, accordingly, makes it possible to carry out / receive voice services for a subscriber through SGSN.
The above subsystem (BSSAP +) is based on the underlying levels of the SS7 stack and, together with the other subsystems in the general scheme of SSN7 / SS7, is presented in the figure to the right.
Network operation mode
The NOM parameter is configured on the side of the base station controller (BSC) and basically determines how paging procedures for the subscriber will go through the "standard" GSM channels, or the paging channels of the packet subnet will be activated.
There are several basic Network Operation Modes:
- NOM I
In this mode, signaling for packet- and circuit-switched subscriber data can be carried out both through the GSM paging channel PCH [Paging CHannel], and through the GPRS paging channel - PPCH [Packet Paging CHannel]. This provides the ability to simultaneously use voice services along with packet data. Moreover, if the PPCH is used, then the signaling necessarily passes through the SGSN to the MSC / VLR via the G s interface, which is also used for GSM / GPRS attach procedures and Location Updates procedures.
If the voice subnet needs to be paged at a time when the subscriber has a GPRS / EDGE session open, the mobile terminal will be notified via the PACCH [Packet Associated Control CHannel] channel, to which the circuit-switched subnet is not has access.
If the GPRS / EDGE session is not activated, then paging will go through the “standard” GSM PCH or PPCH channel. NOM I is the only mode where the G s interface can be enabled . - NOM II
This is the simplest of the three modes, it is assumed that there is no direct signaling connection between the packet- and circuit-switched operator subnets and therefore the PPCH channel cannot be used. In this case, the subscriber will not be able to receive / make a call with an active GPRS / EDGE session. - NOM III
In this mode, it is assumed that the G s interface is not activated and, accordingly, circuit-switched paging should be carried out through the PCH channel. Also in this mode, a common GPRS control channel is available - PCCCH [Packet Common Control CHannel], along with its subchannels: PPCH, PRACH [Packet Random Access CHannel], and PAGCH [Packet Access Grant CHannel], which means that all the paging for the packet part will be implemented through "own" GPRS channels.
This mode may be preferred over NOM II mode, in some cases, to reduce the load on the PCH GSM channel.
To notify subscriber terminals which of the GPRS modes is currently used on the operator’s network, the GSM Broadcast BCCH [Broadcast Common Control Channel] is used.
You can find out which mode the operator is currently using, using special applications, for example, Netmonitor .
Mobile Terminal Class
The second important point that allows us to make / receive voice services is the terminal class. Below are the main classes of mobile terminals.
- Class A
In class “A” there is the possibility of transmitting voice traffic while receiving / transmitting data over GPRS. - Class B A
mobile phone with GPRS class “B” can automatically switch GPRS and GSM services, ie when the GPRS connection is on with incoming or outgoing SMS - the connection is temporarily interrupted, after the call ends - the GPRS connection is automatically resumed. Also happens with incoming and outgoing calls. - Class C
It is impossible to use both GSM and GPRS service, i.e. with an active GPRS session, for example, if you are connected to the Internet it is impossible to receive calls or send SMS.
Most of the devices currently in use by users are “representatives” of class B and, accordingly, without a G s interface they will not be able to receive / make voice services. In this situation, the subscriber trying to reach you will receive a reply, “The subscriber is out of the network coverage area ...”, and until you end the GPRS / EDGE session, you will be out of the zone for voice services (including SMS).
Summary
So, what did we find out from this article ... For the simultaneous operation of voice services and packet data transfer, several conditions must be met:
- the subscriber’s device must not be lower than class B, because class C will not allow you to simultaneously use voice services and packet transmission
- operator must use NOM I mode on base station controllers
- G s interface * must be configured on the operator’s side , allowing you to provide the ability to make / receive voice services
* - such a situation may arise (of course, purely hypothetically :) when not all the equipment of the operator supports the ability to implement the G s interface. For example, not all switchboards (MSC / VLR) of an operator due to different vendors, or due to different software versions (or due to the use of outdated hardware) can provide a connection between SGSN and MSC / VLR, and you will inadvertently find yourself at the border service areas of different switches, then on the one hand they will be able to reach you with an active GPRS / EDGE session, and on the other (where there is no way to organize a G s interface) - no.
If these conditions are not met, it is not possible to call (send SMS / establish CSD communication) to a subscriber who has a GPRS / EDGE session open.
Small helper:
AP - Access Point
BSC - Base Station Controller
BSSAP - Base Station System Application Part
BSSGP - Base Station System GPRS Protocol
BTS - Base Transceiver Station
CAMEL - Customized Applications for Mobile Enhanced Logic
CAP - CAMEL Application Part
CS - Circuit Switched
eUTRAN / E-UTRA - Evolved UTRAN
GAN - Generic Access Network
GANC - GAN Controller
GERAN- GSM EDGE Radio Access Network
LA - Location Area
MAP - Mobile Application Part
MM - Mobility Management
MS - Mobile Station
RA - Routing Area
RANAP - Radio Access Network Application Part
RNS - Radio Network Controller
SCCP - Signaling Connection Control Part
SCP - Service Control Point
TCAP - Transaction Capabilities Application Part
UMA - Unlicensed Mobile Access
UTRAN - UMTS Terrestrial Radio Access Network
Related links:
