General packet service via radio
The general packet radio service, in English: General Packet Radio Service (GPRS), It was created in the 80's A GPRS connection is established by reference to your Access Point Name (APN). With GPRS you can use services such as Wireless Application Protocol (WAP), Short Message Service (SMS), Multimedia Messaging System (MMS), Internet and for communication services such as email and the World Wide Web (WWW). To set up a GPRS connection for a wireless modem, a user must specify an APN, optionally a username and password, and very rarely an IP address, all provided by the network operator. GPRS data transfer is charged per volume of information transmitted (in kilograms or megabytes), while traditional circuit-switched data communication is charged per minute of connection time, regardless of whether the user uses all the channel capacity or is in an idle state. For this reason, the switched connection is considered more suitable for services such as voice that require constant bandwidth during transmission, while packet services such as GPRS are oriented towards data traffic. GPRS technology, as its name indicates, is a service oriented to radio links that gives better performance to packet switching in said radio links.
Technology used
Channel access used in GPRS is based on frequency divisions over full duplex and TDMA. During the connection, the user is assigned a physical channel, formed by a temporary block on a specific carrier. That channel will be upload or download depending on whether the user is going to receive or send data. This is combined with time-domain statistical multiplexing, allowing multiple users to share the same physical channel, either uplink or downlink. The packets have a constant length, corresponding to the GSM time slot. The downstream channel uses a First in, first out (FIFO) queue for waiting packets, while the upstream channel uses a scheme similar to that of ALOHA with reservation.
In summary, a system similar to slotted ALOHA is used during the contention phase, and TDMA with a FIFO queue during the data transmission phase.
Services offered
GPRS technology improves and upgrades GSM with the following services:
- Network applications for devices through WAP protocol
- Instant messaging,
- Possibility of using the device as USB modem,
- Short Message Service (SMS),
- Multimedia messaging service (MMS),
- Services Peer-to-peer (P2P) using the IP protocol.
GPRS technology can be used for services such as Wireless Application Protocol (WAP) access, SMS and MMS, Internet access and email.
The typical charging method for data transfers using GPRS is payment per megabyte of transfer, while payment for traditional circuit-switched communication is charged per connection time, regardless of whether the user is using the channel or it is inactive. This last method is not very efficient because it maintains the connection even when data is not being transmitted, thus preventing other users from accessing the channel. The method used by GPRS makes possible the existence of applications in which a mobile device connects to the network and remains connected for a long period of time without greatly affecting the amount billed by the operator.
Device classes
There are three classes of mobile devices considering the possibility of using GSM and GPRS services simultaneously:
- Class A
These devices can simultaneously use GPRS and GSM services.
- Class B
They can only be connected to one of the two services at a time. While using a GSM service (voice calls or SMS), the GPRS service is suspended, which is automatically restarted when the GSM service ends. Most mobile phones are of this type.
- Class C
They alternately connect to one or the other service. Switching between GSM and GPRS must be done manually.
For a Class A device to transmit on two frequencies at the same time, it would need two radios. To solve this costly problem, a GPRS mobile usually implements the feature known as "dual transfer mode" (Dual Transfer Mode, DTM). A DTM mobile can use both the data channel and the voice channel, since it is the network that coordinates and ensures that it is not required to transmit on two different frequencies at the same time. DTM mobiles are considered class A, but simplified.
Transfer rate
Depending on the technology used, the transfer speed varies considerably. The following table shows the upload and download data for each type of technology.
Technology Download (kbit/s) Up (kbit/s) CSD 9.6 9.6 HSCSD 28.8 14,4 HSCSD 43.2 14,4 GPRS 80 20 (Class 8 & 10 and CS-4) GPRS 60 40 (Class 10 and CS-4) EGPRS (EDGE) 236,8 59.2 (Class 8, 10 and MCS-9) EGPRS (EDGE) 177.6 118.4 (Class 10 and MCS-9)
To compare GPRS with GSM the SMS transmission speed is normally used. On a GPRS network, approximately 30 SMS can be sent per minute, compared to the 6 or 10 SMS that GSM allows.
- Data transfer.
- Closure or release of the connection.
When communication is established between the 2 terminals, a physical or logical step is created between them (thus occupying a communication channel), which is kept reserved until the connection is released. These circuit-switched networks are responsible for taking the bits from their point of origin to the destination, without identifying PDUs (Protocol Data Unit, data packets) or dealing with their structure.
Packet Switching (GPRS)
The transmitting entity segments the message to be transmitted into independent PDUs (data packets) of appropriate size. The receiving entity is in charge of reconstructing them (reassembling them) until obtaining the complete original message. Each data packet is transferred from one node to another as a single unit. They contain control information (source and destination addresses, identifier, etc.) that allows their management on the network.
The PDU is temporarily stored in each of the nodes it passes through while it waits to be sent to the next one. This entails an increase in delay depending on the volume of existing traffic and the capacity of the link. All of the PDUs that make up the data are related to one another, but the way in which they travel and are regrouped varies. The network itself can fragment the PDUs if their length is greater than the maximum transfer unit (MTU) of the network.