1、 毕业设计说明书（英文翻译） 3 General Packet Radio Service General packet radio service (GPRS) is a packet oriented mobile data service available to users of the 2G cellular communication systems global system for mobile communications (GSM), as well as in the 3G systems. In 2G systems, GPRS provides data rates
2、of 56-114 kbit/s. GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is using the capacity or is in an idle state. GPRS is a best-eff
3、ort packet switched service, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection for non-mobile users. 2G cellular systems combined with GPRS are often described as 2.5G, that is, a technology between the second (2G) and third (3G) generation
4、s of mobile telephony. It provides moderate speed data transfer, by using unused time division multiple access (TDMA) channels in, for example, the GSM system. Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM sta
5、ndard, so that GSM is the only kind of network where GPRS is in use. GPRS is integrated into GSM Release 97 and newer releases. It was originally standardized by European Telecommunications Standards Institute (ETSI), but now by the 3rd Generation Partnership Project (3GPP) . GPRS was developed as a
6、 GSM response to the earlier CDPD and i-mode packet switched cellular technologies. Technical overview Services offered GPRS extends the GSM circuit switched data capabilities and makes the following services possible: Always on internet access Multimedia messaging service (MMS) Push to talk over ce
7、llular (PoC/PTT) Instant messaging and presencewireless village Internet applications for smart devices through wireless application protocol (WAP) Point-to-point (P2P) service: inter-networking with the Internet (IP) If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per m
8、inute may be achieved. This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute. Protocols supported GPRS supports the following protocols: internet protocol (IP). In practice, mobile built-in browsers use IPv4 since IPv6 is not
9、yet popular. point-to-point protocol (PPP). In this mode PPP is often not supported by the mobile phone operator but if the mobile is used as a modem to the connected computer, PPP is used to tunnel IP to the phone. This allows an IP address to be assigned dynamically to the mobile equipment. X.25 c
10、onnections. This is 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 doing this requires either a network based router to perform encapsulation or intelligence built in to the e
11、nd-device/terminal； e.g., user equipment (UE). When TCP/IP is used, each phone can have one or more IP addresses allocated. GPRS will store and forward the IP packets to the phone during cell handover (when you move from one cell to another). TCP handles any packet loss (e.g. due to a radio noise in
12、duced pause) 毕业设计说明书（英文翻译） 4 resulting in a temporary throttling in transmission speed. Hardware Devices supporting GPRS are divided into three classes: Class A : Can be connected to GPRS service and GSM service (voice, SMS), using both at the same time. Such devices are known to be available today.
13、 Class B : Can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time. During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded. Most GPRS mobile devic
14、es are Class B. Class C : Are connected to either GPRS service or GSM service (voice, SMS). Must be switched manually between one or the other service. A true Class A device may be required to transmit on two different frequencies at the same time, and thus will need two radios. To get around this e
15、xpensive requirement, a GPRS mobile may implement the dual transfer mode (DTM) feature. A DTM-capable mobile may use simultaneous voice and packet data, with the network coordinating to ensure that it is not required to transmit on two different frequencies at the same time. Such mobiles are conside
16、red pseudo-Class A, sometimes referred to as simple class A. Some networks are expected to support DTM in 2007. USB GPRS modems use a terminal-like interface USB 2.0 and later, data formats V.42bis, and RFC 1144 and external antennas. Modems can be added as cards (for laptops) or external USB device
17、s which are similar in shape and size to a computer mouse. 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 maximum capability of the mobile device expressed as
18、a GPRS multislot class the channel encoding used summarised in the following table. The least robust, but fastest, coding scheme (CS-4) is available near a base transceiver station (BTS), while the most robust coding scheme (CS-1) is used when the mobile station (MS) is further away from a BTS. Usin
19、g the 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 automati
20、cally depending on the mobile location. In addition to GPRS, there are two other GSM technologies which deliver data services: circuit-switched data (CSD) and high-speed circuit-switched data (HSCSD). In contrast to the shared nature of GPRS, these instead establish a dedicated circuit (usually bill
21、ed per minute). 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. Coding scheme Speed (kbit/s) CS-1 8.0 CS-2 12.0
22、CS-3 14.4 CS-4 20.0 毕业设计说明书（英文翻译） 5 Technology Download (kbit/s) Upload (kbit/s) TDMA Timeslots allocated CSD 9.6 9.6 1+1 HSCSD 28.8 14.4 2+1 HSCSD 43.2 14.4 3+1 GPRS 80.0 20.0 (Class 8 & 10 and CS-4) 4+1 GPRS 60.0 40.0 (Class 10 and CS-4) 3+2 EGPRS (EDGE) 236.8 59.2 (Class 8, 10 and MCS-9) 4+1 EGPR
23、S (EDGE) 177.6 118.4 (Class 10 and MCS-9) 3+2 Multiple access schemes The multiple access methods used in GSM with GPRS are based on frequency division duplex (FDD) and TDMA. During a session, a user is assigned to one pair of up-link and down-link frequency channels. This is combined with time doma
24、in statistical multiplexing； i.e., packet mode communication, which makes it possible for several users to share the same frequency channel. The packets have constant length, corresponding to a GSM time slot. The down-link uses first-come first-served packet scheduling, while the up-link uses a sche
25、me very similar to reservation ALOHA (R-ALOHA). This means that slotted ALOHA (S-ALOHA) is used for reservation inquiries during a contention phase, and then the actual data is transferred using dynamic TDMA with first-come first-served scheduling. Addressing A GPRS connection is established by refe
26、rence to its access point name (APN). The APN defines the services such as wireless application protocol (WAP) access, short message service (SMS), multimedia messaging service (MMS), and for Internet communication services such as email and World Wide Web access. In order to set up a GPRS connectio
27、n for a wireless modem, a user must specify an APN, optionally a user name and password, and very rarely an IP address, all provided by the network operator. Usability The maximum speed of a GPRS connection offered in 2003 was similar to a modem connection in an analog wire telephone network, about
28、32-40 kbit/s, depending on the phone used. Latency is very high； round-trip time (RTT) is typically about 600-700 ms and often reaches 1 s. GPRS is typically prioritized lower than speech, and thus the quality of connection varies greatly. Devices with latency/RTT improvements (via, for example, the extended UL TBF mode feature) are generally available. Also, network upgrades of features are available with certain operators. With these enhancements the active round-trip time can be reduced, resulting in significant increase in application-level throughput speeds.
