A Look At GPRS, HSCSD, and EDGE

Technologies like GPRS (General Packet Radio Service), High Speed Circuit Switched Data (HSCSD) and EDGE fulfill the requirements for packet data service and increased data rates in the existing GSM/TDMA networks. I'll talk about EDGE separately under the section "Migration To 3G". GPRS is actually an overlay over the existing GSM network, providing packet data sevices using the same air interface by the addition of two new network elements, the SGSN and GGSN, and a software upgrade. Although GPRS was basically designed for GSM networks, the IS-136 Time Division Multiple Access (TDMA) standard, popular in North and South America, will also support GPRS. This follows an agreement to follow the same evolution path towards third generation mobile phone networks concluded in early 1999 by the industry associations that support these two network types.

The General Packet Radio Service (GPRS)

The General Packet Radio Service (GPRS) is a wireless service that is designed to provide a foundation for a number of data services based on packet transmission. Customers will only be charged for the communication resources they use. The operator's most valuable resource, the radio spectrum, can be leveraged over multiple users simultaneously because it can support many more data users. Additionally more than one time slots can be used by a user to get higher data rates.

GPRS introduces two new major network nodes in the GSM PLMN:

Serving GPRS Support Node (SGSN) - The SGSN is the same hierarchical level as an MSC. The SGSN tracks packet capable mobile locations, performs security functions and access control. The SGSN is connected to the BSS via Frame Relay.

Gateway GPRS Support Node (GGSN) - The GGSN interfaces with external packet data networks (PDNs) to provide the routing destination for data to be delivered to the MS and to send mobile originated data to its intended destination. The GGSN is designed to provide inter-working with external packet switched networks, and is connected with SGSNs via an IP based GPRS backbone network.

A packet control unit is also required which may be placed at the BTS or at the BSC. A number of new interfaces have been defined between the existing network elements and the new elements and between the new network elements. Theoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today's fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. Actually we may not see speeds greater than 64 kbps however it would be much higher than the speeds possible in any 2G network. Also, another advantage is the fact that the user is always connected and is charged only for the amount of data transferred and not for the time he is connected to the network.

Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours.

Already many field trials and also some commercial GPRS implementations have taken place. GPRS is the evolution step that almost all GSM operators are considering. Also, coupled with other technologies like WAP, GPRS can act as a stepping stone towards convergence of cellular service providers and the internet service providers.

HSCSD (High speed circuit swiched data).

HSCSD (High speed circuit swiched data) is the evolution of circuit switched data within the GSM environment. HSCSD will enable the transmission of data over a GSM link at speeds of up to 57.6kbit/s. This is achieved by cocatenating, i.e. adding together, consecutive GSM timeslots, each of which is capable of supporting 14.4kbit/s. Up to four GSM timeslots are needed for the transmission of HSCSD. This allows theoretical speeds of up to 57.6 kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel. HSCSD is part of the planned evolution of the GSM specification and is included in the GSM Phase 2 development. In using HSCSD a permanent connection is established between the called and calling parties for the exchange of data. As it is circuit switched, HSCSD is more suited to applications such as video conferencing and multimedia than 'bursty' type applications such as email, which is more suited to packet switched data. In networks where High Speed Circuit Switched Data (HSCSD) is deployed, GPRS may only be assigned third priority, after voice as number one priority and HSCSD as number two. In theory, HSCSD can be preempted by voice calls- such that HSCSD calls can be reduced to one channel if voice calls are seeking to occupy these channels. HSCSD does not disrupt voice service availability, but it does affect GPRS. Even given preemption, it is difficult to see how HSCSD can be deployed in busy networks and still confer an agreeable user experience, i.e. continuously high data rate. HSCSD is therefore more likely to be deployed in start up networks or those with plenty of spare capacity since it is relatively inexpensive to deploy and can turn some spare channels into revenue streams.

An advantage for HSCSD could be the fact that while GPRS is complementary for communicating with other packet-based networks such as the Internet, HSCSD could be the best way of communicating with other circuit switched communications media such as the PSTN and ISDN. But one potential technical difficulty with High Speed Circuit Switched Data (HSCSD) arises because in a multi-timeslot environment, dynamic call transfer between different cells on a mobile network (called "handover") is complicated unless the same slots are available end-to-end throughout the duration of the Circuit Switched Data call.

Because of the way these technologies are evolving, the market need for high speed circuit switched data and the market response to GPRS, the mobile infrastructure vendors are not as committed to High Speed Circuit Switched Data (HSCSD) as they are to General Packet Radio Service (GPRS). So, we may only see HSCSD in isolated networks around the world. HSCSD may be used by operators with enough capacity to offer it at lower prices, such as Orange. [1] believes that every GSM operator in Europe will deploy GPRS, and by 2005 GPRS users will almost match the number of voice only users. Right now there are 300 million wireless phones in the world. By 2005 we expect one billion.

Before I proceed, a quick look at the table below would help you appreciate and understand clearly the technology characterizations as 2nd generation, 2.5 generation and 3G. We have looked into 2G and some 2.5G technologies so far.

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