22 GPRS

Suchit Purohit

epgp books

Learning Objectives:

  • Understanding the fundamentals of GPRS
  • GPRS characteristics
  • GPRS system architecture
  • GPRS protocol architecture

Introduction to Module

 

GPRS module is an extension to GSM and specially used for higher data transfer rate. This module describes the fundamental of GPRS along with its unique characteristics. Next section highlights GPRS system architecture along with the working of important elements. GPRS transmission plane protocol architecture is explained with details of various protocols used. In last section, features of GPRS are listed.

 

Overview of GPRS

 

GPRS stands for General Packet Radio Service. GPRS technology was established to provide higher data rate compared to GSM. GPRS is the packet – based and continuous wireless internet service which offers data rates from 56 kbps to 172 kbps for computer and mobile users.

 

The standardization of GPRS technology was done by the European Telecommunications Standard Institute (ETSI). The important point for GPRS technology is that it works with packet switched data rather than circuit switched data and packet switching efficiently utilized available capacity of the system. GPRS also supports unicast, broadcast and multicast services. It provides more efficient and inexpensive packet transfer service for typical internet applications that depends on packet transfer. ‘Always on’ characteristic is the key advantage for GPRS user which does not require to setup connection prior to data transfer.

 

For the new GPRS radio channels, the GSM system can assign between one to eight time slots within a TDMA frame. Time slots are dynamic, they are not pre-determined or pre- fixed. In GPRS technique, at least one time slot is reserved by the operator to provide good QoS and to guarantee a minimum data transfer speed. Data transfer rate also depends on the capacity of Mobile Station (MS) as all devices are not able to handle sending and receiving of data at same time. Following Table – I represents data rates available with GPRS in combination with GSM.

Table – I: GPRS Data rate (kbps)

Coding Standard 1 Slot 2 Slots 3 Slots 4 Slots 5 Slots 6 Slots 7 Slots 8 Slots
CS – 1 9.05 18.2 27.15 36.2 45.25 54.3 63.35 72.4
S – 2 13.4 26.8 40.2 53.6 67.0 80.4 93.8 107.2
CS – 3 15.6 31.2 46.8 62.4 78.0 93.6 109.2 124.8
CS – 4 21.4 42.8 64.2 85.6 107.0 128.4 149.8 171.2

As per ETSI standard, GPRS offers point-to-point (PTP) packet transfer service in phase

  1. PTP connection oriented network service (PTP-CONS) is one of the PTP version. PTP- CONS is able to maintain a virtual circuit upon the change of cell within GSM network, which is associated with X.25. Another offered PTP version is PTP connectionless network service (PTP-CLNS) to support Internet Protocol (IP) based applications. In phase 2, GPRS offers point-to-multipoint (PTM) service as multicasting.

Characteristicsof GPRS

 

Following are the key characteristics of GPRS technology:

  • Packet switching operation:
    • GPRS technology works with packet switching technique and is quite advantageous compared to GSM which works on circuit switching technique. Packet switching is more efficient and can maximum utilize available capacity of system as well it also allows high commonality in internet techniques.
  • Always-on connection:
    • As GPRS works with packet switching technique, and in packet switching charges are on the base of amount of data carried as it uses service provider’s capacity. While in circuit switching technique, charges are calculated on the base of time for which circuit is used or for the time of call.
  • Speed:
    • The main reason to establish GPRS is to provide higher data rate over GSM. GPRS can provide data rate upto 172 kbps, which is quite higher compared to GSM.
  • Support more applications:
    • Always on characteristics and packer switching along with higher data rate allows GPRS to work with many new applications. One of the greatest advancement in GPRS is Blackberry form of PDA and mobile.
  • CAPEX and OPEX:
    • Capital Expenditure (CAPEX) and Operational Expenditure (OPEX) are important for GPRS as they are concerned with finance. As GPRS is an upgradation to GSM and mostly in the form of Software upgrade, so it does not require dedicated networks and other resources. GPRS can easily operate and maintain remotely. Hence the capital expenditure for GPRS is not as high as newly setup network. Simultaneously, operational expenditure is also very low, as operators don’t have to maintain networks separately it can be handled and managed remotely. Also the base – station infrastructure will be same so it will not affect OPEX.
  • Quality of Service (QoS):
    • GPRS guarantees good Quality of Service (QoS) based on high, normal and low service precedence, reliability class and delay class of the transmission and user data throughput. Reliability class is used for greater error tolerance, error-detection and error-correction. While delay class is used for channel access delay, coding for error correction and transfer delay in GPRS network.
  • Security:
    • GPRS provides various security services like authentication, user identity confidentiality, user information confidentiality and access control.

GPRS System Architecture:

 

GPRS system architecture contains 2 new elements, i.e., GSN (GPRS Support Nodes), which are also known as routers, and SGSN (Serving GPRS Support Nodes). Figure – 1 displays GPRS system architecture. GPRS architecture includes following important components:

  • MS
  • BSS
  • GGSN
  • SGSN
  • MSC
  • HLR / GR
  • VLR
  • EIR
  • Interfaces

Figure 1 – GPRS System Architecture

 

Mobile Station (MS) comprises of users’ hardware and software to interact with GPRS network. Base Station Subsystem (BSS) is responsible to maintain radio connection with MS and coding / decoding of voice. Every BSS of network is controlled by Base Station Controller (BSC). BSS is connected with MS via Um interface. All GSNs are integrated with GSM architecture and new interfaces have been defined in GPRS architecture. Gateway GPRS Support Node (GGSN) is an inter-working component between external packet data network (PDN) and GPRS network. GGSN maintains GPRS user’s routing information and also performs address conversions. GGSN is connected with PDN via Gi interface and transfer information in terms of packets via Gn interface. SGSN is connected with MS and BSS via Gb interface while it is connected with other SGSN through Gn interface. SGSN request Home Location Register (HLR) or GPRS Register (GR) to track location of individual MS and collecting billing information. SGSN can request to Equipment Identity Register (EIR) for identification and authentication related tasks. SGSN can also request to Visitor Location Register (VLR) via Mobile Switching Center (MSC) for bilingual information if MS is not available in HLR. As displayed in figure 1, information in packet data format is transferred from external network to MS via GGSN, SGSN and BSS respectively. In GPRS architecture, MSC is used for signaling process.

GPRS Protocol Architecture:

 

Figure 2 – GPRS Transmission Plane Protocol Architecture

 

GPRS transmission plane protocol architecture is depicted in figure 2. As per the architecture stack, all data inside GPRS backbone is transferred through GPRS Tunneling Protocol (GTP). GTP uses both protocols, either TCP – for reliable communication or UDP – for non-reliable communication. Generally, TCP is used for transferring X.25 packets while UDP is used for IP packets. To adapt different characteristics of underlying layers networks, Subnetwork Dependent Convergence Protocol (SNDPC) is used between SGSN and MS. LLC protocol is used for higher reliability during packet transfer between SGSN and MS. To provide routing and Quality of Service (QoS) related information, Base Station Subsystem GPRS Protocol (BSSGP) is used between SGSN and BSS. But BSSGP does not perform error-correction and it works on top of a frame relay (FR) network. At last to transfer data to MS via Um interface, Radio Link Protocol (RLC) is used. RLC provides a reliable link, while the MAC controls access with signaling procedures for the radio channel and the mapping of LLC frames onto the GSM physical channels.

 

Features of GPRS:

 

GPRS includes following important features:

  • Short Message Service (SMS):
    • SMS is a special-purpose protocol designed for text message communication. SMS contains upto 160 characters to transfer text message.
  • Enhanced Message Service (EMS):
    • Enhanced Message Service offers larger message size compared to SMS, which contains 760 characters in single transaction of data transfer.
  • Multimedia Message Service (MMS):
    • Multimedia Message Service allows transmission of multimedia contents like JPG, GIF and short videos.
  • Wireless Application Protocol (WAP):
    • Wireless Application Protocol is a special purpose communication protocol used for mobile browsers.

Summary:

 

GPRS is a packet-switched radio service, established to provide higher data rate compared to GSM. GSM focuses on voice service while GPRS concentrate on data transmission. GPRS is an extension to GSM, so it does not require special hardware, instead it requires only software installations and can provide service on same wireless network. GPRS also guarantee reliability and good QoS with the help of its protocol stack.

you can view video on GPRS

Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.