In-depth discussion of key technologies of mainstream GPRS routing in the world

With the rapid development of GPRS routing in the world, it also promotes the update and upgrade of wireless routing technology. Here we mainly analyze the mainstream GPRS routing technology. With the development of communication technology and personalized and diversified user needs, people have put forward higher requirements for services provided by mobile networks. However, the existing second-generation mobile network (GSM) far from meeting the data business needs of users and mobile operators.

For users, due to the slow data transmission speed and long connection establishment time, the service cost of data services is still too expensive for most users. From the technical point of view, the existing wireless data service is based on circuit switching. In the GSM system, a service channel (TCH) is allocated only when the user needs it to improve the spectrum resource utilization. Once used, the channel is released. This brings great difficulties to the transmission of abrupt and large data volumes. If you can allocate TCH for abrupt data transmission on a wireless channel as needed to bind multiple time slots, the channel utilization and transmission speed will be significantly improved. For mobile operators, the current billing method is for voice services, measured by the call duration. This method is not suitable for the development of data services.

In view of the shortcomings in the GSM network, the International Telecommunication Union (ITU) proposed to develop the third generation mobile communication IMT-2000, to achieve broadband data services to achieve seamless global coverage, fundamentally solve the problems existing in the data application field of GSM. Although the research on the second generation of mobile communication has made great progress, the final application standards are still not unified. GPRS, as a new bearer service of GSM, greatly improves and simplifies the way the wireless network enters the grouped data network. It adopts the new encoding and multi-time-slot bundling technology on the wireless channel, at the same time, the grouping technology is used in the backbone network to better overcome the above problems in the GSM network. The advantage of using GPRS routing is that it can shorten the network access time and achieve a higher data transmission speed.

In GSM, the connection is usually established for a long time, and the data transmission speed does not exceed 9.6Kb/s. The connection time required for GPRS routing is shorter than that for GSM. The actual data transmission speed obtained by using CS1 and CS2 encoding can reach dozens of Kbit/s. In addition, the cost of using GPRS is only related to the amount of data transmitted on the Internet and has nothing to do with the time on the Internet. This feature ensures that the user attaches the data to the Internet for a long time, to protect the interests of users. In short, GPRS routing can effectively improve the utilization of wireless and network resources, achieve a higher data transmission speed, and access the grouped data network from the wireless data network in a simpler way, it also provides a traffic-based billing system.

GPRS mobility management

In GPRS, the mobility management process is similar to the GSM network. Each GPRS location zone (LA) can contain several routing zones (RA), and each routing zone is composed of one or several residential areas. MS (mobile station) in GPRS network, usually in IDLE (IDLE), STANDBY (STANDBY) or READY (READY) one of three states, the conversion process of MS. When MS is in the STANDBY and READY statuses, there are internal timers to monitor these two statuses. If no data packet is transmitted, MS can pass through the STANDBY state by the READY state, enter the IDLE state. Between IDLE and READY, you can initiate GPRS attach or GPRS to manage the mobility of MS. There are two types of location updates: Cell-level mobility management, it involves the process of updating the location of MS in different cells in the same routing area; the second is the update of mobility in the routing area (RA), which involves the process of updating the location of MS in different routing areas. Whether the system can effectively manage these two conditions is closely related to the current status of MS. When MS is in the IDLE (IDLE) state, it indicates that it is not in the GPRS network coverage area, and SGSN (Service GSN) does not have the valid location and routing information of MS. Therefore, user-related mobility management cannot be performed. If you want to change from the MS empty network status to the ready state, you must perform the GFR attach operation. If the operation succeeds, the MS enters the ready state.

When MS is in READY (READY) state, MS can perform mobility management in a small area. When cell routing is updated in MS, SGSN is automatically notified. When MS enters the new routing area, a route update request is sent to the corresponding SGSN. The message contains the old RAI, And the BSS sends the New Cell ID (CI) to the SGSN, SGSN generates a new RAI for SGSN. If MS is in the STANDBY status, it indicates that it has been successfully attached to the GPRS network, and MS only performs Mobility Management in the RAI routing area. When updating the routing area in MS, the SGSN should be notified. For cell updates in the same routing area, new cell messages can be obtained only when MS enters the READY state by sending a paging message through SGSN.

No matter whether MS is in the ready or STANDBY state, when MS enters the new routing zone, a route update request is sent to the corresponding SGSN, which contains the old RAI, the BSS sends the New Cell ID (CI) to the SGSN, which generates a new rai for the sgsn. In short, there are two kinds of GPRS Route Area updates: one is that the new and old routing areas are under the same SGSN jurisdiction, because the new and old RAI are in the same SGSN, SGSN has saved the necessary user documentation and does not have to notify other network element devices such as SGSN (Gateway GPRS Support Node) and HLR (Location Register), just assign a new P-TMSI to the user. Second, the new and old routing regions have different sgsns. The new SGSN changes MS and requires the old SGSN to send a PDP context to MS, then, the old SGSN notifies the GGSN about the new route. In addition, the HLR and MSC/VLR should be notified about the route information changes during the process. To sum up, GPRS mobility management involves two levels of management: HLR, MSC/VLR, and SGSN tracking associated with MS, trace the cells and routing Areas attached to MS.

GPRS wireless resource management

On the physical layer, GSM adopts the FDMA/TDMA access mode. The uplink and downlink use different frequency bands with an interval of 45 MHz. The 890-915MHz is used for uplink data transmission, and the 935-960MHz is used for downstream data transmission of BTS. the uplink and downlink bandwidths account for 25 MHz, and are divided into 124 carrier frequencies with a carrier interval of 200 kHz. A physical channel can be defined by frame numbers, time slots, and frequency hopping sequences. The basic wireless resource is a time slot with a length of 576.9 μs and a modulation rate of 270.833 kbit/s. Therefore, each time slot contains 156.25bit. Different from GSM, a GPRS route can be allocated with 1 to 8 time slots for one MS in a TDWA, And the upstream and downstream channels are separated, that is to say, GPRS allows a mobile phone to use multiple time periods for data transmission. This is a flexible channel allocation method and is conducive to the transmission of asymmetric data services.

In traditional GSM communication, MS occupies a fixed channel. However, GPRS routes only apply for channel allocation during data transmission. After the transfer is complete, the channel will be released. This allows for better allocation of wireless channel resources for burstable data services. Based on this feature, multiple users can share the same time slot. If a residential area supports GPRS, multiple physical channels can be allocated to the GPRS Service. The physical channel is called the grouped data channel (PDCH ). PDCH can be allocated from all channels in a cell, and GPRS can be dynamically adjusted based on the current service load and priority. In addition, these cells can support both GPRS and non-GPRS bearer services.

GPRS Logical Channel

Logically, a series of logical channels can be defined for physical channels to implement functions, such as signaling channels, broadcast channels, and paging channels. PDTCH is used for data service transmission. Each Mobile Phone can use multiple PDTCH at the same time. PDTCH is a one-way, point-to-multiple-point signaling channel from BSS to MS. BSS transmits specific information to the GPRS mobile phone in the residential area through this channel. In addition to GBPS information, PBCCH can also broadcast important circuit exchange service information, so GPRS/GSM mobile phones do not have to accept BCCH channel messages. PCCCH is a two-way, point-to-Multi-Point signaling channel for transmission of MS incoming signaling, such as the allocation of wireless resources and paging messages, it consists of PRACH, PAGCH, PPCH, and pn4subsignaling. PRACH is used to request one or more pdtchs for MS and to respond to paging messages. PAGCH is used to allocate one or more pdtchs to MS. PPCH is used to send paging messages to MS, the message priority is higher than that of downstream data services. PNCH is used to notify MS of transmission of PIM messages.

The independent control channel is a two-way, point-to-point signaling channel, including PACCH and PTCCH. The PACCH is always allocated to one or more PDTCH together with one or more PDTCH, And the signaling message is sent, for example, confirm or transmit power control information to a specific MS. PTCCH is used for adaptive timing synchronization. The coordination between circuit switching and group switching logical channels is very important. In a cell, when PCCCH is ineffective in the cell, MS can use the gsm ccch channel to send grouped data packets. When PBCCH is not available, MS can obtain wireless network information through the BCCH channel in the GSM network.

Ing between logical channels and physical Channels

The ing between logical channels and physical channels involves two aspects: frequency ing and time ing. Frequency ing is based on the TDMA frame number, and the frequency is pre-allocated to BTS. Time ing defines a complex Frame Structure Based on TDMA frames. A pdch consists of 52 TDMA frames, and four consecutive TDMA frames form a block. There are 12 blocks numbered as B0-Bll, two of which are retained to PTCCH, the remaining two TDMA frames are idle. How the Logical Channel maps to the B0-Bll is determined by the parameters in the Broadcast Channel in PBCCH. In addition to complex frames composed of 52 TDMA frames, GPRS routes also contain complex frames consisting of 51 TDMA frames, such as PCCCH and PBCCH carried by PDCH.

GPRS billing policy

GPRS allows GSM to be connected to external dedicated or Internet like a LAN. GPRS routing users can be attached to the Internet for a long time, but they can occupy or do not occupy network resources for a certain period of time. If the traditional long-time-based billing principle is adopted, the user will be redirected as soon as the data is sent, which limits the use of the user. GPRS adopts the new traffic-based billing principle. In order to better meet the needs of users, GPRS billing should be based on this principle, it is necessary to do further research on the following aspects. The upstream and downstream data volumes adopt different billing principles. GPRS can clearly know whether the data is from the user to the network, or from the network to the user, which provides us with a new billing policy, such as charging only downstream data, however, no fees are charged for upstream access. Based on the QoS billing principles. Users can get different discounts based on different service quality provided by the system. The service quality of the network is evaluated by indicators such as reliability, priority, throughput, and latency. For most users, it is very difficult to determine the discount rate based on the quality of service obtained by MS. They often only care about whether the service can be used in the Internet, rather than the quality of service (QoS ). However, some large communication companies will put forward some requirements on the quality of network operation. Once these requirements are not met, they will ask the operation companies to offer discounts accordingly.

Whether it is billed Based on the transmitted content. Currently, the GPRS routing network can only identify the user access name (APN), such as an Internet port, but the content transmitted from this port is usually unknown. In theory, although we can use a rough division of the APN to achieve this goal, it is actually very difficult, A better way is to use TCP/IP-based application software to further analyze the content. Billing is based on the MS location information. In traditional GSM billing methods, charging based on location information is an important standard. However, due to the long time of being attached to the Internet, there are many changes in the routing area. If you want to charge based on the location information, the system will have higher real-time requirements. Billing by time period. Different charging policies are adopted for different time periods, which can reduce the pressure on wireless channels when data and voice services are busy at the same time, it helps reduce the impact of the conversation audio service. GPRS billing should form a reasonable billing method based on the above factors. Compared with GSM phone numbers, ETSI defines a wide range of GPRS call details. The further question is whether the existing GSM billing system and the GPRS billing system can be integrated to adapt to new services, this requires in-depth research.