The main interface of GPRS

1 GPRS wireless interface between UM MS and BTS need to transfer a large number of user data and control signaling, different kinds of information from different logical channel transmission, logical channel mapping to the physical channel. It includes the following set of logical channels for the transmission of public control signaling.

Packet random access channel (Prach, Packet randem access Channel):

Only exists with Uplink, MS is used to initiate uplink transmission data and signaling information. Packet access burst and extended packet access burst use the channel.

Group Paging channel (PPCH, Packet paging Channe1):

Only exists on the downlink link. Used for paging MS before downlink data transfer. Can be used for paging circuit switching business.

Packet Access permission channel (Pagch,packet access Grant Channel):

Only exists on the downlink link. Before sending a group, the network sends resource allocation information to MS during the group transfer establishment phase.

Packet Notification channel (Pnch,packet Notification Channel):

Only exists on the downlink link. The network uses the channel to send notification information to MS before the send point to Multicast (ptm-m) grouping.

(2) Packet broadcast control channel (PBCCH, Packet broadcast controls Channel)

Only exists on the downlink link. Broadcast packet data-specific system information.

(3) Packet Service channel (Ptch, Pachet traffic Channe1)

Packet Data Service channel (Pdtch,pachet-Traffic Channe1:

Used to transfer grouped data. In ptm-m mode, the channel can only belong to a MS or a group of MS at a certain time. In a multiple-time gap operation, one MS can transmit a single grouping in parallel with several pdtch. All data packet channels are one-way, for mobile-initiated transmission is uplink (pdtch/u), for mobile termination packet transmission is downlink (PDTCH/D).

Group-related control channel (PACCH, Packet Associate Controls Channel):

It carries signaling information about a specific Ms. These signaling information includes confirmation, power control, and so on. It also carries resource allocation and redistribution messages, including the capacity of the allocated pdtch and the capacity of the pacch to be allocated. When PACCH is shared with Pdtch, it is a resource that has been allocated to MS when it is shared. In addition, when a MS is in the packet transmission, it is possible to use PACCH for the transmission of circuit switching services. (Computer science)

In summary, the GPRS system defines a logical channel optimized for grouping data, as shown in table 5-1.

2. Wireless Interface um

GPRS wireless interface um can be described using the graph 5-1 GPRS ms-network Reference Model. The communication between MS and network involves several levels of physical RF (RF), physical link, Wireless Link control/media access control (RLC/MAC), logical link control and subnet dependency.

Figure 5-1

GPRS ms-Network Reference Model data link layer includes RLC and Mac two sub layers. The RLC/MAC layer provides services for transmitting information through the GPRS wireless interface. These features include a back-error correction process. The MAC layer provides a way for multiple MS access to shared media. The RLC/MAC layer uses the services provided by the physical link layer and provides services to the upper level (LLC). The physical RF layer is defined by the GSM05 series standard, including the following: characteristics of carrier frequency and GSM channel structure, modulation mode of transmitting waveform and data speed of GSM channel, characteristics and requirements of transmitter and receiver.

(2) Physical link layer

The physical link layer runs above the physical RF layer, providing physical links between the MS and the network. The goal is to transmit information through GSM's wireless interface, including the RLC/MAC layer. The physical link layer supports multiple MS sharing a physical channel.

The physical link layer's control function provides the services needed to maintain communication capabilities. In GPRS does not use the network control the more area to switch, but is by the MS Execution Community's afresh choice.

Layer function

A. Physical link-level responsibilities

Forward error-correcting code, detection and correction of the code word sent and provide error code word instructions, block interleaving, in the TDMA frame four consecutive bursts of orthogonal interleaving and provide detection of physical link layer congestion process.

B. Control function of physical link layer

The synchronization process includes the determination and adjustment of MS Timing advance;

Wireless Link signal quality monitoring and evaluation process;

The process of community selection and re-election;

The power control process of the transmitter;

Battery power management processes, such as discontinuous receive (DRX) processes.

Wireless block structure

Transmission of different data and control information has different wireless block structure, block structure contains the MAC layer head, RLC data block or RLC/MAC layer control block, in general, including 4 normal bursts, as shown in Figure 5-2.

Figure 5-2

The RLC head of the wireless block structure contains different control fields in the uplink and downlink direction, and the RLC is variable length. The RLC data field contains one or more LLC PDU data characters (8 bits). The Block check sequence (BCS) is used for error detection.

The Rlc/mac Control information field contains a Rlc/mac control information.

Channel coding

The packet data channel defines 4 kinds of packet data coding schemes, CS-1 to CS-4. The encoding block structure is shown in Figure 5-3 and figure 5-4. In addition to Prach, ptach/u, all other packet control channels generally use CS-1. For Prach access bursts, two coding schemes are specified. Ms must provide all of the coding schemes, while the network end simply provides CS-1.

Figure 5-3 Block structure of CS-1, CS-2 and CS-3

Figure 5-4 the CS-4 block structure defines 4 coding schemes for the wireless block carrying RLC.

The first step in the coding process is to attach a block check sequence (BCS).

For CS-1, CS-2 and CS-3, the second step includes the uplink status flag (USF) pre-coding (except for the CS-1), with a 4-bit tail code, a half rate convolutional code, which is truncated to provide the desired encoding rate.

For CS4, error-correcting codes are not encoded.

They use the CS-1 encoding scheme.

C. Parch Channel coding

Parch can use two bursts: 8-bit information to access burst or 11-bit information for an extended access burst. MS supports two kinds of bursts.

For 8-bit bursts, the channel coding scheme uses the same encoding scheme as the random access channel defined in GSM05.03.

The 11-bit burst coding scheme uses a 8-bit-coded truncation code scheme. The time advance process is used to derive the correct time advance value so that MS can transmit the wireless block on the uplink. It consists of two parts: initial time advance estimate and continuous time advance update.

The initial time estimate is based on a single access burst that carries a packet channel request. The estimated time value is carried to the MS after the packet uplink assignment or the packet downlink assignment. MS uses this value to transmit over the uplink until the continuous-time advance update process provides a new value.

Continuous advance update in packet transfer mode MS uses continuous time advance update process. The continuous-time advance update process is carried by PTCCH assigned to Ms. In the uplink (downlink) packet transmission, in the packet Uplink (downlink) Link allocation information, assigns to the MS time advance instruction and the PTCCH. (3) The Media intervention control and the Wireless Link Control layer (MAC/RLC) RLC function defines the process of selective retransmission of RLC data blocks that are not successfully sent. The Rlc/mac feature offers two modes of operation, which are not validated and validated.

Layer functionality provides efficient data and signaling multiplexing functions on both uplink and downlink, and reuse control is left to the network end. In Downlink, multiplexing is controlled according to the scheduling mechanism; In the uplink, multiplexing is controlled by allocating the media to a single user.

For mobile-initiated channel access, competition rulings, including conflict detection and recovery, are conducted.

For mobile termination of the channel access, including packet access queues.

Priority processing.

Operation mode

Generally, in GPRS, multiple MS and network share media resources, namely PDCH. GPRS wireless interface consists of asymmetric and independent uplink and downlink links. Downlink is the transmission from the network to multiple MS, and no competition rulings are required. The uplink is between media shared between multiple MS and requires a competitive adjudication process.

PLMN allocation of wireless resources and MS use these resources can be divided into two parts: PLMN in a symmetrical manner to GPRS allocation of wireless resources (uplink and downlink). Wireless resources for point-to-point, point-to-point, multicast, and group-call services using uplink and row links are independent, and uplink and downlink-related allocations are allowed to support the transfer of data simultaneously in two directions by a few Ms. A MS can also be assigned several pdtch.