Gprs Channels

GPRS CHANNELS

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GPRS Channels 

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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 (ondemand). The PDCHs are taken from the common pool of all channels available in the cell 2

GPRS Channels contd.., 

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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 multislot class

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GPRS Channels contd.., 

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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 deallocated. 4

GPRS Channels contd.., 

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EDGE can carry data speeds up to 236.8 kbit/s for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots) in packet mode 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. 5

GPRS Logical Channels

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GPR S CH ANNE LS 

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Pa cket Broad cast Co ntro l Ch ann 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. Pa cket Commo n C ontrol Chan nel (PCC CH ) A control channel service for signaling for the packet data:    

Packet Packet Packet Packet

Random Access Channel (PRACH) Paging Channel (PPCH) Access Grant Channel (PAGCH) Notification Channel (PNCH)

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GPRS Channels contd., 

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PRACH (P acket Ra ndom Acc es s Ch annel) : 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. Packet Acc ess Gr an t Ch ann el ( PA GCH): This channel is used in the packet transfer establishment phase to send channel reservation acknowledgements to a MS prior to packet transfer. 8

GPRS Channels contd., 

PPCH (Pa cket Pagin g Cha nnel) : PPCH is used by BSS to find out the location of a MS (paging) prior to down-link packet transfer.

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PNCH ( Pa cket N oti fi cati on Cont rol Ch ann el) : PNCH is used to inform the MS of incoming PTM message (multicast or group call).

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GPRS Channels contd., 

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Pack et D ata Tra ffi c Channe l (PD TCH) 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. 10

GPRS Channels contd., Pa cket -Ded ica te d Contr ol Cha nn el (PDC CH) Pa cket Asso ci at ed Co ntrol C han nel (PAC CH ) 

An uplink and downlink function used to carry signaling information to and from the mobile station

Pa cket Timin g Advan ce Contr ol Cha nne l/Up lin k (P TCCH/U L)

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Used for estimation of timing advance of one mobile station

Pa cket Timin g Advan ce Contr ol Cha nne l/DL (PTCC H/DL )

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11 Used to transmit timing advance information to several

GPRS Logical Channels Group

Channel

Packet data Traffic channel

PDTCH

Packet broadcast control channel

PBCCH

Packet common Control Channel (PCCCH)

Packet Dedicated Control Channels

Function Data Traffic

Direction MS

BSS

Broadcast Control

MS

BSS

PRACH

Random Access

MS

BSS

PAGCH

Access Grant

MS

BSS

PPCH

Paging

MS

BSS

PNCH

Notification

MS

BSS

PACCH

Associated Control

MS

BSS

PTCCH

Timing Advance Control

MS

BSS 12

Factors which affect practical GPRS performance    

Available network resources Concurrent usage - user population Type of application Mobile movement (fading)

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How do reduced bandwidth and increased delay affect applications? 

Some applications will work fine.

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Some applications will be very slow

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Some applications will not work at all (“unfortunate” timers).

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The impact of low bandwidth 

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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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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 

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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 images could waste up to 5 seconds just waiting for replies from the server. To overcome such an overhead, data should be collected into one packet and transmitted within one single operation 17

Data Compression 

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With GPRS limited bandwidth, compression of data can improve the achieved bandwidth for data transfers. Compression & decompression depend on the mobile stations processing capability and memory storage.

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Local caching of frequently used information 

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What can be cached ?  e.g. files, databases, web pages, IP addresses. What communication applications could benefit from caching information?  Browsers  Any software using information on a server.

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