专业术语-GPRS
General Packet Radio Service (GPRS) is a mobile data service available to users of GSM mobile phones. It is often described as "2.5G", that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused TDMA channels in the GSM network. Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that is the only kind of network where GPRS is in use. GPRS is integrated into GSM standards releases starting with Release 97 and onwards. First it was standardised by ETSI but now that effort has been handed onto the 3GPP.
GPRS basics
GPRS is different from the older Circuit Switched Data (or CSD) connection included in GSM standards releases before Release 97 (from 1997, the year the standard was feature frozen). In CSD, a data connection establishes a circuit, and reserves the full bandwidth of that circuit during the lifetime of the connection. GPRS is packet-switched which means that multiple users share the same transmission channel, only transmitting when they have data to send. This means that the total available bandwidth can be immediately dedicated 1 to those users who are actually sending at any given moment, providing higher utilisation where users only send or receive data intermittently 3. Web browsing 4, receiving e-mails as they arrive and instant messaging are examples of uses that require intermittent 2 data transfers, which benefit from sharing the available bandwidth.
Usually, GPRS data are billed per kilobytes of information transceived while circuit-switched data connections are billed per second. The latter is to reflect the fact that even during times when no data are being transferred, the bandwidth is unavailable to other potential users.
GPRS originally supported (in theory) IP, PPP and X.25 connections. The latter has been typically used for applications like wireless 5 payment terminals although it has been removed as a requirement from the standard. X.25 can still be supported over PPP, or even over IP, but doing this requires either a router to do encapsulation or intelligence built into the end terminal.
GPRS speeds and profile
Packet-switched data under GPRS is achieved by allocating 6 unused cell bandwidth to transmit data. As dedicated voice (or data) channels are setup by phones, the bandwidth available for packet switched data shrinks. A consequence of this is that packet switched data has a poor bit rate in busy cells. The theoretical limit for packet switched data is approx. 160.0 kbit/s (using 8 time slots and CS-4). A realistic bit rate is 30–80 kbit/s, because it is possible to use max 4 time slots for downlink. A change to the radio part of GPRS called EDGE allows higher bit rates of between 160 and 236.8 kbit/s. The maximum data rates are achieved only by allocation of more than one time slot in the TDMA frame. Also, the higher the data rate, the lower the error correction capability 7. Generally, the connection speed drops logarithmically with distance from the base station. This is not an issue in heavily populated areas with high cell density 8, but may become an issue in sparsely 9 populated/rural areas.
The GPRS Capability Classes
Class A
Can be connected to GPRS service and GSM service (voice, SMS), using both at the same time. No such devices are known to be available today.
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 devices 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.
GPRS Multislot Classes
GPRS speed is a direct function of the number of TDMA time slots assigned, which is the lesser 10 of (a) what the particular cell supports and (b) the maximum capability of the mobile device expressed as a GPRS Multislot Class.
The most common GPRS Multislot Classes are:
Class 2
Minimal 11 GPRS implementation 12
Class 4
Modest GPRS implementation, 50% faster download than Class 2
Class 6
Modest implementation, but with better uploading than Class 4
Class 8
Better implementation, 33% faster download than Classes 4 & 6
Class 10
Better implementation, and with better uploading than Class 8, seen in better cell phones and PC Cards
Class 12
Best implementation, with maximum upload performance, typically seen only in high-end PC Cards
GPRS Coding
Transfer speed depends also on the channel encoding used. The least robust 13 (but fastest) encoding scheme (CS-4) is available near the Base Transceiver Station (BTS) while the most robust encoding scheme (CS-1) is used when the Mobile Station (MS) is further away from the BTS.
Using 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 14 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.
Speed (kbit/s)
CS-1
8.0
CS-2
12.0
CS-3
14.4
CS-4
20.0
Download (kbit/s)
Upload (kbit/s)
CSD
9.6
9.6
HSCSD
28.8
14.4 (2+1)
HSCSD
43.2
14.4 (3+1)
GPRS 4+1
80.0
20.0 (Class 8 & 10 and CS-4)
GPRS 3+2
60.0
40.0 (Class 10 and CS-4)
Note: Like CSD, HSCSD establishes a circuit and is usually billed per minute. For an application such as downloading, HSCSD may be preferred, since circuit-switched data are usually given priority over packet-switched data on a mobile network, and there are relatively 15 few seconds when no data are being transferred.
GPRS is packet based. When TCP/IP is used, each phone can have one or more IP addresses allocated 16. GPRS will store and forward the IP packets to the phone during cell handover (when you move from one cell to another). A radio noise induced pause can be interpreted by TCP as packet loss, and cause a temporary throttling 17 in transmission speed.
GPRS Services
GPRS upgrades GSM data services providing:
Point-to-point (PTP) service: internetworking with the Internet (IP protocols 18) and X.25 networks.
Point-to-multipoint (PT2MP) service: point-to-multipoint multicast and point-to-multipoint group calls.
Short Message Service (SMS): bearer for SMS.
Anonymous 19 service: anonymous access to predefined services.
Future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.
GPRS in practice
Telephone operators have priced GPRS relatively cheaply (compared to older GSM data transfer, CSD and HSCSD) in many areas, such as Finland. Most mobile phone operators don't offer flat rate access to the Internet (with the notable exceptions of T-Mobile and Cingular in the United States), instead basing their tariffs 20 on data transferred, usually rounded off per 100 kilobyte.
Typical rates vary wildly, ranging from EUR
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