GPRS - Quick Guide
GPRS - Overview
GPRS stands for General Packet Radio System. GPRS provides packet radio access for mobile Global System for Mobile Communications (GSM) and time-division multiple access (TDMA) users.
GPRS is important as a migration step toward third-generation (3G) networks and allows network operators to implement an IP-based core architecture for data applications, which will continue to be used and expanded for 3G services for integrated voice and data applications.
GPRS is a new bearer service for GSM that greatly improves and simplifies wireless access to packet data networks, e.g., to the Internet. It applies a packet radio principle to transfer user data packets in an efficient way between GSM mobile stations and external packet data networks. Packets can be directly routed from the GPRS mobile stations to packet switched networks.
Networks based on the Internet Protocol (IP) (e.g., the global Internet or private/corporate intranets) and X.25 networks are also supported in the current versions of GPRS.
Who owns GPRS?
The GPRS specifications are written by the European Telecommunications Standard Institute (ETSI), the European counterpart of the American National Standard Institute (ANSI).
The following three key features describe wireless packet data:
- The always online feature: Removes the dial-up process, making applications only one click away.
- An upgrade to existing systems: Operators do not have to replace their equipment; rather, GPRS is added on top of the existing infrastructure.
- An integral part of future 3G systems: GPRS is the packet data core network for 3G systems EDGE and WCDMA.
Goals of GPRS:
GPRS is the first step toward an end-to-end wireless infrastructure and has the following goals:
- Open architecture
- Consistent IP services
- Same infrastructure for different air interfaces
- Integrated telephony and Internet infrastructure
- Leverage industry investment in IP
- Service innovation independent of infrastructure
Benefits of GPRS:
Higher Data Rate:
Users of GPRS benefit from shorter access times and higher data rates. In conventional GSM, the connection setup takes several seconds and rates for data transmission are restricted to 9.6 kbit/s. GPRS in practice offers session establishment times below one second and ISDN-like data rates up to several ten kbit/s.
GPRS packet transmission offers a more userfriendly billing than that offered by circuit switched services. In circuit switched services, billing is based on the duration of the connection. This is unsuitable for applications with bursty traffic. The user must pay for the entire airtime, even for idle periods when no packets are sent (e.g., when the user reads a Web page).
In contrast to this, with packet switched services, billing can be based on the amount of transmitted data. The advantage for the user is that he or she can be "online" over a long period of time but will be billed based on the transmitted data volume.
To sum up, GPRS improves the utilization of the radio resources, offers volume-based billing, higher transfer rates, shorter access times, and simplifies the access to packet data networks.
GPRS - Applications
GPRS enables a variety of new and unique services to the mobile wireless subscriber. These mobile services have unique characteristics that provide enhanced value to customers. These characteristics include the following:
Mobility: The ability to maintain constant voice and data communications while on the move.
Immediacy: Allows subscribers to obtain connectivity when needed, regardless of location and without a lengthy login session.
Localization: Allows subscribers to obtain information relevant to their current location.
The combination of these characteristics provides a wide spectrum of possible applications that can be offered to mobile subscribers. In general, applications can be separated into two high-level categories: corporate and consumer. These include:
Communications: E-mail, fax, unified messaging and intranet/Internet access, etc.
Value-added services: Information services and games, etc.
E-commerce: Retail, ticket purchasing, banking and financial trading, etc.
Location-based applications: Navigation, traffic conditions, airline/rail schedules and location finder, etc.
Vertical applications: Freight delivery, fleet management and sales-force automation.
Advertising: Advertising may be location sensitive. For example, a user entering a mall can receive advertisements specific to the stores in that mall.
It is also possible to send SMS messages over GPRS. In addition, it is planned to implement supplementary services, such as call forwarding unconditional (CFU), call forwarding on mobile subscriber not reachable (CFNRc), and closed user group (CUG).
GPRS - Architecture
GPRS is a data network that overlays a second-generation GSM network. This data overlay network provides packet data transport at rates from 9.6 to 171 kbps. Additionally, multiple users can share the same air-interface resources simultaneously.
Following is the GPRS Architecture diagram:
GPRS attempts to reuse the existing GSM network elements as much as possible, but to effectively build a packet-based mobile cellular network, some new network elements, interfaces, and protocols for handling packet traffic are required.
Therefore, GPRS requires modifications to numerous GSM network elements as summarized below:
|GSM Network Element||Modification or Upgrade Required for GPRS.|
|Mobile Station (MS)||New Mobile Station is required to access GPRS services. These new terminals will be backward compatible with GSM for voice calls.|
|BTS||A software upgrade is required in the existing base transceiver site.|
|BSC||The base station controller (BSC) requires a software upgrade and the installation of new hardware called the packet control unit (PCU). The PCU directs the data traffic to the GPRS network and can be a separate hardware element associated with the BSC.|
|GPRS Support Nodes (GSNs)||The deployment of GPRS requires the installation of new core network elements called the serving GPRS support node (SGSN) and gateway GPRS support node (GGSN).|
|Databases (HLR, VLR, etc.)||All the databases involved in the network will require software upgrades to handle the new call models and functions introduced by GPRS.|
GPRS Mobile Stations:
New Mobile Stations are required to use GPRS services because existing GSM phones do not handle the enhanced air interface or packet data. A variety of MS can exist, including a high-speed version of current phones to support high-speed data access, a new PDA device with an embedded GSM phone, and PC cards for laptop computers. These mobile stations are backward compatible for making voice calls using GSM.
GPRS Base Station Subsystem:
Each BSC requires the installation of one or more Packet Control Units (PCUs) and a software upgrade. The PCU provides a physical and logical data interface to the base station subsystem (BSS) for packet data traffic. The BTS can also require a software upgrade but typically does not require hardware enhancements.
When either voice or data traffic is originated at the subscriber mobile, it is transported over the air interface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call. However, at the output of the BSC, the traffic is separated; voice is sent to the mobile switching center (MSC) per standard GSM, and data is sent to a new device called the SGSN via the PCU over a Frame Relay interface.
GPRS Support Nodes:
Following two new components, called GPRS support nodes (GSNs), are added:
Gateway GPRS support node (GGSN):
The Gateway GPRS Support Node acts as an interface and a router to external networks. The GGSN contains routing information for GPRS mobiles which is used to tunnel packets through the IP based internal backbone to the correct Serving GPRS Support Node. The GGSN also collects charging information connected to the use of the external data networks and can act as a packet filter for incoming traffic.
Serving GPRS support node (SGSN):
The Serving GPRS Support Node is responsible for authentication of GPRS mobiles, registration of mobiles in the network, mobility management, and collecting information for charging for the use of the air interface.
The internal backbone is an IP based network used to carry packets between different GSNs. Tunnelling is used between SGSNs and GGSNs, so the internal backbone does not need any information about domains outside the GPRS network. Signalling from a GSN to a MSC, HLR or EIR is done using SS7.
GPRS introduces the concept of a routing area. This is much the same as a Location Area in GSM, except that it will generally contain fewer cells. Because routing areas are smaller than Location Areas, less radio resources are used when a paging message is broadcast.
GPRS - Protocol Stack
Following diagram shows the GPRS protocol stack and end-to-end message flows from the MS to the GGSN. The protocol between the SGSN and GGSN using the Gn interface is GTP. This is a Layer 3 tunneling protocol.
One of the most important things to note here is that the application communicates via standard IP, which is carried through the GPRS network and out through the gateway GPRS looks like a normal IP sub-network to users both inside and outside the network.
Also notice that packets travelling between the GGSN and the SGSN use the GPRS tunnelling protocol, so the internal backbone network does not have to deal with IP addresses outside the GPRS network. This GTP is run over UDP and IP.
Between the SGSN and the MS a combination of SubNetwork Dependent Convergence Protocol and Logical Link Control is used. SNDCP compresses data to minimize the load on the radio channel. The LLC provides a safe logical link by encrypting packets. The same LLC link is used as long as a mobile is under a single SGSN.
When the mobile moves to a routing area that lies under a different SGSN, the LLC link is removed and a new link is established with the new Serving GSN X.25. Services are provided by running X.25 on top of TCP/IP in the internal backbone.
GPRS - Quality of Service
The Quality of Service QoS requirements of typical mobile packet data applications are very diverse. For example, different GPRS applications like realtime multimedia, Web browsing, and e-mail transfer need a different QoS support. This QoS becomes a very important feature of GPRS services.
GPRS allows defining QoS profiles using the parameters service precedence, reliability, delay, and throughput. These parameters are described below:
The service precedence is the priority of a service in relation to another service. There exist three levels of priority: high, normal, and low.
The reliability indicates the transmission characteristics required by an application. Three reliability classes are defined which guarantee certain maximum values for the probability of loss, duplication, mis-sequencing, and corruption of packets.
The delay is defined as the end-to-end transfer time between two communicating mobile stations or between a mobile station and the Gi interface to an external packet data network.
This includes all delays within the GPRS network, e.g., the delay for request and assignment of radio resources and the transit delay in the GPRS backbone network. Transfer delays outside the GPRS network, e.g., in external transit networks, are not taken into account.
The throughput specifies the maximum/peak bit rate and the mean bit rate.
Using these QoS classes, QoS profiles can be negotiated between the mobile user and the network for each session, depending on the QoS demand and the current available resources.
The billing of the service is then based on the transmitted data volume, the type of service, and the chosen QoS profile.
GPRS - MS Classes
The handset is probably the most well-known piece of equipment, because this is the part we use to make phone calls and to access data services. When we talk about advanced services, the handset is commonly called an MS, which consists of terminal equipment (TE) and a mobile terminal (MT).
TE is the device that hosts the applications and the user interaction, while the MT is the part that connects to the network.
In the following example, Palm Pilot is TE and Mobile phone is MT.
In order to take advantage of the new GPRS services, we need new GPRS enabled handsets. There are three different classes of GPRS terminal quipments:
Class A terminals can handle packet data and voice at the same time. In other words, we need two transceivers because the handset has to send and/or receive data and voice at the same time. This situation makes class A terminals significantly more expensive to manufacture than class B and C terminals.
Class B terminals can handle both packet data and voice, but not at the same time. In other words, you can use the same transceiver for both, keeping the cost of the terminals down.
In practice, the GPRS session (like WAP browsing, file transfer, and so on) is suspended when a GSM voice call is started. How this information is presented to the user is up to the device manufacturer, but one way is to give the user the choice between receiving an incoming call and maintaining the data session. That way, a user who is transferring money between his or her accounts by using a WAP service does not have to stop that transaction just because someone calls.
Class C terminals can only handle either voice or data. Examples of class C terminals are GPRS PCM/CIA cards, embedded modules in vending machines, and so on.
Due to the high cost of class A handsets, most handset manufacturers have announced that their first handsets will be class B. There is currently work going on in 3GPP to standardize a lightweight A class in order to make handsets with simultaneous voice and data available at a reasonable cost.
GPRS - PDP Context
The PDP addresses are network layer addresses (Open Standards Interconnect [OSI] model Layer 3). GPRS systems support both X.25 and IP network layer protocols. Therefore, PDP addresses can be X.25, IP, or both.
Each PDP address is anchored at a Gateway GPRS Support Node (GGSN), as shown in Figure below. All packet data traffic sent from the public packet data network for the PDP address goes through the gateway (GGSN).
The public packet data network is only concerned that the address belongs to a specific GGSN. The GGSN hides the mobility of the station from the rest of the packet data network and from computers connected to the public packet data network.
Statically assigned PDP addresses are usually anchored at a GGSN in the subscriber's home network. Conversely, dynamically assigned PDP addresses can be anchored either in the subscriber's home network or the network that the user is visiting.
When a MS is already attached to a SGSN and wants to begin transferring data, it must activate a PDP address. Activating a PDP address establishes an association between the mobile's current SGSN and the GGSN that anchors the PDP address.
The record kept by the SGSN and the GGSN regarding this association is called the PDP context.
It is important to understand the difference between a MS attaching to a SGSN and a MS activating a PDP address. A single MS attaches to only one SGSN; however, it may have multiple PDP addresses that are all active at the same time.
Each of the addresses may be anchored to a different GGSN. If packets arrive from the public packet data network at a GGSN for a specific PDP address and the GGSN does not have an active PDP context corresponding to that address, it may simply discard the packets. Conversely, the GGSN may attempt to activate a PDP context with a MS if the address is statically assigned to a particular mobile.
GPRS - Data Routing
One of the main requirements in the GPRS network is the routing of data packets to and from a mobile user. The requirement can be divided into two areas: data packet routing and mobility management.
Data Packet Routing:
The main functions of the GGSN involve interaction with the external data network. The GGSN updates the location directory using routing information supplied by the SGSNs about the location of an MS. It routes the external data network protocol packet encapsulated over the GPRS backbone to the SGSN currently serving the MS. It also decapsulates and forwards external data network packets to the appropriate data network and collects charging data that is forwarded to a charging gateway (CG).
There are three important routing schemes:
Mobile-originated message: This path begins at the GPRS mobile and ends at the host
Network-initiated message when the MS is in its home network: This path begins at the host and ends at the GPRS mobile
Network-initiated message when the MS roams to another GPRS network: This path begins at the host of visited network and ends at the GPRS mobile
The GPRS network encapsulates all data network protocols into its own encapsulation protocol called the GPRS tunnelling protocol (GTP). The GTP ensures security in the backbone network and simplifies the routing mechanism and the delivery of data over the GPRS network.
The operation of the GPRS is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions to increase efficiency and to make optimum use of free GSM resources (such as unallocated time slots).
An MS can be any of the following three states in the GPRS system. The three-state model is unique to packet radio. GSM uses a two-state model either idle or active.
Data is transmitted between an MS and the GPRS network only when the MS is in the active state. In the active state, the SGSN knows the cell location of the MS.
Packet transmission to an active MS is initiated by packet paging to notify the MS of an incoming data packet. The data transmission proceeds immediately after packet paging through the channel indicated by the paging message. The purpose of the paging message is to simplify the process of receiving packets. The MS listens to only the paging messages instead of to all the data packets in the downlink channels. This reduces battery usage significantly.
When an MS has a packet to transmit, it must access the uplink channel (i.e., the channel to the packet data network where services reside). The uplink channel is shared by a number of MSs, and its use is allocated by a BSS. The MS requests use of the channel in a random access message. The BSS allocates an unused channel to the MS and sends an access grant message in reply to the random access message.
In the standby state, only the routing area of the MS is known. (The routing area can consist of one or more cells within a GSM location area).
When the SGSN sends a packet to an MS that is in the standby state, the MS must be paged. Because the SGSN knows the routing area of the MS, a packet paging message is sent to the routing area. On receiving the packet paging message, the MS relays its cell location to the SGSN to establish the active state.
In the idle state, the MS does not have a logical GPRS context activated or any packet-switched public data network (PSPDN) addresses allocated. In this state, the MS can receive only those multicast messages that can be received by any GPRS MS. Because the GPRS network infrastructure does not know the location of the MS, it is not possible to send messages to the MS from external data networks.
When an MS that is in an active or a standby state moves from one routing area to another within the service area of one SGSN, it must perform a routing update. The routing area information in the SGSN is updated, and the success of the procedure is indicated in the response message.
A cell-based routing update procedure is invoked when an active MS enters a new cell. The MS sends a short message containing the identity of the MS and its new location through GPRS channels to its current SGSN. This procedure is used only when the MS is in the active state.
The inter-SGSN routing update is the most complicated routing update. The MS changes from one SGSN area to another, and it must establish a new connection to a new SGSN. This means creating a new logical link context between the MS and the new SGSN and informing the GGSN about the new location of the MS.
GPRS - Access Modes
The GPRS access modes specify whether or not the GGSN requests user authentication at the access point to a PDN (Public Data Network). The available options are:
Transparent: No security authorization/authentication is requested by the GGSN.
Non-transparent: In this case, GGSN acts as a proxy for authenticating.
The GPRS transparent and non-transparent modes relate only to PDP type IPv4.
Transparent access pertains to a GPRS PLMN that is not involved in subscriber access authorization and authentication. Access to PDN-related security procedures are transparent to GSNs.
In transparent access mode, the MS is given an address belonging to the operator or any other domain's addressing space. The address is given either at subscription as a static address or at PDP context activation as a dynamic address. The dynamic address is allocated from a Dynamic Host Configuration Protocol (DHCP) server in the GPRS network. Any user authentication is done within the GPRS network. No RADIUS authentication is performed; only IMSI-based authentication (from the subscriber identity module in the handset) is done.
Non-transparent access to an intranet/ISP means that the PLMN plays a role in the intranet/ISP authentication of the MS. Non-transparent access uses the Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) message issued by the mobile terminal and piggybacked in the GTP PDP context activation message. This message is used to build a RADIUS request toward the RADIUS server associated with the access point name (APN).
GPRS Access Point Name:
The GPRS standards define a network identity called an access point name (APN). An APN identifies a PDN that is accessible from a GGSN node in a GPRS network. In GPRS, only the APN is used to select the target network. To configure an APN, the operator configures three elements on the GSN node:
Access point: Defines an APN and its associated access characteristics, including security (RADIUS), dynamic address allocation (DHCP), and DNS services.
Access point list: Defines a logical interface that is associated with the virtual template.
Access group: Defines whether access is permitted between the PDN and the MS.
GPRS - Processes
This chapter gives a brief description of the basic processes used in GPRS networks:
Attach process: Process by which the MS attaches (i.e., connects) to the SGSN in a GPRS network
Authentication process: Process by which the SGSN authenticates the mobile subscriber
PDP activation process: Process by which a user session is established between the MS and the destination network
Detach process: Process by which the MS detaches (i.e., disconnects) from the SGSN in the GPRS network
Network-initiated PDP request for static IP address: Process by which a call from the packet data network reaches the MS using a static IP address
Network-initiated PDP request for dynamic IP address: Process by which a call from the packet data network reaches the MS using a dynamic IP address
GPRS - Billing
As packet data is introduced into mobile systems, the question of how to bill for the services arises. Always online and paying by the minute does not sound all that appealing. Here, we describe the possibilities but it totally depends on different service providers how they want to charge their customers:
The SGSN and GGSN register all possible aspects of a GPRS user's behavior and generate billing information accordingly. This information is gathered in so-called Charging Data Records (CDR) and is delivered to a billing gateway.
The GPRS service charging can be based on the following parameters:
Volume: The amount of bytes transferred, i.e., downloaded and uploaded.
Duration: The duration of a PDP context session.
Time: Date, time of day, and day of the week (enabling lower tariffs at offpeak hours).
Final destination: A subscriber could be charged for access to the specific network, such as through a proxy server.
Location: The current location of the subscriber.
Quality of Service: Pay more for higher network priority.
SMS: The SGSN will produce specific CDRs for SMS.
Served IMSI/subscriber: Different subscriber classes (different tariffs for frequent users, businesses, or private users).
Reverse charging: The receiving subscriber is not charged for the received data; instead, the sending party is charged.
Free of charge: Specified data to be free of charge.
Flat rate: A fixed monthly fee.
Bearer service: Charging based on different bearer services (for an operator who has several networks, such as GSM900 and GSM1800, and who wants to promote usage of one of the networks). Or, perhaps the bearer service would be good for areas where it would be cheaper for the operator to offer services from a wireless LAN rather than from the GSM network.
GPRS - Summary
In this tutorial, we have taught you all the basic concepts related to GPRS technology. Hope, now you have basic understanding of GPRS Technology.
You have learnt about GPRS basic overview, its architecture, a short description of GSM protocol stack and available GPRS applications. We also told you how you can charge GPRS services.
A list of all the important GPRS Acronyms has been given for your quick reference. So you can book mark this page for future reference.
What is Next ?
We have now seen that GPRS is a crucial step in the mobile evolution, and it opens endless possibilities for application developers and users. The next step after GPRS can be either EDGE or UMTS (or both).
Enhanced Data rate for GSM Evolution (EDGE): using a new modulation scheme to provide up to three times higher throughput (for HSCSD and GPRS)
Universal Mobile Telecommunication System (UMTS): a new wireless technology using new infrastructure deployment.
If you are not aware of GSM technology, then our Simple GSM tutorial will give you a very good start up.
Now, if you need more detail about GPRS technology, then I would recommend you to go through other GSM resources listed in GPRS Useful Resources chapter.
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