Mobile Internet with GPRS 

General Packet Radio Service, or GPRS, is a relatively new technology that offers packet-based radio service to mobile networks. Designed to supplement the existing mobile technologies, like GSM, CDMA, TDMA etc, GPRS aims to provide anytime-anywhere “always-on” network connections to mobile devices. GPRS also enables mobile networks to provide speeds much higher (theoretically up to 171.2 kbps) than those offered by GSM alone (9.6/ 14 kbps). Here is how it works with GSM networks. 

Why

GPRS?



Though GSM uses circuit switching like your telephone line, data transfer over GSM is not quite the same as that over the PSTN line. For each of your data requests (over GSM), first, a connection is established with your network, your request sent, the data received and then, and this is important, that call is disconnected. Thus, you have a dedicated connection as long as a transaction lasts, but not in between transactions. This is designed to overcome its bandwidth wastage limitation and ensure that your connectivity costs don’t spiral up as there can be, and generally are, long periods of inactivity between consecutive transactions. 

The savings in costs, however, are more than compensated by the long delays associated with each transaction and subsequent data transfer–a major part of which is the time spent in trying to establish a connection with the network. Add to it the fact that GSM offers a maximum speed of 14 kbps only, or 1/4th speed of your dial-up

connect, and the reason why mobile internet hasn’t taken off, becomes clear.

GPRS, though, has the potential to change that. Using GPRS and its packet switching technique, both the mobile device and the network send packets as and when they have to, without having to wait for the connection to be established. 

Also GPRS can theoretically offer speeds up to three times your dial-up speeds or 171.2 kbps (wherein all timeslots are allocated to a single user). Such high speeds though are unlikely to be seen,

as networks would share the available bandwidth between multiple users.

GPRS enables an existing GSM network to communicate with any application and/or network that uses packet switching.

Thus it allows mobile networks to connect to the Internet (and other IP and X.25 networks) and provide their subscribers the facility to access any of the related applications–Web browsing, mail, FTP, telnet etc.

Implementing GPRS



Integrating GPRS into a GSM network does not involve any major changes to the GSM architecture. A new class of nodes, GSN (GPRS Support Nodes) has been introduced and two types of GSNs defined. A SGSN (Serving GPRS Support Node) interacts with the mobile devices and is responsible for delivery of data packets from and to these devices. The SGSN communicates with the registers (same as in GSM) to access the required information. A network would have as many SGSNs as necessary to GPRS enable the required network.

A GGSN (Gateway GSN) is an interface between the GPRS enabled network and the external packet switching network

The GGSN is responsible for all the conversions necessary for transport of data from the external network to the internal one and vice versa. This involves converting all the data packets as well the source/destination addresses from one standard to the other. The SGSNs communicate with the GGSN to provide access to the external network to their subscribers. A network would have as many GGSNs as the number of external packet switching data networks linked to it.

Next step



Also known as 2.5G GPRS is considered to be the next step towards the 3G networks. The market is flooded with GPRS enabled cell phones like the Nokia 8310. In India, though, paying that extra bit for a GPRS enabled phone may not make sense–not yet at least. Not many cell phone operators in India have implemented GPRS as yet and thus you may not be able to get the most out of your set. 

As it offers pretty good speeds, GPRS has the potential to move beyond cell phones and move into the PC world. Handhelds and PDAs having GPRS functionality built in, or the ability to communicate with a GPRS phone via a serial port, are the next logical extension. Laptops connected to GPRS phones would offer speeds as fast, and maybe even faster, than your PC connected to your landline. Before we end, let’s see what circuit switching and packet switching are.

Circuit switching vs Packet switching



While using circuit switching the sender seeks a connection right up to the receiver before it begins transmitting. Data transmission begins only after the sender has verified that a dedicated connection to the receiver exists. 

This means that circuit-switching networks suffer from “hunting delays,” the time spent waiting to establish a connection with the receiver. But once the connection has been established, communication is, for all practical purposes, instantaneous. This connection is maintained–even if no data transmission is taking place–until either side explicitly terminates it. Thus, all information travels the same path from the sender to the receiver and arrives in the same order as it was sent.

Using packet switching, however, eliminates the need to establish a connection before transmission can begin. The information to be sent is broken down into packets (of fixed size) and sent onto the network one after the other. Each packet completes the journey to the receiver independently of the others–routing equipment along the way decide which path the packet must take, depending upon the current network conditions and other factors. 

This means that packets may take different paths in reaching the receiver and may even arrive out of order–the job of reassembling them in the proper order is that of the receiver. Thus, though hunting delays are absent from packet switching networks, they do

suffer from performance related problems.

A key advantage that packet switching networks have is that of dynamic allocation of bandwidth–packets are sent to the physical medium as and when there is data to be sent. This allows the bandwidth to be used for other purposes during “pauses” between “conversations.” Contrast this with the circuit switching technique wherein the path between the sender and receiver is reserved for their exclusive use, which means that the bandwidth is wasted during “moments of silence”. The table in the previous page compares differences between the two techniques.

Kunal Dua