FACH and cell DCH

Hi sir,

'RRC State' refer to various different phases in which UE/Network be after RRC Connection Setup and before RRC Release. In most case, these states occurs after Radio Bearer Setup. RRC State Change refers to the process of switching between these states.

Roughly there are three different stages (DCH, FACH, PCH).. but in more detail you can classify them into four different stages as shown below. As you see below, in most case you can jump from one states to any other stages by single step, but there are a couple of cases you cannot switch with a single step. For example, you cannot switch from CELL_PCH to URA_PCH directly. You can switch from DCH to PCH directly, but you cannot switch from PCH to DCH directly.

https://www.sharetechnote.com/image/UMTS_RrcStateChange_01.png

Let's think about what can happen in each of these states.

DCH : You can call this state as 'Normal traffic mode'. When you make any connection for traffic (e.g, voice call and data call), in most case you (UE/Network) establish DCH state and most of traffic (Voice data, packet data) are being transmitted and received in this stage.

FACH : This is the stage in which UE can still send and receive user data but at much lower data rate comparing to DCH. For the detailed understanding on this stage, you have to understand the detailed channel mapping. But it is out of scope of this page. see Cell FACH Channel Mapping for R99/R5/R6 and R7.

PCH : In terms of mode of operation, PCH is very similar to IDLE mode. UE cannot send and receive the user data, it can only monitor/recieve SIBs and Paging. The difference between PCH and IDLE mode is that PCH is still a kind of 'RRC Connected' stage. So it would require less steps of RRC procedure to go back to FACH/DCH for user data transaction.

Hello friend!

The CELL_DCH state is characterised by:

•A dedicated physical channel is allocated to the UE in uplink and downlink.

•The UE is known on cell level according to its current active set.In this state, the UE shall•Use the connected mode measurement control information received in other states until new measurement controlinformation has been assigned to the UE

•perform measurements and transmit measurement reports according to the measurement control information

•depending on UE capabilities, monitor the FACH to receive any broadcast messages

•monitor a DSCH in downlink for user data and signalling messages when instructed by UTRAN.

The CELL_FACH state is characterised by:

•Neither an uplink nor a downlink dedicated physical channel is allocated to the UE.

•The UE the continuously monitors a FACH in the downlink

•The UE is assigned a default common or shared transport channel in the uplink (e.g. RACH) that it can use anytimeaccording to the access procedure for that transport channel

•The UE is known on cell level according to the cell where the UE last made a cell update.In this state, the UE shall

•Use C-RNTI assigned in the current cell as the UE identity on common transport channels unless when a new cell isselected

•monitor a FACH to receive signalling messages or user data addressed to the UE or any broadcast messages

•acquire system information on the BCH and use the common physical channel and transport channel configurationand measurement control information according to that system information when no UE dedicated commonphysical channel and transport channel configuration and measurement control information has been assigned to theUE

•by default, use RACH for transfer of signalling messages or user data in the uplink according to the random accessprocedure

•use transport channels of type CPCH or USCH (TDD only) for transfer of signalling messages or user data in theuplink accordong to the access procedures when instructed by the UTRAN

•perform cell reselection and upon selecting a new UTRA cell, initiate a cell update procedure

•upon selecting a new cell belonging to another radio access system than UTRA, enter idle mode and make an accessto that system according to its specifications

•perform measurements and transmit measurement reports according to the measurement control information

Regards,

Hello, dear. 

DCH (dedicated channel) dedicated channel. DCH frame protocol provides the following services: - transmitting transmission block set through Iub and IUR interfaces - transmitting outer loop power control information between SRNC and node B - supporting transmission channel synchronization mechanism - supporting node synchronization mechanism - transmitting DSCH TFI from SRNC to node B - transmitting receiving time offset from node B to SRNC (TDD) - transmitting wireless interface parameters from SRNC to node B DCH frame protocol provides the following  services:- Transport  of TBS across Iub and Iur interface.- Transport of outer loop power control information between the SRNC and the Node B.- Support of transport channel synchronisation mechanism.- Support of node synchronization mechanism.- Transfer of DSCH TFCI from SRNC to Node B.- [3.84 Mcps TDD - Transfer of Rx timing Device from the node B to the SRNC.] - transfer of radio interface parameters from the SRNC to the node B dedicated transmission channel is used to send all information above the physical layer of a specific user, including data of actual service and high-level control information. Because the information content sent on DCH is invisible to the physical layer, the same processing method is adopted for the high-level control information and user data. Of course, UTRAN has different physical layer parameter settings for control information and data. The main characteristics of the dedicated transmission channel include: fast power control, fast data rate change frame by frame, and transmitting to a specific part of a cell or sector by changing the antenna weight of the adaptive antenna system. The dedicated channel also supports soft handoff.

Fach (forward link access channel) is a forward access channel. A downlink channel for transmitting a small amount of data without closed-loop power control. Common downlink channel without closed-loop power control used for transmission of relatively small amount of data. It is also possible to send packet data in fach. A cell can have multiple FACHS, but one of them must have a lower bit rate so that all terminals in the cell can receive it; that is, other FACHS can also have a higher data rate. Fach does not use fast power control, and messages sent must include in band identification information to ensure correct reception. RACH (random access channel) random access channel. A contention uplink channel used to transmit a small amount of data, such as initial access or non real time control or service data. A content based uplink channel used for transmission of relatively small amounts of data, e.g. for initial access or non real time differentiated control or traffic data. It can also be used to send a small amount of packet data from the terminal to the network. Normal system operation requires that the random access channel can be received in the whole expected cell coverage. Therefore, it means that the actual data rate must be low enough, at least for the system initialization access and other control processes. When RRC is just connected, it can be set up on private channel or public channel!

The establishment process is as follows.