A paper on the cellular technology
General Packet Radio Service
by
Rajvikram Singh
Contents
1. Introduction
2. Some abbreviations and acronyms used in this paper
3. Reasoning behind development of GPRS networks
4. GPRS architecture: Changes over 2nd Generation GSM
4.1 SGSN (Serving GPRS Service Node)
4.2 GGSN (Gateway GPRS Service Node)
4.3 BTS (Base Transceiver System)
4.4 BSC (Base Station Controller)
4.5 Databases
4.6 Mobile Stations (MS)
5. Other key concepts of a GPRS System
5.1 Mobility Management
5.2 Quality of Service (QoS)
5.3 High data speeds in GPRS
6. Limitations of a practical GPRS system
6.1 Upper bound on the cell capacity
6.2 Practical throughput much lower than expected
6.3 Limited GPRS terminal abilities
6.4 Transit Delays due to inherent broadcasting
6.5 No Store and Forward
7. Some observations and recommendations
7.1 Operating systems upgrade
7.2 Parallel processing systems
7.3 Network Management tools
8. References
1. Introduction
The omnipresent cellular phone technologies seem to have captured the fancy of the consumer in a big way. The need to constantly be in touch with friends, colleagues or relatives has been the driving force behind the success of such wireless technologies. The telephone is widely regarded as one of the greatest inventions of the last century and even Alexander Graham Bell himself would never have had imagined the impact of his creation on the modern society. But in spite of the popularity of Bell’s "electrical speech machine," one thing was still missing – and that was that a person still had to go to a phone to make a call. The dream of having a communication device on a person, which allows him to communicate “anytime, anywhere” has led to a very sophisticated evolution of the telephone.
This ‘evolution’, like all other modern technologies, has been a result of significant advancement in other related fields. Research aimed at building more efficient wireless circuits, better antennas, more robust coding and error-correcting techniques, more compact and efficient batteries and power cells, designing of higher bandwidth land networks, faster and energy efficient microprocessors etc have all contributed towards the development of the cellular technology as we see in its present form. The most popular mobile technologies like the GSM, CDMA or Iridium are considered as extremely complex systems, which have layers and layers of logic embedded in them but at the same time provide a transparent interface to the user. These technologies are evolving by the day and the market is soon expected to see the 3rd generation of cellular technologies like EDGE and UMTS that will provide the user with services and option never before imagined.
In this paper I shall try to present a brief but an insightful look in to the General Packet Radio Service (GPRS). The GPRS is considered to be an important stepping-stone in moving towards 3rd generation systems. It is also called as the 2.5th generation of cellular phones in this respect and is a huge leap in terms of the services and bandwidths that were offered to the 2nd generation users of GSM networks. GPRS is not a system, which has been designed from scratch – Instead it can be considered as an add-on system to the present GSM networks. The reasons behind such an approach will be discussed in detail later in this paper. But the reader can think of it as a very clever way to boost the capacity of the existing GSM infrastructure taking into consideration the time, money and effort that has already gone in setting up the present 2nd generation GSM networks. GPRS provides many classes of services and for the first time in cellular networks implements the concept of QoS (Quality of Service).
We’ll also discuss the various components of a GPRS network as defined by ETSI and other institutions. We also try to highlight the difficulties involved with setting up such a complex system, which not only has to work under stringent network performance “guarantees” but also has to mutually co-exist with the older GSM infrastructure without disrupting the efficiency of the ‘host’ network.
2. Some abbreviations and acronyms used in this paper
Though I’ll clarify and explain any abbreviation or notation used in the subsequent sections, the reader is encouraged to go through this section, as these abbreviations will be used generously throughout the paper. The following list gives the ones used most often in this paper.
AuCAuthentication Centre
AUT(H) AUThentication
BSCBase Station Controller
BSSBase Station System
BTS Base Transceiver Station
CFB Call Forwarding on mobile subscriber Busy supplementary service
CRC Cyclic Redundancy Check
CSPDNCircuit Switched Public Data Network
DCE Data Circuit terminating Equipment
DLData Link (layer)
DTE Data Terminal Equipment
ETSI European Telecommunications Standards Institute ( )
FEC Forward Error Correction
GGSNGateway GPRS Serving node
GMMGlobal Mobility Management
GSMGlobal System for Mobile Communication
GPA GSM PLMN Area
GPRSGeneral Packet Radio Service
GTPGPRS Tunneling Protocol
HDLC High level Data Link Control
HLR Home Location Register
HLR Home Location Register
IMEI International Mobile station Equipment Identity
IMSI International Mobile Subscriber Identity
ISDN Integrated Services Digital Network
ISO International Organization for Standardization
LAN Local Area Network
LAPBLink Access Protocol Balanced
LLC Low Layer Compatibility (Also stands for Logical Link Control layer – in a protocol stack on the SGSN)
MSMobile Station.
MSC Mobile-services Switching Centre, Mobile Switching Centre
NMC Network Management Centre
NMSI National Mobile Station Identification number
O&M Operations & Maintenance
OSI Open System Interconnection
PLMN Public Lands Mobile Network
PDPProtocol Data Packet
PSTNPublic Switched Telephone Network
RR Radio Resource
RTOSReal Time Operating Systems
SDU Service Data Unit
SGSNServing GPRS Support Node
TMSI Temporary Mobile Subscriber Identity
VLR Visitor Location Register
Note:ETSI, which is the body controlling and maintaining the GPRS standards, has prepared a reference document (GSM 01.04 version 8.0.0) for specifying the various abbreviations and acronyms used in the standards’ documents.
The reader is advised to have a quick perusal of the document before reading any of the GPRS standards.
3. Reasoning behind development of GPRS networks
The GPRS network can be described as a supplement to the GSM networks for enabling the implementation of a packet switched network over the circuit switched architecture of GSM. One of the main concerns was that already a lot of investment had gone into setting up the GSM networks in many countries all over the world. So the designers of GPRS have tried to use the existing GSM infrastructure as much as possible and have added extra nodes in the network to allow packet switching over GSM. The GSM networks were originally designed as digital systems intended for carrying voice for users at a rate of 9.6 kbps plus a small amount of data.
The digital-data carrying capability of GSM is very small as user data is piggybacked along with the voice in the network. This data piggybacking is termed as the SMS or the Short Messaging Service and is not very useful by itself for transferring large amounts of data to and from the mobile terminal. And though service providers have been using SMS for implementing simple services like alphanumeric paging for stock market updates, weather forecasts, to even implementing WAP (Wireless Application Protocol), GSM in a true sense can never be considered suitable for ‘packet switched’ services because of its extremely small bandwidth and lack of basic services for digital data transfer.
With the explosion of information technology and with data communication exceeding the amount of voice carried over networks, it was inevitable that the market would demand high speed and reliable data transfers to and from the mobile handsets. Yes it is true that one can connect to the Internet using a laptop, modem and a cell phone on a basic GSM network. But the data rates and reliability of the connection leaves much to be desired. The GPRS networks are intended to provide the users with data rates from 9.6 kbps to 171 kbps. And since it also implements Quality of Service (QoS) within the mobile network, the network can now provide network performance guarantees to the users. Thus finally it will be possible to implement bandwidth hungry real-time multimedia applications such as video-conferencing or video streaming on mobile terminals.
Another attractive feature for the consumer is that since GPRS implements a packet switched network, which has been designed for handling bursty traffic, users can exploit this feature and can efficiently share the network bandwidth at the same time. So the users only have to pay for the ‘amount’ of data transacted and not for the ‘time’ for which they are connected to the network. A GPRS subscriber is always connected to the network but is billed only when data is sent or received over the connection. The GPRS network can be considered akin to a mobile LAN in which the user is always online and unlike circuit switched network, does not have to establish a connection, whenever he needs to access the network.
And also since the GPRS provides an upgrade to the GSM networks, it is backward compatible with the old GSM handsets i.e. the users can still use their old GSM cell-phones in a GPRS network albeit only for voice communication. So if they want to use the GPRS services, they’ll have to buy a completely new terminal. GPRS specifies three classes of handsets, which allow users to use the packet switched services.
One of the final goals has been to ultimately give at least one IP-address to every mobile handset in the network, so that the handset becomes a part of the Internet. Thus all the applications and protocols that have been developed for the World Wide Web will also be available on a mobile terminal. And the users have access to virtually infinite amounts of information from their handset. The applications on such a system is left totally to one’s imagination. Apart from accessing the Internet, the corporate consumer is also interested in applications involving private intranets. GPRS has been designed taking into consideration the possibilities of providing a range of such applications. The prospective applications can be broadly classified as the following: -
Communications:
These include services pertaining to typical applications on the Internet private intranets of companies. This kind might prove to provide the maximum number of applications ranging from private/public email services with a unified messaging system (like MIME) to ensure the consistency of data across platforms. Other applications can be multimedia applications like video streaming, video/tele-conferencing, etc, which require real time guarantees from the network.
Value Added Services:
The term “value added services” is used to denote the services offered by the network service providers, to increase the value of their services to the consumer. These kinds of services typically include options like stock market or weather updates at regular intervals; secure transactions for carrying out business deal or any other financial trading. These applications require a strong security support from the underlying network for transmission of sensitive data.
Location-Based Services:
Location-based services provide the ability to link push or pull information services related to a user’s location. Examples include hotel and restaurant finders, roadside assistance, and city-specific news and information. This technology also has interesting corporate applications such as workforce management and vehicle tracking.
Vertical Applications:
In the mobile environment, vertical applications apply to systems utilizing mobile architectures to support the carrying out of specific tasks within the value chain of a company, as opposed to applications that are then being offered for sale to a consumer.
Examples of vertical applications include:
• Sales support—Provision of stock and product information for sales staff, as well as integration of their use of appointment details and the remote placing of orders
• Dispatching—Communication of job details such as location and scheduling; permitting interrogation of information to support the job
• Fleet management—Control of a fleet of delivery or service staff, monitoring their locations and scheduling work
• Parcel delivery—Tracking the locations of packages for feedback to customers and performance monitoring
Advertising:
Advertising services will be offered as a push type information service. The push services are services, which can be best explained as unsolicited data that can be sent to mobile handsets without requiring the users consent or intervention.
4. GPRS architecture: Changes over 2nd Generation GSM
In this sub-section, we will have a look at the extra components that need to be added to a 2nd generation GSM network to upgrade it to carry packet switched data. We are assuming that the reader is aware of the general concepts and architecture of a GSM network. Fig. 1 shows a GPRS network built on top of a GSM network. The gray colored components denote the original GSM architecture and the blue ones denote the extra components that had to be added to upgrade the system to a GPRS network.
The various interfaces as specified by ETSI are as shown in the figure, the important ones are briefly described below:
GdInterface between the SMS GMSC (Short Messaging Service Gateway MSC) and the SGSN
GnBetween the SGSN and the GGSN. Uses the GTP (GPRS tunneling protocol) to shuttle data between the two nodes. IP routing is used for transferring information, as the link between the two nodes may not be direct. This is because the SGSN and the GGSN can have a many-to-many relationship between them i.e. there could be more than one GGSN supporting multiple SGSNs.
Gb Defines the interface between the SGSN and the BSC. Uses frame-relay at the data-link level to transfer packets. Provides logical link control and defines virtual circuits and virtual pipes to distinguish the data intended for different BSCs and BTSs in its domain.
GpInterface between the SGSN and an external GGSN (of some other GPRS network).
GiBetween the SGSN and the host PDN network (typically the Internet). Uses TCP/IP for transferring data to and from external hosts. The SGSN here acts as a proxy. It is supposed to take care of any address translation before letting the data through.
Apart from the apparent hardware that needs to be added, the software on almost all the present nodes has to be upgraded so that they can handle user and signaling information for packet-switched data. The ETSI standards define many such software interfaces as protocol stacks. An uppercase letter and a lower-case subscript denote each interface. The notations besides the links, between the various components, represent the interfaces.
Enabling GPRS on a GSM network requires the addition of two main modules:
4.1 SGSN (Serving GPRS Service Node)
The SGSN provides packet routing to and from the SGSN service area for all users in that service area. The SGSN can be considered to be on the same level of hierarchy as a MSC in a GSM network. In fact it can be considered as a packet-switching MSC, which is responsible for delivering the data to the various Mobile Stations in its domain. Some other responsibilities are:
SGSNs send queries to home location registers (HLRs) to obtain profile data of GPRS subscribers.
SGSNs detect new GPRS MSs in a given service area, process registration of new mobile subscribers, and keep a record of their location inside a given area. Therefore, the SGSN performs mobility management functions such as mobile subscriber attach/detach and location management.
The SGSN is connected to the base-station subsystem via a Frame Relay connection to the PCU in the BSC.
4.2 GGSN (Gateway GPRS Service Node)
As the word Gateway in its name suggests, the GGSN acts as a gateway between the GPRS network and Public Data Networks such as IP and X.25. GGSNs also connect to other GPRS networks to facilitate GPRS roaming. Thus it broadly takes care of the following:
GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular MSs
Network and subscriber screening and address mapping. One (or more) GGSNs may be deployed to support multiple SGSNs.