gprs final report

8/6/2019 Gprs Final Report

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The GSM system originally offers the following 4 features:

Voice service.

SMS Services.

Fax service.

Data service , maximum Transfer rate 9.6Kbps

Limited capacity, 9.6 kbps.

Uplinkand downlinkchannels were allocated for the entire calltime.

The user consequently paid a lot, as charging was based on

connection time rather than being volume based.

Connection set-up was slow, typically 20-25 secs.

The fact that GSM was designed for voice and not data, meant that,

on average, 50% of radio resources were wasted as the transfer of

data was optimized for speech.

To increase the data transmission rates, in GSM phase 2+ new

bearer services with rates comparable to or higher as ISDN are

developed:

HSCSD (High Speed Circuit Switched Data)

GPRS (General Packet Radio Services)

EDGE (Enhanced Data Rates for the GSM Evolution)


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HSCSD is a circuit switched data service (only point-to-point) for

applications with higher bandwidth demands and continuous data

stream, e.g. motion pictures or video telephony. The higher bandwidth

is achieved by combining 1-8 physical channels for one subscriber.

Additionally, the data transmission codec was changed such that a

maximum of14.4 kbit/s instead of9.6 kbit/s can be transmitted per

physical channel. In this way, HSCSD theoretically enables

transmission rates up to 115.2 kbit/s.

With GPRS it is possible to combine 1-8 physical channel for one

user, just as with HSCSD. Various new coding schemes withtransmission rates of up to 21.4 kbit/s per physical channel enable

theoretical transmission rates up to 171.2 kbit/s. Opposite to HSCSD,

GPRS is a packet-switched bearer service, meaning that the same

physical channel can be used for different subscribers. GPRS is

resource efficient for applications with a short-term need for high data

rates (e.g. surfing the Internet, E-mail, ...). GPRS also enables point-

to-multipoint transmission and volume dependent charging.

Extensions of the GSM network and protocol architecture are

necessary for GPRS implementation.

Fig1


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EDGE is able to realize up to 69.2 kbit/s per physical channel though

the change of the GSM modulation procedure (8PSK instead of

GMSK). Theoretically, transmission rates of up to 553.6 kbit/s(meeting 3G requirements) would be possible by combining up to 8

channels. A combination of GPRS and EDGE could offer optimum

usage of Inter- and Intranet, ensuring highest economy in frequency

resource utilization at the same time.

For circuit-switched communication, the network sets up an air

interface connection by allocating one radio channel to an MS when

data is to be transmitted through the network. Even if only smallamounts of data are transferred, the MS occupies the radio channel

during the connection. The user must pay for the totalconnection

time. Circuit-switched communication is suitable for data traffic when

one or more of the following cases apply:

Constant bandwidth data flow.

Data is sensitive to even small connection delays.

For example circuit-switched communication should be chosen for

videoconferences and notification, the former because of its sensitivity

to connection delays, and the latter because of its almost constantbandwidth.

For packet-switched communication, the network delivers a packet

with data when the need arises. Thus, for the air interface, one radio

channel can be shared between several MSs simultaneously. When an

MS generates a data packet, the network forwards the packet to its

addressee on the first available radio channel. Since data traffic oftenconsists ofbursts of data, the radio channels will be used efficiently.

Address information is included with each packet to enable the packet

to find its addressee. Packet-switched communication is suitable for

data traffic when one or more of the following cases apply:

Data is sent in bursts. (e.g.-mail)

Data is sensitive to errors.(e.g.-Telementry)

For example packet-switched communication should be chosen for

telemetry applications and e-mail, the former because of its sensitivity

to errors and the latter because the data is sent in bursts.


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General Packet Radio Services (GPRS) is a standardized packet-

switched data service for GSM enabling mobile use ofinternet.

The GPRS system provides a basic solution for Internet Protocol

(IP) communication between Mobile Stations (MS) and InternetService Providers (ISP) or a corporate LAN (Local Area Network).

GPRS establishes an end-to-endIP connection from the mobile

terminal to the servers at the ISP. The packet data transmission is

thus carried out on an end-to-end basis, including the air interface.

GPRS users can remain on-line without continuously occupying a

specific radio channel.. The same physical channels will be used

but in a more efficient way since several GPRS users will be able

to share one channel. Thus giving a better channel utilization. In

addition, GPRS channels are allocated only when data is sent orreceived.

GPRS is an extension of the GSM architecture; packet data traffic

runs on a new backbone IP network and is separate from the

existing GSM core networkthat is used for circuit switched traffic.

GPRS stands out as one major development in the GSM standard

that benefits from packet switched techniques to provide mobile

subscribers with the much-needed high bit rates for bursty datatransmissions. It is possible theoretically for GPRS subscribers to

use several time slots (packet data channels) simultaneously

reaching a bit rate of about 170kbit/s.

Volume-based charging is possible because channels are allocated

to users only when packets are to be sent or received.

Overall the GPRS offers the following:

Enhancement of GSM data transfer capabilities.

A new set of bearer services.

Anew kind of data-pipe.

Focus on IP-networking.

Quality of Service categories.

Packet switching technology.

Efficient use of air interface resources.

Volume based traffic.

Always connected. High data rate.


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Fig2

This Figure roughly illustrates areas ofbursty and/or bandwidth

consuming communication. Burstiness and bandwidth requirements

affect the type of communication chosen - circuit-switched, packet-

switched, or e.g. SMS (Short Message Service) communication.

Burstiness applications are those that require too many bursts of the

same or similar signals, therefore low bandwidth. Bandwidth

applications are those that require large bandwidth to convey the

information but a few bursts are used at the beginning of the sessionto establish a connection.


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GPRS is, on the one hand, intended to provide the possibility of

transmitting large volumes of data in a very short time.

On the other hand it is meant to ensure effective management of

available resources, which will increase the number of users and

reduce the costs arising for the individual user (volume-oriented

fees).

Another positive consequence of the introduction ofGPRS is its

direct access to the Intra- and Internet and the possibility to use

point-to-point and point-to-multipoint services side by side.

Fig3

GPRS Services are defined to fall in one of two categories: PTP

(Point-To-Point) and PTM(Point-To-Multipoint) services.

PTP (Point-To-Point) Services:-

GPRS will support applications based on IP. Applications basedon the Connection Oriented Network Protocols are also defined

to be supported. The X.25 protocol was initially mentioned but

has been dropped in recent standard developments

PTM (Point-To-Multipoint) Services:-

The PTM services provide the subscribers with the capability to

send data to multiple destinations within one single service

request. With the exception ofPTM-M (Point-To-Multi-point

Multicast) services, groups must be defined and members arerequired tojoin an ongoing call to become participants.


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It is probable that once a wireless Internet infrastructure is in place, it

will be used in ways that we cannot predict. This is all the more likely

as parallel developments in technology will, for example, raise thecomputing power of a wireless terminal by a factor of hundreds.

With GPRS a new complete solution to mobile computing is

available.

It is possible to build cars with GPRS communication facilities.

This would provide GPRS subscribers

with the opportunity to utilize their

laptops while traveling on the bus ortrain.

GPRS will make the work ofjournalists and photographers easier

and cheaper.

For introducing GPRS, the logical GSM architecture is extended

by two functional units:

The Serving GPRS Support NodeSGSN is on the same

hierarchic level as MSC and has functions comparable to

those of a Visited MS (VMSC).

The Gateway GPRS Support NodeGGSN has functions

comparable with those of a Gateway MSC (GMSC) andoffers interworking functions for establishing contact

between the GSM/GPRS-PLMN and external packet data

networks PDN.

A GPRS Support Node GSN includes the central functions

required to support the GPRS. One PLMN can contain one or more

GSNs.


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In addition to GSN, extensions of functions in other GSM functional

units are necessary:

In theBSS a Packet Control UnitPCU ensures thereception/adaptation of packet data from SGSNintoBSS and

vice versa.

Fig4

Fig5

The Architecture with Interfaces

Terminal Equipment TE

Mobile Terminal MT

Mobile Station MS

Base Station Controller BSCBase Transceiver Station BTS


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Gateway Mobile Services

Switching Center

GMSC

Mobile Services Switching

Center

MSC

Visitor Location Register VLRHome Location Register HLR

Authentication Center AUC

Equipment Identity

Register

EIR

Serving GPRS Support

Node

SGSN

Gateway GPRS Support

Node

GGSN

GPRS Interfaces Gx

Serving GPRS Support Node (SGSN) Functions:-SGSN is on the same hierarchic level as an MSC and handles

many functions comparable to a Visited MSC (VMSC)

Is the node serving GPRS mobile stations in a region

assigned to it.

Traces the location of the respective GPRS MSs(Mobility

Management functions).

Is responsible for the paging of MS.

Performs security functions and access control(authentication/cipher setting procedures).

Has routing/traffic-management functions.

Collects data connected with fees/charges.

Realizes the interfaces to GGSN (Gn), PCU (Gb), other

PLMNs (Gp), HLR (Gr), VLR (Gs), SMS-GMSC (Gd),

EIR (Gf).


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Gateway GPRS Support Node (GGSN) FunctionsGGSNrealizes functions comparable to those of a gateway MSC.

GGSN

Is the node allowing contact/interworking between a GSM

PLMN and a packet data networkPDN (realization Gi-interface).

Contains the routing information for GPRS subscribers

available in the PLMN.

Has a screening function.

Can inquire about location information's from the HLR via the

optional Gc interface.

Transfers data/signaling to SGSN via Gn interface.

Fig7

SGSN and GGSN functions, respectively, can be located within the

same physical unit or at different locations in different physical units.

SGSN and GGSN include the internet protocol (IP) routing function

and can be linked together/Interconnected with IP routers (IP-based

GPRS backbone network for Gn). The same holds for the Gp interface

(SGSN and GGSN in different PLMNs).


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HLR (GPRS Register GR)HLR includes the GPRS subscriber information (GPRS Register GR)

and routing information.

Access to HLR is possible from SGSN via Gr and from GGSN via Gc

interface.

Packet Control Unit PCU:-In the BSS, the PCU serves

For the management ofGPRS radio channels (Radio

Channel Management functions), e.g. power control,

congestion control, broadcast control information

.

For the temporal organization of the packet data transfer for

uplinkand downlinkit has channel access control functions,e.g. access request and grants.

It serves for converting protocols from the Gb interface to

the radio interface Um.

Three options for positioning the PCU are provided

Option A: In the BTS

Option B: in the BSC

Option C: In spatial connection with the SGSN.

Fig9

Channel Codec Unit CCU:-The CCU contains the following functions:

Channel coding, including forward error correction FEC and

interleaving.

Radio channel measurements, including received quality and signal

level, timing advance measurements.


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Terminal Equipment (TE)TE is the computer terminal on which the end-user works. This is the

component used for the GPRS system to transmit and receive end-user

packet data. For example, the TE could be a laptop computer. The

GPRS system provides IP connectivity between the TE and an InternetService Provider or Corporate LAN (Local Area Network) connected

to the GPRS system. From the TE point of view, you could compare

the Mobile Terminal to a modem, connecting the TE to the GPRS

system.

Mobile Terminal (MT)The MT communicates with a TE, and over the air with a BTS. The

MT must be equipped with software for GPRS functionality when

used in conjunction with the GPRS system.

Mobile Station (MS):-A GPRS MS can work in three different operational modes. The

operational mode depends on the service an MS is attached to (GPRS

or GPRS and other GSM services) and on the mobile stations

capacity of simultaneously handling GPRS and other GSM services.

Class A:- mode of operation allows an MS to have a circuit-

switched connection at the same time that it is involved in apacket transfer.

Class B:- mode of operation allows an MS to be attached to

both CS and PS but it cannot use both services at the same

time. However, MS that is involved in a packet transfer can

receive a page for circuit-switched traffic. The MS can then

suspend the packet transfer for the duration of the circuit-

switched connection and afterwards resume the packet transfer.

Fig10

Class C:- mode of operation allows an MS only to be attached to one

service at the time. An MS that only supports GPRS and not circuit-switched traffic will always work in class C mode of operation.


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Hint About GPRS Subscriber Profile:-

The GPRS Subscriber Profile is the description of the services a

subscriber is allowed to use. Essentially, it contains the description

of the packet data protocol used.

A subscriber may also use different packet data protocols (PDPs),or one PDP with different addresses. The following parameters are

available for each PDP:

The packet network addressis necessary to identify the subscriber

in the public data net. Either dynamically assigned (temporary)

addresses or (in the future) static addresses are used in case of IP.

The quality of service QoS: QoS describes various parameters. The

subscriber profile defines the highest values of the QoS parametersthat can be used by the subscriber.

The screening profile: This profile depends on the PDP used and

on the capacity of the GPRS nodes. It serves to restrict acceptance

during transmission/reception of packet data.

Fig11

In GPRS on the other hand, a group of cells is called a RA (Routing

Area). The SGSN controls a service area containing several RAs.

There may not be a direct mapping between SGSN and MSC/VLR

service areas but a RA is a subset of one, and only one, LA. GPRS has

chosen a different layout from GSM (i.e., RAs instead of LAs) to

allow for signaling and paging over geographically smaller areas and

thus, a better optimization of radio resources. One possible

implementation of GPRS in the existing GSM network ofFigure A isshown in Figure B. The example suggests 3 SGSN service areas to


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span over 11 RAs. The reader should be aware that the example is

simplified to illustrate the difference between GSM and GPRS service

areas. In a real network implementation, the layout is decided by the

operator of the network..

GSM Network Service Area GPRS Network Service Area

The air interface makes it necessary to introduce protocols, which

adopt the size of the packets. They perform segmentation/re-

assembly depending on the direction of the packets to be able to

send IP packets via an air interface which consists of bursts which

a fixed bit structure.

One of the main advantages ofGPRS compared to HSCSD is that

it is packet switched. This can only be done by introducing newnetwork elements using new hardware/protocols and by changes in

the protocol structure on Um to enable packet switching. The latter

is done by the MAC protocol.

The GPRS protocol stack from a BSS perspective is shown in next

Figure. The layers are described in the following list. The first

layer is implemented in the BTS. The Packet Control Unit (PCU),

which is new hardware in the BSC, handles the other BSS protocol

layers.


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Fig12

L2, L1: L2 and L1 are the link layer and physical layer of the

external networks connected via the Gi-interface to the GSM-GPRS-

PLMN.

GTP (GPRS Tunneling Protocol):-

GPRS Tunneling Protocol (GTP) tunnels user data and signaling

between GSN in the GPRS backbone network. The GTPencapsulates all Point-To-Point (PTP) Packet Data

Protocol (PDP) Packet Data Units (PDUs). GTP provides

mechanisms for flow control between GSNs if required.

IP is the GPRS backbone network protocol used for routing user data

and signaling.

The SubNetwork Dependent Convergence Protocol (SNDCP) is

situated below the network layer and above the Logical Link Control(LLC) layer in the MS and the SGSN,

SNDCP performs the following subfunctions:

Multiplexing of data packets from one or several

applications onto one logical link.

Compression of redundant protocol control

information and user data. This may include e.g.

TCP/IP header compression and V.42 bis datacompression.


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Segmentation and reassembly. The output of the

compression subfunctions are segmented to

maximum-length LLC frames, also called LLC Packet

Data Unit (LLC PDU)

The LLC protocol provides a reliable logical linkbetween the MS

and its SGSN. LLC provides the services necessary to maintain a

ciphered data linkbetween an MS and an SGSN.

The relay function is different depending on whether it is in BSS or

in SGSN. In BSS it relays LLC PDUs between the Um and Gb

interfaces. In SGSN it relays PDP PDUs between the Gb and Gn

interfaces.

Base Station System GPRS Protocol (BSSGP) conveys routing-,

QoS (Quality of Service)-related between BSS and SGSN.

Network Service (NS) transports BSSGP PDUs.

RLC/MAC(Radio Link Control/Medium Access Control) contains

two functions. The RLC function provides a radio-solution-

dependent reliable link. The MAC function controls the access

signaling (request and grant) procedures for the radio channel, andthe mapping of LLC frames onto the GSM physical channel.

GSM Radio Frequency (GSM RF) TSs makes up the Time-

Division Multiple Access (TDMA) frame.

The channel allocation in GPRS is different from GSM. GPRS allows

a single mobile station to transmit on multiple time slots of the same

TDMA frame (multi-slot operation).

This results in a very flexible channel allocation: one to eight time

slots per TDMA frame can be allocated for one mobile station.


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Channel coding starts with the division of digital information into

transferable blocks.

These radio blocks, i.e. the data to be transferred (prior to encoding)comprise:

A header for the Medium Access Control MAC (MAC

Header).

Signaling information (RLC/MAC Signaling Block) or user

information (RLC Data Block).

A Block Check Sequence BCS.

Fig14

The RLC/MAC layer implements two functions, Radio Link Control

(RLC) and Medium Access Control (MAC) functions. RLC provides a

radio-solution-dependent reliable link. MAC controls the access

signaling (request and grant) procedures for the radio channel, and the

mapping of LLC frames onto the GSM physical channel (also referred

to as GSM RF).

Multiframe Structure


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In addition to the nine existing logical radio channels used for

signaling (BCCH, SCH, FCCH, PCH, RACH, AGCH as well as

SDCCH, SACCH and FACCH) and the Traffic Channel (TCH) forcircuit switched user information, a new set of logical channels was

defined for GPRS.

Fig15

Packet Data Traffic Channel (PDTCH):-Is employed for the transfer ofuser data. It is assigned to one mobile

station (or in the case of PTM to multiple mobile stations). One

mobile station can use several PDTCHssimultaneously.

Packet Broadcast Channel (PBCCH):-

Is a unidirectional point-to-multipoint signaling channel from the BSS

to the mobile stations. It is used to broadcast specific information

about the organization of the GPRS radio networkto all GPRS mobile

stations in a cell.

Packet Common Control Channel (PCCCH):-

Is a bi-directional point-to-multipoint signaling channel that transports

signaling information for network access management.

PRACH: Packet Random Access Channel, used by the

mobile to request one or more PDTCHs.

PAGCH: Packet Access Grant Channel, used to allocate oneor more PDTCHs to the mobile station.


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PPCH: Packet Paging Channel, used by the BSS to page

(find out the location of a mobile station) the mobile prior to

downlink packet transmission.

PNCH: Packet Notification Channel, used inform a mobile

station of incoming PTM messages (multi-cast or group

call).

Dedicated Control Channels:-

Are bi-directional point-t-point signaling channels. There are two

channels:-

PACCH: Packet Associated Control Channel, alwaysallocated in combination with one or morePDTCH that are

assigned to one mobile station. It transports signaling

information related to one specific mobile station (e.g.,

power control information, ACK/NACK messages & packet

resource assignment messages).

PTCCH: Packet Timing advance Control Channel, used for

adaptive frame synchronization.

Summary of Logical Channels in GPRS


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The GPRS packet data traffic is arranged in 52-type multiframes.

52 TDMA frames in each case are combined to form one GPRS

traffic channel multiframe, which is subdivided into 12 blocks with 4TDMA frames each. One block (B0-B11) contains one radio block

each (4 normal bursts, which are related to each other by means of

convolutional coding). Every thirteenth TDMA frame is idle. In the

idle frame the PTACCH is sent. The idles frames are used by the MS

to be able to determine the various base station identitycodes BSIC, to

carry out timing advance updates procedures or interference

measurements for the realization of power control.

Fig17

In the diagram below, the three MSs have TSs 1 to 4 assigned to them

for uplink packet transfer. Since they are sharing the same TSs, then

each TS must have a different name for each MS. This name orindicator is called the USF. Hence TSs 1 to 4 are indicated to by USFs

1 to 4 for MS1, USFs 5 to 8 for MS2 and USFs 9 to 12 for MS3. On

the downlink the three MSs will read the USF value and the one that

has that value will use the corresponding TS(s) to send uplink packets.

In the uplink Direction the reason for having a USF along with the

TFI, is that MSs are sending packets to one BSC/PCU, so collision

would happen between two MSs on the same TS(s). Where as in the

downlink direction, MSs are receiving packets from one BSC/PCU.

The uplinkTFI value would refer to the block flow to a certain MSand the USF would determine on which TS(s) will it be sending on.


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In the downlink direction, packets will be received by all MSs

listening to that TS and the MS with the matching downlink TFI will

handle that packet.

Fig18

The PDCHs are allocated to the PCU. The PCU is responsible for

assigning channels to the different GPRS MSs. The PDCHs can be

allocated in different ways:

Dedicated PDCHs are allocated and released by operator

command.

On-demand PDCHs, serving as temporary dynamic GPRS

resources, are allocated and released depending on GPRS

traffic demand.

Master PDCH:-

A Master PDCH (MPDCH), is a PDCH carrying a PBCCH and a

PCCCH, as well as GPRS traffic. The PCCCH carries all the

necessary control signaling to initiate packet transfer. In the

standard, the MPDCH is called the PDCH carrying the

PBCCH.

The first dedicated PDCH that is allocated according to the

operators preferences regarding non-hopping BCCH will be

configured as an MPDCH. The following PDCHs that are


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allocated will only carry GPRS traffic and associated signaling. If

the operator decreases the number of dedicated PDCHs, the

MPDCH is kept as long as there is at least one dedicated PDCH

in the cell. In a cell with no MPDCH (no dedicated PDCH

allocated) the ordinary control channels, like BCCH, RACH etc,will handle the broadcasting and signaling to the GPRS mobiles.

There are three GPRS mobility management states, listed below. See

also. The SGSN knows the state ofall MSs that are in standby or

ready state

Fig20

Idle state:

The MS is turned on but not GPRS attached. The MS is

invisible to GPRS, e.g. if the MS is outside the coverage area

for GPRS.

Standby state:

The MS is GPRS attached and sends routing area updates to the

SGSN every time it changes Routing Area.


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Ready state:

A packet transfer is ongoing or has recently ended. A ready

timer defines how long time the MS shall remain in ready state

after a transfer. The time is decided by SGSN and can take

values from zero to infinity; i.e. the MS shall never go back toStandby state.

Fig21

The network may provide coordination ofpaging for circuit-switched

and packet-switched services and also a coordination of RA/LA

updates done by the mobile station.

Three network operation modes then are defined:

Mode I:-The Gs interface is present and the Master PDCH may

exist.

Mode II:- The Gs interface is not present and the Master PDCH

doesnt exist.

Mode III:-TheGs interface is not present and the Master

PDCH may exist.


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