WCDMA OSI MODEL Radio Resource Control (RRC) Logical channels RLC RLC RLC (What is transmitted) Medium Access Control (MAC) Transport channels (How it is transmitted) Physical Layer Physical channels (Code,frequency,etc)
OSI reference model WCDMA protocol stack is based on OSI (open system interconnection) reference model OSI reference model ,developed by ISO, is a “set of seven layers”. Each of this layer performs certain tasks on the data to make it travel from one device to another over a network. These tasks at each layer, prepare it for the next layer. Layer 7: Application - This is the layer that actually interacts with the user. For example if the user chooses to transfer files or read messages , application layer interfaces with the operating system. Layer 6: Presentation - Layer 6 takes the data provided by the Application layer and converts it into a standard format that the other layers can understand. Layer 5: Session - Layer 5 establishes, maintains and ends communication with the receiving device.
Layer 4: Transport - This layer maintains flow control of data, means that if data is coming from more than one application, transport layer integrates it into a single stream for the physicalnetwork. It also provides error checking and recovery of databetween the devices.Layer 3: Network – This layer determines the way that the data willbe sent to the recipient. Logical protocols, routing and addressing arehandled here.Layer 2: Data link-This layer transforms the physical layer from araw transmission facility to a reliable link..Layer 1: Physical - This is the level of the actual hardware. It definesthe physical characteristics such as connections, voltage levels andtiming.
For WCDMA the lower 3 layers are most important:Layer 1: Physical layer- This layer is responsible for transporting dataacross the available physical path (radio link). Main jobs of physical layerare :1. Forward error correction(FEC)2. Interleaving3. Transport channel multiplexing4. Channel quality measurements5. Synchronization6. Spreading , scrambling, modulation
Layer 2 : Data link layer – It is the upper neighbor of the physical layer. 3GPP divides it into the radio link control (RLC) and the medium access control(MAC) layer. It delivers data block by block to layer 1 via transport channels (or fetches the data when receiving). In the other direction, layer 2 communicates with the adjoining network layer via Logical channels. The RLC and MAC layers have the following essential functions: 1. Mapping of logical channels on transport channels 2. Selection of the transport format based on current needs 3. Data encryption 4. Error protection as a function of the desired QOS 5. Acknowledgement of data blocks and requests for retransmission in case of errors
Layer 3 : Network Layer(RRC Layer): RRC layer is directly above the layer 2 and is responsible for radio resource control. The main RRC tasks include : 1. Broadcasting of system information 2. Establishment and clear down a RRC connection 3. Management of radio connections 4. Cell selection in IDLE mode 5. Querying and evaluation of physical layer measurements
WCDMA CHANNELS : Channels are simply a tool for specifying and describing the various communication and data related services. Various type of data is exchanged between UE and base station over the radio channel. To characterize this different type of data , we refer to different channels. The channels carried are categorized into three: logical, transport and physical channels. The logical channels define the way in which the data will be transferred, the transport channel along with the logical channel again defines the way in which the data is transferred, the physical channel carries the payload data and govern the physical characteristics of the signal. There are 3 different types of channels in WCDMA –
1. Logical channels : Used for communication between Layer 3 (RRC) & Layer 2 (RLC/MAC). Depending on the information content ,logical channels are characterized into traffic & Control channels. Control Channel (Signaling) Traffic Channel (User Data) Broadcast Control Channel (BCCH) Dedicated Traffic channels (DTCH) Paging Control Channel (PCCH) Dedicated Control Channel (DCCH) Common Traffic Channel (CTCH) Common Control Channel (CCCH)
Control channels: Broadcast control channel ( BCCH) : Point to multipoint ,DL It is a broadcasting channel that supplies all the UE’s with basic cell and network parameters(eg. Frequency lists , code lists etc). Paging control channel(PCCH) : Point to multipoint ,DL It transports paging information. All the UE’s are required to regularly read the information on the BCCH and on the PCCH Dedicated control channel (DCCH) : Point to point ,UL/DL This channel is used to carry dedicated control information in both directions. DCCH is provided in parallel to a DTCH for point to point signaling during an active connection Common control channel ( CCCH) : Point to multipoint ,UL/DL This bi-directional channel is used to transfer control information.
TRAFFIC CHANNELS: Dedicated traffic channel (DTCH): Point to point ,UL/DL DTCH is used to transport user data from Node B to a specific UE and vice versa. This means it is “dedicated” to a specific subscriber. This is a bidirectional channel used to carry user data or traffic. Common traffic channel (CTCH) : Point to multipoint ,DL It transfers dedicated user information for all , or a group of specified UE’s
Transport Channels: Characterizes “HOW” data is transported over the air interface . Each transport channel is accompanied by the Transport format indicator(TFI). The physical layer combines the TFI information of different transport channels to the Transport format combination indicator(TFCI). Organize & Pack data from different services from the higher layers for suitable transportation. Unpack incoming data and sort for delivery to upper layers Offers flexible data speeds and channel encoding Efficient usage of radio resources. They are all unidirectional. Transport channels are divided into common channels and dedicated channels.
Dedicated Transport ChannelDCH (Dedicated Channel) This is used to transfer data to a particular UE. Each UE has its own DCH in each direction Common Transport ChannelBCH (Broadcast Channel) This channel broadcasts information to the UEs in the cell to enable them to identify the network and the cell.FACH (Forward Access Channel) Downlink; Transports small amount of data from BCCH/CCCH/DCCH/CTCH to either a specific UE or over the entire cellPCH (Paging Channel) Downlink; Transports data from PCCH to UE over entire cell. This channel carries messages that alert the UE to incoming calls, SMS messages, data sessions or required maintenance such as re-registration.RACH (Random Access Channel) This channel carries requests for service from UEs trying to access the systemCPCH (Common Packet Channel) This channel provides additional capability beyond that of the RACH and for fast power control.
Mapping Logical To Transport Channels In The UEBCCH PCCH DCCH CCCH CTCH DTCHBCH PCH CPCH RACH FACH DSCH DCH In The Node BBCCH PCCH DCCH CCCH CTCH DTCHBCH PCH CPCH RACH FACH DSCH DCH
Physical Channels OverviewChan Name Uplink/ Descriptionnel Downlin kCPIC Common CPICH Downlin Reference channel for estimation of receivedH Channel k signal strength & quality; Fixed bit pattern which is stored in Physical layer; Determines coverage of a cellP- Primary Common Downlin This channel continuously broadcastsCCPC Control Physical k system identification and access controlH Channel information.S- Secondary Downlin This channel carries the ForwardCCPC Common Control k Access Channel (FACH) providingH Physical Channel control information, and the Paging Channel (PACH) with messages for UEs that are registered on the network.PRAC Physical Random Uplink Used by the UE for call setup; UE sends aH Access Channel preamble to Node B and on receipt, Node B sends an immediate ACK or NACK on
PCPC Physical Common Uplink Used by UE for packet data transfer involvingH Packet Channel small quantitiesDPDC Dedicated Uplink / Used for dedicated mode transfer of user dataH Physical Data Downlink (from DCH) & associated control data (from Channel DCCH) to & from UEDPCC Dedicated Uplink / Used for dedicated signaling to and from UE; ToH Physical Control Downlink ensure coherent detection it contains both Channel necessary pilot symbols & commands for fast power controlP/S - Primary / Downlink Used for WCDMA cell search and timingSCH Secondary synchronization; Never spread or scrambled Synchronization ChannelPDSC Physical Downlink Downlink This channel shares control information toH Shared Channel UEs within the coverage area of the node B.
PICH Paging Indication Downlin Paging messages are indicated to the Channel k intended UE by use of bit masks; On positive identification, UE starts reading S-CCPCH to find pageAICH Acquisition Downlin Determines whether system can be Indication k accessed or not; Uses simple bit Channel sequences for operationAP-AICH Access Preamble Downlin UTRAN uses this to confirm reception of - AICH k acquisition preamble sent by the UE on the PCPCHCSICH CPICH Status Downlin Channel informs UE about current Indicator Channel k availability of CPCH uplink transport channel for packet data useCD/CA - ICH Collision Downlin UTRAN uses this channel to inform UEs Detection / k about any packet collisions on the uplink Channel & also about the current assignment of Assignment the packet data channel. Indicator Channel
Mapping Transport To Physical Channels BCH PCH CPCH RACH FACH DSCH DCH PHYSICALP- S- PICH PCPCH PRACH PDSCH DPDCHCCPCH CCPCH AP-AICH AICH P/S - DPCCH CSICH SCH CD/CA-ICH