GPRS inside. Part 1
In this series of articles, I would like to tell the habrew community about packet data transmission technologies in the networks of mobile operators. We will review the circuit diagrams of the Packet Switched (PS) Core Network, look at the stack of protocols used for communication between different network elements, and also take a closer look at the functions of the main elements that allow us to use packet transmission in mobile networks. Specifically, in this article we will discuss the most common at the moment GPRS / EDGE technologies.
So, what did we have in terms of data transmission at the beginning of the development of mobile networks of operators.
Let's start our "countdown" with the so-called. CSD [Circuit Switched Data]. This technology appeared in the GSM standard and allowed to establish connections using a modem built in or connected to the subscriber's machine, while the subscriber at the base station transmitter was allocated only one timeslot (TS), the transmission rate does not exceed 9.6 kbit / s.
Data transmission using CSD is practically no different from a normal voice call, because at the time of the call, you completely occupy the channel and therefore the charging of such a connection is made minutely and naturally at the dawn of the development of mobile networks was by no means small.
The next stage in the development of data transmission in mobile networks was the improvement of CSD technology - the HSCSD technology (en) [High Speed ​​CSD] appeared. The use of this technology made it possible to increase the data transfer rate by combining 4 TS +, the throughput of one channel was increased to 14.4 Kbps by using “simplified” error correction methods. Thus, the maximum throughput for HSCSD was 57.6 Kbps.
Despite the low transfer rate and per minute billing, this technology continues to be popular for transferring small amounts of data in systems, such as burglar alarms (meter readings, indicators), primarily because of its ease of use on modern devices.
Everything changed with the advent (Phase 1 specifications appeared in 2000/2001) of packet data transmission technology - GPRS [General Packet Radio Service], which significantly increased the capacity of the data transmission channel (the maximum transmission speed, using 8 TS - 171, 2 kbit / s), and also used packet switching, as opposed to circuit switching in CSD / HSCSD, which allowed more efficient use of resources at base stations, but at the same time, this technology “required” the addition of additional elements to the network structure - SGSN , GGSN.
In principle, the EDGE [Enhanced Data Rates for GSM Evolution] technology is practically no different from GPRS, since can be implemented on an existing network. Changes in the implementation of EDGE relate to changes in the coding schemes at the radio interface, as well as changes in software on network elements. The maximum speed that EDGE can provide is 473.6 kbps (8 time slots x 59.2 kbps).
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Thus, GPRS / EDGE technologies, compared with circuit-switched technologies (CSD / HSCSD), made it possible to provide the end user:
The emergence of a new principle of data transmission in mobile networks, has required changes to the architecture of the GSM network itself, so let's take a look at the network architecture ...
What is the so-called? PS Core Network? Let's take a look at the concept of GSM architecture.
Explanation of the scheme:
AuC - Authentification Center
BSC - Base Station Controller
BTS - Base Transceiver Station
CGF - Charging Gateway Function
EIR - Equipment Identification Register
GGSN - Gateway GPRS Support Node
GMSC - Gateway MSC
HLR - Home Location Register
ISDN - Integrated Services Digital Network
MSC - Mobile Switching Center
PSDN / PDN - Public Switched Data Network / Packet Data Network
PSTN - Public Switched Telephone Network
SGSN - Serving GPRS Support Node
VLR - Visiting Location Register
The main element in the network architecture of GPRS is SGSN . As you can see from the scheme, SGSN is connected by various interfaces with most elements of the GSM network architecture. SGSN’s integral “satellite” in the operator’s packet network is GGSN , which is a bridge between the operator’s IP Backbone and other Packet Data Networks (PDN). The GGSN can be a “regular” Cisco router, but there are also separate solutions from Nokia Siemens Networks (NSN), Huawei, etc. vendors. In most cases, there are several similar elements on the operator’s network, which in turn is determined by the network capacity and the load on the territory.
Functionally SGSN allows:
The possible interfaces connecting the SGSN to the various network elements are shown in the figure below.
Now we will not consider all the interfaces of SGSN, but dwell only on a few key points.
All interfaces that are associated with SGSN'om refer to the letter "G" (please do not confuse with the appropriate point ), many of them are mandatory, others introduce some functionality and are auxiliary. In particular, the Gs interface (between MSC and SGSN) allows you to receive and receive voice services while using GPRS services. The Gd interface (between SMS-GMSC and SGSN) allows you to send SMS messages through a packet network *.
* - by the way, the SMS sending service is a vivid example of how operators do not like to reduce their revenues, because sending SMS through a packet network is almost 2-3 times cheaper for the end user, operators often “refuse” to use this functionality, but at the same time it can be noted that GPRS coverage is not everywhere. Usually in the device, the SMS sending function is configured via: Messages -> Message settings -> Text messages -> Use packet data.
The above two interfaces are optional, but add some functionality to the operator’s network. FR / IP / ATM may be the technologies used for data transfer between different interfaces, but recently there is a tendency to switch to IP Backbone, both the most technically easily implemented and cost-effective transport for data exchange.
In short, the principle of the packet network can be described as follows:
1. the allocation of resources for packet transmission on the side of the base station controller *
* - this takes into account the priority of voice services.
2. carrying out the subscriber authentication procedure (GPRS Attach), including the identification of the subscriber terminal, the so-called. IMEI Check *
* - is optional.
3. updating subscriber location information in HLR
4. negotiation of stream encryption keys
5. establishing communication between the subscriber's terminal device and the PS Core Network, which in the terminology of the GPRS / EDGE architecture is called PDP (Packet Data Protocol) Context activation and depends on the type of data requested - Mobile internet / Intranet / Wap / MMS / SMS_over_IP
6. after the end of the use of packet transmission services, the subscriber is disconnected - the PDP Context is deactivated
7. if the subscriber’s terminal device is not configured for a permanent connection with the packet network (you can check it on most devices in Menu -> Settings -> Device connection -> Packet data -> Packet connection -> On demand / Permanent access), This will be the operation performed, the inverse of the initial authentication and authorization of the subscriber, GPRS Detach.
We will look at this section in more detail in the second part of the article. Determine what data is transmitted during the authorization process, as well as what data is stored on the subscriber’s side / SGSN, let's touch on some encryption algorithms used in the GPRS / EDGE architecture.
Promising technologies that can improve GPRS / EDGE technologies are their direct “heirs” - EGPRS2 / Evolved EDGE, which are supported by vendors such as Nokia Siemens Networks (NSN) and Nortel (the technologies were standardized by 3GPP Rel-7).
To move to Evolved EDGE, it is enough to upgrade the software on the existing EDGE network, while the suppliers promise that the Evolved EDGE technology can more than double the efficiency of spectrum utilization when compared to how it was done in EDGE.
In particular, after the transition to the new standard, users will be able to download data from the network at speeds up to 1.2 Mbps (Downlink is the direction from the base station to the subscriber), transfer data to the base station (Uplink) at up to 473 kbps /with.
At the request of manufacturers, this evolution of GSM technology will provide operators with a cost-effective transition to next-generation technologies, in particular, LTE and full service compatibility between GSM and next-generation mobile data transmissions.
At the end of the article I wanted to write that this is my first topic on Habré, so that they would not kick much and so on ... but then decided not to write, because how can you understand the article like it or not, if there is no criticism from the readers. Therefore, questions / comments / clarifications / threats are welcome, if you like the article, then all this will be taken into account in the following papers.
Related Links:
Story
So, what did we have in terms of data transmission at the beginning of the development of mobile networks of operators.
Let's start our "countdown" with the so-called. CSD [Circuit Switched Data]. This technology appeared in the GSM standard and allowed to establish connections using a modem built in or connected to the subscriber's machine, while the subscriber at the base station transmitter was allocated only one timeslot (TS), the transmission rate does not exceed 9.6 kbit / s.
Data transmission using CSD is practically no different from a normal voice call, because at the time of the call, you completely occupy the channel and therefore the charging of such a connection is made minutely and naturally at the dawn of the development of mobile networks was by no means small.
The next stage in the development of data transmission in mobile networks was the improvement of CSD technology - the HSCSD technology (en) [High Speed ​​CSD] appeared. The use of this technology made it possible to increase the data transfer rate by combining 4 TS +, the throughput of one channel was increased to 14.4 Kbps by using “simplified” error correction methods. Thus, the maximum throughput for HSCSD was 57.6 Kbps.
Despite the low transfer rate and per minute billing, this technology continues to be popular for transferring small amounts of data in systems, such as burglar alarms (meter readings, indicators), primarily because of its ease of use on modern devices.
Everything changed with the advent (Phase 1 specifications appeared in 2000/2001) of packet data transmission technology - GPRS [General Packet Radio Service], which significantly increased the capacity of the data transmission channel (the maximum transmission speed, using 8 TS - 171, 2 kbit / s), and also used packet switching, as opposed to circuit switching in CSD / HSCSD, which allowed more efficient use of resources at base stations, but at the same time, this technology “required” the addition of additional elements to the network structure - SGSN , GGSN.
In principle, the EDGE [Enhanced Data Rates for GSM Evolution] technology is practically no different from GPRS, since can be implemented on an existing network. Changes in the implementation of EDGE relate to changes in the coding schemes at the radio interface, as well as changes in software on network elements. The maximum speed that EDGE can provide is 473.6 kbps (8 time slots x 59.2 kbps).
')
Thus, GPRS / EDGE technologies, compared with circuit-switched technologies (CSD / HSCSD), made it possible to provide the end user:
- high transfer rate
- less time opening session
- more favorable rates of use
- billing according to the volume of transmitted data, but not by the minute
- do not occupy the entire channel during data transfer
The emergence of a new principle of data transmission in mobile networks, has required changes to the architecture of the GSM network itself, so let's take a look at the network architecture ...
Network layout
What is the so-called? PS Core Network? Let's take a look at the concept of GSM architecture.
Explanation of the scheme:
AuC - Authentification Center
BSC - Base Station Controller
BTS - Base Transceiver Station
CGF - Charging Gateway Function
EIR - Equipment Identification Register
GGSN - Gateway GPRS Support Node
GMSC - Gateway MSC
HLR - Home Location Register
ISDN - Integrated Services Digital Network
MSC - Mobile Switching Center
PSDN / PDN - Public Switched Data Network / Packet Data Network
PSTN - Public Switched Telephone Network
SGSN - Serving GPRS Support Node
VLR - Visiting Location Register
The main element in the network architecture of GPRS is SGSN . As you can see from the scheme, SGSN is connected by various interfaces with most elements of the GSM network architecture. SGSN’s integral “satellite” in the operator’s packet network is GGSN , which is a bridge between the operator’s IP Backbone and other Packet Data Networks (PDN). The GGSN can be a “regular” Cisco router, but there are also separate solutions from Nokia Siemens Networks (NSN), Huawei, etc. vendors. In most cases, there are several similar elements on the operator’s network, which in turn is determined by the network capacity and the load on the territory.
Functionally SGSN allows:
- provide subscribers with the ability to send and receive packet data
(mobile internet / wap / mms / intranet) - authenticate and authorize subscribers
- provide billing data to the operator
- send SMS_over_IP
- provide interfaces for government agencies
- monitor and update subscriber data in the HLR / MSC, i.e. Mobility management
- manage user sessions
The possible interfaces connecting the SGSN to the various network elements are shown in the figure below.
Now we will not consider all the interfaces of SGSN, but dwell only on a few key points.
All interfaces that are associated with SGSN'om refer to the letter "G" (
* - by the way, the SMS sending service is a vivid example of how operators do not like to reduce their revenues, because sending SMS through a packet network is almost 2-3 times cheaper for the end user, operators often “refuse” to use this functionality, but at the same time it can be noted that GPRS coverage is not everywhere. Usually in the device, the SMS sending function is configured via: Messages -> Message settings -> Text messages -> Use packet data.
The above two interfaces are optional, but add some functionality to the operator’s network. FR / IP / ATM may be the technologies used for data transfer between different interfaces, but recently there is a tendency to switch to IP Backbone, both the most technically easily implemented and cost-effective transport for data exchange.
Principle of operation
In short, the principle of the packet network can be described as follows:
1. the allocation of resources for packet transmission on the side of the base station controller *
* - this takes into account the priority of voice services.
2. carrying out the subscriber authentication procedure (GPRS Attach), including the identification of the subscriber terminal, the so-called. IMEI Check *
* - is optional.
3. updating subscriber location information in HLR
4. negotiation of stream encryption keys
5. establishing communication between the subscriber's terminal device and the PS Core Network, which in the terminology of the GPRS / EDGE architecture is called PDP (Packet Data Protocol) Context activation and depends on the type of data requested - Mobile internet / Intranet / Wap / MMS / SMS_over_IP
6. after the end of the use of packet transmission services, the subscriber is disconnected - the PDP Context is deactivated
7. if the subscriber’s terminal device is not configured for a permanent connection with the packet network (you can check it on most devices in Menu -> Settings -> Device connection -> Packet data -> Packet connection -> On demand / Permanent access), This will be the operation performed, the inverse of the initial authentication and authorization of the subscriber, GPRS Detach.
We will look at this section in more detail in the second part of the article. Determine what data is transmitted during the authorization process, as well as what data is stored on the subscriber’s side / SGSN, let's touch on some encryption algorithms used in the GPRS / EDGE architecture.
Perspectives
Promising technologies that can improve GPRS / EDGE technologies are their direct “heirs” - EGPRS2 / Evolved EDGE, which are supported by vendors such as Nokia Siemens Networks (NSN) and Nortel (the technologies were standardized by 3GPP Rel-7).
To move to Evolved EDGE, it is enough to upgrade the software on the existing EDGE network, while the suppliers promise that the Evolved EDGE technology can more than double the efficiency of spectrum utilization when compared to how it was done in EDGE.
In particular, after the transition to the new standard, users will be able to download data from the network at speeds up to 1.2 Mbps (Downlink is the direction from the base station to the subscriber), transfer data to the base station (Uplink) at up to 473 kbps /with.
At the request of manufacturers, this evolution of GSM technology will provide operators with a cost-effective transition to next-generation technologies, in particular, LTE and full service compatibility between GSM and next-generation mobile data transmissions.
Conclusion
At the end of the article I wanted to write that this is my first topic on Habré, so that they would not kick much and so on ... but then decided not to write, because how can you understand the article like it or not, if there is no criticism from the readers. Therefore, questions / comments / clarifications / threats are welcome, if you like the article, then all this will be taken into account in the following papers.
Related Links:
Source: https://habr.com/ru/post/80951/
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