HSCSD and GPRS: GPRS vs High-Speed Circuit Switched Data

High-Speed Circuit Switched Data (HSCSD) and General Packet Radio Service (GPRS) are network enhancements made to the second-generation (2G) GSM networks. The HSCSD technology is now obsolete; however, GPRS is still around and can be seen on mobile phones as the “G” symbol when 2G GSM networks serve a phone.

HSCSD (High-Speed Circuit-Switched Data) was an enhancement to Circuit Switched Data (CSD), a mobile data technology in GSM networks that offered peak speeds of up to 64 kbps. GPRS (General Packet Radio Service) is a packet-switched mobile data technology that offers peak speeds of up to 171.2 kbps.

Difference between HSCSD and GPRS technologies

When 2G GSM networks (Global System for Mobile Communications) initially started, they could only deliver voice calling and text messaging. Mobile data capability required for enabling the internet on the phone through cellular networks was not available at the time.

Circuit-Switched Data (CSD) technology was later introduced to allow users to access basic internet connectivity through their mobile phones. CSD, like other circuit-switched services in GSM, including voice and text, relied on dedicated timeslots for enabling the internet. It essentially meant that if a user wanted to access the internet services through CSD, a timeslot would be occupied for the entire duration of their data session.

While this approach was not efficient, it was the only way to allow customers to access the internet through their GSM phones at the time. CSD offered maximum download speeds of up to 9.6 kbps through a single timeslot.

High-Speed Circuit-Switched Data (HSCSD) was an enhancement to the existing Circuit-Switched Data (CSD) technology to improve the data rates. HSCSD achieved higher data rates than CSD by employing superior error-coding techniques to extract higher bit rates out of a single timeslot.

As a result, HSCSD increased the data rates from 9.6 kbps per timeslot to 14.4 kbps per timeslot. Unlike CSD, HSCSD could use multiple timeslots, which allowed it to offer a maximum download speed of up to 64 kbps. However, the use of multiple timeslots required upgrades to GSM mobile phones.

General Packet Radio Service (GPRS) is a packet-switched technology and a successor of HSCSD. While HSCSD technology is now obsolete, GPRS is still available on GSM mobile networks. GPRS can offer peak download speeds of up to 171.2 kbps using packet-switched technology. The introduction of GPRS networks required significant changes to the GSM network architecture.

HSCSD uses circuit-switched technology to enable mobile data

At the start of their journey, second-generation GSM networks were based on circuit-switched technology to enable voice calls and text messages (SMS – Short Message Service). However, the circuit-switched technique does not efficiently utilise the radio network resources because it permanently engages the resources for the entire session duration.

The switching capability was provided by the Mobile Switching Centre (MSC) only, which was connected to external telephone networks like PSTN (Public Switched Telephone Network) and ISDN (Integrated Services Digital Network). In addition, another network entity SMSC (Short Message Service Centre), enabled users to send and receive text messages or SMS (Short Message Service).

HSCSD or High-Speed Circuit-Switched Data was an enhancement to the Circuit-Switched Data (CSD) technology that offered mobile data (internet) through a single dedicated timeslot in GSM networks.

Since HSCSD could utilise the existing circuit-switched network, it was a quick win for mobile operators as it provided them with the opportunity to offer peak download speeds of up to 64 kbps. Mobile operators could offer HSCSD in GSM networks through minor modifications to the existing GSM network architecture.

However, since HSCSD uses multiple timeslots to provide mobile data, a multi-slot capability was also required in GSM phones to receive communication through multiple timeslots.

GSM networks use a combination of FDMA (Frequency Divison Multiple Access) and TDMA (Time Division Multiple Access) for wireless connectivity. The frequency channel in GSM networks is called ARFCN (Absolute Radio Frequency Channel Number), which has eight (8) timeslots. Regular voice calls on a mobile phone can be established on a single timeslot at any given time.

A single timeslot for mobile data services (internet) could enable up to 9.6 kbps with CSD and 14.4 kbps after the HSCSD enhancement. When using all eight timeslots, the HSCSD technology could potentially deliver a maximum data rate of 14.4 kbps x 8 timeslots = 115.2 kbps. However, due to radio network limitations, HSCSD could only offer a maximum throughput of up to 64 kbps.

GPRS uses packet-switched technology to enable mobile data

The HSCSD enhancement was a quick win for mobile operators because it required minor modifications to the existing circuit-switched network to deliver mobile data. However, from an operational perspective, HSCSD was not efficient as it permanently engaged the radio network resources for the entire data session duration.

Since a mobile user does not download or upload all the time during a session, dedicating timeslots for the whole session duration was unnecessary and operationally expensive. Since the charging and billing were linked to the connection duration, the solution was expensive for customers also.

A paradigm shift was needed to overcome this challenge which led to the introduction of the packet-switched technology in GSM networks. This was achieved through General Packet Radio Service (GPRS) in second-generation GSM networks a few years before the launch of 3G networks.

The GPRS technology that still exists today (2022) is aligned with the bursty nature of internet services. Generally, internet users download and upload intermittently, but there are also times when they are idle, i.e. neither downloading nor uploading at any given time. GPRS supports this bursty internet usage pattern as it does not permanently engage the radio network resources for the entire session duration.

GPRS required GSM core network architecture changes and introduced two additional nodes Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). GPRS can enable peak download data rates of up to 171.2 kbps when using all eight timeslots.

After GPRS, another enhancement EDGE (Enhanced Data rates for Global Evolution) was introduced to upgrade the packet-switched GPRS and circuit-switched HSCSD. Within the scope of EDGE, Enhanced Circuit Switched Data (ECSD) improved the data rate per timeslot for HSCSD.

EDGE also included the E-GPRS (Enhanced GPRS) enhancement for GPRS to improve the peak data rates to up to 384 kbps. I have written a dedicated post on GPRS, E-GRPS and EDGE that provides a detailed comparison of these technologies.

Conclusion

HSCSD and GPRS are two different technologies introduced in the second-generation (2G) GSM networks to deliver mobile internet. HSCSD stands for High-Speed Circuit Switched Data, and it was an enhancement to the original GSM mobile data technology, Circuit Switched Data (CSD). As a multi-slot data technology, HSCSD could deliver a maximum download speed of up to 64 kbps. On the other hand, GPRS stands for General Packet Radio Service, and it introduced the packet-switched technology in GSM networks. GPRS is highly efficient compared to HSCSD and enables peak download data rates of up to 171.2 kbps.

Here are some helpful downloads

Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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