导图社区 09-GPRS
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GPRS General Packet Radio Service
The GPRS is a packet data service.
A number of TDMA slots are assigned to the GPRS data channels and then shared dynamically in uplink and downlink by different UEs.
Therefore statistical multiplexing of multiple flows on the same physical resources (time slots) is adopted.
The fixed part of the network is an basically IP(Internet protocol) network.
Core network elements (SGSN and GGSN) are basically IP routers.
GPRS/EDGE: Radio Interface
GPRS
GPRS uses the same TDMA scheme of GSM and can share radio sources with it.
Some TDMA carriers can be allocated to GPRS.
In the same carrier, some slots can be used by GPRS and other used by GSM.
GPRS use the same physical bursts of GSM.
EDGE
It is just an evolved version of GPRS.
It uses higher layer modulation scheme.
Physical and Logical Channels
The physical channel used by GPRS is PDCH.
PDCH: Packet Data Channel
The PDCH occupies one slot per frame.
Multiple PDCHs are managed by GPRS on the different time slots.
The minimum transmission unit is Radio Block.
Radio block is just 4 noraml brusts transmitted on the same time slot in consecutive frames, in EDGE, it can be 2.
On the PDCH, there are 3 logical channels are mapped.
PDTCH: Packet Data Traffic Channel (Up and down)
PACCH: Packet Associated Control Channel
PTACCH: Packet Timing Advanced Control Channel
Optional Logical Channels
GPRS can choose some common control channels.
PCCCH: Packet Common Control Channel
PPCH: Packet Paging Channel
PRACH: Packet Random Access Channel
PAGCH: Packet Access Grant Channel
PNCH: Packet Notification Channel
PBCCH: Packet Boardcast Control Channel
If no present, GPRS uses the equivalent GSM control channel.
Mapping of Logical Channels
A multi-frame of 52 frames is used.
48 frames are used for transmitting 12 Radio Blocks.
2 for PTACCH (Packet Timing Advanced Control Channel).
2 for IDLE.
Radio Blocks are shared dynamically between users who assigned to the same PDTCH and between PDTCH and PACCH.
UEs can simultaneously monitor multiple PDCH (up to 6) on different time slots in up/downlink.
The Medium Access Control can assign multiple block in the same or different time slots for EUs to transmit or receive.
The allocation of Uplink and downlink is independent.
Resource Assignment
Transmission resource assignment can be static or dynamic (more commonly).
The UE has to monitor the time slots assigned in the downlink during association phase.
The set of Up/dpwnlink slots can be modified by the network with a reconfiguration command on the PACCH.
A TFI (Temporary Flow Identifier) is also allocated to UE that can identify the header and downlink blocks which of them must be received and processed.
For the assignment of Uplink blocks, UE has to request via RACH/PRACH.
In the reply message on the AGCH/PAGCH, an USF (Uplink State Flag) is temporary assigned to UE.
USFs are transmitted in the downlink blocks and used by PCU to assign uplink blocks to UEdynamically.
TFI and USF of downlink blocks are in general not associated to the same UE (uplink and downlink scheduling are independent).
Protocols User Plane→ ←Control Plane
MAC (Medium Access Control)
It allows resource sharing of GPRS channels at the radio interface.
Manages packet multiple access in the uplink.
and dynamic multiplexing in the downlink.
RLC (Radio Link Control)
It implements a ARQ scheme on RLC blocks (selective reject).
LLC (Logical Link Control)
It is a layer 2 protocol similar to LAPD that is used to implement a virtual and encrypted point-to-point connection between SGSN and UE.
SNDCP (Simple Network Dependent Convergence Protocol)
It has basically the task of adapting the upper layer protocol (typically IP) to the transport offered by GPRS.
BSSGP (BSS GPRS Protocol)
It is also a convergence protocol that provides a virtual connection between PCU and SGSN over the network service (typically IP).
GTP (GPRS Tunneling Protocol)
It allows the transparent transfer of packets (IP) between GGSN and SGSN crossing the IP network of the GPRS provider; it is similar to tunneling protocols of IP world (like IPSec) but it makes use of a transport layer (TCP or UDP).
GTP-C
GTP for the control plane.
GMM (GPRS Mobility Management)
it used for mobility management procedures.
SM (Session Management)
It is used for session setup and tear down.
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GPRS mobile states
IDLE
not attached to GPRS.
UE is not reachable.
READY
UE can transmit/receive packets and do not need paging.
UE is traced on a cell basis.
STANDBY
UE attached to GPRS.
UE can receive packets through paging only.
UE can transmit packets (this generates a transition to READY state. )
UE is traced on a Routing Area basis.
Session management: PDP context activation
For starting a data session, PDP context must be activated.
A PDP context is a logical connection to the Internet, in particular, with a gateway identified by an APN (Access Point Name).
During the activation, the UE is assigned an IP addresses and have some optional quality parameters.
Mobility management
Mobility management in GPRS follows the same principles of that in GSM, with some differences.
In STANDBY state, UE is traced on a Routing Area basis with its RAI (Routing Area Identifier).
In READY state, UE is traced on a cell basis, but there is no handover procedure but just cell reselection operated by UE (there is also a NACC – Network Assisted Cell Change mode).
Cell Update
When a UE in READY state detects a new cell within its current RA, it performs a cell update procedure by sending any LLC frame containing its identity.
Routing Area Update
RA update can be between RAs of the same SGSNs or of different SGSNs.
Packet forwarding
IP packet forwarding in GPRS, and also all subsequent mobile technologies, is based on tunnels.
In the specific case of 2G and 3G technologies there are two tunnels.
A level 2 tunnel managed by the LLC protocol between UE and SGSN.
A level 4 tunnel managed by the GTP protocol between SGSN and GGSN.
Mobility management and tunnels
Tunnels are also used to modify packet forwarding according to UE mobility.