10.3 Internetworking Solution

For the interworking with other networks, the first thing to consider is the service interworking. Presently, the services provided by the operators include fixed telephone service, IP access service, IP toll service and mobile phone service. In addition, the multimedia video phone service may be a key service of the 3G networks, for which the interworking should be considered.
For the interworking with other networks, the second thing to consider is the allocation of key resources to services, which affects the QoS. For the voice communication, it is required to consider the proper configuration of EC resources while preventing other services from wasting the EC resources.
For the interworking with other networks, it is required to consider the security and reliability of the whole network. It is better for the gateway exchange construction to follow the principles of large capacity and paired construction.
For the interworking with other networks, it is better to access the nearby network to affectively avoid the alternative routes of the mobile subscribers. The remote network may also be accessed for this purpose, which is useful for the charging settlement and the full utilization of EC functions.

10.3.1  Numbering Plan

Now, the numbering plans adopted by the mobile communication include the network numbering plan and the PSTN numbering plan.
The net numbering plan is the numbering plan adopted in current China mobile communication system. Both China Mobile and China Unicom adopt this kind of numbering plan. No further description is given for the network numbering plan in this document.
The PSTN numbering plan is to number the mobile subscribers and equipment with the original PSTN coding scheme. The plan is consistent with the PSTN as to the dialing scheme with a short number and easy availability of the number.
Advantages of the PSTN numbering plan:
l   The types of the numbers are the same, so such original equipment of PSTN as the tandem exchange and the toll exchange may be still adopted by only adding relevant data configurations.
l   Compared with the network numbering plan, its number length is shorter for the convenience of dialing by fixed subscribers.
However, the following problems may occur in this kind of numbering plan:
l   Alternative routes may occur when the called is roaming.
l   After the separation of North Telecom and South Telecom, this plan may obtain different number resources from different regions, or be different from the dialing habits of subscribers. It is not certain whether subscribers can accept such a plan.
The situation of severe competitions among the operators in the China domestic communication market comes into being. The operators build their independent networks, which interconnect via gateway exchanges. The adoption of the PSTN numbering plan will greatly increase the data configuration pressure of some gateway exchanges, such as the international incoming calls or the international roaming.
With MAP routing analysis and call routing analysis, the adoption of the net numbering plan is helpful for the network selection with simple data configurations. It is better to adopt this numbering plan for the call among some international gateway exchanges or some operators, while the PSTN numbering plan is useful to select the geographical position of the call or MAP signaling routes.

10.3.2  Gateway Exchange Solution

1. Requirements for gateway exchange construction

1)      Have such features as large-capacity trunks, powerful signaling processing capacity and abundant voice resources, thus satisfying the development trend and construction requirements of the gateway exchange for “large capacity & few offices”.
2)      Have a variety of interfaces (at least E1 interfaces available) to support diverse services and access modes; and provide ISP with the PRI interface to adapt to the fast development of the Internet.
3)      Have echo cancellation function to reduce the echo resulting from the two-wire/four-wire transformation between mobile calls and fixed calls with the relevant echo canceller configured in the gateway exchange.
4)      Have the built-in SDH to provide the 155M interfaces for implementing the centralized traffic, few office directions and large trunk groups in the gateway exchange, so as to reduce the networking cost, the utilization area of equipment room and the system power consumption.
5)      Have the ability to analyze the calling number of the incoming trunk.
6)      Support flexible networking: For the local network without LSTP or enough LSTP capacity, the gateway exchange may be served also as LSTP; for the mobile network providing abundant services, it is required to support 2M signaling or multi-signaling point technology to satisfy the high capacity requirement of the signaling links; For the exchange with over 4096 circuits in a single office direction, the switching equipment must support multi-signaling point technologies.
7)      Have the SSP/IP function, the CAMEL and upgrading capabilities for the convenience of CAP capability expansion according to the service requirements.

2. Gateway exchange construction solution

The networking of the GMSC is very simple. The GMSC is usually the external interface exchange of an operator. For the outgoing calls, it is to transfer the voice channels out directly; for the incoming calls, it needs to inquire the MSRN of the called, and then connect the call according to the MSRN. At the early stage of construction, generally the GMSC is also served as the VMSC. With the increase of VMSCs, independent GMSC construction gradually begins (if there are more than three VMSCs) to reduce alternative voice channels between these VMSCs.
For the former mobile operators, construction of the mobile gateway exchange is to build new ones and upgrade old gateway exchanges. It is required to consider the networking setting problems of the gateway exchange, that is, the planning of the gateway exchange.
For the operators that own fixed networks, the following solutions are available for the construction of their mobile gateway exchange:
1)      Solution 1: The fixed gateway exchange is integrated with the mobile GMSC by upgrading the fixed network gateway exchange
The following functional upgrading should be carried out for the fixed gateway exchange:
l   Increase MAP functional modules for route enquiry.
l   Increase EC functional modules.
l   Increase the functions of CAMEL3 SSPs.
l   Increase relevant billing functions.
l   Enhance the number analysis function.
If only partial upgrading can be achieved for the current fixed gateway exchange, some local networks should be set separately and some may be integrated. For the office that can be upgraded, it is also necessary to check the current traffic load of the gateway exchange. If the traffic load is heavy, the switch should be checked to find whether capacity expansion can solve the load problem. For this reason, the following requirements should be satisfied before carrying out the upgrading:
l   The current traffic load is light.
l   The current traffic load is heavy but it may be solved through capacity expansion.
l   The equipment vendors ensure the exchange can be upgraded smoothly.
Advantages of the solution:
l   The GW and GMSC integration is in accordance with the network construction trend of large capacity and few offices. Especially at the early stage of networking, the existing resources of the fixed gateway exchange are utilized to carry out efficient networking, thus shortening the construction cycle and reducing the construction cost.
Disadvantages of the solution:
l   It requires to upgrade the fixed gateway exchange, which may affect the existing network.
l   Part of the gateway exchange cannot be upgraded, which makes the networking structure unclear.
2)      Solution 2: Build a new 3G mobile gateway exchange for the fixed network
This solution is to build a new 3G mobile gateway exchange separated from the fixed gateway exchange.

Advantages of the solution:
l   There is no change to the old gateway exchange, no risks from the upgrading, and no impact on the existing fixed network from the traffic load.
l   The networking structure is clear, which is helpful for future network optimization and upgrading.
l   It is helpful for the integrated operators to carry out independent operation management and independent accounting of the mobile and fixed networks.
l   The EC allocation and utilization is clear, which is helpful to reduce the EC costs.
Disadvantages of the solution:
l   At the early stage of construction, it causes the trunk waste; and compared with the integrated gateway exchange solution, it increases the constructions costs and operation costs of the office and its auxiliary equipment.
l   Although the integrated operators have fixed telephone networks, they treat their mobile phone networks the same as those of other mobile operators with two GWs passed, which increases the links of the call and affects the network quality.
Generally, independent accounting is required for each internal network of the integrated operators. At present, the interconnection and accounting of the internal networks are carried out by building their respective gateway exchanges. Thus, each private network needs its own independent gateway exchange, which results in repeated investment and does no good to the unified internal accounting.

10.3.3  Echo Canceller (EC) Configuration Solution

The EC configuration is critical for the interconnection between the 3G mobile network and the PSTN. The EC has the automatic detection function to detect the modern signals and can disable the EC function automatically, so it will not affect the data service functions. However, the equipment cost is fairly high. To configure EC resources where unnecessary will increase the costs of network construction.
EC should be configured as near as possible to the fixed telephone. Generally, the EC can cancel 64 ms echo of the toll calls within about 6500 km. Echo problems occur if the distance is more than this figure.

1. Access mode of the EC

MSC provides two types of echo cancellers: Embedded echo canceller and independent echo canceller.
The embedded echo canceller is based on the concept of exclusive use of resources, that is, one trunk circuit exclusively occupies one echo canceller. When the E1 signals access the MSC, the signal echo cancellation function is implemented.
The independent echo canceller is based on the concept of resources sharing. All the echo cancellers are placed in the ECPOOL (Echo Cancellation Resources Pool). The echo canceller is occupied upon request and then released after use for reoccupation by other connections. The number of independent echo cancellers may be configured flexibly according to the traffic and the number of trunk equipment generating the echo to maximize the resources sharing.
From the perspective of system resources occupation, the ECPOOL should occupy the interface frame slot. For the call connection that requires echo cancellation, if an independent echo canceller is adopted, then four switching timeslots will be occupied in the central switching network: One is connected to the incoming E1 timeslot, one to the outgoing E1 timeslot, and the other two to the echo canceller as shown in the following figure. And if an embedded echo canceller is adopted, then the connection is the same as the common call connection with only two switching timeslots occupied: One is connected to the incoming E1 timeslot, and the other to the outgoing E1 timeslot shown in the following figure. That is, the independent echo canceller realizes the global sharing of echo cancellation resources by occupying more system resources than the embedded echo canceller. In particular, the EC is usually configured on the GMSC equipment with great capacity. The shared EC will greatly reduce the number of E1s that can access the GMSC.

Although the embedded EC can save the switching resources, sometimes the ECs are wasted. For example, the data services (including dial-up access and multimedia calls) are converted with IWF equipment and need no EC to cancel echo any more. In such a case, EC will not affect these services, but it is a waste of the EC processing resources. The embedded EC is suitable for the GSM network, in which there are few such services and little waste of EC resources.
In the other case, PSTN calls the mobile subscriber and then forwards the call to another PSTN subscriber, thus forming an end-to-end PSTN-to-PSTN call and causing a waste of the embedded EC resources.
The adoption of ECPOOL may cause a waste of EC resources, because the TUP protocol and ISUP protocol on the network do not support the EC processing. In this way, the GMSC with ECPOOL resources does not know whether the exchanges in the call route have applied for EC. Incorrect judgment will cause excessive or insufficient EC resources to be requested, thus lowering the communication quality.

2. Configuration of data service channels

The data service, multimedia service and fax service need no EC equipment. PSTN calls the mobile network, and the call is forwarded to the PSTN again without passing the EC equipment. When interconnecting with the PSTN, the data service and the voice service may have their respective trunk groups or office directions, with one for speech call and one for data service or multimedia service. This requires that the GMSC should have the ability to select different routes for the data service and the voice service.

If the data or multimedia traffic is not large, then data services and voice services may not be separated. In the current GSM network, they are usually not separated. However, the 3G multimedia service is considered as a key service with wide application, so it may be separated to some extent.

10.3.4  Routing Mode

The routing mode is considered generally from the perspective of the caller.

1. Access to the nearby network is recommended for internal calls of the operator

Considering the fact of internal calls and all revenues to the same operator, technically the mode of accessing the nearby network is recommended.
The advantage of this mode is that it may effectively reduce the alternative routes of the mobile services. Its disadvantage is that it does no good to the fully play of the EC function when the mobile network calls the toll fixed network.

2. Access to the remote network is recommended for the calls among operators (a subscriber of the local network calls a subscriber of other networks)

In this mode, the toll service adopts its own toll network, thus achieving the maximum benefits during internetwork settlement.

10.3.5  R4 Interworking

The 3G R4 network is characterized by the separation of the MSC server from the MGW and the use of ATM/IP as the transmission bearer. Just like R99, the R4 network needs the interworking of the bearer connections and the conversion of the user plane media stream formats, as well as the echo control and the interworking of control layer protocols. As to the service interworking mode, R99 and R4 are almost the same.
On the whole, the interaction between the 3G R4 core network and the PSTN is completed via the ISUP signaling and trunk signaling. Just like the traditional core network, 3G R4 should be configured with a dedicated gateway exchange to fulfill the interworking requirements as described above, so as to simplify the internetwork interactive connections and reduce the number of interactive points for the convenience of internetwork settlement and interworking resources management. Based on the architecture of separating the 3G R4 core network bearer from control, it is clear that the GMSC server undertakes the signaling interworking in the application layer of the control plane. The signaling bearer interworking is completed by the independent signaling gateway SG equipment or GMSC server, while the interworking function between the bearer connection and the multimedia layer depends on the MGW equipment.

In the traditional mobile network and the PSTN interworking model, the interworking between the signaling control plane and the bearer plane is implemented by the centralized GMSC with large capacity. In the 3G R4 model where bearer is separate from control, the same MSC server with large capacity may control several MGWs, so the interworking may be more flexible according to the MGW capability and the peripheral networking conditions, for instance, the independent gateway MSC server or the integrated MSC server (that combines the local exchange and gateway exchange functions) controls the interworking between the multiple local MGWs that provide TDM interfaces and the local PSTN network.

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