Lots of factors should be considered in the WCDMA network construction. For example, the operators should carefully consider the following issues: The investment of construction capitals, the utilization of the existing network resources, the planning of the networking form, the smoothness of network upgrading, the implementation difficulty etc.., as described below.
10.2.1 CS Domain Construction Solution
The construction of the CS domain in the core network is always the focus no matter you upgrade the GSM network to the WCDMA network or upgrade the versions of different protocols in the WCDMA network. Presented below is about the solution of establishing a new WCDMA CS domain network.
The network resources of the current operators are abundant: TDM networks, IP networks and ATM networks. The bearer used for networking is decided by the condition of the specific bearer resources.
1. Solution 1: Building the CS network in the R99 protocol mode
When this solution is adopted, the original TDM transmission network can still be used for transmission of the R99 CS domain. As for the gateway and toll tandem equipment, you can upgrade the original GW or establish WCDMA equipment through stacking. The advantages and disadvantages of the two modes are already given in the previous chapter.
With the R99 construction solution, we can make the best of the resources such as transmission resources, gateway office and tandem office, the good compatibility with the original network equipment can be well guaranteed, and voice services can be provided with good QoS. Therefore, this solution is an economical and quick method to introduce the 3G systems.
2. Solution 2: Building the CS network in the R4 protocol mode
In building the CS domain through the R4 network which separates MGW from the MSC Server, you can use ATM/IP in the internal core network for transmission and use the GMSC for the conversion of voice codec as well as the conversion of ATM/IP to TDM.
The advantages of building the network through R4 are given as follows:
l It makes networking very flexible. You can configure the network capacity flexibly through MGW according to the local traffic and conduct centralized management and configuration through the MSC Server.
l It helps evolve to the future packet-based network. It is also helpful to save the bandwidth for transmission.
l With Trfo technology, the voice quality is improved and the voice codec equipment can be saved.
The disadvantages of building the network through R4 are as follows:
l It is not easy to reuse the original PSTN equipment, because the ATM/IP technology is also used in the transmission of the signaling and it is difficult to use the original signaling network. We need the signaling gateway to interwork with PSTN.
l With the separate architecture, we need to take the interworking between the MSC Server and MGW into consideration. However, the test of the compatibility may lead to the delay of the network construction.
3. Solution 3: Building the R99 network through R4 (with bearer and control separated)
In building the CS domain through the MGW and MSC Server of R4, the MGW and MSC SERVER are located in the same place when it comes to the construction; and the MSC Server accesses the MGW via the LAN.Other characteristics are the same as that of the R99 network. Therefore, solution 3 has the same advantages and disadvantages as solution 1.
Compared with solution 1, the major advantage of solution 3 is that it facilitates the transition to the R4 architecture as these facilities have been equipped with ATM/IP interfaces and the signaling processing capability for the R4 networking. A gradual transition is available through replacing boards or adding the corresponding MGW to enable the transition from the R99 to the R4.
10.2.2 PS Domain Construction Solution
The construction of the PS domain comprises two aspects: 1) the construction of NEs such as SGSN, GGSN, CG, and DNS; 2) the construction of the GPRS backbone network, namely the construction of the WCDMA PS domain backbone network.
1. Construction of the WCDMA backbone network
There are several construction methods for the WCDMA backbone network:
l By using the existing IP network.
l By using the existing ATM network.
l By using the private line network.
l By using the above methods for hybrid networking according to the local conditions.
Please note that if we use the existing IP network to build the backbone network, we need VPN and firewall for security, while if we use ATM, the firewall is unnecessary.
Normally in the PS domain of the WCDMA system, we can use the PVC of ATM or IP for bearing on the Gn interface.
l As for the provincial Gn interface, if we use the ATM network, it is enough to achieve high security without adding a firewall at the Gn interface, while if we use the IP network, the cost is relatively low but extra methods are needed to ensure the security, such as a firewall and IPSec encryption.
l As for the interconnection with external networks, the IP mode is always needed. However, for the interconnection with the GPRS backbone network, a firewall must be set.
2. Construction of NEs in the PS domain
The NE form of the PS domain and the basic functions are just the same no matter it is the GPRS network, or the R99/R4/R5 network of WCDMA.
As to new operators of WCDMA, they should establish a new WCDMA PS domain network.
As to the operators of the existing GPRS network, there are two solutions for building the PS domain NEs.
Solution 1: Upgrading the existing GPRS network to the WCDMA network:
As the difference between the GPRS network and the WCDMA PS domain network lies in their access networks, the SGSN-related interface modules need to change their Gb interface into the lu-PS interface.
l SGSN should support the access of the original BSS and the new RAN at the same time.
l SGSN, GGSN and CG should support the relevant procedures and services of GPRS and WCDMA.
l It is required to support the integrated billing of GPRS and WCDMA.
l It is not difficult for upgrading. You can save part of your investment.
l The original equipment has been tested on the network, so it enjoys higher stability as compared with the new equipment.
l The existing packet network architecture can be maintained to keep the overall stability of the existing network.
l It has little impact on the facilities in the original equipment room. We need not reconsider building any new equipment room or placing the facilities.
l Upgrading the old one may impact the existing network.
l Service availability after the upgrading varies with the original GPRS platform.
In fact, the difficulty of implementing this solution and the upgrading smoothness depend on the building ability of the GPRS network equipment. If the original
is developed on the basis of the GPRS protocol without considering the
subsequent transition to WCDMA, it will be very difficult to upgrade the GPRS
NEs, especially to upgrade it smoothly. You can do nothing but establish a new
suite of WCDMA PS domain equipment. On the contrary, if the original GPRS NEs
have powerful functions with good foresight and universal architecture, it can
be upgraded smoothly and save a lot of investment. GPRS NE
Solution 2: Establishing a new WCDMA packet network
If the original GPRS network cannot be upgraded smoothly, or it is not worthwhile to upgrade it, the operators can choose to establish a new WCDMA packet network.
The new WCDMA network can coexist with the original GPRS network at the initial stage, but it shall gradually switch the GPRS subscribers to the WCDMA packet network.
The disadvantage of this solution is that investment is needed in building the new equipment. It does not allow us to utilize the original equipment, and we still have to consider building new equipment rooms and placing these equipment.
10.2.3 Signaling Network Solution
1. R99 signaling network solution
1) Principles for signaling network construction
l High reliability of the equipment to ensure high security of the signaling network.
l Supporting dual backup of the network without single point failure to ensure high security of the signaling network.
l Powerful processing capability to adapt to the expansion of network scale and the growth of services.
l Low delay to ensure the service connection speed.
l Even load distribution to evenly plan the flow of load on the signaling network.
The load of the trunk signaling is light, and if each site has a direct signaling link, there will be too many signaling links; therefore, in most regions the signaling link is available only for the interconnection between the TMSC and the STP equipment, while the interaction between the TMSC and other sites is completed via the STP.
Generally, the dual-net dual-plane networking mode is adopted for the signaling network to ensure high security of the signaling network. STP equipment should have powerful processing capability to adapt to the network scale expansion and the service growth. It also should have low delay to ensure the service connection speed. In addition, the signaling links in the 3G mobile networks need to be organized carefully to avoid too many signaling transfer points. At present, the networking modes of the fixed and mobile signaling networks are the same. A direct signaling link should be set between two SPs with a large information volume, especially between the MSC/VLR and the local HLR when the transmission condition permits. There are two SCCP addressing modes: GT and DPC addressing modes. The GT addressing mode is adopted for inter-province networks while the DPC + SSN addressing mode for intra-province networks. In this way, the work amount of GT translation of the STP equipment can be greatly reduced.
2) Difference between the mobile signaling network and the fixed signaling network
The hierarchical signaling network is suitable for the fixed signaling network. At present, for the fixed signaling network a pairs of independent HSTPs are generally set in the provincial capital to form a dual-plane mesh network. For each of the local networks, a pair of LSTPs are available to transfer the PSTN signaling and the intelligent network information. An important application of the LSTP is to complete the inter-office conversion from mesh signaling network to hierarchical signaling network, which greatly reduces the amount of direct signaling links and improves the reliability. However, the mobile network features high capacity of local SPs, few SP sites and large signaling flow between local SPs, so it is better to adopt direct links between SPs, and in that case the signaling link convergence functionality of STP is not applicable to the mobile network, rather, the signaling networking of mesh topology is recommended for the local mobile network.
If the STP network of PSTN directly serves as the mobile signaling network, it functionally makes no difference except the following issues:
l The mobile communication system use the GT addressing mode largely and the processing of GT code is in the SCCP layer, so the STP network of PSTN need to be upgraded if not support SCCP function.
l The signaling of the local mobile network usually adopts direct links and the signaling to other local networks should pass the STP. However, the LSTP of the current PSTN is only used to transfer the signaling of the local network, thus all the signaling information to the LSTP should be transferred from the HSTP, which increases the load of the HSTP (that is, it has to transfer both the domestic roaming signaling and the intra-area signaling).
l There exist multiple intra-area signaling transfers, which causes time delay of the connection and affects the QoS to some extent.
3) Signaling network construction solution
Two networking solutions are available for the mobile signaling network:
Solution 1: The mobile equipment of each local network is only connected with the LSTP, which is used to transfer the intra-province signaling and hand over the inter-province signaling to the HSTP.
Advantages of solution 1 are simple structure and easy capacity expansion. Its disadvantage is also obvious: The inter-province signaling passes LSTP and this increases the load of the LSTP and the time-delay of inter-province roaming and call signaling processing.
Solution 2: The mobile equipment of each local network is connected with both the LSTP and the HSTP. The mobile equipment of the local networks recognize the intra-province signaling and the inter-province signaling to forward the signaling to different STPs for processing.
The advantage of solution 2 is that it reduces the load of LSTP and the signaling processing delay. Its disadvantage is that the capacity expansion is not convenient. With the increase of the network nodes, the utilization of the signaling links to the HSTP will be inefficient to some extent.
To fully capitalize on the existing HSTP resources and create new LSTPs to transfer the inter-province signaling, solution 1 with minimum changes may be adopted for networking.
4) Introducing high speed signaling links to the mobile network
At present, the SS7 network of the TDM-based mobile network aims to providing transmission bandwidth and reliability for the signaling network and further improving the signaling network management functions. From the perspective of network development, the capability of 3G signaling networks is much greater than that of GSM. That is, with the expansion of the signaling network capacity and scale, and the improvement of the signaling network reliability, the signaling protocol needs to be upgraded further, so as to enrich the services provided to the subscribers.
At present, the SS7 mobile signaling network generally adopts 64Kbit/s signaling links. Restricted by the SS7 protocol, the maximum signaling bandwidth between the nodes of the mobile signaling network is only 1024K (64K´16), which cannot satisfy the requirements for the signaling bandwidth between the MSC and the HLR. The mobile signaling network needs to adopt 2Mbit/s high speed signaling links as soon as possible to increase the bandwidth between the nodes.
To further improve the network reliability, the transmission paths of the signaling network should be dispersed as much as possible and the signaling network management functions should be enhanced.
2. R4 signaling network construction
In the 3G R4 stage, the bearer of call-independent signaling can only be the TDM-based SS7. It may be upgraded to the optional IP-based SIGTRAN or still adopt the TDM-based SS7. According to the 3GPP specifications, the SCCP/TCAP-based MAP/CAP signaling must adopt the M3UA/SCTP SIGTRAN bearer mode, and M3UA provides the upper SCCP with the primitive interface completely equal to the MTP3 protocol, so no matter the 3G R4 signaling bearer is TDM/SS7 or IP/SIGTAN, it is required to construct a hierarchical private roaming signaling network with network layer signaling transfer capability, that is, the STP network.
If the hierarchical structure matches and the TMSC server processing delay is enough, the IP STP equipment may be integrated with the TMSC server equipment physically, so as to effectively reduce the networking investment of the operators.