Analysis of GPRS protocol stack function

Zhang Bing Tao, is currently the Guangzhou Jinpeng Group Engineering Technology Center Technical Support Department Manager and senior system engineer. April 1998 graduated from Northeastern University, the state Key laboratory of continuous rolling automation, and received a master's degree in engineering. The same year joined Jinpeng Group Co., Ltd., mainly responsible for GSM, GPRS, CDMA pre-sales, after-sale technical support work and system network optimization work, has been responsible for the Jiangxi Unicom GSM/CDMA Network of multi-stage engineering construction, network optimization and project management work and Jinpeng cdma1x Shanxi Taiyuan Test Bureau design, Engineering construction, system testing and acceptance work. Currently studying in Huazhong University of Science and Technology Electronics and communications Engineering master.

A general load model of mobile data service

GPRS, WCDMA, CDMA2000 all belong to roaming mobile users of the host network, both in mobile users and remote data network to provide data connectivity. This article will be a mobile data service carrier network and based on GSM GPRS technology from the point of view of the protocol stack in-depth analysis.

Second, GPRS network structure and network element function

(1) Network topology structure of GPRS

(2) GPRS function of each network element

PCU function: The functional entity can be combined with BSC, or as a separate network element, and the GB interface between the SGSN and the standard interface defined by the specification, and the interface between BSC is the internal interface. Responsible for handling the data service of wireless channel; wireless data channel management and allocation, allowing multiple users to access the same wireless resources, user data compression, encryption and forwarding, but also has power control, quality control and selection channel coding scheme function, it to the GPRS data core network shielding the use of wireless technology. (Computer science)

SGSN function: It is the important component part of GPRS backbone network, it is the core part of packet switching, which is connected by frame Relay and PCU. function is similar to the MSC/VLR function of GSM system, SGSN not only handles signaling transmission in packet switching, but also carries out packet processing and transmission. Mobile management, security management, intervention control and routing function are implemented for Ms. That is, the information about the mobile data users currently active in the SGSN area is logged. such as location information, the current user information can be modified, deleted, etc., responsible for data users attach and detach, location updates, paging, authentication, encryption, etc. responsible for the establishment of logical links between MS and SGSN, Maintenance and release, responsible for routing selection and information storage and forwarding, generating original billing data.

GGSN function: GGSN internal maintenance GPRS backbone network, external can connect a number of data network such as Internet, Enterprise network, X.25 network, GPRS backbone network and external Data network gateway; In the GPRS data network position is similar to the traditional GSM network in the status of Gmsc , responsible for generating the original billing data of the data business.

Three, GPRS each interface stack of functions

The protocol stack is the process of encapsulating and marshaling information multiple times so that information can be transmitted between different entities.

Mac media access control: The MAC defines and assigns the logical channel of the air interface, and controls the mobile station to access these shared logical channels; the MAC-defined logical channel has a public control channel PCCCH: Control signaling; packet broadcast channel PBCCH: Broadcast system message; Packet Service Channel Ptch: Transfer packet data ; Packet Special control channel: Power control, timing and so on.

RLC Wireless Link Control: RLC to the Upper-level LLC Data Segmentation and reorganization for transmission on the logical channel, RLC through selective retransmission, to the upper layer to provide a reliable link, which according to the Wireless Link transmission capacity of 1527 bytes (the maximum length of the LLC frame) by CS (coding Scheme) to cut into different small blocks for convolution, interleaving, and other channel coding.

LLC Logical Link Control: LLC is an agreement between MS and SGSN. Based on HDLC Wireless Link Protocol, the transmission capacity of 1520 bytes, plus 7 bytes of overhead, a total of 1527 bytes; LLC provides one or more highly reliable, error-free, encrypted logical links (GMM,SM,SNDCP, etc.) to the upper level, which is distinguished by SAPI. LLC has nothing to do with the wireless interface protocol used by the lower level, isolating the wireless network down; the change of the lower GPRS wireless interface protocol will not affect the upper layer protocol and the network subsystem; The broken frame can be detected and recovered; a one-to-many address can be implemented (send messages to multiple MS) , the transmitted information can have different priority, that is, sequential control, flow control and so on. The LLC realizes the upward unification of the different protocols and is a key point to ensure smooth transition to 3G. Together with the SNDCP protocol, it is similar to the function of the PPP protocol between MS and PDSN in the CDMA1X network.

SNDCP Subnet Aggregation protocol: the various protocols of the extranet (IPV4, Ipv6, X.25, etc.) format unified as a protocol format, using NSAPI to distinguish between different applications; the IP packets or X.25 packets between the mobile station and the SGSN are split into multiple SNDC packet units to accommodate the MTU of the LLC, the packet unit is placed inside the LLC frame, the data is segmented and reload, and the data is compressed to save Air interface Bandwidth, responsible for TCP/IP head compression, data encryption and so on.

NS Network Service Protocol: Network Service protocol is based on frame relay network, including network layer and link layer function, transmission of BSSGP Protocol data unit between PCU and SGSN; PCU and SGSN may pass multi-hop Frame Relay network The logical Frame Relay link between BSS and SGSN is established through nsvc identification, and the LLC-PDU of multiple users are reused on these virtual circuits by load sharing; The layer is also responsible for NS-VC lock, unlock, reset, test and other NS-VC management functions. The mapping relationship between Nsvci and GBL and DLCI needs to be defined in the PCU and SGSN databases.

BSSGP (BSS GPRS Application Protocol): provides connectionless links between PCU and SGSN, carries upper-layer Data Information, LLC information, provides RLC/MAC-useful wireless-related information to BSS in the downlink direction, and provides SGSN from the upstream rlc/ Wireless related information obtained by Mac function, host routing information and QoS information between SGSN and PCU, support signaling management and group confirmation between SGSN and PCU, identify the community through Bvci, and Nsei by PCU, so through nsei+ Bvci can uniquely identify each BSSGP virtual connection.

BSSGP's business model

GMM Business Access point function: Responsible for GPRS mobility management. Indicates BSS paging MS; request SGSN to send MS Current wireless access capability to BSS; BSS sends SGSN status information about wireless interface such as poor link quality, MS Attachdetach operation etc.

NM Network management functions: Complete the Fluch operation, when mobile phone from a cell to another cell, SGSN notify BSS delete the original Bvci related data, and send data to the new Bvci; BVC lock, Unlock and reset, and the flow control of each BVC BSS sends information about traffic control to SGSN, SGSN control traffic to BSS.

L2 protocol: It can be Ethernet, ATM, and so on.

IP Network Interconnection Protocol: Network Interconnection Protocol, mainly completes the routing function, uses in the user data and the signaling route. The current use is IPv4. The openness of IP makes the application of the upper layer and the load-carrying network of the lower level not necessarily connected, making the application service and the carrying network technology develop in accordance with its independent direction, speeding up the speed of technology development.

TCP Transmission Control Protocol: Provides a reliable connection-oriented data transmission link that needs to be established before data is transferred. TCP is used to host GTP PDUs that require reliable data links, such as X.25. TCP provides the function of traffic control.

UDP User Datagram Protocol: provide non-connection-oriented, unreliable data transmission link, before data transmission without the need to establish a connection, the reliability of transmission data needs the upper application software to ensure. UDP is used to host GTP PDUs that do not require reliable data links, such as IP. UDP does not provide traffic-control functionality.

GTP is the GPRS Tunneling Protocol: The GPRS backbone composed of SGSN and GGSN is a pure IP network, the GPRS backbone can be constructed by the same method as the Internet, or by VPN on the existing IP network. GPRS backbone network, GTP can pass the user grouped data, can pass on the tunnel establishment, maintenance, release of the tunnel management information, you can pass on the response request, response, version does not support the management of information, these GTP signaling message is dependent on each message has a serial number, Each request message guarantees the reliability of the transmission due to response messages and associated timer. GTP allows multiple protocol packages, such as IP packets or X.25 packets, to be passed through in a tunnel between GSN components. Because each user can only have one pipe at a time, the TID used to differentiate between different GTP must be distinguished from the IMSI of different users and the NSAPI distinguishing between different applications. In the existing IP network through VPN to build GPRS backbone network can ensure that the IP message from outside the network is not in the backbone of the route, increased the system's concealment and message confidentiality; Because the tunnel can encapsulate arbitrary data, this can realize GPRS backbone network and a variety of external data network interoperability. In the GPRS backbone network, GTP relies on the lower UDP/IP protocol to realize, the port number is 3386, the interface with the GTP protocol has the same PLMN internal GSN node's GN interface and the GP interface between PLMN nodes between different GSN, as well as the GA interface between GSN and billing network management CG.

Four, GPRS application situation and the evolution route

As the 2.5G GPRS network download transmission rate is far lower than the expected rate, GPRS theoretically can reach the maximum rate of 171.2Kbps, but this is in all using CS-4 coding method and MS has 8 time slots simultaneously received, in the actual application of this is impossible. On the one hand, because the mobile terminal does not support the use of too much time slot (currently China's GPRS mobile phone generally support 3 of the next row gap), in addition, although GPRS introduced 4 kinds of coding schedeme coding scheme, but for the CS-4 code rate of nearly 1:1, There is little tolerance information for error detection and correction, that is, there is no error-correcting ability, which is impossible for wireless transmission, so the current actual rate of use is about 20Kbps, in the wireless environment than the poor and network busy sometimes compared to the speed of fixed telephone dialing. GPRS air interface Modulation technology is also relatively backward, still gmsk way, and so on. All these factors from the technical point of view restricts the development of GPRS.

3GPP standardization of WCDMA, TD-SCDMA is the evolution direction of GPRS. WCDMA, TD-SCDMA on the wireless interface and wireless access network speaking, is new. Wireless access network will be based on ATM or IP transmission technology, instead of the current based on frame Relay transmission technology, wireless interface using WCDMA, TD-SCDMA, instead of the current single TDMA technology, that is, the UM interface will occur "revolutionary" changes, all the load plane will have WCDMA or td- SCDMA protocol stack Instead, and the network layer and its upper application do not change. At the same time, in order to meet the higher rate requirements, the GB interface will introduce more advanced, faster ATM or IP transmission technology to replace the current FR technology. At the same time the data core network is basically unchanged. It can be seen that in the process of network evolution, the basic structure and function of the protocol stack is invariable, only introducing more advanced technology at each protocol level of various interfaces, the function has been further enhanced, and the efficiency has been further mentioned.

3GPP2 standardized cdma2000-1x, compared to IS-95 wireless interface and wireless access network are "evolution" type. 3GPP2 developed cdma1x release 0 and 3GPP R99 similar to the implementation of the circuit and packet business separation, the introduction of PCF (Group control function) and PDSN (Packet Data Service node) and other new network elements and corresponding interfaces, wireless side using cdma2000-1x technology, are compatible with IS-95 because they are all based on CDMA technology.

From the current situation of the global telecommunications operators, CDMA1X development is very fast. This not only depends on the technology and standards more mature, but also to a greater extent, depending on the cdma1x compatible IS-95, can be smooth upgrade on the IS-95 network, not only save investment, but also protect the interests of users. The reason for the postponement of WCDMA commercialization is not only that the technology is more complex, but also that the wireless side can not be compatible with GSM and GPRS. It is necessary to invest a large amount of money for network transformation, but also to introduce a dual-mode mobile phone to achieve the switch with 2G network.

V. Concluding remarks

Thus, we can understand the structure and function of GPRS protocol stack, understand the essence of GPRS to 3G evolution, and the interface and protocol layer changing in the process of evolution, and it is very helpful to understand the function Unit and protocol layer of CDMA1X network.