GPRS
General Packet Radio Service, also called 2.5G, is a mobile data standard that is part of the 2G cellular communication network Global System for Mobile Communications. Networks and mobile devices with GPRS started to roll out around the year 2001; it offered, for the first time on GSM networks, seamless data transmission using packet-switched data for an "always-on" connection, eliminating the need to dial up, providing improved Internet access for web, email, Wireless Application Protocol services, Short Message Service, Multimedia Messaging Service and others.
Up until the rollout of GPRS, only circuit-switched data was used in cellular networks, meaning that one or more radio channels were occupied for the entire duration of a data connection. On the other hand, on GPRS networks, data is broken into small packets and transmitted through available channels. This increased efficiency also gives it theoretical data rates of 56–114 kbit/s, significantly faster than the preceding Circuit Switched Data technology. GPRS was succeeded by EDGE which provided improved performance and speeds on the 2G GSM system.
Technical overview
The GPRS core network allows 2G, 3G and W-CDMA mobile networks to transmit IP packets to external networks such as the Internet. The GPRS system is an integrated part of the GSM network switching subsystem.GPRS is a best-effort service, implying variable throughput and latency that depend on the number of other users sharing the service concurrently, as opposed to circuit switching, where a certain quality of service is guaranteed during the connection. It uses unused time-division multiple access channels in the GSM system for efficiency. Unlike older circuit-switched data, GPRS was sold according to the total volume of data transferred instead of time spent online, which is now standard.
Services offered
GPRS extends the GSM Circuit Switched Data capabilities and makes the following services possible:- SMS messaging and broadcasting
- MMS
- "Always on" Internet access
- Push-to-talk over cellular
- Instant messaging and presence—wireless village
- Internet applications for smart devices through WAP
- Point-to-point service: inter-networking with the Internet
- Point-to-multipoint service: point-to-multipoint multicast and point-to-multipoint group calls
Frequencies
As the GPRS standard is an extension of GSM capabilities, the service operates on the 2G and 3G GSM frequencies. GPRS devices can typically use of the frequencies within one of the frequency bands the radio supports. Depending on the device, location, and intended use, regulations may be imposed either restricting or explicitly specifying authorised frequency bands.GSM-850 and GSM-1900 are used in the United States, Canada, and many other countries in the Americas. GSM-900 and GSM-1800 are used in Europe, the Middle East, Africa and most of Asia. In South Americas these bands are used in Costa Rica, Brazil, Guatemala, El Salvador. There is a more comprehensive record of
Protocols supported
GPRS supports the following protocols:- Internet Protocol. In practice, built-in mobile browsers use IPv4 before IPv6 is widespread.
- Point-to-Point Protocol is typically not supported by mobile phone operators, but if a cellular phone is used as a modem for a connected computer, PPP may be used to tunnel IP to the phone. This allows an IP address to be dynamically assigned to the mobile equipment using IPCP rather than DHCP.
- X.25 connections are typically used for applications like wireless payment terminals, although it has been removed from the standard. X.25 can still be supported over PPP, or even over IP, but this requires either a network-based router to perform encapsulation or software built into the end-device/terminal.
Hardware
Devices supporting GPRS are grouped into three classes:;Class A: Can be connected to GPRS service and GSM service simultaneously.
;Class B: Can be connected to GPRS service and GSM service, but using only one at a time. During GSM service, GPRS service is suspended and resumed automatically after the GSM service has concluded. Most GPRS mobile devices are Class B.
;Class C: Are connected to either GPRS service or GSM service and must be switched manually between one service and the other.
Because a Class A device must service GPRS and GSM networks together, it effectively needs two radios. To avoid this hardware requirement, a GPRS mobile device may implement the Dual Transfer Mode feature. A DTM-capable mobile can handle both GSM packets and GPRS packets with network coordination to ensure both types are not transmitted at the same time. Such devices are considered pseudo-Class A, sometimes referred to as "simple class A".
Image:Huawei E220.jpg|right|thumb|Huawei E220 3G/GPRS Modem
USB 3G/GPRS modems have a terminal-like interface over USB with V.42bis, and data formats. Some models include an external antenna connector. Modem cards for laptop PCs, or external USB modems are available, similar in shape and size to a computer mouse, or a pendrive.
Addressing
A GPRS connection is established by reference to its Access Point Name. The APN defines the services such as WAP, SMS, MMS, and Internet services such as email and World Wide Web access.In order to set up a GPRS connection for a wireless modem, a user must specify an APN, optionally a user name and password, and very rarely an IP address, provided by the network operator.
Coding schemes and speeds
The upload and download speeds that can be achieved in GPRS depend on a number of factors such as:- the number of BTS TDMA time slots assigned by the operator
- the channel encoding used.
- the maximum capability of the mobile device expressed as a GPRS Multislot Class
Multiple access schemes
Channel coding
The channel coding process in GPRS consists of two steps: first, a cyclic code is used to add parity bits, which are also referred to as the Block Check Sequence, followed by coding with a possibly-punctured convolutional code. The Coding Schemes CS-1 to CS-4 specify the number of parity bits generated by the cyclic code and the puncturing rate of the convolutional code. In Coding Schemes CS-1 through CS-3, the convolutional code is of rate 1/2; i.e., each input bit is converted into two coded bits. In Coding Schemes CS-2 and CS-3, the output of the convolutional code is punctured to achieve the desired code rate. In Coding Scheme CS-4, no convolutional coding is applied. The following table summarises the options.| GPRS Coding scheme | Bitrate including RLC/MAC overhead | Bitrate excluding RLC/MAC overhead | Modulation | Code rate |
| CS-1 | 9.20 | 8.00 | GMSK | 1/2 |
| CS-2 | 13.55 | 12.00 | GMSK | ≈2/3 |
| CS-3 | 15.75 | 14.40 | GMSK | ≈3/4 |
| CS-4 | 21.55 | 20.00 | GMSK | 1 |
The least robust, but fastest, coding scheme is available near a BTS, while the most robust coding scheme is used when the mobile station is further away from a BTS.
Using CS-4 it is possible to achieve a user speed of 20.0 kbit/s per time slot. However, using this scheme the cell coverage is 25% of normal. CS-1 can achieve a user speed of only 8.0 kbit/s per time slot, but has 98% of normal coverage. Newer network equipment can adapt the transfer speed automatically depending on the mobile location.
In addition to GPRS, there are two other GSM technologies which deliver data services: Circuit Switched Data and High-Speed Circuit-Switched Data. In contrast to the shared nature of GPRS, these instead establish a dedicated circuit. Some applications such as video calling may prefer HSCSD, especially when there is a continuous flow of data between the endpoints.
The following table summarises some possible configurations of GPRS and circuit switched data services.
Multislot Class
The multislot class determines the speed of data transfer available in the uplink and downlink directions. It is a value between 1 and 45 which the network uses to allocate radio channels in the uplink and downlink direction. Multislot class with values greater than 31 are referred to as high multislot classes.A multislot allocation is represented as, for example, 5+2. The first number is the number of downlink timeslots and the second is the number of uplink timeslots allocated for use by the mobile station. A commonly used value is class 10 for many GPRS/EGPRS mobiles which uses a maximum of 4 timeslots in downlink direction and 2 timeslots in uplink direction. However simultaneously a maximum number of 5 simultaneous timeslots can be used in both uplink and downlink. The network will automatically configure for either 3+2 or 4+1 operation depending on the nature of data transfer.
Some high end mobiles, usually also supporting UMTS, also support GPRS/EDGE multislot class 32. According to 3GPP TS 45.002, Table B.1, mobile stations of this class support 5 timeslots in downlink and 3 timeslots in uplink with a maximum number of 6 simultaneously used timeslots. If data traffic is concentrated in downlink direction the network will configure the connection for 5+1 operation. When more data is transferred in the uplink the network can at any time change the constellation to 4+2 or 3+3. Under the best reception conditions, i.e. when the best EDGE modulation and coding scheme can be used, 5 timeslots can carry a bandwidth of 5*59.2 kbit/s = 296 kbit/s. In uplink direction, 3 timeslots can carry a bandwidth of 3*59.2 kbit/s = 177.6 kbit/s.