2.5G (GPRS - General Packet Radio Service) - 1.2.5.1

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The core innovation of GPRS was the shift from **circuit-switched data (CSD)** to **packet-switched data**. In the CSD model, a dedicated circuit was reserved for the entire duration of a data connection, even during periods of inactivity, similar to a traditional phone call. This was highly inefficient for the bursty nature of internet traffic and led to slow speeds and time-based billing. GPRS fundamentally changed this: data was broken down into small **packets**, and these packets were then sent independently over shared network resources. This meant that radio capacity was only utilized when packets were actually being transmitted, leading to a much more efficient use of the valuable radio spectrum.

This shift brought about two immediate and significant benefits for the user experience. Firstly, GPRS enabled **"always-on" connectivity**. Users no longer had to manually "dial up" for an internet connection; once activated, their device maintained a continuous logical link to the network, allowing for instant sending and receiving of data. Secondly, the billing model transitioned from per-minute charges to **volume-based billing** (e.g., per kilobyte or megabyte). This made mobile data far more affordable and predictable, as users only paid for the data they actually consumed, which greatly spurred adoption.

GPRS also delivered a notable increase in data speeds. While theoretical peaks could reach around 115 kilobits per second by allowing a single mobile phone to use multiple TDMA time slots simultaneously, real-world speeds typically ranged from 30 to 50 kbps. These speeds, though modest by today's standards, were a vast improvement over CSD and made new services practical.

To support GPRS, new core network elements were introduced. The **Serving GPRS Support Node (SGSN)** managed the packet data for mobile devices within its area, handling mobility and security. The **Gateway GPRS Support Node (GGSN)** acted as the bridge, connecting the GPRS network to external packet data networks, most notably the Internet.

These technical advancements allowed for the widespread adoption of services like **Multimedia Messaging Service (MMS)**, enabling the sending of photos and audio clips, and significantly improved the experience of **WAP Browse** and mobile email. GPRS was instrumental in demonstrating the mass market demand for mobile data, laying the essential groundwork for the high-speed mobile internet of subsequent generations.

Key Concepts

• Packet Switching: The core technical shift.

• Always-On & Volume Billing: The main user benefits.

• SGSN & GGSN: The new network components.

• MMS: A key new service enabled.

• Bridge to 3G: Its role in mobile data evolution.

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• Examples

• Before GPRS (CSD): Imagine needing to physically dial a number and connect your computer to the phone line every time you wanted to send an email, and being charged for every minute the line was open, even if you were just typing the email.

• With GPRS: You could leave your phone "connected" to the internet, and receive emails or news alerts in the background. You only paid for the size of the email or the news article, not the whole time you were connected.

• MMS Photo: Sending a picture of your cat to a friend from your phone, something impossible with SMS alone.

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• Flashcards

• Term: GPRS (2.5G)

• Definition: Packet-switched data for GSM, "always-on," volume-billed.

• Term: Packet Switching

• Definition: Data sent in independent packets, sharing resources.

• Term: Always-On

• Definition: Continuous data connection without constant re-establishment.

• Term: SGSN

• Definition: Serving GPRS Support Node, handles mobile packet data and mobility.

• Term: GGSN

• Definition: Gateway GPRS Support Node, connects GPRS to external networks (e.g., Internet).

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• Memory Aids

• GPRS: "Get Packet Radio Speed\!"

• "2.5G: The 'Half-Step' to Mobile Internet."

• Think "Post Office" for Packet Switching: You send letters (packets) individually, and they share delivery trucks (network resources). You pay per letter (volume), not per hour the truck is driving.

• "SGSN: Serving Goes with Subscriber; GGSN: Gateway Goes to Global (Internet)": Helps remember the roles of SGSN and GGSN.

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