Having established the requirement for data services on mobile phones with GPRS, the next evolution was GSM EDGE, Enhanced Data rates for GSM Evolution.
This GSM EDGE evolution provided a higher data rate of 384 kbps primarily by using a new form of modulation, along with a number of other enhancements.
In view of the further evolution, GSM EDGE was often referred to as being 2.5G - it formed the bridge between 2G and the 3G technology that was starting to be developed at the time.
What was EDGE? - the basics
GSM EDGE cellular technology was an evolutionary upgrade to the existing GSM / GPRS networks, and could often be implemented as a software upgrade to existing GSM / GPRS networks.
EDGE evolved the technology of the two previous cellular schemes:
- GSM: GSM was the foundational technology on which GSM EDGE was based. It provided the basic voice standard that was the mainstay of mobile phone systems of the time.
Note on 2G GSM:
GSM - Global System for Mobile communications was one of the second generation mobile phone systems. Using digital technology transmitting voice via data, it became the most widely used 2G system, gaining billions of subscribers.
Read more about 2G GSM.
- GPRS: GPRS was the first major evolution of GSM which provided the packet data capability, enabling data to be carried over the network via a usable channel.
Note on 2G GPRS:
GPRS - General Packet Radio System introduced packet data to the GSM cell phone system. Although data was slow by current standards, it enabled data to become a main part of the system.
Read more about 2G GPRS.
This made the EDGE evolution a particularly attractive option providing virtually basic 3G data rates for a small upgrade to an existing GPRS network. The level of capital expenditure required was very low.
GSM EDGE evolution was able to provide data rates of up to 384 kbps, and this meant that it offered a significantly higher data rate than GPRS.
There were a number of key elements in the evolution from GSM or GPRS to EDGE. The GSM EDGE technology required a number of new elements to be added to the system:
- Use of 8PSK modulation: In order to achieve the higher data rates within GSM EDGE, the modulation format was changed from GMSK to 8PSK. This provided a significant advantage in being able to convey 3 bits per symbol, thereby increasing the maximum data rate. This upgrade required a change to the base station. Sometimes hardware upgrades were required, although it was often simply a software change.
- Base station: Apart from the upgrade to incorporate the 8PSK modulation capability, other small changes were required to the base station. These were normally relatively small and could often be accomplished by software upgrades.
- Upgrade to network architecture: GSM EDGE provided the capability for IP based data transfer. As a result, additional network elements were required. These were the same as those needed for GPRS and later for UMTS. In this way the introduction of EDGE technology is part of the overall migration path from GSM to UMTS.
The two main additional nodes required for the network were the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). The GGSN connected to packet-switched networks such as the Internet and other GPRS networks. The SGSN provided the packet-switched link to mobile stations.
- Mobile stations: It was necessary to have a GSM EDGE handset that is EDGE compatible. As it was not possible to upgrade handsets, this meant that the user had to buy a new GSM EDGE handset.
Despite the number of changes that need to be made, the cost of the upgrade to move to GSM EDGE cellular technology was normally relatively small. The elements in the network required for GPRS may already have been in place and therefore not needed fort he EGE upgrade.
The new network entities were also needed for 3G UMTS and therefore they were on the overall evolution path. Other changes to the base stations were comparatively small and could often be achieved very easily.
GSM EDGE evolution highlights
It is worth summarizing the key parameters of GSM EDGE cellular technology.
|GSM EDGE specification highlights
|Multiple Access Technology||FDMA / TDMA|
|Channel Spacing||200 kHz|
|Slots per channel||8|
|Frame duration||4.615 ms|
|Latency||Better than 100 ms|
|Overall symbol rate||270 k symbols / s|
|Overall modulation bit rate||810 kbps|
|Radio data rate per time slot||69.2 kbps|
|Max user data rate per time slot||59.2 kbps (MCS-9)|
|Max user data rate when using 8 time slots||473.6 kbps *|
* A maximum user data rate of 384 kbps is often seen quoted as the data rate for GSM EDGE. This data rate corresponds to the International Telecommunications Union (ITU) definition of the data rate limit required for a service to fulfil the International Mobile Telecommunications-2000 (IMT-2000) standard(i.e. 3G) in a pedestrian environment.
The 2G GM EDGE evolution provided a significant improvement in performance over GPRS, although it was still short of the performance of 3G UMTS even in its basic form.
For many years, the GSM EDGE evolution provided a fall-back when 3G, and later even 4G coverage was not available. Although very much slower, it did provide some form of connectivity and as such remained in use for many years.
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