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GPRS
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=== Channel encoding === The channel encoding 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]].<ref name="TS45001">{{cite web |url=http://www.3gpp.org/ftp/Specs/html-info/45001.htm |title=3GGP TS45.001: Technical Specification Group GSM/EDGE Radio Access Network; Physical layer on the radio path; General description |author=3rd Generation Partnership Project |version=12.1.0 |date=November 2014 |access-date=2015-12-05}}</ref> 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.<ref name="TS45001"/> 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.<ref name="TS45001"/> In Coding Schemes CS-2 and CS-3, the output of the convolutional code is [[punctured code|punctured]] to achieve the desired code rate.<ref name="TS45001"/> In Coding Scheme CS-4, no convolutional coding is applied.<ref name="TS45001"/> The following table summarises the options. {| class="wikitable" align=center ! GPRS<br />Coding scheme ! Bitrate including RLC/MAC overhead{{efn|name=gross|This is rate at which the RLC/MAC layer [[protocol data unit]] (PDU) (called a radio block) is transmitted. As shown in TS 44.060 section 10.0a.1,<ref name="TS44060-section-10.0a.1">{{cite web |url=http://www.3gpp.org/ftp/Specs/html-info/45001.htm |title=3GGP TS45.001: Technical Specification Group GSM/EDGE Radio Access Network; Mobile Station (MS) - Base Station System (BSS) interface; Radio Link Control / Medium Access Control (RLC/MAC) protocol; section 10.0a.1 - GPRS RLC/MAC block for data transfer |author=3rd Generation Partnership Project |version=12.5.0 |date=June 2015 |access-date=2015-12-05}}</ref> a radio block consists of MAC header, RLC header, RLC data unit and spare bits. The RLC data unit represents the payload, the rest is overhead. The radio block is coded by the convolutional code specified for a particular Coding Scheme, which yields the same PHY layer data rate for all Coding Schemes.}}{{efn|name=usf|Cited in various sources, e.g. in TS 45.001 table 1.<ref name="TS45001"/> is the bitrate including the RLC/MAC headers, but excluding the uplink state flag (USF), which is part of the MAC header,<ref name="TS44060-section-10.2.1">{{cite web |url=http://www.3gpp.org/ftp/Specs/html-info/45001.htm |title=3GGP TS45.001: Technical Specification Group GSM/EDGE Radio Access Network; Mobile Station (MS) - Base Station System (BSS) interface; Radio Link Control / Medium Access Control (RLC/MAC) protocol; section 10.2.1 - Downlink RLC data block |author=3rd Generation Partnership Project |version=12.5.0 |date=June 2015 |access-date=2015-12-05}}</ref> yielding a bitrate that is 0.15 kbit/s lower.}}<br />(kbit/s/slot) ! Bitrate excluding RLC/MAC overhead{{efn|name=net|The net bitrate here is the rate at which the RLC/MAC layer payload (the RLC data unit) is transmitted. As such, this bit rate excludes the header overhead from the RLC/MAC layers.}}<br />(kbit/s/slot) ! [[Modulation]] ! Code rate |- align=center | CS-1 | 9.20 | 8.00 | GMSK | 1/2 |- align=center | CS-2 | 13.55 | 12.00 | GMSK | β2/3 |- align=center | CS-3 | 15.75 | 14.40 | GMSK | β3/4 |- align=center | CS-4 | 21.55 | 20.00 | GMSK | 1 |} {{notelist}} The least robust, but fastest, coding scheme (CS-4) is available near a [[Base Transceiver Station|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. 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 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|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 billed 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. :{| class="wikitable" align=center ! Technology ! Download (kbit/s) ! Upload (kbit/s) ! TDMA timeslots allocated (DL+UL) |- style="text-align:center;" | CSD | 9.6 | 9.6 | 1+1 |- style="text-align:center;" | HSCSD | 28.8 | 14.4 | 2+1 |- style="text-align:center;" | HSCSD | 43.2 | 14.4 | 3+1 |- style="text-align:center;" | GPRS | 85.6 | 21.4 (Class 8 & 10 and CS-4) | 4+1 |- style="text-align:center;" | GPRS | 64.2 | 42.8 (Class 10 and CS-4) | 3+2 |- style="text-align:center;" | [[EGPRS]] (EDGE) | 236.8 | 59.2 (Class 8, 10 and MCS-9) | 4+1 |- style="text-align:center;" | [[EGPRS]] (EDGE) | 177.6 | 118.4 (Class 10 and MCS-9) | 3+2 |}
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