GPRS from the inside out. Part 1

    With this series of articles, I would like to tell the habitation community about the technologies of packet data transmission in the networks of mobile operators. We will examine the concepts of Packet Switched (PS) Core Network, look at the protocol stack used for communication between various 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, this article will focus on the most common GPRS / EDGE technologies at the moment.


    History

    So, what did we have in terms of data transfer 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 made it possible to establish connections using a modem built-in or connected to the subscriber’s device, while the subscriber on the base station transmitter was allocated only one time slot (TS), the transmission speed does not exceed 9.6 kbit / s.
    Data transfer using CSD is practically no different from a regular voice call, because for the duration of the call, you completely occupy the channel and therefore the tariffication of such a connection is carried out every minute and naturally at the dawn of the development of mobile networks it was by no means small.
    The next stage in the development of data transmission in mobile networks was the improvement of CSD technology - HSCSD (en) [High Speed ​​CSD] technology appeared . Using this technology allowed to increase the data transfer rate due to the combination of 4 TS +, the throughput of one channel was increased to 14.4 Kbps due to the use of “simplified” error correction methods. Thus, the maximum throughput for HSCSD was 57.6 Kbps.
    Despite the low transmission speed and per-minute billing, this technology continues to be popular for the transfer of small amounts of data in systems, for example, security alarms (meter readings, indicators), primarily due to the 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 data transmission bandwidth (maximum transmission speed, provided that 8 TS is used - 171, 2 kbit / s), and also used packet switching, unlike channel switching in CSD / HSCSD, which allowed more efficient use of resources at base stations, but at the same time this technology “required” the introduction of additional elements in the network structure - SGSN GGSN.
    Fundamentally EDGE Technology[Enhanced Data rates for GSM Evolution] is practically no different from GPRS, because can be implemented on an existing network. Changes in the implementation of EDGE relate to changes in coding schemes on the radio interface, as well as software changes 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, in comparison with circuit switched technologies (CSD / HSCSD), allowed to provide the end user with:
    • high speed transmission
    • less time to open a session
    • more favorable usage rates
    • charging according to the amount of data transferred, not per minute
    • do not occupy the entire channel during data transfer

    The emergence of a new principle of data transfer in mobile networks required changes in the architecture of the GSM network itself, so let's take a look at the network architecture ...

    Network diagram

    What is the so-called PS Core Network? Let's take a look at the concept of GSM architecture.
    image

    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 A

    key element in the GPRS network architecture is SGSN . As you can see from the diagram, SGSN is connected by various interfaces to most elements of the GSM network architecture. SGSN’s “satellite” in the operator’s packet network is GGSN, which is a kind of bridge between the IP Backbone of the operator and other Packet Data Networks (PDN). The GGSN iron version may be a “regular” Cisco router, but there are also separate solutions from vendors of Nokia Siemens Networks (NSN), Huawei, etc. 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 you to:
    • 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 to government agencies
    • monitor and update subscriber data in HLR / MSC, the so-called Mobility management
    • manage user sessions

    Possible interfaces connecting SGSN with various network elements are presented in the figure below.
    image
    Now we will not consider all the SGSN interfaces, but focus only on a few key points.

    All interfaces that are associated with SGSN are designated with the letter "G" ( please do not confuse with the corresponding dot ), many of them are mandatory, others introduce certain 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 over the packet network *.
    * - by the way, the SMS sending service is a vivid example of how operators do not like to reduce their income, because sending SMS through a packet network is almost 2-3 times cheaper for the end user, then 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 function of sending SMS is configured through: Messages -> Message Settings -> Text Messages -> Use packet data.
    The two above-mentioned interfaces are optional, but add some functionality to the operator’s network. The technologies used to transfer data between different interfaces can be FR / IP / ATM, but recently there has been a tendency to switch to IP Backbone, both the most easily implemented in technical terms and cost-effective transport for data exchange.

    Principle of operation

    In short, the principle of operation of a packet network can be described as follows:
    1. 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 identification of the subscriber terminal, the so-called IMEI Check *
    * - is optional.
    3. updating the location information of the subscriber in the HLR
    4. matching the encryption keys of the stream
    5. establishing communication between the subscriber’s terminal device and the PS Core Network, which in the terminology of GPRS / EDGE architecture is called PDP (Packet Data Protocol) Context activation and depends on the type the requested data - Mobile internet / Intranet / Wap / MMS / SMS_over_IP
    6. after the end of using packet transmission services, the subscriber is disconnected - PDP Context is deactivated
    7. if the subscriber’s terminal device is not configured for permanent connection with the packet network (you can check this on most devices in Menu -> Settings -> Connection devices -> Packet data -> Packet connection -> On demand / Permanent access), then the operation will be performed, the reverse of the initial authentication and authorization of the subscriber, the so-called GPRS Detach.

    We will consider this section in more detail in the second part of the article. We will determine what data is transferred during the authorization process, as well as what data is stored on the subscriber / SGSN'a side, we will touch on a few encryption algorithms used in the GPRS / EDGE architecture.

    Prospects

    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 upgrade to Evolved EDGE, it is enough to upgrade the software on an existing EDGE network, while suppliers promise that the technology Evolved EDGE can more than double the efficiency of spectrum use when compared with how it is 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 - direction from the base station to the subscriber), transfer data in the direction to the base station (Uplink) at a speed of up to 473 kbps /from.
    According to manufacturers, this evolution of GSM technology will provide operators with a cost-effective transition to next-generation technologies, in particular, LTE and full compatibility in services between GSM and the next generation of mobile data.

    Conclusion

    At the end of the article I wanted to write that this is my first topic on the hub, so as not to kick much and all that ... but then I decided not to write, because how can you understand whether the article was liked or not, if there is no criticism from readers. Therefore, questions / comments / clarifications / threats are welcome, if you like the article, then all this will be taken into account in the following works.

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