Ch7

Mobile Computing

Chapter 7
GPRS
Asoke K Talukder
Hasan Ahmed
© Tata McGraw Hill

GPRS

 General Packet Radio Service
 Step to efficiently transport high-speed data over the current
GSM and TDMA-based wireless network infrastructures
 Deployment of GPRS networks allows a variety of new
applications ranging from mobile e-commerce to mobile corporate
VPN access
 GPRS allows for data speeds of 14.4 KBps to 171.2
KBps, which allow for comfortable Internet access
 Allows for short ‘bursty’ traffic, such as e-mail and web
browsing, as well as large volumes of data

GPRS

 No dial-up modem connection is necessary
 Offers fast connection set-up mechanism to offer a perception
of being ‘always on’ or ‘always connected’
 Immediacy is one of the prime advantages of GPRS

QoS in GPRS

 Allows definition of QoS profiles using the parameters of
service precedence, reliability, delay and throughput
 Service precedence is the priority of a service in relation to
another service which can be either high, normal or low
 Reliability indicates the transmission characteristics required
by an application and guarantees certain maximum values for the
probability of loss, duplication, mis-sequencing and corruption of
packets
 Delay parameters define maximum values for the mean delay
and the 95-percentile delay
 Throughput specifies the maximum/peak bit rate and the mean
bit rate

GPRS Network Architecture

 GPRS uses the GSM architecture for voice
 To offer packet data services through GPRS, a new class of
network nodes called GPRS support nodes (GSN) are introduced
 GSNs are responsible for the delivery and routing of data
packets between the mobile stations and the external packet data
networks (PDN)
 Two main GSNs are Serving GSN (SGSN) and Gateway GSN
(GGSN)

SGSN

 SGSN is at the same hierarchical level as the MSC and so,
whatever MSC does for voice, SGSN does for packet data
 SGSN’s tasks include packet switching, routing and transfer,
mobility management, logical link management, authentication
and charging functions
 SGSN processes registration of new mobile subscribers and
keeps a record of their location inside a given service area
 Location register of the SGSN stores location information (like
current cell, current VLR, etc.) and user profiles of all GPRS users
registered with this SGSN
 SGSN sends queries to HLR to obtain profile data of GPRS
subscribers

GGSN

 GGSN acts as an interface between the GPRS backbone
network and the external packet data networks and functions like
a router in a LAN
 GGSN maintains routing information that is necessary to
tunnel Protocol Data Units (PDUs) to the SGSNs that service
particular mobile stations
 GGSNs convert the GPRS packets coming from the SGSN into
the appropriate packet data protocol (PDP) format for the data
networks like Internet or X.25
 GGSN stores the current SGSN address of the user and user’s
profile in its location register while performing authentication and
charging functions related to data transfer

GPRS Network Enhancements

 Base Station System (BSS) needs enhancement to recognize
and send packet data and this includes BTS upgrade to allow
transportation of user data to the SGSN. BTS, too, needs to be
upgraded to support packet data transportation between BTS and
MS (mobile station).
 HLR needs enhancement to register GPRS user profiles and
respond to queries originating from GSNs regarding these
profiles.
 MS (mobile station) for GPRS is different from that of GSM.
 SMS-GMSCs and SMS-IWMSCs are upgraded to support
SMS transmission via the SGSN.

Channel Coding

 Channel coding is used to protect the transmitted data packets
against errors
 Channel coding technique in GPRS is quite similar to the one
employed in conventional GSM
 Under very bad channel conditions, reliable coding scheme is
used where redundant bits are added to recover from burst errors
 Under good channel conditions, no encoding scheme is used
resulting in a higher data rate

Transmission Plane Protocol Architecture

Signaling Plane
 Protocol architecture of the signaling plane comprises
protocols for control and support of the functions of the
transmission plane and includes GPRS attach and
detach, PDP context activation, control of routing paths and
allocation of network resources.
 Between SGSN and HLR as well as between SGSN and
EIR, an enhanced Mobile Application Part (MAP) is
employed which is a mobile network specific extension of
the Signaling System SS#7 used in GSM and transports the
signaling information related to location updates, routing
information, user profiles and handovers.
 MAP messages are exchanged over Transaction
Capabilities Application Part (TCAP) and Signaling
Connection Control Part (SCCP) while BSSGP is an
enhancement of GSM’s BSSAP.

GPRS Backbone

 It includes the transmission plane between SGSN and GGSN.
 User data packets and signaling information within GPRS
networks are encapsulated using GPRS Tunneling Protocol (GTP)
which is also used in both intra-PLMN (between SGSN and
GGSN within one PLMN) and inter-PLMN (between SGSN and
GGSN of different PLMNs).
 GTP protocol tunnels the user data packets through GPRS
backbone by adding GPRS specific routing information in the
form of GTP packets which can carry data packets from both IP
and X.25 data networks.
 Finally, GPRS backbone has an IP/X.25-over-GTP-over-
UDP/TCP-over-IP transport architecture.

BSS-SGSN Interface

The BSS-SGSN interface is divided into four layers:
1. Sub-Network Dependent Convergence Protocol (SNDCP)
which transfers data packets between SGSN and
MS, multiplexes several connections of the network layer onto
one virtual logical connection of the underlying LLC layer and
does segmentation, compression-decompression of user data.
2. Logical Link Control (LLC) is data link layer protocol for
GPRS which functions similar to Link Access Procedure-D
(LAPD) and assures the reliable transfer of user data across a
wireless network.

BSS-SGSN Interface

3. Base Station System GPRS Protocol (BSSGP) delivers routing
and QoS related information between BSS and SGSN.
4. Network Service layer manages the convergence sub-layer
that operates between BSSGP and Frame Relay Q.922 Core
by mapping BSSGP’s service requests to the appropriate
Frame Relay services.

Air Interface

 Air interface of GPRS comprises data link layer and physical
layer.
 Data link layer between MS and BSS is divided into three
sublayers: the logical link control (LLC) layer, the radio link
control (RLC) layer and the medium access control (MAC) layer.
 Physical layer between MS and BSS is divided into two
sublayers: the physical link layer (PLL) and the physical RF layer
(RFL).

LLC Layer

 Logical Link Control (LLC) layer provides a reliable logical
link between an MS and its assigned SGSN as its functionality is
based on HDLC (High Level Data Link Control) protocol and
includes sequence control, in-order delivery, flow
control, detection of transmission errors and retransmissions.
 Encryption is used.
 Variable frame lengths are possible and both
acknowledged and unacknowledged data transmission
modes are supported.

RLC Layer

 Radio Link Control (RLC) layer establishes a reliable link
between MS and BSS.
 It also does segmentation and reassembly of LLC frames into
RLC data blocks and ARQ of uncorrectable data.

MAC Layer

 Medium Access Control (MAC) layer controls the access
attempts of an MS on the radio channel shared by several MSs by
employing algorithms for contention resolution, multi-user
multiplexing on a packet data traffic channel (PDTCH) and
scheduling and prioritizing based on the negotiated QoS.

PL Layer

 Physical Link Layer (PLL) provides services for information
transfer over a physical channel between the MS and the network.
 Its functions include data unit framing, data coding and
detection and correction of physical medium transmission errors.
 Physical Link Layer uses the services of the Physical RF Layer.

PRF Layer

 Physical RF Layer (RFL) performs the modulation of the
physical waveforms based on the sequence of bits received from
the Physical Link Layer above.
 It also demodulates received wave forms into a sequence of
bits that are transferred to the Physical Link layer for
interpretation.

Radio Resource Management

 On the radio interface, GPRS uses a combination of FDMA
and TDMA.
 A series of logical channels are defined to perform functions
like signaling, broadcast of general system
information, synchronization, channel assignment, paging or
payload transport.
 Such channels can be divided into two categories: traffic
channels and signaling channels.
 GPRS traffic channels are allocated when data packets are sent
or received and they are released after the transmission of data.
 GPRS allows a single mobile station to use multiple time slots
of the same TDMA frame for data transmission which is known as

Radio Resource Management

 Uplink and downlink channels are allocated separately which
efficiently supports asymmetric data traffic like Internet.
 Physical channels to transport user data packet are called
Physical Data Traffic Channel (PDTCH) which are taken from a
common pool of all channels available in a cell.
 Mapping of physical channels to either packet switched data (in
GPRS mode) or circuit switched data (in GSM mode) services are
performed dynamically depending on demand.
 Demand-wise, the number of channels allocated for GPRS can
be changed. For example, physical channels not currently in use
by GSM can be allocated as PDTCHs to increase the bandwidth
of a GPRS connection.

Security

 GPRS security is similar to the existing GSM security.
 SGSN performs authentication and cipher setting procedures
based on the same algorithms, keys and other criteria of GSM.
 GPRS uses a ciphering algorithm optimized for packet data
transmission.

Attachment and Detachment in GPRS

 MS registers itself with SGSN of GPRS network through a
GPRS attach which establishes a logical link between the MS and
the SGSN.
 Network checks if MS is authorized to use the services; if so, it
copies the user profile from HLR to SGSN and assigns a Packet
Temporary Mobile Subscriber Identity (P-TMSI) to the MS.
 To exchange data packets with external PDNs after a successful
GPRS attach, an MS must apply for an address which is called
PDP (Packet Data Protocol) address.
 For each session, a PDP context is created which contains PDP
type (e.g. IPv4), PDP address assigned to the mobile station (e.g.
129.187.222.10), requested QoS and address of the GGSN that
will function as an access point to the PDN.

Attachment and Detachment in GPRS

 Such a context is stored in MS, SGSN and GGSN while with
an active PDP context, the MS is ‘visible’ to the external PDN.
 A user may have several simultaneous PDP contexts active at a
given time and user data is transferred transparently between MS
and external data networks trough GTP encapsulation and
tunneling.
 Allocation of the PDP address can be static or dynamic.
 In case of static address, the network operator permanently
assigns a PDP address to the user while in other case, a PDP
address is assigned to the user upon the activation of a PDP
context.

PDP Context Activation
 Using the message ‘activate PDP context request’, MS informs
the SGSN about the requested PDP context and if request is for
dynamic PDP address assignment, the parameter PDP address will
be left empty.
 After necessary security steps, if authentication is
successful, SGSN will send a ‘create PDP context request’
message to the GGSN, the result of which is a confirmation
message ‘create PDP context response’ from the GGSN to the
SGSN, which contains the PDP address.
 SGSN updates its PDP context table and confirms the
activation of the new PDP context to the MS.
 Disconnection from the GPRS network is called GPRS detach
in which all the resources are released.

Mobility Management

 Mobility Management functions are used to track its location
within each PLMN in which SGSNs communicate with each other
to update the MS’s location in the relevant registers.
 Profiles of MSs are preserved in VLRs that are accessible to
SGSNs via the local MSC.
 A logical link is established and maintained between the MS
and the SGSN at each PLMN.
 At the end of transmission or when a mobile station moves out
of area of a specific SGSN, the logical link is released and the
resources associated with it can be reallocated.

Routing
 Routing is the process of how packets are routed in GPRS.
 Here, the example assumes two intra-PLMN backbone
networks of different PLMNs. Intra-PLMN backbone networks
connect GSNs of the same PLMN or the same network operator.
 These intra-PLMN networks are connected with an inter-
PLMN backbone while an inter-PLMN backbone network
connects GSNs of different PLMNs and operators. However, a
roaming agreement is necessary between two GPRS network
providers.
 Gateways between PLMNs and external inter-PLMN backbone
are called border gateways which perform security functions to
protect the private intra-PLMN backbones against malicious
attacks.

Routing

 Let’s say that GPRS MS located in PLMN1 sends IP packets to
a host connected to the IP network (e.g. to a Web server connected
to the Internet).
 SGSN that the MS is registered with encapsulates the IP
packets coming from the mobile station, examines the PDP
context and routes them through the intra-PLMN GPRS backbone
to the appropriate GGSN.
 GGSN de-encapsulates the packets and sends them out on the
IP network, where IP routing mechanisms are used to transfer the
packets to the access router of the destination network and
finally, delivers the IP packets to the host.

Routing
 Let us also say that home-PLMN of the mobile station is
PLMN2.
 An IP address has been assigned to MS by the GGSN of
PLMN2 and so, MS’s IP address has the same network prefix as
the IP address of the GGSN in PLMN2.
 Correspondent host is now sending IP packets to the MS onto
the IP network and are routed to the GGSN of PLMN2 (the home-
GGSN of the MS). The latter queries the HLR and obtains the
information that the MS is currently located in PLMN1.
 It encapsulates the incoming IP packets and tunnels them
through the inter-PLMN GPRS backbone to the appropriate
SGSN in PLMN1 while the SGSN de-encapsulates the packets
and delivers them to the MS.

Routing
 HLR stores the user profile, the current SGSN address and the
PDP addresses for every GPRS user in the PLMN.
 When the MS registers with a new SGSN, HLR will send the
user profile to the new SGSN.
 Signaling path between GGSN and HLR may be used by the
GGSN to query a user’s location and profile in order to update its
location register.

Communicating with IP Networks
 A GPRS network can be interconnected with Internet or a
corporate intranet and supports both IPv4 and IPv6.
 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 usual IP router.
 Each registered user who wants to exchange data packets with
the IP network gets an IP address which is taken from the address
space of the GPRS operator maintained by a Dynamic Host
Configuration Protocol (DHCP) server.
 Address resolution between IP address and GSM address is
performed by the GGSN using the appropriate PDP context.

 Domain Name Server (DNS) managed by the GPRS operator
or the external IP network operator is used to resolve host name.
 To protect the PLMN from unauthorized access, a firewall is
installed between the private GPRS network and the external IP
network.
 Thus, GPRS can be seen as a wireless extension of the Internet
all the way to a MS or mobile computer as mobile user has a
direct connection to the Internet.

Data Services in GPRS
 Any user is likely to use either of the two modes of the GPRS
network: application mode or tunneling mode.
 In application mode, user uses the GPRS mobile phone to
access the applications running on the phone itself. The phone
here acts as the end user device.
 In tunneling mode, user uses GPRS interface as an access to
the network as the end user device would be a large footprint
device like laptop computer or a small footprint device like PDA.
The mobile phone will be connected to the device and used as a
modem to access the wireless data network.

GPRS Handsets
 GPRS terminal can be one of the three classes: A, B or C.
 Class A terminal supports GPRS data and other GSM services
such as SMS and voice simultaneously. This includes
simultaneous attach, activation, monitoring and traffic. As such, a
class A terminal can make or receive calls on two services
simultaneously while supporting SMS.
 Class B terminal can monitor GSM and GPRS channels
simultaneously, but can support only one of these services at any
time. Therefore, a Class B terminal can support simultaneous
attach, activation, and monitoring but not simultaneous traffic.
Users can make or receive calls on either a packet or a switched
call type sequentially but not simultaneously. SMS is supported in
class B terminals.

GPRS Handsets
 Class C terminal supports only non-simultaneous attach. The
user must select which service to connect to. Therefore, a class C
terminal can make or receive calls from only the manually
selected network service (and so, the service that is not selected is
not reachable). The GPRS specifications state that support of SMS
is optional for class C terminals.
 Each handset will have a unique form factor. So, terminals will
be available in the standard form factor with a numeric keypad
and a relatively small display. Other types of phones with different
form factors, color displays, cameras are common apart from the
latest smart phones.

Bearers in GPRS

 Bearer services of GPRS offer end-to-end packet switched data
transfer.
 GPRS supports two different kinds of data transport services:
point-to-point (PTP) services and point-to-multipoint (PTM)
services.
 GPRS continues to support SMS as a bearer.
 Wireless Application Protocol is a data bearer service over
HTTP protocol, supported by GPRS.
 Multimedia Messaging Service, too, is supported by GPRS.

Applications of GPRS
 Chat
 Multimedia Services
 Virtual Private Network
 Personal Information Management
 Job Sheet Dispatch
 Unified Messaging
 Vehicle Positioning
 Location based services and Telematics

Limitations of GPRS
 Limited cell capacity for all users
 Lower access speed in reality
 No support of GPRS Mobile Terminate Connection for a
mobile server

Billing and Tariffing
Minimum charging information that must be collected are:
 Destination and source addresses
 Usage of radio interface
 Usage of external Packet Data Networks
 Usage of the packet data protocol addresses
 Usage of general GPRS resources and location of the Mobile
Station

Billing and Tariffing
 Various business models exist for charging customers as billing
of services can be based on the transmitted data
volume, the type of service, the chosen QoS profile, etc.
 GPRS call records are generated in the GPRS Service
Nodes.
 Packet counts are passed to a Charging Gateway that
generates Call Detail Records that are sent to the billing
system.

Next Chapter

Wireless Application Protocol

Thanks

Ch7

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