Mostly Asked GPRS (General Packet Radio Service) Interview Preparation Guide

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Applications can choose different profiles for throughput. There are 2 distinctions in class, peak and mean. Peak throughput class is used mainly for bursty transmissions with a variable in octets per second describing the throughput required for burst of specified size. Mean is the average data transfer rate over a period of time measured in octets per hour.

Applications can request differing levels of reliability for its data depending on its tolerance to data loss.

Applications can request predictive delay classes which guarantee an average and 95- percentile delay. There are 4 classes, 1 being the fastest.

An application can be assigned a Precedence Class 1, 2 or 3. If an application has a higher precedence (1) than another (3) then its traffic will be given a higher priority.

The Telecommunications Standard Institute has defined 3 new coding schemes for Radio Interface. When the GPRS device talks to the base station they can use 1 of the 4 schemes. The schemes are CS - 1 through CS - 3 where CS - 1 is the same as standard GSM. In simple terms CS - 1 is highly redundant but because of this is slow, 2 and 3 have less redundancy, whilst 4 has the least - removing all forward error control - but is capable of maximum throughput. If radio quality is bad then coding scheme 1 is used, as the quality improves less error control is needed.

Just because GPRS uses many of the components of a standard GSM network it would be foolhardy to assume that the same standards should apply. Things to be taken into account include provider general network architecture, radio interface and throughput. Here are some of the key elements briefly explained.

GPRS devices are not as straightforward as you may think. There are in fact 3 different classes of device.
Class A:
Class A terminals have 2 transceivers which allow them to send / receive data and voice at the same time. This class of device takes full advantage of GPRS and GSM. You can be taking a call and receiving data all at the same time.
Class B:
Class B devices can send / receive data or voice but not both at the same time. Generally if you are using GPRS and you receive a voice call you will get an option to answer the call or carry on.
Class C:
This device only allows one means of connectivity. An example would be a GPRS PCMCIA card in a laptop.

A mobile device is programmed with one or more Access Point Names which are commonly referred to as the APN's. An APN consists of a fully qualified DNS name e.g. globalguideline.com When a GPRS device wants to talk to globalguideline.com, the SGSN does a DNS look up and resolves the name to the correct GGSN. You could have multiple APN's programmed into your phone so you are not limited to a single service or GGSN.

The second means of addressing is dynamic addressing. This is where a mobile device does not have its own IP address stored in the HLR. Instead the IP address is assigned to the GGSN domain. The method is also a type of dynamic IP addressing in which the IP address is assigned by RADIUS servers normally situated inside an IP network outside the mobile network, an example of this being when you dial up to an ISP from your home PC.

Fixed IP addresses for mobile devices are not widely used due to shortages of Ipv4 addresses (see below). This information is stored in the HLR.

The Serving GPRS Support Node, or SGSN for short, takes care of some important tasks, including routing, handover and IP address assignment.
The SGSN has a logical connection to the GPRS device. As an example, if you where in a car travelling up the M1 on a long journey and were browsing the Internet on a GPRS device, you will pass through many different cells. One job of the SGSN is to make sure the connection is not interrupted as you make your journey passing from cell to cell. The SGSN works out which BSC to "route" your connection through.
If the user moves into a segment of the network that is managed by a different SGSN it will perform a hand-off of to the new SGSN, this is done extremely quickly and generally the user will not notice this has happened. Any packets that are lost during this process are re-transmitted. The SGSN converts mobile data into IP and is connected to the GGSN via a tunneling protocol.

GPRS is packet switched. Higher connection speeds are attainable at around 56-118 kbps, a vast improvement on circuit switched networks of 9.6 kbps. By combining standard GSM time slots theoretical speeds of 171.2 kbps are attainable. However in the very short term, speeds of 20-50 kbps are more realistic

GPRS is an always-on service. There is no need to dial up like you have to on a home PC for instance. This feature is not unique to GPRS but is an important standard that will no doubt be a key feature for migration to 3G. It makes services instantaneously available to a device.

GSM:
☆ Uses one among 7 slots
☆ Connecting resources to each unit from remote location to the back office is done through a direct dial up
☆ Circuit switched mode of operations
☆ Dedicated channel all the way to the destination is provided to the customer
GPRS:
☆ Uses as many as 4+1 time slots
☆ Does not claim any resources until some data is sent. The information is divided into packets
☆ Packet switched mode of operations
☆ One or more dedicated channels are assigned by the operator specifically for shared use

GPRS uses 2.5 generation of GSM signals
The radio interface is the same that of GSM.

The following depicts the architectural differences between GPRS and GSM:
☆ Mobile Station: New mobile station is needed for accessing GPRS services. They are backward compatible with GSM for voice calls
☆ Base Transceiver Station: Software upgrade is needed in the existing BTS
☆ Base Station Controller: Software upgrade and new hardware, Packet Control Unit are needed for GPRS.
☆ GPRS Support Nodes: Installation of new core network elements, known as serving GPRS support node, and gateway GPRS support node are needed to deploy GPRS
☆ Databases: Requires software upgrade to handle new models and functions to handle databases involved in the network

☆ GPRS provide gross data rate of 22.80 KBPS / time slot
☆ Voice uses only 13 KBPS/ time slot and the rest is consumed by Error Correction Code
☆ Error connection code is utilized for detecting and correcting the transmission errors
☆ As voice is compressed, the usage of Error connection code is mandatory

☆ Speed of data transfer available in the Uplink and Downlink directions is determined by Multislot Class.
☆ The speed varies between 1 and 45 for allocating by the network to allocate radio channels in the uplink and downlink channels.
☆ The representation of multi slot class is 5+2, for example, where the first number is the number of down link time slots and the second number is the number of up link time slots that are allocated to use by a mobile station.

☆ Channel Encoding Scheme is based on a convolution code at various code rates and GMSK modulation
☆ The table below summarizes the options:
☆ Coding Scheme Speed
CS-1 8.0
CS-2 12.0
CS-3 14.4
CS-4 20.0
☆ The least robust and fasted coding scheme is CS-4 and is available near a Base Transceiver Station
☆ The most robust coding scheme is CS-1 and is used in case of mobile station is further away from a BTS
☆ A speed of 20.0 Kbit/s/time slots can be achieved with CS-4 and has 25% normal cell coverage
☆ A speed of 8.0 Kbit/s/time slots can be achieved with CS-1 and has 98% of normal coverage

☆ Multiple access scheme is used in GPRS based on the FDD and TDMA
☆ One pair of up-link and down-link frequency channels are assigned to a user during a session
☆ The Multiple Access Coding Scheme is combined with statistical multiplexing [packet mode communication], that allows several uses to share the same frequency channel
☆ Down-link uses first-come first-served packet scheduling and up-link uses a scheme that is similar to reservation ALOHA.
☆ Slotted ALOHA is utilized for reservation inquiries in a contention phase, followed by transferring data by utilizing dynamic TDMA with first-come, first-served scheduling

☆ The GPRS services are classified into 12 service classes as per the duration of the time slots occupied / frame.
☆ Usually 5 time slots per frame are occupied including transmission time slots and reception time slots
☆ Increase in the number of occupied time slots certainly causes the increase in the entire duration of the occupied time slots, so that increased amount of data transmission and receipt are enabled
☆ All the 12 service classes are further classified into higher and lower classes as per the relative duration of time slots
☆ Higher speed data communication can be transmitted with high-class GPRS service by setting high transmission power and setting the duration of the time slots long.

☆ GPRS uses 2.5 generation of GSM signals
☆ The radio interface is the same that of GSM
☆ GPRS uses 900 / 1800 Mhz, frequency band and GMSK modulation
☆ The bit rates are EGPRS, similar to EDGE
☆ Separate hardware and ports need to be added and availed.

A base transceiver station (BTS) or cell site is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. UEs are devices like mobile phones (handsets), WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc. The network can be that of any of the wireless communication technologies like GSM, CDMA, WLL, WAN, WiFi, WiMAX etc. BTS is also referred to as the radio base station (RBS), node B (in 3G Networks) or, simply, the base station (BS). For discussion of the LTE standard the abbreviation eNB for enhanced node B is widely used.

The General Packet Radio Service (GPRS) system is used by GSM mobile phones, the most common mobile phone system in the world, for transmitting IP packets. The GPRS core network is the centralized part of the GPRS system. It also provides support for WCDMA based 3G networks. The GPRS core network is an integrated part of the GSM network switching subsystem.

Access point name (APN) identifies an IP packet data network (PDN), that a mobile data user wants to communicate with. In addition to identifying a PDN, an APN may also be used to define the type of service, (eg connection to wireless application protocol (WAP) server, multimedia messaging service (MMS)), that is provided by the PDN. APN is used in 3GPP data access networks, eg general packet radio service (GPRS), evolved packet core (EPC).