GPRS Session2
GPRS Session2
GPRS Session2
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GPRS Channels
The GPRS system uses the physical radio
channels as defined for GSM.
A physical channel used by GPRS is called a
Packet Data Channel (PDCH).
The PDCHs can either be allocated for GPRS
(dedicated PDCH) or used by GPRS only if no
circuit-switched connection requires them
(on-demand).
The PDCHs are taken from the common pool
of all channels available in the cell
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GPRS Channels contd..,
The radio resources of a cell are shared by all
GPRS and all non-GPRS mobiles in the cell.
The mapping of physical channels to either
GPRS or GSM usage can be performed
dynamically, based on:
Capacity on demand principle
Depending on the current traffic load, priority of
service, and the multi slot class
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GPRS Channels contd..,
The number of channels allocated to GPRS
can be changed according to current demand.
Physical channels not currently in use by
conventional GSM, can be allocated as PDCHs
to increase the GPRS quality of service.
When there is a resource demand for services
with higher priority, PDCHs can be de-
allocated.
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GPRS Channels contd..,
4 possible channel coding schemes
(CS1….CS4). depending on the quality
of the radio link.
Data rates supported per timeslot are
9.06, 13.4, 15.6, and 21.4 kb/s.
When all 8 timeslots are available,
throughput can reach 8 x 21.4 kb/s =
171.2 kb/s.
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GPRS Logical Channels
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GPR S CH ANNE LS
Pa ck et Broad cas t Con trol C hann el (PB CCH)
The packet broadcast control channel (PBCCH) is a
unidirectional point to-multipoint signaling channel from the BSS
to mobile stations.
Used by the BSS to broadcast configuration data about the
GPRS network to all GPRS mobile stations.
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GPRS Channels contd.,
PR ACH (Packe t Ran dom Acces s Ch an nel ):
MS uses this channel whenever they need to access
the network to initiate an uplink packet transfer or to
respond to paging messages in order to initiate a
downlink packet transfer.
Pa cket Acce ss Gran t Ch an nel (PA GCH ):
This channel is used in the packet transfer
establishment phase to send channel reservation
acknowledgements to a MS prior to packet transfer.
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GPRS Channels contd.,
PPCH (P acket Pagin g Cha nnel) :
PPCH (Packet Paging Channel) is used by BSS
to find out the location of a MS (paging) prior
to down-link packet transfer.
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GPRS Channels contd.,
Packet Da ta T raf fi c Ch ann el ( PDT CH )
The traffic channel is an up and downlink
function used for user data traffic transfer.
PDTCH is temporarily dedicated to a user or
group of users.
PDTCH for uplink and PDTCH for downlink
are unidirectional and assigned separately to
support asymmetric user traffic flow.
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GPRS Channels contd.,
Pack et -Ded icat ed Co ntr ol Ch an nel (PD CC H)
Packet Associated Control Channel (PACCH)
An uplink and downlink function used to carry signaling
information to and from the mobile station
Pack et Timi ng Adv an ce Co ntr ol Ch an nel /Up lin k
(PT CC H/UL )
Used for estimation of timing advance of one mobile
station
Pack et Timi ng Adv an ce Co ntr ol Ch an nel /DL
(PT CC H/DL )
Used to transmit timing advance information to several
mobile stations 11
GPRS Logical Channels
Group Channel Function Direction
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Factors that affect Applications
Performance
High and varying delays.
Low and varying bandwidth.
Periods of no connection at all i.e. lost
connection.
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Movement
Concurrent Usage
Shadowing
Handover
GPRS
Bandwidth
GPRS Client
Delay
Some server
Disconnection
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How do reduced bandwidth and increased
delay affect applications?
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The impact of delays
Delays cause:
Interactive computing to get sluggish.
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The impact of low bandwidth
Transfers of larger amounts of data
takes a long time.
Certain kinds of applications do not
work well or at all.
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The impact of temporary
disconnection
Temporary disconnection is caused by
radio shadow and slow handover
between base stations and can cause :
Applications to freeze temporarily.
Tasks might “time-out” - redo whole
operation.
Decreased performance.
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What can be done to overcome
GPRS’s limitations?
Actions that increase actual performance
Avoid Small Talk
Data Compression
Pipelining
Local Caching of Info.
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Small Talk
Since GPRS has a high latency (according to
specification, up to 500ms), the number of
requests/replies should be kept as low as
possible
e.g. loading web page containing 10 small
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Pipelining
Facilitates multiple requests to be sent
without waiting for a response.
Multiple requests and responses can be
contained in a single TCP segment.
Used to cut down on round trip delays,
improving performance, and reducing the
number of packets even further.
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Local caching of frequently used
information
What can be cached ?
e.g. files, databases, web pages, IP
addresses.
What communication applications could
benefit from caching information?
Browsers
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GPRS Protocols
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General Packet Radio Service or GPRS
Protocol Stack is a packet switched
technology, based on GSM.
The radio and network resources are only
accessed when data actually needs to be
transmitted between the mobile user and the
network.
GPRS Protocol Stack facilitates instant
connections whereby information can be sent
or received immediately as the need arises.
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The manner in which the data associated with the
application running on the laptop gets encapsulated
depends on the operating system.
For example Windows CE/ 95/98/NT, palm OS and
Linux all have a built-in TCP/IP stack. Therefore,
such equipment will work with GPRS.
The data will be further encapsulated by means of
the Point-to Point protocol before transmission to the
GPRS-enabled mobile station.
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Upon reception of the data from the laptop
decapsulation is performed up to the IP level.
The data is then encapsulated for
transmission over the air interface by some
GPRS-specific protocols. Some of them are:
Sub -Netw ork Depend ent Conve rgenc e
Prot ocol (S NDCP )
LLC
RLC
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GPRS Air Interface
The physical channel dedicated to GPRS
traffic is called a Packet Data Channel
(PDCH).
Capacity on demand
PDCH(s) are increased or decreased
according to demand
Load supervision is done in MAC Layer
(provides flexibility)
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SGSN
MS BSS
GMM/SM
GMM/SM
Application LLC
LLC
Relay
Application RLC BSSGP BSSGP
RLC
MAC Network
MAC Network service layer
GSM/RF GSM RF Physical layer Phy Layer
Um Gb
GMM/SM:GPRS Mobilty Management and session Management Protocol
GSM/RF:GSM physical layer(radio interface)
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The RLC function offers a reliable radio link to
the upper layers.
The MAC function handles the channel
allocation and the multiplexing, i.e. the use of
physical layer functions.
The RLC/MAC layer together form the open
system interconnection (OSI) Layer 2 protocol
for the Um interface and uses the services of
the physical link layer
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LLC
The LLC layer is responsible for handling the virtual
connection between the SGSN and the GPRS mobile
station and exists even when no physical resources
are available between the two.
It supports peer-to-peer data transfer between the
SGSN and the GPRS mobile station.
For each Temporary Logical Link Identifier (TLLI) the
LLC offers various services using the Network Service
Access Point Identifier (NSAPI).
These services may include Quality of Service classes
for user data, GMM/SM signaling information and/or
SMS data.
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LLC
In addition to managing the logical link, the
LLC layer ensures user data confidentiality
using ciphering/encryption features.
The transmission of LLC PDUs is possible in
the acknowledged and unacknowledged
mode.
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SNDCP
The Sub- Ne twor k D epe ndent
Converg ence P rot oc ol (SNDCP) layer is
located above the LLC layer in the SGSN and
GPRS mobile station.
Its central task is to improve the channel
efficiency.
To do so, the SNDCP layer compresses
header information and user data using
separate algorithms to minimize the amount
of information transmitted over the air
interface.
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SNDCP
So, the sub-network-dependent convergence protocol
(SNDCP) mainly is a mapping and compression function
between the network layer and lower layers.
It also performs segmentation, reassembling and
multiplexing.
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BSSGP (BSS GPRS Protocol)
The BSSGP carries out the flow control from
the SGSN to the BSS.
Upon reception at the SGSN the data is
formatted for efficient transfer over the public
data network.
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EDGE
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Enhanced Data Rates for GSM Evolution, is a
cost-efficient upgrade to existing GSM/GPRS
EDGE operates in existing spectra and boosts
the speed over the air interface.
You can think of EDGE as a mechanism that
squeezes in more capacity into each resource
(time slot) over the air interface.
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While a GPRS upgrade mainly consists of new nodes
in the core network, EDGE accelerates speeds over
the air.
EDGE provides nine different coding schemes known
as MCS (compared to the four that GPRS uses), and
you can switch a connection between different
schemes.
Each MCS state may use either GMSK (low data rate)
or 8-PSK (high data rate) modulation for network
access, depending on the instantaneous demands of
the network and the operating conditions.
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The choice of coding scheme is dynamic and
depends on the received signal quality.
As the signal quality goes down, EDGE
switches to a coding scheme that is more
robust but that also gives a lower throughput.
This adaptive capability to select the best air
interface is called “increme nta l
redu nda ncy ”.
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EDGE is also called
EGPRS.
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