Gprs Channels
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GPRS CHANNELS
1
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 2
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
3
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 deallocated. 4
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. 5
GPRS Logical Channels
6
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. Pa ck et Co mmon C ontrol Ch annel (PCCCH) 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) 7
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.
8
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.
PNCH (Packet Notification Control Channel) is used to inform the MS of incoming PTM message (multicast or group call).
9
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. 10
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
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 that affect Applications Performance
High and varying delays. Low and varying bandwidth. Periods of no connection at all i.e. lost connection.
13
Movement Radio interference
Weather conditions
Concurrent Usage Shadowing Handover
GPRS Bandwidth GPRS Client
Delay
Some server
Disconnection
14
How do reduced bandwidth and increased delay affect applications?
Some applications will work fine.
Some applications will be very slow
Some applications will not work at all (“unfortunate” timers).
15
The impact of delays Delays cause: Interactive computing to get sluggish.
16
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.
17
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.
18
What can be done to overcome GPRS’s limitations?
Actions that increase actual performance
Avoid Small Talk Data Compression Pipelining Local Caching of Info.
19
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 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 20
Data Compression
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.
21
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.
22
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 Any software using information on a server.
23
GPRS Protocols
24
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. 25
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.
26
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 27
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) 28
SGSN
BSS
MS
GMM/SM
GMM/SM
LLC
Application LLC Application RLC MAC GSM/RF
RLC
MAC GSM RF
Relay BSSGP
BSSGP
Network service
Network layer
Physical layer
Phy Layer
Um
Gb
GMM/SM:GPRS Mobilty Management and session Management Protocol GSM/RF:GSM physical layer(radio interface)
Signalling Plane:MS to SGSN 29
RLC/MAC
The RLC/MAC layer provides services for information transfer over the physical layer of the GPRS radio interface. The RLC is responsible for segmentation and reassembly of the LLC packets. The segmentation results in RLC blocks. Control information is added to each RLC block to allow Backward Error Correction (BEC). 30
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 31
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. 32
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.
33
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. 34
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.
35
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.
36
EDGE
37
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.
38
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. 39
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 ”. 40
EDGE is also called EGPRS.
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 41
1
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 2
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
3
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 deallocated. 4
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. 5
GPRS Logical Channels
6
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. Pa ck et Co mmon C ontrol Ch annel (PCCCH) 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) 7
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.
8
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.
PNCH (Packet Notification Control Channel) is used to inform the MS of incoming PTM message (multicast or group call).
9
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. 10
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
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 that affect Applications Performance
High and varying delays. Low and varying bandwidth. Periods of no connection at all i.e. lost connection.
13
Movement Radio interference
Weather conditions
Concurrent Usage Shadowing Handover
GPRS Bandwidth GPRS Client
Delay
Some server
Disconnection
14
How do reduced bandwidth and increased delay affect applications?
Some applications will work fine.
Some applications will be very slow
Some applications will not work at all (“unfortunate” timers).
15
The impact of delays Delays cause: Interactive computing to get sluggish.
16
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.
17
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.
18
What can be done to overcome GPRS’s limitations?
Actions that increase actual performance
Avoid Small Talk Data Compression Pipelining Local Caching of Info.
19
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 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 20
Data Compression
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.
21
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.
22
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 Any software using information on a server.
23
GPRS Protocols
24
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. 25
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.
26
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 27
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) 28
SGSN
BSS
MS
GMM/SM
GMM/SM
LLC
Application LLC Application RLC MAC GSM/RF
RLC
MAC GSM RF
Relay BSSGP
BSSGP
Network service
Network layer
Physical layer
Phy Layer
Um
Gb
GMM/SM:GPRS Mobilty Management and session Management Protocol GSM/RF:GSM physical layer(radio interface)
Signalling Plane:MS to SGSN 29
RLC/MAC
The RLC/MAC layer provides services for information transfer over the physical layer of the GPRS radio interface. The RLC is responsible for segmentation and reassembly of the LLC packets. The segmentation results in RLC blocks. Control information is added to each RLC block to allow Backward Error Correction (BEC). 30
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 31
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. 32
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.
33
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. 34
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.
35
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.
36
EDGE
37
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.
38
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. 39
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 ”. 40
EDGE is also called EGPRS.
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 41
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