General packet radio service (GPRS) in Mobile Telephone
1. GPRS in Mobile Telephone
GPRS (General packet radio service) is a packet based communication service for mobile devices that allows be sending and receiving across a mobile telephone network it delivered as a network overlay for GSM, CDMA and TDMA networks.
From a high level, GPRS can be though of as an overlay network onto a 2G GSM network enabling packet data transport at rates from 9.6 to 171 kbps. GPRS attempts to reuse the existing GSM network elements as much as possible, but in order to effectively build a packet based mobile cellular network, some new network elements, interfaces, and protocols that handle packet traffic are required . The new class of network nodes, called GPRS support nodes (GSN), is responsible for the delivery and routing of data packets between the mobile stations (MS) and the external packet data networks (PDN). The communication between the GSN nodes is based on IP tunnels through the use of the GPRS Tunneling Protocol (GTP).
1.1. What is GPRS?
¡ GPRS:
l Stands for General Packet Radio Service
l Is the major GSM Phase 2+ enhancement and an important step to 3G
l Aims at providing data services to mobile users with high bandwidth efficiency and “always on” connectivity
GPRS System
2.1. Why introducing GPRS?
¡ The percentage of people accessing the Internet as part of their every day activities has known an explosive growth during the last decade
¡ This fact combined with the impressive dispersion of mobile telephony in the last few years, has triggered a global interest towards mobile access to IP-based applications (e.g. e-commerce) and services
¡ GSM could not support data services with traffic patterns similar to those that exhibit Internet applications.
Shortcomings of GSM
¡ Technical point of view
l GSM deploys circuit-switched mechanisms,
l Which are inappropriate for the bursty
l characteristics of Internet traffic
l impose a time-oriented charging scheme
¡ Users’ point of view
l Data rates are too slow; up to 9.6 kbps
l Connection setup takes too long
l The service is too expensive for end users
2.2. GPRS Characteristics
¡ GPRS:
l supports high data rates; up to 172 kbps
l offers fast call setup times
l provides “always on” connectivity
l supports QoS aware mechanisms
l integrates IP infrastructure into the GSM network
l deploys packet-switched mechanisms, which
allow for a volume-based charging discipline
l result in more efficient resource utilization when bursty traffic is concerned
2.3 GPRS Network Architecture
General Packet Radio Service (GPRS) is a packet oriented Mobile Data Service available to users of GSM and IS-136 mobile phones. It provides data rates from 56 up to 114 kbit/s. Architecture of both GSM & GPRS Network is shown above. The basic differences of GPRS and GSM are Circuit Switched and Packet Switched. Data as a packet switched and voice as a circuit switched is using the same resource of several equipments in the GSM Network. From Mobile Station (MS) or Mobile Equipment (ME) to Base Transceiver Station is using one network.
Why GPRS is needed?
1. GPRS make possible to get always connected connection
2. GPRS is charged by bit transferred
3. GPRS give a lower prices rather than High Speed Circuit Data
4. Higher Speed
5. Faster Session Start
The logical structure for GPRS Network is shown here-
2.4. GPRS Transmission plane
Transmission plan
Providing transmission of user data and its associated signaling, e.g., for flow control, error detection, and error correction.
3.1. GPRS Backbone: SGSN GGSN — As mentioned earlier, user data packets are encapsulated within the GPRS backbone network. The GPRS Tunneling Protocol (GTP) tunnels the user data packets and related signaling information between the GPRS supports nodes (GSNs). The protocol is defined both between GSNs within one PLMN (Gn interface) and between GSNs of different PLMNs (Gp interface). In the transmission plane, GTP employs a tunnel mechanism to transfer user data packets. In the signaling plane, GTP specifies a tunnel control and management protocol. The signaling is used to create, modify, and delete tunnels.
GTP packets carry the user’s IP or X.25 packets. Below GTP, the standard protocols TCP or UDP are employed to transport the GTP packets within the backbone network. X.25 expects a reliable data link, thus TCP is used. UDP is used for access to IP-based packet data networks, which do not expect reliability in the network layer or below. IP is employed in the network layer to route packets through the backbone. Ethernet, ISDN, or ATM-based protocols may be used below IP.
To summarize, in the GPRS backbone we have IP/X.25-over-GTP-over-UDP/TCP-over-IP transport architecture.
Sub network Dependent Convergence Protocol — The Subnetwork Dependent Convergence Protocol (SNDCP) is used to transfer data packets between SGSN and MS. Its functionality includes:
Multiplexing of several connections of the network layer onto one virtual logical connection of the underlying LLC layer. Compression and decompression of user data and redundant header information.
Air Interface — In the following, we consider the data link layer and the physical layer at the air interface Um.
Data Link Layer: The data link layer between the MS and the network is divided into two sublayers: the LLC layer (between MS-SGSN) and the RLC/MAC layer (between MS-BSS).
The logical link control (LLC) layer provides a highly reliable logical link between an MS and its assigned SGSN. Its functionality is based on the well known HDLC protocol and includes sequence control, in-order delivery, flow control, detection of transmission errors, and retransmission (automatic repeat request (ARQ)). The data confidentiality is ensured by ciphering functions. Variable frame lengths are possible. Both acknowledged and unacknowledged data transmission modes are supported. The protocol is mainly an adapted version of the LAPDm protocol used in GSM.
The RLC/MAC layer at the air interface includes two functions. The main purpose of the radio link control (RLC) layer is to establish a reliable link between the MS and the BSS. This includes the segmentation and reassembly of LLC frames into RLC data blocks and ARQ of uncorrectable code words. The medium access control (MAC) layer controls the access attempts of an MS on the radio channel shared by several MSs. It employs algorithms for contention resolution, multiuser multiplexing on a PDTCH, and scheduling and prioritizing based on the negotiated QoS. The GPRS MAC protocol is based on the principle of slotted Aloha.In the RLC/MAC layer, both the acknowledged and unacknowledged modes of operation are supported.
Physical Layer: The physical layer between MS and BSS is divided into the two sublayers: the physical link layer (PLL) and the physical RF Layer (RFL). The PLL provides a physical channel between the MS and the BSS. Its tasks include channel coding (detection of transmission errors, forward error correctio(FEC),indication of uncorrectable codewords), interleaving, and detection of physical link congestion. The RFL operates below the PLL. Among other things, it includes modulation and demodulation. BSS SGSN Interface — The BSS GPRS Application Protocol (BSSGP) delivers routing and QoS-related information between BSS and SGSN. The underlying Network Service (NS) protocol is based on the Frame Relay protocol.
3.2GPRS Signal plane
The protocol architecture of the signaling plane comprises protocols for control and support of the functions of the transmission plane, e.g., GPRS attach and detach PDP context activation, control of routing paths, and allocation of network resources.
Fig: – Signaling plane: SGSNHLR, SGSNEIR.
Between MS and SGSN, the GPRS Mobility Management and Session Management (GMM/SM) protocol supports mobility and session management when performing functions such as GPRS attach/detach security functions, PDP context activation, and routing area updates.
The signaling architecture between SGSN and the registers HLR, VLR, and EIR use the same
protocols as conventional GSM and extends them with GPRS-specific functionality. Between SGSN and HLR as well as between SGSN and EIR, an enhanced Mobile Application Part (MAP) is employed. The MAP is a mobile network-specific extension of the Signaling System SS#7. It transports the signaling information Related to location updates, routing information, user Profiles and handovers. The exchange of MAP messagesis accomplished over the transaction capabilities application part (TCAP) and the signaling connection control part (SCCP). The base station system application part (BSSAP+) includes functions of GSM’s BSSAP. It is applied to transfer signaling information between the SGSN and the VLR (Gs interface). This includes signaling of the mobility management when coordination of GPRS and conventional GSM functions is necessary (e.g., combined GPRS and non-GPRS location update, combined GPRS/IMSI attach, or paging of an MS via GPRS for an incoming GSM call).The signaling architecture between SGSN and the registers HLR, VLR, and EIR uses the same protocols as conventional GSM and extends them with GPRS-specific functionality. Between SGSN and HLR as well as between SGSN and EIR, an enhanced Mobile Application Part (MAP) is employed. The MAP is a mobile network-specific extension of the Signaling System SS#7. It transports the signaling information related to location updates, routing information, user profiles, and handovers. The exchange of MAP messages is accomplished over the transaction capabilities application part (TCAP) and the signaling connection control part (SCCP). The base station system application part (BSSAP+) includes functions of GSM’s BSSAP. It is applied to transfer signaling information between the SGSN and the VLR (Gs interface). This includes signaling of the mobility management when coordination of GPRS and conventional GSM functions is necessary (e.g., combined GPRS and non-GPRS location update, combined GPRS/IMSI attach, or paging of an MS via GPRS for an incoming GSM call).
4. Protocols
Here we show how a GPRS network can be interconnected with an IP-based packet data network, such as the Internet or intranets. GPRS supports both IPv4 and IPv6.
Figure 3 – GPRS system architecture and routing example.
As shown in Fig. the Gi interface is the interworking point with IP networks. From outside, i.e., from an external IP network’s point of view, the GPRS network looks like any other IP sub network, and the GGSN looks like a U heregives an example of how a GPRS network may be connected to the Internet. Each registered user who wants to exchange data packets with the IP network gets an IP address, as explained earlier. The IP address is taken from the address space of the GPRS operator. In order to support a large number of mobile users, it is essential to use dynamic IP address allocation (in IPv4). Thus, a DHCP server (Dynamic Host Configuration Protocol) is installed. The address resolution between IP address and GSM address is performed by the GGSN, using the appropriate PDP context. The routing of IP packets and the tunneling through the intra-PLMN backbone (using the GPRS Tunneling Protocol GTP) has been explained in prior sections. Moreover, a domain name server (DNS) managed by the GPRS operator or the external IP network operator can be used to map between external IP addresses and host names. To protect the PLMN from unauthorized access, a firewall is installed between the private GPRS network and the external IP network. With this configuration, GPRS can be seen as a wireless extension of the Internet all the way to a mobile station or mobile computer. The mobile user has direct connection to the Internet
5.GPRS Air Interface Um.
Air Interface — Physical Layer
Multiple Access and Radio Resource Management Principles
On the physical layer, GSM uses a combination of FDMA and TDMA for multiple accesses. As shown in Fig. two frequency bands 45 MHz apart have been reserved for GSM operation: 890 915 MHz for transmission from the mobile station, i.e., uplink, and 935 960 MHz for transmission from the BTS, i.e., downlink. Each of these bands of 25 MHz width is divided into 124 single carrier channels of 200 kHz width. A certain number of these frequency channels, the so-called cell allocation, is allocated to a BTS, i.e., to a cell.
Each of the 200 kHz frequency channels carries eight TDMA channels by dividing each of them into eight time slots. The eight time slots in these TDMA channels form a TDMA frame. Each time slot of a TDMA frame lasts for duration of 156.25 bit times and, if used, contains a data burst. The time slot lasts 15/26 ms = 576.9 µs; so a frame takes 4.613 ms. The recurrence of one particular time slot defines a physical channel. A GSM mobile station uses the same time slots in the uplink as in the downlink.
The channel allocation in GPRS is different from the original GSM. GPRS allows a single mobile station to transmit on multiple time slots of the same TDMA frame (multisport operation). These results in a very flexible channel allocation: one to eight time slots per TDMA frame can be allocated for one mobile station. Moreover, uplink and downlink are allocated separately, which efficiently supports asymmetric data traffic (e.g., Web browsing).
In conventional GSM, a channel is permanently allocated for a particular user during the entire call period (whether data is transmitted or not). In contrast to this, in GPRS the channels are only allocated when data packets are sent or received, and they are released after the transmission. For bur sty traffic this results in a much more efficient usage of the scarce radio resources. With this principle, multiple users can share one physical channel.
A cell supporting GPRS may allocate physical channels for GPRS traffic. Such a physical channel is denoted as packet data channel (PDCH). The PDCHs are taken from the common pool of all channels available in the cell. Thus, the radio resources of a cell are shared by all GPRS and non-GPRS mobile stations located in this cell. The mapping of physical channels to either packet switched (GPRS) or circuit switched (conventional GSM) services can be performed dynamically (capacity on demand principle), depending on the current traffic load, the priority of the service, and the multislot class. A load supervision procedure monitors the load of the PDCHs in the cell. According to the current demand, the number of channels allocated for GPRS (i.e., the number of PDCHs) can be changed. Physical channels not currently in use by conventional GSM can be allocated as PDCHs to increase the quality of service for GPRS. When there is a resource demand for services with higher priority, PDCHs can be de-allocated.
Figure – GSM carrier frequencies, duplexing, and TDMA frames.
6.Channels.
Logical channels in GPRS
On top of the physical channels, a series of
logical channels are defined to perform a
multiplicity of functions, e.g., signaling,
broadcast of general system information,
synchronization, channel assignment, paging,
or payload transport. Table lists the packet
data logical channels defined in GPRS .
Fig: – Logical Channels in
GPRS
As with conventional GSM, they can be divided into two categories: traffic channels and signaling (control) channels. The packet data traffic channel (PDTCH) is employed for the transfer of user data. It is assigned to one mobile station (or in the case of PTM to multiple mobile stations). One mobile station can use several PDTCHs simultaneously.
The packet broadcast control channel (PBCCH) is a unidirectional point-to-multipoint signaling channel from the base station subsystem (BSS) to the mobile stations. It is used by the BSS to broadcast specific information about the organization of the GPRS radio network to all GPRS mobile stations of a cell. Besides system information about GPRS, the PBCCH should also broadcast important system information about circuit switched services, so that a GSM/GPRS mobile station does not need to listen to the broadcast control channel (BCCH). The packet common control channel (PCCCH) is a bidirectional point-to-multipoint signaling channel that transports signaling information for network access management, e.g., for allocation of radio resources and paging. It consists of four sub-channels:
The packet random access channel (PRACH) is used by the mobile to request one or more PDTCH. The packet access grant channel (PAGCH) is used to allocate one or more PDTCH to a mobile station. The packet paging channel (PPCH) is used by the BSS to find out the location of a mobile station (paging) prior to downlink packet transmission. The packet notification channel (PNCH) is used to inform a mobile station of incoming PTM messages (multicast or group call).
The dedicated control channel is a bidirectional point-to-point signaling channel. It contains the channels PACCH and PTCCH:
The packet associated control channel (PACCH) is always allocated in combination with one or more PDTCH that are assigned to one mobile station. It transports signaling information related to one specific mobile station (e.g., power control information). The packet timing advance control channel (PTCCH) is used for adaptive frame synchronization. The coordination between circuit switched and packet switched logical channels is important. If the PCCCH is not available in a cell, a mobile station can use the common control channel (CCCH) of conventional GSM to initiate the packet transfer. Moreover, if the PBCCH is not available, it will listen to the broadcast control channel (BCCH) to get informed about the radio network.
Mobile station requests radio resources for uplink transfer by sending a “packet channel request” on the PRACH or RACH. The network answers on the PAGCH or AGCH, respectively. It tells the mobile station which PDCHs it may use. A so-called uplink state flag (USF) is transmitted in the downlink to tell the mobile station whether or not the uplink channel is free.Mapping of Packet Data Logical Channels onto Physical Channels the mapping of logical channels onto physical channels has two components:-mapping in frequency and mapping in time. The mapping in frequency is based on the TDMA frame number and the frequencies allocated to the BTS and the mobile station. The mapping in time is based on the definition of complex multiframe structures on top of the TDMA frames. A multiframe structure for PDCHs consisting of 52 TDMA frames. our consecutive TDMA frames form one block (12 blocks, B011), two TDMA frames are reserved for transmission of the PTCCH, and the remaining two frames are idle frames.
The mapping of the logical channels onto the blocks B0 B11 of the multiframe can vary from block to block and is controlled by parameters that are broadcast on the PBCCH. Besides the 52-multiframe, which can be used by all logical GPRS channels, a 51-multiframe structure is defined. It is used for PDCHs carrying only the logical channels PCCCH and PBCCH and no other logical channels.
Channel Coding: Channel coding is used to protect the transmitted data packets against errors. The channel coding technique in GPRS is quite similar to the one employed in conventional GSM. An outer block coding, an inner convolution coding, and an interleaving scheme is used.
7. GPRS Packet Transfer
GSM phones use a technology called CSD (Circuit Switched Data) to transfer data. CSD requires the phone to make a special connection to the network before it can transfer data (like making a voice call) which can take up to 30 seconds. Once connected, the data is sent or received and the user is billed for the time spent online. Data transfer is relatively slow: 14.4 kbps (kilobits per second) for GSM 1800 networks (Orange and T-Mobile) and 9.6 kbps for GSM 900 networks (Vodafone and O2).
GPRS (General Packet Radio Service) is a method of enhancing 2G phones to enable them to send and receive data more rapidly. With a GPRS connection, the phone is “always on” and can transfer data immediately, and at higher speeds: typically 32 – 48 kbps. An additional benefit is that data can be transferred at the same time as making a voice call. GPRS is now available on most new phones.
GPRS is part of a series of technologies that are designed to move 2G networks closer to the performance of 3G networks. The key characteristic of a 3G network is its ability to transfer large amounts of data at high speed (up to 2 Mbps), enabling applications like video calling, video downloads, web browsing, email, etc. By increasing the speed of a 2G network, some of these applications become possible, e.g. web browsing and sending or receiving emails with large attachments. These technologies are called 2.5G and include enhancements to the CSD technology, such as HSCSD and EDGE.
8.GPRS Class Types
The class of a GPRS phone determines the speed at which data can be transferred. Technically the classes refers to the number of timeslots available for upload (sending data from the phone) or download (receiving data from the network). The timeslots used for data are in addition to the slot that is reserved for voice calls. These timeslots are available simultaneously, so the greater the number of slots, the faster the data transfer speed. Because GPRS transmits data in packets, the timeslots are not in use all the time, but are shared amongst all users of the network. That increases the overall data capacity of the network, and it also means that you are billed for the quantity of data transmitted, not the time that you are online. It may mean that during busy times, data transfer rates slow down, because the network will give priority to voice calls.
The most common GPRS classes in use are as follows:
GPRS Class Slots Max. data transfer speed
Class 2 3 8 – 12 kbps upload / 16 – 24 kbps download
Class 4 4 8 – 12 kbps upload / 24 – 36 kbps download
Class 6 4 24 – 36 kbps upload / 24 – 36 kbps download
Class 8 5 8 – 12 kbps upload / 32 – 40 kbps download
Class 10 5 16 – 24 kbps upload / 32 – 48 kbps download
Class 12 5 32 – 48 kbps upload / 32 – 48 kbps download
Generally speaking, the higher the GPRS class, the faster the data transfer rates.
9. Mobility management
GPRS Mobility Management (GMM) is a GPRS signaling protocol that handles mobility issues such as roaming, authentication, and selection of encryption algorithms. GPRS Mobility Management, together with Session Management (GMM/SM) protocol support the mobility of user terminal so that the SGSN can know the location of a mobile station (MS) at any time and to activate, modify and deactivate the PDP sessions required by the MS for the user data transfer.
10.GPRS Routing.
GPRS provides highly effective wide area data networking where communications are initiated from the remote node for example, when collecting email using a notebook PC or hand held device.
Unfortunately GPRS networks are not as effective when a communications session needs to be initiated from the centre, for example, in remote control applications.
• MS from PLMN-2 is visiting PLMN-1
• IP address prefix of MS is the same as GGSN-2
• Incoming packets to MS are routed to GGSN-2
• GGSN-2 queries HLR and finds that MS is currently in PLMN-1
• It encapsulates the IP packets and tunnels them through the GPRS backbone to the appropriate SGSN of PLMN-1
• SGSN encapsulates and delivers to the MS
11.GPRS QoS.
• Each GPRS subscription is associated with one QoS profile (HLR); consists of 4 parameters:
– precedence: operator defined priority; 3 classes
– delay: includes radio access delay (uplink) or radio scheduling delay (downlink), radio transit delay, GPRS-network transit delay; upto 4 classes supported
– reliability: error/loss rates/probabilities; upto 4 classes supported
– throughput: specified by maximum bit rate and mean bit rate
• SGSN will negotiate QoS for the flow
– Based on subscribed default in HLR, requested profile from MN and current availability of GPRS resources
– SGSN does admission control to each PDP context activation
– SGSN can re-negotiate QoS with MN even during run time
• Four traffic classes
– conversational, streaming, interactive, background – they differ in delay sensitivity
• (1) conversational, streaming: for carrying real-time flows
– for telephony and video
– forward error correction
• (2) interactive, background: for traditional internet traffic
– interactive class has higher response
– better error recovery using retransmissions
• Requirements set by human perception
• Assumed to be relatively non-bursty
• Real time, low delay – Voice
• Characterized by
– maximum bit rate
– guaranteed bit rate
– guaranteed transfer delay
• rest optional, but usually specified
• lower classes specify fewer parameters
12.GPRS Capacity.
• Difficult to estimate actual bandwidth available to the GPRS user – will vary a lot
– depending on time of day
– total number of active users
– current geographical location and others…
• Technical Limitations to capacity
– Allocation of time slots – between GSM and GPRS and which multislot classes available
– Restrictions in terminals
– Availability of coding schemes
Channel coding schemes
The later coding schemes reduce the amount of forward error correction, so they need a strong signal. In practice, carriers may never commit to these coding schemes.
• Maximum data transmission rate (radio) is 4 timeslots at 13.4 kbps (53.6 kbps)
• data rates will be further restricted due to
– number of active GPRS users
– amount of retransmissions
– quality of service
– level of compression
• indicative value for average transmission rate seem to be around 30kbps at radio level. (GSM World)
13.GPRS Security.
• User must have a SIM card
• Network can request a password from the user using either CHAP or PAP protocols
• For privacy GPRS encrypts the airlink
• Between GGSN and the external networks carriers can optionally use IPSec
• Also since GPRS runs on IP, end-to-end security can be obtained using VPNs
GSM v/s GPRS
INITIAL CALL PROCESS TIME (s)
• GSM Call 4
• TrainModem 30
• Login and Authenticate 11
• Download mail 180
Total 3 min 45s
SUBSEQUENT CALL
Repeat Above 3 min 45s
INITIAL CALL PROCESS TIME (s)
• GPRS Call 4
• Login and Authenticate 11
• Download mail 180
Total 3 min 15s
SUBSEQUENT CALL
• Not applicable –
Permanent Virtual Circuit 0min 0s