What is mobile phone GPRS?

General Packet Radio Service is abbreviated as GPRS. Put in simple terms, it is a technology that allows the GSM networks to transfer data at speeds of about 56 to 114 kbps (kilobits per second).

Confused? Let's take it slowly. In order to understand GPRS, it is necessary to have a basic knowledge of GSM (Global System for Mobile Telecommunication). GSM is a standard that is used for mobile phone communication, both voice and data (SMS). It uses circuit switching for data (including voice) transferring. Head to What is GSM for a better understanding of GSM and circuit switching.

Getting back to - what is GPRS...

GPRS is a standard or protocol which uses the existing GSM networks to send and receive data, but at faster speeds than the original circuit switching network. It transfers data in the form of packets. Hence it is a packet-switching topping over the existing circuit switching network. As aforementioned, it is capable of achieving speeds of up to 114 kbps. Practically, lower speeds are achieved. But these speeds are higher than that of the circuit switched data (CSD). A mobile phone that is GPRS enabled does not have to set up a connection with the network. It is in an "always connected" state. However, unlike CSD, consumers are NOT charged based on how much time they are connected to the network. Instead they are charged based on how much data they transfer (per kilobit or per Megabit).

Additional Requirements for GPRS...

However, in-order to add GPRS ability to the existing network, additional hardware and software was required. Refer the diagram above. A mobile phone voice call, goes through BTS - BSC - MSC - PSTN/BSC. This is circuit switched network. If the mobile phone is accessing the internet, the path takes is BTS - BSC (PCU) - SGSN - GGSN - Internet. Therefore, for the implementation of GPRS, new components like PCU, SGSN, GGSN and more came into the picture.

The SGSN, GGSN and PCU are additional hardware required for GPRS.

SGSN - Serving GPRS Support Node. This unit handles authentication of GPRS mobile phones, billing information, registration details and so on.

GGSN - Gateway GPRS Support Node. This unit interfaces with the external network. It maintains routing details for moving data to the correct SGSN. In case the use of any network data is chargeable, the GGSN collects such charging information too.

The internal backbone network is just concerned with pushing data between the GSNs.

The Mobile Switching Center routes the call destined to a landline phone to the Public Switched Telephone Network (PSTN) or routes the call destined to another mobile phone to another BSC.

Thus for a voice call or data transfer, the path shared is common from the handset till the BSC. At the BSC, there is a new hardware called Packet Control Unit (PCU) that has been added, which sorts out the data packets and sends them to the SGSN to get serviced. It is from the PCU that the actual packet switched network comes into picture.

Packet switching is dynamic...

GPRS transfers data in the form of packets. It is not necessary that a dedicated path be established between the source and destination. Let's take the London to Edinburgh example. Let's assume that a GPRS mobile phone in London wants to access some information that's present in a server (computer) in Edinburgh. The mobile phone user sends the link or address of that particular information from his mobile phone. This request is converted into packets and is routed to the destination.

Say there are 3 packets of data. It is not necessary that all 3 packets take the same route to reach the Edinburgh server. The route that is to be taken by the packets is determined on the fly. Hence packet switching has a dynamic path, in contrast with circuit switching, which has a static path for a particular session. The routers in packet switched network should thus be equipped with additional intelligence to decide which route would be the best to send each packet on. Since packets can take different routes to reach the destination, there is no guarantee that the packets will reach in the same order as they are sent. See the diagram above.... Packet 2 might get delivered earlier than Packet 1.

Streaming and buffering...

Packets arriving in different order can be tolerated in case of data (internet pages) traffic. You might have noticed that a web page does not open up at once. Certain sections appear first and slowly the rest of the page gets filled up with details. But in case of audio or video, it is an absolute necessity that all packets be put in proper order before the user can hear the audio or see the video files. Hence, an assembler is required to collect the packets and put them in proper order at the destination. This is an additional requirement for packet switched networks. In circuit switched networks, all data is sent and received in order because of the dedicated path. For video and audio files, a process called streaming is done, to ensure timely arrival of the data packets. Most of the times, buffering is also done to enable playing the audio or video file continuously at the destination.

Packet Switching advantage...

Since there is no dedicated path set up for data transfers in packet switching, multiple communication sessions can be handled at the same time. Thus, there is more efficient usage of the network with packet swithing than with circuit switching. Refer the figure....

Let's assume that there are 4 nodes (intermediate routers) handling calls in a circuit switched network. Let's also assume that each node can handle a maximum of 2 calls.

Node A is serving 2 calls (red and green).

Node B is serving 2 calls (red and blue).

Node C is serving 2 calls (blue and green).

Node D is serving 2 calls (blue and red).

If another call has to be served, it can happen only if one of these existing calls terminate.

But in a similar environment with a packet switched network, as soon as a packet is sent from one node to the next, the line is free to handle another packet-transfer. Thus there is efficient network utilisation plus ability to serve more users at the same time in packet switched networks.

The higher generations of mobile communication, like 2.5G (like EDGE) and 3G(like HSDPA, HSUPA) are built upon GPRS and offer much higher speeds of data transfers. While GPRS is not very suitable for handling voice calls, 3G offers higher speeds of up to 7.2Mbps, making it more suitable for handling voice calls using packet switching. Head to Packet Switching vs Circuit Switching for more differences between the 2 technologies.

Examples used here are just for illustrative purposes.

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