- 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 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).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.
.
Figure – Example of a GPRS
Internet connection.
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.
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,
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:
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
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
handhelddevice.
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
–
•
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
|