A very big company, Facebook, recently released a new feature, Live Videos, that works on every device which supports Facebook application. People can now go live from anywhere they want. And we mostly see reports from the local places, like streets and events. Also, we often see people uploading instant photos about what they are doing- even outside home. Some check urgent emails and some search meaning of words online, on the go.
The key to this is, prominent use of cellular data network. Cellular data network is a network that uses same resources from a telecommunication network that individuals subscribes to for using telecom service mostly for mobile phones. The Global System for Mobile (GSM) is a standard of wireless telecommunication network developed to elaborate the protocols of second generation 2G digital networks, mostly used by mobile phones. With the addition of General Packet Radio Services(GPRS) in GSM, the use of cellular data has been made feasible.
How do you think making a phone call and using cellular data through a single device has been possible? Let us have an inner insight in order to understand the technology, architecture and protocols behind it.
The Global System for Mobile (GSM) Association estimated in 2010 that GSM had 80% of the global mobile market, with more than 5 billion subscriptions, making it the most pervasive cellular data network. Contrary to 1G, which is the first generation wireless telecommunication technology using analog signals for communication, GSM uses the second generation digital (2G) cellular networks.
GSM was originally introduced to digitalize the analog signals. It also introduced the SMS services. But it expanded over time to include data communications, first by circuit-switched transport, then by packet data transport through GPRS. Moreover, improvisation were made with the introduction of GPRS and EDGE (Enhanced Data rates for GSM Evolution) that expanded the potential of GSM networks. Multimedia messaging was added to its list of features allowing subscribers to audio clips, send pictures, and also short video snaps to each other. EDGE resulted in increment in the speed of mobile internet browsing to Dial-up speeds.
The cellular data network uses GPRS which is a packet oriented mobile data service. GPRS is a 2.5G network. It has implemented packet switched domain in addition to the circuit switched domain. When it came to cellular data in GSM, packaging of data was introduced. Packet refers to the process of transmitting data by breaking it up into small chunks. Packet data is how most data travels over the internet, and over cellular data network.
Network Signaling and connectivity:
For every internet connection, there is a network architecture behind it. In this case, the cellular data network in GSM uses the same signaling architecture as the GSM wireless signal telecommunication service. There is an inbuilt modem in mobile devices that receives, modulates and demodulates the signal transmitted by base stations within each cell. The network and its signal is distributed in land areas that are called cells. The cells are defined by each of the base stations that provide network signaling and coverage that can transmit voices and data. Each of these base stations are plotted in such a way that there is network coverage in almost all possible geographical regions. The antennas in the base station are generally plotted in 6 sector at 60 degrees in order provide assurance of network coverage. To avoid interference from other cells, each of these cell use different set of frequencies.
In every network design, there is a channel for communication within nodes or base stations. Channels are the resources that any network use for the flow of data. We will be looking at the channels that are allocated in GPRS system.
Data network in GPRS uses Packet Data Channel (PDCH) for communication. PDCH is the channel used by GPRS. Before the introduction of GPRS, the channel access method in GSM was Time Division Multiple Access (TDMA) to communicate with base station over air. But in cellular data network, the GPRS uses several physical and logical channels like PDCH and Packet Timing Advance Control Channel (PTCCT) The packet data channel.
With all of channel allocation and linking data to the Base Station in Data Link Layer of the Open System Interconnection (OSI) model, the Base Station Controller (BSC) allocates PDCHs to specific time slots. The OSI model is a conceptual model of a network that has several layers with its specific functions. The data link layer in that model would link the data using Packet Data Protocol (PDP) that a client is trying to send as a packet, to the base station where the signal is being transmitted to and from, mostly through a medium or channel. There will be some times when the PDCH is not active. This allows the mobile device to look for some other base stations and check on their network coverage strength. This will then enable the network to judge weather or not a handover is required. Handover is a method of acknowledging hands off from one base station and let the switching center switch the signal in mobile device to another base station that is near and strong. The Base station uses GPRS slot to judge the time delay using a logical channel known as the Packet Timing Advance Control Channel (PTCCT) for packet data switching.
GPRS Core Network and Protocols:
‘GPRS core network’ is the capital technology of GPRS that has the utmost significance. GPRS core network allows 2G and 3G networks to transmit IP cellular data packets to external internet connections. This work is carried out by Network Switching Subsystem (NSS). NSS allows mobile phones to have access to services such as Wireless Application Protocol (WAP) and the Internet. WAP is an internet standard that is used in mobile phone internet connections enabling phones to use Internet Protocol and web applications. The WAP protocol stack assures the handshake between the gateway server and the WAP client. It also maintains data flow, encryption and authentication, checks data integrity, and also ensures the compatibility to different network providers.
GPRS Tunneling Protocol (GTP), is what defines the IP based protocol in GPRS core network. The two main type of GPRS tunneling protocols are- first, GTP U, where the ‘U’ stands for transfer user data using separate tunnels. They use the Packet Data Protocol Context (PDP Context) for each of it. A PDP context is a data structure that is present in both the Serving GPRS support node (SGSN) and Gateway GPRS support node (GGSN). It contains the subscriber’s internet session information when the session is started. SGSN is responsible for delivery of data packets to and from the mobile stations within its sector and GGSN is the gateway for external network routing. The session contained by PDP context is then transmitted to the IP and WAP to establish a stable data connection.
The second type of GTP is GTP C, where ‘C’ stands for control. From the data connection session establishment to the closing of it, GTP C is required. Two important GTP C elements are SGSN and GGSN.
Here, the SGSN is similar to the Mobile Switching Center(MSC) in GSM. The fundamental work of SGSN is to convert Internet Protocol (IP) to RF. It also authenticates GPRS users and does the work of routing of data when connection to external network is required. It communicates with NSS to allow access to WAP and retrieve subscription information. SGSN encrypts down-link data, and decrypts up-link data. Communication from base station to the nodes is down-link whereas the communication from note to the tower is up-link. SGSN does traffic statistics collections for network management purposes.
Likewise, GGSN is the main gateway to external network. When a subscriber has change in its location, GGSN provides link through gateway connection to to another SGSN. It Interfaces to external IP networks and deals with security issues. It is also involved in the establishment of tunnels with the SGSN and with other external networks and Virtual Private Networks (VPN).
GPRS Tunneling Protocol is a protocol that allows GPRS users to go from one place to another as they continuously connect to the Internet, almost as if they are connected from one location at the GGSN. In the Network Layer of the OSI model, this happens as a result of carrying the subscriber’s data from the subscriber’s current SGSN to the GGSN which has been handling the subscriber’s current internet session.
A main gateway between a GPRS cellular network and other internet network is called an Access Point Name (APN). When a mobile data user wants to communicate with a Packet Data Network, the APN identifies it. Apart from identifying a PDN, an APN is also used to define the type of the network, or type of the service, for example, WAP server; that is provided by the PDN.
The other protocols supported by the GPRS are the Internet Protocol (IP) that enables the usage of internet browsing and web application, Point to Point Protocol (PPP) that is used for tunneling IP to the phone if mobile is used as a modem connected to the computer and X.25 protocol for packet switching in WAN clouds.
To sum up how cellular data is integrated in wireless GSM network, the introduction of GPRS in existing GSM network was, and has been the key. For use of cellular data network, the transmission of signal and receiving of signal for connectivity between base stations and the GSM mobile client is basically the same connection technology that is used in connection for telecommunication services. However, the channels and its’ allocation for transmission of data is what deviates the GSM telecommunication from GPRS cellular data network. The use of PDCH and PCTCCT for transmission of data using the packet switched technology makes packet data internet usage possible. Once the data is ready to be transmitted, the PDP protocol and PDP context that contains session information enables the browsing and makes IP access possible. For the core GPRS technology to be operated, GPRS tunneling protocol has the highest magnitude of significance. The GTP allows users to take their mobile phones anywhere without losing their internet session and The Serving GPRS node enables the conversion of IP into Radio Frequency for data transmission and gateway GPRS node acts as the gateway to external internet connections. All in all, this is how the usage of cellular data network in wireless GSM technology is implemented.
GPRS has changed the way we use and imply internet in our daily lives. The prominent use of cellular data network has helped many people to connect, talk to their loved ones, even from places where internet connection seemed unviable. All thanks to the packet switched GPRS technology that has been a highly significant entity of mobile use in rural as well as urban places around the globe.
However, in the current context, use of GPRS in GSM, the 2G wireless technology has been fading out. The invention of third generation (3G) and fourth generation (4G) of wireless communication technology has seen the decline of usage in 2G digital networks. Though the existing GPRS 2G network has upgraded to 3GPRS, the 3G technology has seen to be efficacious in the Code Division Multiple Access (CDMA) networks, which is a different wireless telecommunication technology. For the first time, an Australian company named “Telstra” has completely shut down its 2G network services in on December 1, 2016 and the next company to shut down its 2G services would be AT & T from USA in 1st January 2017. Singapore will phase out 2G services by April 2017.
Despite the phasing out of 2G services, it is a necessity that technologists should have the knowledge about how the analog signals were digitalized and how Multimedia messaging, use of cellular data network was possible through GSM and GPRS as I believe it was one of the most congenial thing that has happened to the internet and mobile phones.
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