GPRS and EDGE
<伊落丹> illidan.modeler [at] gmail.com
Northern Capital, Republic of Pandaren

Of the Net, by the Net, for the Net

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 the 2G systems, GPRS provides data rates of 56-114 kbit/s.2G cellular systems combined with GPRS are often described as 2.5G. It provides moderate speed data transfer, by using unused time division multiple access (TDMA) channels.

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-effort packet switched service, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection for non-mobile users.

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 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 GSM response to the earlier CDPD and i-mode packet switched cellular technologies.

The diagram that depicts the interfaces and key components is shown below;

Interfaces in the GPRS network

Gb
    Interface between the base station subsystem and the SGSN the transmission protocol could be Frame Relay or IP.
Gn
    IP Based interface between SGSN and other SGSNs and (internal) GGSNs. DNS also shares this interface. Uses the GTP Protocol.
Gp
    IP based interface between internal SGSN and external GGSNs. Between the SGSN and the external GGSN, there is the border gateway (which is essentially a firewall). Also uses the GTP Protocol.
Ga
    The interface servers the CDRs (accounting records) which are written in the GSN and sent to the charging gateway (CG). This interface uses a GTP-based protocol, with modifications that supports CDRs (Called GTP' or GTP prime).
Gr
    Interface between the SGSN and the HLR. Messages going through this interface uses the MAP3 protocol.
Gd
    Interface between the SGSN and the SMS Gateway. Can use MAP1, MAP2 or MAP3.
Gs
    Interface between the SGSN and the MSC (VLR). Uses the BSSAP+ protocol. This interface allows paging and station availability when it performs data transfer. When the station is attached to the GPRS network, the SGSN keeps track of which routing area (RA) the station is attached to. An RA is a part of a larger location area (LA). When a station is paged this information is used to conserve network resources. When the station performs a PDP context, the SGSN has the exact BTS the station is using.
Gi
    IP based interface between the GGSN and a public data network (PDN) either directly to the Internet or through a WAP gateway.
Ge
    The interface between the SGSN and the service control point (SCP); uses the CAP protocol.
Gx
    The on-line policy interface between the GGSN and the charging rules function (CRF). It is used for provisioning service data flow based charging rules. Uses the diameter protocol.
Gy
    The on-line charging interface between the GGSN and the online charging system (OCS). Uses the diameter protocol (DCCA application).
Gz
    The off-line (CDR-based) charging interface between the GSN and the CG. Uses GTP'.
Gmb
    The interface between the GGSN and the broadcast-multicast service center (BM-SC), used for controlling MBMS bearers..

Enhanced Data Rates for GSM Evolution (EDGE)

Transmission techniques

In addition to Gaussian minimum-shift keying (GMSK), EDGE uses higher-order PSK/8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.

EDGE can carry data speeds up to 236.8 kbit/s (with end-to-end latency of less than 150 ms) for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots) in packet mode. This means it can handle four times as much traffic as standard GPRS. EDGE meets the International Telecommunications Union's requirement for a 3G network, and has been accepted by the ITU as part of the IMT-2000 family of 3G standards. It also enhances the circuit data mode called HSCSD, increasing the data rate of this service.

1. [Web] "GPRS Core Network." Wikipedia, The Free Encyclopedia. 25 Apr 2009, 11:47 UTC. 28 Apr 2009 <http://en.wikipedia.org/w/index.php?title=GPRS_Core_Network&oldid=286027496>.
2. [Web] "Enhanced Data Rates for GSM Evolution." Wikipedia, The Free Encyclopedia. 16 Apr 2009, 22:13 UTC. 17 Apr 2009 <http://en.wikipedia.org/w/index.php?title=Enhanced_Data_Rates_for_GSM_Evolution&oldid=284298553>.