EGPRS: GPRS/EDGE
8.5 Layered Overview of the Radio Interface
8.5.6 Channel Sharing
In GPRS there are two distinct features different from GSM:
• Several timeslots in PTCH on one carrier frequency may be allocated to one user – this is known as ‘bundling’ of timeslots. Timeslots can be bundled on the UL and the DL. The allocation of timeslots may also be asymmetric. For instance, a subscriber who wants to download some data from the internet will receive more timeslots for the DL than for the UL.
• One timeslot is not reserved exclusively for one subscriber; that is, a timeslot may be shared by several subscribers. As a GPRS connection is packet switched, the packet transmissions from several users can be multiplexed through the same timeslots on the Um. In the case that several subscribers share a UL timeslot and all of them want to transmit, if none of them has a higher priority than the others the PCU allows each MS in turn to transmit some data. So the mobiles transmit one after the other until each transmission is complete.
There are two parameters that are important for the allocation of GPRS radio resources:
• the Temporary Flow Identifier (TFI)
• the Uplink State Flag (USF)
These parameters are sent to the mobile as part of the MAC protocol header and are used to give information about channel usage to the mobiles that are sharing a timeslot.
Any data transfer for a single user is associated with the establishment of Temporary Block Flow (TBF), as described in the next section.
8.5.6.1 Downlink Radio Channel
Several subscribers may share a radio channel on the DL. Therefore, in each DL radio block (see Section 8.5.7) a TFI is necessary for determining the owner of each packet.
A TFI has 5 bits, so 32 different values are possible; that is, up to 32 subscribers may the- oretically share a DL radio channel. Several mobiles may also share a radio channel on the UL. These mobiles must be informed when it is their turn to send. Therefore, an addi- tional parameter is sent in each DL radio block: the USF. It indicates which subscriber can send next on the relevant
8.5.6.2 Uplink Radio Channel
An USF has 3 bits, so eight different values are possible; that is, up to eight subscribers may theoretically share a UL timeslot. If one UL timeslot has been configured (reserved) as a PRACH (Packet Random Access Channel – used by an MS to request a connection) then USF=111 is reserved to identify the PRACH and the other seven values (000 to 110) remain to identify up to seven subscribers on this UL timeslot.
8.5.7 TBF
A TBF is a physical connection used by the two Radio Resource entities to support the unidirectional transfer of LLC PDUs on packet data physical channels. The TBF is assigned a radio resource on one or more PDCHs and comprises a number of RLC/MAC blocks carrying one or more LLC PDUs. A TBF is a temporary resource and is main- tained only for the duration of the data transfer. The major function of TBF is an orderly transfer of a number of radio blocks (RLC/MAC) constituting a customer data packet across the air interface.
Each TBF is assigned aTemporary Flow Identity(TFI) by the network. The TFI is assigned in a resource assignment message and is part of the first octet of the RLC/MAC block. The TFI allows several MSs to share one RTSL.
INACTIVE PDP phase(Packet Idle state)
In packet idle mode, no temporary block flow exists.
ACTIVE PDP Phase(Packet Transfer Mode)
In packet transfer mode, the mobile station is allocated a radio resource providing a temporary block flow on one or more physical channels. Continuous transfer of one or more LLC PDUs is possible. Concurrent TBFs may (but do not have to) be established in opposite directions (as mentioned, TBFs are unidirectional and are not connected to the TBF in the opposite direction).
8.5.7.1 TBF Establishment
The schematic procedure for the data transfer session is shown in Figure 8.11.
8.5.7.2 DL TBF Establishment
The SGSN has to know the cell of the MS. It will send the LLC PDUs to the correct PCU.
The PCU allocates one or more PDTCHs for the TBF and indicates that and the TFI to the MS in an assignment message.
114 Introduction to Mobile Network Engineering
Ready State
Request over RACH
Establish Immediate Assignment
Get PDCH Requested Time Slot(s)
TBF session
Release TBF
Connection Data transfer
Figure 8.11 MS initiated DL TBF establishment.
ADL TBFmay be established:
• onPACCHwhen a concurrent TBF exists (or timer is running in the MS).
The PCU sends a PACKET_DOWNLINK_ASSIGNMENT or PACKET_TIMESLOT _RECONFIGURE message. The TBF mode (GPRS/EGPRS) is always the same as the existing UL TBF.
• onPCCCHwhen it is supported and no DL TBF exists.
The PCU allocates one or more PDTCH for the TBF and send a PACKET_
DOWNLINK_ASSIGNMENT message to the MS.
• onCCCHwithout PCCCH and no DL TBF exists.
First, the PCU allocates one PDTCH and sends a IMMEDIATE_ASSIGNMENT mes- sage. The possible multislot allocation is done later with a reallocation message.
The schematic signalling flow is shown in Figure 8.12. The reservation of time slots (groups of four) for radio blocks is only given as long as there is information stored in the buffer memory on the transmit side. As soon as all the buffered data bits have been successfully transferred (including possible ARQ retransmissions) the channel alloca- tion is released and can be used by other connections. When a new burst of data arrives to the buffer, a new reservation of time slots must be made.
P-Immediate Assignment
P-Immediate Assignment Ack
Immediate Assignment (CCCH)
Packet Polling request Packet Polling request (PACCH)
Packet Control Ack (PACCH)
Packet Control Ack Packet Power Control/Timing
Advance Packet Power Control/Timing
Advance
Figure 8.12 Downlink TBF assignment, MS monitors CCCH.